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Reactive dyes have become very popular by virtue of brilliancy, ease of application, good all-round fastness properties and comparatively low cost. However, they suffer from the drawback that during application under alkaline conditions, substantial quantity of the dye reacts with water and the hydrolyzed dye thus formed becomes ineffective for reaction with cellulose. In order to overcome this drawback, bi-functional and poly-functional reactive dyes have been marketed. These dyes by virtue of more than reactive group in the dye molecule are likely to combine with more than one hydroxyl group in cellulose chain. Hetero-bifunctional reactive dyes (monochlorotriazine/vinyl sulphone) have gained considerable popularity because of the differences in reactivity of the two reactive groups present in the same dye molecule which facilitates the optimisation of the dyeing conditions in a wide temperature range. The stability of the bond between the two different types of reactive groups and cellulose is different under alkaline and acidic conditions of treatment; dyeings obtained with these dyes show high stability to different conditions to storage and washing. There is hardly any information available on all in method of dyeing in presence of alkali buffer system Sheth et al.
The present work was undertaken to evolve a simplified method for application of hetero-bifunctional and vinyl sulphone reactive dyes on knitted cotton fabric by exhaust method by making suitable modifications with a liquid alkali buffer system (Sorbecol ASA) and levelling agent (Sorbecol CDL). Dyeings obtained by modified method were evaluated for colour value and fastness characteristics.
Experimental
Dyes
Five hetero-bifunctional reactive dyes viz, Yellow 160 and 145, Red 198, Red 195, Orange 122, Blue 222 and Black HFGR
Five vinyl sulphone reactive dyes viz Yellow 15 and 16, Orange 107, Violet 5, Blue 220 and 21, and Black 5.
Fabric
Single jersey mill bleached cotton knitted fabric was used.
Method of dyeing
Exhaust dyeing in shaker bath at 600C with the sequence of exhaustion followed by fixation: Dyeing was carried out in soft water (60 ppm) as well as in hard water (480 ppm). Fabric and required amount of dye solution to obtain 2% shade or 4% shade was taken in a flask and dyeing was continued at ambient temperature in shaker bath for 15 min. At the end of 15 min, required amount of common salt (50 g/l for 2% shade and 60 g/l for 4% shade) and 1 g/l Sorbecol CDL were added and temperature was raised to 600C. Exhaustion was continued for 45 min at 600C. Sodium carbonate (20 g/l) was added at 600C after 45 min and dyeing was continued for another 60 min. At the end of dyeing, fabric was given a thorough wash with cold water and then with boiling water containing 2 g/l. Sorbecol PNN to remove the unfixed dye followed by washing with cold water.
Simultaneous exhaustion and fixation at 600C (All-in method) : Dye (2% or 4% shade) was taken in the flask and dye solution was exhausted for 15 min at ambient temperature. At the end of 15 min, required amount of common salt (for 2% shade 50 g/l, and 4% shade 60 g/l) along with required amount of Sorbecol ASA (2 g/l Sorbecol ASA for 2% shade, or 3 g/l Sorbecol ASA for 4% shade) and 1 g/l Sorbecol CDL were added together.
Temperature was raised from room temperature to 600C and continued dyeing for 75 min. At the end of dyeing, thorough washing was given as illustrated in the first method.
Colour strength measurement
Colour strength expressed as K/S was calculated from reflectance measurement carried out on Spectrascan 5200 using Kubelka Munk equation:
K/S = (1-R)2/2R
Fastness characteristics
Wash fastness of dyed fabric was determined equivalent to ISO Test No 2. Dry and wet rub fastness was determined according to AATCC Test Method 8 - 1989.
Results and discussion
In the present study, alkali buffer (Sorbecol ASA) has been developed with the view to make simplified process as well as a reduction in time and total dissolved solid. Further, the quantity of alkali required is very low (1/7th to 1/10th of Na2Co3) as compared to conventially used sodium carbonate. Results of studies indicated that dyeing wih hetero-bifunctional reactive dyes to cotton knit fabric for both the depth of shade, the colour value in presence of Sorbecol ASA and all-in method dyeing showed comparable to that of conventional method of dyeing for both the depth of shade in presence of hard water (480 ppm) or soft water (60 ppm) (Tables 1 and 2). Further, the time required during dyeing is less and quantity of alkali required is also less as compared with conventional method dyeing. This is due to the fact that all-in method in presence of buffer alkali system does not only require very low quantity but it also reduces the hydrolysis of dye and thereby colour value is not affected. The addition of Sorbecol CDL gave level dyeing of knitted fabric. Sorbecol CDL does not allow dye to rush onto the fabric thereby gives uniform dyeings. In case of sulphatoethyl ester types of (vinyl sulphone) dye, similar results were obtained to that of hetero-bifunctional dyes with respect to colour value for different depth of shade in hard as well as soft water (Tables 3 and 4 and Figs 1 and 2). Fastness characteristics in case of all-in method were found to be comparable to that of fabric dyed by conventional method.
Wash fastness equivalent to ISO-2 showed comparable results in case of fabric dyed by all-in method to that of fabric dyed by conventional method with respect to change in shade and staining of white fabric. Wet rub fastness of dyed fabric showed comparable results to that of conventional method of dyed fabric (Tables 5 and 6).
Conclusions
Simplified method of dyeing of knitted cotton fabric with sulphato ethyl ester dye as well as hetero-bifunctional reactive dyes in presence of Sorbecol ASA not only reduces time but also quantity of alkali as compared with conventional method of dyeing with soda-ash. Sorbecol ASA is not affected in presence of hard water (480 ppm). Addition of Sorbecol CDL during dyeing showed level dyeing. Dyeings obtained by this method showed comparable fastness charachteristics to that of conventiional method.
Reference
1. G N Sheth and V J Fernandes, Book of Papers of 31st Joint Technological Conference held in NITRA (1990) p 1.
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