Introduction
Reactive Dyes are a most popular dye class for dyeing of cellulosic fabrics. These dyes offers several advantages such as low cost, ease of application, wide gamut of colours and good fastness to light, washing and rubbing. However, in case of reactive dyes, high concentrations of salt are required during dyeing for effective exhaustion of dye onto the cotton surface. Although use of salt is economically viable, it increases total dissolved solid in the effluent stream. Further, high electrolyte concentrations of dye-bath discharges are undesirable as increased salinity in rivers upsets the delicate balance of aquatic flora and fauna.

Different approaches adopted to reduce salt during dyeing such as modification of the dye structure to it more substantive to cellulose at low salt dyeing¹, cationisation of cellulose through deposition or chemical reaction with polymeric compounds containing cationic groups^2,3 or controlled dosing of dye and salt during exhaustion process have been recommended⁴.

Hardly, any information is available in the literature for the replacement of salt by alternative biodegradable products during dyeing of cotton fabric with reactive dyes. The present work was undertaken with a view to explore possibility for the replacement of salt by an eco-friendly product during dyeing of cotton fabric with hetero-bifunctional reactive dyes.

Experimental

Dyes : Five hetero-bifunctional reactive dyes viz. C. I. Reactive Yellow 186, Yellow 145 A, Yellow 145, Orange 122, Red 195, Blue ME2BF, Blue 222, Blue 221.

Fabric : Mill bleached mercerized cotton fabric with reeds x picks, 96 x 60 threads/inch and counts 30^s X 30^s.

Method of dyeing

Following two methods of application have been examined for the dyeing of cotton with vinylsulphone and hetero-bifunctional reactive dyes.

a. Exhaust dyeing in beaker dyeing machine at 60⁰C with sequence of exhaustion followed by fixation has been carried out. Dye bath was set at 60 ± 1⁰C at liquor: material ratio 20:1. Dye (2% and 4% owf.) and required amount of common salt or Ecosalt E was added at the same temperature and dyeing was continued further for 60 min. At the end of 60 min, 20 g/l soda ash was added at the same temperature and dyeing was continued further for 60 min. At the end of dyeing, fabric was given a thorough wash with cold water and then with 2 g/l Sorbecol PNN at 80-85⁰C for 20 min to remove the unfixed dye, followed by washing with cold water. This treatment is highly effective for the removal of unreacted as well as hydrolysed dye.

b. Simultaneous exhaustion and fixation at 60⁰C (all-in method). In this method, dye, Ecosalt E (10-20 g/l) and soda ash (20 g/l) were added together and dyeing was continued at 60⁰C for 120 min. At the end of dyeing, fabric was given a wash with 2 g/l Sorbecol PNN as mentioned in method 'a'.

Colour strength measurement: Colour strength expressed as K/S was calculated from reflectance measurement carried out on Spectrascan 5100 using Kubelka Munk equation.

K/S = (1-R)²/2R

Fastness charachteristics:

Wash fatness of dyed fabric was determined equivalent to ISO test no. 3. Dry and wet rub fastness determined according to AATCC test method 8-1989.

Result and discussion

Cellulosic fibres acquire negative charge in aqueous solution due to the dissociation of hydroxyl group. Reactive dyes also on ionization acquire nagative charge. Thus both dye and fibre being negatively charged repel each other and thereby approach of the dye onto fabric surface is restricted. In order to exhaust dye onto the fabric, the large amount of electrolyte are required during dyeing. The electrolyte suppresses the negative charge at the fabric surface and facilitates the approach of dye onto the fabric. However, electrolytes increase total dissolved in the effluent streams. In the present study, a speciality product has been developed to replace conventionally used electrolytes in the dyeing process. The product develops positive zeta potential at the fabric surface and thereby facilitates transfer of dye from the bath on to the dye.

In this study, conventional method of dyeing was carried out in presence of salt as well as in presence of the speciality product. Further, dyeing was also carried out by an all-in method where dye, speciality product and alkali were added together and dyeing carried out for a stipulated time. Results obtained indicated that, exhaustion of the dye in presence of conventionally used salt or speciality product are more or less the same when dyeing was carried out by the all-in method or conventional method (Figure 1 and 2). Similarly fixation was also found to be the same or more at both 2 and 4% depth of shade (Tables 1 and 2). Fixation of dye was found to be increased when concentration of Ecosalt E was increased 10 to 20 g/l as compared with that during conventional method of dyeing in presence of Ecosalt E as salt substitute show higher color value than that during conventional method of dyeing in presence of salt. Fixation of dye was found to be higher for C. I. Reactive Yellow 160, Orange 122 and blue 222 as compared with that of conventional method. The sulphatoethyl sulphone group in the dye molecule has negative charge whereas the vinyl sulphone group formed in the dye molecule results in greater dye execution as indicated by Fugioka and Abeta. Thus in the all-in method when alkali was added along with Ecosalt E, the vinyl sulphide form of the dye is formed which has higher exhaustion which in turn showed higher exhaustion and fixation on cotton fabric.

Conclusions

It is possible to replace conventionally used salt by Ecosalt E during dyeing of cotton fabric with hetero-bifunctional reactive dyes without affecting color value as well as fastness charachteristics of dyes. Also product is biodegradable and decrease in effluent stream.

References

1. A. J. Murgatroyd, AATCC Book of papers, 241
2. M. R. El-shishtwy and S. H. Nasar, Color Technol, 118 (2002)
3. B. N. Bandyopadhyay, G. N. Sheth and M. M. Moni, Int. 183 (11), 39 (1998)
4. C. S. Whewell, Rev. Progress, Coloration 14, 157
5. Faheem U. Int. Dyer 188 (9), 28 (2003)
6. Fujioka S. and Abeta S., Dyer and Pigments,3,287