The influence of fibre treatment on the performance of coir-polyester composites

Abstract

Surface modifications of coir fibres involving alkali treatment, bleaching, and vinyl grafting are made in view of their use as reinforcing agents in general-purpose polyester resin matrix. The mechanical properties of composites like tensile, flexural and impact strength increase as a result of surface modification. Among all modifications, bleached (65°C) coir-polyester composites show better flexural strength (61.6 MPa) whereas 2% alkali-treated coir/polyester composites show significant improvement in tensile strength (26.80 MPa). Hybrid composites comprising glass fibre mat (7 wt.%), coir fibre mat (13 wt.%) and polyester resin matrix are prepared. Hybrid composites containing surface modified coir fibres show significant improvement in flexural strength. Water absorption studies of coir/polyester and hybrid composites show significant reduction in water absorption due to surface modifications of coir fibres. Scanning electron microscopy (SEM) investigations show that surface modifications improve the fibre/matrix adhesion.

Introduction

Growing environmental awareness throughout the world has triggered a paradigm shift towards designing materials compatible with the environment. The use of biofibres, derived from annually renewable resources, as reinforcing fibres in both thermoplastic and thermoset matrix composites provide positive environmental benefits with respect to ultimate disposability and raw material utilization [1]. The advantages of natural lignocellulosic fibres over traditional reinforcing materials such as glass fibres, talc and mica are acceptable specific strength properties, low cost, low density, nonabrasivity, good thermal properties, enhanced energy recovery, and biodegradability. The main bottle necks in the broad use of these natural fibres in various polymer matrixes are poor compatibility between the fibres and the matrix, and the inherent high moisture absorption, which brings about dimensional changes in the lignocellulosic based fibres [2]. The efficiency of a fibre-reinforced composite depends on the fibre/matrix interface and the ability to transfer stress from the matrix to the fibre. This stress transfer efficiency plays a dominant role in determining the mechanical properties of the composite.

In natural lignocellulosic fibres, cellulose is the main component. The elementary unit of a cellulose macromolecule is anhydro-d-glucose, which contains three hydroxyls (−OH). These hydroxyls form hydrogen bonds inside the macromolecule itself (intra-molecular) and also with hydroxyl groups from moist air. Therefore, all natural fibres are hydrophilic in nature and their moisture content can reach 3–13% [3]. The internal adhesion can be improved by modifying the surface topology of fibres by a suitable pre-treatment or by selecting the proper components of the bonding system.

Coir is an important lignocellulosic fibre obtained from coconut trees which grow extensively in tropical countries. Because of its hard-wearing quality, durability and other advantages, it is used for making a wide variety of floor furnishing materials, yarn, rope etc [4]. However, these traditional coir products consume only a small percentage of the potential total world production of coconut husk. Hence, research and development efforts have been underway to find new use areas for coir, including utilization of coir as reinforcement in polymer composites [5], [6], [7], [8], [9], [10], [11]. Unfortunately, the performance of coir as a reinforcement in polymer composites is unsatisfactory and not comparable even with other natural fibres due to its low cellulose content (36–43%), high lignin content (41–45%) and high microfibrillar angle. Morphological studies of coir fibres show the outer sheath of lignin that develop the cellulose ultimates. The removal of this surface layer of lignin usually results in a better and more stable bond. Keeping in view the above facts in our present investigation, we have chosen alkali treatment, bleaching and graft copolymerization as surface treatments for coir fibres in order to obtain better adhesion of these fibres with the polyester resin. The mechanical properties like tensile, flexural and impact strength of coir-polyester and hybrid composites are determined. The improved fibre-matrix adhesion has been examined by scanning electron microscopy (SEM).