Indian Journal of Fibre & Textile Research

Vol. 29, December 2004, pp. 454-461

Sulphur dyeing using non-sulphide reducing agents

s R Shukla" & Roshan S Pai

Department of Fibres and Textile Processing Technology, Institute of Chemical Technology,

University of Mumbai, Matunga, Mumbai 400 019, India

Received I I August 2003; revised received and accepted 1 9 January 2004

Cotton hanks were dyed with three different sulphur dyes using non-sulphide reducing agents such as glucose, fructose, invert sugar and molasses. The alkalies employed for reduction were sodium carbonate and sodium hydroxide. Invert sugar as a reducing agent and sodium carbonate as alkali proved to be better as this combination gave results comparable to those obtained with the conventional sodium sulphide dyeing with respect to depth and tone of dyeing. The fastness properties were also found to be comparable.

Keywords: Cotton, Dyeing, Non-sulphide reducing agent, Sulphur dyes

IPC Code: lnt. Cl.7 D06P 1 130, D06P 3/60

1 Introduction

In the colouration of cellulosic fibres, sulphur dyes hold a promise, particularly for workwear and polyester/cellulosic blends. The fastness characteristics of sulphur dyes as a class are very close to vat dyes, except for the brilliance of shades. Being considerably cheaper than vat dyes, the sulphur dyes are in much demand for medium to heavy depths, especially black, blue and brown, of reasonably good fastness at an economical price l . These dyes are available in powders, prereduced liquids, grains, dispersed powders, water-soluble brands, etc.

Sulphur dyes are a class of dyes which in their final form of application are macromolecular compounds characterised by the disulphide and polysulphide bonds (-S-S-)o between aromatic residues2• They are produced by the thionation of organic compounds containing nitro and amino groups; the basic chromophore being thiazole ring. Due to quinonoid structure, the reversible oxidation and reduction are possible; the reduced phase being alkali soluble. Majority of the conventional sulphur dyes are of unknown and variable constitution. The

a To whom all the correspondence should be addressed. Phone: 24 1456 16 ; Fax: +91 -22-�4 1456 14; E-mail: sanjeevrshukla@rediffmail.com

lack of reliable structural formula for these dyes hinders scientific control for industrial processes and discourages the research worker from any investigation on reaction mechanism3.

Application of sulphur dyes involves reduction of dye molecule to attain a water-soluble leuco form having high substantivity to cellulosic fibres. Sodium sulphide is the largest consumed reducing agent, even for the liquid sulphur dyes. A part of sodium sulphide is used for the reduction but the excess amount causes an odour problem which is dangerous to the life and also it damages sewer structure due to the bacterial oxidation of liberated hydrogen sulphide to sulphuric acid. The effluent arising from sulphur dyeing process contains sulphides, the concentration of which depends on the dyeing method and the applied depth. Hydrogen sulphide is highly toxic, leading to a number of ecological and human health problems4. The modification in the application procedure of the sulphur dyes to replace the process of reduction by sodium sulphide with different techniques can lead to ecofriendly dyeing5- 1 I . Chavan and Vhanbattel2 used molasses to obtain reducing sugar for reduction of a number of sulphur dyes. They measured the redox potential and dye uptake, which were higher for reducing sugar than for sodium sulphide.

The present paper reports the use of a few reducing sugars for the reduction of sulphur dyes followed by dyeing on cotton.

SHUKLA & PAl: SULPHUR DYEING USING NON-SULPHIDE REDUCING AGENTS 455

2 Materials and Methods

2.1 Materials

Cotton hanks (scoured and bleached) ready for dyeing were used.

The dyes used were powder forms of Sulphur Military Green VI, Sulphur Slate Grey Conc. and Sulphur Red Brown Conc., supplied by Mafatlal Dyes and Chemicals Ltd, Mumbai.

The non-sulphide reducing agents used for the reduction of sulphur dyes were D (-) glucose, D (-) fructose and molasses. Sucrose was used to prepare invert sugar, which is a reducing sugar. All the chemicals used were of LR grade. Sodium sulphide flakes, sodium hydroxide pellets and sodium carbonate used were of AR grade. 2.2 Methods

2.2.1 Preparation of Non-sulphide Reducing Agents

Glucose and fructose powders were dissolved in water to give 10 % (w/v) solutions. Molasses was available as a syrupy liquid.

