Nanocellulose-Polymer Composites for Applications ...

Polymer-Plastics Technology and Engineering

ISSN: 0360-2559 (Print) 1525-6111 (Online) Journal homepage: http://www.tandfonline.com/loi/lpte20

Nanocellulose-Polymer Composites for Applications in Food Packaging: Current Status, Future Prospects and Challenges A. K. Bharimalla, S. P. Deshmukh, N. Vigneshwaran, P. G. Patil & V. Prasad To cite this article: A. K. Bharimalla, S. P. Deshmukh, N. Vigneshwaran, P. G. Patil & V. Prasad (2017) Nanocellulose-Polymer Composites for Applications in Food Packaging: Current Status, Future Prospects and Challenges, Polymer-Plastics Technology and Engineering, 56:8, 805-823, DOI: 10.1080/03602559.2016.1233281 To link to this article: http://dx.doi.org/10.1080/03602559.2016.1233281

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Date: 28 March 2017, At: 23:31

POLYMER-PLASTICS TECHNOLOGY AND ENGINEERING 2017, VOL. 56, NO. 8, 805–823 http://dx.doi.org/10.1080/03602559.2016.1233281

Nanocellulose-Polymer Composites for Applications in Food Packaging: Current Status, Future Prospects and Challenges A. K. Bharimallaa, S. P. Deshmukhb, N. Vigneshwaranc, P. G. Patila, and V. Prasada,c a

Technology Transfer Division, ICAR-Central Institute for Research on Cotton Technology, Mumbai, India; bDepartment of General Engineering, Institute of Chemical Technology, Mumbai, India; cChemical and Biochemical Processing Division, ICAR-Central Institute for Research on Cotton Technology, Mumbai, India ABSTRACT

KEYWORDS

Nanocellulose has potential applications across the several industrial sectors and addresses a lot of issues related to environmental concern. As biodegradable filler in composite manufacturing, coating, and self-standing thin films, it offers novel and promising properties. Very few available reviews report on nanocellulose-impregnated composite materials for food packaging. Nanocellulose reinforcement is found to be promising for mechanical and barrier properties of composite for biopolymer and synthetic polymer. In this paper, we provide a thorough review of recent advances of nanocellulose synthesis and its application as a filler material for production of nanocomposites to be used for food packaging.

Biodegradable composite; biopolymer; food packaging; green composite; nanocellulose

GRAPHICAL ABSTRACT

Introduction Packaging plays the most viable and catalytic role in a modern economy. In the world scenario, the total turnover of packaging industry is about $550 billion, where Indian share is about $24.6 billion per annum[1]. The future of global packaging industry is projected to increase at a rate of 3.3% on an average per annum until 2018, reaching a value of $975 billion[2]. The packaging industry is poised to grow rapidly led by the increasing

use of innovative packaging equipment and the rising flexible packaging market. Of the total packaging sector, the food and beverages occupy the largest share, accounting for 85% followed by plastic packaging market which is expanding rapidly with a growth of 20–25% per annum and is valued at 6.8 million ton. The paper packaging industry stands at 7.6 million ton, and 10% is occupied by pharmaceuticals. The global market value of food packaging is US$161 billion, beverage packaging

CONTACT Virendra Prasad vprasadbhu@gmail.com Chemical and Biochemical Processing Division, ICAR-Central Institute for Research on Cotton Technology, Adenwala Road, Matunga (East), Mumbai 400019, India. Color versions of one or more of the figures in this article can be found online at www.tandfonline.com/lpte. © 2017 Taylor & Francis

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A. K. BHARIMALLA ET AL.

is US$76 billion, pharmaceutical packaging is US$21 billion, cosmetic packaging is US$13.3 billion, and others is US$153 billion[3]. The substantial market growth potential for sustainable packaging materials is likely to change the way we look at demand for raw materials, how they are chosen, and how much is processed and consumed. This is due to the direct and indirect links of raw materials to overall sustainability, energy efficiency, and especially security of supply. In recent years, the application of biobased raw materials in preparation of composites has increased due to its availability as renewable materials and ecological benefits[4]. More use of these biobased materials will not only benefit the ecosystem, but would also lead to economic development for farming and rural areas in developing countries. Application of biopolymers as eco-friendly foodpackaging materials has been limited because of their poor mechanical and barrier properties, which may be improved by adding reinforcing compounds[5]. Polyethylene or copolymer-based materials are used as packaging materials for more than 50 years. These materials are not only safe, inexpensive, and versatile but also flexible. The strain and stress of environmental balance imposed by these conventional packaging materials have been a strong motivation for replacing plastic packaging materials by biodegradable materials from renewable sources during the last decades. Moreover, improving the barrier and tensile properties is a major challenge that has directed the focus toward nanotechnology and the potential of forming bionanocomposites by incorporation of nanosized reinforcing fillers in biopolymer matrices. The application of nanotechnology may open new possibilities for improving not only the properties but also the cost–price efficiency[6]. This paper summarizes a thorough review of recent advances of nanocellulose synthesis and its application as a filler material for production of nanocomposites to be used for food packaging[7]. In food packaging, decreasing water vapor permeability is another critical issue in the development of biopolymers as sustainable packaging materials. To improve the physical properties of packaging materials and increase the mechanical strength, thermal stability, physicochemical, and recyclability properties, a promising technology ‘Nanocomposite’ is used[6]. The properties of nanocomposites depend less upon their individual components, which are dissimilar on the nanoscale to control and develop new and improved structures and properties than whole materials[8]. Nanoclays, kaolinite, carbon nanotubes, and graphene nanosheets that are used as fillers were shown to have potentials to improve the ability of plastic packaging

against migration of gases and flavor compounds as well as boosting shelf life[9]. The recent increase in environmental awareness has contributed toward the development of new biobased edible packaging materials to extend shelf life and improve the quality of foods with reduced packaging waste[10]. Starch, as an example, has received considerable attention as a biodegradable thermoplastic polymer. ‘Active packaging’ a new innovation involves the combination of food-packaging materials with antimicrobial substances such as the incorporation of antibacterial nanoparticles into polymer films to control microbial surface contamination of foods. Viable edible films and coatings have successfully been produced from whey proteins, their ability to serve other functions, such as carrier of antimicrobials, antioxidants, or other nutraceuticals, without significantly compromising the desirable primary barrier and mechanical properties of packaging films, will add value for eventual commercial applications in food industries[11]. In addition, active packaging technology has attracted much attention from the food industry because of the increase in consumer demand for minimally processed, preservative-free products. The preservative agents may be applied to packaging materials in such a way that only low levels of preservatives come into contact with the food[12].

