«EUROPEAN COMMISSION Integrated Pollution Prevention and Control (IPPC) Reference Document on Best Available Techniques for the Textiles Industry July 2003 ...»
Four-bowl machines are normally used if the scouring process is to incorporate a simultaneous insect-resist (IR) treatment. Bowls 1 & 2 are charged as above for scouring, bowl 3 contains clean water for rinsing, and bowl 4 is adapted for insect-resist application. Bowl 4 may be of the low volume type (100 - 200 litres), designed specifically for the treatment of yarn with insectresist agent in order to minimise the volume of the process liquor and the resulting emissions.
In these installations, insect-resist agent is applied by a process of “continuous exhaustion” rather than physical impregnation and the active substance is stripped from the bath by the yarn, equilibrium bath concentration being maintained by continuous chemical metering at a rate proportional to yarn throughput.
Application is carried out at 50 – 60 ºC under acidic conditions (approximately pH 4.5 by either formic or acetic acid) to promote rapid uptake of the active substance in the short yarn residence times available.
The insecticide content of the bowl is such that it cannot be discharged to drain and therefore storage tanks are used to retain the liquor between treatment cycles. Heavy contamination of the liquor with dyestuff removed from the yarn would lead to a change of shade in subsequent yarn lots and so a simple adsorptive filter system may be used to remove dyestuff before storage.
This consists of a quantity of wool fibre packed into a filter housing and through which the liquor can be circulated. The liquor is preheated to a minimum of 70 ºC to assist effective dyestuff removal. Operating with this liquor renovation system permits re-use of the liquor without the need to discharge to drain.
In the absence of these abatement systems the spent treatment liquor can be pumped from the scouring machine and added to a dark shade dyeing, where uptake at the high dyeing temperatures minimises emissions of active substance. Both loose fibre and yarn dyeing can be done in this way.
A third abatement option uses chemical hydrolysis of the active ingredient to destroy residual insecticide. Spent liquor is pumped from the machine and treated in a separate tank at 98 ºC with sodium hydroxide (4g/l) for 60 minutes. The ester and cyano-ester linkages in permethrin and cyfluthrin undergo rapid hydrolysis under these conditions and more than 98 % abatement is achieved. The primary degradation products are at least one order of magnitude less toxic to aquatic invertebrates when compared to the parent molecule. Liquors treated in this way are normally discharged to drain, where the high alkali-content is neutralised by acids from dyeing processes.
More information about these techniques is reported in Section 4.8.4.
Chemical twist setting Five-bowl machines are normally employed if chemical twist setting is to be carried out at the same time as scouring. In this mode Bowls 1 and 2 contain sodium metabisulphite (10 to 20 g/l) in addition to detergent and alkali and Bowl 4 may be charged with hydrogen peroxide (5 to 10g/l) to neutralise any residual bisulphite. In all other respects the process is similar to that described above.
Hanks leave the final mangle of the scouring line with a moisture content of approximately
0.8 litres per kg (dry weight). If the material is to receive no further wet processing, this residual moisture is further reduced by centrifugal extraction before evaporative drying in a hot air dryer.
Scouring in hank form may also be carried out using batch solvent processing equipment, although this practice is now less common. Perchloroethylene is the solvent of choice, and these machines operate on the totally enclosed principle, washing, rinsing and drying being accomplished sequentially within a horizontal drum. All machines are fitted with solvent recovery systems to distil used solvent and recover solvent vapour during drying.
Hank and package dyeing processes Traditionally, carpet yarn dyeing is carried out in hank form, where liquor circulation in the dyeing machine produces a yarn with a characteristic physical property, often described as loft or fullness. Hank dyeing machines are mostly of the Hussong type.
In other sectors of the textile industry it is common to dye yarn in package form – wound onto a perforated centre, through which dye liquor can be circulated under pressure. This process has considerable cost advantage over hank dyeing in that it requires no reeling operation to form the hank and consequently no winding of the hank back onto cones in preparation for weaving or 132 Textiles Industry
Chapter 2tufting. With wool and wool-blend yarns the extension applied during package winding results in the yarn being set in a “lean” condition and the resultant yarn does not have the required physical characteristics for carpet manufacture. There are, however, a number of ways of overcoming these objections and package dyeing is slowly gaining credence in the carpet yarn
dyeing industry. Three basic types of machines may be used for package dyeing wool yarns:
horizontal or vertical spindle machines or tube type machines.
