Chemically known as polychloroprene but often referred to by the trade name Neoprene®, chloroprene was one of the first synthetic materials developed as an oil-resistant substitute for natural rubber. Neoprene’s molecular structure closely mirrors that of natural rubber, with the exception that a chlorine atom has replaced a methyl (CH3) sidegroup. The presence of a chlorine atom in each repeating unit increases the compound’s polarity and improves its resistance to hydrocarbon fluids despite the presence of a double bond in the main chain. Because the chlorine atom essentially deactivates the double bond, chloroprene is more resistant to oxygen, ozone, and UV light than similarly unsaturated polymers.

Due to the similarity of their structures, natural rubber and chloroprene are generally comparable in their good strength, abrasion resistance, resilience, elongation, and strain crystallization characteristics. Both also offer a similar low fatigue property, low heat build up, low temperature flexibility, and high bondability. Chloroprene surpasses natural rubber in its resistance to aging, heat, oils, ozone, and solvents. Chloroprene has also gained FDA approval for use in the food and beverage industries.

Best known by the trade name Hypalon®, chlorosulfonated polyethylenes are valued for their attractive combination of physical resilience and chemical resistance to corrosives, heat, oil, oxygen, ozone, and weather. Both heat resistance and low temperature flexibility can be enhanced through the use of lower amounts of chlorine, but oil resistance is only fair in such formulations. Higher chlorine levels lead to greater oil resistance at the expense of heat resistance and low temperature flexibility. CSM can also be compounded to retain permanent bright colors. On the downside, CSM suffers from poor compression set resistance.

Epichlorohydrin is an oil resistant compound available in three formulations: as a homopolymer (CO), as a copolymer of epichlorohydrin and ethylene oxide (ECO), and as a terpolymer of epichlorohydrin, ethylene oxide, and a cure site monomer (GECO).

Epichlorohydrin combines low gas and solvent permeability with high resistance to hydrocarbon oils and fuels. Epichlorohydrin is very resistant to weathering and ozone due to complete saturation within the polymer chain.

Epichlorohydrin also remains stable during low to high temperature cycling. Because it exhibits less sub-zero stiffness, epichlorohydrin is often used in place of nitrile and chloroprene (Neoprene®) for applications requiring resistance to low temperatures.

Epichlorohydrin has only fair compression set resistance at high temperatures (i.e. 250° to 275° F).

Better known as Vamac®, ethylene acrylic is a copolymer of ethylene and methyl acrylate. Vamac also has a third, acid-containing monomer to cure the polymer chain’s active groups. Vamac offers exceptional resistance to ozone, sunlight, and heat, as well as low gas permeability and moderate oil swell resistance. Flex life is good, as are tear, abrasion, and compression set properties.

Ethylene propylene is a copolymer of ethylene and propylene (EPM), or, in some cases, a terpolymer due to the addition of a diene monomer (EPDM). This additional diene monomer can be important because it includes unsaturation to facilitate sulfur crosslinking.

In use since 1961, ethylene propylene is primarily valued for its outstanding resistance to Skydrol® and other phosphate ester type hydraulic fluids (including Pydraul® and Fyrquel®), as well as for its typical temperature range (-65° to +300° F, -54° to +149° C).

Also referred to as fluoroelastomers, fluorocarbon compounds are thermoset elastomers containing fluorine. FKM makes excellent general purpose O-rings due to their exceptional resistance to chemicals, oil, and temperature extremes (-15° to +400° F). Specialty compounds can further extend the low temperature limit down to about -22° F for dynamic seals and about -40° F in static applications.

FKM usually has good compression set resistance, low gas permeability, and good resistance to ozone and sunlight. Over the last five decades, this remarkable combination of properties has prompted the use of FKM seals in a variety of demanding industries. Though they were initially formulated for use in aerospace applications, FKM compounds are now widely used in the automotive, appliance, fluid power, and chemical processing industries.

Fluorosilicone is the common name for fluorovinylmethyl silicone rubber. Fluorosilicones combine the best properties of fluorocarbons and silicones. Fluorosilicones resist solvents, fuel, and oil (similar to fluorocarbons). They also have high and low temperature stability (as with silicones). Fluorosilicones are resilient, with low compression set characteristics. Though widely used in aerospace fuel systems and auto fuel emission controls, fluorosilicones are really only good as static seals. High friction tendencies, limited strength, and poor abrasion resistance disqualify them from dynamic uses.

