Biodegradable polymers/bamboo fiber biocomposite with bio-based coupling agent

Abstract

Effects of lysine-based diisocyanate (LDI) as a coupling agent on the properties of biocomposite from poly (lactic acid) (PLA), poly (butylene succinate) (PBS) and bamboo fiber (BF) were investigated. Tensile properties, water resistance, and interfacial adhesion of both PLA/BF and PBS/BF composites were improved by the addition of LDI, whereas thermal flow became somewhat difficult due to cross-linking between polymer matrix and BF. Crystallization temperature and enthalpy in both composites were increased and decreased with increasing LDI content, respectively. The heat of fusion in both composites was decreased by addition of LDI, whereas there was no significant change in melting temperature. Thermal degradation temperature of both composites was lower than those of pure polymer matrix, but the composites with LDI showed higher degradation temperature than those without LDI. Enzymatic biodegradability of PLA/BF and PBS/BF composites was investigated by Proteinase K and Lipase PS, respectively. Both composites could be quickly decomposed by enzyme and the addition of LDI delayed the degradation.

Introduction

In recent years, the development of biocomposites from biodegradable polymers and natural fibers have attracted great interests in the composite science, because they could allow complete degradation in soil or by composting process and do not emit any toxic or noxious components [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16].

Among the biodegradable polymers, in particular, poly (lactic acid) (PLA) and poly (butylene succinate) (PBS) are of increasing commercial interest. PLA can be synthesized by the condensation polymerization of the lactic acid or ring opening polymerization of the cyclic lactide dimer. Advanced industrial technologies of polymerization have been developed to obtain high molecular weight PLA that leads to a potential for structural materials with enough lifetime to maintain mechanical properties without rapid hydrolysis even under humid environment, as well as good compostability. PLA is transparent and crystalline polymer with relatively high melting point and has brittle properties, i.e. high strength and low elongation at break [17].

PBS is a white crystalline thermoplastic with melting point of about 90–120 °C (similar to LDPE), glass transition temperature of about −45 to −10 °C (between PE and PP), tensile strength between PE and PP, and stiffness between LDPE and HDPE. PBS has excellent processing capabilities and can be processed on polyolefin processing machines at temperatures of 160–200 °C, into various products, such as infected, extruded and blown ones [1]. Furthermore, its raw materials, butanediol and succinic acid, may be soon available from bio-based renewable resources [18], [19], [20], [21], [22], [23].

Natural fibers can be a renewable and cheaper substitute for synthetic fibers, such as glass and carbon and have numerous advantages, such as low cost, low density, high toughness, acceptable specific strength properties, ease of separation and biodegradability. So, there is much research on natural fiber-reinforced composites [24], [25], [26], [27], [28], [29], [30], [31], [32], [33]. However, the main drawback of natural fiber may be their hydrophilic nature, which decreases the compatibility with hydrophobic polymeric matrix. In these composite fields, therefore, most of the research has focused on improving interfacial properties between the polymer matrices and natural fillers in order to enhance the physical and mechanical properties of the end products.

The purpose of this study was to develop the biocomposites with designable interfacial properties from biodegradable polymers, PLA and PBS, and bamboo fiber by using lysine-diisocyanate as a bio-based coupling agent.

So far, several conventional isocyanates, such as methylene diisocyanate (MDI), toluene diisocyanate (TDI), 4-4′-methylenedicyclohexyl diisocyanate (hydrogenated MDI), and hexamethylene diisocyanate have been used as a coupling agent [34], [35], [36], [37], [38]. For example, Wang et al. reported the effect of MDI on the properties of PLA/starch blend. The addition of MDI resulted in an enhancement of mechanical properties and water resistance [39], [40].

However, these isocyanates have found limited use as a biocompatible material because their ultimate hydrolysis products, i.e. their corresponding diamines, such as 4,4′-methylenedianiline and 2,4-diaminotoluene have been found to be a cancer suspect agent or produce hepatitis in man. So, the use of nontoxic materials should be expected as a coupling agent, in order to synthesize fully biodegradable biocomposites without emitting toxic or noxious components.

Lysine-diisocyanate (LDI) is based on lysine with two amino groups and one carboxyl group, which is one of natural amino acids. LDI can react with hydroxyl or carboxyl groups in PLA or PBS, producing urethane bonds that can be easily and completely hydrolyzed into raw materials [41], [42], [43], [44], [45]. For example, the polyurethane that synthesized from LDI, glycerol, and ascorbic acid can be completely degraded in aqueous solution and yield the nontoxic breakdown products of lysine, glycerol, and ascorbic acid [46]. In fact, our interest in LDI as a bio-based coupling agent stems from these facts, because ecotoxicity is currently a key point in biocomposites.