Elastomer Types and Chemical Compatibility in CIP Applications

Elastomer types and properties

Elastomer types are often referred to under standard designations/acronyms listed in ASTM D1418 or ISO 1629. A brief description of each elastomer type and their typical characteristics follows:

ACM (Polyacrylic or Polyacrylate): ACM or simply acrylate rubber consists of a polymerized ester and a curing monomer. Ethyl acrylate rubber has a good resistance to heat and mineral oil; on the other hand butyl acrylate has a better cold flexibility. Polyacrylate has a good resistance to mineral oil, oxygen and ozone even at high temperatures. The water compatibility and cold flexibility of ACM are significantly worse than with NBR.

AEM (Ethylene Acrylic): These elastomers are terpolymers of ethylene, methyl acrylate and a cure site monomer. AEM elastomers offer good resistance to heat ageing, weathering, aliphatic hydrocarbons and good low temperature performance. They show poor resistance to strong acids, hydrolyzing agents and some polar fluids. AEM applications are similar to those of ACM elastomers, but AEM has the advantage where low temperature flexibility is concerned. 

AU / EU (Polyester and Polyether Urethane): AU (polyester urethane) and EU (polyether urethane) are the two types of polyurethane elastomer used in the manufacture of seals, comprising the U-Group with carbon, oxygen and nitrogen in the main chain. Polyurethane is synthetically produced by reacting an isocyanate with a polyol in the presence of a catalyst and can be used for products as diverse as foam mattresses to car dashboards to varnish. By carefully selecting the isocyanate and polyols, the properties of the final material compound can be aligned with the requirements for sealing products; such materials have excellent wear and extrusion resistance, high tensile strength and high elasticity in comparison with all other elastomers.

CR (Chloroprene): Chloroprene Rubber (CR) is a synthetic rubber and has a good balance of properties, including good chemical stability and usefulness over a wide temperature range. It is mostly applied to bond structures or different materials. To improve the performance, oil-soluble phenolic resin is added to make chloroprene rubber phenolic adhesive, which sets at room temperature and is applied to bond varieties of metal and nonmetal materials including steel, aluminum, copper, ceramic, cement products, plastic and hard fiberboards. CR are widely used in general engineering applications, such as in the production of dipped articles (e.g. gloves), wraps and sheets, in various molded goods, in cable coatings, transmission/conveyor belts and profiles and primarily as the material for Gaskets, tubing, O-Rings, Weather stripping and hose.

CSM (Chlorosulphonylpolyethylene or Chlorosulphonatedpolyethylene): CSM is a synthetic rubber based on polyethylene. The CSM rubber is suitable for continuous use up to about 130 degree celcius. Chlorosulfonated polyethylene (CSM) is known for its excellent resistance to atmospheric conditions and good resistance to chemicals such as chromic, nitric, sulfuric and phosphoric acids and alkalis like chlorine dioxide, and hydrogen peroxide. CSM is often used in products which are in contact with sulfuric acid or nitric acid due to its excellent acid resistance. It’s resistance to acids and alkalis make it a great choice for chemical processing applications. Chlorosulfonated polyethylene is used in a variety of industrial applications that require high performance and have to withstand extreme weather conditions.

ECO (Epichlorohydrin): Epichlorohydrin rubber, or ECO for short, is the umbrella term for copolymers of epichlorohydrin and ethylene oxide. These compounds are cured with metal oxides or metal salts, with the addition of diamines or ethylene thiourea. ECO and polychloromethyl oxirane (CO) are often blended in order to achieve specific material properties. Blends with other rubbers are much less common. The main advantages of ECO lie in its low-temperature flexibility, good mineral oil and ozone resistance, acid and base stability and low gas permeability.

EPR/EPDM (Ethylene-Propylene): These rubbers are mainly available in two structures – as the copolymer (EPR), or as the terpolymer (EPDM). The properties for both types of rubber are very similar with the polymers exhibiting outstanding resistance to weathering, ozone, water and steam. These rubbers have good chemical resistance and are particularly recommended for use with phosphate ester based hydraulic systems. They are typically used in the production of window and door seals, wire and cable insulations, waterproofing sheets and hoses. They are not suitable for use with mineral oils or petroleum based fluids. These rubbers can either be sulphur or peroxide-cured, in general sulphur-cured grades have superior mechanical properties and inferior high temperature properties and vice-versa for peroxide cured grades.

