The effect of urea, thiourea and ammonium thiocyanate on various dye solutions has been examined. It has been found that generally urea and thiourea disaggregate the dye in solution whilst ammonium thiocyanate causes aggregation. Exceptions to these findings do occur. Thus urea enhances the aggregation of C.I. Acid Red 138, hut not other dyes of this class. Thiourea reacts preferentially with CI. Reactive Reds 1 and 2, giving products which are relatively inert in reactions with wool. Such products, whilst not reducing dye exhaustion, reduce fixation considerably.
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Asquith et al. (1979) studied this question.
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