Elastomers
Elastomeric materials are described as having non-linear, viscoelastic behaviour, this means that they exhibit elastic recovery, time dependent behaviour and the relationship between load and deflection is not linear. Elastomers used in sealing are often described as compounds, meaning that they are a mixture of ingredients manufactured under specific conditions. A compound typically comprises:
- Polymer backbone – a long chain of molecules made up of one or more monomeric units, this governs the basic thermal, chemical and physical properties of a compound. ISO and ASTM classifications define families of elastomer such as NBR, FKM etc.
- Cross-link – polymer chains are tied together by cross links, short chains of molecules e.g. sulphur, to prevent chain slippage and create elastic behaviour. Different cross link systems will fundamentally change thermochemical or physical properties
- Fillers – organic or inorganic solid particles with specific shapes and chemistries that tailor physical properties such as tensile strength, hardness, elongation at break, modulus and compression-set
- Other ingredients used to achieve specific manufacturing, application
or cost requirements A typical HNBR 70 Shore A compound may have 20 ingredients and may contain only 30% polymer by weight. Therefore it is important not just to specify the family of polymer backbone and hardness, but to specify an individual compound/grade in order to achieve consistent performance.
Nitrile (NBR)
Nitrile (often referred to as Buna-N) is the most commonly used elastomer in the seal industry and is a copolymer of two monomers; acrylonitrile (ACN) and butadiene. The properties of this elastomer are ruled by the ACN content which is broken down into three
classifications:
High Nitrile: >45% ACN content
Medium Nitrile: 30 – 45% ACN content
Low Nitrile: <30% ACN content
The higher the ACN content, the better the elastomers resistance to hydrocarbon oils. With lower ACN content, the material offers better flexibility at low temperatures. Medium nitrile is, therefore, the most widely specified due to its good overall balance in most applications. Typically, nitrile rubber can be compounded to work over a temperature range of -35°C to +120°C and is superior to most other elastomers in regard to Compression set, tear and abrasion resistance. Nitrile rubbers posses excellent resistance to oil-based fluids, vegetable oils, greases, water and air.
Hydrogenated Nitrile (HNBR)
The properties of Hydrogenated Nitrile Rubber (HNBR) are dependent upon the acrylonitrile content and the degree of hydrogenation of the butadiene copolymer. They have a better oil and chemical resistance than nitrile rubber and can withstand much higher temperatures. HNBR has excellent resistance to glycol based coolants, hot water and ozone. Physical properties (e.g. tensile and tear strength, elongation, abrasion resistance, compression set, etc.) are also excellent and compounds display strong dynamic behaviour at elevated temperatures. HNBR can either be cured with sulphur or peroxide, depending upon which properties are the most important. Typical applications include accumulator bladders, diaphragms, gaskets and seals. Limitations include poor electrical properties, poor flame resistance and attack by aromatic oils..
Silicone (VMQ)
Silicone elastomers are commonly used for extreme temperature ranges (-90°C to +230°C) and offer good low temperature flexibility. They also offer good resistance to ultra violet radiation (UV), oxygen and ozone. Silicone is best suited to non-dynamic applications, as this elastomer type possess relatively low tear strength and abrasion resistance, although higher strength grades are available. They are also compliant with engine and transmission oils, vegetable oils and some brake fluids.
Polychloroprene (Neoprene Rubber, CR)
Polychloroprene rubbers are homopolymers of chloroprene (chlorobutadiene), and were among the earliest synthetic rubbers used to produce seals. CR has good ageing characteristics in ozone and weather environments, along with abrasion and flex-cracking resistance. Most elastomers are either resistant to deterioration from exposure to petroleum based lubricants, or, to oxygen; CR is unusual, in offering a degree of resistance to both. CR also offers resistance to refrigerants, ammonia, Freon® (e.g. R12, R13, R21, R113, R114, R115, R134A), silicone oils, water, ozone, vegetable oils and alcohols. This, combined with a broad temperature range and moderate cost, accounts for its desirability in many seal applications. CR is not effective in aromatic oils and offers only
Polyurethane (AU, EU, PU)
Polyurethane is a polymer formed from a chain of organic units joined by urethane links. Polyurethanes are produced by the addition reaction of a polyisocyanate with a polyalcohol (polyol) in the presence of a catalyst and other additives. Polyurethane demonstrates excellent resistance to weathering and oxidation. They resist hydrocarbon fuels and mineral oils, however some grades degrade (hydrolyse) in hot water. Polyurethane also offers some of the best resistance to abrasion, and are therefore often specified for use in dynamic seals.
Acrylic Elastomers (ACM, AEM Vamac®)
There are generally two forms of acrylicbased elastomer available: Polyacrylates (ACM) and ethylene-acrylates (AEM, Vamac®). Polyacrylates offer good resistance to lubricating oils and high temperatures, and are commonly used where the two are found in combination. ACM elastomers show excellent resistance to engine oils (semi- and fully-synthetic), petroleum based lubricants, transmission fluids, aliphatic hydrocarbons, ozone and ultraviolet radiation. Ethylene acrylic elastomers (AEM) are terpolymers of ethylene, acrylic and a cure-site monomer, supplied by DuPont™ under the tradename of Vamac®. AEM elastomers exhibit mechanical properties similar to ACM, although they can operate over a wider temperature range than ACM and hydrogenated nitriles (HNBR).
