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## **Meet the editor**

Matheus Poletto is professor and researcher at Universidade de Caxias do Sul. He is an expert in composite science, working with thermoplastic composites and cellulosic materials. He has published over 80 scientific articles and conference papers and several book chapters. He obtained his bachelor's degree in Chemical Engineering and master's degree in Materials Science and Engineering

from Universidade de Caxias do Sul, Brazil, and his PhD in Materials Engineering from Universidade Federal do Rio Grande do Sul, Brazil. Professor Poletto currently works with cellulosic and lignocellulosic materials, studying the effect of fiber composition on the thermal and mechanical properties of polymer composite materials. He is also member of the Polymer Laboratory Research Group at Universidade de Caxias do Sul.

Heitor Luiz Ornaghi Júnior is professor and researcher in PGMAT (Programa de Pós-Graduação em Engenharia e Ciências dos Materiais) at Universidade de Caxias do Sul (UCS). He obtained his bachelor's degree in Polymer Technology from Universidade de Caxias do Sul, Brazil, and master's degree and PhD in Materials Engineering from Universidade Federal do Rio Grande do Sul (PPGEM), Brazil. He is an expert in polymeric composite materials, working mainly with thermosetting resins reinforced with cellulosic vegetal and/or synthetic fibers. He has published over 23 scientific papers and several book chapters.

 He currently works with mechanical, thermal dynamic mechanical, thermal and rheological properties of composites and nanocomposites, degradation kinetics of polymeric and cellulosic materials, polymer processing, and condensed matter physics of composite materials.

### Contents

### **Preface XI**


Chapter 6 **Microbial Cellulose — Biosynthesis Mechanisms and Medical Applications 133**

Wilton R. Lustri, Hélida Gomes de Oliveira Barud, Hernane da Silva Barud, Maristela F. S. Peres, Junkal Gutierrez, Agnieszka Tercjak, Osmir Batista de Oliveira Junior and Sidney José Lima Ribeiro

Chapter 7 **Crystalline Nanocellulose — Preparation, Modification, and Properties 159**

Mikaela Börjesson and Gunnar Westman


### Preface

Chapter 6 **Microbial Cellulose — Biosynthesis Mechanisms and Medical**

Chapter 7 **Crystalline Nanocellulose — Preparation, Modification, and**

Chapter 8 **Current Trends in the Production of Cellulose Nanoparticles**

Chapter 9 **Cellulose - Chitosan Nanocomposites - Evaluation of Physical, Mechanical and Biological Properties 229**

López-Dellamary, María E. Hérnandez and José A. Silva

**and Nanocomposites for Biomedical Applications 193**

Guillermo H. Riva, Joaquín García-Estrada, Brenda Vega, Fernando

Tamilselvan Mohan, Silvo Hribernik, Rupert Kargl and Karin Stana-

Mikaela Börjesson and Gunnar Westman

John Rojas, Mauricio Bedoya and Yhors Ciro

Chapter 10 **Nanocellulosic Materials in Tissue Engineering**

**Applications 251**

Kleinschek

Wilton R. Lustri, Hélida Gomes de Oliveira Barud, Hernane da Silva Barud, Maristela F. S. Peres, Junkal Gutierrez, Agnieszka Tercjak, Osmir Batista de Oliveira Junior and Sidney José Lima Ribeiro

**Applications 133**

**VI** Contents

**Properties 159**

Environmental concerns and global warming are the two main reasons for developing new materials mainly from renewable sources and studying the chemical and energy trends in‐ volved. Due to this fact, after more than 170 years of the discovery of the "sugar of the plant cell wall", there is an increasing demand of cellulose products from consumers, industries, and governments. The importance of cellulose as a potential material is widely recognized, and research in this field is expanding around the world. At this point, one question can be proposed: What makes cellulose such an important material? Cellulose is the most abundant biopolymer on earth and presents unique properties associated with its specific structure. Therefore, a solid knowledge is necessary to transform cellulose into useful materials and chemicals. Owing to this, this book intends to develop a deeper understanding about the fundamental aspects and current applications of cellulose and its derivatives in the interest‐ ed reader. *Cellulose - Fundamental Aspects and Current Trends* consists of ten chapters related to the cellulose and the nanocellulose structure, modification, production, dissolution, and application. This is a useful book for readers from diverse areas, such as physics, chemistry, biology, materials science, and engineering. It is hoped that this book will expand the read‐ er's knowledge about this fascinating polymer.

### **Matheus Poletto**

Center of Exact Sciences, Nature and Technology (CENT), Caxias do Sul University (UCS) Brasil

### **Heitor Luiz Ornaghi Júnior**

Materials Science and Engineering Graduate Program (PGMAT), Caxias do Sul University (UCS) Brasil

**Chapter 1**

### **From Cellulose Dissolution and Regeneration to Added Value Applications — Synergism Between Molecular Understanding and Material Development**

Poonam Singh, Hugo Duarte, Luís Alves, Filipe Antunes, Nicolas Le Moigne, Jan Dormanns, Benoît Duchemin, Mark P. Staiger and Bruno Medronho

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/61402

### **Abstract**

Modern society is now demanding "greener" materials due to depleting fossil fuels and increasing environmental awareness. In the near future, industries will need to become more resource-conscious by making greater use of available renewable and sustainable raw materials. In this context, agro-forestry and related industries can in‐ deed contribute to solve many resource challenges for society and suppliers in the near future. Thus, cellulose can be predicted to become an important resource for ma‐ terials due to its abundance and versatility as a biopolymer. Cellulose is found in many different forms and applications. However, the dissolution and regeneration of cellulose are key (and challenging) aspects in many potential applications. This chap‐ ter is divided into two parts: (i) achievements in the field of dissolution and regenera‐ tion of cellulose including solvents and underlying mechanisms of dissolution; and (ii) state-of-the-art production of value-added materials and their applications includ‐ ing manmade textile fibers, hydrogels, aerogels, and all-cellulose composites, where the latter is given special attention.

**Keywords:** Cellulose, dissolution and regeneration, fiber, hydrogels, all-cellulose composites

### **1. Introduction**

Cellulose was isolated for the first time by the French chemist Anselme Payen in 1838 [1], who extracted it from green plants and reported its elemental composition four years later [2]. Cellulose is the main component of the cell wall in higher plants, typically combined with

© 2015 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

lignin, hemicelluloses, pectins, proteins, and water. Apart from higher plants, cellulose can be synthesized by bacteria or be found in algae and tunicates. This readily available and renew‐ able biopolymer is widely used in many applications such as paper, textiles, membranes, or packaging [3]. Cellulose is the most abundant and studied biorenewable material with an estimated annual production of 7.5 x 1010t [4]. After more than 170 years of research into the "sugar of the plant cell wall", consumers, industries, and governments are increasingly demanding products from renewable and sustainable resources that are biodegradable, nonpetroleum based, carbon neutral, and, at the same time, generating low environmental, animal/ human health and safety risks [5].

Regarding its basic structure, cellulose is a linear syndiotactic homopolymer composed of D-anhydroglucopyranose units (AGU), that are connected by β(1–4)-glycosidic bonds (Figure 1) [5]. increasingly demanding products from renewable and sustainable resources that are biodegradable, non-petroleum based, carbon neutral, and, at the same time, generating low environmental, animal/human health and safety risks [5]. Regarding its basic structure, cellulose is a linear syndiotactic homopolymer composed of D-anhydroglucopyranose

units (AGU), that are connected by β(1–4)-glycosidic bonds (Figure 1) [5].

Figure 1. Molecular structure of cellulose (n = value of DP). **Figure 1.** Molecular structure of cellulose (n = value of DP).

macroscopic (e.g. solubility) properties of cellulose.

The size of the cellulose molecules can be defined by the average degree of polymerization (DP). The average molecular weight is estimated from the product of the DP and the molecular mass of a single AGU. Each AGU bears three hydroxyl groups (one primary and two secondary moieties that represent more than 30 % by weight), with the exception of the terminal ones. These structural features make cellulose surface chemistry quite intriguing and opens a broad spectrum of potential reactions, which typically occur in the primary and secondary hydroxyl groups [3]. From the single AGU up to the cellulose micro- and macrofibrils that constitute the cell walls, cellulose organizes in a rather dense and highly hierarchical fashion where an extended intra- and intermolecular network of hydrogen bonds is believed to constitute the basis of cohesion between cellulose molecules [6]. At the beginning of the biosynthesis, cells in The size of the cellulose molecules can be defined by the average degree of polymerization (DP). The average molecular weight is estimated from the product of the DP and the molecular mass of a single AGU. Each AGU bears three hydroxyl groups (one primary and two secondary moieties that represent more than 30 % by weight), with the exception of the terminal ones. These structural features make cellulose surface chemistry quite intriguing and opens a broad spectrum of potential reactions, which typically occur in the primary and secondary hydroxyl groups [3].

higher plants are surrounded by a very thin outer wall, the primary (P) wall (0.1–0.5 μm thick), which envelops the cytoplasm. After completion of the elongation growth stage of the cells, the thickness of the cell wall increases significantly by the successive deposition of concentric inner layers, constituting the secondary (S1) and (S2) walls (0.1– 0.3 μm and 1–10 μm thick, respectively). The last layer deposited, called the tertiary layer (T) in the case of wood fibers and (S3) layer in the case of cotton, is not always present and is very thin (< 100 nm). At the end of the biosynthesis, the cytoplasm dies, and the resulting central channel within the cells, so-called the lumen, is more or less narrow depending on the maturity of the fibers. In this layered structure, the different chemical components are distributed and organized, forming a complex and tri-dimensional composite microstructure [4, 6]. Cellulose microfibrils are reasonably oriented and hold together due to the cooperative function of the hemicelluloses, lignin, and pectins that act as matrix and adhesive components. The resulting fibrillar cells, also called elementary fibers, are usually gathered in fiber bundles as for wood or flax and hemp stems, or can be eventually found individualized in the elementary fibers as is in the case of cotton fibers. A complementary perspective highlights the amphiphilic nature of cellulose [7–12]; the equatorial direction of a From the single AGU up to the cellulose micro- and macrofibrils that constitute the cell walls, cellulose organizes in a rather dense and highly hierarchical fashion where an extended intraand intermolecular network of hydrogen bonds is believed to constitute the basis of cohesion between cellulose molecules [6]. At the beginning of the biosynthesis, cells in higher plants are surrounded by a very thin outer wall, the primary (P) wall (0.1–0.5 µm thick), which envelops the cytoplasm. After completion of the elongation growth stage of the cells, the thickness of the cell wall increases significantly by the successive deposition of concentric inner layers, constituting the secondary (S1) and (S2) walls (0.1–0.3 µm and 1–10 µm thick, respectively). The last layer deposited, called the tertiary layer (T) in the case of wood fibers and (S3) layer in the case of cotton, is not always present and is very thin (< 100 nm). At the end of the biosynthesis, the cytoplasm dies, and the resulting central channel within the cells, so-called

glucopyranose ring has a hydrophilic character because all three hydroxyl groups are located on the equatorial positions of the ring. On the other hand, the axial direction of the ring is hydrophobic since the hydrogen atoms of C–H bonds are located on the axial positions of the ring. Thus, cellulose molecules have an intrinsic structural anisotropy and due to intra- and intermolecular hydrogen bonding, there is a formation of rather flat ribbons, with sides that differ markedly in their polarity [10, 13, 14]; this is expected to considerably influence both the microscopic (e.g. interactions) and

As a semi-crystalline polymer, cellulose is able to adopt different forms in the cell wall of a plant such that amorphous regions (lower order) coexist with crystalline domains (higher order) [4]. The degree of crystallinity of cellulose, usually in the range of 40–60 %, depends on the origin and pre-treatment of the sample [6]. Interestingly, the parallel arrangement found in nature (so called cellulose I), is not the most stable structure for a cellulose crystal. Thus, when cellulose I is dissolved and recrystallized, cellulose chains may adopt an anti-parallel arrangement known as the cellulose II type crystal [15, 16]. This intriguing process is still not understood in detail. However, it has been postulated that the transition from cellulose I to cellulose II crystals does not require full chain swelling and coiling in solution; this transition could be reached by a simple translational movement of molecules, in particular during mercerization [4].

the lumen, is more or less narrow depending on the maturity of the fibers. In this layered structure, the different chemical components are distributed and organized, forming a complex and tri-dimensional composite microstructure [4, 6]. Cellulose microfibrils are reasonably oriented and hold together due to the cooperative function of the hemicelluloses, lignin, and pectins that act as matrix and adhesive components. The resulting fibrillar cells, also called elementary fibers, are usually gathered in fiber bundles as for wood or flax and hemp stems, or can be eventually found individualized in the elementary fibers as is in the case of cotton fibers.

A complementary perspective highlights the amphiphilic nature of cellulose [7–12]; the equatorial direction of a glucopyranose ring has a hydrophilic character because all three hydroxyl groups are located on the equatorial positions of the ring. On the other hand, the axial direction of the ring is hydrophobic since the hydrogen atoms of C–H bonds are located on the axial positions of the ring. Thus, cellulose molecules have an intrinsic structural anisotropy and due to intra- and intermolecular hydrogen bonding, there is a formation of rather flat ribbons, with sides that differ markedly in their polarity [10, 13, 14]; this is expected to considerably influence both the microscopic (e.g. interactions) and macroscopic (e.g. solubility) properties of cellulose.

As a semi-crystalline polymer, cellulose is able to adopt different forms in the cell wall of a plant such that amorphous regions (lower order) coexist with crystalline domains (higher order) [4]. The degree of crystallinity of cellulose, usually in the range of 40–60 %, depends on the origin and pre-treatment of the sample [6]. Interestingly, the parallel arrangement found in nature (so called cellulose I), is not the most stable structure for a cellulose crystal. Thus, when cellulose I is dissolved and recrystallized, cellulose chains may adopt an anti-parallel arrangement known as the cellulose II type crystal [15, 16]. This intriguing process is still not understood in detail. However, it has been postulated that the transition from cellulose I to cellulose II crystals does not require full chain swelling and coiling in solution; this transition could be reached by a simple translational movement of molecules, in particular during mercerization [4].

### **2. Cellulose dissolution**

### **2.1. Solvents**

lignin, hemicelluloses, pectins, proteins, and water. Apart from higher plants, cellulose can be synthesized by bacteria or be found in algae and tunicates. This readily available and renew‐ able biopolymer is widely used in many applications such as paper, textiles, membranes, or packaging [3]. Cellulose is the most abundant and studied biorenewable material with an estimated annual production of 7.5 x 1010t [4]. After more than 170 years of research into the "sugar of the plant cell wall", consumers, industries, and governments are increasingly demanding products from renewable and sustainable resources that are biodegradable, nonpetroleum based, carbon neutral, and, at the same time, generating low environmental, animal/

Regarding its basic structure, cellulose is a linear syndiotactic homopolymer composed of D-anhydroglucopyranose units (AGU), that are connected by β(1–4)-glycosidic bonds

**Cellobiose based unit**

units (AGU), that are connected by β(1–4)-glycosidic bonds (Figure 1) [5].

**O HO O**

Figure 1. Molecular structure of cellulose (n = value of DP).

**Figure 1.** Molecular structure of cellulose (n = value of DP).

**OH**

**OH HO O**

macroscopic (e.g. solubility) properties of cellulose.

increasingly demanding products from renewable and sustainable resources that are biodegradable, non-petroleum based, carbon neutral, and, at the same time, generating low environmental, animal/human health and safety risks [5].

Regarding its basic structure, cellulose is a linear syndiotactic homopolymer composed of D-anhydroglucopyranose

**3**

**4**

**OH OH**

**OH**

**O**

**<sup>H</sup> <sup>O</sup> HO**

The size of the cellulose molecules can be defined by the average degree of polymerization (DP). The average molecular weight is estimated from the product of the DP and the molecular mass of a single AGU. Each AGU bears three hydroxyl groups (one primary and two secondary moieties that represent more than 30 % by weight), with the exception of the terminal ones. These structural features make cellulose surface chemistry quite intriguing and opens a broad spectrum of

**OH**

**2**

**5 6**

**OH HO O**

**1**

From the single AGU up to the cellulose micro- and macrofibrils that constitute the cell walls, cellulose organizes in a rather dense and highly hierarchical fashion where an extended intra- and intermolecular network of hydrogen bonds is believed to constitute the basis of cohesion between cellulose molecules [6]. At the beginning of the biosynthesis, cells in higher plants are surrounded by a very thin outer wall, the primary (P) wall (0.1–0.5 μm thick), which envelops the cytoplasm. After completion of the elongation growth stage of the cells, the thickness of the cell wall increases significantly by the successive deposition of concentric inner layers, constituting the secondary (S1) and (S2) walls (0.1– 0.3 μm and 1–10 μm thick, respectively). The last layer deposited, called the tertiary layer (T) in the case of wood fibers and (S3) layer in the case of cotton, is not always present and is very thin (< 100 nm). At the end of the biosynthesis, the cytoplasm dies, and the resulting central channel within the cells, so-called the lumen, is more or less narrow depending on the maturity of the fibers. In this layered structure, the different chemical components are distributed and organized, forming a complex and tri-dimensional composite microstructure [4, 6]. Cellulose microfibrils are reasonably oriented and hold together due to the cooperative function of the hemicelluloses, lignin, and pectins that act as matrix and adhesive components. The resulting fibrillar cells, also called elementary fibers, are usually gathered in fiber bundles as for wood or flax and hemp stems, or can be eventually found individualized in the elementary fibers as is in the case of

A complementary perspective highlights the amphiphilic nature of cellulose [7–12]; the equatorial direction of a glucopyranose ring has a hydrophilic character because all three hydroxyl groups are located on the equatorial positions of the ring. On the other hand, the axial direction of the ring is hydrophobic since the hydrogen atoms of C–H bonds are located on the axial positions of the ring. Thus, cellulose molecules have an intrinsic structural anisotropy and due to intra- and intermolecular hydrogen bonding, there is a formation of rather flat ribbons, with sides that differ markedly in their polarity [10, 13, 14]; this is expected to considerably influence both the microscopic (e.g. interactions) and

As a semi-crystalline polymer, cellulose is able to adopt different forms in the cell wall of a plant such that amorphous regions (lower order) coexist with crystalline domains (higher order) [4]. The degree of crystallinity of cellulose, usually in the range of 40–60 %, depends on the origin and pre-treatment of the sample [6]. Interestingly, the parallel arrangement found in nature (so called cellulose I), is not the most stable structure for a cellulose crystal. Thus, when cellulose I is dissolved and recrystallized, cellulose chains may adopt an anti-parallel arrangement known as the cellulose II type crystal [15, 16]. This intriguing process is still not understood in detail. However, it has been postulated that the transition from cellulose I to cellulose II crystals does not require full chain swelling and coiling in solution; this transition could be reached by a simple translational movement of molecules, in particular during mercerization [4].

potential reactions, which typically occur in the primary and secondary hydroxyl groups [3].

**O OH**

**OH**

**n-3**

**Non-reducing end group Anhydroglucose unit Reducing end group**

The size of the cellulose molecules can be defined by the average degree of polymerization (DP). The average molecular weight is estimated from the product of the DP and the molecular mass of a single AGU. Each AGU bears three hydroxyl groups (one primary and two secondary moieties that represent more than 30 % by weight), with the exception of the terminal ones. These structural features make cellulose surface chemistry quite intriguing and opens a broad spectrum of potential reactions, which typically occur in the primary and secondary hydroxyl

From the single AGU up to the cellulose micro- and macrofibrils that constitute the cell walls, cellulose organizes in a rather dense and highly hierarchical fashion where an extended intraand intermolecular network of hydrogen bonds is believed to constitute the basis of cohesion between cellulose molecules [6]. At the beginning of the biosynthesis, cells in higher plants are surrounded by a very thin outer wall, the primary (P) wall (0.1–0.5 µm thick), which envelops the cytoplasm. After completion of the elongation growth stage of the cells, the thickness of the cell wall increases significantly by the successive deposition of concentric inner layers, constituting the secondary (S1) and (S2) walls (0.1–0.3 µm and 1–10 µm thick, respectively). The last layer deposited, called the tertiary layer (T) in the case of wood fibers and (S3) layer in the case of cotton, is not always present and is very thin (< 100 nm). At the end of the biosynthesis, the cytoplasm dies, and the resulting central channel within the cells, so-called

human health and safety risks [5].

2 Cellulose - Fundamental Aspects and Current Trends

(Figure 1) [5].

**HO**

groups [3].

cotton fibers.

The first attempts to dissolve cellulose or cellulose-containing materials were described about 150 years ago [3]. In many important cellulose applications, it is necessary to dissolve the biopolymer and normally this is a challenging step. Due to the complexity of such a biopoly‐ meric network, the partial crystalline structure and the extended non-covalent interactions among molecules, chemical processing of cellulose is rather difficult. Cellulose is neither meltable nor soluble in water or a large range of organic compounds [17]. However, this biopolymer is soluble in more exotic media with no apparent common properties [18].

Cellulose solvents are usually divided in two main groups: non-derivatizing and derivatizing solvents [19]. Historically, derivatization came first. As the name suggests, the derivatizing solvent group comprises all the systems where cellulose dissolution occurs via covalent modification giving an "unstable" ether, ester, or acetal intermediate, such as cellulose nitrate, cellulose xanthate or cellulose carbamate. The viscose process (NaOH + CS2) is by far the most common derivatizing cellulose solvent system used [20].

The "non-derivatizing solvent" class, comprise systems capable of dissolving cellulose only via physical intermolecular interactions. This class of solvents is quite relevant for the organic chemistry of cellulose under homogeneous conditions. Historically significant, and of practical relevance for analytical purposes, is the system introduced by Schweizer in 1857 [21]. It was found that copper salts and concentrated ammonia effectively dissolve cotton. Among the most popular is cuprammonium hydroxide or simply "cuam". Fifty years later, ethylenedia‐ mine was found to be a good alternative to ammonia and also new complexing solvents were designed, such as the cadmium hydroxide in aqueous ethylenediamine, "cadoxen", or nickel oxide in the same aqueous ethylenediamine, "nioxen" [18, 22, 23]. Similar alternative systems have been reported using mainly other transition metals (such as palladium and zinc) and an amine or ammonium compound. However, none of these systems have been able to achieve full commercial success [18].

Since the work of Sobue et al., it is known that cellulose is soluble in aqueous NaOH below -5 °C within a specific concentration range of NaOH (7–10 %) [24, 25]. This system is potentially cheap, non-polluting, easy to handle and uses common chemicals. The aqueous alkali systems often, however, do not completely disrupt the semicrystalline regions of cellulose and the solubility is limited to cellulose of relatively low DP. The apparent solubility also depends on the degree of crystallinity and crystal type. Pre-treatments, such as steam explosion of the dissolving pulp, have been successfully used [26]. Other aqueous bases, such as LiOH [27], or quaternary ammonium hydroxides are also capable of dissolving cellulose [4, 18, 28]. More recently, the combination of aqueous NaOH solutions with other additives such as polyethy‐ lene glycol, PEG [29], urea [30], and thiourea [31] has been explored.

In 1932, Letters et al. investigated the swelling and dissolution of cellulose in aqueous zinc chloride; dissolution was only observed at concentrations as high as 63 % (w/w) [32]. However, in later publications only three water/salt systems were described as effective cellulose solvents in more detail: Ca(SCN)2/H2O, LiSCN/H2O, and ZnCl2/H2O. Among such classes of salts, Ca(SCN)2 3H2O showed a strong swelling and solvent action on cellulose [33]. Only the mixture of NaSCN/KSCN with Ca(SCN)2 3H2O or dimethyl sulfoxide (DMSO) was found to be able to dissolve cellulose. In addition, molten LiSCN 2H2O is described as cellulose solvent [19]. Recently, Leipner et al. reported that LiClO4 3H2O is a very effective solvent where transparent cellulose solutions can be obtained within a few minutes without pre-treatment or activation [34]. Moreover, mixtures of LiClO4 3H2O with Mg(ClO4)2 H2O or MgCl2.6H2O are also regarded as promising solvents.

Non-aqueous systems are also suitable to dissolve cellulose. Again, the solvent spectrum is broad and the number of possible combinations is high. From a historical point of view, combinations of some simple inorganic compounds, such as SO2 and NH3 with a suitable ammonium salt, can indeed be considered as the origin of two large groups of non-derivatizing non-aqueous solvent systems [35]. The first group worthy of mention generally comprises mixtures of a polar organic liquid, SO2, and primary, secondary, or tertiary aliphatic or secondary alicyclic amines [36]. Alternatively, the sulfur component may be modified to SOCl2. Suitable polar liquids employed are for instance N,N-dimethylformamide (DMF), DMSO, N,N-dimethylacetamide (DMAc) or formamide. From the wide variety of possible mixtures, DMSO/SO2/diethylamine is one of the most versatile [35].

solvent group comprises all the systems where cellulose dissolution occurs via covalent modification giving an "unstable" ether, ester, or acetal intermediate, such as cellulose nitrate, cellulose xanthate or cellulose carbamate. The viscose process (NaOH + CS2) is by far the most

The "non-derivatizing solvent" class, comprise systems capable of dissolving cellulose only via physical intermolecular interactions. This class of solvents is quite relevant for the organic chemistry of cellulose under homogeneous conditions. Historically significant, and of practical relevance for analytical purposes, is the system introduced by Schweizer in 1857 [21]. It was found that copper salts and concentrated ammonia effectively dissolve cotton. Among the most popular is cuprammonium hydroxide or simply "cuam". Fifty years later, ethylenedia‐ mine was found to be a good alternative to ammonia and also new complexing solvents were designed, such as the cadmium hydroxide in aqueous ethylenediamine, "cadoxen", or nickel oxide in the same aqueous ethylenediamine, "nioxen" [18, 22, 23]. Similar alternative systems have been reported using mainly other transition metals (such as palladium and zinc) and an amine or ammonium compound. However, none of these systems have been able to achieve

Since the work of Sobue et al., it is known that cellulose is soluble in aqueous NaOH below -5 °C within a specific concentration range of NaOH (7–10 %) [24, 25]. This system is potentially cheap, non-polluting, easy to handle and uses common chemicals. The aqueous alkali systems often, however, do not completely disrupt the semicrystalline regions of cellulose and the solubility is limited to cellulose of relatively low DP. The apparent solubility also depends on the degree of crystallinity and crystal type. Pre-treatments, such as steam explosion of the dissolving pulp, have been successfully used [26]. Other aqueous bases, such as LiOH [27], or quaternary ammonium hydroxides are also capable of dissolving cellulose [4, 18, 28]. More recently, the combination of aqueous NaOH solutions with other additives such as polyethy‐

In 1932, Letters et al. investigated the swelling and dissolution of cellulose in aqueous zinc chloride; dissolution was only observed at concentrations as high as 63 % (w/w) [32]. However, in later publications only three water/salt systems were described as effective cellulose solvents in more detail: Ca(SCN)2/H2O, LiSCN/H2O, and ZnCl2/H2O. Among such classes of salts, Ca(SCN)2 3H2O showed a strong swelling and solvent action on cellulose [33]. Only the mixture of NaSCN/KSCN with Ca(SCN)2 3H2O or dimethyl sulfoxide (DMSO) was found to be able to dissolve cellulose. In addition, molten LiSCN 2H2O is described as cellulose solvent [19]. Recently, Leipner et al. reported that LiClO4 3H2O is a very effective solvent where transparent cellulose solutions can be obtained within a few minutes without pre-treatment or activation [34]. Moreover, mixtures of LiClO4 3H2O with Mg(ClO4)2 H2O or MgCl2.6H2O are also regarded

Non-aqueous systems are also suitable to dissolve cellulose. Again, the solvent spectrum is broad and the number of possible combinations is high. From a historical point of view, combinations of some simple inorganic compounds, such as SO2 and NH3 with a suitable ammonium salt, can indeed be considered as the origin of two large groups of non-derivatizing non-aqueous solvent systems [35]. The first group worthy of mention generally comprises

lene glycol, PEG [29], urea [30], and thiourea [31] has been explored.

common derivatizing cellulose solvent system used [20].

4 Cellulose - Fundamental Aspects and Current Trends

full commercial success [18].

as promising solvents.

Relevant mixtures of two-component solvents containing DMSO [19] such as DMSO/methyl‐ amine, DMSO/KSCN, DMSO/CaCl2, DMSO/formaldehyde, and DMSO/substituted ammoni‐ um fluorides, such as tetrabutylammonium fluoride, TBAF, a recently discovered powerful solvent capable of dissolving cellulose of reasonably high DP (650) within a few minutes without any pre-treatment at room temperature [37]. Non-aqueous solutions based on lithium salts are also very relevant for the analysis of cellulose and for the preparation of a wide variety of derivatives. In this regard, the DMAc/LiCl mixture, developed by McCormick in the late 1970s is noteworthy [38]. N-methylmorpholine-N-oxide (NMMO) emerged as the best of the amine-oxides in the late 1970's [3, 39]. Solutions up to 23 wt. % of cellulose were obtained by dissolving the polymer in NMMO/water mixtures, subsequently removing water under vacuum. This constitutes the basis of the Lyocell process whose commercial potential has been demonstrated and is now applied in large scale. Two main problems are still connected with the NMMO process; the instability of the solvent (demands costly investments for safety reasons) and the tendency of the regenerated fiber towards fibrillation [40].

Ionic liquids (ILs) are a very promising group of compounds in cellulose dissolution. These systems were first employed by Graenacher, in the form of N-alkylpyridinium salts, for the dissolution of cellulose and as media for homogeneous chemical reactions [41]. Almost seventy years passed before the work of Swatloski et al. where several low melting ionic liquids (below 100 °C) were reported as cellulose solvents [42]. This work triggered a new and exciting field in cellulose research and, since then, a huge variety of ILs has been developed. Note that the number of potential ion combinations available is estimated to provide around 1012 ILs [43]. Presently, the most efficient ILs for cellulose dissolution are mainly composed of a salt with halide [44], phosphonate [45], formate [46] or acetate [47] as anion, and imidazolium [48], pyridinium [6], choline [49], or phosphonium [50] as cation.

### **2.2. Swelling and dissolution mechanisms: From the cell wall structure down to the molecular level**

### *2.2.1. Evidence of a heterogeneous swelling and role of the solvent quality*

When placed in contact with a solvent, synthetic polymers dissolve in three main steps, *(i)* the solvent swells the solid phase that goes above its glass transition Tg, *(ii)* the swelling increases up to the disentanglement of the polymer chains, and finally *(iii)* the chains move out of the swollen phase to the solvent phase and the dissolution is completed [51, 52]. The dissolution occurs step by step from the outside to the inside of the bulk of the polymer, being the kinetics considerably different regarding the amorphous and crystalline zones.

Due to their complex hierarchical structures, cellulose fibers show a different picture charac‐ terized by a heterogeneous swelling and dissolution. The most peculiar effect of this hetero‐ geneous swelling is the ballooning phenomenon, in which swelling occurs in specific zones along the fibers (Figure 2). The ballooning phenomenon has been observed and described long ago, first in 1864 by Nägeli [53], then by Pennetier [54], Flemming and Thaysen [55, 56], Rollins and Tripp [57, 58], Hock [59], Warwicker et al. [60]. According to these authors, this phenom‐ enon is assumed to be caused by the swelling of the cellulose contained in the secondary wall of the fibers that leads to the bursting of the primary wall. As the cellulose swells, the primary wall rolls up in such a way as to form collars, rings, or spirals that restricts the uniform expansion of the fiber forming balloons.

Further studies of Chanzy et al. [61] and Cuissinat and Navard [62] showed that the swelling and dissolution mechanisms are strongly influenced by the solvent quality. Based on optical microscopy observations of the dissolution of wood and cotton fibers in a wide range of solvent qualities (i.e. in NMMO at 90 °C with increasing amount of water), the authors identified four main dissolution modes: *(i)* fast dissolution by fragmentation in good solvent (NMMO - < 17 % w/w water), *(ii)* swelling by ballooning and full dissolution in moderate solvent (NMMO - 19 to 24 % w/w water), *(iii)* swelling by ballooning and no complete dissolution in bad solvent (NMMO - 25 to 35 % w/w water), *(iiii)* low homogeneous swelling and no dissolution in nonsolvent (NMMO - > 35 % w/w water).

**Figure 2.** Wood fiber swollen by "ballooning" in NMMO – 20 % w/w water at 90 °C (magnification of 200 times). (Pic‐ tures were taken by NLM at CEMEF - MINES ParisTech)

These mechanisms have also been observed in NaOH-water [63], ionic liquids [64], and for a wide range of other plant fibers [65] and some cellulose derivatives prepared by heterogeneous derivatization [66]. All these studies pointed out the leading roles of the solvent quality and the fiber microstructure on the dissolution mechanisms. As long as the native cell wall structure of cellulosic fibres is preserved, the swelling and dissolution mechanisms are found to be similar.

### *2.2.2. Gradient of dissolution within the cell wall structure and role of the chemical environment of cellulose chains*

The role of the cell wall structure was investigated in detail by Le Moigne et al. [67, 68] by studying the dissolution mechanisms of cotton fibers extracted from cotton bolls at successive growth stages. When observing fiber dissolution in various solvent qualities, the authors showed that the primary P wall and the secondary S1 wall and S2 wall, which are successively deposited in concentric layers during biosynthesis, behave very differently upon dissolution. In good solvents (NMMO - 16 % w/w water), the whole fiber breaks into rod-like fragments and dissolves fast. In contrast, cellulose fibres in moderate solvent (NMMO – 20 % w/w water) dissolve by ballooning following the steps (1, 2, and 3) illustrated in Figure 3. Three main areas are implied in the ballooning phenomenon: *(i)* the fragments that are made of the secondary S2 wall and dissolve easily and quickly within the balloons, *(ii)* the membrane of the balloons that is made of the swollen secondary S1 wall and dissolves slowly, and *(iii)* the helices and collars surrounding the balloons that are made of the primary P wall do not dissolve easily and break under the expansion of the secondary wall. These observations revealed a centripetal radial gradient of the dissolution capacity from the inside to the outside of the fiber, being the inside layers from the secondary S2 wall easier to dissolve.

geneous swelling is the ballooning phenomenon, in which swelling occurs in specific zones along the fibers (Figure 2). The ballooning phenomenon has been observed and described long ago, first in 1864 by Nägeli [53], then by Pennetier [54], Flemming and Thaysen [55, 56], Rollins and Tripp [57, 58], Hock [59], Warwicker et al. [60]. According to these authors, this phenom‐ enon is assumed to be caused by the swelling of the cellulose contained in the secondary wall of the fibers that leads to the bursting of the primary wall. As the cellulose swells, the primary wall rolls up in such a way as to form collars, rings, or spirals that restricts the uniform

Further studies of Chanzy et al. [61] and Cuissinat and Navard [62] showed that the swelling and dissolution mechanisms are strongly influenced by the solvent quality. Based on optical microscopy observations of the dissolution of wood and cotton fibers in a wide range of solvent qualities (i.e. in NMMO at 90 °C with increasing amount of water), the authors identified four main dissolution modes: *(i)* fast dissolution by fragmentation in good solvent (NMMO - < 17 % w/w water), *(ii)* swelling by ballooning and full dissolution in moderate solvent (NMMO - 19 to 24 % w/w water), *(iii)* swelling by ballooning and no complete dissolution in bad solvent (NMMO - 25 to 35 % w/w water), *(iiii)* low homogeneous swelling and no dissolution in non-

**Figure 2.** Wood fiber swollen by "ballooning" in NMMO – 20 % w/w water at 90 °C (magnification of 200 times). (Pic‐

These mechanisms have also been observed in NaOH-water [63], ionic liquids [64], and for a wide range of other plant fibers [65] and some cellulose derivatives prepared by heterogeneous derivatization [66]. All these studies pointed out the leading roles of the solvent quality and the fiber microstructure on the dissolution mechanisms. As long as the native cell wall structure of cellulosic fibres is preserved, the swelling and dissolution mechanisms are found to be

*2.2.2. Gradient of dissolution within the cell wall structure and role of the chemical environment of*

The role of the cell wall structure was investigated in detail by Le Moigne et al. [67, 68] by studying the dissolution mechanisms of cotton fibers extracted from cotton bolls at successive growth stages. When observing fiber dissolution in various solvent qualities, the authors

**50 µm**

expansion of the fiber forming balloons.

6 Cellulose - Fundamental Aspects and Current Trends

solvent (NMMO - > 35 % w/w water).

tures were taken by NLM at CEMEF - MINES ParisTech)

similar.

*cellulose chains*

**Figure 3.** Optical microscopy observation of a cotton fiber in NMMO – 20 % w/w water at 90 °C: (left) description of the zones implied in the ballooning phenomenon; (right) successive dissolution steps (1, 2, and 3). (Pictures were taken by NLM at CEMEF - MINES ParisTech)

Considering that the degree of polymerization and the crystallinity of cellulose increase during the biosynthesis of the secondary S2 wall [69, 70], the dissolution capacity of cellulose fibers does not depend, in a first instance, on thermodynamics (molar mass), or kinetic (crystalline or amorphous) parameters. The gradient in dissolution capacity of the successively deposited cell wall layers has to be related with the specific hierarchical and multi-component structure of cellulose fibres. The primary wall is indeed composed of cellulose and hemicellulose as well as pectins and proteins, while the secondary S2 wall contains almost only cellulose and hemicelluloses [71]. Dynamic FTIR spectroscopy experiments showed that all these compo‐ nents are strongly linked together [72]. As described by Klemm et al. [4], the cellulose micro‐ fibrils are also differently arranged and packed within the cell wall structure. The dissolution capacity of cellulose fibers must thus be considered at two structural levels, *(i)* the cell wall structure that needs to be dismantled for improving dissolution, and *(ii)* the chemical envi‐ ronment of cellulose chains. In this sense, it has been shown that the dismantlement of the cell wall layers induced by steam explosion, acidic hydrolysis, or a simple shearing of the solution eases the dissolution of wood pulps in bad quality solvents such as NaOH-water mixtures [73– 76]. Based on a selective separation by centrifugation of insoluble and soluble cellulose fractions in cellulose-NaOH-water mixtures and further analyses by size exclusion chroma‐ tography, Le Moigne et al. [76] also showed that cellulose chains of similar molar masses can be either dissolved or remain as insoluble fractions. Some of the cellulose chains are thus less accessible than others and embedded in structural regions difficult to dissolve. The carbohy‐ drate composition analysis of the various insoluble fractions revealed that it may contain higher amount of non-cellulosic components than the soluble ones [76, 77]. Beyond thermo‐ dynamic considerations, the dissolution capacity of cellulose chains is thus highly influenced by their localization in the cell wall structure. Additionally, their chemical environment has to be regarded as an important parameter in the dissolution efficiency.

### *2.2.3. Molecular mobility and cellulose/solvent interactions*

The dissolution of cellulose substrates implies that the solvating molecules have sufficient accessibility within the fiber structure to approach cellulose chains, and that the latter are able to gain conformational entropy to be further dissolved. However, the spinning process used for regenerated cellulose fibers as well as the naturally occurring bio-deposition mechanism in native cellulose fibers imply that cellulose chains are well aligned and strongly oriented in an extended conformational state, out of equilibrium. Considering the large number of hydroxyl groups available per AGU, this results in a strong intra- and intermolecular hydrogen bond network [7] that links all the cellulose chains together achieving a long range order. As postulated by O'Sullivan [1], even the chains in non-crystalline regions most likely still possess a certain degree of order. All these features strongly restrict the molecular mobility of cellulose chains.

Evidence of this long range order is the large contraction usually observed during swelling and dissolution of cellulose fibres in moderate quality solvent [68], which can be interpreted as the release of the stresses stored within the fiber structure during fiber processing or biosynthesis, and the return to the equilibrium conformational of individual chains that force the whole fiber structure to contract. If this contraction mechanism is restricted by holding the fiber under tension [78], it has been shown that full dissolution can be prevented and a large decrease of the chemical derivatization efficiency was also observed. The acetylation of Lyocell fibers without tension indeed led to degree of substitution (DS) values up to 1.3, while under tension, lower DS values of ca. 0.2–0.5 were estimated. Thus, the local molecular motions needed for the solvatation and the chemical reactions of cellulose chains were inhibited under tension. These observations highlight the key role of the molecular mobility and the need to find efficient ways to break the abovementioned intra- and intermolecular hydrogen bond network and prevent its reformation [79, 78] for improving cellulose dissolution and reactivity.

Apart from the hydrogen bonding role, there are also several indications regarding cellulose amphiphilicity, and thus a careful examination of the interactions involved suggests that hydrophobic interactions may also play a significant role in governing cellulose solubility. In fact, hydrophobic interactions have been shown to markedly contribute to the crystal-like structure of cellulose and its stability over a hypothetical solution state; from free energy simulations in oligomers it has been estimated that there is a 2.0 kcal/mol/residue contribution for hydrophobic stacking, while the estimated hydrogen bonding contribution is about eight times less [7].

From a thermodynamic point of view, the dissolution of a polymer, such as cellulose, in a solvent is, of course, governed by the free energy of mixing [18]. A negative value of the free energy change on mixing means that the mixing process will occur spontaneously. Charging up a polymer is always expected to help solubility in many solvents; thus dissociated coun‐ terions contribute strongly to the translational entropy of mixing. This seems to be the main reason why cellulose tends to be more soluble or be more penetrated by water at either high or low pH. However, the pK values are such that rather extreme conditions are needed for either deprotonation or protonation of the hydroxyls; a pKa of ca. 13.3 has been found assuming that only one hydroxyl group per AGU dissociates [80].

fractions in cellulose-NaOH-water mixtures and further analyses by size exclusion chroma‐ tography, Le Moigne et al. [76] also showed that cellulose chains of similar molar masses can be either dissolved or remain as insoluble fractions. Some of the cellulose chains are thus less accessible than others and embedded in structural regions difficult to dissolve. The carbohy‐ drate composition analysis of the various insoluble fractions revealed that it may contain higher amount of non-cellulosic components than the soluble ones [76, 77]. Beyond thermo‐ dynamic considerations, the dissolution capacity of cellulose chains is thus highly influenced by their localization in the cell wall structure. Additionally, their chemical environment has to

The dissolution of cellulose substrates implies that the solvating molecules have sufficient accessibility within the fiber structure to approach cellulose chains, and that the latter are able to gain conformational entropy to be further dissolved. However, the spinning process used for regenerated cellulose fibers as well as the naturally occurring bio-deposition mechanism in native cellulose fibers imply that cellulose chains are well aligned and strongly oriented in an extended conformational state, out of equilibrium. Considering the large number of hydroxyl groups available per AGU, this results in a strong intra- and intermolecular hydrogen bond network [7] that links all the cellulose chains together achieving a long range order. As postulated by O'Sullivan [1], even the chains in non-crystalline regions most likely still possess a certain degree of order. All these features strongly restrict the molecular mobility of cellulose

Evidence of this long range order is the large contraction usually observed during swelling and dissolution of cellulose fibres in moderate quality solvent [68], which can be interpreted as the release of the stresses stored within the fiber structure during fiber processing or biosynthesis, and the return to the equilibrium conformational of individual chains that force the whole fiber structure to contract. If this contraction mechanism is restricted by holding the fiber under tension [78], it has been shown that full dissolution can be prevented and a large decrease of the chemical derivatization efficiency was also observed. The acetylation of Lyocell fibers without tension indeed led to degree of substitution (DS) values up to 1.3, while under tension, lower DS values of ca. 0.2–0.5 were estimated. Thus, the local molecular motions needed for the solvatation and the chemical reactions of cellulose chains were inhibited under tension. These observations highlight the key role of the molecular mobility and the need to find efficient ways to break the abovementioned intra- and intermolecular hydrogen bond network and prevent its reformation [79, 78] for improving cellulose dissolution and reactivity. Apart from the hydrogen bonding role, there are also several indications regarding cellulose amphiphilicity, and thus a careful examination of the interactions involved suggests that hydrophobic interactions may also play a significant role in governing cellulose solubility. In fact, hydrophobic interactions have been shown to markedly contribute to the crystal-like structure of cellulose and its stability over a hypothetical solution state; from free energy simulations in oligomers it has been estimated that there is a 2.0 kcal/mol/residue contribution for hydrophobic stacking, while the estimated hydrogen bonding contribution is about eight

be regarded as an important parameter in the dissolution efficiency.

*2.2.3. Molecular mobility and cellulose/solvent interactions*

8 Cellulose - Fundamental Aspects and Current Trends

chains.

times less [7].

A reasonable solvent for cellulose dissolution must be able to overcome the low entropy gain by favorable solvent/polymer interactions, and better dissolution results are obtained using amphiphilic solvents that are not only able to eliminate hydrogen bonding but also eliminate hydrophobic interactions. Both the amorphous and the crystalline regions can be affected by the solvent. However, the amorphous domains, due to a higher free energy, are preferentially and more easily accessed [8]. Therefore, it is not surprising that quite often, prior to dissolution, different activation processes are applied to mainly transform the more ordered and less accessible (crystalline) domains of cellulose into disordered and more accessible regions. It is argued that these alterations of the cellulose structure facilitate the solvent molecules to get access to the cellulose chains [81]. The crystallinity effect in dissolution is still controversial despite some supportive evidences of its effect. For instance, it is argued that sisal pulp (e.g. fibers extracted from the leaves of the *Agave sisalana* plant) dissolves more readily than cotton linters in a particular solvent as its crystallinity index and crystallite size are smaller than that of the latter. After pre-treatment of cotton linter (e.g. mercerization), dissolution was consid‐ erably improved and this observation was related to the decrease in both the crystallinity index and average crystal size [82].

In the majority of the cases, cellulose is not dissolved down to a molecular level but rather forms stable colloidal dispersions where ordered cellulose aggregates of, at least, several hundred chains, are present. The structure in solution has been proposed to consist of aggregates of "fringed micelle" type characterized by a highly ordered cylindrical core of aligned chains, which is insoluble in the solvent, and two spherical coronas surrounding the core ends [83]. Reaching molecularly dispersed systems has been challenging for nearly all known solvent systems. Recently, Cohen et al. showed that ionic liquids are able to dissolve cellulose down to a molecular level [84]. Typically, the cations of IL are bulky species with amphiphilic properties. Proof of this is that most literature agrees on the formation of aggre‐ gates or micelles of ionic liquids in water, similar to a surfactant behavior [85]. Such amphi‐ philicity is normally not considered when discussing the mechanism of dissolution of cellulose. This is particularly relevant since crystalline cellulose has an amphipathic-like structure; hydrophobic surfaces consisting of pyranose ring hydrogens and hydrophilic regions arising from the hydroxyl groups directed towards the sides of the ring. In fact, this also follows the earlier discussion on the effect of additives such as PEG, urea, and thiourea on NaOH solutions. Recent molecular dynamics simulation reveals that urea is preferentially adsorbed on the hydrophobic faces of the anhydroglucose rings but has the same affinity as water to the hydroxyl groups. Thus, the simulations suggest that urea acts primarily by mitigating the effect of the hydrophobic portions of the cellulose molecule [86, 87]. In the same direction, a recent molecular dynamics simulations study carried out on cellulose oligomers and 1-ethyl-3 methylimidazolium acetate (C2mimOAc) indeed suggest that the cations are in close contact with the cellulose through hydrophobic interactions [88].

