Saturday, June 20, 2009

Natural dyes

Natural dyes comprise those colourants that are obtained form animal or vegitable matter without chemical processing. Natural dyes are generally non-substantive and hence must be used in conjunction with mordants.


Based on chemical structure:
Indigoids: -
The dyestuff is extracted from Indigofera tinctoria, a bush of the pea family. The dye is extracted from the leaves of the plant.
Anthraquinones: -
Red dyes are based on anthraquinone structure. These dyes are characterized by good fastness to light.
Alpha napthaquinones: -
Lawsone or Henna is the most prominent member of this class. It is obtained from the leaves of Lawsonia inermis. Another similar dye is Juglone obtained from the shell of unripe walnuts.
Flavones: -
The yellow colours are derivatives of hydroxy and methoxy substituted flavones or isoflavones. Eg: - jackfruit bark
Dihydropyrans: -
These are principal colouring matters of logwood and they give dark shades on cotton, silk, and wool.
Anthocyanidins: -
This class includes carajurin, obtained from the leaves of Bignonia chica and Awabanin. It dyes silk in blue shades.
Carotenoids: -
This class includes the orange pigment carotene found in carrots. The dyes based on carotenoid structure are annatto and saffron.

Based on Color
Blue Dyes: Natural indigo, sulphonated natural indigo and the flowers of the Japanese “Tsuykusa”
Red Dyes: The colour index lists 32 red natural dyes. The prominent among them are madder (Rubia tinctorum L), Manjeet (Rubia cordifolia), Brazil wood/Sappan wood (Caesalpina sappan L), Al or Morinda (Morinda citrifolia L).
Yellow Dyes: Colour index lists 28 yellow dyes. Some of the important yellow dyes are, black oak (quercus velutina), tumeric (curcuma longa), and weld
(reseda luteola) and Himalayan rhubarb (rheum emodi).

Natural dyes

CI Natural No. of Dyes
Yellow 28
Orange 6
Red 32
Blue 3
Green 5
Brown 12
Black 6

Structures of only 67 are known
Many dyes have more than one colouring compound
Some dyes have identical structures
Some dyes have structures similar to synthetic dyes.

Extraction: -
In general the extraction of the dye is carried out by boiling the contents in water for optimum time (found out by optimization of parameters) which is 45 to 60 minutes in most cases. The solution is filtered and cooled. The filtrate is used as a dye.
Apart form this there are two other methods of extraction of dyes. These are 1) solvent extraction e.g., Alkamin Natural Red 24,
and 2) supercritical fluid extraction.

Natural dyes are either substantive, needing no mordant, or adjective requiring one. The majority of natural dyes need a chemical in the form of a metal salt to create an affinity between the fibre and the dye these chemicals are called as mordants, thus mordant is a chemical, which can fix itself on the fibre and also combines with the dyestuff. A link is therefore formed between the dyestuff and the fibre, which allows certain dyes with no affinity to be fixed on to the fibre. Tannins, metallic salts and oils are used as mordants.

In the dyeing of textiles with natural dyes, tannins are used as natural mordants. These are high molecular weight compounds (between 500 to 3000) containing phenolic hydroxyl groups to enable them to form effective cross-links between proteins and other macromolecules.
The stability of the tannin treated fibre depends upon the pH, ionic strength and metal chelators. Tannins may be further classified into two on the basis of their chemical structure as:
- Hydrolysable tannins obtained from myrobalan fruit, oak bark, gallnuts,
pomegranate rind, sumac leaves.
- Condensed tannins like catechin obtained from acacia catechu.

Metallic mordants
There are several different metal salts that can be used for mordanting. The most effective ones are: -
Alum – potassium aluminum sulphate
Copper – copper sulphate
Chrome – potassium dichromate
Iron – ferrous sulphate
Tin – stannous chloride, stannic chloride.

Oil –mordants are mainly used in the dyeing of Turkey Red Colour from madder. The main function of the oil-mordant is to form a complex with alum used as the main mordant. Since alum is soluble in water and does not have affinity for cotton it is easily washed out from the treated fabric. The naturally occurring oils contain fatty acids such as palmitic, stearic, oleic, ricinolic etc. and their glycerides. The –COOH groups of fatty acids react with metal salts and get converted into –COOM, where M denotes the metal, for instance in the case of alum it would be Al. subsequently, it was found that the treatment of oils with concentrated sulphuric acid produces sulphonated oils which possess better metal binding capacity than the natural oils due to the introduction of sulphonic acid group, -SO3H. The sulphonic acid can react with metal salts to produce –SO3M. The bound metal can then form a complex with the mordant dye such as madder to give Turkey Red colour of superior fastness and hue.

