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A Review on Preparation and Properties of Cellulose Nanocrystal-Incorporated Natural Biopolymer

  • Sujosh Nandi
  • Proshanta Guha
Review Paper
  • 451 Downloads

Abstract

The emergence of green clean technology has escalated the use of eco-friendly biopolymer by substituting petroleum-derived polymers. Over the years, natural biopolymers’ properties have been evaluated to check their applicability in food packaging industries. However, most of the biopolymers show poor barrier and mechanical properties compared to petroleum-derived polymers. Recently, cellulose nanocrystal has acquired attention of the researchers as potential prospective filler, which can overcome the drawbacks of biopolymer films. The basic advantages of cellulose nanocrystal are its high strength and stiffness, high surface area, biodegradability, etc. Therefore, lignocellulosic biomass, animal, and bacterial cellulose have been used as feedstock of cellulose nanocrystal, and then incorporated into the biopolymer films. This review provides detailed procedure of cellulose nanocrystal extraction in a stepwise manner and discusses the changes in film properties after reinforcement into different biopolymers.

Keywords

Cellulose nanocrystal Acid hydrolysis Ionic liquid hydrolysis Biopolymer Mechanical properties Barrier properties 

Introduction

Packaging may be defined as an encasement of a commercial product designed for specific purposes such as protection from external environment and pilferage, providing statutory and commercial information to the consumer, and attracting the consumers. According to [1], packaging can be defined as “a means of safely and cost effectively delivering products to the consumer in accordance with the marketing strategy of the organization”. In the ancient time, products were packed into pottery and containers made of various materials such as wood, glass, metal, and paper for storage and transportation. However, flexible packaging stormed into the market for easy handling, light weight, and cost-effectiveness during post-World War II. Among different flexible packaging materials, synthetic plastics have been used widely over the years owing to good barrier properties, transparency, and to provide detailed information of the food products such as date of manufacture, best before use, nutritional values, and methods of use or preparation. This enhances consumer acceptability in all segments of food industries.

Packaging industries play a major role in the advancement of modern civilization and economic growth of any nation. This is because packaging industry is a multibillion dollar industry all over the world which provides employment to lakhs of workers and helps other concerned industries to grow, like the industries which produce raw materials for packaging. Currently, the global plastic production has crossed 300 million tonnes, which depicts the enormous application of petrochemical sources (around 4% of total petrochemical feedstock) in packaging industries [2]. Till date, different crude oil-derived polymers have  captured the world plastic market because of their easy availability, low cost, good mechanical and barrier properties, ease of handling and mouldability. Despite these advantages, non-biodegradable nature of the petroleum-derived plastic films causes severe environmental pollution [3, 4, 5] through various means, such as food, drinks, soil, water and even air (after burning).

Among total plastic film production, half is being utilized for packaging of disposable items and out of that about 20% is used for packaging of food items [6]. This enormous usage of petroleum-based disposable plastics is significantly generating municipal solid waste every year if not disposed of in ocean. Therefore, plastic industries are being urged to have eco-friendly plastic production to meet the desired functional and environmental attributes such as compostability, non-toxicity, and good mechanical and barrier properties. The rate of degradation, however, depends on microorganisms, environmental factors (light, oxygen and temperature), and mode of disposal, viz. landfills, terrestrial and marine environments [7]. Therefore, researchers are trying to develop the biodegradable films with the functional properties for packaging of food articles. Moreover, governmental and non-governmental organizations are also working hard to create awareness among the people about the detrimental effects of non-biodegradable plastics. They are also trying to promote bio-plastic production and its use.

Bioplastics can be biodegradable or non biodegradable produced from bio sources or petrochemical sources (Table 1). For instance, polybutyrate (PBAT) is fossil fuel-derived biodegradable polymer, whereas bio-based PE, PP or PET is non-biodegradable [2]. It has been reported that bioplastics are a blend of an equal proportion of renewable materials with additives and synthetic polymer to enhance overall desirable properties, viz. mechanical, barrier and optical properties [8]. Therefore, a bioplastics can be termed as a completely biodegradable polymer only if raw materials and additives are obtained from renewable and biological sources, which is further known as natural biopolymer.
Table 1

Classification of biodegradable and non-biodegradable plastic based on their sources

 

Biodegradable

Non-biodegradable

Bio based

PLA, PHA, PBS, starch blends, etc.

PE, PET, PA, PTT, etc.

Fossil based

PBAT, PCL, etc.

PE, PP, PET, etc.

These biopolymers have their own limitations as packaging material of food products. First, bio-based film produced from starch and protein depicts high water absorption property, which, therefore, significantly decreases the mechanical properties. Second, in some particular cases, biopolymers exhibit poor mechanical properties than petrochemical polymers [9]. Moreover, high cost and low availability of the biopolymer films prevent their consumer acceptability [4].

In view of the above limitations, extensive researches have been conducted to enhance the desirable properties of biodegradable polymer to make it suitable for industries, viz. food and beverages [10], medicine [11] and optics [12]. In the process of improvement, researchers have come out with an idea of “filler” as a potential tool, which can overcome the fiasco of biopolymer applicability in industrial scenario and day-to-day life [13]. In the past years, different fillers (clay, metal and metal oxide, cellulose, etc.) have been incorporated into polymer matrix to improve the overall properties of the film. However, filler which has nanoscale dimension in any one of the sides (nanofiller) substantially improves the characteristics of biopolymer as compared to the macro-filler [14]. Along with filler size, the amount of filler also needs to be considered [15].

Based on dimension, three types of nano-fillers have been dispersed in polymer matrix to obtain the desirable features: (1) iso-dimensional nano-particles with three side in nano-metric dimension (e.g., spherical silica nano-particles and semiconductor cluster); (2) nano-tubes or whiskers with two sides in nano-metric dimension (e.g., carbon nano-tube or cellulose whiskers); and (3) polymer-layered crystalline nanocomposites with one side in nano-metric dimension [16]. The types of nano-filler that have been used to cast nanocomposite film are listed in Table 2. Among all nano-fillers, cellulose nanocrystal (CNC) has acquired significant attention of the various research groups for wide availability, renewability, superior mechanical properties, high aspect ratio, large specific area and low cost [17, 18, 19]. Hence, in this article, the preparation of CNC and their incorporation in natural resource-based biopolymers are discussed along with different properties of the biocomposite films incorporated with CNC.
Table 2

Types of nano-filler used for development of composite film

Classification

Type of fillers

Examples

References

Organic

Clay/layered silicate

Montmorillonite

[20, 21]

  

Halloysite

[22]

  

Cloisite Na+

[23, 24]

  

Kaolinite

[25, 26, 27]

  

KH-MTa

[28]

  

I30Pb

[28]

  

I30Ec

[29, 30]

  

Cloisite 30B

[31, 32]

  

Cloisite 20A

[28, 33]

  

Cloisite 10A

[34]

 

Nano-biopolymer

Cellulose

[35, 36]

 

Starch

[37]

 

Chitosan

[38, 39]

Inorganic

Metal

Silver

[40, 41]

  

Copper

[42, 43]

  

Gold

[44]

  

Platinum

[45]

 

Metal oxide

Silver oxide

[46]

  

Titanium oxide

[47]

  

Iron oxide

[48, 49]

  

Magnesium oxide

[50]

  

Silicon oxide

[51, 52]

  

Zinc oxide

[53, 54]

aQuaternary alkylamine-modified montmorillonite

bOctadecylamine modified montmorillonite

cOnium ion-modified montmorillonite

Structure and Morphology of Cellulose

Cellulose is a primary component available in natural fiber along with hemicellulose and lignin. It is synthesized from lignocellulosic biomass, animals (algae, tunicates) and few bacterial sources worldwide [55, 56]. Lignocellulosic biomass, viz. wood, cotton, flax, hemp, sisal and agricultural residues have been considered as primary and affordable natural resources for cellulose fiber [57]. From ancient period, cellulose fiber has been utilized in various construction sectors besides paper, textile and automobile industries owing to its superior strength [58].

