Elastomer Material Types
Elastomers represent a family of materials where the main characteristic is the high elasticity, ie: the ability to return to its original shape once removed from the source of the stress. For this reason they are considered the best material for seals.
ACM elastomers offer excellent heat resistance; they can typically be used at temperatures of 150°C (up to 175°C for limited periods). They provide high resistance to oxygen, ozone and industrial oils. Resistance to water is generally poor, and compression set and low temperature flexibility depend on base polymer and compounding choice. ACM elastomers are used primarily used where combined resistance to heat and oils is required.
AEM rubber materials offer an unusual combination of physical properties; high heat resistance (up to 160°C), excellent ozone and weather resistance, moderate resistance to mineral oils, low temperature flexibility to –30°C, good resistance to hot water and high tensile strength. AEM elastomers have the advantage of low temperature flexibility.
EPDM elastomers have a fair tensile strength and excellent resistance to weathering and ozone, and chemical attack. They also exhibit excellent electrical insulation properties. Peroxide cured elastomers exhibit excellent heat ageing and resistance to compression set, sulphur cured even more so. They are resistant to a wide range of media, including hot water and steam to 288°C (in the absence of air), but are not considered compatible with mineral and synthetic lubricants, and hydrocarbon fuels.
FKM (also known as FPM)
FKM seal materials include copolymer, terpolymer and tetrapolymer grades and bisphenol and peroxide cure systems.
FKM elastomers are highly fluorinated polymers containing few compounding ingredients. They exhibit very low gas permeability and are stable at very high temperatures (they can withstand 250°C / 482°F indefinitely, in service). By comparison, conventional elastomers would become brittle in 24 hours at this temperature, in air. FKMs in general, have outstanding resistance to oxygen, ozone, weather, flame and oxidative chemicals, and excellent resistance to swelling in a wide variety of media. However, they are not compatible with polar solvents (e.g. M.E.K.), some organic acids (e.g. Formic acid), certain methanol and ester based hydraulic fluids (e.g. Skydrol), ammonia and some amines. They are suitable in high-energy radiation environments up to about 106 Rads. Special grades of FKM may be required for use in hot water and steam applications.
FKM elastomers provide high compression set resistance if compounded with bisphenol cure systems. Generally they are serviceable down to –30°C, but specialist grades (such as PPE’s Endura® V91A) can provide effective sealing down to –51°C. Electrical insulation properties are not particularly outstanding, but would be adequate for sheathing where elevated temperatures, ozone, chemical and flame resistance are required (e.g. shaft seals, O-rings and gaskets, diaphragms and cable sheathing).
|TYPE||FLOURINE CONTENT||ADVANTAGES / DISADVANTAGES|
|Copolymer||65% - 65.5%||Contains two monomers (simple molecules from which polymers are built).
General purpose, most common, most widely used for sealing. Best compression set and very good fluid resistance (compression set is very important for O-rings).
Often referred to as 'A' and 'E' type grades.
These are normally the least expensive types of compound.
|Terpolymer||67%||Contains three monomers.
Better fluid and oil/solvent resistance than copolymer but at the expense of poorer compression set resistance.
Often referred to as 'B' types grades.
|Tetrapolymer||67% - 69%||Contains four monomers.
Improved fluid, acid, solvent resistance over other types. Compression set better than terpolymers.
These are sometimes known as 'G' grades.
In addition, certain tetrapolymers have good low-temperature flexibility.
Tetrapolymers are the most expensive of the three types listed here.
FEPM (also known as TFE/P)
The base polymer is solely produced by the Asahi Glass company, and sold under the name ‘Aflas’. FEPM materials exhibit similar thermal stability to FKM elastomers, but better electrical resistance and a different chemical resistance profile (e.g. sour gas, acids and alkalis, ozone and weather, steam and water, all hydraulic and brake fluids, alcohols and high energy radiation). However, they are not compatible to aromatic hydrocarbons, chlorinated hydrocarbons (e.g. M.E.K. and acetone), organic acetates and organic refrigerants.
The low temperature flexibility of FEPM is relatively poor and FKM elastomers exhibit better compression set. It is always recommended to carry out functional tests under working conditions. FEPM elastomers are commonly used in oil-field operations and chemical processing as O-rings, seals and gaskets.
FFKM elastomers are fully fluorinated polymers that provide almost universal chemical resistance, they are effectively a rubber form of PTFE. FFKMs display other properties which prove most valuable in applications where purity, high temperatures and retention of sealing force are paramount.
FVMQ elastomers are modified silicone rubbers, with superior fluid resistance, but limited to about 200°C (392°F).
NBR (BUNA N)
NBR elastomers are available in five basic grades; based on acrylonitrile content, giving proportional physical and chemical properties. NBR’s typically have (depending on increased ACN content), decreasing low temperature flexibility, increasing compression set, gas permeability, improved heat ageing and ozone resistance, improved tensile and abrasion strength, hardness and density. NBR’s are used where good resistance is required, to aromatic hydrocarbons at –40 to +120°C (e.g. gaskets and seals, hoses and cable jacketing), typically in the oil and gas industry.
High Nitrile: >45% ACN content
Medium Nitrile: 30-45% ACN content
Low Nitrile: <30% ACN content
The higher the ACN content, the higher the resistance to aromatic hydrocarbons.
The lower the ACN content, the better the low temperature flexibility.
The best overall balance for most applications is medium ACN content.
The hydrogenation process of NBR elastomers provides excellent heat and ozone resistance. Peroxide cured HNBR’s have the best compression set and heat resistance, and high-nitrile (ACN) HNBR elastomers have better resistance to mineral oils. HNBR’s combine best resistance and low temperature flexibility, although they are more expensive than NBR’s. HNBR’s are useful where resistance is required to ozone and weather, ageing in hot air and industrial lubricants, hot water and steam to 150°C, amine based corrosion inhibitors and sour gas (H2S), and high-energy radiation. HNBR’s fill the gap between NBR’s and FKM’s in many areas of application where resistance to heat and aggressive media are required simultaneously, and may therefore provide a lower cost alternative to FKM elastomers.
Natural rubber (tapped from the cultivated rubber tree) exhibits high tensile strength, abrasion resistance, resilience, tear strength and low hysteresis.
The chemcially similar polyisoprene has lower strength properties than the natural form but better low-temperature properties. Both rubbers are susceptible to degradation by weathering characteristics, and both show poor resistance to mineral and petroleum-based oils and fuels.
Typical long-term operating range is from -30°C to 70°C.
Main applications for natural rubber materials, apart from tyres, are for vibration mounts, springs and bearings.
The distinctive properties of silicone rubbers are outstanding resistance to ozone and corona, outdoor weather and sunlight. Special compounds will withstand up to 300°C; however, in the absence of air, silicones can revert to a paste, even at lower temperatures. The usual high temperature limit, quoted for continuous service, is 200°C. Silicones have an excellent reputation for their low temperature flexibility (some compounds to –90°C) and electrical insulation, which are maintained fairly constant at the full range of service temperatures. Electrically conductive compounds are also available.
Silicones have a low level of combustible components; even when exposed to flame, the elastomer is reduced to a non-conducting silica ash. Silicones also exhibit excellent compression set and high physiological inertness (tasteless, odourless and completely non-toxic). Silicones are resistant to bacteria, fungi, a wide range of media including high energy radiation to 106 Rads, and excellent release properties (except to glass). The main limitations are low tensile properties and poor resistance to acids, alkalis, and steam above 120°C. Silicone elastomeric parts are used for electrical insulation, gaskets and O-rings (static or low dynamic applications only), food and pharmaceutical goods.