Biodegradation and Thermal Properties of Crosslinked Chitosan/Corn Cob Biocomposite Films by Electron Beam Irradiation

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Biodegradation and Thermal Properties of Crosslinked Chitosan/Corn Cob Biocomposite Films by Electron Beam Irradiation

Ming Yeng Chan ,¹Seong Chun Koay²

1HELP College of Arts and Technology (HELP CAT), Centre for Engineering Programmes, Kuala Lumpur 55200, Malaysia

2School of Engineering, Taylor’s University Lakeside Campus, Subang Jaya, 47500, Selangor, Malaysia

Thermal and biodegradation properties of chitosan (CS)/corn cob (CC) biocomposite films and their irradiation-crosslinked were tested. The CS/CC biocomposite films after irradiation showed better thermal stability and lower weight loss in enzymatic and soil biodegradation in comparison with unirradiated CS/CC biocomposite films due to the formation of new bonds (radiation-induced crosslinks). The surface erosion for biodegraded biocomposite films were examined by scanning electron microscope. Furthermore, the formation of new bonds in irradiated biocomposite films were analyzed by Fourier transform infrared spectroscopy. POLYM. ENG. SCI., 00:000–000, 2018.V^C2018 Society of Plastics Engineers

INTRODUCTION

To date, biopolymer materials have played an important role as a functional polymer materials [1]. This is due to the environ- mental impacts of nonbiodegradable petroleum-based plastic materials wastes that has increased global concerns [2]. Chitosan is widely applied in medical and packaging materials for its biocompatible, nontoxic, and biodegradable properties [1].

Chitosan (CS) is a well-known biopolymer, which is prepared from chitin by deacetylation [3–6]. CS has a higher potential in various applications as compared to chitin because CS contains a greater amount of free NH₂ groups [7]. In general, CS is insoluble in water, but soluble in aqueous organic acids such as formic, acetic, citric, and lactic acid to produce a vis- cous CS solution [8]. Because of these excellent properties (e.g., biocompatible; biodegradable; nontoxic; antimicrobial), CS has potential in the packaging application [9].

Nowadays, many researchers are interested to utilize renewable resources in their researches [10–13]. These renewable resources such as low value plants, energy crops and product from food crops, sawmills, palm oil production, marine waste, and food waste are new materials to produce composites [14].

Normally, natural fillers, or known as renewable raw materials have unlimited availability. These natural fillers promote several advantages, such as low density, renewability, biodegradability, recyclability, and cost effectiveness [9, 10, 15, 16].

Corn (Zea mays) is a Poaceae (grass family) and one of the top three cereal crops grown worldwide [17]. In 2013, the annual world corn production was about 964 million tons, which gener- ated about 204 million tons of corn residues [18]. Corn cob is a corn residues. To reduce this agricultural waste, the corn cob was

utilized as the natural filler in this study. The inclusion of natural fillers into plastics is mainly due to their advantages, such as lower production cost and density, low energy requirements for processing, ease of preparation, and biodegradability.

Over the past few decades, radiation processing was used for polymer modification. Accordingly, the irradiation of polymeric materials with ionizing radiations, such as gamma rays, X-rays, ion beam, and accelerated electrons, lead to the formation of reactive intermediates, ions, free radicals, and excited states. Conse- quently, these intermediates can follow some reaction paths, resulting in disproportion, hydrogen abstraction, arrangements, and/or the formation of new bonds, to enhance the properties of the polymeric materials [19]. The crosslink by using electron beam irradiation effects on the properties of polymers were inves- tigated by some researchers [20, 21]. This radiation technology is not only for surface grafting, but also for reactive compatibilisation to improve the properties of polymers. There are several ben- efits of radiation application to polymer such as: (i) formation of strong bridges between macromolecules; (ii) compatibilisation of polymer blend by high energy radiation; and (iii) presence of mul- tifunction monomer and inomer to accelerate and increase the crosslinking degree [20, 22]. Normally, the radiation process cov- ers radiation crosslinking, radiation induced polymerization, and degradation of polymers. Many researchers reported that the electron beam irradiation has improved the tensile properties of chitosan biocomposite films, such as konjac glucomannan/chitosan [21], starch/chitosan [23], polyvinyl alcohol (PVA)/carboxylme- thylated chitosan [24], and polyaniline grafted chitosan [25].

