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.

Experimental evidence and published results show that when dyes are faded they are usually oxidised, probably by atmospheric oxygen. In certain systems reduction may occur, e.g. when the substrate is more readily oxidised than the dye or under anaerobic conditions in the presence of a hydrogen donor.

Several anthaquinone and azo disperse dyes adsorbed on films of polyester and nylon 6. 6 and dissolved in a mixture of acetone and water have been faded by various lines in the visible spectrum, and the quantum yields (φ) determined by means of the Parker‐Hatchard potassium ferrioxalate actinometer. φ for adsorbed dye lies between ca 10‐4 and 10‐6, decreasing with increase in exposure time, probably owing to the presence of associated dye, and with increase in wavelength of illumination. Values for dye in solution are 20–60 times as high as those for adsorbed dye. The low values for adsorbed dye are the result of a combination of several factors, including association of dye, retardation of gaseous diffusion by the substrate, and the shield or cage effect of the surrounding polymer substrate.

Measurements have been made of the ratio of extinction of the short-wave (y) and the long-wave ( x ) band in the visible region of spectra of four non-ionic (disperse) dyes sorbed in films of cellulose acetates, nylon, and polyester. The changes in the ratio with dye concentration and also with fading on exposure to light at given concentrations have been measured. The results are consistent with the presence of dye particles in a range of small sizes ; association of dye appears to occur during the dyeing process. The smaller sizes, probably molecularly disperse dye, predominate, especially in polyester, and these fade the most rapidly. The nature of the change in ratio with time of exposure suggests that dye molecules become more resistant to photochemical breakdown when associated together.

A series of monoazo compounds based on phenyl azo-β-naphthol and phenyl azo-8-hydroxyquinoline were applied to polyvinyl chloride and poly(ethylene terephthalate), respectively. From the relation between the electron attracting or withdrawing properties of the substituents in the benzene nucleus and relative fading rates, the conclusion was that the fading reaction in both the dye-fiber systems is probably oxidative. This conclusion agrees with that from an earlier analysis of fading products of anthraquinone disperse dyes on polyester. It is therefore clear that both polyvinyl chloride and poly(ethylene terephthalate) conform with the general observation that fading on any nonprotein is oxidative, provided the oxygen has access to the dye.Lightfastness of phenyl azo-β-naphthol dyes on polyvinyl chloride and that of phenyl azo-8-hydroxyquinoline dyes on poly(ethylene terephthalate) was very low. Also, in both the systems, the dyes in the substrate were in the form of a collection of particles of many different sizes.The fading of dyed materials by light has long been a subject of investigation, yet surprisingly little is known of the fundamental photochemical reactions involved [ 10,12,13], mainly because of the complex nature of dye-fiber systems. The reactions are not necessarily the same as those of dyes in solution [9], which are probably simpler and have been more widely studied [9,10,12,19]. Calvert and Pitts [4] have stated that &dquo;In general ... solid-phase photochemistry has not received the amount of quantitative effort afforded the liquid and gaseous systems,&dquo; and they have suggested that one reason for this neglect lies in the difficulty of dealing with the very high light absorption of many solids.This difficulty can be overcome if dyes are adsorbed in transparent films of solid polymers and changes caused by irradiation readily measured spectrophotometrically. Even then, however, many complicating factors are present, e.g., the physical state of the adsorbed dye, that is, the size and location of its associated particles, structure of the dye, extent of penetration of the dye, diffusion of air, moisture, gases, etc. Because of these and many other factors, the lightfastness of a dye differs when applied to different textile substrates.A dye may show very good lightfastness on one fiber and poor on another one. For example, the same anthraquinone dyes show moderate to good lightfastness on nylon and cellulose acetate but poor on polyester [ 13]. A number of studies have attempted to account for such variations in lightfastness. One line of work has ascertained the nature of chemical reactions taking place during fading of dyes, mainly to show oxidation and reduction, and another has reported on the physical state of dyes in fibers. Kienle et al. [ 16] were probably the first to study quantitatively the fading rates of a series of benzene azo-R-acid dyes on wool and gelatin. They found that electron attracting groups (e.g., -N02) accelerate fading and electron repelling ones (e.g., ...