In the year 1838, the French chemist Anselme Payen first mentioned about the presence of cellulose in plant tissues as a resistance fibrous solid material. He discovered the method of cellulose breakdown into simple sugar (dextrose) using acid and ammonia treatment [59]. Since this discovery, several researchers have focused on cellulose and its derivates. In the year of 1868, the first polymeric material was prepared from cellulose nitrate and camphor (softener) by the Hyatt Manufacturing Company [60] and encouraged the industrial level production of cellulose from lignocellulosic sources. However, cellulose acquired the attention of the academicians in the realm of polymer chemistry when Staudinger in 1926 elucidated the polymeric structure of cellulose [17].

Cellulose is primarily the group of cellobiose units which are produced from elimination of water molecules from two β-d-glucopyranose units. The glycosidic link is formed between C-1 of one d-glucose unit and C-4 of another one. Then, the group of cellobiose units are joined together to form a crystalline structure of cellulose called elementary fibrils. The elementary fibrils are bundled together to form micro-fibrils which in turn produced macro-fibrils or cellulose fiber in plant cell wall [19]. Therefore, the characteristics of cellulose depend on degree of polymerization (DP) and the number of glucose units grouped together to form cellulose [61]. The schematic diagram of cellulose fiber is given in Fig. 1.
Fig. 1

Schematic diagram of wood cellulose fiber morphology

Cellulose Nanocrystal (CNC)

Cellulose nanocrystal is a rod-shaped nano-particle of cellulose extracted from hydrolysis of lignocellulosic biomass. In the past decades, several biomasses have been opted for extraction of CNC using mineral acid hydrolysis process that produced nano-size cellulose crystal of different dimensions and crystallinities (Table 3). The dimension of CNC varied from 100 to 1000 nm in length and 4–25 nm in diameter depending on raw materials.
Table 3

Physical properties of cellulose nanocrystal derived from various sources

Source

Length (nm)

Diameter (nm)

Aspect ratio (L/D)

Crystallinity (%)

References

Chili fibre

90–180

4–6

26

78.5

[63]

Oil palm trunk

361.7–397

7.7–8

69.61

[64]

Potato peel waste

410

10

41

85

[65]

Pea hull fibre

240–400

7–12

32.2–36

[18]

Grass of Korea

180–300

< 10

18–30

[66]

Pineapple leaf

249.7 ± 51.5

4.5 ± 1.41

60

73

[67]

coconut husk

80–500

6

20–60

[68]

Mulberry fiber

400–500

20–40

73.4

[69]

Sisal

250 ± 100

4 ± 1

60

[70]

Tunicate

1160–2000

15

77–133

[71]

Corn stover

356

6.9

54.7

45

[72]

Soy hulls

122.7 ± 39.4

2.8

44

73.5

[73]

Corncob

210.8 ± 44.2

4.15

53.4 ± 15.8

83.7

[74]

Mengkuang leaves

50–400

5–25

10–20

69.5

[75]

Alfa

200

20

10

[76]

Phormium tenax

100–200

15

[77]

Rice straw

117–270

11.2–30.7

8.8–10.5

91.2

[78]

Sweet potato residue

20–40

72.53

[79]

Microcrystalline cellulose

200–400

10

20–40

[80]

Cotton linter

25–320

6–70

[81]

Avicel

35–265

3–48

[81]

Flax fiber

100–500

10–30

[82]

Sugarcane bagasse

275 ± 73

5

55

78

[83]

Cassava bagasse

360–1700

2–11

54.1

[84]

Tomato peel

135 ± 50

3.3

80.8

[85]

Fundamentally, the native cellulose is a bundle of micro-fibrils which comprises crystalline region and amorphous region. The crystalline region consists of more or less parallel ordered arrangement of cellulose chain by hydrogen bonding between three –OH groups of pyranose unit. However, it is impossible for long cellulose chains to maintain a perfect arrangement throughout its length, and hence form amorphous regions. In the amorphous regions, cellulose chains are arranged in random order which makes them less durable and weaker than crystalline region. This amorphous region of cellulose chain is more prone to link with water, radical degrading agent, and dye molecules. Moreover, the bonds present in the amorphous regions and between amorphous and crystalline zone are quite sensitive to acid–alkali treatment. Therefore, the hydrolytic treatment has been preferred for effective isolation of high-purity CNC from cellulose fiber [62]. However, the effluent generated from hydrolysis process is hazardous to our environment. Recently, researchers have observed that different ionic liquids have the potential to dissolve the hemicelluloses and take part in the hydrolysis reaction of the cellulose micro-fibrils. In the following section, the step of the extraction of cellulose nanocrystal has been reviewed.

Extraction Procedure of Cellulose Nanocrystal

In 1950s, for the first time, Rånby prepared the colloidal solution of cellulose from wood using 2.5 N sulfuric acid for accomplishing hydrolysis process. The hydrocellulose obtained from the acid hydrolysis became turbid after several water washes [86]. The turbid solution was analyzed using transmission electron microscopy (TEM) and revealed a rod-like structure of cellulose. Further, electron diffraction analysis revealed that molecular structure of the suspended material was similar to the native fibers [87]. Since then, several mineral acids (sulfuric acid, hydrochloric acid, phosphoric acid, hydrogen bromide acid) have been considered for the hydrolysis of cellulose fiber and among them sulfuric acid has become the most accepted mineral acid by the research groups. The reason behind that is the presence of sulfate groups on the surface of CNC which generate 84 m-eq kg−1 surface charge [88]. Moreover, time-independent behavior of the CNC solution enhances its acceptance percentage. The acid hydrolysis method has also been experimented over different sources of raw material to produce CNC having high crystallinity and aspect ratio. Besides acid hydrolysis, microwave-assisted enzymatic hydrolysis and ionic liquid hydrolysis have also been performed to produce cellulose nanocrystal. In this review, acid and ionic liquid hydrolysis processes are described in detail. In the following section, the procedure of isolation of cellulose nanocrystal is schematically described (Fig. 2) and discussed in detail.
Fig. 2

