«EUROPEAN COMMISSION Integrated Pollution Prevention and Control (IPPC) Reference Document on Best Available Techniques for the Textiles Industry July 2003 ...»
The relatively high investment cost (no precise information given) of this sophisticated equipment makes this technique more appropriate for large-volume print houses. Nevertheless, digitally-controlled printing and processing offers several economic advantages. First of all this technique allows the flexibility necessary to satisfy customer and retailer demands with just-intime delivery (quick reaction to customer’s orders, alterations easily made). Equally, the stocking of finished goods becomes unnecessary as goods are produced to order. As designs are stored in an electronic format there is no need for a large screen storage facility [171, GuT, 2001].
Reference plants Many plants already use digital jet printing machines.
As already stated in Section 2.8.2, the latest developments of this technique are already applied on commercially available machines, such as the Zimmer’s Chromojet and the latest Milliken’s Millitron machine.
Reference literature [171, GuT, 2001].
4.7.9 Ink-jet digital printing for flat fabric Description A technique such as the jet printing technique (see Sections 2.8.2 and 4.7.8), where the colour is injected deep into the face of the fabric, is applicable for carpet and bulky fabrics but is not suitable for light fabrics such as those that are typically printed in the textile finishing sector.
Ink-jet printing (see also Section 2.8.2) seems to be the most appropriate technique in this case because the colour has to be applied to the surface (similar to paper).
Although great improvements have been made in ink-jet printing, production speeds are still low, which prevents this technique from yet replacing traditional analogue printing techniques.
Nevertheless it can already offer significant advantages in the production of short runs (typically less than 100 m), where the system losses in analogue printing are often comparable with or can even exceed the amount of paste printed on the fabric.
Urea (for dissolving highly concentrated dyes) and a thickening agent are needed. These auxiliaries cannot be injected through the needles, due to viscosity problems and the reduced size of the needles. Therefore the first operation in the printing process is to cover the substrate (woven or knitted fabric) with the urea and the thickening agent.
After printing, the fabric generally has to be first dried and then fixation takes place. The fabric is then washed and finished.
A suitable set of dyes with affinity for the fabric is required. Companies like Ciba, Dystar and Brookline have developed prints with acid, reactive and disperse dyes. Pigment formulations have also recently been made available.
Main achieved environmental benefits Dye residues are minimised. The application of the “colour-on-demand” principle means that the printing paste preparation stage is no longer needed. Therefore there are no dye residues and no printing paste preparation tanks to be cleaned at the end of each run. The environmental advantage over conventional analogue printing techniques is particularly noticeable when processing small lots, when the printing paste losses are particularly high for analogue machines.
In pigment printing, the digital technique is even more advantageous because no afterwashing is needed. This means no residues or waste water from the process and also higher productivity.
Pigment printing accounts for a large share of printed goods production and the omission of afterwashing makes this technique likely to become rapidly competitive with conventional printing methods.
Operational data This technique is constantly being improved. However, the current speed of commercial ink-jet textile printing machines is in the range of 20 to 40 m2 of fabric per hour.
It should emphasised that digital printing machines can work 24 h/day and, unlike with analogue printing, no extra time needs to be spent in cleaning operations when changing from one product to another.
Injector maintenance is still crucial.
Cross-media effects Ink-jet printing is considered a clean technology, but it cannot be considered as such when ink wasted (continuous ink printing technologies) or when jets are flushed out with solvent to prevent blocking when the printer is not in use.
Applicability Ink-jet printing is suitable for flat fabrics.
Ink-jet printing is often considered a technique only applicable to the production of samples.
However, using digital printing for sampling and analogue printing (screen-printing) for industrial production implies that the sample obtained by digital printing matches the characteristics of the product obtained from analogue printing. This is currently very difficult for various reasons. Therefore the future aim of ink-jet printing in the textile sector is industrial scale production. The problem is that at the current performance speed, the process becomes economically attractive only at runs below 100 m (short runs). In conclusion, the ink-jet technique should be considered as BAT for the production of short runs rather than for the production of samples (with the aim of mass customisation where the sampling machine will be the production machine) [281, Belgium, 2002].
Economics Information on investment cost was not made available. It is reported that the higher flexibility and promptness in satisfying customer and retailer demands compensate the machine [180, Spain, 2001].
Reference plants Many plants.
Reference literature [180, Spain, 2001], [204, L. Bettens, 2000].
4.8 Finishing 4.8.1 Minimisation of energy consumption of stenter frames Description Stenters are mainly used in textile finishing for heat-setting, drying, thermosol processes and finishing. It can be roughly estimated that, in fabric finishing, each textile substrate is treated on average 2.5 times in a stenter.
Energy savings in stenters can be achieved by applying the following techniques.
A) Optimising exhaust airflow through the oven The main energy requirements for a stenter are for air heating and evaporation. It is therefore fundamental that the fabric moisture content should be minimised before the fabric enters the stenter and that exhaust airflow within the oven is reduced.
Water content on the incoming fabric can be minimised using mechanical dewatering equipment such as vacuum extraction systems, optimised squeezing rollers, etc. (the latter is less efficient, but less energy consuming). Up to 15 % energy savings in the stenter (depending on the type of substrate) can be obtained if moisture content of the fabric is reduced from 60 % to 50 % before it enters the stenter.
