Stability of the Vulcanized Cross-Link in Butyl Rubber: Theory and Application

When rubberlike polymers are vulcanized with sulfur, the process involves two competing phenomena. The competing reactions are cross-linking and degradative in nature, and the conditions of vulcanization, as well as the extent, govern the rate at which these actions take place. At lower temperatures (110 to 150° C) vulcanizate degradation is held at a minimum, while higher temperatures usually accelerate the reversion process. A recent stoichiometric study of sulfur addition during Butyl rubber vulcanization revealed that the cross-link was composed of two atoms of sulfur. Calculations from analytical data placed the number of atoms at 1.7 to 2.2 per cross-link. This degree of constancy was maintained, regardless of the extent of vulcanization, until the onset of reversion. Reversion is the degradative process opposite to cross-linking. It can be pictured as the breakdown of the sulfur bridges between polymer molecules, and in Butyl it is evident at vulcanization temperature above 350° F. in 20 minutes. The type of rubber plays an important part in the relative way the two competing reactions occur. In addition, the whole process of cross-linking and reversion may be complicated by oxidative phenomena in predominantly diene type polymers. For example, when the vulcanization of natural rubber is prolonged, softening of the structure is followed by a hardening of the network, attributed to oxidation or cyclization. With Buna type rubbers, the softening action is barely evident before continued heating leads to a harder less extensible material As shown by Andrews, Tobolsky, and Hanson, the stress of a GR-S vulcanizate under intermittent strain increased on aging at 130° C. The stress of a Butyl sample remained essentially unchanged under the same conditions for about 10 hours, whereupon noticeable stress relaxation ensued. The use of an elastomer with a chemical unsaturation of 1 to 2 mole per cent of that found in polyisoprene reduces the opportunities for cyclization reactions. Observations on the stability of the sulfur cross-link can be made with less interference.