After a compound has been formulated, it must still be processed into a useful form (such as the lip of a shaft seal). Under normal conditions, an elastomer’s amorphous chain segments are free to move relative to one another. This is not true only when the chains meet mechanical entanglement (as with the spaghetti effect), or when the separate chains are chemically connected. Vulcanization (also known as cure) is a heat-induced process whereby the long chains of the rubber polymers are permanently cross-linked to one another, thus forming three-dimensional elastic structures (see Figure 14). Aided by curing agents in the original compound, vulcanization transforms soft, weak, non-cross-linked materials into strong elastic products. In addition to making the compound stronger, the vulcanization process is also generally the point at which the material is molded into a useful shape that it can retain thanks to its memory.

Though every effort has been made to simplify the preceding discussion, it’s important to realize that putting together an elastomeric compound can get quite complex. Decisions made in compounding will ultimately impact the processing and performance of any seals produced from the compound. Depending on the type and degree of additives in use, a single base polymer can generate hundreds of different compounds, each with unique characteristics.

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