Fluorocarbon Rubber (FKM, Viton®)
FKMs (sometimes known as FPMs in Europe) are frequently used to resist extreme temperatures and harsh chemicals. The strong carbon-fluorine bonds that make up the polymer structure provide high thermo-chemical resistance, giving excellent ageing characteristics shown by low compression set at elevated temperatures. FKMs offer excellent resistance to mineral oils and greases, aliphatic, aromatic and some chlorinated
hydrocarbons, petrol and diesel fuels, silicone oils and greases. However FKMs show poor resistance to ethers, esters and amines. FKMs are available as a copolymer (two monomers), terpolymer (three monomers) or as a tetrapolymer (four monomers). Each type determines both fluorine content and chemical structure which in turn significantly impact the chemical resistance and Temperature performance of the polymer. More recent innovations include the development of FKM materials for use in low temperature applications, where, with a glass transition of -40°C, it is possible to use FKMs down to -51°C in service.
Types of Flourocarbon Rubber
PTFE
PTFE (polytetrafluoroethylene) is a synthetic, thermoplastic polymer which offers exceptional chemical resistance over a wide range of temperatures, and offers extremely low levels of friction. PTFE lacks elasticity which prevents its use as an elastomeric-type sealing ring, however, it is commonly used for anti-extrusion as a back-up ring, and for non-stick requirements. Owing to its low friction and excellent chemical resistance, it is also commonly used for applications such as bearings, gears, rotary seals etc. Non-filled (virgin) grades are stable up to +260°C and are quite flexible and resistant to breaking under tensile and compressive stresses. PTFE is also available with fillers added, to enhance its physical characteristics.
Typical fillers include:
- Glass fillers for improved deformation and wear
- Inorganic fillers (e.g. calcium silicate, wollastonite) are used in a similar manner to glass fillers, with reduced abrasiveness.
- Carbon-filled for considerable wear and deformation improvement, and increased thermal conductivity.
- Graphite or molybdenum disulphide (MoS2) filled to lower the coefficient of friction.
- Bronze filled for excellent wear, deformation strength, thermal conductivity (reduced chemical resistance)
- Stainless steel fillers are used to increase wear resistance, and increased chemical resistance compared to bronze filled grades.
- Polyester filled for improved high temperature and wear resistance, for applications where running surfaces are non-hardened.
- Polyphenylenesulphide (PPS) filled for improved extrusion and deformation resistance
- Polyimide (PI) fillers are used to increase wear and abrasion resistance, being polymeric, the abrasion of running surfaces is reduced.
- Combinations of some of the above are also often used to offer optimal performance in service.
Fabric / Phenolic Resin Composites
Phenolic resins, also known as phenol formaldehyde resins (PF), are synthetic thermosetting resins created by the reaction of phenols with formaldehyde. These thermosets perform well in most engineering applications such as: hydraulic fluids, oil, glycols, phosphate esters, silicone oils and brake fluids etc. Phenolic resins demonstrate high dimensional stability and abrasion resistance, and are commonly used in wear-ring applications as fabric resin composites.
Polyamide (Nylon)
Nylon is a generic designation for a family of synthetic thermoplastic polymers known generically as polyamides, developed in 1938. Nylons are condensation copolymers formed by reacting a diamine and a dicarboxylic acid. Chemical elements included are carbon, hydrogen, nitrogen, and oxygen. The numerical suffix specifies the numbers of carbons donated by the monomers; the diamine first and the diacid second.
The most common variant is Nylon 6-6 which refers to the fact that the diamine and the diacid each donate 6 carbons to the polymer chain. The levels of these monomers has an influence on the chemical resistance as well as the mechanical properties. Nylon offers excellent mechanical properties in combination with good sheer strength, deformation and wear resistance. Chemical resistance is generally broad, with good resistance to most chemicals, although Nylon can be susceptible to damage when exposed to moisture.
PEEK (PolyEtherEther-Ketone)
PolyEtherEther-Ketone, or PEEK, is a high performance thermoplastic material that offers chemical and water resistant properties to seals. Peek seals are manufactured with peek material, and operate at much higher temperatures than other seals. Alfa’s peek seals can be used consistently to a temperature of 500F without losing any physical properties, and since they are made with thermoplastic materials, they can be heated, cooled, and heated again without degradation. When high temperature and high pressure demand a high performance thermoplastic seal, peek seals meet the needs of various applications. A low co-efficient of friction, and a high wear resistance without lubricants, peek seals are insoluble in solvents such as oils, salts, and acids. Since the properties and materials that peek seals are manufactured from are important to their performance, peek seal materials must resist hardening, and maintain a long service life. Alfa can provide custom peek seals that ensure optimal fluid control while eliminating leakage of your specific products. Our seals are engineered for static and dynamic applications and we utilize the latest research, technologies, and finest materials to provide our customers with optimum performance during demanding application conditions. Our peek sealing products comprise o-rings, lip seals, piston rings, wipers, and wear rings.