«EUROPEAN COMMISSION Integrated Pollution Prevention and Control (IPPC) Reference Document on Best Available Techniques for the Textiles Industry July 2003 ...»
SBR foam coating Foam coating methods consist in the application of a foam layer onto a pre-coated carpet, as the following figure shows.
Figure 2.31: Foam-coated tufted carpet The foam finish is carried out in two steps: foam application and foam solidification through drying.
The lattice is foamed with air and then applied by means of a doctor-blade onto the precoated carpet.
The SBR foam must be stabilised until it is solidified in the vulcanisation oven. For this
stabilisation, two methods are used:
· the non-gel process, which uses surfactants as foam stabilisers · the gel process, which uses ammonium acetate (AA gel system) or silicium fluoride (SF gel system) as gelling agents.
The overall process is schematised in Figure 2.32.
Figure 2.32: Representation of the SBR foam coating process [63, GuT/ ECA, 2000]
The foam is composed of:
· the SBR colloidal dispersion · a paste, which contains a number of active additives · inactive fillers (mainly chalk, which is added to the ready-compounded paste) · water · thickeners (e.g. polyvinyl alcohol, methyl cellulose, polyacrylates) · colourants and pigments · anti-oxidants and ozone stabilisers.
Some of the active components of the paste are responsible for the environmental impact of this
coating method. In order to identify better the emission sources they can be divided as follows:
- UV stabilisers
- antistatic agents
- flame retarding agents (e.g. Al2O3).
PU foam coating Polyurethane is another method for foam coating. The ICI polyurethane coating process is the most commonly applied. The carpet is prepared by steaming and then reaches the spray chamber where the components of the polyurethane (diisocyanate and an alcohol) are sprayed.
The CO2 produced during the chemical reaction is embedded into the foam. The coating is reinforced in an infrared heating field and in a subsequent reaction field. The process is schematically represented in the following diagram.
Textile back coating Textile backing consists in the application of a textile fabric onto the pre-coated carpet. The connection between the carpet and the textile fabric is obtained through the application of a
· laminating glue · melting glue.
Figure 2.34: Textile backing [63, GuT/ ECA, 2000] Laminating glue In this process an x-SBR latex is applied to the carpet by slop-padding.
After the application of the textile fabric, the final reinforcement of the latex is carried out by means of heat treatment (Figure 2.35). The latex composition is similar to that used for pre-coating, with a higher share of polymer dispersion in order to allow a higher adhesive power.
Figure 2.35: Textile backing by means of the laminating glue process [63, GuT/ ECA, 2000] Melting glue This system uses thermoplastic polymers (mainly polyethylene) which are meltable by means of heat.
In powder lamination (and in particualr in powder scattering lamination) polyethylene powder is evenly sprinkled onto the back of the carpet. Subsequently the polymer is melted in an infrared field. In the next stage the fabric is pressed into the melting glue. Through subsequent cooling, the melting glue becomes permanently connected between the textile fabric and the bottom side of the carpet. The process is represented in Figure 2.36.
Figure 2.36: Textile backing by means of the powder lamination (melting glue) [63, GuT/ ECA, 2000] Another textile backing process by means of melting glue is the so-called AdBac process.
In this case the carpet is constructed using a primary cloth (carrier layer) with low melting point additives. In the next stage the secondary cloth (also with a low melting point) is brought into contact with the back of the carpet before this enters the heating zone. The higher temperature melts the cloths, which are then forced together by nip rolls at the exit of the heating zone. The carpet is then cooled. A scheme of a carpet produced with the AdBac process is reported in Figure 2.37.
Figure 2.37: Carpet manufactured with the AdBac process [63, GuT/ ECA, 2000] Heavy coating Heavy coating is mainly used for the coating of self-lying (SL) tiles.
The coating process consists in the application of the coating material by means of slop-padding or doctor blade and subsequent reinforcement. In most cases the coating material is applied into layers (two-coat technique). After the first layer, which may also serve as a pre-coating layer, a glass-fibre web may be added. The second coating application follows. The following coating materials are
· APO (abbreviation for “atactical polyolefin”) · bitumen (enriched with inorganic and organic additives) · PVC (polyvinylchloride) · EVA (ethylen vinyl acetate).
The process principle is schematised in Figure 2.38.
Figure 2.38: Representation of the heavy coating process [63, GuT/ ECA, 2000]
2.12 Washing 2.12.1 Washing with water
Important factors in washing are:
· water characteristics · choice of soaps and detergents · hydromechanical action · temperature and pH · rinsing stage.
