Saminathan Ratnapandian

Figure 2.2 Indigo formation in plants

Chemically identical synthetic indigo, introduced by BASF in 1897, is a purer product with a higher degree of crystallinity [13, 112, 113]. Synthetic indigo has developed into a global multibillion dollar industry [112].

The commercially available natural indigo used in this study was extracted from Indigofera tinctoria.

Mordant dyes
Among mordant dyes, tannins are the most abundant chemical groups with an estimated global production of 1,800,000 tonnes annually [28, 47]. This yield is obtained from different plant species belonging to families such as Fagaceae, Pinaceae,
Anacardiaceae, Combretaceae and Leguminosae. Plant tannins are extracted from leaves, nuts, bark, seed pods and heartwood. Tannins are classified as hydrolysable and condensed (non-hydrolysable) tannins based on how they are made up and their decomposition products. Hydrolysable tannins contain at least one carbohydrate molecule and yield gallic or ellagic acid on decomposition. Condensed tannins do not contain any carbohydrate and yield anthocyanidins on decomposition [114].

Two commercial dyes, Thar (CI Natural brown 3) and Caspian (CI Natural brown 3:3), both condensed tannins derived from Acacia catechu and Acacia nilotica respectively, were chosen to represent the mordant dyes. Both plants belong to the Leguminosae family. The dye is extracted from the heartwood of the first plant, while for the second it is from the bark [28]. The main components present in these dyes are prorobinetidine and profisenitidine shown in Figure 1.2. These multi-dentate polyphenols generally exist as dimers and trimers [28, 115]. As seen in Figure 2.3 and Figure 2.2, different stereo isomers of the dimers and at times even oligomers exist. The exact polymer formed and the final set of polyphenols present are subject to natural variation [115].

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Figure 2.3 Four possible stereo-isomeric dimers of profisetinidine

Profisetinidin trimer Prorobinitidine oligomer