Exposure Studies of Exterior House Paints Containing Kaolin Clay Pigments

By Thad T. Broome

In the 12 months ending September 1996, 2,775 million liters (733 million gallons) of architectural coatings were manufactured in the U.S.,1 of which exterior latex house paints represented a substantial part. While other extender pigments, such as silica and calcium carbonate, are probably most widely used in these products, there are, to the author’s knowledge, a substantial number of commercial exterior latex house paints manufactured in the U.S. that utilize pigments derived from kaolin clay.

A literature search found support for such usage in several extensive studies reported about 30 years ago. These include works by Liberti,2 Vannoy,3,4 the Los Angeles Society for Coatings Technology,5 and Brooks, et al.6 Brooks specifically concentrated on varieties of clay pigments in vinyl acetate copolymer latex, acrylic latex, and linseed oil formulations exposed on 500 white pine panels with calcium carbonate and talc pigments, as well as several commercial paints as controls.

In single-extender formulations, clay pigments chalked more rapidly than control extenders, but outperformed controls in mildew resistance and general appearance; a delaminated kaolin having large diameter platelets showed best overall performance among all extenders. In mixed extender formulations, a combination of two-thirds delaminated kaolin and one-third coarse calcium carbonate had the best tint retention in the study while also showing good mildew resistance, no iron staining, and enhanced opacity.