For critical components in mechanical engineering, such as O-rings, the question generally arises as to which material should be used. Developers usually use a very high-quality material, such as FFKM, for the initial equipment. This has excellent resistance to most media and its physical properties guarantee optimum sealing. However, the costs for this material are usually higher than planned, which can lead to an uncompetitive price or low margins for the end product.
It is therefore essential to carefully examine the material selection process in order to choose the optimum sealing solution for the respective requirement.
Four requirement profiles must be checked before selecting the material
- Operating temperature: In which temperature range should the O-ring be used? What is the minimum and maximum temperature? Are these short-term peaks or continuous use of the seal in these temperature ranges?
- Chemical resistance: Against which media must the O-ring seal and be resistant? Are there interactions, e.g. use in both acids and alkalis? What are the temperatures of the media to be sealed? Are oils or greases used during assembly?
- Mechanical properties: How is the O-ring used? Is it a static, static seal or a non-static, dynamic seal? For dynamic seals: How high is the mechanical stress? Is the seal moved rarely, regularly or permanently?
- Approvals: Which guidelines and approvals apply to the respective production process and must also be fulfilled by the sealing materials used?
O-rings made from polyurethane rubber are characterized by their particularly high performance: Polyurethane O-rings have high mechanical values, such as tear and abrasion resistance, very good rebound elasticity and high gas tightness. The fuel resistance and resistance to many common technical oils, especially those with a higher aromatic content, is excellent. Due to the good temperature resistance (up to +125 °C) and good low-temperature flexibility (down to -30 °C) as well as excellent resistance to oxygen and ozone, polyurethane rubber seals have a long life expectancy.
The area of application for this rubber is where high resistance to hot water and steam is required of the seal used. EPDM O-rings also have very good resistance to ageing and ozone. Compared to the usual types of synthetic rubber, the cold resistance can be described as good. Their resistance to oils, lubricating greases and solvents is similar to that of butadiene styrene rubber. EPDM seals have very good chemical resistance, even against oxidizing agents. Temperature range from -50 °C to +140 °C depending on the type used.
FKM O-rings and FPM O-rings are exceptionally resistant to the effects of mineral oils, aliphatic and aromatic hydrocarbons, chlorinated hydrocarbons, concentrated and diluted acids and weak alkalis. Excellent temperature resistance up to +200 °C and low temperature resistance down to -40 °C, depending on the type used, as well as high mechanical values and outstanding ageing resistance place FKM/FPM rubber (including Viton®) far above conventional synthetic rubbers. A wide variety of sealing geometries can be produced from FKM, FPM or Viton®. The FKM sealing material Viton® is a trade name for the basic elastomer FKM/FPM. Viton® is a registered trademark of Dupont Performance Elastomers.
O-rings made of HNBR are characterized by an outstanding range of properties: High resistance to technical oils containing additives; low vapor and gas permeability; good low-temperature flexibility of up to -30°C depending on the type used; good ozone resistance and high abrasion resistance. HNBR gaskets are heat-resistant up to +150°C.
Natural rubber is a highly elastic material with excellent physical properties. Despite the variety of available synthetic rubber types with their special characteristics, O-rings made of natural rubber still find a significant area of application in numerous niche areas. The operating temperature ranges from -45 °C to +100 °C, depending on the type used.
The area of application of this rubber results from its excellent temperature resistance (-55 °C up to +200 °C), which, however, must not be transferred to hot water or steam. Although the oil resistance of O-rings made of silicone rubber comes close to that of NBR, the good physical-mechanical properties of this material are not achieved.
NBR rubber is primarily resistant to the effects of mineral oils, in particular hydraulic oils, lubricating greases, petrol and other aliphatic hydrocarbons, diluted acids and alkalis. Good physical values such as high abrasion resistance and stability and favorable temperature resistance (-25 °C up to +120 °C, sometimes even down to -40 °C) ensure a wide range of applications for NBR O-rings.
PTFE O-rings have universal resistance except against liquid alkali metals and fluorine gas under pressure. Good sliding properties, low wear. Temperature resistance from -200 °C up to +260 °C. However, as PTFE has a hardness of approx. 95 °Shore, the installation of O-rings in split grooves is recommended, and slit or encapsulated versions are preferable due to their low elasticity.
The chemical and physical properties of CR rubber are similar to those of NBR. Although its resistance to mineral oil is somewhat lower, its resistance to ageing, ozone, acids and alkalis is excellent. O-rings made of chloroprene rubber can cope with a temperature range of -40 °C to +120 °C, depending on the type used.
The peroxide cross-linking results in excellent resistance to hot water and steam. EPM O-rings have very good resistance to other atmospheric conditions such as UV radiation, ozone and moisture as well as good resistance to acids and alkalis. Temperature range from -20 °C to +150 °C, in some cases up to +180 °C.
Polyether urethane rubber is characterized by particularly high performance, including high mechanical values such as tear and abrasion resistance, very good rebound resilience and high gas tightness. The fuel resistance and resistance to many common technical oils, especially those with a higher aromatic content, is excellent. Due to their good temperature resistance and excellent resistance to oxygen and ozone, O-rings made of polyurethane rubber have a long life expectancy. EU materials have better hydrolysis resistance than AU materials and are also more resistant when used with aqueous media.
FEPM is a modified fluororubber that surpasses the outstanding chemical resistance of normal fluororubber. Well-known trade names here are Viton® Extreme and Aflas®.
The polymer structure of Viton® Extreme-ETP significantly reduces swelling in solvents and strongly alkaline media; heat resistance and low-temperature flexibility are retained and are in the range from -10 °C to +200 °C. This material is used wherever particularly aggressive chemicals require outstanding resistance properties.
Aflas® is a specialty rubber and belongs to the group of fluoroelastomers. Materials based on Aflas show exceptionally good resistance to a large number of specific media and chemicals, such as hot water, water vapor, acids, alkalis, ammonia, sour (H2S), bleaching agents, etc: Hot water, steam, acids, alkalis, ammonia, bleaching agents, sour (H2S) gases and oils as well as amines, especially media with amine-containing additives and corrosion inhibitors, alloyed engine and gear oils, brake fluids and oxidized media. There are restrictions with regard to aromatic hydrocarbons and fuels. The operating temperatures are similar to those of fluoroelastomers.
Perfluoroelastomers (FFKM) are terpolymers consisting of tetrafluoroethylene (TFE), perfluoromethyl vinyl ether (PMVE) and a crosslinking monomer (cure site monomer = CSM). FFKM O-rings are versatile and can be used in industrial processes that place the highest demands on the properties of elastomer seals.
Tetrafluoroethylene (TFE) provides the chemical resistance as a base monomer, while the elastic properties are achieved by crosslinking with PMVE and the perfluorinated crosslinking monomer (CSM). For many applications, there is no alternative to a perfluoroelastomer, as this material is extremely resistant, even with changing media. FFKM/FFPM advantages at a glance:
- Highest chemical resistance of all elastic sealing materials
- High temperature stability up to 325 °C, depending on the type used
- Low compression set
- Excellent vacuum behavior
- Flexible in application
- Suitable materials for a wide range of requirements
- Large number of approvals
In addition to the typical properties of normal silicone rubber, FVMQ O-rings have significantly improved resistance to oils, fuels and solvents. This applies in particular to aromatic and chlorinated hydrocarbons and alcohols. O-rings made of fluorosilicone rubber can therefore be used where resistance is required over a wide temperature range from -60 °C to +200 °C with simultaneous exposure to aggressive media such as petrol, alcohol mixtures, aromatic and naphthenic oils and a range of chlorinated solvents.