Tensile Properties of Chitosan/Corn Cob Biocomposite Films by Crosslinking with Adipic Acid

*Chan Ming Yeng1), Salmah Husseinsyah2), Sam Sung Ting3)

1), 2) Division of Polymer Engineering, School of Materials Engineering, Universiti Malaysia Perlis, 02600 Jejawi, Perlis, Malaysia.

3) School of Bioprocess Engineering, Universiti Malaysia Perlis, 02600 Arau, Perlis, Malaysia.

2) chanmingyeng@hotmail.my

ABSTRACT

The utilization of adipic acid (ADP) as a crosslinking agent had enhanced the tensile properties of chitosan/corn cob biocomposite films. This paper covered the effect of ADP content on tensile properties and morphology study of chitosan (CS)/corn cob (CC) biocomposite films. The CS/CC biocomposite films were prepared by solvent casting method. The tensile properties of modified CS/CC biocomposite film with ADP is higher than unmodified biocomposite films. This is due to the presence of ADP, which brought a better interfacial interaction as confirmed by morphology study. The tensile strength and elongation at break of modified CS/CC biocomposite film reduced as increasing of ADP content. The optimum increment of tensile strength and elongation at break is at 1 % of ADP. On the other hand, the modulus of elasticity increased with ADP content. Keyword: Chitosan, corn cob, adipic acid, crosslinking, biocomposite, films. 1. INTRODUCTION

     Chitosan is a polyssacharide polymer that is composed mainly of β-(1, 4)-linked 2-deoxy-2-amino-D-glucopyranose and partially of β-(1, 4)-linked 2-deoxy-2-acetamido-D-glucopyrose (Khan 2013; Srinivasa 2007). Due to the biodegradability, non-toxicity and biocompatibility of chitosan recently much attention has been paid to chitosan as a potential material to replace synthetic polymer. Chitosan can be becomes cationic polyssacharide when it dissolve in acidic solution (e. g. acetic acid, lactic acid or formic acid). This is due to the presence of protonation of amino groups of the pyranose ring (Miao 2008). __________________ 1) Graduate Student 2) Associate Professor 3) PhD

Fig. 1 shows the proposed protonation reaction between chitosan and acetic acid. Generally, the chitosan film prepared by solvent evaporation, chemical cross-linking, or physical interaction with protein (Porta 2011). Chitosan can form transparent films, which can apply in a variety of packaging application (Srinivasa 2007).

Fig.1 Proposed protonation reaction between chitosan and acetic acid (Yeng 2013a).

 

Many paper have recently been published on the utilization of agricultural waste as a filler in polymer composites. In previous studies, there have been published the study of agricultural filled polymer composites such as corn cob (Chun 2013a; Yeng 2013b, c), cocoa pod husk (Chun 2013b), and coconut shell (Chun 2012). These fillers gave several benefits including cost effectiveness, being recyclable, biodegradability, low density, and being renewable. In this paper, corn cob was utilized as a filler in chitosan film. Corn (Zeamays) is cultivated around the worth. Corn cob being as waste of corn industry. Thus, corn cob has a potential as natural filler filled in polymer that could have economic advantages and low environment impact. Unfortunately, chitosan/corn cob biocomposite films inherent unfavorable tensile properties. Consequently, to obviate this problem, chemical modification on biocomposite films by crosslinking agent can be improve the tensile properties (Yeng 2013b). Normally, the crosslinking reaction occurred between the active groups of crosslinking agent (such as acid groups, aldehyde groups and epoxy groups) and amino groups of chitosan. Adipic acid is dicarboxylic acid, which perform as crosslinking agents through the formation of amide linkages by the carboxylic groups of adipic acid and amino groups of chitosan (Cai 2013). Currently, many investigations have been concentrated on improving the properties by crosslinking method (Cai 2013; Mitra 2012; Beppu 2007; Monteiro 1999; Mathew 2007).

The present study emphasizes that the preparation of chitosan/corn cob biocomposite films using adipic acid as cross-linking agent. The effect of adipic acid content on tensile properties and morphology of biocomposite films were investigated.  

2. METHODOLOGY

2.1 Materials      Chitosan (used as Matrix) was purchased from Hunza Nutriceuticals Sdn. Bhd. (Malaysia) with degree of deacetylation (DD) of 90 %. Corn cob (used as filler) was collected from Kodiang Plantations, Kedah (Malaysia), with an average size of 38 µm. Adipic acid (used as crosslinking agent) was supplied by Merck, Germany.  

2.2 Formulations All CS/CC biocomposite films were prepared through solvent casting method in ratios CS: CC of 80:20 with and without addition of adipic acid (ADP) as crosslinking agent. Table 1 shows formulations of unmodified and modified CS/CC biocomposite films.

