Photoinitiated Cross-Linking of Polyethylenes and Diene Copolymers

Abstract

During recent years we have studied the photoinitiated crosslinking of various synthetic polymers, e.g. polyethylenes (HD, LLD and LD), ethylene-propylene-diene elastomers (EPDM) and unsaturated polyesters, using a UV-absorbing initiator (mainly aromatic ketones) and a multifunctional crosslinker (e.g. an allyl ether) as additives. The crosslinking is largely related to hydrogen abstraction from polymer and crosslinker, giving free radicals which form crosslinks by combination. High degrees of crosslinking have been obtained (measured as gel content by extraction) after short irradiation times (10 to 60 sec). Kinetic studies of the crosslinking reaction indicate a second order reaction at the early stages and a lower order reaction with increasing gel content of the polymer. The mechanism of photocrosslinking of EPDM elastomers has been studied in more detail using model compounds for the dienes: ethylidenenorbornane (1), dihydrodicyclopentadiene (2) and 2-heptene (3). ESR studies of the spin trapped radicals formed has established that EPDM elastomers containing (1) and (2) dienes are crosslinked mainly by hydrogen abstraction to allyl radicals via combination. Only EPDM elastomers containing (3) crosslink by abstraction like (1) and (2) as well as free radical addition to the double bonds. The reported crosslinking reactions are rapid and convenient and require only inexpensive equipment. Therefore, photoinitiated crosslinking is promising for industrial applications both for polyethylenes and EPDM elastomers.