Rubber materials / elastomer types
PPE offers an unrivalled range of over 300 high performance seal materials from the 17 types of elastomer (rubber) shown below. Our Materials Technologists are constantly developing new seal materials to meet the increasing demands of today's engineering applications.
Chart showing operating temperature range of each elastomer type.
Table showing general overview of chemical resistance for most common rubber materials.
How to select the correct seal materials
Resistant to the same fluid types as EPDM, the distinctive properties of IIR rubber materials are very low gas and moisture permeability, excellent insulating properties, good ozone and weathering resistance, and resistance to a great many organic and inorganic media. IIR elastomer materials can be polymerised with various halogens (e.g. Chlorine / Bromine) to improve resistance to certain chemical media, but at the expense of electrical insulation and moisture resistance. They can be used from –40 to +120°C typically, and are mostly used in the production of tyre inner tubes, seals and gaskets, vacuum seals and membranes, and pharmaceutical goods.
CR seal materials typically display good resistance to ozone, heat aging and chemicals. Good resistance to refrigerants, aliphatic hydrocarbons, mineral oils and greases. Typical trade name: Neoprene (DuPontTM)
Chlorobutadiene rubbers contain chlorine in the polymer to reduce the reactivity to many oxidising agents, as well as to oil and flame. CR elastomers also have good resistance to ozone cracking, heat ageing and chemical attack. Some of the important applications of CR elastomers include Vee-belts, coated fabrics, cable jackets, tyre-sidewalls, seals and gaskets in contact with refrigerants, mild chemicals and atmospheric ozone.
CSM grades contain 24-43% chlorine content to provide excellent ozone and weather resistance, discoloration by sun and ultraviolet light, high resistance to many oxidising and corrosive chemicals, good resistance to dry heat to 150°C, low flammability and gas permeability, and also good resistance to hot water (when cured with lead oxide). The low temperature properties are generally limited, depending on the chlorine content of the CSM grade used, and the compression set is not very good. CSM elastomers are generally useful in electrical applications, weather resistant membranes, hoses and acid resistant tank linings.
Generally resistance to high temperatures, oils, ozone, and flame with gas resistance comparable to NBR’s. The temperature range for continuous use is –40 to +120°C, but are generally unsuitable for rubber to metal bonding (they are corrosive to metals). ECO elastomers are suitable for use in seals, gaskets, diaphragms, cable jackets, belting etc, for a wide range of media. However, they are unsuitable for use with ketones and esters, alcohols, phosphate ester hydraulic fluids, sour gas, water and steam.
AEM rubber materials offer an unusual combination of physical properties; high heat resistance (up to 175°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 applications are similar to ACM elastomers, but has the advantage where low temperature flexibility is concerned. They are typically moulded into O-rings seals, boots and ignition wire jackets.
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 from –40 to +150°C, more so if sulphur cured. They are resistant to a wide range of media, including hot water and steam to 200°C (in the absence of air), but are not considered compatible with mineral and synthetic lubricants, and hydrocarbon fuels. They are typically used in the production of window and door seals, wire and cable insulations, waterproofing sheets and hoses, and seals, O-rings and gaskets.
| || ADVANTAGES || DISADVANTAGES |
Much better heat resistance than sulphur-cured elastomers.
Better compression set resistance.
Peroxide-cured materials are normally better where retained sealing force is important. Examples are O-rings and seals.
Peroxide cured parts are considered cleaner for food and pharmaceutical applications.
Poorer tensile strength, elongation at break and tear strength than sulphur-cured elastomers.
These properties are not nromally the most important for sealing elements like O-rings.
Better tensile strength, elongation at break and tear strength than peroxide-cured elastomers.
Poorer compression set resistance.
Lower heat resistance.
Outstanding resistance to heat, weather, ozone, oxygen and oxidisers. Very low gas permeability. PPE FKM (FPM) 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 are stable at very high temperatures (they can withstand 200°C / 400°F indefinitely, in service). By comparison, conventional elastomers would become brittle in 24 hours at this temperature, in air. FKM vulcanisates, 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 Endura® V91A) can provide effective sealing down to –45°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).
| || |
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.
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.
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.
FVMQ elastomers are modified silicone rubbers, with superior fluid resistance, but limited to about 175°C (347°F).
The hydrogenation process of NBR elastomers provide 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 -50°C to 70°C.
Main applications for natural rubber materials, apart from tyres, are for vibration mounts, springs and bearings.
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.
This is the most chemically resistant elastomer available and is effectively a rubber form of PTFE. It displays other properties which prove most valuable in applications where purity, high temperatures and retention of sealing force are paramount.
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.
AU / EU
These elastomers generally show outstanding tensile strength, tear and abrasion resistance, and give excellent protection against oxygen and ozone (except in hot climates, due to greater risk of microbiological attack in AU types, and ultraviolet light in the case of EU types. EU elastomers have a better low temperature flexibility (-35°C typically) and both have excellent resistance to high-energy radiation (106 Rads).
Polyurethane rubbers are used where high abrasion resistance and oil / solvent resistance are required together, e.g. hydraulic seals and gaskets, diaphragms, hoses and roller-skate and skateboard wheels. In all applications, consideration should be given to hydrolysis and limited heat resistance.
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 also 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.
Produced as a substitute for NR (Natural Rubber); in general, SBR’s can be used in similar applications as NR / IR elastomers, except in severe dynamic applications (e.g. requiring low heat build-up on flexing). SBR’s can be used in tread compounds on car tyres, but not truck tyres. SBR’s have inferior weathering and chemical resistance to most other elastomers.
TFE/P (also known as FEPM)
The base polymer is solely produced by the Asahi Glass company, and sold under the name ‘Aflas’. TFE/P vulcanisates 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.
FKM elastomers exhibit better compression set, and the low temperature flexibility is relatively poor (except in certain fluids that would plasticise the rubber to some extent. It is always recommended to carry out functional tests under working conditions. TFE/P elastomers are finding wide application; mainly in oil-field operations and chemical processing as o-rings, seals and gaskets, cable insulating and jacketing, and hose liners.