Radiation crosslink on chitosan had been widely reported by researchers. However, the research on electron beam irradiation crosslink chitosan/corn cob biocomposite films is not found in any literature study.

The development of biodegradable polymers from renewable resources has increased in recent years, especially for the packaging and disposable applications to maintain the sustainable development of economical and ecological attractive technology [26].

Biodegradation can be denoted as degradation that occurs in a biological environment [27]. The utilization of a variety of microorganisms and enzymes to degrade polymers is classified as the biodegradation method of polymers. This research article is focused on two biodegradation methods such as enzymatic hydrolysis and soil biodegradation. Commonly, enzymatic hydrolysis of biopolymers is a heterogeneous process. The heterogeneous process is affected by the interaction between enzymes and polymeric chains [27]. In soil degradation method, soil microbes can initiate the depolymerization of biopolymers, such as polysaccharide, cellulose, and hemicellulose. These soil microbes secrete different

Correspondence to: M. Yeng Chan; e-mail: [email protected] DOI 10.1002/pen.24854

Published online in Wiley Online Library (wileyonlinelibrary.com).

VC2018 Society of Plastics Engineers

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enzymes into the soil water and then begin to breakdown the polymers.

Our previous work, in producing irradiation-crosslinked of chitosan (CS)/corn cob (CC) biocomposite films was successfully and the tensile properties of irradiated CS/CC biocomposite films were improved compared to unirradiated biocomposite films [6]. Hence, the objective of the present work was to inves- tigate the effect of electron beam irradiation of CS/CC biocomposite films on the functional characteristics of biocomposite films, such as thermal stability, gel fraction, enzymatic, and soil biodegradation. The characterization of CS/CC biocomposite films by thermogravimetric analysis (TGA), scanning electron microscope (SEM) and Fourier transform infrared spectroscopy (FTIR) were performed. Currently, chitosan/corn cob biocomposite films are proposed for packaging application, such as plastic bag and nursery poly bag in our university, as shown in Fig. 1.

These products are successfully produced in Universiti Malaysia Perlis (UniMAP) via solvent casting method.

METHODOLOGY Materials

Chitosan (CS) powder (used as polymer matrix) was purchased from Hunza Nutriceutical Sdn. Bhd., Malaysia. The degree of deacetylation (DD) and average particle size of CS were 90% and 80mm, respectively. The CS is in irregular shape. Corn cob (CC) was obtained from the Kodiang Corn Plantation, Kedah, Malaysia.

The CC was cleaned, crushed, and manually grounded to turn into powder form. The Malvern particle size analyzer was used to determine the average CC powder particle size which was 38mm.

Acetic acid was used as the solvent to dissolve the CS powder, which was purchased from Aldrich, Penang, Malaysia.

Preparation of Chitosan (CS)/Corn Cob (CC) Biocomposite Films The CS/CC biocomposite films were prepared by solvent casting method. A 1.5% (w/v) of CS solution was prepared by dispers- ing CS powder into 1% (v/v) of acetic acid and then stirred for 30 minutes using a mechanical stirrer. The CC powder was next added and stirred for 15 minutes. The prepared CS/CC biocomposite solution was poured into a plastic mold with dimension of

20320 cm and dried at room temperature for 48 h. The formula- tions of CS/CC biocomposite films were prepared in ratios CS:

CC of 100:0; 90:10; 80:20; 70:30; and 60:40.

Electron Beam Irradiation Process

The electron beam irradiation process was performed in the Malaysia Nuclear Agency by using an EPS-3000 electron beam machine. The prepared biocomposite films were irradiated at an ambient temperature and a dose per pass of 10 kGy. The accel- erating voltage and beam current used were 2 MeV and 2mA, respectively.

Thermogravimetric Analysis (TGA)

The Perkin-Elmer Pyris Diamond Thermogravimetric analyzer was used to measure the thermal stability of the biocomposite films. About 762 mg of samples were heated at a heating rate of 108C/min. The measurement was scanned from 308C to 6008C and nitrogen flow was kept at a constant rate of 50 mL/min.Tdmax, residues at the temperatures of 2008C, 5008C, and 6008C was measured through the TGA thermogram.

