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New sustainable processes in the textile industry: Extraction of natural dyes
Published in Gianni Montagna, Cristina Carvalho, Textiles, Identity and Innovation: In Touch, 2020
P.M.S. Silva, T.R. Fiaschitello, R.S. Queiroz, S.A. Costa, S.M. Costa
The fermentation extraction method is based on the use of enzymes produced by the microorganisms present in the atmosphere and the vegetal material to aid in the extraction of the natural dye, which is reduced to its water-soluble leuco form (John, 2009; Saxena & Raja, 2014). Indigo species are the main examples of this type of extraction, such as the genus Isatis and Indigofera (John, 2009; Saxena & Raja, 2014). In this process, the leaves of these species are crushed, soaked in warm water and allowed to stand for a few hours or days, when enzymatic fermentation occurs (John, 2009; Saxena & Raja, 2014). The glucoside indicant that is present in the plant leaves is broken down into glucose and indoxyl during fermentation (Saxena & Raja, 2014). After fermentation, the yellow liquor containing the indoxyl is agitated in a vat. Then, the indoxyl is oxidized by air to insoluble blue indigotin, which precipitates itself (Saxena & Raja, 2014). This precipitate is then washed, squeezed to remove excess water and stored in solid-state for later use (Saxena & Raja, 2014). The disadvantages of this process are the its long duration, the need to perform the extraction right after the harvest of the plant material and the bad smell produced during the fermentation (Saxena & Raja, 2014).
Mao berry as a green reducing agent in natural indigo dyeing on cotton fabrics
Published in The Journal of The Textile Institute, 2022
Laksanawadee Saikhao, Jantip Setthayanond, Thitinun Karpkird, Potjanart Suwanruji
Natural indigo is the oldest vat blue dye that is mainly applied on cellulosic fibres, especially for denim dyeing. One of the indigo plants is Indigofera tinctoria usually cultivated in tropical countries such as India, China and Thailand (Duangbubpa & Pratumnet, 2015; Vuorema, 2008). Leaves and twigs of Indigofera tinctoria contain indican which can be hydrolysed by enzymes into glucose and indoxyl. By aeration, indoxyl is oxidized to blue indigo pigment (Chakraborty & Chavan, 2004). The insoluble blue indigo extracted from the plants needs to be reduced in an alkaline solution into its water soluble leuco form before use in dyeing. The leuco indigo dyes have substantivity towards cellulosic fibres and can exist in different ionic forms in the dyebath depending on pH. Leuco indigo in the mono-ionic form is predominant at pH 11.5, whereas the di-anion form is the main at a higher pH. The former one exhibits much higher substantivity for cellulose than the latter one, resulting in higher colour yield (Etters, 1989). On exposure to air, the leuco indigo impregnated in the fibre is oxidized to the original water insoluble form (Figure 1).