Because most shaft seals feature an elastomeric lip bonded (during molding) to a metallic (or, in some cases, plastic) case, the ability of a rubber material to bond fully (and permanently) is very important to the life span of the seal. As described in ASTM D 429, adhesion tests for rubber bonded to a rigid substrate (such as metal) are typically performed using the same type of tester used to stretch dumbbells and gauge tensile strength (see Figure 20). In an adhesion test, the rubber portion of a rubber-bonded-to-metal sample is pulled at a constant rate until either the rubber peels away from the metal (the bond fails) or the rubber itself ruptures (though the bond stays intact). An example of adhesion testing is shown in Figure 30.

Because adhesion testing cannot be performed on completed seals, some manufacturers have devised ad-hoc methods for testing the adhesion properties of finished parts. One such method uses pliers to grip the sealing lip and pull it away from the case. If the lip separates cleanly, the bond is not strong enough. If rubber residue is left behind on the case, the bond was satisfactory. Under normal circumstances, a sealing lip in service will never be subjected to stresses so great, but adhesion tests can be good indicators of bond strength.

As important as they are, the physical properties of a given material are not the end of the story. Chemical properties are also critical, so let’s take a closer look at them next.

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