Fourier Transform Infrared (FTIR)

FTIR spectra were measured using a Perkin-Elmer model L1280044 FTIR spectrometer. The spectrometer was operated with a resolution of 4 cm²¹. 16 scans in the wavelength ranging of 4,000–600 cm²¹ were recorded. The attenuated total reflec- tance method was used.

Gel Fraction

The gel fraction of biocomposite films was estimated by following the method as reported by Ramaprasad et al. [28]. The specimens were swelled in 1% of acetic acid for 24 h. The remains (gel) were filtered and oven dried at 508C for 24 h. The percentage of gel fraction was calculated by using the following Equation:

% GF5 w Wo

3100 (1)

where,

% GF5Percentage of gel fraction w5Weight of gel

Wo5Weight of specimen before swelling

Enzymatic Biodegradation

A buffer solution of pH 7.3 was prepared by adding 4.8 mL of 0.2 M acetic acid into 45.2 mL of 0.2 M sodium acetated solution.

Then, 10 mg ofa-amylase was added to the mixture. The samples were immersed into the mixture for 14 days at 378C. The samples were taken out every 2 days and then rinsed with distilled water to remove excessa-amylase on the sample surface. Next, the specimens were oven dried at 508C for 24 h. The weight loss of the enzymatic biodegradation was calculated usingEq.2:

%B5 wi2w1

w_i

3100 (2)

FIG. 1. Proposed packaging application from CS/CC biocomposite films.

[Color figure can be viewed at wileyonlinelibrary.com]

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where,

%B5Percentage weight loss of specimen on biodegradation wi5Initial weight of dried specimen before biodegradation w15Weight of dried specimen after biodegradation

Morphology Study

The biocomposite film surface with and without undergoing the enzymatic biodegradation were examined using a SEM, Model JEOL JSM-6460 LA. The surfaces were coated with pal- ladium to avoid electrostatic charging and poor image resolution during examination.

Soil Biodegradation

Soil degradation test was performed according to ASTM D 5988. The samples were cut into rectangular shapes of dimen- sions of 23330.1 mm (width3length3thickness), respectively. The specimens were oven dried at 508C for 24 h and weighted. The test was performed at 25638C with humidity at 40%–45% to ensure aerobic conditions for degradation. The humidity of soil was measured by using Digital soil tester mater every day. The samples were taken out from soil every two weeks and washed thoroughly with distilled water to remove the remaining soil before conditioning them in an oven at 508C for 24 h. The weight loss of soil degradation can be calculated usingEq.2.

RESULTS AND DISCUSSION Thermogravimetric Analysis (TGA)

The thermogravimetric (TGA) and derivative thermogravimetric analysis (DTG) curves of unirradiated and irradiated CS/CC biocomposite films at different contents of CC are shown in Figs. 2 and 3, respectively. Table 1 summarizes the data of TGA and DTG of unirradiated and irradiated CS/CC biocomposite films. The unirradiated and irradiated CS films exhibited two weight loss steps: (i) decomposition of absorbed moisture (from 508C to 1008C); (ii) decomposition of chitosan, vaporization and elimination of volatile products (from 2008C to

3408C). Whereas, both unirradiated and irradiated CS/CC biocomposite films showed three degradation steps: (i) evaporation of moisture (from 508C to 1008C); (ii) pyrolysis of polysaccharide and decomposition of hemicellulose from CC (from 2008C to 4008C); and (iii) decomposition of cellulose and lignin from CC (from 4208C to 6008C). From Table 1, it can be observed that the T_dmax of both irradiated neat CS film and CS/CC biocomposite films shifted to a high temperature in comparison with unirradiated films. This indicated that the formation of an irradiation-induced crosslink has enhanced the thermal stability of the biocomposite films. Furthermore, the residue at temperatures 2008C and 5008C of unirradiated CS/CC was reduced as CC content increased. However, the irradiated CS/CC biocomposite films showed an opposite trend, the percentage of residue was increased with CC content. This observation was evidence of crosslinking formation between CS and CS chains and CC and CS chains upon radiation, which resulted in a high thermal degradation. The presence of these irradiation-induced crosslin- kages between chains, which have to be broken by high energy before the stepwise degradation of the chain, occurred on pyrolysis. Furthermore, these crosslink promotes char formation at high temperature in pyrolysis of biocomposite films. Lomakin et al. [29] claimed that the tendency of polymer to char was increased by altering its molecular structure. They also noted that the crosslinked polymer formed more char than uncros- slinked polymer, consequently improved the thermal stability of the polymer. Moreover, the formation of char at high temperature acted as barrier to slowdown polymer volatile diffusion and

