Water-Base Coatings

The purpose of this work was to develop water-based analogs of existing solvent-based epoxy primer and polyurethane topcoat systems. The approach was to prepare separate aqueous emulsions of epoxy resin and curing agent that could be mixed just before application and isocyanate prepolymer adducts that could be emulsified without reaction with water. Solvent-free, stable emulsions of epoxy resins with average particle diameter of 0.2 micro m and below were prepared using the anionic sodium lauryl sulfate-cetyl alcohol or the cationic hexadecyltrimethyl-ammonium bromide-cetyl alcohol combinations as emulsifier. Also, cationic emulsions were prepared from the curing agent. The emulsification of these epoxy resins and curing agents was improved by the substitution of n-decane for the cetyl alcohol in the mixed emulsifier. In order to increase the curing rate of the epoxy films at room temperature, some chemical additives were incorporated either in the epoxy resin or the curing agent prepolymer prior to the emulsification process. The solvent-stripped cationic epoxy resin and curing agent emulsions were mixed in various proportions and cured at room temperature, or 55 C. A mechanism was established for the curing of emulsion-cast epoxy-curing agent films by determining the morphological properties and the dynamic mechanical properties of these films. The results demonstrated that the diffusion of the curing agent molecules into the epoxy resin particles is controlled by the degree of interfacial crosslinking which takes place between the coalesced epoxy resin and curing agent particles.