Photocrosslinking of polypropylene: the effect of different photo-initiators and coagents

Abstract

Crosslinking of polypropylene (PP) induced by photoreduced (benzophenone. xanthone and 2-methyl anthraquinone) and photofragmentated (halogenated sulphur, phosphorus and hydrocarbon derivatives) substances was studied. It was found that high gel content could be obtained in PP using sulphur monochloride, sulphuryl chloride, trichlorethylene and tetrachlorethylene as photoinitiators. The gel yield in the presence of photoreduced initiators increases if acryloyloxy polyfunctional monomers are used as PP photocrosslinking promoters, whilst the effect of allylic monomers is comparatively small. Temperature increase was shown mainly to depress PP crosslinking efficiency and to enhance photothermal degradation, although increased gel yield was attained if PP containing photoreduced initiators combined with allylic monomers was exposed to UV light at temperatures up to 140 °C. DSC studies showed that the melting temperature and the heat of fusion of PP photocrosslinked in the molten state or recrystallized after crosslinking were decreased substantially with gel content, especially if photofragmented chlorides are used as initiators. Investigation of photocrosslinked PP tensile properties at ambient temperatures revealed the increase of Young's modulus and yield stress along with catastrophic decrease of strain at break. At increased testing temperatures ( T > 80 °C) the ductility of photocrosslinked PP is enhanced and its tensile properties noticeably surpass those of uncrosslinked samples.