Schematic diagram of extracting nanocrystalline cellulose from lignocellulosic biomass

Removal of Extractive Components from Biomass

Despite different structural components (cellulose, hemicelluloses and lignin), wood and non-wood fiber consist of extractives. The name extractive refers to the chemical substance present in wood which can be removed by solvent. The extractive comprises monomers, dimmers and polymers of fat, fatty acid, waxes, terpenes, terpenoids, flavonoids, rosin, resin, tannins, free sugar, etc. [89]. Therefore, to obtain a pure holocellulose (hemicelluloses + cellulose), extractives need to be removed from biomass. The extractives present in biomass comprise about 1% of inorganic components and about 10% organic components on dry weight basis. Solvent extraction technique has been preferred to remove the extractives. Various polar and non-polar solvents are capable of solubilizing the extractives. Hence, a certain type of solvents and their combination, i.e., acetone–alcohol [90], n-hexane–ethanol [91], ethanol–water [92], ethanol–benzene [93], toluene–ethanol [94] have been used for the removal of extractives [89] prior to chemical treatments.

Alkali Treatment

Hemicellulose is the naturally occurring hetero-polysaccharide in wood fiber comprising hexose, pentose or deoxyhexose and other units of sugars, viz. xylose, mannose, glucose, galactose, arabinose and rhamnose. Structurally hemicellulose can be classified as highly branched side chain (high water soluble) and more uniform or low-branched side chain (less water soluble) [95] which are needed to be dissolved. Additionally, entrenched pectin with hemicellulose also creates difficulty for extraction of pure cellulose from lignocellulosic materials. Thus, alkali treatment has been provided to dissolve hemicellulose partially and pectin completely. During alkali treatment, the carboxylic bond of pectin is converted to its corresponding carboxylate and thus, solubilizes it readily [66]. On the other hand, splitting of ester link present in hemicellulose leads to increased crystallinity [93] and reduces the degree of polymerization than native fiber [96]. Moreover, alkali, viz. sodium hydroxide (NaOH) [97], potassium hydroxide (KOH) [93] and sodium bisulfate [98] have also been used to dissolve protein, extractive residue, soluble mineral salt, silica and ash [73, 99]. In brief, the process involves heating (80–130 °C) of chopped raw material at particular bath ratio (material: solution) in different alkali concentration, and then wash with distilled water until neutral pH is achieved. Generally, cellulose fibers are subjected to 2–12% concentrated alkali solution with around 4 h of heating treatment for effective removal of hemicelluloses. Recently, microwave-assisted alkali treatment has also been tried to reduce the time of treatment. Jute stalk was subjected to microwave treatment for 45 min at 2.5 M NaOH solution. The results showed around 36% removal of hemicellulose along with lignin [100]. The effect of microwave temperature on the yield of hemicellulose and its molecular weight was analyzed, and it was observed that with increasing temperature from 130 to 210 °C, yield increased from 10 to 30%. However, molecular weight of obtained hemicelluloses reduced above 180 °C [101]. Further experiments are required on microwave-assisted alkali pretreatment to optimize time of operation. Also, a comparison of energy consumption between microwave heating and conventional hot water bath heating treatment needs to be studied. Further, the work on optimization of alkali concentration and type of alkali effective for pretreatment of CNC extraction is not yet studied well.

Steam explosion is another promising eco-friendly technique used as a pretreatment to obtain cellulose from lignocellulosic material [102]. The process involved an application of high pressure (14–16 bar) and temperature (200–270 °C) for short time (20 s to 20 min) followed by a sudden release of pressure. Breakdown of the hemicelluloses into oligomers and sugars and de-polymerization of lignin components by exertion of thermo-mechanical pressure occurs due to sudden release of pressure. Therefore, water-soluble portions of hemicellulose and lignin are dissolved in solution and then separated by water extraction procedure. However, after that, bleaching treatment is required for complete removal of lignin, and subsequently obtain cellulose moieties [61, 103, 104]. In addition of having eco-friendly characteristics, steam explosion technique is quite effective for hardwood in comparison to softwood [105].

Delignification Process

Delignification process denotes mainly removal of lignin and hemicellulose residues from lignocellulosic biomass. Lignin is a three-dimensional aromatic polymer and structural material of tissues present in hardwood, softwood and agricultural residues. Lignin is made up of three basic components—guaiacyl, syringyl, and p-hydroxyphenyl moieties. The lignin content in hardwood and softwood is in the range of 18–25 and 25–35%, respectively. Lignin forms lignin–carbohydrate complexes with hemicelluloses, which is resistant to hydrolysis. So, there is a need to remove that complex.

In general, several oxidative and reducing bleaching agents (Table 4) are available to accomplish delignification process in industries. Among them, sodium hypochlorite [72], sodium chlorite [63, 64, 68] and hydrogen peroxide [79, 106] have been preferred for bleaching treatment of lignocellulosic biomass. This process involves splitting of phenolic α-o-4 linkages, splitting of non-phenolic β-o-4 linkages and removal of residual lignin fractions either by cleavage of C–C linkages or by carbohydrate degradation [107].
Table 4

List of bleaching agents [108]

Bleaching agents

Oxidative agents

Reductive agents

Peroxy compounds

Chlorine-based compounds

(a) Hydrogen peroxide

(b) Sodium hypochlorite

(c) Sodium perborate

(d) Potassium permanganate

(e) Peracetic acid

(a) Bleaching powder

(b) Sodium hypochlorite

(c) Lithium hypochlorite

(d) Sodium chlorite

(a) Sulfur dioxide

(b) Sodium hydrosulfite

(c) Sulfoxylates

(d) Acidic sodium sulphite

(e) Sodium bisulfites

Sodium hypochlorite is one of the strong oxidizing agents and liberates hypochlorous acid (HOCl). However, in acidic condition, chlorine gas is formed due to decomposition of HOCl which does not have any bleaching effect. Therefore, sodium hydroxide (NaOH) is added to attain a pH of 9–11.5, where HOCL decomposes to OCL ions and shows bleaching property [108]. Another widely used bleaching agent is sodium chlorite (NaClO2) where chlorine dioxide (ClO2) is responsible for bleaching process. During bleaching process, sodium chlorite liberates ClO2 enormously at between pH 1–2 which is toxic and corrosive in nature. Moreover, at low pH, the balance between generation and emission of ClO2 gets affected which eventually creates hindrance on bleaching process [108]. Therefore, it is recommended to keep the pH in the range of 3.5–4.0 adjusting by acetic acid or formic acid with sodium di-hydrogen phosphate buffer solution. Hydrogen peroxide (H2O2) is also being considered as an effective bleaching agent at optimum alkali medium (pH 11.5–11.6) generally known as alkaline hydrogen peroxide (AHP) [109]. It was reported that at low-alkaline medium, H2O2 can solubilize most of the hemicelluloses and half of the lignin present in agricultural biomass, viz. wheat straw, corn stover at 25 °C [110]. Same author also stated that the primary lignin-oxidizing substances are hydroxyl radical (HO·) and O2 instead of H2O2 and perhydroxyl ion (HOO) generated during decomposition of H2O2.
$${\text{H}}_{2} {\text{O}}_{2}\; { \leftrightharpoons }\;{\text{H}}^{ + } + {\text{HOO}}^{ - }$$
$${\text{H}}_{2} {\text{O}}_{2} + {\text{HOO}}^{ - } \to {\text{HO}} \cdot + {\text{O}}_{2}^{ - } + {\text{H}}_{2} {\text{O}}$$

Hydrolysis Process

Cellulose produced by the above-mentioned steps is subjected to hydrolysis process at the controlled condition to disrupt the amorphous structure and extract cellulose nanocrystal. During hydrolysis process, the disruption of the amorphous region is initiated by the hydronium ions that cause the formation of rod-like crystals. The disruption of which is mainly due to the cleavage of glycosidic bonds. The hydrolysed suspension is then diluted with distilled water, washed and neutralized with distilled water and pure CNC is obtained.

