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

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


(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)

Dyeing Wool with Metal-free Dyes –

Sodium hydrosulphite

Sodium hydroxide solution (38°Bé)

Dispersing agent (Detergent NA-B) (g/L)

Method (Dyed in Turbomat 30 mins at 60°C)

Conc. Sodium Hydrosulphite (g/L)

> Dyed with 2g/L SBH; 8 g/L Bisulphite; 6 ml/L 38°Bé NaOH

for 30 mins in the Turbomat)

result in a lower level of fibre damage, as discussed later.

Dispersing Agent (Detergent NA-B) (g/L)

Conc. NaOH (ml/L of 38°Bé)

The Use of Sodium Borohydride for the Application of Vat Dyes to Wool 25

**Reagent/Conditions A B C D** 

pH after fabric added (40°C) 11.8 11.8 11.9 11.9 pH after 30 min at 60°C 11.3 11.2 11.5 11.5 Colour of fabric after 30 min at 60°C Pink/Purple Pink/Purple Purple Purple Colour of liquor after 30 min at 60°C Purple Purple Purple Purple Absorbance of final dyebath at 548 nm 1.08 0.88 0.43 0.71

> Water rinse pH 11.0 11.0 11.1 11.2 Soap off 2g/L Detergent NA-B pH 9.5 ammonia

Final soap off pH 10.0 9.9 10.0 10.0

Final pH of Dyebath

0.25 9.5 6.5 4.6 4.6

3 7 Nil 11.3 3.4 4.4 4.3 3 7 0.25 11.2 4.0 4.4 4.3 5 12 Nil 11.5 4.3 4.3 4.1 5 12 0.25 11.5 3.9 4.4 3.7

Table 6. Comparison of Colour Yield (K/S Values) and Rubbing Fastness of Samples Dyed with Vat Red 45 (1% oww) by the Hydrosulphite/NaOH and SBH Methods at 60°C (Dyed

Table 5. Vat Dyeing with Vat Red 45 (1% oww) by the Conventional Hydrosulphite/NaOH

Table 6 shows K/S and rubbing fastness data for samples dyed with Vat Red 45 and various concentrations of hydrosulphite and sodium hydroxide. The best results were obtained with the highest concentrations of chemicals (5g/L hydrosulphite and 12 ml/L NaOH solution) without a dispersing agent. Table 6 also shows that when Vat Red 45 was applied to wool by the optimised SBH method, the colour yield and rubbing fastness were superior to the values obtained by the conventional hydrosulphite procedure. Another advantage of the SBH method was the lower pH of the dye liquors at the end of the dyeing cycle (pH 8.5 - 9.0 compared with pH 11.5 for the hydrosulphite/NaOH method). This would be expected to

(g/L) 3 3 5 5

(ml/L) 7 7 12 12 pH of vat 12.2 12.2 12.3 12.3 Colour of vat Purple Purple Purple Purple

0.25 nil 0.25 nil

20 mins at 100°C

K/S Value at 520 nm

Rubbing Fastness Dry Wet

Of the four dispersing agents tested, Kieralon DB gave the worst results and was not investigated further. The other three products gave similar results. For these, only a very small concentration of dispersing agent was required in order to counteract the adverse effect of a low final dyebath pH. High concentrations of dispersing agent tended to reduce the colour yields. The optimum concentrations of Albigen A and PVP were 0.05 g/L, whereas for Detergent NA-B, the highest colour yields were obtained with 0.25g/L.

For all the dispersing agents studied, addition of sodium sulphate (5% oww) to the dyebath slightly decreased the colour yield. It appears, therefore, that with the SBH reducing system, there is a marked benefit with some vat dyes in using a small concentration of dispersing agent in the dyebath, in order to avoid the necessity of maintaining a high liquor pH throughout the dyeing cycle. The pH always dropped to some extent when the dyebath, set with the vatted dye, contacted the wool fabric. It was observed that the amount the pH changed varied from dye to dye. Although the reason for this variability is not known, it is possible that the finishing agents used in the formulation of the dyes give a buffering effect, in some cases. It was found that not all vat dyes showed this pH sensitivity when applied to wool. However, in order to offset any adverse effects caused by unpredictable pH changes, a dispersing agent was added to all dye liquors, as a standard part of setting the dyebath.

#### **3.3 Application of vat dyes to wool by the conventional sodium hydrosulphite/sodium hydroxide method**

In order to compare the SBH/bisulphite method with the conventional vat dyeing procedure, a series of fabric samples were dyed with either Vat Red 45 or Vat Green 1 (1% oww), following reduction to the respective leuco compound with sodium hydrosulphite and sodium hydroxide. As the dyestuff manufacturers' pattern cards give only very general information on the amounts of sodium hydrosulphite and sodium hydroxide required for effective reduction, the dyes were vatted by the method described in Section 2.2.1 with the two concentrations of the chemicals shown in Table 5. All the dye liquors contained sodium sulphate (5% oww), in accordance with normal practice on wool. Two sets of fabric samples were dyed with each formulation: one without a dispersing agent and one containing 0.25g/L Detergent NA-B. All the samples were soaped off in a similar manner with Detergent NA-B and ammonia.

