Dyeing of cotton, acetate, regenerated cellulose, polyamide fibre and their blands

Dyeing of coir
Method as for jute – dyeing is usually carried out using direct or acid dyes under mildly acidic conditions (cationic dyes have inadequate light fastness). Coir is also dyed before spinning using oilsoluble dyes in the spinning oil.

Cotton fibre dyeing
The affinity of cotton for dye varies according to its history. The adsorption capabilities of cotton are increased by caustic solutions or mercerization.
There is a huge selection of dyestuffs available for cotton: substantive dyes, vat dyes, sulphur dyes, leuco-ester vat dyes, reactive dyes, naphthol dyes and phthalocyanine dyes. Of less importance are pigment dyes and chrome dyes. Cationic, aniline black and mineral khaki dyes do not have any significant application.
There is a clear trend towards dye groups with high fastness properties. A continual problem for the finisher is the proportion of immature or dead fibres (dead cotton) which give rise to the appearance of bright spots in coloured fabrics. These faults are significantly reduced by treating the fibre with caustic solution or by mercerization.

Dyeing of cotton and acetate or cotton and triacetate fibre blends,
I. Acetate or triacetate white, cotton coloured: dyeing is usually carried out with selected substantive dyes which reserve the acetate and triacetate. The dyeing temperature must not exceed a maximum of 80°C with acetate since staining of the fibre occurs at the boil.
Acetate which has been scoured at too high a temperature and which is partially saponified will no longer be reserved. Even for acetate and triacetate which have been delustered in spinning a good reserve is more difficult to obtain.
II. Cotton white, acetate or triacetate coloured: this is a considerably less favourable combination as disperse dyes stain the cotton to a greater or lesser extent. An afterclear treatment is necessary with sodium dithionite, sodium hypochlorite or potassium permanganate.
III. Cotton/acetate or triacetate dyed tone-in-tone: usually carried out by single-bath dyeing with substantive and disperse dyes. Two-bath method : a) the acetate or triacetate is dyed with diazotizable disperse dyes; b) then using a fresh bath for the acetate or triacetate, the cotton is filled in with substantive dyes. All the above variants are carried out by exhaust dyeing; pad dyeing is only used in exceptional cases.

Ccotton and regenerated cellulose fibre blend dyeing
Cotton/regenerated cellulose blends are dyed using the same classes of dyes as for cotton. The achievement of tone-in-tone uniformity between cotton and regenerated fibres is problematic due to the greater dye affinity of the latter (greatest with cupro fibres and weakest with high-wet-modulus fibres). The dyeing method, especially for substantive dyes, must be appropriately adapted with regard to salt additions and temperature, i.e. dyeing must be carried out with less salt or no salt at all and at lower temperatures. If necessary, dyeing is started at the boil with a portion of the dye, and the rest is added to the bath after cooling down.
Padding methods are more suitable than exhaust methods for vat, leuco-vat ester and naphthol dyes. Causticizing before dyeing improves the affinity of the cotton. However, despite all these measures, tone-in-tone dyeing is not achievable with dark shades.

Cotton and regenerated cellulose with polyamide blends
Tone-in tone dyeings with a single class of dyes are only achieved with great difficulty. In most cases, one or other of the fibre components will have to be filled in with specifically appropriate dyes.

Exhaust methods: selected substantive and vat dyes (for which the semi-pigmentation process is particularly suitable) are capable of producing tone-in-tone dyeing in pale shades by using appropriate dyeing methods. A single-bath method with substantive and metal-containing disperse dyes can be used here. For dark shades, the polyamide must be dyed first with metal-containing disperse dyes followed by the cellulose in the same bath with substantive dyes. A single bath two-stage method may also be used with selected reactive dyes. In this case, either the cellulose is dyed first under alkaline conditions followed, after acidifying the bath, by the polyamide or, the polyamide is dyed first at pH 5 and followed, after neutralization, by the cellulose. Two-bath two-stage method: metal-containing disperse or 1 : 2 metal complex and reactive or vat dyes. Pad dyeing: pigments may be used for pastel shades. Where high fastness is required, each fibre component must be dyed separately by the methods customarily used for it. For the polyamide component, metal-containing disperse dyes are by far the most important, and vat, sulphur or reactive dyes for the cellulosic components. Selected acid dyes may be applied together with vat dyes by a single-stage method, e.g.: both classes of dye are padded together, followed by acid steaming, drying, thermofixation, padding with caustic/sodium dithionite, then steaming, oxidizing and washing. Phthalocyanine dyes may be employed for turquoise shades.

Dyeing of flammé effects
This process is mainly carried out on hank yarns. This is effectively done by tying off the hanks individually at the sites where they are to be dyed in one particular colour first. Dyeing is then carried out in the customary way with the liquor just reaching the ties. The hanks are withdrawn and thoroughly rinsed by immediately spraying them with cold water in an upward direction. The ties are undone (after an intermediate drying step if necessary), rehung and tied again for the next dyeing with another dye, as already described, and rinsed down again etc. until the whole hank is dyed in the various colours desired. Two, three, four or more different colours may be involved and these may be interspersed with non-dyed areas or the colours may be adjacent to each other or even overlapping.

Instead of obtaining flame effects on non-dyed yarn, hanks can be pre-dyed in a certain shade and then dyed in certain places as described above. Instead of using the technique described above, dyeing is also carried out using a pulley lifting system without tying off the hanks. Õ Dyeing of ombré or shaded effects.

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