The following paragraphs briefly review the various elastomers currently available for use in O-rings and other elastomeric seals.

1. Acrylonitrile- Butadiene (NBR)

Nitrile Rubber (NBR) is the general term for acrylonitrile butadiene terpolymer. The acrylonitrile content of nitrile sealing compounds varies considerably (18% to 50%) and influences the physical properties of the finished material.

The higher the acrylonitrile content, the better the resistance to oil and fuel. At the same time, elasticity and resistance to compression set is adversely affected. In view of these opposing realities, a compromise is often drawn, and a medium acrylonitrile content selected. NBR has good mechanical properties when compared with other elastomers and high wear resistance. NBR is not resistant to weathering and ozone.

Heat Resistance
Up to 100°C (212°F) with shorter life @ 121°C (250°F).

Cold Flexibility
Depending on individual compound, between -34°C and -57°C
(-30°F and -70°F)

Chemical Resistance
Aliphatic hydrocarbons (propane, butane, petroleum oil, mineral oil and grease, diesel fuel, fuel oils) vegetable and mineral oils and greases.
HFA, HFB and HFC fluids
Dilute acids, alkali and salt solutions at low temperatures.
Water (special compounds up to 100°C)

Not compatible with:
Fuels of high aromatic content (for flex fuels a special compound must be used.)
Aromatic hydrocarbons (benzene)
Chlorinated hydrocarbons (trichloroethylene)
Polar solvents (ketone, acetone, acetic acid, ethylenester)
Strong acids
Brake fluid with glycol base.
Ozone, weather and atmospheric aging.

2. Carboxylated Nitrile (XNBR)

Carboxylated Nitrile (XNBR) is a special type of nitrile polymer that exhibits enchanced tear and abrasion resistance. For this reason, XNBR based materials are often specified for dynamic applications such as rod seals and rod wipers.

Heat resistance
Up to 100°C (212°F) with shorter life @ 121°C (250°F).

Cold Felxibility
Depending on individual compound, between -18°C and -48°C
(0°F and -55°F)

Chemical resistance
Aliphatic hydrocarbons (propane, butane, petroleum oil, mineral oil and grease, diesel fuel, fuel oils) vegetable and mineral oils and greases.
HFA, HFB and HFC fluids
Many diluted acids, alkali and salt solutions at low temperatures
Water (special compounds up to 100°C)

Not compatible with:
Fuels of high aromatic content (for flex fuels a special compound must be used)
Aromatic hydrocarbons (benzene)
Chlorinated hydrocarbons (trichloroethylene).
Polar solvents (ketone, acetone, acetic acid, ethyleneester)
Strong acids
Brake fluid with glycol base.

3.Ethylene Acrylate (AEM)

Ethylene acrylate is a mixed polymer of ethylene and methyl acrylate with the addition of a small amount of carboxylated curing monomer. Ethylene acrylate rubber is not to be confused with ethyl acrylate rubber (ACM).

Heat resistance
Up to 149°C with shorter life up to 163°C

Cold flexibility
Between -29°C and -40°C

Chemical resistance
Ozone
Oxidizing media
Moderate resistance to mineral oils

Not compatible with:
Ketones
Fuels
Brake fluids

4.Ethylene Propylene Rubber (EPDM, EPM)

EPM is a copolymer of ethylene and propylene. Ethylene propylene-diene rubber (EPDM) is produced using a third monomer and it particularly useful when sealing phosphate-ester hydraulic fluids and in brake systems that use fluids having a glycol base.

Heat resistance
Up to 150°C (max 204°C) in water and /or steam)

Cold Flexibility
Down to approximately -57°C

Chemical resistance
Hot water and steam up to 149°C with special compounds up to 204°C
Glycol based brake fluids up to 149°C
Many organic and inorganic acids
Cleaning agents, soda and potassium alkalis.
Phosphate- ester based hydraulic fluids (HFD-R)
Silicone oil and grease
Many polar solvents (alcohols, ketones, esters).
Ozone, aging and weather resistant.

Not compatible with:
Mineral oil products (oils, greases and fuels)

5.Butyl Rubber (IIR)

Butyl (isobutylene, isoprene rubber, IIR) is produced by many companies in different types and varies widely in isoprene content. Isoprene is necessary for proper vulcanization. Butyl has a very low permeability rate and good electrical properties

Heat resistance
Up to app. 121°C

Cold flexibility
Down to app. -59 °C

Chemical resistance
Hot water and steam up to 121°C
Brake fluids with glycol base
Many acids
Salt solutions
Polar solvents, eg. Alcohols, ketones and ester
Poly-glycol based hydraulics fluids (HFC fluids) and phosphate-ester bases (HFD-R fluids)
Silicone oil and grease
Ozone, aging and weather resistant

Not compatible with:
Mineral oil and grease
Fuels
Chlorinated hydrocarbons

6. Butadiene Rubber (BR)

Polybutadiene (BR) is mostly used in combinations with other rubbers to improve cold flexibility and wear resistance. BR is primarily used in the tire industry, for sure drive belts and conveyor belts and is not suitable as a sealing compound.

