**Abstract**

The chapter describes the chemical structure and hierarchical organization of cellulose fibers, characteristics of non-wood plant raw materials (NWPM), and methods for preparing pulp and nanocellulose (NC). NWPM have the necessary reserves and properties to make up for a possible shortage of wood fiber for pulp production. The methodology for evaluating the efficiency of the delignification processes of plant raw materials is presented. A two-stage technology for producing pulp for the preparation of NC by environmentally friendly organosolvent methods of NWPM delignification is proposed. Methods for preparing nanocellulose are described. The technological parameters of the extraction of NC from pulp are discussed. The influence of NC on the properties of composite materials is analyzed. Areas of use for NC from NWPM are shown.

**Keywords:** non-wood plant, wheat straw, flax, kenaf, miscanthus, pulp, nanocellulose, paper, cardboard, thermoelectric material, composite

#### **1. Introduction**

In recent years, there has been a growing interest in the development of new biodegradable materials from environmentally friendly renewable plants. They are able to replace materials made from exhaustible natural resources—oil, gas, coal. Polymers from these fossils take hundreds of years to decompose, causing irreparable damage to the environment. Plastic accounts for 85 percent of all waste in the world's oceans, half of which are disposable plastic products [1, 2].

The European Parliament in March 2019 approved a new law banning singleuse plastic products such as plates, cutlery, straws, plastics and food containers and expanded polystyrene cups [3]. Scientists and civil society organizations are working together to create new consumption patterns that meet the needs of all people, while eliminating waste and overconsumption, where the production of consumer goods is less dependent on the use of natural resources and makes the most of recycled materials [4, 5]. The use of natural polymers from cellulosic plant materials is being seen as an alternative to plastics and could be a viable approach to reducing deforestation, increasing the use of agricultural surplus and developing biodegradable materials. The development of environmentally friendly technologies of processing renewable plant sources contribute to the sustainable development of society, solving economic and environmental problems in the production

of consumer goods [6–8]. The processing products of such renewable plant materials are widely used in the chemical, pharmaceutical, paper, medicine, textile and electronic industries [9, 10].

The main component of all plants is cellulose, which is the most abundant renewable biopolymer in nature with an estimated annual production of 1.5 × 1012 ton [11]. Cellulose is a structural component of the cell walls of softwood and deciduous wood, stalks and leaves of non-wood plants. A source of cellulose can be also bacteria, algae, and fungi [12, 13]. Cellulose (C6H10O5)n is a stereoregular, semicrystalline polysaccharide consisting of a linear chain from several hundred to several tens of thousands of repeating units of β-D-glucopyranose (*n*), covalently linked by 1–4 glycosidic bonds (**Figure 1**).

The number of repeating units *n* is highly dependent on the source of the original cellulose (e.g. 10,000 in natural wood, 15,000 in cotton and 44,000 in the genus Valonia). To a lesser extent, the number of repeating unit *n* depends on the methods of preparation and purification (e.g., *n* = 250–500 in regenerated cellulose and *n* = 1000 in bleached kraft pulp) [11, 14]. The β-D-glucopyranose ring of the middle units of cellulose macromolecules contains three hydroxyl groups, which determine the chemical reactions, the ability to form intramolecular and intermolecular between different chains hydrogen bonds and properties of cellulose - solubility, thermal stability and mechanical properties [13, 15]. In the process of biosynthesis due to enzymatic polymerization of glucose monomers, glucan chains are formed, which independently form chains through van der Waals forces and are held together through hydrogen bonds, forming elementary fibrils. It was found that 36 cellulose chains lead to the production of elementary cellulose fibrils, the cross section of which has a size of 3–5 nm [16]. These elementary fibrils or nanofibrils have highly ordered regions (crystallites) that alternate with less organized (amorphous regions) [16, 17]. Then multiple elementary fibrils are brought together into larger units called microfibrils with a diameter of ~20–30 nm and length of several micrometers [18, 19]. The hierarchical organization of cellulose macromolecules in elementary fibrils and microfibrils of the plant cell wall is shown in **Figure 2**.

Cellulose is in the form of microfibrils, consisting of amorphous and crystalline domains in combination with other substances such as lignin, hemicelluloses, proteins, extractives and minerals, which constitute the main structural unit of plant cell walls [11], as shown schematically in the **Figure 2**. The proportion of plant fiber constituents depends on parameters like botanical origin, maturation time, climatic conditions, age of the plant etc. [20].

