Binder System for Water-based epoxy
Water-based epoxies used in conventional protective coating formulations are based on two principal technologies, i.e., liquid epoxy self-emulsifying systems and solid epoxy dispersion systems.With epoxy equivalent weight ranging from 175 to 240,liquid epoxy self-emulsifying systems are very rich in epoxide groups. The combination of concentrated epoxides and amine hydrogens in the same emulsion particle provides coatings with high crosslink density and good chemical and abrasion resistance, but very short usable pot life, low flexibility and low impact resistance. These films are often too brittle for use on metal substrates. Since the film formation of liquid epoxy emulsion is dependent upon chemical reaction to increase viscosity, the dry time is very slow, often taking more than six hours to reach a tack-free state.These liquid epoxy coating does exhibit good handling, flow and coalescence without the aid of co-solvents, and can often be formulated to very low or zero VOC.
A new and innovative solid epoxy dispersion topcoat, cured with a modified water-soluble amine binder system, incorporates the best characteristics of liquid epoxy emulsion and solid epoxy dispersion technologies and can be formulated to zero VOC. It is based on a bisphenol-A “1”-type solid epoxy (EEW 500) using an internal surfactant built into the epoxy resin. When reacted, the internal surfactant becomes epoxy functional and is non-leachable, resulting in cured films with very good water resistance. A unique and patented high-speed dispersion process used for production of the epoxy dispersion yields a very small particle size (0.4 microns) without adding solvents or reactive diluents, providing good film formation, good storage stability properties and low to zero VOC formulation capability. The new zero-VOC water-based epoxy topcoat formulated with this binder system exhibits long pot-life (5.5 h @ 77 °F/50% R. H.), short dry time (2 h tack free @ 77 °F/50% R. H.), very good corrosion resistance (4,000 h salt fog) and good chemical resistance.