Effects of environment on the properties of low-density polyethylene composites reinforced with pineapple-leaf fibre

Abstract

The influence of water environment on the sorption characteristics of low-density polyethylene composites reinforced with short pineapple-leaf fibers (PALF/LDPE) has been studied by immersion in distilled water at 28, 50 and 70°C. The effects of fiber loading, temperature and chemical treatment on the sorption behavior are also evaluated. Water uptake is found to increase with fiber loading owing to the increased cellulose content. Weight change profiles for the composites at high temperature indicated that the diffusion is close to Fickian. All of the treated composites showed lower uptake than the unmodified composites. Parameters like diffusion, sorption and permeability coefficients were determined and activation energies were calculated. The thermodynamic constants such as entropy, enthalpy and first-order kinetic rate constant have been evaluated. A correlation between theoretical and experimental sorption results was evaluated. The effect of water uptake on uniaxial tensile properties has also been studied. Mechanical properties decreased after exposure to water, depending on time of immersion, fiber loading and chemical treatment. Finally, studies were carried out on the flexural properties of PALF/LDPE composites exposed to ultraviolet radiation.

Introduction

During service, polymeric materials are subjected to a variety of environmental conditions such as moisture, solvents, oil, temperature, mechanical loads and radiation. Moisture effects in fiber-reinforced-plastic composites have been studied widely during the last two decades.1, 2, 3The rate at which water is absorbed by a composite depends on many variables including fiber type, matrix, temperature, the difference in water distribution within the composite, reaction between water and the matrix, among others.[2]Both the rate of water pick-up and the total amount of moisture absorbed depend on the chemical structure of the resin and crosslinking agent together with the temperature and relative humidity (RH). The moisture absorbed not only plasticizes the matrix resin, but also changes the state of stress in favor of cracking through swelling.[4]At the fiber/matrix interface, moisture may reduce the bond strength by breaking the bonds.[5]

At lower temperature, the translational freedom of water molecules is slowly hindered by the stiffness of polymer chain segments. Moisture diffuses into a polymer to varying degrees depending upon molecular and microstructural aspects such as polarity, the extent of crystallinity of thermoplastics and the presence of residual hardeners or other water-attracting species.[6]In the absence of radiation, the environmental degradation of the mechanical properties of polymers is generally associated with plasticization, weight losses, micromechanical damage, and blistering induced by the sorbed solvents.[2]Loss of material by leaching after longer time periods also leads to effective damage.

The rate at which the adhesive bonds lose strength in the presence of water depends primarily upon the rate at which moisture enters the interface by diffusion through the adhesive, the diffusion rates in permeable adherends and transport along the adhesive/adherend interfaces and through any cracks if present. Joint strength may then decrease as a result of changes in the properties of the adhesives owing to plasticization, hydrolysis or cracking. Coupling agents such as silane are used to promote adhesion between fibers (glass) and matrix (e.g. epoxy resin) and these may protect the bond from moisture. However, it has been shown that they may dissolve in water and even promote higher moisture uptake and leaching.[5]Vega and Ramos[7]studied the degree of sorption and swelling obtained with different liquids in the case of henequen fibers. The swelling behavior was correlated to the polar and hydrogen bonding components of the solubility parameters of the liquid. Recently, the dimensional stability and mechanical properties of low-density polyethylene (LDPE) composites reinforced with CTDIC-treated sisal fibers were studied by Thomas and co-workers.[8]Several authors report the effect of moisture on the dynamic mechanical and electrical properties of filled polymer systems.9, 10Changes in the macroscopic mechanical properties reflect changes to the microstructure. Therefore an insight into the mechanism of moisture uptake in composites can be gained by studying the effect of moisture and temperature on the overall properties of samples.

Very recently, in this laboratory, high-performance composites have been developed from pineapple-leaf fibers (PALF) and low-density polyethylene by George et al.11, 12, 13We have already reported the effect of pineapple-leaf fiber on the mechanical, thermal, viscoelastic and melt rheological properties of low-density polyethylene.11, 12, 13, 14The influence of fiber length, loading, orientation, processing methods (i.e. melt mixing and solution mixing), etc. on the properties was examined. In this paper we present the results of our investigation on the moisture absorption characteristics of PALF/LDPE composites. The moisture diffusion profile is described in terms of weight changes with exposure (immersion) time. The dependence on fiber loading and the influence of chemical treatment have been analysed in detail. From the swelling data, the entropy and enthalpy of sorption and kinetic rate constants have been estimated. The experimental sorption data have been compared with theoretical predictions. The effect of moisture on uniaxial stress/strain properties is also evaluated.

Besides moisture, exposure to different types of radiation produces considerable changes in polymer systems.15, 16If artifical light sources are used, main-chain scission and crosslinking can occur and this will depend on wavelength, the nature of the initiator, and the concentration of oxygen at the site of absorption.17, 18Therefore the effect of ultraviolet (UV) radiation on the mechanical properties (i.e. flexural properties) of PALF/LDPE composites has also been studied.