In world practice, the main consumers of cellulose are the pulp and paper industry for the production of paper and cardboard, and the chemical industry for the production of cellulose derivatives. Recently, cellulose has also attracted considerable interest as a source of raw materials for the production of nanocellulose (NC). Nanocellulose belongs to a group of nanomaterials consisting of the nanosized cellulose particles. The NC exhibit unique properties, such as high elastic modulus, high specific surface area, optical transparency, low thermal expansion

**63**

*Preparation, Properties and Use of Nanocellulose from Non-Wood Plant Materials*

coefficient, and chemical reactivity [21–23]. NC has high transparency, biodegradability and biocompatibility, a low lightweight and production cost in comparison

*The hierarchical organization of cellulose macromolecules in elementary fibrils and microfibrils of the plant* 

The main raw material for cellulose production in the world pulp and paper industry is wood. For countries that do not have large reserves of free wood, alternative sources of fibrous raw materials may be non-wood plant raw materials (NWPM) - annual and perennial plants and fibrous waste from agricultural production. For example, in 2014, 172.6 million tons of pulp were produced from wood and only 13 million tons from non-wood fibers [26]. At the same time, in the world, forests occupy 3937 million hectares and agricultural plants 4932 million hectares [27]. World reserves of NWPM are estimated at 2.527 billion tons [28]. Almost half of all NWPM stocks are cereal stalks (1250 million tons), of which about half are wheat straw [29]. Non-wood fibers have a wide range of properties that are used for

*DOI: http://dx.doi.org/10.5772/intechopen.94272*

with synthetic polymers [24, 25].

*cell wall (adapted from [15, 19]).*

**Figure 2.**

the production of cellulose-containing products [30].

which include: cotton linter, flax, hemp, kenaf, etc.

bamboo, esparto, natural herbs, etc.;

In general, NWPM can be divided into two broad categories [31]:

a.common non-forest plants, which are considered as an alternative to deciduous wood, which include: straw of cereals, corn stalks, sorghum, bagasse, reeds,

b.special types of plants that are considered as an alternative to coniferous wood,

The first category includes the predominant absolute reserves of non-wood plants. It contains 35 ˗ 62% cellulose, 10 ˗ 25% lignin and 18 ˗ 36% pentosanes.

**Figure 1.** *Chemical structure of cellulose.*

*Preparation, Properties and Use of Nanocellulose from Non-Wood Plant Materials DOI: http://dx.doi.org/10.5772/intechopen.94272*

**Figure 2.**

*Novel Nanomaterials*

electronic industries [9, 10].

linked by 1–4 glycosidic bonds (**Figure 1**).

conditions, age of the plant etc. [20].

of consumer goods [6–8]. The processing products of such renewable plant materials are widely used in the chemical, pharmaceutical, paper, medicine, textile and

The main component of all plants is cellulose, which is the most abundant renewable biopolymer in nature with an estimated annual production of 1.5 × 1012 ton [11]. Cellulose is a structural component of the cell walls of softwood and deciduous wood, stalks and leaves of non-wood plants. A source of cellulose can be also bacteria, algae, and fungi [12, 13]. Cellulose (C6H10O5)n is a stereoregular, semicrystalline polysaccharide consisting of a linear chain from several hundred to several tens of thousands of repeating units of β-D-glucopyranose (*n*), covalently

The number of repeating units *n* is highly dependent on the source of the original cellulose (e.g. 10,000 in natural wood, 15,000 in cotton and 44,000 in the genus Valonia). To a lesser extent, the number of repeating unit *n* depends on the methods of preparation and purification (e.g., *n* = 250–500 in regenerated cellulose and *n* = 1000 in bleached kraft pulp) [11, 14]. The β-D-glucopyranose ring of the middle units of cellulose macromolecules contains three hydroxyl groups, which determine the chemical reactions, the ability to form intramolecular and intermolecular between different chains hydrogen bonds and properties of cellulose - solubility, thermal stability and mechanical properties [13, 15]. In the process of biosynthesis due to enzymatic polymerization of glucose monomers, glucan chains are formed, which independently form chains through van der Waals forces and are held together through hydrogen bonds, forming elementary fibrils. It was found that 36 cellulose chains lead to the production of elementary cellulose fibrils, the cross section of which has a size of 3–5 nm [16]. These elementary fibrils or nanofibrils have highly ordered regions (crystallites) that alternate with less organized (amorphous regions) [16, 17]. Then multiple elementary fibrils are brought together into larger units called microfibrils with a diameter of ~20–30 nm and length of several micrometers [18, 19]. The hierarchical organization of cellulose macromolecules in elementary fibrils and microfibrils of the plant cell wall is shown in **Figure 2**.

