PHOTO-CROSS-LINKING OF POLYPROPYLENE - THE EFFECT OF DIFFERENT PHOTO-INITIATORS AND COAGENTS

Crosslinking of polypropylene (PP) induced by photoreduced (benzopheno ne, xanthone and 2-methyl anthraquinone) and photofragmentated (haloge nated sulphur, phosphorus and hydrocarbon derivatives) substances was studied. It was found that high gel content could be obtained in PP us ing sulphur monochloride, sulphuryl chloride, trichlorethylene and tet rachlorethylene as photoinitiators. The gel yield in the presence of p hotoreduced initiators increases if acryloyloxy polyfunctional monomer s are used as PP photocrosslinking promoters, whilst the effect of all ylic monomers is comparatively small. Temperature increase was shown m ainly to depress PP crosslinking efficiency and to enhance phototherma l degradation, although increased gel yield was attained if PP contain ing photoreduced initiators combined with allylic monomers was exposed to UV light at temperatures up to 140 degrees C. DSC studies showed t hat the melting temperature and the heat of fusion of PP photocrosslin ked in the molten state or recrystallized after crosslinking were decr eased substantially with gel content, especially if photofragmented ch lorides are used as initiators. Investigation of photocrosslinked PP t ensile properties at ambient temperatures revealed the increase of You ng's modulus and yield stress along with catastrophic decrease of stra in at break. At increased testing temperatures (T > 80 degrees C) the ductility of photocrosslinked PP is enhanced and its tensile propertie s noticeably surpass those of uncrosslinked samples.