THE KINETICS OF FADING: OPAQUE PAINT FILMS PIGMENTED WITH ALIZARIN LAKE AND TITANIUM DIOXIDE
Ruth Johnston-Feller, Robert L. Feller, Catherine W. Bailie, & Mary Curran
PRECISE SPECTROPHOTOMETRIC reflectance measurements, made before and during the exposure of paint films to radiant energy, coupled with computer color-matching calculations based on the Kubelka-Munk equations, can be used to compute the percentages of pigment remaining after exposures for various lengths of time. With such quantitative data regarding the concentration of colorants at any particular time in the course of fading, the principles of chemical kinetic analysis may be applied. Using the artists' pigment, alizarin lake, as a model chromatic pigment in mixture with rutile titanium dioxide white, we have shown that the fading of alizarin lake, in an opaque, high-gloss paint film made with poly(vinyl acetate) as the vehicle, approximates first-order behavior: the amount of colorant faded in any given interval of time is a constant percentage of the amount of alizarin lake pigment present. Within experimental error, this relationship is independent of the relative alizarin lake-titanium white concentration and the pigment volume concentration. Exposure was carried out in a xenon-arc Fade-ometer¯, filtered to approximate solar radiation through window glass in the near ultraviolet and visible spectral regions.
Spectrophotometric curve analysis also revealed that the red alizarin lake pigment tends to fade to a yellow intermediate before fading to a colorless form. By means of Munsell Notation and CIE color-difference calculations, curves were developed to show how the orderly change in concentration relates in a non-linear fashion to the perceived color of the paint. The inadequacy of visual evaluations for quantifying the changes in pigment concentrations is thus illustrated.
The techniques described are equally applicable to the study of the fading of other pigments, alone or in mixture.