Washing is normally carried out in hot water (40 – 100 ºC) in the presence of wetting agent and detergent. The detergent emulsifies the mineral oils and disperses the undissolved pigments.
The choice of the surfactants may vary also depending on the type of fibre. Mixtures of anionic and non-ionic surfactants are commonly used. An important factor in the selection of a surfactant is its effectiveness in strong alkaline conditions.
Washing always involves a final rinsing step to remove the emulsified impurities.
Fabric washing can be carried out in rope form or open-width, and both in discontinuous or in continuous mode. The most commonly used technique is continuous mode in open-width.
2.12.2 Dry cleaning Industrial solvent washing is sometimes necessary especially for delicate fabrics. In this case the impurities are carried away by the solvent, which is usually perchloroethylene. In the same step, softening treatments may also be carried out. In this case, water and surfactant-based chemicals are added to the solvent.
Solvent washing may be carried out continuously in full width (for woven or knitted fabric) or discontinuously with yarn or fabrics in rope form (generally for knitted fabric).
Solvent plants have a built-in solvent treatment and recovery system in which the solvent is purified by distillation and re-used for the next washing process. Residual sludge from distillation must be disposed of as hazardous waste in case of high concentration of solvent.
After distillation, the solvent must be cooled before re-use and thus high amounts of cooling water are required. This water is never contaminated by solvent and can therefore be re-used. In mills having both solvent and water washing facilities, warm water from the cooling plant may
be used for water washing treatments, allowing water and energy savings. In many cases, however, this water is not re-used and it is discharged together with the other effluents.
Both closed and open airflow circuits can be used for the removal of solvent from fabric.
In open circuit machines, when the washing cycle is over, large amounts of air are taken from the external environment, warmed up with a steam heat exchanger and introduced into the machine, thus obtaining the evaporation of the organic solvent. This process goes on until the solvent is almost completely eliminated from clean fabrics. Solvent-rich-air is then sent to a centralised activated charcoal filtering system. Filters require regular regeneration to ensure optimal cleaning performance. Most modern filters allow discharge into the atmosphere below 3 – 4 mg/m3.
In closed circuit machines the volume of air used to carry out the drying process, instead of being filtered and released into the atmosphere, is internally treated. Such treatment consists in recovering the solvent by condensation using a chiller. When the solvent has been removed from air and recovered, solvent-poor-air is heated by a heat exchanger and then sent again inside the machine. Recovered solvent is sent to a centralised plant, where it is distilled and purified.
Closed circuit machines do not require an active carbon filter.
Apart from the above-mentioned air emissions in open-circuit machines, possible emissions during washing operations may result from machine losses (which can be eliminated or reduced by hermetic sealing of the machinery) and from solvent attached to the dried fabric and ultimately released in the atmosphere. Most modern machines have a built-in control system which makes it impossible to open the machine hatch if the solvent concentration in the machine is greater than values established by national regulations.
Other potential sources of emissions are represented by the solvent contained in the residual sludges and active carbon filters.
Figure 2.39, Figure 2.
40 and Figure 2.41 show the solvent and the air circuits in open loop and closed loop solvent washing machines (the solvent circuit is always closed) Figure 2.39: Solvent washing: representation of the solvent circuit [66, CRIT, 1999]
Figure 2.40: Solvent washing: representation of the air circuit in a open-loop washing machine [66, CRIT, 1999] revised by [318, Sperotto Rimar, 2002] Figure 2.
41: Solvent washing: representation of the air circuit in a closed-loop washing machine [66, CRIT, 1999] revised by [318, Sperotto Rimar, 2002]
2.13 Drying Drying is necessary to eliminate or reduce the water content of the fibres, yarns and fabrics following wet processes. Drying, in particular by water evaporation, is a high-energyconsuming step (although overall consumption may be reduced if re-use/ recycling options are adopted).
Drying techniques may be classified as mechanical or thermal. Mechanical processes are used in general to remove the water which is mechanically bound to the fibre. This is aimed at improving the efficiency of the following step. Thermal processes consist in heating the water
and converting it into steam. Heat can be transferred by means of:
· convection · infrared radiation · direct contact · radio-frequency.
In general, drying is never carried out in a single machine, normally drying involves at least two different techniques.
