Latex Blends: An Approach to Zero VOC Coatings

By Mitchell A. Winnik, Jianrong Feng

An attractive strategy for zero VOC coatings involves blends of hard and soft latex particles. A dispersion of a latex with a low minimum film-forming temperature will form a transparent film upon evaporation, even when blended with a substantial amount of high Tg latex. The low Tg latex deforms to fill space as the dispersion dries, and the high Tg component imparts good block resistance and mechanical properties to the final film.

Transparency requires that the hard latex be sufficiently small and well dispersed in the final film. In addition, the two components must have similar refractive indices. Two of our most surprising observations are that dispersions of soft latex particles dry more slowly than those of high Tg latex, and that mixtures of the two particles dry even more slowly.

Consideration of these results leads to a new model for the drying mechanism of latex dispersions that takes into account the propagating drying front. Dynamic mechanical analysis indicates that the hard particles in blends improve significantly the mechanical properties for the films.