**3. Results and discussion**

#### **3.1 Determination of optimum concentration of SBH/bisulphite**

Important requirements for a satisfactory vat dyeing are:


Sodium bisulphite was dissolved in 250mL of water at room temperature, followed by the addition of the aqueous solution of sodium borohydride (SBH) diluted with ten times its volume of water. After 2 minutes, an aqueous solution of sodium hydroxide (320 g/L or 38°Bé) was added and the mixture stirred until the effervescence had ceased (usually 5-10 minutes). Stirring was continued while the vat dye was added slowly and also while the mixture was heated at 2°C/min to the vatting temperature, where it was maintained for 30

After diluting to the required volume, the vatted liquor was added to the dyeing pot containing the fabric. The liquor ratio was 20:1 The liquor was heated at 1.5°C per minute to the required temperature (usually 60°C or 70°C), where it was held for 30 minutes. The fabric was overflow rinsed (cold) for one minute, followed by two five minute rinses at 40°C. Oxidation of the leuco compound to the vat pigment was carried out by treatment with hydrogen peroxide (1g/L) for 10 minutes at 50°C. The fabric was then soaped off (the normal procedure with vat dyes (Latham, 1995; Trotman, 1984; Bird, 1947)) with Detergent NA-B (2g/L), adjusted to pH 9.5 with ammonium hydroxide, for 20 minutes at 98°C. After cooling, the fabric was rinsed and removed from the dyeing machine. It was then rinsed, with hand stirring, for 5 minutes in a beaker containing a solution of Detergent NA-B (1 g/L) at 50°C (liquor ratio 50:1). This treatment was considered to simulate the process of washing-off a fabric in a scouring machine or back-washing wool top after dyeing. It was noted that the gentle mechanical action involved in this step removed a small amount of

Dyebath exhaustion levels were determined by measuring the absorbance of the dyebath on a Jasco V530 UV-Vis Spectrophotometer at the wavelength of maximum absorbance of the dye. Colour yields were determined by measuring the K/S values of the dyed samples on a Datacolor Texflash Spectrophotometer at the wavelength of maximum reflectance of the dye. Dry and wet rubbing fastness was assessed by *IWS Test Method 165 – Fastness to Rubbing*.

Grey scale staining and colour changes were measured on a Datacolor Texflash Spectrophotometer. The software supplied with the instrument (Datacolor Iris Version 2.3)

Wet burst strength was measured according to *Australian Standard AS2001.2.4A-90, Determination of Burst Pressure of Textile Fabrics, Hydraulic Diaphragm Method* (which is

iii. maintaining the leuco compound in a soluble form during the dye exhaustion phase.

oxidised, insoluble pigment from between the fibres and yarns in the fabric.

Alkaline perspiration was assessed by *ISO-105-EO4 – Fastness to Perspiration.* Washing fastness was assessed by ISO-105-CO2 – Colour Fastness to Washing.

equivalent to ASTM D3787-01 but also includes procedures for wet testing).

i. complete reduction of the dye to the leuco compound during vatting;

**3.1 Determination of optimum concentration of SBH/bisulphite** 

ii. prevention of premature oxidation of the leuco compound;

Important requirements for a satisfactory vat dyeing are:

enabled ratings to be quoted to 0.1 of a greyscale unit.

**3. Results and discussion** 

**2.4.2 Sodium borohydride/bisulphite method** 

minutes.

**2.5 Fabric dyeing** 

**2.6 Measurements** 

A conventional vat dyeing system uses a mixture of sodium hydrosulphite and sodium hydroxide to reduce the dyestuff to its leuco compound. It has been estimated that the stoichiometric relationship between sodium hydrosulphite and SBH is that 1g/L hydrosulphite is equivalent to 0.44 g/L of solid SBH (Rohm and Haas Technical Information, 2007). This, however, provides only an approximate guide to the amount of SBH required for vat dyeing, as the dye manufactures' pattern cards contain only general information on the amount of hydrosulphite required (Weber, 1951).


Table 1. Effect of Concentration of Sodium Borohydride and Sodium Bisulphite in the Application of Vat Red 45 (1% oww) to Wool (Dyed in the Turbomat for 30 mins at 60°C)

The optimum concentrations of SBH, sodium bisulphite and sodium hydroxide required to produce a satisfactory dyeing were, therefore, determined experimentally. A ratio of sodium bisulphite to SBS solution of 4:1 was used because, as discussed previously, this has been found to be suitable for the application of indigo to cotton. Table 1 shows the various concentrations of SBH, sodium bisulphite and sodium hydroxide used to reduce the dye Vat Red 45 to its leuco compound prior to exhaustion onto wool. The samples were soaped off after dyeing with 2g/L Detergent NA-B at pH 9.5 (obtained with ammonium hydroxide) for 20 min at 98°C. Although all the formulations reduced the dye to its purple, soluble leuco form, the three mixtures containing the highest concentrations of reagents gave solutions that were more stable than the one containing the least amounts of the chemicals. The stability was judged by observing the formation of partially oxidised (green) pigment on the liquor surface, after stirring had been stopped. The fully oxidised pigment was red in colour. Other experiments with the lowest concentration of chemicals (i.e. 1 g/L SBH and 4 g/L sodium bisulphite) showed some variability in the reproducibility of the vatting process, in particular with respect to the sensitivity to stirring rate. Furthermore, with liquors vatted with 1g/L SBH and 4 g/L sodium bisulphite, there was a tendency for slight oxidation to occur during the dyeing cycle.

