**3. Modification of cellulose and cellulose derivatives**

Cellulose is often modified via physical and chemical techniques to increase their functionality responsive to the film application and industry. Physical methods refer to approaches, which do not largely rely on the use of chemical compounds to modify cellulose fibers. The physical modification section covers the use of techniques that utilize mechanical, thermal, electrical, and other high-energetic (e.g., gamma irradiation, UV light) processes. The chemical modifications section covers common techniques to obtain traditional and novel cellulose derivatives including hydrolysis, esterification, acetylation, etherification, silylation, carbamylation, TEMPO oxidation. A summary of the modification methods for obtaining cellulose derivatives is illustrated in **Figure 1**.

#### **3.1 Physical modifications**

*Thermal treatment* can be used as a pre-treatment or post-treatment to remove undesired substances, enhance the oxygen-carbon ratio, and improve crystallinity of cellulose fibers. These modifications can improve the gas barrier properties and increase the acidity of the fiber surfaces used to produce films. For example,

heating (up to 145°C) tempo-oxidized cellulose nanofiber films was shown to significantly reduce the water retention and oxygen permeability as compared to original films prepared without post-thermal treatment [15]. Moreover, heating can help in inducing the closure of pores between cellulose nanofibers. This can be explained with the reorientation of amorphous and paracrystalline regions into the crystalline regions [16].

*Plasma surface treatment* can be used to enhance the hydrophilicity to cellulose fibers, and subsequently to provide functional properties to their films, such as better oxygen and air permeability. This technique utilizes electric discharge to generate vacuum-based ionized gas at ambient temperatures. The ionized gas can be generated via low pressure, low temperature, atmospheric glow discharge, atmospheric pressure plasma jet technique. The ionized gas includes a mixture of free radicals, heavy particles, and electrons, which promote the modification cellulose fibers and fiber aggregates as in films. The modifications can be in the form of defibrillation, remove low molecular weight impurities from nanocellulose fibers, and enhance microcrystalline cellulose [17]. Plasma treatment can increase the hydrophilicity of the fibers by generating polar functional groups, which provides good adhesion of cellulose coating layers. Plasma treatment was also shown to increase the crosslinking and entanglements of cellulose nanofibrils, increase the extent of impermeable crystals, which hinders the molecular diffusion of oxygen to increase the oxygen barrier properties of microfibrillar cellulose films [18].

*The corona discharger* process is similar to plasma surface treatment and involves the use of a bundle of charged particles, such as ions and electrons, accelerated through an electric field. The ionization process involves a shade of plasma that occurs at the tip of the particle generator that is named corona. This method has several advantages such as high efficiency, continuous processing and being environmentally friendly [19]. Cellulose fibers exposed to electromagnetic field with high voltage become excited and available for further surface modification via oxygenation. Electric discharge induces oxygenation of ozone into carbonyl groups that brings about crosslinking between fibers and the media to improve the mechanical and thermal properties of cellulose fiber films. The corona treatment process was shown to improve the interfacial adhesion between cellulose fiber and polypropylene in laminated film applications in addition to improved film strength and rigidity [19]. In parallel, corona treatment was also shown to enhance the strength and thermal resistance of cellulose films [20].

*Gamma irradiation* is a non-thermal and environmentally friendly process that modifies the chemical structure of cellulose fibers via high energetic gamma rays. High energy gamma irradiation results in decomposition and crosslinking, thus strengthening the structure of cellulose fibers [21]. Gamma rays typically are generated by using Cobalt-60 or Cesium-137 sources at specific irradiation facilities, which constitute a major limitation for its availability and wide-spread use. The irradiation process has several advantages over traditional treatment methods, such as no need for catalysts, less atmospheric pollution, minimum time for the process and continuous operation. Irradiation may cause initial structure disruption and irreversible bond cleavage, thus fragmentation of the polymers into nanofibrils. The high energetic of the gamma irradiation can result in crosslinking of cellulose nanofibrils into fiber networks and gels without the use of chemical reagents.