Though the double bonds within nitrile’s butadiene segments are needed for cross-linking, they are also the main attack sites for heat, chemicals, and oxidation. As part of an ongoing effort to engineer more resistant compounds, a new class of nitrile was developed in the 1980s. Initially known as highly saturated nitrile (HSN), this class is now more commonly called hydrogenated nitrile butadiene rubber (HNBR), or just hydrogenated nitrile.

As you might guess, hydrogenated nitrile results from the hydrogenation of standard nitrile. Hydrogenation is the process of adding hydrogen atoms to the butadiene segments. Adding hydrogen greatly reduces the number of carbon-to-carbon double bonds that would otherwise be weak links in the polymer chain. Why are double bonds weak? It stems from valence, or the ability of an atom to form one or more energy bonds with neighboring atoms. A carbon atom can form four distinct covalent bonds. Because carbon has this valence of four, it is most “satisfied” when it has actually formed four single bonds (a state known as saturation) rather than two single bonds and a double bond. A satisfied, saturated atom is more stable, so a compound composed largely of saturated carbons is less reactive and more resistant to chemical attack.

Also known as polyisoprene, natural rubber is vulcanized from the latex of the Hevea brasiliensis tree. Natural rubber was the sole O-ring polymer before the development of synthetic elastomers in the 1930s. Though its use has since sharply declined, natural rubber offers many excellent characteristics, including low heat build up, high resilience and elongation, good abrasion resistance and excellent low temperature flexibility.

Natural rubber has both high tensile strength and good tear strength due to its tendency to strain crystallize. It also undergoes low compression set. Its chief drawback is its poor resistance to oils and solvents. The double bond in its main polymer chain also makes natural rubber susceptible to attack by oxygen, ozone and UV light.

Nitrile rubber, often called Buna-N, is the most commonly used elastomer for O-rings and other sealing devices. Nitrile is a copolymer of butadiene and acrylonitrile (ACN). The name Buna-N is derived from butadiene and natrium (the Latin name for sodium, the catalyst used in polymerizing butadiene). The “N” stands for acrylonitrile.

The butadiene segment imparts elasticity and low temperature flexibility. It also contains the unsaturated double bond that is the site for crosslinking, or vulcanization. This unsaturated double bond is also the main attack site for heat, chemicals, and oxidation.

Most commercial perfluoroelastomers are terpolymers of tetrafluoroethylene (TFE), perfluoromethylvinyl ether (PMVE), and a cure site monomer (CSM). The fully-fluorinated monomers contained in perfluoroelastomers are the reason they exhibit superior chemical resistance. As with fluorocarbon elastomers, the bonds between carbon and fluorine atoms are extremely strong, making the chemical structure virtually unbreakable. Also, polymers with high levels of fluorine (as opposed to hydrogen) have proven to be more stable and less chemically reactive. Perfluoroelastomers also enjoy immunity from chemical attack due to saturation along the polymer’s backbone. There are no double bonds to be attacked by degradants such as oxygen, ozone, UV light, or harsh chemicals.
Perfluoroelastomers can trace their lineage back to the late 1960s, when chemists at DuPont pioneered what came to be known as Kalrez®. In so doing, they combined the chemical resistance of polytetrafluoroethylene (Teflon®) and the elasticity of flouroelastomer (Viton®) into a fully-fluorinated polymer that could be cross-linked. Differences in perfluoroelastomer performance are often due to the manner in which the material is cross-linked.

Polyacrylate is a copolymer which offers good resistance to petroleum fuels and oils. The auto industry uses polyacrylate O-rings as seals in automatic transmissions and power steering designs. Resistant to flex cracking, polyacrylate also resists damage from oxygen, sunlight, and ozone (due to main chain saturation).

Use of a butyl acrylate instead of an ethyl acrylate rubber can greatly improve low temperature flexibility. Though it is marginally more resistant to hot air than nitrile, polyacrylate falls short in strength and compression set resistance, as well as in resistance to water and low temperatures.

Polyurethane is the toughest, most extrusion-resistant, and most abrasion-resistant of all elastomeric sealing materials. Polyurethane O-rings can withstand pressures up to 5,000 psi with a .010” extrusion gap. Polyurethane is also very resistant to explosive decompression and has excellent properties over a wide temperature range. Polyurethane O-rings are used in a wide variety of products, including quick-disconnect hydraulic fittings, hydraulic cylinders and valves, pneumatic tools, CO2 firearms, or for applications requiring extreme abrasion or extrusion resistance.