### *2.2.4. Improving cellulose dissolution: Towards targeted activation treatments*

The challenge faced by generations of scientists and numerous companies is to find a simple, cheap, and non-polluting cellulose dissolution process. As pointed out in the previous sections, one of the crucial aspects in cellulose dissolution is the capacity of the fibers to be accessible for reagents in order to perform the subsequent transformation stages. Consequently, several chemical and physical methods, so-called ''activation", have been developed to ease cellulose dissolution, e.g. acid hydrolysis [24, 89], ball or vibration milling [4, 90], steam explosion [73– 75], and electron beam irradiation [91–93]. The main goal is to disrupt the cell wall structure and the hydrogen bond network in order to increase the accessibility of the solvent within the fiber structure, while preserving the original macromolecular structure of cellulose chains. Although such methods improve cellulose dissolution, typically these activation treatments are accompanied by a strong degradation of the cellulose chains leading to low DPs, ca. 600– 1200 for dissolving wood pulps and ca. 100–200 for cellulose powders. Such reduction in molecular weight may adversely compromise the properties of the final cellulose-based products [4]. This is mainly due to the lack of selectivity of the activation treatments that strongly affect the integrity of cellulose substrates at the different scales of their structure depending on the procedure employed. Thus, there is a need for more targeted activation methods.

In this respect, the recent advances in biotechnology with the use of enzymes are expected to bring new insights for the development of targeted activation treatments. As it was previously discussed, the chemical environment of the cellulose chains, as well as their localization within the cell wall structure, plays a key role in their dissolution capacity. Due to their specificity, enzymes should thus be an alternative strategy to improve the selectivity of activation treatments towards "non-dissolving" areas within the fiber structure. Several studies have showed that enzymatic treatments can increase cellulose solubility basically due to changes in DP and hydrogen bond density [94, 95]. However, in these studies, the potential specific action of enzymes within the fiber structure for the improvement of their dissolution capacity was not exploited.

Based on the observations that the external cell wall layers, i.e. the primary and S1 walls, prevent the efficient dissolution of cellulose fibers [67], Le Moigne et al. have recently inves‐ tigated the possibility to perform a targeted enzymatic peeling treatment in order to remove the external walls and to improve the dissolution capacity of cellulose fibers without degrading their whole structure [96]. The alkaline solubility of the different enzymatically treated samples was investigated in NaOH–water solution. The results showed that the enzymatic peeling has two effects at short treatment times, *(i)* a digestion of the primary wall that was assessed by the near absence of ballooning and *(ii)* a degradation of the fiber structure that was supported by the decrease in the DP. At longer treatment times, the external walls were totally digested and the fiber structure totally dismantled. As expected, a direct correlation was found between the cellulose solubility and its DP and degree of crystallinity. Nevertheless, the removal of the external walls and the macrostructural disruption of the fibers must be regarded as important factors for the improvement of cellulose dissolution. The alkaline solubility of wood pulps having similar intrinsic viscosity values was indeed two times higher when the external walls were removed by the enzymatic peeling.

Interesting results were also obtained by the use of endopectinase and endoglucanase mixtures to improve dissolution of wood pulps in NaOH-water solution [97]. Although the mechanisms are not fully elucidated, it was suggested that endopectinases have a targeted hydrolyzing action on the pectic network present in the primary and S1 walls. This resulted in an enhanced accessibility and diffusion of the solvent through the external walls of the fibers and a significant increase in their swelling and dissolution capacity, while having a limited effect on the cellulose DP.

The use of enzymes for the improvement of cellulose dissolution is thus of great interest due to their high specificity and potential targeted action on the fiber structure that should allow to preserve the solid state of cellulose while improving its processing and the properties of final cellulose based products. As underlined by Koivula et al. [98], the future role of enzymes in lignocellulosic polysaccharide processing will depend on their added value and economical feasibility, as well as the wide industrial acceptance of biotechnologies.

### **3. Understanding the cellulose regeneration process**

hydroxyl groups. Thus, the simulations suggest that urea acts primarily by mitigating the effect of the hydrophobic portions of the cellulose molecule [86, 87]. In the same direction, a recent molecular dynamics simulations study carried out on cellulose oligomers and 1-ethyl-3 methylimidazolium acetate (C2mimOAc) indeed suggest that the cations are in close contact

The challenge faced by generations of scientists and numerous companies is to find a simple, cheap, and non-polluting cellulose dissolution process. As pointed out in the previous sections, one of the crucial aspects in cellulose dissolution is the capacity of the fibers to be accessible for reagents in order to perform the subsequent transformation stages. Consequently, several chemical and physical methods, so-called ''activation", have been developed to ease cellulose dissolution, e.g. acid hydrolysis [24, 89], ball or vibration milling [4, 90], steam explosion [73– 75], and electron beam irradiation [91–93]. The main goal is to disrupt the cell wall structure and the hydrogen bond network in order to increase the accessibility of the solvent within the fiber structure, while preserving the original macromolecular structure of cellulose chains. Although such methods improve cellulose dissolution, typically these activation treatments are accompanied by a strong degradation of the cellulose chains leading to low DPs, ca. 600– 1200 for dissolving wood pulps and ca. 100–200 for cellulose powders. Such reduction in molecular weight may adversely compromise the properties of the final cellulose-based products [4]. This is mainly due to the lack of selectivity of the activation treatments that strongly affect the integrity of cellulose substrates at the different scales of their structure depending on the procedure employed. Thus, there is a need for more targeted activation

In this respect, the recent advances in biotechnology with the use of enzymes are expected to bring new insights for the development of targeted activation treatments. As it was previously discussed, the chemical environment of the cellulose chains, as well as their localization within the cell wall structure, plays a key role in their dissolution capacity. Due to their specificity, enzymes should thus be an alternative strategy to improve the selectivity of activation treatments towards "non-dissolving" areas within the fiber structure. Several studies have showed that enzymatic treatments can increase cellulose solubility basically due to changes in DP and hydrogen bond density [94, 95]. However, in these studies, the potential specific action of enzymes within the fiber structure for the improvement of their dissolution capacity was

Based on the observations that the external cell wall layers, i.e. the primary and S1 walls, prevent the efficient dissolution of cellulose fibers [67], Le Moigne et al. have recently inves‐ tigated the possibility to perform a targeted enzymatic peeling treatment in order to remove the external walls and to improve the dissolution capacity of cellulose fibers without degrading their whole structure [96]. The alkaline solubility of the different enzymatically treated samples was investigated in NaOH–water solution. The results showed that the enzymatic peeling has two effects at short treatment times, *(i)* a digestion of the primary wall that was assessed by the near absence of ballooning and *(ii)* a degradation of the fiber structure that was supported

with the cellulose through hydrophobic interactions [88].

10 Cellulose - Fundamental Aspects and Current Trends

methods.

not exploited.

*2.2.4. Improving cellulose dissolution: Towards targeted activation treatments*

Typically, the regeneration of cellulose occurs when contacting the cellulose solution with a coagulation medium. The polymer profile at the point of precipitation exhibits a very high interfacial concentration, thus favoring the formation of a dense polymer "skin". The bulk of the sample is at near the initial concentration and is in a fluid state. Thus, a rapid inflow of the coagulant can take place through the weak points at the skin interface. Rapid growth of fingerlike voids in the fluid region is expected to occur due to the moving interface created by the coagulant (less viscous) and solution (more viscous). The kinetics of regeneration is mainly controlled by the relative velocities of the counter-diffusion process; diffusion of the solvent fromthe solutionintothe coagulationbathandthe coagulantfromthebathintothe solution[99].

The exchange of solvent with non-solvent leads to a desolvation of the cellulose molecules and supposed reformation of the intra- and intermolecular hydrogen bonds [100]. The regenerated mechanical and surface chemical properties are known to depend strongly on the type of cellulose solvent and coagulant. The reader is directed to the work of Isobe et al. and references therein [101].

Regenerated cellulose is highly wettable and has been identified as one of the most hydrophilic polymers. The contact angles of water droplets on regenerated cellulose such as cellophane and cuprophane are 11.6° and 12.2°, respectively. In contrast, the wetting angle of commodity polymers such as poly(styrene) are 83 and 108.5°, respectively. The high hydrophilicity of cellulose is due to 3 hydroxyls per glucose unit, although this alone is not sufficient to explain the high wettability. A further reason for high hydrophilicity could be due to the uniplanar orientation of the (1 1 ¯ 0) crystal plane when parallel to the material surface as many hydroxyl groups are on this plane [102]. Despite its hydrophobic nature, the structural anisotropy of cellulose may provide regenerated cellulose with hydrophilic properties [10]. As described by Yamane et al. [103] through wide angle X-ray diffraction (WAXD) analysis, glucopyranoses in the (1 1 ¯ 0) crystal plane are stacked together by hydrophilic interactions and van der Waals forces. As a result, the density of hydroxyls on this plane surface is very high, resulting in the highest surface energy of crystal planes among cellulose I and cellulose II polymorphs. Molecular dynamics (MD) simulations are often used to elucidate on the mechanistic aspects of cellulose regeneration. Cousins and Brown have [104] suggested that a molecular sheet is the starting structure for the formation of native cellulose. Additionally, it is stated that the potential energy of the molecular sheets formed by hydrophobic interaction is far lower than that of hydrogen-bonded molecular sheets in water and therefore the former case was suggested to constitute the initial structure of crystallization.

Cellulose chains exist in a folded form and once chains are packed into particles their length can be more than ten-fold larger than their diameter [105]. Therefore, it is possible that the regenerated cellulose chains are also folded and, in the case of the cellulose II polymorph, chains align in an anti-parallel fashion. Studying the cellulose chain folded structure and ring conformers through MD, Yamane et al, proposed a mechanism for the structural formation of regenerated cellulose from a polar solution based on three stages, as schematically illustrated in Figure 4 [106]. and ring conformers through MD, Yamane et al, proposed a mechanism for the structural formation of regenerated cellulose from a polar solution based on three stages, as schematically illustrated in Figure 4 [106].

Figure 4. Schematic model of the structural formation of regenerated cellulose in an aqueous environment. The dark grey symbols schematically represent cellulose molecules, while the light grey represent water molecules. a) Glucopyranose rings stacking by hydrophobic interactions, forming a sheet-like structure; b) ordered and less ordered domains; c) regenerated material with a mixture of crystalline and amorphous regions (adapted from reference [106]). **Figure 4.** Schematic model of the structural formation of regenerated cellulose in an aqueous environment. The dark grey symbols schematically represent cellulose molecules, while the light grey represent water molecules. a) Glucopyr‐ anose rings stacking by hydrophobic interactions, forming a sheet-like structure; b) ordered and less ordered domains; c) regenerated material with a mixture of crystalline and amorphous regions (adapted from reference [106]).

Firstly, when the aqueous cellulose solution is set under unstable conditions (i.e. addition of a coagulant or via marked changes in temperature), cellulose molecules appear to aggregate side by side, with glucopyranose rings stacking by hydrophobic interactions, forming a sheet-like structure (Figure 4.a). Secondly, as the coagulation process proceeds, many sheet-like structures are progressively stacked by hydrogen bonds to form thin planar crystals incorporating amorphous regions. These structural domains are simultaneously laid down parallel to the surface of the shrinking cellulose gel due to their sheet-like shapes. Some aggregates that are tightly stacked with each other and free of defects transform into the crystalline regions (Figure 4.b on the left), while those that incorporate defects in their structure become less ordered domains or amorphous regions (Figure 4.b on the right). This mechanism suggests that the amorphous regions in regenerated cellulose could be defined as being composed of molecular sheets with distances that have substantial distributions, as originally described by Hermans in the late 1940s [107]. In the final stage, these randomly dispersed structural units make contact with other units and, by a diffusion cluster-cluster aggregation mechanism, form a regenerated material with a mixture of crystalline and amorphous regions (Figure 4.c) [108, 109].

As previously mentioned, in the course of coagulation the solvent is squeezed out from the precipitating cellulose gel, resulting in shrinkage, which causes the uniplanar orientation of the (1 1� 0) crystal plane. There is also recent experimental evidence for the molecular sheet formation process in cellulose regeneration [101]. The regeneration of cellulose triggered either by a coagulant or upon heating was followed in an aqueous alkali-urea solvent and monitored by time-resolved synchrotron X-ray scattering. It is suggested that when the medium surrounding the cellulose molecules becomes energetically unfavorable for molecular dispersion, the regeneration starts, and the initial process would consist of stacking the hydrophobic glucopyranoside rings (driven by hydrophobic interactions) to form monomolecular sheets, which then would line up by hydrogen bonding to form Na-cellulose type IV crystallites, a hydrate form of cellulose II (Figure 4) [110]. This may constitute one of the first experimental findings that is evidence of the development of hydrophobically stacked monomolecular sheets as hypothesized first by Hermans [107], later by Hayashi [111], and also discussed by Yamane et al. The work of Isobe et al. not only elucidates the regeneration mechanism of cellulose but also provides an alternative vision to the typical regeneration mechanism found in literature that essentially focuses on the reformation of the broken inter- and intramolecular hydrogen bonds among cellulose molecules [101]. Independently of the solvent system used, different studies concluded that both the coagulation medium and the post-treatment influence the regenerated cellulose films regarding the pore size distribution and crystallinity [112, 14, 113, 114]. Furthermore, the crystallinity, porosity and density of regenerated cellulose depend on the temperature and solvent concentration of the coagulation bath, which affect the diffusion rate and slower regeneration leads to higher crystallinity and density [115, 116]. The precipitation method affects cellulose formation as well, with remarkable differences between immersion and vapor precipitation. Song et al. show that crystalline cellulose

spherulites can grow from concentrated cellulose solution in IL by very slow precipitation in vapor [117].

Firstly, when the aqueous cellulose solution is set under unstable conditions (i.e. addition of a coagulant or via marked changes in temperature), cellulose molecules appearto aggregate side by side, with glucopyranose rings stacking by hydrophobic interactions, forming a sheet-like structure (Figure 4.a). Secondly, as the coagulation process proceeds, many sheet-like struc‐ tures are progressively stacked by hydrogen bonds to form thin planar crystals incorporating amorphous regions. These structural domains are simultaneously laid down parallel to the surface of the shrinking cellulose gel due to their sheet-like shapes. Some aggregates that are tightly stacked with each other and free of defects transform into the crystalline regions (Figure 4.b on the left), while those that incorporate defects in their structure become less ordered domains or amorphous regions (Figure 4.b on the right). This mechanism suggests that the amorphous regions in regenerated cellulose could be defined as being composed of molecu‐ lar sheetswithdistances thathave substantialdistributions, asoriginallydescribedbyHermans in the late 1940s [107].In the final stage,these randomly dispersed structural units make contact with other units and, by a diffusion cluster-cluster aggregation mechanism, form a regenerat‐ ed material with a mixture of crystalline and amorphous regions (Figure 4.c) [108, 109].

cellulose is due to 3 hydroxyls per glucose unit, although this alone is not sufficient to explain the high wettability. A further reason for high hydrophilicity could be due to the uniplanar

groups are on this plane [102]. Despite its hydrophobic nature, the structural anisotropy of cellulose may provide regenerated cellulose with hydrophilic properties [10]. As described by Yamane et al. [103] through wide angle X-ray diffraction (WAXD) analysis, glucopyranoses in

Cellulose chains exist in a folded form and once chains are packed into particles their length can be more than ten-fold larger than their diameter [105]. Therefore, it is possible that the regenerated cellulose chains are also folded and, in the case of the cellulose II polymorph, chains align in an anti-parallel fashion. Studying the cellulose chain folded structure and ring conformers through MD, Yamane et al, proposed a mechanism for the structural formation of regenerated cellulose from a polar solution based on three stages, as schematically illustrated

crystalline and amorphous regions (adapted from reference [106]).

**Figure 4.** Schematic model of the structural formation of regenerated cellulose in an aqueous environment. The dark grey symbols schematically represent cellulose molecules, while the light grey represent water molecules. a) Glucopyr‐ anose rings stacking by hydrophobic interactions, forming a sheet-like structure; b) ordered and less ordered domains;

c) regenerated material with a mixture of crystalline and amorphous regions (adapted from reference [106]).

Firstly, when the aqueous cellulose solution is set under unstable conditions (i.e. addition of a coagulant or via marked changes in temperature), cellulose molecules appear to aggregate side by side, with glucopyranose rings stacking by hydrophobic interactions, forming a sheet-like structure (Figure 4.a). Secondly, as the coagulation process proceeds, many sheet-like structures are progressively stacked by hydrogen bonds to form thin planar crystals incorporating amorphous regions. These structural domains are simultaneously laid down parallel to the surface of the shrinking cellulose gel due to their sheet-like shapes. Some aggregates that are tightly stacked with each other and free of defects transform into the crystalline regions (Figure 4.b on the left), while those that incorporate defects in their structure become less ordered domains or amorphous regions (Figure 4.b on the right). This mechanism suggests that the amorphous regions in regenerated cellulose could be defined as being composed of molecular sheets with distances that have substantial distributions, as originally described by Hermans in the late 1940s [107]. In the final stage, these randomly dispersed structural units make contact with other units and, by a diffusion cluster-cluster aggregation mechanism, form a regenerated material with a mixture of crystalline and amorphous regions (Figure 4.c) [108, 109].

As previously mentioned, in the course of coagulation the solvent is squeezed out from the precipitating cellulose gel, resulting in shrinkage, which causes the uniplanar orientation of the (1 1� 0) crystal plane. There is also recent experimental evidence for the molecular sheet formation process in cellulose regeneration [101]. The regeneration of cellulose triggered either by a coagulant or upon heating was followed in an aqueous alkali-urea solvent and monitored by time-resolved synchrotron X-ray scattering. It is suggested that when the medium surrounding the cellulose molecules becomes energetically unfavorable for molecular dispersion, the regeneration starts, and the initial process would consist of stacking the hydrophobic glucopyranoside rings (driven by hydrophobic interactions) to form monomolecular sheets, which then would line up by hydrogen bonding to form Na-cellulose type IV crystallites, a hydrate form of cellulose II (Figure 4) [110]. This may constitute one of the first experimental findings that is evidence of the development of hydrophobically stacked monomolecular sheets as hypothesized first by Hermans [107], later by Hayashi [111], and also discussed by Yamane et al. The work of Isobe et al. not only elucidates the regeneration mechanism of cellulose but also provides an alternative vision to the typical regeneration mechanism found in literature that essentially focuses on the reformation of the broken inter- and intramolecular hydrogen bonds among cellulose molecules [101]. Independently of the solvent system used, different studies concluded that both the coagulation medium and the post-treatment influence the regenerated cellulose films regarding the pore size distribution and crystallinity [112, 14, 113, 114]. Furthermore, the crystallinity, porosity and density of regenerated cellulose depend on the temperature and solvent concentration of the coagulation bath, which affect the diffusion rate and slower regeneration leads to higher crystallinity and density [115, 116]. The precipitation method affects cellulose formation as well, with remarkable differences between immersion and vapor precipitation. Song et al. show that crystalline cellulose

spherulites can grow from concentrated cellulose solution in IL by very slow precipitation in vapor [117].

suggested to constitute the initial structure of crystallization.

¯ 0) crystal plane are stacked together by hydrophilic interactions and van der Waals forces. As a result, the density of hydroxyls on this plane surface is very high, resulting in the highest surface energy of crystal planes among cellulose I and cellulose II polymorphs. Molecular dynamics (MD) simulations are often used to elucidate on the mechanistic aspects of cellulose regeneration. Cousins and Brown have [104] suggested that a molecular sheet is the starting structure for the formation of native cellulose. Additionally, it is stated that the potential energy of the molecular sheets formed by hydrophobic interaction is far lower than that of hydrogen-bonded molecular sheets in water and therefore the former case was

¯ 0) crystal plane when parallel to the material surface as many hydroxyl

**a)**

**b)**

**c)**

orientation of the (1 1

12 Cellulose - Fundamental Aspects and Current Trends

the (1 1

in Figure 4 [106]. and ring conformers through MD, Yamane et al, proposed a mechanism for the structural formation of regenerated cellulose from a polar solution based on three stages, as schematically illustrated in Figure 4 [106]. As previously mentioned, in the course of coagulation the solvent is squeezed out from the precipitating cellulose gel,resulting in shrinkage, which causes the uniplanar orientation ofthe (11 ¯ 0) crystalplane.There is alsorecent experimental evidence forthemolecular sheetformation process in cellulose regeneration [101]. The regeneration of cellulose triggered either by a coagulant or upon heating was followed in an aqueous alkali-urea solvent and monitored by time-resolved synchrotron X-ray scattering. It is suggested that when the medium surround‐ ing the cellulose molecules becomes energetically unfavorable for molecular dispersion, the regeneration starts, and the initial process wouldconsist of stacking the hydrophobic glucopyr‐ anoside rings (driven by hydrophobic interactions) to form monomolecular sheets, which then would line up by hydrogen bonding to form Na-cellulose type IV crystallites, a hydrate form of cellulose II (Figure 4) [110]. This may constitute one of the first experimental findings that is evidence of the development of hydrophobically stacked monomolecular sheets as hypothe‐ sized first by Hermans [107], later by Hayashi [111], and also discussed by Yamane et al. The workofIsobeetal.notonlyelucidates theregenerationmechanismof cellulosebutalsoprovides an alternative vision to the typical regeneration mechanism found in literature that essential‐ ly focuses on the reformation of the broken inter- and intramolecular hydrogen bonds among cellulose molecules [101]. Independently of the solvent system used, different studies conclud‐ edthatboththe coagulationmediumandthepost-treatmentinfluence the regeneratedcellulose films regarding the pore size distribution and crystallinity [112, 14, 113, 114]. Furthermore, the crystallinity, porosity and density of regenerated cellulose depend on the temperature and solvent concentration of the coagulation bath, which affect the diffusion rate and slower regenerationleadstohighercrystallinityanddensity[115,116].Theprecipitationmethodaffects cellulose formation as well, with remarkable differences between immersion and vapor precipitation. Song et al. show that crystalline cellulose spherulites can grow from concentrat‐ ed cellulose solution in IL by very slow precipitation in vapor [117].

> Figure 4. Schematic model of the structural formation of regenerated cellulose in an aqueous environment. The dark grey symbols schematically represent cellulose molecules, while the light grey represent water molecules. a) Glucopyranose rings stacking by hydrophobic interactions, forming a sheet-like structure; b) ordered and less ordered domains; c) regenerated material with a mixture of Typically, water is the most suitable coagulation agent but often the presence of certain additives (such as salts and amphiphiles) is required in order to tune the mechanical and morphological properties of the regenerated material.

### **4. Brief overview of some cellulose applications**

### **4.1. Fibers for textile and non-woven applications**

The forest industry is facing a paradigm shift, where fast growing short fibers from the southern hemisphere are claiming more room in a matured paper pulp market. Margins are under pressure and profitability will inevitably decrease. Simultaneously, paper consumption is ever decreasing. For the world leaders within the area of forestry and refining of forest raw materials, identifying new products and processes has become paramount. One very interest‐ ing alternative is the use of cellulose for the production of man-made fibers. Even though regenerated cellulose fiber has confirmed its position as one of the most dynamic segments of the century, achieving an average annual growth rate of 5.4 % since the year 2000, the demand for cellulose fibers is predicted to exceed the available supply by 3.3 million tons in the year 2020! This is due to population growth and increased prosperity megatrends, along with stagnating cotton production. Currently the global fiber consumption within the textile industry is about 80 million tons where only 5 % of the fibers are originated from dissolving pulp. Since petrochemically-derived fibrs are not sustainable in the long run and the cotton production is being increasingly questioned, sourcing sustainable raw materials has become increasingly difficult for global players in the textile market. In this respect, regenerated cellulose is regarded to potentially play a leading role as a replacement for cotton and synthetic fibres for textile and non-woven applications. Cellulose fibres are known to have good strength properties and a relatively high stiffness, while being only of moderate density, due to the hollow nature of the fiber composition. While the properties of natural fibers have an inherent variability in properties due to differences in species, as well as growing and harvesting conditions, the properties of regenerated cellulose fibers are homogeneous and can be adjusted by processing parameters. However, the successful use of this rapidly renewable, abundant, and biodegradable material will depend on the development of economically and environ‐ mentally advantageous processing technologies. Historically, one of the first solvent systems developed for cellulose was based on aqueous complexing agents (such as cupper salts in concentrated ammonia solutions, also referred to as cuprammonium solution). This discovery by Schweizer essentially triggered the cellulose fiber industry [21]. Three years later, H. Despassie developed another process to produce fibers from the same solvent system (aqueous cuprammonium solution-Cupro). A few years later, with the "stretch-spinning process" invention by Thiele, the process became commercialized. A competitor to this process, was the viscose route developed by Cross, Bevan, and Beadke in 1891 that is based on a reaction between cellulose and carbon disulfide (CS2) in alkali media. A labile and soluble intermediate in aqueous sodium hydroxide solution is formed (e.g. cellulose xanthogenate) and during precipitation of the shaped product, the substituent is cleaved off, and high purity nonmodified cellulose is regenerated [102, 118]. The viscose method is still the most important procedure for cellulose shaping and dominates the supply of regenerated cellulose fiber accounting for more than 90 % of the annual production volume.

This route is technologically complex, requiring reasonably high-quality dissolving pulp and leading to problematic environmental loads from the use of CS2, heavy metal compounds in the precipitation process, and resultant by-products [119]. Due to the emergence of the synthetic fiber industry in the 1940s and to the environmental pollution during the production of regenerated cellulose fibers, caused by the generation of harmful gases, the production of fibers based on Viscose and Cupro processes decreased in the 1960s. In order to overcome these disadvantages, many new solvents have been discovered and studied [120, 121]. A reasonably successful example in the commercial scale production is the Lyocel, a type of rayon fiber [39,122]. The underlying process has been in industrial use for several years, but the production is relatively small. One of the reasons is the thermal instability of the solvent (N-methylmor‐ pholine-N-oxide), which demands major investments in safety technology [40]. A very interesting alternative based on the viscose spinning technology is the CarbaCell process, which does not require the use of sulfur-containing compounds and has been developed to commercial maturity [119]. In this process, a new reaction variant employs the urea to convert cellulose into cellulose carbamate, which can be subsequently processed on existing viscose spinning systems.

As discussed before, the use of ionic liquids to dissolve and produce cellulose based fibers seems very attractive. An example of a recently synthesized ionic liquid with great potential for fiber spinning is the 1,5-diaza-bicyclo[4.3.0]non-5-enium acetate ([DBNH][OAc]), which is described as a powerful direct cellulose solvent [123]. The development of a process designated as Ioncell-F produces regenerated cellulose fiber with properties comparable (or even superior) to Lyocell [124].

### **4.2. Bioethanol**

**4. Brief overview of some cellulose applications**

accounting for more than 90 % of the annual production volume.

This route is technologically complex, requiring reasonably high-quality dissolving pulp and leading to problematic environmental loads from the use of CS2, heavy metal compounds in

The forest industry is facing a paradigm shift, where fast growing short fibers from the southern hemisphere are claiming more room in a matured paper pulp market. Margins are under pressure and profitability will inevitably decrease. Simultaneously, paper consumption is ever decreasing. For the world leaders within the area of forestry and refining of forest raw materials, identifying new products and processes has become paramount. One very interest‐ ing alternative is the use of cellulose for the production of man-made fibers. Even though regenerated cellulose fiber has confirmed its position as one of the most dynamic segments of the century, achieving an average annual growth rate of 5.4 % since the year 2000, the demand for cellulose fibers is predicted to exceed the available supply by 3.3 million tons in the year 2020! This is due to population growth and increased prosperity megatrends, along with stagnating cotton production. Currently the global fiber consumption within the textile industry is about 80 million tons where only 5 % of the fibers are originated from dissolving pulp. Since petrochemically-derived fibrs are not sustainable in the long run and the cotton production is being increasingly questioned, sourcing sustainable raw materials has become increasingly difficult for global players in the textile market. In this respect, regenerated cellulose is regarded to potentially play a leading role as a replacement for cotton and synthetic fibres for textile and non-woven applications. Cellulose fibres are known to have good strength properties and a relatively high stiffness, while being only of moderate density, due to the hollow nature of the fiber composition. While the properties of natural fibers have an inherent variability in properties due to differences in species, as well as growing and harvesting conditions, the properties of regenerated cellulose fibers are homogeneous and can be adjusted by processing parameters. However, the successful use of this rapidly renewable, abundant, and biodegradable material will depend on the development of economically and environ‐ mentally advantageous processing technologies. Historically, one of the first solvent systems developed for cellulose was based on aqueous complexing agents (such as cupper salts in concentrated ammonia solutions, also referred to as cuprammonium solution). This discovery by Schweizer essentially triggered the cellulose fiber industry [21]. Three years later, H. Despassie developed another process to produce fibers from the same solvent system (aqueous cuprammonium solution-Cupro). A few years later, with the "stretch-spinning process" invention by Thiele, the process became commercialized. A competitor to this process, was the viscose route developed by Cross, Bevan, and Beadke in 1891 that is based on a reaction between cellulose and carbon disulfide (CS2) in alkali media. A labile and soluble intermediate in aqueous sodium hydroxide solution is formed (e.g. cellulose xanthogenate) and during precipitation of the shaped product, the substituent is cleaved off, and high purity nonmodified cellulose is regenerated [102, 118]. The viscose method is still the most important procedure for cellulose shaping and dominates the supply of regenerated cellulose fiber

**4.1. Fibers for textile and non-woven applications**

14 Cellulose - Fundamental Aspects and Current Trends

Cellulose dissolution is typically required for one of the following reasons: analytical charac‐ terization of cellulose; shaping in a different format (such as fibers); homogenous chemical modification or degradation to obtain the elementary sugars. In the latter case, it is therefore possible to obtain ethanol from cellulose. In theory, this can be simply achieved by hydrolysis of cellulose-based materials using, for instance, enzymes to break cellulose into simple sugars, followed by fermentation and distillation. In reality, the process is not that straightforward. A pretreatment is typically necessary as cellulose usability is limited due to its structural rigidity, partial crystallinity, and complex network of interactions. This pretreatment also liberates cellulose from the lignin so it becomes more accessible for hydrolysis [125]. During enzymatic hydrolysis, the cellulose chains are broken into small oligomers or even into the elementary sugar units through cellulase enzymes in a similar process as occurring in the stomach of some ruminants. Some species of bacteria, such as *Clostridium thermocellum*, can promote an immediate conversion of cellulose into ethanol. These species break down cellulose and synthesize ethanol. Research in this area has also focused on genetically engineering bacteria that are capable of optimizing ethanol production and inhibit the formation of side products [126]. The environmental benefits of cellulosic ethanol are considerable as it can reduce carbon dioxide emissions to nearly zero and it can impressively reduce green house gas emissions compared with fossil fuels [127]. For instance, corn ethanol reduces green house gas emissions by 13 %, while this value exceeds 85 % if ethanol is of cellulosic origin [128]. The long-predicted arrival of cellulose-based fuels might be very close as several companies are initializing the production of ethanol from biomass while others are producing efficient enzymes.

### **4.3. Aerogels**

Aerogels are low-density solids (0.004–0.15 g/cm3) with high porosity (90–99.8 % air) and a large internal surface area [129–131]. Being one of the most attractive materials, aerogels are also referred to as frozen smoke, solid smoke, or solid air. Some of the most interesting applications include NASA missions, where silica aerogels are used to collect comet particles, insulators in semi-transparent roofs, thickening agents in paints and cosmetics, thermal protections for clothes and materials, drug delivery systems and water purification systems. The low density and high porosity (air takes up the majority of the space in an aerogel) permit the aerogel to be almost weightless. Most aerogels exhibit a slight color due to Rayleigh scattering and they can show a thermal conductivity smaller than the gas they contain, due to the Knudsen effect [132]. Aerogel shows superior insulation and can be used as alternative to the current home insulation. One windowpane (2.5 cm thick) composed of silica aerogel is equivalent to the insulation provided by 20 windowpanes of glass [133]. This will have a strong impact on energy saving to keep houses warm. Additionally, it has obvious impact in the worldwide production of carbon dioxide and other greenhouse gases. Aerogels can be extremely tough, able to hold up to 4,000 times their weight. These systems are typically prepared by a sol-gel process, involving crosslink formation between the components. Firstly, a colloidal suspension or solution of particles with diameter in the range of 1–1,000 nm are subjected to polymerization reactions and/or crosslinking that leads to the formation of a gel with a three dimensional network. The pores of the network contain the solvent of the suspension, usually water. The solvent in the gel can then be removed by exchanging with a CO2 miscible liquid such as ethanol, followed by liquid CO2 and then tuning CO2 above its critical point. An alternative method is to direct inject the supercritical CO2 into the gel. The pore solvent is replaced with air without altering the gel network structure or the volume of the gel. An alternative way to remove the solvent from the pores is by freeze drying the suspension. Such materials are typically called "cryogels" [134]. Normal evaporation of the pore solvent is inadequate to obtain common aerogels with high porosity: such methods result in xerogels that exhibit large amounts of shrinkage after drying, since the magnitude of the surface tensions of the liquid-solid interfaces can destroy the network.

Aerocellulose is a biodegradable aerogel composed of cellulose. The first aerocellulose known was prepared in 1971 from cellulose pulp [135]. The water from the wet cellulose pulp was exchanged with ethanol and then with CO2, which was removed by critical point drying, as described above. A cellulose aerogel with a specific surface area of 206 m2 /g was obtained. Since then many aerocellulose derivatives were reported. Tan et al. have developed cellulose acetate aerogels [136]. Jin et al. have developed highly porous cellulose aerogels consisting of cellulose nanofibrils with an approximate pore size of 500 nm [132]. Transparent aerocellulose that combine mechanical toughness and good heat insulation was reported by Kobayashi et al. [137]. Novel aerogels (or aerocellulose) based on all-cellulose composites were prepared by partially dissolving microcrystalline cellulose (MCC) in LiCl/DMAc solution [138]. The inclusion of nanocellulose as a building block for aerogels has also motivated several studies in particular because nanocellulose is able to provide mechanical reinforcement to the material among other advantages [139–145]. Also aerogels of regenerated cellulose prepared through molecular dissolution processes have attracted attention [146, 147]. It is clear that the interest in these materials is considerable and this is probably motivated from the vast range applica‐ tions and unique features of aerogels. Thus, it is not surprising that these materials have already started being produced on an industrial scale.

### **4.4. Hydrogels**

**4.3. Aerogels**

16 Cellulose - Fundamental Aspects and Current Trends

Aerogels are low-density solids (0.004–0.15 g/cm3) with high porosity (90–99.8 % air) and a large internal surface area [129–131]. Being one of the most attractive materials, aerogels are also referred to as frozen smoke, solid smoke, or solid air. Some of the most interesting applications include NASA missions, where silica aerogels are used to collect comet particles, insulators in semi-transparent roofs, thickening agents in paints and cosmetics, thermal protections for clothes and materials, drug delivery systems and water purification systems. The low density and high porosity (air takes up the majority of the space in an aerogel) permit the aerogel to be almost weightless. Most aerogels exhibit a slight color due to Rayleigh scattering and they can show a thermal conductivity smaller than the gas they contain, due to the Knudsen effect [132]. Aerogel shows superior insulation and can be used as alternative to the current home insulation. One windowpane (2.5 cm thick) composed of silica aerogel is equivalent to the insulation provided by 20 windowpanes of glass [133]. This will have a strong impact on energy saving to keep houses warm. Additionally, it has obvious impact in the worldwide production of carbon dioxide and other greenhouse gases. Aerogels can be extremely tough, able to hold up to 4,000 times their weight. These systems are typically prepared by a sol-gel process, involving crosslink formation between the components. Firstly, a colloidal suspension or solution of particles with diameter in the range of 1–1,000 nm are subjected to polymerization reactions and/or crosslinking that leads to the formation of a gel with a three dimensional network. The pores of the network contain the solvent of the suspension, usually water. The solvent in the gel can then be removed by exchanging with a CO2 miscible liquid such as ethanol, followed by liquid CO2 and then tuning CO2 above its critical point. An alternative method is to direct inject the supercritical CO2 into the gel. The pore solvent is replaced with air without altering the gel network structure or the volume of the gel. An alternative way to remove the solvent from the pores is by freeze drying the suspension. Such materials are typically called "cryogels" [134]. Normal evaporation of the pore solvent is inadequate to obtain common aerogels with high porosity: such methods result in xerogels that exhibit large amounts of shrinkage after drying, since the magnitude of the

surface tensions of the liquid-solid interfaces can destroy the network.

described above. A cellulose aerogel with a specific surface area of 206 m2

Aerocellulose is a biodegradable aerogel composed of cellulose. The first aerocellulose known was prepared in 1971 from cellulose pulp [135]. The water from the wet cellulose pulp was exchanged with ethanol and then with CO2, which was removed by critical point drying, as

Since then many aerocellulose derivatives were reported. Tan et al. have developed cellulose acetate aerogels [136]. Jin et al. have developed highly porous cellulose aerogels consisting of cellulose nanofibrils with an approximate pore size of 500 nm [132]. Transparent aerocellulose that combine mechanical toughness and good heat insulation was reported by Kobayashi et al. [137]. Novel aerogels (or aerocellulose) based on all-cellulose composites were prepared by partially dissolving microcrystalline cellulose (MCC) in LiCl/DMAc solution [138]. The inclusion of nanocellulose as a building block for aerogels has also motivated several studies in particular because nanocellulose is able to provide mechanical reinforcement to the material among other advantages [139–145]. Also aerogels of regenerated cellulose prepared through molecular dissolution processes have attracted attention [146, 147]. It is clear that the interest in these materials is considerable and this is probably motivated from the vast range applica‐

/g was obtained.

Renewable raw materials have immense potential for the development and research of new products. Development of new techniques together with the knowledge of traditional methods can facilitate the production of, for example, super-sorbent gels or hydrogels [148]. Cellulosebased hydrogels are biocompatible and biodegradable materials which are promising for a large number of industrial uses, in particular the cases where environmental issues are concerned, as well as in biomedical applications. Several water-soluble cellulose derivatives can be used, alone or combined, to form robust hydrogel networks possessing highly specific properties in terms of swelling capability and sensitivity to external stimuli. The current trend in the design of cellulose hydrogels mostly related to the use of non-toxic crosslinking chemicals or treatments, to further improve the safeness of both the end product and the productions processes. Structurally, due to the extended crosslinked network of polymers (intimately related to previously discussed aerogels), hydrogels are similar to the extracellular matrix of the human body [149]. These structures may be modified or designed in accordance to the external physical and chemical factors that may influence their activity during a certain application. Hydrogels are generally categorized into functional and synthetic on the basis of their formation [150]. Self-assembled natural polymers are termed as functional hydrogels whereas synthetic hydrogels are programmed to have desired functionality with multiple interactions.

The acrylamide-based hydrogels are currently all over the market but such systems raise serious environmental and societal concerns [148]. As stated above, cellulose is naturally available and abundant resource in nature. Even though cellulose is a biocompatible, biode‐ gradable, and sustainable polymer, cellulose itself cannot be reabsorbed by human cells due to the lack of cellulases.

The stability of hydrogels is altered by changing the type of cross-linker (in the case of chemical hydrogels) and number of cross-linking sites per unit volume. The free hydroxyl groups present on the cellulose backbone enable a rich surface chemistry and cellulose derivatives that are typically water-soluble [151]. Common examples are carboxymethyl cellulose (CMC), hydroxypropyl methylcellulose (HPMC), and hydroxyethyl cellulose (HEC) and these systems have the capacity to produce ultra fine gel networks, particularly when mixed with other polymers, resulting in high water retaining capacities and remarkable mechanical properties [152]. The water holding capacity is determined by the chemical composition and the manu‐ facturing procedures of the starting material. The "smart" behavior of some cellulose-based systems regarding response to physiologically relevant variables (e.g. pH, ionic strength, temperature) makes the resulting hydrogels particularly appealing for in vivo applications.

### *4.4.1. Hydrogels in agriculture*

A common concern nowadays in agriculture is related to the adequate supply and use of pesticides, herbicides, and fungicides, and the use of non-effective agrochemical products since these are devastating the agricultural economy. Additionally, the crop uptake of micronutrient should be balanced. Encapsulation in agriculture is a delivering method that seems to hold a promising targeted approach. This will enable, for instance, the decrease of losses of agrochemicals by erosion, being advantageous from an economic and sustainability point of view. Hydrogels can be specifically synthesized to give a sustainable delivery of water, nutrients, or agrochemicals in adequate amounts in an undisturbed fashion [153]. In this respect, cellulose-based systems (e.g. CMC- and HEC-based hydrogels) can be, for example, effective water reservoirs in agriculture. For instance, tomatoes have been successfully cultivated using these hydrogels in soil without any other water supply. The water absorption and conserving capacity seems to be enhanced. The development of super absorbent polymer (SAP) gels aim at reducing the release of water from the network. It has been reported that CMC and starch, when cross-linked with aluminium sulphate octadecahydrate, produce super absorbent gels [154] that can even be used in extreme drought conditions [155]. Other suc‐ cessful applications in agriculture include the so-called root-targeted delivery vehicle systems. These systems can be formed by dissolving CMC, iron, and calcium salts and are used, for instance, to grow wheat in nutrient depleted soil media. Additionally, when mixed with fertilizers, an efficient and increased growth rate is often observed. Another promising cellulose-based hydrogel is composed of methyl cellulose, polyacrylamide, and calcium montmorillonite that has been shown to be an efficient nutrient carrier. The addition of urea can make the hydrogel more porous, thus enhancing its capability for absorption [156].

### *4.4.2. Hydrogels in biomedicine*

Biomedical, healthcare, and pharmaceutical industries extensively use cellulose derivatives as release matrices, tablets, granules, delivery systems, stabilizers, semi solid gelling agents, artificial wound dressing, for cell encapsulation, and in many other applications [148]. A cellulose hybrid hydrogel having doped phosphor (PP) and epichlorohydrin as a cross-linker was reinforced in a lightly alkaline solution. This PP-based cellulose hydrogel was found to have great potential for bioimaging applications [157]. This system is particularly promising due to its high intensity in green florescence and extended glowing periods. This is relevant because it not only avoids the use of harmful radiation but can also be detected under the skin and/or in the stomach. A hydrophobic hydrogel network formed by cellulose nanowhiskers (extracted crystalline fractions of cellulose), acrylamide, and stearyl methacrylate possess unique properties of self-healing and remolding. This system has a wide extensibility and good mechanical strength making it highly suitable for different applications within the biomedical field [158]. Zhou et al. found that cellulose extracted from bamboo (*Phyllostachys heterocycla)* shows high swelling rates at human body temperature and intestinal pH. Thus, hydrogels based on this cellulose exhibit an immense potential as an oral drug model [159]. According to Sannino et al., edema problems can also be treated using cellulose-based hydrogels as these systems have improved water absorbing and holding capacity [160]. In several examples, no arachidonic acid release is identified confirming that inflammation does not occur and the cellulose-based hydrogel is biocompatible. Different studies have also given the prospect of using cellulose-based hydrogels as encapsulating agents for probacteria and its sustainable delivery after incorporation in different food matrix systems [161–163].

### **4.5. Cellulose-based composite materials**

these are devastating the agricultural economy. Additionally, the crop uptake of micronutrient should be balanced. Encapsulation in agriculture is a delivering method that seems to hold a promising targeted approach. This will enable, for instance, the decrease of losses of agrochemicals by erosion, being advantageous from an economic and sustainability point of view. Hydrogels can be specifically synthesized to give a sustainable delivery of water, nutrients, or agrochemicals in adequate amounts in an undisturbed fashion [153]. In this respect, cellulose-based systems (e.g. CMC- and HEC-based hydrogels) can be, for example, effective water reservoirs in agriculture. For instance, tomatoes have been successfully cultivated using these hydrogels in soil without any other water supply. The water absorption and conserving capacity seems to be enhanced. The development of super absorbent polymer (SAP) gels aim at reducing the release of water from the network. It has been reported that CMC and starch, when cross-linked with aluminium sulphate octadecahydrate, produce super absorbent gels [154] that can even be used in extreme drought conditions [155]. Other suc‐ cessful applications in agriculture include the so-called root-targeted delivery vehicle systems. These systems can be formed by dissolving CMC, iron, and calcium salts and are used, for instance, to grow wheat in nutrient depleted soil media. Additionally, when mixed with fertilizers, an efficient and increased growth rate is often observed. Another promising cellulose-based hydrogel is composed of methyl cellulose, polyacrylamide, and calcium montmorillonite that has been shown to be an efficient nutrient carrier. The addition of urea can make the hydrogel more porous, thus enhancing its capability for absorption [156].

Biomedical, healthcare, and pharmaceutical industries extensively use cellulose derivatives as release matrices, tablets, granules, delivery systems, stabilizers, semi solid gelling agents, artificial wound dressing, for cell encapsulation, and in many other applications [148]. A cellulose hybrid hydrogel having doped phosphor (PP) and epichlorohydrin as a cross-linker was reinforced in a lightly alkaline solution. This PP-based cellulose hydrogel was found to have great potential for bioimaging applications [157]. This system is particularly promising due to its high intensity in green florescence and extended glowing periods. This is relevant because it not only avoids the use of harmful radiation but can also be detected under the skin and/or in the stomach. A hydrophobic hydrogel network formed by cellulose nanowhiskers (extracted crystalline fractions of cellulose), acrylamide, and stearyl methacrylate possess unique properties of self-healing and remolding. This system has a wide extensibility and good mechanical strength making it highly suitable for different applications within the biomedical field [158]. Zhou et al. found that cellulose extracted from bamboo (*Phyllostachys heterocycla)* shows high swelling rates at human body temperature and intestinal pH. Thus, hydrogels based on this cellulose exhibit an immense potential as an oral drug model [159]. According to Sannino et al., edema problems can also be treated using cellulose-based hydrogels as these systems have improved water absorbing and holding capacity [160]. In several examples, no arachidonic acid release is identified confirming that inflammation does not occur and the cellulose-based hydrogel is biocompatible. Different studies have also given the prospect of using cellulose-based hydrogels as encapsulating agents for probacteria and its sustainable

delivery after incorporation in different food matrix systems [161–163].

*4.4.2. Hydrogels in biomedicine*

18 Cellulose - Fundamental Aspects and Current Trends

Composite materials have gained tremendous interest over the last decades and are found in numerous lightweight, structural applications ranging from aerospace to automotive and sports equipment. A large body of research has focused on replacing the traditional reinforce‐ ment of composites, such as glass fibers, with lignocellulosic fibers [164–167]. The most commonly used natural fibers are flax, hemp, sisal, jute, and wood [168, 167]. The high specific strength and stiffness of bast fibers, such as hemp, flax, and ramie, give natural fiber reinforced composite properties that compare favorably with glass fiber reinforced composites [169]. The advantages of natural fibers over synthetic fibers, include lower energy usage during produc‐ tion, reduced hazards during handling, less abrasion on machinery, lower density, and a neutral CO2-balance [169, 164]. However, a drawback of natural fiber is the large inherent scatter in properties due to differences in growing and harvesting conditions, necessitating rigorous quality management to ensure consistent composite properties [167].

