Crosslink Density Evaluation of EPDM Vulcanizates

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Abstract
Crosslink density of EPDM vulcanizates, both gum and filled, has been evaluated from stress-strain measurements on both dry and swollen samples, by plotting the data according to the Mooney-Rivlin diagram. Values of the C1 constant of swollen specimens were generally found to be much higher than those of the dry ones, and the difference was ascribed to the effect of entanglements, which probably act as stable crosslinks in swollen networks. The Flory-Huggins equation has also been studied and its applicability to EPDM vulcanizates examined ; a large degree of uncertainty was found for the polymer-solvent interaction parameter, μ, that suggests the use of this equation with reservation in the case of EPDM elastomers. Many data of the Mooney-Rivlin C2 constant being available, its dependence on various parameters has been investigated and the hypothesis has been confirmed that it is associated with entanglements and intermolecular structure. The stress-strain curves of samples filled with both mineral and black fillers were analysed using the Mullins and Tobin “strain amplification factor.” This technique, which allows one to describe the behavior of the rubber phase in filled vulcanizates, has given good results that endorse the use of the Mooney-Rivlin plot for the characterization of rubbery networks. The Mooney-Rivlin constants also gave very interesting results for the investigation of the networks obtained from curing elastomer blends.

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