Water sorption in oil palm fiber reinforced phenol formaldehyde composites

Abstract

Water sorption kinetics in oil palm fiber reinforced phenol formaldehyde (PF) composites and oil palm/glass hybrid fiber reinforced PF composites was investigated. The influence of fiber loading, relative volume fractions of fibers in hybrid composites and fiber surface modifications on the kinetic and thermodynamic parameters of water sorption by the composites were also studied. Water sorption at four different temperatures was analysed and compared. Composite with 10 wt% fiber loading exhibits maximum water uptake. Hybridisation of the oil palm fiber with glass considerably decreased the water sorption by the composite. The concentration dependency of the diffusion coefficient was analysed and discussed.

Introduction

Water sorption in polymer composites receives considerable attention owing to its theoretical and practical significance, as they are important for many applications such as wastewater treatment, packaging, and building industry. Water sorption have effects on the physical properties of the composites and can affect the matrix structure and the fiber–matrix interface resulting in changes of bulk properties such as dimensional stability, mechanical and electrical properties. The important factors that affect water sorption of composites are the hydrophilicity of the individual components and the structural arrangement of the fibers within the matrix.

Natural fibers being lignocellulosic are highly hydrophilic in nature and are permeable to water. Incorporation of natural fibers into polymeric matrices thus generally increases the water sorption ability. Aditya and Sinha reported that moisture diffusivities of epoxy–jute composites were observed to be eight times as high as that of equivalent epoxy–glass laminates [1]. Oil palm fiber is lignocellulosic having 65% cellulose and 19% lignin content. The fiber is highly hydrophilic due to its polarity owing to the free hydroxyl groups from cellulose and lignin. These hydroxyl groups can hold water molecules by hydrogen bonding. Cellulose is a hydrophilic glucan polymer consisting of linear chain of 1,4-β anhydro glucose units. The hydroxyl groups on crystalline regions can form hydrogen bonds between parallel chains thereby reducing the water sorption. Studies on water sorption behaviour of oil palm fibers at different temperature in distilled water, mineral water and salt water were carried out in this lab and is reported elsewhere [2]. Scanning electron microscopic studies of the oil palm fiber surface reveals micropores on its surface (Fig. 1). This facilitates the capillary action during sorption. The mechanism of absorption is different in fiber incorporated composite material. This can be illustrated with the help of a schematic model (Fig. 2). Water enters through the interface and can diffuse through the porous structure of the fibers as seen in the figure. The cross-sectional area of the composite becomes the main absorption face. Each fiber is found to be surrounded by the phenol formaldehyde (PF) resin as they can penetrate through individual fibers. Three dimensionally cross-linked thermoset matrix have negligible water sorption. The water penetration and diffusion are mainly through the fiber–matrix interfacial region and the cross-sectional portions of the fiber by capillary mechanism. This mechanism involves flow of water molecules along the fiber–matrix interface, followed by diffusion from the interface into the matrix and fibers [3]. Extent of fiber–matrix adhesion is an important factor in determining the sorption behaviour of the composite. The cross-sectional structure of the oil palm fiber is evident from the scanning electron micrographs of the fracture surfaces of the embedded fibers in PF matrix (Fig. 3). It shows a lacuna like portion in the middle surrounded by porous tubular structures. This can be a major access to the penetrating water. Karmaker et al. [4] reported that in jute fiber reinforced polypropylene composites, a gap surrounding the fiber results from the thermal shrinkage of the matrix. In the case of thermoset matrix, this thermal shrinkage during curing is comparatively high and result in decohesion between oil palm fiber and PF matrix resin. This is evident from the scanning electron micrograph of the cross-section of an embedded oil palm fiber in PF matrix (Fig. 4). Due to the high hydrophilic nature of phenolic resin and oil palm fiber, higher compatibility is achieved at this interfacial region. Thus, the gap width is comparatively lower in phenolic composites. This gap is shown in the schematic model (Fig. 2). The mechanism of water sorption is schematically shown in the model. Water sorption by the fibers results in an increase of the fiber diameter and fills the gap surrounding the fibers. This results in a better stress transfer at the interface and thus the interfacial properties of the composite enhanced at swollen stage. This is discussed later in this paper.

Moisture is sorbed in the polymer composite mainly by the dissolution of water in the polymer network, moisture sorption onto the free volume if any present in the glassy structure and by the hydrogen bonding between hydrophilic groups of water and the components of the composite. Microcracks can also pave way to moisture transport involving flow and storage of water within the cracks. The present article summarises the water sorption behaviour of untreated and treated oil palm fiber reinforced PF composites and oil palm fiber/glass hybrid PF composites. The effects of fiber loading, fiber treatment and relative volume fractions of fibers in hybrid composites on the sorption kinetics are highlighted in this study. The mole percent uptake of water by the composites at four different temperatures, i.e. 30, 50, 70 and 90 °C were analysed. The kinetic parameters of water diffusion were determined in order to understand the mechanism of sorption. The values of diffusion coefficient, sorption coefficient and permeability coefficient were also calculated. Activation energy values and thermodynamics of water sorption were studied. The concentration dependency of the diffusion coefficient at room temperature was analysed.