Understanding the interfacial compatibility and adhesion of natural coir fibre thermoplastic composites

An integrated physical–chemical–micromechanical approach is implemented to investigate the fibre–matrix interfacial compatibility and adhesion of natural coir fibre composites. Wetting measurements of the fibres and the matrices are carried out to obtain their static equilibrium contact angles in various liquids, and these are used to estimate the surface energies comprising of different components. The work of adhesion is calculated for each composite system, accordingly. Also, fibre surface chemistry is examined by X-ray photoelectron spectroscopy (XPS) to have more information about functional groups at the fibre surface, which assists in a deeper understanding of the interactions at the composite interfaces. Single fibre pull-out tests and transverse three point bending tests are performed on UD composites to measure interfacial shear strength and interfacial tensile strength respectively. The results suggest that the higher interfacial adhesion of coir fibres with polyvinylidene fluoride compared with polypropylene can be attributed to higher fibre–matrix physico-chemical interaction corresponding with the work of adhesion. Whilst the improvement of interfacial adhesion for coir fibres with maleic anhydride grafted polypropylene compared with polypropylene can probably be attributed to a chemical adhesion mechanism. In agreement with the interface evaluations, the flexural strength in longitudinal direction of the composites is largely correlated with their interfacial adhesion.