Novel Waterborne Epoxy Resin Technology for Zero-VOC 2K Coatings
This article discusses performance properties, including handling properties, corrosion resistance, and flexibility, as well as Electrochemical Impedance Spectroscopy (EIS) evaluation of barrier properties.
Waterborne epoxy coatings have evolved into two fundamentally different technologies commonly identified as Type I and Type II systems. Type I systems are based on liquid epoxy resin. Typically, the amine curing agent serves as the emulsifier for the epoxy and the emulsion is formed when the A and B components of the paint are mixed together. The more recent Type II technology uses solid epoxy resin pre-dispersed in water and co-solvent. In Type I systems, the dispersed resin particles contain both epoxy and curing agent. In Type II systems, the dispersed resin particles contain only solid epoxy resin so that the curing agent must migrate from the aqueous phase into the dispersed epoxy particles for reaction to occur.
Type I systems can often be formulated to zero VOCs, primarily because liquid epoxy exhibits good handling, flow and coalescence without the aid of co-solvents. However, liquid epoxy systems cannot reach high viscosity through water evaporation alone, but must depend on chemical reaction to increase viscosity. As a result, liquid epoxy systems are very slow to dry, often taking more than six hours to reach tack free.
As mentioned, Type I systems use amine curing agents to emulsify the liquid epoxy forming dispersed resin particles, which contain both epoxy and amine. In addition, liquid epoxy (epoxide equivalent weight 190) is very rich in epoxide groups. The combination of concentrated epoxides and amine hydrogens in the same dispersed particle leads to very short pot lives.