Recent developments with two-component waterborne polyurethane coatings for plastic and wood applications have clearly demonstrated that these systems are excellent candidates for achieving significant reductions in volatile organic compounds (VOC) while maintaining satisfactory coating performance. Two-component waterborne polyurethane coatings can be based on either hydrophilic or hydrophobic polyisocyanates. However, the hydrophilic group that is attached to a polyisocyanate to make it water compatible also reduces the functionality and imparts an undesirable water sensitivity to the coating. Coatings based on a hydrophobic polyisocyanates do not suffer from these problems. Recent studies have demonstrated that systems based on hydrophobic polyisocyanates can produce coatings with excellent properties, that are comparable to those of a solvent based two-component polyurethane coatings. Unfortunately, hydrophobic polyisocyanates generally have poor compatibility with water dispersed resins and may require high shear mixing equipment or large amounts of a co-solvent in order to'prepare an acceptable waterborne system using these polyisocyanates. We have evaluated a number of low viscosity hydrophobic polyisocyanates that have excellent compatibility with waterborne resins. These systems are based on an extension of the work carried out at King. These polyisocyanates require neither high shear mixing equipment nor large amounts of co-solvent for dispersion. They also yield systems with a long pot-life and excellent coating performance such as gloss and solvent resistance. This paper will present the results of our work on these new two-component waterborne polyurethane systems based on low viscosity hydrophobic polyisocyanates.
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