In a previous paper, the improvement in wet fastness of acid dyes on nylon fiber by aftertreatment with tannic acid (backtanning) was investigated. From the results obtained by the diffusion of dyes in the nylon film treated with tannic acid prior to the experiment, it was deduced that the improvement might be attributed to suppression of dye diffusion by the electrostatic repulsion between dye and tannic acid and the filling up of voids in nylon by tannic acid.
In this paper, the effect of electrostatic interaction between dye and tannic acid on the dye fixation was investigated by measurement of desorption rate of dye from nylon 6 fiber backtanned after dyeing in practical convention and further by microscopic observation of distributions of dye and tannic acid in the fiber. Four disperse, 10 acid, 1 direct and 4 basic dyes were used in this experiment.
The informations obtained are as follows:
1) The wet fastness of these dyes on the fibers is improved by backtanning and further by a successive treatment with tartar emetic (full backtanning).
2) The anionic (acid and direct) dyes, which are adsorbed in skin layer of the fiber, are moved into core by backtanning and then tannic acid is adsorbed in the skin layer. Consequently, tannic acid is concentrated at surface layer of the skin, and the dyes, especially those having two or more sulfonic groups, are fixed effectively. This may support the favorable effect of electrostatic repulsion between the dye anion and tannic acid on the dye fixation.
3) In the case of nonionic (disperse) dye, considerable amount of tannic acid easily permeates into the dyed fiber and simultaneously the dye appreciably bleeds out during the backtanning because of the absence of electrostatic interaction between the dye and tannic acid. Therefore, the fixation effect for nonionic dye seems to be not so remarkable.
4) The cationic (basic) dye forms a lake with tannic acid by electrostatic attraction and it is deposited in the skin layer of the dyed fiber. It brings unfavorable desorption of dye and decreases fairly the fixation effect.
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