Effect of Molecular Weight on the Rolling Behaviour of Polypropylene Sheets

Abstract

Rolling of polypropylene (PP) sheets was carried out at the roll temperature of 110°C. Sheets prepared from four resins having different melt indices and weight average molecular weights were used in this study. Particular attention had been paid to the influence of molecular weights on the rolling behaviour of PP sheets. A stable stretching process of sheets was conducted experimentally using a singlepass rolling technique. The rolled PP sheets at higher thickness reduction had smooth surfaces and showed high transparency. The mechanical properties and molecular orientation in the rolled sheets were studied by tensile tests, sonic propagation method, birefringence, and dynamic viscoelastic measurements. The crystallinity of sheets decreased with the thickness reduction. Rolled PP sheets with higher molecular weight showed lower crystallinity. Changes in average molecular orientaion produced during rolling were influenced slightly by the molecular weights of sheets. PP sheets with higher molecular weights showed somewhat higher orientation. Yield strength of sheets increased markedly at thickness reduction higher than 60% and was affected by the molecular weight of PP.