FIG. 2. TGA curve of unirradiated and irradiated neat CS and CS/CC biocomposite films. [Color figure can be viewed at wileyonlinelibrary.com]

FIG. 3. DTG curve of unirradiated and irradiated neat CS and CS/CC biocomposite films. [Color figure can be viewed at wileyonlinelibrary.com]

TABLE 1. TheTdmaxand percentage residue of unirradiated and irradiated neat CS film and CS/CC biocomposite films at different temperature.

Residue (%) Biocomposite films Tdmax(8C) T200 T500 T600

Unirradiated neat CS 293 83 40 38

Unirradiated CS/CC (80:20) 285 82 29 27

Unirradiated CS/CC (60:40) 272 79 24 20

Irradiated neat CS 295 80 36 34

Irradiated CS/CC (80:20) 289 82 32 30

Irradiated CS/CC (60:40) 281 83 37 35

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thus improved the thermal stability of polymer composites. Nev- ertheless, the char remaining at a temperature of 6008C for the irradiated neat CS was lower than the unirradiated CS. This indicated that the irradiated neat CS was unstable at high decomposition temperatureas compared to the unirradiated neat CS due to less free amino groups in the CS films, provided a stabilizing effect at high degradation temperature.

Fourier Transform Infrared (FTIR) Analysis

The irradiation effect on FTIR spectra for the comparison of unirradiated and irradiated neat CS film and CS/CC biocomposite film is shown in Figs. 4 and 5, respectively. The major spectra of unirradiated and irradiated neat CS film and CS/CC biocomposite film are presented in Table 2. For the unirradiated CS/CC biocomposite film at 20 wt% of CC content, the broad peak at 3,283 cm²¹ was related to theAOH andANH stretching groups. The peak at 2,920 cm²¹ was explained by CAH stretching. The peak at 2,868 cm²¹ was ascribed to ACH₃ aliphatic groups of the CS film. The bands at 1,634 cm²¹ and 1,550 cm²¹presented the C@O stretching and NAH bending of NH2, respectively. Nevertheless, there was no peak that exhibited the free aldehyde group near 1,720 cm²¹. The peak at 1,408 cm²¹ was related to the ACH2 deformation vibration from cellulose or CAH deformation in lignin. The band at 1,328 cm²¹attributed the deformation of CAH groups. Besides that, the sharp peak at 1,151 cm²¹ was ascribed to the ether bond of chitosan CAOAC. Moreover, the peaks at the range of 1,000–1,150 cm²¹ corresponded to the CAOAC and CAO groups, respectively. The presence of peaks at 1,151, 1,063, 1,022, and 897 cm²¹ represented the chitosan saccharide structure. The peak at 897 cm²¹ attributed the CAH bending vibration.

Nevertheless, both irradiated CS film and irradiated CS/CC biocomposite films at 20 wt% of CC content showed evidence that the most obvious difference occurred at the regions of 3,260 and 1,550 cm²¹ (Fig. 4), 3,224 and 1,542 cm²¹ (Fig. 5), which was related to the AOH and ANAOA bonds, respectively. The intensity of the peaks at 3,260 cm²¹ (Fig. 4) and 3,224 cm²¹ (Fig. 5) was reduced. This is due to the fact that the AOH groups of the films was reduced upon irradiation.

