«EUROPEAN COMMISSION Integrated Pollution Prevention and Control (IPPC) Reference Document on Best Available Techniques for the Textiles Industry July 2003 ...»
The carpet is accumulated into a J-box, and is then steamed and brushed. When it reaches the printing table it is stopped. The jets are mounted on a sliding frame that can itself be moved in the direction of the warp while the carpet remains stationary during the printing process.
Ink-jet printing is another digital printing technique with its origins in paper printing technology that is now also increasingly used in the textile industry. In ink-jet printing, colour is applied to the surface of the substrate without variation in firing time, pressure or velocity. For this reason it can only be applied for flat light fabrics, especially silk (see also Section 4.7.9).
The TAK printing system can still be found in the carpet industry. With this technique irregular patterns can be produced. The carpet, previously dyed with a ground shade, is provided with coloured spots through dripping. The size and the frequency of the coloured spots can be varied by adjusting the overflow groove placed along the carpet width.
Figure 2.26: Schematic representation of the TAK system [63, GuT/ ECA, 2000] 2.
8.3 Environmental issues
Emission sources typical of printing processes are:
· printing paste residues · waste water from wash-off and cleaning operations · volatile organic compounds from drying and fixing.
Printing paste residues Printing paste residues are produced for different reasons during the printing process and the amount can be particularly relevant (Section 184.108.40.206.5 provides information about consumption and emission levels). Two main causes are, for example, incorrect measurements and the common practice of preparing excess paste to prevent a shortfall.
Moreover, at each colour change, printing equipment and containers (dippers, mixers, homogenizers, drums, screens, stirrers, squeegees, etc.) have to be cleaned up. Print pastes adhere to every implement due to their high viscosity and it is common practice to use dry capture systems to remove them before rinsing with water. In this way these residues can at least be disposed of in segregated form, thus minimising water contamination.
Another significant, but often forgotten source of printing paste residues is the preparation of sample patterns. Sometimes they are produced on series production machines, which means high specific amounts of residues produced.
There are techniques available that can help to reduce paste residues (see Section 4.7.4) and techniques for recovery/re-use of the surplus paste (see Sections 4.7.5 and 4.7.6). Their success is, however, limited due to a number of inherent technological deficiencies of analogue printing technology. Most of these deficiencies are related to the analogue transfer of the pattern, the unavoidable contact between the surface of the substrate and the applicator (screen) and the need for thickeners in the formulation (paste rheology), which limits the ultimate potential for paste re-use. Digital printing offers a solution to these problems (see Sections 4.7.8 and 4.7.9).
Waste water from wash-off and cleaning operations Waste water in printing processes is generated primarily from final washing of the fabric after fixation, cleaning of application systems in the printing machines, cleaning of colour kitchen equipment and cleaning of belts.
Waste water from cleaning-up operations accounts for a large share of the total pollution load, even more than water from wash-off operations.
Emission loads to water are mainly attributable to dyestuff printing processes because in the case of pigment printing, although considerable amounts of waste water arise from cleaning operations, pigments are completely fixed on the fibre without need for washing-off.
Pollutants that are likely to be encountered in waste water are listed in the table below.
Table 2.17: Pollutants that are more likely to be encountered in waste water from printing processes Volatile organic compounds from drying and fixing Drying and fixing are another important emission source in printing processes.
pollutants may be encountered in the exhaust air [179, UBA, 2001]:
· aliphatic hydrocarbons (C10-C20) from binders · monomers such as acrylates, vinylacetates, styrene, acrylonitrile, acrylamide, butadiene · methanol from fixation agents · other alcohols, esters, polyglycols from emulsifiers · formaldehyde from fixation agents · ammonia (from urea decomposition and from ammonia present, for example, in pigment printing pastes) · N-methylpyrrolidone from emulsifiers · phosphoric acid esters · phenylcyclohexene from thickeners and binders.
A more comprehensive list of pollutants potentially present in the exhaust air from heat treatment after printing, with an indication of the potential source, is given in Section 12.
2.9 Finishing (functional finishing) 2.9.1 Finishing processes The term "finishing" covers all those treatments that serve to impart to the textile the desired end-use properties. These can include properties relating to visual effect, handle and special characteristics such as waterproofing and non-flammability.
