Epoxy is a substance that is noted for its strength, durability, chemical resistance and wide range of applications. However, before it can be used, epoxy must be mixed, initiating a chemical reaction. Heat can be used to change the rate of this reaction, and high temperatures also have a significant effect on the properties of cured epoxy.
"Epoxy" can refer to several types of resins with similar properties and chemical structures. The term "epoxy" comes from the epoxy groups found in the chemical structure of the components of epoxy resins. This substance is hard and notably inert and is primarily used for adhesives, protective coatings and molds, although epoxy resins have a number of uses. Epoxy resins are also notable in that they can be used to glue metals together.
Epoxies are formed from two parts that are mixed together, a polymer with epoxy groups at either end and a diepoxy. When these two chemicals are mixed, they begin to undergo a chemical reaction, which is why epoxies do not come premixed and must be mixed immediately before they are going to be used. Epoxies do not set via evaporation; they cure, or the chemicals within the epoxy finish reacting, forming an inert solid. The products of this chemical reaction are the epoxy resin and heat, which is the energy released during the chemical reaction. Epoxy cures faster when the ambient temperature is warm, and the heat of the chemical reaction in epoxy can help it cure quickly.
The chemical reaction that results in epoxy is exothermic. That is, the reaction creates heat; epoxy that is curing can generate enough heat to burn skin and melt plastic. Adding more heat to this reaction makes the reaction proceed more quickly. In general, temperatures below 50 degrees Fahrenheit are too cold for epoxy to cure properly. Temperatures around 70 degrees Fahrenheit are ideal. In addition, if epoxy is spread in a thin layer, the chemicals are distributed evenly, the heat dissipates quickly, and the heat released by the reaction doesn't affect the curing process significantly. If epoxy is kept in a container or spread in a thick layer, the heat from the reaction dissipates more slowly, and the heat of the chemical reaction can speed its own curing process. (see Reference 4)
Cured epoxy is typically hard and rather brittle. However, it will soften under high heat, usually around 140 degrees Fahrenheit. This can be advantageous, as this allows for epoxy to be removed. For instance, epoxy can be removed from an item by heating it with a heat gun. The epoxy rehardens to its original state when it starts to cool. Some epoxies are designed to withstand very high temperatures, however. These epoxies have chemical structures designed specifically to withstand high heat.