7. Chlorobutyl Rubber (CIIR)

Chlorobutyl (CIIR) is produced by chlorinating butyl polymer. Is chlorine content is approximately 1.1% to 1.3%. Apart from the properties of butyl rubber (IIR), chlorobutyl (CIIR) shows improved compression set properties and can be compounded with other materials

8. Chloroprene Rubber (CR)

Chloroprene was the first synthetic rubber developed commercially and exhibits generally good ozone, aging and chemical resistance. It has good mechanical properties over a wide temperature range

Heat resistance
Up to app. 121°C

Cold flexibility
Down to app. -40 °C

Chemical resistance
Paraffin base mineral oil with low DPI, eg: ASTM oil No.1
Silicon oil and grease
Water and water solvents at low temperature
Refrigerants
Ammonia
Carbondioxide
Improved ozone, weathering and aging resistance compared with NBR

Limited compatibility
Naphthalene based mineral oil (IRM 902 and IRM 903 oils)
Low molecular aliphatic hydrocarbons (propane, butane, fuels)
Glycol based brake fluids

Not compatible with:
Aromatic hydrocarbons (benzene)
Chlorinated hydrocarbons (trichloroethylene)
Polar solvents (ketones, esters, ethers, acetones).

9. Chlorosulfonated Polyethylene (CSM)

The polyethylene polymer contains additional chlorine and sulfur groups. Chlorine gives the mineral resistance to flame and mineral oil and also improves the cold flexibility

Heat resistance
Up to 121°C

Cold flexibility
Down to app. -29°C

Chemical resistance
Many acids
Many oxidizing media
Silicon oil and grease
Water and water solvents
Ozone, aging and weathering resistance

Limited compatibility
Low molecular aliphatic hydrocarbons (propane, butane, fuel)
Mineral oil and grease
Limited swelling in aliphatic oil (ASTM oil No.1)
High swelling in naphthene and aromatic base oils (IRM 902 and IRM 903 oil)
Polar solvent (acetone, methyl ether, ketone, ethyl acetate, diethyl ether, dioxane)
Phosphate-ester based fluids

Not compatible with:
Aromatic hydrocarbons (bezene)
Chlorinated hydrocarbons (trichloroethylene)

10. Epichlorohydrin (CO, ECO)

Epichlorohydrin is available in 2 types: the homopolymer (CO) and the copolymer (ECO). Both CO and ECO have good resistance to mineral oils, fuels and ozone. The high temperature resistance is good. Compression set and the tendency to corrode metal sealing faces increase at 150°C. ECO has a good cold flexibility. CO has a high resistance to gas permeability

Heat resistance
Up to app. 135°C

Cold flexibility
Down to app. -40°C

Chemical resistance
Mineral oil and grease
Aliphatic hydrocarbons (propane, butane, fuel)
Silicone oil and grease
Water at room temperature
Ozone, aging and weather resistance

Not compatibility with:
Aromatic and chlorinated hydrocharbons
Ketones and esters
Non-flammable hydraulic fluids in the groups HFD-R and HFD-S.
Glycol based brake fluids

11. Fluorocarbon (FKM)

Fluorocarbon (FKM) has excellent resistance to high temperatures, ozone, oxygen, mineral oil, synthetic hydraulic fluids, fuels, aromatics and many organic solvents and chemicals. Low temperature resistance is normally not favourable and for static applications is limited to approximately -26°C, although in certain situations it is suitable down to -40°C. Under dynamic conditions, the lowest service temperature is between -15°C and -18°C.

Gas permeability is very low and similar to that of butyl rubber. Special FKM compounds exhibit and improved resistance to acids, fuels, water and steam.

Heat resistance
Up to 204°C and higher temperatures with shorter life expectancy.

Cold flexibility
Down to -26°C (some up to -40°C)

Chemical resistance
Mineral oil and grease, low swelling in ASTM oil No.1, and IRM 902 and IRM 903 oils
Non-flammable hydraulic fuels in the group HFD.
Silicone oil and grease
Mineral and vegetable oil and grease
Aliphatic hydrocarbons (fuel, butane, propane, natural gas)
Aromatic hydrocarbons (benzene, toluene)
Chlorinated hydrocarbons (trichloroethylene and carbon tetrachloride)
Fuels, also fuels with methanol contents
High vacuum
Very good ozone, weather and aging resistance

Not compatible with:
Glycol based brake fluids
Ammonia gas, amines, alkalis
Superheated steam
Low molecular organic acids (formic and acetic acids)

12. Fluorosilicone (FVMQ)

FVMQ contains trifluoropropyl groups next to the methyl groups. The mechanical and physical properties are very similar to VMQ. FVMQ offers improved fuel and mineral oil resistance but poor hot air resistance when compared with VMQ.

