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2 environmental issues in dyeing Document Transcript

  • 1. 2.7.8 Environmental issues Potential sources and types ofemissions associated with dyeing processes are summarisedin the followins table. Table 2.1 1: Overview of the typical emissions generatedin dyeing processesAs the table shows, most of the emissions are emissions towater. Due to the low vapour pressure of the substances inthe dye bath, emissions to air are generally not significantand can be regarded more as problems related to theworkplace atmosphere (fugitive emissions fromdosing/dispensing chemicals and dyeing processes in "open"machines). A few exceptions are the thermosol process,pigment dyeing and those dyeing processes where carriersare employed. In pigment dyeing the substrate is notwashed after pigment application and therefore the
  • 2. pollutants are quantitatively released to air during drying.Emissions from carriers are to air and water. In the first partof the following discussion the environmental issues relatedto the substances employed will be described, while in thesecond part the environmental issues related to the processwill be mentioned.2.7.8.1 Environmental issues related to thesubstances employed Water-polluting substances in theabove-mentioned emissions may originate from:78 • the dyes themselves (e.g. toxicity, metals, colour) • auxiliaries contained in the dye formulation • basic chemicals (e.g. alkali, salts, reducing and oxidising agents) and • auxiliaries used in dyeing processes • contaminants present on the fibre when it enters the process sequence (residues of pesticides on wool are encountered in loose fibre and yarn dyeing and the same occurs with spin finishes present on synthetic fibres). Dyes • Spent dye baths (discontinuous dyeing), • residual dye liquors and • water from washing operations always contain a percentage of unfixed dye.The rates of fixation vary considerably among the differentclasses of dyes and may be especially low for reactive dyes(in the case of cotton) and for sulphur dyes. Moreover, largevariations are found even within a given class o fcolourants.This is particularly significant in the case of reactive dyes.Fixing rates above 60 % cannot be achieved, for example, inthe case of copper (sometimes nickel) phthalocyaninereactive dyes especially turquoise green and some marineshades. In contrast the so called anchor reactive dyes canachieve extremely high rates of fixation (see Sections 4.6.10and 4.6.11).
  • 3. The degree of fixation of an individual dye varies accordingto type of fibre, shade and dyeing parameters. Thereforefixation rate values can be given only as approximations.However, they are useful to give an idea of the amount ofunfixed dyes that can be found in waste water. Informationfrom different authors is given in the table below.Table 2.12 % of non-fixed dye that may be discharged inthe effluent for principle classes of dyes79As stated earlier, as a consequence of incomplete fixation apercentage of the dyestuff used in the process ends up inthe waste water. Dyestuffs are not biodegradable inoxidative conditions, although some of them may degradeunder other conditions (e.g. azo dyes may cleave underanoxic and anaerobic conditions).
  • 4. Less water-soluble dyestuffs molecules (typically, disperse,vat, sulphur, some direct dyestuffs and pigments) can belargely bio-eliminated from waste water by coagulation/precipitation or absorption/ adsorption to the activatedsludge.The quantity of activated sludge in the waste watertreatment plant and the quantity of dyestuff to be eliminatedare key factors in determining the efficiency of removal of adyestuff from the effluent.Another factor to take into consideration is the colourstrength of the colourant. For example, with reactivedyestuffs a lower amount of colourant is needed to achieve agiven shade compared to other classes of dyes (e.g. direct,vat and sulphur dyes). As a result a lower amount ofdyestuff will need to be removed from the waste water.Dyestuffs that are poorly bio-eliminable (unless they aresubmitted to destructive treatment techniques) will passthrough a biological waste water treatment plant and willultimately end up in the discharged effluent.The First noticeable effect in the receiving water is thecolour. High doses of colour not only cause aesthetic impact,but can also interrupt photosynthesis, thus affecting aquaticlife.Other effects are related to organic content of the colourant(normally expressed as COD and BOD, but could be betterexpressed as organic carbon, using TOC, DOC asparameters), its aquatic toxicity and the presence in themolecule of metals or halogens that can give rise to AOXemissions. These issues are discussed in more detail foreach class of dyestuff in Section 9.Only some general key issues are considered in this section.AOX emissionsVat, disperse and reactive dyes are more likely to containhalogens in their molecule. The content of organically boundhalogen can be up to 12 % on weight for some vat dyes. Vat
