«EUROPEAN COMMISSION Integrated Pollution Prevention and Control (IPPC) Reference Document on Best Available Techniques for the Textiles Industry July 2003 ...»
With synthetic knitted fabric, one of the advantages of carrying out the washing step before thermofixation is that air emissions from the stenter can be significantly reduced without the need for air emission abatement equipment.
Operational data The alternative hydrosoluble knitting oils described in this section produce emulsions which remain stable for three days [295, Spain, 2002].
Cross-media effects Provided that the water-soluble knitting oils used in substitution of the conventional ones are biodegradable and that the fabric is processed in high-efficiency washing machines, a net environmental benefit is achieved.
Applicability The technique is applicable to new and existing plants. However, corrosion problems have been observed in some existing plants.
As for the types of fibres, the referenced hydrosoluble oils are suitable for knitted fabrics made of cellulose fibres and blends, as well as fabrics made of synthetic fibres, mainly polyester and polyamide, and their blends with natural and synthetic fibres, including elastane.
248 Textiles Industry
Some synthetic knitted fabrics need to be thermofixed before washing. In this case another option is the “dry route”: the fabric is submitted to thermofixation before washing and the resulting fumes are treated in an electrostatic precipitator with subsequent recovery of the oil.
The advantage is that the recovered oil can be disposed of separately, thereby reducing the contamination of the water effluent. Energy recovery is another feature of this technique. More details are given in Section 4.10.9.
The overall cost balance of this technique is comparable with the conventional one. The referenced water-soluble knitting oils are more expensive than the mineral oil-base ones, but this extra cost is compensated by the higher productivity and by the higher treatability of the effluent resulting from pretreatment.
Driving force for implementation The strict limits set by environmental legislation for emissions to air and water favour the implementation of this technique.
Reference plants Many plants.
Reference literature [295, Spain, 2002].
Description Sizing agents are applied to warp yarn in order to prevent thread breakage during weaving. For subsequent processing, the sizing agents need to be removed almost completely from the fabric.
This is done in the desizing step, often with a large amount of water and the addition of auxiliaries. The resulting effluent is responsible not only for high COD/BOD load (about 30 to 70 % of the overall COD load), but also for the presence of difficult to degrade substances that may pass through the final treatment. In some cases sizing agents may be recovered from the desizing liquor. More often, however, the effluent is treated in the waste water treatment plant.
Along with low add-on techniques such as prewetting (see Section 4.2.5), targeted selection of sizing agents may also contribute significantly to the reduction of the environmental impact of this operation.
Environmentally-optimised sizing agents should be:
· highly efficient with low add-on · completely and easily removed from the fabric · readily biodegradable or bioeliminable (80 % after 7 days according to OECD-test 302 B).
It is now accepted that readily biodegradable/ bioeliminable sizing agent formulations are available, covering all needs. Modified starches, certain galactomannans, polyvinyl alcohol and certain polyacrylates satisfy this requirement.
Furthermore, latest generation-polyacrylates are able to fulfil all the requirements listed above.
Firstly, use of these high efficiency synthetic sizes instead of conventional modified starches allows a reduction in size add-on without any decline in weaving efficiency (in some cases, increased weaving efficiency is observed). Secondly, new generation-polyacrylates are easy to wash out and can be removed with little water and without additional auxiliaries.
The new polyacrylates can be applied as almost universal sizing agents for all kinds of fibres.
Exceptions include filament polyester and some specific finishing treatments where the use of polyacrylates may give rise to quality problems. For example, technical problems in the finishing stage were experienced when using polyacrylates on cotton fibre that had to be submitted to pre-shrink finishing [281, Belgium, 2002].
For cotton, polyacrylates are applied in combination with other sizes, usually PVA, in order to increase the viscosity of the system Main achieved environmental benefits The application of biodegradable/bioeliminable sizing agents leads to significant reduction of the COD-load that may pass un-degraded through the waste water treatment plant and be discharged to natural waters.
Additional advantages are achievable when using the highly efficient, easy-to-wash sizing agents. Lower add-on means reduced COD load in the discharged effluent, while the fact that they are easily washed out means that significant savings can be obtained in chemicals, water and energy consumption. With advanced efficient washing machines only low quantities of water are needed to remove the size, without additional auxiliaries (e.g. emulsifiers) or long cycle times (lower energy consumption).