In addition to bio-based reinforcements, there has been much effort on the development of naturally derived biopolymers as matrix systems [166, 170] in order to achieve a completely bio-based, biodegradable, and CO2-neutral material. Composites that are a mix of bio-based and synthetic materials are referred to as *biocomposites* or *eco-composites*, while those completely composed of bio-based materials are elevated to *green composites*.

### *4.5.1. All-cellulose composite materials*

The replacement of synthetic fiber by natural fiber is a promising advancement. However, chemical incompatibility may exist between a hydrophilic fiber reinforcement and hydropho‐ bic polymer matrix that leads to weak fiber-matrix bonding. Poor fiber-matrix bonding leads to inefficient load transfer between the two phases such that the high mechanical properties of natural fibers are not fully exploited [168]. The relatively poor mechanical performance of biocomposites is significantly improved by chemically treating the fiber and/or matrix. Common procedures include chemical grafting, corona discharge and silane or alkaline treatments. However, such procedures add process complexity and costs [171–173].

Single polymer composites (SPCs) overcome the issue of chemical incompatibility by using the same polymer for both fiber and matrix. The main advantage of using the same polymer for both phases is the high level of bonding, resulting in high interfacial strength and efficient load transfer. A further advantage of SPCs is the ease of recycling as the fiber and matrix components do not require extraction and separation. An overview of the field of SPCs is presented in several reviews [174–176].

All cellulose composites (ACCs) are green composites that have been developed using the SPC concept. In ACCs, both the reinforcing and matrix phases are based on cellulose, leading to high chemical compatibility at the fiber-matrix interface. Although high performance ACCs have recently appeared in the literature [177–179], there are historically a number of materials that could be categorized as ACCs. For example, currently used materials such as vulcanized fiber and paper were patented in 1859 [180], cellophane was patented in 1918 [181], and the processing of vegetable parchment is a technology dating back to the 19th century [182].

### *4.5.2. Processing of all-cellulose composites*

Generally, ACCs are processed *via* one of two different pathways, as illustrated in Figure 5. The first pathway involves two distinct processing steps in which (i) cellulose is first dissolved in a solvent and then (ii) regenerated in the presence of an undissolved cellulose reinforcement. The first example of this two-step method was reported by Nishino et al. [177], in which unidirectional ramie fiber was combined with fully dissolved wood pulp. The regeneration of the wood pulp created a matrix phase that provided adhesion between the undissolved ramie fibers. The second pathway is essentially a single processing step that involves wetting a cellulose reinforcement with a solvent and partially dissolving the cellulose. Regeneration of the dissolved portion of cellulose leads to the *in situ* formation of a matrix phase that binds together the undissolved fraction of cellulose. Gindl and Keckes [183] were the first to report the use of the one-step method for creating an ACC, and it has been variously referred to as partial dissolution [184], surface-selective dissolution [185] and natural fiber welding [186]. The one-step method results in ACCs with a relatively high volume fraction of fibers (e.g. ≤ 88 vol. %) [179]. Thus, the volume fraction of fibers in an ACC approaches the theoretical maximum of 90.7 % that exists for a hexagonal packing arrangement [187].

**Figure 5.** Schematic of processing pathways of ACCs: (a) One-step method and (b) Two-step method. Adapted from Nishino et al. [177].

A number of different cellulose solvents and cellulose types have been used for processing ACCs. The most commonly used solvents are NMMO, LiCl/DMAc, IL, and NaOH aqueous solution [188], of which NMMO, IL, and NaOH based solvents are advantageous because they do not require an activation step before dissolution.

*4.5.2. Processing of all-cellulose composites*

20 Cellulose - Fundamental Aspects and Current Trends

Nishino et al. [177].

Generally, ACCs are processed *via* one of two different pathways, as illustrated in Figure 5. The first pathway involves two distinct processing steps in which (i) cellulose is first dissolved in a solvent and then (ii) regenerated in the presence of an undissolved cellulose reinforcement. The first example of this two-step method was reported by Nishino et al. [177], in which unidirectional ramie fiber was combined with fully dissolved wood pulp. The regeneration of the wood pulp created a matrix phase that provided adhesion between the undissolved ramie fibers. The second pathway is essentially a single processing step that involves wetting a cellulose reinforcement with a solvent and partially dissolving the cellulose. Regeneration of the dissolved portion of cellulose leads to the *in situ* formation of a matrix phase that binds together the undissolved fraction of cellulose. Gindl and Keckes [183] were the first to report the use of the one-step method for creating an ACC, and it has been variously referred to as partial dissolution [184], surface-selective dissolution [185] and natural fiber welding [186]. The one-step method results in ACCs with a relatively high volume fraction of fibers (e.g. ≤ 88 vol. %) [179]. Thus, the volume fraction of fibers in an ACC approaches the theoretical

**Figure 5.** Schematic of processing pathways of ACCs: (a) One-step method and (b) Two-step method. Adapted from

A number of different cellulose solvents and cellulose types have been used for processing ACCs. The most commonly used solvents are NMMO, LiCl/DMAc, IL, and NaOH aqueous

maximum of 90.7 % that exists for a hexagonal packing arrangement [187].

The choice of cellulose source mainly determines the type of composite that will be obtained. Isotropic ACCs have been created from micro- or nanofibrillated cellulose [183, 189–192], wood pulp [193], bacterial cellulose [194, 195] and filter paper [189]. Uni- or multidirectional ACCs have been prepared from natural fibre including ramie [177, 178, 185] and flax [196], and regenerated cellulose fibers including Lyocell [196], Cordenka [184, 197], and Bocell [179].

The use of man-made regenerated fiber in ACCs is beneficial for several reasons: man-made regenerated cellulose fiber can be synthesised to be highly consistent in terms of purity, microstructure, morphology, and mechanical properties [198, 199]. Moreover, the homogene‐ ous microstructure of man-made regenerated cellulose fiber results in a uniform dissolution from the skin to the core [200]. In contrast, the dissolution of natural fiber can be inhomoge‐ neous, with ballooning, contraction, and rotation of cotton and wood fibers as an example [63, 68]. Furthermore, the structural integrity of the cell wall components of natural fiber is severely affected by partial dissolution, which is detrimental to the mechanical properties of flax fiberbased ACCs [196]. ACCs based on regenerated fibers (Lyocell) have a higher mechanical performance, and their mechanical properties compare favorably with epoxy-based compo‐ sites [196].

The structural form (or polymorph) of the precursor cellulose may undergo a phase transfor‐ mation during the processing of ACCs. An obvious example is the transformation of cellulose I to cellulose II following dissolution and regeneration. Thus, it is possible for a mixture of different polymorphs to be present in the final ACC. However, the transformation of cellulose I following dissolution and regeneration may follow different pathways depending on the starting materials and processing conditions. For example, Duchemin et al. reported that the partial dissolution and regeneration of MCC leads to the formation of a paracrystalline matrix phase, rather than the expected cellulose II [201].

The nature of the interface between different polymorphs within an ACC has not been studied in detail. Interestingly, Qin et al. observed improved mechanical properties of ramie fiberbased ACCs after a mercerisation treatment of the composites [178]. During mercerisation, cellulose I is transformed to cellulose II, which is concomitant with a decrease in the mechanical properties of cellulose [202]. Therefore, it is supposed that the resulting increase in mechanical properties is due to improved chemical compatibility between the cellulose II in the fiber and that of the matrix formed after partial dissolution. However, several other factors may play a role including the removal of hemicellulose, fiber strengthening, or elimination of defects [178].

Pullawan et al. investigated the micromechanics of the fiber-matrix interface of all-cellulose nanocomposites [203, 204]. Interestingly, the overall crystallinity of the cellulose increased from 26 % in the pure matrix film to 48 % in the ACC with the addition of 1 vol. % of highly crystalline cellulose nanowhiskers. However, a simple calculation based on the rule of mixtures shows that the upper limit of crystallinity of the nanocomposite is 27 %. Thus, the presence of nanowhiskers appears to assist in the nucleation of crystallites during the regen‐ eration of the dissolved cellulose [203]. Chemical compatibility between cellulose I and II is also suggested by the successful stress transfer from fibre to matrix [203, 204].

### *4.5.3. Developing industrial scale manufacturing pathways for all-cellulose composites*

The majority of literature studies of ACCs have produced and characterized films of ACCs with thicknesses that are ca. 0.5 mm. However, the expansion of ACCs into different applica‐ tions will likely require greater thicknesses of material. The manufacture of ACCs relies on wet processing and necessitates a washing and drying step. The removal of solvent and subsequent drying result in a volumetric shrinkage approximately equal to the ratio of cellulose to solvent. ACC films are typically cast from 5 to 25 vol. % cellulose and hence a shrinkage of > 80 % is to be expected, that is significantly higher than the shrinkage of 1 to 3 % observed during injection moulding of thermoplastics [205]. Furthermore, the removal of solvent from the dissolved portion of cellulose also results in *differential* shrinkage due to large differences in the shrinkage of the reinforcing and regenerated matrix phases. Differential shrinkage is problematic for two reasons: (i) internal residual stresses are generated that compromise the mechanical performance of the composite; and (ii) dimensional stability of the material is decreased following the final drying step (i.e. warpage occurs). Hence, it is clear that extensive experimental studies and development of predictive models of shrinkage in ACCs are still required in order to meet the requirements of the composite industry. Finally, the disposal and/or recycling of the solvent and identification of cost-effective sources of cellulose are important aspects in the context of industrial manufacturing that require further research and development.

Processes based on the one-step method seem more promising for larger scale manufacture of ACCs, due to the higher cellulose/solvent ratio and minimal transformation of cellulose precursor to matrix, which limits the overall and differential shrinkage. Industrial-scale composite manufacturing techniques (e.g., compression moulding, resin infusion) have been explored and adapted for larger scale one-step manufacturing of ACCs, although the technol‐ ogy is still in its infancy. A common characteristic of these processing routes is the application of pressure to consolidate the material during all stages of processing so as to manage the shrinkage and ensure dimensional stability. Conventional compression moulding of compo‐ sites involves the use of a rigid double-sided mould through which pressure and heat are applied to consolidate the reinforcement and matrix materials [206]. Compression moulding of ACC laminates was carried out by Huber et al. [197]. Initially, several layers of a woven regenerated cellulose fiber textile were impregnated with an ionic liquid as the solvent, followed by stacking and compression of the layers. The application of heat and pressure (110 °C, < 2.5 MPa, 80 min) leads to the partial dissolution of the fibers within the textile layers, resulting in the in situ formation of the matrix phase. The compression-moulded ACC laminates were formed into dimensionally-stable, flat sheets with a final thickness of 2 to 8 mm, tensile strength of 70 MPa, and Young's modulus of 2.5 GPa.

Vacuum-assisted resin transfer moulding (VARTM) is a liquid moulding process that is used to fabricate complex-shaped, high quality composite laminate parts [207, 208]. Typically, a woven textile preform is placed on a one-sided rigid mould that is then covered with a vacuum bag. A low pressure vacuum forces resin to flow through the textile preform, while also acting to remove voids and compact the laminate stack. As an adaptation of the VARTM process, a method called solvent infusion processing (SIP) was developed for the fabrication of dimen‐ sionally-stable ACC laminates. During SIP, a cellulosic textile is homogeneously wetted by a solvent (rather than resin). Further compaction and partial dissolution of the cellulose reinforcement are then achieved using external pressure and heat. SIP results in ACC laminates with a high fiber volume fraction (70 to 90 vol. %) with minimal void content [184]. The thickness of the ACC laminates is easily adjusted by varying the number of textile layers. The mechanical properties of the final laminate can be tailored through the choice of precursor (i.e. fiber preform). ACCs based on rayon fiber processed *via* SIP have been shown to exhibit high tensile and flexural strength of 95 MPa and 135 MPa, respectively, and outstanding impact resistance with a puncture impact strength of 2 kN mm-2 and an unnotched Charpy impact strength of 42 kJ m-2 [184, 209].

### *4.5.4. Mechanical properties of all-cellulose composites*

eration of the dissolved cellulose [203]. Chemical compatibility between cellulose I and II is

The majority of literature studies of ACCs have produced and characterized films of ACCs with thicknesses that are ca. 0.5 mm. However, the expansion of ACCs into different applica‐ tions will likely require greater thicknesses of material. The manufacture of ACCs relies on wet processing and necessitates a washing and drying step. The removal of solvent and subsequent drying result in a volumetric shrinkage approximately equal to the ratio of cellulose to solvent. ACC films are typically cast from 5 to 25 vol. % cellulose and hence a shrinkage of > 80 % is to be expected, that is significantly higher than the shrinkage of 1 to 3 % observed during injection moulding of thermoplastics [205]. Furthermore, the removal of solvent from the dissolved portion of cellulose also results in *differential* shrinkage due to large differences in the shrinkage of the reinforcing and regenerated matrix phases. Differential shrinkage is problematic for two reasons: (i) internal residual stresses are generated that compromise the mechanical performance of the composite; and (ii) dimensional stability of the material is decreased following the final drying step (i.e. warpage occurs). Hence, it is clear that extensive experimental studies and development of predictive models of shrinkage in ACCs are still required in order to meet the requirements of the composite industry. Finally, the disposal and/or recycling of the solvent and identification of cost-effective sources of cellulose are important aspects in the context of industrial manufacturing that require further

Processes based on the one-step method seem more promising for larger scale manufacture of ACCs, due to the higher cellulose/solvent ratio and minimal transformation of cellulose precursor to matrix, which limits the overall and differential shrinkage. Industrial-scale composite manufacturing techniques (e.g., compression moulding, resin infusion) have been explored and adapted for larger scale one-step manufacturing of ACCs, although the technol‐ ogy is still in its infancy. A common characteristic of these processing routes is the application of pressure to consolidate the material during all stages of processing so as to manage the shrinkage and ensure dimensional stability. Conventional compression moulding of compo‐ sites involves the use of a rigid double-sided mould through which pressure and heat are applied to consolidate the reinforcement and matrix materials [206]. Compression moulding of ACC laminates was carried out by Huber et al. [197]. Initially, several layers of a woven regenerated cellulose fiber textile were impregnated with an ionic liquid as the solvent, followed by stacking and compression of the layers. The application of heat and pressure (110 °C, < 2.5 MPa, 80 min) leads to the partial dissolution of the fibers within the textile layers, resulting in the in situ formation of the matrix phase. The compression-moulded ACC laminates were formed into dimensionally-stable, flat sheets with a final thickness of 2 to 8

Vacuum-assisted resin transfer moulding (VARTM) is a liquid moulding process that is used to fabricate complex-shaped, high quality composite laminate parts [207, 208]. Typically, a woven textile preform is placed on a one-sided rigid mould that is then covered with a vacuum

mm, tensile strength of 70 MPa, and Young's modulus of 2.5 GPa.

also suggested by the successful stress transfer from fibre to matrix [203, 204].

*4.5.3. Developing industrial scale manufacturing pathways for all-cellulose composites*

research and development.

22 Cellulose - Fundamental Aspects and Current Trends

The mechanical properties of ACCs strongly depend on the type of reinforcement and processing conditions. One of the main factors is the dissolution time and its effect is best explained with the example of a unidirectional ACC. With increasing dissolution times, a higher fraction of the fibers is transformed into the matrix, which decreases the longitudinal tensile strength due to the reduction of the fiber cross-sectional area, and simultaneously leads to an increase in transverse strength, due to a higher matrix-volume fraction and better interfacial adhesion [177, 185]. The dissolution time has a pronounced effect on the micro‐ structure and crystallinity of ACCs, as well as the DP of the processed cellulose [185, 190]. A similarly important parameter is the regeneration rate, with ACCs formed by slow precipita‐ tion displaying higher crystallinity, Young's modulus, and tensile strength [190].

The mechanical properties of ACCs can also be influenced by wet drawing. Stretching a regenerated, but still wet ACC leads to a preferred orientation of the cellulose crystallites in the direction of stretching [210, 192]. The crystalline orientation is maintained after drying and a linearly increasing relationship of tensile strength and Young's modulus with the applied draw ratio was found. Applying a draw ratio of 1.5 leads to an increase in tensile strength from 202 to 428 MPa and Young's modulus from 9.9 to 33.5 MPa [210]. Similarly, the orientation of nanowhiskers within an ACC can be influenced by a magnetic field to achieve an increase in mechanical properties in a preferred direction [211, 212].

The comparison of mechanical properties of ACCs in the literature is problematic due to variations in cellulose type and processing parameters. A broad overview of the mechanical properties of ACCs was presented by Huber et al. [188]. A comparison of ACCs with other biocomposites shows that ACCs are highly competitive in terms of their properties (Figure 6a). The Young's modulus of isotropic ACCs is relatively high when compared to conventional biocomposites; this can be attributed to the high properties of the regenerated cellulose matrix [188]. The situation is different for unidirectional composites, where the Young's modulus of ACCs falls in the same range as that of biocomposites (Figure 6b). This may be due to either a decrease in the fiber modulus caused by solvent interaction in the processing of ACCs or the modulus of biocomposites is mainly determined by the fiber modulus and the interfacial strength is of little influence [188]. When it comes to ultimate tensile strength, ACCs perform significantly better than isotropic and unidirectional biocomposites.

**Figure 6.** Comparison of mechanical properties of (a) isotropic and (b) unidirectional biocomposites and ACCs. Under‐ lying data is presented in the review by Huber et al. [188].

In terms of viscoelastic behavior, ACCs exhibit a storage modulus that decreases with increasing temperatures. Nevertheless, the storage modulus remains considerably high, up to a temperature of around 250 °C (Figure 7), indicating that ACCs may have relatively high thermal performance as polymer-based composites [190, 213]. The thermal stability strongly depends on the cellulose source. For example, ramie-based ACCs have a higher thermal resistance [177] than flax and Lyocell-based ACCs [196]. Dynamic mechanical analysis of cellulose reveals three separate glass transitions, labeled as α1, α2, and α3 (Figure 7). The α<sup>3</sup> transition occurs at ca. 30 °C and has been assigned to cooperative motions of cellulose chains and water molecules in the non-crystalline regions. The transitions α2 and α1 are observed around 140 °C and 300 °C, respectively, and have been assigned to micro-Brownian motions of polymer segments in non-crystalline regions [214]. Duchemin et al. observed that the α<sup>1</sup> transition is composed of two contributions, α1,1, and α1,2 [213]. Interestingly, the amplitude of the α1,2 viscoelastic relaxation at 303 ± 2 °C decreases with increasing crystallinity in ACCs made by partial dissolution of MCC. This relaxation was assigned to the molecular motion of non-crystalline domains during thermal decomposition. Composites with a higher crystallin‐ ity thus showed a better retention of the dynamic storage modulus when tested at 300 °C, which translates to a better thermo-mechanical stability in this temperature range.

**Figure 7.** Storage modulus (solid line) and damping (dashed line) of dry all-cellulose composites prepared by partial dissolution of 15 wt. % MCC in DMAc/LiCl for 8 h and followed by precipitation in water. Adapted from Duchemin et al. [213].

### *4.5.5. Biodegradability*

modulus of biocomposites is mainly determined by the fiber modulus and the interfacial strength is of little influence [188]. When it comes to ultimate tensile strength, ACCs perform

**Figure 6.** Comparison of mechanical properties of (a) isotropic and (b) unidirectional biocomposites and ACCs. Under‐

In terms of viscoelastic behavior, ACCs exhibit a storage modulus that decreases with increasing temperatures. Nevertheless, the storage modulus remains considerably high, up to a temperature of around 250 °C (Figure 7), indicating that ACCs may have relatively high thermal performance as polymer-based composites [190, 213]. The thermal stability strongly depends on the cellulose source. For example, ramie-based ACCs have a higher thermal resistance [177] than flax and Lyocell-based ACCs [196]. Dynamic mechanical analysis of cellulose reveals three separate glass transitions, labeled as α1, α2, and α3 (Figure 7). The α<sup>3</sup> transition occurs at ca. 30 °C and has been assigned to cooperative motions of cellulose chains and water molecules in the non-crystalline regions. The transitions α2 and α1 are observed around 140 °C and 300 °C, respectively, and have been assigned to micro-Brownian motions of polymer segments in non-crystalline regions [214]. Duchemin et al. observed that the α<sup>1</sup> transition is composed of two contributions, α1,1, and α1,2 [213]. Interestingly, the amplitude of

lying data is presented in the review by Huber et al. [188].

significantly better than isotropic and unidirectional biocomposites.

24 Cellulose - Fundamental Aspects and Current Trends

A key advantage of green composites that are based on biopolymers is their inherent biode‐ gradability. However, many synthetic biopolymers (e.g. polylactides, PLA) require energyintensive composting temperatures (ca. 60 °C) for biodegradation to occur rapidly [215, 216]. In contrast, native cellulose and regenerated cellulose can degrade rapidly at ambient tem‐ peratures (i.e. 20–30 °C) due to the hydrophilicity of cellulose [217, 218].

Kalka et al. were the first to demonstrate the rapid biodegradation of ACCs using soil burial trials. Initially, the ACC laminates increased in mass due to moisture uptake, although this was followed by significant mass loss as the material degraded. The percentage mass loss of the ACCs following 70 days of soil burial varied between 39 and 73 % at temperatures of 20.8 and 33.5 °C, respectively. Comparatively, the mass loss of a PLA-matrix composite using the same rayon fiber as reinforcement was minimal (< 2.4 %) under the same conditions [219].

Kalka et al. observed delamination and discoloration of the surface of the ACCs as evidence of microbial activity during the soil burial trials. Furthermore, the surface of ACCs was found to be colonized by filamentous bodies of cellulose decaying fungi and spherical fruit bodies. Microscopic analysis also revealed that clusters of fibers had separated from the composite. The mechanism of fiber-matrix debonding may be the preferential degradation of a more disordered matrix phase that permits greater accessibility to microorganisms compared with that of the original rayon fiber [219].

The rapid biodegradation of ACCs is a distinct advantage over other bio-based composites. However, the high potential for degradation also limits the use of ACCs in some applications. The development of bulk modifications or surface treatments are likely to be required to protect the high mechanical properties of ACCs from the influence of water. For example, Yousefi et al. found that immersion treatment of ACCs with a silane coupling agent (3 wt. % dodecyltriethoxysilane solution) leads to an increase in water contact angle from 59 to 93° and reduces the percentage water uptake from > 4 % to < 1 %. These results demonstrate the efficacy of a silane treatment for the protection of ACCs from moisture ingress, while additionally resulting in higher mechanical properties due to a lower equilibrium moisture content of treated samples and the closure of voids by the silane [220].

### **5. Conclusions**

The dependence of mankind on depleting fossil resources is a problematic issue, and based upon its criticism, a high demand for more sustainable engineering materials has arisen. Cellulose is a decidedly interesting alternative raw material to petrochemical-derived mate‐ rials due to its high strength and stiffness, wide availability, and biodegradability. Due to the complexity of the biopolymeric network, as well as the partially crystalline structure and extended noncovalent interactions among molecules, chemical processing of cellulose is rather difficult. A wide variety of suitable solvents for cellulose is already available. Nevertheless, most solvent systems have important limitations and there is an intense activity in both industrial and academic research aiming to optimize existing solvents and develop new ones. The problem of obtaining a picture of molecular processes is not trivial since cellulose solvents are of remarkably different nature and thus the understanding of the subtle balance between the different interactions involved becomes non-trivial. The dissolution and regenerative precipitation of cellulose appear to both depend upon a subtle interplay of forces, among which hydrogen bonds are only one of the important factors, and do not seem to be the most significant driving force. Key factors instead include hydrophobic interactions, due to the amphiphilic character of cellulose, as well as the entropy of the counterions for the case that cellulose molecules have a net charge; this net charge can arise from protonation or deproto‐ nation as well as association with charged species. Cellulose can find applications in many and different areas as the ones briefly discussed in this treatise. However, it becomes clear that the understanding of the interplay between all the interactions is very important not only from a fundamental point but particularly for the creation and development of enhanced materials.

### **Acknowledgements**

The authors acknowledge support from the Portuguese Foundation for Science and Technol‐ ogy (project PTDC/AGR-TEC/4049/2012, PTDC/AGR-TEC/4814/2014, PhD and post-doc grants assigned to Luis Alves: SFRH/BD/80556/2011 and Bruno Medronho: SFRH/BPD/ 74540/2010, respectively). Jan Dormanns thanks the University of Canterbury for funding a Doctoral Scholarship. Poonam Singh acknowledges the received funding from the People Programme (Marie Curie Actions) of the European Union's Seventh Framework Programme FP7/2007-2013/ under REA grant agreement n° 606713.

### **Author details**

disordered matrix phase that permits greater accessibility to microorganisms compared with

The rapid biodegradation of ACCs is a distinct advantage over other bio-based composites. However, the high potential for degradation also limits the use of ACCs in some applications. The development of bulk modifications or surface treatments are likely to be required to protect the high mechanical properties of ACCs from the influence of water. For example, Yousefi et al. found that immersion treatment of ACCs with a silane coupling agent (3 wt. % dodecyltriethoxysilane solution) leads to an increase in water contact angle from 59 to 93° and reduces the percentage water uptake from > 4 % to < 1 %. These results demonstrate the efficacy of a silane treatment for the protection of ACCs from moisture ingress, while additionally resulting in higher mechanical properties due to a lower equilibrium moisture content of

The dependence of mankind on depleting fossil resources is a problematic issue, and based upon its criticism, a high demand for more sustainable engineering materials has arisen. Cellulose is a decidedly interesting alternative raw material to petrochemical-derived mate‐ rials due to its high strength and stiffness, wide availability, and biodegradability. Due to the complexity of the biopolymeric network, as well as the partially crystalline structure and extended noncovalent interactions among molecules, chemical processing of cellulose is rather difficult. A wide variety of suitable solvents for cellulose is already available. Nevertheless, most solvent systems have important limitations and there is an intense activity in both industrial and academic research aiming to optimize existing solvents and develop new ones. The problem of obtaining a picture of molecular processes is not trivial since cellulose solvents are of remarkably different nature and thus the understanding of the subtle balance between the different interactions involved becomes non-trivial. The dissolution and regenerative precipitation of cellulose appear to both depend upon a subtle interplay of forces, among which hydrogen bonds are only one of the important factors, and do not seem to be the most significant driving force. Key factors instead include hydrophobic interactions, due to the amphiphilic character of cellulose, as well as the entropy of the counterions for the case that cellulose molecules have a net charge; this net charge can arise from protonation or deproto‐ nation as well as association with charged species. Cellulose can find applications in many and different areas as the ones briefly discussed in this treatise. However, it becomes clear that the understanding of the interplay between all the interactions is very important not only from a fundamental point but particularly for the creation and development of enhanced materials.

The authors acknowledge support from the Portuguese Foundation for Science and Technol‐ ogy (project PTDC/AGR-TEC/4049/2012, PTDC/AGR-TEC/4814/2014, PhD and post-doc grants assigned to Luis Alves: SFRH/BD/80556/2011 and Bruno Medronho: SFRH/BPD/

that of the original rayon fiber [219].

26 Cellulose - Fundamental Aspects and Current Trends

**5. Conclusions**

**Acknowledgements**

treated samples and the closure of voids by the silane [220].

Poonam Singh1 , Hugo Duarte2 , Luís Alves1 , Filipe Antunes1 , Nicolas Le Moigne3 , Jan Dormanns4,5, Benoît Duchemin6 , Mark P. Staiger4,5 and Bruno Medronho2\*

\*Address all correspondence to: bfmedronho@ualg.pt

1 University of Coimbra, Department of Chemistry, Coimbra, Portugal

2 Faculty of Sciences and Technology, Centre for Mediterranean Bioresources and Food, University of Algarve, Campus de Gambelas, Faro, Portugal

3 Centre des Matériaux des Mines d'Alès (C2MA), Ecole des Mines d'Alès, France

4 Department of Mechanical Engineering, University of Canterbury, Christchurch, New Zealand

5 MacDiarmid Institute for Advanced Materials and Nanotechnology, Wellington, New Zea‐ land

6 Laboratoire Ondes et Milieux Complexes, Normandie Université, Le Havre, France

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### **An Assessment of Surface Properties and Moisture Uptake of Nonwoven Fabrics from Ginning By-products**

Vince Edwards, Paul Sawhney, Alvin Bopp, Alfred French, Ryan Slopek, Michael Reynolds, Chuck Allen, Brian Condon and Joseph Montalvo

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/61329

### **Abstract**

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10.1016/j.compositesa.2013.12.012.

Greige (raw) cotton by-products resulting from cotton ginning and mill processes have long been bleached for use in absorbent nonwoven products. The potential to use greige cotton by-products as an economical source for absorbent nonwoven blends is explored. The nonwoven hydroentanglement of greige cotton lint with cot‐ ton gin motes and comber noils blends was analyzed for fiber surface polarity, swel‐ ling, and absorbance to assess properties with potential usefulness in absorbent nonwovens. The electrokinetic analysis of the fabric surface gives a composite picture of the relative hydrophilic/hydrophobic polarity absorbency and swelling properties. Nonwoven fabrics made with cleaned greige cotton lint separately blended with comber noils and ginning motes at 40:60 and 60:40 blend ratios demonstrated charge, swell, and percent moisture uptake profiles that are characteristic of the fabrics' crys‐ talline/amorphous cellulosic content with some variance in swelling properties. How‐ ever, cellulose crystallite size varied. X-ray diffraction patterns of the three different cotton constituents displayed similar crystalline cellulose compositions. An electro‐ chemical double-layer analysis of charge based on a pH titration (ζplateau) was em‐ ployed to measure the relative fiber and fabric surface polarity which varied slightly between -21 and -29 mV. A relationship of fiber swelling (∆ζ) and percent moisture content is apparent when greige cotton lint and other fibers are blended. The blended nonwoven materials possess absorbent properties characterized by similar moisture uptake (7.1-9.5 %) and fiber polarity, but some variation in swelling is based on the by-product additive and its percent content. The crystallinity, electrokinetic, and wa‐ ter binding properties of the nonwoven by-product materials are discussed in the con‐ text of the molecular features water, cellulose, and greige cotton components that enhance potential uses as absorbent nonwoven end-use products.

**Keywords:** nonwoven cotton, surface properties, cellulose, water of hydration, elec‐ trokinetic, by-products

© 2015 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

### **1. Introduction**

### **1.1. Cotton and cotton by-products in nonwovens**

In recent years, the preference to use cotton fibers in nonwoven absorbent products has increased. Cotton fiber is naturally renewable and biodegradable. Cotton's characteristic soft hand, hypoallergenic properties, absorbency, and cellulosic composition have been histori‐ cally utilized mostly in woven fabric products. Cotton's current use in nonwovens is estimated to be approximately 2 % (by volume/weight) of the total fiber consumption in nonwovens. Most of the cotton used at present in absorbent nonwovens is bleached cotton, including lint, gin motes, linters, comber noils, and the so-called other cotton textile processing wastes. However, the potential to use greige (nonbleached) cotton in nonwoven absorbent products has received increased attention based on innovations in cotton cleaning and nonwovens processes that open and expose the hydrophilic cellulosic component of greige cotton fiber to water absorption [1-3]. This affords an economical source of highly cleaned absorbent greige cotton nonwovens with the retention of properties inherent to the traditional cotton fabrics that generally require costly and eco-sensitive chemical scouring and bleaching processes.

Griege (raw) cotton gin motes are just one of several by-products (viz., cotton seeds, linters, motes, sticks, burrs, gin trash, among others) of the "cotton ginning process" that mechanically separates the (long) cotton fibers (greige lint) from the harvested seed cotton. The cotton ginning by-products are used in numerous applications [4-9]. The cotton gin motes typically have fibers up to ½ inch long and thus can be classified as textile fibers. However, limited use of cotton gin motes is made in traditional textiles made with spun yarns. Although cotton gin motes and textile comber noils have been used in cotton socks, their relatively lower cost and apparent differences in structural and surface properties make these cotton coproducts/fibers of interest to explore a broader nonwoven and high value end use. In addition, the processing innovations of modern nonwovens provide a facile conduit for efficiently blending these types of discounted by-products with greige cotton lint to explore new, value-added cotton-blend nonwoven products.

### **1.2. Absorbent applications**

Highly cleaned greige cotton fiber retains most of its natural, native protective membrane or surface coating of waxes and pectin (native to the greige cotton fiber). In combination with surface-exposed cellulose from nonwoven hydroentanglement process conditions, unique fiber properties are retained when compared to scoured and bleached cotton. The amphiphilic surface character in nonwoven greige cotton, which is a combination of the polarity balance between the hydrophilic and the hydrophobic elements of the cotton material, is suitable for the application to the material layer components of incontinence absorbent products and wipes [10].

The potential for the topsheet application of nonwoven greige cotton for incontinence and wound dressing's materials is based on its suitability as it facilitates a mechanism of optimal polar gradients between the layers surrounding the absorbent core. During fluid transport, an optimal charge gradient between the layers from topsheet to backsheet helps facilitate better performance of wettable and absorbent materials and is associated with lower rewet, strike‐ through, and acquisition/distribution to the absorbent core [11]. Nonwoven greige cotton compares well with other commercial materials when analyzed for its performance as an incontinence layer surrounding the absorbent core. Moreover, when combined with synthetic fibers like polyester and polypropylene, greige cotton/synthetic blends possess an added scope of utilization in this regard [10]

### **1.3. Fabric surface properties and electrokinetic analysis**

**1. Introduction**

nonwoven products.

[10].

**1.2. Absorbent applications**

**1.1. Cotton and cotton by-products in nonwovens**

46 Cellulose - Fundamental Aspects and Current Trends

In recent years, the preference to use cotton fibers in nonwoven absorbent products has increased. Cotton fiber is naturally renewable and biodegradable. Cotton's characteristic soft hand, hypoallergenic properties, absorbency, and cellulosic composition have been histori‐ cally utilized mostly in woven fabric products. Cotton's current use in nonwovens is estimated to be approximately 2 % (by volume/weight) of the total fiber consumption in nonwovens. Most of the cotton used at present in absorbent nonwovens is bleached cotton, including lint, gin motes, linters, comber noils, and the so-called other cotton textile processing wastes. However, the potential to use greige (nonbleached) cotton in nonwoven absorbent products has received increased attention based on innovations in cotton cleaning and nonwovens processes that open and expose the hydrophilic cellulosic component of greige cotton fiber to water absorption [1-3]. This affords an economical source of highly cleaned absorbent greige cotton nonwovens with the retention of properties inherent to the traditional cotton fabrics that generally require costly and eco-sensitive chemical scouring and bleaching processes.

Griege (raw) cotton gin motes are just one of several by-products (viz., cotton seeds, linters, motes, sticks, burrs, gin trash, among others) of the "cotton ginning process" that mechanically separates the (long) cotton fibers (greige lint) from the harvested seed cotton. The cotton ginning by-products are used in numerous applications [4-9]. The cotton gin motes typically have fibers up to ½ inch long and thus can be classified as textile fibers. However, limited use of cotton gin motes is made in traditional textiles made with spun yarns. Although cotton gin motes and textile comber noils have been used in cotton socks, their relatively lower cost and apparent differences in structural and surface properties make these cotton coproducts/fibers of interest to explore a broader nonwoven and high value end use. In addition, the processing innovations of modern nonwovens provide a facile conduit for efficiently blending these types of discounted by-products with greige cotton lint to explore new, value-added cotton-blend

Highly cleaned greige cotton fiber retains most of its natural, native protective membrane or surface coating of waxes and pectin (native to the greige cotton fiber). In combination with surface-exposed cellulose from nonwoven hydroentanglement process conditions, unique fiber properties are retained when compared to scoured and bleached cotton. The amphiphilic surface character in nonwoven greige cotton, which is a combination of the polarity balance between the hydrophilic and the hydrophobic elements of the cotton material, is suitable for the application to the material layer components of incontinence absorbent products and wipes

The potential for the topsheet application of nonwoven greige cotton for incontinence and wound dressing's materials is based on its suitability as it facilitates a mechanism of optimal polar gradients between the layers surrounding the absorbent core. During fluid transport, an The zeta (ζ) potential is a measure of the charge and charge density on the surface of a particle or fiber and can be used to gauge the stability of a colloidal or the ability of a material to absorb liquid. The ζ potential can be either positive or negative depending on the surface chemistry. The ζ potential analysis is useful with absorbent materials. The fluid dynamics of the electro‐ chemical double-layer model enables measurement of functional properties similar to those that occur at the solid-liquid interface of incontinence materials [12]. The ζ potential decrease (a change in electrostatic potential at the shear plane of the electrochemical double layer) is caused by the swelling of the fibers and the outward movement of the aqueous shear plane where ions are in contact with the outer Helmholtz plane on the fiber surface [13]. Thus, a concomitant increase in the swelling of the fiber occurs for most fibers as the shear plane moves out to the more diffuse layer of ions, causing a decrease in ζ potential [14].

This chapter examines the electrokinetic properties of hydroentangled nonwoven materials made by blending clean greige cotton lint with greige cotton by-product fibers with a view to understanding the similarities the materials possess. The three constituent cotton fibers employed in the nonwovens were evaluated for their relative cellulosic crystallinity in relation to their moisture uptake properties. Some of the blends also included polyester fibers for comparison as were previously examined [10]. Two different measures of moisture uptake are also reported. One method [15] is simple, portable, easy to use, and involves an infrared lamp to dry the samples. All weight loss is attributed to moisture. The other method is the recent ASTM Karl Fischer titration method for water content developed for lint cotton, raw and processed. Water absorbency tests were also conducted. We report here the preparation, characterization, and electrokinetic analysis of cleaned greige cotton [16] in combination with gin motes and comber noils at two different blend ratios of the greige cotton and the byproducts as a measure of the fabric polarity, swelling, and absorbent properties achievable with these fabric blends. The properties of the cotton materials are discussed in light of their water binding properties related to potential absorbent applications.

### **2. Experimental materials and methods**

A commercially available bale of precleaned greige cotton was acquired from T. J. Beall, LLC. A quantity of cotton gin mote fibers was also obtained from T. J. Beall.

### **2.1. Hydroentanglement of fibrous webs into nonwoven fabric structures**

The needle-punched webs of the different fiber blends were uniformly hydroentangled using a Fleissner MiniJet system (Figure 1). The system is equipped with one low water pressure jet head that wets the incoming feed web material on its top face, while two high water pressure jet heads alternatively impact the wetted substrate on either face. For all the fabrics, the low water pressure head was set to inject the water at 50 bars, and the two high water pressure heads were set at 125 bars. The fabric production speed was 5 m per minute. The resulting hydroentangled fabric was dried using a meter-wide, gas-fired drum dryer and wound onto a cardboard tube to form a compact fabric roll. The hydroentangling line was flushed and cleaned after each fabric production trial.

**Figure 1.** Diagram of a nonwovens hydroentanglement line. An outline schematic of the Fleissner MiniJet system used in the study.

### **2.2. Fabric absorbency measurements**

The AATCC drop test measures the time it takes for one drop of water applied to a fabric held in an embroidery hoop to be absorbed (when the sheen disappears). The ATSM method uses a sample of fabric that is 76 mm wide and cut to a length that equals 5.0 ± 0.1 g. The sample is rolled into a cylindrical shape, upon itself, and placed in a basket of standardized weight and size. The basket is dropped from a height of 25 mm into a water bath, and the time it takes the sample and basket to sink is measured as sink time. After sinking, the sample is then allowed to remain submerged in water for 10 s. The basket is removed and allowed to drain for 10 s, and the sample is weighed to determine its water content. The weight of the water is reported as the absorptive capacity (grams of water held by 1 g of fabric).

The test methods used in the study are as follows:


### **2.3. Measurement of fabric polarity, charge, relative composition, swelling, and porosity**

Streaming zeta potential experiments are carried out with an electrokinetic analyzer, which is manufactured in Ashland VA, USA, using the cylindrical cell developed for the measurement of fibrous samples. For each measurement, a fiber plug is placed between the Ag/AgCl hollow cylindrical electrodes of the cylindrical cell. The pH dependence of the zeta potential is investigated with the background electrolyte of 1 mM KCl solution. The swelling behavior of the incontinence products is measured using the Anton Paar analyzer with the cylindrical cell template. A 0.65-g sample is loaded into the cell and quickly rinsed with electrolyte solution. The flow rate is adjusted into the range of 60-100 mL/min by compression of the sample to remove trapped air. The pH of the sample is about 5.5 and was adjusted to 9.0 by adding 0.1 N NaOH solution.

### **2.4. Moisture uptake by ASTM D629-99**

**2.1. Hydroentanglement of fibrous webs into nonwoven fabric structures**

cleaned after each fabric production trial.

48 Cellulose - Fundamental Aspects and Current Trends

**2.2. Fabric absorbency measurements**

as the absorptive capacity (grams of water held by 1 g of fabric).

**•** ASTM D6242—mass per unit area (weight) of nonwoven textile fabrics

**•** ATSM D5035—breaking force and elongation of textile fabrics (strip method)

The test methods used in the study are as follows:

**•** AATCC 79—absorbency of textiles (drop test)

in the study.

The needle-punched webs of the different fiber blends were uniformly hydroentangled using a Fleissner MiniJet system (Figure 1). The system is equipped with one low water pressure jet head that wets the incoming feed web material on its top face, while two high water pressure jet heads alternatively impact the wetted substrate on either face. For all the fabrics, the low water pressure head was set to inject the water at 50 bars, and the two high water pressure heads were set at 125 bars. The fabric production speed was 5 m per minute. The resulting hydroentangled fabric was dried using a meter-wide, gas-fired drum dryer and wound onto a cardboard tube to form a compact fabric roll. The hydroentangling line was flushed and

**Figure 1.** Diagram of a nonwovens hydroentanglement line. An outline schematic of the Fleissner MiniJet system used

The AATCC drop test measures the time it takes for one drop of water applied to a fabric held in an embroidery hoop to be absorbed (when the sheen disappears). The ATSM method uses a sample of fabric that is 76 mm wide and cut to a length that equals 5.0 ± 0.1 g. The sample is rolled into a cylindrical shape, upon itself, and placed in a basket of standardized weight and size. The basket is dropped from a height of 25 mm into a water bath, and the time it takes the sample and basket to sink is measured as sink time. After sinking, the sample is then allowed to remain submerged in water for 10 s. The basket is removed and allowed to drain for 10 s, and the sample is weighed to determine its water content. The weight of the water is reported

The moisture content of the cotton samples is measured using modified ASTM D629-99 and AATCC 20A-2000 methods. (The modification involved using an infrared lamp to dry the materials rather than a laboratory oven as called for in the standard methods.) The sample is conditioned overnight in a humidity chamber with the hygrometer reading at 70 % and a room temperature of 23°C. The moisture measurements are based on weight loss and are taken with an infrared moisture balance (Kett FD 240, manufactured by Kett Electric Laboratory in Tokyo, Japan). The balance is set for automatic wet-based moisture with a drying temperature of 110°C. Approximately, a 1-g sample was used for each measurement on the Kett FD 240.

### **2.5. Water content by Karl Fischer titration ASTM D7795-12**

In order to determine the water content of cotton fibers via Karl Fischer titration (KFT), fiber samples must be conditioned to standard testing conditions, 21.1°C and 65 % relative humidity. The samples were weighed into 0.1-g specimens to four decimal places, placed in the Karl Fischer glass vials, and immediately crimped with septa caps. Each blend was formulated directly in the glass KFT vials by weight basis. All open vials were equilibrated to moisture equilibrium, sealed with a septum cap, and analyzed by standard KFT (ASTM D7785, 2012). The enclosed samples were then placed into mason jars that had been acclimated in the conditioned lab. The samples were then encapsulated in the jar right up until testing to maintain the environment. Prior to KFT testing cotton samples, blank vials were used for quality control measures. During testing, the vial is lowered into a 150°C single sample oven for approximately 5 min, while dry nitrogen (< 20 ppb water) is bubbled into the sample at 60 mL per minute. The water from the cotton sample from the vial is released and driven into the titration cell from which the percentage of moisture present is calculated from the volume of reagent consumed. The water content in the blank vial was 0.1 %. After analysis, a visual observation of the containers revealed all samples were white; there was no discoloration or scorching. Additionally, the polyester (PES) fibers had not melted.

### **2.6. Standard oven drying ASTM D2495-01**

The typical method to measuring moisture or weight loss of cotton fibers involves using an oven where samples are heated to 105-110°C for 24 h. Empty sample bottles were weighed and then filled with 1 g samples and reweighed. Cottons were heated in a Yamato DKN 600 mechanical convection oven with a 150-L capacity and a mean flow rate of approximately 1.3 L/s. All weights were made in a standard conditioned laboratory.

### **2.7. X-ray diffraction**

Wiley-milled samples of the comber noils, gin motes, and UltraClean cotton were pressed into pellets and placed in a powder diffractometer with Cu Kα radiation in reflection mode. The sample was secured in a paraffin base, which had a small effect on the comber noil pattern. For comparison, a pattern was simulated with the Mercury software [17] based on a modified crystal information file from the cellulose Iβ crystal structure [18]. The modification was a short increase in the *a*-axis of the unit cell to 7.906 from the original 7.784 value to better position the (200) peak because of the small differences in the cotton and tunicate unit cell [19]. A peak width at half maximum height of 1.85° 2*θ* was used. Additionally, the preferred orientation induced by pressing the sample pellet was compensated by a facility in the Mercury software, with a March-Dollase parameter of 1.2. The Scherrer formula was used to convert the peak width at half maximum (pwhm) to crystallite sizes perpendicular to the large (200) peak with a shape constant of 1.0. The Segal Crystallinity Index [20] was used to calculate the degree of relative crystallinity.

### **2.8. Environmental scanning electron microscope**

A Philips XL-30 environmental scanning electron microscope (ESEM) was used to image the specimens, operating at 10-13 kV. The samples were mounted on standard Cambridge 1/200 SEM stubs using double-stick photo adhesive tabs. They were coated with 60/40 % gold/ palladium in a Technics' Hummer II sputter coater to a thickness of 20-30 nm.

### **3. Results and discussion**

### **3.1. Preparation of the various fibrous materials for their respective nonwovens**

The fibers utilized in this study were hand blended in ratios, as indicated in Table 1, before being converted into fibrous webs of nominal density (60-90 g/m2 ). Fiber qualities of materials used in this study, including fiber length and properties illustrative of the fiber nomenclature, were consistent with those previously reported for greige cotton, gin motes, and comber noils [21]. UltraClean cotton, which is a form of greige cotton [2], was separately combined with the cotton gin motes and comber noils, whereupon the blends were carded, crosslapped, and subjected to light needle punching prior to their separate hydroentanglement at 50 bar wetout water pressure and 125 bar hydroentangling water pressure. Figure 1 diagrams the process of hydroentanglement. This approach to greige cotton-based nonwoven production has previously been shown to increase absorbency while still retaining some of the cotton's native waxes and pectin [3]. Thus, depending on the hydroentangling process parameters and conditions [1-3], this approach increases hydrophobicity of the greige cotton nonwoven compared to an equivalent scoured and bleached cotton nonwoven product.