Mineral Acid Hydrolysis

The mineral acid hydrolysis is a widely used method to prepare cellulose nanocrystal. The mineral acids, namely sulfuric acid, hydrochloric acid, hydro-bromic acid, phosphoric acid and their combination have been reported to use for hydrolysis process [111]. The time and temperature of acid hydrolysis play an imperative role in characteristics of cellulose nanocrystal. It was reported that longer the hydrolysis time, the shorter the length of CNC [112]. On the other hand, the properties of CNC, namely degree of crystallinity and dimension depend on raw materials also [88]. A group of researchers have examined the impact of acid-to-pulp ratio and hydrolysis time on properties of CNC [113] and found that length of CNC was indirectly proportional to acid-to-pulp ratio. However, no significant difference was observed in surface charge and sulfur content with changes in reaction time at an optimum acid-to-pulp ratio. So, the conclusion was made that temperature had a greater influence on surface charge due to de-esterification of sulfate group at higher temperature. A similar conclusion has been drawn for soy hull [73]- and corncob [74]-derived cellulose nanocrystal. In another study, the CNC was extracted from pea hull fiber to evaluate the effect of hydrolysis time on dimension and aspect ratio of crystal particles. It was found that length and diameter decreased with time and in the case of aspect ratio, it was first increased and then diminished after 8 h of reaction [18].

Besides size and degree of polymerization (DP) of CNC, the stability of aqueous suspension of CNC is also correlated with different acids used for hydrolysis [11]. The stability of CNC suspension is denoted by phase separation. The phase separation is observed due to changes in poly-dispersity, surface charge, nano-particles’ dimension and the ionic strength of the various suspensions. Therefore, different mineral acid-assisted hydrolysis processes have been tried to obtain stable aqueous suspension of CNC. During hydrolysis using H2SO4, sulfate group (SO42−) of H2SO4 accumulates on the surface of cellulose nanocrystal which creates negative charges. As a result, a stable aqueous suspension of CNC is formed. However, in case of hydrochloric acid-assisted hydrolysis process, a CNC aggregate was developed due to the presence of hydrogen bond on the surface of CNC [88, 114, 115]. Despite high stability in the aqueous solution, cellulose nanocrystal produced by sulfuric acid hydrolysis exhibits low thermal stability which restricted the use of CNC incorporation in the bio-nanocomposite films. Hence, numerous experiments have been conducted on the enhancement of thermal stability of the CNC. For instance, Wang and his group [116] have conducted a combined hydrochloric acid and sulfuric acid hydrolysis process to improve thermal stability. In that experiment, micro-fibrillated cellulose was hydrolyzed with the acid mixture of 30% H2SO4 and 10% HCl and then neutralized by two different methods. In the first method, neutralization was done by repetitive washing with distilled water followed by dialysis process and in the second method the obtained CNC was first washed with distilled water for more than 1 month and then neutralized by 1% (w/w) NaOH. The result showed that thermal degradation was at lower temperature for CNC with acid sulfate group, whereas for the second method CNC degraded at higher but narrow temperature range. On the other hand, phosphoric acid (H3PO4) was also considered for hydrolysis process. The obtained CNC revealed improved thermal stability than partially sulfated cellulose nanocrystal and better dispersibility in organic solvents [117]. The 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO)-mediated oxidation has also been done to improve the thermal stability of tunicin whiskers after HCl hydrolysis treatment. The resulted whiskers showed negative surface charge due to the selective conversion of hydroxymethyl groups into carboxylic groups. Moreover, it did not appear to be flocculated in solution and exhibited birefringence. However, strong mineral acid-assisted hydrolysis process has numerous drawbacks (corrosive, hazardous and bio-incompatibility) which restrict the production of cellulose nanocrystal in industrial level [61].

Ionic Liquid (IL) Hydrolysis

Ionic liquid is a “green” solvent being extensively used for the dissolution of lignocellulosic materials. It comprises one cation and one anion which join and form low-boiling point, non-volatile, highly thermally stable and easily recyclable solvent. In the past years, ILs have been extensively used for the dissolution of cellulose, but the application of ILs for isolation of CNC was limited. Man and his group [118] extracted cellulose nanocrystal from microcrystalline cellulose (MCC) by ionic liquid hydrolysis (1-butyl-3-methylimidazolium hydrogen sulfate). They obtained cellulose nanocrystal of about 50–300 nm length and 14–22 nm diameters with low thermal stability than MCC. In another study, a combined ionic liquid and acid hydrolysis process treatment was given to jute stalk to evaluate the physiochemical characteristics of cellulose nanocrystal. The average diameter and length of CNC were 10–12 and 92 nm, respectively. They observed that acid hydrolysis process combined with ultrasonication accelerated the cleavage of hydrogen and glycosidic bonds more than hydrolysis using ionic liquid [100]. The effect of various ILs and hydrolysis time on dimension of CNC, viscosity and stability of aqueous solution of CNC is not well established. Moreover, it is required to examine the behavior nano-biocomposite films incorporated with CNC derived from ionic liquids hydrolysis process.

Ultrasonication Process

The neutralized aqueous suspension of CNC obtained after centrifugation requires to be subjected to ultrasonication treatment for disrupting aggregates of crystals, and then store it for further uses. The principle of ultrasonic treatment is formation, expansion, and collapse of cavities. Due to a sudden collapse of cavities, kinetic energy released and thus, creates stress on the surface of materials. Similarly, when the aqueous suspension of CNC is exposed to ultrasonic wave, disintegration of CNC lumps takes place. A comparative study was performed to evaluate the effect of ultrasonic treatment on fiber with acid-treated fiber [119] and reported that ultrasonic treatment produced fibrils from fiber, while acid-treated fiber disintegrated into particles, mostly aggregates of elongated units. A similar result was also obtained when ultrasonic treatment was given to ramie fiber CNC after hydrolysis treatment [120]. The viscosity in cupriethylenediamine was measured to evaluate the effect of ultrasound treatment on degree of polymerization. No change in DP was observed between treated and untreated samples which revealed that ultrasonic treatment only cleaved the side-by-side hydrogen-bonded particles keeping intact the primary valence structure of cellulose.

