Valorisation of sulphonated lignin as a dye for the sustainable colouration of wool fabric using sustainable mordanting agents: enhanced colour yield, colourfastness, and functional properties

Biodegradable sulphonated lignin (SL) can be used as a natural dye for the sustainable colouration of wool fabrics, producing light to dark brown shades, but the colourfastness to washing is only moderate, and the colour strength is poor. In this work, wool fabrics were dyed with various concentrations of water-soluble sulphonated lignin using several sustainable mordanting agents, such as calcium (Ca), aluminium (Al), iron (Fe) and copper (Cu) salts, to improve the colour yield and also the colourfastness to washing along with introducing new functional properties. The colour yield of wool fabrics dyed with SL using various mordanting agents was measured using a reflectance spectrometer. The Al and Ca mordants produced a brown colour but the Cu and Fe mordants produced dark olive and dark greyish brown colours, respectively. The wool fabrics dyed with SL using the various mordanting agents exhibited considerably higher colour yield and enhanced colourfastness to washing compared to fabrics dyed with SL without using any mordanting agent. The colour yield considerably increased with an increase in the lignin concentration. Of the mordants studied, Cu and Fe mordanting agents provided the best results. Wool fabric treated with 1% the weight of fibre (owf) SL using Cu mordant, produced better colour yield compared to the fabric treated with 5% (owf) SL without using any mordant. The highest colour strength (4.21) was produced by wool fabric treated with 5% (owf) SL using Cu mordant. The colourfastness increased to grade 4-5 for the fabric dyed with SL using Cu mordant with 35% the weight of SL compared to grade 3 achieved for the fabric dyed with SL without using any mordant. The results suggest that the developed method can be used for the sustainable dyeing of wool fabrics in the textile industry with high colour yield and colourfastness to washing, with a variety of colours. Wool fabric was coloured using sulphonated lignin as a natural dye and a range of sustainable mordanting agents that considerably improved colourfastness to washing and introduced new functional properties.