«EUROPEAN COMMISSION Integrated Pollution Prevention and Control (IPPC) Reference Document on Best Available Techniques for the Textiles Industry July 2003 ...»
Pad processes encompass the following techniques:
· pad-steam · pad-roll · cold pad-batch · pad-jig process (the material is padded with the dye and then passed through a salt liquor in a jigger).
In all processes the material is rinsed at the end with cold water.
With increasing depth of colour the wet fastness can decrease to such an extent that aftertreatment must generally be carried out [186, Ullmann's, 2000]. Two methods exist:
1. removing the unfixed dye by washing with complexing agents or surfactants with a dispersing effect
2. reducing the solubility of the dye by blocking the hydrophilic groups (“enlargement of the molecule”).
Various techniques can be applied to achieve this enlargement of the molecule. Namely, the
dyed textile can be treated with:
· fixative cationic agents: these are complex substances that form with the anionic dye a saltlike compound less soluble than the original dye. Quaternary ammonium compounds with long hydrocarbon chains, polyamines and polyethyleneimine derivatives can be used for this purpose · metal salts: copper sulphate and potassium dichromate can form with certain azo dyes metal-complex with higher light fastness · agents based on formaldehyde condensation products with amines, polynuclear aromatic phenols, cyanamide or dicyandiamide (the use of these condensation products leads to the formation of sparingly soluble adducts with the dye molecules) · diazotised bases: after dyeing, the material is submitted to diazotisation and is then coupled with aromatic amines or phenols that must not contain hydrosolubilising groups [186, Ullmann's, 2000].
Environmental concerns arise when after-treating with formaldehyde condensation products or metal salts. The method using fixative cationic agents is, therefore, the most frequently applied.
However, quaternary ammonium compounds are often non-biodegradable, fish-toxic and contain nitrogen.
Vat dyes Vat dyes have excellent fastness properties when properly selected and are often used for fabrics that will be subjected to severe washing and bleaching conditions (towelling, industrial and military uniforms, etc.).
Vat dyes are normally insoluble in water, but they become water-soluble and substantive for the fibre after reduction in alkaline conditions (vatting). They are then converted again to the original insoluble form by oxidation and in this way they remain fixed into the fibre.
When applying vat dyes in batch processes the textile is dyed very rapidly and unevenly due to
the high affinity of the dye. Nevertheless, level dyeing can be achieved by:
· adding levelling agents · increase of the temperature under a controlled profile ("High Temperature" process and "Semi-pigmention" method) · impregnation of the textile with the dye as water-insoluble dispersion, followed by addition of the reductive agent in a subsequent step (pre-pigmentation process).
In all cases, oxidation and after-treatment follow. After-treatment consists in washing the material in a weakly alkaline bath with a detergent at boiling temperature.
Continuous processes are used almost exclusively for dyeing woven fabrics and to only a small extent for knitwear. The most commonly applied continuous process is the pad-steam process.
The textile is padded with the aqueous dye dispersion in the presence of anti-migrant (polyacrylates, alginates, etc.) and dispersing/wetting agents, if required. After drying, the fabric is passed through a chemical padder, which contains the required amount of alkali and reducing agent and is fed immediately to a steamer. The material is finally rinsed, oxidised and soaped in an open-width washing machine.
A more rapid, one-step process is also possible (see Section 4.6.4), but only for pastel to pale shades.
Voluminous open fabrics can be dyed according to a wet-steam process. Unlike the pad-steam process, this process does not require intermediate drying before steaming.
The following chemicals and auxiliaries are applied in vat dyeing:
· reducing agents: mainly sodium dithionite (hydrosulphite) and sulphoxylic acid derivatives (Zn-sulphoxylate). The latter, in particular, is used when the pad-steam process is applied.
Sulphur-free organic reducing agents such as hydroxyacetone are also now available for some applications · oxidising agents, such as hydrogen peroxide, perborate, or 3-nitrobenzenesulphonic acid · alkali (caustic soda) · salt · dispersing agents: they are already present in the dye formulation and are further added in the subsequent steps of the dyeing process · levelling agents: they form adducts with the dye, thus retarding its absorption onto the fibre.
Sulphur dyes Sulphur dyes are used in piece dyeing (cellulose and cellulose-polyester blends), yarn dyeing (sewing thread, warp yarn for denim fabric, yarn for coloured woven goods), dyeing of flock, card sliver (wool-man-made fibres blends) [186, Ullmann's, 2000].
Like vat dyes, sulphur dyes are insoluble in water, and, under alkaline conditions, are converted into the leuco-form, which is water-soluble and has a high affinity for the fibre. After adsorption into the fibre the colourant is oxidised and converted to the original insoluble state. The
reducing agent, salts, alkali and unfixed dye are finally removed from the fibre by rinsing and washing.
