**3.4 Effect of using a buffered dyebath**

26 Textile Dyeing

The results obtained when Vat Green 1 was applied by the conventional

**Reagent/Conditions A B C D** 

pH After fabric added (40°C) 11.4 11.4 11.5 11.5 Final dyebath pH 11.0 11.0 11.5 11.5 Colour of fabric after 30 min at 70°C Green Green Blue Blue/Green Colour of liquor at after 30 min at 70°C Green Green Blue Green Absorbance of final dyebath at 558 nm 1.09 0.63 2.87 1.16

Final soap off pH 9.9

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

Water rinse pH 10.5 10.6 10.8 10.9 pH of Oxidation 9.5 9.2 9.9 9.9

Table 7. Vat Dyeing with Vat Green 1 (1% oww) by the Hydrosulphite Method (Dyed for 30

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

3 7 Nil 11.0 7.9 4.4 3.4 3 7 0.25 11.0 5.1 3.8 2.6 5 12 Nil 11.5 7.5 4.2 3.2 5 12 0.25 11.5 9.8 4.4 3.5

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

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

Final pH of Dyebath

(g/L) 3 3 5 5

(ml/L) 7 7 12 12 pH of vat 11.9 11.9 12.0 12.0 Colour of vat Blue Blue Blue Blue

0.25 Nil 0.25 Nil

20 mins at 100°C

0.25 8.6 11.3 4.6 3.8

K/S Value at 640 nm

Rubbing Fastness

Dry Wet

hydrosulphite/NaOH method are shown in Tables 7 and 8.

Sodium Hydrosulphite

NaOH (38°Bé)

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

mins in the Turbomat at 70°C)

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

mins in the Turbomat)

Conc. NaOH (ml/L of 38°Bé)

Conc. Sodium Hydrosulphite (g/L)

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 was used in all further dyeings.


Table 9. Effect of Buffering the Dyebath with Trisodium Phosphate on the Colour Yield and Rubbing Fastness of Vat Green 1 (1% oww) Applied to Wool by the SBH Method (2g/L SBH; 8g/L sodium bisulphite; 6 ml/L of 38° Bé NaOH) (Dyed in Turbomat for 30 min at 60°C; 0.05 g/L Albigen A added to dyebath)

Dyeing Wool with Metal-free Dyes –

hydrosulphite.

Vat Dye (Colour Index Number)

Red 1

Red 10

Red 45

Green 1

Green 3

Yellow 5

Violet 18

Orange 5

Brown 24

Methods

(Table 11 is shown in colour in the on-line version of the paper)

Table 11. Wool Fabrics Dyed with Vat Dyes (1% oww dye) by the SBH and Hydrosulphite

wool by the hydrosulphite/caustic soda method.

The Use of Sodium Borohydride for the Application of Vat Dyes to Wool 29

using sodium hydrosulphite and sodium hydroxide. Table 10 presents results for colour yield and fastness to washing, alkaline perspiration and rubbing for 9 vat dyes applied by the optimised SBH method. For comparison, results are also shown for the dyes applied to

The data in Table 10 confirm the results obtained with Vat Red 45 and Vat Green 1, discussed above. Thus, for all nine dyes, the SBH/bisulphite system gave better colour yields than were obtained by the conventional method using sodium hydrosulphite and sodium hydroxide. The differences in colour yields can also be seen in Table 11. Furthermore, Table 10 also shows that accompanying the higher colour yields, the SBH/bisulphite system gave similar or slightly better overall fastness properties than

