A Beginner’s Guide to Elastomers

The choice of elastomer might seem like a minor decision, but it can have a major impact. Seal material that is a poor fit for the application can cause any number of problems, from swelling to peeling or cracking.

To help avoid these issues, analyze the elastomer already being used in the same or a similar application. Variables such as chemical compatibility, temperature, pressure, and mechanical wear can make it challenging to predict with certainty what the best material will be. In most cases, several materials will work, but some elastomers will have a longer life than others. If a gasket is performing well and is low in cost, there is usually no need to change materials.

When an elastomer must be selected for a brand new application, there are several important guidelines to consider.

STANDARDS

Consider the standards that must be met. It’s easy to think that EPDM is EPDM, but there’s more to the story. An EPDM o-ring could be perfectly safe for sanitary processing or, depending on how the o-ring is made, it could contain any number of potentially harmful components. It’s possible for these components to leach into finished goods.

Understand which specific standards are important for an individual application. Are the components considered food grade? Do they meet Food and Drug Administration (FDA) standards? Biopharmaceutical processors often require that seals meet USP Class VI standards.

Sometimes, a choice in curing compounds is offered. For instance, some silicone is cured using peroxide. However, most food, beverage, or pharmaceutical companies can’t allow peroxides to contaminate their product, so a silicone cured with platinum is typically preferred in sanitary applications.

TYPES OF SEALS

The job the elastomer will perform determines the type of seal needed. A static seal that sees no movement may have different needs than a dynamic seal. An abrasive product will be harder on a dynamic seal (like a valve seat or o-ring on a moving component) than on something passive, like a Tri-Clamp gasket.

Sugar syrups can be tricky with dynamic seals. In general, they are very benign and work well with any elastomer. However, sugar crystals can build up on seals, sometimes called “plating.” On a valve seat, the crystals can behave like sandpaper when the seat closes, quickly wearing away the seat. When abrasion is a concern, PTFE, also known as Teflon, is often the best choice.

Another consideration for the type of seal is the resilience of the elastomer. Buna and EPDM have good resilience and “bounce back” more like natural rubber when compressed. They’re great for a good seal, especially after repeated compression (such as in a butterfly valve seat or a gasket for a fitting that is often removed). Viton can be somewhat flexible and resilient but becomes less flexible at lower temperatures.

PTFE, known as Teflon, has no memory and can “cold flow,” causing it to change shape when compressed. Often it provides an adequate seal on the first use, but if a Tri-Clamp fitting is removed and replaced with the same gasket, there might be some leakage. For this reason, envelope-style gaskets were created. These are PTFE gaskets with a core of a more resilient material like EPDM, combining the mechanical and chemical toughness of PTFE with the resilience of rubber.

CHEMICAL COMPATIBILITY

Chemical compatibility is another important factor. Here are some general guidelines to follow:

Oils or High-Fat Products

Buna, while usually the least expensive option, is often the top performer when it comes to oil-based products. EPDM might be more expensive than Buna, but when it comes to fats and oils, it is a poor choice and inferior to Buna. Viton works well with most oils, as does PTFE. Either can come in handy when your oils are at a high temperature. 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.

Acids and Caustics

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.

In general terms, Buna is usually 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. Silicone can be finicky – it’s best to consult a compatibility chart. Teflon has great chemical resistance, and there are 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.

TEMPERATURE

A final consideration is the temperature that your elastomer will experience. As with the other success factors for an elastomer, we can only speak in general terms until we try an elastomer in the specific application. Time at temperature, variations in temperature, and the combination of temperature with other chemical and mechanical stresses can all affect elastomer life.

As a guideline, here are rough high-temperature limits for some common elastomers:

EXOTIC MATERIALS

You may sometimes run into an elastomer outside of the usual suspects. Any number of brand name and specialized materials are on the market and might be offered as options for some types of sanitary equipment. You might see names like Kalrez, PEEK, or Delrin on a brochure from time to time. Each has strengths and weaknesses, and there are a few situations in the sanitary processing world that call for specialized materials.

For example, steam-in-place operations can cause headaches with traditional elastomers. Although they can resist the heat, silicone tears and deforms easily, while PTFE can lead to leaks when it cold flows. Even envelope gaskets can wear quickly in this tough environment. Several gasket manufacturers now offer a material made of PTFE and fine particles of stainless steel. The gasket is inflexible, but has the same temperature resistance as PTFE and resists cold flow.

Other specialty elastomers include special colors, which can aid in visibility, and metal-detectable elastomers, allowing your in-line metal detectors to catch a broken gasket as well as a lost nut or bolt.

PRICE

The cost of a gasket often influences a decision. A gasket that costs less than 50 cents in Buna could run $1.50 as an envelope gasket with Viton and PTFE, or $5.00 in a PTFE /stainless compound like “Silverback” or “Tuf-Steel.” The goal is to find the lowest-cost material that will meet your needs.

The only way to be sure of which elastomer will work best is to experiment, starting with the lowest-cost option that meets application requirements. There might be some trade-offs; Viton might provide a longer life than EPDM, but it may not be worth the added cost. A higher-cost elastomer is not always a better choice, depending on the application.

In many cases, Buna is a low-cost option, while EPDM can be a higher priced. PTFE, platinum-cured silicone, and Viton tend to be in a higher price class, with Viton usually the highest of the three. Envelope gaskets and specialty compounds can be higher still. Prices can change, depending on the availability of an elastomer.

If you are stumped on how to choose the best elastomer, check out the main page for gaskets along with the accompanying elastomer guide, or call a CSI representative at 417.831.1411 for help with the decision-making process.