  • 5. dyes, however, usually show a very high degree of fixation.In addition, they are insoluble in water and the amount thatreaches the effluent can be eliminated with high efficiency inthe waste water treatment plant through absorption on theactivated sludge.Reactive dyes, on the contrary, may have low fixationdegrees (the lowest level of fixation is observed withphthlocyanine in batch dyeing) and their removal from wastewater is difficult because of the low biodegradability and/orlow level of absorption of the dye onto activated sludgeduring treatment. The halogen in MCT (monochlorotriazines)reactive groups is converted into harmless chloride duringthe dyeing process. In calculating the waste water burden itis therefore assumed that the MCT reactive groups reactcompletely by fixation or hydrolysis so that they do notcontribute to AOX emissions. However, many commonlyused polyhalogenated reactive dyes, such as DCT(dichlorotriazine), DFCP (difluorochloropyrimidine) and TCP(trichloropyrimidine) contain organically bound halogen evenafter fixation and hydrolysis. Bound halogen is also found indischarges of dye concentrates (pad, kitchen) and non-exhausted dye baths that may still contain unreacteddyestuff. For the other classes of colourants the AOX issue isnot relevant because, with few exceptions, halogen contentis usually below O.I %. PARCOM 97/1 recommends strictlimits for AOX. Even stricter limits are set by the EU-Ecolabel and German legislation. Extensive investigation ofAOX in textile effluents was performed, but AOX as anindicator remains a matter of discussion. 7980 Dyestuffs containing organically bound halogens (exceptfluorine) are measured as AOX. The only way to limit AOXfrom dyeing is by dye selection, by more efficient use ofdyes or by treating the resulting effluent by decolouration.Effluent decolouration can be achieved using destructivetechniques, such as the free radical oxidation or non-
  • 6. destructive techniques (e.g coagulation, adsorption).However, it should be noted that AOX from dyes do not havethe same effect as the AOX derived from chlorine reactions(haloform reaction, in particular) arising from textileprocesses such as bleaching, wool shrink-resist treatments,etc.Dyestliffs are not biodegradable compounds and thehalogens in their molecule should not give rise the haloformreaction (main cause of hazardous AOX). In this respect it isinteresting to consider that PARCOM 97/1 does not set ageneral discharge limit value For AOX. but rather allowsdiscrimination between hazardous and non-hazardous AOX[50,OSPAR, 1997].Heavy metal emission Metals can be present in dyes for tworeasons. First, metals are used as catalysts during themanufacture of some dyes and can be present as impurities.Second, in some dyes the metal is chelated with the dyemolecule, forming an integral structural element.Dye manufactutrers are now putting more effort intoreducing the amount of metals present as impurities. Thiscan be done by selection of starting products, removal ofheavy metal and substitution of the solvent where thereaction takes place.ETAD has established limits in the content of heavy metal indyestuffs. The values have been set to ensure that emissionlevels from a 2 % dyeing and a total dilution of the dye of1:2500, will meet the known waste water requirements [64,BASF, 1994].Examples of dyes containing bound metals are copper andnickel in phthalocyanine groups, copper in blue copper-azo-complex reactive dyes and chromium in metal-complex dyesused for wool silk and polyamide. The total amount ofmetallised dye used is decreasing, but there remain domains(certain shades such as greens, certain levels of fastness tolight) where phthalocyanine dyes,, for example, cannot beeasily substituted.
