TEXTILE CHAIN
2.7 Dyeing
2.7.3 Cellulose fibres dyeing
liquor. The substrate absorbs an amount of dye solution before leaving the dip trough through rollers that control the pick-up of the dye. Surplus stripped dye flows back into the dye bath. In the carpet industry (and for open goods that must pick-up and retain large volumes of liquor), thickening agents are added to the pad liquor to prevent dye migration. Moreover, special application systems are also encountered, where the dyestuff is poured, jet-sprayed, injected or applied in the form of foamed liquor (see Section 10.4.2).
Dye fixation is usually achieved in a subsequent stage using chemicals or heat (steam or dry heat). The final operation is washing, which is usually carried out in washing machinery at the end of the same line.
The only difference between continuous and continuous processes is the fact that in semi-continuous dyeing the application of the dye is performed semi-continuously by padding, while fixation and washing are discontinuous.
In general, dyes with low affinity are favoured in continuous dyeing to prevent tailing (attributable to undesirable exhaustion of the padding solution) and to make washing-off of the unfixed dye easier.
In continuous and semi-continuous processes the liquor ratio is not of practical importance and it is not used as a parameter. In these processes the factor to be taken into account is the wet pick-up % (grams of liquor picked up by 100 grams of substrate) and the concentration of the dye.
An overview of the most common techniques and machinery utilised in continuous and semi-continuous processes is given in Table 2.3.
Make-up Process Equipment
Rope Continuous Padding machine for piece in rope form + J-box or conveyor + washing machine Pad-batch
(or Carp-O-Roll for carpet)
Padding machine + washing machine
Pad-roll (or Carp-O-Roll for carpet)
Padding machine + washing machine
Semi-continuous
Pad-jig Padding machine + jigger + washing machine
Pad-steam Padding machine (1) + steamer + washing machine
Pad-dry Padding machine (1)+ stenter frame + washing machine
Woven &
knitted fabric, tufted
carpet Open-width
Continuous
Thermosol Notes:
(1) different applicators are used to dye carpets on continuous ranges (see also Section 10.4.2) Table 2.3: Semi-continuous and continuous dyeing processes and equipment
Reactive dyes
One third of dyes used for cellulose fibres today are reactive dyes. They are mostly applied according to the pad-batch and continuous processes for woven fabric, while batch processes are the most common for knitted fabric, loose stock and yarn.
In batch dyeing, dye, alkali (sodium hydroxide or sodium carbonate or bicarbonate) and salt are added to the dye bath in one step, at the start of the process, or stepwise. In the stepwise process the alkali is added only after the dye has absorbed to the fibre. Its amount is determined by the reactivity of the system and the desired depth of shade (cold dyers are applied at lower pH compared to warm and hot dyers). Salt is added to improve bath exhaustion: the concentration employed depends on the substantivity of the dye and on the intensity of the shade. Higher concentrations are required for deep shades and low-affinity dyes, as shown in the table below.
Shade High-affinity dyes Low-affinity dyes
<0.5 % 10 - 30 g/l NaCl Up to 60 g/l NaCl
>4 % ~50 g/l NaCl Up to 80 - 100 g/l NaCl Source: [186, Ullmann's, 2000], [11, US EPA, 1995]
Table 2.4: Salt concentration required for reactive dyes
After dyeing, the liquor is drained off and the material is rinsed and then washed off with the addition of auxiliaries.
In pad dyeing processes dye and alkali can be added together to the dye liquor or in separate steps into two separate padders (or other types of application systems). When all the chemicals are applied in one step, the stability of the pad liquor is important. In fact with increasing reactivity of the dye there is a risk that the dye, after a long dwell time in the pad box, is hydrolysed by the alkali, before reacting with the fibre. For this reason dye and alkali are commonly metered separately into the padder. In addition, pad boxes are now constructed so that the liquor volume is as low as possible, so that it is replaced on average within 5 minutes [186, Ullmann's, 2000].
Among semi-continuous processes the cold pad-batch is by far the most important one for reactive dyes. After the textile has been padded with dye and alkali, it is rolled up into batches.
Fixing takes place during storage.
In continuous processes, padding, fixing, washing-off and drying are carried out in the same process line. Fixation is commonly achieved either by dry-heating or by steaming. The following processes are commonly used:
• pad-steam processes (one common method is the pad-dry-pad-steam process which includes dye application by padding - intermediate drying - alkali application by padding - dye fixation with saturated steam - washing - drying)
• pad-dry thermofix processes (dye and alkali are padded at the same time; then the material can be dried and fixed in a single step or it can be thermofixed after an intermediate drying stage).
