The effect of alpha, beta crystalline structure on the mechanical properties of polypropylene

In order to study the effect of the alpha, beta crystalline structure of po lypropylene (PP) on its mechanical properties, it is necessary to prepare s amples with variable alpha/beta -phase contest but with constant crystallin ity and constant spherulite size. With this objective, heat treatment was f irst defined to be applied to an isotactic PP containing a beta nucleating agent in order to achieve these conditions. Then study of the effect of the beta -phase content on the tensile properties and fracture behaviour has b een done at room temperature. The mechanical properties at fracture were as sessed by three-point bending tests and were analysed on the basis of the " Essential Work of Fracture'' (EWF). The results show that the elongation at fracture under tensile stress and the ''near" Plane-Strain Essential Work of Fracture, w(Ie), increase substantially with the beta -phase content. Be sides, Young's modulus and the yield stress in tensile tests decrease slowl y with the beta -phase content. Finally, these results are analysed taking account the differences in structure of the beta and spherulites.