This disclosure relates to methods for making scrub and stain-resistant coating compositions and articles comprising the same.
Opacifying agent particles are useful for conferring opacity to plastic articles and films, and particularly to coating compositions. In coating compositions, the opacifying agent particles can be one of the most expensive components. It is an ongoing objective of the coatings industry to achieve the desired degree of opacity while minimizing the amount of opacifying agent particles in the coating composition. The optimal way to make efficient use of opacifying agent particles is to disperse them as well as possible. If not adequately dispersed, opacifying agent particles agglomerate into larger particles, which tend to settle out of the coating composition as “grit”. When grit is present, the opacifying agent particles make less than their full potential contribution to the coating opacity and depth of color. In addition, even low levels of grit can significantly detract from the appearance of the coated article. In cases of severe grit, the dried coating film will display a very rough surface texture with an almost sand paper-like appearance.
The hiding power of the coating composition is a function of the spacing of the opacifying agent particles in the dried coating. Maximum light scattering occurs when the opacifying agent particles are spaced far apart from each other, on the order of a few particle diameters, so that there is minimal interference between the light scattered from neighboring particles. In a coating, the distribution of the opacifying agent particles best approaches that of a random distribution. As a result, there exist a substantial number of opacifying agent particles in close proximity to each other, and possibly in direct physical contact with each other. This phenomenon is known as “crowding”, and it diminishes light scattering. In order to reduce grit formation, high shear force manufacturing processes are employed. In addition, significant quantities of surface-active dispersing, wetting or “coupling” agents are employed to reduce grit formation and crowding.
Another approach for reducing the crowding of opacifying agent particles is the use of polymer latex binders that adsorb onto opacifying agent particles so that the resulting composite particles are well spaced out. Polymer latex binders with phosphate functional groups are particularly well suited for latex adsorption onto opacifying agent particles. These phosphate-functional polymer latex binders have such a high affinity for surface treated titanium dioxide however, that if the kinetics of adsorption are not controlled, the composite particles will aggregate causing unacceptable viscosity increases, and the formation of undesirable grit. It is therefore desirable to control the kinetics of adsorption of polymer latex binders onto the opacifying agent particles. It is also desirable to have a method for making coating compositions in which the viscosity is stable, and where grit formation and crowding is minimized.