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Dyeing is carried out in continuous or in batch dyeing machines. The use of disperse dyes is dominant.
2.14.5 The Carpet industry 220.127.116.11 Wool and wool-blend carpet yarn dye-house The production of spun dyed yarn can be regarded as a specific sector within the carpet manufacturing industry. Mills can be identified as dyehouses processing mainly wool and woolblend fibres. Different treatments are carried out in order to convert white loose fibre into dyed carpet yarn. Wet processes essentially consist of dyeing and other ancillary operations carried out either in loose fibre or yarn form. Dry processes consist, in turn, in blending, carding, spinning, etc. These processes will not be considered here, since they have already been described in previous sections. Depending on when colouration takes place raw fibre flows through some or all of these processes. As can be seen from Figure 2.46, three basic process sequences are possible.
The dry spinning route, so called because there is no wet processing after yarn formation, begins with colouration at the loose fibre stage. This is followed by yarn formation and finally twist setting. This process sequence is of relatively recent origin and requires the consistent use of wool with a low lanolin content and specialised spinning lubricants which can be left on the yarn without causing subsequent soiling of the carpet. The process is particularly useful in the production of yarn for large volume plain shade carpets and for effect yarns, obtained by blending together fibre dyed to different shades. While this production sequence is the most economical in terms of resource consumption, the selection of clean raw materials and the ongoing maintenance of the mill in a clean condition are essential.
The traditional loose stock dyeing route was originally used to produce large batches of yarn to the same shade for plain carpets. Loose fibre is first dyed and then converted to yarn using what is still sometimes referred to as the “oil spinning” process; this terminology arose from the practice of using spinning lubricants based on emulsions of mineral oil. Even small traces of residual mineral oil would lead to a marked propensity for the carpet to soil in service, and so yarns prepared by this route were thoroughly cleaned by scouring (washing) in hank form (see below). While the use of mineral oil-based lubricants has been largely replaced with watersoluble synthetic products, the practice of scouring the yarn is still judged to be essential by many processors in order to avoid potential claims arising from soiling. Unlike the dry spinning process, this route allows greater flexibility in the purchase of raw materials, so that wool with a higher lanolin content can be used.
In the Yarn Dyeing Route, clean fibre is first converted into yarn before dyeing. This process is particularly suitable for the production of the small coloured lots required for patterned carpet weaving or the bespoke trade, where white yarn can be held in stock and dyed as required to fill orders. The process is, however, by no means restricted to small batches, and dyeing machines with capacities of up to four tonnes are used to produce plain shades for both tufting and weaving.
In the case of integrated yarn manufacturers, it is common to find two or more of these process streams operating side by side and sharing common wet processing equipment. Since the dyeing and finishing techniques used apply equally to all three sequences, they are discussed in the following sections without further considering the different routes mentioned. Variants are described where they occur, and the relevance of any dry process segments is discussed where they have a significant impact on environmental performance or emissions.
18.104.22.168.1 Carpet loose fibre dye-house
Fibre is conventionally dyed in loose form (loose stock) when a large quantity of yarn is required to be of precisely the same shade, for example in a large solid shade (plain coloured) carpet where subtle variations in colour would be visible in service. Single colour batches may be made up of a number of individual dyeings, the dyer adjusting the dye addition to each dyeing in order to achieve the desired final shade of the yarn. Thorough mixing of the individual dyeings in a batch is achieved in a specific mechanical blending operation and during carding.
Loose fibre dyeing, therefore, need not be as level as, for example, yarn dyeing, where there is no possibility of levelling the colour by further mechanical processing.
Dyed loose fibre is also used to achieve multicoloured effects in some yarns. In this process fibre dyed to different shades is blended together to produce a large range of designs, such as the “heather” styles in fashion at the present time. Such blends may contain dyed and undyed natural fibre and undyed and pigmented synthetic fibre.
Where the final yarn will contain a blend of wool and synthetic fibres (typically 80 % wool and 20 % polyamide) the required weights of the two components are normally dyed separately to optimise application conditions and dyestuff selection for each fibre type.
Loose-fibre dyeing processes Scoured wool and new synthetic fibre are presented to the dye-house in a “clean” state and usually require no further treatment to remove contaminants before dyeing. If purchased from outside sources, fibrous raw materials normally arrive on site in the press-packed bales used universally by the textile industry to transport raw fibre.
