Abstract
A few dyes and fluorescent brightening agents of disperse type, in hydrophobic substrates, show anomalous fading characteristics, i.e., reduced lightfastness with in creased dye concentration, a reversal of the normal trend.
Measurements, by the density gradient method, of the change of apparent specific gravity of dyed film material with change in dye concentration reveal a marked difference between the nature of this change as between anomalous and normal dyes. The effect is, therefore, due to physical changes in the substrate, probably in its porosity, produced in the dyeing process. The facts are consistent with the hypothesis that the anomalous dyes at moderate concentrations do not penetrate as deeply into the substrate structure as normal dyes do. They may tend to block the entrance to internal pores whereby the apparent specific gravity is reduced and, by remaining outside these pores, they are more readily accessible to air and moisture.
The wider implication of the results is that changes in lightfastness, even as much as five-fold, can be produced solely by physical changes in the substrate structure.
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