Wetting analysis and surface characterisation of coir fibres used as reinforcement for composites

Vietnam grown coir fibres have been studied for use in composite materials. In order to understand the fibre–matrix interfacial compatibility, it is necessary to investigate the wetting behaviour and surface chemistry of the fibres. The aim of this paper is to develop a wetting measurement procedure to determine stable and reproducible advancing static contact angles, and estimate the fibre surface energy. Dynamic contact angles are measured using the Wilhelmy technique. The experiments are carried out by considering the effects on the contact angle results of irregular wetted perimeter along the fibre and liquid absorption, which commonly appear with natural fibres. Dynamic wetting of coir fibres is then modelled using the Molecular-kinetic theory from which the advancing static contact angle can be approximated by fitting dynamic data. Besides, advancing static contact angle was determined experimentally using an approach derived from the Wilhelmy technique, in which the relaxation of the liquid meniscus was monitored after stopping the fibre movement. There is a good agreement between advancing static angles from the experimental and modelling method. The fibre surface energy comprising polar and dispersive components is estimated following the Owens/Wendt approach using the advancing static contact angle of various test liquids. The fibre surface chemistry examined by XPS shows a carbon rich surface consisting of waxes and a low proportion of lignin and fatty substances. From the results of the wetting measurements and surface characterisation, the coir fibre surface seems to be hydrophobic.