Exhaust airflow optimisation is another determinant factor. Many stenters are still poorly controlled, relying on manual exhaust adjustment and operator estimation of fabric dryness. For optimum performance, exhaust humidity should be maintained between 0.1 and 0.15 kg water/kg dry air. It is not unusual to find stenters with exhaust humidity of only 0.05 kg water/kg dry air, indicating that the exhaust volume is too high and excessive energy is being used to heat air [146, Energy Efficiency Office UK, 1997]. Energy consumption for air heating can reach up to 60 % of the total energy requirement, if airflow is not monitored [185, Comm., 2001].
Equipment is available (variable-speed fans) which will automatically adjust exhaust airflow according to moisture content of the exhaust air or according to moisture content or temperature of the fabric after the process. A reduction of fresh air consumption from 10 kg fresh air/kg textile to 5 kg fresh air/kg textile results in 57 % energy saving [179, UBA, 2001].
B) Heat recovery 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 % ([146, Energy Efficiency Office UK, 1997]). Approximately 30 % savings in energy
can be achieved [179, UBA, 2001]. An aqueous scrubber alone or with subsequent electrostatic filtration can optionally be installed for off-gas cleaning.
C) Insulation 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).
D) Heating systems 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 [146, Energy Efficiency Office UK, 1997].
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, p.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.
E) Burner technology
With optimised firing systems and sufficient maintenance of burners in directly heated stenters, the methane emissions can be minimised. A typical range for an optimised burner is 10 - 15 g methane (calculated as organic carbon)/h, but it has to be taken into account that methane emissions from burners are strongly linked with actual burner capacity.
Stenters should receive general maintenance by specialised companies at regular intervals.
There should also be routine checking of the burner air inlet for blocking by lint or oil, cleaning of pipework to remove precipitates and adjusting of burners by specialists.
F) Miscellaneous techniques 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.
Operational data 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.
Cross-media effects None believed likely.
Applicability All described techniques are applicable to new installations. For existing equipment, the applicability is in some cases limited. For example, improving stenter insulation (see Option C) is not always practicable, although on some older machines, it may be cost-effective to insulate the roof panels. Existing stenters cannot be retrofitted with air-to-air heat exchangers.
Fresh air Heat-setting 11000 20 22000 9.6 33000 6.6 temp.20 °C Source: [179, UBA, 2001] Table 4.35: Return on investment for different processes (textiles drying and heat setting), heat recovery systems (air/water and air/air) and number of shifts per day
The above information does not consider the installation of other measures such as fabric moisture control and exhaust humidity control. If these systems are installed, according to some sources heat recovery may not be cost-effective [146, Energy Efficiency Office UK, 1997].
Driving force for implementation Minimisation of energy consumption (and therefore costs) is the main reason to retrofit optimised stenter technology.
Reference plants The described technologies are in use in many finishing mills in Europe and worldwide. The indirect heating system based on flue-gas/air exchanger is currently due to be installed in several finishing plants.
Reference literature [146, Energy Efficiency Office UK, 1997], [179, UBA, 2001], [185, Comm., 2001].
Description Easy-care finishing is mainly carried out on cellulosic fibres and their blends in order to increase the crease recovery and/or dimensional stability of the fabrics (see Sections 126.96.36.199 and 8.8.1).
Easy-care finishing agents are mainly compounds synthesised from urea, melamine, cyclic urea derivatives and formaldehyde. Cross-linking agents (reactive groups) are composed of free or etherificated N-methylol groups.
Figure 4.28: Chemical structure of cross-linking agents [179, UBA, 2001] Formaldehyde-based cross-linking agents may release free formaldehyde.
Formaldehyde is thought to be carcinogenic and is a threat to the workforce (formaldehyde can also be released, for example, during cutting operations). The presence of free formaldehyde or partly hydrolysable formaldehyde on the finished fabric also represents a potential risk for the final consumer. The European Eco-label scheme sets a threshold of 30 ppm for products that come into direct contact with the skin.
Low-formaldehyde or even formaldehyde-free products are an alternative.
Dimethyloldihydroxyethylene urea (DMDHEU) High Dimethyloldihydroxyethylene urea (DMDHEU) Low derivatives (most commonly used) Modified dimethyldihydroxyethylene urea Formaldehyde-free
Main achieved environmental benefits With low-formaldehyde or formaldehyde-free products, a reduction of formaldehyde emissions in finishing is achieved. Formaldehyde residues on the textiles can be minimised (75 mg/kg textile, or even lower than 30 ppm for consumer requirement). With optimised catalysts, curing temperature and therefore energy consumption can be reduced.
If directly-heated stenters are inefficiently maintained, they may also emit formaldehyde in the exhaust air.
A typical recipe for low-formaldehyde finishing of cotton (woven fabric) is:
· 40 - 60 g/l cross-linking agent · 12 - 20 g/l catalyst · liquor pick-up: 70 % · drying and condensation (150 °C, 3 min).
A typical recipe for a formaldehyde-free finishing of cotton is:
· 80 - 120 g/l cross-linking agent (integrated catalyst) · liquor pick-up: 80 % · acidifying with acetic acid · drying and condensation (130 °C, 1 min).
Cross-linking compounds are often applied in combination with wetting agents, softeners, products which increase rip-resistance etc.
Cross-media effects Like the conventional cross-linking agents, the formaldehyde-free alternative products mentioned above are hardly biodegradable. However, as a fundamental rule, the amount of concentrated liquor should be kept to a minimum by small pad-boxes, and residues should be disposed of separately without draining them to the waste water.
Non-optimised formaldehyde-free products can be odour-intensive.
Applicability In the carpet sector it is always possible to avoid formaldehyde emissions by using formaldehyde-free easy-care finishing agents, whereas in the textile sector the use of formaldehyde-poor agents may be inevitable [281, Belgium, 2002].