Colouration of wool and silk with Rheum emodi

Abstract

Silk and wool fabrics have been dyed with colourant extracted from Rheum emodi in the absence and presence of magnesium sulphate, aluminium sulphate and ferrous sulphate mordants for producing shades of different colours, ranging from yellow to olive green. Colouring component of Rheum emodi has close resemblance with a typical anthraquinonoid type disperse dye; the uptake of such colouring component by the protein fibres remains unaffected within a wide pH range of 4 – 8. Colour uptake, rate of dyeing and affinity of colour are found to be more for silk than that for wool under all the conditions studied. The dyeing mechanism corresponds to the partition mechanism, confirming that this anthraquinonoid-based colourant is adsorbed by silk and wool fibres as a disperse dye and the dyeing process is endothermic. The use of ferrous sulphate and aluminium sulphate produces significant improvement in depth of shade, when both the substrates are treated with such salts prior to application of the colourant. Coloured protein fibres, in general, show a common light fastness and wash fastness ratings of 4 and 3 respectively. Ferrous sulphate, however, improves the colour fastness properties and colour retention on washing of both wool and silk fabrics further.