Heat resistance
Up to 177°C max.

Cold flexibility
Down to app. -73°C

Chemical resistance
Aromatic mineral oils (IRM 903 oil)
Fuels
Low molecular weight aromatic hydrocarbons (benzene, toluene)

13. Hydrogenated Nitrile (HNBR)

Hydrogenated Nitrile is a synthetic polymer that results from the hydrogenation of nitrile rubber (NBR). In this process the molecular double bonds in the NBR primiary polymer chain undergo a hydrogenation process and therefore the term hydrogenated nitrile (HNBR). The allow temperature range extends to 149°C with short periods at higher temperature possible. By following design guidelines effective sealing can be achieved at -32°C for static applications. For dynamic applications however, operating temperatures are limited to above - 23°C. HNBR compounds posses superior mechanical characteristics, particularly their high strength. For sealing applications up to app 159°C, this is an advantages as it prevents extrusion and wear.

Chemical Resistance
Aliphatic hydrocarbons
Vegetable and animal fats and oils
HFA, HFB and HFC fluids
Dilute acids, bases and salt solutions at moderate temperature
Water and stream up to 149°C
Ozone, aging and temperature

Not compatible with:
Chlorinates hydrocarbons
Polar solvents (ketone and ester)
Strong acids

14. Perfluoroelastomer (FFKM)

The name Perfluoroelastomer is somewhat misleading. An actual perfluorinated material with a high molecular weight is polytetrafluoroethylene or PTFE which has the chemical formula (CF2)n. The molecular of the large bonded fluorine atoms. Perfluoroelastomer is produced by the copolymerization of tetrafluoroethylene (TFE) and is perfluorinated ether.

The differing resistance to volume swell of the difference perfluoroelastomers is due to the perfluorinated ether elements, where the side-chain can consist of up to four perfluorinated carbon atoms. The extraordinary chemical resistance is partly due to the fluorine atoms shielding the carbon chain, and partially due to the vulcanization system.

Heat resistance
232°C to 300°C depending on compound

Cold flexibility
-18°C or -26°C

Chemical resistance
Aliphatic and aromatic hydrocarbons
Chlorinated hydrocarbons
Polar solvents (acetone, methylethylketone, ethylacetate, diethylether and dioxane)
Inorganic and organic acids
Water and steam
High vacuum with minimal loss in weight

Not compatible with:
Fluroinated refrigerants (R11, 12, 13, 113, 114, etc)

15. Polyacrylate (ACM)

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.

Heat resistance
Shortened lifetime up to approximately 177°C.

Cold flexibility
Down to approximately -21°C

Chemical Resistance
Mineral Oil (engine, gear box, ATF oil)
Ozone, weather and aging resistance

Not compatible with:
Glycol based brake fluid
Aromatics and chlorinated hydrocarbons
Hot water, steam
Acids, alkalis, amines

16. Polyurethane (AU, EU)

One must differentiate between polyester urethane (AU) and polyether urethane (EU). AU type urethanes exhibit better resistance to hydraulic fluids. Polyurethane elastomers, as a class, have excellent wear resistance, high tensile strength and high elasticity in comparison with any other elastomers. Permeability is good and comparable with butyl.

Heat Resistance
Up to approximately 82°C

Cold flexibility
Down to approximately -40°C

Chemical Resistance
Pure aliphatic hydrocarbons (propane, butane, fuel)
Mineral oil and grease
Silicone oil and grease
Water up to 50°C (EU type)
Ozone and aging resistance

Not compatible with:
Ketones, esters, ethers, alcohols, glycols
Hot water, steam, alkalis, amines, acids

17. Silicone Rubber (Q, MQ, VMQ, PVMQ)

The term silicone covers a large group of materials in which vunyl-methyl-silicone (VMQ) is often the central ingredient. Silicone elastomers as a group have relatively low tensile strength, poor tear and wear resistance. However, they have many useful properties as well. Silicones have good heat resistance up to 232°C, good cold flexibility down to -59°C and good ozone and weather resistance as well as good insulating and physiologically neutral properties.