The mechanism involves the formation of macro-cellulosic radicals by the removal of hydrogen-hydroxyl and disturbance of carbon-carbon bonds through high energy gamma rays. These radicals are very reactive to facilitate cross-linking between cellulose nanocrystals and result in strong cellulose nanocrystal films with desirable oxygen and water vapor resistance [22].

*UV radiation* treatments is the application of electromagnetic radiation at wavelengths from 100 nm to 400 nm. This method uses ultraviolet laser light source to treat cellulose with enough energy to modify the molecular bonds on the surface layer. Briefly, UV radiation increases the formation of carbonyl groups on cellulose fiber surface and increases the surface polarity. In addition to this, the strength and gas barrier ability of cellulose films and sheets can be increased by promoting crosslinking between fibril units. For example, cellulose sheets undergone through UV radiation showed increased tensile strength since UV radiation enables intercross linking between the neighboring cellulose molecules [23].

*Homogenization and related mechanical processes* including spinning, flow-focusing, microfluidization, and ultrasonication can modify the structure of cellulose fibrils into microfibrils and nanofibrils through mechanical breakdown. Cellulose filaments are typically continuous fibers of random length. Cellulose microfibrils, on the other hand, are smaller fiber agglomerates with diameters ranging from 3.5 nm to 35 nm. Nanofibrils are smaller than microfibrils in width nanometric range and the length of micrometer range. Cellulose microfibrils store three different domains, namely, noncrystalline, paracrystalline and crystalline regions. The former chains include regions without regular packing of cellulose chains. The paracrystalline domain has partial crystal distortion and loose molecular packing. As its name implies, the crystalline region takes shape with highly ordered chains. Furthermore, additional networks

#### *Advances in Cellulose-Based Packaging Films for Food Products DOI: http://dx.doi.org/10.5772/intechopen.110817*

such as hydrogen-bonded bundles or microfibrils can be observed on microfibril surfaces due to hydroxyl groups. Cellulose micro or nanofibril units possess large surface area and strength. Hence, films made from cellulose micro/nano fibrils are known to have low thermal expansion coefficient, high transparency and good mechanical characteristics [24]. Cellulose nanofibril films may even show better oxygen and water vapor barrier ability than some petroleum-based polymers [25]. Furthermore, mechanical properties are important for handling and packaging applications to interpret the durability and sustainability of materials. Cellulose nanofibrils mixed with other polymers can exhibit good mechanical features such as high elasticity and tensile strength [26]. Application of high-shear and high-pressure processes (e.g., homogenization and microfluidization) on cellulose dispersions dates back to couple of decades. These techniques can be used during extraction to obtain microfibers. For example, They can provide sufficient mechanical force to breakdown native cellulose fibers in the pulp into microfibrillated cellulose gels [27]. Another research demonstrated the use of microfluidization in combination with enzyme treatment to obtain nanofibrillated cellulose from palm fruit peels [28, 29]. Extrusion and flow spinning were used in a similar way to modify cellulose fibers in their solution, which is extruded through a spinneret and rotating godets with a certain velocity and pressure. The cellulose solution pushed through the axial direction, generates a highly oriented form. These cellulose filaments contain highly crystalline and dense microfibrillar structure leading to strong and moisture resistant film materials [30].

## **3.2 Chemical modifications**

#### *3.2.1 Hydrolysis*

Hydrolysis involves partial depolymerization of cellulose by using dilute mineral acid. Microcrystalline cellulose (MCC) constitutes the micron scale highly crystalline regions of cellulose microfibrils. MCC can be generated by the hydrolysis of partially depolymerized α-cellulose using hydrochloric acid. MCC is preferred as a biocomposite in various application areas, such as medicine and pharmacology due to its biodegradability, physicochemical stability, low density, elasticity, and large surface area. MCC was first discovered and commercialized as Avicel® since early twentieth century. Commercial production is conventionally from softwood, hardwood, cotton stalks, soybean husks or rice husks. MCC dispersions are decomposed using sulfuric acid to obtain hydrophobic or hydrophilic CNC depending on the solvent polarity. In parallel to MCC, nanocrystalline cellulose (or cellulose nanocrystals, CNC) are rod-shaped highly crystalline units of cellulose microfibrils at nanoscale dimensions. Processes similar to MCC are involved in production of CNC by hydrolysis of native cellulose commonly from algae, bacteria and cotton linter. CNC enjoys optical transparency, light weight, biocompatibility and low thermal expansion for uses in biomedical and packaging applications. There are examples of manufacturing packaging films from CNC using layer by layer deposition or spray drying techniques. The former method is based upon adsorption of the oppositely charged polyelectrolytes to create uniform nanocellulose multilayers [31].

