CLASSIFICATION OF NATURAL DYES

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).

CLASSIFICATION OF NATURAL DYES

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.

Basic dyeing of cellulosics

The dye bath is set with the following recipe and then the dyeing is performed according to the process given below:


The fabric is loaded in the rota dyeing machine and then the fabric is run for 10min at a temperature of 40C, with the dye solution in the m/c. After 10 minutes pre-dissolved salt solution is added in two lots and dye bath was heated simultaneously at 2° C/min. Finally temperature of the dye bath was set at 85– 90° C and dyeing was continued at this temperature for 60min. After the completion of dyeing the exhausted dye bath was drained and dyed material was rinsed with cold water followed by soaping and washing.


Washing off Treatments After dyeing, different washing off treatments can be carried out, time - temperature profile of those after treatments are given below. All the treatments have done in a MLR of 1:20.

CR=Cold Rinse: The dyed sample after neutralization treated with cold water for 5min at a MLR of 1:30.

HR=Hot Rinse: The dyed sample treated with hot water at 70oC for 5min at a MLR of 1:30.
SB=Soap at Boil: The sample is treated with 1gpl Auxipon NP, at boil for 10min at a MLR of 1:30

Dye carriers

Dye carriers are accelerators that assist in the dyeing of synthetic fabrics in a shorter time at a lower temperature. They are particularly used for dyeing polyesters with disperse dyes, although they were originally developed for cellulose acetate. There are a number of carriers commonly used for polyester fibres. Some of the good dye carriers are o-phenyl phenol and it’s monochloro derivative, p-phenyl phenol, diphenyl, monomethyl naphthalene, trichlorobenzene, dimethyl terephthalate and methyl salicylate. Other dye carriers in common usage are o-dichlorobenzene, diphenyl ether, n-butyl phthalimide, chloromethoxy ethanol, methyl cresotinate, alkyl and aryl benzoates and tetralin. The dye carrier imbibes deep shades on the polyester and blended fabrics. Acrylic and nylon fibers also can be dyed using dye carriers but this approach is more expensive than the other alternatives available.
In practice, the choice of the dye carrier is governed by its general effectiveness, toxicity, economics and environmental acceptance. Some of the phenolics and chloro-derivatives are getting eliminated. In a typical operation for dyeing polyester fiber, the dye carrier suitably emulsified in water is used in both the exhaust method at atmospheric pressure as well as in high temperature beam dyeing. After dyeing, the dye carrier is removed from the fabric by an after-scouring process. Whereas dye carriers like the chlorobenzenes are used by emulsification, the phenyl phenols can be used as their water soluble sodium salts along with an acid-liberating agent like ammonium phosphate. At about 100C, the free phenol liberated forms the active carrier which is later removed by alkaline scouring. Butyl benzoate is another successful carrier for use in polyester dyeing at the boil at atmospheric pressure.
The dye carriers are generally used with a leveling agent on polyester/wool mixtures in a high temperature process. The formulations usually consist of around 70% active ingredients (one or more dye carriers), 15% leveling agent and 15% solubilising agent. Depending on the color and shade, the dye carrier used is in a variable range of 2-10% of the textile weight.

author : Laxmikant S. Jawale

Concept of pH in Dyeing

A variety of factors can influence the quality of dyeing and batch-to-batch reproducibility, but pH ranks as one of the most important factors. The pH of dye bath solution is most critical because of its effect on the dyeing cycle with respect to level and reproducible dyeing5. Even if the pH of the dye bath has been adjusted prior to the dyeing process, it may then be affected by various factors such as the absorption of acid by the fiber itself, increased alkalinity by boiling temporarily hard water and reduce alkalinity by loss of ammonia where ammonium salts are present in the bath at elevated temperatures in open-dyeing systems.

The control of pH in dyeing is ensured by three fundamentally different techniques.

A) Maintenance of a relatively high degree of acidity or alkalinity.
B) The control of pH within narrow tolerance mainly near the neutral region.
C) The gradual shifting of pH as dyeing proceeds.


Approach A is normally easiest to control and is used in the application of leveling type of acid dyes and 1:1 metal complex dyes to wool and nylon, and of the reactive, sulphur, vat dyes to cellulose. The agents traditionally used are the acids such as sulphuric, hydrochloric, formic acid, and alkalies like sodium bicarbonate and caustic soda.

Approach B needs a greater awareness of the factors that not only determine pH, but also stabilize it against interferences. Most of the dye-fibre systems requiring this approach are operated in the near neutral region (pH 4-9) and are much more sensitive to minor changes in pH. The pH of water supply may vary, or drift, during heating.

Approach C is particularly useful for non-migrating acid dye on wool and nylon. More recently, similar systems have been proposed for reactive dyes on cellulosics. In this case, pH control involves a deliberate shift of pH during processing in a consistent direction, rather than randomly.

pH control has received considerable attention in dyeing processes because of its critical role in quality assurance8,9. In dyeing, pH exhibits strong nonlinearity and time varying behavior. Some dyeing processes such as acid dyes on nylon, etc., are very dependent on pH. In dyeing process, pH not only responds to the addition of acids or base, but also varies as the temperature increases.

General Concept of pH and Buffer

pH

Pure water contains, in addition to H₂O molecules, a very small proportion of electrically charged ions formed by electrolytic dissociation, according to the equation

Where H ⁺ is a hydrogen ion, hydrion or proton, produced from a hydrogen atom by the loss of an electron and hence carrying positive charge; and OH ^- is a hydroxyl ion, which is negatively charged.

In pure water the two ions are present in equal numbers, since the water is electrically neutral, and at 25⁰C. their concentration is 10^-7 gm-ions per litre. It can be shown (from the law of mass action) that, in any dilute aqueous solution, the product of concentrations of the hydrogen ions and hydroxyl ions is constant and is equal to 10^-14 gm-ions per litre. i.e.,

[H ⁺] X [OH ^-] = 1.0 X 10^-14

When an acid is added to water it introduces hydrogen ions. Hydrochloric acid, for example, introduces hydrogen ions and chloride ions through electrolytic dissociation

Consequently, hydroxyl ions in the water are reduced so that the product of the hydrogen ion concentration and hydroxyl ion concentration remains equal to 10^-14. For example, if the concentrations of hydrogen ions is increased to 10^-3, the concentration of hydroxyl ions must fall to 10^-11. Similarly, alkalies such as caustic soda introduce hydroxyl ions.

and so cause a reduction in the number of hydrogen ions present in the water. Solutions are acid when the number of hydrogen ions exceeds the number of hydroxyl ions, and alkaline when the hydroxyl ions are in excess of the hydrogen ions. The degree of acidity or alkalinity depends entirely on the relative proportions of the two ions.

In pure water at 25⁰C, the hydrogen ion concentration (H ⁺) = 10^-7gm-ions per litre, or –log₁₀ (H⁺) = 7. The pH value of water is therefore 7, the relation between pH and hydrogen ion concentration being expressed by the equation –

i.e., the pH value is equal to the logarithm of the hydrogen ion concentration with negative sign.

Methods of mordanting for natural dyeing

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.

Cotton:
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.

Wool:
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.

Silk:

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.