Saminathan Ratnapandian

Research on natural dyes

The growing interest in industrial use of natural dyes has exposed a knowledge gap on how to use natural dyes in modern dyeing facilities. In the past 150 years, these natural dyes have been used for textile colouration in limited quantities. Their reduced use over a long period has resulted in:

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  loss of knowledge regarding extraction and application process due to absence of written records
  
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  extinction of some dye sources and non-identification of new sources
  
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  slowing down of scientific research on natural dyes, leading to stagnation in recipe development, modernisation and optimisation.
  

The new interest has brought about scientific investigations resulting in publications relating to dye sources, extraction and application methods, dye chemistry and

performance of natural dyes [30, 40]. The following sub-sections discuss some of these findings.

1. Sources and extraction

Several authors have collated a wide variety of plant sources for natural dyes across the world [28, 38, 39, 48, 76]. Mohanty et al. [77] report that more than 300 indigenous plant species have been identified in India as traditional dye sources. The plants range from undesirable weeds to those that can be cultivated. They also include rare and exotic species. Different parts of the plants such as leaves, flowers, bark, seed pods and roots may contain maximum amounts of colourant and in some cases yield different colours from each part [38, 39]. Apart from these, the use of byproducts and waste products such as wood chips and sawdust (timber industry), grape pomace (wine making) and olive pomace (oil extraction) as cheap dye sources has been reported [78, 79]. Developments in genetic engineering have been applied to make micro-organisms produce dyes as well [80-83].

Steeping, boiling or fermentation in water of plant material forms the most common dye extraction technique. The extract is purified by simple filtration, settling and evaporation to obtain the final dye in paste, cake, granule or powder form. This process reduces transportation cost of raw materials and also helps to standardise the final product. Bechtold [10] has suggested moving the dye houses near the extraction facilities to eliminate the evaporation step. Spray drying and super-critical carbon dioxide extraction are the new developments in this sector. Commercial quantities of some natural dyes are now available [84, 85].

2. Application techniques

The industrial use of natural dyes declined at a time when long-liquor exhaust dyeing and hand or simple mechanical printing methods were the only colouration techniques available. Such methods were laborious, time consuming and sometimes convoluted compared to present day colouration techniques [11]. Some traditional recipes and techniques that were passed down by word-of-mouth only were lost [13, 28].

Enthusiasts like Dyer [39], Flint [86] and Adrosko and Furry [48] have collated recipes from across the world, simplified them based on their personal experiences and

published books for use by other hobbyists. Interest groups conduct classes to disseminate their knowledge as well. Numerous national and international conventions, seminars and symposiums are being conducted to popularise the natural-dyeing concept. However, a common disclaimer among these protagonists is ‘These are my results. Your results will vary and you are welcome to try your own version.’

Researchers like Bechtold [10], Cardon [28], Chavan [87, 88] Gulrajani and Gupta

[30] and Gulrajani [89] have approached these recipes in a scientific manner. Their efforts have been aimed at understanding the chemistry and then standardizing the dyes, assessing the relative merits of different dyes and evaluating the dyeing process. Variations in the exhaust dyeing method have been addressed [28, 30, 90]. The use of modern machinery for this process has been investigated [91]. Non-conventional methods such as ultrasound, microwave and radio frequency dyeing have been used with natural dyes [62, 92, 93]. Padding using natural dyeing has been reported by only a few researchers [94, 95]. However, the padding parameters mentioned are not in proportion, warranting further investigation. Application of chitosan to improve the uptake of some natural dyes during exhaust dyeing has been studied [96, 97]. However, the large number and variety of natural dyes offer opportunities for continued research [49].

3. Performance of natural dyes

Natural dyes have been used to produce a variety of shades [10, 28, 98]. However, with exceptions such as indigo, cochineal and madder, most natural dyes cannot perform to the fastness standards set for synthetic dyes. There is a constant scientific effort to improve this deficiency. The role of mordants for increasing fastness ratings has been studied [37, 99, 100]. Certain natural dyes have been shown to impart antimicrobial activity and UV protection [101, 102]. Thus research on natural dyes provides scope for innovative developments.

4. Objections to natural dyes

Common resistance to change has been seen with the idea of reintroducing natural dyes to industrial use. The question of whether natural dyes can provide the desired variety of shades has been answered positively by several researchers [10, 28, 32, 98]. The limitations indicated are quantity and quality. Glover [103] expresses doubts about the
agricultural feasibility of replacing synthetic dyes with natural dyes. It should be noted that there is no move for complete replacement and natural dye consumption in Europe would probably constitute only 5% by volume of total dye consumed there. Research has revealed that this can be met by utilizing a small portion of non-agricultural semi- arable land in Europe [84, 104, 105]. Modern farming methods have made dye-crop cultivation a viable commercial proposition in some countries [84]. Queries regarding sustainability, safety and pollution with respect to natural dyes, for example the use and discharge of metal salts for mordanting, have to be considered [103]. The above arguments bring out the knowledge gap mentioned earlier and highlight the need for research.

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