Saminathan Ratnapandian
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- Figure 1.2
- Anthocyanins
Figure 1.2 Typical chemical groups of natural dyes Indigoid dyes These are the blue dyes, especially indigo, derived primarily from the indigofera, isatis and polygonum species [13, 28]. Tyrian purple extracted from murex species of molluscs is another noted member of this group. These were the first vat dyes and are characterised by the two carbonyl (C=O) groups. The conversion to leuco-form was traditionally brought about by lengthy and inconsistent fermentation [10, 28, 29, 41- 43]. Flavonoids These are the largest group of natural dyes which are prominently yellow in colour. Luteolin is the primary colourant in these dyes. Weld, sawwort and fustic are some of the well known examples from this group. The fastness to light is typically poor as is evident from the fading of old tapestries [10]. Anthocyanins These are water soluble flavonoids responsible for the red and blue colours of flowers and fruits. The colours are sensitive to pH and light. Their use as watercolours is well established [44]. In the case of anthocyanidins the glycosides of the anthocyanins are replaced by hydroxyl groups. Logwood is an excellent example of this class of dyes. Carotenoids These colourants are named carotenoids because they were initially isolated from carrots. Other common sources are tomatoes and sweet peppers while saffron and turmeric are exotic sources. A yellow to orange-red colour is obtained using these direct dyes [28]. Anthracenes This class of dyes is extracted from plants (flowers, bark and roots), lichens and insects. The two main groups of this class are 1) anthraquinones with yellow, pink and red pigments [45]; and 2) napthoquinones with brown, pink or purple pigments [46]. Safflower, madder and cochineal are noted examples of this class of dyes. Chlorophyll This is the green pigment present in plants, algae and cyanobacteria that absorbs light energy during photosynthesis. Four types of chlorophyll have been identified although the commercial product is mainly made up of chlorophylls a and b. It is generally not used for textile colouration because of difficulties in application and browning on exposure to light, resulting in poor light fastness [29]. Tannins This is the term introduced by Seguin in 1790 to describe plant extractions that can convert raw hide to leather [47]. In addition, they have the character of astringency (a term used in leather manufacturing to describe the reactivity of the tannin for the hide) to touch and taste and the propensity to give a greenish or bluish-black hue in the presence of iron salts. In spite of substitution by minerals and synthetics, annual global production of plant tannins was estimated to be 1,800,000 tonnes in 2007 [28, 47]. Tannins are identified chemically as polyphenols and are broadly classed as hydrolysable (gallo-tannins and ellagi-tannins) and condensed (proanthocyanidins) tannins. Their common characteristic reaction with iron salts is utilised for the manufacture of iron-gall inks. Galling or sumaching is a textile application of this reaction, wherein the tannin serves as a mordant. At times they serve the dual purpose of mordant and colourant due to their close association with some of the other colouring groups such as flavonoids and quinones [28, 47]. Yüklə 2,14 Mb. Dostları ilə paylaş:
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