Within an integrated manufacturing site, for example, one with its own wool scouring facilities, loose fibre may be transferred between the scouring department and blending department or loose fibre dyehouse by pneumatic conveyer or as individual low-density bales from intermediate warehousing.
Special opening machinery is not usually necessary when dealing with previously scoured wool and with new synthetic fibre. Bales are, therefore, often simply weighed and then brought into the dyehouse, opened at the side of the dyeing machine and the required quantity of (dry) fibre loaded manually into the dyeing vessel. Alternatively, fibre may be wet prior to packing in order to facilitate more even machine loading.
Various types of machines are used for dyeing wool and synthetic fibres in loose form. These include conical pan, pear shaped and radial flow machines (see Section 10). Loose fibre is typically packed into these machines manually.
Dyestuffs are dissolved in hot water before being added to the circulating bath. Typical dyestuffs and chemicals for wool and wool-blends are employed (see Sections 2.7.4 and 2.7.6).
In the majority of cases all chemical and dyestuff additions are made manually to the open dyeing machine. Less frequently, or if “pressure” dyeing machinery is being utilised (for synthetic fibres, because wool is normally dyed at atmospheric pressure), pre-dissolved chemicals and dye are introduced to the circulating dye bath from special addition tanks.
The dye bath is typically run for 10 - 15 minutes to ensure even penetration of the liquor through the fibre pack before commencing the heating cycle, raising the temperature of the dye liquor to 98 ºC at a rate of 1 – 2 ºC per minute. On reaching top temperature, dyeing may continue for up to 60 minutes, during which time the dye bath pH may be checked and adjusted by adding further acid to achieve maximum dye uptake. Progress of the dyeing is normally judged by eye and fibre samples are then removed for comparison with a standard.
A dyeing which is judged to be on shade will be terminated and the machine drained. A dyeing which is not of the required colour may have further additions of one or more dyestuffs, the dye bath being returned to the boil after each addition. Because of the blending operation which follows loose fibre dyeing, it is uncommon for there to be more than one shade addition unless the machine load is the only fibre in a batch.
Dyeing is followed by rinsing with cold water, to remove any surface-bound dyestuff and to cool the dyepack before manual unloading. The machine may be filled with cold water and then run for 10 - 15 minutes before draining. The use of “flood rinsing” in which the dye bath is allowed to refill and then run continuously to drain during the rinsing operation is now much less common due to increases in water charges and effluent disposal costs.
Liquor from both the dyeing and rinsing process may be recycled for further use. In this case the machine must be fitted with an external holding tank. The dye bath may be recycled if a number of dyeings of the same shade are being performed to make up a bigger dye lot. In this case the dye bath is pumped to the reserve tank and dropped back to the dyeing vessel when required for the next dyeing. There are, however, severe limitations to the use of this process because dye uptake is temperature-dependent and starting the dyeing at too high a temperature can result in an unacceptable rate of strike and unlevel application. The selection of dyestuffs and dyeing
conditions which promote maximum uptake of dye are required for the successful operation of this process.
In such circumstances it is more usual to recycle the rinse liquor, as the temperature of this liquor is lower and more compatible with dyeing start temperatures. Depending on the design of the machine a reserve tank may not be required for the operation of this process, as the fibre carrier can often be removed with the dyeing vessel full. Both these alternatives conserve water, and to a lesser degree, thermal energy (see also Section 4.6.22).
Application of functional finishes
A number of functional finishes may be applied to the loose fibre, either during the dyeing process itself or by application from an additional bath following dyeing/rinsing. This is particularly relevant in the “dry spinning” route where there will be no further wet processing after yarn formation. Finishes applied at the loose fibre stage include insect-resist treatments, antistatic treatments, anti-soiling treatments and treatments to counteract yarn/carpet colour change due to light exposure in service (see Section 8.8).
For expediency these finishes are combined with dyeing whenever possible, aftertreatments only being used when the chemistry of the two finishes is incompatible or if they require widely differing conditions of temperature and pH. Co-application with the dyes is simply accomplished by adding the product to the dye bath, usually with the dyeing auxiliaries.
Aftertreatments may require a fresh bath of clean water, or alternatively the rinse bath may be clean enough for re-use.
