The technical performance of five selected commercial photochromic dyes applied by screen-printing on textiles was evaluated using a colour measurement methodology that has previously been established and validated. The results of wash fastness assessments were distinctly unusual. With the selected spirooxazine dyes, the degree of photocoloration increased with initial washing and decreased with subsequent washings, while the naphthopyrans behaved more normally, showing a consistent marginal decrease in photocoloration with repeated washing. An explanation is proposed consistent with scanning electron microscopy examination of the binder film degradation and with the structural differences between the dye classes. The dyes in this application showed limited photostability. The incorporation of ultraviolet absorbers was found to increase photostability only to an extent specific to particular ultraviolet absorber ⁄ dye combinations. However, the presence of the ultraviolet absorber led to a consistent reduction in the degree of photocoloration of the dyes. In contrast, the incorporation of hindered amine light stabilisers significantly enhanced the photostability of the dyes, providing an increase in resistance to photodegradation of up to fivefold.

The performance of five commercial photochromic dyes applied by screen printing on textiles was evaluated using a colour measurement methodology which has previously been established and validated. Printing as disperse dyes on polyester gives the strongest photochromic effect when the dye is located near the surface. However, the dyes are applied more effectively using an adaptation of a pigment printing method. Several factors influencing the photochromic properties of the printed textiles were evaluated. The ultraviolet reflecting properties of cotton give a higher level of photocoloration than on polyester. In the dyes studied, the spirooxazines show slightly faster colour development and fade much more rapidly than the naphthopyrans, and the latter show a residual colour after fading. The effect of the ultraviolet irradiation wavelength profile on the photochromic response may be correlated with the ultraviolet absorption spectra of the dyes.

Commercial photochromic dyes change rapidly and reversibly from colourless to coloured when activated by ultraviolet irradiation. This paper contributes towards the growing interest in the development of chromic textiles for creative design, functional or smart purposes. The optimisation of a methodology to evaluate the colouristic properties of photochromic textiles using traditional ultraviolet illumination source and colour measurement equipment is described. The experimental factors requiring careful control, including temperature and the time interval between ultraviolet irradiation and measurement, are identified. On the basis of a statistical evaluation of validity, a consistent, reproducible semi‐quantitative methodology is described, which is appropriate for evaluation of the performance of commercial photochromic dyes applied to textiles, a detailed investigation of which will form the basis of subsequent papers in this series.

Two commercial photochromic dyes, a spirooxazine and a napthopyran, were successfully incorporated into polypropylene fibres by melt extrusion to produce fibres that showed intense photochromism. The technical performance of the photochromic dyes was investigated using a methodological approach that has been established and validated for the dyes applied by screen printing onto textiles in previous publications in this series. These parallel investigations allowed a comparison of the properties of the dyes in polypropylene and screen printed on cotton. The photocolorability of both dyes was found to be significantly higher in polypropylene compared with screen prints on cotton when applied at the same dye concentration. Of particular interest was the observation of positive solvatochromism, providing evidence that the photochromism is due to the colorants in solution in both media. The colour development rates during UV exposure of the dyes applied by screen printing cotton and by extrusion into polypropylene were similar. However, differences were observed in the thermal fading rates after removal of the UV source, in that the spirooxazine dye reverted more slowly in polypropylene, while the reverse was true in the case of the naphthopyran. Both dyes showed better photostability in polypropylene than in screen-printed textiles. A UV absorber and two hindered amine light stabilisers were found significantly to enhance the photostability of the dyes in polypropylene, although the presence of the UV absorber reduced the degree of photocoloration.