Hydrosulphite Method SBH Method

#### **3.5 Optimisation of dye fastness**

When cotton is dyed with vat dyes, the dyed samples are soaped off to remove oxidised pigment from the fibre surface and to aggregate the pigment particles inside the fibre. Both these effects improve overall fastness properties (Latham, 1995; Trotman, 1984; Bird, 1947; McNeil et al, 2005). It was observed during the early part of this work that some loose pigment remained on the fabric surface, even after wool fabrics had been soaped off for 20 minutes at 98°C. It was also found that this had an adverse effect on fastness properties. Unlike normal wool dyes, vat pigments are insoluble in water after they have been oxidised. It is suggested that a dyeing machine such as the Turbomat is not very effective in removing surface pigment, because its circulation action, which involves pumping liquor through the fabric, will tend to filter any pigment particles removed in the wash off. Washing-off in equipment such as a scouring machine would be expected to be more effective in removing pigment particles trapped within the yarns. In the present study, in order to produce dyed fabrics with optimum fastness properties, after soaping-off in the dyeing machine, all fabrics were rinsed with hand stirring in a beaker containing 1 g/L Detergent NA-B, as described in Section 2. This treatment was considered to provide a laboratory simulation of fabric scouring for piece goods, or backwashing in the case of wool that had been top dyed. Such a treatment should be part of any procedure for applying vat dyes to wool by the new SBH method.


SC- Shade Change; W – Stain on Wool; C – Stain on Cotton; N – Stain on Nylon

Table 10. Colour Yield and Fastness to Wet Treatments and Rubbing of Vat Dyes Applied by the SBH and Hydrosulphite/NaOH Methods at 60°C (1% oww dye).

#### **3.6 Colour yield and fastness of vat dyes applied by the SBH and hydrosulphite methods**

The data in Section 3.3 for Vat Red 45 and Vat Green 1 show that the SBH method gave better colour yields and better, or similar rubbing fastness than the conventional method

When cotton is dyed with vat dyes, the dyed samples are soaped off to remove oxidised pigment from the fibre surface and to aggregate the pigment particles inside the fibre. Both these effects improve overall fastness properties (Latham, 1995; Trotman, 1984; Bird, 1947; McNeil et al, 2005). It was observed during the early part of this work that some loose pigment remained on the fabric surface, even after wool fabrics had been soaped off for 20 minutes at 98°C. It was also found that this had an adverse effect on fastness properties. Unlike normal wool dyes, vat pigments are insoluble in water after they have been oxidised. It is suggested that a dyeing machine such as the Turbomat is not very effective in removing surface pigment, because its circulation action, which involves pumping liquor through the fabric, will tend to filter any pigment particles removed in the wash off. Washing-off in equipment such as a scouring machine would be expected to be more effective in removing pigment particles trapped within the yarns. In the present study, in order to produce dyed fabrics with optimum fastness properties, after soaping-off in the dyeing machine, all fabrics were rinsed with hand stirring in a beaker containing 1 g/L Detergent NA-B, as described in Section 2. This treatment was considered to provide a laboratory simulation of fabric scouring for piece goods, or backwashing in the case of wool that had been top dyed. Such a treatment should be part of

any procedure for applying vat dyes to wool by the new SBH method.

Red 1 Hydrosulphite 5.3 3.7 5.0 4.8 4.9 4.2 3.4

Red 10 Hydrosulphite 4.5 4.2 4.9 4.7 4.8 4.3 3.5

Red 45 Hydrosulphite 3.9 4.7 4.9 4.7 3.3 4.4 3.7

Green 1 Hydrosulphite 9.8 2.4 4.9 4.8 4.8 4.4 3.5

Green 3 Hydrosulphite 1.9 4.5 4.9 4.8 4.8 4.4 4.1

Yellow 5 Hydrosulphite 1.5 2.8 4.9 4.8 4.9 4.5 4.3

Violet 18 Hydrosulphite 8.8 2.9 4.9 4.8 4.9 4.5 3.9

Orange 5 Hydrosulphite 6.1 3.5 4.9 4.8 4.9 4.3 3.8

Brown 24 Hydrosulphite 0.75 3.4 4.9 4.8 4.7 4.4 4.1

SC- Shade Change; W – Stain on Wool; C – Stain on Cotton; N – Stain on Nylon

the SBH and Hydrosulphite/NaOH Methods at 60°C (1% oww dye).