Although the machinery employed in hank and package-dyeing processes is different, the dyeing procedures and techniques are essentially the same and are described together.
Considerable care is required to obtain a level (even) dyeing on yarn as there are no opportunities to even up the colour by mechanical blending, as is the case with loose fibre dyeing. Faulty dyeings must be corrected by manipulation in the dye bath, by either removing or adding colour to achieve the final shade. This process can add significantly to the resources consumed in yarn dyeing.
In comparison to synthetic fibres, the rate of dyeing and the extent of dye uptake is less predictable when dyeing wool, as natural variations in the physical and chemical composition of the fibre have a marked effect on these important parameters.
The dyeing of carpet yarns predominantly composed of a blend of wool and polyamide fibre further compromises the dyer because the two fibres have markedly different dyeing properties and special dyeing auxiliaries must be used to achieve a commercially acceptable product.
Problems associated with level dyeing are further compounded by the fact that very few shades can be achieved with one single dyestuff; most shades require the simultaneous application of a number of colours in various proportions and which may have different rates of uptake.
The usual approach is to carry out trial laboratory dyeing on a sample of the particular fibre blend and then to apply 5 – 10 % less dye in the full scale dyeing, the final shade being 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.
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 practice of last resort in most dyehouses.
This shade matching procedure is an essential part of the dyeing processes as most dyeing is carried out to an agreed standard, either for internal use in the case of an integrated site or by agreement with the customer. Shade matching is predominantly carried out by eye, the dyer comparing the dyed material with a reference pattern under standard illumination.
In other sectors of the textile industry it is common to use colour matching spectrophotometers to determine the reflectance spectra of the dyed material for comparison with a numerical standard. In some instances these measurements may also be used to generate the dyeing recipe from the standard. These techniques are less successful with carpet yarn because a sample of yarn prepared to represent the cut pile of a carpet, viewed end on, must be used for the result to be meaningful. Despite these difficulties a number of manufacturers do use this technology, claiming significant improvements in batch-to-batch matching and subsequent reductions in material wastage.
Hank dyeing machines may be loaded with either dry or wet yarn. In the latter case the yarn may be carrying moisture from the scouring operation or may have been deliberately wetted out to facilitate even packing. This technique is often applied when loading large hanks of yarn with a high twist factor. Package dyeing machines are loaded dry.
Textiles Industry 133 Chapter 2 Dyestuffs and chemicals typical of wool and polyamide fibres are employed (see Sections 2.7.4 and 126.96.36.199). Preparation for dyeing normally consists of filling the machine with water at 15 - 30 ºC and adding acids, salts and dyeing auxiliaries as required by the recipe. With hank dyeing machines it is conventional to raise the lid and yarn from the dye bath before adding predissolved dyestuffs. In closed package dyeing machines dyestuffs are added from linked transfer tanks.
The dye liquor is circulated for 10 - 15 minutes at 15 – 30 ºC before commencing the heating programme, raising the temperature of the dye bath, according to the dyeing programme in order to maximise exhaustion.
At this stage the dyer will obtain a sample of the dyed yarn for comparison with a standard, in the case of hank dyeing by raising the load from the dye bath, or with package dyeing equipment, through a sampling port in the machine case. A dyeing which is judged to be on shade at this stage is terminated and the dye bath drained. If further additions of dyestuff are required the dye bath may be cooled, in the case of hank dyeing machines by partial draining and refilling with cold water or in package dyeing machines by circulating cooling water through an internal heat exchange core.
Following addition of dyestuff, the dye bath will be returned to the boil and boiled for 30 - 60 minutes before a further yarn sample is taken for shade matching. This operation may be repeated several times before the dyer is satisfied that the bulk material matches the standard.
The spent dye bath is then drained and the yarn rinsed in clean water at 15 – 30 ºC for 10 - 20 minutes before finally being allowed to drain, ready for unloading.
In some instances the spent rinse bath may contain little or no residual colour. As the temperature of this liquor is compatible with dyeing start temperatures, it may be retained in the dyeing machine and used for a subsequent dyeing. This practice reduces water usage by up to 50 %.
Application of functional finishes A number of functional finishes may be applied, either with the dyestuffs or from additional baths of clean water following dyeing. These include insect-resist treatments, flame-retardant treatments and antistatic treatments.