#### *3.2.2 Esterification*

Esterification or acylation process involves the combination of organic acid and alcohol to form an ester and water. Esterification reactions can occur on cellulose

polymer chains or the outer surface of cellulose fibers by aromatic and aliphatic reagents. Cellulose esters can be classified as organic (e.g., cellulose acetate, cellulose acetate butyrate and cellulose acetate propionate) and inorganic (cellulose nitrate, cellulose phosphate, cellulose sulfate, and cellulose xanthate) esters.

*Cellulose acetate* is a well-known and one of the most versatile cellulose derivatives. It can be obtained by the reaction of cellulose with acetic acid, acetic anhydride and sulfuric acid as catalysts. Degree of substitution with the acetate groups is the average number of acetyl groups replacing the hydroxyl groups per glucose unit, and it determines the solubility of cellulose acetate in water. The maximum degree of acetylation is three, when all OH groups are replaced by acetyl groups, which has the highest water solubility. Cellulose acetate has a great commercial appeal due to its versatility related to its solubility in various solvents and toughness for mechanical strength. Therefore, it found applications in distinct industry segments which require either flexible and resistant material as in photography films or rigid and durable materials produced by molding such as in glass frames, combs, tool handles, solvent reservoirs, etc. Therefore, it is not surprising to find packaging applications of cellulose acetate due its high chemical and thermal stabilities, dense and strong structure, and low cost [32]. The versatility of cellulose acetate allows its adoption to different film production technologies [6]. Electrospinning was used a promising and affordable method to manufacture packaging material. This technique relies on the application of an electrical potential to create thin cellulose fibers extruded through a nozzle [33].

*Cellulose sulfate* is an inorganic derivative produced with the addition of sulfuric acid, sulfur trioxide or chlorinated sulfonic acid to cellulose fiber. Esterification can occur via two strategies. Heterogeneous sulfation includes SO3 complexes such as DMF as solvent, pyridine as sulfation agent and H2SO4 as the reactant. The sulfation of cellulose results in chain cleavage and produces cellulose sulfate with a high degree of substitution. Cellulose sulfate produced with heterogeneous method may have non-uniform distribution of crystalline and amorphous regions and limited solubility in water. In contrast, homogeneous sulfation requires N2O4-DMF and SO3 complexes for the synthesis of cellulose sulfate. It generates even distribution of crystalline and amorphous regions, which results in higher solubility in water and low-chain degradation. When used in packaging manufacturing, cellulose sulfate can represent antimicrobial characteristics. In addition, cellulose sulfate films are recognized as biodegradable and biocompatible, different from cellulose acetate.

*Cellulose phosphate* is an inorganic cellulose derivative generated by phosphorylation of cellulose fibers using phosphor-containing esters such as phosphonites, phosphites, or phosphinates. Similar to sulfation, phosphorylation may take place under heterogeneous or homogeneous conditions. Heterogeneous phosphorylation of cellulose fiber occurs in the presence of solvents such as *N*-methylmorpholine *N*-oxide or DMF. The homogeneous approach results in complete or partial esterification of cellulose fiber to yield a soluble product. Unlike cellulose sulfate, cellulose phosphate is largely water-insoluble due to polymer chain crosslinking. This may result in coagulation in the solution and serves as a limitation for applications that require a free-flowing melt, such as packaging [7].

