Fastness properties

Fastness to light is basically a feature of the actual dye molecule itself; it is, in fact, a measure of the ability of the molecule to absorb radiation without being destroyed, the absorbed radiation being dissipated in other ways. In a dye with poor light-fastness the molecule will be broken down by the absorbed radiation. No dye is completely fast to light, but it should be emphasised that this is not of real importance. All that matters is the colorant should not fade appreciably during the life of the article which it colours, and today colorants exist which fulfill this condition.

Generally speaking, the more efficient a detergent is in removing dirt or stains, the better it is in breaking down the physical bonds which bind the dye to the fibre. In the case of a serve wash at the boil, given (for example) to men’s shirts, this poses a considerable problem, particularly if the garment has coloured stripes, as the dye loosened by the detergent is liable to stain the adjacent white.

Fastness to washing is dependent not so much upon the structure of the dye molecule as upon the tenacity with which it holds on to the substrate. Unfortunately this tenacity is under constant attack during washing by the action of detergents especially as the detergent industry is constantly introducing new, more efficient products.