Table 1: Formulations of unmodified and modified CS/CC biocomposite films. Materials Unmodified CS/CC

biocomposite films Modified CS/CC

biocomposite films Chitosan (CS) 80 wt% 80 wt% Corn Cob (CC) 20 wt% 20 wt%

Adipic acid (ADP) (1 w/v %) - 1, 2, 3 % 2.3 Preparation of unmodified and modified CS/CC biocomposite films Firstly, CS powder was dissolved in acetic acid (1 v/v %) to produce 1.5 w/v % of CS solution and stirred for 30 minutes. Then, the CC powder was added and stirred until homogenous solution was obtained. However, for modified CS/CC biocomposite films with ADP, the ADP powder was first dissolve in methanol to produce 1 w/v % of ADP solution. After that, different content of ADP solution (1, 2, and 3 %) were mixed with CS solution, and CC filler was added. Both of biocomposite solution were poured into acrylic mould and dried at room temperature for 48 hours. 2.4 Tensile testing The tensile properties (tensile strength, elongation at break and modulus of elasticity) were carried out by using an Instron Universal Testing System, Model 5569, following ASTM D 882 method. The biocomposite films were cut into rectangular shape with size of 100 x 15 mm. The tensile test was performed at 15 mm/min of cross-head speed under room temperature (25 ± 3 oC) condition.

2.5 Morphology study The fracture surface of biocomposite films were analyzed by a scanning electron microscope (SEM), Model JEOL JSM-6460 LA, at an accelerating voltage of 5 kV. The biocomposite films were coated with a thin layer of palladium before testing.

3. RESULTS AND DISCUSSION Fig. 2 shows the effect of ADP content on tensile strength of CS/CC biocomposite films. It can be observed that the tensile strength of modified CS/CC biocomposite films with ADP is higher as compared to unmodified biocomposite films. However, at 1 % of ADP showed the highest tensile strength. This may facilitate that the formation of amide linkages between carboxylic groups of ADP and amino group of CS, which enhanced the interfacial adhesion between CS matrix and CC filler. Up to 2 % of ADP content, the tensile strength of CS/CC biocomposite films were reduced due to the increasing of crosslinkages, which led to low tensile strength. This phenomena was supported by morphology study. The SEM micrograph of CS exhibited a homogeneous surface and matrix tearing (Fig. 3). Additionally, the SEM micrograph of unmodified CS/CC biocomposite at 20 wt % of CC content is illustrated in Fig. 4. From Fig. 4, it can be seen that rough surface, CC filler pulled out and detached from CS matrix. This micrograph indicates poor wettability and interfacial interaction between CS matrix and CC filler, resulting in a low tensile strength. Alternately, modification CS/CC biocomposite films with ADP had enhanced the adhesion between matrix and filler with less filler pulled out and detached, as shown in Fig. 5. The proposed schematic crosslinking reaction between CS and ADP is illustrated in Figure 6.

Fig. 2 Tensile strength of unmodified and modified CS/CC (80:20) biocomposite films with

different ADP content.

Fig. 3 SEM micrograph of neat CS

Fig. 4 SEM micrograph of unmodified CS/CC (80:20) biocomposite film.

Fig. 5 SEM micrograph of modified CS/CC (80:20) biocomposite film with 1 % of ADP.

Fig. 6 Proposed schematic cross-linking reaction between chitosan and adipic acid.

The elongation at break of unmodified and modified CS/CC biocomposite films with different ADP content is displayed in Fig. 7. Obviously, the elongation at break of modified CS/CC biocomposite films increased with optimum increment at 1 % of ADP. However, at high content of ADP (3 %), the elongation at break is lower than unmodified biocomposite film. This may attributed to the presence of ADP provided the plasticizing effect to biocomposite films. But at high content of ADP, it increased the rigidity of CS/CC biocomposite films, resulting in a low elongation at break.

Fig. 7 Elongation at break of unmodified and modified CS/CC (80:20) biocomposite films

with different ADP content. Fig. 8 illustrates modulus of elasticity of unmodified and modified CS/CC biocomposite films. It was found that the presence of ADP was improved the stiffness of biocomposite films. This attributed to the formation of amide linkages between CS and ADP, leading to enhance interfacial interaction and stiffness of CS/CC biocomposite films.

Fig. 8 Modulus of elasticity of unmodified and modified CS/CC (80:20) biocomposite films

with different ADP content.

3. CONCLUSION

The presence of adipic acid as cross-linking agent improved the tensile strength, elongation at break and modulus of elasticity of CS/CC biocomposite films. However, at

same content of CC, modification with 1% of ADP showed the optimum increment in tensile strength and elongation at break. The improvement of tensile properties is due to formation of amide bonds between CS and ADP, owing to better interfacial interaction between CS and CC, as proven by morphology study. ACKNOWLEDGEMENT

This project was supported by Ministry of Higher Educational (MOHE) for providing Fundamental Research Grant Schemes (FRGS). No. Project: 9003-00402. REFERENCES Beppu, M. M., Vieira, R. S., Aimoli, C. G., and Santana, C. C. (2007), “Crosslinking of

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