Chap._300033 - m S CH 2-CH 2-CH-CH 2 CH-CH=CH-CH 2 CH...

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* Crosslinking (Vulcanization) of Elastomers Based on Dienes Crosslinking is an absolute requirement if elastomers are to have their essential property of rapidly and completely recovering from deformations. Crosslinking or vulcanization of diene containing polymers can be accomplished by using sulfur, peroxides, etc. The commercial crosslinkig of diene containing polymers is carried out by heating with surfur. Although sulfur vulcanization has been studied since its discovery in 1839 by Goodyear, its mechanism is not well understood. Recent studies indicate that the reaction proceeds by ionic route Hydride transfer regenerates the polymer carbenium ion CH 2 -CH=CH-CH 2 Sm CH 2 -CH-CH 2 CH 2 CH-CH=CH-CH 2 + Sm CH 2 -CH-CH-CH 2 CH-CH=CH-CH 2 CH 2 -CH=CH-CH 2 + m S CH-CH=CH-CH 2 S 8 The polymeric (allylic) carbenium ion undergoes crosslinking with surfur and then by addition to a polymer double bond + +
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Unformatted text preview: m S CH 2-CH 2-CH-CH 2 CH-CH=CH-CH 2 CH 2-CH=CH-CH 2 (sulfonium ion) reacts with a polymer by hydride abstraction + m S CH 2-CH CH-CH 2 heat-+ n m S S S 8 CH 2-CH=CH-CH 2 Vulcanization of a diene containing polymer by heating with sulfur alone is a very inefficient process, wasted by the formation of long polysulfide crosslinks. Commercial sulfur vulcanization are carried out in the presence of various additives (accelerators and activators), including organo-sulfur compounds, zinc oxide and fatty acid which greatly increase the rate and efficiency of the process. Due to the low diene concentration in both butyl and EP rubbers, the halogenated polymers conctaining allylic halide significantly improve the crosslinking reactivity. Brominated butyl rubber shows higher vulcanization rates than chlorinated and normal butyl rubber....
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This note was uploaded on 10/16/2010 for the course MATSE 441 taught by Professor Chung,tze-chiang during the Fall '10 term at Pennsylvania State University, University Park.

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