History of packaging industry In earlier days, packaging materials used were leaves, hollowed-out tree limbs, grounds, skins, reed baskets, and earthenware vessels as containers. With the development of civilization, more complex containers were used to meet specific needs followed by large ceramic vessels, amphorae from 1500 B.C. to 500 A.D. to ship wine and other products commercially throughout the Mediterranean. Since early man first used a variety of locally available natural containers to store and eat foods, significant developments in food-packaging materials have provided the means to lower the growth of microbes as well as protect foods from external microbial contamination. Packaging materials were developed over the years to prevent the deterioration of foods by microbes resulting from exposure to air, moisture, or pH changes associated with the food or its surrounding atmosphere[13]. Glassmaking that began in 7000 B.C. was first industrialized in Egypt in 1500 B.C. Paper (from stems of papyrus in ancient Egypt) is the oldest form of what is referred to as ‘flexible packaging’. Sheets of treated mulberry bark were reported to be used as a flexible packaging material by the Chinese to wrap foods as early as the first or second century B.C. and during

POLYMER-PLASTICS TECHNOLOGY AND ENGINEERING

the next 1500 years, the paper-making technique was refined and transported to the Middle East, then Europe, and finally into the United Kingdom[14]. Metal containers as packaging materials started from ancient boxes and cups, made from silver and gold, which were too valuable for common use. Cheaper metals, stronger alloys, thinner gauges, and coatings were eventually developed and mass produced. In the 1950s, the pop top/tear tab appeared and now tear tapes that open and reseal are popular[15]. The packaging industry is the largest user of plastics as more than 90% of flexible packaging is made of plastics, compared to only 17% of rigid packaging. Barrier resins are generally being used for plastic containers by modifications to improve product protection and make them more cost effective. Plastic materials made up of large, organic (carbon-containing) molecules are formed into a variety of useful products as they are fluid, heat sealable, easy to print, and can be integrated into production processes[16]. The use of plastics in packaging has increased worldwide with an estimate of 280 metric tons[17]. For packaging, molded deodorant squeeze bottles were introduced in 1947 and in 1958; heat shrinkable films were developed from blending styrene with synthetic rubber. Cellulose acetate was first derived from wood pulp in 1900 and developed for photographic uses in 1909. Moreover new manufacturing processes were developed using various methods such as forming, molding, casting, and extrusion to churn out plastic products in vast quantities[18]. Other cellophanes and transparent films have been used as outer wrappings to maintain their shape when folded. The polyethylene terephthalate (PETE) container became available during the last two decades with its use for beverages entering the market in 1977. By 1980, foods and other hot-fill products such as jams could also be packaged in PETE. In 1986, aluminum trays were replaced by plastic, microwavable trays. To reduce food waste, metallocene-catalyzed polyolefin was introduced in 1996. In 2000, polylactic acid from corn entered the packaging market signaling the return of biobased plastic[19]. There are new innovations to encourage active packaging which involves the combination of food-packaging materials with antimicrobial substances such as the incorporation of antibacterial nanoparticles into polymer films to control microbial surface contamination of foods. It was observed for both migrating and nonmigrating antimicrobial materials, an intensive contact between the food product and food-packaging material is required and therefore potential food applications include vacuum or skin-packaged products,

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e.g., vacuum-packaged meat, fish, poultry, or cheese. Nanocomposites are known to exhibit increased barrier properties, increased mechanical strength, and improved heat resistance compared to their neat polymers and conventional composites[6]. Cellulose and polylactic acid have received attention as sustainable, biocompatible, biodegradable materials with good mechanical and optical properties. Lactic acid, the monomer of PLA, may easily be produced by fermentation of carbohydrate feedstock such as corn. A successful polymer nanotechnology in food packaging will have to take into consideration the complete life cycle of the packaging material[20]. The life cycle assessment considers the overall impact on the environment from all the stages of raw materials sourcing to the production process, transportation, and delivery until it reaches end users and finally being disposed[21].

Polymer matrix Materials in a fibrous state exhibit good strength due to their bonding by a suitable polymer matrix[22]. The matrix holds the reinforcement fillers in place and acts as a binding agent. A good matrix possesses the ability to deform easily under applied load, transfer the load onto the fibers, and evenly distribute stress concentration. It also isolates the fibers from one another to prevent abrasion and formation of new surface flaws. Moreover the matrix redistributes the load to surrounding fiber fractures and laterally supports the fiber buckling in compression[23]. Polymer matrices are broadly divided into two categories, i.e., thermoset and thermoplastics. Thermoset polymer matrix Thermoset polymer matrices are materials that undergo a chemical reaction and transform from liquid to a solid. In its uncured form, the material has very small, unlinked molecules (monomers). The molecules crosslink during the reaction and form a longer molecular chain, leading to solidification (Figure 1). This change is permanent and irreversible and subsequent exposure to high heat will cause the material to degrade, not melt. This is because these materials generally degrade at a temperature below the melting point. Thermosets have lower raw material costs and often provide easy wetting of reinforcing fiber. Thermoset polymers are the most widely used matrix material in composite materials. These include polyesters, vinylesters, epoxies, bismaleimides, and polyamides. Thermosetting polyesters are commonly used in fiberreinforced plastics, and epoxies make up most of the current market for advanced composites resins. The

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matrices attractive to high-volume industries such as the automotive industry. Currently, thermoplastics are used primarily with discontinuous fiber reinforcements such as chopped glass or carbon/graphite. However, there is great potential for high-performance thermoplastics reinforced with continuous fibers. For example, thermoplastics could be used in place of epoxies in the composite structure of the next generation of fighter aircraft[24].