Fiber ID:

for approximately 5 min, while dry nitrogen (< 20 ppb water) is bubbled into the sample at 60 mL per minute. The water from the cotton sample from the vial is released and driven into the titration cell from which the percentage of moisture present is calculated from the volume of reagent consumed. The water content in the blank vial was 0.1 %. After analysis, a visual observation of the containers revealed all samples were white; there was no discoloration or

The typical method to measuring moisture or weight loss of cotton fibers involves using an oven where samples are heated to 105-110°C for 24 h. Empty sample bottles were weighed and then filled with 1 g samples and reweighed. Cottons were heated in a Yamato DKN 600 mechanical convection oven with a 150-L capacity and a mean flow rate of approximately 1.3

Wiley-milled samples of the comber noils, gin motes, and UltraClean cotton were pressed into pellets and placed in a powder diffractometer with Cu Kα radiation in reflection mode. The sample was secured in a paraffin base, which had a small effect on the comber noil pattern. For comparison, a pattern was simulated with the Mercury software [17] based on a modified crystal information file from the cellulose Iβ crystal structure [18]. The modification was a short increase in the *a*-axis of the unit cell to 7.906 from the original 7.784 value to better position the (200) peak because of the small differences in the cotton and tunicate unit cell [19]. A peak width at half maximum height of 1.85° 2*θ* was used. Additionally, the preferred orientation induced by pressing the sample pellet was compensated by a facility in the Mercury software, with a March-Dollase parameter of 1.2. The Scherrer formula was used to convert the peak width at half maximum (pwhm) to crystallite sizes perpendicular to the large (200) peak with a shape constant of 1.0. The Segal Crystallinity Index [20] was used to calculate the degree of

A Philips XL-30 environmental scanning electron microscope (ESEM) was used to image the specimens, operating at 10-13 kV. The samples were mounted on standard Cambridge 1/200 SEM stubs using double-stick photo adhesive tabs. They were coated with 60/40 % gold/

The fibers utilized in this study were hand blended in ratios, as indicated in Table 1, before

). Fiber qualities of materials

palladium in a Technics' Hummer II sputter coater to a thickness of 20-30 nm.

**3.1. Preparation of the various fibrous materials for their respective nonwovens**

being converted into fibrous webs of nominal density (60-90 g/m2

scorching. Additionally, the polyester (PES) fibers had not melted.

L/s. All weights were made in a standard conditioned laboratory.

**2.6. Standard oven drying ASTM D2495-01**

50 Cellulose - Fundamental Aspects and Current Trends

**2.7. X-ray diffraction**

relative crystallinity.

**3. Results and discussion**

**2.8. Environmental scanning electron microscope**

UC = UltraClean—unbleached precleaned cotton staple

PES = polyester staple fiber (1.5 denier, 3.8 cm (1.5") long)

GM = cotton gin motes

CM = cotton textile comber noils

**Table 1.** Hydroentangled fabrics of different fiber blends

### **3.2. Fabric surface polarity**

The moisture uptake ( % MC) and electrokinetic results, including the fabric surface polarity **(ζplateau**), swelling (∆ζ), rate of swelling (*k*), isoelectric point (IEP), and related material density for the cotton/by-product blends, are shown in Table 2 for the cotton/by-product blends. The ζ potential is taken from a titration as previously shown [12-14] that assesses the surface charge based on pH and the planar portion along the *x*-axis of the plot is designated as the zeta plateau value (ζplateau), which is a reflection of the relative hydrophilic versus hydrophobic character of the fabric. The ζ potentials reported here are consistent with the previously reported ζ potential titrations for both absorbent incontinence layers and greige cotton nonwoven incontinence topsheet and acquisition distribution layer prototypes [10,11].

### **3.3. Swelling and moisture uptake**

Figure 2 shows the relationship between % moisture content and swelling behavior (Δζ) of the cotton blends. The swelling of the fiber blends results in the expansion of the electrochemical shear plane formed near the fibrous surfac [11,13] and is manifested as a decrease in the absolute value of ζ. The electrokinetic data presented in Figure 2 includes samples of the cotton blends investigated in this study. The percent moisture versus the ∆ζ for all ratios of cotton fiber reflects a correlation between the amount of moisture the material is prone to absorb and its degree of swelling. As the ∆ζ value (*x*-axis) of the cotton fabrics increases, the moisture uptake ( % M) remains between 7.5 % and 9 %. KFT water content correlates with mean ∆ζ with an *R*<sup>2</sup> = 0.88. These two properties (moisture uptake and swelling) promote fluid transport. The increased swelling and relatively constant value for water uptake shown in Figure 2 can be contrasted with absorbency properties, as shown for the cotton blends in Table 3. As seen in Table 3, the ASTM sink time measurement (an indirect measurement of the material's swelling properties) principally correlates with material density in contrast with previous observations in that it correlates with swelling in greige cotton/polyester blends [10]. Swelling and rate of swelling of fabric blends are noticeably increased by blending cleaned greige cotton with by-products at a 60/40 ratio. On the other hand, the moisture uptake was the highest in the 100 % Gin Motes and the lowest in 60/40 UC/GM. This is consistent with the absorbency capacity (Table 3) being highest in the UC/GM blends. The gin motes have lower density and higher surface area and thus promote hydrophilic transport of water in the fabric. In addition, as discussed below, the smaller cellulose crystallite size of the gin motes is consistent with this and plays a role at a molecular level in the increased water absorption capacity observed in the cotton by-product nonwovens.

**Figure 2.** Plot of delta Zeta versus percent moisture content for the cotton blends of this study.

It is also notable that 60 % UC blended with 40 % GM, and CM results in the greatest swelling of the material. The hydrophobicity of the outer greige cotton fiber present in the UltraClean cotton may begin to contribute to the overall swelling of the material at 60 % UC present in the blend. There is little data on the presence of waxes and pectin in comber noils and gin motes, so a relative comparison of cotton cuticle contributions is not possible. However, recent contact angle measurements of greige cotton nonwovens have shown that approximately 32 % of the water droplet coverage is with wax-coated fiber [22]. Thus, the increased swelling due to increasing the ratio of UC may be a result of an additive contribution of waxes from the greige cotton, which are expected to contribute hydrophobicity to the fiber surface analogous to more hydrophobic fibers like polyester. Consistent with this observation, it is important to note that the greige cotton/polyester blend had the highest water absorption capacity. In addition, the similarity of the isoelectric points (IEPs) among the UltraClean cotton samples is consistent with the composition of the samples being cellulosic [13].

shear plane formed near the fibrous surfac [11,13] and is manifested as a decrease in the absolute value of ζ. The electrokinetic data presented in Figure 2 includes samples of the cotton blends investigated in this study. The percent moisture versus the ∆ζ for all ratios of cotton fiber reflects a correlation between the amount of moisture the material is prone to absorb and its degree of swelling. As the ∆ζ value (*x*-axis) of the cotton fabrics increases, the moisture uptake ( % M) remains between 7.5 % and 9 %. KFT water content correlates with mean ∆ζ

The increased swelling and relatively constant value for water uptake shown in Figure 2 can be contrasted with absorbency properties, as shown for the cotton blends in Table 3. As seen in Table 3, the ASTM sink time measurement (an indirect measurement of the material's swelling properties) principally correlates with material density in contrast with previous observations in that it correlates with swelling in greige cotton/polyester blends [10]. Swelling and rate of swelling of fabric blends are noticeably increased by blending cleaned greige cotton with by-products at a 60/40 ratio. On the other hand, the moisture uptake was the highest in the 100 % Gin Motes and the lowest in 60/40 UC/GM. This is consistent with the absorbency capacity (Table 3) being highest in the UC/GM blends. The gin motes have lower density and higher surface area and thus promote hydrophilic transport of water in the fabric. In addition, as discussed below, the smaller cellulose crystallite size of the gin motes is consistent with this and plays a role at a molecular level in the increased water absorption capacity observed in

**Figure 2.** Plot of delta Zeta versus percent moisture content for the cotton blends of this study.

= 0.88. These two properties (moisture uptake and swelling) promote fluid transport.

with an *R*<sup>2</sup>

the cotton by-product nonwovens.

52 Cellulose - Fundamental Aspects and Current Trends


Fiber codes: UC = UltraClean; PES = polyester; CB = comber noils; GM = gin motes.

KFT = Kraft Fisher Test; IEP = isoelectric point; plateau potential refers to the flat portion of the zeta potential titration curve; Swell test *k* refers to the rate of swelling; and **Δζ** refers to the change in zeta potential.

**Table 2.** Electrokinetic data for the hydroentangled fabric samples made with the different fibers and their blends

### **3.4. Bound versus free water and crystallite size in cotton blends**

It is an understatement to say that the nature of the binding of water to cotton plays a role in the swelling of the blended fabrics as are examined here. The microstructure of cotton fibers allows the penetration of water, in the case of the greige cotton nonwovens studied here. Wateraccessible sites of cellulose are formed when cotton contacted by the high-pressure water jets during the nonwoven hydroentanglement process, which enhances the exposure of the primary and secondary cell wall of the fiber to an aqueous environment and results in an increase in cellulose-bound water. A subsequent loosening of the fiber cuticle resulting in the exposure of the cellulosic portion of the fiber is evidenced in the SEM image of nonwoven greige cotton shown in Figure 3. Thus, the hydroentanglement process promotes the disrup‐ tion of the fiber cuticle that retains some wax and pectin while exposing cellulose fibrils and microfibrils to water penetration.

**Figure 3.** SEM image of hydroentangled greige cotton.

The 7-9.5 % moisture uptake observed for the cotton fabrics of this study and correlated in Figure 2 with swelling is consistent with previous studies that show moisture regain of dried cotton in this range [23]. It is also interesting to speculate how variation in cellulose crystallite size may affect binding of water to the cellulosics of this study. The X-ray diffraction patterns for the individual gin motes, comber noils, and greige cotton (UCC) are shown in Figure 4. The spectra all show the profile characteristic of cellulose I [18, 19]. Table 4 gives the percent crystallinity and cellulose crystallite size of the different types of cotton fibers compared with bleached cotton. The crystallinity index for each sample is as high as can be expected [20] for cotton and reflects nearly complete crystallinity. The calculated X-ray diffraction pattern is shown in Figure 3. The calculated pattern matches the observed patterns [28], especially that of the gin motes, fairly well despite the lack of any consideration of amorphous material in the calculation. The Segal Crystallinity Index values arise almost completely from the overlap of the wide observed peaks (20).

On the other hand, the crystallite size of cellulose in the gin motes and comber noils was larger than cellulose crystallites in greige cotton linters. It is important to note that bleached cotton also was found to have larger crystallite size, but the much higher density of the sample tested excludes it from comparison with the other samples. The relationship between crystallinity

An Assessment of Surface Properties and Moisture Uptake of Nonwoven Fabrics from Ginning By-products http://dx.doi.org/10.5772/61329 55

**Figure 4.** X-ray powder diffraction patterns of comber noil, gin motes, and UltraClean cotton. The comber noil pattern is affected by added intensity near 20° 2*θ*, but it does not seem to affect the crystallinity measurement. Also shown is a calculated pattern for the modified crystal structure of Nishiyama et al.18

and moisture uptake has received some attention in the literature over the years (24-26). Relevant to crystallinity, crystallite size, and moisture content, Nelson and O'Connor have previously noted that for two celluloses of the same crytallinity but different crystallite size, the cellulose with the smaller crystallite size will exhibit higher moisture regain (27). Obser‐ vation of this phenomenon is based on more accessible water binding sites, i.e., hydroxyls on smaller crystallites, and is consistent with what is observed in this study (Table 4) in samples of similar density (Table 1) that exhibit differences in crystallite size, i.e., there is a parallel of crystallite size to absorption capacity in the cellulosic greige cotton, gin motes, and comber noils. Thus, as the crystallite size decreases, all things being equal, the absorption capacity of the fabrics consisting of 100 % greige cotton, gin motes, and comber noils increases. The calculated surface-to-volume ratio of smaller cellulose crystallites, as observed with the cotton by-products, is higher than the greige cotton, which infers more accessible hydroxyls for bound water. Thus, an increased capacity to hold water is observed as well in the relatively higher absorption capacity of the cotton by-product nonwovens, which have smaller cellulose crystallite size than the greige cotton (Table 4).

### **3.5. Moisture determinations**

during the nonwoven hydroentanglement process, which enhances the exposure of the primary and secondary cell wall of the fiber to an aqueous environment and results in an increase in cellulose-bound water. A subsequent loosening of the fiber cuticle resulting in the exposure of the cellulosic portion of the fiber is evidenced in the SEM image of nonwoven greige cotton shown in Figure 3. Thus, the hydroentanglement process promotes the disrup‐ tion of the fiber cuticle that retains some wax and pectin while exposing cellulose fibrils and

The 7-9.5 % moisture uptake observed for the cotton fabrics of this study and correlated in Figure 2 with swelling is consistent with previous studies that show moisture regain of dried cotton in this range [23]. It is also interesting to speculate how variation in cellulose crystallite size may affect binding of water to the cellulosics of this study. The X-ray diffraction patterns for the individual gin motes, comber noils, and greige cotton (UCC) are shown in Figure 4. The spectra all show the profile characteristic of cellulose I [18, 19]. Table 4 gives the percent crystallinity and cellulose crystallite size of the different types of cotton fibers compared with bleached cotton. The crystallinity index for each sample is as high as can be expected [20] for cotton and reflects nearly complete crystallinity. The calculated X-ray diffraction pattern is shown in Figure 3. The calculated pattern matches the observed patterns [28], especially that of the gin motes, fairly well despite the lack of any consideration of amorphous material in the calculation. The Segal Crystallinity Index values arise almost completely from the overlap of

On the other hand, the crystallite size of cellulose in the gin motes and comber noils was larger than cellulose crystallites in greige cotton linters. It is important to note that bleached cotton also was found to have larger crystallite size, but the much higher density of the sample tested excludes it from comparison with the other samples. The relationship between crystallinity

microfibrils to water penetration.

54 Cellulose - Fundamental Aspects and Current Trends

**Figure 3.** SEM image of hydroentangled greige cotton.

the wide observed peaks (20).

The water content results via Karl Fischer titration (KFT), following ASTM 7795, track the moisture results based on the Kett moisture determination balance that utilizes an infrared lamp (Table 3). The differences between KFT and Kett are due, in part, to the different nature of the two methods. The Kett measures moisture weight loss after drying under a large infrared heat lamp which, minimizes scorching.15 On the other hand, scorching may contribute to oxidation, thus increasing weight loss (29). Because KFT is specific to water and is carried out under nitrogen, oxidation is eliminated. As a check, standard oven drying was used on the UC


Fiber codes: UC = UltraClean; GM = gin motes; CB = comber noils; PES = polyester staple; BC = bleached cotton. (A sample of bleached cotton prepared by similar process was not available for the testing. This sample is provided for information and not comparison due to its higher density).

#### **Table 3.** Absorbency characteristics of the various hydroentangled fabrics

cotton and polyester fibers, generating a moisture content of 7.3 % and 0.43 %, respectively. The polyester water content value is consistent with that found in literature of fiber at standard testing conditions, around 0.4 % (30). Also, note that the KFT water content (Table 3) for polyester is 0.53 %, which is expected since the carrier gas is dry nitrogen. In addition, the small range of KFT values of the cellulosic blends is due to the specificity of KFT to water compared to the weight loss as measured with the Kett infrared heating method.15 The tight range of crystallinities (Table 4) of the cellulosic is consistent with the KFT findings.


**Table 4.** Crystallinity index and crystallite size of the different fibers

### **3.6. Cellulose-water binding considerations**

The binding of water to cotton has been characterized in three states, including (1) strongly bound or nonfreezing water, (2) anisotropically constrained or perturbed water, and (3) unperturbed water, or water undergoing isotropic motion (24-26). Consideration of the phenomena of water binding to cellulosic fiber, from crystalline to fibrillar state, crystalline cellulose (crsytallites of 36 cellulose chains or more) has been characterized as low water binding (24-26, 31). However, ordered microfibrillar cellulose, which is composed of cellulose crystallites, possesses surface hydroxyls that present accessible water binding sites where penetrating water may form a monolayer (termed nonfreezing water) at a level of (0.1 g/g cotton) (24,31).

French et al. illustrated a cotton fiber moisture model based on the cotton fiber as a solid cylinder of cellulose with a central lumen (length 28 mm, diameter 15 µm) having crystallites at a size of 4 nm by 28 mm, a crystallite density of 1.63 g/cm3 , and a water monolayer of 0.25 nm thickness (25, 28). This model is contrasted with experimental models, including one based on TEM images of a water swollen cotton fiber to image water-accessible surfaces. In the cotton fiber water model at a moisture level of 5 %, a monolayer of water only covers 30 % of the surface, and this scenario accounts for one water molecule per cellobiose unit in the cellulose chains of the crystallite. This is somewhat consistent with a recent report on cotton water sorption based on sorption isotherms of cotton by Yakumin et al., showing a capacity of 41.5 g water/g of cellulose (32). They report that 37 % of the dry cotton cellulose is accessible to water binding at equilibrium moisture adsorption. These authors also point out that it is insufficient to use the cellulose crystallinity alone in calculating accessible water surfaces since water molecules can also enter defective regions in the crystallites formed during drying. This feature is also implicated in the incomplete removal of water being located in regions of the crystalline phase that are disorganized or "defective crystallites" upon drying of cotton. Nonetheless, the presence of a strongly bound monolayer of water on cotton is consistent historically with thermal calculations (90 cal/g), approximating that of the heat of fusion for ice and validating the hydrogen bonding forces of water to cellulose (31,33). From this state, further water sorption then assumes the character of a capillary-like condensation and has been characterized as free water, i.e., perturbed and unperturbed water. Thus, based on numerous past studies that characterize the role of water binding to moisture in cotton, the results of this study suggest that the cotton blends, which possess an average of 8 % moisture content, probably have most of the water strongly bound as nonfreezing water under ambient conditions. This is likely as well as it has previously been shown that the density of interfacial water (strongly bound water) on cellulose is increased when it is perturbed. This property improves the wettability of the cotton blends and may be seen as contributing synergistically to improving the swelling properties in the 60 % UC blends.

### **4. Conclusion**

cotton and polyester fibers, generating a moisture content of 7.3 % and 0.43 %, respectively. The polyester water content value is consistent with that found in literature of fiber at standard testing conditions, around 0.4 % (30). Also, note that the KFT water content (Table 3) for polyester is 0.53 %, which is expected since the carrier gas is dry nitrogen. In addition, the small range of KFT values of the cellulosic blends is due to the specificity of KFT to water compared to the weight loss as measured with the Kett infrared heating method.15 The tight range of

Fiber codes: UC = UltraClean; GM = gin motes; CB = comber noils; PES = polyester staple; BC = bleached cotton. (A sample of bleached cotton prepared by similar process was not available for the testing. This sample is provided for

**Carded Carded Carded Carded**

60.1 <1 69.4 6.87

74.8 <1 79.0 6.12

66.5 5.4 26.4 7.25

59.5 <1 40.2 7.13

75.4 >60 113.8 7.05

66.6 >60 241.0 7.97

100% BC 103.6 <1 2.25 6.46

100% UC 54.6 <1 40.8 4.97 100% GM 51.9 2.4 32.1 6.88 100% CB 73.9 6.6 45.0 6.71 100% PES 68.6 >60 >300 0.14

(sec.) ASTM Sink time (s) ASTM Absorbency Capacity (g H2O

per g of fabric)

Fiber/fiber

60% UC/ 40% CB

40% UC/ 60% CB

60% UC/ 40% GM

40% UC/ 60% GM

60% UC/ 40% PES

40% UC/ 60% PES

blend WT (g/m2) AATCC Drop Test

56 Cellulose - Fundamental Aspects and Current Trends

information and not comparison due to its higher density).

**Table 3.** Absorbency characteristics of the various hydroentangled fabrics

**Sample Crystallinity index Crystallite size (Å)**

UltraClean cotton 88.77 53.48 Comber noils 87.25 47.08 Gin motes 88.73 46.06 Bleached cotton 87.95% 65.30

crystallinities (Table 4) of the cellulosic is consistent with the KFT findings.

**Table 4.** Crystallinity index and crystallite size of the different fibers

This study has shown that the ability of cotton gin mote fibers to modulate swelling and moisture uptake is beneficial in absorbent products. Depending on the end-use application, a hydroentangled nonwoven fabric made by using a blend of cotton mote fibers and greige (bleach-less) cleaned cotton lint gave optimal swelling and reasonably good moisture uptake. We compare properties of the nonwoven fabrics made with precleaned greige cotton that were blended in different proportions with cotton gin motes and comber noils. This approach opens up considerations for a more eco-friendly and economical use of cotton by-products, i.e., for wettable topsheet use and fluid transport layer in absorbent nonwovens. Here we contrast cotton by-product blends in greige cotton nonwovens with a similar previous study where polyester was blended with precleaned greige cotton lint (10). The results have shown that the cotton gin mote fibers, compared to the polyester fibers, yield improved moisture uptake while giving comparable swelling attributes. The cellulose-water interactions have been discussed in light of these findings. This moisture uptake/swelling property could assist toward pro‐ moting greater utilization of cotton and cotton motes in absorbent nonwoven products where end-use applications require comparable moisture uptake to cotton and similar swelling properties to polyester. Also, the water content by KFT seems to correlate with the ζ potential. Balanced material surface polarity, swelling, density, and moisture uptake is key to optimizing absorbent nonwovens for use in hygiene, incontinence, and even wound care applications, and the results of this study illustrate how these properties may be tuned in with cotton by products used in combination with greige cotton lint. Thus, it is demonstrated that the use of less expensive cotton gin motes—the by-product of cotton ginning process—in blends with cleaned greige cotton lint can potentially be useful and competitive for many nonwoven enduse products where absorbency or moisture uptake, swelling, biodegradability, and sustain‐ ability are desirable. This study demonstrates the versatility of nonwoven greige cotton when combined with cotton by-products as putative economical substitutes for synthetic fibers in absorbent applications. It further demonstrates the merit of focusing on material construction and analysis of fiber surface properties with novel by-product fibers at solid-liquid interfaces and the value of considering the molecular factors that influence properties of wettability and fluid transport as they exist in topsheet and layer components useful in absorbent prototypes, i.e., fibers that present green alternatives to petro-based approaches while demonstrating structure/function value. Future studies will focus on specific functional applications for incontinence hygiene and wound care nonwovens.

### **Author details**

Vince Edwards1\*, Paul Sawhney1 , Alvin Bopp2 , Alfred French1 , Ryan Slopek1 , Michael Reynolds1 , Chuck Allen1 , Brian Condon1 and Joseph Montalvo1


### **References**

hydroentangled nonwoven fabric made by using a blend of cotton mote fibers and greige (bleach-less) cleaned cotton lint gave optimal swelling and reasonably good moisture uptake. We compare properties of the nonwoven fabrics made with precleaned greige cotton that were blended in different proportions with cotton gin motes and comber noils. This approach opens up considerations for a more eco-friendly and economical use of cotton by-products, i.e., for wettable topsheet use and fluid transport layer in absorbent nonwovens. Here we contrast cotton by-product blends in greige cotton nonwovens with a similar previous study where polyester was blended with precleaned greige cotton lint (10). The results have shown that the cotton gin mote fibers, compared to the polyester fibers, yield improved moisture uptake while giving comparable swelling attributes. The cellulose-water interactions have been discussed in light of these findings. This moisture uptake/swelling property could assist toward pro‐ moting greater utilization of cotton and cotton motes in absorbent nonwoven products where end-use applications require comparable moisture uptake to cotton and similar swelling properties to polyester. Also, the water content by KFT seems to correlate with the ζ potential. Balanced material surface polarity, swelling, density, and moisture uptake is key to optimizing absorbent nonwovens for use in hygiene, incontinence, and even wound care applications, and the results of this study illustrate how these properties may be tuned in with cotton by products used in combination with greige cotton lint. Thus, it is demonstrated that the use of less expensive cotton gin motes—the by-product of cotton ginning process—in blends with cleaned greige cotton lint can potentially be useful and competitive for many nonwoven enduse products where absorbency or moisture uptake, swelling, biodegradability, and sustain‐ ability are desirable. This study demonstrates the versatility of nonwoven greige cotton when combined with cotton by-products as putative economical substitutes for synthetic fibers in absorbent applications. It further demonstrates the merit of focusing on material construction and analysis of fiber surface properties with novel by-product fibers at solid-liquid interfaces and the value of considering the molecular factors that influence properties of wettability and fluid transport as they exist in topsheet and layer components useful in absorbent prototypes, i.e., fibers that present green alternatives to petro-based approaches while demonstrating structure/function value. Future studies will focus on specific functional applications for

incontinence hygiene and wound care nonwovens.

, Chuck Allen1

\*Address all correspondence to: vince.edwards@ars.usda.gov

, Alvin Bopp2

1 Southern Regional Research Center, US Department of Agriculture, USA

2 Chemistry Department, Southern University of New Orleans, USA

, Brian Condon1

, Alfred French1

and Joseph Montalvo1

, Ryan Slopek1

,

**Author details**

Michael Reynolds1

Vince Edwards1\*, Paul Sawhney1

58 Cellulose - Fundamental Aspects and Current Trends


tance/fourier transform infrared spectroscopy, colorimetry, and particulate matter formation. *Textile Research Journal*. 2014; 84:157-73.

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[16] Malkan SR and Wadsworth LC. Polymer-laid systems. In: Turbak A, (ed.). *Nonwo‐ vens: Theory, Process, Performance, and Testing*. Atlanta: TAPPI Press; 1993, p. 171-92. [17] Macrae CF, Bruno IJ, Chisholm JA, et al. Mercury CSD 2.0—new features for the vis‐ ualization and investigation of crystal structures. *Journal of Applied Crystallography*.

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[21] Sawhney P, Allen C, Reynolds M, Slopek R and Condon B. Whiteness and absorben‐ cy of hydroentangled cotton-based nonwoven fabrics of different constituent fibers

[22] Buisson YL, Rajasekaran K, French AD, Conrad DC and Roy PS. Qualitative and quantitative evaluation of cotton fabric damage by tumble drying. *Textile Research*

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[24] French AD, Goynes WR, Rousselle MA and Thibodeaux DP. Cotton fiber and mois‐ ture—some of the basics. *2004 Beltwide Cotton Conferences*. San Antonio, TX: Cotton

[25] Venkatraman P, Ashbaugh H, Johnson GP and French AD. Simulation studies of the wetting of crystalline faces of cotton cellulose. *2010 Beltwide Cotton Conferences*. New

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### **Cellulose Grafting by Atom Transfer Radical Polymerization Method**

### Nevin Çankaya

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/61707

### **Abstract**

Increased public awareness on environmental issues, fluctuations in raw material prices, and global difficulties in raw material supplies have made it necessary to find new, sustainable, environment-friendly, and inexpensive natural polymer sources. Cellulose and its derivatives, at this point, have become an important research matter once again, because of its easy accessibility, abundance, price, minimal effect on the environment, and new properties discovered with the help of technology.

This study explains cellulose and its modification, and gives information about ATRP, which is a controlled radical polymerization. Later, detailed information on grafting of cellulose by ATRP in both, the author's studies and other existing researches is provided.

**Keywords:** Cellulose, Grafting, Graft Copolymer, Atom Transfer Radical Polymerization (ATRP), Controlled Radical Polymerization (CRP)

### **1. Introduction**

Cellulose is the most abundant natural polymer, which is used raw or substituted in a number of applications, for instance in paper, packaging, or lacquer technologies. Moreover, cellulose is biodegradable and renewable, which makes it preferable from an environment point of view. On the other hand, since its utilization would be limited in its natural form, cellulose has to be modified. This modification is mostly achieved by reaction of hydroxyl groups, yielding to cellulose esters or ethers. Besides, cellulose backbone can be grafted with synthetic polymers by either "grafting from" or "grafting onto" it, using various polymerization methods [1].

© 2015 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Thus, different materials with many different properties, such as elasticity, ion exchange ability, thermal stability, and mechanical properties can be obtained by grafting. Grafting is mostly realized via free-radical polymerization initiated with redox systems, mostly based on ceric or ferrous salts or sodium hydrogen sulfite systems together with peroxides. Using these methods, many new cellulose–backbone graft copolymers have been synthesized but key properties like number, density, length, and molecular mass distribution were out of control. In addition to the vinyl monomers, heterocyclic lactones can be grafted from cellulose or its derivatives by ring-opening polymerization (ROP), giving, in principle, biodegradable polymeric materials [2]. Also, nitroxide-mediated polymerization (NMP) [3] or reversible addition–fragmentation chain transfer process (RAFT) [4] have recently been applied into controlled grafting of cellulose with synthetic polymers. In recent years, a couple of papers reporting controlled cellulose grafting using ATRP have been published [5-6].

This chapter provides information about cellulose, ATRP and cellulose grafting by ATRP method. Later, some research conducted on cellulose grafting by ATRP are discussed.

### **2. Cellulose**

Natural polymers are polymers that are biologically produced and have unique functional attributes. Polysaccharides (cellulose, starch, chitin, chitosan, dextran, inulin, levan etc.), proteins (collagen, gelatin, elastin, actin, etc.), and polynucleotides (DNA and RNA) are natural polymers. Increasing attention is being given to more complex carbohydrate polymers produced by bacteria and fungi, especially to polysaccharides such as xanthan, curdlan, pullulan, and hyaluronic acid.

Cellulose is regarded as the most abundant and renewable biopolymer in nature and the most common organic natural polymer, representing about 1.5 x 1012 tons of the annual biomass [7]. Together with its abundancy, cellulose has several other factors that make it a superior material, such as its renewability; high chemical stability; low price; high tenacity in the wet state; ability to yield to thin membranes; ease of the control of pore size, ranging from 1 to 100 nm in diameter; and controllable porosity [8].

### **2.1. Structure of cellulose**

Natural and unprocessed cellulose in crystal form is called cellulose I (natural-raw cellulose). Crystal form of cellulose changes with external effects like chemical processes and tempera‐ ture, which yields to various crystalline modifications depending on these processes. Cellulose II is obtained by mercerization of cellulose with concentrated alkaline solution, while cellulose III is obtained by inflating and decomposing of cellulose I or II with liquid ammonia (under -30°C). Cellulose IV is a high-temperature form and is prepared by heating cellulose I, II, or III in glycerol. When various crystalline forms of cellulose are heated up to 250°C, intramo‐ lecular hydrogen bonds loosen, which results in a different crystal structure. Cellulose V is obtained by processing cotton or paper with strong phosphoric acid or hydrochloric acid [9]. It is also one of the most promising raw materials for the modern industry, because of its low cost and convertibility to various functional materials. Cellulose is a carbohydrate homopol‐ ymer consisting of β-D-glucopyranose units joined together by β-1,4-glycosidic linkages (Fig. 1). Unlike starch, glucose units in cellulose have long and unbranched chains caused by – CH2OH groups alternating above and below the plane of rings. Lack of side chains lets cellulose molecules form organized structures. Cellulose has both crystallized (higher packing density) and amorphous (lower packing density) regions. Hydrogen bonds connect cellulose chains in the crystallized region, as well as van der Waals forces, which is also important in the lattice energy. Cellulose chains can either have parallel or antiparallel orientation, which is the source of the names of cellulose I and II, respectively. These two forms of cellulose have different structures. Other polymorphic forms are called cellulose III and IV. X-ray diffraction meas‐ urements prove that cotton cellulose contains about 60% crystalline cellulose I and 40% amorphous cellulose [10]. By delignification, removal of noncellulosic polysaccharides and low molecular mass cellulose components (called β- and γ-cellulose) the raw materials containing cellulose can be transformed to pure long-chain cellulose called α-cellulose. Pulp analysis of cellulose shows that it contains 44% carbon, 6.2% hydrogen, and 49% oxygen. Pure cellulose yields to approximately 95% D-glucose (C6H12O6) when hydrolyzed [11]. **Lists of Figures** 

Thus, different materials with many different properties, such as elasticity, ion exchange ability, thermal stability, and mechanical properties can be obtained by grafting. Grafting is mostly realized via free-radical polymerization initiated with redox systems, mostly based on ceric or ferrous salts or sodium hydrogen sulfite systems together with peroxides. Using these methods, many new cellulose–backbone graft copolymers have been synthesized but key properties like number, density, length, and molecular mass distribution were out of control. In addition to the vinyl monomers, heterocyclic lactones can be grafted from cellulose or its derivatives by ring-opening polymerization (ROP), giving, in principle, biodegradable polymeric materials [2]. Also, nitroxide-mediated polymerization (NMP) [3] or reversible addition–fragmentation chain transfer process (RAFT) [4] have recently been applied into controlled grafting of cellulose with synthetic polymers. In recent years, a couple of papers

This chapter provides information about cellulose, ATRP and cellulose grafting by ATRP method. Later, some research conducted on cellulose grafting by ATRP are discussed.

Natural polymers are polymers that are biologically produced and have unique functional attributes. Polysaccharides (cellulose, starch, chitin, chitosan, dextran, inulin, levan etc.), proteins (collagen, gelatin, elastin, actin, etc.), and polynucleotides (DNA and RNA) are natural polymers. Increasing attention is being given to more complex carbohydrate polymers produced by bacteria and fungi, especially to polysaccharides such as xanthan, curdlan,

Cellulose is regarded as the most abundant and renewable biopolymer in nature and the most common organic natural polymer, representing about 1.5 x 1012 tons of the annual biomass [7]. Together with its abundancy, cellulose has several other factors that make it a superior material, such as its renewability; high chemical stability; low price; high tenacity in the wet state; ability to yield to thin membranes; ease of the control of pore size, ranging from 1 to 100

Natural and unprocessed cellulose in crystal form is called cellulose I (natural-raw cellulose). Crystal form of cellulose changes with external effects like chemical processes and tempera‐ ture, which yields to various crystalline modifications depending on these processes. Cellulose II is obtained by mercerization of cellulose with concentrated alkaline solution, while cellulose III is obtained by inflating and decomposing of cellulose I or II with liquid ammonia (under -30°C). Cellulose IV is a high-temperature form and is prepared by heating cellulose I, II, or III in glycerol. When various crystalline forms of cellulose are heated up to 250°C, intramo‐ lecular hydrogen bonds loosen, which results in a different crystal structure. Cellulose V is obtained by processing cotton or paper with strong phosphoric acid or hydrochloric acid [9]. It is also one of the most promising raw materials for the modern industry, because of its low

reporting controlled cellulose grafting using ATRP have been published [5-6].

**2. Cellulose**

pullulan, and hyaluronic acid.

64 Cellulose - Fundamental Aspects and Current Trends

**2.1. Structure of cellulose**

nm in diameter; and controllable porosity [8].

**Fig.1.** Cellobiose unit: two β-D-glucopyranose units joined together by β-1,4-glycosidic **Figure 1.** Cellobiose unit: two β-D-glucopyranose units joined together by β-1,4-glycosidic linkage [12]

linkage [11] Polymerization degree of natural cellulose is strongly related with cellulose source and separation/purification methods that are used. Cotton cellulose has the highest molecular mass which is around 800 000, while wood and other sources of cellulose have molecular masses around 160 000. Hydroxyl groups cause intense intra- and intermolecular hydrogen bonds in cellulose in solid form. These hydrogen bonds and high molecular mass make cellulose insoluble in classic solvents. Dissolving becomes possible when appropriate solvents (like Schweitzer solution –copper-II-hydroxide with concentrated ammonium hydroxide mixture) are used, or by modification of cellulose into derivatives like cellulose triacetate and cellulose nitrate [13-15].

**Fig.2.** Intra and intermolecular hydrogen binding of cellulose [15]

When the distance between oxygen and hydrogen atoms in a cellulose molecule is less than 3 Å, these molecules interact with each other and form intra- or intermolecular hydrogen bonds (Fig. 2) [16].

**Figure 2.** Intra and intermolecular hydrogen binding of cellulose

Equatorial orientation of the hydroxyl group together with the linear structure of cellulose results in both intermolecular and intramolecular hydrogen bonds. Sheet-like structure and crystalline form of the polymer are a result of intermolecular hydrogen bonds. Microfibrils formed by the cellulose molecules (Fig. 3) stack together and make up the fibrils, which gives the cellulose fibers. Hydrophilic property of the polymer is a result of hydroxyl groups. This makes it readily adsorb water [17].

### **2.2. Modification of cellulose and its derivatives**

Its specific structure makes cellulose attractive. Cellulose consists of repetitive glucose units, which makes it more specific, architecturally diverse, reactive, and multifunctional. Some parameters that make cellulose a unique material to study are: isolation process itself, amount of hydrogen bonds, chain length and distribution, crystallinity, and functional group distri‐ bution in a repeating unit. They are also the factors that determine the reactions and properties of cellulose [7].

The first question that comes to mind: why should cellulose derivatives be established?

Compounds obtained by chemical changes of cellulose are called **cellulose derivatives**. While cellulose transforms into its derivatives, hydroxyl groups react and yield esters with organic and inorganic acids, ethers with some alcohols, alcoholates with bases and oxidation products with acids. They also react with halides, amines, and some complexes. Most important industrial derivatives of cellulose are cellulose esters and ethers. Producing alkali cellulose is a starting process to obtain these two products [18].

Cellulose Grafting by Atom Transfer Radical Polymerization Method http://dx.doi.org/10.5772/61707 67

**Figure 3.** Structure of cellulose fibers and fibrils

When the distance between oxygen and hydrogen atoms in a cellulose molecule is less than 3 Å, these molecules interact with each other and form intra- or intermolecular hydrogen bonds

Equatorial orientation of the hydroxyl group together with the linear structure of cellulose results in both intermolecular and intramolecular hydrogen bonds. Sheet-like structure and crystalline form of the polymer are a result of intermolecular hydrogen bonds. Microfibrils formed by the cellulose molecules (Fig. 3) stack together and make up the fibrils, which gives the cellulose fibers. Hydrophilic property of the polymer is a result of hydroxyl groups. This

Its specific structure makes cellulose attractive. Cellulose consists of repetitive glucose units, which makes it more specific, architecturally diverse, reactive, and multifunctional. Some parameters that make cellulose a unique material to study are: isolation process itself, amount of hydrogen bonds, chain length and distribution, crystallinity, and functional group distri‐ bution in a repeating unit. They are also the factors that determine the reactions and properties

The first question that comes to mind: why should cellulose derivatives be established?

Compounds obtained by chemical changes of cellulose are called **cellulose derivatives**. While cellulose transforms into its derivatives, hydroxyl groups react and yield esters with organic and inorganic acids, ethers with some alcohols, alcoholates with bases and oxidation products with acids. They also react with halides, amines, and some complexes. Most important industrial derivatives of cellulose are cellulose esters and ethers. Producing alkali cellulose is

(Fig. 2) [16].

66 Cellulose - Fundamental Aspects and Current Trends

**Figure 2.** Intra and intermolecular hydrogen binding of cellulose

**2.2. Modification of cellulose and its derivatives**

a starting process to obtain these two products [18].

makes it readily adsorb water [17].

of cellulose [7].

Substitute groups in cellulose molecules are revealed when preparing cellulose derivatives. This process results in changes in physical properties, which makes cellulose derivatives industrially practical. This effect is revealed by both natural substitute groups and degree of substitution. Mechanical and physical properties of cellulose and cellulose derivatives differ with respect to average molecular mass. Increase in molecular mass also increases resistivity values; however, this effect becomes less significant after a certain degree [18, 19]. **Fig.3.** Structure of cellulose fibers and fibrils **2.2. Modification of Cellulose and its Derivatives** 

Although it has many useful properties, it is short of some properties that synthetic polymers have. Graft copolymerization is a significant way to alter the physical and chemical properties of cellulose like heat resistance, elasticity, resistance to abrasion and wear, ion-exchange capabilities, oil and water repellency, and antibacterial activity [20]. In the preparation of cellulose derivatives, making them soluble is often the decisive quality criterion, because cellulose is insoluble in organic solvents such as water, alcohol, acetone, benzene, chloroform etc. The usefulness of cellulose as a starting material for edible and biodegradable polymer is extended by chemical modification. Additionally, high solution viscosity is a desired property. Its specific structure makes cellulose attractive. Cellulose consists of repetitive glucose units, which makes it more specific, architecturally diverse, reactive, and multifunctional. Some parameters that make cellulose a unique material to study are: isolation process itself, amount of hydrogen bonds, chain length and distribution, crystallinity, and functional group distribution in a repeating unit. They are also the factors that determine the reactions and properties of cellulose [7].

Therearethreeways tochangethechemicalpropertiesof cellulose,asgiveninother sources [21]: The first question that comes to mind: why should cellulose derivatives be established?

6

Compounds obtained by chemical changes of cellulose are called *cellulose derivatives*. While cellulose transforms into its derivatives, hydroxyl groups react and yield esters with organic


The most significant cellulosic applications are in the paper, wood products, textiles, film, and fiber industries. However, recently it has also attracted significant interest as a source of biofuel production [23]. Some usages of soluble cellulose are given in Table 1 [24].


**Table 1.** Usage of soluble cellulose

### **2.3. Nanocellulose**

**1.** By preparing an ester or ether derivative of cellulose, **ethers** *,* e.g., ethyl cellulose (EC), methyl cellulose (MC), propyl cellulose (PC), hydroxypropyl cellulose (HPC), hydroxy‐ propyl methyl cellulose (HPMC), hydroxylethyl cellulose (HEC), hydroxylethyl methyl cellulose (HEMC), methyl hydroxypropyl cellulose (MHPC), ethyl hydroxylethyl cellulose (EHEC), carboxymethyl cellulose (CMC), benzyl cellulose (BC); **esters** like cellulose acetate (CA) and cellulose xanthate, which are used to process cellulose into either fiber or film forms, during which the cellulose is regenerated by controlled hydrol‐ ysis; **acetals,** especially the cyclic acetal formed between secondary hydroxyl groups and butyraldehyde cellulose acetate, cellulose propionate, and cellulose acetate-butyrate [22].

**3.** By preparing a graft copolymer of cellulose, namely, a branched derivative of cellulose

The most significant cellulosic applications are in the paper, wood products, textiles, film, and fiber industries. However, recently it has also attracted significant interest as a source of biofuel

liquids, etc.

polymerization emulsions, etc.

explosives, cement, plates, etc.

nonconductive parts

retainers), eye lenses, stabilizers, etc.

HEC (hydroxyethlycellulose) Dyes, chemicals, liquid detergents, rubber, oil wells,

Filaments Garments, threads, furniture, packaging, etc.

*Nitrates (Nitrocellulose) (inorganic ester)* Automotive, textiles, dyes, lacquers, polishes, films,

Tows (linen and cannabis fibers) Textiles, yarns, cigarette filters, etc.

*Nitriles* Food and other packaging

Foods, dyes, drugs, detergents, cosmetics, textiles, lacquers, finishing, inks, glues, electrical insulators, fire extinguishers, electrical appliances, water retainers, stabilizers, borehole

Foods, cosmetics, textiles, drugs, paper, detergents, glues, etc.

Foods, drugs, papers, plastics, ceramics, gels (as water

Textiles, films, plates, sheaths, coils, isolation, and

**2.** By preparing a cross-linked derivative of cellulose.

68 Cellulose - Fundamental Aspects and Current Trends

(This study reports about cellulose grafting by ATRP).

**Cellulose derivatives Areas of utilization**

*Ethers* MC-EC

*Esters*

CMC-NaCMC

(carboxymethylcellulose-

(methylcellulose- ethylcellulose)

sodiumcarboxymethylcellulose) (E-466)

HPC-HPMC (hydroxypropylcellulosehydroxypropylmethylcellulose)

**Table 1.** Usage of soluble cellulose

Acetates (organic esters: cellulose acetate, cellulose triacetate, cellulose propionate, cellulose acetate propionate, propionate, cellulose acetate butyrate, etc.

Plastics Various

production [23]. Some usages of soluble cellulose are given in Table 1 [24].

Mechanical properties of natural materials like elasticity or endurance could differ from nanoscale to macroscale. First-generation cellulosic engineering products like cotton and wood have been used in various fields depending on their structural sizes. However, some vital properties like functionality, durability, and homogeneity could not have been obtained in following generation cellulose-based products by using conventional cellulosic materials. Although conventional forestry products are still being widely used, these products have failed to meet the requirements of high-performance materials. Deep research interest in forestry products and by-products triggered by the necessity of sustainability has gone into a further phase after the invention of cellulose nanoparticles. Since cellulose extracted at nanoscale did not have compositional errors at hierarchical dimension like first-generation cellulose, this new method has paved the way to the development of next-generation cellulose based on composite materials. Although wood pulp is commonly used, nanocellulose can be produced from any cellulose material. Fibrils are isolated from wood-based fibers by using high-pressure homogenizer. Amorphous part in natural cellulose is hydrolyzed by acid hydrolyzation and crystal part at nanoscale is expelled by centrifuge (Fig. 4) [14, 25].

**Figure 4.** Schematics of (a) repeating single cellulose chain unit with the directionality of the -4 linkage and intra chain hydrogen bonding (dotted line), (b) idealized cellulose microfibril illustrating a suggested configuration of the crystal‐ line and amorphous regions, and (c) cellulose nanocrystals with disordered regions dissolved in acid

### **3. Atom Transfer Radical Polymerization (ATRP)**

Ostu, et al. (1982) invented of the term "Living/controlled radical polymerization" in their work on the iniferter mechanism [26]. Various studies have proven that Controlled Radical Polymerization (CRP) is the most versatile method for surface-initiated polymerization (SIP) on model surfaces, and it includes methods such as Atom Transfer Radical Polymerization (ATRP), nitroxide-mediated and iniferter-based (INItiators-transFER-terminaTER agent) polymerizations (NMP) and Reversible Addition–Fragmentation chain Transfer (Radical Addition Fragmentation Transfer) (RAFT) polymerization [27]. The polymerization is ob‐ tained by implication of techniques generally based on the reversible homolytic cleavage of (a) a dormant chain end group into the corresponding polymeric radical (which induces chain propagation) and (b) a stable, persistent radical that cannot undergo addition to monomer [27, 28]. The radical-dormant adduct equilibrium can be reached by the application of thermal energy, light, or the addition of a catalyst as a function of the particular method considered [27].

ATRP, which is quite versatile in yielding to polymers with low polydispersity and controlled molecular weight, is probably the most widely studied CRP [29]. Moreover, ATRP has proven to be useful in synthesizing graft copolymers with well-defined structures utilizing a variety of monomers. The application of ATRP to cellulose could prove to be attractive when preparing novel cellulose derivatives with well-defined side-chain structures. Besides, reports on ATRP and the synthesis of cellulosic graft copolymers show that the method is promising [30-32].