Drying Process

The drying of aqueous CNC solution is an optional process after ultrasonic treatment. Generally, water is used as a carrier for the aqueous solution of CNC after ultrasonic process because strong hydrogen bond interaction between water and CNC keeps the solution thermally or kinetically stable irrespective of water content. However, the presence of water affects the processing of nanocomposite films in extrusion or injection molding process because these are thermal melting processes which develop foam due to evaporation of water. Furthermore, the presence of free water can affect the amide or ester bonds of a non-polar polymer during the thermal melting process. Therefore, the researchers have emphasized on the development of appropriate drying techniques to obtain dried CNC which simultaneously reduce transportation cost also [111, 121, 122].

The four different methods, viz. oven drying (OD), freeze drying (FD), spray drying (SD) and supercritical drying (SCD) have been experimented to obtain appropriate drying methods [121]. In the oven drying method, the aqueous solution was exposed to 105 °C for 24 h and water evaporated in three steps: (a) removal of free water or constant rate drying, (b) first falling period, and (c) second falling rate period. During the second falling period, water diffusion has occurred from the interior of cellulose nanocrystal suspension due to combined effect of capillary and diffusion forces. As a result, the distance among nanocrystals gradually decreased which creates strong hydrogen bonding interaction forces between CNCs. This strong hydrogen bond interaction developed a bulk network structure of CNC. Therefore, the dimension of crystal nano-particles increased and varied between hundreds of micrometer and millimeter. To avoid this problem, freeze drying has been tried by several researchers [122]. During freeze drying process, lateral agglomeration occurred instead of forming bulk fiber network. The probable reason for lateral arrangement of CNC was diffusion force or hydrogen bonding interaction. However, the morphological characteristic of the freeze dried sample was similar to the oven-dried sample and exhibited thickness of CNC in nanoscale dimension and length in microns to millimeter range. On the other hand, the CNC could not be dried using supercritical drying process because of incomplete removal of water with ethanol.

Spray drying, another well-established drying process, has been opted for drying of the aqueous CNC suspension [123]. The primary reason for opting the spray drying technique was low labor and maintenance cost. However, this drying technique produced spherical-shape CNC particles which exhibited nanoscale dimension in small amount. The spray-dried sample also showed agglomeration behavior owing to high polarity of CNC. Therefore, surface modification has to be done before spray drying process. Otherwise, improper dispersion of CNC into non-polar polymer matrix can reduce the tensile strength and increase brittleness of nanocomposite films [122].

Natural Biopolymer Film

The rapid development of civilization from last 100 years has enhanced the use of non-biodegradable packaging films which causes environmental contamination. To avoid the environmental contamination, the utilization of biodegradable plastic has been reported by several researchers. Plastics are certified as biodegradable if degradation is performed by the microbes present in the environment generating non-toxic or eco-friendly residues. Generally, biodegradable polymers are broadly classified into three types based on their sources: (1) natural biopolymer, (2) synthetic biodegradable polymer, and (3) biopolymer produced from microbial fermentation [6]. Among the three types, natural biopolymers have been considered as the potential substitute of non-biodegradable plastic, and it can be extracted from different plant and animal sources [4, 124, 125]. Moreover, these natural biopolymers have several beneficial characteristics, viz. edibility, biodegradability, capability of improving shelf life of food products, ability to enhance organoleptic properties (appearance, odor and flavor), and capability of incorporating various additives such as antimicrobial and antioxidant agents. In addition, the polymer matrix developed from polysaccharide and protein exhibits good mechanical and gas barrier properties, but shows inferior moisture diffusion property. On the other hand, matrix developed by lipids has comparatively low water vapor permeability, but elucidates poor mechanical properties than polysaccharide and protein-based films. Therefore, multi-layer films have been developed in combination of hydrophilic (carbohydrate, protein, etc.) and hydrophobic polymers (lipids) to obtain good mechanical as well as barrier properties [126]. Despite that, different additives have also been incorporated into the matrix and feasibility was assessed to overcome the inferior properties of the biopolymer films. Additives such as synthetic polymers, cross-linking agents (Ca and Zr salts), fatty acids, protein, and fiber have been reported to provide better resistance to water vapor permeation and gas diffusion across film and simultaneously to enhance mechanical properties. Recently, incorporation of nano-fillers (nano-clay, metal oxide, metal and nano-biopolymer) has emerged as a superior process towards development of nano-biocomposite film. Among different nano-fillers, cellulose nanocrystal has shown great potential owing to its inherent properties such as biocompatibility, biodegradability, high aspect ratio, and superior mechanical properties (high stiffness, tensile strength, Young’s modulus and low thermal expansion co-efficient).

Natural Biopolymer–CNC-Based Composite Film

Cellulose nanocrystal-incorporated biopolymer films are emerging composite films in the realm of bio-nanocomposite films. Tunicin cellulose whisker was first incorporated into co-polymer of styrene and acrylate to manufacture nanocomposite film [127]. After that, various biopolymers were chosen to reinforce the CNC after extracting from plant or animal sources. For instance, poly(Ɛ-caprolactone) [128], poly(β-hydroxyoctanoate) [129], poly(vinyl acetate) [130], poly(butyl methacrylate) [131], poly(oxy-ethylene) [132], ethylene oxide–epichlorohydrin co-polymers [133], polyurethane [134, 135], polyvinyl alcohol [136, 137], polyvinyl chloride [138], xylan [139], starch [82], agar [140], alginate [141] and chitosan [142, 143] have been characterized after CNC incorporation. However, growing awareness about environmental pollution compels the researcher to choose natural biopolymer as a primary matrix for reinforcement of CNC (Table 5). There are different types of CNC-based biopolymer composite films. Some of these are reviewed below.
Table 5

Natural biopolymer–cellulose nanocrystal-based composite film and their properties

Sources of biopolymer

CNC source

Filler content (%)

Tensile strength (MPa)

Young modulus (MPa)

Water vapor permeability (g m−1 s−1 Pa−1)

Water uptake (%)

References

Pea starch

Flax fiber

5, 10, 15, 20, 25, and 30

3.9–11.9 (increased)

31.9–498.2 (increased)

[82]

Maize starch

Waxy maize starch nanocrystalline cellulose/microcrystalline cellulose

1, 2.5, and 5

1.0–1.6 (increased)

2.5–7.5 (increased)

[148]

Wheat starch

Microcrystalline cellulose

0, 1, 2, 3, 4, and 5

3.15–10.98 (increased)

(5.7–3.4) × 10−10 (decreased)

[35]

Maize starch

Tunicin whisker

5, 10, 15, 20, 25, and 30

~ 5–42 (increased)

208–833 (increased)

 

[144]

Pectin

Crystalline nanocellulose

2, 5, and 7

7.1–13.1 (increased)

 

(1.2–0.93) × 10−10 (decreased)

[149]

