| 摘要 |
A low-solvent or solvent-free polyurethane dispersion having pyrazole-blocked isocyanate groups is provided to have storage stability and form coating films having excellent optical characteristics, superior chemical resistance, and pendulum hardness more than 80 seconds. A method for preparing blocked polyurethane prepolymer dispersions includes the steps of: (I) preparing a blocked polyurethane prepolymer by reacting (a) 100% by equivalent of at least one polyisocyanate with (b) 50-90% by equivalent, based on the isocyanate-reactive groups, of a thermally removable blocking agent, (c) 5-45% by equivalent, based on the isocyanate-reactive groups, of a monohydroxycarboxylic acid as a hydrophilicizing agent, (d) 0-25% by equivalent, based on the isocyanate-reactive groups, of a polyhydroxycarboxylic acid as a hydrophilicizing agent, and (e) 0-15% by equivalent, based on the isocyanate-reactive groups, of a chain extender component which is bifunctional or polyfunctional and has a molecular weight of 32-350 g/mol; (II) at least partially, deprotonating the carboxylic acid groups with a base before, during, or after the step (III); (III) dispersing the polyurethane prepolymer solution of the step (I) in water; and (IV) distilling the dispersion to remove an organic solvent during or after the step (III).
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