«EUROPEAN COMMISSION Integrated Pollution Prevention and Control (IPPC) Reference Document on Best Available Techniques for the Textiles Industry July 2003 ...»
Dyeing is a method for colouring a textile material in which a dye is applied to the substrate in a uniform manner to obtain an even shade with a performance and fastness appropriate to its final use. A dyestuff is a molecule which contains a chromophoric group (conjugated system) capable of interacting with light, thus giving the impression of colour.
Textile dyeing involves the use of a number of different chemicals and auxiliaries to assist the dyeing process. Some of them are process-specific, while others are also used in other operations. Some auxiliaries (e.g. dispersing agents) are already contained in the dyestuff formulation, but more commonly auxiliary agents are added at a later stage to the dye liquor.
Since auxiliaries in general do not remain on the substrate after dyeing, they are ultimately found in the emissions.
Textiles Industry 57 Chapter 2
Various dyeing techniques exist:
· mass dyeing/gel dyeing, in which a dye is incorporated in the synthetic fibre during its production (this technique is the most commonly applied process for PP fibres and is of interest also for PAC, but will not be described in this document) · pigment dyeing, in which an insoluble pigment, without affinity for the fibre, is deposited onto the textile substrate and then fixed with a binder · dyeing processes which involve the diffusion of a dissolved or at least partially dissolved dye into the fibre.
This last group of processes is the one that will be discussed in more detail in the following
sections. From a molecular point of view, four different steps are involved:
1. first, the dye, previously dissolved or dispersed in the dye liquor, diffuses from the liquor to the substrate
2. the second step consists in the accumulation of the dye on the surface of the textile material.
This process is controlled by the affinity (substantivity) of the colourant for the fibre
3. the dye diffuses/migrates into the interior of the fibre until this is uniformly dyed. This step is much slower than the transport of the dissolved dye within the dye bath. The penetration of the dye into the fibre requires that the fibre itself is accessible. In the case of hydrophilic fibres the dye penetrates through the available micro-pores, while in hydrophobic fibres, whose molecular structure does not allow a continuous water phase, cavities need to be developed in order to make the penetration of the dye possible. In general, access to the fibre is enhanced by temperature. Hydrophobic fibres can only be penetrated by the dye above the glass transition temperature, which sometimes lies above 100 ºC. During the diffusion into the micro-pores an electrostatic barrier, which develops on the surface of the fibre, still has to be overcome. In some cases large amounts of salt have to be added to the dyeing bath in order to reduce the electrostatic forces on the surface of the fibre and promote an even penetration of the dye
4. the dye must be anchored (fixation) to suitable places within the substrate. Different mechanisms of fixation are known, ranging from chemical reaction of the dye with the fibre to form a covalent bond (reactive dyes) to formation of Van der Waals and other short range forces between the fibre and the dye (direct dyes). An important role is also played by hydrogen bonding responsible for long-, medium-, short-distance interactions between fibre and dyestuff, dyestuff to dyestuff, water to fibre and water to soluble substances present in the liquor, such as surfactants. This is more fully explained in Section 9.
2.7.2 Dyeing processes Textiles can be coloured at any of several stages of the manufacturing process so that the
following colouring processes are possible:
· flock or stock dyeing · top dyeing: fibres are shaped in lightly twisted roving before dyeing · tow dyeing: it consists in dyeing the mono-filament material (called tow) produced during the manufacture of synthetic fibres · yarn dyeing · piece (e.g. woven, knitted and tufted cloths) dyeing · ready-made goods (finished garments, carpet rugs, bathroom-sets, etc.).
Dyeing can be carried out in batch or in continuous/semi-continuous mode. The choice between the two processes depends on the type of make-up, the chosen class of dye, the equipment available and the cost involved. Both continuous and discontinuous dyeing involve the
· preparation of the dye · dyeing · fixation · washing and drying.
Batch- dyeing In batch dyeing (also called exhaustion dyeing) a certain amount of textile material is loaded into a dyeing machine and brought to equilibrium with a solution containing the dye and the auxiliaries over a period of minutes to hours.
The dyeing process starts with the absorption of the colourant onto the external surface of the fibre, then the diffusion and migration of the colourant through the fibre takes place. The use of chemicals and controlled temperatures accelerates and optimises exhaustion and fixation (rate/level) of the dye. When the dyeing is judged to be the right shade, the spent dye bath is drained and the textile material is washed to remove unfixed dyes and chemicals. Washing is usually carried out in the same equipment. However, separate washing machines can also be used in the case of fabric.