Cellulose

Figure 1. Polymerized thermoset polymer.

different types of thermoset polymer matrices used in composites are: bismaleimides, epoxy (Epoxide), phenolic (PF), polyester (UP), polyimide, polyurethane (PU), and silicone[24]. Thermoplastic polymer matrix Thermoplastic polymer matrices, also called engineering plastics are used with nanocellulose materials to offer important advantages of biodegradability and high fracture toughness[25]. Thermoplastic polymer mainly includes some polyesters, polyetherimide, polyamide imide, polyphenylene sulfide, polyetheretherketone, and liquid-crystal polymers. A thermoplastic matrix is melt processable plastic which is solid at room temperature, but melts upon heating and solidifies on cooling (Figure 2). This melting–solidification sequence is infinitely repeatable without any chemical change to the polymer[26]. In reality, each heating cycle degrades the polymer to some extent. This makes thermoplastic

The term “cellulose” was first used in 1839 in a report of the French Academy on the work of Payen[27]. In 1838, the French chemist Anselme Payen described a resistant fibrous solid that remains behind after treatment of various plant tissues with acids and ammonia[28]. He determined the molecular formula as C6H10O5 by elemental analysis and observed the isomerism with starch. This plant constituent was first termed as cellulose. For thousands of years before the discovery, cellulose was used in the form of woods, cotton, and other plant fibers as an energy source, for building materials, and for clothing[29]. Cellulose is a long chain natural polymer made by linking of smaller molecules (Figure 3). These links in the cellulose chain consist of sugar, b-D-glucose[30]. It occurs in almost pure form in cotton fiber. However, in wood, plant leaves, and stalks, it is found in combination with other materials, such as lignin and hemicelluloses. Cellulose can be obtained from many natural sources such as lignocellulosic biomass like wood, agricultural residues, cotton, flax, hemp, sisal, and especially from by-products of different plants. Other examples of agricultural by-products which might be used to derive cellulose include those obtained from the residues of wheat and cereals[31], jute[32], soybean husk[33] flax fibers and flax straw[34], sugar cane bagasse[35], corn, sorghum, barley, pineapple, sorghum, bananas and coconut crops, and other lignocellulosic biomass[36]. Although it is widely used in fiber, paper, films, and polymer industries, the utilization of this natural biomass for processing of novel material applications has recently attracted growing interest due to its ecological and renewable characteristics. One of these applications has been the development of nanocellulose in virtue of its super functionalities, due to its extremely large, active surface area, and low cost[37]. Nanocellulose

Figure 2. Polymerized thermoplastic polymer.

The term ‘nanocellulose (NC)’ first evolved publicly in the early 1980s[38]. Whereas, the terminology microfibrillated nanocellulose (MFC) was first used by Turbak,


809

Figure 3. Structural organization of the plant cell wall.

Snyder, and Sandberg in the late 1970s at the ITT Rayonier labs in Whippany, NJ, USA to describe a product prepared as a gel-type material by passing wood pulp through a Gaulin-type milk homogenizer at high temperatures and high pressures followed by ejection impact against a hard surface. Nanocellulose is one of the most promising innovations for the modern industry. In the near future, they are better replacement for synthetic fillers to reinforce polymer composites used in automotive industry, packaging, and furniture production[31]. The word ‘nanocellulose’ generally refers to cellulosic materials with one dimension in the nanometer range and look like gel if it is prepared from wood pulp as shown in Figure 4. On the basis of their dimensions, functions, and preparation methods, which in turn depend mainly on the cellulosic source and on the processing conditions, nanocellulose may be classified into three main subcategories[39]. The three main types of nanocellulose are cellulose nanocrystal (CNC), nanofibrillated cellulose (NFC), and bacterial nanocellulose (BNC). The nomenclature used by Klemm et al. is given in Table 1.

chemical and chemomechanical methods as shown in Figure 5. However, BNC is synthesized by the bottomup method from glucose by a family of bacteria, referred to as Gluconoacetobacter xylinus[39] but here we mainly focus our attention on nanocellulose produced with the top-down method from wood or agricultural/forest crops or residues, i.e., lignocellulosic biomass. Previously several approaches for preparing highly purified nanocellulose from cellulosic materials have been reported, such as steam explosion treatment[42], acid or alkaline hydrolysis [43,44], enzyme-assisted hydrolysis[45] as well as a combination of two or several of the aforementioned methods[46]. High-pressure homogenization (HPH) is an efficient technology for biomass refining due to its

Preparation and properties of nanocellulose Nanocellulose has great potential as a strength enhancer in paper, as additive in composites, in emulsions, and as oxygen barriers for food-packaging and in biomedical devices. In general, nanocellulose is produced from several raw materials through top-down approach using

Figure 4. Image of nanocellulose prepared from wood pulp.

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Table 1.


Classification of nanocellulose[39–41].

Type of nanocellulose

Synonyms

Typical sources

Average size

Cellulose nanocrystal

Nanocrystalline cellulose, whiskers, rod like cellulose, microcrystals

Wood, cotton, hemp, flex, wheat straw, rice straw, mulberry bark, ramie, MCC, Avicel, tunicin, algae, bacteria, etc.

Diameter: 5–70 nm Length: 100–250 nm (from plant); 100 nm-several micrometers (from cellulose of tunicates, algae, bacteria)

Nanofibrillated cellulose

Nanofibrils, microfibrils, nanofibrillated cellulose, microfibrillated cellulose

Wood, sugar beet, potato, tuber, hemp, flex etc.

Diameter: 5–60 nm Length: several micrometers

Bacterial nanocellulose

Microbial cellulose, biocellulose

Low-molecular-weight sugars and alcohols

simplicity, high efficiency, and no requirement of organic solvents[47]. The schematic of high pressure is presented in Figure 6 for better understanding. Microfibrillated nanocellulose is typically obtained as a suspension in liquid, usually water. During homogenization, the suspension changes from a low-viscosity to a high-viscosity medium. Normally, a 2% fiber

Figure 5. Flow diagram for synthesis of nanofibrils and nanocrystals by mechanical and chemical routes.