However, intensity peak at 1,550 cm²¹(Fig. 4) and 1,542 cm²¹ (Fig. 5) was increased due to the CS formed crosslinks within itself. According to the literature study, the steps of transition from linear macromolecule to continuous network structure due to irradiation took place [30]: (i) linear macromolecules; (ii) branched macromolecules; (iii) branched molecules with indi- vidual two dimensional cycles to branched molecules with frag- ments of three dimensional network; (iv) three dimensional network. A proposed schematic crosslinking reaction between CS and itself when exposed to irradiation is demonstrated in Fig. 6. Besides, when the irradiation exposed on the biocomposite film, the radicals from water radiolysis might attack doubles of CC to form CC radicals. Then, these CC radicals might attack CS long chain to form a new CSACC bond. Therefore, the induced-crosslinks of irradiated CS/CC biocomposite films were increased upon irradiation.

Gel Fraction

Accordingly, the crosslinks of polymer can be calculated from gel fraction determination [31]. Therefore, the gel fraction of irradiated biocomposite films can explain the presence of crosslinks in the biocomposite films. The gel fractions of the unirradiated and irradiated CS film and CS/CC biocomposite films are summarized in Table 3. It can be observed that the gel

FIG. 4. FTIR spectra of unirradiated and irradiated neat CS films.

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fraction of both unirradiated and irradiated CS and CS/CC biocomposite films increased as the CC content increased. This finding was possibly because the natural filler that could not dissolve in acetic solvent. A higher CC content will cause a higher CS/CC biocomposite films gel fraction. The neat CS film was partially dissolved in acetic acid during the swelling process because CS giving a semi crystalline structure in solid state [32]. However, the irradiated neat CS film and CS/CC biocomposite films showed a higher gel fraction than the unirradiated neat CS film and CS/CC biocomposite films. This is due to the fact that the presence of crosslinks by radiation will induce free radical and consequently increased the gel fraction of biocomposite films.

Enzymatic Biodegradation

Figure 7 displays the weight loss for the unirradiation and irradiation CS and CS/CC biocomposite films as a function of incubation time in the presence ofa-amylases. The weight loss data on enzymatic biodegradation of unirradiated and irradiated neat CS film and CS/CC biocomposite films after 14 days are shown in Table 4. The irradiated neat CS film exhibited 14.29%

lower weight loss on enzymatic biodegradation as compared to the unirradiated neat CS film. Besides, it was found that the weight loss in enzymatic biodegradation of unirradiated CS/

CC biocomposite films increased with increasing CC content because CS/CS biocomposite films that will be hydrolyzed by the a-amylases, leading to micro voids and eroded surface.

TABLE 2. The major spectra of unirradiated and irradiated neat CS film and CS/CC biocomposite films.

Biocomposite films Frequency (cm²¹) Bond Intensity (%)

Unirradiated CS/CC biocomposite film 3,285 Hydroxyl (AOH), amine (NAH) —

2,919 CAH stretching

2,878 –CH3aliphatic groups

1,643 C@O stretching

1,551 NAH bending

1,409 C@C stretching

1,332 CH2bending

1,151 CAOAC bridge Saccharide structure of CS

1,063 CAO stretching

1,026 CAO stretching

893 CH bending

Irradiated neat CS film 3,260 Hydroxyl (AOH) Decreased (0.95)

1,550 ANAO bonds Increased (3.15)

Irradiated CS/CC biocomposite film 3,224 Hydroxyl (AOH) Decreased (1.7)

1,542 ANAO bonds Increased (5.0)

FIG. 5. FTIR spectra of unirradiated and irradiated CS/CC biocomposite films at 20 wt% of CC content.

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Conversely, the weight loss of irradiated biocomposite films decreased with the increase of the CC content. The weight loss of the irradiated biocomposite films was lower at around 24.91% than the unirradiated biocomposites films due to the formation of radiation-induced crosslinks in biocomposite films.

The formation of these crosslinks was resistant to be hydrolyzed by a-amylases and prevented the penetration of a-amylases into CS/CC biocomposite films. Furthermore, the presence of radiation-induced crosslinks had restricted and made the area

exposed to enzyme hydrolysis became smaller, resulting in a lower degradation rate by digestive enzymes.

Figure 8a displays the SEM micrograph of unirradiated neat CS film before undergoing enzymatic biodegradation, while Fig.