  • 7. The presence of the metal in these metallised dyes can beregarded as a less relevant problem compared to thepresence of free metal impurities. Provided that highexhaustion and Fixation levels are achieved and thatmeasures are taken to minimise losses from handling,weighing, drum cleaning, etc., only a little unconsumed dyeshould end up in the waste water. Moreover, since the metalis an integral part of the dye molecule, which is itself non-biodegradable, there is very little potential for it to becomebio-available.It is also important to take into account that treatmentmethods such as Filtration and adsorption on activatedsludge, which remove the dye from the waste water, alsoreduce nearly proportionally the amount of bound metal inthe final effluent. Conversely, other methods such asadvance oxidation may free the metal.Toxicitv Dyestuffsshowing aquatic toxicity and/ or allergenic effects arehighlighted in Section 9. Here it is also important to mentionthat about 60 % to 70 % of the dyes used nowadays are azodyes [77, ELJRATEX. 2000]. Under reductive conditions,these dyes may produce amines and some of them arecarcinogenic. A list of carcinogenic amines that can beformed by cleavage of certain azo dves is shown in the Table2.13. SO81
  • 8. Table 2.13: List of carcinogenic aminesThe use of azo-dyes that may cleave to one of the 22potentially carcinogenic aromatic amines listed above isbanned according to the 19th amendment of Directive76/769/EWG on dangerous However, more than 100 dyeswith the potential to form carcinogenic amines are stillavailable on the market [77, EURATEX, 2000].Auxiliaries contained in dye formulationsDepending on the dye class and the application methodemployed (e.g. batch or continuous dyeing, printing)different additives are present in the dye formulations. Sincethese substances are not absorbed/ fixed by the fibres, theyare completely discharged in the waste water. Typicaladditives are listed in the table below.82
  • 9. Table 2.14: Ecological properties of dye formulationsadditivesWhile these additives are not toxic to aquatic life, they are ingeneral poorly biodegradable and not readily bioeliminable.This applies in particular to the dispersants present in theformulations of vat, disperse and sulphur dyes. These dyesare water-insoluble and need these special auxiliaries inorder to be applied to the textile in the form of aqueousdispersions. These dispersants consist mainly of naphthalenesulphonate-formaldehyde condensation products and ligninsulphonates, but sulphomethylation products derived fromthe condensation of phenols with formaldehyde and sodiumsulphite can also be found.Other not readily eliminable additives are acrylate and CMC-based thickeners and anti-foam agents.The difference between liquid and powder formulationsshould also be mentioned. Dyes supplied in liquid formcontain only one third of the amount of dispersing agentnormally contained in powder dyes (see Table 2.15). Thereason for this difference stems from the manufacturing
  • 10. process of powder dyes: the very small particles generatedduring grinding must be protected during the subsequentdrying process and this is possible only by adding highproportions of dispersing agents.Table 2.15: Proportion of additives and dye in powderand liquid dyes83 Note that liquid formulations include liquid dispersions andtrue solutions (solutions without solubilising aids), whereaspowder dyes can be supplied as dusting, free-flowing, non-dusting powders or granulates.Basic chemicals and auxiliaries used in the dyeingprocessRegarding the environmental concerns associated with thechemicals and auxiliaries used in dyeing processes it isworth mentioning the following key issues.Sulphur containing reducing agentsWaste water from sulphur dyeing contains sulphides used inthe process as reducing agents. In some cases the sulphideis already contained in the dye formulation and in someother cases it is added to the dye bath before dyeing. In theend, however, the excess of sulphide ends up in the wastewater. Sulphides are toxic to aquatic organisms andcontribute to increasing COD load. In addition, sulphideanions are converted into hydrogen sulphide under acidicconditions, thereby giving rise to problems of odour andcorrosivity. .