Images

Bacterial synthesis Diameter: 20–100 nm

suspended in aqueous is used for the preparation of MFC. At higher concentrations, the increased viscosity during processing becomes too high to process further. Therefore, low concentration of fibers in aqueous is considered to be an appropriate homogeneous media for nanocellulose preparation because of its nonvolatility, thermal stability, and recyclablity[52]. Schematic overview approaches to produce nanocellulosic materials from fibers are given below[53,54]. Pretreatment of cellulose such as steam explosion, microfluidizer processor or other methods is essential before homogenization to minimize the size of the cellulose fibers to avoiding clogging during homogenization[48–50]. Recently, room temperature ionic liquids have received much attention as agent for pretreatment as they have excellent dissolution ability for cellulose[51]. Microfibrillated nanocellulose has many interesting intrinsic properties such as low density, high chemical reactivity, high strength and modulus, and high

Figure 6. Typical flow diagram of pressure homogenizer.


Table 2. Sl. No.

Mechanical properties of various reinforcements. Plant fiber/materials

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

Cellulose micro/nanofibril CNC Aramid fibers (Kevlar) Cotton Kapok Bamboo Flax Hemp Jute Aluminum wire Steel Kenaf Ramie Abaca Banana Pineapple

17 18

Sisal Coir

19 20 21

Ramie E-glass Carbon

Tensile strength (GPa) 10 10 3–3.5 0.3–0.7 0.093 0.57 0.5–0.9 0.31–0.75 0.2–0.45 0.62 0.54 0.29–1.19 0.91 0.012 0.53–0.92 0.413– 1.627 0.08–0.84 0.106– 0.175 0.4–0.94 2–3.5 4

Modulus (GPa)

Reference

150 150 130 6–10 4 27 50–70 30–60 20–55 73 200 22–60 23 41 27–32 60–82

59 60 61 62 62 62 62 62 62 63 63 63 62 62 62 62

9–22 6

62 62

61.4–128 70 230–240

62 64 64

transparency that make it attractive for many applications[55–58]. Nanocellulose has attracted a great deal of interest recently as potential filler for use in nanocomposites. Nanocellulose has been reported to improve the mechanical properties by incorporating into a wide range of polymer matrices, including poly(3-hydroxybutyrate), hydroxypropyl cellulose, poly(L-lactide), waterborne polyurethane, poly(3,4-ethylenedioxythiophene), polyvinyl acetate, and poly(o-ethoxyaniline). It is observed from Table 2 that the tensile strength and stiffness of nanocellulose are comparable with that of aramid fiber (Kevlar) and better than that of glass fiber, both of which are used commercially to reinforce plastics. Films made from nanocellulose have high strength (over 200 MPa), high stiffness (around 20 GPa), and high strain (12%). Its strength/weight ratio is 8 times higher than the stainless steel[65]. Therefore, nanocellulose composites may be used as coatings and films, paints, foams, and packaging materials[66].

Nanocellulose–polymer composite films The field of nanocomposites has offered a rapidly expanding area of research generating new materials with novel properties[67]. Nanocomposites are mixture of polymers with nanosized inorganic or organic fillers with particular size, geometry, and surface chemistry properties (Figure 7). The polymers used are normally hydrocolloids, such as proteins, starches, pectins, polysaccharides, and synthetic polymers. Various inorganic nanoparticles have been recognized as possible additives

811

to enhance the polymer performance[68]. A polymer composite is a combination of a polymer matrix and a strong reinforcing phase, or filler. Polymer nanocomposites (PNC) are polymers (thermoplastics, thermosets or elastomers) that have been reinforced with small quantities (less than 5% by weight) of nanosized particles having high aspect ratios (L/D > 300)[69]. Preparation of nanocellulose-based composite films Petrochemical-based polymers predominate in food packaging due to their easy processing, excellent barrier properties, and low cost[70]. The use of nanocellulose may extend the food shelf life and can also improve the food quality as they can serve as carriers of some active substances, such as antioxidants and antimicrobials. The NC fiber-based composites have great potential in the preparation of cheap, lightweight, and very strong nanocomposites for food packaging[71]. In the recent years, the vigorous development of polymeric science and extensive utilization of polymeric materials in technology has led to increased interest in the preparation and characterization of polymer and its composite films. In situ polymerization In situ polymerization was the first method used to synthesize nanocomposites. Nowadays, it is the process that is conventionally used to synthesize thermoset nanocomposites. For thermosets such as epoxies or unsaturated polyesters, a curing agent or peroxide is added to initiate the polymerization. For thermoplastics, the polymerization can be initiated either by the addition of a curing agent or by increase in temperature (Figure 8)[72]. Solvent casting method Solvent casting method is one of the easiest methods for the preparation of polymer nanocomposites as it needs simple equipment and is less time-consuming, but in this method it is difficult to manufacture films unless it is sandwiched with another film for support. Furthermore, due to their rigidity, the film produced cracks very easily, and this causes easy peeling off into thin layers like mica with its low interlayer peel strength. Films formed by the extrusion and tubular process, where the liquid-crystalline polymer (LCP) is substantially melted give rise to the problem of vertical anisotropy and interlayer peeling because of the peculiar alignment characteristics[73]. Therefore, in such case, if the solvent casting method is used, where after

812

NCC NFC

Polycaprolactone þMC matrix

Kappa-carrageenan

CNF

PP

Nanocomposite films

Transparent nanocomposite films

Glossy and transparent nanocomposite films

Cellulose nanofiber–PLA nanocomposite films

Biohybrid nanocomposite films BC/NFC based nanocomposite films Cellulose nanocomposite film

CNC–Starch nanocomposite films Nanocomposite films MFC based nanocomposite films MFC-coated papers

NC–MC nanocomposite biodegradable films NCC-reinforced nanocomposite antimicrobial diffusion films NFC–kappa-carrageenan based nanocomposite films Nanocomposite films

NCC–chitosan nanocomposite films/biodegradable films

Products

55 45 4.09–8.76

43

27.8

31

3.6 GPa

21 1584 36

1550

—

500

71

12.8

15

30 140

17.3

7.3 � 0.5

14 —

326.1

480

—

124.1 � 14.6

—

4430 (Increased)

YM (MPa)

257

47 � 4

68 —

3.5–8.2

18.03–9

26

18.7 � 3.7

—

55.3 245 (Increased)

TS (MPa)

— — 2.66 to 1.67

—

—

—

—

—

—

—

—

— —

—

—

—

50 kGy

Decreased

GBP (gm m 2 day kPa)