Mostly continuous dyeing methods are applied, although batch dyeing (in jigger, jet, and winch beck) is also possible.
In continuous processes the material is impregnated with dye, reducing agent and wetting agent through a one-bath or a two-bath procedure. With the one-bath procedure (pad-steam process) the reducing agent and the dye are added at the same time. With the two-bath procedure (paddry/pad-steam) the material is padded in the liquor containing the dye and the wetting agent, while the reducing agent is applied, if necessary, in a second step, after intermediate drying. The material is then submitted to air-free steaming. After that, rinsing, oxidation and re-rinsing are carried out.
Because the exhaustion is not too high, it is possible to re-use dyeing baths in continuous processes.
Chemicals and auxiliaries applied to the substrate during the dyeing process are:
· reducing agents: sodium sulphide, sodium hydrogensulphide and thiourea dioxide are the most commonly employed (although their use has decreased over the past decade [281, Belgium, 2002]). Binary systems made of glucose and sodium dithionite, hydroxyacetone and glucose or formamidine sulphinic acid and glucose are also used as alternative reducing agents (see Section 4.6.6) · alkali (caustic soda) · salt · dispersing agents (they are necessary in the process steps in which the pigment has not yet been reduced or has been re-formed by oxidation) · complexing agents: EDTA or polyphosphates are used in some cases, especially in circulating-liquor dyeing to avoid the negative effects of alkaline-earth ions on dyeing · oxidising agents: mainly hydrogen peroxide and halogen-containing compounds such as bromate, iodate and chlorite.
Azoic dyes (naphthol dyes) Naphthol AS dyes allow colours with outstanding fastness, but their popularity has declined because of application costs and the complexity of the process for the preparation of the colourant [77, EURATEX, 2000].
Dyeing with azoic colourants is a complex process which involves a number of delicate steps:
· preparation of the naphtholate solution by the hot solution process (the naphthol is dissolved by boiling with caustic soda) or by the cold solution process (the naphthol is solubilised with alcohol or cellosolve, caustic soda and cold water). For certain naphthols the addition of formaldehyde is also necessary to prevent the formation of free naphthol · application of the naphtholate to the fibre by batch or padding techniques · preparation of the diazotized base by reaction with sodium nitrite and hydrochloric acid (this step can be avoided when using fast colour salts) · formation of the azoic dye into the fibre, by passing the textile, previously impregnated with the naphtholate solution, through a bath containing the diazotized base or the fast colour salt (addition of buffering agents is necessary to control the pH, in order to increase the coupling capacity) · after-treatment by rinsing the material to remove the excess naphthol from the fibre.
2.7.4 Wool dyeing
Wool can be dyed with the following dyestuffs:
· acid (metal-free) · chrome · 1:1 and 1:2 metal complex · reactive.
Acid dyes Acid dyes are typically applied in acidic conditions, but the pH range used varies depending on the type of acid dye. The greater the affinity of the dyestuff for the fibre, the more the hydrophobic interaction must be repressed by applying the dye at higher pH.
Therefore level-dyeing colourants are applied under strongly acidic conditions (1 – 3 % formic acid) in the presence of sulphate ions (5 – 10 % sodium sulphate) to assist migration and levelling. HCOOH and HSO4- compete with the sulphonated dyes acting as levelling agents. As a result, other levelling agents are not generally required and the rate of dye uptake is controlled initially by increasing the temperature of the dye bath slowly (1 ºC/min.) and then extending the time at the boil to allow the dye to migrate from areas of high initial uptake.
Fast acid dyes (also known as half-milling dyes or perspiration-fast dyes) exhibit superior fastness properties to level-dyeing acid dyes, while retaining some of the migration properties.
Application is from a moderately acidic dye bath (1 – 3 % acetic acid) in the presence of sodium sulphate (5 – 10 %) and levelling agent to assist migration.
Acid milling dyes (including 1:2 metal-complex dyes mentioned later) have good affinity for the fibre and do not migrate well at the boil. They are therefore applied at more neutral pH (5 - 7.5 with acetic acid) in the presence of sodium acetate (2g/l) or ammonium sulphate (4 %) and levelling agent (1 – 2 %). Sodium sulphate is usually avoided as it has little effect on migration and can promote uneven dye adsorption.
Levelling agents play an important role in acid dyeing. A number of non-ionic, cationic, anionic and amphoteric surfactants belong to this category.
Chrome dyes A number of techniques have been developed for the application of chrome dyes. The Chrome mordant process relies on chroming the fibre prior to dyeing with a chromable dye; the Metachrome process applies both dye and chromium salt simultaneously. Both processes have been largely superseded by the Afterchrome process in which the dye is applied first and the fibre is then chromed in a separate step, utilising the exhausted dye bath and thus conserving water.
