Low density polyethylene–chitosan composites: A study based on biodegradation

Abstract

In recent years, there has been a marked increase in the interest in biodegradable polymer materials for the use in packaging, agriculture, medicine, and other areas. Biodegradable composite films has been fabricated, via peroxide initiated melt-compounding process, by blending low density polyethylene (LDPE) and chitosan, in a rotor disperser. Chitosan, maleic anhydride (MA), dicumyl peroxides (DCP) and oleic acid constituent in palm oil have been used as biodegradable filler, coupling agent, free radical initiator and Lewis acid catalyst, respectively. The palm oil serves the additional function of a plasticizer as indicated by the mechanical property studies. The hydrophilicity, thermal, biodegradablity and dielectric properties of the biocomposite system have also been investigated. Biodegradability of the samples has been studied by inoculating the films with Aspergillus niger (A. niger) on a potato dextrose agar media and incubated at surrounding temperature (25 °C) for 21 days. After incubation, the films were again subjected to morphological and tensile studies. Hydrophilicity and biodegradation rate has been found to increase with increase in chitosan loading in the matrix. The plasticized samples showed better biodegradablity rate and hydrophilicity compared to the unplasticized ones. The addition of palm oil retains the thermal stability of the present biocomposite film. Chitosan and palm oil hybrid reinforced LDPE has proved to be a novel combination with increased biodegradable rate of LDPE. The improved dielectric property of the composite has also enhanced the antibacterial properties. The tailor-made biodegradable plastic film proves to be eco-friendly alternatives to synthetic plastics and have invoked potential applications in food-packaging, bio-separations and drug delivery.

Introduction

Conventional polyethylene products are nonbiodegradable and will remain hundreds of years in soil. The increased cost of solid waste disposal methods as well as the potential hazard from waste incineration such as dioxin emission from thermoplastics is a severe problem in waste management [1]. Recycling of polymeric waste proves to be a better solution for this problem but is very costly. Moreover, the recycled products have poor mechanical properties [2]. Major research is currently underway to develop materials with better biodegradability and more environment-friendly nature [3].

Biodegradable polymers are designed so that degradation happens by the enzymatic action of living micro-organisms such as bacteria or fungi to produce carbon dioxide, water and nontoxic biomass [4]. Polymer blends and composites with natural polymers as one of the components have been developed by many researchers [5], [6]. These types of systems are easily processable and can be commercialized. One of the major disadvantages of blending or reinforcing natural polymers with synthetic polymer is their incompatibility with the matrix, which is caused by the immiscibility of the hydrophilic natural polymers with the hydrophobic synthetic polymers [7].

Low density polyethylene (LDPE) is a low-cost material with good processability, excellent electrical insulation properties, chemical resistance, high toughness and flexibility even at a low temperature [8]. Reasonable transparency of thin films, free from odor and toxicity, better ductility, low water vapor permeability and heat seal ability are also the peculiarities of LDPE [4], [9]. It is used for packaging applications, making trays and plastic bags for food and non-food items. It is also used as a protective coating on paper, textiles and other plastics. The biodegradability resistance of LDPE is one of the major concerns these days [10]. This problem can be overcome by proper introduction of natural polymers such as starch, chitosan or cellulose in the matrix of polyethylene [11]. Among these natural polymers, chitosan (which is the second most plentiful natural biopolymer) is of great interest due to its unique antimicrobial activity, and has the advantages of biodegradability, biocompatibility and excellent film-forming ability [12], [13]. Chitosan produced from chitin, is a natural polysaccharide found in crab, shrimp, lobster, coral, jellyfish, butterfly, ladybug, mushroom and fungi. Marine crustacean shells are widely used as primary sources for the production of chitosan [14]. A common method for the synthesis of chitosan is the deacetylation of chitin using sodium hydroxide in excess as a reagent and water as a solvent. Chitosan acts as a biodegradable additive and gives antimicrobial properties to the blend [15]. It has been reported in literature that highly deacetylated chitosan is more antimicrobial than acetylated chitosan [16].

The biodegradability enhancement of polyethylene is one of the major research areas, which has been actively pursued [17], [18]. In all these studies, significant biodegradability enhancement of polyethylene has not been achieved by chemical or physical modifications. The synthetic plasticizers cannot be used as they are non-biodegradable, irritating, corrosive and even toxic. Generally used plasticizers include mineral oils, synthetic esters and some of the natural products [19]. Glycerol, ethylene glycol (EG), propylene glycol (PG), diethylene glycol (DEG) and triethylene glycol (TEG) are the commonly used plasticizers in LDPE. Due to their volatility or UV susceptibility some of the problems such as leaching, migration and insufficient lubrication at sub-zero temperatures and suspected carcinogenic effects on a number of living organisms have been reported [20]. Glycerol is the most used plasticizer for LDPE based biodegradable composite films. However, glycerol causes the composite to have high water vapor permeability and often migrates from biocomposite due to its high sensitivity towards moisture. Ethylene glycol is very toxic and many poisoning cases have been reported throughout the globe [19], [20].

The fast depleting petroleum resources call for exploration of alternative materials. The vegetable oils are the potential substitutes for mineral oils in this regard. Its renewable nature and presence of various fatty acids make them more important. Vegetable oils in general were used as a coupling agent for improvement of filler-rubber interaction in carbon black reinforced rubber [21]. Studies on the concentration effect of stearic acid on natural rubber were reported by Coran [22]. He found that the specific rate of vulcanization decreased on increasing stearic acid concentration. Barton and Hart [23] reported that mechanical properties of polymer composite improved as the lauric acid content increases. Epoxidized soybean oil (ESBO) has been already approved for use as a PVC plasticizer in gaskets and metal caps, such as for baby food jars [24]. Fatty acids obtained from palm oil processing consist of a mixture of myristic, palmitic, stearic, lauric, oleic and linoleic acids [25]. These acids are long stearic chain compounds containing an even number of carbon atoms, C₁₀–C₁₈. Hence palm oil can improve flexibility, processability and filler dispersion of the composite films.

LDPE has dielectric properties like low dielectric constant, loss factor and high-volume resistivity and is used as high-frequency insulators. It is found that the introduction of polar components into polyethylene matrix increased the dielectric constant and dielectric loss of the blends [26]. The cationic properties of chitosan offer good opportunities to take advantage of electron interactions with numerous compounds and incorporate specific properties into the material [27]. The antibacterial activity of chitosan arising from its polycationic (polar acetate and ${\text{NH}}_3^{+}$ ions) nature is well known for a variety of bacteria and fungi [28]. The interaction between the positively charged chitosan and the negatively charged microbial cell wall leads to its damage and the leakage of the intracellular constituents. The binding of chitosan with DNA and inhibition of mRNA synthesis occurs by the penetration of chitosan molecules into the nuclei of the microorganisms and interfering with the synthesis of mRNA and proteins [29]. In this paper, we have studied the effect of chitosan loading on dielectric constant as a function of frequency.

The present work aims at improving the biodegradability of LDPE by using palm oil as a plasticizer and chitosan as filler. Palm oil being a renewable resource is biodegradable, nontoxic and a more effective coupling agent. In addition the envisaged filler, chitosan has antimicrobial activity against a wide range of target organisms. The combination of filler (chitosan) with plasticizer (palm oil) is proposed to improve the antibacterial properties of the composite for packaging applications. The present paper investigates the morphology, mechanical, thermal and hydrophilicity effect of fatty acid oil (palm oil) on LDPE/Chitosan composites.