**2.4.1 Hydrosulphite method**

The various amounts of sodium hydrosulphite and sodium hydroxide were dissolved in 250 mL of water at room temperature and the solution stirred while the powdered vat dye was slowly added. Stirring was continued while the mixture was heated at 2°C /min to the vatting temperature (usually 60-70°C), where it was maintained for 30 minutes.

Dyeing Wool with Metal-free Dyes –

The Use of Sodium Borohydride for the Application of Vat Dyes to Wool 21

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

> **Reagent/Conditions A B C D**  SBH solution (12%) (g/L) 1 2 4 5 Sodium Bisulphite (g/L) 4 8 16 20 NaOH solution (38° Bé) (ml/L) 3 6 12 15 Vatting conditions 30 min at 60°C

> > Colour of vat Purple Purple Purple Purple

pH after fabric added (40°C) 11.4 11.4 11.5 11.5 Dyebath pH at end of dyeing 8.8 8.8 9.5 10.2 Colour of dyebath after 30 min at 60°C Purple Purple Purple Purple Absorbance of dye liquor after 30 min at 60°C 0.15 0.16 0.27 0.36

> pH of oxidation liquor 9.5 9.2 9.9 9.9 Final soap off pH 9.9 9.9 9.9 9.9

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)

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

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

information on the amount of hydrosulphite required (Weber, 1951).

### **2.4.2 Sodium borohydride/bisulphite method**

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 minutes.

#### **2.5 Fabric dyeing**

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

#### **2.6 Measurements**

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*.

Alkaline perspiration was assessed by *ISO-105-EO4 – Fastness to Perspiration.*

Washing fastness was assessed by ISO-105-CO2 – Colour Fastness to Washing.

Grey scale staining and colour changes were measured on a Datacolor Texflash Spectrophotometer. The software supplied with the instrument (Datacolor Iris Version 2.3) enabled ratings to be quoted to 0.1 of a greyscale unit.

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