FEP/PFA (Fluoroethylene Propylene-Perfluoroalkoxy): These chemically modified fluorocarbon copolymers (fluoropolymers) appear more like plastic than rubber, they are extremely resilient and show excellent chemical resistance. Mechanical properties are very good even at high temperatures. Non-stick characteristics are excellent and abrasion resistance can be classified as moderate. The effective continuous temperature range is from -100°C/-148°F to +200/250°C/ +392/482°F for FEP/ PFA respectively. Typical applications include door seals and sealing systems in diaphragm pumps, cryogenic plants, sealed filter units, corrosive fluid plants, relief and emergency valves and pneumatics. Fluoropolymers are often used to encapsulate other elastomers to produce composite seals.

FEPM or TFE/P (Tetrafluoroethylene/Propylene): Tetrafluoroethylene propylene (FEPM, TFE/P) is a partially fluorinated elastomeric polymer consisting of propylene and tetrafluoroethylene repeat units. Its thermo-physical properties are similar to those of polytetrafluoroethylene (PTFE) but somewhat inferior. For example, it has excellent thermal, electrical and chemical resistance. However, unlike PTFE resin, it is melt-processable using conventional injection molding and extrusion equipment. FEPM is ideally suited for applications where broad chemical resistance, high durability over a wide temperature range, and excellent dielectric properties are required. It provides some of the best all-around chemical resistance and is mainly used for the manufacture of seals operating in harsh environments.

FFKM/FFPM (Perfluoroelastomer): Perfluoroelastomer (FFKM/FFPM) parts resist over 1,800 different chemicals, including concentrated nitric acid, sodium hydroxide, ethylene diamine and steam while offering the high temperature stability of PTFE (327°C). FFKM parts and seals also help prevent semiconductor process contamination by reducing particulates, lowering extractables, and resisting degradation in harsh plasma. Perfluorelastomers provide low outgassing in vacuum-sealing applications. FDA-compliant seals are also available for food, beverage or pharmaceutical processes. FFKM parts are used in highly aggressive chemical processing, semiconductor wafer fabrication, pharmaceutical, oil and gas recovery, and aerospace applications. The long-term, proven performance of FFKM parts can mean less frequent seal changes, repairs and inspections, increasing process and equipment uptime for greater productivity and yield.

FKM/FPM (Fluoroelastomer or Fluorocarbon): FKM (also known as FPM in Europe) rubber compounds 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, such aslow compression set at elevated temperatures. FKM offers excellent resistance to mineral oils and greases, aliphatic, aromatic and some chlorinated hydrocarbons, fuels, silicone oils and greases. However, FKMs shows poor resistance to bases.

Copolymer, Terpolymer or Tetrapolymer: Fluoroelastomer or fluorocarbon (FKM/FPM) materials are available in three general types depending on their fluorine content and the number of monomers contained within the polymer.

FVMQ (Fluorosilicone): FVMQ combines the good high- and low-temperature properties of silicone with limited fuel and oil resistance. Fluorosilicones provide a much wider operational temperature range than Fluorocarbon rubbers. Primary uses of fluorosilicone O-rings are in fuel systems at temperatures up to +177°C (+350°F) and in applications where the dry-heat resistance of silicone O-rings are required. Fluorosilicone O-rings may also be exposed to petroleum-based oils and/or hydrocarbon fuels. In some fuels and oils; however, the high-temperature limit in the fluid list is more conservative because fluid temperatures approaching 200°C (390°F) may degrade the fluid, producing acids that attack fluorosilicone O-rings. For low-temperature applications, fluorosilicone O-rings seal at temperatures as low as -73°C (-100°F).

HNBR (Hydrogenated Nitrile): HNBR is made via selective hydrogenation of the NBR butadiene groups which improves the temperature and ozone resistance considerably. HNBR is widely known for its physical strength and retention of properties after long-term exposure to heat, oil, and chemicals. HNBR has better heat resistance; oxidation resistance; tensile strength and abrasion resistance than standard nitrile (NBR). Also an excellent choice for automotive refrigerant service. Both o-rings and gaskets can be made from Nitrile materials.