450 94 10%

600

—

675

—

12.8

—

—

1.2 � 0.1

7 —

38

13

—

—

—

47

EB (%)

Mechanical properties

0.2 66 —

0.06

0.02

0. 099

—

5.2

—

—

—

3.3 6.39

8.68

14

—

—

6.34 (Decreased)

3.31 to 2.23/0.23 � 10 11/12.91 (Decreased)

WVP (gm m 2 day) Applications

Reference

Barrier and protective film in food packaging Food packaging, water and milk bottles, degradable plastic bags/green-based packaging materials/shortterm food packaging/ Film applications, like bread and frozen food bags, flexible lids, squeezable food bottles Bottles for milk, juice and water; margarine tubs, cereal box liners Thin film coatings/Yogurt containers and margarine tubs, microwavable packaging Films for enhancing shelf life Edible films for packaging —

Functional packaging

Antimicrobial action for application in the foodpackaging Flexible packaging

Biobased packing/edible packaging Food packaging Edible films, Transparent films

Food packaging

Food packaging

[90] [90] [90]

[90]

[121]

[120]

[119]

[118]

[117]

[116]

[108]

[114] [115]

[113]

[112], [90]

[111]

Transparent functional [90, 91, 109] packaging/edible food packaging/nanoreinforcer materials in biodegradable packaging/edible films for enhancing shelf life Biodegradable cellulosic based [87] packaging Vegetable packages [110]

TS, tensile strength; YM, Young’s modulus; GBP, gas barrier properties; EB, elongation at break; WVP, water vapor permeability; NCC, nanocrystalline cellulose; NC, nanocellulose; NFC, nanofibrillated cellulose; CNC, cellulose nanocrystal; MFC, microfibrillated cellulose; MFC, microfibrillated cellulose; HDPE, high-density polypropylene; PVC, polyvinyl chloride; CNF, carbon nanofiber.

CNF Nanocomposite film CNF Nanocomposite films Cellulose nanofiber Nanocomposite films

CNF

HDPE

PVC Pea starch Nanocomposite

CNF

Cellulose nanofibres Cellulose nanofiber

Nanocellulose fibers BC or NFC

MFC

NC MFC

LDPE

vermiculite nanoplatelets þnanocellulose fiber Thermoplastic starch/ chitosan Sugarcane bagasse nanofibers Polylactic acid (PLA)

Glucomannan, pectin, gelatin Hydroxypropyl methylcellulose Caffeine þbase paper

Starch Kenaf/ corn starch (CS)

Cellulose nanoparticles CNC

NC

Methylcellulose (MC)

Alginate biopolymer/alignate

NCC

Type of NC

Application of nanocellulose-based composites and their properties.

Chitosan/chitosan sodium þcaseinate þnanocellulose þchitosan

Polymer

Table 3.


813

Figure 7. Preparation of polymer–nanocellulose composite.

Figure 8. Process for preparation of nanocomposite through in situ polymerization.

dissolution in a solvent, the solvent is eliminated to obtain the product, a nonanisotropic film can be obtained and the film could be produced without melting the LCP as depicted in Figure 9. Thus, solventcasted films from LCP move from an amorphous state to the film through treatment in such a way that the anisotropy in the processing of LCP films does not occur.

Melt intercalation process The melt intercalation process was first reported in the year 1993 by Vaia, Ishii, and Giannelis[74]. It consists of blending a molten thermoplastic with nanoparticle to optimize the polymer–nanomaterial interactions. The mixture is then annealed above the glass transition temperature of the polymer and then forms a nanocomposite (Figure 10). The melt intercalation process has become increasingly popular because of its great potential for application in industry. The melt intercalation is more flexible, requires no solvents, and chemical reaction and improve the matrix–filler interactions by reducing the interfacial tension[75,76].

Ring opening polymerization

Figure 9. Preparation process and image of LCP solventcasting film.

Ring opening polymerization (ROP) is a wellestablished technique to polymerize cyclic monomers such as lactones and lactides. An alcohol is generally used as initiator for ROP, which makes it interesting to utilize cyclic monomers for the polymer modification of cellulose or cellulose derivatives[77]. ROP operates through different mechanisms depending on which monomer, initiator, and catalytic systems are utilized (Figure 11). ROP has been used for nanocellulose-based polymer composite preparation by different scientists[78,79]. Very few studies have been reported dealing with the processing of cellulose nanofiber-reinforced nanocomposites by extrusion methods. Some reviews related to the preparation of composite films are highlighted below.

814


Figure 10. Melt intercalation process forpreparation of nanocomposite.

The hydrophilic nature of cellulose causes irreversible agglomeration during drying and aggregation in nonpolar matrices because of the formation of additional hydrogen bonds between amorphous parts of the nanoparticles. Therefore, Oksman et al. prepared cellulose whisker-reinforced PLA nanocomposites by the melt extrusion method. In this method, the preparation was performed by pumping the suspension of nanocrystals into the polymer melt during the extrusion process[80]. The produced film composites possess higher tensile strength as the proportion of modified cellulose increases and higher elongation at break as the proportion of PLA increases. So modified cellulose plays vital role in increasing the tensile strength of film composites. Hence these films can be used as a packaging to protect food from oxidation reaction and moisture[81]. Bruce et al. prepared composites based on MFC obtained from swede root and different resins including four types of acrylic and two types of epoxy resins. All the composites were significantly stiffer and stronger

than the unmodified resins. The main merit of the study was that it demonstrated the potential for fabricating nanocomposites with good mechanical properties from vegetable pulp in combination with a range of resins[82]. Another useful feature of NC is their low coefficient thermal expansion (CTE), which can be as low as 0.1 ppm/K and comparable with that of quartz glass[83]. This low CTE combined with high strength and modulus could make NC a potential reinforcing material for fabricating flexible displays, solar cells, electronic papers, panel sensors, and actuators.[83]. Nogi and Yano[84] prepared a foldable and ductile transparent nanocomposite film by combining low YM transparent acrylic resin with 5% BC to obtain low CTE and high Young’s modulus. PU-MFC composite materials were prepared recently using a film stacking method in which the PU films and nonwoven cellulose fibril mats were stacked and compression molded[85]. Wan et al.[86] tested BC as a potential reinforcing material in PVOH for medical device applications. These authors developed a PVOH-BC nanocomposite with mechanical