It can be seen from Table 5 that all the concentrations of sodium hydrosulphite and NaOH reduced Vat Red 45 to the purple leuco compound. However, although the two dyebaths set with the lower concentrations of these chemicals remained purple up to the end of the exhaustion phase, the fabrics changed to a pink/purple during the dyeing cycle. This showed that some oxidation of the leuco compound to the pigment form of the dye had occurred during dye exhaustion, which indicates that insufficient hydrosulphite had been used. The results show that the most stable system regarding resistance to premature oxidation was the one containing 5 g/L hydrosulphite and 12 ml/L of the sodium hydroxide solution. This was considered to be the optimum concentrations of these chemicals for Vat Red 45, because in this case the dye was not oxidised until after the end of the exhaustion/fibre penetration phase of the dyeing cycle. These concentrations are very similar to those recommended for the application of vat dyes to wool/cotton blends (Lemin & Collins, 1959).

Of the four dispersing agents tested, Kieralon DB gave the worst results and was not investigated further. The other three products gave similar results. For these, only a very small concentration of dispersing agent was required in order to counteract the adverse effect of a low final dyebath pH. High concentrations of dispersing agent tended to reduce the colour yields. The optimum concentrations of Albigen A and PVP were 0.05 g/L, whereas for Detergent NA-B, the highest colour yields were obtained with

For all the dispersing agents studied, addition of sodium sulphate (5% oww) to the dyebath slightly decreased the colour yield. It appears, therefore, that with the SBH reducing system, there is a marked benefit with some vat dyes in using a small concentration of dispersing agent in the dyebath, in order to avoid the necessity of maintaining a high liquor pH throughout the dyeing cycle. The pH always dropped to some extent when the dyebath, set with the vatted dye, contacted the wool fabric. It was observed that the amount the pH changed varied from dye to dye. Although the reason for this variability is not known, it is possible that the finishing agents used in the formulation of the dyes give a buffering effect, in some cases. It was found that not all vat dyes showed this pH sensitivity when applied to wool. However, in order to offset any adverse effects caused by unpredictable pH changes, a dispersing agent was added to all

**3.3 Application of vat dyes to wool by the conventional sodium hydrosulphite/sodium** 

In order to compare the SBH/bisulphite method with the conventional vat dyeing procedure, a series of fabric samples were dyed with either Vat Red 45 or Vat Green 1 (1% oww), following reduction to the respective leuco compound with sodium hydrosulphite and sodium hydroxide. As the dyestuff manufacturers' pattern cards give only very general information on the amounts of sodium hydrosulphite and sodium hydroxide required for effective reduction, the dyes were vatted by the method described in Section 2.2.1 with the two concentrations of the chemicals shown in Table 5. All the dye liquors contained sodium sulphate (5% oww), in accordance with normal practice on wool. Two sets of fabric samples were dyed with each formulation: one without a dispersing agent and one containing 0.25g/L Detergent NA-B. All the samples were soaped off in a similar manner with

It can be seen from Table 5 that all the concentrations of sodium hydrosulphite and NaOH reduced Vat Red 45 to the purple leuco compound. However, although the two dyebaths set with the lower concentrations of these chemicals remained purple up to the end of the exhaustion phase, the fabrics changed to a pink/purple during the dyeing cycle. This showed that some oxidation of the leuco compound to the pigment form of the dye had occurred during dye exhaustion, which indicates that insufficient hydrosulphite had been used. The results show that the most stable system regarding resistance to premature oxidation was the one containing 5 g/L hydrosulphite and 12 ml/L of the sodium hydroxide solution. This was considered to be the optimum concentrations of these chemicals for Vat Red 45, because in this case the dye was not oxidised until after the end of the exhaustion/fibre penetration phase of the dyeing cycle. These concentrations are very similar to those recommended for the application of vat dyes to wool/cotton blends (Lemin

dye liquors, as a standard part of setting the dyebath.

0.25g/L.

**hydroxide method** 

Detergent NA-B and ammonia.

& Collins, 1959).


Table 5. Vat Dyeing with Vat Red 45 (1% oww) by the Conventional Hydrosulphite/NaOH Method (Dyed in Turbomat 30 mins at 60°C)

Table 6 shows K/S and rubbing fastness data for samples dyed with Vat Red 45 and various concentrations of hydrosulphite and sodium hydroxide. The best results were obtained with the highest concentrations of chemicals (5g/L hydrosulphite and 12 ml/L NaOH solution) without a dispersing agent. Table 6 also shows that when Vat Red 45 was applied to wool by the optimised SBH method, the colour yield and rubbing fastness were superior to the values obtained by the conventional hydrosulphite procedure. Another advantage of the SBH method was the lower pH of the dye liquors at the end of the dyeing cycle (pH 8.5 - 9.0 compared with pH 11.5 for the hydrosulphite/NaOH method). This would be expected to result in a lower level of fibre damage, as discussed later.