  • 11. Sodium hydrosulphite (also called sodium dithionate) isanother sulphur-containing reducing agent, which iscommonly used not only in sulphur and vat dyeingprocesses, but also as reductive after-cleaning agent in PESdyeing. Sodium hydrosulphite is less critical than sodiumsulphide. However, during the dyeing process sodiumdithionite is converted into sulphite (toxic to fish andbacteria) and in some cases this is further oxidised intosulphate. In lhe waste water treatment plant sulphite isnormally oxidised into sulphate, but this can still causeproblems. Sulphate, in fact, may cause corrosion of concretepipes or may be reduced under anaerobic conditions intohydrogen sulphide.Hydroxyacetone, although it produces an increase in CODload, is recommended to lower the sulphur content in wastewater, but it cannot replace hydrosulphite in all applications.New organic reducing agents with improved reducing effectshave been developed (see Section 4.6.5 and Section 4.6.6for further details).Consumption of the reducing agent by the oxygen present inthe machine (partially-flooded dyeing machines) needs alsoto be taken into account. Instead of applying only theamount of reducing agent required for the reduction of thedyestuff, a significant extra amount of reducing agent oftenneeds to be added to compensate for the amount consumedby the oxygen contained in the machine. This obviouslyincreases oxygen demand of the effluent.Oxidising agents Dichromate should no longer be used inEurope as an oxidising agent when dyeing with vat andsulphur dyes, but it is still widely used for the fixation ofchrome dyes in wool dyeing. Chromium III exhibits lowacute toxicity, while chromium VI is acutely toxic and hasbeen shown to be carcinogenic towards animals. During thedyeing processes with chrome dyes, Cr VI is reduced to CrIII if the process is under control. Nevertheless, emissions ofCr VI may still occur due to inappropriate handling of
  • 12. dichromate during dye preparation (care must be taken asdichromate is carcinogenic and may cause health problemsfor workers handling it).Emissions of trivalent chromium in the waste water can beminimised (see Section 4.6.15), but cannot be avoided,unless alternative dyestuffs are applied (see Section 4.6.16).The use of bromate, iodate and chlorite as oxidising agentsin vat and sulphur dyeing processes and the use ofhypochlorite as stripping agent for decolouring faulty goodsor for cleaning dyeing machines (e.g. before subsequentlighter-coloured dyeing) may produce AOX emissions.However, only hypochlorite and elemental-chlorine-containing compounds (e.g. certain chlorite products thatcontain Cl2 or use chlorine as activator for formation ofchlorine dioxide gas) are likely to give rise to hazardous AOX8384SaltSalts of various types are used in dyeing processes fordifferent purposes (e.g to promote level dyeing or topromote dye erhaustion).. In particular, large amounts ofsalt are used in cotton batch dyeing processes with reactivedyes. The amount of salt employed is quite significantcompared to other classes of dyestuffs, for example directdyes (Table 2.16) and efforts have been made by dyemanufacturers to solve this problem (see Section 4.6.1 1).Table 2.16: Amount of salt employed in cotton batchdyeing processes with reactive and direct dyes
  • 13. In addition to the use of salt as raw material, neutralizationof commonly used acids and alkalis produces salts as a by-products. Salts are not removed in conventional waste watertreatment systems and they are therefore ultimatelydischarged in the receiving water.Although the mammalian and aquatic toxicity of thecommonly employed salts are very low, in arid or semi-aridregions their large-scale use can produce concentrationsabove the toxic limit and increase the salinity of thegroundwater. Countries have set emission limits at 2000ppm or below. River quality standards must also be takeninto account.Carriers The use of these auxiliaries, which were widelyemployed in the past, has now been reduced due toecological and health problems. They are still an issue indyeing of polyester in blend with wool.Carriers may already be added to the dyes bymanufacturers. In this case textile finishers will have littleknowledge of the loads discharged ([4, Tebodin, 1991] and[61, L. Bettens, 1999]).Carriers (sec Section 8.6.7) include a wide group of organiccompounds, many of them steam volatile, poorlvbiodegradable and toxic to humans and aquatic life.However, as the active substances usually have high affinityfor the fibre (hydrophobic types), 75 - 90 % are absorbed bythe textile and only the emulsifiers and the hydrophilic-typecarriers such as phenols and benzoate derivatives are foundin the waste water.The carriers that remain on the fibre after dyeing andwashing, are partially volatilised during drying and fixingoperations and can give rise to air emissions. Traces can stillhe found on the finished product, thus representing apotential problem for the consumer. Alternative options aredescribed in Sections 4.6.1 and 4.6.2.Other auxiliaries of environment interest
