Chapter 2
A Perspective on Resins for Aqueous Coatings John L . Gardon
Downloaded by NANYANG TECH UNIV LIB on November 3, 2014 | http://pubs.acs.org Publication Date: April 1, 1997 | doi: 10.1021/bk-1997-0663.ch002
Research and Development, Akzo Nobel Coatings, Inc., Troy, MI 48007
The binders in traditional aqueous coatings are latexes made by free radical polymerization in water. The modern seeded semibatch synthesis allows precise particle size control and versatile modification of the morphology of latex particles. Core-shell structures provide unique film formation characteristics, gloss, rheology and crosslink acceptance. These advances in the classical latex technology have a sophisticated scientific foundation. In contrast, various other rapidly evolving classes of aqueous resins are based on largely empirical developments. Polyurethane latexes are the products of the reaction between isocyanate terminated prepolymers and aqueous diamines and, just as classical latexes, have very high molecular weights. A great variety of other aqueous dispersion polymers are synthesized in nonaqueous media, have low molecular weights and are dispersed into water by mechanical means. This class of dispersion polymers encompass alkyds, polyesters, acrylics and epoxies and forms films only when crosslinked. Novel crosslinking chemistry includes two component aqueous urethanes and epoxies, and systems for crosslinking carboxyl, carbonyl and acryloyl functional polymers.
The need for solvent reduction has led to an exciting technology competition between aqueous, high solids, powder, radiation cure and coil coatings (1-4). For example, modem high solids automotive topcoats are being partially replaced by aqueous and even by powder coatings. Discussion of the competition between the five low VOC technologies is beyond the scope of this paper. It should suffice to describe below issues associated with the rapidly advancing technologies of aqueous binder resins. This subject matter excludes additives, thickeners, and reactive diluents. Resins which dissolve in water are ignored because all modem aqueous binders belong to one of three major classes of polymeric colloids: classical latexes, urethane latexes and dispersion polymers.
© 1997 American Chemical Society
In Technology for Waterborne Coatings; Glass, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1997.
Downloaded by NANYANG TECH UNIV LIB on November 3, 2014 | http://pubs.acs.org Publication Date: April 1, 1997 | doi: 10.1021/bk-1997-0663.ch002
28
TECHNOLOGY FOR WATERBORNE COATINGS
Classical latexes are made by aqueous free radical polymerization. Modem synthesis methods now allow precise particle size control and well defined modification of the morphology of the particles. Urethane latexes are similar to classical latexes in that they have high molecular weights (>100,000) achieved by reactions in aqueous media and their particle sizes are defined primarily by the chemistry of the aqueous reactions. However, they are products of polycondensation between isocyanate terminated prepolymers and aqueous diamines and are in this respect fundamentally different from classical latexes. Dispersion polymers differ greatly from latexes. They have low (
