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The absorption of the dye increases with decreasing pH. Dyeing conditions vary from weakly acidic by addition of ammonium sulphate and acetic acid to neutral or moderately alkaline for
high-affinity dyes. For high-affinity dyes amphoteric or non-ionic levelling agents are usually added.
Reactive In principle, the reactive dyes used for wool are also suitable for polyamide. The dyeing process is carried out in weakly acidic conditions (pH 4.5 - 5). The process is started at 20 – 45 ºC and then temperature is increased near to boiling. Non-ionic surfactants and sodium bicarbonate or ammonia are used in the after-treatment step.
Table 2.7: Summary of the most common dyestuffs and dyeing techniques for polyamide fibres 2.
7.6.2 Polyester fibres Articles made of pure PES are dyed almost exclusively using batch dyeing techniques and among these, dyeing under high-temperature conditions is the most commonly applied.
Dyeing polyester fibres under atmospheric conditions (below 100 ºC) was also frequently done in the past with the aid of carriers. Since these substances are ecologically harmful (see Section 184.108.40.206 and 8.6.7), dyeing below 100 ºC is no longer in use today for pure PES fibres, unless carrier-free dyeable fibres are employed (see Section 4.6.2).
Concerning high-temperature dyeing, the process is usually carried out in acidic conditions (pH 4 - 5) with addition of acetic acid under pressure at 125 – 135 ºC. In these conditions levelling agents are necessary to prevent excessively rapid absorption.
Provided that alkali-stable dyes are used, dyeing in alkaline medium (pH 9 - 9.5) is also possible. This technique has been developed in order to counteract the migration and deposition of oligomers typical of PES fibres (see Section 220.127.116.11). In fact, oligomeric components (cyclic trimers of ethylene terephthalate are especially harmful) tend to migrate out of the fibre during dyeing, thus forming with the dye agglomerates that can deposit on the textile or on the dyeing equipment. To achieve level effects, ethoxylated products are used as levelling agents.
70 Textiles Industry
Chapter 2The thermosol process is another applied technique, although it is primarily used for PES/cellulose blends. The dye is padded on the textile together with an anti-migration agent. A drying step at 100 – 140 ºC is carried out. Then the dye is fixed (200 – 225 ºC for 12 - 25 seconds).
For light shades, the material needs only to be rinsed or soaped after dyeing. For dark shades, in order to ensure high light fastness, an after-clearing step is normally necessary. This usually consists of an alkaline reductive treatment followed by post-rinsing in weakly acidic conditions.
Information about alternative processes is reported in Section 4.6.5.
PES fibres can be dyed with cationic dyestuffs, provided that acidic components (e.g. sulphated aromatic polycarboxylic acid) are used as co-monomers during the manufacturing of the fibre (creation of anionic sites).
Table 2.8: Summary of the most common dyestuffs and dyeing techniques for polyester fibres 2.
7.6.3 Acrylic fibres So called PAC fibres are hydrophobic and contain anionic groups in the molecule. As a result, they can be dyed with disperse and cationic dyes. With the introduction of cationic comonomers in the polymer, the fibre can also be dyed with acid dyes.
Batch dyeing is commonly applied for cable or stock (package dyeing), yarn in hank form or packages and for fabric. Piece dyeing can be performed on beam, overflow, paddle (for knitwear, ready-made bath sets), or drum (socks).
Stock, cable and top can be also dyed on special machine, using the pad-steam process, preferably with pressurised steam to obtain short fixing times. Piece goods, especially upholstery material (velour), are also dyed according to the pad-steam process, but in this case fixing is carried out with saturated steam. This implies longer fixing times, which means that rapidly diffusing cationic dyes and dye solvents are required.
Disperse dyes are used to produce light to medium-deep shades. The dyeing techniques correspond to those used on polyester fibres. However, dyeing can be performed at temperatures 100 ºC without carriers. Furthermore, due to the good migration properties of disperse dyes, levelling agents are not required.
Cationic dyes Typical recipes used in batch dyeing include an electrolyte (sodium acetate or sodium sulphate), acetic acid, a non-ionic dispersant and a retarding agent. Dyeing is conducted by controlling the temperature in the optimum range for the treated fibre. Finally the bath is cooled down and the material is rinsed and submitted to after-treatment.
