Radical and Sulfur Curing

The problem of cross-linking or grafting low unsaturated rubbers (e. g. EPDMs or EPTMs) or thermoplastics (e. g. ethylene-butadiene copolymer is fundamentally the same. Radical mechanisms are believed to be operative when peroxides or sulfur-based formulations are used, even though in the latter case ionic mechanisms also seem to contribute to the curing process  

While a lot of technolo cal work has been done in the past to find the best applications for a number of new polymers, relatively little information has been obtained about the basic chemistry of cross-linking processes. 

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It is generally believed that sulfur vulcanization is promoted by a free radical mechanism, although some literature suggests a polar mechanism of vulcanization. There is also evidence that both free radical and polar mechanisms are operative depending on the formulation and curing conditions. A major source of the uncertainty has arisen from the intractable nature of the cured vulcanizate. Cured elastomers are insoluble this eliminates most analytical techniques for examining polymeric structures. 

Funatsu et al. examined the change in free radical concentration of sulfur and peroxide cured cA-l,4-polyisoprene rubber (IR) vulcanizates during tensile deformation. Figure 25.32 shows the first derivative ESR curves determined... 

The system Cl-butyl-cis-polybutadiene has been studied in some detail because it was suitable for the developed differential swelling technique and because this system of blends vulcanized with zinc oxide, sulfur, and thiuram disulfide first revealed the presence of interfacial bonds. This curative system has the feature of a flat cure —i.e.y the two homophases are vulcanized rapidly, and the crosslinked density does not increase radically as vulcanization time is prolonged. This is observed in Table IV by swelling and extractable levels of a series of crosslinked networks cured at increasing times and swollen in a common solvent, cyclohexane. 

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