Methods of mordanting
The three methods used for mordanting are: -
- Pre-mordanting: - The substrate is treated with the mordant and then dyed.
- Meta - mordanting: - The mordant is added in the dye bath itself.
- Post-mordanting: - The dyed material is treated with a mordant.
The methods have different effects on the shade obtained after dyeing and also on the fastness properties. It also depends upon the dye and the substrate. It is therefore necessary to choose a proper method to get the required shade and fastness by optimisation of parameters.
Since metallic mordants are soluble in water and are loosely held by the cotton fibres, these mordants have to be precipitated on the fabric by converting them into insoluble form, or by first treating the fibres with oil or tannic acid and then impregnating treated fabric with solution of mordant, whereby the metallic mordants are held on to cotton via oil or tannic acid.
Unlike cotton, wool is highly receptive towards mordants. Due to its amphoteric nature wool can absorb acids and bases equally effectively. When wool is treated with a metallic salt it hydrolyses the salt into an acidic and basic component. The basic component is absorbed at –COOH group and the acidic component is removed during washing.
Wool also has a tendency to absorb fine precipitates from solutions. These precipitates are superficially sorbs onto surface of fibres and the dye attached to these gives poor rubbing fastness.
Like wool, silk is also amphoteric and can absorb both acids as well as bases. However, wool has thiol groups (-SH) from the cystine amino acid, which act as reducing agent and can reduce hexavalent chromium of potassium dichromate to trivalent form. The trivalent chromium forms the complex with the fibre and dye. Therefore potassium dichromate cannot be used as mordant effectively.

Effects of mordanting
 Some of the natural dyes can form metal-complexes and thereby yield different colours with different metal salts.
 Mordanting improves the wash fastness of the dyes as the dyes are fixed on to the textile substrate.
 Treatment with tannins makes the dyeings dull. Alternatively, if a water-soluble salt such as alum is applied on to a cotton substrate and is then insolubilised by treatment with alkali then the insoluble salt of aluminum is deposited in the material. This provides metal chelating areas for the natural mordant dye. This kind of treatment can form the basis to get bright shades of natural metal complexing dyes.

Environmental problems posed by mordants: -
There is a tendency to use all types of metal salts for the purpose of mordanting disregarding the restrictions laid on the permissible quantities of different metals by the German ban. Accordingly the indicative maximum permissible quantities of different metals in the ultimate product are as follows: -

Arsenic 1.0
Lead 1.0
Cadmium 2.0
Chromium 2.0
Cobalt 4.0
Copper 50.0
Nickel 4.0
Zinc 20.0
Mercury 0.02

The upper limits of the presence of metals vary from product to product and are different for different eco-marks. However there is no upper limit on aluminum, iron and tin. Hence one can use these salts for complexing and mordanting.
According to eco-standards copper and chrome are red- listed and hence are not to be used. Another problem posed by a mordant is that a substantial proportion of it is left unexhausted in the residual dyebath and may cause serious effluent problems Researchers claim that the use of heavy metals is not necessary because the resulting shades can be obtained from other natural dyes and also advocate the use of environmental friendly mordants by craft workers. Aluminum and iron are relatively innocuous; they are abundantly available and produce excellent dyeings.


1) Health and safety aspects of natural dyes: Though all natural dyes are not 100% safe they are less toxic than their synthetic counterparts. Many of the natural dyes like turmeric, annatto and saffron are permitted as food additives. Many natural dyes have pharmacological effects and possible health benefits.
2) They are obtained from renewable sources.
3) Natural dyes cause no disposal problems, as they are biodegradable.
4) Practically no or mild reactions are involved in their preparation.
5) They are unsophisticated and harmonized with nature.
6) Many natural dyes have the advantage that even though they have poor wash fastness ratings, they do not stain the adjacent fabrics in the washing process because of the non-substantive nature of the dye towards the fabric. An exception to this is turmeric, which shows substantivity for cotton.
7) Natural dyes are cost effective
8) It is possible to obtain a full range of colours using various mordants.


The limitations of natural dyes that are responsible for their decline are: -

 Availability
 Colour yield
 Complexity of dyeing process
 Reproducibility of shade
Besides these there are other technical drawbacks of natural dyes: -
These are: -
 Limited number of suitable dyes
 Great difficulty in blending dyes
 Non-standardized
 Inadequate degree of fixation
 Inadequate fastness properties
 Water pollution by heavy metals and large amounts of organic substances.

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