ELASTOMER Type Details
Details are provided here for some of the more common o-ring sealing elastomers. General descriptions are provided in the text. Additional details on several of the elastomers are provided in the tables. Further information regarding compatibility with specific chemical species can be obtained in the chemical compatibility tables in the next section.
NBR is a very commonly used material for o-rings because of its good mechanical properties, its resistance to lubricants and greases and its relatively low cost. The physical and chemical resistance properties of NBR materials are determined by the acrylonitrile (ACN) content of the base polymer which can vary between 18% and 50%. Low ACN content ensures good flexibility at low temperatures, but offers limited resistance to oils and fuels. As the ACN content increases, the low temperature flexibility reduces and the resistance to oils and fuels improves.
Physical and chemical resistance properties of NBR materials are also affected by the cure system of the polymer. Peroxide-cured materials have improved physical properties, chemical resistance and thermal properties as compared to sulfur-donor-cured materials.
Standard grades of NBR are typically resistant to mineral oil-based lubricants and greases, many grades of hydraulic fluids, aliphatic hydrocarbons, silicone oils and greases and water to about 80°C.
NBR is generally not resistant to aromatic and chlorinated hydrocarbons, fuels with a high aromatic content, polar solvents, glycol-based brake fluids and non-flammable hydraulic fluids (HFD). NBR also has low resistance to ozone, weathering and aging, but in many applications this has no negative effect.
HNBR is obtained by partially or fully hydrogenating NBR. This leads to considerable improvement of the resistance to heat, ozone and aging, and gives it very good mechanical properties. The media resistance compares to that of NBR.
EPDM materials generally have a high resistance to hot water, steam, aging and chemicals, and are suitable for a wide range of application temperatures. They are divided into sulfur-cured and peroxide-cured types. Peroxide-cured compounds are suitable for higher temperatures and have much lower compression sets.
EPDM has good resistance to hot water and steam, detergents, caustic potash solutions, sodium hydroxide solutions, silicone oils and greases, many polar solvents and many diluted acids and chemicals. Special formulations are excellent for use with glycol-based brake fluids.
EPDM materials are totally unsuitable for use with all mineral oil products—lubricants, oils, fuels.
Silicone rubbers are noted for their ability to be used over a wide temperature range and for excellent resistance to ozone, weathering and aging. Compared with most other sealing elastomers, the physical properties of silicones are poor. Generally, silicone materials are physiologically harmless so they are commonly used by the food and drug industries.
Standard silicones are resistant to water (to 100°C), aliphatic engine and transmission oils and animal and plant oils and fats.
Silicones are generally not resistant to fuels, aromatic mineral oils, steam (short term to 120°C possible), silicone oils and greases, acids or alkalis.
Although fluorosilicone elastomers have the same mechanical properties as silicones, they are far more resistant to oils and fuels. The temperature range of applications is somewhat more restricted than that of silicones.
FKM materials are noted for their very high resistance to heat and a wide variety of chemicals. Other key benefits include excellent resistance to aging and ozone, very low gas permeability and the fact that the materials are self-extinguishing.
Standard FKM materials have excellent resistance to mineral oils and greases, aliphatic, aromatic and chlorinated hydrocarbons, fuels, non-flammable hydraulic fluids (HFD) and many organic solvents and chemicals.
In addition to the standard FKM materials, a number of specialty materials with different monomer compositions and fluorine content (65% to 71%) are available that offer improved chemical or temperature resistance and/or better low temperature performance.
FKM materials are generally not resistant to hot water, steam, polar solvents, glycol-based brake fluids and low molecular weight organic acids.
Polyurethanes differ from classic elastomers in that they have much better mechanical properties. In particular they have a high resistance to abrasion, wear and extrusion, a high tensile strength and excellent tear resistance. Polyurethanes are generally resistant to aging and ozone, mineral oils and greases, silicone oils and greases, nonflammable hydraulic fluids HFA & HFB, water up to 50°C and aliphatic hydrocarbons.