8b shows the SEM micrograph of unirradiated neat CS film after enzymatic biodegradation. Before biodegradation, the unirradiated neat CS film exhibited a smooth surface. However, after 14 days of enzymatic biodegradation, it was observed that enzyme attack has taken place on the unirradiated neat CS film surface, which was accompanied by the weight loss of oligomer, leading to an eroded surface. Besides that, the formation of micro voids was also observed on the neat CS film surface after the enzymatic biodegradation. As discussed earlier, the CS chain will be hydrolyzed by thea-amylases, owing to the micro voids and eroded surface. These SEM micrograph reflected the results obtained from the weight loss of enzymatic biodegradation.

The SEM micrographs of unirradiated CS/CC biocomposite films with 20 wt% and 40 wt% of CC content before enzymatic biodegradation are shown in Fig. 9a and b, respectively. Both unirradiated CS/CC biocomposite films showed CC distribution and agglomeration at CS matrix on the surface of the biocomposite film. The SEM micrographs of both biocomposite films after biodegradation are represented in Fig. 9c and d, respectively. It is clearly shown that enzyme consumption caused surface erosion in biocomposite film and the surface was rougher

FIG. 6. Proposed schematic reaction between CS and CS chain upon irradiation.

TABLE 3. Gel fraction of unirradiated and irradiated neat CS film and CS/

CC biocomposite films.

Biocomposite films Gel fraction (%)

Unirradiated neat CS 44.39

Unirradiated CS/CC (80:20) 50.34

Unirradiated CS/CC (60:40) 57.51

Irradiated neat CS 63.16

Irradiated CS/CC (80:20) 65.14

Irradiated CS/CC (60:40) 71.58

FIG. 7. Weight loss of unirradiated and irradiated CS/CC biocomposite films on enzymatic biodegradation. [Color figure can be viewed at wileyonlinelibrary.com]

TABLE 4. Weight loss of unirradiated and irradiated neat CS film and CS/

CC biocomposite films after 14 days on enzymatic biodegradation.

Biocomposite films

Weight loss of unirradiated and irradiated CS/CC

biocomposite films after 14 days (%) on enzymatic biodegradation

Unirradiated Neat CS 12.3260.47

Unirradiated CS/CC (80:20) 14.7660.51

Irradiated Neat CS 10.5660.42

Irradiated CS/CC (80:20) 12.5260.50

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than the neat CS film. There were some poles found on the biocomposite film surface. The biocomposite film with high CC content (40 wt%) showed more poles as compared to the biocomposite film with a low CC content (20 wt%). This indicated that the enzyme attacked more on the CC filler than the CS matrix. Hence, the CC degradation was higher than the CS matrix, which resulted in the CC filler to enhance the biodegradability of the neat CS film.

Figure 10a illustrates the SEM micrograph on the surface of irradiated neat CS film. The SEM micrograph showed a

homogeneous surface of the irradiated neat CS film. It can be seen that the surface erosion occurred on the surface of the irradiated film after enzymatic biodegradation, as exhibited in Fig.

10b. Figure 11a and b display the surface of irradiated CS/CC biocomposite films at 20 and 40 wt% of CC content, respectively. Both SEM micrographs indicated that a better filler- matrix adhesion with the filler was covered by the matrix before enzymatic biodegradation. Nevertheless, it can be observed that there were micro voids and surface erosions on the surface of the biocomposite film, were shown in Fig. 11c and d,

FIG. 8. SEM micrograph on surface of unirradiated neat CS film before enzymatic biodegradation; (b). SEM micrograph on surface of unirradiated neat CS film after enzymatic biodegradation.

FIG. 9. SEM micrograph on surface of unirradiated CS/CC biocomposite film at 20 wt% of CC content before enzymatic biodegradation; (b) SEM micrograph on surface of unirradiated CS/CC biocomposite film at 40 wt% of CC content before enzymatic biodegradation; (c) SEM micrograph on surface of unirradiated CS/CC biocomposite film at 20 wt% of CC content after enzymatic biodegradation; (d) SEM micrograph on surface of unirradiated CS/

CC biocomposite film at 40 wt% of CC content after enzymatic biodegradation.