IIR (Butyl): Butyl Rubber has excellent resistance to oxygenated solvents such as Ketones and alcohols. It is also able to counteract the effects of alkalis, flexing, and abrasion. This material is also able to withstand exposure to oxygen/ ozone and chemicals. A typical working temperature range for this material would be between -50 and 120o. A copolymer of isobutylene and a small amount of isoprene, this synthetic rubber is regularly used in precision engineering as rubber sealants and in the industrial sector for everything from damp proofing and roofing to tyre inners and protective clothing.

IR (Polyisoprene): Synthetic version of natural rubber; its strengths and uses are similar, but its relative purity means that IR materials tend to crystallise less at low temperatures. Consequently, it has better performance at lower temperatures but, at normal temperatures, its performance is inferior to natural rubber.

NBR (Nitrile or Acrylonitrile Butadine): Acrylonitrile or Nitrile as it’s more commonly referred to is a copolymer of Acrylonitrile and Butadiene. The Nitrile content varies in commercial products from 18% to 50%, the higher the Nitrile content the better the resistance to petroleum oils and hydrocarbon fuels, but low temperature flexibility decreases.

As Nitrile has an excellent resistance to petroleum products, and an ability to be compounded for service over a wide temperature range, Nitrile is the most widely used elastomer in the seal industry today. Nitrile compounds are superior to most elastomers with regard to compression set, tear, and abrasion resistance but have limited resistance to ozone, sunlight, or weather. To obtain good resistance to lower temperatures, it is often necessary to sacrifice some high temperature resistance. Nitrile should not be kept near electric motors or other ozone generating equipment and should not be kept in direct sunlight.

NR (Natural rubber): Through tapping rubber trees planted in plantations a white watery milk (latex) is obtained which contains natural rubber. NR has a very high tensile strength, elasticity, cold flexibility and excellent dynamic characteristics which in this combination can hardly be reached by synthetic elastomers and therefore make NR still today essential for some applications. Without the corresponding equipment with protective agents the ageing and ozone resistance is very low, and NR is not resistant to mineral oils and greases.

PTFE (Polytetrafluoroethylene): polytetrafluoroethylene (PTFE), a strong, tough, waxy, nonflammable synthetic resin produced by the polymerization of tetrafluoroethylene. Known by such trademarks as Teflon, Fluon, Hostaflon, and Polyflon, PTFE is distinguished by its slippery surface, high melting point, and resistance to attack by almost all chemicals. These properties have made it familiar to consumers as the coating on nonstick cookware; it is also fabricated into industrial products, including bearings, pipe liners, and parts for valves and pumps.

SBR (Styrene-Butadiene): SBR is the highest volume general-purpose synthetic rubber in production today. This general-purpose synthetic rubber is manufactured from a copolymer of styrene and butadiene, and in many cases is used as a cost-efficient alternative to natural rubber. SBR rubber is highly regarded for its excellent abrasion resistance, crack endurance and positive aging characteristics. It also delivers good compression set and water resistance, as well as favorable bonding and heat-aging properties. SBR rubber is not suitable for applications involving chemicals, ozone, strong acids, oils, greases, fats and most hydrocarbons. Its solvent resistance and weathering properties are inferior to most other elastomers.

VMQ/PVMQ (Silicone): These elastomers, which include the phenyl substituted silicones are noted for their high and low-temperature applications (phenyl silicones offer exceptionally low-temperature flexibility). They have excellent resistance to ozone and weathering and good resistance to compression set at high temperatures. They do, however, have poor tensile strength, low tear, and abrasion resistance, and high gas permeability. Silicones have a low level of combustible components; even when exposed to flame, the elastomer is reduced to non-conducting silica ash. Silicones also exhibit excellent compression sets and high physiological inertness (tasteless, odorless and completely non-toxic). Silicones are also resistant to bacteria, fungi, and a wide range of media including high energy radiation and excellent release properties (except to glass). Platinum-cured silicones offer enhanced levels of purity and low extractables making them ideal for pharmaceutical, biomedical, and food & drink applications.

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