Viton and EPDM are both common elastomers used for O-rings. Elastomers are a type of rubber polymer with high yield strength. Yield strength refers to the point at which your material won’t return to its original shape after being stressed. Basically, they can be stretched a great deal without breaking. The most common usage for such elastomers are as a gasket material. Pumps, valves and other related equipment use such elastomers to prevent leakage.
Viton is a fluoroelastomer manufactured by DuPont. It has a temperature range of -15 to 400 degrees Fahrenheit. It is very resistant to damage caused by compression, heat, acids, alkalis, petrol-based substances, sunlight, salt solution and ozone. It has poor resistance to acetones and ketones.
EPDM is ethylene-propylene-diene-monomer. It has a temperature range of -70 to 300 degrees Fahrenheit. It is very resistant to damage caused by compression, heat, sunlight, alkalis and ozone. It has a poor resistance to petrol-based substances, acids and hydrocarbons.
The basic difference is range. Viton can handle higher temperatures, whereas EPDM can handle lower temperatures. Viton is also more resistant to chemicals and oils, but if you are to have acetones present, you will want to choose EPDM.