  • 14. Other substances that may be encountered in the dyeingauxiliaries and that may give rise to water pollution are:85 • fatty amine ethoxylates (levelling agent) • alkylphenol ethoxylates (levelling agent) • quaternary ammonium compounds (retarders for cationic dyes) • polyvinylpyrrolidone (levelling agent for vat, sulphur and direct dyes) • cyanamide-ammonia salt condensation products (auxiliaries for fastness improvement) • Acrylic acid-maleic acid copolymers (dispersing agent) • Ethylene diamine tetra acetate (EDTA) • diethylenetriaminepentaacetate (DTPA) • ethylenediaminetetra(methylenephosphonic acid) (EDTMP) • diethylenetriaminepenta(methylenphosphonic acid) (DTPMP)These are water-soluble hard-to-biodegrade compoundswhich can pass untransformed or only partially degraded,through waste water treatment systems. In addition, someof them are toxic (e.g. quaternary amines) or can give riseto metabolites which may affect reproduction in the aquaticenvironment (APEO).2.7.8.2 Environmental issues related to the processBoth water and energy consumption in dyeing processes area function of the dyeing technique, operating practices andthe machinery employed.Batch dyeing processes generally require higher water andenergy consumption levels than continuous processes. Thisis due number of different factorsThe higher liquor ratios involved in batch dyeing representone of these factors. As previously mentioned in Section2.7.2, higher liquor ratios mean not only higher water andenergy uses, but also a higher consumption of thosechemicals and auxiliaries that are dosed based on the
  • 15. volume of the bath. Consistently with the quality of thedifferent types of substrates, all equipment manufacturersnow can offer machines with reduced liquor ratios. Termslike "low liquor ratioand "ultra-low liquor ratio" are nowcommonly used to define the performance/ features ofmodern machines, for dyeing fabric in rope form.Nominal reference values for "low liquor ratio machines" arein the range of 1:5-1:8 for cotton and correspondingly1:3-1:4 for PES. The liquor ratio can be higher for othertypes of substrates/fibres.The term "Ultra-low liquor ratio" is used to define machinesthat can be operated at liquor ratios as low as the minimumvolume required to completely wet out the substrate andavoid cavitations of the pumps. This term applies only tomachines for dyeing fabric in rope form.It is important to show the difference between the nominaland real liquor ratios. As already stated in Section 2.7.2, thenominal liquor ratio is the liquor ratio at which a machinecan be operated when it is loaded at its maximum/ optimalcapacity. It is often the case that the machine isunderloaded compared to its optimal capacity. This oftenoccurs in commission companies where a high productionflexibility is required to serve variable lot sizes according tocustomers demands. Modern machines can still be operatedat approximately constant liquor ratio whilst being loaded ata level as low as 60 % of their nominal capacity (or even 30% of their nominal capacity with yarn dyeing machines - seeSection 4.6.19). In this way the same benefits achievablewith low liquor ratios can be kept even with reduced loading.It is obvious however, that when a machine is loaded farbelow its optimal capacity (e.g. below 60 % of its nominalcapacity for fabric dyeing machines) the real liquor ratio willdiffer greatly from the nominal liquor ratio. This will resultnot only in lower environmental performances (higher water,energy and chemicals consumptions), but also in higheroperating costs.86
  • 16. In conclusion, the use of low liquor ratio machinery, orselection of the most adequate machine for the size of thelot to be processed, is fundamental to the resultantenvironmental performance of the process. Having said that.high energy and water consumption in batch dyeing is notonly the result of high liquor ratios.Another factor to take into consideration is thediscontinuous nature of the batch dyeing operating mode,especially with regard to operations such as cooling,heating, washing and rinsing.Furthermore, shade matching can be responsible for higherwater and energy consumption, especially when dyeing iscarried out without the benefit of laboratory instruments. Ina manual regime the bulk of the dyestuff is normally appliedin the first phase to obtain a shade which is close to thatrequired in the final product. This