Cellulose is in the form of microfibrils, consisting of amorphous and crystalline domains in combination with other substances such as lignin, hemicelluloses, proteins, extractives and minerals, which constitute the main structural unit of plant cell walls [11], as shown schematically in the **Figure 2**. The proportion of plant fiber constituents depends on parameters like botanical origin, maturation time, climatic

In world practice, the main consumers of cellulose are the pulp and paper industry for the production of paper and cardboard, and the chemical industry for the production of cellulose derivatives. Recently, cellulose has also attracted considerable interest as a source of raw materials for the production of nanocellulose (NC). Nanocellulose belongs to a group of nanomaterials consisting of the nanosized cellulose particles. The NC exhibit unique properties, such as high elastic modulus, high specific surface area, optical transparency, low thermal expansion

**62**

**Figure 1.**

*Chemical structure of cellulose.*

*The hierarchical organization of cellulose macromolecules in elementary fibrils and microfibrils of the plant cell wall (adapted from [15, 19]).*

coefficient, and chemical reactivity [21–23]. NC has high transparency, biodegradability and biocompatibility, a low lightweight and production cost in comparison with synthetic polymers [24, 25].

The main raw material for cellulose production in the world pulp and paper industry is wood. For countries that do not have large reserves of free wood, alternative sources of fibrous raw materials may be non-wood plant raw materials (NWPM) - annual and perennial plants and fibrous waste from agricultural production. For example, in 2014, 172.6 million tons of pulp were produced from wood and only 13 million tons from non-wood fibers [26]. At the same time, in the world, forests occupy 3937 million hectares and agricultural plants 4932 million hectares [27]. World reserves of NWPM are estimated at 2.527 billion tons [28]. Almost half of all NWPM stocks are cereal stalks (1250 million tons), of which about half are wheat straw [29]. Non-wood fibers have a wide range of properties that are used for the production of cellulose-containing products [30].

In general, NWPM can be divided into two broad categories [31]:


The first category includes the predominant absolute reserves of non-wood plants. It contains 35 ˗ 62% cellulose, 10 ˗ 25% lignin and 18 ˗ 36% pentosanes.

The fibers in it are shorter than the fibers of the second category and softwood, the length of the fibers of which is 0.3 ˗ 2 mm. Fibers of the second category of plants contain 55 ˗ 85% of cellulose, 1 ˗ 10% of lignin and have stronger and longer (larger than 5 mm) fibers [32, 33]. For most annuals, the average fiber length is close to the length of wood fibers of some deciduous species, but less than the average length of coniferous wood fibers and some industrial crops. The fiber width of annual plants is 2 ˗ 3 times thinner than the fibers of coniferous wood, but the ratio of the length of the fibers to their width in annual plants and in wood have the same order [34].

The chemical composition of the main components of NWPM differs from coniferous and deciduous wood. The content of cellulose, as the main component of raw materials, varies in wide ranges of values from 26% (bamboo) to 98% (cotton). Nonwood plant materials are distinguished by a high content of hemicellulose, especially pentosans to 30% [35]. Most NWPM have a lower lignin content from 6% (hemp) to 24% (bagasse) compared to wood (to 34%), which indicates the possibility of their use for pulp extraction [36]. Lignin of NWPM consists of guaiacylpropane, syringylpropane and oxyparaphenylpropane structural units, connected by simple ether and carbon–carbon bonds [37]. NWPM contain more minerals, but less lignin than wood, which a priori gives reason to expect a lower consumption of reagents for their delignification in comparison with the production of pulp from wood.

#### **2. Preparation of nanocellulose**

Properties of NC particles depend on the properties of plant raw materials and methods used for their production [38]. In world practice, for the production of NC, pulp with a minimum content of lignin, mineral and extractive substances, is usually used as a feedstock [39–42]. In this case, such pulp is obtained either by traditional methods of cooking with subsequent bleaching [43], or by environmentally friendly organosolvent methods of delignification [44].

