Defect free coatings from two-pack isocyanate curable acrylic dispersions

The combination of isocyanate crosslinking groups and water as continuous p hase in coatings provides a real challenge to the coatings industry. Especi ally the ability of the crosslinker to react with water and liberate carbon dioxide is a problem. CO2 will be entrapped in the film, resulting in poor aesthetic properties and porous films with reduced resistance properties. In this paper it will be shown that the choice of acid monomer and the acid value have a significant effect on the physical drying rate of the films' on the level of water remaining in the film after cure, and on the rate of decrease of the NCO concentration and CO2 blister formation. Increasing the acid value will result in faster NCO loss and more CO2. The same effects a re reached when acrylic acid is chosen over methacrylic acid, which in turn yields a faster NCO concentration decrease and more CO2 blisters than beta CEA. A faster decrease of the NCO group concentration does not necessarily lead to a faster tack free time. By careful selection of the type of acid defect free films without CO:, blisters can be formed with a thickness of a t least 300 mu m (dry). CO2 blister formation is caused by the presence of water soluble poly-acid which remains in the film enabling the reaction of the isocyanate group with water. (C) 1999 Elsevier Science S.A. All rights reserved.