Figure 11. Ring opening process for preparation of nanocomposite.


properties over a broad range, thus making it appropriate for replacing different tissues. Khan et al.[87] prepared methylcellulose (MC)-based films casted from its 1% aqueous solution containing 0.5% vegetable oil, 0.25% glycerol, and 0.025% Tween80. They also reported the effect of gamma radiation on the NC containing MC-based composites and it was revealed that mechanical properties of the films were slightly increased at low doses because of NC fibers reorientation, whereas barrier properties were further improved. Dufresne et al.[88,89] prepared potato starchbased nanocomposites, while preserving the biodegradability of the material through the addition of MFC. MFC significantly reinforced the starch matrix, regardless of the plasticizer content, and the increase in YM as a function of filler content was almost linear. Nanocomposites from wheat straw nanofibers and thermoplastic starch from modified potato starch were prepared by the solution casting method[31]. He observed that the TS and YM were significantly enhanced in the nanocomposite films, which could be explained by the uniform dispersion of nanofibers in the polymer matrix. Azeredo et al.[90] developed NC-reinforced chitosan films with different NC and glycerol (plasticizer) content. Pereda et al.[91] developed sodium caseinate films with NC by dispersing the fibrils into film forming solutions, casting, and drying. Composite films have been reported to be less transparent and had a more hydrophilic surface than neat sodium caseinate films. Caseinate films produced an initial increase in the water vapor permeability (WVP) and then decreased as filler content increased.

Characterization of composite films Nanocomposite films are characterized for their surface morphology, mechanical properties, thermal properties, optical properties, and permeability to water and gases using various instrumental techniques. Microscopy plays an important role in understanding the distribution of NC in polymer matrix and bonding between interface. Permeability Film permeability can be determined from diffusion cell experiment[92]. The mass transport rate can be expressed by diffusing specimens across a membrane as per Fick’s first law assuming linear concentration drop within the film (two-film theory). Using a partition coefficient K, the bulk concentrations in the diffusion cell chambers can be related to the concentrations at the film surfaces. Another assumption made is that the concentration of the diffused specimens present within the film is negligible compared to the total

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chamber concentrations. The whole derivation can be seen in the work of van den Mooter et al.[92]. It is important to note that the permeability is scaled against the film thickness, resulting in the unit m2/s. Scanning electron microscopy Scanning electron microscopy is used to determine the morphology and microstructure of samples. A focused electron beam scan across the sample surface results in signals that are converted to an image on a computer screen. The most widely used signals for imaging are secondary electrons which are electrons that are excited from the sample molecules by the scanning electron beam[93]. The secondary electrons give information about surface texture, and dark regions in the obtained image mean that the secondary electrons are prevented from reaching the detector. In addition, there are also other signals obtained when beam strikes the sample; for example, backscattered electrons, Auger electrons, and X-rays[93]. Thermal analysis Thermal analysis is defined as the measurement of physical and chemical properties of materials as a function of temperature. The two main thermal analysis techniques are thermogravimetric analysis (TGA), which automatically records the change in weight of a sample as a function of either temperature or time, and the differential thermal analysis (DTA), which measures the difference in temperature between a sample and an inert reference material as a function of temperature. DTA therefore detects change in heat content. A technique closely related but modified version of DTA is differential scanning calorimetry (DSC). Dynamic mechanical analysis Dynamic mechanical analysis (DMA) is a method to determine short-time mechanical behavior of materials. The sample when subjected to a small sinusoidal stress of a certain frequency responds with a sinusoidal strain of the same frequency. A purely elastic material responds immediately and therefore remain in phase with the oscillating stress. If the material is purely viscous, the responding strain will be out of phase with the sinusoidal applied stress and the phase angle between these will be 90°. Thus, DMA can be used to determine the viscoelasticity of a material[94]. Through DMA, it is possible to determine shear storage and shear loss modulus, which represent the shear modulus for the elastic and viscous parts of the material[95].

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Differential scanning calorimetry Differential scanning calorimetry determines the energy changes within a material during constant heating rate. These energy changes correspond to either chemical reactions occurring in the sample or physical changes such as glass transition, crystallization, melting of crystals, or sample decomposition[96]. The instrument has two sample pans, one is for the sample and another is a reference pan, which is left empty. The two pans are heated at a constant rate and the instrument measures the difference in heat flow needed to keep the pans at the same temperature[95]. On melting, the energy is either liberated or needed, resulting in a peak in the heat flow through the sample. The position of the peak gives the temperature at which the melting/crystallization occurs, and the area of the peak gives the change in heat flow during the process[96]. Thermal stability The thermal stability of polymeric materials is usually studied by TGA. The weight loss due to the formation of volatile products after degradation at high temperature is monitored as a function of temperature. When the heating occurs under an inert gas flow, a nonoxidative degradation occurs, while the use of air or oxygen allows oxidative degradation of the samples. In general, the incorporation of clay into the polymer matrix was found to enhance thermal stability by acting as a superior insulator and mass transport barrier to the volatile products generated during decomposition[97].

Properties of polymer nanocomposites The blending of nanocellulose and polymer matrix at low filler loading showed outstanding mechanical properties[98]. Under suitable condition, mechanically percolating stiff network of nanoparticles can form within the polymer matrix that supports the mechanical solicitation. The stiffness of the percolating CNC was found to increase with the aspect ratio of the nanocrystals[99]. The higher aspect ratio of CNC is more interesting from a mechanical point of view, because it first induces a decrease in the critical percolation threshold and stiffens the formed continuous network. When the formation of this percolating nanoparticle network is inhibited, only the high stiffness of crystalline cellulose at nanoscale dimensions, high aspect ratio, and filler–matrix interactions are involved in the reinforcing phenomenon.