In the global practice of pulp and paper industry, the dominating technologies to obtain pulp are sulphate and sulfite methods, which lead to environmental pollution [45]. Cooking pulp from NWPM in alkaline liquor is predominant because lignin from NWPM has a lower molecular weight than softwood lignin [37]. During cooking in an alkaline solution, the main ingredient of hemicellulose, xylan, is easily dissolved, which also opens additional channels for the cooking solution to penetrate into lignin, thereby facilitating the removal of lignin from the cell wall [46]. Alkaline methods for producing pulp from non-wood plants also include the NACO method based on alkaline oxygen cooking and the SAICA method. Experiments have shown that NACO-derived straw pulp has a lower yield and is inferior to the physico-mechanical properties of soda pulp and pulp obtained by the SAICA method [47, 48].

Alkaline methods include the neutral-sulfite method, which is used to obtain high yield pulp from hardwood and annual plants. Neutral sulfite pulp in comparison with sulphate pulp with the same degree of delignification has a 3–5% higher yield from plant raw materials due to less destruction of hemicelluloses and, therefore, is easier to grind [49]. It should be noted that among the main problems of organizing the process of obtaining pulp from NWPM by alkaline methods is the high content of silicates in them, which turn into black liquor during cooking and require additional technological solutions [45, 46].

Increased environmental requirements to the quality of wastewater and gas emissions of industrial enterprises requires the development of new technologies for processing of plant raw materials with the use of different organic solvents [50–52]. Organic solvents used in organosolvent methods of delignification differ

**65**

*Preparation, Properties and Use of Nanocellulose from Non-Wood Plant Materials*

example, from 20 to 30 kW/kg to 0.5 kW/kg of sulfite pulp [38].

depend on the cellulosic source and processing conditions [69].

Different terminologies have been used for the various types of NC. The Technical Association of the Pulp and Paper Industry (TAPPI) proposed standard terms and their definitions for cellulose nanomaterial WI 3021, based on the NC size [76]. NC is categorized into following kinds, such as cellulose nanofibrils (CNF), cellulose nanocrystals (CNC), amorphous nanocellulose, and cellulose nanoyarn [78]. CNF consist of a network of intertwined elementary nanofibrils, consisting of alternating crystalline and amorphous areas. CNF particles are

in the chemistry of interaction with the components of plant raw materials and technological parameters of the pulp cooking process. The most developed organosolvent methods of plant delignification include the following methods: ASAE [53], ALCELL [54], Acetosolv [55], MILOX [56], Chempolis [57] and CIMV [58]. Each of them has its own advantages and disadvantages, but they are all united by relative

Methods for producing NC include mechanical, chemical, oxidative and enzymatic treatment of cellulose fibers [59–61]. The essence of the mechanical methods is an application of different forces to reduce the size of the natural cellulose fibers to nanoscale. For this, various mechanical processing is used: homogenization, grinding, microfluidization, ultrasonic treatments, ball milling, and cryocrushing [42, 62]. The use of mechanical methods for obtaining nanocellulose is characterized by significant energy consumption, for example, with multiple passages of the cellulose fibers through a high-pressure homogenizer, it is above 25 kW/kg [38]. In [63] have shown that the homogenization process is the most expensive method for nanomaterial isolation. To reduce energy consumption and fiber damage during mechanical processes, various pretreatments of cellulose are used: enzymatic treatment, alkaline treatment and chemical oxidation. As a result of the rupture of strong interfibrillar hydrogen bonding, the power required for the production of NC is significantly reduced, for

Chemical methods are based on the cleavage of 1–4 glycosidic bonds of cellulose chains and isolation of cellulose nanocrystals with the removal of a part of the amorphous cellulose under the action of acids [64, 65]. For these purposes, the different acids are used: sulfuric, hydrochloric, phosphoric, maleic, hydrobromic, nitric, formic, p-toluenesulfonic [66–68]. Sulfuric acid is the most widely used acid for making NC. It reacts with the surface hydroxyl groups of cellulose to form negatively charged sulfonic groups and a stable gel. Otherwise, upon hydrolysis with hydrochloric acid, uncharged nanocellulose particles tend to flocculate in aqueous dispersions [69]. Recently, oxidizing agents such as 2,2,6,6-tetramethylpiperidine-1-oxyl radical (TEMPO) and phthalimide-N-oxyl (PINO) have been used to obtain NC. They improve the environmental friendliness and shorten the duration of the nanocellulose production process compared to hydrolysis, but have a higher cost than the

Enzymatic methods are based on the biosynthesis from monosaccharides or decreasing the size of the cellulose fibers by the fermentation. The enzymatic methods are time-consuming and require reagents that are more expensive. However, preliminary treatment of cellulose by enzymes before the mechanical grinding can decrease the energy consumption required for preparation of NC [73, 74]. For these reasons, a pre-treatment of the fibrous material is usually performed in order to decrease the size of the cellulose fibers and to ease the fibrillation and the process of nanocellulose preparation. The method of NC production by combining mechanical, chemical or biological pretreatment with homogenization treatment can not only reduce energy consumption, but also obtain NC with controllable size [75]. The various types of NC can be classified into different subcategories based on their shape, dimension, function, and preparation method, which in turn primarily

*DOI: http://dx.doi.org/10.5772/intechopen.94272*

environmental safety.

above acids [70–72].