Specific techniques have been devised to minimise the concentration of mothproofing agents present in the spent liquors from loose fibre dyeing. The formulated commercial product is added at the beginning of the dyeing cycle and dyeing carried out as normal. At the end of the dyeing cycle the pH of the dye bath is lowered with the addition of formic acid and boiling is continued for a further 20 - 30 minutes. These strongly acidic conditions promote uptake of any active ingredient not adsorbed by the wool fibre under normal dyeing conditions and residual concentration can be reduced by up to 98 %.
Rinsing the fibre at moderate temperatures is known to cause desorption of mothproofer bound on or close to the surface of the wool fibre. Active ingredient concentrations in the spent rinse bath may consequently be significantly higher than those present in the dye bath. Techniques to minimise the impact of rinse desorption have been developed, in which the rinse bath is recycled, forming the next dye bath, thus eliminating all residues from the rinse liquor and reducing overall water consumption by 50 % (see Section 4.8.4 for further details).
Fibre in a drained carrier will contain up to 2 litres/kg of residual water (dry fibre weight). This is initially reduced by either centrifugal extraction or by mangling before evaporative drying in a hot air dryer.
22.214.171.124.2 Carpet yarn dye-house On integrated sites the spun undyed yarn may be held in a bulk store as either hanks, wound onto cones or wound onto the special centres compatible with package dyeing equipment.
Batches of suitable size are drawn from this material to fill individual orders. Commission yarn processors generally receive hanks baled in conventional wool bales.
In hank-based processes the bales are normally brought into the dyehouse and opened at the side of the scouring or dyeing machine ready for manual loading
Yarn scouring Scouring is generally carried out as a semi-continuous process in which batches of yarn are transported through a series of aqueous baths containing detergent and alkali or rinse water. As shown in Figure 2.46 scouring can be carried out both on dyed and undyed yarn. To prevent cross contamination with dyestuffs, integrated yarn manufacturers may operate two scouring machines, one being reserved for scouring white yarn prior to dyeing and the second for the scouring of coloured yarn.
Yarn may be scoured using either hank scouring or package to package (sometimes referred to as single end) processing machinery.
In tape scouring machines (Figure 2.47) hanks are transported through the machine trapped between an upper and lower set of nylon tapes which run in an endless belt through each bowl and mangle set, guided by intermediate rollers in the bottom of each bowl. Bowl working volume is typically between 1200 and 1800 litres. Throughput capacity typically ranges from 500 to 1500 kg/hour. Residence time in each bowl varies between 20 and 45 seconds. Heating is provided by either closed coils in the base of the machine or live steam injection.
Figure 2.47: Schematic layout of a hank-scouring machine [32, ENco, 2001] Each bowl is initially charged with the required chemicals and further additions are made during processing, either manually or with a metering device.
In machines used only for scouring, the process liquor may flow from bowl four towards bowl one, thus providing a simple countercurrent extraction system. Specific water consumption varies widely, depending on the quantity of yarn processed through the machine before dropping the liquor for cleaning and the extent of any flowdown to drain from the scouring bowls. Values between 2 and 7 litres of water per kg yarn are common.
"Package to package" scouring machines (Figure 2.48) are less common and are of more recent design. With this machinery the whole process may be automated, including drying. Coiling devices take yarn from a number of individual cones and form this into an endless blanket of overlaid coils, laid down automatically onto a moving conveyer belt. The conveyer passes through each of the scouring and rinse bowls. The yarn is transferred to a second conveyer, which then passes through the dryer. The yarn blanket is then uncoiled and the yarn finally rewound onto cones.
Figure 2.48: Schematic diagram of a "Package to Package" yarn scouring installation [32, ENco, 2001] The scouring bowls are of larger volume (3500 litres) than tape scour machines and heating may be by direct gas firing.
Most machines are equipped with dual yarn coilers, giving an overall capacity of up to 500 kg/hour.
Both hank and single end machines may be utilised only for scouring or the process may be modified to include simultaneous chemical setting of yarn twist and the application of insectresist (IR) agents.
Scouring to remove lubricant When the machines are operated only to remove lubricant, the first two bowls are charged with detergent and alkali and operate at 50 – 60 ºC, while the remaining bowls serve to rinse the yarn with clean water at 20 – 30 ºC. Chemical additions are made initially to set the bath concentration at a predetermined level, which is then maintained by further additions during processing.
Scouring and insect-resist treatment