Chemical, physical and mechanical properties evolution in electron beam irradiated isotactic polypropylene

Abstract

Isotactic Polypropylene 3 mm thick tensile samples, prepared by compression molding, were subject to electron beam irradiation with doses 0, 20, 40, 60, 100, 200 and 300 kGy. These samples were characterized by spectroscopic methods (UV spectroscopy), X-ray diffraction and mechanical tests (tensile tests and instrumented indentation), with the aim to investigate the ability of the instrumented indentation test to detect the changes in the macroscopic properties which arise from the changes in the chain structure. The use of larger irradiation doses compared with the commercial levels led to an unexpected behavior. At the smaller doses (up to 60 kGy), as expected, sample crystallinity decreases, characterizing irradiation induced amorphization. For the 100 kGy dose, however, the sample recrystallizes, returning to crystal/amorphous phase ratios similar to the ones observed for the pristine material. These changes correlated with the progressive production of –C=O and –C=C– chromophores in the chain and with a loss in yield strength and Young modulus up to 200 kGy (the sample subjected to 300 kGy is brittle). In spite of this, the indentation test showed limited sensitivity to the changes in the macroscopic tensile properties.