In this article we will discuss about:- 1. Introduction 2. Research and Technology 3. Safeguarding the Environment 4. Fermented Dyes 5. Classification – Natural Dyes.
Introduction:
Natural dyes are a class of colorants extracted from vegetative matter and animal residues. These are considered as mordant dyes as they require the inclusion of one or more metallic salts of aluminium, iron, chromium, copper and others for ensuring reasonable fastness of the colour to sunlight and washing. These metallic salts combine with the dyestuff to produce dye aggregates, which cannot be removed from the cloth easily.
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Natural dyes were extensively used for the dyeing of all natural fibers until the middle of the nineteenth century. The discovery of methods of synthesizing alizarin and indigo spelt the death knell of the indigenous industry. Due to the ease of application, the bright shades obtained and the hard sell of the colonial rulers, hand weavers started to opt for synthetic dyes without a clear understanding of the after effects of using these.
The baleful influence of the synthetic dyes affected not only the hand weaver but also the agriculturist. Some of the natural dyes like indigo were leguminous in nature and played a valuable role in the crop rotation of rice. With the advent of synthetic indigo, natural indigo seeds were no longer collected and the fields lost out a valuable input by which nitrogen from the atmosphere was fixed in the soil.
Use of myrobalan and pomegranate rind in dyeing encouraged the use of bio wastes and cleaned up the environment. Lac was an inset dye derived from the shell of the lac insect, providing gainful employment to hundreds of forest dwellers. Cultivation and collection of lac helped in the coppicing of trees like Som and Arjun by way of clearing of dead branches. More important than the commercial value of the individual natural dye concerned was the empathy between man and nature.
This empathy had fostered in mankind a healthy respect for the biosphere, which was broken once the dyes started being synthesized in the laboratory. The use of natural dyes has petered out except in few pockets like Kalahasti of Andhra Pradesh and few places in the tribal belt of Madhya Pradesh, Orissa and Rajasthan.
The unbridled use of synthetic dyes and the non-treatment of effluents contained in the wastewaters of the dyeing process have led to horrendous results. In a few places like Erode in Tamil Nadu, cocktails of lethal chemicals have been injected into the water table by the hundreds of dye houses spewing out non-degradable chemicals, so much so that the water from the bore well has an evil smell and is coloured reddish brown. The Bandi River in Rajasthan is dying. Flowing through various villages of Rohet tehsil in Pali district, its water has a reddish hue. It can no longer be used for irrigation or drinking. Even animals do not drink this water.
Research and Technology:
Researches carried out have led to the development of processes for the extraction of natural dyes from some abundantly occurring plant materials of forest origin. These dyes may be used for imparting different shades on silk, wool and cotton using common mordants like alum, salts of iron, tin and chrome. Different combinations of mordants impart unique shades.
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All over the world debates are on regarding the viability of natural dyes and if they could be a potential dye source for mass production in the textile industry. But it is rightly said by Hill that “The amount of research effort devoted to natural dyes is negligible. If there had been significant research on the use of natural dyes, it is probable that they would already be much more widely used than they currently are. As there is much catching up to do after 150 years of neglect, there is plenty of scope for more rapid developments. This applies to the technique of agricultural production and processing as well as to dyeing itself.”
It is true that natural dyes will not replace synthetic dyes but designers can very effectively utilize natural dyes as a designer tool and naturally dyed, hand printed fabrics will have the exclusive look of a hand-woven Jamdani shawl and an original patola saree. The review of existing literature reveals that non-availability of the natural dyes in the standardized form; shortage of trained dyers and dearth of books on the technology of dyeing with natural dyes pose a challenge today in the arena of textiles.
The processes employed for the extraction of dyes are simple, low-cost and eco-friendly. It is now possible to produce marketable natural dyes, which would provide eye-catching shades, with the simple workable techniques. The forest biomass can be used for the production of dye of cottage scale, thus leading to employment generation for the people through value addition to the non-wood forest products along with creating an additional source of revenue.
The cost of raw materials varies from Rs.2 to Rs.40 per kg depending upon the material used for isolation of dyes and the site of requirement. Processes have also been standardized to use the dye raw materials directly for dyeing different fabrics.