All these operations can be carried out with different degrees of automatisation. In fully automated dyehouses all steps going from the preparation of recipes and laboratory trials to dyestuffs and chemicals feeding, material transportation, loading and unloading of the machines and control of dyeing parameters (e.g. level, heating, injection at selected speed, pH, temperature, etc.) are performed under computer guidance.
In a manual regime the dyestuffs and chemicals are dosed and fed to the machines manually. A manual dyeing approach used for wool is to carry out trial laboratory dyeing on a sample of the particular fibre and then to apply 5 – 10 % less dye in the full scale dyeing. The final shade is achieved by adding additional dye in small portions to achieve the final shade. Depending on the dyestuffs, it may be necessary to cool the dye bath for each of these additions in order to promote even migration of the added dye. Shade matching is carried out by eye, the dyer comparing the dyed material with a reference pattern under standard illumination.
Dyeings which are “overshade” can be corrected by stripping dyestuff from the fibre using an excess of levelling agent or reducing conditions, and then adding further colour to achieve the correct shade. This is a very costly and polluting practice and is only used as a last resort in most dyehouses.
An important parameter in discontinuous dyeing is the liquor ratio of the equipment. This is the weight ratio between the total dry material and the total liquor. So, for example, a liquor ratio of 1:10 means 10 litres of water on 1 kg textile material.
This parameter not only influences the amount of water and energy consumed in the dyeing process, but also plays an important role in the level of exhaustion of the dye and in the consumption of chemicals and auxiliaries.
The liquor ratio is related to the exhaustion level of the bath through the equation: E = K/(K+L),
K (affinity) = 50 to 1000 for various dye/fibre combinations L (liquor ratio) = 5 to 50 for various machines E (exhaustion) = 0.5 to 1 (50 to 100 % exhaustion) From this equation it can be inferred that when L increases, E decreases and less dye is absorbed onto the fibre when the equilibrium is reached. The effect is more pronounced on low-affinity dyes.
As stated earlier, the liquor ratio also has an influence on the consumption levels of chemicals and auxiliaries. Most are dosed on the basis of the amount of bath (o.w.b) rather than the weight of the fibre (o.w.f). For example, in a 1:5 bath ratio, 50 g/l of salt will mean 250g/kg of fibre, but at 1:40 liquor ratio, the same 50 g/l of salt correspond to 2 kg/kg of fibre.
Textiles Industry 59 Chapter 2 Dyeing machines vary greatly in their liquor ratios, depending also on the type of substrate to be dyed and its hydrophilicity. Equipment manufacturers provide a range of nominal liquor ratios for each type of machine. This is defined as the range of liquor ratios at which the machine can be operated when it is loaded at its optimum/ maximum capacity. In each range the lowest values normally refer to synthetic fibres (PES is usually taken as reference), while the highest figures apply to cotton. This is due to the lower amount of liquor retained by synthetic fibres compared to cotton.
Table 2.2 shows typical ranges of nominal liquor ratios for each type of machine.
It should also be noted that each type of machine has its own limitations and range of applicability.
The features of a number of typical machines are described in more detail in Sections 10.1 to 10.4.1.2, whereas the latest developments in selected types of machines are reported in Sections 4.6.19 to 22.214.171.124.
(1) [32, ENco, 2001] (2) [294, ETAD, 2001] (3) The typical range is 1:15 – 1:25 as reported in the Comment from BCMA [208, ENco, 2001]. A L.R. of 1:12 has been reported for hank carpet wool (semi-worsted) [281, Belgium, 2002] (4) [171, GuT, 2001] (5) According to one major supplier (THEN) and textile finishing companies [209, Germany, 2001] (6) [3, RIZA, 1998] (7) [293, Spain, 2002] Table 2.2: Discontinuous dyeing equipment and liquor ratios Continuous and semi-continuous dyeing In continuous and semi-continuous dyeing processes, the dye liquor is applied to the textile either by impregnation (by means of foulards) or by using other application systems. Most commonly, textiles are fed continuously in open width through a dip trough filled with dye 60 Textiles Industry
Chapter 2liquor. 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 semi-continuous processes is the fact that in semicontinuous dyeing the application of the dye is performed 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 semicontinuous processes is given in Table 2.3.
(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 2.7.3 Cellulose fibres dyeing
Cellulose fibres can be dyed with a wide range of dyestuffs, namely:
· reactive · direct · vat · sulphur · azoic (naphtol).
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.
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).
62 Textiles Industry
Chapter 2Urea 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.