Finishing in textile industry
The essence of textile finishing is giving fabrics the visual, physical and aesthetic properties which consumers demand. The main processes involved are bleaching, dyeing (of yarn, fabric, ready-made garments), printing, coating / impregnating and the application of various functional finishings. In most cases, the textile finishing process is combined with a manufacturing process which gives the final product its particular shape (BAT for the Textiles Industry, July 2003).
2. FIELD OF APPLICATION
The main product categories cover clothing textiles, interior textiles (furnishing fabrics, curtains and carpets), household textiles (bed/bath and table linen) and technical textiles (automotive fabrics, geo- and medical textiles) (BAT for the Textiles Industry, July 2003).
3. DESCRIPTION OF TECHNIQUES, METHODS AND EQUIPMENT
- Functional finishing: (BAT for the Textiles Industry, July 2003)
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 (f. ex., 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 pollution 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 typical 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 incompability 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 propertied are applied in a single bath rather than in different steps.
- Chemical finishing treatments: (BAT for the Textiles Industry, July 2003)
a) Easy-care treatments
Easy-care finishings are applied to cellulose-containing fibres to impact 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
- additives (softeners, hand builder most commonly, but also water-repellents, hydrophilizing agents, etc.)
- surfactants as wetting agent
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.
b) 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).
c) 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 semi-continous processes the impregnated fabric is dried in the stenter frame.
d) 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 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.
There are other approaches available to produce textile products with flame-retardant properties including:
- 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, mattresses), whereby a fire-resistant layer is attached to one side of the finished textile.
e) 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.
f) 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.
g) 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.
- Anti-felt treatments: (BAT for the Textiles Industry, July 2003)
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 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).
a) Oxidising treatmentsIn 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 agents:
- 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 anti-chlorine 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 peroxysulpahte, sodium sulphate is then added as an anti-oxidant 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.
b) 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.
The polymer must have a high substantivity for wool. Cationic polymers are the most suitable for this treatment because, after the previous oxidative and reductive pre-treatment, the wool surface becomes anionic.
The polymer may be, in some case, sufficiently effective on its own to make pre-treatment unnecessary. However, the combination of subtractive and additive processes has the greatest technical effect.
c) Combined treatments: Hercosett process
The oldest combination process is the so called Hercosett process (by C.S.I.R.O), which consists in chlorine pre-treatment followed by application of a polyamide-epichloridrine resin.
Whilst the Hercosett process can be carried out in batch or continuous mode, the latter is predominant nowadays.
Whilst the Hercosett process can be carried out in batch or continuous mode, the latter is predominant nowadays.
The continuous process consists of the following steps (see Figure 2.27):
- chlorine treatment in acid medium (using chlorine gas or sodium hypochlorite)
- reduction of chlorine using sulphite in the same bath
- neutralisation with sodium carbonate
- resin application
- softener application
- drying and polymerisation
The Hercosett process has been widely used for years as anti-felt finishing of wool in different states (loose fibre, combed top, yarn, knitted and woven fabric) due to its low cost and high quality effects. However, the effluent shows high concentrations of COD and AOX. The formation of AOX is attributable not only to the oxidant, but also to the resin. In fact, the typical resin applied in the Hercosett process is a cationic polyamide whose manufacturing process involves the use of epichloridrine, which is another source of the chlorinated hydrocarbons in the effluent.
Alternative resins have been developed, based on polyethers, cationic aminopolysiloxanes, synergic mixtures of polyurethanes and polydimethylsiloxanes, but they all have some limitations concerning their applicability.
New processes have also been developed, but so far the results achieved with the Hercosett process cannot be fully matched by any alternative, which is why it is still the preferred process particularly for treatments such as the anti-felt finishing of combed tops.