Continuous processes commonly applied are:
· pad-steam process (fixation with pressurised steam at more than 100 ºC) - this process has the advantage of reducing fixing time. Pad liquor typically contains a steam-resistant cationic dye, acetic acid and a dye solvent · pad-steam process (fixation with saturated steam at 100 - 102 ºC) - this process requires a longer fixing time. Rapidly diffusing cationic dyes and dye solvents, which exhibit a carrier effect, are required.
When dyeing with basic dyes, special levelling agents (also called retarding agents) are widely used to control the absorption rate of the colourant on the fibre, thus improving level dyeing.
Dyestuff Chemicals and auxiliaries/ typical application conditions Technique
Table 2.9: Summary of the most common dyestuffs and dyeing techniques for polyacrylic fibres 2.
7.6.4 Cellulose acetate (CA) and cellulose triacetate (CT) In contrast to the other regenerated cellulose fibres, CA and CT are hydrophobic and therefore they can be dyed with disperse dyes under conditions which are very similar to those applying to PES fibres.
Cellulose acetate is dyed by the exhaustion method with disperse dyes in the presence of nonionic or anionic dispersing agents in weakly acidic conditions (pH 5 - 6). Dyeing is normally done at 80 – 85 ºC. However, a series of less wetfast dyes already absorb onto the fibre at 50 - 60 ºC, whereas more wetfast dyes require temperatures up to 90 ºC.
Compared to CA, CT dyeing and finishing characteristics are more similar to purely synthetic fibres. CT, like CA, is dyed with disperse dyes in a weakly acidic medium in the presence of
levelling auxiliaries. Applied dyeing techniques for CT are:
· batch dyeing process, usually at 120 ºC, but if these conditions are not possible a dyeing accelerant (based on butyl benzoate or butyl salicylate) is required · thermosol process.
2.7.7 Fibre blends dyeing Natural/synthetic fibre blends are becoming more and more important in the textile industry because this allows combining the favourable technological properties of synthetic fibres with the pleasant feel of natural fibres.
Of the worldwide consumption of PES fibres, 55 – 60 % is used in blends with cellulose fibres or wool. About 40 % of polyamide is used in blends, while 50 % of polyacrylic fibres is used especially in blends with wool for knitwear [186, Ullmann's, 2000].
Fibre blends can be produced according to three different methods:
· fibres of different types in the form of staple fibres are mixed at the yarn manufacturing stage, during spinning · fibres of different types are separately spun and the resulting yarns are wound together to give a mixed yarn · fibres of different types are separately spun and combined together only at the weaving stage where one or more fibre yarns are used as warp and the other ones as weft.
Dyeing of blend fibres is always longer and more difficult as an operation compared to pure fibre dyeing. Despite these disadvantages, dyeing tends to be placed as close as possible towards the end of the finishing process. In fact this enables the dyer to satisfy the requests of the market without the need to store large amounts of material already dyed in flock or yarn form in all available shades.
When dyeing blend fibres, the following methods can be applied:
· the two fibres are dyed in the same tone ("tone on tone") or in two different shades using the same dyes · only one fibre is dyed (the colourant is not absorbed by the other ones) · the different fibres are dyed in different tones.
For "tone on tone" dyeing, it is sometimes possible to use the same dye for the different fibres.
When dyes of different classes have to be employed, the dyeing process is easier to control when the selected colourants have affinity only for one fibre and not for the other one. In reality, however, this situation is exceptional and the dyeing of fibre blends remains a complex operation.
Blend fibres dyeing can be done in batch, semi-continuous and continuous processes. Batch
· dyeing in one bath and one step (all dyes are added in the same bath in one single step) · dyeing in one bath and in two steps (dyes are added to the same bath in subsequent steps) · dyeing in two baths (dyes are applied in two steps in two different baths).
The most common fibre blends will be discussed in the following sections.
Polyester-cellulose blends A large part of the entire production of PES (ca. 45 %) is used to make this mixture. Polyestercellulose blends are used for all types of clothing and for bed linen. The cellulose component is usually cotton, but viscose staple fibres and occasionally linen are also used. The preferred mixing ratio is 67:33 PES: cellulose (for textiles worn close to the skin), 50:50 and 20:80 [186, Ullmann's, 2000].
In dyeing PES-cellulose mixtures, disperse dyes are used for the polyester component, while the cellulose portion is usually dyed with reactive, vat and direct dyes. Pigment dyeing is also commonly used for light shades.
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 padsteam, 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.
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 18.104.22.168), 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.
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].