*Cellulose nitrate* is the poly-nitrate ester of cellulose obtained by replacing hydroxyl groups with nitrates using nitrating acid mixtures. Cellulose nitrate demonstrates odorless and tasteless characteristics and is typically soluble in organic solvents (e.g., alcohols), which allow it to be used in various areas, such as packaging film, membrane filters, pharmaceutical, and optical instruments. The degree of

#### *Advances in Cellulose-Based Packaging Films for Food Products DOI: http://dx.doi.org/10.5772/intechopen.110817*

nitration is an important parameter for cellulose nitrate that shows the volume of nitric oxide release when 1 g of cellulose nitrate is completely decomposed at 0°C and 1 atm. It influences the physicochemical properties of cellulose nitrate such as viscosity, solubility, heat of formation and combustion and thermal stability [34, 35]. Generally, lower degree of nitration (10–11%) is preferred for film manufacturing as it provides thermoplastic behavior and solubility in organic solvents, which are essential for film manufacturing [36].

*Cellulose xanthate* is a sodium hydroxide-soluble cellulose derivative. The control over the process is critical since industrial viscose rayon fabric (known as viscose) and cellophane are formed from xanthate. It is obtained by stepwise xanthation reactions, which involve alkalization using sodium hydroxide, followed by ripening to bring the degree of polymerization to 0.5 (i.e., to adjust the viscosity). After ripening, the solution is passed through a spinneret to create soft filaments [37]. Finally, the soft filaments are exposed to sulfuric acid and zinc, and sodium sulfate to obtain the rayon fiber filaments. Rayon fabric shows good plastic behavior with high mechanical strength, elasticity and strong moisture absorption. Degree of ripening and polymerization determine the strength of rayon fibers: higher DP and DR increased the strength of the films prepared by fiber spinning [38]. The high moisture absorption rate is related to their lower crystallinity that serves as a drawback for packaging films applications. Rayon fabric is commercially produced from wood pulp. The physicochemical features of the rayon fabric are close to fabrics produced from natural cellulose (e.g., cotton or linen). The viscose fibers show high resistance to acid and alkali conditions, and resistant to fabric and film manufacturing conditions [39]. Versatility and affordability allow viscose for its use in large-scale industrial textiles and food packaging, such as sausage casings [40].

#### *3.2.3 Etherification*

Etherification of cellulose involves homogeneous or heterogeneous reactions with etherifying agents, such as epoxides, halogenated alkanes, alkyl, silyl chlorides, bromides, or vinyl compounds. Cellulose ethers are classified as silyl ethers, ionic alkyl ethers and nonionic alkyl ethers and have alkyl halide structures with hydroxyl groups. They are among the most abundant cellulose derivatives and manufactured on large scales. The commercial forms used in various industries include methylcellulose, carboxymethyl cellulose, ethyl cellulose, hydroxyethyl cellulose, and hydroxypropyl cellulose. Cellulose ethers represent non-toxic, tasteless and non-flammable and water-soluble features. These properties enable them to be used as a thickener, water binder, processing aids in food formulations; as excipients in pharmaceutical formulations; and textile and packaging applications [41]. Their solubility depends on the pH of the solution: soluble in alkaline conditions with decrease in solubility with decreasing pH. They are also thermally stable up to 100°C, and form crosslinking network with slight degradation at 130–150°C [42].

*Methyl cellulose* is the nonionic alkyl ether that is produced with methyl chloride or dimethyl sulfate in an alkaline medium. Methyl cellulose demonstrates high solubility in water and organic solvents, good gelling capabilities and emulsifying power. Therefore, they are common ingredients in sugar-based syrups, oil emulsions, creams, food gels, baked goods, fried foods, and soups. Moreover, methyl cellulose has a potential in food packaging film industry as a plasticizer since they display low cost, transparency, high strength, easy handling and environmentally friendly structure that are important properties for packaging films [43].

*Carboxymethyl cellulose (CMC)* is an anionic cellulose ether derivative obtained by the reaction between alkali cellulose and sodium chloroacetate, where hydroxyl groups are replaced with carboxymethyls at 2nd, 3rd, and 6th carbon of a glucose unit. CMC is another traditional food additive as a thickener, stabilizer, emulsifier, bulking agent, and water binder related to its affordability, water solubility, chemical stability, and non-toxicity. It is a common ingredient in food gels, such as processed meat products, dry pet foods, yoghurt, beverages, ice cream and dessert. In addition to these, CMC can provide control to the viscosity of film-forming solution, and protective barrier when used in food packaging film applications [44].