#### *Preparation, Properties and Use of Nanocellulose from Non-Wood Plant Materials DOI: http://dx.doi.org/10.5772/intechopen.94272*

in the chemistry of interaction with the components of plant raw materials and technological parameters of the pulp cooking process. The most developed organosolvent methods of plant delignification include the following methods: ASAE [53], ALCELL [54], Acetosolv [55], MILOX [56], Chempolis [57] and CIMV [58]. Each of them has its own advantages and disadvantages, but they are all united by relative environmental safety.

Methods for producing NC include mechanical, chemical, oxidative and enzymatic treatment of cellulose fibers [59–61]. The essence of the mechanical methods is an application of different forces to reduce the size of the natural cellulose fibers to nanoscale. For this, various mechanical processing is used: homogenization, grinding, microfluidization, ultrasonic treatments, ball milling, and cryocrushing [42, 62]. The use of mechanical methods for obtaining nanocellulose is characterized by significant energy consumption, for example, with multiple passages of the cellulose fibers through a high-pressure homogenizer, it is above 25 kW/kg [38]. In [63] have shown that the homogenization process is the most expensive method for nanomaterial isolation. To reduce energy consumption and fiber damage during mechanical processes, various pretreatments of cellulose are used: enzymatic treatment, alkaline treatment and chemical oxidation. As a result of the rupture of strong interfibrillar hydrogen bonding, the power required for the production of NC is significantly reduced, for example, from 20 to 30 kW/kg to 0.5 kW/kg of sulfite pulp [38].

Chemical methods are based on the cleavage of 1–4 glycosidic bonds of cellulose chains and isolation of cellulose nanocrystals with the removal of a part of the amorphous cellulose under the action of acids [64, 65]. For these purposes, the different acids are used: sulfuric, hydrochloric, phosphoric, maleic, hydrobromic, nitric, formic, p-toluenesulfonic [66–68]. Sulfuric acid is the most widely used acid for making NC. It reacts with the surface hydroxyl groups of cellulose to form negatively charged sulfonic groups and a stable gel. Otherwise, upon hydrolysis with hydrochloric acid, uncharged nanocellulose particles tend to flocculate in aqueous dispersions [69].

Recently, oxidizing agents such as 2,2,6,6-tetramethylpiperidine-1-oxyl radical (TEMPO) and phthalimide-N-oxyl (PINO) have been used to obtain NC. They improve the environmental friendliness and shorten the duration of the nanocellulose production process compared to hydrolysis, but have a higher cost than the above acids [70–72].

Enzymatic methods are based on the biosynthesis from monosaccharides or decreasing the size of the cellulose fibers by the fermentation. The enzymatic methods are time-consuming and require reagents that are more expensive. However, preliminary treatment of cellulose by enzymes before the mechanical grinding can decrease the energy consumption required for preparation of NC [73, 74]. For these reasons, a pre-treatment of the fibrous material is usually performed in order to decrease the size of the cellulose fibers and to ease the fibrillation and the process of nanocellulose preparation. The method of NC production by combining mechanical, chemical or biological pretreatment with homogenization treatment can not only reduce energy consumption, but also obtain NC with controllable size [75].

The various types of NC can be classified into different subcategories based on their shape, dimension, function, and preparation method, which in turn primarily depend on the cellulosic source and processing conditions [69].

