The Influence of the Physical State of Dyes upon their Light Fastness *

From a study of the quantitative relationship between light fastness and dye concentration conclusions are drawn regarding the physical state of dyes in air‐dry substrates, on the basis of a simple theoretical model representing the nature of particle growth with increase in concentration. Only one type of dye‐substrate system, viz. a water‐insoluble merocyanine dye in solid solution in collodion, has been found to obey the normal photochemical laws for ideal systems. All other systems, whether involving water‐insoluble or water‐soluble dyes, behave non‐ideally: i.e. the dye appears to exist in the form of aggregates or crystals (usually submicroscopic) and to obey a fading law in which the rate‐affecting factor is the extent of the dye‐air interface. Treatments which cause microscopically visible growth of such particles, e.g. soaping of azoic‐dyed cellulose, raise the light fastness as anticipated. Also in accordance with prediction, the light fastness even of water‐soluble dyes is raised by increasing the porosity of the substrate; this is believed to result from the formation of larger aggregates of dye in the dry substrate. Thus the light fastness of direct cotton dyes is higher on highly porous (“gel”) Cellophane film than on normal film, and on normal viscose rayon fibre than on highly oriented viscose rayon fibre; and acid wool dyes have higher fastness on the sulphuric‐acid anodic film on aluminium than on the less porous chromic‐acid film. A number of other examples of changes in light fastness, believed to be due to similar physical factors, are discussed. The following two observations are reportedndash( a ) undried “gel” Cellophane film slowly reverts to the normal form on keeping, even when wet; and ( b ) there is a linear relationship between the logarithm of percentage reflectance and the logarithm of total dye content for direct dyes on various viscose rayon fabrics.