| Vat dye is mainly used to dye cellulose fibers and considered to be an important dye for cotton fabrics dyeing. As known to all, it is necessary to turn vat dyes into leuco by reducing agent and then they can go to cotton fabric with good affinity. The traditional methods always use sodium thiosulfate as a reducing agent, which will produce a large number of sulfur-containing pollutants and cannot meet the requirements of green chemistry production. With respect to this, we attempt to use electrochemical reduction dyeing as a potential and friendly environment method. But the studies published show that the electrochemical reduction dyeing has low current efficiency, slow dyeing speed and less color depth compared with those dyed in traditional process. Moreover, the voltage of work bath is not stable, which leads to the problems of actual production.In this study, we developed a new indirect electrochemical reduction dyeing method, using Fe3+/Fe2+ion pairs in cycle redox system and Fe(Ⅲ)-oxalic acid-calcium gluconate complex as media system.The personally designed with multi-cathode and double anode electrolyzer of indirect electrochemical reduction dyeing was proved to greatly improve the electrolysis efficiency and the effect of indirect electrochemical reduction dyeing. The main contents and conclusions are as follows:Firstly, comparing the cyclic voltammograms of Fe(Ⅲ)-oxalate complexes, Fe(Ⅲ)-calcium gluconate complexes and Fe(Ⅲ)-oxalate-calcium gluconate complex, we dedicated the possible electrochemical behavior of Fe(Ⅲ)-oxalate-calcium gluconate complex, and found that Fe(Ⅱ)-gluconate could be regarded as the critical substance in the electrochemical reduction process.Secondly, using Fe (Ⅲ)-oxalate-calcium gluconate complex as the dyeing mediator in the indirect elecochemical reduction dyeing, we successfully dyed cotton fabric with Vat Brilliant Green FFB. The effects of various factors on the oxidation peak current and K/S values were discussed by orthogonal and single factor experiments. The optimal reduction dyeing conditions of Fe (Ⅲ)-oxalate-calcium gluconate complex were shown as follows:reduction time10min, electrolytic time15min, electrolysis voltage4.5V, dye concentration1.5g/L, C2H2O410.08g/L, Ca(C6H11O7)217.92g/L, Fe2(SO4)38.0g/L and NaOH45g/L.Under these conditions, the dyed cotton fabric had high dye yield, good levelness and favorable color stability. Then, we selected some other vat dyes, dyed with the optimum dyeing process and analyzed their dyeabilities.At last, we compared Fe(Ⅲ)-oxalic acid-calcium gluconate media system with another common Fe(Ⅲ)-TEA(triethanolamine)-calcium gluconate complex. The results showed that the two media systems had the similar electrochemical behavior on the electrode surface, and Fe(Ⅲ)-oxalic acid-calcium gluconate complex achieved more negative redox potential than that of Fe(Ⅲ)-TEA-calcium gluconate media system under the same mole ratio, but the dyeing effect of Fe(Ⅲ)-oxalic acid-calcium gluconate complex was not so good. Combined Fe(Ⅲ)-oxalic acid-calcium gluconate complex with4g/L hydrosulfite, the dyeing bath potential can maintain more negative than-945mV, K/S value was significantly improved, dyeing fastness achieved grade4-5or better, and the levelness index of SDE reduced to below0.1. The effect of the optimized Fe(Ⅲ)-oxalic acid-calcium gluconate complex dyeing method is not only better than that of Fe(Ⅲ)-TEA-calcium gluconate complex, but also close to traditional dyeing process. Concerned TEA is irritant to skin, Fe(Ⅲ)-oxalic acid-calcium gluconate complex seems more suitable as an indirect electrochemical reduction dyeing mediator. This new method will give some reference for reducing the dosage of hydrosulfite in vat dyes dyeing and the environment pressure. |
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