Studies of High Fastness to Light in Coloring Matters in Hydrophilic Substrates

Abstract

A variety of dyes of high lightfastness, vis., metal-complex and mordanted dyes on protein and cellulosic substrates, basic dye lakes, and reactive dyes, on cellulose, have been examined by fading-rate and CF (characteristic fastness) measurements. Ap parently in all cases the dye is highly aggregated.Normally with water-soluble dyes lightfastness improves with rise in concentration ; theoretically this should not occur with pigments applied in substance, but in practice it does do so. The cause is shown to be aggregation of particles by capillary attraction before the fluid matrix sets to a solid.It is shown that, on account of the relation between the association of a dye and its absorption spectrum, fading may tend to cause a reddening of shade of water-soluble dyes, and a blueing of water-insoluble ones.The heat of illumination can break down dye aggregates and thus cause an apparent rise in depth of color. This effect, which occasionally masks fading in the early stages. is sometimes favored by dry conditions. The accelerated fading produced by rise in atmospheric humidity is due in some cases partly to the absence of this compensating factor ; but it is also probably due to the mass action effect of an increase in concentration of the moisture that enters into the fading reaction.