Different terminologies have been used for the various types of NC. The Technical Association of the Pulp and Paper Industry (TAPPI) proposed standard terms and their definitions for cellulose nanomaterial WI 3021, based on the NC size [76]. NC is categorized into following kinds, such as cellulose nanofibrils (CNF), cellulose nanocrystals (CNC), amorphous nanocellulose, and cellulose nanoyarn [78]. CNF consist of a network of intertwined elementary nanofibrils, consisting of alternating crystalline and amorphous areas. CNF particles are

*Novel Nanomaterials*

The fibers in it are shorter than the fibers of the second category and softwood, the length of the fibers of which is 0.3 ˗ 2 mm. Fibers of the second category of plants contain 55 ˗ 85% of cellulose, 1 ˗ 10% of lignin and have stronger and longer (larger than 5 mm) fibers [32, 33]. For most annuals, the average fiber length is close to the length of wood fibers of some deciduous species, but less than the average length of coniferous wood fibers and some industrial crops. The fiber width of annual plants is 2 ˗ 3 times thinner than the fibers of coniferous wood, but the ratio of the length of the fibers to their width in annual plants and in wood have the same order [34]. The chemical composition of the main components of NWPM differs from coniferous and deciduous wood. The content of cellulose, as the main component of raw materials, varies in wide ranges of values from 26% (bamboo) to 98% (cotton). Nonwood plant materials are distinguished by a high content of hemicellulose, especially pentosans to 30% [35]. Most NWPM have a lower lignin content from 6% (hemp) to 24% (bagasse) compared to wood (to 34%), which indicates the possibility of their use for pulp extraction [36]. Lignin of NWPM consists of guaiacylpropane, syringylpropane and oxyparaphenylpropane structural units, connected by simple ether and carbon–carbon bonds [37]. NWPM contain more minerals, but less lignin than wood, which a priori gives reason to expect a lower consumption of reagents for their

delignification in comparison with the production of pulp from wood.

tally friendly organosolvent methods of delignification [44].

require additional technological solutions [45, 46].

Properties of NC particles depend on the properties of plant raw materials and methods used for their production [38]. In world practice, for the production of NC, pulp with a minimum content of lignin, mineral and extractive substances, is usually used as a feedstock [39–42]. In this case, such pulp is obtained either by traditional methods of cooking with subsequent bleaching [43], or by environmen-

In the global practice of pulp and paper industry, the dominating technologies to

obtain pulp are sulphate and sulfite methods, which lead to environmental pollution [45]. Cooking pulp from NWPM in alkaline liquor is predominant because lignin from NWPM has a lower molecular weight than softwood lignin [37]. During cooking in an alkaline solution, the main ingredient of hemicellulose, xylan, is easily dissolved, which also opens additional channels for the cooking solution to penetrate into lignin, thereby facilitating the removal of lignin from the cell wall [46]. Alkaline methods for producing pulp from non-wood plants also include the NACO method based on alkaline oxygen cooking and the SAICA method. Experiments have shown that NACO-derived straw pulp has a lower yield and is inferior to the physico-mechanical properties of soda pulp and pulp obtained by the

Alkaline methods include the neutral-sulfite method, which is used to obtain high yield pulp from hardwood and annual plants. Neutral sulfite pulp in comparison with sulphate pulp with the same degree of delignification has a 3–5% higher yield from plant raw materials due to less destruction of hemicelluloses and, therefore, is easier to grind [49]. It should be noted that among the main problems of organizing the process of obtaining pulp from NWPM by alkaline methods is the high content of silicates in them, which turn into black liquor during cooking and

Increased environmental requirements to the quality of wastewater and gas emissions of industrial enterprises requires the development of new technologies for processing of plant raw materials with the use of different organic solvents [50–52]. Organic solvents used in organosolvent methods of delignification differ

**2. Preparation of nanocellulose**

SAICA method [47, 48].

**64**

10–60 nm in width, 500–2000 nm in length, and high aspect ratio L/D > 50 [77]. CNF is usually obtained by some kind of mechanical treatment of softwood pulp without any pretreatment or after chemical or enzymatic pretreatment. CNC particles are extracted from pulp, usually by hydrolysis, and have a diameter from 4 to 20 nm, a length 100–300 nm, and low aspect ratio L/D > 5 [78].

The hydrolysis of cellulosic materials remains the most common commercialscale CNC production method. The CNC yield after acid hydrolysis of pulp is 30–50% [62], and CNC films have brittle and rigid characteristics, which limits its use, for example, in flexible electronics. CNF has higher yield, good strength and good elasticity [79]. The combination of the intrinsic strength of CNF particles with the strong interaction between nanoparticles during drying makes it possible to obtain a more rigid and flexible film from CNF than from CNC [80].

Typically, higher acid concentrations, longer reaction times, and higher temperatures lead to higher surface charge and narrow sizes, but to lower yield and decreased crystallinity and thermal stability of cellulose nanocrystals [81].
