Polyisobutylene cold flow

Although polyisobutylene described earlier is a nonrubbery polymer exhibiting high cold flow, the copolymer containing about 2% isoprene can be vulcanized with a powerfiil accelerated sulfur system to give rubbery polymers. Being almost saturated, they are broadly similar to the EPDM rubbers in many properties. 

High molecular weight polyisobutylene has fair tensile strength, but suffers from the disadvantage of considerable cold flow. A copolymer of butylene with some isoprene for crosslinking is therefore used as a commercial elastomer and called butyl rubber. The isoprene is present in the copolymer in only minor proportions (1.4-4.5%). The uncrosslinked material is very similar to polyisobutylene. Copolymers of isobutylene with other dienes are also called butyl rubbers. They can also be terpolymers, where the third component may be cyclopentadiene for improved ozone resistance. 

Thomas reduced the cold flow of polyisobutylene by blending it with about 10% of natural rublter in the early 1930 s. However, the copolymer of isobutylene and isoprene was not produced until the late 1930 s, after many years of researdi efforts. This important copolymer was patented by Thomas and Sparks [U.S. Pat. 2,356,128 (1940)]. This new elastomer was also described in articles by R.M. Thomas and co-workers in 1940-41 in several journals including Ind. Eng. Chem.. 32. 1283 (1940). 

The same pigments and fillers commonly used with other rubbers can be compounded with butyl rubber and polyisobutylene, and the general principles of selection are the same. Very fine pigments increase cohesive strength and stiffness, reduce cold flow, and also reduce tack. Platy pigments such as mica, graphite, and talc are preferred for acid and chemical resistance and low gas permeability. Some of the coarser pigments increase tack. 

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