Relationship between photochemical properties and colourfastness due to light-related effects on monoazo reactive dyes derived from H-acid, γ-acid, and related naphthalene sulfonic acids

Abstract

Photofading of 15 reactive azo dyes derived from H- (seven), J-, L-, R-, 2R-, γ-acids (three), pyrazoline and a reactive copper-complex dye on cellophane under wet conditions was examined to estimate the essential properties: photo-oxidisability ( k 0: the reactivity with molecular singlet oxygen), photosensitising ability ( f) and photo-reductivity. The relationships between the essential properties of these dyes and the lightfastness (LF) as well as the perspiration-lightfastness (PLF) on cotton fabrics are discussed. A linear correlation between the logarithmic product, log ( f k 0), and the LF ratings of dyes on dry cotton fabric was found when plots of reactive dyes with low LF and those of the vat dyes examined in a previous paper were added because the ratings of 11 red dyes out of 15 dyes examined had a narrow range. It showed that LF of monoazo red reactive dyes on dry cotton fabric is very dependent on photo-oxidative fading. A linear correlation was also found between logarithms of f-values and the relative initial rates of photo-reductive fading, except for dyes with very high photosensitising ability. A conflicting relationship between the k 0-values and the photo-reductivity could be shown for dyes derived from H- and γ-acids, an unsolved restriction on synthesis of azo dyes with excellent LF as well as PLF. Azo dyes derived from γ- and 2R-acids possess smaller f but larger k 0 values than those derived from H-acids. The photofading of dyes on cotton fabrics in the presence of artificial perspiration was explained in terms of the essential properties. The substrates promoted photo-reduction and suppressed photo-oxidation.