Table 2 shows data for colour yield (K/S) and fastness to dry and wet rubbing. There was no significant difference in rubbing fastness between any of the samples (rubbing fastness gives an indication of the amount of oxidised dye remaining on the fibre surface). There was also no significant difference between the K/S values obtained with the two lowest concentrations of SBH and bisulphite. However, the K/S values decreased with increasing concentration of chemicals above these levels. This was possibly caused by destruction of the chromaphore by over-reduction of the dye. The highest colour yield consistent with

Dyeing Wool with Metal-free Dyes –

The Use of Sodium Borohydride for the Application of Vat Dyes to Wool 23

this is very sensitive to pH within the range used in these experiments (in this study we

In order to test this hypothesis, fabric samples were dyed with Vat Green 1 in the presence of a dispersing agent. Detergent NA-B was used because it was considered that this compound would be an effective dispersing agent for vat dyes as it is recommended for soaping off vat-dyed cotton after oxidation. Table 3 shows that in the presence of 0.25 g/L of Detergent NA-B, a high colour yield was obtained, even when the pH of the dyebath had dropped to pH 8.6 at the end of the dyeing cycle. The results also show that, with a dispersing agent in the dyebath, maintaining the pH to a high value (pH 11.6) by alkali addition had little effect on colour yield. Following these results, the effect of Detergent

Table 4 shows the effect of various dispersing agents added to the dyebath on the colour yield of Vat Green 1. Polyvinylpyrrolidone (PVP) was included in the evaluation because it is the main constituent of the commercial product Albigen A. The results from Table 3 for

None 11.0 60 5 Blue 0.68 9.5 11.0 None 10.8 60 5 Blue 0.21 9.1 6.0 0.05 A 11.1 70 0 Blue/Green 0.82 9.0 12.3 0.1 A 11.0 70 0 Blue/Green 1.29 8.9 11.2 0.1 A 11.1 70 5 Blue 0.88 8.7 10.1 0.2 A 11.0 70 0 Blue/Green 1.72 8.9 9.7 0.25 A 11.0 70 0 Blue/Green 1.67 8.9 10.3 0.5 A 11.0 70 0 Blue/Green 3.11 8.8 8.3 0.05 PVP 11.1 70 0 Blue 1.40 8.7 10.6 0.1 PVP 11.1 70 0 Blue 2.24 8.6 10.0 0.1 PVP 11.2 70 5 Blue 2.25 8.7 9.5 0.15 PVP 11.0 70 0 Blue 2.73 8.5 8.3 0.20 PVP 11.0 70 0 Blue 3.30 8.4 7.1 0.25 NA-B 11.0 70 0 Blue/Green 0.96 8.9 13.2 0.25 NA-B 11.2 70 5 Blue 0.70 8.6 10.9 0.5 NA-B 11.5 70 0 Blue 1.05 9.1 10.7 1.0 NA-B 11.5 70 0 Blue 0.53 9.2 6.1 0.25 KDB 11.0 70 0 Blue/Green 0.53 8.9 8.6 0.5 KDB 11.6 70 0 Blue 1.51 9.4 8.6 0.5 KDB 11.1 70 5 Blue 0.68 8.7 7.1 1.0 KDB 11.5 70 0 Blue 3.16 9.2 7.1

Final Dyebath Colour

Absorbance at 552 nm

Final Dyebath pH

K/S (at 640 nm)

two samples dyed without a dispersing agent are included for comparison purposes.

Sodium Sulphate in dyebath (% oww)

tried to minimise alkaline damage to the wool by keeping the pH as low as possible).

NA-B and other dispersing agents was examined further.

Dyeing Temp.

(Dyed for 30 mins in the Turbomat at various temperatures).

A is Albigen A; KDB is Kieralon DB; NA-B is Detergent NA-B; PVP is polyvinyl pyrrolidone.

Table 4. Effect of Various Dispersing Agents Added to the Dyebath (1% oww Vat Green 1)

**3.2 Effect of dispersing agent in the dyebath** 

Initial Dyebath pH

Dispersing agent (g/L)


good stability of the vatted liquor was obtained with a concentration of 2g/L SBH and 8g/L sodium bisulphite.

Table 2. Colour Yield (K/S Values) and Rubbing Fastness of Wool Fabrics Dyed with Vat Red 45 (1% oww) by the Sodium Borohydride/Bisulphite Method

The combination of 8 g/L sodium bisulphite and 2 g/L SBH, found to be the optimum amounts to effectively reduce Vat Red 45, was used with the dyestuff Vat Green 1. The data in Table 3 show that the blue leuco form of the dye was maintained until the end of the exhaustion phase. This confirmed that these concentrations of SBH and bisulphite were also satisfactory for this dyestuff.