Heat Resistance
Up to approximately 204°C (special compound up to 232°C )

Cold Flexibility
Down to approximately -59°C to -54°C with special compounds down to -115°C

Chemical resistance
Engine and transmission oil (eg: ASTM oil No.1)
Animal and vegetable oil and grease
Brake fluid (non-petroleum base)
Fire-resistance hydraulic fluid, HFD-R and HFD-S
High molecular weight chlorinated aromatic hydrocarbons
Moderate water resistance
Diluted salt solutions
Ozone, aging and weather resistance

Not compatible with;
Superheated water steam over 121°C
Acids and alkalis
Low molecular weight chlorinated hydrocarbons
Aromatic mineral oil
Hydrocarbon based fuels
Aromatic hydrocarbons (benzene, toluene)

18. Styrene-Butadiene (SBR)

SBR probably is better known under its old names Buna S and GRS. SBR was first produced under government control between 1930 and 1950 as a replacement for natural rubber. The basic monomers are butadiene and styrene, with styrene content approximately 23.5%. About one third of the world output of SBR is used in tire production. SBR is mostly used in seals and non-mineral oil based brake fluid applications

Heat Resistance
Up to approximately 107°C

Cold flexibility
Down to approximately -57°C

Compatible with:
Water, alcohol, glycol and certain ketones (acetone)
Non-mineral oil based brake fluid.
Silicon oil and grease
Diluted water solutions, weak acids

Not compatible with:
Mineral oils
Petroleum greases and fuels
Aliphatic hydrocarbons like benzene, toluene, xylol.
Chlorinated hydrocarbons- such as chloroform, trichloroethylene, carbon tetrachloride
Oxidizing, media like nitric acids, chromic acid, hydrogen peroxide, chlorine, bromine.

19. Tetrafluoroethylene-Propylene (AFLASR)

This elastomer is a copolymer of tetrafluoroethylene (TFE) and propylene. Its chemical resistance is excellent across a wide range of aggressive media.

Heat Resistance
Up to approximately 232°C

Cold Flexibility
Down to approximately -4°C

Compatible with:
Bases
Phosphate esters
Amines
Engine Oils
Steam
Pulp and paper liquors

Not compatible with:
Aromatic fuels
Ketones
Carbon Tetrachloride

Selection of Base Polymer

System operating temperatures and compatibility with the media to be sealed are the most important parameters which must be considered when selecting a base polymer. Only when these two factors are identified (including any lubricants and potential cleaning fluids), can a reliable recommendation be given concerning selection of the proper elastomer base. For the seal designed, a compromise often has to be made between specifying high quality, sealing grade materials and cheaper commercial products (which usually contain less polymer and more inexpensive fillers)

The application temperatures given as shown below chart refer to long-term exposure to non-aggressive media. At higher temperatures, new crosslink sites may be formed between the polymer chains and lead to a loss of seal flexibility. The stiffness in the polymer chains may be observed as excessive compression set in highly filled compounds. This condition prevents an O-ring cross-section from re-turning to its original, pre-compressed shape after deformation forces are removed. During compression, a seal changes its original shape to effect a seal and over time, and with excessive temperature, elastic memory loss in the elastomer seal element can cause leakage. Exceeding the normal maximum temperature limit for a given compound always result in reduced service life.

Practically all elastomers undergo a physical or chemical change when in contact with a sealed medium. The degree of change depends on the chemistry of the medium and on the system temperature. An aggressive medium becomes more active with increasing temperature. Physical changes are caused by two mechanisms which can work concurrently when:

a. The elastomer absorbs a medium
b. Plasticizers and other components of the compound are dissolved and extracted or leached out by the media.

The result is volume changes, i.e, swelling or shrinkage of the elastomer seal. The degree of volume change depends on the type of medium, molecular structure of the rubber compound, system temperature, geometrical seal shape, and the stressed conditions of the rubber part. When deformed and exposed to a medium, rubber, when confined in a gland, swells significantly less than in free state (up to 50%) due to a number of factors including lessened surface area in contact with the medium.

The limit of permissible volume change varies with the application. For static seals, a volume change of 25% to 30% can be tolerated. Swelling leads to some deterioration of the mechanical properties, and in particular, those properties which improve extrusion resistance.

In dynamic applications, swelling leads to increased friction and a higher wear rate. Therefore, a maximum swell of 10% should generally not be exceeded. Shrinkage should also be avoided because the resulting loss of compressive force will increase the risk of the leakage.

The extraction of plasticizer from a seal material is sometimes compensated for by partial absorption of the contact medium. This situation however, can still lead to unexpected shrinkage and resultant leakage when an elastomer dries out and the absorbed fluids evaporate.

A chemical reaction between sealed or excluded medium and the elastomer can bring about structural changes in the form of further crosslinking or degrading. The smallest chemical change in an elastomer can lead to significant changes in physical properties, such as embrittlement.

The suitability of an elastomer for a specific applications can be established only when the properties of both the medium and the elastomer are known under typical working conditions. If a particular seal material suits a medium, it is referred to as begin compatible with the medium.