### **3.1. Mechanism of ATRP**

In 1995, two different research groups separately reported very similar controlled radical polymerization techniques, the ATRP, both based on catalytic systems used for the atom transfer radical addition reaction (ATRA), or in other words, Kharasch reaction, a method of forming carbon-carbon bonds between organic halides and alkenes known to be productive. The other system, reported by Matyjaszewski, *et al.*, is the polymerization of styrene under catalyzation of CuCl/ 2, 2'-bipyridyl (bpy) with 1-phenylethyl chloride as an initiator. Since the outset of reports, many reports have been published on ATRP of styrene, acrylates, methacrylates, and acrylonitrile by different transition metal complexes. ATRP proves to be versatile when compared to other controlled radical polymerization methods, providing control in the polymerization of various monomers under different reaction conditions and enabling to prepare polymers with a wide range of architectures. Some examples are blocks, grafts, gradient copolymers, stars, combs, branched, and hyper-branched. ATRP is one of the fastest growing subjects in chemistry, with the number of publications approximately dou‐ bling each year [26].

The mechanism of ATRP can be seen in Scheme 1. In or der to generate a growing radical or an active species (Pn ⋅ ), a transition metal complex (Mn-Y/L) undergoes one-electron oxidation together with abstraction of a halogen (X) from an initiator or a dormant species (Pn-X). The active species initiate monomers (CH2=CH2R1) to yield new growing radicals. This reaction may continue until the release of halogen atoms from the oxidized metal back to form dormant species. This turns the polymer chain ends from a dormant to a propagating and active state. In this process, the oxidized metal complexes (X-Mn+1-Y/L) are utilized as persistent radicals and reduce the rippling concentration of growing radicals in order to terminate by generation of a steady low concentration of active radical chain ends with short life span. This mechanism has the advantage of keeping concentration of radical intermediates in the reaction medium low before adding new monomers, because of fast but reversible transformation into nonactive species. Speedy initiation and the fast reversible deactivation provide uniform growth in all chains, which in turn allows achieving narrow polydispersities. Besides, it leads to suppression of termination reactions. An ATRP reaction is considered to be successful if it has a small contribution of terminated chains, and a uniform growth of all the chains, managed by fast initiation and rapid reversible deactivation. This process, shown in Scheme 1, takes place with a rate constant of activation (*k*a) and deactivation (*k*d). The growth of polymer chains comes by the addition of the intermediate radicals to monomers in a similar manner to a constant of propagation (*k*p). Termination reactions (*k*<sup>t</sup> ) would also occur through radical coupling and disproportionation in ATRP in a great amount [5, 24, 29, 31, 32-39].

**Sheme 1.** Mechanism of metal-catalyzed ATRP

Polymerization (CRP) is the most versatile method for surface-initiated polymerization (SIP) on model surfaces, and it includes methods such as Atom Transfer Radical Polymerization (ATRP), nitroxide-mediated and iniferter-based (INItiators-transFER-terminaTER agent) polymerizations (NMP) and Reversible Addition–Fragmentation chain Transfer (Radical Addition Fragmentation Transfer) (RAFT) polymerization [27]. The polymerization is ob‐ tained by implication of techniques generally based on the reversible homolytic cleavage of (a) a dormant chain end group into the corresponding polymeric radical (which induces chain propagation) and (b) a stable, persistent radical that cannot undergo addition to monomer [27, 28]. The radical-dormant adduct equilibrium can be reached by the application of thermal energy, light, or the addition of a catalyst as a function of the particular method considered [27].

ATRP, which is quite versatile in yielding to polymers with low polydispersity and controlled molecular weight, is probably the most widely studied CRP [29]. Moreover, ATRP has proven to be useful in synthesizing graft copolymers with well-defined structures utilizing a variety of monomers. The application of ATRP to cellulose could prove to be attractive when preparing novel cellulose derivatives with well-defined side-chain structures. Besides, reports on ATRP and the synthesis of cellulosic graft copolymers show that the method is promising [30-32].

In 1995, two different research groups separately reported very similar controlled radical polymerization techniques, the ATRP, both based on catalytic systems used for the atom transfer radical addition reaction (ATRA), or in other words, Kharasch reaction, a method of forming carbon-carbon bonds between organic halides and alkenes known to be productive. The other system, reported by Matyjaszewski, *et al.*, is the polymerization of styrene under catalyzation of CuCl/ 2, 2'-bipyridyl (bpy) with 1-phenylethyl chloride as an initiator. Since the outset of reports, many reports have been published on ATRP of styrene, acrylates, methacrylates, and acrylonitrile by different transition metal complexes. ATRP proves to be versatile when compared to other controlled radical polymerization methods, providing control in the polymerization of various monomers under different reaction conditions and enabling to prepare polymers with a wide range of architectures. Some examples are blocks, grafts, gradient copolymers, stars, combs, branched, and hyper-branched. ATRP is one of the fastest growing subjects in chemistry, with the number of publications approximately dou‐

The mechanism of ATRP can be seen in Scheme 1. In or der to generate a growing radical or

together with abstraction of a halogen (X) from an initiator or a dormant species (Pn-X). The active species initiate monomers (CH2=CH2R1) to yield new growing radicals. This reaction may continue until the release of halogen atoms from the oxidized metal back to form dormant species. This turns the polymer chain ends from a dormant to a propagating and active state. In this process, the oxidized metal complexes (X-Mn+1-Y/L) are utilized as persistent radicals and reduce the rippling concentration of growing radicals in order to terminate by generation of a steady low concentration of active radical chain ends with short life span. This mechanism has the advantage of keeping concentration of radical intermediates in the reaction medium

), a transition metal complex (Mn-Y/L) undergoes one-electron oxidation

**3.1. Mechanism of ATRP**

70 Cellulose - Fundamental Aspects and Current Trends

bling each year [26].

an active species (Pn

⋅

### **3.2. Ligands and initiators commonly used in ATRP**

Various transition metals, usually in the form of salts, have been used in ATRP together with various complexing ligands. Copper is the most common of them, due to its low cost and versatility. Ligand's duty is to solubilize the metal ion, and this affects the reduction potential of the transition metal ion too. Although alkyl iodides have been used too, alkyl bromides and chlorides are more common initiators [40]. Some common ligands and alkyl halides are 2,2' bipyridine and ligands bound nitrogen. For example: N,N,N/ ,N//,N///–pentamethyl diethylene triamine (PMDETA), tetramethyl ethylene diamine (TMEDA), 1,14,7,10,10- hexamethyl triethylene tetramine (HMTETA), hexahexyl triethylenetetramine (HHTETA) [41], tris [2 diamethylamino ethylamine (Me6-TREN)], alkyl pyridyl methanimine [34]. Some general copper containing ligands are given in Fig. 5 [39].

### **3.3. Catalysts commonly used in ATRP**

The catalyst is arguably the most important component in ATRP, since it determines the position of the atom transfer equilibrium and the dynamics of exchange between the dormant and active species. A transition metal catalyst needs to fulfill some requirements in order to be considered as efficient. These are: the metal having at least two oxidation states separated by one electron; the metal center having reasonable affinity toward a halogen; and the coordination sphere around the metal being expandable upon oxidation to accommodate a (pseudo)-halogen; and the ligand being relatively strongly. Moreover, the dynamics and position of the ATRP equilibrium is expected to be appropriate for the particular system [31].


**Figure 5.** Examples of some ligands used in copper mediated ATRP 13

### **3.4. Halide exchange effect in ATRP**

Replacing a chloride catalyst with a bromide catalyst is an effective way to achieve initiation rates higher than propagation. This results in halide exchange giving predominantly Cl-ended polymer and bromide catalyst by a mechanism (shown in Fig. 6). Utilizing a bromide catalyst with a soluble chloride salt would yield the same effect too. The ongoing radical effect brings two activation- deactivation equilibria involving P⋅ with equilibrium constants K1 and K2, K<sup>2</sup> being greater than K1 since Kex >> 1. The halide exchange is not great with the initiator when compared with the polymer as a result of the lower concentration of R⋅ than that would have been established by the regular persistent radical effect because of the capture by the monomer, so the initiator remains a bromide, while the polymer becomes a chloride predominantly. Initiation becomes faster than propagation as a result of the weaker C–Br bond in the initiator compared to the stronger C–Cl bond in the polymer. On the other hand, although majority of the polymers are Cl- ended, most of the deactivation events occur by the faster process of Br atom transfer, inasmuch as kda2 > kda1, resulting in narrow molecular weight distribution polymers. Nevertheless, its slower frequency of activation results in P–Cl build up. Since P– Cl being more stable than P–Br, the end group loss due to side reactions at higher conversions becomes much less [39].

**Figure 6.** Halide exchange using bromide initiator and chloride catalyst

**Figure 5.** Examples of some ligands used in copper mediated ATRP 13

72 Cellulose - Fundamental Aspects and Current Trends

### **4. Some studies about cellulose grafting by atom transfer radical polymerization method**

Graft copolymers of cellulose are obtained by binding synthetic polymers to cellulose – a natural polymer – with covalent bonds. This process – called grafting – changes behaviors and properties of cellulose. Numerous researchers have studied this subject. Below are some researches on grafting by ATRP method. Other than grafting studies on cellulose, there are other natural polymers grafting studies.

Coşkun M. et al grafted styrene, methyl methacrylate, methacrylamide and acrylomorpholine on pulverized-raw cellulose by using ATRP method. They prepared cellulose chloroacetate as macroinitiator by reacting chloroacetyl chloride with OH group of primary alcohol in pulv‐ erized cellulose. CuBr and 1,2-dipiperidinoethane was used as catalyst and transient metal compound. The reactions were followed by FT-IR and mass increase in cellulose chloroacetate. Dye affinity and dye absorption values of cellulose, macroinitiator, and graft polymers were determined by acidic alizarin yellow and alkaline bromocresole green, while humidity, water, and dye absorption capacities were measured by thermal stability [35].

Our research group grafted cellulose by using two different methods, through free-radical graft copolymerization and atom transfer radical polymerization (ATRP). N-(4-nitrophenyl) acrylamide (4NPA), and N-cyclohexylacrylamide (NCA) monomers were synthesized by the researchers, while some commercial monomers were used for this grafting. In the first method, primary OH groups in powder cellulose were first transformed to ester groups with metha‐ cryloyl chloride and cellulose methacrylate was obtained. This cellulose methacrylate was grafted with the synthesized monomers of N-(4-nitrophenyl) acrylamide (4NPA) and Ncyclohexylacrylamide (NCA); 4-vinylpyridine (4VP), acrylamide (AM), methacrylamide (MAM), diacetone acrylamide (DAAM), and methylmethacrylate (MMA) monomers, in the presence of 2,2'-azobis (isobutyronitrile) in acetonitrile as solvent. In the second method, with the ATRP, primary OH groups of powder-raw cellulose were firstly reacted with chloroacetyl chloride and cellulose chloroacetate was obtained, where cellulose chloroacetate was used as macroinitiator. 4NPA, NCA, 4VP, AM, and DAAM monomers reacted in the presence of copper (I) chloride and 2,2'-bipyridine as ligands at 130°C in the N,N-Dimethylformamide (DMF), where cellulose chloroacetate was used as macroinitiator. In the third method, Poly [Cell-g-(N-(4-nitrophenyl) acrylamide-co-methyl methacrylate)] {Poly [Cell-g-(4NPA-co-MMA)]} was prepared in the presence of CuCI/2,2'-bipyridine. In the fourth method, Poly [Cell-g-(N-cyclohexylacrylamide)-co-methyl methacrylate)] {Poly [Cell-g-(NCA-co-MMA)]} was prepared similarly. The monomer reactivity ratios of both graft copolymerization reactions were calculated by using Fineman-Ross (F–R), Inverted Fineman-Ross (I–F–R), Yezrielev-Brokhina-Roskin (Y–B–R), Kelen-Tüdos (K–T), and Extended Kelen-Tüdos (E–K–T) methods. All of the graft copolymers were characterized by elemental analysis, IR, TGA techniques. Water and metal ion (by Atomic Absorption Spectrum) uptake properties of the graft copolymers obtained with the first method were determined. Electrical conductivity of the graft copolymers by ATRP obtained with the second method was determined. The results are discussed below [24].

Cankaya et al. synthesized the graft copolymers of cellulose methacrylate by free- radical polymerization. Cellulose methacrylate was prepared by esterification of primary -OH group on raw-powder cellulose with methacryloyl chloride with a 21.3% yield by mole. The vinyl monomers such as 4NPA, NCA, 4VP, AM, MAM, DAAM, and MMA were grafted into the cellulose methacrylate via free-radical polymerization using 2,2'-azobis (isobutyronitrile) as an initiator in acetonitrile. The graft copolymers were characterized by FT-IR spectra, elemental analysis, and thermal analysis. Thermal stabilities of the graft copolymers were determined by TGA method and thermal stability of the copolymers decreased with grafting. Wateruptake capacities increased the grafting and its metal ion sorption tendency (Ni2+, Co2+, Cu2+, Cd2+, Pb2+, Fe3+ and Cr3+) improved with the grafting. –The study was conducted by using 8 graft copolymers and 7 metals and no comparison between graft copolymers and metals was possible, which is an expected result. It could be concluded that +3 valence metals were held by graft copolymers more than +2 valence metals, and cellulose methacrylate holds metals more than cellulose [49].

Cankaya et al. synthesized graft copolymers of cellulose chloroacetate by ATRP. Cellulose chloroacetate was prepared by esterification of primary alcoholic OH groups on raw-pow‐ dered cellulose. Cellulose was first esterified with chloroacetyl chloride, yielding cellulose chloroacetate which behaves as a macroinitiator because of chloroacetyl groups on it (Scheme 2). The graft copolymers of cellulose with some monomers such as NCA, 4VP, and DAAM were prepared by means of these groups using the Cu(I)/2,2'-bipyridine complex as a catalyst in DMF at 130o C by ATRP. Reactions of grafting on cellulose are indicated in Scheme 3. Formed homopolymers were removed from graft copolymers. The electrical conductivity of the graft copolymers was measured as a function of temperature. The electrical conductivity of the copolymers increased with increase in temperature, and this indicates that the studied copolymers exhibit semiconducting behavior. The electrical and optical properties of all the copolymers changed with grafted monomers. The cellulose exhibits the highest conductivity and lowest optical absorption edge with 4VP monomer [32].

**Fig.9.** Synthesis of cellulose chloroacetate macroinitiator **Sheme 2.** Synthesis of cellulose chloroacetate macroinitiator

researches on grafting by ATRP method. Other than grafting studies on cellulose, there are

Coşkun M. et al grafted styrene, methyl methacrylate, methacrylamide and acrylomorpholine on pulverized-raw cellulose by using ATRP method. They prepared cellulose chloroacetate as macroinitiator by reacting chloroacetyl chloride with OH group of primary alcohol in pulv‐ erized cellulose. CuBr and 1,2-dipiperidinoethane was used as catalyst and transient metal compound. The reactions were followed by FT-IR and mass increase in cellulose chloroacetate. Dye affinity and dye absorption values of cellulose, macroinitiator, and graft polymers were determined by acidic alizarin yellow and alkaline bromocresole green, while humidity, water,

Our research group grafted cellulose by using two different methods, through free-radical graft copolymerization and atom transfer radical polymerization (ATRP). N-(4-nitrophenyl) acrylamide (4NPA), and N-cyclohexylacrylamide (NCA) monomers were synthesized by the researchers, while some commercial monomers were used for this grafting. In the first method, primary OH groups in powder cellulose were first transformed to ester groups with metha‐ cryloyl chloride and cellulose methacrylate was obtained. This cellulose methacrylate was grafted with the synthesized monomers of N-(4-nitrophenyl) acrylamide (4NPA) and Ncyclohexylacrylamide (NCA); 4-vinylpyridine (4VP), acrylamide (AM), methacrylamide (MAM), diacetone acrylamide (DAAM), and methylmethacrylate (MMA) monomers, in the presence of 2,2'-azobis (isobutyronitrile) in acetonitrile as solvent. In the second method, with the ATRP, primary OH groups of powder-raw cellulose were firstly reacted with chloroacetyl chloride and cellulose chloroacetate was obtained, where cellulose chloroacetate was used as macroinitiator. 4NPA, NCA, 4VP, AM, and DAAM monomers reacted in the presence of copper (I) chloride and 2,2'-bipyridine as ligands at 130°C in the N,N-Dimethylformamide (DMF), where cellulose chloroacetate was used as macroinitiator. In the third method, Poly [Cell-g-(N-(4-nitrophenyl) acrylamide-co-methyl methacrylate)] {Poly [Cell-g-(4NPA-co-MMA)]} was prepared in the presence of CuCI/2,2'-bipyridine. In the fourth method, Poly [Cell-g-(N-cyclohexylacrylamide)-co-methyl methacrylate)] {Poly [Cell-g-(NCA-co-MMA)]} was prepared similarly. The monomer reactivity ratios of both graft copolymerization reactions were calculated by using Fineman-Ross (F–R), Inverted Fineman-Ross (I–F–R), Yezrielev-Brokhina-Roskin (Y–B–R), Kelen-Tüdos (K–T), and Extended Kelen-Tüdos (E–K–T) methods. All of the graft copolymers were characterized by elemental analysis, IR, TGA techniques. Water and metal ion (by Atomic Absorption Spectrum) uptake properties of the graft copolymers obtained with the first method were determined. Electrical conductivity of the graft copolymers by ATRP obtained with the second method was determined. The results

Cankaya et al. synthesized the graft copolymers of cellulose methacrylate by free- radical polymerization. Cellulose methacrylate was prepared by esterification of primary -OH group on raw-powder cellulose with methacryloyl chloride with a 21.3% yield by mole. The vinyl monomers such as 4NPA, NCA, 4VP, AM, MAM, DAAM, and MMA were grafted into the cellulose methacrylate via free-radical polymerization using 2,2'-azobis (isobutyronitrile) as an initiator in acetonitrile. The graft copolymers were characterized by FT-IR spectra, elemental

and dye absorption capacities were measured by thermal stability [35].

other natural polymers grafting studies.

74 Cellulose - Fundamental Aspects and Current Trends

are discussed below [24].

(Celluse choloroacetate) O CH2OC OH O OH O CH2 Rn O CH2OC OH O OH O CH2 Cu-CI/ bpy ka kd 2 Cl-Cu-CI/ bpy Cl 1 Cankaya et al. prepared graft copolymers with 4NPA and MMA of cellulose chloroacetate, using cellulose chloroacetate/Cu(I)/2,2'-bipyridine complex as a catalyst in DMF at 130o C, by ATRP. Cellulose chloroacetate behaved as a macroinitiator because of chloroacetyl groups on it (Scheme 4). Poly [Cell-g-(N-(4-nitrophenyl) acrylamide-co-methyl methacrylate)] {Poly [Cell-g-(4NPA-co-MMA)]} graft copolymers were characterized by elemental analysis, FT-IR spectra, and thermal analysis. FT-IR spectra of Cell-*g*-(4NPA-*co*-MMA) graft copolymer showed that as 4NPA content decreased, 1670 cm-1 amide carbonyl peak sharpness decreased and as MMA content increased, 1745 cm-1 ester sharpness increased. Also, 1745 cm-1 ester band in the cellulose chloroacetate macroinitiator was observed for each graft copolymer (Fig.7). Thermal stabilities of the graft copolymers were determined by TGA method and thermal

R1:

R2:

R3:

**Fig.10.** The grafting of cellulose chloroacetate with some monomers by ATRP

Cell-g-NCA

Cell-g-4VP N

> C H

H2C

C O

CH3

CH2 CH3

C CH3 O

NH C

Cell-*g*-DAAM

H C N H

O

H2C C

H2C CH

**Fig.9.** Synthesis of cellulose chloroacetate macroinitiator

**Fig.10. Sheme 3.** The The grafting of cellulose chloroacetate with some monomers by ATRP grafting of cellulose chloroacetate with some monomers by ATRP

stability of the copolymers increased with the increase in amount of MMA, while it decreased with the increase in amount of 4NPA (Fig.8). In order to investigate the effect of 4NPA with MMA monomers interactions on grafting, the graft copolymerization was also studied using different feed compositions ranging from 0.15 to 0.85. This study was undertaken in order to determine of the monomer reactivity ratios in the grafting of cellulose with 4NPA and MMA by ATRP, with linear methods such as the F–R, Inverted F–R, Y–B–R, K–T, and Extended K– T methods. The reactivity ratios of 4NPA and MMA in the graft copolymerization found was r1=0.017–0.116 and r2=1.209–1.472, with various methods (Table 2). According to these meth‐ ods, the r1 and r2 values were 0.011 and 1.263; r1.r2= 0.014, respectively. The values calculated by all methods were found to be equal to each other. Two monomer mixtures on cellulose had a tendency to form alternative copolymers because the r1.r2 was close to zero. It could be concluded that 4NPA monomer with amide group and larger molecule size was less reactive on grafting onto cellulose with respect to MMA monomer with ester group and smaller molecule size [30].

Cellulose Grafting by Atom Transfer Radical Polymerization Method http://dx.doi.org/10.5772/61707 77

Cell-*g*-(4NPA-co-MMA)

**Fig.11.** Synthesis of the serials Cell-*g*-(4NPA-*co*-MMA) **Sheme 4.** Synthesis of the serials Cell-*g*-(4NPA-*co*-MMA)

stability of the copolymers increased with the increase in amount of MMA, while it decreased with the increase in amount of 4NPA (Fig.8). In order to investigate the effect of 4NPA with MMA monomers interactions on grafting, the graft copolymerization was also studied using different feed compositions ranging from 0.15 to 0.85. This study was undertaken in order to determine of the monomer reactivity ratios in the grafting of cellulose with 4NPA and MMA by ATRP, with linear methods such as the F–R, Inverted F–R, Y–B–R, K–T, and Extended K– T methods. The reactivity ratios of 4NPA and MMA in the graft copolymerization found was r1=0.017–0.116 and r2=1.209–1.472, with various methods (Table 2). According to these meth‐ ods, the r1 and r2 values were 0.011 and 1.263; r1.r2= 0.014, respectively. The values calculated by all methods were found to be equal to each other. Two monomer mixtures on cellulose had a tendency to form alternative copolymers because the r1.r2 was close to zero. It could be concluded that 4NPA monomer with amide group and larger molecule size was less reactive on grafting onto cellulose with respect to MMA monomer with ester group and smaller

**Sheme 3.** The The grafting of cellulose chloroacetate with some monomers by ATRP grafting of cellulose chloroacetate with some monomers by ATRP

O CH2OC

O

CH2 Rn

O

H C N H

O OH

R1:

R2:

R3:

OH

H2C C

H2C CH

Cell-g-NCA

Cell-g-4VP N

> C H

H2C

C O

CH3

CH3

C CH3

2

Cl-Cu-CI/ bpy

O

CH2

NH C

Cell-*g*-DAAM

**Fig.9.** Synthesis of cellulose chloroacetate macroinitiator

Cu-CI/ bpy

Cl 1

ka

kd

(Celluse choloroacetate)

O OH

O CH2OC

O

CH2

76 Cellulose - Fundamental Aspects and Current Trends

OH

molecule size [30].

**Fig.10.**

**Figure 7.** FT-IR spectra of the serials Poly [Cell-g-(4NPA-co-MMA)] graft copolymers

**Figure 8.** Thermal Gravimetric Analysis curves of the serials Poly [Cell-g-(4NPA-co-MMA)] graft copolymers


**Table 2.** Comparison of monomer reactivity ratios for Poly [Cell-g-(4NPA-co-MMA)] graft copolymers with five methods

The same method was utilized to graft NCA and MMA monomers on cellulose. For this purpose, poly [Cell-g-(N-cyclohexylacrylamide-co-methyl methacrylate)] {Poly [Cell-g- (NCA-co-MMA)]} graft copolymers were synthesized (Scheme 5). Cellulose graft copolymers were characterized by elemental analysis, FT-IR spectra, and thermogravimetric analysis. The FT-IR spectra of the graft copolymers are presented in Fig. 9. In the FT-IR spectra, the ester band of cellulose chloroacetate macro initiator at 1745 cm-1 was observed for each graft copolymer. Also, the FT-IR spectra of Cell-*g*-(NCA-*co*-MMA) showed that as the NCA content decreased in the graft copolymer, the amide carbonyl peak sharpness (band at 1670 cm-1) decreased too, and the 1745 cm-1 ester sharpness increased with MMA content. (Fig.9). Thermal stabilities were compared considering the thermogravimetric curves. Residual percentage in the graft copolymers decreased at 500 °C, while the MMA amount decreased (Fig.10). Also, the monomer reactivity ratios in the grafting of cellulose with NCA and MMA were calculated by using 7 different feed compositions ranging from 0.15 to 0.85 by ATRP. Reactivity ratios were determined by linear methods, such as the F–R, Inverted F–R, Y–B–R, K–T, and Extended K–T methods, which were r1 = 0.004-0.128 for NCA cellulose graft copolymer and r2 = 0.657-0.907 for MMA cellulose graft copolymer (Table 3). Two monomer mixtures on cellulose had a tendency to form an alternating copolymer, because the value of r1, r2, and r1.r2 ap‐ proached zero. It was concluded that MMA monomer with ester group and smaller molecular size was more reactive on grafting onto cellulose with respect to NCA molecule with amide group and larger molecular size [1]. graft copolymers

**Fig13.** Thermal Gravimetric Analysis curves of the serials Poly [Cell-g-(4NPA-co-MMA)]

Cell-*g*-(NCA-co-MMA)

**Sheme 5. Fig.14.** Synthesis of the serials Cell*-g-*(NCA-*co*-MMA) Synthesis of the serials Cell-g-(NCA-co-MMA)

**Figure 8.** Thermal Gravimetric Analysis curves of the serials Poly [Cell-g-(4NPA-co-MMA)] graft copolymers

**Table 2.** Comparison of monomer reactivity ratios for Poly [Cell-g-(4NPA-co-MMA)] graft copolymers with five

The same method was utilized to graft NCA and MMA monomers on cellulose. For this purpose, poly [Cell-g-(N-cyclohexylacrylamide-co-methyl methacrylate)] {Poly [Cell-g- (NCA-co-MMA)]} graft copolymers were synthesized (Scheme 5). Cellulose graft copolymers were characterized by elemental analysis, FT-IR spectra, and thermogravimetric analysis. The FT-IR spectra of the graft copolymers are presented in Fig. 9. In the FT-IR spectra, the ester band of cellulose chloroacetate macro initiator at 1745 cm-1 was observed for each graft copolymer. Also, the FT-IR spectra of Cell-*g*-(NCA-*co*-MMA) showed that as the NCA content decreased in the graft copolymer, the amide carbonyl peak sharpness (band at 1670 cm-1) decreased too, and the 1745 cm-1 ester sharpness increased with MMA content. (Fig.9). Thermal stabilities were compared considering the thermogravimetric curves. Residual percentage in the graft copolymers decreased at 500 °C, while the MMA amount decreased (Fig.10). Also, the monomer reactivity ratios in the grafting of cellulose with NCA and MMA were calculated by using 7 different feed compositions ranging from 0.15 to 0.85 by ATRP. Reactivity ratios were determined by linear methods, such as the F–R, Inverted F–R, Y–B–R, K–T, and Extended

**Methods r4NPA rMMA r1.r2** FR 0.096 1.365 0.130 IFR 0.011 1.263 0.014 YBR 0.017 1.209 0.021 KT 0.116 1.455 0.169 EKT 0.109 1.472 0.160

78 Cellulose - Fundamental Aspects and Current Trends

methods

**Figure 9.** FT-IR spectral area of the serials Poly [Cell-g-(NCA-co-MMA)] graft copolymers

**Figure 10.** TGA curves of the serials Poly [Cell-g-(NCA-co-MMA)] graft copolymers


**Table 3.** Comparison of monomer reactivity ratios for Poly [Cell-g-(4NPA-co-MMA)] graft copolymers by different methods

Billy M. et al. obtained more flexible and hydrophilic graft copolymers with various hydro‐ philic/hydrophobic balances by implanting methyldiethylene glycolmethacrylate (MDEGMA) on cellulose acetate with 2-bromoisobutyrilbromine initiator in cyclopentanone solvent with ATRP method. It was observed that MDEGMA copolymerization is in line with Hanns Fischer's kinetic modeling with radical persistency. Then ATRP grafting of two different cellulose acetate with different ATRP initiator group numbers were studied with respect to macroinitiator, which yielded two graft copolymers with different nanostructures, one being a short graft copolymer, and the other, a longer graft copolymer with the same mass ratio. Their morphologies were analyzed by X-ray diffraction method and it was found that phase separation depended on the number and length of poly (MDEGMA) grafts. As a result, cellulose acetate copolymers yielded to powerful films that could be utilized in membrane applications [50].

Chun-xiang L. et al. obtained cellulose graft poly (methylmethacrylate) copolymers in 1 allyl-3-methylimidazolium chloride (BMIMCl) which is an ionic liquid, using 2,2/ -bipyridine and CuBr as catalysts. Cellulose chloroacetate as macroinitiator was synthesized by direct acetylation of cellulose with chloroacetyl chloride in BMIMCl in the absence of catalyst. Copolymerization was done in ionic liquid BMIMCl in the absence of homopolymer byproduct. When an ionic liquid was used, polymers separated from the catalysts easily. Grafted polymers were characterized by 1 H-NMR, GPC (gel permeation chromatography), and AFM (atomic force microscope) methods. According to the findings from AFM, cellulose-grafted copolymers in the solution merged into a sphere-like shape [51].

Glaied O. et al. grafted cationic poly [2-(methacryloyloxy)ethyl]-trimethylammonium chloride (PMeDMA) on cellulose fibers in aqueous solution by using ATRP. Binding 2-bromoisobutyryl bromide – which is an efficient initiator – on hydroxyl groups on cellulose surface, cellulose macroinitiator, was synthesized. Obtained fiber/ PMeDMA complex was characterized by infrared, SEM, and XPS (X-ray photoelectron spectroscopy), which yielded some evidences that showed the polymerization was on the surface of the cellulose. Different initiators were added and withdrawn in order to better define the polymers and it was proven that polymer‐ ization was under control in this heterogeneous media by SEC (size exclusion chromatogra‐ phy). It was concluded that cellulose modified by cationic PMeDMA grafting had better mechanical properties [52].

**Figure 10.** TGA curves of the serials Poly [Cell-g-(NCA-co-MMA)] graft copolymers

80 Cellulose - Fundamental Aspects and Current Trends

methods

**Methods rNCA rMMA r1.r2** FR 0.022 0.696 0.015 IFR 0.128 0.907 0.116 YBR 0.004 0.657 0.003 KT 0.066 0.832 0.055 EKT 0.065 0.837 0.054 Average 0.057 0.786 0.049

**Table 3.** Comparison of monomer reactivity ratios for Poly [Cell-g-(4NPA-co-MMA)] graft copolymers by different

Billy M. et al. obtained more flexible and hydrophilic graft copolymers with various hydro‐ philic/hydrophobic balances by implanting methyldiethylene glycolmethacrylate (MDEGMA) on cellulose acetate with 2-bromoisobutyrilbromine initiator in cyclopentanone solvent with ATRP method. It was observed that MDEGMA copolymerization is in line with Hanns Fischer's kinetic modeling with radical persistency. Then ATRP grafting of two different cellulose acetate with different ATRP initiator group numbers were studied with respect to macroinitiator, which yielded two graft copolymers with different nanostructures, one being a short graft copolymer, and the other, a longer graft copolymer with the same mass ratio. Their morphologies were analyzed by X-ray diffraction method and it was found that phase separation depended on the number and length of poly (MDEGMA) grafts. As a result,

Meng T. et al. synthesized methylmethacrylate and styrene graft copolymers by direct acetylation of cellulose with ATRP method in 1-alyl-3-methylimidazoliumchloride (AMIMCl) – which is an ionic liquid at room temperature – in the absence of catalysts and chemical preservative by using 2-bromoisobutyrylate as macroinitiator. Synthesized cellulose graft copolymers were characterized by using FT-IR, 1 H-NMR, 13C-NMR, and GPC methods. It was concluded by TEM (transmission electron microscopy) and static and dynamic laser scattering measurements that cellulose graft copolymers in the solution could aggregate into a spherelike polymer structure [53].

Yan Q. et al. grafted cellulose-g-poly (N,Ndimethlyamino ethylmetacrylate)-g-poly (*ε*caprolactone) (EC-g-PDMAEMA-g-PCL) by using ring opening and ATRP methods. Unlike others, (EC-g-PDMAEMA-g-PCL) brush copolymers show unique physicochemical proper‐ ties and have multifunctional structure. These bio adaptive copolymers formed mycelia in water and these mycelia merged more with altered pH value. Therefore, it was concluded that these mycelia could be used as perfect nanocarriers for controlled drug release. It was also concluded that crystal structure and crystal morphology of the copolymers could be controlled by altering the length of side chains [54].

Yi J. et al. synthesized temperature-dependent cellulose nanocrystals-g-(N,N-dimethylamino ethylmethacrylate) (CNC-g-PDMAEMA) with ATRP method. Graft copolymers were charac‐ terized by FT-IR, TGA, GPC methods. AFM characterization showed that the width of the nanocrystals was 10–40 nm, while their length was 100–400 nm. The liquid-crystal ratio of the graft copolymers was studied by polarized optical microscopy. It was observed that graft copolymers showed lyotropic property and change of copolymer chains with respect to temperature studied [55].

Xu Q et al. synthesized a new amphoteric polymer by grafting azo polymers to cellulose nanocrystals that showed liquid crystal behavior both in solvent and during heating process. This new crystal prepared by hydrolyzing filter paper with acid was defined by AFM. Liquid crystal polymers poly {6-[4-(4-methoxyphenylazo) phenoxy] hexyl methacrylate} (PMMAZO) were successfully grafted to cellulose nanocrystals by ATRP. The structure and thermal properties of PMMAZO-grafted nanocrystals were investigated by FT-IR and TGA methods; phase structure and transition were studied by DSC and polarized optical microscopy (POM) methods. Experiment results showed that PMMAZO-grafted cellulose nanocrystals showed both thermotropic and lyotropic properties of liquid crystal formation [56].

Bhut B.V. et al. grafted poly (2-dimethylaminoethylmetacrylate) on cellulose successfully by ATRP method. Surface topography, porous membrane morphology as a result of modification, and changes in chemical functions was characterized by ATR-FTIR AFM and SEM. In order to evaluate protein binding capacity of the graft copolymer Bovine serum albumin was utilized and it was concluded that graft has increased in time [57].

Yan L. and Tao W. synthesized graft copolymer of cellulose with N,N-dimethyl acrylamide (DMA) in homogeneous media by ATRP method. First, cellulose was dissolved in N,Ndimethylacetoamide (DMAc)/LiCl system and 2-bromoisobutyloylbromide (B*i*BBr)-Cell-B*i*B macroinitiator was formed. Then DMA was polymerized with cellulose chain under existence of Cell-B*i*B at dimethyl sulfoxide. FT-IR, NMR, and GPC measurements showed that Cell-PDMA graft copolymer was formed. Protein adsorption studies proved that modified cellulose membranes prepared as Cell-PDMA had good protein adsorption resistivity [58].

Sui X. et al. synthesized cellulose-*g*-poly (*N*,*N*-dimethylamino-2-ethylmetacrylate) (cellulose*g*-PDMAEMA) graft copolymer with ATRP method. Macroinitiator was obtained by direct acetylation of cellulose with 2-bromopropionyl bromide in 1-alyl-3-methylimidazoliumchlor‐ ide, which is an ionic liquid at room conditions. Copolymers were obtained by undertaking DMAEMA into ATRP process in the presence of CuBr/PMDETA (pentamethyldiethylenetri‐ amine) –as catalyst in DMF solvent without homopolymer by-product. Grafted copolymers were characterized by 1 H-NMR, FT-IR, and TGA, and the results proved that PDMAEMA bonded to cellulose with covalent bond. Moreover, cellulose grafted PDMAEMA in aqua was studied under various concentration, temperature, and pH values with UV, TEM, AFM, and DLS (Dynamic light scattering) methods, and was observed that copolymers showed proper‐ ties expected from PDMAEMA to temperature and pH. It is thought that the method used in this study could be used in preparation of polysaccharides in various biomaterials [59].

Kang H. et al. synthesized ethyl cellulose-g-poly (2-hydroxyethylmetacrylate) (EC-g-PHEMA) in methanol by ATRP method and characterized with GPC and 1 H-NMR. Kinetic study showed that polymerization was controllable and (EC-g-PHEMA) aggregated in aqua as mycelia. Morphology of mycelia was determined by DLS and TEM and its formation was discussed [60].

Kang H. et al. synthesized ethyl cellulose-g-poly (acrylic acid) (EC-g-PAA) and ethyl celluloseg-*tert*-butylacrylate (EC-g-P*t*BA) copolymers by ATRP method. Using 2-bromoisobutyrilbro‐ mide as initiator, hydroxyl groups in ethyl cellulose were esterified and in existence of ethyl cellulose as macroinitiator EC-g-PAA and EC-g-P*t*BA graft copolymers were synthesized and were characterized by FT-IR, 1 H-NMR, and GPC methods. Molecular mass of the copolymers increased with polymerization while polydispersity decreased. This study showed that polymerization had kinetics of first degree [61].

graft copolymers was studied by polarized optical microscopy. It was observed that graft copolymers showed lyotropic property and change of copolymer chains with respect to

Xu Q et al. synthesized a new amphoteric polymer by grafting azo polymers to cellulose nanocrystals that showed liquid crystal behavior both in solvent and during heating process. This new crystal prepared by hydrolyzing filter paper with acid was defined by AFM. Liquid crystal polymers poly {6-[4-(4-methoxyphenylazo) phenoxy] hexyl methacrylate} (PMMAZO) were successfully grafted to cellulose nanocrystals by ATRP. The structure and thermal properties of PMMAZO-grafted nanocrystals were investigated by FT-IR and TGA methods; phase structure and transition were studied by DSC and polarized optical microscopy (POM) methods. Experiment results showed that PMMAZO-grafted cellulose nanocrystals showed

Bhut B.V. et al. grafted poly (2-dimethylaminoethylmetacrylate) on cellulose successfully by ATRP method. Surface topography, porous membrane morphology as a result of modification, and changes in chemical functions was characterized by ATR-FTIR AFM and SEM. In order to evaluate protein binding capacity of the graft copolymer Bovine serum albumin was utilized

Yan L. and Tao W. synthesized graft copolymer of cellulose with N,N-dimethyl acrylamide (DMA) in homogeneous media by ATRP method. First, cellulose was dissolved in N,Ndimethylacetoamide (DMAc)/LiCl system and 2-bromoisobutyloylbromide (B*i*BBr)-Cell-B*i*B macroinitiator was formed. Then DMA was polymerized with cellulose chain under existence of Cell-B*i*B at dimethyl sulfoxide. FT-IR, NMR, and GPC measurements showed that Cell-PDMA graft copolymer was formed. Protein adsorption studies proved that modified cellulose

Sui X. et al. synthesized cellulose-*g*-poly (*N*,*N*-dimethylamino-2-ethylmetacrylate) (cellulose*g*-PDMAEMA) graft copolymer with ATRP method. Macroinitiator was obtained by direct acetylation of cellulose with 2-bromopropionyl bromide in 1-alyl-3-methylimidazoliumchlor‐ ide, which is an ionic liquid at room conditions. Copolymers were obtained by undertaking DMAEMA into ATRP process in the presence of CuBr/PMDETA (pentamethyldiethylenetri‐ amine) –as catalyst in DMF solvent without homopolymer by-product. Grafted copolymers

bonded to cellulose with covalent bond. Moreover, cellulose grafted PDMAEMA in aqua was studied under various concentration, temperature, and pH values with UV, TEM, AFM, and DLS (Dynamic light scattering) methods, and was observed that copolymers showed proper‐ ties expected from PDMAEMA to temperature and pH. It is thought that the method used in this study could be used in preparation of polysaccharides in various biomaterials [59].

Kang H. et al. synthesized ethyl cellulose-g-poly (2-hydroxyethylmetacrylate) (EC-g-PHEMA)

showed that polymerization was controllable and (EC-g-PHEMA) aggregated in aqua as mycelia. Morphology of mycelia was determined by DLS and TEM and its formation was

in methanol by ATRP method and characterized with GPC and 1

H-NMR, FT-IR, and TGA, and the results proved that PDMAEMA

H-NMR. Kinetic study

membranes prepared as Cell-PDMA had good protein adsorption resistivity [58].

both thermotropic and lyotropic properties of liquid crystal formation [56].

and it was concluded that graft has increased in time [57].

temperature studied [55].

82 Cellulose - Fundamental Aspects and Current Trends

were characterized by 1

discussed [60].

Vlcek P. et al. acetylated cellulose diacetate (CDA) with 2-bromoisobutyryl bromide or dichloroacetyl chloride and obtained a new macroinitiator with functionality for ATRP method for different reaction conditions and synthesized new graft copolymers of styrene (St), MMA and butyl acrylate (BuA) with it. Poly (CDA-g-St) and poly (CDA-g-MMA) were used as macroinitiator of BuA polymer and consequently poly [CDA-g-(St-b-BuA)] and poly [CDA-g-(MMA-b-BuA)] diblock graft polymers were obtained, showing that different graft copolymer macroinitiators and different monomers could change graft polymers' length and density [62].

Shen D. et al. grafted polystyrene copolymer on ethyl cellulose (EC) by ATRP method and characterized mycelium properties of the graft copolymer by using DLS, AFM and TEM. Increased concentration resulted in increased mycelia. TEM and AFM images revealed that mycelia had spherical shapes and nucleus-crust structure. All of the macroinitiator was used; molecular mass of the graft copolymers increased and polydispersity decreased. Kinetic study showed that polymerization was of first degree [63].

Shen D. et al. synthesized graft copolymers of ethyl cellulose with polystyrene and polyme‐ thylmethacrylate by ATRP method. OH groups in ethyl cellulose reacted with 2-bromoisobu‐ tyrilbromide, which is known as an effective ATRP initiator. Functionalized ethyl cellulose has been synthesized as ATRP initiator in toluene, where CuBr/N,N,N′,N′′,N′′- pentamethyldie‐ thylenetriamine was used as catalyst. Molecular mass of the macroinitiators copolymer increased, and polydispersity decreased. Kinetic studies showed that polymerization took place in the first minutes. Graft copolymers were characterized by LLS (laser light scattering) and approved by AFM [64].

Shen D. et al. grafted polymethylmethacrylate (PMMA) on cellulose diacetate by using ATRP method and characterized by 1 H-NMR and GPC. OH groups in CDA reacted with 2-bromoi‐ sobutyryl bromide, which is known to be an effective initiator for ATRP and was used as functional CDA macroinitiator in MMA graft copolymer. Polymerization was performed under 70◦C and in N,N,N′,N′′,N′′- pentamethyldiethylenetriamine/CuBr/1,4-dioxane system. All of the macroinitiator was used, molecular mass increased, and polydispersity decreased. Kinetic studies showed that polymerization was of first degree [65].

Wang et al. [66] reported that pH responsive poly [ethyl cellulose-grafted-(2-diethylamino) ethyl methacrylate] (EC-g-PDEAEMA) synthesized by ATRP can be used in the pH-responsive release of rifampicin (RIF). In addition, another graft product obtained by ATRP, which is graft copolymer of ethyl cellulose with azobenzene-containing polymethacrylates [67], has been reported to be used in some applications such as sensors and optical materials.

### **5. Conclusion**

Cellulose graft copolymers can be obtained either in homogeneous or heterogeneous media. Most widely used methods for graft confirmation are FT-IR, NMR, TGA, SEM, TEM, XRD, and XRF. Monomer grafted on cellulose can give a hydrophilic or hydrophobic character to the new copolymer, which can pave the way to various applications. Reactions in homogeneous media usually show better controllability and yield to higher number of grafts per cellulose chain. CRP methods, such as ATRP, make it possible to obtain pre-designed copolymers, which make grafting a promising research area.

### **6. Abbreviations list**

CRP: Controlled Radical Polymerization ATRP: Atom Transfer Radical Polymerization NMP: Nitroxide-Mediated Polymerization RAFT: Reversible Addition–Fragmentation Chain Transfer Process ATRA: Atom Transfer Radical Addition Reaction SIP: Surface-Initiated Polymerization ROP: Ring-Opening Polymerization TEM: Transmission Electron Microscopy AFM: Atomic Force Microscope XPS: X-Ray Photoelectron Spectroscopy SEC: Size Exclusion Chromatography TGA: Thermal Gravimetric Analysis DLS : Dynamic Light Scattering GPC: Gel Permeation Chromatography GPC: Gel Permeation Chromatography LLS: Laser Light Scattering C6H12O6: D-glucose EC: Ethyl Cellulose MC: Methyl Cellulose PC: Propyl Cellulose

BC: Benzyl Cellulose

**5. Conclusion**

make grafting a promising research area.

84 Cellulose - Fundamental Aspects and Current Trends

CRP: Controlled Radical Polymerization

ATRP: Atom Transfer Radical Polymerization

ATRA: Atom Transfer Radical Addition Reaction

RAFT: Reversible Addition–Fragmentation Chain Transfer Process

NMP: Nitroxide-Mediated Polymerization

SIP: Surface-Initiated Polymerization ROP: Ring-Opening Polymerization

TEM: Transmission Electron Microscopy

XPS: X-Ray Photoelectron Spectroscopy

SEC: Size Exclusion Chromatography

TGA: Thermal Gravimetric Analysis

GPC: Gel Permeation Chromatography GPC: Gel Permeation Chromatography

DLS : Dynamic Light Scattering

LLS: Laser Light Scattering

C6H12O6: D-glucose EC: Ethyl Cellulose

MC: Methyl Cellulose

PC: Propyl Cellulose

AFM: Atomic Force Microscope

**6. Abbreviations list**

Cellulose graft copolymers can be obtained either in homogeneous or heterogeneous media. Most widely used methods for graft confirmation are FT-IR, NMR, TGA, SEM, TEM, XRD, and XRF. Monomer grafted on cellulose can give a hydrophilic or hydrophobic character to the new copolymer, which can pave the way to various applications. Reactions in homogeneous media usually show better controllability and yield to higher number of grafts per cellulose chain. CRP methods, such as ATRP, make it possible to obtain pre-designed copolymers, which HPC: Hydroxypropyl Cellulose

HPMC: Hydroxypropyl Methyl Cellulose

HEC: Hydroxylethyl Cellulose

HEMC: Hydroxylethyl Methyl Cellulose

MHPC: Methyl Hydroxypropyl Cellulose

EHEC: Ethyl Hydroxylethyl Cellulose

CMC: Carboxymethyl Cellulose

CA: Cellulose Acetate

Pn: An Active Species

Mn-Y/L: Transition Metal Complex

X: Halogen

Pn-X: Initiator or a Dormant Species

CH2=CH2R1: Active Species Initiate Monomers

X-Mn+1-Y/L: Oxidized Metal Complexes


PMDETA: N,N,N/ ,N//,N///–Pentamethyl Diethylene Triamine

TMEDA: Tetramethyl Ethylene Diamine

HMTETA: 1,14,7,10,10-Hexamethyl Triethylene Tetramine

Me6-TREN: Tris [2-diamethylamino ethylamine]

*o*-phen: 1,10-phenanthroline

bpy: 2,2'-bipyridine

dNbpy: 4,4'-di-(5-nonyl)-2,2'-bipyridine

NPPMI: N-(n-propyl)pyridylmethanimine

NOPMI: N-(n-octyl)pyridylmethanimine

PMDETA: N, N, N', N'', N'''-pentamethyldiethylenetriamine

HMTETA: 1,1,4,7,10,10-hexamethyltriethylenetetramine

TPMA: Tris[(2-pyridyl)methyl]amine

TPEN: N, N, N',N'-tetrakis(2-pyridylmethyl)ethylenediamine

4NPA: N-(4-Nitrophenyl) acrylamide

NCA: N-Cyclohexylacrylamide

4VP: 4-Vinylpyridine

AM: Acrylamide

MAM: Methacrylamide

DAAM: Diacetone acrylamide

MMA: Methylmethacrylate

F–R: Fineman-Ross method

K–T: Kelen-Tüdos method

E–K–T: Extended Kelen-Tüdos method

I–F–R: Inverted Fineman-Ross method

Y–B–R: Yezrielev-Brokhina-Roskin method

DMF: N,N-Dimethylformamide

DMF: N,N-Dimethylformamide

### **Acknowledgements**

The author thanks M. M. Temüz who was the author's PhD thesis advisor in Fırat University.