Method K/S Washing Fastness Rubbing

SBH 5.7 4.4 5.0 4.7 4.8 4.6 3.9

SBH 5.5 4.3 4.9 4.7 4.7 4.5 4.0

SBH 6.5 4.8 4.9 4.6 3.8 4.6 4.6

SBH 11.3 3.0 4.9 4.8 4.9 4.6 3.8

SBH 2.9 4.1 4.8 4.8 4.8 4.5 4.3

SBH 7.6 3.1 4.9 4.8 4.9 4.4 4.3

SBH 12.9 2.7 4.8 4.7 4.6 4.6 4.0

SBH 12.4 4.2 4.9 4.8 4.9 4.4 3.4

SBH 2.2 3.9 4.9 4.7 4.6 4.5 4.3

**3.6 Colour yield and fastness of vat dyes applied by the SBH and hydrosulphite** 

Table 10. Colour Yield and Fastness to Wet Treatments and Rubbing of Vat Dyes Applied by

The data in Section 3.3 for Vat Red 45 and Vat Green 1 show that the SBH method gave better colour yields and better, or similar rubbing fastness than the conventional method

Fastness

SC W C N Dry Wet SC W C N

Alkaline Perspiration Fastness

All Rated 5

**3.5 Optimisation of dye fastness** 

Vat Dye (Colour Index Number)

**methods** 

using sodium hydrosulphite and sodium hydroxide. Table 10 presents results for colour yield and fastness to washing, alkaline perspiration and rubbing for 9 vat dyes applied by the optimised SBH method. For comparison, results are also shown for the dyes applied to wool by the hydrosulphite/caustic soda method.

The data in Table 10 confirm the results obtained with Vat Red 45 and Vat Green 1, discussed above. Thus, for all nine dyes, the SBH/bisulphite system gave better colour yields than were obtained by the conventional method using sodium hydrosulphite and sodium hydroxide. The differences in colour yields can also be seen in Table 11. Furthermore, Table 10 also shows that accompanying the higher colour yields, the SBH/bisulphite system gave similar or slightly better overall fastness properties than hydrosulphite.


(Table 11 is shown in colour in the on-line version of the paper)

Table 11. Wool Fabrics Dyed with Vat Dyes (1% oww dye) by the SBH and Hydrosulphite Methods

Dyeing Wool with Metal-free Dyes –

be destroyed by over reduction.

Dyeing Method

**4. Conclusions** 

disulphide and peptide covalent bonds (Lewis, 1989).

Dyeing Temp. (°C)

Hydrosulphite/NaOH and SBH Methods

had a bursting force of 286N (equivalent to a strength retention of 86%).

Final Dyebath pH

Table 13. Wet Burst Strength of Wool Fabrics Dyed with Vat Dyestuffs by

Undyed ---- ---- 330 100 Hydrosulphite 60 11.5 193 58 SBH 60 8.9 256 78 Hydrosulphite 70 11.5 223 67 SBH 70 8.9 242 73

The feasibility of using a reducing system based on sodium borohydride and sodium bisulphite to apply vat dyes to wool has been demonstrated. For a range of nine vat dyes, better colour yields and overall better fastness properties were obtained by the

The Use of Sodium Borohydride for the Application of Vat Dyes to Wool 31

As can be seen from Table 12, the addition of extra reducing agent improved the colour yield of samples dyed in the Mathis machine. The significance of this result is that it provides an indication of the possible behaviour of the SBH system when wool is dyed with vat colours in different types of machines. Thus, the results obtained in the Turbomat can be considered to relate to the behaviour of top, loose stock, package and beam dyeing equipment, where liquor is pumped through the substrate and very little mixing with air occurs. The action of the Mathis, however, can be considered to simulate that of a jet, winch or hank dyeing machine, all of which allow a high degree of contact between the dyebath and atmospheric oxygen. It is clear from these results that when this type of equipment is used, the bath sharpening technique should be employed. It is inadvisable to use extra reagents during the dyestuff vatting step, because with some vat dyes the chromophore can

