EPDM Vulcanization and High Temperature Compression Set

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Abstract
The data presented are suggestive of the following conclusions regarding the vulcanization behavior and high temperature compression set properties of low unsaturation EPDM polymers: 1. The olefinic residue derived from ENB is more reactive in conventional sulfur vulcanization than is the residue from MNB. However, the ENB residue is more prone to reversion reactions. 2. The structure of the olefinic residue present can have important bearing on the exact nature at the crosslinks produced at a given crosslink density. 3. The conversion of high to lower sulfur rank crosslinks appears to occur partly by a thermally induced interconversion process and partly by concerted reactions in which crosslink bond scission does not occur as a discrete step. Both the nature and the concentration of accelerator present should have important bearing on the competitive balance between these two processes as should the presence of free sulfur. 4. Since the presence of free sulfur can procrastinate the sulfur crosslink rank reduction process, the observed low rate (relative to highly unsaturated elastomers) of conversion of polysulfidic to lower rank crosslinks must be associated with the absence of a large “sink” for sulfur. Thus with a given curing system, stable crosslinks should be generated faster the higher the level of unsaturation present and the lower the sulfur concentration. 5. High temperature compression set performance is influenced not only by the formation of new crosslinks during heating under compression but also by the interconversion of polysulfidic linkages present in the initial vulcanizates. Thus two polymers containing different olefin residues can exhibit identical set properties but for different reasons. 6. In practical terms both crosslink density and stability can be maximized by a combination of a short, high temperature forming cure followed by a long heat soak at a lower temperature. This observation should have important bearing on the economics of production of quality mechanical goods from low unsaturation elastomers via injection molding, the heat soaking step being paid for by the reduction in accelerator cost and molding cycle time.

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