Invert sugar was prepared according to the method reported by HorwitzJ3• Sucrose ( 1 0 g) was accurately weighed and dissolved in water to which 5 mI of conc. HCl was added. The solution was diluted to - 1 00 mI, stored at room temperature for 3 days and then neutralised using I N NaOH solution. It was finally diluted to 1 litre. 2.2.2 Estimation of Reducing Power

Lane-Eynon volumetric method was used for estimating the reducing power of the agents in terms of dextrose equivalentl4.

Two solutions were prepared separately by dissolving 34.639 g of copper sulphate in 500 mI of water and 1 73 g of sodium potassium tartrate plus 50 g of sodium hydroxide in 500 mI of water. These solutions were mixed in exactly equal amounts to get 1 0 mI of Soxhlet reagent and diluted to 50 ml. Standard solution of glucose (0.25% w/v) was prepared such that a titre value between 1 5 and 50 is obtained while reducing all copper. The glucose solution was gradually added to the moderately boiled Soxhlet reagent. Methylene Blue solution ( 1 rnl of 0.2%) was added as an indicator and titration was completed within total boiling time of 3 min by small additions of solution to decolourise blue colour of the solution. After complete reduction of copper, Methylene Blue was reduced to colourless compound and the solution resumes cuprous oxide colour, which

it had before the addition of indicator. The volume of the glucose solution required to reduce all copper was noted and the Soxhlet solution was thus standardized with respect to glucose.

The reducing power of other reducing agents with respect to glucose was also found out using the same procedure and expressed as dextrose equivalent. If the titre value for a reducing sugar of 0.25% (w/v) concentration is x mI, then its dextrose equivalent is:

23 .2x O.25 x

where 23.2 is the titre value for 0.25% (w/v) glucose against 1 0 mI of Soxhlet solution.

In all the cases, except molasses, the concentrations of reducing agents taken for titration were identical (0.25% w/v). Molasses was diluted to 200 times in order to get titration reading in the desired range. 2.2.3 Estimation of Effect of Temperature on Reducing Power

The effect of temperature on reducing power was studied by heating 50 mI of standard reducing sugar solutions (0.25% w/v) to different temperatures between 30 °C and 90 °C for 30 min, cooling and then estimating reducing power by similar procedure. 2.2.4 Reduction of Sulphur Dyes

2.2.4. 1 Sodium Sulphide Reducing Agent

One gram of dye was mixed with 3 g sodium sulphide and a paste was prepared with 5 mI of 1 0% soda ash solution. The mixture was boiled for l min, diluted to 1 00mI with hot water and filtered to ensure the removal of any unreduced dye particlel5•

2.2.4.2 Non-sulphide Reducing Agent

The reduction of sulphur dyes was carried out using non-sulphide reducing agents in different amounts with respect to the dye powder at 70 °C in alkaline condition using different alkalies. In one method, soda ash was used to get the pH of 1 0.5, whereas in the other method, caustic soda was used to get the pH of 1 2.5 . The time required was 20-25 min. The solution was then made up to 100 mI and filtered to ensure the removal of any unreduced dye particle.

2.2.5 Dyeing

The required amount of reduced dye was measured into the dye pot. The wetted cotton hank ( 1 g) was introduced into the dye bath at a material­to-liquor ratio of 1 :50. The dyeing was started at

456 INDIAN J. FIBRE TEXT. RES., DECEMBER 2004

room temperature and the temperature was then raised to boil gradually. The dyeing was continued at this temperature for 30rnin. Glauber's salt ( 1 0% owf) was added for exhaustion and dyeing was continued for another 30rnin. After dyeing, the hanks were squeezed, rinsed with water, oxidised in air, washed and dried. In the case of sodium sulphide, dyeing was carried out for 1 -5% shade, whereas with nonsulphide reducing agents, it was carried out only for 5% shade. 2.2.6 Estimation ol Colour Values

The estimation of colour values (KIS, L *, a* and b*) was done using a D65 source and a 1 00 observer angle on a computerized colour matching system (CCM) Spectraflash-300 of DataColor, USA. 2.2.7 Wash and Crock Fastness Tests

Wash fastness test was carried out according to ISO 3 method. Dry and wet crock fastness tests were carried out by using crock meter.

