Crosslinked polyurethane emulsion, process and prepolymer for its production and process for forming coatings from said emulsion

摘要

Stable crosslinked polyurethane aqueous emulsions capable of forming water-resistant plastic coatings without heating are prepared by(1) forming in an inert solvent a branched prepolymer with at least 0.4% (0.095 meq/g) of free isocyanate from(a) an alcohol having two or three hydroxyl groups,(b) a hydrophobic polymeric diol or triol,(c) a polyisocyanate, the amounts of (a), (b) and (c) being such that the equivalent ratio of total isocyanate to total hydroxyl is less than 1.4; and(d) optionally a urethane-forming catalyst;(2) admixing the reaction products from (1) supra with a fatty, tertiary amine additive with molecular weight of less than 1,000 in an amount up to 1% by weight of the prepolymer in (1) supra;(3) adding the mixture from (2) with vigorous stirring to water acidified to pH 1.5 - 5.0 with an inorganic acid or a non-carboxylic organic acid that retains its acidic character up to 200°C, to form a crosslinked polyurethane emulsion; and(4) removing the inert solvent and a portion of the water so that the emulsion contains 60 to 75% solids but less than 1% of residual acid.