Crosslinking of Rubbers

Abstract

Many different methods have been proposed for the crosslinking (vulcanization) of rubbers but relatively few have come into commercial use. The method to be used with any particular rubber depends, of course, on what reactive groups are present in the polymer backbone. Much the largest class of rubbers, in terms of volume usage, are the high-unsaturation, diene hydrocarbon rubbers. These comprise the butadiene-based rubbers, polybutadiene (BR) and styrene–butadiene rubber (SBR); the polyisoprene-based rubbers, natural (NR) and synthetic (IR) polyisoprene; and polypentenamer rubber. Except for a minority of special applications, they are always vulcanized with sulfur or sulfur-donor compounds. Sulfur vulcanization is also used extensively for the other high-unsaturation diene rubbers, acrylonitrile–butadiene rubber (NBR) and polychloroprene (CR), but the latter is more often crosslinked through its small proportion of allylic chlorine atoms, using a combination of zinc and magnesium oxides.1, 2 The diene rubbers containing low concentrations of alkenic unsaturation, butyl or isobutene–isoprene rubber (IIR) and ethylene–propylene–diene copolymer rubber (EPDM), are usually vulcanized with very active sulfur systems but other very reactive methods of crosslinking are in use: phenol–formaldehyde resols3–5 and a combination of a quinonedioxime and an oxidizing agent6, 7 for IIR; organic peroxides for EPDM.8 The chlorinated and brominated butyl rubbers (CIIR and BIIR), like CR, may also be vulcanized with metal oxides.9–11 Finally, the large group of specialized rubbers which contain no alkenic unsaturation may be crosslinked using organic peroxides or using reagents specific for their particular functional groups.12, 13 These rubbers include ethylene–propylene rubber (EPM), the various types of silicone (QR) and epoxy (CO, ECO, GPO) rubbers, chlorosulfonated polyethylene (CSM), acrylate rubbers (ACM, EAM), polyurethane rubbers (AU, EU), polysulfide rubbers (TR), and fluorinated rubbers (FKM).