Table 3. Vat Dyeing with Vat Green 1 (1% oww) by the SBH Method (Dyed for 30 mins in the Turbomat at various temperatures)

Table 3 shows, however, that in contrast to the result obtained with Vat Red 45, the colour yield for Vat Green 1 was dependent on the final dyebath pH, with the highest value obtained when the pH was greater than 9.5. Diffusion into the fibre did not appear to be a factor, as increasing the dyebath temperature above 60°C did not improve the colour yield. It is also unlikely that this effect was due to premature oxidation of the dyebath, because all the liquors remained blue (indicative of the reduced leuco form) throughout the whole exhaustion stage. Furthermore, the poor colour yields cannot be explained by lower levels of dyebath exhaustion, because the absorbance values in Table 3 show that the samples with the lower colour yields had higher dyebath exhaustions. A possible explanation is that the aggregation state of the leuco compound is an important factor; and that with some dyes

good stability of the vatted liquor was obtained with a concentration of 2g/L SBH and 8g/L

1 4 8.8 6.6 4.7 4.3 2 8 8.8 6.5 4.6 4.6 4 16 9.5 5.2 4.6 4.3 5 20 10.2 4.1 4.5 4.8 Table 2. Colour Yield (K/S Values) and Rubbing Fastness of Wool Fabrics Dyed with Vat

The combination of 8 g/L sodium bisulphite and 2 g/L SBH, found to be the optimum amounts to effectively reduce Vat Red 45, was used with the dyestuff Vat Green 1. The data in Table 3 show that the blue leuco form of the dye was maintained until the end of the exhaustion phase. This confirmed that these concentrations of SBH and bisulphite were also

None 60 Blue 11.0 9.5 Blue 0.68 11.0 None 60 Blue 10.8 9.1 Blue 0.21 6.0 None 70 Blue 10.7 9.1 Blue 0.18 7.8 None 80 Blue 11.0 9.0 Blue 0.21 4.8 None 90 Blue 10.9 9.1 Blue 0.08 4.0 0.25 70 Blue 11.2 8.6 Blue 0.70 10.9 0.25 70 Blue 11.0 11.6 Blue 1.66 10.9 Vatted with : 2 g/L SBH; 8 g/L sodium bisulphite; 6 ml/L caustic soda (38°Bé). The dyebaths contained sodium sulphate (5% oww). The samples were soaped off for 20 min at 100°C with 2g/L Detergent NA-B at pH 9.5. Table 3. Vat Dyeing with Vat Green 1 (1% oww) by the SBH Method (Dyed for 30 mins in

Table 3 shows, however, that in contrast to the result obtained with Vat Red 45, the colour yield for Vat Green 1 was dependent on the final dyebath pH, with the highest value obtained when the pH was greater than 9.5. Diffusion into the fibre did not appear to be a factor, as increasing the dyebath temperature above 60°C did not improve the colour yield. It is also unlikely that this effect was due to premature oxidation of the dyebath, because all the liquors remained blue (indicative of the reduced leuco form) throughout the whole exhaustion stage. Furthermore, the poor colour yields cannot be explained by lower levels of dyebath exhaustion, because the absorbance values in Table 3 show that the samples with the lower colour yields had higher dyebath exhaustions. A possible explanation is that the aggregation state of the leuco compound is an important factor; and that with some dyes

Dyebath pH with Fabric (40°C)

K/S Value at 520 nm

> Final Dyebath pH

Final Dyebath Colour

Rubbing Fastness Dry Wet

> Absorb -ance of final dyebath

K/S at 640 nm

Final Dyebath pH

Red 45 (1% oww) by the Sodium Borohydride/Bisulphite Method

Vat Colour

sodium bisulphite.

satisfactory for this dyestuff.

Dyeing Temp (°C)

the Turbomat at various temperatures)

Conc. Dispersing Agent NA-B (g/L)

Conc. Sodium. Bisulphite (g/L)

Conc. SBH (12%) (g/L)

this is very sensitive to pH within the range used in these experiments (in this study we tried to minimise alkaline damage to the wool by keeping the pH as low as possible).

In order to test this hypothesis, fabric samples were dyed with Vat Green 1 in the presence of a dispersing agent. Detergent NA-B was used because it was considered that this compound would be an effective dispersing agent for vat dyes as it is recommended for soaping off vat-dyed cotton after oxidation. Table 3 shows that in the presence of 0.25 g/L of Detergent NA-B, a high colour yield was obtained, even when the pH of the dyebath had dropped to pH 8.6 at the end of the dyeing cycle. The results also show that, with a dispersing agent in the dyebath, maintaining the pH to a high value (pH 11.6) by alkali addition had little effect on colour yield. Following these results, the effect of Detergent NA-B and other dispersing agents was examined further.
