**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 disulphide and peptide covalent bonds (Lewis, 1989).

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


Table 13. Wet Burst Strength of Wool Fabrics Dyed with Vat Dyestuffs by Hydrosulphite/NaOH and SBH Methods
