7/8/12

Dyeing of acrylic fibres

All fibres dyed the same colour: dye with a combination of direct-dyeing union dyes and acetate dyes. Dyeing of acrylic fibres These fibres can exhibit very different dye affinities. Acrylic staple fibres are more easily dyed and in greater depths than filaments. The dyeing of acrylic fibres has proved difficult to the extent that whilst polyacrylonitrile fibres only begin to exhibit adequate affinity at relatively high temperatures of 70–85°C (depending on origin), the rate of dyeing is virtually doubled with an increase in temperature of around 3–4°C. Moreover, only a very low degree of migration takes place at the usual dyeing temperatures, i.e. as a rule, uneven dyeings cannot be levelled out adequately.

I. Cationic dyes: suitable for pale to dark shades; pale to medium depth shades are usually dyed using cationic or anionic retarders at the boil. The HT process is only suitable for acrylic filaments and fibres of low affinity. Continuous methods are used for loose stock, tow and combed sliver as well as, to some extent, acrylic/cellulose blends (fixation medium = saturated steam). The thermosol process is of practically no significance (levelling problems, unsatisfactory reproducibility and effect on handle).

II. Disperse dyes: the material is introduced into the dye bath at 70°C, brought to the boil quickly and dyed at the boil for 1 h. The thermosol and pad-steam processes are of secondary importance. Dyeing of acrylic/wool blends Dyeing methods single-bath single-stage, single-bath two-stage or a two-bath process. Cationic dyes are used to dye the acrylic component and acid, 1 : 2 metal-complex and reactive dyes are used to dye the wool component.

When dyeing acrylic fibres and wool, the cationic dyes stain the wool component first. As the boil is approached during dyeing, they transfer slowly from the wool on to the acrylic fibre and if the proportion of wool is at least 30%, retarders are not necessary. The transfer of cationic dyes from the wool is dependent on the quantity of dye, dyeing time, dyeing temperature and the pH. In the case of deep shades, a dyeing time of 45–60 min. at the boil is necessary to achieve a good wool reserve. With falling pH, the wool reserve increases. A good wool reserve is a prerequisite for the achievement of optimum wet and light fastness properties.

Dyeing of cellulose fibres: Dyeing of cotton, Dyeing of linen, Dyeing of jute, Dyeing of regenerated cellulose. 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 oil soluble dyes in the spinning oil.

Dyeing of cotton 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.

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