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Material Selection and Chemical Resistance

Chemical Resistance Chemical Resistance

Philippe Grenier Production Coordinator at Elasto Proxy

Last week, we learned how a failed seal on a diesel tank’s access door traveled through a fuel line and clogged an engine. Believed to be made of Buna, a synthetic rubber known for its chemical resistance, the gasket disintegrated from the splash of diesel fuel after just a few weeks. What can technical buyers learn from this experience, and how can seal fabricators help them source the right rubber compounds?

Charts and Chemical Resistance

Rubber manufacturers publish charts that describe the suitability of various elastomers for service with specific chemicals, including diesel fuel. When evaluating rubber materials, suppliers consider factors such as service temperature, service condition, material grade, and durometer. The material properties of the rubber compound itself are also important to consider.

As a rule, higher temperatures increase the effects of chemicals on rubber compounds. Although these increases vary with both the material and the chemical, all polymers are affected by high temperatures. In the case of our failed door seal then, was the gasket exposed to unusually high heat? This is different than service condition, which refers to the gasket’s suitability for static or dynamic sealing.

Grades, Fillers, and Costs

Door seals are dynamic seals in that one surface (the door) is in motion relative to the other (the door frame). To prevent fluid leakage, a specific amount of compression is required to maintain contact with the sealing surfaces. Just as different rubber materials can withstand different amounts of compression, different grades of the same rubber may have different compression and recovery characteristics.

If material properties don’t meet all of an application’s requirements, gaskets may not seal properly. The use of fillers to reduce costs or alter material properties may also play a role. Pure Buna costs more and has different characteristics than Buna that contains fillers. Durometer (duro), a measure of hardness, may also be a factor  since harder compounds generally provide greater chemical resistance.

Lettered Rating Systems

To express chemical resistance in a standardized format, rubber manufacturers use a lettered rating system to indicate a material’s suitability for use with a specific chemical. The letter A indicates that the rubber is recommended with only minor effects from the chemical. The letter B indicates that there are minor to moderate effects, but that parts made of the material are still useful in many applications.

This lettered system also indicates if a compound is potentially problematic, unsuitable, or unknown in its chemical resistance. The letter C denotes that the chemical can have moderate to severe effects on the material, and that rubber parts are useful only in limited applications. The letter U means that the rubber is not recommended, or that the chemical’s effects on the material are still unknown.

Is there more to selecting the right rubber than reading a chart? Yes, of course! Although the reasons behind the failed fuel-tank door seal are still in doubt, I’m certain about one thing. It’s critical to work with partners that you trust, who ask questions about your application, and who analyze the answers before recommending sealing solutions.

Do you have questions about material selection and chemical resistance? Do you need help balancing application requirements against material costs? How can we help you? Please comment on this blog entry, or contact us today.