Some Observations on the Kinetics of Dye Fading

Abstract

All lightfading rates of dyed or pigmented materials can be divided into five main classes and some subgroups, according to the shape of the arithmetic plot of rate of loss of dye. The physical condition and environment of the coloring matter appear to be principal factors determinipg the shape; a change in concentration does not usually change the class of fade. Class I fades are approximately first-order curves and are indicative of dye dispersed as single molecules or very small associated units. Class II fades are initially class I but later become linear (zero order), probably because a high proportion of coloring matter either is firmly embedded in the substrate molecules or is present as relatively large associated units. Class III fades resemble the later portion of Class II for similar reasons. Class IV fades have a point of inflection, with a more rapid fade following an initial slow change or even an apparent negative fade, i.e., an increase in depth of color; class V fades accelerate continuously from the start. Both classes IV and V are believed to be the result of combined effects of light and heat on aggregated particles of color. A theoretical treatment of these conditions is given.