A Beginner's Guide to Elastomers
Whether for a gasket, valve seat, or O-ring, the choice of elastomer might seem like a minor decision, but it can have a major impact. When choosing an elastomer, your best bet is always to look at what is already being used in the same or a similar application. But what if you are starting from scratch?
First, consider the standards you must meet. The components of an elastomer can vary widely, and yet the end product can still be called the same thing. EPDM, for instance, may or may not contain all food-grade components, or be FDA-compliant. This can make a huge difference.
Types of Seals
Next, think about the job the elastomer will perform. A static seal that sees no movement might have different needs than a dynamic one. If you have a dynamic seal like a valve seat or an O-ring on a moving component, consider if the product flowing through the system might be abrasive to the seal. Gritty or sugary products may call for the use of PTFE (Teflon), which has high abrasion resistance.
Another consideration for the type of seal is the resilience of the elastomer. Buna and EPDM have good resilience and "bounce back" when compressed, more like natural rubber. They're great for a good seal, especially after repeated compression, such as a butterfly valve seat or gasket for a fitting that is often removed. Viton is somewhat flexible and resilient, mainly at higher temperatures.
PTFE has no memory and can "cold flow", causing it to change shape when compressed. This leads to a poor seal after multiple adjustments. Envelope style gaskets (PTFE gaskets with a resilient core material like EPDM) offer the mechanical and chemical toughness of PTFE with the resilience of rubber.
For oils and high-fat products, Buna is often the top performer despite its low cost. EPDM might be more expensive, but in these applications it is a poor choice. Viton and PTFE work well with most oils, and can come in handy in high-temperature situations. Silicone is not always a good choice with oils. If you don't have prior experience with it in your application, check a compatibility chart first.
As far as acids and caustics go, it's usually best to consult a chemical compatibility chart. Because an elastomer might react differently depending on the type of chemical and concentration, it's difficult to predict compatibility.
Buna is generally acceptable up to about 2% caustic or 0.5% acid. EPDM does fairly well with both. Viton works better with caustics than acids, but can be acceptable for either. For silicone, it's best to consult a compatibility chart. Teflon has great chemical resistance, and there few products that won't work with it.
Since there is an endless list of chemicals out there, it can be helpful to consult a chemical compatibility chart to inform your decision. There are quite a few available on the Internet or supplied by vendors. These charts can be a great guideline but there is no substitute for real world testing.
A final consideration is the temperature that your elastomer will experience. We can only speak in general terms until we try an elastomer in the specific application, and this should be a key point in your decision.
As with anything, you're in good shape if you can find the lowest cost item that will meet your needs. Trial and error can reveal the life expectancy of a gasket in your particular application, and help determine if a higher cost elastomer is worth the price. Remember, a higher-cost material will not always perform better than a lower cost alternative.
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