The Mechanism of Phenolic Resin Vulcanization of Unsaturated Elastomers

It is well known that during the sulfur curing of unsaturated rubbers, two competing reactions occur: (a) crosslinking or vulcanization, and (b) reversion or devulcanization. In the case of butyl rubber, these two competing reactions have been summarized in earlier reports. Tire curing bag (bladder) compounds are usually made of butyl rubber (IIR), a copolymer of isobutene and isoprene, with typically 1–5% of the diene monomer. Curing bags were originally manufactured using sulfur cures. The high temperatures (140–180°C) employed in tire curing caused reversion, however, and these bladders had very short service lives. The deterioration of the IIR bladders was evidenced by a gradual softening of the surface. A major technical advancement for increasing the service life of curing bladders was the development of phenol/formaldehyde (resole) resins for vulcanizing IIR. These resins can give IIR cures with very thermally stable crosslinks. The vulcanizates are essentially immune to reversion, even at the high use temperatures of tire curing operations. The basic curing resins used are generally 2,6-dihydroxymethyl-4-alkylphenols 1 or their condensation polymers 2 (Scheme 1). These materials are produced via the base-catalyzed reaction of the p-substituted phenol with formaldehyde. R is typically methyl, t-butyl, or t-octyl in commercial resins. The use of a blocking substituent in the para position maximizes the formation of o-hydroxymethyl groups. R′ is either methylene (—CH2—) or dibenzylether (—CH2—O—CH2—), depending on the conditions of the resin synthesis or the cure.