**Dyeing Wool with Metal-free Dyes – The Use of Sodium Borohydride for the Application of Vat Dyes to Wool**

John A. Rippon, Jackie Y. Cai and Shaun M. Smith *CSIRO Materials Science and Engineering, Australia* 

#### **1. Introduction**

16 Textile Dyeing

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Vat dyestuffs are pigments that must be pre-treated with a reducing agent, such as sodium hydrosulphite, to make them water-soluble immediately before they are used for dyeing (Latham, 1995; Trotman, 1984). The reduction step converts the pigment form into a leuco compound. This owes its name to the Greek word for colourless, because many vat dyes are very pale in colour when in the reduced state, due to the lower level of conjugation of double bonds. A schematic of this reaction for the dyestuff Vat Red 1 is shown in Figure 1.

Fig. 1. Structure of Vat Red 1 and formation of the sodium salt of the leuco compound.

In strongly alkaline conditions, the leuco forms of vat dyestuffs are anionic and soluble in water. They can be exhausted onto cotton from long liquors under alkaline conditions in the presence of an electrolyte, such as sodium chloride or sodium sulphate (Latham, 1995; Trotman, 1984). After adsorption by the substrate, the leuco form of the dye is oxidised back to the insoluble coloured pigment inside the fibre. This can be done by exposure to air, or with an oxidising agent such as hydrogen peroxide. Wool is usually dyed with anionic dyestuffs from acidic dyebaths, where the amphoteric wool fibres are positively charged as a result of protonation of amino and carboxyl groups. Under alkaline conditions, however, fewer amino groups are protonated and, depending on the pH, the net charge on the fibres may be neutral or even negative. The effect of this is that conventional acid, premetallised and reactive wool dyes have a lower substantivity for wool under alkaline conditions than under acidic conditions. In contrast to this behaviour, however, even under strongly alkaline conditions, the anionic leuco form of a vat dyestuff has a relatively high substantivity for wool (Bird, 1947; Hug, 1948; Luttringhaus, Flint & Arcus, 1950; Weber, 1951) and wool/cotton blends (Lemin & Collins, 1959). This results in high levels of dyebath exhaustion at pH values as high as pH 9 and above.

Dyeing Wool with Metal-free Dyes –

method with sodium hydrosulphite.

The following dispersing agents were used:


dye samples of wool fabric. The details are described below.

pumping the liquor through the fabric tube (in to out).

**2.4 Preparation of the dye vat by reduction** 

**2. Experimental** 

**2.2 Dyes and chemicals** 

**2.1 Fabric** 


**2.3 Equipment** 

*Ahiba Turbomat.* 

*Mathis Labomat* 

was mixed with the liquor.

**2.4.1 Hydrosulphite method** 

The Use of Sodium Borohydride for the Application of Vat Dyes to Wool 19

in which results obtained with a borohydride/bisulphite reducing system are compared with those obtained with a method based on the application of vat dyes using the traditional

A commercial sample of sodium borohydride (SBH) was supplied by Rohm and Haas as a

Two types of laboratory machines with different methods of liquor circulation were used to

Fabric samples were wound onto perforated spindles. The dyebaths were circulated by

With this machine, the fabric samples and dye liquors were sealed inside pots. Circulation was achieved by tumbling the pots inside a heated box. This resulted in both fabric and liquor moving together. As the pots were not completely filled with liquor, the air in the pot

The vat pigments were converted to the water-soluble leuco form by the following methods.

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.

12% solution in sodium hydroxide. Other chemicals were of laboratory grade.


A 100% wool, plain weave fabric (weight 193 g/m2) was used in this study.

Commercial samples of the following nine vat dyes were used:

Vat dyes are amongst the oldest colouring materials used for textiles, and for many years selected vat dyes were used on both cotton and wool for products requiring very high levels of wet fastness and light fastness. Vat dyes are still used on cotton, where the highly alkaline conditions employed in their application do not damage the fibre. In the case of wool, however, the propensity for alkaline damage during dyeing makes their use less attractive. This resulted in them being replaced by chrome and premetallised dyes, which also give high levels of fastness. Furthermore, chrome and premetallised dyes are applied under pH conditions where fibre damage is less likely to occur. The more recent introduction of reactive dyes for wool also enables excellent wet fastness properties to be achieved with little fibre damage.

Pressure from environmental lobby groups and some major retailers has raised the possibility that wool products that are coloured with metal-containing dyestuffs may become increasingly unacceptable because of concerns about the possible effects of heavy metals on the environment. Although metal-free reactive dyes can be used on wool to give products with high wet fastness, with some shades lightfastness can be a problem. Furthermore, heavy black and navy shades are difficult for many mills to achieve with reactive dyes. This paper investigates the feasibility of using vat dyes as alternatives to reactive dyes to obtain shades with high fastness properties on wool.

The traditional method of preparing the leuco form of a vat dye employs the reducing agent sodium dithionite (sodium hydrosulphite; Na2S2O4) and sodium hydroxide. Sodium hydrosulphite has a sufficiently negative reduction potential for it to effectively reduce all vat dyes. Other reducing agents have also been used, but these have not found wide acceptance. Sodium borohydride has been evaluated but, on its own, reacts too slowly with vat dyes for practical usage (Latham, 1995). It has been claimed, however, to improve the stability against atmospheric oxidation of vat dyes reduced with sodium hydrosulphite (Neale, 1961; Harrison & Hinckley, 1963; Medding, 1980; Vivilecchia, 1966), but other workers have disputed this claim (Baumgarte & Keuser, 1966; Nair & Shah, 1970).

A technique has recently been developed for producing sodium hydrosulphite in situ by mixing sodium borohydride and sodium bisulphite (Rohm and Haas Technical Information, 2007) (Figure 2).

NaBH4 + 8NaHSO3 4Na2S2O4 + NaBO2 + 6H2O

Fig. 2. Reaction between sodium borohydride and sodium bisulphite

Sodium borohydride is supplied commercially as an aqueous solution containing sodium borohydride (12%), stabilised with sodium hydroxide (NaOH). It has been found that a mixture of sodium bisulphite and the sodium borohydride solution in the ratio 4:1 is suitable for the application of indigo to cotton under alkaline conditions (Rohm and Haas Technical Information, 2007; Schoots, 2007). Hydrosulphite produced in this way is claimed to be virtually free of the by-products that result from its decomposition during storage (Rohm and Haas Technical Information, 2007). Furthermore, this reducing system has been found to be more efficient than hydrosulphite alone and it has been claimed to give a dyestuff saving of around 15% in the application of indigo to cotton warps (Schoots, 2007). A borohydride/bisulphite mixture has also been found to be very effective for the reductive bleaching of wool under acid to neutral conditions (Technical Manual, Australian Wool Innovation, 2010; Schoots & Stevens, 2007).

Based on the findings on cotton, it was considered that this reducing system may provide the basis of a new method of dyeing wool with vat dyes. This study describes an evaluation in which results obtained with a borohydride/bisulphite reducing system are compared with those obtained with a method based on the application of vat dyes using the traditional method with sodium hydrosulphite.
