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Abstract The work presented in this thesis is undertaken with a view of : (a) studying factors affecting the dyeability of wool, nylon-6, cotton and mercerized cotton fabrics with direct dyes in presence and absence of redox systems, (b) understanding the role of re~ox systems as well as the mode of interaction, and (c) improving the dyeing properties of direct dyes on the given substrates. Results obtained along with conclusions arrived at from this thesis are summarized and given below under three main headings : (I) Utilization of redox systems for improving fixation of direct dyes on wool fabric. (II) The role of redox systems in dyeing nylon-6 fabric with direct dyes, and (III) Factors affecting the dyeability of cotton and mercerized cotton fabrics with direct dyes in presence of redox systems. I. Utilization of Redox Systems for Improving Fixation of Direct Dyes on Wool Fabric With a view to enhance the dyeability of wool fabric with direct dyes, attempts have been made to incorporate different redox systems in the dyeing bath. Redox systems used include ammonium persulphate as oxidant and glucose, thiourea or potassium pyrosulphite as reductant. Given below dre the main conclusions arrived at from these studies - vi - 1. The colour strength, expressed as K/5, is much greater in the presence than in the absence of the redox system. This is observed irrespective of the reductant used. Nevertheless, for a glven set of dyeing conditions, the magnitude of colour strength is determined by the kind of reductant used along with (NH4) 2N2o8, and it follows the order (NH4)2 s2o8/c6H12o6 > (NH4 ) 2s 2os~cH 4N 2S > (NH 4) 2s 2os~K 2s 2o 5 > (NH4) 2s2o8 ~None, 2. Presence of c6H12o6 along with (NH4) 2s2o8 (0.025 mol/1 each) enhances significantly the colour strength. 3. Dyeing proceeds much faster in presence than in absence of (NH4)2 s2o8/C6H12o6 redox system. Needless to say that the colour strength increases initslly very fast then slows down with time. 4. Raising the dyeing temperature (from 50° to 90°C) is accompanied by a signi ficnat enhancement in colour strength of dyeings. 5. At lower liquor ratios the availability of the dye, redox system in the vicinity of the wool macromolecules is greater thereby leading to higher dyeing. 6. Increasing the concentration up to 2~o (owf) is accompanied by a significant improvement in the colour strength of the wool fabric. Moreover, the highest colour strength is obtained in the presence of (NH4) 2s2o8; c6H12o6 redox system. 7. The enhancement in the dye fixation before and after DMF extraction as well as the improvement in the fasters properties of dyeings via- vii - incorporation of the redox system in the dyeing formulation indicates that presence of the redox system alters the mode of dye attachment to the wool substrate. 8. In the presence of the redox system, i.e. (NH4l 2s2o8;c6 H12o6 ,it is postulated that dyeing involves covalent bonds and that there is a freeradical mechanism in bonding between wool and direct dye in addition to their conventional attachment. II. The Role of Redox System in Dyeing Nylon-6 Fabric with Direct Dyes Factors affecting dyeing of nylon-6 fabric with direct dyes in the absence and presence of certain redox systems were investigated under different conditions. The variables studied were : type and concentration of redox system, dyeing conditions, i.e. time and temperature, material to liquor ratio 8S well as type and concentration of direct dye. The obtained results indicate that : 1. The colour strength is outstandingly higher in presence than in absence of the oxidant, i.e. (NH4) 2s2o8, alone or along with reductant. A comparison between the colour strength values of such dyeings obtained along with four reductants would call for the following order : (NH4) 2s2o8;c6H12o6 > (NH4) 2s2o8 > (NH4)2s2DgiCH4N2S > (NH4) 2s2o8/K2s2o5 >(NH4) 2s2o8/Na 2s2o3. 5H 2D ~None. 2. In absence of the redox system, dyeing proceeds via electrostatic bonds between the dye anion and the protonated amino group, hydrogen bonds and/or Van der Waals fources, but in presence of redox system, free radical are supposed to be formed in both the fibre and the dye and the inter- viii - action between these free radicals brings about covalent fixation beside the usual physical bonds. This rather supported by the significant value of colour strength after DMF extraction for dyeings obtained using (NH4) 2s2o8/C6H12o6 (0.025/0.025 mol/1) redox system. 3. The rate of dyeing increases initially very fast then slows down with time to level off. A perusal of result indicates that, 80°C constitutes the optimal temperature for dyeing nylon-6, for a given set of dyeing conditions. Nevertheless, the rate of dyeing is far greater in presence than in absence of (NH4) 2s2o8;c6H12o6 redox system. 4. Increasing material to liquor ratio up to 1 : 40 is accompanied by a significant decrease in colour strength. 5. The enhancement of colour strength as the dye concentration increases could be attributed to greater availability of dye molecules and/or dye free radicals in the vicinity of nylon-6 immobile macroradicals. 6. The magnitude of the colour strength and the fastness properties of the dyeings are determined by the nature of the dye, i.e. size, shape, configuration, number of dye sites, the fibre dye capacity, as well as by the mode of interaction. III. Factors Affecting the Dyeability of Cotton and Mercerized Cotton Fabrics with Direct Dyes in Presence of Redox Systems The work was further extended with a view of studying factors affecting the dyeability of cotton and mercerized cotton fabrics with direct dyes and understanding the mode of interaction in presence of certain redox systems.- ix - Results obtained led to the following conclusions : 1. Presence of c6 H 12o6 along with (NH4) 2s2o8 in the dyeing bath enhances significantly the colour strength. This suggests that (a) glucose assists decomposition of (NH4 ) 2s2o8 , (b) glucose radicals takes part in creating a reactive sites located along the cellulose backbone as well as on the dye capable of combination to form covalent bond in addition to their conventional attachment via physical bond. 2. Mercerized cotton showed higher extent of dyeing than unmercerized cotton. This is a a direct consequence of increased accessibility and availability of the cellulose active centers for dye adsorption, i.e. hydroxyl groups, under the influence of mercerization. 3. The rate of dyeing is much better in presence than in absence of the redox system. On the other hand, raising the dyeing temperature from 50° to 70°C is accompanied by a considerable increase in colour strength irrespective of the substrate used. This could be ascribed to the greater activiation energy, and the greater mobility of the macromolecular chains in the fibre as well as the greater solubility and mobility of the dye. 4. The colour strength, expressed as K/S, decreases by increasing the material-to-liquor ratio up to 1 : 40. Nevertheless, the colour strength is much better in presence than in absence of the redox system as well as with mercerized than unmercerized cotton fabric as already emphasized. 5. The colour strength increases initially very fast by increasing the dye concentration up to 1% owf, then almost levels off by further increase- X - in concentration irrespective of the substrate used. On the other hand, the nature of direct dye exerts a considerable effect on the magnitude of the colour strength as well as the fastness properties of the dyeings. |