Doug Sharpe
President of Elasto Proxy
Do people say that you look like your father? How about your mother? Maybe you resemble both of your parents, but in different ways. Human genetics can be complex, but so can polymer chemistry. Like DNA strands, long-chain polymeric molecules consist of base units (monomers) and groups. The double-helix structure of DNA is well-known, but the tangled configurations of polymer molecules are difficult to describe. That’s not the hard part, however. The real challenge is creating a plastic alloy from them.
Because of their complex structures, many polymers do not lend themselves to blends. Human history includes a Bronze Age and an Iron Age, but have you read any books that call our current era the Age of Long-Chain Polymer Blends? Through advances in material science, however, mixers are able to create compounds from rubbers such as silicone and EPDM. Blending these long-chain polymers is challenging, but imparting the best characteristics from each is essential.
Let’s look at the advantages – and disadvantages – of silicone and EPDM. Next, we’ll learn how mixers make silicone/EPDM blends, and how creating a blend for an application means getting the recipe for rubber just right.
Silicone Rubber
Silicone resists ozone, sunlight, oxidation, and weathering while providing excellent electrical insulation and flexibility at low temperatures. Although silicone is valued for its low compression set and superior color stability, this synthetic rubber is best known for its usefulness at high or low service temperatures. Silicone is more expensive than EPDM, but silicone seals generally last longer.
Silicone isn’t ideal for all applications, however. Because of its chemical properties, this type of rubber isn’t recommended for use with acids, alkalis, solvents, oil, or gasoline. Silicone lacks strong resistance to abrasion and tearing, too. Although many excellent silicone compounds are available, pure silicone isn’t the right choice for applications that require high tensile strength.
EPDM Rubber
EPDM also has its strengths and weaknesses. A popular polymer, this synthetic rubber is used in belts, hoses, and O-rings for the automotive and mass transit industries. With its excellent resistance to aging, ozone, and weathering, EPDM also provides strong resistance to acids, alkalis, and some solvents. EPDM parts are color stable, remain flexible at low temperatures, and resist both water and steam.
EPDM isn’t perfect however. In fact, this synthetic rubber is no match for mineral oils or aromatic hydrocarbons. EPDM compounds aren’t recommended for applications involving petroleum derivatives then. Though suitable for use as a high voltage insulation material, EPDM can conduct electricity if carbon black is added to improve weathering.
Silicone EPDM Blend
To produce blends with the best properties of silicone and EPDM, compounders add the two rubbers in proportions such as 50:50 or 70:30 to a two-roll mixing mill. Typically, dicumyl peroxide is used as the vulcanizing agent. After the materials are compression-molded into sheets and cured, the mixers test the blends and measure properties such as dielectric strength, tensile strength, and percentage elongation at break.
According to test results published by the IEEE, increasing the proportion of silicone improves a blend’s electrical insulation. By increasing the weight percentage of EPDM, mixers can boost mechanical strength instead. As with other compounds, there are tradeoffs. For example, silicone-modified EPDM blends can withstand higher temperatures than EPDM alone, but provide less temperature resistance than pure silicone. EPDM/silicone blends are tougher than silicone, but not as tough as EPDM alone.
Expansion, Resistance – and Conversation
Do you need to source expansion joint systems for bridges and highways? How about electrically insulating materials for outdoor environments with solid airborne particles? Let’s talk about how a silicone EPDM blend might meet your application requirements. For over 20 years, Elasto Proxy has been solving challenges and providing solutions. Join the conversation today.
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