In all cases, after fixation the material is always carefully washed off in open width or in a rope washing machine to remove completely the hydrolysed colourant and is then dried.
In pad-dry thermofix processes, urea is usually added to the padding liquor to act as a solvent for the dye during fixation. Urea melts at 115°C and binds water above 100 °C. It can therefore be used as solvent for the dye in dry heat. A recently developed dyeing process is now available that does not require the addition of urea (see Section 4.6.13).
Urea is also sometimes used in pad-batch processes as dyeing solvent to increase the solubility of the dye. As early as 1992 the use of urea as dyeing solvent was already in decline [61, L.
Bettens, 1999]. New highly soluble reactive dyes have been introduced in the market which do not need urea even for deep dyeing in highly concentrated dye liquor.
Direct dyes
Direct dyes are also quite important in cellulose fibres dyeing: 75 % of the total consumption of these colourants is used, in fact, to dye cotton or viscose substrates [186, Ullmann's, 2000].
Direct dyes are applied directly from the dye bath together with salt (sodium chloride or sodium sulphate) and auxiliary agents, which ensure a thorough wetting and dispersing effect. Mixtures of non-ionic and anionic surfactants are used for this purpose.
In the batch process the dye is made into paste, then dissolved in hot water and added to the dye bath. The electrolyte is then added to the dye bath. After the dye bath has been drained, the fabric is washed with cold water and generally subjected to after-treatment.
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 after-treatment 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 salt-like 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 (pad-dry/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.
Dyestuff Chemicals and auxiliaries/ typical application conditions Technique Batch
Pad-batch
Pad-steam Reactive - pH 9.5 - 11.5 by addition of sodium carbonate and/or sodium hydroxide
- Salt is used to increase dye bath exhaustion: higher concentrations are used for low-affinity dyes and for deep shades
- Application temperatures vary from 40 ºC to 80 ºC depending on the class of the dyestuff
- In padding processes urea or cyanoguanidine is usually added to the pad liquor (the Econtrol® process described in Section 4.6.13 does not need urea)
- After dyeing, the material is soaped and then washed off with addition of
surfactants to remove unfixed dye Pad-dry
Batch Pad-batch Pad-jig Direct - Salt is used to increase bath exhaustion
- Mixtures of non-ionic and anionic surfactants are used as wetting/dispersing agents
- After-treatment is usually necessary to improve wet-fastness (possible
use of fixative cationic agents, formaldehyde condensation products) Pad-steam Batch Vat - Alkali and reducing agents (sodium dithionite, sulphoxylic acid
derivatives, thiourea dioxide, and other organic reducing agents) are applied to convert the dye to the sodium leuco form
- Poorly degradable dispersants are present in the dye formulation and are further added in other steps of the process
- Levelling agents are sometimes necessary
- Temperature and the amount of salt and alkali required vary according to the nature of the dye (IK, IW, IN)
- Dye is fixed to the fibre by oxidation, generally using hydrogen peroxide, but halogen-containing oxidising agents can also be used
- After-treatment takes place in a weakly alkaline detergent liquor at boiling temperature
- In continuous processes, anti-migration and wetting agents are used
Pad-steam
Batch
Pad-steam Sulphur - Reducing agents (Na2S, NaHS, glucose-based combination of reducing
agents) and alkali are applied to convert the dye into soluble form, unless ready-for-use dyes are used
- Dispersants and complexing agents are used in batch dyeing
- In batch dyeing, the dye generally absorbs at 60 - 110 ºC, while in the pad-steam process the material is padded at 20 - 30 ºC and then subjected to steaming at 102 - 105 ºC
- Oxidation is carried out mainly with hydrogen peroxide, bromate and iodate
Pad-dry/ pad-steam Batch Azoic - Preparation of the naphtholate (caustic soda and, in some cases, addition
of formaldehyde is required to stabilise the naphthol on the fibre) - Impregnation of the naphtholate by exhaustion or padding processes - Preparation of the diazotised base (with NaNO2 and HCl)
- Developing stage (the textile is passed through the cold developing bath or the developing solution is circulated through the stationary textile in the dyeing machine)
Padding methods
Table 2.5: Summary of the most common dyestuffs and dyeing techniques applied for cellulose fibres