2.13.1 Loose fibre drying The water content of the fibre is initially reduced by either centrifugal extraction or by mangling before evaporative drying.
184.108.40.206 Centrifugal extraction Textile centrifugal extractors (hydroextractors) are essentially a more robust version of the familiar domestic spin dryer, and normally operate on a batch principal, although machines capable of continuous operation may be used in very large installations.
When using conventional batch hydroextractors, fibre is unloaded from the dyeing machine into specially designed fabric bags which allow direct crane loading of the centrifuge. An extraction cycle of 3 - 5 minutes reduces residual moisture content to approximately 1.0 l/kg dry fibre (in the case of wool).
Pneumatically loaded mangles may be used to reduce the water content of dyed loose fibre.
Such equipment is often associated with a fibre opening hopper which is designed to break up the dyepack and present the fibre to a continuous dryer as an even mat. Mangling is invariably less efficient than centrifugal extraction.
220.127.116.11 Evaporative drying
All hot air evaporative dryers are of essentially similar design consisting of a number of chambers through which hot air is fan circulated. Consecutive chambers operate at different temperatures, fibre passing from the hottest into progressively cooler chambers. Fibre may be transported on a brattice or conveyer belt or may be carried through the machine on the surface of a series of “suction drums”. High efficiency dryers with perforated steel conveyer belts have been developed which even out the air pressure drop across the fibre matt. This design results in more even drying and lower thermal energy requirements.
While the majority of dryers are steam heated, a number of manufactures supply radio frequency dryers. Fibre is conveyed on a perforated polypropylene belt through the radio frequency field and air flow is fan assisted. With these machines the fibre is not subjected to such high temperatures and the moisture content of the dried material can be controlled within fine limits.
Radio frequency dryers are reported to be significantly more energy efficient than steam heated chamber dryers. However, the higher efficiency is not always gained if a more global analysis is made, comparing the primary energy needed for production of electric power with methane gas consumed for thermal energy production. Radio frequency dryers are mainly used where the cost of electricity is low.
2.13.2 Hanks drying 18.104.22.168 Centrifugal extraction Drained hanks from the dyeing machine can contain (in the case of wool) up to 0.75 kg water per kg of dry fibre (or higher depending on the hydrophilicity of the fibre). Moisture content is normally reduced by centrifugal extraction prior to evaporative drying using equipment identical to that described for loose fibre, above. Yarn is normally unloaded from the dyeing machine into fabric bags held in round carts to facilitate direct crane loading of the centrifuge.
Hydroextraction reduces the moisture content to approximately 0.4 litres/kg dry weight.
22.214.171.124 Evaporative drying Evaporative dryers consist of a number of heated chambers with fan assisted air circulation, through which the hanks pass suspended on hangers or poles or supported on a conveyer.
The hank sizes employed in carpet yarn processing require a slow passage through the dryer to ensure an even final moisture content, and a residence time of up to 4 hours is not uncommon.
Air temperature is maintained below 120 ºC to prevent yellowing (wool yellows above the boiling temperature).
All designs are capable of continuous operation. Thermal input is normally provided by a steam heated exchanger and many designs incorporate air-to-air heat exchangers on the dryer exhaust to recover heat.
Less commonly, hanks may be dried by employing a dehumidifying chamber. Moisture is recovered by condensation, using conventional dehumidification equipment. In comparison to evaporative dryers, yarn residence time tends to be longer, but energy consumption is lower.
2.13.3 Yarn packages drying The moisture content of dyed packages is initially reduced by centrifugal extraction. Specially designed centrifuges, compatible with the design of the dyeing vessel and yarn carriers are employed.
Traditionally packages were oven dried, very long residence times being required to ensure adequate drying of the yarn on the inside of the package. Two methods are currently used, rapid (forced) air drying and radio frequency drying, the latter sometimes being combined with initial vacuum extraction. Forced air dryers generally operate by circulating hot air from the inside of the package to the outside at a temperature of 100 ºC, followed by conditioning, in which remaining residual moisture is redistributed in a stream of air passing from the outside to the inside of the package. Radio frequency dryers operate on the conveyer principle and are perhaps more flexible than the types mentioned above. Lower temperatures can be used and energy efficiency is said to be high (comments made for evaporative drying of loose fibre apply in this case, too).
2.13.4 Fabric drying The drying process for fabric usually involves two steps: the first one is aimed at removing water which is mechanically bound to fibres, while the second one is necessary to dry completely the fabric.