Effect of alkali treatment on interfacial and mechanical properties of coir fiber reinforced poly(butylene succinate) biodegradable composites

Abstract

The poly(butylene succinate) (PBS) biodegradable composites reinforced with coir fibers were developed. The effect of alkali treatment on the surface morphology and mechanical properties of coir fibers, interfacial shear strength (IFSS) and mechanical properties of coir fiber/PBS composites was studied. The effect of fiber mass content varying from 10% to 30% on the mechanical properties of coir fiber/PBS composites was also investigated. The coir fibers which are soaked in 5% sodium hydroxide solution at room temperature (RT) for 72 h showed the highest IFSS with 55.6% higher than untreated coir fibers. The mechanical properties of alkali-treated coir fiber/PBS composites are significantly higher than those of untreated fibers. The best mechanical properties of alkali-treated coir fiber/PBS composite were achieved at fiber mass content of 25% in this study, which showed an increase of tensile strength by 54.5%, tensile modulus by 141.9%, flexural strength by 45.7% and flexural modulus by 97.4% compared to those of pure PBS resin. The fiber surface morphologies and fractured surface of the composites exhibited an improvement of interfacial fiber–matrix adhesion in the composites reinforced with alkali-treated coir fibers.

Introduction

In the past decade, natural fiber composites based on petroleum-based thermoplastics or thermosets matrices have been used in various industrial sectors, especially in automobile industry such as door panels, seat backs, headliners, package trays, dashboards, and interior parts [1], [2]. However, these natural fiber composites are not fully environmentally friendly because matrix resins are non-biodegradable [3]. Therefore, biodegradable composites based on natural fibers and biodegradable polymeric matrix made from cellulose, starch, and other natural resources are called “green composites” and have been developed because of their environmentally beneficial properties [4], [5], [6], [7], [8]. In general, the research and development of natural fiber biodegradable composites from renewable resources for a wide range of applications is increasing due to their advantages, such as eco-friendliness, lightweight, carbon dioxide reduction and biodegradable characteristics.

The commercial natural fibers such as henequen, hemp, jute, kenaf, sisal, flax, bamboo, coir, banana, palm, silk, cotton and wood are renewable resources in many developing countries. These fibers offer specific benefits such as low cost, low density, low pollutant emissions, acceptable specific properties, renewable characteristics, enhanced energy recovery, and complete biodegradability [9], [10], [11], [12]. They are considered as strong candidates to replace the conventional glass fibers due to eco-friendliness, low cost, renewable resources and biodegradability. Among the natural fibers, plant fibers which contain strongly polarized hydroxyl groups are hydrophilic in nature [13]. These fibers are inherently incompatible with hydrophobic thermoplastics. Furthermore, due to the presence of pendant hydroxyl and polar groups in various constituents of fibers, moisture absorption of fibers is very high and leads to poor interfacial bonding with the hydrophobic matrix polymers. Therefore, it is necessary to decrease the moisture absorption and hydrophilic character of fibers by suitable surface chemical modification [14], [15], [16], [17].

Among the plant fibers, coir fibers are nowadays extensively used in many industrial applications. Coir is a versatile lignocellulosic fiber extracted from the tissues surrounding the seed of coconut palm (Cocos nucifera). Coir consists of cellulosic fibers with hemicellulose and lignin as the bonding materials for the fibers. Table 1 summarizes several physical, chemical and mechanical properties of coir fiber compared with other typical natural fibers such as flax, hemp, jute, ramie and sisal [2], [11], [18]. Coir fiber has low cellulose and hemicellulose, high lignin content and high microfibrillar angle compared with other natural fibers (Table 1). As a result tensile strength and Young’s modulus of coir fiber are lower than those of other plant fibers. Coir fiber has low moduli due to high microfibrillar angle [19]. Besides, elongation at break of natural fibers increases with increasing microfibrillar angle, thus the elongation at break of coir is the highest among typical natural fibers [20]. This property of coir fiber is certainly useful in cushion applications. An example of the application to seat cushion for automobiles is reported in [21]. The high lignin content in coir fiber is responsible for other useful properties such as weather, fungal, and bacterial resistance [20]. The lignin content in coir fiber is quite high, so the fiber becomes stiffer and tougher.

Due to hardwearing quality, durability and other advantages, coir is used for marking a wide variety of floor-furnishing materials, yarn, rope, etc. However, these traditional coir products consume only a small percentage of the potential total world production of coconut husk. According to official website of International Year for Natural Fibres 2009, about 500,000 tonnes of coir are produced annually, mainly in India and Sri Lanka followed by Thailand, Vietnam, the Philippines and Indonesia. Its total value is estimated at $100 million. Hence, the research and development efforts have been underway to find new utilization of coir as a reinforcement in polymer composites, such as coir-polypropylene and coir based polyester green composites [15], [16], [17], [21], [22], [23], [24], [25].

A fully biodegradable composite reinforced by natural fibers is usually made from completely biodegradable polymeric matrix. Among the completely biodegradable polymers which have been frequently studied as biodegradable polymer matrices in the biocomposites, polylactic acid (PLA) and poly(butylene succinate) (PBS) are increasing commercial interest [26]. However, PBS is commercially available at lower cost than PLA. PBS can be naturally degraded into the environment by bacteria and fungi [27], [28]. Furthermore, PBS has excellent biodegradability in nature, such as in soil, lake, sea, and compost [29]. It can be completely combustible by fire without evolving toxic gases as described in [30]. It has comparable mechanical properties with several thermoplastics such as polyethylene, polypropylene and polystyrene. Therefore, PBS can be a good candidate material for the matrix of biodegradable composites.

The combination of coir fibers and PBS resin can produce the environment-friendly biodegradable composite. In the present work, tensile properties of untreated and alkali-treated coir fibers were reported. The effect of alkali treatment on the interfacial shear strength (IFSS) of coir fiber/PBS system was evaluated by single fiber pull-out test. The PBS biodegradable composites reinforced with untreated and alkali-treated coir fibers were fabricated by compression molding method. The effect of alkali treatment and fiber content on mechanical properties of coir fiber/PBS biodegradable composites was studied. Coir fiber surface morphology and fractured surfaces of untreated and alkali-treated coir fiber/PBS composites were investigated by scanning electron microscope (SEM) providing the information for the evaluation of interfacial fiber–matrix adhesion.