Agar

Paper mulberry bast pulp

1, 3, 5, and 10

33.3–41.3 (increased)

0.8–1.1 (increased)

(0.7–1.6) × 10−9 (increased)

[150]

Agar

Microcrystalline cellulose (MCC)

2.5, 5, and 10

18.2–20.8 (increased)

118–87.46 (decreased)

(1.6–2.1) × 10−10 (increased)

[140]

Alginate

Commercial bleached softwood

kraft pulp

1, 3, 5,, and 8

57–78 (increased)

(1.8–3.2) × 103 (increased)

(6.4–4.1) × 10−11 (decreased)

[141]

Alginate

Mulberry pulp

2, 4, and 6

(45.6–54.9) at 4% (increased)

(1.6–1.8) × 103 at 4% (increased)

(1.3–1.9) × 10−9 (increased)

[150]

Chitosan

Eucalyptus

wood pulp

120 mg

45–108 (increased)

(1.2–3.7) × 103 (increased)

[142]

Chitosan

Commercial bleached softwood

kraft pulp

1, 3, 5, and 10

79–98 (increased)

1590–2971 (increased)

(3.8–2.6) × 10−11 (decreased)

[143]

Chitosan

Bleached bagasse kraft pulp

5, 10, 15, and 20

9.03–15 (increased)

123–733 (increased)

[151]

Chitosan

Cotton linter

pulp

5, 10, 15, 20, 25, and 30

85–120 (dry state)

9.9–17.3 (wet state) (increased)

71–40 (decreased)

[152]

Polyvinyl chloride/carboxymethyl cellulose

Sugarcane bagasse

0.5, 2.5, 5, and 10

64.9–118.7 (increased)

1138.7–2744.2 (increased)

(3.2–0.6) × 10−11 (decreased)

[83]

Soy protein isolate

Commercial CNC

1.23

3.13–4.8 (increased)

21.2–48.4 (increased)

96.4–79.6 (decreased)

[153]

Soy protein isolate

Modified CNC

1.23

3.1–5.6 (increased)

21.2–66.3 (increased)

96.4–68.4 (decreased)

[153]

Soy protein isolate

Cotton linter pulp

0, 5, 10, 15, 20, 25, and 30

5.8–8.1 (increased)

44.7–133 (increased)

40–25 (decreased)

[154]

Oxidized natural rubber

Commercial CNC

5

1.3–8.4 at 3% CNC (increased)

1.7–2.4 at 2% CNC (increased)

[125]

Natural rubber

Soy hull

0, 1, 2.5, and 5

0.59–3.03 (increased)

0.6–18.1 (increased)

[155]

Cariflex-IR rubber latex

Chili leftover

1, 3, and 6

0.2–0.4 (increased)

4.8–24.6% (increased)

[63]

Starch–CNC-Based Composite Film

The native starch and its derivative thermoplastic starch show a promising structural compound for manufacturing eco-friendly packaging films. The characteristics of starch such as copious in nature, renewability, cheap, biodegradability, and processability endorse it to be selected as a base material for incorporating into CNC [82, 144].

The maize starch-based film reinforced with tunicin whiskers was developed to evaluate the effect of nano-size cellulose crystal and relative humidity of surrounding environment on prepared nanocomposite [145]. It was observed that at low water activity (aw < 0.35), the water content was unchanged with varying filler content. However, at water activity above 0.35, water absorption capacity of the film increased even at low filler content. For instance, at 25% filler content, water uptake by composite film was 40% while unfilled starch absorbed 60% water. Besides the filler content, the water uptake capacity of the film also depended on formation of porosity and transcrystalline zone around cellulose whisker during drying of the film. Improvement of glass transition temperature (Tg) was also reported after addition of filler into plasticized starch matrix. On the other hand, mechanical properties of composite film are highly essential to be characterized. The same research group has developed starch film with glycerol as a plasticizer and tunicin whisker as filler. They stated that amount of filler has a significant contribution towards improvement of mechanical properties (tensile strength and elongation at break) of the films above moisture content of 35%. However, the properties remained unchanged at moisture content above 43% even after increased the filler percentage [146].

In another study, flax cellulose nanocrystal (FCN) was reinforced in pea starch (PS)-based film by solvent casting method to examine the mechanical and water resistance properties [82]. The authors reported that increasing the FCN content from 0 to 30% (dry weight), tensile strength escalated from 3.9 to 11.9 MPa and Young’s modulus from 31.9 to 498.2 MPa which implied production of a strong and less deformed plastic. This improvement of mechanical properties was explained by two terms: (1) homogeneous dispersion of FCNs into PS matrix, and (2) strong hydrogen bonding between FCNs and PS matrix. On the other hand, water uptake was reduced continuously with increasing FCN content into PS matrix.

Nanocomposite films have been prepared with glycerol-plasticized wheat starch (GPS) and cellulose nano-particles (CN) to examine mechanical properties, thermal stability and water vapor permeability of the composites [35]. During the isolation of CN, microcrystalline cellulose was first kept into NaOH/urea/H2O solution at “minus 10 °C” and then hydrolyzed with a solution made of 36.5% concentrated HCl and ethanol (30:170 v/v). They prepared GPS/CN films by incorporating 0, 1, 2, 3, 4 and 5 wt % of CN into 5 g of wheat starch solution. It was observed that tensile strength of the casted film increased from 3.15 to 10.98 MPa and elongation at break decreased with increasing filler content. The result depicted a good matrix–filler interaction because of identical primary structure of cellulose and starch. Moreover, water vapor diffusion across the film, which is considered as the most important characteristic of food packaging films, was reduced from 5.75 × 10−10 to 3.43 × 10−10g−1 s−1 Pa−1 with increasing filler content up to 3%. The reason behind the diminishing water vapor permeability (WVP) was development of tortuous path which could be developed by proper dispersion of cellulose nano-particles into polymer matrix. On the contrary, excess addition of CN (> 3%) formed agglomerate and thus, reduced WVP of the films.

Chen and his group [65] have manufactured potato starch-based composite films reinforced with waste potato peel (PCN) and cotton (CCN)-derived cellulose nano-particles. It was observed that thermal properties were independent of the sources of CNC. The change of water vapor permeability with CNC variant and loading rate was insignificant. On the other hand, Young’s modulus for both thermoplastic starch and PVA was higher for the addition of PCN than CCN. However, for both the cases the enhancement of Young’s modulus was escalated with increasing amount of CNC.

In another study, potato starch-based thermoplastic films were prepared with cellulose and cellulose nanocrystal (CNC) as a filler [147]. Thermal, mechanical, permeability, and water solubility properties were tested. Solubility of the composite films decreased, whereas melting temperature, melting enthalpy, tensile strength and elastic modulus increased due to cellulose and CNC incorporation. The improvement in melting temperature attributed proper interactions between the starches and fillers and escalation of crystalline arrangement of filler into composite films. They have also reported improved mechanical properties after addition of cellulose and CNC, but improvement was better for CNC than cellulose. Water vapor permeability of the films was decreased with addition of cellulose and CNC. However, CNC incorporation ascribed less WVP than its counterparts because of nano-size dimension of CNC. Therefore, clearly, the CNC dispersed evenly into the matrix and created tortuous path, reducing the water transmission rate [35].