Finishing may involve mechanical/physical and chemical treatments. Moreover, among chemical treatments one can further distinguish between treatments that involve a chemical reaction of the finishing agent with the fibre and chemical treatments where this is not necessary (e.g. softening treatments).
Some finishing treatments are more typical for certain types of fibre (for example, easy-care finishes for cotton, antistatic treatment for synthetic fibres and mothproofing and anti-felt treatments for wool). Other finishes have more general application (e.g. softening).
In this document particular attention is given to chemical finishes because these are the processes with the most significant polluting potential.
In the case of fabric (including carpets in piece form), the finishing treatment often takes place as a separate operation after dyeing. However, this is not a rule: in carpets, for example, mothproofing can be carried out during dyeing and, in pigment dyeing, resin finishing and pigment dyeing are combined in the same step by applying the pigment and the film-forming polymer in the dyeing liquor.
In more than 80 % of cases, the finishing liquor, in the form of an aqueous solution/dispersion, is applied by means of padding techniques. The dry fabric is passed through the finishing bath containing all the required ingredients, and is then passed between rollers to squeeze out as much as possible of the treating solution before being dried and finally cured. Washing as final step, tends to be avoided unless absolutely necessary.
In order to reduce the pick-up, other so-called minimum application techniques are gaining
importance. These are topical application methods like:
· kiss-roll (or slop-padding) application (the textile is wetted by means of a roller, which is immersed in a trough and which applies a controlled amount of liquor on only one side of the textile) · spray application · foam application.
In the case of foulard application the pick-up is approximately 70 %, while with minimum application systems this can be about 30 %. In the minimum application techniques, however, the liquors are more concentrated by a factor of 2 to 3 in order to allow the same amount of active ingredient to be applied.
In the wool yarn carpet sector the functional finishes are applied to the yarn or to the loose fibre either during the dyeing process or in the subsequent rinsing or finishing bath.
Apart from particular cases where there are problems of incompatibility between the different auxiliaries, both with padding and long liquor application techniques (batch processes), all the finishing agents necessary to give the textile material the desired properties are applied in a single bath rather than in different steps.
2.9.2 Chemical finishing treatments 220.127.116.11 Easy-care treatments Easy-care finishings are applied to cellulose-containing fibres to impart characteristics such as easy-to-wash, creasing resistance during wash and wear, no ironing or minimum ironing. These properties are now required for cellulose fibres to allow them to compete with synthetic fibres such as polyamide and polyester.
Easy-care recipes consist of various ingredients:
· cross-linking agent · catalyst · additives (softeners, hand builder most commonly, but also water-repellents, hydrophilizing agents, etc.) · surfactants as wetting agent.
Information about the typical substances used can be found in Section 8.8.1. In the easy-care process the fabric, after being padded, is dried in open-width in a stenter frame and is finally cured. The most common curing method is the dry cross-linking process, in which the fabric is cured in a dry state in a curing apparatus or on the stenter immediately after drying.
18.104.22.168 Water-repellent treatments (hydrophobic treatments) Water-repellent treatments are applied to fabrics for which waterproofing properties are required but which also need air and water-vapour permeability.
This may be obtained by:
· precipitation of hydrophobic substances such as paraffin emulsions together with aluminium salts (e.g. wax-based repellents) · chemical transformation of the surface of the fibre by addition of polymers that form a cross-linked water-repellent film (e.g. silicone repellents, resin-based repellents, fluorochemical repellents).
The characteristics of the substances used as water-repellents are described in Section 8.8.5.
22.214.171.124 Softening treatments Softeners are used not only in finishing processes, but also in batch dyeing processes, where they are applied in the dyeing baths or in the subsequent washing baths.
The application of softening agents does not involve curing processes. In continuous or semicontinuous processes the impregnated fabric is dried in the stenter frame.
The substances used as softening agents are described in Section 8.8.6.
126.96.36.199 Flame-retardant treatments Flame-retardant finishing has become more and more important and is compulsory for some articles. Flame-retardant treatments should protect the fibre from burning, without modifying the handle, the colour or the look of the fabric.
They are generally applied to cotton and synthetic fibres (e.g. they are important in the furniture sector for upholstery fabric). In some specific cases, in particular in the carpet sector (e.g.
contract market, aviation), they can also be required for wool, even though this fibre is already inherently flame resistant.