Application of the dyestuff takes place from a moderately acid dye bath (1 % acetic acid) often with an addition of formic acid towards the end of the boiling period to promote exhaustion of the dye. The dye bath is then cooled back from the boil to approx. 80 ºC, the pH is lowered to approx. 3.5 with formic acid and the pre-dissolved chromium salt added. The dye bath is then returned to the boil and boiling continued for 20 - 30 minutes. Chromium is added to the dye bath as either sodium or potassium dichromate. In solution the chroming species present vary according to pH, with the dichromate anion Cr2O72- predominating at pH between 3 and 7.
Although it is the chromium (VI) anion which is initially adsorbed by the wool, the dye complex is formed with chromium (III), formed by the action of reducing groups in the fibre itself. Strong acids have an activating effect on this process; organic acids (tartaric, lactic, formic acid) or thiosulphate can also be used to enhance the degree of conversion of Cr VI (see also Section 4.6.15).
Metal-complex dyes Application of 1:1 metal-complex dyes is carried out at pH 1.8 - 2.5 with sulphuric acid or at pH 3 – 4 with formic acid, in the presence of sodium sulphate (5 - 10 %) and other organic levelling agents. Owing to these particular operative conditions, this class of dyestuffs is particularly suitable for piece-dyeing of carbonised wool.
1:2 metal-complex dyes form the most important group in this class and may be divided into two sub-groups: weakly polar and strongly polar 1:2 complexes. Application is usually carried
out in moderately acidic conditions:
- pH range 4 - 7 (acetic acid) in the presence of ammonium acetate, for weakly polar complexes
- pH range 5 - 6 (acetic acid) in the presence of ammonium sulphate, for strongly polar complexes.
The use of levelling agents is very common when dyeing with metal-complex dyes. The groups of substances used for acid dyes also apply in this case.
Reactive dyes Reactive dyes are generally applied at pH values of between 5 and 6, depending on the depth of shade, in the presence of ammonium sulphate and specialised levelling agents (amphoteric substances, which form complexes with the dye at low dye bath temperatures and then break down as the dyeing temperature increases).
The dyeing method may include a temperature hold step at 65 – 70 ºC, during which the dye bath is maintained at this temperature for 30 minutes in order to allow the dye to migrate without fibre reaction. When dyeing of the very highest fastness is required, the fibre is rinsed under alkali conditions (pH 8–9 with ammonia) to remove unreacted dyestuff.
2.7.5 Silk dyeing Silk is dyed with the same dyes as wool, excluding 1:1 metal complex dyes. In addition, direct dyes can be used. The dyeing pH is slightly higher than with wool. For reactive dyeing 20 - 60 g/l of salt and 2 - 5g/l of soda ash is needed for fixation [294, ETAD, 2001].
For more a detailed description, reference is made to the “Wool dyeing” section.
2.7.6 Synthetic fibres dyeing 184.108.40.206 Polyamide fibres Polyamide fibres (PA 6 and PA 6,6) are easily dyed with various types of dyes. Due to their hydrophobic characteristics, they can be dyed with disperse dyes (non-ionic), whereas thanks to
the presence of the groups NH-CO- and NH2- in the polymer chain, acid, basic, reactive and 1:2
metal-complex dyes (ionic) can also be used. However, in practice acid levelling dyes are increasingly used.
Before dyeing, fabrics must generally be pre-fixed to compensate for material-related differences in affinity and to reduce the sensitivity to creasing during the dyeing process. Prefixing can be performed in a stenter frame.
Disperse dyes Disperse dyes used for polyamide fibres are mainly azo compounds and anthraquinones. They are applied especially for lighter shades.
The material is dyed in acidic conditions (pH 5) by acetic acid. A dispersing agent is always added to the liquor.
Acid dyes As with acid dyeing of wool, with increasing dye affinity, the hydrophobic interaction in the initial phase must be repressed to achieve uniform absorption. This means that for high-affinity dyes the liquor must be sufficiently alkaline at the start and then slowly decreased to optimise exhaustion. The level of acidity of the liquor is regulated either by dosing with acids during dyeing or by adding acid-donors (e.g. ammonium sulphate, sodium pyrophosphate or esters of organic acids) at the start of the process (see also Section 4.6.14).
Optimal exhaustion and uniform dyeing can also be achieved by controlling the temperature profile.
Auxiliary agents (anionic, cationic, non-ionic surfactants) are normally used to improve the levelling effect.
The wet-fastness of dyeing with acid dyes on polyamide fibres is often unsatisfactory. Aftertreatment with syntans (synthetic tanning agents) is often necessary. The syntans are added to the exhausted bath or to fresh liquor at pH 4.5 by formic or acetic acid. The material is treated at 70 - 80 ºC and is then rinsed.
Metal-complex dyes Among 1:2 metal-complex dyes, molecules containing sulphonic groups are the most suitable for polyamide fibres.