TEXTILE CHAIN
2.7 Dyeing
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 following steps:
• 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), where:
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.
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 Fehler! Verweisquelle konnte nicht gefunden werden. to 4.6.21.3.
Make-up Process Equipment Liquor ratio
Loose/stock fibre (also card sliver
and tow) Loose stock
dyeing Autoclave (loose stock
dyeing) 1:4 - 1:12 (1)
Bobbins/
cones Yarn dyeing Autoclave (package dyeing) 1:8 - 1:15 (2) Yarn
Hank Hank dyeing Hank dyeing machines 1:12 - 1:25 (3)
Winch beck 1:15 - 1:40 (4)
Overflow 1:12 - 1:20 (2)
Jet - for fabric - for carpet
1:4 - 1:10 (4) 1:6 - 1:20 (4) Rope Piece dyeing in
rope form
Airflow 1:2 - 1:5 (5)
Winch (only for carpet) 1:15 - 1:30 (4) Beam dyeing
Beam + washing machine
1:8 - 1:10 (6) 1:10 - 1:15 (7) Woven and knitted
fabric, tufted carpet
Open-width Piece dyeing in open-width form
Jig dyeing
Jigger + washing machine
1:3 - 1:6 (6) 1:10 (7)
Paddle 1:60 (not
exceptional) Ready-made goods (e.g. garments,
rugs, bathroom-sets, etc.) Piece dyeing
Drum Very variable
Source:
(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
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