Continuing with the focus on resins, I will cover epoxy curing agents in this month’s column. When it comes to great adhesion, chemical resistance, and corrosion resistance, an epoxy is hard to beat. That is why it has been the mainstay of industrial primers for decades. An epoxy system is the thermoset reaction of an epoxide ring and a proton donor, most commonly from an amine or amide hardener. However, there are different epoxy curing agents, and each one provides different properties that can be suited for a wide variety of end-use applications. Epoxy resins can also react with other chemistries at high temperatures, such as carboxyl groups, but we will limit this article to common ambient cure systems. For this article, the epoxy part of the two-component system will be known as the resin, and the amine or amide curing agent will be known as the hardener.
Epoxy resins tend to have limited compatibility with some epoxy curing agents, so an induction time is needed. The induction time is the time between the mixing of the resin and hardener, and the application of the coating. This allows the chemical reaction between resin and curing agent to start. As the epoxy and curing agent reaction takes place, the compatibility of the system increases, and there will be fewer coating defects and better final film properties. If proper mixing does not occur, it is possible that the amine or amide component may rise to the surface of the applied coating, creating an uncured layer at the surface known as surface “blush,” which appears as a hazy finish.