Abstract:

Objective It is urgent to adopt the mild decolorization technology for waste cotton fabrics to achieve clean pulping to resolve the problems that the high consumptions of energy, as well as severe degradation of cellulose during preparation of cotton pulp from waste cotton fabrics.

Method Due to the violent reaction when decolorizing cotton fabric with oxidant, the breaking strength of the decolorized cotton fabric is lost more, and the molecular chain is broken seriously. While the reaction when decolorizing cotton fabric with reductant is relatively mild. A method for decolorizing was conducted to black mercerized cotton fabric, named as the sodium hydroxide-sodium hydrosulfite system through orthogonal experiment and single factor experiment. The role of decolorization parameters, influence of decolorization treatment on the structure and properties of cotton fabrics and mild decolorization mechanism were investigated.

Results Sodium hydroxide can improve the stability of sodium hydrosulfite (Fig. 1), and can hydrolyze and destroy the covalent bond between reactive dyes and black mercerized cotton fabric, so that the dyes on cotton fabric can hydrolyze and diffuse into the decolorization solution(Fig. 2). At the same time, the dye chromophore group on the cotton fabric and in the solution will be destroyed under the reduction of sodium hydrosulfite (Fig. 3). Because sodium hydrosulfite itself will undergo invalid decomposition, sodium hydrosulfite is added in a two-step process decomposes more active substances for decolorization and shorten the time of decolorization reaction (Fig. 5 and Fig. 6), and the lightness value of the decolorized cotton fabric is increased from 16.52 to above 70.00 by adopting the best two-step decolorization process. Almost no dye remains on the cotton fibers under the synergistic effect of sodium hydroxide and insurance powder (Fig. 7). The decolorization system has been proved to be able to reduce the strength loss of the cotton fabric after bleaching, and the break strength retention rate of the cotton fabric after decolorization in both warp and weft directions is more than 98% by adopting the best two-step decolorization process(Fig. 8). The chemical structure and crystal structure of cotton fabric are hardly affected during the decolorization process. Among which, sodium hydrosulfite will destroy the chromophore group of the dye without destroying the covalent bond between the dye and the cotton fiber. sodium hydroxide will hydrolyze the covalent bond between the dye and the fiber, and the hydrosulfite will promote the hydrolysis of the covalent bond by sodium hydroxide (Fig. 9). Only a small amount of the crystal area of cotton cellulose has been damaged, and most of the crystal area and crystal structure are not affected during the decolorization process of sodium hydroxide-sodium hydrosulfite system (Fig. 10), and the position of the diffraction peak of cotton cellulose has not changed before and after decolorization. Finally, it has been found that the molecular weight and its distribution of cotton fiber were almost unchanged before and after decolorization by gel liquid chromatography (Tab. 3).

Conclusion In summary, sodium hydroxide can promote the hydrolysis of covalent bond between reactive dyes and cotton fabric, and transfer dyes on cotton fabric to decolorization solution; and sodium hydrosulfite can destroy the chromophore group of the dye to achieve the purpose of decolorization. The coexistence of sodium hydroxide and sodium hydrosulfite in the decolorization solution can promote each other, thereby realizing the decolorization of cotton fabrics. Thus, the sodium hydroxide-sodium hydrosulfite system, which be adopted to decolorize cotton fabric under normal pressure and high temperature, can achieve mild decolorization of waste cotton fabric without affecting subsequent pulping.

Key words: waste cotton fabric, decolorization mechanism, sodium hydroxide, sodium hydrosulfite, pulp, mild decolorization technology