An experiment was conducted to prepare coating material using carboxymethyl cellulose (CMC) and starch blend and CNC as filler and applied it on cellulosic paper [156]. They evaluated the mechanical properties such as tensile index, tear index, folding endurance and burst index along with air permeability of cellulosic paper. The rheological behavior of nanocomposite film was also checked to obtain an idea about interaction between polymer and other ingredients. It was observed that CNC addition into starch/CMC matrix showed shear-thinning behavior. Initially, viscosity of composite films enhanced due to strong interaction between CMC/starch and CNC and a hindrance was created in flow. After that, gradual decrease in viscosity was observed with increasing shear rate because of network break down of network structure. At shear rate 100 S−1, viscosity increased from 7.06 to 12.79 mPa-s at 0.3 wt % of CNC loading. The mechanical properties and air permeability properties also improved after incorporation of CNC. However, detrimental effect was observed in all the properties when percentage of CNC addition crossed 0.3%.

Maize starch-based nanocomposite films was prepared using waxy starch nanocrystal (WSNC) and cellulose nanocrystal (CNC) as reinforcement material and it was observed that Young’s modulus improved to around 70% at 2.5% reinforcement of WSNC or CNC [148]. The probable reason behind improvement of Young’s modulus was identical chemical configuration and strong hydrogen bonding between maize starch matrix and WSNC. However, no significant change was observed when filler content increased from 2.5 to 5% due to the development of ineffective percolated network. On the other hand, WSNC reinforcement did not change WVP but oxygen transmission rate decreased significantly for 2.5 and 5% WSNC. In case of CNC, no improvement was observed for overall barrier properties.

In view of the above outcomes of reported articles, it is difficult to say that CNC reinforcement into starch matrix can improve barrier properties along with mechanical properties. The hydrophilic nature of starch and cellulose restricts the improvement of barrier properties. Therefore, addition of hydrophobic compounds into starch–CNC composite films could be a possible solution to improve barrier properties.

Chitosan–CNC-Based Composite Film

Chitosan is an abundantly available polysaccharide in nature with molecular structure of 1, 4-linked 2-amino-deoxy-β-d-glucan. Chitosan has been preferred for food article packaging owing to antimicrobial and antifungal activities [157]. Moreover, biodegradability, biocompatibility, bio-functional and non-toxic characteristics of chitosan increase its acceptability by industrialists [143]. Unfortunately, chitosan-based film also depicts poor mechanical and barrier properties. Thus, CNC has also been embedded in chitosan matrix to enhance the properties.

Li et al. [152] prepared a chitosan film reinforced with cellulose whiskers isolated from cotton linter pulp by solvent casting method. Tensile strength of the composite film was improved from 85 to 120 MPa and elongation at break was decreased from 20 to 6% by increasing the cellulose whisker percentage 0–20. However, cellulose whisker reinforcement above 20% elucidated significant drop of the overall mechanical properties due to micro-phase separation. The water uptake properties of composite films also decreased from 71 to 40% at 30% of cellulose whisker loading due to strong filler–matrix interaction. In case of thermal property, glass transition temperature (Tg) was increased up to 10% loading of cellulose whiskers which ascribed proper interaction between chitosan and cellulose whiskers. However, the detrimental result was observed due to agglomeration of cellulose whisker, above 10% loading.

Another work on chitosan-based nanocomposite film was carried out to evaluate the effect of CNC incorporation on the thermal, mechanical and barrier properties of the film [143]. Only with 5% loading of CNC, mechanical properties and WVP were improved by 26 and 27%, respectively. On the other hand, only 41% increase of TS was observed after 15–20% incorporation of cotton linter pulp CNC into chitosan matrix [69]. The reason behind such small improvement in the TS after threefold increase in the filler content was the dimension of CNC. The reported dimension of CNC by the first group of researchers [143] was four times shorter than cotton linter pulp CNC. However, 10% addition of CNC provided optimum barrier to water vapor diffusion and above that, diffusion rate again increased. Similar result was also obtained when central composite design model was performed to optimize concentration of CNF and plasticizer [158]. In both the experiments, thermal properties did not vary significantly with different CNC concentrations.

Chitosan–CNC composite film was made by covalent linkage, where CNC was isolated from eucalyptus wood pulp [142]. The resultant film showed significant enhancement in mechanical properties (150%). Water vapor barrier property was also improved significantly for CH–c–CNC nanocomposite.

Therefore, it can be concluded that chitosan films have the potential to improve mechanical and barrier properties of nanocomposite films. However, application-related works are needed to be explored in future for packaging of food articles.

Alginate–CNC-Based Composite Film

Alginate is a polysaccharide, derived from brown sea wood, namely Macrocytis pyrifera, Laminaria hyperborea, Ascophyllum nodosum, and Laminaria digitata. The primary molecular structure of alginate consists of 1–4-linked β-d-mannuronic acid and α-l-glucuronic acid [159]. From the last few decades, alginate has been preferred to produce packaging films owing to its distinctive colloidal properties such as thickening, gel forming, stabilizing agent for emulsion, and good film forming properties along with high mechanical strength and transparency [160]. However, commercial application is restricted for poor water vapor barrier. Therefore, to improve mechanical and barrier properties, 1, 3, 5 and 8% CNC were blended with 3% alginate [141]. It was concluded that loading of 5% CNC to alginate matrix increased tensile strength by 37% and WVP decreased around 31%. The gel swelling properties of the alginate–CNC (5%) film was also decreased by 57% than only alginate-based film.

Recently, alginate-based film was reinforced with mulberry pulp-extracted cellulose nanowhiskers (CNW) and cellulose fiber (CF) [161]. The CNW and CF at different concentrations (2, 4 and 6%) were blended into polymer matrix and tested for mechanical properties, WVP and transparency of the films. The mechanical properties, viz. tensile strength (TS) of the alginate films increased from 45.6 to 54.9 MPa after 4% addition of CNW. However, the TS value was decreased to 47.0 MPa when enforcement level increased to 6%. The probable reason for decreasing the TS value was formation of CNW agglomerate in the matrix. Similar pattern was observed in case of elastic modulus. The WVP property of the both composite films increased from control sample owing to hydrophilic nature of CF and CNW. Further, alginate/CF depicted higher WVP than alginate/CNW. In case of transparency of the films, composite films showed poor light transmission capability than alginate films. In addition, transparency of the composite films varied with filler types and cellulose content. However, the transparency of the alginate/CNW composite films was comparatively higher than alginate/CF composite films.

It is very interesting to note that mechanical properties of the alginate-based composite films increased irrespective of sources of CNC but positive change of WVP is not significant with sources of CNC. Still, extensive research is required in the realm of alginate–CNC composite films to obtain suitable alginate-based composite films for food packaging.

