Sag control of high solid polyurethane clearcoats by urea thixotrope/acrylic microgel systems

A sprayable polyurethane clearcoat composition which produces a durable, high gloss surface finish when applied to and cured over a suitable substrate, said composition comprising

• (1) a urethane binder resin which is the reaction product of an aliphatic polyisocyanate and a stoichiometric excess with respect to the polyisocyanate of polyether polyols,
• (2) an acid catalyzed melamine cross-linking agent, and
• (3) a sufficient amount of a flow control system consisting of a combination of urethane-urea polymer and a transparent filler of polymethyl methacrylate microgel,
• said flow control system reducing sagging of the urethane clearcoat when applied to vertically oriented substrates, said urethane-urea polymer being a reaction product of an isocyanate-terminated prepolymer prepared by the reaction of one or more polyols with an excess of a diisocyanate and an ethanolamine selected from the group consisting of monoethanolamine, diethanolamine, and triethanolamine, said polymer thus being capable of forming polar and hydrogen bonds with molecules of said binder resin for improving said sagging property but not entering into the cross-linking reactions with said binder resin molecules when said binder resin is cured by the crosslinking constituent.

A sprayable polyurethane clearcoat composition comprising at least about 50 weight percent nonvolatile constituents which composition produces a durable, surface finish when applied to and cured over a suitable substrate, said composition comprising

• (1) a urethane binder resin being a reaction product of an aliphatic polyisocyanate and a stoichiometric excess with respect to the polyisocyanate of polyether polyols,
• (2) an acid catalyzed melamine cross-linking agent, and
• (3) a flow control system consisting of a combination of urethane-urea polymer having number average molecular weight of between 500 and 5000 and a transparent filler of polymethyl methacrylate microgel having particle size between 1 and 50 microns in a ratio between 1:5 to 5:1,
• said flow control system when added in an amount of up to 20 wt% reducing sagging of the urethane clearcoat when applied to vertically oriented substrates without adversely affecting the viscosity and gloss properties, said urethane-urea polymer contained in the flow control system being a reaction product of an isocyanate-terminated prepolymer prepared by the reaction of one or more polyols with an excess of a diisocyanate and an ethanolamine selected from the group consisting of monoethanolamine, diethanolamine, and triethanolamine, said polymer thus being capable of forming polar and hydrogen bonds with molecules of said binder resin for improving said sagging property but not entering into the cross-linking reactions with said binder resin molecules when said binder resin is cured by the cross-linking constituent.