*Ethyl cellulose* is a nonionic cellulose derivative obtained by the reaction between alkali cellulose and ethyl chloride between 90 and 150°C, where hydroxyl groups are converted into ethyl ether groups. This results in less polarity than CMC and methyl cellulose. Ethyl cellulose is soluble in various organic solvents such as hydrocarbons, alcohols, esters and chlorinated solvents. The physicochemical properties and applicability of ethyl cellulose depend on the degree of etherification. The conversion of hydroxyl groups into ethyl ether units provides water-solubility, adhesive features as well [45].

*Hydroxyethyl cellulose* is a nonionic derivative obtained by the reaction of cellulose with sodium hydroxide and ethylene oxide, where ethyl groups are attached to hydroxyl units. They show non-toxic, compressible, and water-soluble characteristics attractive for tissue engineering and biomedical applications. They are also widely used as a thickening, emulsifying agent and stabilizer in personal care and cosmetic products. It is not permitted to be used in food formulations due to its chemical structure, but it might be an alternative for food packaging films [46].

*Hydroxypropyl cellulose* is an alkali cellulose derivative obtained with the substitution of 2-hydroxypropyl chloride with hydroxyl groups. Similar to methyl cellulose, it shows gelling ability, however, at higher temperatures. Hydroxypropyl cellulose can be used as gelling agent, emulsifier or thickener in salad dressing, sorbet, and confectionary product fillings. Hydroxypropyl cellulose can also contribute to gas barrier properties oil-resistance of packaging films [47, 48].

#### *3.2.4 Silylation*

Silylation is based on incorporation of multifunctional silane compounds (i.e., methyltrimethoxysilane, aminopropyltriethoxysilane, methacrylopropyltrimethoxysilane alkoxysilane) into cellulose fibers or micro- and nano-crystals. Silyation provides unique characteristics depending on the specific groups in the glucose unit (i.e., ∙OH, ∙CH or ∙COOH). For example, cellulose nanocrystals can be partially silylated using n-dodecyldimethylchlorosilane as a silylating agent and acetone as the solvent to allow surface modification. The silylation process can effectively enhance crystallinity, crosslinking, and strength of cellulose fibers. For example, partially silylated cellulose nanocrystals exhibited improved degree of crystallinity and tensile strength [49]. Similar to other cellulose derivatives, when used in film formulations, silylation can improve their functional properties, such as water vapor barrier and thermal resistance [50].

#### *3.2.5 Carbamylation*

Carbamylation of cellulose occurs by the reaction of hydroxyl group of cellulose with isocyanate. Carbamylation of nanofibers can increase their reactivity and

thermal stability. The carbamylation of cellulose modifies the polarity of cellulose fibers. For example, hydrophobicity of cellulose nanocrystals increased by crosslinking phenyl isocyanate with hydroxyl groups, while crystallinity and other physicochemical characteristics of cellulose nanofibers remained unchanged [51].

A major limitation in food and packaging applications is related to the use of isocyanate that can create toxic byproducts during the carbamylation process.

#### *3.2.6 TEMPO oxidation*

The free radical TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl) is a water soluble and relatively stable nitroxyl radical (i.e., half-life in minutes at room temperature). It can catalyze the oxidation of primary hydroxyl groups of cellulose fibrils to carboxyl form. TEMPO reaction requires oxidizers such as NaClO or NaClO2 in the presence of NaBr at alkaline conditions. This process can provide electrostatic repulsion between cellulose fibers by preventing hydrogen bonding. TEMPO-oxidized cellulose fibers can be used to produce films with improved network structure, optical features, gas barrier and thermomechanical characteristics. For instance, the films prepared from TEMPO-oxidized wood cellulose nanofibers exhibited improved degree of fibrillation and low oxygen permeability, desirable optical transparency, and high tensile strength and Young's modulus in their films [52]. Similarly, softwood and hardwood celluloses oxidized by TEMPO displayed transparent, flexible and low thermal expansion coefficient with higher crystallinity compared with untreated cellulose nanofiber films [53].
