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Drying and Collapse of Hollow Latex

Christine M. Cardinal, Lorraine F. Francis, and L. E. Scriven. Chemical Engineering and Materials Science, University of Minnesota, 151 Amundson Hall, 421 Washington Ave, Minneapolis, MN 55455

A conventional paint contains water, inorganic pigments, soft polymer binder to hold inorganic particles together, quality enhancing additives, and titanium dioxide, an expensive white pigment that provides opacity. Hollow latex is an inexpensive alternative white pigment to titanium dioxide for paints and paper coatings. When dispersed, each hollow particle is filled with water. As the paint dries water vacates the latex, leaving an air-filled void within each particle sized to scatter light (~0.5 μm). Examinations of dried coatings reveal that hollow particles can collapse as they dry, decreasing their light scattering efficiency. To determine the drying stage in which particles collapse, cryogenic scanning electron microscopy (cryo-SEM) was used. Images suggest latex voids empty after air invades into the coating interstitial space and collapse occurs in the last drying stage. The effects of temperature (10 C 60 C), humidity (20% - 80%), and binder concentration (0 wt% - 30 wt%) on particle collapse were also studied through SEM of coating dried surfaces. High drying temperature, high humidity and low binder concentrations promoted collapse. If the hollow latex particle shell wall is porous, temperature and humidity had little effect, whereas binder increased collapse. From these results, a theoretical model is proposed in which the diffusion of water from the particle pulls a vacuum on the latex. If favorable, a bubble nucleates from the dissolved air within the particle that can relieve the vacuum. Otherwise, collapse occurs.