Basic Concept of Dyeing of Animal Fibres

Dyeing of Animal Fibres
Wool and silk are the two important natural textile fibres next to cotton. Both are protein fibres obtained from animal sources. Depending on the species, they are available in wide varieties having varied qualities and costs. It is difficult to recommend specific processing conditions for them; the conditions are to be changed depending on the quality of the specific lot of fibre. Wool is obtained from sheep, while silk is obtained from silkworms. Both need careful rearing. High qualities of wool and silk are very costly and need very careful handing. They have complex chemical structure. They are delicate and very sensitive to alkali and heat.
Dyeing of Animal Fibres
Silk and wool fiber
These fibres can be dyed with various classes of dyes, the most important being:
  1. Acid dyes
  2. Chrome dyes
  3. Premetallised or metal-complex dyes
  4. Reactive dyes
Less important classes, which were once popular, but now are of theoretical interest, are direct dyes, basic dyes, solubilised vat dyes, etc.

The breakdown of traditional wool coloration processes is estimated to be as follows:
  • Loose stock dyeing -............................. 16%
  • Top dyeing -.......................................... 16%
  • Yarn dyeing (hank and package) -.......... 40%
  • Piece and garment dyeing -.................... 28%
Quick response has become an important factor in the wool-processing pipeline, and the industry has identified late-stage coloration as a means of delaying production commitments until the final possible stage. Current emphasis is on improving application techniques in package dyeing, piece dyeing and garment dyeing to reduce stock holding and shorten delivery times.

Fibre damage in wool dyeing has been minimised in new machine designs, incorporating controlled liquor flow, pressure and optimised drying procedures. Radio-frequency technology has become a standard in wool-drying systems based on conveyor belt R.F. dryer.

In yarn coloration, dyeing in hank form is still popular, because of the bulky handle it produces, but the winding and unwinding operations are inefficient and the restricted liquor circulation and channelling can lead to unlevelness. Package dyeing provides a much more uniformly dyed substrate. Horizontal package dyeing machines have the flexibility to keep liquor ratios constant with varying batch sizes.

Woollen yarns, which are to be dyed in package form, are preferably dry spun with a minimum quantity of lubricant. Water-soluble lubricants can be removed by simple rinse – extensive scouring in package form is expensive and difficult. Yarns for weaving, knitting and carpets are dyed with acid milling and metal-complex dyes at pH 5.5-6.0 in the presence of a levelling agent and optionally a moth-proofing agent such as Eulan WA (DyStar, formerly Bayer). Wool fibres, which have been given a shrink-resist treatment in sliver form, are generally dyed with reactive dyes.

In fabric dyeing, the limited liquor circulation found in winch machines has restricted their use to applying levelling and Sandolan MF half-milling dyes. However, with modern jet dyeing machines milling acid dyes can be applied in a level manner.

The four major stages in woollen fabric processing are carbonising, scouring, milling and dyeing. The processing stages can be carried out in various sequences. Grey carbonising allows maximum removal of cellulosic contaminants, but unless selected lubricants (mostly saponifiable types) are used, the acid solution is contaminated. Least damage of wool is claimed, but long storage in highly acidic condition may cause damage and yellowing. Acid treatment, followed by an alkaline treatment without care, can cause serious damage. Scouring before carbonising means that a larger number of lubricating oils can be used by the spinners, but burr removal may be difficult, especially for highly contaminated materials, due to consolidation of fabric during scouring. Acid and alkaline milling techniques give fabrics of different quality; the former gives denser felting. When carbonisation is done before dyeing, neutralisation is not necessary for levelling acid dyes and 1:1 metal-complex dyes. The following sequence gives significant economics in acid consumption.

Scour carbonise semi-neutralise acid mill dye with levelling acid dyes.

Discharge printing is popular on silk material. To prepare discharge ground shades, selected direct dyes are used. Substantive direct dyes are also suitable for covering component fibre. These are also used for covering cellulosic components coming from dust in the weaving mill in heavy silk fabric (e.g. shantung) and also in schappe blends. Direct dyes are applied in dyeing under the same conditions as those for acid dyes.

The affinity of basic dyes for wool and silk is probably due partly to electrostatic attraction between the basic group in the dye and the carboxylic groups in these fibres and partly to non-polar Van der Waals’ forces. In general, the affinity is not very high, so they show only moderate exhaustion. Basic dyes are now rarely used for wool and silk dyeing, more so since acid-modified Rhodamine and triphenylmethane dyes for very brilliant shades in green and turquoise blue hues are available. Certain basic dyes are used as discharge-resistant illuminating colour in discharge printing. Basic dyes may be applied on silk material at 85ºC using 0.5 g/l acetic acid. Fastness can be improved by treating the dyed material with 1% (o.w.m.) tannic acid for 20 minutes at 60ºC or alternatively for overnight at cold. After squeezing, the material is further treated with a cold or lukewarm solution having 0.5 g/l tartar emetic, followed by washing.

