Cellulose Nanocrystals - American Chemical Society

Chapter 17

Downloaded by UNIV OF ILLINOIS URBANA on October 1, 2014 | http://pubs.acs.org Publication Date (Web): July 22, 2014 | doi: 10.1021/bk-2014-1162.ch017

Cellulose Nanocrystals: A Potential Nanofiller for Food Packaging Applications Prodyut Dhar, Umesh Bhardwaj, Amit Kumar, and Vimal Katiyar* Department of Chemical Engineering, Indian Institute of Technology Guwahati, Guwahati-781039, Assam, India *E-mail: [email protected].

This chapter focuses on emerging technologies developed for the fabrication of cellulose nanocrystal (CNC) based composite films for food packaging applications. Due to its strong reinforcing effect, CNC is a promising smart material for application in several fields such as healthcare, biomedical engineering, packaging etc. These bionanoparticles possess several attractive properties such as biodegradability, non-toxicity and bio-based origin. Fabrication of CNC based polymeric films via industrially viable approaches is a challenging task. Therefore, novel strategies for surface modification and innovative fabrication techniques need to be developed to effectively disperse CNCs into polymeric matrices. CNCs, due to their bio-origin and renewability, are emerging nanomaterials for the 21st century, whose demand will continue to grow in the near future. In response, robust, cost effective, high volume industrial scale production processes for CNCs are required to meet the growing demand.

Introduction Cellulose is a unique, abundantly available biomaterial which possesses several favorable properties such as renewability, biodegradability and non-toxicity. It is a polysaccharide consisting of repeating β-D-glucopyranose © 2014 American Chemical Society In Food Additives and Packaging; Komolprasert, V., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2014.


units with three hydroxyl groups per anhydrous glucose unit giving it a high functionality. Cellulose has widely been used either in pure form or as derivatives for fabrication of wide array of products in several industries such as food, pharmaceutical etc (1). Aqueous colloidal solution of cellulose micelles was first obtained by Ranby in 1950s through sulfuric acid treatment (2). TEM images of dried powdered nanocellulose having needle shaped morphology were first reported by Mukherjee et al. in 1953 (3). The terminology “nanocellulose” was first coined by Turbak, Snyder and Sandberg in the late 1970s at the ITT Rayonier Lab in Whippany, New Jersey, USA to describe the gel-like product formed by wood pulp homogenization at high temperature and high pressure (1, 4). In the early 1980s, several patents were granted to ITT Rayonier detailing the preparation and application of these cellulosic nanomaterials (1). Acid hydrolysis of cellulose fibers has been found to produce rod-like cellulose particles of nanometer dimensions, called cellulose nanocrystals (CNCs). Several terms are employed in the literature when referring to CNCs such as nanowhiskers, nanocrystals, nanoparticles, micro crystallites or nanofibers. These rod-like cellulose nanoparticles possess many unique morphological aspects such as nanoscale dimension, high aspect ratio, high surface area as well as favorable properties such as high specific strength, unique optical transparency etc. Due to its attractive physicochemical and structural properties, CNC has received considerable interest from academia and industry. Therefore, at present, significant research is being undertaken on CNC based technologies. When incorporated into polymer matrix, CNCs can improve the mechanical properties of the neat polymer. Moreover, CNCs can also intercalate into the polymer matrix, resulting in the improvement of the water vapor and oxygen barrier properties. This may be attributed to the creation of torturous pathways by the dense polymer-CNC network that hinders the diffusion of small gas molecules. Biopolymers and biodegradable polymers (biopolymers that are biodegradable) are currently being explored as alternatives to conventional polymers which usually possess high water and/or gas permeability which may be undesirable for several applications. Incorporation of CNCs into such biopolymers is expected to significantly improve their gas barrier properties without altering the biodegradability. In fact, CNCs have been reported to decrease the oxygen permeability of biopolymers such as poly (lactic acid) (PLA) significantly which makes CNCs a potential filler for use in packaging applications (5). In the twentieth century, there have been significant advancements in the packaging industry and a rapid increase in plastic use has been observed, especially in food packaging applications. During the storage of raw or minimally processed food for long periods of time, there is a risk of biofilm formation due to microbial contamination, oxidation, surface dehydration etc. The safety and quality of polymer based packaged food may be compromised not only by significant permeation of oxygen, water vapor and other gases, but also by migration of potentially toxic chemicals from the packaging material to the food product. Biodegradable polymers are defined as those that undergo mineralization by microbial chain scission under specific conditions in terms of pH, temperature, humidity etc. Such environment friendly polymers can be synthesized from petrochemical precursors (e.g. polycaprolactone), obtained from bio-sources 198 In Food Additives and Packaging; Komolprasert, V., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2014.


such as corn, wood etc. (e.g. cellulose) or synthesized by bacterial fermentation (e.g. polyhydroxyalkanoates). Figure 1 provides a classification of biodegradable polymers based on their source of origin.

