Interfaces in Natural Fibre Composites: Effect of Surface Energy and Physical Adhesion

An integrated physical-chemical-mechanical approach was used to study the effect of adhesion on the mechanical strength of composites reinforced with bamboo and coir fibres, and three thermoplastic matrices (Polypropylene (PP), maleic anhydride grafted polypropylene (MAPP), and polyvinylidene fluoride (PVDF). Surface chemical analysis was conducted by X-ray photoelectron spectroscopy, XPS. Surface physical-chemistry was studied by determining surface energy components of fibres and matrices, by performing contact angle measurements with various known test fluids and employing a 3-component model for surface energy. Work of physical adhesion was calculated. Transverse 3-point bending tests on uni-directional composites were conducted to study interface strength and composite strength was determined by longitudinal flexural tests. Results show that increase in physical adhesion can explain the improved interface and longitudinal strength in bamboo-PVDF and coir-PVDF composites, as compared to the PP composites. For MAPP, no increase in physical adhesion is predicted, but particularly for coir a strong increase in mechanical interface strength and consequently an increase in longitudinal strength was observed, which may be attributed to a chemical adhesion mechanism.