Discovery of the first direct dye, depended on various techniques for depositing insoluble colorants inside the amorphous regions of the cellulosic polymer. For a further four decades, this remained the only feasible method of achieving dyeings of high fastness to washing on cellulosic textiles.
During this period much fundamental work on the structure of cellulose and the morphology of cellulosic fibres was undertaken and numerous ethers and esters of cellulose were prepared. It would be surprising if none of these had involved the formation or attachment of coloured sidechain substituents on the cellulose chain. Although such necessarily complex and esoteric reactions did indeed confirm the formation of covalent bonds between typical chromophoric groups and the hydroxy groups in the cellulose molecule, they remained essentially of academic interest only.
Surprisingly, few attempts were made to adapt such reactions or to develop appropriate reagents that would allow such derivatives to be formed under typical dyehouse conditions. Some of the drawbacks of the treatments applied in this period, apart from their multi-stage complexity or the use of costly and hazardous solvent media, were degradative attack of the cellulose chains by some of the vigorous reagents or reaction conditions necessary, or sensitivity of the colorant–fibre bond to hydrolytic attack during subsequent handling or storage of the coloured product.
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