An investigation of alternatives to reductive clearing in the dyeing of polyester

Abstract

Reduction clearing is commonly carried out as an after-treatment to remove deposits of disperse dye and other residual impurities from the surface of dyed polyester. The research described in this thesis establishes the positive effect of conventional reduction clearing using aqueous sodium dithionite under alkaline conditions on the colour and fastness properties of polyester dyed with a series of selected commercial disperse dyes at a range of depths of shade. Because of certain environmental, technological and economic disadvantages associated with traditional reduction clearing, there is industrial interest in alternative processes. This research also describes a study of some alternative approaches to clearing by means of the development of an understanding of the principles of clearing and its effect on the properties of polyester dyed with disperse dyes. The study involved the use of three organic reducing agents, formamidine sulphinic acid (thioureadioxide), hydroxyacetone and glucose, and also a previously-reported method using a simple detergent wash-off procedure. Besides the organic reducing agents, this study presents investigations concerning the use of enzymes and electrochemical methods in the reduction clearing of polyester. The relative merits of the alternative processes, in terms of the efficiency of surface dye removal, the effect on fastness and colour properties, the biochemical oxygen demand and chemical oxygen demand of the residual treatment liquors are discussed. The redox potential values of the reducing agents under application conditions have also been compared. The results throughout correlate closely with the efficiency of surface dye removal as assessed by acetone extraction of the dyed samples. Scanning electron microscopic investigations of dyed samples before and after reduction clearing are qualitatively consistent with the observations. The outcome of the clearing process varies with the particular dye used. Explanations have been suggested based on mechanisms proposed for the clearing processes in relation to specific characteristics of the molecular structures of the dyes.