is followed by a number ofmatching operations, during which small quantities of dyeare applied to achieve the final shade. Shades which aredifficult to match may require repeated shade additions withcooling and reheating between each addition [32, ENco,2001].Increased energy and water consumption may also becaused by inappropriate handling techniques and/or poorlyperforming process control systems. For example, in somecases displacement spillage may occur during immersion ofthe fibre in the machine, while the potential for overfillingand spillage exists where the machines are only equippedwith manual control valves, which fail to control the liquorlevel and temperature correctly (see also Section 4.1.4).Continuous and semi-continuous dyeing processesconsume less water, but this also means a higher dyestuffconcentration in the dye liquor. In discontinuous dyeing thedye concentration varies from O.I to I g/l, while incontinuous processes this value is in the range of 10 to 100g/l. The residual padding liquor in the troughs, pumps andpipes must be discarded when a new colour is started. Thedischarge of this concentrated effluent can result in a higher
  • 17. pollution load compared with discontinuous dyeing,especially when small lots of material are processed.However, modern continuous dyeing ranges have steadilyimproved in recent years. The use of small pipes and pumpsand small pad-bath troughs help to reduce the amount ofconcentrated liquor to be discharged.In addition, it is possible to minimise the discard ofleftovers, by using automated dosing systems, which meterthe dye solution ingredients and deliver the exact amountneeded (see also Sections 4.1.3 and 4.6.7 for more detailedinformation about recent improvements).In hoth continuous and batch dyeing processes, finalwashing and rinsing operations are water- intensive stepsthat need to be taken into consideration. Washing andrinsing operations actually consume greater quantities ofwater than dyeing itself (see Sections 4.9.1 and 4.9.2 forwater and energy conservation techniques in batch andcontinuous processing and Sections 4.1.4 and 4.6.19 forequipment optimisation in batch processing).Printing2.8.3 Environmental issues Emission sources typical ofprinting processes are:• printing paste residues• waste water from wash-off and cleaning operation• volatile organic compounds from drying and fixingprocesses97 Printing paste residues Printing paste residues areproduced for different reasons during the printing processand the amount can be particularly relevant (Section3.3.3.5.5 provides information about consumption andemission levels). Two main causes are, for example,
  • 18. incorrect measurements and the common practice ofpreparing excess paste to prevent a shortfall. Moreover, ateach colour change, printing equipment and containers(dippers, mixers. homogenizers, drums, screens, stirrers,squeegees, etc.) have to be cleaned up. Print pastes adhereto every implement due to their high viscosity and it iscommon practice to use dry capture systems to removethem before rinsing with water. In this way these residuescan at least be disposed of in segregated form, thusminimising water contamination. Another significant, butoften forgotten source of printing paste residues is thepreparation of sample patterns. Sometimes they areproduced on series production machines, which means highspecific amounts of residues produced. There are techniquesavailable that can help to reduce paste residues (see Section4.7.4) and techniques for recovery/re-use of the surpluspaste (see Sections 4.7.5 and 4.7.6). Their success is,however, limited due to a number of inherent technologicaldeficiencies of analogue printing technology. Most of thesedeficiencies are related to the analogue transfer of thepattern, the unavoidable contact between the surface of thesubstrate and the applicator (screen) and the need forthickeners in the formulation (paste rheology), which limitsthe ultimate potential for paste re-use. Digital printing offersa solution to these problems (see Sections 4.7.8 and 4.7.9).Waste water from wash-off and cleaning operationsWaste water in printing processes is generated primarilyfrom final washing of the fabric after fixation, cleaning ofapplication systems in the printing machines, cleaning ofcolour kitchen equipment and cleaning of belts. Waste waterfrom cleaning-up operations accounts for a large share ofthe total pollution load. even mOre than water from wash-offoperations. Emission loads to water are mainlyattributable to dyestuff printing processes because in thecase of pigment printing, although considerable amounts ofwaste water arise from cleaning operations, pigments arecompletely fixed on the fibre without need for washing-off.