Solubilised vat dyes can also be used as discharge-resistant illuminating colour in discharge printing of silk. Most of the solubilised vat dyes are suitable for application on wool. Indigosol O, derived from indigo, behaves like milling acid dyes, but dyeing is followed by strong acid treatment to regenerate the parent vat dye by hydrolysis and oxidation, and then by soaping to develop the true shade and maximum fastness properties of the dye. The method, mainly of interest for loose wool and slubbing, is to start dyeing at 40ºC with 5% ammonium sulphate, the temperature is raised to boil and kept for 30 minutes. 1-6% formic acid (85%) is added and dyeing is continued for a further 30 minutes. If exhaustion is incomplete, 2% sulphuric acid is added further. After dyeing and rinsing, the colour is developed by oxidation for 30 minutes at 80ºC in a bath containing 5-7½% ammonium persulphate and 10-20% sulphuric acid, all percentages are on the weight of material (o.w.m.). With Indigosol O, oxidation may be carried out with sodium nitrite and sulphuric acid. Rinsing, neutralising and soaping for 15 minutes at 70ºC, in a bath containing 0.5 g/l Lissapol N (ICI) completes the process.

Acid dyes are comparatively cheaper and produce bright dyeing, while dyeing with the chrome mordant works out to be expensive. These dyes they produce dull shades of good all round fastness. The major objection to their application is the release of objectionable chromium compounds in the effluent water. Metal-complex or premetallised dyes are easy to apply and produce dyeing of reasonably good fastness. Reactive dyes find application on these fibres nowadays and produce bright dyeing with good fastness properties.

One of the major problems of wool dyeing is the uneven dyeing or skittery dyeing. If the wool cuticle is damaged by localised chemical attack or by abrasion, more rapid strike of dyes occurs on the exposed cortical cells. At low dyeing temperature, all dyes are taken up preferentially by damaged fibres and fibre tips. At higher temperature, the more polar the dye is, more strongly it is absorbed by the damaged fibres. Dyes of low molecular weight cover damaged fibres well and they produce less skittery dyeing by subsequent migration. With dyes of higher molecular weight, particularly sulphonated 1:2 metal-complex dyes, good coverage can be achieved by the use of cationic auxiliary products. They form hydrophobic complexes with anionic dyes, and these complexes are less sensitive to differences in dyeability of wool fibres. Moreover, wool fibres are very porous and entrapped air in these pores causes the fibres to float on the dyeing liquor during dyeing. Sufficient precautions are to be taken so that the material is dipped under the liquor during dyeing.

Since 1960s, after the advent of the domestic washing machines, our requirements for clothing have changed. In the past, formal attire was probably of great concern. Now, we live to a far extent in a wash-n-wear culture. The synthetic fibres and cellulosic natural fibres were able to conform to new culture, whereas wool had a marked disadvantage – it has a tendency to shrink even in mild hand or machine washing. Shrink-resist processes modify the wool fibre to restrict its natural ability to undergo felting shrinkage. An additional application of a polymer resin renders wool knitwear fully machine washable. This modified fibre is not only more receptive to dye, but also exhibits a markedly lower colourfastness.

In blends of wool with synthetic fibres, the good handle, comfort and drape properties of wool have been complemented by the additional easy-care properties of synthetic fibres, which impart strength, washability and stability to a blend. This has been further enhanced by the introduction of synthetic microfibres. Blends of wool with nylon are common in knitwear and footwear and blends with polyester are common in woven articles. Elastomeric fibres, such as spandex, are also becoming important.

These developments led to a need for faster dyes that could withstand domestic washing, even in deep shades. As a result, new dye ranges of higher colour fastness are demanded and 1:2 metal-complex and reactive dyes find more application on wool at present. However, the higher colour fastness of 1:2 metal-complex and reactive dyes is to some extent negated by their poor level-dyeing properties, and as such, their use is restricted to dyeing loose fibre and slubbing. Further developments are required so that they can be applied in yarn or garment forms.

Levelling agents are always used in wool dyeing. Glauber’s salt is the main levelling agent. It promotes migration with levelling acid dyes, but may precipitate milling and 1:2 metal-complex dyes of large molecular weight due to aggregation. Fibre-substantive cationic type levelling agents are very popular. They reduce dye-uptake by complex formation and promote migration of adsorbed dye. Fibre-substantive amphoteric type levelling agents, such as Lyogen TP (Clariant), are also popular. They improve coverage of the inherent variations of wool (such as tippiness) and increased yield with some dyes.

About the Editor-in-Chief:

Mazharul Islam Kiron is a textile consultant and researcher on online business promotion. He is working with one European textile machinery company as a country agent. He is also a contributor of Wikipedia.

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