Flame-retardant properties are achieved by the application of a wide range of chemicals, which either react with the textile or are used as additives. Substances that are usually used as flameretardant finishing treatments are described in Section 8.8.4.
There are other approaches available to produce textile products with flame-retardant properties
· the addition of specific chemicals in the spinning solution during fibre manufacturing · the development of modified fibres with inherent flame-retardant properties · back-coating of finished textile-covered articles (e.g furniture, matresses), whereby a fireresistant layer is attached to one side of the finished textile.
188.8.131.52 Antistatic treatments The process consists in treating the fabric with hygroscopic substances (antistatic agents) which increase the electrical conductivity of the fibre, thus avoiding the accumulation of electrostatic charge.
These finishing treatments are very common for synthetic fibres, but they are also applied to wool in the carpet sector for floorcoverings that have to be used in static-sensitive environments.
The substances commonly used as antistatic agents are described in Section 8.8.3.
184.108.40.206 Mothproofing treatments The mothproofing of wool and wool-blends is mainly restricted to the production of textile floorcoverings, but some high-risk apparel is also treated (for example military uniforms). For apparel application, mothproofing is usually carried out in dyeing. Floorcoverings may be mothproofed at different stages of the production processes, such as during raw wool scouring, spinning, yarn scouring, dyeing, finishing or later in the backing line.
The biocides used in the mothproofing treatments are described in Section 8.8.2.
220.127.116.11 Bactericidal and fungicidal treatments These finishes may be applied to chemicals (to preserve auxiliaries and dye formulations) and to apparel, for example in odour suppressant for socks and for the treatment of floorcoverings for the healthcare sector and to provide anti dust-mite finishes. Close analysis shows that more and more textile products (clothing and underwear) are being treated with anti-microbial agents.
The products used are biocides: they are mentioned in Section 8.8.2.
18.104.22.168 Anti-felt treatments Anti-felt finishing is applied in order to provide anti-felt properties to the good. This will prevent shrinking of the finished product when it is repetitively washed in a laundry machine.
Two treatments, which are also complementary, are applied:
· oxidising treatment (subtractive treatment) · treatment with resins (additive treatment).
These treatments can be applied at any stage of the process and on all different make-ups. They are most commonly applied on combed tops for specific end-products (e.g. underwear).
Oxidising treatments In the oxidising treatment the specific chemicals used attack the scales of the cuticles and chemically change the external structure of the fibre.
This treatment has traditionally been carried out using one of the following chlorine-releasing
· sodium hypochlorite · sodium salt dichloroisocyanurate · active chlorine (no longer used).
The oldest process is the one using sodium hypochlorite. However, since the development of active chlorine is difficult to control, wool fibre characteristics can be deeply changed, also giving irregular results. Dichloroisocyanurate is more advantageous here because it has the ability to release chlorine gradually, thereby reducing the risk of fibre damage.
The process with dichloroisocyanurate (Basolan process licensed by BASF) consists in impregnating the material in a bath (35˚C) containing the oxidant, sodium sulphate and an auxiliary (surfactant). After 20 - 30 min the material is rinsed, then it is submitted to an antichlorine treatment with 2 – 3 % of sodium bisulphite and rinsed again.
All these chlorine-based agents have recently encountered restrictions because they react with components and impurities (soluble or converted into soluble substances) in the wool, to form absorbable organic chlorine compounds (AOX).
Alternative oxidising treatments have therefore been developed. In particular, peroxysulphate, permanganate, enzymes and corona discharge come into consideration. However, the only alternative to chlorine-based agents readily available today is peroxysulphate.
The process with peroxysulphate compounds is quite similar to the chlorine treatment, but does not involve the use of chlorine and does not generate chloroamines. The material is treated with the oxidising agent in acid liquor at room temperature until the active oxygen has been largely consumed.
Both with chlorine-based agents and peroxysulphate, sodium sulphite is then added as an antioxidant to the same liquor at slightly alkaline pH. This is a reductive aftertreatment to avoid damage and yellowing of the wool fibre at alkaline pH.
The goods are subsequently rinsed. If necessary, they are treated with a polymer (see treatments with resins below).
Treatments with resins (additive processes) In additive processes, polymers are applied to the surface of the fibre with the aim of covering the scales with a "film". However, this treatment must be regarded as a pseudo felt-free finishing process, as it is not the felting propensity that is reduced, but merely the effect thereof.