EFFECT OF TANNIC ACID ON THE DIFFUSION OF MONOAZO ACID DYES IN NYLON 6 FILM

Abstract

In order to investigate the mechanism of the improvement in wet fastness of acid dyes on nylon by after-treatment with tannic acid (Chinese gallotannin), the diffusion of monoazo acid dyes, Orange II (C. I. Acid Orange 7) and Diacid Supra Red 3B (C. I. Acid Red 35), in the nylon 6 films containing various amounts of tannic acid up to 4.46%, was measured by the film roll method.Diffusion profiles obtained were divided into ionic (type I) and nonionic (type N) parts, and the diffusion coefficients of both parts at zero concentration of dyes, D0I and D0N, were estimated by extrapolation of respective profiles, and the fraction of ionic part, I/I+N, was calculated as a measure of the tendency of ionic diffusion of dye in nylon.With increase of tannic acid content in nylon, the uptake of both dyes at its surface decreased more rapidly than the decrease of amino endgroup content of nylon, and the diffusion coefficients of both dyes decreased markedly.The I/I+N value for Orange II was increased apparently from nearly 0 in untreated nylon to about 70% in nylon containing only 0.44% tannic acid, but was almost unchanged for further adsorption, whereas that for Diacid Supra Red 3B which may be taken as more ionic than Orange II was 100% throughout in all cases.From these results, it seems most reasonable to conclude that the main reason for improvement in wet fastness properties of monoazo acid dyes is lowering of the rate of diffusion of dye by two effects, i.e., the electrostatic repulsion between dye anion and the negative charge of dissociated hydroxyl group of tannic acid and filling up of the pores with tannic acid in nylon.