Figure 1. Potential sources of degradable polymers. This chapter focuses on the recent developments in fabrication of biopolymer/CNC based (green) nanocomposites, and discusses the possibility of their application in food packaging. The chapter begins with a general discussion of biopolymer nanocomposites having potential food packaging applications. Next, various methods of acid based CNC synthesis and surface modification of CNCs are described. Thereafter, several biopolymer/CNC nanocomposites are discussed and properties of CNCs relevant to food packaging applications are detailed. Next, a technical discussion on different methods for CNC based film preparation and their potential for scale-up to industrial scale is discussed. Finally, different scale-up strategies and troubleshooting practices for CNC production are discussed keeping in mind that industrial scale production of the material will be necessary in the next few decades.

Biopolymers for Food Packaging Applications The success of conventional non-biodegradable polymer nanocomposites coupled with the problems associated with their proper disposal has stimulated new research on bio-based nanocomposites having a biodegradable polymer matrix. Innovations in the development of economically and ecologically 199 In Food Additives and Packaging; Komolprasert, V., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2014.

attractive green materials from biodegradable polymers and their widespread adoption will lead to preservation of fossil based raw materials. Moreover, use of such polymers will lead to complete biological degradation of bioplastics in due period of time through natural ecological cycle. So far, the most studied biodegradable polymers for the fabrication of nanocomposites are PLA, starch, cellulose, PHB, chitosan etc. Potential of these bio-based materials towards fabrication of bio-based nanocomposites for food packaging applications will be briefly discussed next.


Starch-Based Nanocomposites Starch is a promising bio-based, renewable raw material because of its easy availability, low cost and complete biodegradability (6). When mixed with synthetic polymers, it has the ability to promote the degradation of the article. Starch does not form films with high mechanical strength and requires proper plasticization agent or chemical modifications. Glycerol and other low molecular weight poly- hydroxyl compounds, polyether, urea and water are common plasticizers for processing of starch. Thermomechanically extruded starch in the presence of plasticizers is referred to as thermoplastic starch (TPS). Starch suffers from several drawbacks such as hydrophobicity, change in water content and performance changes during processing (7). To overcome these limitations, several different film fabrication techniques, starch nanoparticle synthesis, and chemical modification of starch have been reported (8–10). Starch based composites, in the form of films or bag, can be employed as packaging for fruits and vegetables, snacks, dry products or as adsorbent pads for meat exudation due to its hygroscopic nature. De Carvalho et al. were the first to provide insight into the preparation and characterization of thermo plasticized starch-kaolin composites by melt intercalation technique (11). Starch and cellulose based poly(L-lactic acid) (PLLA) nanobiocomposites are of interest to researchers because of the improvement in degradation properties over neat PLLA (12). Park et al. (13) compared the thermal degradation of PLLA and PLLA/starch nanobiocomposites, and observed a shift in the thermal degradation range upon addition of starch. For neat PLLA, the onset of degradation was observed at 310°C and degradation was complete by 400°C. After addition of starch, the degradation temperature of PLLA/starch nanobiocomposite decreased to 220-230°C while near-complete degradation was observed between 280°C and 340°C. Further, increasing the starch content increased the moisture absorption capacity (6-8% compared to 1% for neat PLLA) and crystallinity indicating that starch acts as a nucleating agent. Changing the plasticizer used in PLLA/starch composite modifies the mechanical properties such as tensile strength, percentage elongation at break, and modulus of the nanobiocomposite (14). Hydrophobicity of starch based films and their poor mechanical properties can also be improved by fabrication of nanobiocomposite of TPS and nanocellulose fibers (NCF) (12, 15). Savadekar et al. (16) observed that the tensile strength of the base polymer film increased upon addition of NCF. Maximum tensile strength of the film was observed at 0.4% NCF loading. Significant decrease in water vapor 200 In Food Additives and Packaging; Komolprasert, V., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2014.

transmission rate (WVTR) was observed in 0.4% NCF/TPS films (4.3 × 10−4 g/h/sq m as compared to 7.8 × 10−3 g/h/sq m for neat TPS films). Moreover, oxygen transmission rate (OTR) reduced by 93% in 0.4% NCF/TPS films compared to neat TPS films.


Cellulose-Based Nanocomposites Cellulose is one of the most promising natural raw material and constitutes the most abundant renewable polymer resource available today. Cellulosic materials when subjected to acid hydrolysis (Figure 2) yield defect-free, crystalline CNC residues. CNCs possess many favorable characteristics such as nanoscale dimension, high specific strength and modulus, high surface area, unique optical properties etc. These properties make them a promising material for various applications such as fabrication of polymer nanocomposite materials and films, drug delivery, protein immobilization and metallic reaction template (17). Polymer matrix gets transformed when pooled with cellulose nanocrystals. The resulting nanobiocomposite has enhanced mechanical, thermal, barrier and antibacterial properties along with greater ease of degradability.

