Epoxy resins are well known as high-performance materials in various construction and metal coating applications, providing the combined properties of chemical resistance, adhesion, corrosion resistance, mechanical strength and also high flexibility in some cases. Figure 1 illustrates how these favorable properties can be directly related to the resin chemical structure, which is available most commonly in the form of bisphenol A diglycdidyl ether (BADGE). BADGE is formed from the reaction of bisphenol A and epichlorohydrin, which when further reacted with an appropriate curing agent will eventually form a thermoset polymer, with aromatic groups from the bisphenol A component evenly distributed throughout the entire structure. They are one of the best materials available when comparing performance against relative cost, and this has in turn led to epoxies being the substance of choice for use in many high-performance polymeric applications. It is only their disadvantage of poor UV stability that prevents them from having a more dominant position.
Epoxy resins can be reacted with a range of different curing agents, and a particular resin and curing agent combination is often referred to as a system. Figure 2 shows the two different points of reactivity in a typical resin molecule, namely the end epoxy ring and the side hydroxyl group. Both of these functional groups can react at either ambient or elevated temperature, thus allowing the formulation of both one-pack and two-pack curing systems from the majority of epoxy resins available commercially. For a one-pack system the resin and curing agent can be combined together well in advance of its actual application, as the system will either not react at all or react very slowly at ambient temperature. After application of these one-pack systems, when heat is applied, the curing reactions occur much faster and the final polymer is formed. For a two-pack epoxy system, the resin and curing agent have to be kept apart until just prior to their application, where they are combined together, leading to an initiation of chemical reactions that form the final polymer. Therefore, the applicator has a finite time period after mixing the resin and curing agent to use and apply the material before the growing molecular weight of the system becomes too high that it cannot be used anymore. This time period is commonly referred to as the pot life, and two-pack epoxy systems are often further formulated by adding pigments, fillers, additives, etc. for use in coating and civil applications.