The Nature of Azo Dye Light Fading in Polyester and the Physical State of the Dye

Abstract

A series of azo compounds based on benzeneazo- β-naphthol were applied to polyester film by dyeing from dispersions, and their fading rates were determined. From the relation between the electron-attracting or withdrawing properties of the substituents in the benzene nucleus and the relative fading rates, it is concluded that the fading reaction is oxidative. This conclusion agrees with that from earlier analyses of fading products of anthraquinone disperse dyes on polyester. It is therefore clear that polyester conforms with the general rule that fading on any nonprotein is oxidative, provided that oxygen has access to the dye. The shape of the fading rate curves in the present experiments indicates that the dyes are present in the polyester at least partly as aggregates. The linear curve of relative fading rate vs. the Hammett σ-value of the substituent in the benzene nucleus has a lower slope than that for the same dyes on polypropylene. It is suggested that the polar nature of the substrate reduces the influence of a substituent group on the reactivity of the active center (the azo group) in the dye molecule.