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respectively. However, less micro voids and eroded surface were observed as compared to CS/CC biocomposite film without irradiation. This indicated that the formation of radiation- induced crosslinks in biocomposite films had restricted the penetration of enzyme, resulting in weight loss on the enzymatic biodegradation of irradiated biocomposite films was lower than the unirradiated biocomposite films. The SEM results were in line with the results of enzymatic biodegradation.

Soil Biodegradation

Figure 12 shows the soil biodegradation of unirradiated and irradiated neat CS and CS/CC biocomposite films. Table 5 summarizes the weight loss data on the soil biodegradation of unirradiated and irradiated CS/CC biocomposite films. The biocomposite films were reduced into small particles for a minimal damage to the environment. As seen from the figure, the biodegradation rate of the unirradiated CS/CC biocomposite films

FIG. 10. SEM micrograph on surface of irradiated neat CS film before enzymatic biodegradation; (b) SEM micrograph on surface of irradiated neat CS film after 14 days enzymatic biodegradation.

FIG. 11. SEM micrograph on surface of irradiated CS/CC biocomposite film at 20 wt% of CC content before enzymatic biodegradation; (b) SEM micrograph on surface of irradiated CS/CC biocomposite film at 40 wt% of CC content before enzymatic biodegradation; (c) SEM micrograph on surface of irradiated CS/CC biocomposite film at 20 wt% of CC content after 14 days enzymatic biodegradation; (d) SEM micrograph on surface of irradiated CS/CC biocomposite film at 40 wt% of CC content after enzymatic biodegradation.

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increased with the increase of the CC content. However, after 14 weeks, the CS/CC biocomposite films with 20 wt% and 40 wt%

of CC content were left at about 10% and 20%, respectively. The CS/CC biocomposite films weight loss occurred was higher than the neat CS film. The increment was expected because a higher cellulose content of CC natural filler led to higher water absorp- tion that had a synergetic effect on the biocomposite films degradation. However, lignins are considered as three dimensional natural polymers, which have a macromolecular framework that is difficult to degrade even by microorganisms; only lignolytic microorganisms can do it [33, 34]. Conversely, the irradiated CS/

CC biocomposite films exhibited an opposite trend due to the crosslinks formation between CS and CS or CS and CC had occurred when exposed to irradiation, the microorganisms were restricted from attacking the film. Moreover, the irradiated neat CS film showed a lower weight loss than the unirradiated film. It can be explained that the presence of radiation-induced crosslink- structure in neat CS film and CS/CC biocomposite films had hin- dered the diffusion of microorganisms into CS/CC biocomposite films. Therefore, the irradiated neat CS film and CS/CC biocomposite films were more resistant to degrade in soil as compared to unirradiated films.

CONCLUSIONS

In summary, the results from the thermal analysis revealed that the thermal stability and residues at 2008C, 5008C, and

6008C of the irradiated neat CS film and CS/CC biocomposite films were higher than the unirradiated films. This was due to the fact that the presence of radiation-induced crosslinks improved the thermal stability of the biocomposite films. After being exposed to radiation, the neat CS film and CS/CC biocomposite films exhibited lower weight loss in enzymatic and soil biodegradation. This indicated that the biocomposite films were resistant to biodegrade upon radiation. Furthermore, the irradiated CS/CC biocomposite films at lower dose has a potential in a packaging application as compared to the higher dose.

This is because when a high irradiation dose is applied to the CS/CC biocomposite films, it would improve the brittleness of the biocomposite films.

ACKNOWLEDGMENT

Authors would like thank to Malaysian Nuclear Agency for providing the electron beam irradiation facility. Besides, the authors also thank to Prof. Dr. Salmah Husseinsyah for her guidance along the research.

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FIG. 12. Weight loss of unirradiated and irradiated neat CS film and CS/

CC biocomposite films on soil biodegradation. [Color figure can be viewed at wileyonlinelibrary.com]

TABLE 5. Weight loss of unirradiated and irradiated neat CS film and CS/

CC biocomposite films after 14 weeks on soil biodegradation.

Biocomposite films

Weight loss of unirradiated and irradiated CS/CC biocomposite

films after 14 weeks on soil biodegradation (%)

Unirradiated Neat CS 71.6865.62

Irradiated Neat CS 66.2963.69

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