**3.8 Wool fibre damage caused by SBH and hydrosulphite/NaOH vat dyeing methods**  Wool fibre damage was assessed by comparing the wet burst strength (Lewis, 1989) of fabrics after dyeing with vat dyes, by either the optimised SBH or optimised hydrosulphite/NaOH methods, with the values for the undyed fabrics. In a wet wool fabric, ionic interactions (salt linkages) and hydrogen bonds within fibres are largely disrupted and their stabilising effect on wool structure is considerably diminished. Wet burst strength is, therefore, particularly useful because it provides an indication of cleavage of both

The data in Table 13 show that at dyeing temperature of 60°C and 70°C, the optimised SBH method produced less fibre damage than the standard procedure using hydrosulphite and NaOH. This can be attributed largely to the much lower final dyebath pH obtained with SBH. Fibre damage is, however, determined by both temperature and pH; and at a dyeing temperature of 70°C, the difference in fibre damage was lower than at 60°C. As it has been shown that dyed fabrics with good colour yield and fastness properties can be obtained at 60°C, it is concluded that this temperature should be used for the application of vat dyes to wool by the SBH/bisulphite system. For comparison purposes, a fabric sample was also dyed with a pre-metallised dye (1% oww) at pH 5 for 45 minutes at 98°C. The dyed fabric

> Wet Burst Strength Bursting Force (N) Strength Retention (%)

#### **3.7 Effect of type of dyeing machine**

In a long liquor dyeing process, interchange of the dyebath liquor with the substrate is important in order to ensure a constant supply of dyestuff molecules to the fibre surface. This can be achieved either by pumping the liquor through a stationary material, moving the substrate though the liquor, or moving both the liquor and material through the machine. Anionic wool dyes applied under acidic conditions have a high substantivity for wool as a result of ionic attraction between the anionic dye molecules and protonated amino groups in the fibre. For this reason, the type of liquor circulation used in the dyeing machine is not an important factor in the uptake of most types of wool dyes. Vat dyes, however, are applied to wool at a relatively high pH, where the fibre is negatively charged. Thus, in this case the substantivity of the dye will be dependent largely on non-polar/hydrophobic interactions rather than on ionic attraction. It is possible, therefore, that the more efficient liquor interchange in the Turbomat, involving pumping the liquor through the fabric, may result in better dyebath exhaustion than in a machine such as the Mathis Labomat, where liquor and fabric are tumbled around together. Another factor that may be important in the Mathis machine is that the constant mixing of air with the liquor could result in premature oxidation of the leuco compound. This could result in precipitation of the dyestuff in the dyebath and, consequently, a lower colour yield.

In order to compare the effects of the SBH vat dyeing system in machines with different actions, fabric samples were dyed with Vat Green 1 by the optimised SBH method in both the Turbomat and Mathis laboratory dyeing machines. The first two sets of data in Table 12 show that the colour yield of the sample dyed in the Mathis was much lower than the one dyed under similar conditions in the Turbomat. Although the exhaustion was slightly better in the Turbomat than in the Mathis, the difference was not great enough to account for the large difference in colour yield. A second possibility, discussed above, is that the reducing power of the SBH system had been adversely affected by oxidation resulting from mixing the dyebath with air during agitation in the Mathis machine. In order to test this possibility, two further samples were dyed in the Mathis machine. Extra SBH, sodium bisulphite and caustic soda were added to one of the pots when the vatted dye liquor was diluted immediately before the fabric was added. This technique, called "sharpening the bath", is used when concentrated stock vats are prepared and then diluted for use over a few days. The extra reducing agent replaces losses due to air oxidation.


(a) Bath Sharpened with 1 g/L SBH / 4 g/L sodium bisulphite / 3 ml/L caustic soda (38°Bé ) All samples soaped off for 20 mins at 98°C in 2 g/L Detergent NA-B

Table 12. Effect of Dyeing Machine Type and of Sharpening the Bath on Dyeing Vat Green 1 (1% oww) by the SBH/Bisulphite Dyeing System (Dyed for 30 min at 70°C; Liquor ratio 25:1; 5% oww Sodium sulphate; 0.05 g/L Albigen A added to the dyebath).

