Synthetic polyisoprene rubbers green strength

The copolymerization of butadiene in trans configuration with suitable comonomers represents a second route for obtaining a wide range of strain induced crystallizable elastomers, with melting point tailorable in a wide range of temperatures. These copolymers can be used, in particular, in blends with other crystallizable rubbers (e.g. synthetic cis-l,4-polyisoprene) in order to improve their "green strength". 

Initial attempts to make synthetic rubber similar to NR date back to mid 1800s. Modern synthetic polyisoprene is designed to be similar to natural rubber in structure and properties. Although it has lower green strength, slower cure rates, lower hot tear, and... 

A true synthetic natural rubber was introduced in the mid-1960s with the exact same chemical structure as latex tapped from a tree. The difference is that natural rubber comes with a variety of other ingredients in the latex that can both add and detract from performance, while polyisoprene is considered relatively pure. In addition, there are some differences in molecular weight distribution that impact performance. Available in both latex and solid forms, this elastomer can be directly substituted for natural rubber in many applications. Adhesives which are not cured tend to have higher creep values than natural rubber, but also exhibit lower tack and green strength properties. Vulcanized adhesive products perform equal to cured natural rubber adhesive products. 

Dr. N.R. Legge, 1987 Charles Goodyear medalist and, at that time, director of the synthetic polyisoprene program at Shell Development Company, recalled that it was in attempting to provide a solution to this problem of poor "green strength in alkyl lithium polymerized polyisoprene that the styrenic block copolymers were first synthesized. The functional use of the first block copolymers was not as an identifiable monolithic rubber structure but provided a vital function to another identifiable material and lost its identity in this process. 

This also shows that the lower compound green strength associated with the synthetic polyisoprenes relative to natural rubber is not an insurmountable problem. However, the incipient problem has been generally avoided by not making a total replacement of the natural rubber in tyre compounds which are subject to substantial deformation during building operations. 

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