### **Author details**

Nevin Çankaya\*

Address all correspondence to: nevin.cankaya@usak.edu.tr; nevincankaya@hotmail.com Uşak University, Faculty of Science and Art, Department of Chemistry, Uşak, Turkey

### **References**

HMTETA: 1,1,4,7,10,10-hexamethyltriethylenetetramine

TPEN: N, N, N',N'-tetrakis(2-pyridylmethyl)ethylenediamine

TPMA: Tris[(2-pyridyl)methyl]amine

86 Cellulose - Fundamental Aspects and Current Trends

4NPA: N-(4-Nitrophenyl) acrylamide

NCA: N-Cyclohexylacrylamide

4VP: 4-Vinylpyridine

MAM: Methacrylamide

DAAM: Diacetone acrylamide

MMA: Methylmethacrylate

F–R: Fineman-Ross method

K–T: Kelen-Tüdos method

E–K–T: Extended Kelen-Tüdos method

I–F–R: Inverted Fineman-Ross method

DMF: N,N-Dimethylformamide

DMF: N,N-Dimethylformamide

**Acknowledgements**

**Author details**

Nevin Çankaya\*

Y–B–R: Yezrielev-Brokhina-Roskin method

The author thanks M. M. Temüz who was the author's PhD thesis advisor in Fırat University.

Address all correspondence to: nevin.cankaya@usak.edu.tr; nevincankaya@hotmail.com

Uşak University, Faculty of Science and Art, Department of Chemistry, Uşak, Turkey

AM: Acrylamide


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### **Chemical Functionalization of Cellulosic Materials — Main Reactions and Applications in the Contaminants Removal of Aqueous Medium**

Roosevelt D.S. Bezerra, Paulo R.S. Teixeira, Ana S.N.M. Teixeira, Carla Eiras, Josy A. Osajima and Edson C. Silva Filho

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/61431

### **Abstract**

The cellulose is the most abundant biopolymer in the world and presents a higher chemi‐ cal variability for presence of several hydroxyl groups. These hydroxyl groups allow sur‐ face modification of biomaterials, with insertion of several chemical groups which change cellulose characteristics. This natural biopolymer and its derivatives have been used a lot as adsorbent, from several contaminants of aqueous medium due to biocompatibility, chemical degradability, and variability. Therefore, this chapter has the objective to review the literature about several cellulose surfaces or cellulosic material (incorporation of car‐ boxymethyl, phosphorus, carboxyl, amines, and sulfur), presenting the main characteris‐ tics of reactions and showing its adsorption in application of aqueous medium (metals, dyes, and drugs), locating the main interactions between biomaterial/contaminant.

**Keywords:** cellulose, chemical modification, interations

### **1. Introduction**

The cellulose (Figure 1) is the most abundant polysaccharide on the earth, being the main structural component of plants and seaweed cell walls. The cellulose is formed from the repeated units of D-glucose, which are linked by glycosidic linkages β(1 → 4). This natural polysaccharide has become the more used material due to its physical and structural properties and its biocompatibility. These properties arise from multiple interactions of hydrogen, which result in semicrystalline polymer, containing highly structured crystalline regions, and also, in materials with high tensile strength [1].

© 2015 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

**Figure 1.** Molecular structure of cellulose.

The cellulose is mainly obtained from four resources: forest, agriculture, industrial, animal waste. The biomass which is obtained from all sources has three main components: cellulose, hemicellulose, and lignin, with its percentage depending highly on the obtained source. Therefore, the biomass has been extracted and processed in order to separate different components and to isolate cellulose [1].

The cellulose has lot of hydroxyl groups which can bond to several functional groups through variability of chemical modifications [2]. These chemical modifications provoke the formation of covalent bond through interaction between the modifier agent and the interactive center of solid surface, where insertion of organic molecules, in the surface of solid support, gives it advantageous and additional properties that differ from the original support [3].

The cellulose derivatives modified by several chemical reactions have various application possibilities in different fields. Among the various possibilities, one can be highlighted the usefulness of cellulosic derivatives in contaminants removal of aqueous medium. For example, the modified cellulose with diethylenetriamine was applied in the adsorption of Cu (II) and Pb (II) [4]. The cellulose modification with aminoethanethiol was more efficient with adsorp‐ tion of red reactive dye RB [3] than pure cellulose. The phosphated cellulose presented higher adsorption capacity of ranitidine drug [6] than the pure cellulose [7]. Both the phosphated bacterial cellulose as well as the bacterial cellulose containing quaternary salt were efficient in protein adsorption.

Therefore, this chapter's goal is to perform a review of the literature about the main chemical reactions on cellulosic material surfaces and its applications in contaminants removal of aqueous medium.

### **2. Modifications and applications in contaminants removal**

### **2.1. Carboxymethylation**

The sodium carboxymethylcellulose (CMC) is a polyelectrolyte formed when the chloroacetic acid, or its sodium salt, reacts with alkaline cellulose. The CMC is a copolymer of two units: β-D-glucose and salt β-D-glucopyranose 2-*O*-(carboxymethyl)-monosodium, not spread randomly around macromolecules which are linked by β-1,4-glycosidic linkages [8].

The CMC is used in a lot of industrial fields, such as in food industry as thickener or viscosity modifier for stabilizing emulsion*,* in the petroleum industry as ingredient of drilling mud. Moreover, it is a component of a lot of products, such as detergents, toothpaste, water-based paints, textile sizing, and several paper products. Lately, it has been applied in green synthesis study and stabilization of silver nanoparticles [9].

The CMC synthesis is divided in two stages: (I) alkalinization and etherization (II). In stage (I), the cellulose is dispersed in sodium hydroxide solution and ethanol. In stage (II), the sodium chloroacetic is added to solution; the mixture is hectic at 353 K, according to Figure 2 [9,10,11].

**Figure 2.** Preparation scheme of the CMC.

<sup>O</sup> <sup>O</sup>

H

n

H H H OH

OH

H OH

The cellulose is mainly obtained from four resources: forest, agriculture, industrial, animal waste. The biomass which is obtained from all sources has three main components: cellulose, hemicellulose, and lignin, with its percentage depending highly on the obtained source. Therefore, the biomass has been extracted and processed in order to separate different

The cellulose has lot of hydroxyl groups which can bond to several functional groups through variability of chemical modifications [2]. These chemical modifications provoke the formation of covalent bond through interaction between the modifier agent and the interactive center of solid surface, where insertion of organic molecules, in the surface of solid support, gives it

The cellulose derivatives modified by several chemical reactions have various application possibilities in different fields. Among the various possibilities, one can be highlighted the usefulness of cellulosic derivatives in contaminants removal of aqueous medium. For example, the modified cellulose with diethylenetriamine was applied in the adsorption of Cu (II) and Pb (II) [4]. The cellulose modification with aminoethanethiol was more efficient with adsorp‐ tion of red reactive dye RB [3] than pure cellulose. The phosphated cellulose presented higher adsorption capacity of ranitidine drug [6] than the pure cellulose [7]. Both the phosphated bacterial cellulose as well as the bacterial cellulose containing quaternary salt were efficient in

Therefore, this chapter's goal is to perform a review of the literature about the main chemical reactions on cellulosic material surfaces and its applications in contaminants removal of

The sodium carboxymethylcellulose (CMC) is a polyelectrolyte formed when the chloroacetic acid, or its sodium salt, reacts with alkaline cellulose. The CMC is a copolymer of two units: β-D-glucose and salt β-D-glucopyranose 2-*O*-(carboxymethyl)-monosodium, not spread

The CMC is used in a lot of industrial fields, such as in food industry as thickener or viscosity modifier for stabilizing emulsion*,* in the petroleum industry as ingredient of drilling mud.

randomly around macromolecules which are linked by β-1,4-glycosidic linkages [8].

**2. Modifications and applications in contaminants removal**

advantageous and additional properties that differ from the original support [3].

**Figure 1.** Molecular structure of cellulose.

94 Cellulose - Fundamental Aspects and Current Trends

components and to isolate cellulose [1].

protein adsorption.

aqueous medium.

**2.1. Carboxymethylation**

After the introduction of carboxymethyl group, the cellulose presents an anionic feature and there occurs an increase in its water solubility [12]. This carboxymethylation changes com‐ pletely the electric charge of cellulose surface, turning it in ion exchanger. And the increase of carboxymethyl groups also promotes the increase of zeta potential value on CMC on to pure cellulose [10]. After carboxymethylation, these new characteristics acquired by celluloses are very important in the application in contaminants removal of aqueous medium, because the adsorption process is extremely affected by interactions which can occur between adsorbate and adsorbent, and they can be affected by pH of the medium [5,6,7].

For example, in the removal study of the methylene blue cationic dye (MB) (Figure 3 (a)), the pH is an important factor in the adsorption process of CMC. In acid conditions, the carboxy‐ methyl group of CMC is protonated, and there occurs an ion exchange (-CH2COONa turning into -CH2COOH), and thus adsorption of MB is a disadvantage. In alkaline conditions, the carboxymethyl group is totally deprotonated (-CH2COO– ), furthering the MB dye adsorption. These results indicate which adsorption mechanism of the MB in CMC is due to ion exchange reactions, according to Figure 3 (b) [11].

The adsorption of the metal strontium (II) (Sr2+) in CMC proved efficient and independent of pH. The ion exchange is the main reason for adsorption of ions Sr(II) in carboxymethyl groups of the CMC. In acidic pH, i.e., pH less than 2.0, the excess ions will compete to the Sr (II) ions, for match to –COO– , as shown in Eq.1. Thus, as the pH of the medium increase, there will be an increase in the polymer hydrolysis, and hence it will grow the available sites for interaction to the strontium ions (II) [13].

$$\left[\text{n(-COO)Sr}\right]^{(2-n)+} + \text{nH}^+ \leftrightarrow \text{nCOOH} + \text{Sr}^{2+} \left(n = 1, 2\right) \tag{1}$$

The adsorption of Sr (II) in CMC is highly influenced by ionic force from the medium. As the concentration of KCl increases, the ionic force of the solution grows, and the ion adsorption Sr

**Figure 3.** (a) Molecular structure of methylene blue dye. (b) Adsorption mechanism of the MB in CMC is due to ion exchange. **Figure 3.** (a) Molecular structure of methylene blue dye. (b) Adsorption mechanism of the MB in CMC is due to ion exchange.

(II) in CMC decreases. The ionic force effect can be explained by two factors: (I) despite the presence of KCl in adsorption system which can improve the electrostatic attraction among – COO– groups present in the CMC and the Sr (II) ions, the K+ ions destroy the ionic bond among –COO– groups and the Sr (II) ions, similar to H+ (Eq.2) ions, (II) the affinity growth between KCl and water, after adding KCl, decreases the metal mobility, which provokes the decrease in adsorption [13].

$$\left[\text{n(-COO)Sr}\right]^{(2-n)^{+}} + \text{nK}^{+} \leftrightarrow \text{nCOOK} + \text{Sr}^{2+} \left(n = 1, 2\right) \tag{2}$$

#### **2.2. Phosphating**

The chemical incorporation of phosphate in cellulose structure changes its properties mean‐ ingfully, since the synthetized biomaterial starts showing the characteristics of the phosphate group*.* This phosphated biomaterial is used in textile industry such as flames delayer to cellulose based. Also, it is used as a biomaterial for the disease treatment about calcium ions transport [6].

Due to importance of the phosphated group, a lot of methods to cellulose phosphating have been developed, either by free hydroxyl groups in positions C2, C3, or C6 or by shift from the ester or ether group already in the cellulose. The latest groups are usually used in homogene‐ ous reaction using toxic reagents and mainly organic solvents which promote partial or total break of cellulose fibers. The synthesis of phosphate cellulose derivatives have been performed in several solvents [dimethylsulfoxide (DMSO)-methylamine, DMAc-LiCl, SO3-triethylamine, formic acid, trifluoroacetic acid, N,N-dimethylformamide (DMF)-N2O2, paraformaldehyde, DMF, molten or aqueous urea, NaOH] with various phosphorus-containing compounds (phosphoric acid and phosphinic acid, phosphorus oxyacids, phosphorus pentoxide, amido‐ phosphates, alkyl or aryl derivatives of phosphorous acid) [14].

The phosphoric acid (H3PO4) is the main phosphate precursor used in cellulose phosphating reaction. The reaction using phosphoric acid, such as phosphate precursor, can be performed by two manners (heterogeneously and homogeneously), and depending on the way followed, the reaction will produce different phosphated products [14, 15].

The cellulose reaction with phosphoric acid in aqueous medium (heterogeneously) is performed through adding H3PO4 (85%) in a system which has only cellulose. The system temperature is raised to 373 K and maintained at this temperature for 30 min, as shown in Figure 4. In this reaction, the phosphoric acid is linked to cellulose chain under the form phosphorous acid group, by ester linkage, through reactions from cellulose hydroxyl group. The product formed in this reaction also established a tautomeric equilibrium, due to the presence of hydrogen atom linked directly to phosphorus, as can be viewed on Figure 4 (a and b) [14].

(II) in CMC decreases. The ionic force effect can be explained by two factors: (I) despite the presence of KCl in adsorption system which can improve the electrostatic attraction among –

(b)

**Figure 3.** (a) Molecular structure of methylene blue dye. (b) Adsorption mechanism of the MB in CMC is due to ion exchange. **Figure 3.** (a) Molecular structure of methylene blue dye. (b) Adsorption mechanism of the MB in CMC is due to ion

N

(a)

MB+ <sup>O</sup> <sup>O</sup>

N+ CH3

Cl-

H3C

H H H OH

H

n

<sup>+</sup> Na+

OCCH2OOMB

H OH

N S

CH3

H3C

O O

OCCH2OONa

H

n

H H H OH

H OH

96 Cellulose - Fundamental Aspects and Current Trends

The chemical incorporation of phosphate in cellulose structure changes its properties mean‐ ingfully, since the synthetized biomaterial starts showing the characteristics of the phosphate group*.* This phosphated biomaterial is used in textile industry such as flames delayer to cellulose based. Also, it is used as a biomaterial for the disease treatment about calcium ions

Due to importance of the phosphated group, a lot of methods to cellulose phosphating have been developed, either by free hydroxyl groups in positions C2, C3, or C6 or by shift from the ester or ether group already in the cellulose. The latest groups are usually used in homogene‐ ous reaction using toxic reagents and mainly organic solvents which promote partial or total break of cellulose fibers. The synthesis of phosphate cellulose derivatives have been performed in several solvents [dimethylsulfoxide (DMSO)-methylamine, DMAc-LiCl, SO3-triethylamine, formic acid, trifluoroacetic acid, N,N-dimethylformamide (DMF)-N2O2, paraformaldehyde, DMF, molten or aqueous urea, NaOH] with various phosphorus-containing compounds

 groups and the Sr (II) ions, similar to H+ (Eq.2) ions, (II) the affinity growth between KCl and water, after adding KCl, decreases the metal mobility, which provokes the decrease

( ) ( ) ( ) 2–n + <sup>+</sup> 2+ é ù n -COO Sr + nK nCOOK + Sr = 1,2 « *<sup>n</sup>* ë û (2)

ions destroy the ionic bond among

groups present in the CMC and the Sr (II) ions, the K+

COO–

exchange.

–COO–

in adsorption [13].

**2.2. Phosphating**

transport [6].

**Figure 4.** Reaction scheme of cellulose phosphorylation with H3PO4 in aqueous medium, and possible tautomeric products (a and b) of phosphated cellulose structure.

The cellulose reaction as phosphoric acid in molten urea (homogeneously) is initially per‐ formed merging urea at 413 K. Then, it adds cellulose the suspension, water, and phosphoric acid. The reaction takes place for 30 min at 423 K, as shown in Figure 5. In this reaction, in addition to the listed structures (Figure 4 (a and b)), the phosphoric acid can form ester products disubstituted and trisubstituted from cellulose [14].

The cellulose phosphorylation using phosphoric acid and urea can also be performed using DMF as solvent. Initially, in this reaction, it puts cellulose in contact with urea. After 1 hour,

**Figure 5.** Reaction scheme of cellulose phosphorylation H3PO4 in molten urea and the possible gained products.

it adds phosphoric acid and the mixture are stirred for 4 h at 403 K. The reaction products are monophosphated cellulose and water, as shown in Figure 6 [16,17].

**Figure 6.** Reaction scheme of cellulose phosphorylation H3PO4 in urea using DMF as the solvent.

The cellulose phosphorylation increases the capacity of cellulose ionic exchange, because the introduction of phosphated group in the structure promotes the growth of the active groups. This characteristic was noticed by addition of cellulosic solvents [rice straw (37.4% of cellu‐ lose, 44.9% of hemi-cellulose, 4.9% of lignin and 13.1% of ashes) and phosphate rice straw] in contact with a 1 mol L–1 de NaCl solution. The ionic exchange reaction occurs by substitution of H+ of the adsorbents for Na+ which is in solution, generating HCl in solution, according to Figure 7 [18].

**Figure 7.** Ionic exchange solution from the phosphated solution.

The ionic exchange capacity is very important in the application in adsorption of heavy metals from aqueous medium. Surveys have showed which bacterial cellulose has no adsorption capacity of lanthanides metals (La3+, Sm3+ e Ho3+), since phosphated bacterial cellulose presents adsorption capacity of these metals. This adsorption process is influenced by the pH of medium. This pH dependence in adsorption indicates the pH mechanism is cation exchange. In addition, the bacterial phosphated cellulose has high adsorption capacity of lanthanides ions "stiff acid", based on the high affinity of the phosphoric acid group "stiff base" by the principle of base acid stiff acid (HSAB) [16].

The phosphated cellulose also proved efficient in the removal of drug ranitidine (Figure 8). Surveys have shown that after phosphating reaction, a growth of negative charge of cellulose occurred, due to the presence of phosphate group. It made the drug ranitidine reaction higher on modified cellulose surface than on pure cellulose surface. The ranitidine adsorption mechanism on the phosphated cellulose surface was through electrostatic interaction, where the negative charge of phosphate groups interacts with positive charge on the nitrogen atom of drug. As the same way, the ΟΗ– ions can interact with the positive ions of drug, stopping drug interaction with phosphated cellulose, as reported above [6,7].

**Figure 8.** Molecular structure of ranitidine.

### **2.3. Acylation**

it adds phosphoric acid and the mixture are stirred for 4 h at 403 K. The reaction products are

OH Cel

**Figure 5.** Reaction scheme of cellulose phosphorylation H3PO4 in molten urea and the possible gained products.

P OH OH O O H

Cel

O H

or Cel O P O O O Cel

O P H OH O N H

H

O NH2

Cel

or


(NH2)2CO DMF

The cellulose phosphorylation increases the capacity of cellulose ionic exchange, because the introduction of phosphated group in the structure promotes the growth of the active groups. This characteristic was noticed by addition of cellulosic solvents [rice straw (37.4% of cellu‐ lose, 44.9% of hemi-cellulose, 4.9% of lignin and 13.1% of ashes) and phosphate rice straw] in contact with a 1 mol L–1 de NaCl solution. The ionic exchange reaction occurs by substitution

2Na<sup>+</sup> 2H<sup>+</sup>

<sup>O</sup> <sup>O</sup>

H

n

+ H2O

<sup>O</sup> <sup>O</sup>

H

n

H

O NaO P ONa O

H H

H OH

OH

H H H OH

O P OHOH O

H OH

which is in solution, generating HCl in solution, according to

monophosphated cellulose and water, as shown in Figure 6 [16,17].

+ H3PO4

**Figure 6.** Reaction scheme of cellulose phosphorylation H3PO4 in urea using DMF as the solvent.

<sup>O</sup> <sup>O</sup>

<sup>O</sup> <sup>O</sup> H H H OH

H

n

Cel O P OH OH <sup>O</sup> or Cel O P O OH O Cel

Cel O P OH

H3PO4, Ureia 423 K, 30 min

H OH

OH

98 Cellulose - Fundamental Aspects and Current Trends

H

n

<sup>O</sup> <sup>O</sup>

H

n

H

**Figure 7.** Ionic exchange solution from the phosphated solution.

O P OHOH O

H H H OH

OH

H OH

of the adsorbents for Na+

H H

H OH

OH

of H+

Figure 7 [18].

The acylation with cyclic organic anhydrides of five members is a well-established reaction, which at first it involves all three hydroxyl groups of the cellulose unity (acts as nucleophiles), for getting a steady ester and reactive groups from carboxyl acids. In the practical use, these acylation processes are used in covering, cosmetics, in the pharmaceutical and food industries, as membranes and antibody filters, enzyme, protein, ion separation, etc. [19].

The cellulose derivatives which have free carboxyl groups are prepared by cellulose reaction or cellulosic material with succinic, maleic, or phthalic in the solvent absence. This process consists of heating cellulose with the amount of each anhydride up to anhydride melting fusion. For each case, the cellulose/anhydride proportion is 1/10, the mixture is stirred for 20 min and the reaction is interrupted by addition of DMA [20]. The full scheme of reaction is shown in Figure 9.

The three reactions shown above have covalent incorporation of carboxyl groups. This incorporation can be confirmed by the growth of the amount of carbon in the cellulose [20,21]. The cellulose reaction with maleic anhydride presents a high incorporation level, due to absence of solvent in this synthesis, which is well-proportioned by high temperature and by easy availability of reagents. The maleic anhydride presents incorporation, which after

**Figure 9.** Cellulose reaction scheme or cellulose material with anhydride (a) **Figure 9.** Cellulose reaction scheme or cellulose material with anhydride (a) maleic, (b) phthalic, and (c) succinic.

maleic, (b) phthalic, and (c) succinic.

opening the ring forms two isomers of the cellulose: the cellulose maleate (cis) and cellulose fumarate (trans). This possibility to form both isomers is due to the presence of withdrawing groups near to unsaturation, which can affect isomerization according to the potential effect for removing electrons, and also due to impediment caused by these groups. In addition, the reaction α,β-unsaturation, with hydroxyl C(2), C(3), and (C6), through Michael's addition can produce other composts as sub products [21].

The cellulose reaction with phthalic anhydride also has a high level of anhydride incorporation by the same reasons quoted above in the cellulose reaction with maleic anhydride. The reaction using this aromatic anhydride can create some attractive interactions involving the aromatic rings of the anhydride, aromatic–aromatic interactions (Ar–Ar). These interactions can be strong enough to disturb the β-(1 → 4) bonds [22]. The cellulose reaction with succinic anhydride is similar to cellulose reaction with maleic anhydride, the only difference being no formation of cis–trans isomer, due to the absence of unsaturation in succinic anhydride.

These cellulose derivatives are enough in the removal of several contaminants from the aqueous medium. For example, the cellulose derivatives modified with maleic and phthalic anhydride were applied in adsorption of green malachite dye (Figure 10 (a)), and were shown to be more effective than pure cellulose. The dye adsorption was influenced by initial pH of medium. The addition of green malachite dye, on the surface of two cellulosic derivatives, increased as the pH increased until pH 6, where adsorption was established. This happened because in low pH, the carboxyl groups, of modified biopolymers, can be protonated, due to high proton concentration, decreasing the amount of adsorbed cationic dye. Also, this survey has shown the higher amount of carboxyl groups in cellulose, higher is the adsorption of green malachite dye [23]. The cellulose derivative modified with maleic anhydride showed effective in adsorption of divalent metals (Co2+ and Ni2+), where the adsorption mechanism in this system is complexation among basic centers of carboxyl groups and divalent cations, as shown in Figure 10(b). For facilitating this complexation, and any lower minimization of pH, the protons of carboxyl groups were substituted by sodium. Figure 10 (b) is shows a counter-ion for neutralizing free cationic charge, where the cation can interact with a carboxyl group or two distinct basic centers [21].

opening the ring forms two isomers of the cellulose: the cellulose maleate (cis) and cellulose fumarate (trans). This possibility to form both isomers is due to the presence of withdrawing groups near to unsaturation, which can affect isomerization according to the potential effect for removing electrons, and also due to impediment caused by these groups. In addition, the reaction α,β-unsaturation, with hydroxyl C(2), C(3), and (C6), through Michael's addition can

**Figure 9.** Cellulose reaction scheme or cellulose material with anhydride (a)

The cellulose reaction with phthalic anhydride also has a high level of anhydride incorporation by the same reasons quoted above in the cellulose reaction with maleic anhydride. The reaction using this aromatic anhydride can create some attractive interactions involving the aromatic rings of the anhydride, aromatic–aromatic interactions (Ar–Ar). These interactions can be strong enough to disturb the β-(1 → 4) bonds [22]. The cellulose reaction with succinic

produce other composts as sub products [21].

maleic, (b) phthalic, and (c) succinic.

<sup>O</sup> <sup>O</sup>

H

<sup>O</sup> <sup>O</sup> H H H OH

H

<sup>O</sup> <sup>O</sup>

H

H H H OH

OH

H OH

H OH

OH

n

326 K

(a)

(b)

(c)

**Figure 9.** Cellulose reaction scheme or cellulose material with anhydride (a) maleic, (b) phthalic, and (c) succinic.

O O

O

<sup>n</sup> 404 K

<sup>n</sup> 392 K

<sup>O</sup> <sup>O</sup> <sup>O</sup> <sup>O</sup> <sup>O</sup>

<sup>O</sup> <sup>O</sup> <sup>O</sup> <sup>O</sup> <sup>O</sup>

H H H OH

<sup>O</sup> <sup>O</sup> H H H OH

H

n

OH O

H OH

H H H OH

O O

H

n

H OH

O <sup>O</sup> COOH

O

H

n

O OH O

H OH

H H H OH

OH

100 Cellulose - Fundamental Aspects and Current Trends

H OH

**Figure 10.** (a) Molecular structure of green malachite cationic dye. (b) Possible structures for formation of divalent cation complex and modified cellulose with maleic **Figure 10.** (a) Molecular structure of green malachite cationic dye. (b) Possible structures for formation of divalent cati‐ on complex and modified cellulose with maleic anhydride.

anhydride.

The cellulose derivatives modified with succinic and phthalic anhydrides proved efficient in adsorption of metal Cu2+. In these cases, the adsorption is favored by growth of pH, where the sorption is favored in solutions with high pH to the point of zero charge (pHpzc) for each modified material (modified cellulose with phthalic anhydride pHpzc= 5.4 and with succinic anhydride pHpzc= 5.7). Under acid conditions, the chemically modified biopolymers can be protonated, due to high concentrations of H+ ions, decreasing the amount of adsorbed cation. On the other hand, the growth of pH deprotonates carboxyl group, a condition which favors chelating capacity, and consequently, the capacity of adsorbed Cu2+ increases [20]. The cotton fibers and other cellulosic derivatives have an improvement about 40% in the adsorption of blue methylene cationic dye (Figure 3(a)) after the chemical modification with succinic anhydride. The presence of carboxyl groups in biomaterials means significant rise in the adsorption, which can result from electrostatic interactions among these groups and dye reactive groups [24].

### **2.4. Amination**

The molecule incorporation which has basic centers (primarily nitrogen, oxygen, and sulfur) in the cellulose structure raises its contaminants' adsorption capacity [3]. Thus, the nitrogen incorporation in cellulose or in the cellulosic materials is an important chemical modification which allows the insertion of active sites in this biomaterial. These sites can interact in aqueous medium with contaminants molecules, in heterogeneous system in which the interactive processes are defined by solid/liquid interface [25].

An example of incorporation of basic centers in cellulose structure is the reaction with ethylene-1,2-diamine. The first stage of reaction consists in allogeneic derivative synthesis. In the halogenation, chlorine addition is preferable, since it is the most effective allogeneic, whose preferential sequence is: chlorine > bromine, iodine > fluorine [25]. The first step of reaction series consists in the cellulose chlorination, a favored process by nucleophilic attack from thionyl chloride on hydroxyl group in the biopolymer backbone, resulting in chlorine atom pending in polymer structure. This replaced atom is more reactive than the original hydroxyl group, where a total substitution of hydroxyl in C6 by chlorine can happen. It happens due to hydroxyl group which is in C6 being more reactive than other hydroxyl groups, following order C6>> C3≈C2, as shown in Figure 11 [26,27].

**Figure 11.** Reaction of cellulose chlorination.

The second step of reaction consists in chlorinated cellulose reaction to react with ethylene-1,2 diamine under reflux for 3 h in the absence of solvent, according to Figure 12 [28]. When the ethylene-1,2-diamine reacts with chlorinated cellulose and replaced chlorine, the inter- and intramolecular reactions present in biomaterial which are very much responsible for its organization and crystalline arrangement become amorphous. In addition, the molecule incorporation of ethylene-1,2-diamine provides the growth of the amount of carbon in the structure, the increasing C7 and C8 [29].

**Figure 12.** Reaction of chlorinated cellulose with ethylene-1,2-diamine.

The cellulose derivatives modified with succinic and phthalic anhydrides proved efficient in adsorption of metal Cu2+. In these cases, the adsorption is favored by growth of pH, where the sorption is favored in solutions with high pH to the point of zero charge (pHpzc) for each modified material (modified cellulose with phthalic anhydride pHpzc= 5.4 and with succinic anhydride pHpzc= 5.7). Under acid conditions, the chemically modified biopolymers can be protonated, due to high concentrations of H+ ions, decreasing the amount of adsorbed cation. On the other hand, the growth of pH deprotonates carboxyl group, a condition which favors chelating capacity, and consequently, the capacity of adsorbed Cu2+ increases [20]. The cotton fibers and other cellulosic derivatives have an improvement about 40% in the adsorption of blue methylene cationic dye (Figure 3(a)) after the chemical modification with succinic anhydride. The presence of carboxyl groups in biomaterials means significant rise in the adsorption, which can result from electrostatic interactions among these groups and dye

The molecule incorporation which has basic centers (primarily nitrogen, oxygen, and sulfur) in the cellulose structure raises its contaminants' adsorption capacity [3]. Thus, the nitrogen incorporation in cellulose or in the cellulosic materials is an important chemical modification which allows the insertion of active sites in this biomaterial. These sites can interact in aqueous medium with contaminants molecules, in heterogeneous system in which the interactive

An example of incorporation of basic centers in cellulose structure is the reaction with ethylene-1,2-diamine. The first stage of reaction consists in allogeneic derivative synthesis. In the halogenation, chlorine addition is preferable, since it is the most effective allogeneic, whose preferential sequence is: chlorine > bromine, iodine > fluorine [25]. The first step of reaction series consists in the cellulose chlorination, a favored process by nucleophilic attack from thionyl chloride on hydroxyl group in the biopolymer backbone, resulting in chlorine atom pending in polymer structure. This replaced atom is more reactive than the original hydroxyl group, where a total substitution of hydroxyl in C6 by chlorine can happen. It happens due to hydroxyl group which is in C6 being more reactive than other hydroxyl groups, following

The second step of reaction consists in chlorinated cellulose reaction to react with ethylene-1,2 diamine under reflux for 3 h in the absence of solvent, according to Figure 12 [28]. When the

DMF <sup>O</sup> <sup>O</sup>

H H H OH

Cl

H

n

+ SO2 + HCl

H OH

reactive groups [24].

102 Cellulose - Fundamental Aspects and Current Trends

processes are defined by solid/liquid interface [25].

order C6>> C3≈C2, as shown in Figure 11 [26,27].

<sup>+</sup> SOCl2 353 K

<sup>O</sup> <sup>O</sup>

H

**Figure 11.** Reaction of cellulose chlorination.

n

H H H OH

OH

H OH

**2.4. Amination**

Another important reaction of amination is organic molecule incorporation of 2- aminome‐ thylpyridine. As the latest example, initially cellulose is chlorinated and, then it reacts with 2-aminomethylpyridine under reflux for 4 h [25], according to Figure 13. The amount of incorporated nitrogen in cellulose by this reaction is low, when compared to similar patterns. This fact can be explained by steric impediment which is caused by molecule hydrophobicity [30].

**Figure 13.** Reaction of chlorinated cellulose with organic 2-aminomethylpyridine.

These cellulosic derivatives containing basic centers (in this case nitrogen) are extremely used in the contaminants removal of the aqueous medium. For example, the modified cellulose 2-aminomethylpyridine is proved to be effective in divalent cations removal (Cu2+, Co2+, Ni2+, and Zn2+). In this case, the adsorption mechanism is based on the complexation process of a cation in two basic centers, which indicates that cellulose chain acts as a bidentate chelating agent. The scheme of metal complexation is shown in Figure 14 (a), where the available nitrogen centers are coordinated to divalent cations, with charge being counterbalanced for counter-ions. The modified cellulose with ethylene-1,2-diamine also proved to be efficient in divalent cations removal quoted above. In this case, the adsorp‐ tion order was: Co2+ > Cu2+ > Zn2+ > Ni2+. Its adsorption mechanism is similar to that quoted above, where there is transfer of cations from solutions to available basic centers in ethylene-1,2-diamine anchored in the cellulose, by cations complexation through available amine groups, as shown in Figure 14 (b) [28].

(a)

**Figure 14.** (a) Proposed scheme of complexation for divalent cations with a modified cellulose with 2-aminomethylpyridine. (b) Proposed scheme of complexation for **Figure 14.** (a) Proposed scheme of complexation for divalent cations with a modified cellulose with 2-aminomethylpyr‐ idine. (b) Proposed scheme of complexation for divalent cations with the modified cellulose with ethylene-1,2-diamine.

divalent cations with the modified cellulose with ethylene-1,2-diamine. Another amination example is the introduction of ethylene-1,2-diamine in the cellulose structure through a rusty cellulosic intermediate. The first stage consists in nanocrystalline cellulose synthesis (NCC) through hydrolysis with sulfuric acid. In the second stage, the carbon C2 and C3 of the cellulose is oxidised with sodium periodate (NaIO4), in the absence of light at 313 K, forming nanocrystalline dialdehyde from cellulose. After this, ethylene-1,2-diamine is added, the mixture is continuously stirred for 6 h at 303 K. Finally, with imine intermediate reduction through addition of NaBH4 at room temperature, as shown in Figure 15 [31], the zeta potential of the final product of this reaction is formed, which is highly influenced by pH. In lower pH, the zeta potential was positive due to amine protonation (NCC–NH3 + ). By the pH growth, the zeta potential decreases and it becomes negative in alkaline region, resulting in amine group deprotonation (NCC–NH2) and dissociation of sulfate groups on surface [31].

**Figure 15.** Oxidation with sodium periodate and reaction of nanocrystalline cellulose amination.

These cellulosic derivatives containing basic centers were efficient in dye removal. For example, the modified cellulose with ethylene-1,2-diamine, having as intermediate an oxidized cellulose derivative, was efficient in adsorption of the red anionic acid dyes GP, congo red 4 BS, and pale yellow reactive K-4G (Figure 16). The pH affected adsorption of the three dyes on the structure of modified cellulosic biomaterial with ethylene-1,2-diamine. The isoelectric point (pHpzc) of this material is about 8, which indicates its surface at pH < 8 is positively charged, whereas at pH > 8 is negatively charged. Hence, in acid conditions the interaction occurs between the protonated amine group of the cellulosic material and the active anionic site of dye, which favors the adsorption. Whereas in alkaline regions, the surface charge of biomaterial becomes negative, and, consequently, it narrows interaction with dye anionic active site, due to electrostatic repulsion [31].

### **2.5. Sulfonation**

bidentate chelating agent. The scheme of metal complexation is shown in Figure 14 (a), where the available nitrogen centers are coordinated to divalent cations, with charge being counterbalanced for counter-ions. The modified cellulose with ethylene-1,2-diamine also proved to be efficient in divalent cations removal quoted above. In this case, the adsorp‐ tion order was: Co2+ > Cu2+ > Zn2+ > Ni2+. Its adsorption mechanism is similar to that quoted above, where there is transfer of cations from solutions to available basic centers in ethylene-1,2-diamine anchored in the cellulose, by cations complexation through available

n

O O

H

n

+

). By the pH

H H H OH

H OH

NH

H H

N

<sup>O</sup> <sup>O</sup> H H H OH

N

H

O O

<sup>N</sup> <sup>M</sup>2+

H

H H H OH

H OH

(a)

NH

<sup>n</sup> <sup>n</sup>

**Figure 14.** (a) Proposed scheme of complexation for divalent cations with a modified cellulose with 2-aminomethylpyridine. (b) Proposed scheme of complexation for

In lower pH, the zeta potential was positive due to amine protonation (NCC–NH3

divalent cations with the modified cellulose with ethylene-1,2-diamine.

H H

H OH

(b)

**Figure 14.** (a) Proposed scheme of complexation for divalent cations with a modified cellulose with 2-aminomethylpyr‐ idine. (b) Proposed scheme of complexation for divalent cations with the modified cellulose with ethylene-1,2-diamine.

Another amination example is the introduction of ethylene-1,2-diamine in the cellulose structure through a rusty cellulosic intermediate. The first stage consists in nanocrystalline cellulose synthesis (NCC) through hydrolysis with sulfuric acid. In the second stage, the carbon C2 and C3 of the cellulose is oxidised with sodium periodate (NaIO4), in the absence of light at 313 K, forming nanocrystalline dialdehyde from cellulose. After this, ethylene-1,2-diamine is added, the mixture is continuously stirred for 6 h at 303 K. Finally, with imine intermediate reduction through addition of NaBH4 at room temperature, as shown in Figure 15 [31], the zeta potential of the final product of this reaction is formed, which is highly influenced by pH.

CH2

<sup>M</sup>2+

N

H

amine groups, as shown in Figure 14 (b) [28].

104 Cellulose - Fundamental Aspects and Current Trends

O O

<sup>M</sup>2+

H

H H H OH

H OH

N

H

N

H H O3 - N

O3 - N

> The cellulose functionalization with sulfur incorporation is more used in contaminants removal of the aqueous medium. Several reactions are studied and their products are applied in contaminants adsorption. Among reactions that have been studied is the cellulose oxidation forming a dialdehyde, reacting with sodium bisulfite and creating a sulfonated cellulosic material [32]. In this reaction, initially the cellulosic material oxidation occurs with sodium periodate (NaIO4), as shown in Figure 15. Then, the cellulosic material 2,3-dialdehyde formed is dispersed in water and it is treated with sodium bisulfite (NaHSO3), creating a sulfonated cellulosic material, according to Figure 17. Afterward, sulfonation and oxidation of the surface morphology of cellulosic material are modified. In this case, the cellulose nanospheres are

(c)

**Figure 16.** Chemical structure of the red acid dyes GR (a), congo red 4BS (b), and pale **Figure 16.** Chemical structure of the red acid dyes GR (a), congo red 4BS (b), and pale yellow reactive K-4G (c).

gradually deformed, losing their spherical forms with oxidation/sulfonation growth. Howev‐ er, these changes in the nanosphere molds were stretched after sulfonation, with formation of rods aggregates. In addition, the presence of sulfonic group has provided an increasing value of water retention in the cellulosic material structure [32].

**Figure 17.** Sulfonation of cellulosic material.

yellow reactive K-4G (c).

The sulfonated cellulosic derivative (obtained from wheat straw) modified similar to the latest reaction proved efficient in heavy metal lead removal (Pb2+). The sulfonic group introduction in the cellulosic biomaterial structure provokes the growth of electron density of the bioma‐ terial, increasing its affinity for metal ions. In lower levels of pH, the sulfonic groups are protonated, which results in bad ions adsorption Pb2+. Since the increase in pH provides adsorption growth, because sulfonic groups are deprotonated, and consequently in its ionic form, the soft acids form higher covalent complex than ionic with binders containing sulfur. Thus, in the beginning of adsorption, ions can bond with two binders to form complexes, soon, in lower concentration, the ions (all Pb2+) can interact with two linkage locals, having a higher adsorption. Another possible mechanism is ionic exchange with adjacent carboxyl group, but as happens with other materials, there is possibility both mechanisms can participate together during the adsorption, since there are several different active groups on adsorbent surface [33].

Another example of sulfur molecule incorporation is chemical modification of cellulosic biomaterials, derivative from mesocarp and epicarp from babaçu coconut, with ethylenesul‐ fide. In this reaction, the cellulosic biomaterials are put in contact with ethylenesulfide, for 3 h at 323 K, as shown in Figure 18 (a). The amount of incorporated sulfur in these materials was lower than the amount of incorporated sulfur in chitosan, which occurs due to amino reactive center free of chitosan, being higher than hydroxyl groups of carbon 6, of these cellulosic biomaterials [34,35]. Another methodology used to incorporate ethylenesulfide in cellulose chain is by reaction with available amine group, which is from the reaction ethylene-1,2 diamine (Figure 12). Since this amine group has been introduced by reaction with chlorine cellulose (Figure 11) for increasing the cellulose surface reactivity, in this reaction, after amination and chlorination, the cellulose containing amine groups are placed in contact with ethylenesulfide for 4 h at 328 K, according to Figure 18 (b) [19].

The cellulosic biomaterials derivative from mesocarp and epicarp from babaçu coconut proved efficient in adsorption of copper divalent cation (Cu2+). This adsorption process was influenced by pH, where the increase in pH promoted the increasing ions adsorption, with maximum adsorption at pH 6 for two biomaterials. This occurs because the surface of bio-adsorbents changes its polarization with the value of pH of solution and with pH(pzc) of solid. The pH(pzc) is the parameter that indicates the pH value in which a solid exhibit electrically neutral surface so the number of positive charges equals the number of negative charges. The pH(pzc) of two modified cellulosic biomaterials is 5.6, then in pH lower of this value the biomaterial surface is positively charged, which disfavors the adsorption of cation ions Cu2+. Whereas in pH values above pH(pzc), the biomaterials surface is negatively charged, and consequently fostering cation adsorption occurs [34].

gradually deformed, losing their spherical forms with oxidation/sulfonation growth. Howev‐ er, these changes in the nanosphere molds were stretched after sulfonation, with formation of rods aggregates. In addition, the presence of sulfonic group has provided an increasing value

**Figure 16.** Chemical structure of the red acid dyes GR (a), congo red 4BS (b), and pale

**Figure 16.** Chemical structure of the red acid dyes GR (a), congo red 4BS (b), and pale yellow reactive K-4G (c).

(c)

CH3

NH2 N

N

NH

N Cl

S O-Na<sup>+</sup> <sup>O</sup>

> S O Na <sup>O</sup> <sup>+</sup> O-

O

106 Cellulose - Fundamental Aspects and Current Trends

N N

(a)

(b)

N

N

Cl O S O O-Na<sup>+</sup>

NH

N N H2N SO3 - Na<sup>+</sup>

> S O O O-Na<sup>+</sup>

S O <sup>O</sup> <sup>O</sup>-Na<sup>+</sup>

N N

N N HO CH3

Cl

NaHSO3 Sulfonation

O n

O OH

OH OH

H

O

H

SO3 - O3 - S

of water retention in the cellulosic material structure [32].

O

n

H

OH

H

yellow reactive K-4G (c).

**Figure 17.** Sulfonation of cellulosic material.

O O

The modified cellulose with ethylenesulfide, according to Figure 18 (b), also proved to be efficient in removal of several divalent cations (Pb2+, Cd2+, Ni2+, Co2+, Cu2+, and Zn2+). This modified biomaterial presents higher capacity in extracting cations from aqueous solution, due to presence of nitrogen and sulfur acids, which are highly reactive adsorption sites which can coordinate with metal cations. The cations adsorption order was: Pb2+ > Cd2+ > Ni2+ > Co2+ > Cu2+ > Zn2+. The copper is less preferred by modified cellulose than nickel and cobalt, because they have favorable complexation equilibrium constants [29].

Another reaction of sulfur incorporation in cellulose structure is its reaction with aminoetha‐ nethiol, using the cellulose chlorination with intermediate reaction. At first, chlorine cellulose

**Figure 18.** (a) Cellulosic biomaterials reaction with ethylenesulfide. (b) Cellulose reaction, containing amine groups with ethylenesulfide. **Figure 18.** (a) Cellulosic biomaterials reaction with ethylenesulfide. (b) Cellulose reaction, containing amine groups with ethylenesulfide.

is made, as shown in Figure 11. Then, the chlorine cellulose is placed in contact with aminoe‐ thanethiol at 363 K, after the triethylamine is used for removing excess of HCl, according to Figure 19. The molecule introduction of aminoethanethiol in the cellulose structure provokes changes in inter- and intramolecular interactions in biomaterial; these interactions are respon‐ sible by bigger part of biomaterial organization, by biomaterial crystalline arrangement, and, consequently, if some disturbance occurs due to molecule introduction in biomaterial, it becomes amorphous, such as with molecule introduction of aminoethanethiol [3].

**Figure 19.** Reaction of incorporation of aminoethanethiol group in the chlorinated cellulose structure.

is made, as shown in Figure 11. Then, the chlorine cellulose is placed in contact with aminoe‐ thanethiol at 363 K, after the triethylamine is used for removing excess of HCl, according to Figure 19. The molecule introduction of aminoethanethiol in the cellulose structure provokes changes in inter- and intramolecular interactions in biomaterial; these interactions are respon‐ sible by bigger part of biomaterial organization, by biomaterial crystalline arrangement, and, consequently, if some disturbance occurs due to molecule introduction in biomaterial, it

**Figure 18.** (a) Cellulosic biomaterials reaction with ethylenesulfide. (b) Cellulose reaction, containing amine groups

(b)

**Figure 18.** (a) Cellulosic biomaterials reaction with ethylenesulfide. (b) Cellulose

363 K, 4 h triethylamine

<sup>2</sup> + HCl

<sup>O</sup> <sup>O</sup>

O O

OCH2CH2SH

H

n

H H H OH

H OH

H H H OH

NH

S

HN

HS

H

n

H OH

328 K, 4 h <sup>O</sup> <sup>O</sup>

NH

H

n

H

S

SH

H H OH

H OH

becomes amorphous, such as with molecule introduction of aminoethanethiol [3].

**Figure 19.** Reaction of incorporation of aminoethanethiol group in the chlorinated cellulose structure.

<sup>O</sup> <sup>O</sup>

H

n

<sup>O</sup> <sup>O</sup>

H

n

+

<sup>O</sup> <sup>O</sup> H H H OH

H

n

reaction, containing amine groups with ethylenesulfide.

