Colorant-induced exterior discoloration of latex paints

Exterior discoloration of latex paint films has been attributed to known causes such as tannin and stain bleeding, mildew and sap staining, color fading and chalking due to certain pigments, and yellowing due to mildewcides and alkyd modification. This paper refers to exterior discoloration of latex paint films containing specific colorant(s). A simple and quick method was adopted to screen paint formulations that may discolor upon exposure to sunlight and water. The exterior discoloration of latex paint films tinted with raw umber colorant is primarily dependent on the manganese oxide(s) present as impurity in the colorant. The role of titanium dioxide in discoloration of raw umber containing latex paint films is also demonstrated. Using this method, the grades of titanium dioxide could be differentiated that facilitate exterior discoloration. A clear correlation was established between the types of surface treatment on titanium dioxide pigment (alumina, zirconia, or silica) versus colorant-dependent discoloration. The investigation demonstrates that the colorant-dependent discoloration is independent of binders, pigment volume concentrations, and mildewcide used in a formulation.