Cellulose–CNC-Based Composite Film

Cellulose is another abundant available polymer in nature but its properties are not suitable for making commercial films at pure state. Therefore, an attempt was made to improve mechanical properties of cellulose acetate butyrate film by incorporating native and trimethyl silylated crystal. Differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA) and transmission electron microscopy (TEM) were used to analyze the adhesion characteristics of cellulose nanocrystal and polymer matrix. The DSC analysis implied better adhesion properties between matrix-native cellulose nanocrystals than matrix-trimethyl silylated cellulose nanocrystal. However, the amount of native crystals did not affect the melting point because of weak filler–matrix interaction compared to trimethyl silylated crystal–matrix composites [162]. Another attempt was made to improve the characteristics of PVC/CMC-blended films embedding it with sugarcane bagasse cellulose nanocrystals. It was observed that at 5% filler content, tensile strength and tensile modulus enhanced by 141 and 83%, respectively, whereas barrier properties, basically water vapor transmission rate, reduced by 87%. The authors did not observe any significant change in transparency after addition of fillers because of nanoscale dispersion of filler throughout the matrix [83].

Rubber–CNC-Based Composite Film

The sap of rubber tree is the primary source of high molecular weight polymer and commercially available natural rubber (NR) with molecular structure of cis-1, 4-polyisoprene units. Over the years, natural rubber has been used in different fields including tires, adhesives, and eraser. The NR has distinctive mechanical properties of high elasticity. The causes of highly elastic nature of NR are presence of stereo-regular microstructure and free spinning of R-methylenic C–C bonds [163]. Therefore, cellulose nanocrystal has been used to alter elastic property of NR. For instance, soy hull-derived CNC was incorporated into NR to evaluate the changes of mechanical properties occurred due to CNC sedimentation. The NR-based composite films illustrated improvement of tensile modulus by 21-fold than natural rubber. However, high aspect ratio and sedimentation of CNC lead to higher tensile modulus to the lower layer compared to other layer [155]. Another study was made on Cariflex-Isoprene latex with the addition of chili leftover nanocrystal. Incorporation of CNC improved tensile strength, but simultaneously reduced yield strain. It did not show any aggregates of CNC into matrix resulting in appropriate dispersion of high aspect ratio CNC [63].

Other Biopolymer–CNC-Based Composite Films

Cellulose nanocrystal (CNC) was incorporated into pectin to develop a new kind of bio-nanocomposite films for food packaging application. The CNC was loaded at three levels, 2, 5 and 7% with 3 g of pectin at 60 ml distilled water. It was observed that 5% addition of CNC increased the tensile strength by 84% but further addition reduced the TS. Water vapor transmission rate also decreased from 1.28 × 10−10 to 9.3 × 10−11 g/m s Pa at same amount of CNC addition. It was observed that above 5% CNC, dispersibility of CNC into the matrix was affected and thus, formed aggregates of CNC causing detrimental effect to mechanical and barrier properties. The differential scanning calorimeter data revealed that glass transition temperature and melting temperature did not change significantly after incorporation of CNC [149].

Cellulose nanocrystal was also used as filler for agar-based polymer films. It was observed that based on the sources of CNC and incorporation level, the mechanical properties, barrier properties and transparency of nanocomposite films have changed. For instance, paper mulberry pulp nanocellulose incorporation decreased water vapor permeability by 25% after addition of 5% CNC [150], whereas water vapor permeability decreased by only 13% with 10% addition of CNC derived from microcrystalline cellulose [140]. In both the cases, mechanical properties were enhanced due to addition of CNC.

Soy protein [153, 154], soy flour [58], and silk fibroin (protein constituents of silk fiber) [164] were also considered as eco-friendly polymer matrix. Hence, cellulose nanocrystal was incorporated into it to evaluate the composite characteristics. Irrespective of the protein matrix, mechanical properties and thermal stability were improved significantly due to the addition of CNC.

During the current stage of human civilization, rampant use of petroleum by-product-based non-biodegradable plastics overweigh its advantages by pollution caused by it. This necessitates urgent search for alternatives like biodegradable films.

Conclusion

The advent of nano-biocomposite film creates a new era towards producing an eco-friendly packaging system for storage, transportation and serving of food articles. The nano-biocomposite films are primarily biodegradable plastics incorporated with various nano-fillers. The biodegradable plastics produced from natural sources are slowly substituting non-biodegradable plastics in the form of a plastic bag, cup, plate, etc. However, the inherent properties of these biopolymers such as low tensile strength, modulus and high water vapor permeability compared to non-biodegradable petroleum-derived plastic create a question mark of its practical utility. To address these drawbacks, numerous attempts have been made with incorporation of cross-linking agent, coating or blending of hydrophobic materials or blending of other polymers or loading of fillers, and so on. Unfortunately, it has not resulted in any satisfactory biocomposite films so far. However, recently, nanotechnology reveals a promising way to create nano-biocomposite films as an appropriate counterpart of non-biodegradable plastics.

Cellulose nanocrystal, which is a nano-size dimension of cellulose, is considered as a potential natural resource-based biodegradable filler for its superior mechanical properties. However, the interaction between polymer matrix and filler possesses imperative parameter to manufacture nano-biocomposite films. Therefore, several studies have been conducted to produce CNC from different lignocellulosic biomass and to evaluate their compatibility with different natural biopolymers. The chemical pretreatment followed by mineral acid hydrolysis has been preferred over the years for the production of CNC. However, the process, particularly acid hydrolysis, is not suitable for commercial scale because of high processing cost, difficult-to-discard hazardous effluent and maintenance of reactors. Therefore, ionic liquids (IL) have recently been projected as a promising substitute of the strong mineral acid, while low-boiling point of ILs has increased its acceptance among the researchers. Consequently, extensive research is required to choose a suitable IL and optimize the processing conditions.

The incorporation of CNC into different biopolymers such as starch, chitosan, rubber, and protein is reported to enhance the mechanical and barrier properties. However, a contradictory review is reported for barrier properties, particularly for water vapor permeability. The probable reasons would be the similar chemical structure of natural biopolymer and CNC, and inappropriate dispersion of CNC throughout the casted films. The inappropriate dispersion occurs because of agglomeration of CNC into polymer matrix which may reduce the tortuosity. Therefore, the interaction between polymer matrix and filler becomes an important area needed to be studied well in future. Furthermore, an interaction of nano-biocomposite film with food, real-time quality changes of packed foods, and mathematical modeling on how CNC disperses into polymer matrix could be the areas needed to be explored in future to understand the effect of interaction among natural biopolymer and CNC.

Notes

Acknowledgements

The authors are grateful to the Indian Institute of Technology Kharagpur for financially supporting the research and to Dr. Suradeep Basak, former research scholar of Agricultural and Food Engineering Department, IIT Kharagpur, India, for checking the manuscript.

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Copyright information

© Indian Institute of Packaging 2018

Authors and Affiliations

  1. 1.Agricultural and Food Engineering DepartmentIndian Institute of Technology KharagpurKharagpurIndia

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