Polyacrylates, polyvinyl alcohol and modified starch are not only biodegradable/bioeliminable, but are also suitable for size recovery techniques. New polyacrylates have the additional advantage of being applicable as almost universal sizing agents. This means that they are potentially easy to re-use as sizes in weaving firms.
Operational data The bioelimination curves of seven combinations of sizing agents based on modified starches, new generation-polyacrylates, polyvinyl alcohol and certain galactomannans, are shown in Figure 4.3 (see also “Reference Plants”).
Figure 4.3: Bioelimination curves in the modified Zahn-Wellens Test (EN 29888) of seven combinations of different sizing agents which are bioeliminated to more than 80 % after 14 days [179, UBA, 2001] Modified starches are water-removable (less easy than modified polyacrylates), without need for enzymatic or oxidative desizing.
They are bioeliminable, but they give rise to bulky, viscose and filamentous sludge that is difficult to settle.
Polyvinyl alcohol is easy to wash out at mid-range pH and it is recoverable. In alkaline conditions, it swells and becomes highly viscose and difficult to remove. Polyvinyl alcohol is biodegradable only under specific system conditions such as adaptation of the activated sludge, temperatures not below 15 °C and particularly low food to micro-organism ratios (F/M-ratios) (see Section 4.10.1). In alkaline conditions, PVA gives problems with settlement.
As mentioned above, the new polyacrylates are highly efficient with lower add-on (see table below), they are removable with only water (no need for either enzymatic or oxidative desizing) and they also have high stability to alkalis (it is possible to bleach the fabric directly, without a preliminary scour). Unlike classic polyacrylates, the new ones are more than 90 % eliminated under the conditions of the Zahn-Wellens test, even in high concentrations, by adsorption on the activated sludge. Furthermore, they become insoluble by forming complexes with iron. In this way they can be precipitated almost totally with only a small amount of precipitant.
Source: [179, UBA, 2001] with reference to “Steidel, 1998”
(1) typical operating conditions for sizing staple fibre yarn in air jet loom at 650 rpm are as follows:
PVA: 7 kg; modified acrylate (liquid 25 %): 7 kg; wax: 0.4 kg; volume sizing liquor: 100 l; nip pressure: 20kN; sizing speed: 100m/min (2) calculated on an annual production of 4000 t warp yarn Table 4.4: COD reduction after replacement of conventional sizing agent by alternative recipe based on polyacrylates
Cross-media effects The application of sizing agents with higher bioeliminability/biodegradability leads to an increased amount of sludge to be disposed of [179, UBA, 2001]. This sludge can be bulky, filamentous and difficult to settle.
Applicability Although the application of optimised sizing recipes is technically feasible for all sizing departments, the world-wide organisation of the textile chain, makes it difficult for nonintegrated mills and in particular, for commission finishers to influence the up-stream weaving mills [179, UBA, 2001].
Economics In most cases, biodegradable/bioeliminable combinations of sizing agents are no more expensive than others that do not meet the high elimination rate requirement (80 % after 7 days according to OECD-test 302 B) [179, UBA, 2001].
Details are given in the table below, regarding, in particular, the application of alternative synthetic, high-efficiency sizes based on polyacrylates.
Source: [179, UBA, 2001] with reference to “Steidel, 1998”
(1) cost savings are calculated for a typical weaving mill working 8000 h/yr (100 weaving machines, 310 rpm) Table 4.5: Comparison between conventional and high-efficiency sizing agents Driving force for implementation In general the need to minimise discharged COD loads and increasing concern for environmental protection (see also EU Eco-label) are considered as the main driving forces in the selection of biodegradable/bioeliminable sizing agents [179, UBA, 2001].
The selection of biodegradable/bioeliminable sizing agents is also encouraged by initiatives at European level such as EU Eco-label, OSPAR, etc.
In addition, the substitution of conventional size recipes with highly efficient ones is economically motivated in the weaving mills, especially in combination with prewetting techniques (see Section 4.2.5), which can reduce size consumption by 1/3 or even more.
Reference plants Environmentally-optimised sizing agents are largely applied world-wide in weaving mills.
UBA reports about a voluntary initiative in Switzerland of 20 weaving mills. Seven different bioeliminable formulations of sizing agents have been developed (see also Figure 4.3) and currently applied, which can cover all different kinds of substrates and weaving techniques.
They are based on [179, UBA, 2001]:
· starches and starch derivatives · certain polyacrylates, · polyvinyl alcohol · certain galactomannans.
