Studies in high light‐fastness of dyes. The fading of anionic dyes in anionic alumina films

Facbrs influencing the light‐fastness of anionic dyes on (sulphuric acid) anodic oxide films on aluminium have been studied, using three acid wool dyes of widely different light‐fastness rating on textile fibres. The general conclusions are that there are four principal factors influencing the light‐fastness: (i) the normal fastness of the dye on a textile fibre: the higher the fastness on the fibre, trhe higher is it likely to be on the film, because similar chemical and physical effects in both cases are responsible for fastness; (ii) the thickness of the film: fastness rises with thickness (above a minimum) probably because dye deep in a film is better protected; (iii) the amount and valency of other anions in the dyebath: anions, especially those of high valency, tend to reduce fastness, probably by a competition effoct whereby adsorption of micellcs of dye, which have the highest fastness, is prevented; and (iv) the severity of the sealing treatment: the more severe the sealing, the higher is the fastness. It is suggested that sealing blockr off a proportion (not all) of the adsorbed dye and prcvents access to it of atmospheric oxygen and water vapour, so that it cannot fade. Appropriate recommendations are made for achieving the highest light‐fastness in dyed anodic films for architectural use.