+

S

H OH

NH

H2N

S

323 K, 3 h

(a)

H H H OH

OH

108 Cellulose - Fundamental Aspects and Current Trends

H OH

+ SH 2NH

H

Cl

H H OH

with ethylenesulfide.

H OH

**Figure 20.** Molecular structure of reactive red anionic dye RB (a) and scheme of modified cellulose interaction with anionic dye in acid medium (b) and basic medium **Figure 20.** Molecular structure of reactive red anionic dye RB (a) and scheme of modified cellulose interaction with anionic dye in acid medium (b) and basic medium (c).

(c). This modified biomaterial with aminoethanethiol group proved efficient in removal of reactive red anionic dye (Figure 20 (a)). In this survey, it is noted that the adsorption of dye is maximal at pH 8 and 9, which indicate two predominant mechanisms in adsorption: one in acid medium and other in basic medium. In acid medium, the adsorption is favored by electron interaction. The pH(pzc) of modified material is nearest to 6, then, below this pH, the biomaterial surface is positively charged, due to protonation of nitrogen and sulfur acids. These biomaterial positive sites interact with negative ions in the dye favoring the adsorption (Figure 20 (b)). Since in basic medium there will be no protonation on biomaterial surface, adsorption is favored by hydrogen interaction and/or covalent interactions. The first stage of the mechanism involves the group transformation of β-sulfatoethylsulfonic of dye SO2CH2CH2OSO3Na, in a vinylsul‐ fonic group, -SO2CH=CH2. In alkaline conditions, the group divides forming sulfate waste. Then, the vinylsulfonic group interacts with the modified cellulose surface through a covalent bond. In addition, it can have formation of hydrogen bond between sulfur/nitrogen atoms of modified cellulose and the hydrogen sulfonic dye group (Figure 20 (c)) [3].

### **3. Conclusion**

The chemical modification (incorporation of carboxymethyl, phosphorus, carboxyl, amines, and sulfur) of cellulosic materials is very important for the application in the removal of contaminants from aqueous medium. After the chemical modification, cellulosic materials exhibit new chemical properties that are more advantageous than the starting materials. These new chemical properties allow an increase in interaction between the modified cellulosic materials and the contaminants (metals, dyes, and drugs) during adsorption processes in aqueous medium and these interactions are strongly influenced by solution pH. Therefore, the cellulosic materials modified with various chemical groups are promising for application in removing contaminants from aqueous medium of the environment.

### **Acknowledgements**

The authors thank the Council for Scientific and Technological Development (CNPq), Foun‐ dation for Supporting Research of Piauí (FAPEPI), and Coordination Support in Higher Education (CAPES) for financial support. To the Federal University of Piauí (UFPI) and Federal Institute of Piauí (IFPI), to provide work research conditions.

### **Author details**

Roosevelt D.S. Bezerra1\*, Paulo R.S. Teixeira1 , Ana S.N.M. Teixeira1 , Carla Eiras2 , Josy A. Osajima<sup>2</sup> and Edson C. Silva Filho2

\*Address all correspondence to: rooseveltdsb@ifpi.edu.br

1 Federal Institute of Education, Science and Technology of Piaui \_ Campus Teresina Central, Teresina, Brazil

2 Federal University of Piaui \_ LIMAV, Teresina, Brazil

### **References**

positively charged, due to protonation of nitrogen and sulfur acids. These biomaterial positive sites interact with negative ions in the dye favoring the adsorption (Figure 20 (b)). Since in basic medium there will be no protonation on biomaterial surface, adsorption is favored by hydrogen interaction and/or covalent interactions. The first stage of the mechanism involves the group transformation of β-sulfatoethylsulfonic of dye SO2CH2CH2OSO3Na, in a vinylsul‐ fonic group, -SO2CH=CH2. In alkaline conditions, the group divides forming sulfate waste. Then, the vinylsulfonic group interacts with the modified cellulose surface through a covalent bond. In addition, it can have formation of hydrogen bond between sulfur/nitrogen atoms of

The chemical modification (incorporation of carboxymethyl, phosphorus, carboxyl, amines, and sulfur) of cellulosic materials is very important for the application in the removal of contaminants from aqueous medium. After the chemical modification, cellulosic materials exhibit new chemical properties that are more advantageous than the starting materials. These new chemical properties allow an increase in interaction between the modified cellulosic materials and the contaminants (metals, dyes, and drugs) during adsorption processes in aqueous medium and these interactions are strongly influenced by solution pH. Therefore, the cellulosic materials modified with various chemical groups are promising for application in

The authors thank the Council for Scientific and Technological Development (CNPq), Foun‐ dation for Supporting Research of Piauí (FAPEPI), and Coordination Support in Higher Education (CAPES) for financial support. To the Federal University of Piauí (UFPI) and Federal

1 Federal Institute of Education, Science and Technology of Piaui \_ Campus Teresina Central,

, Ana S.N.M. Teixeira1

, Carla Eiras2

,

modified cellulose and the hydrogen sulfonic dye group (Figure 20 (c)) [3].

removing contaminants from aqueous medium of the environment.

Institute of Piauí (IFPI), to provide work research conditions.

and Edson C. Silva Filho2

\*Address all correspondence to: rooseveltdsb@ifpi.edu.br

2 Federal University of Piaui \_ LIMAV, Teresina, Brazil

Roosevelt D.S. Bezerra1\*, Paulo R.S. Teixeira1

**3. Conclusion**

110 Cellulose - Fundamental Aspects and Current Trends

**Acknowledgements**

**Author details**

Josy A. Osajima<sup>2</sup>

Teresina, Brazil


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### **Use of Cellulosic Materials as Dye Adsorbents — A Prospective Study**

Fabrícia C. Silva, Luciano C.B. Lima, Roosevelt D.S. Bezerra, Josy A. Osajima and Edson C. Silva Filho

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/61343

### **Abstract**

Cellulose is the most abundant biopolymer of nature, and it is widely used in the syn‐ thesis of new materials as well as in the adsorption of dye. This study reports a litera‐ ture review (articles) and technology review (patents) about publications and product invention, which contain information on the use of cellulose on the adsorption of dyes in the period 2004–2014. For this work, research database and keywords were used to find articles and patents related to the subject under review. Specific words were used to find articles and patents related to the subject under review. After a demanding re‐ search, 1 patent and 23 articles that contain the words "cellulose," "dye," and "ad‐ sorption or sorption" in their titles were assessed, and annual evolution studies were performed for publications and countries that publish more.

**Keywords:** Cellulose, adsorption, dye, review, prospection

### **1. Introduction**

One of the negative consequences of human development is the large amount of waste discharged into water bodies, with varied compositions that may contain heavy metals, dyes, and/or other undesirable chemical compounds [1, 2].

The problem becomes more serious when it comes to textile industrial wastewater, as they are colorful, produced on a large scale, and do not always receive due treatment before being discharged into the environment, carrying various impurities [2, 3].

The aquatic environment coming into contact with these effluents has modified physical and chemical properties due to their color because such dyes become visible at concentrations up

to 1 mg L-1, resulting in serious aesthetic problems until their toxicity, leading to hazards to human and animal health [2].

Contamination by dyes prevents the development of fish farming and recreation as well as influence the ecological equilibrium affecting the proliferation of aquatic plants by reducing photosynthetic activity caused by the difficulty passage of sunlight. Thus, the removal of these dyes becomes important environmentally [4, 5].

The removal of these dyes are not always easy because of its complex molecular structures that make them stable to light and heat and resistant to biodegradation [6].

Available wastewater treatment processes mainly involve ion exchange [7], chemical precip‐ itation [8], chemical degradation [9], reverse osmosis [10], oxidation/chemical reduction [11], and adsorption [12].

Adsorption particularly stands out because of its high efficiency combined with low cost. It removes chemical pollutants, returning the natural transparency of the medium, and often enables reusing the reuse of the adsorbent. It is a process that is commonly reversible, where the regeneration of the adsorbent becomes possible, further cheapening the practice of this process, beside the use of natural waste in the trash [12, 13].

The adsorption using low-cost adsorbents is recognized as an effective and economical method for the decontamination of water. Because of this, many studies have been conducted em‐ ploying the adsorption process using these adsorbents; among them, one can mention corn cob [14], sugarcane bagasse [15], banana peel [16], sawdust [17], wheat straw, [18], orange peel [19], water hyacinth roots [20], peanut hull [21], eucalyptus leaf [22], rice husk [23], corn kernel [24], coconut husk [25], apple pomace [26], and other cellulosic waste used in natural or modified surface [27].

Cellulose is a biopolymer considered as an almost inexhaustible source of raw materials, and it can be cited, as an example, as a promising natural material that has been extensively explored by researchers in the adsorption area. It is even more attractive when it makes modifications in its structure in order to improve their existing properties or adding new potentialities to this material [12, 28–30].

Modifying the surface of the cellulose can varies some of its properties, such as its hydrophilic or hydrophobic character, elasticity, resistance to microbiological attacks, and thermal and mechanical resistance, may also increase its pollutant adsorption capacity in aqueous and/or nonaqueous solutions [31].

The elucidation of the polymeric structure of cellulose was made by the pioneering work of Hermann Staudinger [32]. Through acetylation and deacetylation, he recognized that the structures did not consist merely of an aggregate of D-glucose units. Rather, glycosidic units were discovered by being linked covalently to one other to form long molecular chains.

Figure 1 shows the molecular structure of cellulose as a polymeric carbohydrate generated by β-D-glucopyranose repeating, which is covalently linked by acetal functions between the equatorial OH group of 4-carbon atom (C4) and the 1-carbon atom (C1). Hence, this resulted in β-1,4-glucan denomination, which is in principle the way in which the cellulose is bioge‐ netically formed. We have the cellulose as a straight-chain extended polymer with a large number of hydroxyl groups: three per anhydroglucose unit (AGU), which is the central unit, see Figure 1, a thermodynamically preferred conformation 4C1 bond between the 4-carbon and the 1-carbon [33].

to 1 mg L-1, resulting in serious aesthetic problems until their toxicity, leading to hazards to

Contamination by dyes prevents the development of fish farming and recreation as well as influence the ecological equilibrium affecting the proliferation of aquatic plants by reducing photosynthetic activity caused by the difficulty passage of sunlight. Thus, the removal of these

The removal of these dyes are not always easy because of its complex molecular structures

Available wastewater treatment processes mainly involve ion exchange [7], chemical precip‐ itation [8], chemical degradation [9], reverse osmosis [10], oxidation/chemical reduction [11],

Adsorption particularly stands out because of its high efficiency combined with low cost. It removes chemical pollutants, returning the natural transparency of the medium, and often enables reusing the reuse of the adsorbent. It is a process that is commonly reversible, where the regeneration of the adsorbent becomes possible, further cheapening the practice of this

The adsorption using low-cost adsorbents is recognized as an effective and economical method for the decontamination of water. Because of this, many studies have been conducted em‐ ploying the adsorption process using these adsorbents; among them, one can mention corn cob [14], sugarcane bagasse [15], banana peel [16], sawdust [17], wheat straw, [18], orange peel [19], water hyacinth roots [20], peanut hull [21], eucalyptus leaf [22], rice husk [23], corn kernel [24], coconut husk [25], apple pomace [26], and other cellulosic waste used in natural or

Cellulose is a biopolymer considered as an almost inexhaustible source of raw materials, and it can be cited, as an example, as a promising natural material that has been extensively explored by researchers in the adsorption area. It is even more attractive when it makes modifications in its structure in order to improve their existing properties or adding new

Modifying the surface of the cellulose can varies some of its properties, such as its hydrophilic or hydrophobic character, elasticity, resistance to microbiological attacks, and thermal and mechanical resistance, may also increase its pollutant adsorption capacity in aqueous and/or

The elucidation of the polymeric structure of cellulose was made by the pioneering work of Hermann Staudinger [32]. Through acetylation and deacetylation, he recognized that the structures did not consist merely of an aggregate of D-glucose units. Rather, glycosidic units were discovered by being linked covalently to one other to form long molecular chains.

Figure 1 shows the molecular structure of cellulose as a polymeric carbohydrate generated by β-D-glucopyranose repeating, which is covalently linked by acetal functions between the equatorial OH group of 4-carbon atom (C4) and the 1-carbon atom (C1). Hence, this resulted

that make them stable to light and heat and resistant to biodegradation [6].

human and animal health [2].

116 Cellulose - Fundamental Aspects and Current Trends

and adsorption [12].

modified surface [27].

nonaqueous solutions [31].

potentialities to this material [12, 28–30].

dyes becomes important environmentally [4, 5].

process, beside the use of natural waste in the trash [12, 13].

**Figure 1.** Cellulose molecular structure showing the numbering of the carbon atoms and functional groups per mono‐ mer of the polymer.

This chapter aims to present a literature and technology review about the use of cellulosic materials in the adsorption of dyes, published and/or deposited during the period 2004 to 2014.

The prospection performed in this work was based on an extensive electronic search in patent databases and articles databases.

For prospecting patent, a search in the free databases available, that is, the European Patent Office (EPO), the National Institute of Industrial Property (INPI), and the United States Patent and Trademark Office (USPTO), was carried out. The keywords *cellulose*, *cellulose and dye*, *cellulose and adsorption*, *cellulose adsorption*, *cellulose and adsorption and dye*, and *cellulose and sorption and dye* were used to specify the search in the EPO and USPTO bases, and the related keywords in Portuguese were used for INPI, including "title."

For the prospection of the articles, a survey was conducted to determine the most published database (Web of Science, Science Direct, Scopus, and Scielo) during the period 2004 to 2014. The same keywords used in the search of patents were used, including "title" and exclusively regular articles publications. Review articles, books, and book chapters were left out of the research. The research was conducted in April 2015. The articles found in database with largest amount of publication were subsequently analyzed.

### **2. Current state of the art in use of cellulosic materials as dye adsorbent**

The investigation of patent by databases showed that the EPO was the database that received more product invention associated with the subject studied during the period investigated compared USPTO and INPI, as shown in Table 1.

Table 1 shows that when the more general term "cellulose" was used, the largest number of invention products in all databases was found. As new words are added and crossed, the


**Table 1.** Total patents registered on the databases EPO, USPTO, and INPI during the period 2004 to 2014.

amount of invention products considerably decreased because the patents containing the word cellulose in their titles have many different purposes.

The patent only found in the databases after refinement was the German patent DE102008026403 (A1)—Dyeing cellulose fibres, e.g., cotton fabric, with reactive vinylsulfone dyes, involves adding dye-bath components in a special sequence to optimize the relation between physical adsorption and chemical reactivity, containing in its title the words "cellu‐ lose", "adsorption," and "dye." This patent brings a dyeing method of cellulose fibers with reactive dyes, intending to optimize the relationship between physical adsorption and chemical reactivity in order to improve dye fixation on cellulose fiber [34].

In the prospection of articles, it was observed that the amount of published articles was higher compared to the number of patents, as shown in Table 2.


**Table 2.** Number of articles published on Web of Science, Science Direct, Scopus, and Scielo databases from 2004 to 2014.

Table 2 shows that the database with the largest number of articles published in the area is Scopus, which showed higher publication when compared with other databases analyzed.

Observing the results obtained in this research using the database Scielo, articles related to the theme in the time period analyzed for their journals are not included. However, 8 articles related to the use of cellulosic materials for adsorption studies have been found. These articles were aimed at removing other species that were not dye. This fact was confirmed when the words "cellulose" and "dye" were used, which were not found articles.

Thus, the research regarding the adsorption of the dye using cellulosic materials as adsorbents was evaluated by analyzing the 23 articles found in this database containing the words "cellulose, adsorption, and dye" and "cellulose, sorption, and dye" in their titles in the last 10 year period from 2004 to 2014.

Figure 2 shows the study of the annual evolution of publications.

**Figure 2.** Annual evolution of articles published.

amount of invention products considerably decreased because the patents containing the word

**Table 1.** Total patents registered on the databases EPO, USPTO, and INPI during the period 2004 to 2014.

**Keywords EPO USPTO INPI** *cellulose* 13,355 823 1 *cellulose* and *dye* 46 5 0 *cellulose* and *adsorption* 19 0 0 *cellulose and sorption* 1 0 0 *cellulose* and *adsorption* and *dye* 1 0 0 *cellulose and sorption and dye* 0 0 0

TOTAL 13,422 828 1

The patent only found in the databases after refinement was the German patent DE102008026403 (A1)—Dyeing cellulose fibres, e.g., cotton fabric, with reactive vinylsulfone dyes, involves adding dye-bath components in a special sequence to optimize the relation between physical adsorption and chemical reactivity, containing in its title the words "cellu‐ lose", "adsorption," and "dye." This patent brings a dyeing method of cellulose fibers with reactive dyes, intending to optimize the relationship between physical adsorption and

In the prospection of articles, it was observed that the amount of published articles was higher

**Articles published between 2004 - 2014**

**Cellulose and adsorption**

*Web of Science* 18,428 165 330 97 15 5 *Science Direct* 4,126 39 120 25 6 3 *Scopus* 13,985 142 356 99 19 4 *Scielo* 154 0 7 1 0 0

**Table 2.** Number of articles published on Web of Science, Science Direct, Scopus, and Scielo databases from 2004 to

Table 2 shows that the database with the largest number of articles published in the area is Scopus, which showed higher publication when compared with other databases analyzed. Observing the results obtained in this research using the database Scielo, articles related to the theme in the time period analyzed for their journals are not included. However, 8 articles related to the use of cellulosic materials for adsorption studies have been found. These articles

**Cellulose and sorption**

**Cellulose and adsorption and dye**

**Cellulose and sorption and dye**

chemical reactivity in order to improve dye fixation on cellulose fiber [34].

**Cellulose and dye**

cellulose in their titles have many different purposes.

118 Cellulose - Fundamental Aspects and Current Trends

compared to the number of patents, as shown in Table 2.

**Cellulose**

**Keywords Databases**

2014.

This result proves that the adsorption of dyes using cellulosic material is a subject that is being currently studied, and the year 2013 showed the highest amount of publication about this topic, presenting a number of 7 from the 23 reviews.

Among the countries that published, China was ranked first, followed by the United Kingdom, India, and Iran, showing that the world's major economies have shown interest in this study (Figure 3).

**Figure 3.** Countries that more published articles.

Adsorption studies often have the aim to evaluate the maximum removal capacity for a given compound (in this case dye) by adsorbent material proposed (in this case, the cellulosic materials). Moreover, there is a wide range of possibility in the variation of experimental parameters and conditions that directly influence in such property, or there are researchers who are concerned with elucidating the various existing mechanisms in this process, making use of theoretical models established or even proposing new theoretical models their investi‐ gations, often making use of computational tools.

Thus, it is observed that the publications about this theme can be divided into two major groups: the experimental conditions and parameters for the maximum removal capacity for a given dye and the theoretical studies to elucidate this process, explaining experimental data existing or proposed theoretical models that can be used in the elucidation of this process in general.

Analyzing the articles relating to this theme found in the Scopus database, note that of the 23 articles found (of which 19 articles were found with the keywords "cellulose," "adsorption," and "dye" and 4 items with words keys "cellulose," "sorption," and "dye"), 7 works bring theoretical studies to understand the adsorption process of given dyes on cellulosic materials or do not make use of parameter variation for study of maximum adsorption capacity for the material proposed, thus using different methodologies to other articles.

One of these studies is about reference [35], which brings the validation of proposed new nonlinear mathematical model of adsorption isotherm using a correlation between the classical data dye adsorption CI Direct Blue 1 in bleached cotton and the data obtained by the nonlinear model proposed, showing that the new model is presented with excellent results setting and brings the possibility of calculation of new parameters on the adsorption process.

The articles in references [36,37] made use of computational methods exploring the quantum mechanical method DFT/BPW91/6-31+G(d) using Gaussian 03 software and isotherm theo‐ retical models for a better understanding of the interaction between adsorbent and adsorbate. These studies evaluated the effect of alkaline treatment using NaOH solutions of different concentrations of cellulose II fibers in the adsorption capacity for the Reactive Red 120 dye in the work of reference [36] and the dye CI Reactive Orange 84 and CI Reactive Red 120 [37].

In references [38,39], the authors published about the adsorption process of xanthene dyes on cellulose granules using a different methodology from other publications evaluated here, which made use of a method by column chromatography. Since the first work of Tabara et al. [38], they showed that the maximum adsorption capacity for the dyes erythrosine, rose bengal, and phloxine were 3.75, 3.42, and 4.74 mg g-1, respectively. In their second study [39], they were concerned to elucidate the adsorption mechanism of such systems.

In article of reference [40], the authors have elucidated the structural factors that control the adsorption of acid dyes at the surface of cellulose; for this, they evaluated the adsorption of 15 different acid dyes on cellulose matrix, correlating with their structures and geometric values as the thermodynamic enthalpy and entropy of binding obtained in each process.

A theoretical and experimental study of the adsorption of a direct trisazo dye in mercerized cotton fiber, which use theoretical model by linear regression and nonlinear for the elucidation of such a process, was investigated in reference [41].

The remaining 15 articles have different methodologies, which aim at obtaining the maximum adsorption capacity for one or more dyes cellulose materials by investigating the influence of parameters such as pH, contact time, temperature, and others. Table 3 shows the main parameters obtained by this work, briefly specifying the proposed material as well as some of the values found for them. The main and most used parameters are better discussed, taking into regard the conclusions observed in these publications, in the following topics.

Adsorption studies often have the aim to evaluate the maximum removal capacity for a given compound (in this case dye) by adsorbent material proposed (in this case, the cellulosic materials). Moreover, there is a wide range of possibility in the variation of experimental parameters and conditions that directly influence in such property, or there are researchers who are concerned with elucidating the various existing mechanisms in this process, making use of theoretical models established or even proposing new theoretical models their investi‐

Thus, it is observed that the publications about this theme can be divided into two major groups: the experimental conditions and parameters for the maximum removal capacity for a given dye and the theoretical studies to elucidate this process, explaining experimental data existing or proposed theoretical models that can be used in the elucidation of this process in

Analyzing the articles relating to this theme found in the Scopus database, note that of the 23 articles found (of which 19 articles were found with the keywords "cellulose," "adsorption," and "dye" and 4 items with words keys "cellulose," "sorption," and "dye"), 7 works bring theoretical studies to understand the adsorption process of given dyes on cellulosic materials or do not make use of parameter variation for study of maximum adsorption capacity for the

One of these studies is about reference [35], which brings the validation of proposed new nonlinear mathematical model of adsorption isotherm using a correlation between the classical data dye adsorption CI Direct Blue 1 in bleached cotton and the data obtained by the nonlinear model proposed, showing that the new model is presented with excellent results setting and

The articles in references [36,37] made use of computational methods exploring the quantum mechanical method DFT/BPW91/6-31+G(d) using Gaussian 03 software and isotherm theo‐ retical models for a better understanding of the interaction between adsorbent and adsorbate. These studies evaluated the effect of alkaline treatment using NaOH solutions of different concentrations of cellulose II fibers in the adsorption capacity for the Reactive Red 120 dye in the work of reference [36] and the dye CI Reactive Orange 84 and CI Reactive Red 120 [37]. In references [38,39], the authors published about the adsorption process of xanthene dyes on cellulose granules using a different methodology from other publications evaluated here, which made use of a method by column chromatography. Since the first work of Tabara et al. [38], they showed that the maximum adsorption capacity for the dyes erythrosine, rose bengal, and phloxine were 3.75, 3.42, and 4.74 mg g-1, respectively. In their second study [39], they

In article of reference [40], the authors have elucidated the structural factors that control the adsorption of acid dyes at the surface of cellulose; for this, they evaluated the adsorption of 15 different acid dyes on cellulose matrix, correlating with their structures and geometric values

A theoretical and experimental study of the adsorption of a direct trisazo dye in mercerized cotton fiber, which use theoretical model by linear regression and nonlinear for the elucidation

brings the possibility of calculation of new parameters on the adsorption process.

material proposed, thus using different methodologies to other articles.

were concerned to elucidate the adsorption mechanism of such systems.

of such a process, was investigated in reference [41].

as the thermodynamic enthalpy and entropy of binding obtained in each process.

gations, often making use of computational tools.

120 Cellulose - Fundamental Aspects and Current Trends

general.



**Table 3.** Adsorptive parameters used in articles according to prospection.

### **2.1. Modification of cellulose**

The presence of free hydroxyl on the surfaces of solids enables the application of a variety of reaction methods, which are explored to obtain new materials exhibited, due to the modifica‐ tion of their surfaces, new properties, or intensifying the existing ones, [54] not only in the reaction developing but also in applicability.

With regard specifically to cellulose, which is a renewable material and low cost, the reactivity is due to the hydroxyl groups, which are in an ideal position, with the primary hydroxyl at carbon 6 (C6), much more reactive than secondary hydroxyl (C2 and C3). Although the reactivity of the hydroxyl depends heavily of the reagents and the reaction conditions, the hydroxyl group present at the 3-carbon is even less reactive. Thus, the cellulose hydroxyl groups have the following reactivity order: C6 >> C2 > C3, this order being explained by the influence of the formation of inter- and intramolecular hydrogen bonds, as well as the degree of crystallinity, by favoring the water absorption in amorphous regions in comparison with the crystalline this material [55]. The main changes occurring in cellulose are due to halogen‐ ation, oxidation, etherification, and esterification.

Among the 23 articles found in the Scopus database, 12 present a proposal for the modification of cellulose aimed at intensification the properties of this biopolymer, a proof of importance of the modification of this promising material, to be involved in most of articles published in this area.

An example is the paper in reference [30], in which the authors modified cellulose incorpo‐ rating cationic and anionic groups from the reaction with compounds of the triazine deriva‐ tives. With the proposed modification, the authors were able to increase the adsorption capacity for the reactive blue dye BF-RN, which was 9.00 mg g-1 for a value of 20 mg g-1 in an unmodified cellulose, justifying the proposed modification.

### **2.2. pH**

**Adsorbent Dye pH Time Temperature Removal Capacity Reference**

Methyl orange 40 °C 78.43 mg g-1

7 240 mim 293 K

Blue B-RN 14.40 mg g-1


Methylene blue 0.030 mmol g-1

7 - 65 °C

The presence of free hydroxyl on the surfaces of solids enables the application of a variety of reaction methods, which are explored to obtain new materials exhibited, due to the modifica‐ tion of their surfaces, new properties, or intensifying the existing ones, [54] not only in the

With regard specifically to cellulose, which is a renewable material and low cost, the reactivity is due to the hydroxyl groups, which are in an ideal position, with the primary hydroxyl at carbon 6 (C6), much more reactive than secondary hydroxyl (C2 and C3). Although the reactivity of the hydroxyl depends heavily of the reagents and the reaction conditions, the hydroxyl group present at the 3-carbon is even less reactive. Thus, the cellulose hydroxyl groups have the following reactivity order: C6 >> C2 > C3, this order being explained by the influence of the formation of inter- and intramolecular hydrogen bonds, as well as the degree of crystallinity, by favoring the water absorption in amorphous regions in comparison with the crystalline this material [55]. The main changes occurring in cellulose are due to halogen‐

Among the 23 articles found in the Scopus database, 12 present a proposal for the modification of cellulose aimed at intensification the properties of this biopolymer, a proof of importance of the modification of this promising material, to be involved in most of articles published in

An example is the paper in reference [30], in which the authors modified cellulose incorpo‐ rating cationic and anionic groups from the reaction with compounds of the triazine deriva‐ tives. With the proposed modification, the authors were able to increase the adsorption capacity for the reactive blue dye BF-RN, which was 9.00 mg g-1 for a value of 20 mg g-1 in an

70 °C 100.29 mg g-1

16.61 mg g-1

0.045 mmol g-1

20.00 mg g-1

[52]

[29]

[53]

[30]

adsorbent Basic fuchsine 6.5 210 min 458.76 mg g-1

Unmodified cellulose 9.00 mg g-1

6 35 mim

Malachite green

Yellow B-4RFN

Methyl orange

RN

**Table 3.** Adsorptive parameters used in articles according to prospection.

Berinjal plant root powder (celulose)

Nano-cellulose hybrid

Magnetic cellulose beads

Modified cellulose with

this area.

triazine derivatives Reactive blue BF-

122 Cellulose - Fundamental Aspects and Current Trends

**2.1. Modification of cellulose**

reaction developing but also in applicability.

ation, oxidation, etherification, and esterification.

unmodified cellulose, justifying the proposed modification.

The study of pH shows that the adsorption depends on the initial pH of the solution. The same process increases the maximum adsorption capacity because it distributes ions on the surface of the adsorbent by modifying their surface charge, and the dye molecules can be ionized, causing the interaction more efficient between adsorbent and adsorbate [56]. This study is very important (12 of 23 articles found on the prospection assessed the influence of this parameter in its work) and should always be performed in the adsorption tests because whether the pH will influence the adsorption process depends on the adsorbent and/or dye used.

Table 3 shows the reference [12] that evaluated the influence of pH on the reactive red dye adsorption on the surface of the modified cellulose with aminoethanethiol, and it presents the maximum adsorption capacity at pH 2 and pH 9. The authors explained that the maximum adsorption is favored by electrostatic interactions at pH 2 and by the formation of hydrogen bonds and/or covalent interactions with the adsorbent at pH 9.

Another study that verified the influence of this parameter is reference [49], which also studied the adsorption of reactive red dye. This work was conducted to modify the cellulose with incorporation of quaternary ammonium groups, with the aim of adding positive charges on their surface and thus improve the adsorption capacity.

It was observed that this material showed maximum adsorption capacity at pH 3. Considering that the dye molecules used in their studies have negative charges, the increase for dye removed at lower pH values than 6 is explained by favoring attraction electrostatic between dye and this adsorbent medium, cell-R-N+ (C2H5)3 SO3 − , but at pH values higher than 6, a decrease in the removal of dye different from the work described in reference [12] was noted. Due to the possibility of covalent interactions, they described good results at pH 9.

### **2.3. Contact time**

Twelve of the 23 articles found in the prospection evaluated the influence of this parameter in its work, showing the importance of this parameter in the study of the adsorption process. By studying the influence of the adsorption contact time, it is possible to establish the adsorption equilibrium, which is reached when the adsorption capacity of the material remains constant over time. This study provides further data to establish the removal rate of the solute solution and how the adsorption takes place, making use of the kinetic studies that assess the theoretical model which best fit the experimental data.

The theoretical models for the kinetic study further reported in the literature are the pseudofirst order [57], the pseudo-second order [58], the intraparticle diffusion [59], and the Elovich equation [60].

In most of the articles investigated in the literature, it was observed that experimental data usually adjust better to the pseudo-second order kinetic model because the adsorption process is dependent on both the concentration of the adsorbate with the strength of the adsorbent [48, 52, 12, 51, 61, 62, 63].

An example of this is the work described in reference [29], in which the influence of contact time in the reactive adsorption of yellow dyes 4RFN B-B and reactive blue-NB in its hybrid material of cellulose showed that data fitted to the pseudo-second-order model. The authors of reference [51] also used this model to explain the adsorption of the dye malachite green and basic fuchsin since the experimental data showed a better linear correlation with the parame‐ ters of the pseudo-second-order model when compared to pseudo-first-order model.

The authors of reference [46] applied the mathematical models already mentioned and also used the kinetic model Bangham [64], but with a different objective from the other authors cited, as they have not sought the model with which the experimental data better adjusted but a comparison between linear and nonlinear form for each model. The results set is the best nonlinear models, based on the values of the statistical parameters.

### **2.4. Temperature**

Studies on the temperature dependence of the dye adsorption process allow obtaining the experimental data enthalpy and entropy related to these processes. Among the evaluated articles, 12 related the study of the influence of temperature on the adsorption process.

The authors of reference [40] made use of this variable to understand the factors that control the adsorption process evaluating 15 different acid dyes cellulose adsorption. The authors observed the enthalpy of adsorption controls this process for most of dyes and came to the conclusion that the enthalpy of adsorption is controlled by the interaction of dye with the surface of material, i.e., a high enthalpy of adsorption will be observed the higher the approx‐ imation (permitted by the structure) of the aromatic nucleus of the dye to the material surface.

Given the dependence of the temperature in a range of 30°C to 70°C, the removal of malachite green dye methyl orange using brinjal plant root powder was evaluated in the paper [52]. For the first dye, the authors have observed an increase in the removal capacity by increasing the temperature, while for the second dye, the opposite trend was observed from 40°C. To explain these trends, the authors make use of the type of interaction between dyes and adsorbent.

The removal of the malachite green is explained by the interaction of the type of Lewis acid– base, thus increasing the temperature and the interaction between the charges of the basic sites of the material and the dye acids. However, in the adsorption of the anionic dye methyl orange, there are only interactions of the type of hydrogen bonds that can be broken easily at elevated temperatures according to the authors.

### **2.5. Dosage**

In this study, 3 of the 23 articles found evaluated the dosage of the adsorbent. Despite the small number of studies among evaluated articles, the dosage of the adsorbent is another very important parameter because once the degree of the adsorption of a material under study is known, it is possible to use the optimal dosage of this adsorbent to determine, for example, the cost of the adsorbent per unit volume.

However, increasing the mass of adsorbent increases the amount of adsorption sites available for the adsorbent–adsorbate interactions on a qualitative analysis, resulting in the increased percentage of dye removal solution [65].

The paper of reference [46] confirmed this fact evaluating cellulose hydrogel dosage in the removal of basic fuchsin dye, finding a good dose of 1.0 g L-1; however, when the dose increased to 2.0 g L-1, the removal percentage continued to increase while the *qe* value (mg g-1) decreased.

In several reports, this factor is explained by the availability of sites for adsorption in the adsorbent because it is found to present limited fixed dye concentration and lower dosage, and therefore, each adsorbent achieves its maximum capacity. By contrast, at high dosages, the articles of adsorbent compete to bind the same dye molecules, thus providing a quantitative measure of the amount of adsorbed dye per unit weight of adsorbent decreasing [46, 62, 63].

### **2.6. Desorption**

An example of this is the work described in reference [29], in which the influence of contact time in the reactive adsorption of yellow dyes 4RFN B-B and reactive blue-NB in its hybrid material of cellulose showed that data fitted to the pseudo-second-order model. The authors of reference [51] also used this model to explain the adsorption of the dye malachite green and basic fuchsin since the experimental data showed a better linear correlation with the parame‐

The authors of reference [46] applied the mathematical models already mentioned and also used the kinetic model Bangham [64], but with a different objective from the other authors cited, as they have not sought the model with which the experimental data better adjusted but a comparison between linear and nonlinear form for each model. The results set is the best

Studies on the temperature dependence of the dye adsorption process allow obtaining the experimental data enthalpy and entropy related to these processes. Among the evaluated articles, 12 related the study of the influence of temperature on the adsorption process.

The authors of reference [40] made use of this variable to understand the factors that control the adsorption process evaluating 15 different acid dyes cellulose adsorption. The authors observed the enthalpy of adsorption controls this process for most of dyes and came to the conclusion that the enthalpy of adsorption is controlled by the interaction of dye with the surface of material, i.e., a high enthalpy of adsorption will be observed the higher the approx‐ imation (permitted by the structure) of the aromatic nucleus of the dye to the material surface.

Given the dependence of the temperature in a range of 30°C to 70°C, the removal of malachite green dye methyl orange using brinjal plant root powder was evaluated in the paper [52]. For the first dye, the authors have observed an increase in the removal capacity by increasing the temperature, while for the second dye, the opposite trend was observed from 40°C. To explain these trends, the authors make use of the type of interaction between dyes and adsorbent.

The removal of the malachite green is explained by the interaction of the type of Lewis acid– base, thus increasing the temperature and the interaction between the charges of the basic sites of the material and the dye acids. However, in the adsorption of the anionic dye methyl orange, there are only interactions of the type of hydrogen bonds that can be broken easily at elevated

In this study, 3 of the 23 articles found evaluated the dosage of the adsorbent. Despite the small number of studies among evaluated articles, the dosage of the adsorbent is another very important parameter because once the degree of the adsorption of a material under study is known, it is possible to use the optimal dosage of this adsorbent to determine, for example,

ters of the pseudo-second-order model when compared to pseudo-first-order model.

nonlinear models, based on the values of the statistical parameters.

**2.4. Temperature**

124 Cellulose - Fundamental Aspects and Current Trends

temperatures according to the authors.

the cost of the adsorbent per unit volume.

**2.5. Dosage**

The possibility of recovery and reuse of biosorbent after going through a desorption process is another relevant factor. Five articles presented results in this parameter; two of them are described below.

The authors of reference [51] evaluated the life cycle of a new material obtained by the modification of cellulose with glycidyl methacrylate and diethylenetriaminepentaacetic acid, observing that, in their dynamic tests adsorption/desorption using a saturated solution of sodium bicarbonate as eluant, the adsorption capacity did not change significantly, showing that the material can be reused in at least four cycles, maintaining the adsorption rate above 85% and 90% for green and basic fuchsin dye malachite, respectively.

Another work that performed desorption testing is the paper of reference [53], in which we evaluated the life cycle of magnetic cellulose beads using NaCl solutions (2 mol L-1) and NaOH (0.05 mol L-1) and found that after three cycles of adsorption, the desorption was maintained at 95% for the dyes methylene blue and methyl orange, on this fact found that the beads can be reused several times.

The paper [52] also conducted a study of desorption, however, for the malachite green dye and methyl orange on the surface of brinjal plant root powder, and by just using water at 80°C, a recovery of 95% for methyl orange and 10% for malachite green was obtained.

The adsorption process involves several attractive interaction forces such as van der Waals forces, hydrogen bonding, covalent, and ionic bonds. Depending on the dye used and the sites available for adsorption on the material, one or more forces will act in the adsorption process, which influences the possibility to reuse the material.

### **3. Conclusion**

Through the presented data, it can be stated that the use of cellulosic materials for the adsorp‐ tion of dyes it is a promising area because of the number of publications found.

The data presented in the patent deposit prospecting shows the database with largest number of deposit among databases searched was EPO, with 12,815 patent deposits in studied range. Even though only a single patent occurs from the refined search referring to the aim of this study, it results in methods for improving fixation of the dye on cellulose fiber.

In the search for articles, a larger number of searches with limiting terms compared to prospecting patents were observed, in which China published more in this area. Other important aspects refer to the year in which most studies that have been done in this area, which was observed that the year 2013, had the highest number of publications showing currently of this theme.

The 23 items found in the Scopus database, using the limiting terms chosen, elucidate the adsorption of one or more dyes on the surface of cellulosic materials. By analyzing these articles, it can be proved that the factors influencing this process are pH, temperature, dosage, contact time, kind of material and modification. These parameters are of most importance in adsorption studies as well as in directly influencing the maximum adsorption capacity for a given material. Together they make an important tool in the elucidation of the possible interactions between adsorbent and adsorbate, helping to explain the potentiality of the material under study.

### **Author details**

Fabrícia C. Silva1 , Luciano C.B. Lima1 , Roosevelt D.S. Bezerra2 , Josy A. Osajima1 and Edson C. Silva Filho1\*

\*Address all correspondence to: edsonfilho@ufpi.edu.br

1 LIMAV, Chemistry Department of Piauí Federal University, Teresina, Piauí, Brazil

2 Federal Institute of Education, Science and Technology of Piaui, Campus Teresina Central, Teresina, Brazil

### **References**


[3] Attia AA, Rashwan WE, Khedr SA. Capacity of activated carbon in the removal acid dyes subsequent to its thermal treatment. Dyes Pigmentes. 2006;69:128–136. DOI: 10.1016/j.dyepig.2004.07.009.

The data presented in the patent deposit prospecting shows the database with largest number of deposit among databases searched was EPO, with 12,815 patent deposits in studied range. Even though only a single patent occurs from the refined search referring to the aim of this

In the search for articles, a larger number of searches with limiting terms compared to prospecting patents were observed, in which China published more in this area. Other important aspects refer to the year in which most studies that have been done in this area, which was observed that the year 2013, had the highest number of publications showing

The 23 items found in the Scopus database, using the limiting terms chosen, elucidate the adsorption of one or more dyes on the surface of cellulosic materials. By analyzing these articles, it can be proved that the factors influencing this process are pH, temperature, dosage, contact time, kind of material and modification. These parameters are of most importance in adsorption studies as well as in directly influencing the maximum adsorption capacity for a given material. Together they make an important tool in the elucidation of the possible interactions between adsorbent and adsorbate, helping to explain the potentiality of the

, Roosevelt D.S. Bezerra2

1 LIMAV, Chemistry Department of Piauí Federal University, Teresina, Piauí, Brazil

2 Federal Institute of Education, Science and Technology of Piaui, Campus Teresina Central,

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currently of this theme.

126 Cellulose - Fundamental Aspects and Current Trends

material under study.

**Author details**

Fabrícia C. Silva1

Teresina, Brazil

**References**

Edson C. Silva Filho1\*

, Luciano C.B. Lima1

\*Address all correspondence to: edsonfilho@ufpi.edu.br

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### **Microbial Cellulose — Biosynthesis Mechanisms and Medical Applications**

Wilton R. Lustri, Hélida Gomes de Oliveira Barud, Hernane da Silva Barud, Maristela F. S. Peres, Junkal Gutierrez, Agnieszka Tercjak, Osmir Batista de Oliveira Junior and Sidney José Lima Ribeiro

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/61797

### **Abstract**

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132 Cellulose - Fundamental Aspects and Current Trends

Currently some principles of sustainability, eco-efficiency and green chemistry are guid‐ ing the development of a new generation of materials as an alternative to conventional polymers based on petroleum. Then, in the field of biodegradable polymers one of the most promising investigations is focused on the use of microbial cellulose (MC), biocellu‐ lose or bacterial cellulose. MC has received substantial interest since it is synthesized from the bacterium *Gluconacetobacter genus* from a variety of carbon sources such as glu‐ cose, fructose, galactose, etc. MC is an interesting emerging biomaterial, with no toxicity, and since its discovery has shown tremendous potential in various fields, because the structural aspect of MC is far superior to those of plant cellulose. Thus, the main focus of the chapter review involves detailed aspects about the biosynthesis and recent advances on microbial production, including mechanism for the biochemistry of the cellulose syn‐ thesis, new sources for culture medium, main aspects about static and air-reactor produc‐ tions and genetic modifications. We also revised microbial cellulose devices for biomedical applications: artificial skin, artificial blood vessels and microvessels, wound dressing of second- or third-degree burn ulcers, scaffolds for tissue engineering, drug de‐ livery systems, dental implants, among others.

**Keywords:** Microbial cellulose, cellulose synthesis, medical applications

### **1. Introduction**

Microbial cellulose (MC) presents the same chemical formula as plant cellulose, however with the fibers in nanometer dimensions; confer different properties to MC [1]. The MC is a type of exopolysaccharides composed of glucose monomers bound by glycosidic β (1-4) linkages, with

© 2015 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

the chemical formula (C6H10O5)n, as can be seen in Figure 1. [2,3]. This biopolymer is produced extracellularly into nanofibers by several genera of bacteria, such as *Gluconacetobacter,* (formerly *Acetobacter*), *Agrobacterium*, *Aerobacter*, *Achromobacter*, *Azotobacter*, *Rhizobium*, *Sarcina*, and *Salmonella* [4].

**Figure 1.** Chemical structure of microbial cellulose.

Historical data show that the MC has been used for a long time in the manufacture of a traditional food in the Philippines, known as the coconut cream [5]. Currently, the MC still remains widely utilized as food in various parts of the world however on the other hand aroused great academic and industrial interest due to its unique properties and diverse opportunity of applications. MC produced by *Acetobacter xylinum* was first reported in 1886 by Brown [6], produced in the presence of oxygen, using glucose as a carbon source.

As proposed by Yamada and colleagues (1997) [7] and subsequently validated by International Journal of Systematic Bacteriology, *Acetobacter xylinum* was reclassified and scientifically cataloged as *Gluconacetobacter xylinus*, due to the characteristics phylogeny based on analysis of partial sequences of 16S ribosomal RNA.

This bacterial species belonging to the family Acetobacteriaceae, being morphologically classified as a bacillus Gran-negative, strictly aerobic, no pathogenic which may be found singly arranged, in pairs or in small sets of chain formation of colonies shiny and smooth in Mannitol Agar. Bacteria belonging to this family are able to oxidize fully various carbon sources such as glucose, fructose, galactose, sucrose, mannitol, glycerol, inositol, among others [3,8] and alcohols such as ethanol [7] and is capable of extracellularly producing pulp at temperatures between 25 and 30 °C and pH 3 to 7. The bacterial cellulose may provide arrangements in parallel via hydrogen bonds and to form a tridimensional network. The morphology of the membrane depends directly on the environment and the interface culture medium / air where MC form a thick film, which can be easily manipulated according to the size of the vial used for cultivation [1,8].

the chemical formula (C6H10O5)n, as can be seen in Figure 1. [2,3]. This biopolymer is produced extracellularly into nanofibers by several genera of bacteria, such as *Gluconacetobacter,* (formerly *Acetobacter*), *Agrobacterium*, *Aerobacter*, *Achromobacter*, *Azotobacter*, *Rhizobium*,

Historical data show that the MC has been used for a long time in the manufacture of a traditional food in the Philippines, known as the coconut cream [5]. Currently, the MC still remains widely utilized as food in various parts of the world however on the other hand aroused great academic and industrial interest due to its unique properties and diverse opportunity of applications. MC produced by *Acetobacter xylinum* was first reported in 1886

As proposed by Yamada and colleagues (1997) [7] and subsequently validated by International Journal of Systematic Bacteriology, *Acetobacter xylinum* was reclassified and scientifically cataloged as *Gluconacetobacter xylinus*, due to the characteristics phylogeny based on analysis

This bacterial species belonging to the family Acetobacteriaceae, being morphologically classified as a bacillus Gran-negative, strictly aerobic, no pathogenic which may be found singly arranged, in pairs or in small sets of chain formation of colonies shiny and smooth in

by Brown [6], produced in the presence of oxygen, using glucose as a carbon source.

*Sarcina*, and *Salmonella* [4].

134 Cellulose - Fundamental Aspects and Current Trends

**Figure 1.** Chemical structure of microbial cellulose.

of partial sequences of 16S ribosomal RNA.

Researchers have sought new bacterial strains capable to produce biopolymers with potential industrial application [9]. Although many organisms are capable to produce cellulose, *Gluconacetobacter xylinus* bacteria are the only known species able to produce cellulose on the industrial scale [10].

In addition *G. xylinus*, other micro-organisms are considered able to produce cellulose, among others they are *Escherichia coli*, *Salmonella spp.* [11] and *Pseudomonas spp*. [12,13]. The cellulose synthesis genes (*MCsA, MCsB, MCsZ and MCsC*) of these species were similar to those in *G. xylinus* [14]. Although these species are also capable to produce the bacterial cellulose, the fact that many of them are potentially pathogenic limits the commercial use of these biopolymers [12,13].

In nature, microorganisms which produce cellulose are usually found in symbiosis with other microorganisms. In the fermentation of Kombucha, for example, *Zygosaccharomyces* yeasts are used as microorganisms symbionts [15]. The Kombucha, also known as tea fungi and *Haipao* [16], is produced in China for over 2000 years, with a widely varied yeast population [17,18], whose function is to convert sucrose to organic acids, carbon dioxide and ethanol, the latter being used for the cellulose-producing bacteria for the production of acetaldehyde and acetic acid [19,20].