4. COMPETITIVE TECHNOLOGIES AND ENERGY SAVING POTENTIALS
- a) Changes in the process
- Plasma Treatment: (http://www.htpunitex.com/NewFiles/plasmai.html)
The plasma treatment is a cost-effective and ecological process able to modify properties of the fabrics surface. It is a “roll to roll” process carried out inside a chamber where vacuum between 50 and 150 Pascal is generated and fabrics, at a speed ranging from 10 up to 40m./min are inlaced amongst a number of electrodes (rolls) connected to a medium-frequency generator which create an electrical field where a gas ( air, oxigen, nitrogen etc) is transformed into “plasma” (the fourth state of matter…). Cold-glow discharge plasma , formed by ions, electrons, UV radiation, free radicals, acts on the surface of the fabrics removing first the organic contaminants and then modifying the chemical structure. Temperature inside the chamber , not higher than 60C°, allows the treatment of any kind of fabric (wool, silk, cotton, polyester, polyammide, kevlar) , plastic films or composite materials avoiding any alteration of mechanical properties of the products (strength, elasticity etc)
AdvantagesIt is possible to modify fabrics surfaces according to the requirements by defining and chosing the appropriate gas and treatment conditions. Plasma treatment is able to increase wettability of those fabrics having, by their nature, a low level of wettability (such as wool, polyester, kevlar, etc) and which may require aggressive chemical agents and/or complicated processing. Considerable increase of adhesion is granted to fabrics treated by coating, impregnating, laminating, coagulating products – this means improvement of mechanical characteristics of the final product on one side in addition to greatly enhanced washing, wear and abrasion resistance (never seen before…) on the other side. Wool antifelting, higher dyeing yield, higher colour fastness, better printing quality are some of the characteristics obtainable by HTP plasma treatment, so allowing manufacturing of higher quality products by simpler and cost effective converting processes. A KPR 180 machine is installed and operative c/o HTP premises to enable trials, demonstration or industrial treatment services for any kind of fabric , woven and non-woven, plastic film or composite material.
- Enzyme catalysed finishing processes: (BAT for the Textiles Industry, July 2003)
Enzymes are proteins that act as biocatalysts activating and accelerating chemical reactions which would otherwise need more energy. Their excellent substrate selectivity allows more gentle process conditions compared to conventional processes. Enzymes are present in bacteria, yeasts and fungi.
At present enzymes are used and under study only for natural fibres, the use of enzymes for man-made fibres is not mentioned in literature. Some enzymes, such as the amylases in the desizing process, have been widely applied for a long time; other enzymes are still the object of investigation. Table 6.1 lists the main enzymatic processes already in use or currently emerging in the textile sector.
Energy savings (lower processing temperature) and lower water consumption (reduced number of rinsing steps) are some of the promising advatages of enzymatic processes, along with the omission, in some cases, of the use of hazardous/harmful substances. Also enzymes can be used in catalytic amounts and as a biocatalyst they can be recycled.
* Reduced water consumption by improved working practices: (BAT for the Textiles Industry, July 2003)
Inappropriate working practises and the absence of automated control systems can lead to significant wastage of water, e.g.:
- during fulling and rinsing, for example, where machines are equipped only with manual water control valves there is potential for overfilling
- displacement spillage during immersion of the fibre in the machine may account for upt tp 20% of total operating volume over the course of a dyeing cycle (this may alos lead to losses of dyes and hazardous chemicals if these are introduced before the displacement takes place).
Well – documented production procedures and training are important. Dyeing machines should at least be fitted with modern process control equipment, capable of accurately controlling both the fill volume and the liquor temperature.
- Reduced water consumption by reducing liquor ratio: (BAT for the Textiles Industry, July 2003)
In continuous dyeing, the dye is applied in the form of a concentrated liquor. The volume of water consumed per kg of processed fabric in the dyeing process is therefore fairly low even when using conventional application systems (e.g. padders). This volume can even be lower in more recently developed application systems (e.g. fluidyer, foam, flexnip application systems etc. – see BAT 2003, Section 10.4.2).
In batch operation the amount of water used per kg of processed substrate is higher, although there has been considerable improvement in this field, too. All major machine manufacturers now have units for dyeing at low liquor ratio. An investment in such units pays because it cuts operating costs (energy, water, chemicals, dyes etc.) and raises productivity by reducing processing times (see also BAT 2003, Section 18.104.22.168 and 4.6.19).
- Reduced water consumption by improving washing efficiency: (BAT for the Textiles Industry, July 2003)
In both batch and continuous processing, water consumption for washing is significantly higher than for the treatment itself (e.g. dyeing etc.) (see also BAT 2003, Section 22.214.171.124). Modern continuous washing machines have greatly improved their washing efficiency. In batch processes, it is not straightforward to achieve a high washing efficiency with little water and in a short time and therefore low liquor ratio does not always correlate with reduced water use as one might expect. Indeed it is not uncommon to find machines able to dye with a liquor ratio of 1:5 and then rinse with a liquor ratio of 1:10. Moreover conventional machines can only handle unloading by increasing the liquor ratio.
