Thermoset elastomers, commonly known as rubber, are the best materials for a wide variety of industrial applications, due to their ability to provide both flexibility and durability. However, elastomers are complex materials, which must be chosen carefully and compounded to fit the part’s function and the molding process required.
Part 1 focuses on an overview of rubber families and the five types of general purpose elastomers and their industrial applications.
While the terms rubber and elastomer are used interchangeably, the materials are different in their chemical compositions and their uses. Elastomers for industrial use are classed according to their application, whether for a general purpose or a high function component, as well as their ability to resist oils and chemicals, maintain biocompatibility, or withstand high and low temperatures during operation.
Generally lower cost and are most appropriate for less demanding applications such as bumpers, where parts are not exposed to chemicals or such extreme temperatures as under-the-hood automotive parts.
Tend to be more expensive but can be vital for high-performance applications and durability under harsh operating conditions. For example, automobile parts, miniature pneumatics, or petroleum processing applications might require the use of any or all of the three high-performance materials.
Range from low to high cost, and are designed to resist swelling and chemical attack without loss of the material’s physical properties when in contact with hydrocarbons or other chemicals.
Medium to high cost, but retain their properties without degradation. They may be used for applications such as nuclear plant equipment and seals that come in contact with pressured steam, allowing them to resist dissolving or oxidation.
Range in cost from low to high and may be used for operating conditions as low as -110°F while retaining their flexing and sealing properties.
Range in cost from low to high and may be used for operating conditions as low as -110°F while retaining their flexing and sealing properties.
When selecting the best material for the application, the rubber molding chemists and engineers will take both price and performance into consideration
EPM is a copolymer of ethylene and propylene.
EPM can only be cured/cross-linked with peroxides. Introducing the third monomer, diene, to the mix will result in a rubber mix to be sulfur cured due to the unsaturation. EPDM’s main properties are its excellent resistance to heat, steam, weather and ozone, and its good electrical insulating properties.
Natural rubber consists of polymers of the organic compound isoprene, with minor impurities of other organic compounds plus water. Forms of polyisoprene that are useful as natural rubbers are classified as elastomers. Natural rubber is based on sap derived from rubber tree plants, which is harvested in the form of latex. The latex is a sticky, milky colloid drawn off by making incisions into the bark and collecting the fluid in vessels in a process called tapping. The latex then is refined into rubber ready for commercial processing.
Natural rubber is very elastic and has a high tensile strength, high elongation values and very good abrasion resistance. Natural rubber is susceptible to aging and oil swell, which is why natural rubber is not highly recommended for seal applications.
In order to improve its properties, the formulation may have to be modified by incorporating antioxidants and antiozonants, as well as blending in other desired elastomers.
Polyisoprene is built up from the linking of multiple isoprene molecules into a synthetic elastomer with characteristics similar to natural rubber in structure and properties.
Synthetic polyisoprene exhibits excellent resilience and good tensile strength. Its color uniformity from one batch to another is easy to maintain because of the uniformity of the polymer. Polyisoprene serves many applications requiring uniform quality and performance, especially where color is concerned.
Polyisoprene has lower green strength, hot tear and a slower curing rate than natural rubber. Polyisoprene has more consistency than natural rubber in strength, cure rate, processing, mixing and molding.
The SBR copolymer consists of styrene and butadiene.
SBR has good abrasion resistance and good aging properties when protected by additives such as antioxidants and antiozonants. SBR has superior water resistance, heat resistance, low-temperature flexibility, and heat aging. SBR also has good electrical insulation, alcohol resistance, oxygenated solvent resistance, and mild acid resistance.
SBR has poor resistance to oils, fuels, hydraulic fluids, strong acids, and greases. SBR can be compounded to improve ozone resistance. SBR is blended with natural rubber for making tires.
Silicone has a completely different type of structure than other elastomers. Silicone rubber consists of a chain of silicon and oxygen atoms, rather than carbon and hydrogen atoms as found in other types of rubber.
In addition to being a general purpose elastomer, silicone rubber offers good resistance to extreme temperatures and can operate normally at temperatures ranging from -55 °C to +230 °C.
In contrast to organic rubber, which has a carbon-to-carbon backbone that can leave it susceptible to ozone, UV, heat and other aging factors, silicone rubber can withstand these elements well. This makes silicone one of the elastomers of choice for many extreme environments. Silicone also has excellent vibration damping and still maintains its dielectric strength. However, silicone has poor tensile strength, tear and abrasion resistance.
Part 2 of this article will cover molded rubber components made from high-performance elastomers, materials that resist oil and hydrocarbons, and those that can withstand extreme temperatures in harsh operating environments.
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