### **2. Biochemical and molecular mechanisms of bacterial cellulose biosynthesis by** *Gluconacetobacter xylinus*

Although several species of microorganisms are capable to produce cellulose, *G. xylinus* is currently considered a model organism for the study these biopolymers [21]. MC biosynthesis consists of a complex process which involves first the polymerization of glucose residues in β1-4-glucan chain [21,22,23], followed by the extracellular secretion of the chains ending the linear arrangement and crystallization of glucan chains through hydrogen bridges and Van der Waals forces hierarchically arranged in strips [22], resulting in formation of a tough three dimensional structure called microfibrils. MC generated by this bacterial species has special characteristics of unidirectional polarity and variable thickness. The crystallization mechanism of the microfibrils in *G. xylinum* can give rise to two cellulose forms, if the microfibrils is oriented parallel arrangement is synthesized cellulose I, while if the arrangement is antiparallel microfibrils is obtained cellulose II [21]. In *G. xylinus*, MC synthesis depends on the cycle of pentoses and of the Krebs cycle [1,21,22] that perform respectively the oxidation function carbohydrates and oxidation of organic acids. A particularity *G. xylinus* is the inability to metabolize glucose anaerobically due to lack of phosphofructokinase-1, an enzyme responsible for catalyzing the reaction of phosphorylation of fructose-6-phosphate, fructose-1,6-bisphos‐ phate, which prevents glycolysis. Thus, MC synthesis by *G. xylinus* results of a metabolic pool hexose phosphate which is produced directly by phosphorylating exogenous hexose or indirectly by the pentose phosphate pathway and gluconeogenesis. The hexose phosphate conversion of cellulose is direct and does not depend on the intermediate divisions carbon skeleton [1,22]. The conversion of glucose, transported from the external environment into the cytoplasm, is catalyzed by four bacterial enzymes, the glucokinase, which is the enzyme responsible for the phosphorylation of the carbon 6 of glucose, yielding glucose-6-phosphate, the phosphoglucomutase, which catalyzes the reaction isomerization of glucose-6-phosphate to glucose-1-phosphate, the UDPG-pyrophosphorylase (also known as glucose-1-phosphate uridylyltransferase), responsible for synthesis of UDP-glucose (UDPG), and cellulose synthase (CS), responsible for the polymerization of cellulose from UDP-glucose. As previously mentioned, the cellulose synthesis can also occur from endogenous sources, for gluconeogen‐ esis. In *G. xylinus*, the synthesis from endogenous sources begins with oxaloacetate, into pyruvate by action of the enzyme pyruvate carboxylase. The transformation of the pyruvate in fosfoenolpiruvado, is produced by action of the enzyme phosphoenolpyruvate carboxyki‐ nase [1,22,24]. MC synthesis reaction is costly to the cell, consuming about 10% of the ATP generated in bacterial metabolism. Thus, the energy used for the synthesis of CB comes from aerobic metabolism. There are different proposals for the substrate used by the CS. [1,21]. The enzymatic complex of synthesis of cellulose, termed as terminal complex (TC) [25,26], consti‐ tute a kind of membrane protein species that, in *Gluconacetobacter spp*., corresponds to cellulose synthase complex [27].

One proposed hypothesis is that the UDP-glucose binds to lipids of the plasma membrane [1, 21]. Another one considers that the soluble precursor interacts directly with the CS [28]. The CS is a protein complex consisting of three (AxCcSAB, AxCcSC and AxCcSD) or four (AxCcSA, AxCcSB, AxCcSC and AxCcSD) protein subunits encoded by genes exist in an operon chromosomal called *MCs*.

The two conserved Asp residues (D) invariably are found in loops at the Carboxyl-terminal (C-terminal) ends of predicted strands, a position frequently observed for catalytic residues [21, 39]. The hydrophobic clusters in domain B are more difficult to interpret in terms of secondary structure. AxCcSA and AxCcSAB have a motif consisting of domain, a single conserved residues Asp (D-D-D), presumably important for catalysis, identified along with the conserved sequence motif Gln (Q) Arg-Trp (R-W) in glucotransferases [1, 29]. Through a functional analysis of CS, it appears that the A subunit of this complex with 83kDa, shows catalytic activity. The B subunit of 90kDa, increases the rate of cellulose synthesis by joining a positive allosteric regulator, cyclic diguanosine monophosphate (c-di-GMP). The C subunit (138kDa) and D (17kDa) appear to structural activity. It has been hypothesized that C subunit related to pore formation and extrusion of the cellulose D subunit appears related to the process decrystalization since mutant strains of the gene which encodes the D subunit production are still able to produce cellulose II [23].

In *G. xylinus* Bis-(3'-5')-cyclic dimeric guanosine monophosphate (c-di-GMP) has been identified as an activator of cellulose biosynthesis [30], considered a second global messenger in bacteria [31]. The free c-di-GMP in the cell is considered to allosterically activate the cellulose synthase BcsA. However, 90% of the cellular c-di-GMP is reversibly bound by the c-di-GMP binding protein BcsB, a membrane protein that is structurally associated with the cellulose synthase [32,33]. It is believed that the spatial proximity is necessary to direct c-di-GMP released from BcsB towards the cellulose synthase. The equilibrium between bound and free c-di-GMP is modulated by the intracellular potassium concentration [34]. The level of free cdi-GMP is regulated by the opposing action of two enzymes, diguanylate cyclase (DGC) that cycles two molecules of GTP under the release of two molecules of PPi, and phosphodiesterase A (PDEA) that degrades c-di-GMP to the inactive GTP under the release of two molecules of PPi, and phosphodiesterase A (PDEA) that degrades c-di-GMP to the inactive 5'-pGpG. *G. xylinus* has three distinct operons each containing PDEA/DGC pair, which contribute at different levels to the c-di-GMP turnover [35, 36] indicating that cellulose biosynthesis underlies various control mechanisms in *G. xylinus*. Figure 2 represents a model of the metabolic pathway for the biosynthesis of cellulose by *G. xylinus*.

carbohydrates and oxidation of organic acids. A particularity *G. xylinus* is the inability to metabolize glucose anaerobically due to lack of phosphofructokinase-1, an enzyme responsible for catalyzing the reaction of phosphorylation of fructose-6-phosphate, fructose-1,6-bisphos‐ phate, which prevents glycolysis. Thus, MC synthesis by *G. xylinus* results of a metabolic pool hexose phosphate which is produced directly by phosphorylating exogenous hexose or indirectly by the pentose phosphate pathway and gluconeogenesis. The hexose phosphate conversion of cellulose is direct and does not depend on the intermediate divisions carbon skeleton [1,22]. The conversion of glucose, transported from the external environment into the cytoplasm, is catalyzed by four bacterial enzymes, the glucokinase, which is the enzyme responsible for the phosphorylation of the carbon 6 of glucose, yielding glucose-6-phosphate, the phosphoglucomutase, which catalyzes the reaction isomerization of glucose-6-phosphate to glucose-1-phosphate, the UDPG-pyrophosphorylase (also known as glucose-1-phosphate uridylyltransferase), responsible for synthesis of UDP-glucose (UDPG), and cellulose synthase (CS), responsible for the polymerization of cellulose from UDP-glucose. As previously mentioned, the cellulose synthesis can also occur from endogenous sources, for gluconeogen‐ esis. In *G. xylinus*, the synthesis from endogenous sources begins with oxaloacetate, into pyruvate by action of the enzyme pyruvate carboxylase. The transformation of the pyruvate in fosfoenolpiruvado, is produced by action of the enzyme phosphoenolpyruvate carboxyki‐ nase [1,22,24]. MC synthesis reaction is costly to the cell, consuming about 10% of the ATP generated in bacterial metabolism. Thus, the energy used for the synthesis of CB comes from aerobic metabolism. There are different proposals for the substrate used by the CS. [1,21]. The enzymatic complex of synthesis of cellulose, termed as terminal complex (TC) [25,26], consti‐ tute a kind of membrane protein species that, in *Gluconacetobacter spp*., corresponds to cellulose

One proposed hypothesis is that the UDP-glucose binds to lipids of the plasma membrane [1, 21]. Another one considers that the soluble precursor interacts directly with the CS [28]. The CS is a protein complex consisting of three (AxCcSAB, AxCcSC and AxCcSD) or four (AxCcSA, AxCcSB, AxCcSC and AxCcSD) protein subunits encoded by genes exist in an operon

The two conserved Asp residues (D) invariably are found in loops at the Carboxyl-terminal (C-terminal) ends of predicted strands, a position frequently observed for catalytic residues [21, 39]. The hydrophobic clusters in domain B are more difficult to interpret in terms of secondary structure. AxCcSA and AxCcSAB have a motif consisting of domain, a single conserved residues Asp (D-D-D), presumably important for catalysis, identified along with the conserved sequence motif Gln (Q) Arg-Trp (R-W) in glucotransferases [1, 29]. Through a functional analysis of CS, it appears that the A subunit of this complex with 83kDa, shows catalytic activity. The B subunit of 90kDa, increases the rate of cellulose synthesis by joining a positive allosteric regulator, cyclic diguanosine monophosphate (c-di-GMP). The C subunit (138kDa) and D (17kDa) appear to structural activity. It has been hypothesized that C subunit related to pore formation and extrusion of the cellulose D subunit appears related to the process decrystalization since mutant strains of the gene which encodes the D subunit production are

synthase complex [27].

136 Cellulose - Fundamental Aspects and Current Trends

chromosomal called *MCs*.

still able to produce cellulose II [23].

**Figure 2.** Hypothetical model of the pathway for the biosynthesis of cellulose by G. xylinus from exogenous sources glucokinase-ATP (1); Phosphoglucomutase (2), glucose-6-phosphate dehydrogenase (3); Phosphoglucoisomerase (4); Fructokinase ATP (5), Aldolase (6); Triosephosphate isomerase (7); Glyceraldehyde 3-phosphate dehydrogenase (8); Phosphoglycerate mutase (9), enolase (10); Pyruvate kinase (11); Pyruvate biphosphate kinase (12), pyruvate dehydro‐ genase(13); 6-phosphogluconate dehydrogenase (14); Phosphorribulose epimeraase (15); Phosphorribulose isomerase (16); Transaketolase (17); Transaldolase (18); Fructokinase (19); Aldehyde dehydrogenase(20); Alcohol dehydrogen‐ ase(21).

### **3. Cellulose Synthase (CS)**

Two bacterial cellulose synthase operons (*bcs*) [23] were identified in the analyzed genome, but only one (*bcs*I) is structurally complete. This operon is composed by seven genes encoding for enzymes endo-1,4-beta-glucanases, a homolog CMCax is a putative beta-glucosidase endoglucanase, CPC is a putative homologues four subunits CS: BCSA, BBRC, BCSC and BCSD and beta-glucosidase. Although the three endoglucanases have been identified [37] its exact function in MC biosynthesis is not well understood. Genomic Analysis showed that *bcs*D gene is conserved as part of the operon and has exactly the same length in all six strains. Interest‐ ingly, this gene encodes for CS subunit D, whose function is still speculative, although their crystal structure has been recently resolved.

In cells actively producing cellulose, approximately 50 cellulose-synthesizing multienzyme complexes are organized in a single row along the longitudinal axis of the bacterial rod whereby each complex secretes approx. 12 to 25 glucan chains which assemble into larger microfibrils at the site of synthesis. This so-called linear terminal complex can be visualized by electron microscopy using freeze fracture as 35 Å pores in the outer membrane or as pits when the outer leaflet is fractured away [1,33].

In *G. xylinus* specie the cellulose synthase complex (BCS) is a multicomponent protein complex encoded in an operon containing at least three genes, *bcs*A, *bcs*B, and *bcs*C, which encodes a transmembrane complex over the cytoplasmic and outer membrane whereby the cellulose synthase (BcsA) and the c-di-GMP binding protein (BcsB) are considered to be localized in the cytoplasmic membrane as shown in Figure 3. [32,33,39]. Cellulose synthesis and transport across the inner bacterial membrane is mediated by a complex of the membrane-integrated catalytic BcsA subunit (green) and the membrane-anchored, periplasmic BcsB domain (blue) and membrane-associated regions BcsB transmembrane anchor (blue). The glycosyltransferase domain is shown brown [40]. BcsC is predicted to form a β-barrel in the outer membrane, preceded by a large periplasmic domain containing tetratricopeptide repeats likely involved in complex assembly [41,42].

The gene corresponding to cellulose synthase BcsA is constituted by a long string that presents between 723-880 amino acid residues, as represented in Figure 3, being the most conserved gene of the operon *MCs* between species, although the amino-terminus portions (N-terminus) and carboxyl-terminus portions (C-terminus) is not so conserved, since the homology is not restricted to the frequently analyzed D, D, D35Q (R, Q) and RW motif, which spans domains A and B [39].

The BcsB protein [Figure 3], related to indirect interaction with c-di-GMP [33] is less well conserved among the species. However, direct comparisons of the MCsB proteins with CelB from *A. tumefaciens* and *R. leguminosarum* bv. *trifolii* revealed significant homology (∼40% similarity) over the entire length of the proteins with several invariable residues. An alanine/ proline rich domain is located at the N-terminus region of all proteins except *A. aeolicus*. One transmembrane domain located at the C-terminus portions has been predicted by various algorithms for all BcsB proteins [39].

**Figure 3.** Schematic representation of cellulose synthase.

**3. Cellulose Synthase (CS)**

138 Cellulose - Fundamental Aspects and Current Trends

crystal structure has been recently resolved.

when the outer leaflet is fractured away [1,33].

in complex assembly [41,42].

algorithms for all BcsB proteins [39].

A and B [39].

Two bacterial cellulose synthase operons (*bcs*) [23] were identified in the analyzed genome, but only one (*bcs*I) is structurally complete. This operon is composed by seven genes encoding for enzymes endo-1,4-beta-glucanases, a homolog CMCax is a putative beta-glucosidase endoglucanase, CPC is a putative homologues four subunits CS: BCSA, BBRC, BCSC and BCSD and beta-glucosidase. Although the three endoglucanases have been identified [37] its exact function in MC biosynthesis is not well understood. Genomic Analysis showed that *bcs*D gene is conserved as part of the operon and has exactly the same length in all six strains. Interest‐ ingly, this gene encodes for CS subunit D, whose function is still speculative, although their

In cells actively producing cellulose, approximately 50 cellulose-synthesizing multienzyme complexes are organized in a single row along the longitudinal axis of the bacterial rod whereby each complex secretes approx. 12 to 25 glucan chains which assemble into larger microfibrils at the site of synthesis. This so-called linear terminal complex can be visualized by electron microscopy using freeze fracture as 35 Å pores in the outer membrane or as pits

In *G. xylinus* specie the cellulose synthase complex (BCS) is a multicomponent protein complex encoded in an operon containing at least three genes, *bcs*A, *bcs*B, and *bcs*C, which encodes a transmembrane complex over the cytoplasmic and outer membrane whereby the cellulose synthase (BcsA) and the c-di-GMP binding protein (BcsB) are considered to be localized in the cytoplasmic membrane as shown in Figure 3. [32,33,39]. Cellulose synthesis and transport across the inner bacterial membrane is mediated by a complex of the membrane-integrated catalytic BcsA subunit (green) and the membrane-anchored, periplasmic BcsB domain (blue) and membrane-associated regions BcsB transmembrane anchor (blue). The glycosyltransferase domain is shown brown [40]. BcsC is predicted to form a β-barrel in the outer membrane, preceded by a large periplasmic domain containing tetratricopeptide repeats likely involved

The gene corresponding to cellulose synthase BcsA is constituted by a long string that presents between 723-880 amino acid residues, as represented in Figure 3, being the most conserved gene of the operon *MCs* between species, although the amino-terminus portions (N-terminus) and carboxyl-terminus portions (C-terminus) is not so conserved, since the homology is not restricted to the frequently analyzed D, D, D35Q (R, Q) and RW motif, which spans domains

The BcsB protein [Figure 3], related to indirect interaction with c-di-GMP [33] is less well conserved among the species. However, direct comparisons of the MCsB proteins with CelB from *A. tumefaciens* and *R. leguminosarum* bv. *trifolii* revealed significant homology (∼40% similarity) over the entire length of the proteins with several invariable residues. An alanine/ proline rich domain is located at the N-terminus region of all proteins except *A. aeolicus*. One transmembrane domain located at the C-terminus portions has been predicted by various

The proposed model considers that after the transfer and addition of glycosyl terminal residue, the glucose molecule rotates around acetyl glucan binding to align the channel as shown Figure 4. It is believed that allosteric interactions guide the direction of rotation, causing rotation feature 180⁰ connecting β-1,4 glucan-glucan between the individual glucose units and intramolecular hydrogen bond between oxygen atoms of the hydroxyl groups of the neigh‐ boring unit [40,43]. This phenomenon may be sufficient to allow the polymer to move into the channel [Figure 4]. Alternatively, for the translocation of the elongated glucan occurs, replace‐ ment is required to UDP-glucose by UDP. Past the channel, induced glucan chain in the BcsA - twist BcssB interface, interaction with BcsB of CBDs (periplasmic carbohydrate binding domains), or aggregation with other glucans may additionally contribute to a unidirectional motion of the polymer [40].

**Figure 4.** Proposed model for cellulose synthesis and translocation (adapted). After glycosyl transfer, the newly added Glc could rotate around the acetyl linkage into the plane of the polymer. The rotation direction would be determined by steric interactions and formation of the β-1,4 glucan characteristic intramolecular O3- H••O5 hydrogen bond. The glucan might translocate into the channel during this relaxation. This process would be repeated with a second UDP-Glc but the rotation direction after glycosyl transfer would be in the opposite direction owing to steric constraints. Al‐ ternatively, the glucan might not translocate into the channel until UDP is replaced by UDP-Glc. Trp 383 and Cys 318 mark the entrance to the transmembrane channel [40].

### **4. Cultivation conditions for production of bacterial cellulose**

The MC production depends of the appropriate cultivation conditions, which include the composition of the culture medium (synthetic and natural media), temperature, pH and methods agitated or static cultivation. The choice of condition cultivation or another depend on the purpose, once these conditions have significant influence on the properties of structure, physical and mechanical MC.

The current methods of MC production are static culture [44], submerged fermentation through aerated or agitated cultivation [45], and the airlift bioreactor [46]. Large scale, semicontinuous and continuous fermentation are dominant to meet commercial demand. In all cases, the main objective is to achieve maximum production of MC with optimum form and suitable properties for the application for which it is intended. After all, a wider application of this versatile biopolymer depends on the practical considerations such as the scale-up capability and production costs. *G. xylinus* has two main operative amphibolic pathways: the pentose phosphate cycle for the oxidation of carbohydrates and the Krebs cycle for the oxidation of organic acids and related compounds [12,21,47,48]. Consequently, several studies have been reported that the composition of the culture medium and the fermentation condi‐ tions significantly affect the order structure of cellulose [49,50,51].

Static cultivation is a relatively simple and widely used method of cellulose production. The medium is placed into shallow tray or bottles, inoculated, and cultivated for several days until the cellulose nearly fills the tray. *G. xylinus* produces a gelatinous MC membrane, which has a denser surface on the side exposed to air, i.e. capable of generating cellulose as an extracel‐ lular product on static media (at the air-medium interface) at temperatures between 25 and 30 °C and pH from 4 to 7 [52]. The traditional static culture represents an expensive way of MC production that may hinder its industrial application since the productivity is low and long cultivation time is required. Consequently, authors have proposed new culture system as strategy to increase the MC productivity to a suitable for commercial applications in simple fed-batch [53], in bioreactor for a semi-continuous production [54], in a modified airlift-type bubble column bioreactor [55].

Nutrients required for the growth of these microorganisms are carbon and nitrogen sources, phosphorus, Sulphur, potassium and magnesium salts [56]. Sometimes a complex medium supplying amino acids and vitamins is also used to enhance the cell growth and production [57]. Between the years 1940 to 1960, researchers at the Hebrew University in Jerusalem intensively investigated the biochemistry of simplified production and quantification of cellulose cellulose for produced by *G. xylinus*. These media, named as HS are widely used nowadays [44]. Typical carbon sources in the production of microbial cellulose include glucose, fructose, sucrose, mannitol, however among others including arabinose, arabitol, citric acid, ethanol, ethylene glycol, diethylene glycol, galactose, glucono lactone, glycerol, inositol, lactose, malic acid, maltose, mannose, methanol, rhamnose, ribose, sorbose, starch, succharide, succinic acid, trehalose, and xylose have been also investigated [58,59,60] to maximize bacterial cellulose production by various *Gluconacetobacter* strains [8]. The best yield was obtained in fed-batch fermentation, 15.3 g/L in 50 hours of cultivation using glucose as carbon source [61] and other examples of different culture conditions can be found in Table 1. The results shown in Table 1 indicate that, glucose seems to be the best carbon source. It demonstrates the validity of the results that various carbon substrates could be converted to monomer glucose by *Gluconacetobacter*, followed by polymerization to MC [60]. Among the factors affecting cost, carbon sources play a major role in fermentation [47,62]. Conversion of 60–80% of the utilized carbon source into crude polymer is commonly found in high yielding polysaccharide fermentations. In order to compensate its low sugar conversion yield and to reduce the feedstock cost of MC production, in recent years, MC has been produced by fermenting the hydrolysates of agricultural wastes such as hemicelluloses [63], konjac powder [62], rice bark [64] and waste cotton fabrics [63,65]. An advantage of using agricultural or industrial residual streams as feedstock is the low or no value of the raw material. Several successful efforts have been made to use certain industrial food wastes as growth medium for the MC producer organisms, which is not only a cheap way but also works as a basin for environmental cleaning [51]. Thin stillage (TS) is a wastewater from rice wine distillery rich in carbon sources and organic acids. [66] discovered that TS, when employ to replace distilled water for preparing Hestrin and Schramm medium (the traditional MC production medium), can enhance the MC production 2.5-fold to a concentration of 10.38 g/L with a sugar-MC conversion yield of 57 % (0.57 g MC/g reducing sugar) after 7 days of static cultivation. In 2012, Ha et al. [48] further improved the MC production, 15.28 g/L of MC was obtained after 15 days of cultivation. Yeast extract and peptone are the most commonly used nitrogen sources in MC production as they provide nitrogen and growth factors for *Gluconacetobacter* strains. Many researchers are trying to find efficient substitutes due to their high cost. Even if various nitrogen sources were added to the HS medium, peptone is found to be the most effective nutrient. However, corn steep liquor (CSL) which produced the second highest production is always chosen as a substitution for the economic viewpoints. Buffering capacity is also important for MC production. Insoluble MC often attaches to pH probe and leads to inaccurate reading [4]. Noro et al. [67] pointed out the buffering capacity of CSL, which could maintain the pH within the optimal range during the production of MC. Jung et al. [68] doubled MC production (from 1.53 to 3.12 g/L) using molasses as carbon source and corn steep liquor as nitrogen source when compared with the results obtained from complex medium. This strategy could not only reduce burden on environment but also achieve the goal of large scale production with low cost. The optimal pH for MC production may vary with carbon source. *G. xylinus* accumulates gluconic acid at low pH, and a preferred environment for both biomass and MC production can be achieved by shifting pH from 4.0 to 5.5 during cellulose production phase in fed-batch cultures [61]. Under static batch cultivation, the pH of the culture medium decreases due to the respiratory metabolism of *G. xylinus*, which involves the ethanol oxidation to acetic acid and the glucose conversion into gluconic acid. This fact makes it very important to control the pH within the optimum range for cell growth and cellulose production [48,69]. *Gluconacetobacter* strains require oxygen as an essential substrate, consequently volumetric oxygen transfer coefficient (kLa) is a key limiting factor in the aerobic fermentation for producing MC. Song et al. [55] investigated the optimum aeration rate for a 50-L spherical type bubble column bioreactor, and it was determined to be 1.0 vvm (30 L/min).

**4. Cultivation conditions for production of bacterial cellulose**

tions significantly affect the order structure of cellulose [49,50,51].

physical and mechanical MC.

140 Cellulose - Fundamental Aspects and Current Trends

bubble column bioreactor [55].

The MC production depends of the appropriate cultivation conditions, which include the composition of the culture medium (synthetic and natural media), temperature, pH and methods agitated or static cultivation. The choice of condition cultivation or another depend on the purpose, once these conditions have significant influence on the properties of structure,

The current methods of MC production are static culture [44], submerged fermentation through aerated or agitated cultivation [45], and the airlift bioreactor [46]. Large scale, semicontinuous and continuous fermentation are dominant to meet commercial demand. In all cases, the main objective is to achieve maximum production of MC with optimum form and suitable properties for the application for which it is intended. After all, a wider application of this versatile biopolymer depends on the practical considerations such as the scale-up capability and production costs. *G. xylinus* has two main operative amphibolic pathways: the pentose phosphate cycle for the oxidation of carbohydrates and the Krebs cycle for the oxidation of organic acids and related compounds [12,21,47,48]. Consequently, several studies have been reported that the composition of the culture medium and the fermentation condi‐

Static cultivation is a relatively simple and widely used method of cellulose production. The medium is placed into shallow tray or bottles, inoculated, and cultivated for several days until the cellulose nearly fills the tray. *G. xylinus* produces a gelatinous MC membrane, which has a denser surface on the side exposed to air, i.e. capable of generating cellulose as an extracel‐ lular product on static media (at the air-medium interface) at temperatures between 25 and 30 °C and pH from 4 to 7 [52]. The traditional static culture represents an expensive way of MC production that may hinder its industrial application since the productivity is low and long cultivation time is required. Consequently, authors have proposed new culture system as strategy to increase the MC productivity to a suitable for commercial applications in simple fed-batch [53], in bioreactor for a semi-continuous production [54], in a modified airlift-type

Nutrients required for the growth of these microorganisms are carbon and nitrogen sources, phosphorus, Sulphur, potassium and magnesium salts [56]. Sometimes a complex medium supplying amino acids and vitamins is also used to enhance the cell growth and production [57]. Between the years 1940 to 1960, researchers at the Hebrew University in Jerusalem intensively investigated the biochemistry of simplified production and quantification of cellulose cellulose for produced by *G. xylinus*. These media, named as HS are widely used nowadays [44]. Typical carbon sources in the production of microbial cellulose include glucose, fructose, sucrose, mannitol, however among others including arabinose, arabitol, citric acid, ethanol, ethylene glycol, diethylene glycol, galactose, glucono lactone, glycerol, inositol, lactose, malic acid, maltose, mannose, methanol, rhamnose, ribose, sorbose, starch, succharide, succinic acid, trehalose, and xylose have been also investigated [58,59,60] to maximize bacterial cellulose production by various *Gluconacetobacter* strains [8]. The best yield was obtained in fed-batch fermentation, 15.3 g/L in 50 hours of cultivation using glucose as carbon source [61]

Attempts to enhance MC production by adding different additives in the fermentation medium have been made. The possible mechanisms of these various additives to enhance MC were also proposed such as reduction of the shear force by increasing the viscosity of medium [70]. Different chemical compounds including alcohols [72], glycerol [60,68], organic acids [68], polysaccharides [72] thin stillage from rice wine distillery [66] and thin stillage from beer culture broth [47,53] have been used as additives to the fermentation medium with the aim of increasing MC production.


**Table 1.** Bacterial cellulose production under different culture conditions

### **5. Microbial cellulose for biomedical applications**

As previously described, intense research has focused on the use of natural biopolymers in a variety of biomedical materials and devices, including wound dressings, medical implants, drug delivery, vascular grafts, and scaffolds for tissue engineering [78]. Consequently, continual efforts from many researchers, led to novel systems that closely mimic the complex and hierarchical structures inherent to the native tissue are sure to emerge.

In the last decade, several nanocellulose-based materials have been created for a diversity of biomedical applications. Some review articles have highlighted the potential applications of cellulose materials [72,79,80,81,82].

MC represents an interesting emerging nanomaterial, with no toxicity, and since its discovery has shown tremendous potential as an effective biopolymer which offers a wide range of applications, especially the biomedical ones, including the use as biomaterial for artificial skin, artificial blood vessels and microvessels, wound dressing of second- or third-degree burn ulcers and dental implants. Other studies with endothelial, smooth muscle cells and chondro‐ cytes have shown that these cells present good adhesion to bacterial cellulose. [83]

### **5.1. Pristine MC based biomaterials**

Attempts to enhance MC production by adding different additives in the fermentation medium have been made. The possible mechanisms of these various additives to enhance MC were also proposed such as reduction of the shear force by increasing the viscosity of medium [70]. Different chemical compounds including alcohols [72], glycerol [60,68], organic acids [68], polysaccharides [72] thin stillage from rice wine distillery [66] and thin stillage from beer culture broth [47,53] have been used as additives to the fermentation medium with the aim of

**mode**

*G. xylinus* (BRC 5) Glucose Fed-batch 2 15.3 Hwang *et al.* (1999)

*G. xylinus* (BPR 2001) Fructose Agitated 3 14.1 Bae *et al.* (2004) [70]

bioreactor

*Gluconacetobacter sp* (F6) Glucose Static 6 4.5 Jahan *et al.* (2012)

As previously described, intense research has focused on the use of natural biopolymers in a variety of biomedical materials and devices, including wound dressings, medical implants, drug delivery, vascular grafts, and scaffolds for tissue engineering [78]. Consequently,

**Time culture (days)**

CSL-Fru Agitated 5 13.0 Cheng *et al.* (2011)

TS-Glu Static 7 10.38 Wu *et al*. (2012)[66]

Glucose Static 14 6.23 Cavka *et al*. (2013)

Syrup Static 14 43.5 Moosavi-Nasab

Glucose Static 15 15.28 Ha and Park (2012)

**MC production (g l-1)**

**Reference**

[61]

[72]

[73]

[74]

[75]

[76]

and Yousefi (2011)

3 5.6 Song et al. (2009)[55]

increasing MC production.

142 Cellulose - Fundamental Aspects and Current Trends

*A. xylinum* (ATCC

*G. xylinus and Trichoderma*

700178)

*G. xylinus* (MCRC 12334)

*ressei*

*G. xylinus* (PTCC, 1734)

*G. xylinus* (ATCC 23769)

**Microorganism Carbon source Cultivation**

*G. xylinus* (KJ1) Saccharified food wastesairlift-type

**Table 1.** Bacterial cellulose production under different culture conditions

**5. Microbial cellulose for biomedical applications**

One of the main direct applications of MC membranes in biomedical field is related to wound dressing. Fontana et al. [84] were the pioneers in describing the use of bacterial cellulose to replace burned skin. Since then, literature shows a great number of papers related to wound dressing. Cellulose dressings are recommended as a temporary covering for the treatment of wounds, including pressure sores, skin tears, venous stasis, ischemic and diabetic wounds, second-degree burns, skin graft donor sites, traumatic abrasions and lacerations, and biopsy sites by the manufacturers [85].

MC based wound dressings are commonly available on the market nowadays, for example: BioFill®, Bioprocess®, XCell® and Gengiflex® (for periodontal diseases reconstruction [86]. The biomembrane BioFill® was one of the first commercial product that fulfills the main prerequisites of an ideal wound dressing, including: low cost, good adherence to the wound, water vapor permeability, elasticity, transparency, durability, it constitutes a physical barrier for bacteria, is hemostatic, it presents easy handling and application with minimum exchanges. BioFill® effectiveness has been proven in more than 300 cases in accelerating the healing process, pain relief, etc. [86,87,88,89].

Despite the analgesic mechanism of action of these dressings has not been fully elucidated, some authors suggest that the healing mechanism involves the capture of ions by means of cellulose hydrogen bonds, or the nano MC 3-D network mimics the skin surface creating optimal conditions for healing or regeneration [87,88,89].

It is important to point out that MC wound dressing clearly shortened the time to heal or wound closure over standard care when applied to non-healing lower extremity ulcers, as observed by many researchers [88,89,90]. As can be seen in Figure 5, novel applications of wet MC as wound dressing in the treatment partial thickness burns were applied by Czaja et al. [88,89] presenting excellent results suggesting that MC as a wound dressing promotes a favorable moist environment for a fast wound cleansing, and consequently for rapid healing. and consequently for rapid healing.

[86,87,88].

healing process, pain relief, etc. [85,86,87,88].

venous stasis, ischemic and diabetic wounds, second-degree burns, skin graft donor sites, traumatic abrasions and lacerations, and biopsy sites by the manufacturers [84].

nowadays, for example: BioFill®, Bioprocess®, XCell® and Gengiflex® (for periodontal diseases reconstruction [85]. The biomembrane BioFill® was one of the first commercial product that fulfills the main prerequisites of an ideal wound dressing, including: low cost, good adherence to the wound, water vapor permeability, elasticity, transparency, durability, it constitutes a physical barrier for bacteria, is hemostatic, it presents easy handling and application with minimum exchanges. BioFill® effectiveness has been proven in more than 300 cases in accelerating the

MC based wound dressings are commonly available on the market

Despite the analgesic mechanism of action of these dressings has not been

fully elucidated, some authors suggest that the healing mechanism involves the capture of ions by means of cellulose hydrogen bonds, or the nano MC 3-D network mimics the skin surface creating optimal conditions for healing or regeneration

It is important to point out that MC wound dressing clearly shortened the time to heal or wound closure over standard care when applied to non-healing lower extremity ulcers, as observed by many researchers [87,88,89]. As can be seen in Figure 5, novel applications of wet MC as wound dressing in the treatment partial thickness burns

a wound dressing promotes a favorable moist environment for a fast wound cleansing,

**Figure 5.** Bacterial cellulose dressing applied on wounded torso and face. (Reprinted with permission from Czaja, W. K. et al. (2007). The future prospects of microbial cellulose in biomedical applications, Biomacromolecules, Vol.8, No.1, pp. 4. Copyright (2007) American Chemical Society).

Other interesting biomedical applications for MC films have emerged. A Brazilian research group had designed and patented a device to manufacture MC contact lens for therapy in cases of regeneration of cornea [91].

In terms of vascular applications, a German group created BASYC® (Bacterial Cellulose Synthetised), consisting of a tubular biomaterial for applications in microsurgery of arteries and veias [92] and [93] applied vascular stents in animals arteries. Bodin et al. [94] obtained MC tubes by modifying the fermentation process of *Acetobacter xylinum* on top of silicon tubes, as shown in Figure 6.

**Figure 6.** MC tubes presenting different sizes and shapes for applications: a) Mc tubes showing different inner diame‐ ters: 1.5mm, 2.4mm, 3.0mm, 4.0 mm and 6.0 mm. b) Branched MC tube fermented on a branched silicone tube. (Re‐ printed with permission from Bodin et al., 2007 Biotechnology and Bioengineering, Vol. 97, No. 2, June 1, 2007 [94])

Lately, Nimeskern et al. [95] designed and fabricated an ear-shaped pristine MC prototype material applying a Magnetic Resonance Imaging (MRI) scanning technique. This study was extremely important to confirm that MC is a promising tissue engineering material with appropriate mechanical properties for ear cartilage replacement. Thereby, it may be used to create patient-specific ear shapes.

### **5.2. Microbial cellulose nanocomposites for biomedical applications**

Beyond the direct uses, MC can be widely and effectively utilized as eitherfunctionalreinforce‐ ments or excellent matrices due to excellent mechanical properties and biocompatibility which

allows it to be engineered in various forms from nano to macro scale. Thus, MC based nanocom‐ posites can be manipulated to improve their properties and/or functionalities becoming one of the reasons that makes MC so exceptional material for biomedical applications.

venous stasis, ischemic and diabetic wounds, second-degree burns, skin graft donor sites, traumatic abrasions and lacerations, and biopsy sites by the manufacturers [84].

nowadays, for example: BioFill®, Bioprocess®, XCell® and Gengiflex® (for periodontal diseases reconstruction [85]. The biomembrane BioFill® was one of the first commercial product that fulfills the main prerequisites of an ideal wound dressing, including: low cost, good adherence to the wound, water vapor permeability, elasticity, transparency, durability, it constitutes a physical barrier for bacteria, is hemostatic, it presents easy handling and application with minimum exchanges. BioFill® effectiveness has been proven in more than 300 cases in accelerating the

healing process, pain relief, etc. [85,86,87,88].

and consequently for rapid healing.

144 Cellulose - Fundamental Aspects and Current Trends

pp. 4. Copyright (2007) American Chemical Society).

of regeneration of cornea [91].

create patient-specific ear shapes.

as shown in Figure 6.

[86,87,88].

MC based wound dressings are commonly available on the market

Despite the analgesic mechanism of action of these dressings has not been

fully elucidated, some authors suggest that the healing mechanism involves the capture of ions by means of cellulose hydrogen bonds, or the nano MC 3-D network mimics the skin surface creating optimal conditions for healing or regeneration

It is important to point out that MC wound dressing clearly shortened the time to heal or wound closure over standard care when applied to non-healing lower extremity ulcers, as observed by many researchers [87,88,89]. As can be seen in Figure 5, novel applications of wet MC as wound dressing in the treatment partial thickness burns were applied by Czaja et al. [87,88] presenting excellent results suggesting that MC as a wound dressing promotes a favorable moist environment for a fast wound cleansing,

**Figure 5.** Bacterial cellulose dressing applied on wounded torso and face. (Reprinted with permission from Czaja, W. K. et al. (2007). The future prospects of microbial cellulose in biomedical applications, Biomacromolecules, Vol.8, No.1,

Other interesting biomedical applications for MC films have emerged. A Brazilian research group had designed and patented a device to manufacture MC contact lens for therapy in cases

In terms of vascular applications, a German group created BASYC® (Bacterial Cellulose Synthetised), consisting of a tubular biomaterial for applications in microsurgery of arteries and veias [92] and [93] applied vascular stents in animals arteries. Bodin et al. [94] obtained MC tubes by modifying the fermentation process of *Acetobacter xylinum* on top of silicon tubes,

**Figure 6.** MC tubes presenting different sizes and shapes for applications: a) Mc tubes showing different inner diame‐ ters: 1.5mm, 2.4mm, 3.0mm, 4.0 mm and 6.0 mm. b) Branched MC tube fermented on a branched silicone tube. (Re‐ printed with permission from Bodin et al., 2007 Biotechnology and Bioengineering, Vol. 97, No. 2, June 1, 2007 [94])

Lately, Nimeskern et al. [95] designed and fabricated an ear-shaped pristine MC prototype material applying a Magnetic Resonance Imaging (MRI) scanning technique. This study was extremely important to confirm that MC is a promising tissue engineering material with appropriate mechanical properties for ear cartilage replacement. Thereby, it may be used to

Beyond the direct uses, MC can be widely and effectively utilized as eitherfunctionalreinforce‐ ments or excellent matrices due to excellent mechanical properties and biocompatibility which

**5.2. Microbial cellulose nanocomposites for biomedical applications**

In relation to biomaterial applications for wound dressing and skin tissue repair several MC based nanobiocomposites were fabricated. Here are some examples: membranes of MC/ collagen [96], MC/gelatin [97], MC/ aloe vera films [98], MC/alginate for temporary dressing material [99].

Further, MC composite with kaolin was proved as short-term and long term wound healing materials [100].

Freeze-drying techniques allowed the preparation MC/poly(ethylene glycol) (PEG) compo‐ sites of by immersing wet MC pellicle in PEG aqueous solution [101]. This same technique was applied by some of us [102] to obtain MC/ silk fibroin (SF) sponge scaffolds. *In vitro* tests proved non-cytotoxic or genotoxic character of these nanobiocomposites. SEM images revealed a greater number of fibroblast cells (L929 cell line) attached at the MC/SF:50% scaffold surface if compare with the surface of pure MC, suggesting that the presence of fibroin improved cell attachment as is possible to see in Figure 7. This could be related to the SF amino acid sequence that act as cell receptors facilitating cell adhesion and growth. Consequently, MC/SF:50% scaffolds configured an excellent option in bioengineering depicting its potential for tissue regeneration and cultivation of cells on nanobiocomposites.

**Figure 7.** To test the hypothesis that the addition of silk fibroin to cellulose scaffolds increases cell adhesion (48 h), L-929 cells were seeded in MC and MC/SF scaffolds. SEM images of the cells attached to MC (a) and MC/SF (b) scaf‐ folds surface; cross-section SEM images of MC (c) and MC/SF (d) evidenced that the cells did not migrate into the scaf‐ folds. (Reprinted with permission from Oliveira Barud et al., 2015, Carbohydrate Polymers, Vol 128, April, 2015 [102]).

Generally, a scaffold provides a foundation for cell attachment, and several materials have been tested as scaffolds to support growth of cells. The need for bio-mimicking scaffolds has led to the exploration of MC as a scaffold material. There is an increased interest in developing adipose tissue as an *in vitro* model for adipose biology and metabolic disease, and to this end, 2D and 3D porous scaffolds of bacterial nanocellulose and alginate were prepared recently [103].

Bäckdahl et al. [104] also developed MC scaffolds with controlled microporosity by placing paraffin wax and starch particles during culture and removing these particles once the cultivation process was finished. The MC scaffolds were then seeded with smooth muscle cells for investigating the potential tissue engineered blood vessel application.

A variety of surface functionalization through biosynthetic or chemical modification was also investigated. Various approaches to the preparation of functional MC-based nanocomposites by incorporating different guest substrates including small molecules, inorganic nanoparticles or nanowires, and polymers on the surfaces of MC nanofibers are exemplified which can improve the functionality of MC nanomaterials and expand its potential application in the biomedical fields.

Nanocomposites were obtained by the association of nanoparticles presenting antimicrobial activities, including silver nanoparticles [105,106,107,108,109,110]. Additionally, Barud et al. [111] also prepared MC/propol membranes that presented good antimicrobial activities to be used as wound dressing material.

In recent years, several controlled release systems based on nanocellulose material for various pharmaceutical applications have been also investigated to delivery Tetracycline [112], benzalkonium chloride [113], topical release of lidocaine [114] and release of proteins with serum albumin [115].

With respect to bone regeneration in defects of rat tibiae, MC-hydroxyapatite (MCHA) nanobiocomposite were prepared to evaluate the biological properties and performance of the material [116]. The MCHA membranes were effective for bone regeneration and accelerated new bone formation. In addition, reabsorption of the membranes was slow, suggesting that this composite takes time to be completely reabsorbed.

### **6. Conclusion**

Microbial cellulose is a natural renewable polymer synthesized from the bacterium *Glucona‐ cetobacter xylinus* that is the only known species capable to produce cellulose on an industrial scale. In an appropriate culture medium the bacteria secretes about 50-80 cellulose microfibrils from 3.0 to 3.5 mm thick, free of lignin and hemicellulose, which aggregate themselves to form strips arranged in a 3-D hierarchical network. Besides that MC configures one of the most promising investigations in the field of biodegradable polymers. Due to this uniform structure and morphology MC is endowed with unique characteristics such as high purity, high crystallinity and remarkable mechanical properties, good chemical stability, high water holding capacity featuring it as a completely biocompatible polymer. Despite its high water content, MC shows a good mechanical performance and it can be produced in almost any shape due to its high moldability during formation. MC is an interesting emerging biomaterial, with no toxicity, and since its discovery has shown tremendous potential as an effective biopolymer in various fields, because the structural aspect of MC is far superior to those of plant cellulose. Thus, this chapter reviewed involved detailed aspects about the biosynthesis and recent advances on microbial production, including mechanism for the biochemistry of the cellulose synthesis, new sources for culture medium, main aspects about static and airreactor productions and genetic modifications. We also revised and presented a great number of different MC based materials that were designed for biomedical applications (dressings, scaffolds, drug delivery systems), among others. Additionally, we hope that this book chapter may aggregate high quality information and may be a benchmark to intensify greater interest of the scientific community in microbial cellulose and related devices and also inspire the development of new materials in this field.

### **Author details**

Generally, a scaffold provides a foundation for cell attachment, and several materials have been tested as scaffolds to support growth of cells. The need for bio-mimicking scaffolds has led to the exploration of MC as a scaffold material. There is an increased interest in developing adipose tissue as an *in vitro* model for adipose biology and metabolic disease, and to this end, 2D and 3D porous scaffolds of bacterial nanocellulose and alginate were prepared recently

Bäckdahl et al. [104] also developed MC scaffolds with controlled microporosity by placing paraffin wax and starch particles during culture and removing these particles once the cultivation process was finished. The MC scaffolds were then seeded with smooth muscle cells

A variety of surface functionalization through biosynthetic or chemical modification was also investigated. Various approaches to the preparation of functional MC-based nanocomposites by incorporating different guest substrates including small molecules, inorganic nanoparticles or nanowires, and polymers on the surfaces of MC nanofibers are exemplified which can improve the functionality of MC nanomaterials and expand its potential application in the

Nanocomposites were obtained by the association of nanoparticles presenting antimicrobial activities, including silver nanoparticles [105,106,107,108,109,110]. Additionally, Barud et al. [111] also prepared MC/propol membranes that presented good antimicrobial activities to be

In recent years, several controlled release systems based on nanocellulose material for various pharmaceutical applications have been also investigated to delivery Tetracycline [112], benzalkonium chloride [113], topical release of lidocaine [114] and release of proteins with

With respect to bone regeneration in defects of rat tibiae, MC-hydroxyapatite (MCHA) nanobiocomposite were prepared to evaluate the biological properties and performance of the material [116]. The MCHA membranes were effective for bone regeneration and accelerated new bone formation. In addition, reabsorption of the membranes was slow, suggesting that

Microbial cellulose is a natural renewable polymer synthesized from the bacterium *Glucona‐ cetobacter xylinus* that is the only known species capable to produce cellulose on an industrial scale. In an appropriate culture medium the bacteria secretes about 50-80 cellulose microfibrils from 3.0 to 3.5 mm thick, free of lignin and hemicellulose, which aggregate themselves to form strips arranged in a 3-D hierarchical network. Besides that MC configures one of the most promising investigations in the field of biodegradable polymers. Due to this uniform structure and morphology MC is endowed with unique characteristics such as high purity, high crystallinity and remarkable mechanical properties, good chemical stability, high water

for investigating the potential tissue engineered blood vessel application.

[103].

biomedical fields.

serum albumin [115].

**6. Conclusion**

used as wound dressing material.

146 Cellulose - Fundamental Aspects and Current Trends

this composite takes time to be completely reabsorbed.

Wilton R. Lustri2 #, Hélida Gomes de Oliveira Barud2 #, Hernane da Silva Barud1,3, Maristela F. S. Peres3 , Junkal Gutierrez4 , Agnieszka Tercjak4 , Osmir Batista de Oliveira Junior2 and Sidney José Lima Ribeiro3

\*Address all correspondence to: hernane.barud@gmail.com

1 University Center of Araraquara, UNIARA, Brazil

2 School of Dentistry/Unesp, São Paulo State University – Unesp, Araraquara-SP, Brazil

3 Institute of Chemistry- São Paulo State University, Araraquara, SP, Brazil

4 Group `Materials + Technologies´, Department of Chemical and Environmental Engineering, University of the Basque Country, UPV/EHU, Donostia-San Sebastián, Spain