Figure 2. Acid hydrolysis to form cellulose nanocrystals.

CNCs improve the barrier properties (such as OTR and WVTR) as well as mechanical properties (such as Young’s modulus and strength) of biodegradable polymers such as PLA (18, 19). A detailed discussion on the effects of CNC reinforcement and barrier properties is provided later in the chapter. Cellulose acetate (CA), cellulose acetate propionate (CAP), and cellulose acetate butyrate (CAB) are thermoplastics produced through esterification of cellulose (20). Among the different derivatives of cellulose, cellulose acetate (CA) is of particular interest because of its biodegradable nature, excellent optical clarity and high toughness. Cellulose ester powders derived from different raw materials such as cotton, recycled paper, wood cellulose and sugarcane in the presence of different plasticizers and additives are melt processed via extrusion to produce commercial pelletized cellulose plastics (21). 201 In Food Additives and Packaging; Komolprasert, V., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2014.


Gelatin-Based Nanocomposites Gelatin is prepared by the thermal denaturation and hydrolytic cleavage of collagen, isolated from the skin of animal and fish, and animal bones using very dilute acid (22). Gelatin contains a large number of glycine, proline and 4-hydroxyproline residues. Gelatin is a heterogeneous mixture of single- or multi-strand polypeptides with extended left handed proline helix conformations of 300-400 amino acids (23) with a typical structure –Ala–Gly–Pro–Arg–Gly–Glu–4Hyp–Gly–Pro–. Gelatin can be used as biopolymer in tissue engineering as well as in edible coatings as it reduces oxygen, moisture and oil migration, and can be loaded with antioxidant or antimicrobial agents. However, the major limitation of gelatin with regards to packaging applications is its poor mechanical strength (22). Grapefruit seed extracts (GFSE) derived from the seed and pulp of grapefruit is a potential additive to gelatin based polymers because it is non–toxic and has been reported to increase shelf life by inhibiting the growth of food borne pathogens. Polymer films coated with GFSE layer using polyamide as binder have shown antimicrobial activity against a variety of microorganisms. These gelatin based GFSE composites find application in packaging of beef and fish products (24). Barley bran (BB), a byproduct of the barley powder manufacturing industry, can be used for protein film preparation because of its low cost. BB films coated with GFSE have potential application in the packing of salmon, since GFSE decreases the peroxide value and thiobarbituric acid content (25). Concentration of the GFSE added does not affect the quality of the food product (25). The thermoreversible nature at its melting point, which is close to body temperature, makes gelatin a good base material for protein films. However, its large scale production possibilities are debatable due to high costs.

Cellulose Nanocrystals: Synthesis and Surface Modification CNCs are fabricated through stringent acid hydrolysis techniques, in which amorphous parts are degraded, leaving behind only the crystalline section of nanometer dimensions. CNCs fabricated from different biomasses have different morphologies and yields. This is because of the different proportions of cellulose, hemicellulose and lignin contents in biomass and interfacial binding between them, which makes it difficult to separate out pure cellulose during pretreatment. There have been several pretreatment procedures reported to date which are listed in Figure 3. The pretreatment process yields relatively pure cellulose pulp with trace amounts of lignin and hemicellulose. As trace amounts of these impurities significantly hinder the CNC fabrication process, proper pretreatment procedure should be selected depending on the biomass type. Figure 4 shows a detailed schematic diagram of CNC production through different pretreatment routes. The different types of acids used for the fabrication of CNCs, significantly alters the stability of the colloidal suspension and physical properties. The choice of biomass source and hydrolyzing acid is important in the optimization of CNC synthesis. 202 In Food Additives and Packaging; Komolprasert, V., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2014.


Figure 3. Different biomass pretreatment methods for cellulose extraction.

Figure 4. Overview of the general pathway followed for CNC production. 203 In Food Additives and Packaging; Komolprasert, V., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2014.


Effect of Bio-Feedstock on Cellulose Hydrolysis

Production of CNC is a complex process which is influenced by several process parameters viz. acid concentration, biomass content, presence of lignin and hemicellulose, process followed for biomass pretreatment, temperature and reaction time. Yield of CNC from cellulose hydrolysis strongly depends on the biomass type, impurities present as well as on the biological origin (26). The degree of crystallinity of cellulose in the biomass/microorganism varies widely with species and natural location and is a critical parameter for the determination of the dimension and yield of nanocellulose. Cellulose, especially from algal and bamboo source, is highly crystalline in nature which hinders the penetration of acid to deep crystalline regions leading to CNCs that are several micrometers long (27). Cellulose from cotton and wood, having lower crystallinity, yields much shorter dimensions of CNCs. Moreover, CNC generation from different biomass and waste products makes it a valuable nano-product with complete biodegradability and recyclability (28). CNCs are most frequently produced at lab scale using filter papers with sulfuric acid (64 %) at room temperature with a yield of

Cellulose Nanocrystals - American Chemical Society

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