Application of Halpin-Tsai equation to microfibril reinforced polypropylene/poly(ethylene terephthalate) composites

While the main goal of the first part of this study was to check the applicability of the Tsai-Hill equation to a polymer-polymer microfibril reinforced composite (MFC), in which the reinforcing elements represented microfibrils with a diameter around 1-3 mu m and aspect ratio of about 100, in the present paper a similar goal involves the Halpin-Tsai equation. In addition, using compatibilised blends an attempt is made to shed light on the mechanism of the microfibril formation during drawing of isotropic polymer blends. For this purpose, injection moulded dog-bone test samples of blends of polypropylene/poly(ethylene terephthalate) (PP/PET) (60/40 wt%) have been prepared starting from highly drawn bristles, also from blends containing 0-9 wt% compatibiliser (ethylene-glycidyl methacrylate). The MFC structure of the test sample is established by X-ray and scanning electron microscope (SEM) analyses. The tensile data are compared with those predicted according to the Halpin-Tsai equation. It is shown that the predicted values are slightly higher than the measured ones and this finding is explained by the presence of a compatibiliser resulting in much lower aspect ratios of the microfibrils. The suggested mechanism of the microfibril formation is based on the coalescence of the individual elongated spheres during drawing. The SEM observations also show that the compatibilised blends contain much shorter microfibrils because the compatibiliser prevents the coalescence process.