**2.1 Fabric**

18 Textile Dyeing

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

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

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

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

NaBH4 + 8NaHSO3 4Na2S2O4 + NaBO2 + 6H2O

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

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

workers have disputed this claim (Baumgarte & Keuser, 1966; Nair & Shah, 1970).

enables excellent wet fastness properties to be achieved with little fibre damage.

reactive dyes to obtain shades with high fastness properties on wool.

Fig. 2. Reaction between sodium borohydride and sodium bisulphite

Innovation, 2010; Schoots & Stevens, 2007).

2007) (Figure 2).

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