At a basic, chemical level, epoxy resin starts with the reaction of bisphenol A, which contains a benzene (carbon) ring and a hydroxyl (OH) functional group, and epichlorohydrin. Polyurethane resin, on the other hand, is based on the reaction of an organic (carbon) polyisocynate and an alcohol.
Epoxy resin dries, or cures, to a very hard, unexpansive surface and is only suitable for indoor use. Polyurethane resin, on the other hand, is up to 50 times more expansive than epoxy resin, making it far more resistant to cracking and therefore suitable for indoor or outdoor use.
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You can install polyurethane resin successfully at temperatures between 20 and 100 degrees Fahrenheit, while epoxy resin requires temperatures above 50 degrees Fahrenheit. If applied as a coating for concrete, polyurethane typically cures in two to four days, while epoxy can take up to eight days to cure.
Volatile Organic Compounds
All polyurethane resins contain moderate levels of volatile organic compounds (VOCs). Some industrial grade epoxies contain low levels of VOCs, but general-use epoxies sold in retail stores may have very high levels.