with a UV–visible spectrometer. The regular light transmittance at 600-nm wavelength was generally reported, and it was observed that films obtained from MFC were optically transparent, if the cellulose nanofibers were densely packed[100], and the interstices between the fibers are small enough to avoid light scattering[55]. However, mechanical compression performed on freeze-dried MFC did not result in transparency. Films prepared by slow filtration, drying, and compression are much more densely packed and are not optically transparent but translucent, probably because of surface light scattering. The transparency of the MFC sheet (thickness 55 mm) reached 71.6% at a wavelength of 600 nm (Figure 12). The transmittance at 600 nm of softwood and hardwood TEMPO-oxidized MFC films was found to be around 90 and 78%, respectively. The lower light transmittance of hardwood cellulose was ascribed to the presence of xylan that was supposed to interfere in part with complete dispersion of the nanofibrils in water[101]. Barrier properties of nanocomposite films Most materials used for food packaging are practically nondegradable petrochemical-based polymers, representing a serious environmental problem. The main reason for their use as food-packaging material is due to their easiness of processability, low cost, and excellent barrier properties[54]. Barrier properties using biobased materials are becoming increasingly desirable in our society to develop environmentally friendly efficient materials in different applications. Moreover, the low permeability of cellulose can be enhanced by the highly crystalline nature of cellulose nanoparticles and their ability to form a dense percolating network. Provided that strong particle-polymer–polymer molecular interactions exist, the smaller particles have a greater ability

Optical properties The optical properties of nanocellulose films can be investigated by determining the regular light transmittance

Figure 12. Light transmittance of microfibrillated cellulose films.[48].


to bond to the surrounding polymer material, thereby reducing the chain segmental mobility and thus the penetrant diffusivity[101]. To improve the gas barrier properties of nanocellulose films at high relative humidity (RH) level, hybrid clay–MFC films can be prepared or chemical modification of the nanoparticles can be performed[102,103]. Coating of polymer films with MFC layers has also been investigated as a new way to produce good barrier materials and as possible solution to retain the advantages of both cellulosic nanoparticles and polymers[104]. Nanocomposite films extend food shelflife, and also improve food quality as they can serve as carriers for active substances such as antioxidants and antimicrobials[105]. Stiffness and strength properties The stiffness and strength properties are highly dependent on the form of nanocellulose. Ruiz et al.[67] observed that composites from NFC and aqueous suspensions of epoxy displayed large aspect ratios and had an ability to associate by means of H-bonds. The reinforcement benefits were linked to strong interactions between the CNCs and epoxy network and the percolating network linked by H-bonds between the NFC. The effect of fiber content on the mechanical and thermal expansion properties of biocomposites based on NFC have also been reported[106]. A linear increase in Young’s modulus was observed at fiber content up to 40% using a phenolic resin. The results also showed a correlation between the CTE relative to fiber content, indicating the effective reinforcement attained by the NFC. In another example, Ruiz et al. found that adding NFC up to about 2% in an epoxy resin increased the mechanical properties but further addition of NFC leading to agglomeration and reduced thermal and mechanical properties[67]. Nakagaito and Yano[106] used a compression molding technique of NFC sheets impregnated with phenol formaldehyde to achieve a Young’s modulus and bending strength of up to 19 GPa and 370 MPa, respectively. Thermal properties The studies on thermal degradation behavior of MFC films are limited. Although this behavior depends mainly on the process of MFC preparation, it also depends on the drying process[54]. Concerning the first point, it has been reported that TEMPO-oxidized cellulose displays multiple degradation peaks[67]. In 225–231°C, the maximum weight loss (60–80%) occur. Quievy et al.[107] studied the influence of the drying process on the thermal stability of MFC obtained by homogenization.

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After freeze drying, MFC gel globally formed a mat corresponding to the microfibril aggregates, in which some of the microfibrils remained distinct. The freezing step at 20°C played an important role in the formation of agglomerates. In addition, this step even changes the rheological properties of the MFC suspension.

Application of polymer nanocomposites in food packaging During the last decade, MFC was essentially used in nanocomposites due to its reinforcement property[108]. Its nanoscale dimension and its ability to form a strong entangled nanoporous network have encouraged the emergence of new high-value applications. Various approaches have been developed to cross-link nanocellulose with other materials to impart multifunctional properties, like mechanical properties, antimicrobial properties, improved coloration and dyeing, tensile strength, water vapor permeability, and barrier properties. Chitosan, a natural linear polysaccharide consisting of 1,4-linked 2-amino-deoxy-b-D-glucan, is the second most abundant natural polysaccharide after cellulose. Chitosan is nontoxic, biodegradable, biofunctional, biocompatible and was reported by several researchers to have strong antimicrobial and antifungal activities[122]. Chitosan films have been successfully used as a packaging material for the quality of preservation of foods[123]. It was observed that nanocrystalline cellulose (NCC) acted as a good reinforcing agent in chitosan and only 3–5% of NCC loading gave the best TS values. Improvement of the mechanical properties was due to the formation of a percolating network and strong filler–matrix interaction. Incorporation of NCC increased the tensile modulus of the chitosan films. NCC also improved the barrier properties of the chitosan by reducing the WVP and swelling property. Surface morphology of the nanocomposite films revealed a homogeneous structure indicating proper dispersion of the NCC into the chitosan matrix. Hence, NCC-reinforced nanocomposite films due to their excellent mechanical and barrier properties would have a promising impact on food packaging in coming years. Similarly, the tensile modulus and strength of composite films increased significantly with increasing cellulose concentration, while the elongation decreased. Boumail et al. developed antimicrobial diffusion films (ADFs) for food applications[124]. ADFs exhibited the highest tensile strength over storage. Further, Savadekar et al.[111] successfully extracted NFC from short stable cotton fibers by chemomechanical process. This study provided an initial insight into the use and characteristics of NFC in Kappa-carrageenan-based biocomposite film.