These problems were recently tackled by machine manufacturers and dyestuff suppliers. Recent technological developments have decreased specific water consumption in batch processing to levels more typical of modern equipment for batch processes. Efficient washing techniques have also been especially developed for batch operations (see BAT 2003 – Section 4.9.1). Furthermore various functions typical of continuous processing have been transferred to batch machines such as (see BAT 2003 – Section 4.6.19).
- in-process separation of the bath from the substrate
- internal separation of process-liquor from the washing liquor
- mechanical liquor extraction to reduce carry-over and improve washing efficiency
- internal counter current flow in the batch washing process
- Reduced water consumption by combining processes: (BAT for the Textiles Industry, July 2003)
Combining and scheduling processes reduces the number of chemical dumps. This is often feasible for pretreatment operations (e.g. scouring/desizing, scouring/desizing/bleaching – see for example BAT 2003 – Section 4.5.3). Combining pretreatment into the colouration stage is also possible in some cases.
- b) Changes in the energy distribution system
- Reduced water consumption by water re-use: (BAT for the Textiles Industry, July 2003)
Batch processes do not easily allow for water recycling. When trying to re-use waste water in batch operations, storage facilities for re-usable waster water must be provided. Other problems associated with re-use of water from batch bleaching and scouring are the non-continuous characters of the waster stream and the higher liquor ratios.
Continuous counter current flow of textiles and water is now also possible in batch processing. Machines are now available with built-in facilities for waste stream segregation and capture. For example, the wash water from a previous load can be recovered and fully used in the bleach bath for the current load, which can then be used to scour the next load. In this way, each bath is used three times.
Some examples of water recycling and re-use are reported in the BAT 2003 – Sector 4.6.22 and 4.7.7).
The internal separation of process-liquor from the washing liquor applied to some modern batch dyeing machines is essential to allow easier bath segregation and re-use, in cases where the characteristics of the liquor make it feasible.
- Heat recovery: (BAT for the Textiles Industry, July 2003)
Exhaust heat recovery can be achieved by using air-to-water heat exchangers. Up to 70% of energy can be saved. Hot water can be used in dyeing. Electrostatic filtration for off-gas cleaning can optionally be installed. Retrofitting is possible.
If hot water is not required, an air-to-air heat exchanger can be used. Efficiencies are generally 50 to 60%. Approximately 30% savings in energy can be achieved. An aqueous scrubber alone or with subsequent electrostatic filtration can optionally be installed for off-gas cleaning.
- Insulation of stenter frame: (BAT for the Textiles Industry, July 2003)
Proper insulation of stenter encasement reduces heat losses to a considerable extent. Savings in energy consumption of 20% can be achieved if the insulation thickness is increased from 120 to 150 mm (provided that the same insulation material is used).
- Improved nozzle systems: (BAT for the Textiles Industry, July 2003)
With optimised nozzles and air guidance systems, energy consumption can be reduced, especially if nozzle systems are installed that can be adjusted to the width of the fabric.
Main achieved environmental benefits
Savings in energy consumption and therefore minimisation of emissions associated with energy production are the main environmental advantages.
Data about achievable energy savings are already indicated for some of the presented techniques. Obviously, for existing plants, the potential for reductions will vary according to the existing technology and energy management policy in the company.
Minimising energy consumption in the stenters, especially if heat recovery systems are installed, requires adequate maintenance (cleaning of the heat exchanger and stenter machinery, checking of control/monitoring devices, adjusting of burners etc.).
Proper scheduling in finishing minimises machine stops and heating-up/cooling-down steps and is therefore a prerequisite for energy saving.
Heat recovery systems are often combined with an aqueous scrubber or electrostatic filtration systems or a combination of these techniques.
Condensed substances (mainly preparation oils) from heat recovery systems have to be collected separately.
- c) Changes in the heat supply system
- Direct and indirect gas firing: (BAT for the Textiles Industry, July 2003)
Direct gas firing is reported to be both clean and cheap. When it was first introduced there was concern that oxides of nitrogen, formed by exposure of air to combustion chamber temperature, would cause fabric yellowing or partial bleaching of dyes. This concern has since been shown to be unjustified.
However, other sources also show the advantages of new (recently developed) indirect gas firing systems. By means of a flue gas/air heat exchanger the heat generated by the burner flame is directly transferred to the circulating air in the stenter (“Monforts, Textilveredlung 11/12, 2001, page 38”). This system has higher efficiency than conventional indirect heating systems using mainly heating oil. Reactions of off-gas compounds with emissions from the textile materials and auxiliaries (especially generation of formaldehyde) are avoided.