Fibre blends dyeing

Disperse dyes stain cellulose fibres only slightly and they can easily be removed by subsequent washing or, if necessary, by reductive aftertreatment. Most of the dyes used for cellulose stain PES only slightly or not at all.

PES-cellulose blends are commonly dyed in continuous processes. Nevertheless, for yarn and knitwear, batch dyeing is of major importance.

In batch dyeing, the application of dyes can be done in one or two steps in one bath or in two different baths in subsequent stages. The disperse dye is generally applied at high-temperature (HT) conditions without the use of carriers.

In the one-bath/ one-step procedure, special auxiliaries, so-called acid donors, are used, which lower the pH when the temperature is increased. In this way, it is possible to fix the reactive dyes in alkaline conditions and then reach the optimal dyeing conditions (pH 5 - 6) for disperse dyes by increasing the temperature. Alternatively, it is advantageous to operate at pH 8 - 10 using alkali-stable disperse dyestuffs, which also avoid oligomer problems.

The one-bath/ one-step procedures are preferred, being more economic, but present more difficulties. For example, the presence of salt increases the tendency of disperse dyes to stain the cotton fibre of the blend. Recently developed low-salt reactive dyes are claimed to show good performance and high reproducibility in this application (see Section 4.6.11).

In continuous processes the dyes are usually applied in one bath. The fabric is subsequently dried and disperse dye is fixed to the PES component by the thermosol process. Afterwards, the second dye is developed according to the procedure typical of each class, using in general pad-steam, pad-jig or pad-batch processes.

Table 2.10 presents a summary of the most frequently applied processes is presented. Dyes are applied according to application conditions typical of their class. For more details regarding a given class of colourant, see the specific section.

Technique ^Disperse

/vat

Disperse/

reactive

Disperse /direct

Pigment

One-bath process Y K W ⁽¹⁾

Two-bath process Y K

Batch

One-bath two-step process Y K Y K Y K

I stage II stage

Thermosol + pad-jig W

Thermosol + pad-batch W

Continuous

Application of all dyes in one bath by padding + drying followed

by: Thermosol +pad-steam W W W

Y = yarn

W = woven fabric K = knitted fabric

(1) Pigment dyeing includes padding with the pigment, a binder and auxiliaries, drying and polymerisation at 140 ºC for 5 min.

Table 2.10: Summary of dyestuff and dyeing techniques for polyester-cellulose blends

Polyester-wool blends

Polyester-wool blends are widely used, especially for woven goods and knitwear. The most frequently found ratio is 55:45 PES: wool.

Wool cannot be dyed at the high temperatures typical of the HT dyeing process for PES fibres and PES-cellulose blends. The dyeing time should also be as short as possible so that the wool is not damaged. For large productions it is therefore preferable to dye wool and PES separately in top, blending the two fibres at the yarn manufacturing stage. However, quick changes in fashion and short-term planning frequently do not allow separate dyeing.

When dyeing polyester-wool blends, disperse dyes are used for polyester and anionic (acid and metal-complex dyes) for wool.

Only disperse dyes that stain wool as slightly as possible or are easily removed by washing can be used for dyeing wool-polyester blends. Disperse dyes, in fact, tend to stain wool and a reductive after-treatment is not always possible (appropriately stable dyes are required).

PES-wool blends are typically dyed according to the following batch processes:

• at boiling temperature with carriers

• at 103 – 106 ºC with little carrier

• at 110 – 120 ºC with the addition of formaldehyde as a wool protective agent and with low amounts of carriers or none at all (HT conditions).

Because of the environmental concerns associated with the use of carriers (see Sections 8.6.7 and 2.7.8.1), the first dyeing technique is avoided when possible. In this respect, carrier-free dyeable fibres are also available, which can be dyed at boiling temperature without the use of carriers (see Section 4.6.2 for more information about carrier-free dyeable fibres and Section 4.6.1 for environmentally optimised carriers).

The one-bath process method is preferred in practice; the two-bath process is applied when deep shades and high fastness are required. The material is first dyed with disperse dyes. A reductive intermediate treatment may be applied before dyeing the wool part. In both dyeing methods, after dyeing, an after-treatment is applied to remove any disperse dye attached to the wool, if the dye used for wool can withstand it. The material is treated with ethoxylated fatty amine in weakly acid liquor at 60 ºC.

Polyamide-cellulose blends

Since PA fibres have an affinity for almost all dyes used for cellulose, different possibilities are available for dyeing this blend:

• direct and disperse dyes (pH 8)

• acid or 1:2 metal-complex dyes (pH 5 - 8)

• vat dyes (exhaust and pad-steam process are used)

• reactive dyes.

Application conditions are those typical of each class of dye. They have already been described in the specific sections.

Polyamide-wool blends

Blends with polyamide/ wool ratios varying from 20:80 to 60:40 are used. This blend is particularly important in the carpet sector. More detailed information is therefore reported in the specific section dedicated to this sector (see Section 2.14.5).

As general information about the dyeing processes suitable for this type of blend, both fibres have affinity for acid and 1:2 metal-complex dyes. However, since PA is more accessible to the dye than wool, it is dyed more deeply, particularly in the case of light colours. To counteract this effect, special levelling agents (also called PA reserving/ blocking agents) are used (mainly aromatic sulphonates). These auxiliaries have a relatively high affinity for the PA fibre and retard the absorption of the colourant on this part of the blend.

Dyeing is performed in the presence of acetic acid and sodium sulphate. Due to limited fastness of acid dyes, 1:2 metal-complex dyes are required for dark shades [186, Ullmann's, 2000].

Acrylic-cellulose blends

PAC-cellulose blends are used for household textiles (drapery and table linen) and imitation fur ("peluche", in which the pile consists of PAC fibres and the back is made of cotton). The percentage of PAC in the mixtures varies between 30 and 80 %.

PAC can be dyed with cationic or disperse dyes, while direct, vat or reactive dyes can be used for the cellulose component.

The following methods are the most commonly used for dyeing this blend:

• continuous dyeing with cationic and direct dyes according to the pad-steam process (to avoid precipitation of cationic and anionic dyes present in the pad liquor at relatively high concentration, combination of anionic and non-ionic surfactants are added to the solution)

• batch dyeing (usually according to the one-bath, two-steps method) with cationic and vat dyes or with cationic and reactive dyes.

Acrylic-wool blends

Among synthetic fibres, PAC fibres are the most suitable for obtaining blends with wool that keep a wool-like character. This makes this blend widely used, especially for knitwear and household textiles. The blending ratio of PAC to wool varies from 20:80 to 80:20.

Metal-complex, acid and reactive dyes are the dyestuffs typically used for the wool part, while PAC is dyed with cationic dyes.

Cationic dyes stain wool fibre. As a matter of fact cationic dyes attach first to wool and then migrate to PAC fibre at higher temperature. Even if well-reserving dyes are selected, dyeing must be conducted for a sufficiently long time (from 60 to 90 minutes) in order to obtain good wool reserve [186, Ullmann's, 2000].

PAC-wool blends can be dyed using the following exhaustion methods:

• one-bath one-step

• one-bath two-step

• two-bath.

The first one allows shorter dyeing times and lower consumption of water. However, it is not always applicable because the simultaneous presence in the dye bath of anionic and cationic compounds can produce the precipitation of the formed adducts on the fibre. Precipitation can be prevented using dispersing agents and selecting adequate dyes.

When dyeing with the one-bath, two-step method the use of reserve agents is not necessary. In fact, wool absorbs the cationic dye and slowly releases it, acting as a retarding agent (exerting a retardant effect on PAC).