ON THE COLOUR STRIPPING

Abstract

Concerning the desorption of dyes from coloured cellulose crystalline regions, the following facts have been revealed;(1) At the first stage of wet treatment, a considerable amount of colour is stripped from a coloured cellulose crystalline region dyed with a selected substantive colour. Especially, a coloured crystalline region which has been dyed in weak alkaline dyebath containing Glauber's salt shows a remarkable desorption of colour at the beginning of hot water treatment. It seems most probable that a great deal of colour is fixed mechanically on the surface of crystalline regions owing to the fact that the dye molecules are too large that they can hardly disperse in compact tissue of crystalline regions.(2) The strippability of colours on crystalline region separated from a cellulose fiber differs considerably from a microstructure of fiber. A coloured crystalline region which has a large rod-like structure, those obtained from cotton fiber, shows the best fastness to various wet treatments. This behaviour is supposed to occur due to the differences of co-ordinate state of dye molecules on crystalline regions.Generally, all the dye molecules fixed along a long axis of crystalline region seem to exist in the best co-ordinate state, and such portions will probably show the better wet fastness.(3) The rate of colour desorption on coloured cellulose crystalline region becomes the slower the greater the frequency of wet treatments. Similar behaviour is observed in the case of coloured cellulose fibers as was shown in the previous report. This behaviour is attributed to a nonuniform microstructure of cellulose fibers, composed of crystalline regions of different degree of crystallinity. It would be proper to infer that the fixed state of dye molecules varies with the degree of crystallinity and that those fixed in better cystallized portions reveal the greater wet fastness as it can hardly spread out.(4) The behaviour of a colour desorption of coloured crystalline region separated from a different cellulose fiber resembles to each other, and the differences in colour strippability of each crystalline region is smaller than that of coloured fiber itself. These differences seem mainly due to the change of dispersed state of dye molecules in microstructure of the fiber.(5) It is assumed that the polar groups on the crystalline regions were responsible for physicochemical adsorption of each dye molecules, due to the close co-ordination between the activated polar groups in the dye molecules and that of the cellulose crystalline regions. The marked differences of colour strippability of coloured cellulose crystalline regions to wet treatments will probably be attributed to the variations in the fixed state of dye molecules in crystalline regions, owing to the wide difference in the physicochemical crystalline organization.