Radiation crosslinking of fluoropolymers—a review

Fluorinated polymers containing hydrogen and copolymers of tetrafluoroethylene and hexafluoropropylene are crosslinked with varying degrees of effectiveness by ionizing radiation. The open literature on the radiolysis of these polymers is summarized in this update of a review originally published about 8 years ago [Lyons, B.J. (1984) The crosslinking of fluoropolymers with ionizing radiation: a review. Second Int. Conf. on Radiation Processing for Plastics and Rubber, Canterbury, Plas. Rub. Inst., London, pp. 4-1 to 4-8]. Surprisingly little can be concluded with certainty about the radiolytic processes occurring in these polymers with the possible exception of poly(vinylidene fluoride) (PVF 2), which has been studied most extensively. An analysis of published gel data for this polymer, irradiated at various temperatures, shows that the sol fraction of some PVF 2 resins varies almost linearly with reciprocal radiation dose (as does the sol of many vinyl terminated polyethylenes). It is postulated that the similar sol behavior of PVF 2 indicates that terminal unsaturation is initially present in these resins and that this endlinks during the initial stages of irradiation, thereby producing a molecular weight distribution which tends to the weight random distribution. A corollary of this postulate is that the published values for G (chain scissions) in this and, by extension, other similar fluoropolymers may be too high. Use of certain unsaturated additives in many instances considerably increases gel formation rates in these polymers during irradiation.