Safeguarding the Environment:
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Azo dyes cause harmful effects during their production and subsequent use. The EEC has promulgated at “EC Control of Substance Hazardous to Health Act, 1989” and published a red list enumerating a number of chemicals, the presence of which in any kind of fabric has been banned. An ordinance in Germany stipulates that no garment or any other article that comes into contact with the skin shall contain any of the twenty aromatic amines named thereunder.
Similar restrictions have been imposed in many other countries. In India, the ban on the use of azoic dyes has been imposed by the Union Ministry of Environment and Forest under section 6(2) (d) of the Environment (Protection) Act, 1986 read with the Rule 13 of the Environment (Protection) Rules, 1986.
Due to environmental awareness the natural dyes obtained from plants and animals are the dyes of 21st century. Their production as well as trade is anticipated to increase manifold in the times to come. The increasing awareness of environmental pollution associated with synthesis, processing and use of synthetic dyes has led to a worldwide interest in dyeing textiles with flora and fauna. Natural dyes procured from natural wealth like plants, minerals and insects are fairly nonpolluting, more challenging, and have rare colour ideas and unlimited scope to generate new shades.
Natural and vegetable dye material cultivation, collection, processing, dye extraction and dyeing of textiles are considered to be agro based rural industry, which helps to promote rural entrepreneurship and thus developing the rural economy. Many of the vegetable dye sources are herbs, shrubs and other plant matters that could be cultivated in dry lands.
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Since the last decade application of natural colorants on cotton, silk, wool, jute, etc., is gaining popularity all over the world. Designers have very effectively utilized natural dyes as a designer tool. The non-reproducibility and non-uniformity of shades makes each creation a unique piece.
The Colour Congress-2002 was the first International Conference organized to address the issues related to the natural dyes and their increasing use worldwide. The conference provided excellent opportunity to interact with scientists, designers and users of natural dyes all over the world.
The amount of research efforts devoted to natural dyes are negligible. If there had been significant research on the use of natural dyes, it is probable that they would already be much more widely used than they currently are. This is a new development taking place in most of the advanced countries as ecological awareness is on the increase. Eco-friendly processes are being insisted upon and eco-standards are laid to identify the effect of toxic and nontoxic dyes and chemicals on the health of living beings.
Scientific study and research on dye yielding plants and their commercial cultivation will not only encourage the use of vegetable dyes but countries like India who have a heritage and knowledge of dyeing and printing with natural dyes will have an edge over the western countries.
Fermented Dyes:
Fermentation is the process activated by microorganisms, which brings about changes of colour and texture in the material to be fermented. The use of enzymes in textile processing and after care, is already the best-established example of the application of biotechnology to textiles. These enzymes usually operate under mild conditions of pH and temperature. Many enzymes exhibit great specificity.
Many microorganisms produce pigments during their growth, which are substantive as indicated by the permanent staining that is often associated with mildew growth on textiles and plastics. Microorganisms, therefore, offer great potential for the direct production of novel textile dyes or dye intermediates by controlled fermentation technique.
Fermentation is the oldest process in use since 2000 B.C. At present traditional dyers of Japan (isolated areas in Nigeria) and some contemporary dyers still use fermentation process. Use of fermentation vats decreased markedly for cotton and linen when the copper’s vat became available in 1750 and for wool when the hydro sulphite vat became available (about 1880).
All natural fibers can be dyed in fermentation vats, but they are generally more suitable for the dyeing of wool than for linen, silk and cotton. One reason is that they must be warm to work well and another is that they are generally weaker with respect to reduced indigo.
Fermentation process relies on bacterial and yeast fermentation to reduce the dye (remove oxygen and reduce the dye). Fermentation process, though somewhat slow, requires considerable expertise and attention and is, responsible for the fine indigo blue found on existing wool specimens.