In a long liquor dyeing process, interchange of the dyebath liquor with the substrate is important in order to ensure a constant supply of dyestuff molecules to the fibre surface. This can be achieved either by pumping the liquor through a stationary material, moving the substrate though the liquor, or moving both the liquor and material through the machine. Anionic wool dyes applied under acidic conditions have a high substantivity for wool as a result of ionic attraction between the anionic dye molecules and protonated amino groups in the fibre. For this reason, the type of liquor circulation used in the dyeing machine is not an important factor in the uptake of most types of wool dyes. Vat dyes, however, are applied to wool at a relatively high pH, where the fibre is negatively charged. Thus, in this case the substantivity of the dye will be dependent largely on non-polar/hydrophobic interactions rather than on ionic attraction. It is possible, therefore, that the more efficient liquor interchange in the Turbomat, involving pumping the liquor through the fabric, may result in better dyebath exhaustion than in a machine such as the Mathis Labomat, where liquor and fabric are tumbled around together. Another factor that may be important in the Mathis machine is that the constant mixing of air with the liquor could result in premature oxidation of the leuco compound. This could result in precipitation of the dyestuff in the

In order to compare the effects of the SBH vat dyeing system in machines with different actions, fabric samples were dyed with Vat Green 1 by the optimised SBH method in both the Turbomat and Mathis laboratory dyeing machines. The first two sets of data in Table 12 show that the colour yield of the sample dyed in the Mathis was much lower than the one dyed under similar conditions in the Turbomat. Although the exhaustion was slightly better in the Turbomat than in the Mathis, the difference was not great enough to account for the large difference in colour yield. A second possibility, discussed above, is that the reducing power of the SBH system had been adversely affected by oxidation resulting from mixing the dyebath with air during agitation in the Mathis machine. In order to test this possibility, two further samples were dyed in the Mathis machine. Extra SBH, sodium bisulphite and caustic soda were added to one of the pots when the vatted dye liquor was diluted immediately before the fabric was added. This technique, called "sharpening the bath", is used when concentrated stock vats are prepared and then diluted for use over a few days.

> Final Dyebath pH

Mathis None 9.6 1.03 1.6 4.2 2.5 Turbomat None 9.0 0.82 12.3 4.5 3.7 Mathis Yes (a) 9.9 1.37 16.1 4.5 3.2 Turbomat Yes (a) 8.6 0.57 14.3 4.4 3.3

Table 12. Effect of Dyeing Machine Type and of Sharpening the Bath on Dyeing Vat Green 1 (1% oww) by the SBH/Bisulphite Dyeing System (Dyed for 30 min at 70°C; Liquor ratio

(a) Bath Sharpened with 1 g/L SBH / 4 g/L sodium bisulphite / 3 ml/L caustic soda (38°Bé )

25:1; 5% oww Sodium sulphate; 0.05 g/L Albigen A added to the dyebath).

Absorb. at 558 nm

K/S at 640 nm

Rubbing Fastness Dry Wet

**3.7 Effect of type of dyeing machine** 

dyebath and, consequently, a lower colour yield.

The extra reducing agent replaces losses due to air oxidation.

Extra SBH/bisulphite and NaOH added

All samples soaped off for 20 mins at 98°C in 2 g/L Detergent NA-B

Dyeing Machine As can be seen from Table 12, the addition of extra reducing agent improved the colour yield of samples dyed in the Mathis machine. The significance of this result is that it provides an indication of the possible behaviour of the SBH system when wool is dyed with vat colours in different types of machines. Thus, the results obtained in the Turbomat can be considered to relate to the behaviour of top, loose stock, package and beam dyeing equipment, where liquor is pumped through the substrate and very little mixing with air occurs. The action of the Mathis, however, can be considered to simulate that of a jet, winch or hank dyeing machine, all of which allow a high degree of contact between the dyebath and atmospheric oxygen. It is clear from these results that when this type of equipment is used, the bath sharpening technique should be employed. It is inadvisable to use extra reagents during the dyestuff vatting step, because with some vat dyes the chromophore can be destroyed by over reduction.
