Both urethane and epoxy are used in many industrial products, including curing agents, paints, adhesive agents, foam and resins. There are similarities between these substances, but their differences are more apparent and each industry seems to have its own preference. In terms of color, chemical resistance, flexibility and texture, urethane and epoxy are very different and it is by balancing these differences that most industries choose which one to use. When it comes to price, both are about the same, so it is not a real factor in their difference.
The color of epoxy and urethane is important to industries such as the paint industry and the resin industry, because the paint should not change color and the resin should remain transparent when it is used. Epoxy is not resistant to ultraviolet light so, while it will not change color immediately, the epoxy will begin to yellow after several months or years. Urethane, which is resistant to ultraviolet light, will remain the same color for a much longer time than epoxy.
Chemical resistance, which is important for just about any industry using epoxy and urethane products, keeps the substance from corroding or reacting with acids or bases that may touch the substance. Epoxy is highly resistant to chemicals and can take many acids and bases without being diluted. Urethane is only moderately resistant and, while it can take diluted bases and acids, it will wear much faster than epoxy.
In adhesive and foam industries, where urethane and epoxy substances are used to bond surfaces together, flexibility and firmness are important, depending on the needs of the industry. Epoxy is considered a higher-strength substance, because it keeps the surface from moving. While strong, it can crack when used on surfaces that move frequently. Urethane is able to remain flexible, which is needed in structures that with get bigger or smaller based on season and temperature.
The texture of urethane and epoxy also makes a difference to different industries, based on whether the substance needs to be chalky or smooth. Epoxy substances, especially after aging, will become chalky and can be brittle to the touch. Urethane will remain smooth throughout its life and is better for surfaces or areas where the substance will be exposed to water.
Chemically, both epoxy and urethane are polymers, or large molecules that contain a repeating structure. Epoxy is made from epoxide and polyamine as a resin and hardener blend. Urethane is composed of organic, or carbon-based, links that react with a monomer, or chemically bonding element.