ANSWER: ETHYLENE PROPYLENE RUBBER (EPM, EPDM, EPR):
This elastomer is a copolymer of ethylene and propylene and is sometimes compounded with a third monomer (EPT). Good to excellent compression set resistance is obtained by the addition of peroxide cures during vulcanisation. Ethylene propylene materials have excellent resistance to phosphate esters such as Skydrol, Fyrquel, Pydraul, water and steam, acids, alkali, salt solutions, ketones, alcohol's, glycol's, and silicone oils and greases. EPR has very poor resistance to petroleum oils and diester-base lubricants. Ethylene propylene is a close contender to Buna-N and butyl in the important sealing properties, except that it does not have the petroleum oil and fuel resistance of Buna-N, nor the low-gas-permeability quality of butyl.
NITRILE OR BUNA-N RUBBER: More nitrile seals are used than all the other elastomers combined, since nitrile is the most versatile material. Nitriles are a copolymer of acrylonitrile and butadiene. As the acrylonitrile content of nitriles increases, the oil and fuel resistance increases while the low-temperature flexibility decreases. Nitrile-based elastomers are usually specified by military MS and AN O-rings when used in oil and fuel applications, but because nitrile compounds vary widely within such a large overall temperature range, particular attention should be paid to specifying physical properties. Materials can be formulated to perform satisfactorily over the temperature range 65 to +300 degrees F, so it is necessary to make sure that the particular nitrile chosen meets the temperature requirements of the application. The nitrile materials are recommended for general-purpose sealing of alkaline and salt solutions, petroleum oils and fluids, vegetable and diester oils, silicone greases and oils, ethylene glycol-base fluids, alcohol's, gasoline's and water. They are not suited for use with strong oxidising agents; chlorinated solvents such as carbon tetrachloride or trichlorethylene, nitrated hydro-carbons such as nitrobenzene or aniline; keytones such as methyl ethyl ketone (MEK) and acetone; and aromatic hydrocarbons. Ozone will will usually attack nitrile materials, but resistance can be greatly improved by the addition of antioxidant compounds.
FLUOROCARBON RUBBER (FKM): Fluorocarbon elastomers have been compounded to meet a wide range of chemical and physical requirements. Under the trade name Viton®, Fluorel, and Kel-F and fluorocarbons seals have been employed where other materials cannot survive severe chemical conditions. The working temperature range of FKM is between -20 and +400 degrees F (-29 and +204 degrees C) and limited temperature spikes of 600 degrees F have been incurred. New compoundings have greatly improved the compression set of fluorocarbon O-ring seals.
SILICONE RUBBER (Si): Silicone elastomers are compounded from dimethy silicone polymers, and thus will deteriorate if used with silicone oils and greases. Various additives have extended the functional temperature range of silicone rubber beyond any other elastomer. Flexibility below -175 degrees F (-114 degrees C) and service above 700 degrees F (371 degrees C) for short periods of time have been demonstrated. High production cost have normally limited the use of silicone seals to applications requiring extreme temperature resistance. Production moulding of silicone seals involves high-temperature secondary cure which results in greater than normal shrinkage. The finished O-ring seal is usually undersized when produced in standard moulds. The designer should be aware of this size difference when designing glands for silicone O-rings. Silicone elastomer have poor resistance to keytone solvents such as MEK and acetone, and poor resistance to most petroleum fluids. They have very poor physical properties that make them unattractive for dynamic applications. Silicone seals are recommended for extreme temperature use with ozone, oxygen, high-aniline point oils, and chlorinated diphenyls.
POLYURETHANE (AU, EU): Polyurethane elastomers are compounds of polyethers and diisocryanates. These materials have excellent physical properties of abrasion resistance and tensile strength, which make them outstanding for dynamic applications. They have excellent resistance to weather, ozone, and oxygen, good resistance to hydrocarbon fuels, petroleum oils, and aliphatic solvents and fair resistance to aromatic hydrocarbons. Acids, keytones, and chlorinated hydrocarbons attack and deteriorate polyurethane. Because polyurethane is available in castable liquids, injection-mouldable pellets and millable gums, it is a very useful material for specialised sealing problems.
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