CRITERION FOR OPTIMUM ADHESION APPLIED TO FIBER-REINFORCED COMPOSITES

The effects of physical adhesion on the mechanical properties of a com posite structure are examined in this work. A criterion for optimum ad hesion between matrix and reinforcing fibres is proposed based on maxi mizing the wetting tension. It is shown that the maximum wetting tensi on criterion best fulfils two im portant requirements for a strong int erface:(i) the physical interactions at the molecular level between th e resin and the fibres must be maximized, and (ii) the liquid resin mu st spontaneously wet the fibre surface in order to minimize the flow d ensity at the interface. The conditions on the surface energy of the v arious phases leading to maximum wetting tension are analysed consider ing three mixing rules: two based on dispersive-polar interactions, an d a third one based on acid-base interactions. The optimum adherend fo r a given adhesive, and the optimum adhesive for a given adherend, are examined. The analysis shows that maximum wetting tension is obtained when the substrate a nd adhesive surface energies are very high a nd equal, so that their polar and dispersive components are equal when th e polar-dispersive mixing rule is used, and e.g. their Lifshitz-van de r Waals' components are equal and the acid component of one phase is e qual to the basic component of the other phase when the acid-base appr oach is considered. It is shown using data from the literature that in terfacial strength correlates with the wetting tension for fibre reinf orced composites. Additional observations show that under poor wetting conditions the voids tend to concentrate at the fibre-resin interface , whereas under favourable wetting conditions they tend to coalesce in regions away from the fibre surface.