3 Results and Discussion

Non-conventional reducing agents, such as glucose, fructose, invert sugar and molasses, were used for reducing different sulphur dyes available commercially in powder form followed by dyeing on cotton hanks. All the sugars were in powder form whereas molasses was a syrupy liquid. Molasses contains mainly sucrose, glucose and fructosel6• In the present work, it was used as such after dilution. Hence, only glucose and fructose were the available reducing sugars from molasses.

Table 1 shows the reducing power of these reducing agents at different temperatures up to 90 °C. It was observed that in all the cases, with the rise in temperature up to 70 DC, there was no change in reduction power. However, beyond 70 DC, it drastically decreased. Therefore, in further studies, the dissolution of sulphur dyes was carried out at about 70 DC followed by dyeing at boil . The reducing powers of glucose, fructose and invert sugar were almost similar, whereas that of molasses was higher by nearly 1 .5 times.

Cotton was dyed at boil with three different sulphur dyes for 1 -5 % shades using the conventional method incorporating sodium sulphide as a reducing agent (3 times the weight of dye) and soda ash as an alkali. The KIS as well as L * a* b* values of the samples were evaluated using the computerised colour matching system. These values for different dyes are given in Table 2. With increase in depth, the

Table 1 - Effect of temperature on reduction power of reducing agents

Reducing agent Conc. Temp. Dextrose % (w/v) °C equivalent

%

Glucose 0.25 30 0.250

50 0.250

60 0.250

70 0.250

80 0. 1 90

90 0. 147

Fructose 0.25 30 0.230

50 0.230

60 0.230

70 0.230

80 0. 1 79

90 0. 142

Invert sugar 0.25 30 0.242

50 0.242

60 0.242

70 0.242

80 0. 1 86

90 0. 1 44

Molasses 0.25 30 0.341

50 0.34 1

60 0.341

70 0.341

80 0.232

90 0. 1 53

KIS values increased whereas L* values decreased for all the three dyes.

The amounts of non-sulphide reducing agents used were varied from 2 times to 5 times with respect to the amount of dye and the reduction was carried out at 70 °C with either soda ash (pH 1 0.5) or sodium hydroxide (pH 1 2.5). Above this temperature, caramel formation was observed, spoiling the dye itself. When dyed for different shades between I % and 5%, it was observed that the depths obtained were much lower as compared to that of the conventionally dyed samples. All the dyes using non-conventional reducing agents were, therefore, used at 5% shade only. The dyed samples were evaluated for KIS as well as differences

SHUKLA & PAl: SULPHUR DYEING USING NON-SULPHIDE REDUCING AGENTS 457

in the colour coordinates I1L *, l1a* and I1b* with respect to the samples dyed using sodium sulphide as reducing agent.

The results of dyeings in terms of KIS values and the differences in colour coordinates with respect to conventional dyeing are shown in Figs 1 -3 , wherein reduction of the dyes was carried out using soda ash as alkali .

Table 2 - Colour values of cotton dyed with different dyes using sodium sulphide as reducing agent

Dye Shade % KIS L* a* h*

Sulphur Military I 1 . 1 8 69.64 -2.50 1 2.99 Green UI 2 2.04 62.87 -3. 1 2 1 5. 4 1

3 2.70 58.05 -3.47 1 4.78

4 5.52 46.38 -3.24 1 3 .97

5 7.46 4 1 .34 -2.99 1 2.75

Sulphur Slate 0.79 64.32 - 1 .32 -7.44 Grey Conc. 2 1 .53 55.34 - 1 .70 -9.01

3 2.37 48.74 - 1 .9 1 -8.96

4 3.40 43. 5 1 - 1 .95 -9.43

5 5.20 36.98 - 1 .56 -8.98

Sulphur Red 1 .8 1 52.04 6.7 1 -6.75 Brown Conc. 2 2.3 1 48.58 8.08 -7.08

3 3.69 4 1 .2 1 6.56 -8.04

4 4.06 39.9 1 9.38 -3.00

5 7.01 3 1 .03 6.86 -6.85

IDi B

1m !IIID

6 IT]

4

2

a '--"" ......... _-...,..