Silicones are primarily based on a strong sequence of silicon and oxygen atoms. This silicon-oxygen backbone is much stronger than a carbon-based backbone, making silicones more resistant to extreme temperatures (-65° to +450° F, -54° to +232° C), chemicals, and shearing stresses.

Due to saturation in the polymer’s main chain, silicones are very resistant to oxygen, ozone, and UV light. This same saturation also demands that the material be peroxide cured since it is not possible to sulfur cure a saturated polymer. In addition to being generally inert (non-reactive), silicones are odorless, tasteless, non-toxic, and fungus resistant. They also have great flexibility retention and low compression set.

There are four primary silicone formulations in use today. Standard methyl silicone is known simply as MQ. By replacing a small number (typically less than 1%) of the pendant methyl (CH3) groups in MQ with vinyl (CH2CH) groups, you arrive at what is known as vinyl methyl silicone, or VMQ. VMQ compounds tend to have better cure properties and undergo lower compression set than standard MQ.

Styrene Butadiene is a copolymer of styrene and butadiene. Though SBR’s low strength properties require the addition of reinforcing agents to make the compound stronger, SBR was widely used as the synthetic substitute for natural rubber during World War II. Like natural rubber, SBR is non oil-resistant.

SBR’s use since WWII has sharply declined, though it is still used in automobile tire production and in assorted molded rubber goods. SBR is unsuitable for some applications because the double bond in the polymer backbone invites attack by oxygen, ozone, and UV light.

Tetrafluoroethylene (PTFE) is a completely fluorinated polymer produced when the monomer tetrafluoroethylene (TFE) undergoes free radical vinyl polymerization. As a monomer, TFE is made up of a pair of double-bonded carbon atoms, both of which have two fluorine atoms covalently bonded to them. Thus the name: “tetra” means there are four atoms bonded to the carbons, “fluoro” means those bonded atoms are fluorine, and “ethylene” means the carbons are joined by a double bond as in the classic ethylene structure.

When TFE polymerizes into PTFE, the carbon-to-carbon double bond becomes a single bond and a long chain of carbon atoms is formed. This chain is the polymer’s backbone. With a ratio of four fluorine atoms to every two carbon atoms, the backbone is essentially shielded from contact. It’s almost impossible for any other chemical to gain access to the carbon atoms. Even if an agent could gain access, the carbon-to-fluorine bonds have high bond disassociation energy, so they’re almost unbreakable. This makes PTFE the most chemically resistant thermoplastic polymer available. PTFE is inert to almost all chemicals and solvents, allowing PTFE parts to function well in acids, alcohols, alkalies, esters, ketones, and hydrocarbons. There are only a few substances harmful to PTFE, notably fluorine, chlorine trifluoride, and molten alkali metal solutions at high pressures.

The FEPM designation was originally directed at copolymers of tetrafluoroethylene (TFE) and propylene (P). TFE/P provides a unique combination of chemical, heat, and electrical resistance. Chemically, TFE/P resists both acids and bases, as well as steam, amine-based corrosion inhibitors, hydraulic fluids, alcohol, and petroleum fluids. TFE/P is also resistant to ozone and weather. TFE/P typically retains its remarkable chemical resistance even in high temperatures (short exposures up to 450° F, 232° C), and tests have shown that electrical resistance actually improves with heat exposure. Nor do physical properties suffer; tensile strength typically approaches 2,500 psi.
The first TFE/P compound to be commercially marketed was Aflas (a product of Asahi Glass). In a sense, Aflas defined the initial boundaries for base-resistant materials. Different grades of Aflas have different molecular weights. Most molded and extruded products are made from Aflas 150P, which has a molecular weight of about 130,000. In comparison, Aflas 100H has a molecular weight of 200,000 and is typically used where high pressures are to be sealed, such as in oil field applications. TFE/P compounds are also widely used in the chemical processing, automotive, and aerospace industries. TFE/P compounds are not as good as standard FKM-A compounds in terms of hydrocarbon resistance, but TFE/P surpasses FKM-A in resistance to strong bases, amines, and polar solvents.