Classification – Natural Dyes:
Quality standards for natural dyes vary widely, so it is necessary to first contact an importer to find out what they are looking for. The problem arises with standardization of the colours, as no two-dye lots are identical. While paint manufacturers might be interested in the uniqueness of each batch of colour produced, technicians in the pharmacology, food and textile industry loathe this lack of consistency. The latter group attempted to standardize natural dyes by imposing a colour index that helps to classify and name them.
Each dye is thus named according to the following pattern:
Natural + base colour + number
These dyes are thereby specifically identified as dyes of the stated colour, but it does not specify whether the dyes are derived from animals or plants. This is because it is a classification based on the dye’s source and colour, and it contains no chemical information, nor does it imply that dyes with similar names but unique numbers are in any way related. It also gives no information about the mechanism by which staining occurs.
This is done in order to authenticate the synthesized organic dyes along with natural dyes under the same universal classification system:
1. Flavones – 90% of all yellow dyes are flavonoids. The fastness of these yellow dyes is greatly affected by the mordant, the surface being dyed and the photosensitivity of the chromophores. Weld is the most lightfast of all yellow dyes. Europe was the traditional growing area, however it grows well in a variety of physiographical ranges. Currently there is no commercial production of weld as a dye plant. Other examples are Quercitron, Fustic, Osage, Chamomile, Tesu, Dolu, Marigold, Cutch.
2. Iso-Quinoline – The only basic dyestuff known from nature, e.g., Barberry.
3. Naphthoquines – Although an array of naphthoquinones occur in nature, only a few are important as dyes, e.g., Henna, Walnut, Alkanet, Pitti.
4. Anthraquinones – Over 95% of the known natural red dyes fall into this category. Anthraquinone dyes surpass all other classes of dyes in their fastness properties, e.g., lac, cochineal, madder (majithro).
5. Benzophyrones. – e.g., Logwood.
6. Indigoids – An unusually small molecule produces this stable blue colour in a vat process. Other naturally occurring indigoids are the shellfish dyes, purpura and murex.
7. Vegetable Tannins – Gallotannins, Ellagitannins, Catechol tannins. All tannins have a large heavy molecular structure which react readily with metallic salts, e.g., wattle, myrobalan, pomegranate, sumach, chestnut, eucalyptus.
Dyeing Process:
The first step of the actual dyeing process is mordanting. A mordant is a chemical that, when ‘cooked’ with the fiber, attaches itself to the fiber molecules. The dye molecule, then, attaches itself to the mordant. Different mordants give different colours when combined with the same dye. For example- the dye, cochineal when used with alum sulfate gives a fuchsia colour; when used with tin, the colour is more scarlet, and when used with copper, it is purplish.
Mordants except for alum and iron are considered toxic and therefore self-defeating. The mordants are toxic to the dyer and the disposal of the bath becomes an environmental problem. Therefore the choice of mordants is limited. Alum compounds and iron are ideal and safe mordants. Treating cotton with tannic acid is useful as it prepares the fabric for effective absorption of the dye.
With protein fibers (wool, mohair, alpaca, angora-animal fibers) alum sulphate is recommended. To mordant, use 10% of the weight of fiber (WOF). Dissolve the alum with boiling water and add this to the dye pot. Use enough water in the pot to amply cover the fiber. Bring the temperature up to 180-190°F. for most protein fibers over half hour. Hold at temp for another half hour. Let the pot cool for several hours or overnight.
The fiber will continue to absorb the mordant as it cools. Squeeze out the excess water, but do not rinse the fiber. Sometimes mordanting is done post dyeing. This is in addition to the pre-dyeing mordanting. Tin and Vinegar are the common post-dyeing mordants used for ‘fixing’ the dye.
1. Weigh, scour and mordant fabric. Weigh out dye. Example – a lighter shade will require a lower amount of dye than a darker shade.
2. Put water into dye pot (use 25 times WOF). Dissolve dye in a small amount of water and add to dye bath.
3. Place damp, pre-mordanted fabric in dye bath. Raise temperature to 120°F and hold for ten minutes, agitating frequently.
4. Raise temperature to 180°F, over a period of ten minutes and continue to stir. Hold temperature at 180° for ten minutes.
5. Remove fabric. Rinse in cold water. Wash fabric in cold water.