Sod. Sulphide Glucose

Fig. I shows the KIS values for cotton dyed with Sulphur Military Green UI. The amounts of non­sulphide reducing agents used were 2-5 times of the dye powder and at all these proportions the complete reduction of dye was obtained as observed by filter paper test for insoluble particles. The invert sugar, used 5 times with respect to dye, gave the KIS value much closer to that obtained using sodium sulphide in the conventional dyeing. In all the other cases, the values were lower, although increased with the increase in proportion of reducing agent with respect to the dye. All the non-sulphide reducing agents were proved to be successful although depth-wise the dyeings were l ighter.

Fig. 2 shows the KIS values for cotton dyed with another dye Sulphur Slate Grey Conc. Here, the invert sugar, used 3 times with respect to dye, gave the KIS value much closer to that of the conventional dyeing. In all the other cases, the values for KIS were lower and there were no tonal variations observed for all the dyeings.

Fig. 3 shows that even with Sulphur Red Brown dye, the invert sugar, used 3 times with respect to dye, gave the KIS value much closer to that with sodium sulphide. In all the other cases, the depths of dyeing were lower but without tonal variation.

Using different amounts of non-sulphide reducing agents from 2 times to 5 times the weight of dye showed the KIS values almost near to each other and

2 limes 3 limes 4 times S limes

Fructose Invert Sugar . Molasses Reducing Agent

Fig. I - Effect of reducing agents on KIS of Sulphur Military Green UI dyed cotton using soda ash

458 INDIAN J. FIBRE TEXT. RES., DECEMBER 2004

small variations were observed in the depth. Within these limits of use, no over or under reduction of the dye was observed, as evident from the absence of any tonal variation.

In another set of experiments, caustic soda (pH 1 2.5) was used for reducing the dyes with non­sulphide reducing agents at about 70 DC and then the dyeings were carried out at boil. The amounts of these reducing agents were varied up to 5 times with

8 !ill m mID

6 ETI

4

2

o L..-----R.LUL __ _

Sod. Sulphide Glucose

respect to the amount of dye, as in the earlier case. The colour values for three different dyes are given in Tables 3-5.

Table 3 shows that for Sulphur Military Green UI, none of the dyeings using non-sulphide reducing agents shows depth comparable to that observed by conventional sulphide dyeing. In the case when fructose was used 2 times with respect to dye, the

2 limes

3 times 4 times 5 times

Fructose Invert Sugar Molasses

Reducing Agent Fig. 2 - Effect of reducing agents on KIS of Sulphur Slate Grey Conc. dyed cotton using soda ash

8

6

CI) � 4

2

o Sod. Sulphide Glucose

!mil 2 limes

1m 3 limes

[ffi] 4 limes

mTI 5 limes:

Fructose Invert Sugar Molasses

Reducing Agent Fig. 3 - Effect of reducing agents on KIS of Sulphur Red Brown Conc. dyed cotton using soda ash

SHUKLA & PAl: SULPHUR DYEING USING NON-SULPHIDE REDUCING AGENTS 459

Table 3 - Colour values of cotton dyed with Sulphur Military Green VI in presence of caustic soda using non-sulphide reducing agents

Reducing agent Amount of reducing agent KIS M* l:1a* M* w.r.t. dye

Sodium sulphide 3 times 7.46

Glucose 2 times 2.52 1 9.03 -0.51 5.07

3 times 2.84 1 6.93 -0.57 4.69

4 times 3.07 14.53 -0.52 3. 1 5

5 times 3.57 1 1 .49 -0.49 2.42

Fructose 2 times 4.64 1 0.43 -0.24 5.56

3 times 3.79 12.61 -0.63 4.25

4 times 3. 1 3 13 . 1 2 -0.78 1 .08

5 times 3 . 17 1 6. 1 1 -0.68 5 . 14

Invert sugar 2 times 4.3 1 9.37 -0.32 3.2 1

3 times 3.75 14. 1 8 -0.30 7.02

4 times 4.33 9.7 1 -0.28 3.66

5 times 4.05 1 3.78 0. 16 7.82

Molasses 2 times 2.90 1 6.01 -0. 5 1 3.62

3 times 3.46 14.41 -0.09 5.48

4 times 3 . 1 8 13 .96 -0.53 3 . 12

5 times 3.78 1 1 .52 -0.50 2.96

Table 4 - Colour values of cotton dyed with Sulphur Slate Grey in presence of caustic soda using non-sulphide reducing agents

