Microfibril reinforced polymer–polymer composites: Application of Tsai-Hill equation to PP/PET composites

In this paper an attempt has been made to check the extent of validity of the Tsai-Hill equation applied to polymer–polymer microfibril reinforced composites (MFCs), in which the reinforcing elements represent microfibrils with diameters around 1 μm and aspect ratios of approximately 100. For this purpose compression moulded plates of polypropylene/poly(ethylene terephthalate: 70/30 by wt%) composites have been prepared and their structures are established by wide angle X-ray scattering and scanning electron microscope (SEM) analysis. The tensile tests of cut specimens at various angles (ϕ with respect to the uniaxially aligned microfibrils) show the degree of agreement with Tsai-Hill predictions. The measured values are slightly higher than the calculated ones and this finding is explained by the higher aspect ratios of microfibrils, their more homogenous distribution and most importantly, by the better matrix/reinforcement adhesion in the case of MFCs as compared to the common composites. The fracture mechanism as determined from the SEM observations on the fracture surfaces is also discussed and a good agreement with the mechanical behaviour is found.