The Anomalous Fading of Some Fluorescent Dyes and Brighteners on Hydrophilic Fibres

The fastness to light of a number of fluorescent yellow dyes and fluorescent brightening agents (FBA) on several fibres at different concentrations has been assessed both visually and by colour-difference measurements. One acid dye on wool, six FBA on wool and two FBA on cotton showed anomalous fading (i.e. the light fastness decreased with increasing concentration). Results of extraction experiments showed that this could be accounted for by the formation of coloured decomposition products from the dye or FBA and, with wool, from the fibre also. With FBA on wool and cotton there appears to be an optimum concentration above which both the fastness to light and the whiteness fall.