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, an O-ring seal made from an oil-derived material can be severely compromised when put in contact with oils or fuels. The most likely result: excessive swelling of the O-ring, resulting in failure of the seal (see Figure 28).

In addition to being resistant to the primary system fluid, the seal must also be resistant to any and all additives which may be encountered during the course of operation. For example, oil-field applications often utilize film-forming amine inhibitors to coat tubular goods and help prevent metal corrosion. Unfortunately, amine inhibitors act as curing agents for some fluorocarbon elastomers, causing seal hardening and failure. In such an application, an O-ring 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 under actual service conditions to accurately determine its appropriateness for an application.

There is no single ASTM test to determine “chemical compatibility.” Rather, compatibility is understood to be a wider concept incorporating changes (or the lack thereof) in a number of material properties, each with their own test methods. 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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