Though the term elastomer was initially used to denote a synthetic form of Natural Rubber, “elastomer” and “rubber” are now more or less synonymous. To be officially considered an elastomer by the American Society for Testing and Materials (ASTM), a polymer must not break when stretched 100%, and it must return to within 10% of its original length within five minutes after being held for five minutes at 100% stretch.

An elastomer is perhaps best described as a visco-elastic material, in that it goes through both a viscous phase and an elastic phase. The visco-elastic behavior of elastomers can be simulated using a spring coupled with a dashpot (damper). The spring illustrates the elastic phase; the dashpot exemplifies the viscous phase (see Figure 15).

But why is an elastomer elastic and resilient, able to undergo high strain and yet recover its original shape? Put simply, it’s the tangled nature of its long molecular chains. When pressure (in the form of either a compressive load or a stretching force) is applied to the elastomer, the chains rotate around their chemical bonds. This rotation tends to uncoil the entangled mass and straighten the chains. When the pressure is removed, the chains coil up again, reverting to their normal state of entanglement. This tendency to return to its original configuration helps explain an elastomer’s rubbery, resilient nature.

Under certain conditions, a few elastomers will have their molecules align and form crystalline regions. An elastomer that crystallizes due to cold temperatures becomes harder and less able to stretch. This can be detrimental to shaft seal performance.

Since choices made during compounding directly determine the properties of an elastomeric seal, let’s look at these physical, thermal, and chemical properties next.

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