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A biobased nanocomposite was developed by Abdollahi et al.[112] by incorporation of cellulose nanoparticles (CN) obtained from sulfuric acid hydrolysis into alginate biopolymer using the solution casting method. The tensile strength value of the composite films increased with increasing NC content from 0 to 5%; but, it decreased with further increase in the filler content. Nevertheless, film transparency decreased with CN incorporation, especially in high level (10%), which suggested the occurrence of partial agglomeration of the fillers at 10% that coincided with microstructural and mechanical results. A novel, technically and economically benign procedure to combine vermiculite nanoplatelets with nanocellulose fiber dispersions into functional biohybrid films was presented by Aulin et al.[116]. Nanocellulose fibers of 20 nm in diameter and several micrometers in length were mixed with high aspect ratio-exfoliated vermiculite nanoplatelets through high-pressure homogenization. The resulting hybrid films obtained after solvent evaporation were stiff (tensile modulus of 17.3 GPa), strong (strength up to 257 MPa), and transparent. The oxygen barrier properties of the biohybrid films showed an oxygen permeability of 0.07 cm3 µm m 2 d 1 kPa 1 at 50% relative humidity. Furthermore, the water vapor barrier properties of the biohybrid films were also significantly improved by the addition of nanoclay. The unique combination of excellent oxygen barrier behavior and optical transparency suggested an alternative in flexible packaging of oxygen-sensitive devices such as thin-film transistors or organic light-emitting diode displays, gas storage applications, and as barrier coatings/laminations in large-volume packaging applications. Polylactic acid (PLA), a very versatile compostable polymer derived from natural source produced either by the polycondensation of lactic acid or by the ring opening polymerization of lactide is a cyclic dimmer prepared by the controlled depolymerization of lactic acid, which is obtained from the fermentation of renewable sugar feedstock, such as corn or sugar beets[125]. The effect of cellulose nanofibrils (CNF) in the PLA matrix was investigated by Jonoobi et al. in terms of mechanical properties and dynamic mechanical properties in consideration of the intended application in food packaging. Increase in tensile strength, Young’s modulus, and improved viscoelastic behavior were observed for nanocomposite films with 5% of CNF. This underlines the success of the melt compounding procedure to prepare cellulose nanocomposites[119]. All cellulose nanocomposite (ACNC) films were produced from sugarcane bagasse nanofibers using N,N-dimethylacetamide/lithium chloride solvent. The

study demonstrated that a very low-value agricultural waste can be converted to a high-performance nanocomposite (tensile strength: 140 MPa). Moreover, WVP of the ACNC film increased relatively to an increased duration of dissolution time. Hence, ACNC can be considered as a multiperformance material with potential for application in cellulose-based food packaging owing to its promising properties such as toughness, biobased, biodegradability, and acceptable levels of WVP. Hence, ACNC has potential for the development of barrier and protective film in food-packaging industries. The tensile properties of ACNC film are at least comparable to better than those of other biodegradable or nonbiodegradable film[118]. Mechanical properties of biocomposites having 6% CNCs in thermoplastic cassava starch (TPCS) showed the highest tensile strength of 8.2 MPa. This suggests the stress transfer and interfacial interactions between the matrix phase and the filler, which is related to the high L/D and efficiency of the fiber treatment. The Kenaf fibers are also found to be compatible with agar and starch made from potato, and films was tested for their potential use in food packaging[113,121].

Future prospects and challenges The stimuli-responsive polymer materials are an emerging field in food packaging. They are an interesting, innovative, and challenging class of materials that can adapt to surrounding environment and regulate the transport of molecules as a reaction to external stimuli[126]. To sustain life and maintain biological function, nature requires selectively tailored molecular assemblies and interfaces that provide a specific chemical function and structure as well as a change in their environment[127]. Nanocellulose reinforcements offer potential advantages in specific properties related to their lower density and other advantages such as low CTE, transparency, and barrier properties. NFCs have begun receiving additional attention as a reinforcement material because of reductions in the energy requirements for breaking down cellulose fibers to NFC[58]. Besides recent progress, there are many challenges remaining to efficiently and economically use it as a reinforcing material in polymer composites. More efficient and effective strategies need to be found for producing nanocellulose with optimal characteristics. Widespread application of these materials will require additional research to address the issues related to their hydrophilic nature in many applications. Techniques and appropriate chemistry are needed to adequately disperse nanocellulose reinforcements or convert them into a useful form for incorporation into a variety of


matrices and strongly bond them to it. More efficient control of structure on multiple scales is needed to tailor performance. Development of new analytical methods is necessary for simulation of processing and also prediction of the mechanical properties of nanocellulose-based structures. The process modeling effort will be required to link the modeling of NFC distributions to optimize the properties. Appropriate applications need to be identified, investigated, and demonstrated. To date, the availability of significant quantities of nanocellulose has prevented more rapid wide-scale research and development efforts related to their use. If high-value applications can be found, it may be possible to integrate nanocellulose production into the materials flow of these biorefineries, where it could potentially help improve economic[128] announcements of commercial and government pilot plant-scale production facilities will likely improve the situation[129].

Conclusion Nanocellulose-based composites made out of CNF and CNC have enormous application in food packaging due to its competency with synthetic material and infinite availability. The studies and experiments reviewed here present that CNF and CNC composites and its coating reduces the oxygen permeability which enhances the shelf life of the packed food. The oxygen permeability of pure CNF and CNC films are highly competitive and even comparable with commercial synthetic polymers. Strong filler–matrix interaction between nanocellulose and polymer composites is crucial to achieve enhanced mechanical properties in polymer nanocomposites. But, taking into account the incompatibility issues of nanocellulosic materials with hydrophobic matrices, it can be envisaged that nanocomposites based on hydrophilic matrix polymers will be easier to manufacture and commercialize in future. The compatibility of nanocellulosic materials with nonpolar materials can be improved by chemical modification of nanocellulose. Although several studies have been performed for chemical modification of nanocellulose, industrially feasible way to produce nanocellulose polymer composites using hydrophobic biopolymers are yet to come. Consequently, more research efforts need to be concentrated targeting novel, environmentally friendly methods of modification as well as an understanding of the mechanism of reactions occurring at the cellulose nanofiber polymer matrix interface is important. Application of nanocellulose leads to improvement in overall performance of polymer composites by improving their mechanical, thermal and barrier

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properties, usually even at very low content. Thus, nanocellulose plays important role to improve the feasibility in use of polymer composites for food-packaging sector by reducing the packaging waste of processed foods and improve the preservation of packaged foods by extending their shelf life. The advantages that nanocomposites offer far outweigh the costs and concerns, and with time the technology will be further refined and more developed. Hence, the reinforcement of nanocellulose in polymer composites will resolve the various problems faced by the food-packaging industry at present.

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