As used by the sealing industry, the term compatibility refers to a seal material’s resistance to having its chemical (and, by extension, its physical) properties degraded (either temporarily or permanently) as a result of contact with a liquid or gas. Because “likes dissolve likes,” the true key to compatibility between the seal and the fluid(s) being sealed is dissimilar chemical structure. For example, a shaft seal lip made from an all-hydrocarbon rubber (such as natural rubber) will be severely compromised when put in contact with petroleum-based oils or fuels.

In addition to being resistant to the primary system fluid, the seal must also be resistant to any and all additives that may be encountered during the course of operation. For example, oil-field applications often utilize film-forming amine inhibitors to coat tubular goods and prevent metal corrosion. Unfortunately, amine inhibitors act as curing agents for some fluoroelastomers, causing seal hardening and failure. In such an application, a shaft seal would need to be resistant to the fluid(s) being sealed and to the added amine inhibitors in order to provide an effective and long-lasting seal.

Even if they do not degrade the elastomeric compound directly, some fluids degrade surfaces adjacent to the seal (as with metal corrosion), thus reducing the effectiveness of the seal itself. You should also keep in mind that while some compounds formulated from a particular polymer may be okay for use in a given fluid, not all compounds of that polymer will be appropriate for use in that fluid. Since a compound’s properties are a direct result of its interactive constituents (e.g. reinforcing agents, plasticizers, etc.), each unique formulation should be tested in service conditions to determine its appropriateness for an application.

There is no single ASTM test method for “chemical compatibility.” Rather, compatibility is understood to be a wider concept incorporating changes (or the lack thereof) in a number of material properties, each of which have its own test method. Hardness, tensile strength, modulus, and elongation can all be compromised if a compound is not compatible with (resistant to) a given fluid. Perhaps the most visible evidence of chemical incompatibility is a change in the material’s volume.

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