Comparison of Color Changes in Paints Based on Ultramarine and Zinc White Resulting from Storage and Thermally Induced Aging

The paper presents the results of measurements of the L*, a*, and b* color coordinates in the CIEL*a*b*76 (D65/10) space of paint layers of zinc white and ultramarine with several primers and binders. The coordinates are measured for naturally aged reference samples and test samples that underwent two thermal shocks. The color differences of samples obtained using three binders and three primers are quantitatively characterized. It is shown that thermally induced changes in the L* coordinate, as distinct from the a* and b* coordinates, are significantly greater than the changes predicted by the artificial aging protocol. However, during subsequent storage, L* partially relaxes to its original values, and the resulting changes in this coordinate can be considered insignificant. The relaxation effect is also characteristic of the a* and b* coordinates for most samples. Specific results are obtained for paint layers with resin-containing binder on glue–chalk primers. When zinc white is used, such samples have greater L* coordinate and smaller b* coordinate, i.e. appear to be lighter and less yellow in comparison with the remaining samples. When ultramarine is used as a pigment, the resin-containing binder and glue–chalk primers provide the maximum depth of blue color (minimum b* value), which remains almost unchanged with aging. However, the crimson hue characteristic of such samples (greater value of the a* coordinate) is intensified due to heating. The results show that two thermal effects on paint layers separated in time can lead to opposite changes in color coordinates.