Table 6. Comparison of Colour Yield (K/S Values) and Rubbing Fastness of Samples Dyed with Vat Red 45 (1% oww) by the Hydrosulphite/NaOH and SBH Methods at 60°C (Dyed for 30 mins in the Turbomat)

Dyeing Wool with Metal-free Dyes –

applied with SBH/bisulphite.

**3.4 Effect of using a buffered dyebath** 

was used in all further dyeings.

Conc. trisodium phosphate in dyebath

at 60°C; 0.05 g/L Albigen A added to dyebath)

system.

The Use of Sodium Borohydride for the Application of Vat Dyes to Wool 27

exhaustion phase of the dyeing cycle. This again indicated that insufficient hydrosulphite had been used. The results show that the most stable system regarding resistance to premature oxidation was again the one containing 5 g/L hydrosulphite and 12 ml/L NaOH. In contrast to the finding for Vat Red 45, in this case, the addition of a dispersing agent produced better results for colour yield and rubbing fastness (Table 8). This is similar to the finding for this dye

Table 8 compares results for Vat Green 1 applied by the hydrosulphite/NaOH method with results obtained by the optimised SBH method. Thus, as found for Vat Red 45, the SBH method gave a much better colour yield and slightly better rubbing fastness than the conventional procedure using hydrosulphite and NaOH. Again, it should be noted that the final liquor pH of the SBH dyebath was significantly less than for the hydrosulphite/NaOH

Results discussed above show that with some dyes (e.g. Vat Green 1) the colour yield can be adversely affected if the pH of the dyebath falls below a certain value during the exhaustion stage. Despite this effect being decreased by addition of a selected dispersing agent to the dyebath, it was considered that the reproducibility/robustness of the system would be improved by buffering the pH of the dyebath. After examining possible alternatives, trisodium phosphate was selected for further study. This compound has been claimed to produce less fibre damage than other alkalis (Bird, 1947). Table 9 shows the colour yields and rubbing fastness results obtained by adding three concentrations of trisodium phosphate to the dyebath. A concentration of 2g/L trisodium phosphate maintained the pH slightly above pH 9.5 and gave the best colour yield. This amount of trisodium phosphate

> Reagent/Conditions **A B C D**  pH of vat 11.9 11.9 11.9 11.9 Colour of vat Blue Blue Blue Blue

pH After fabric added (40°C) 11.2 11.2 11.2 11.2 pH after 30 min at 60°C 9.6 9.7 10.1 10.3 Colour of liquor after 30 min at 60°C Blue Blue Blue Blue Absorbance of final dyebath at 620 nm 1.5 1.6 2.2 2.4

(g/L) 1 2 3 4

Water rinse pH 10.5 10.5 10.6 10.7

Final soap off pH 9.5 9.5 9.4 9.5 K/S at 640 nm 8.8 11.3 9.4 10.2 Dry Rubbing 4.5 4.6 4.5 4.5 Wet Rubbing 4.0 3.8 4.0 3.8

Table 9. Effect of Buffering the Dyebath with Trisodium Phosphate on the Colour Yield and

(2g/L SBH; 8g/L sodium bisulphite; 6 ml/L of 38° Bé NaOH) (Dyed in Turbomat for 30 min

Rubbing Fastness of Vat Green 1 (1% oww) Applied to Wool by the SBH Method

Soap off 2g/L Detergent NA-B pH 9.5 ammonia 20 mins at

100°C


The results obtained when Vat Green 1 was applied by the conventional hydrosulphite/NaOH method are shown in Tables 7 and 8.

Table 7. Vat Dyeing with Vat Green 1 (1% oww) by the Hydrosulphite Method (Dyed for 30 mins in the Turbomat at 70°C)


Table 8. Comparison of Colour Yield (K/S Values) and Rubbing Fastness of Samples Dyed with Vat Green 1 (1% oww) by the Hydrosulphite and SBH Methods at 70°C (Dyed for 30 mins in the Turbomat)

Table 7 shows that all the concentrations of hydrosulphite and NaOH reduced Vat Green 1 to the blue leuco compound. However, as discussed above for Vat Red 45, the dyebaths set with the two lowest concentrations of these chemicals were oxidised to some extent during the exhaustion phase of the dyeing cycle. This again indicated that insufficient hydrosulphite had been used. The results show that the most stable system regarding resistance to premature oxidation was again the one containing 5 g/L hydrosulphite and 12 ml/L NaOH. In contrast to the finding for Vat Red 45, in this case, the addition of a dispersing agent produced better results for colour yield and rubbing fastness (Table 8). This is similar to the finding for this dye applied with SBH/bisulphite.

Table 8 compares results for Vat Green 1 applied by the hydrosulphite/NaOH method with results obtained by the optimised SBH method. Thus, as found for Vat Red 45, the SBH method gave a much better colour yield and slightly better rubbing fastness than the conventional procedure using hydrosulphite and NaOH. Again, it should be noted that the final liquor pH of the SBH dyebath was significantly less than for the hydrosulphite/NaOH system.
