Elastomer materials A-Z
Below is a summary of the different elastomers types available from PPE.
EPDM
EPDM elastomers have a fair tensile strength and excellent resistance to weathering, 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. 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.
FFKM
This is the most chemically resistant elastomer available and is effectively an elastomeric form of PTFE. It displays other properties which prove most valuable in applications where purity, high temperatures and retention of sealing force are paramount. PPE’s range of FFKM elastomers is known by the registered trade name Perlast®.
FKM
FKM seal materials demonstrate outstanding resistance to heat, fuels, lubricants, weather, ozone, oxygen and oxidisers, with low gas permeability. PPE’s FKM seal materials include Type 1, 2 and 3 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. MEK), some organic acids (e.g. formic acid), certain methanol and ester based hydraulic fluids (e.g. Skydrol), ammonia and some amines. Special FKM grades with peroxide cure system are required for use in hot water, steam, acids and diluted alkaline solutions applications.
FKM elastomers provide high compression set resistance if compounded with bisphenol cure systems. Generally, they are serviceable down to –20°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).
Fluoroelastomer or fluorocarbon (FKM/FPM) materials are available in three general types depending on their composition (type of monomers used):
FKM type | Fluorine content | Advantages/disadvantages |
Type 1 | >66% | Contains two monomers (simple molecules from which polymers are built). General purpose, most common, most widely used for sealing. Best compression set and good fluid resistance. These are normally the least cost types of compound. |
Type 2 | 67-70% | Contains three monomers. Better fluid and oil/solvent resistance than Type 1 and varying according to the fluorine content. Some of them (peroxide cured materials) show improved resistance to aqueous environments (water/steam, acids and alkalis). |
Type 3 | 64-66% | Contains three monomers. Improved low temperature performance. Fluid and oil/solvent resistance varying according to the fluorine content. Type 3 FKM can only be cured with a peroxide curing system so they show excellent resistance to aqueous environments (water/steam, acids and alkalis) |
FVMQ
FVMQ elastomers are modified silicone rubbers, with superior fluid resistance, but limited to about 175°C (347°F).
HNBR
The hydrogenation process of NBR elastomers provides excellent heat and ozone resistance. Peroxide cured HNBRs have the best compression set and heat resistance, and high-nitrile (can) HNBR elastomers have better resistance to mineral oils. HNBRs combine best resistance and low temperature flexibility, although they are more expensive than NBRs. HNBRs 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.
HNBRs fill the gap between NBRs and FKMs 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.
NBR (Buna N)
NBR elastomers are available in different grades; based on acrylonitrile content, giving proportional physical and chemical properties. NBRs 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. NBRs 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.
FEPM (TFE/P)
TFE/P vulcanisates exhibit similar thermal stability to FKM elastomers, with superior electrical resistance and a different chemical resistance profile (e.g. sour gas, acids and alkalis, amines, steam and water, all hydraulic and brake fluids, alcohols and high energy radiation). However, they are not compatible to aromatic hydrocarbons, polar solvents (e.g. MEK, acetone and organic acetates) and fluorinated refrigerants.
TFE/P elastomers exhibit inferior compression set performance than FKM materials. In addition, the low temperature flexibility is relatively poor However, TFE/P elastomers have a niche, high-value, in applications such as oil-field operations and chemical processing, in the form of O-rings, seals and gaskets, cable insulating, jacketing and hose liners.
VMQ
The distinctive properties of silicone rubbers are outstanding resistance to ozone and corona, outdoor weather and sunlight. 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, high energy radiation (to 106 Rads) and excellent release properties (except to glass). The main limitations are low tensile properties and poor resistance to fuels, 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.