Reducing agent Amount of reducing agent KIS l:1L* l:1a* M* w.r.t. dye

Sodium sulphide 3 times 5.20

Glucose 2 times 1 .37 1 9.57 0. 1 7 0.89

3 times 1 .48 1 8.66 0.07 0.04

4 times 2.09 14.33 -2.62 3.92

5 times 2 . 10 13 . 10 -0.02 1 .26

Fructose 2 times 1 .55 1 7.61 0.20 1 .41

3 times 2.25 1 1 .69 0.33 2.34

4 times 1 .88 14.64 0. 1 3 1 .69

5 times 2.61 9.9 1 0. 1 5 0.59

Invert sugar 2 times 1 .08 22.8 1 0. 16 2.29

3 times 1 .47 1 8.57 0. 1 0 0.94

4 times 1 . 1 0 22.63 0.00 2.28

5 times 1 .7 1 1 6.35 -0.30 1 .59

Molasses 2 times 0.93 24.76 -0. 1 5 3.5 1

3 times 1 . 13 22.40 -0.20 1 .93

4 times 1 .36 19.63 -0.44 2.50

5 times 1 .7 1 15.86 -0.6 1 4.24

460 INDIAN J. FIBRE TEXT. RES . , DECEMBER 2004

Table 5 - Colour values of cotton dyed with Sulphur Red Brown in presence of caustic soda using non-sulphide reducing agents

Reducing agent Amount of reducing agent KIS W.r.t. dye

Sodium sulphide 3 times 7.01

Glucose 2 times 2.99

3 times 2.71

4 times 3.06

5 times 3.08

Fructose 2 times 2.85

3 times 3.37

4 times 3.28

5 times 3.54

Invert sugar 2 times 3.27

3 times 2.88

4 times 3.27

5 times 3.98

Molasses 2 times 2.30

3 times 2.54

4 times 2.53

5 times 3. 1 9

depth was about 60% of that obtained by using sodium sulphide. In all the other cases, the values were much lower. The !1L * values were significantly higher. Also, the tone became more yellow in many cases. Such effects were not observed when soda ash was used during reduction of dye.

Table 4 shows the colour values for cotton dyed with Sulphur Slate Grey Conc. Here, the use of fructose, 5 times with respect to dye, gave K/S value almost half to that obtained in conventional dyeing. In all the other cases, the values were still lower. Insignificant tonal variations were observed for all the dyeings, as indicated by the smaller values of !1a* and !1b* which were well within the tolerance limits.

In the case of Sulphur Red Brown Conc. (Table 5), the dyeing gave little more than half the

t:J.L* t:J.a* t:J.b*

13 .55 2.3 1 1 .82

15 .03 2. 1 8 1 .46

1 3.06 1 .68 2.34

1 2.7 1 0.89 1 .95

14.29 2.58 3 . 18

1 1 .5 1 1 .57 1 .85

1 1 .92 1 . 5 1 2.26

1 0.79 0.68 1 .26

1 2 . 1 8 2. 1 2 1 .34

13 .81 1 .30 1 .78

1 1 .66 0.87 3.20

8.6 1 -0.35 1 .87

1 7 .35 1 .9 1 2.43

1 6.36 3.43 1 .83

1 5.59 1 .08 3.61

1 1 .90 -0.35 1 .55

depth as compared to that in case of conventional dyeing when the dye was reduced using invert sugar 5 times with respect to the dye. In all the other cases, the KIS values were lower. As far as tonal variations are concerned, it may be observed that the matching of conventional and unconventional dyeing is satisfactory, giving the !1a* and !1b* values within the tolerance limits. All the reducing agents did not show tonal variations for this dyeing.

All the sulphide and non-sulphide dyeings showed comparable wash and crock fastness characteristics.

4 Conclusion

The non-sulphide reducing agents perform well w.r.t. matching of depth and tone of dyeing when

SHUKLA & PAl : SULPHUR DYEING USING NON-SULPHIDE REDUCING AGENTS 461

soda ash is used as an alkali at a pH of 10.5 . Caustic soda (pH 1 2.5) gives larger tonal vanatlOns, sometimes beyond the tolerance limits. Among the different non-sulphide reducing agents used, the invert sugar is found to be better for getting the proper level and tone of dyeing. Although molasses has its reduction power higher by 1 .5 times as compared to other non-sulphide reducing agents. it is not found to be effective in the reduction of sulphur dyes.

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