Reference literature [51, OSPAR, 1994] P003, P004, P005, P008, P047, [179, UBA, 2001], [18, VITO, 1998], [169, European Commission, 2001], [77, EURATEX, 2000].
4.2.5 Minimising sizing agent add-on by pre-wetting the warp yarns Description It is well known that in cotton finishing mills the desizing process accounts for 50 to 70 % of the total COD in the waste water. Minimising the amount of size applied on the warp yarn during fabric processing is one of the most effective pollution prevention techniques for reducing the organic load caused by sizing agents.
On-line monitoring/control of size add-on and prewetting technology are now available options.
The prewetting technology consists in running the warp yarn through hot water before the sizing process. The warp yarn is dipped into the hot water (an additional spraying of hot water may be possible), then a squeeze roller removes the surplus water before the sizing stage. Systems with two dipping and squeezing steps are also in use.
Prewetting allows a more homogeneous sizing effect, increased adhesion of the size and reduced hairiness of the yarn. From experimental analysis it seems that a lower “core-sizing” effect is produced. As a result, a reduced amount of size can be applied to the fibre without affecting weaving efficiency. In some cases, an increase in weaving efficiency can even be observed.
Main achieved environmental benefits A reduced size load on the yarn means that a lower amount of sizing agent is discharged in the water during pretreatment, with immediate positive effects on the organic load of the final effluent.
Operational data Depending on the setting of the warp beam and the type of yarn processed (e.g. density of the yarn, type of fibres in the blend), a reduction of the size add-on of about 20 - 50 % is possible.
Cross-media effects None believed likely.
Applicability Prewetting has now been tested in practice with all types of cotton yarns and blends of cotton/PES and viscose. Best results are achieved with medium to coarse yarns. Application is
possible for batches of more than 5000 m (better 10000 m), both for ring-spun and OE-yarns.
On the contrary, the technique is not applicable to small batches (5000 m) as the add-on cannot be controlled adequately. This is often the case for dyed yarns.
Technical problems may be observed in relation with the efficient measurement and control of high humidity percentages and with the calibration of the wetting device [281, Belgium, 2002].
Existing sizing machines with two sizing boxes can be reconstructed/upgraded by using the first sizing box for prewetting and the second one for sizing.
A high percentage of cotton fabric finished in Europe is actually imported from non EUcountries (e.g. India) where these control techniques are not yet widely used. As a result, from a practical point of view, this pollution-prevention measure is more easily and immediately applicable in integrated than in commission companies.
Sizing equipment with prewetting boxes is approximately EUR 25000 – 75000 more expensive than sizing equipment without a prewetting section. Operating costs are only marginally higher, because the additional expenditure for the prewetting water is compensated by the reduction in sizing liquor consumption.
A direct comparison between sizing with and without prewetting (operational data of an Italian weaving mill) shows cost savings of about 27 %, an increase in sizing machine speed of about 22 %, and an increase in weaving efficiency of about 0.2 % [179, UBA, 2001].
Driving force for implementation Sizing agent savings, increase of weaving efficiency, as well as a reduction of waste water load (environmental cost savings) are driving forces for the implementation of the prewetting technology.
Reference plants Worldwide about 100 prewetting boxes have been sold by Benninger Zell GmbH, Zell, Germany. About 60 prewetting boxes of Deutsche Babcock Moenus Textilmaschinen AG, Mönchengladbach are running in 40 plants. Another supplier for prewetting systems is Karl Mayer Textilmaschinenfabrik GmbH, Obertshausen, Germany [179, UBA, 2001].
Reference literature [179, UBA, 2001] 4.2.6 Use of techniques that allow reduced load of sizing agents on the fibre (compact spinning) Description Normally, in ring spinning, after the draft system a spinning triangle is generated. At the moment, when they pass the spinning triangle, the fibres are not strengthened by twisting. Yarn breakage and yarn hairiness occur. In compact spinning the fibre strands are compressed after the draft system by means of pneumatic devices (application of low pressure) [179, UBA, 2001] with reference to “Artzt, 1995”. A higher yarn quality results (increased fibre strength and elongation, reduced hairiness, increased abrasion resistance).
Main achieved environmental benefits Compared to conventional ring spun yarns, compact yarns have better running properties and less thread breakage during weaving even when the size add-on is reduced by up to 50 %. This leads to a considerable reduction in waste water load in desizing. Due to the reduced hairiness, application of paraffins to yarns is no longer necessary [179, UBA, 2001].