Insect-Resist treatments Traditionally formulated insecticides, based on synthetic pyrethroids or Sulcofuron, were added to the dyeing with the dyestuffs. To minimise residues and control fugitive emissions this basic procedure has been modified. The formulated product is now added to the dyeing at a later stage; to avoid the spillages that occur during yarn lowering and dyeing, auxiliaries are selected which do not interfere with exhaustion. Emissions from dyeings carried out under acidic conditions are normally within permitted limits, but experience has shown that these standards cannot be met when dyeing under more neutral conditions. In this case, the insect-resist agent is applied from a blank aftertreatment bath in the presence of formic acid at a temperature of 70 - 80 ºC (see also Section 4.8.4).
Antistatic finish applied to the pile yarn is mainly based on a cationic surfactant system, which is readily applied to the fibre under mildly alkali conditions. Cationic compounds are not compatible with anionic dyestuffs and these materials cannot, therefore, be incorporated in the dye bath, but must instead be applied as aftertreatments. The process consists of preparing a fresh bath of clean water, adjusting the pH and adding the required quantity of the proprietary product. The liquor is raised to 60 ºC and run at this temperature for 20 - 30 minutes, followed by rinsing in clean water.
Flame-retardant treatments Potassium salts of fluoro complexes of zirconium (potassium hexafuorozirconate) are typically used for wool and wool-blend fibres. Typical application conditions for carpet wool yarn are as
· rinsing is required to remove interfering sulphate and phosphate ions, if present · bath set up at 20 – 30 ºC, pH 3 with hydrochloric acid (10 % o.w.f.) or formic acid (15 % o.w.f.) and citric acid (4 % o.w.f.) · addition of potassium hexafluorozirconate (3 to 8 % o.w.f. depending on the final specification to be achieved and the substrate) dissolved in 10 times its weight of hot water · temperature raised at 1 – 2 ºC per minute to 60 ºC and held at this temperature for 30 minutes · rinsing in cold water for 10 - 20 minutes.
In addition to application of the above functional finishes, which are all invariably carried out in conjunction with colouration, yarn dyeing equipment may be used for other specific yarn preparation or treatment procedures, principally bleaching and twist setting. These are described separately below.
The industry favours the neutral white colour obtained by an oxidation bleach, followed by a
reductive bleach. Typical processing conditions would be:
1. at 40 ºC, run yarn in liquor containing 3 % o.w.f. proprietary stabiliser, 1.5 % o.w.f.
sodium tri-polyphosphate, 20 % o.w.f. hydrogen peroxide (35 %). Raise liquor to 70 ºC, circulate 40 minutes. Drain
2. run in a fresh bath containing 0.2 % o.w.f. formic acid (85 %) and 0.75 % o.w.f. sodium hydrosulphite. Raise to 50 ºC, circulate 20 minutes, drain and rinse in cold water.
Yarn (dye bath) twist setting This process is not always carried out as a separate treatment. In fact, during the hank dyeing of wool yarns the twist inserted during spinning is stabilised by chemical changes within the fibre at the temperatures reached by the boiling dye bath.
Yarn may, however, be twist set in hank form using conventional hank dyeing equipment.
Typical processing conditions would be:
1. raise dye bath to 80 ºC, add 5 % on the weight of yarn sodium metabisulphite, immerse yarn, circulate liquor for 15 minutes, drain machine
2. rinse cold with liquor containing 0.8 % o.w.f. hydrogen peroxide (35 %) for 15 minutes.
188.8.131.52 Integrated Carpet Manufacturing Mills
Fully integrated carpet manufacturers carry out all the mechanical processes, wet processes (pretreatment, dyeing, printing and finishing operations) required to convert natural and synthetic fibres into finished carpet. Such companies may also produce their own synthetic fibres from raw polymer feedstock. Regarding the natural fibres processed they can in some cases select and purchase natural fibres and operate the whole chain of processes from wool scouring to dyeing, yarn spinning and carpet weaving/tufting. However, usually not all of these operations are carried out at the same site.
The conversion of the fibre into finished carpet can follow different routes depending on the style of the carpet to be produced.
Yarn can be manufactured from:
· staple fibres, from both synthetic (PA, PP, PES, PAC) and natural (wool and cotton) fibres · continuous filaments, exclusively from synthetic fibres (mainly PA, PP and PES).
The carrier materials (primary backing) usually consist of:
· PP woven fabrics or webs · PES woven fabrics or webs · jute fabrics.
Finishing of tufted carpets involves:
· dyeing and/or printing · coating · mechanical finishing · chemical finishing.
Dyeing and chemical finishing can be applied on loose fibre, yarn or piece, while the other operations are carried out on the final carpet.