On the effects of processing conditions and interphase of modification on the fiber/matrix load transfer in single fiber polypropylene composites

The scope of this study was the investigation of the effects of both processing conditions (in terms of thermo-mechanical history) and interphase modification (fiber sizing and/or matrix coupling) on the interfacial shear strength (tau(i)) of fiber reinforced isotactic polypropylene (iPP). Fiber/matrix load transfer efficiency was investigated by modified single fiber pullout and microdroplet pulloff test methods, respectively. It was established that tau(i) of the neat microcomposite (unsized fiber/uncoupled matrix) is improved by quenching of the samples rather than by various spherulitic or transcrystalline supermolecular structures set under isothermal crystallization conditions. Enhanced interfacial shear strength for the quenched samples was attributed to a better wetting behaviour and a fine dispersion of the amorphous PP (aPP) fraction formed. An adhesion model was proposed based on which optimum tau(i) is linked to both matrix strength and its wetting behaviour. It was demonstrated that the results from pullout and pulloff tests correlate very well with each other for the particular glass fiber/iPP model composite systems studied. It was shown further that matrix modification (coupling) or fiber sizing enhances tau(i) practically to the same level, whereas a combination of matrix coupling and fiber sizing yields an even higher interfacial shear strength (synergistic effect).