Dyeing of acetate fibres
(Dyeing of triacetate fibres). Disperse dyes are predominantly used both for acetate and triacetate fibres and are mainly used in exhaustion processes. As is the case for polyesters, triacetate must be dyed either with the addition of carriers or under HT conditions. In the case of acetate, selected cationic and naphthol-based dyestuffs can also be considered. There are only limited possibilities for using the padding process as the thermosol dyeing process cannot be used for acetate fibres and can only be used for triacetate fibres when lighter shades are being applied.
Acetate, wool, cotton fibre union dye:
I. Acetate white, union coloured: single-bath dyeing with suitable neutral-dyeing acid, direct or union (mixed fibre) dyes. Two-bath method, this is possible by pre-dyeing the cellulosic fibres with direct dyes at approx. 70°C, rinsing and filling in the wool with acid dyes from an acidic bath.
II. Acetate coloured, union white: it is questionable whether a satisfactory union reserve is possible as even the most suitable acetate dyes cause staining of the wool.
III. Acetate and union in two colours: single-bath method using a combination of dyes according to I. +
II. (if necessary II. + union dyes) in a neutral sodium sulphate bath (Glauber’s salt). Two-bath method by pre-dyeing the acetate, filling in the union and rinsing.
IV. Acetate, union in three colours: single-bath method using dyes which reserve the other fibres completely by dyeing according to dyeing methods I.–II. In a neutral bath. Two-bath method as single-bath with filling in of the vegetable fibre.
V. 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/ cellulosic 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.
Acetate, wool, cotton fibre union dye:
I. Acetate white, union coloured: single-bath dyeing with suitable neutral-dyeing acid, direct or union (mixed fibre) dyes. Two-bath method, this is possible by pre-dyeing the cellulosic fibres with direct dyes at approx. 70°C, rinsing and filling in the wool with acid dyes from an acidic bath.
II. Acetate coloured, union white: it is questionable whether a satisfactory union reserve is possible as even the most suitable acetate dyes cause staining of the wool.
III. Acetate and union in two colours: single-bath method using a combination of dyes according to I. +
II. (if necessary II. + union dyes) in a neutral sodium sulphate bath (Glauber’s salt). Two-bath method by pre-dyeing the acetate, filling in the union and rinsing.
IV. Acetate, union in three colours: single-bath method using dyes which reserve the other fibres completely by dyeing according to dyeing methods I.–II. In a neutral bath. Two-bath method as single-bath with filling in of the vegetable fibre.
V. 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/ cellulosic 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.
Acrylic and wool fibre blend dye
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.
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