  • 19. Pollutants that are likely to be encountered in waste waterare listed in the table below.98 Table 2.17: Pollutants that are more likely to be encounteredin waste water from printing processes Volatile organic compounds from drying and fixingDrying and fixing are another important emission source inprinting processes. The following pollutants may beencountered in the exhaust air [179, DBA, 2001]:• aliphatic hydrocarbons (C10-C20) from binders • Monomers such as acrylates, vinyl acetates, styrene, acrylonitrile, methylol acrylamide, butadiene, • • methanol from fixing agents
  • 20. • • 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 • • phenycyclohexene from thickeners and binders.A more comprehensive list of pollutants potentially presentin the exhaust air from heat treatment after printing with anindication of the potential source, is given in Section 12.Finishing2.9.2.8 Anti felt treatment fo woolAnti-felt finishing is applied in order to provide anti-feltproperties to the good. This will prevent shrinking of thefinished product when it is repetitively washed in a laundrymachine. 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 processand on all different make-ups. They are most commonlyapplied on combed tops for specific end-products (e.g.underwear102 Oxidising treatmentsIn the oxidising treatment the specific chemicals used attackthe scales of the cuticles and chemically change the externalstructure of the fibre. This treatment has traditionally beencarried out using one of the following chlorine-releasingagents:• sodium hypochlorite • sodium salt dichloroisocyanurate• active chlorine (no longer used).
  • 21. The oldest process is the one using sodium hypochlorite.However, since the development of active chlorine is difficultto control, wool fibre characteristics can be deeply changed,also giving irregular results.Dichloroisocyanurate is more advantageous here because ithas the ability to release chlorine gradually, therebyreducing the risk of fibre damage. The process withdichloroisocyanurate (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 itis submitted to an anti- chlorine treatment with 2-3% ofsodium bisulphite and rinsed again.All these chlorine-based agents have recently encounteredrestrictions because they react with components andimpurities (soluble or converted into soluble substances) inthe wool, to form absorbable organic chlorine compounds(AOX). Alternative oxidising treatments have therefore beendeveloped. In particular, peroxysulphate, permanganate,enzymes and corona discharge come into consideration.However, the only alternative to chlorine-based agentsreadily available today is peroxysulphate. The process withperoxysulphate compounds is quite similar to the chlorinetreatment, but does not involve the use of chlorine and doesnot generate chloroamines.The material is treated with the oxidising agent in acid liquorat room temperature until the active oxygen has beenlargely consumed.Both with chlorine based agents and peroxysulphate, sodiumsulphite is then added as an anti- oxidant to the same liquorat slightly alkaline pH. This is a reductive after treatment to
  • 22. avoid damage and yellowing of the wool fibre at alkalinepH. The goods are subsequently rinsed. If necessary, theyare treated with a polymer (see treatments with resinsbelow).Treatments with resins (additive processes) In additiveprocesses, polymers are applied to the surface of the fibrewith the aim of covering the scales with a "film". However,this treatment must be regarded as a pseudo felt-freefinishing process, as it is not the felting propensity that isreduced, but merely the effect thereof.The polymer must have a high substantivity for wool.Cationic polymers are the most suitable for this treatmentbecause, after the previous oxidative and reductivepretreatment, the wool surface becomes anionic. Thepolymer may be. in some case, sufficiently effective on itsown to make pretreatment unnecessary. However, thecombination of subtractive and additive processes has thegreatest technical effect, 102103 Combined treatments (Hercosett Process) The oldest combination process is the so called Hercosettprocess (by C.S.I.R.O), which consists in chlorinepretreatment followed by application of a polyamide-epichloridrine resin. Whilst the Hercosett process can becarried out in batch or continuous mode, the latter ispredominant nowadays.The continuous process consists of the following steps (seeFigure 2.27): 1. chlorine treatment in acid medium (using chlorine gas or sodium hypochlorite) 2. reduction of chlorine using sulphite in the same bath 3. rinsing 4. neutralisation with sodium carbonate
  • 23. 5. rinsing 6. resin application 7. softener application 8. drying and polymerisation. The Hercosett process has been widely used for years asanti-felt Finishing of wool in different states (loose fibre,combed top, yarn, knitted and woven fabric) due to its lowcost and high quality effectsHowever, the effluent shows high concentrations of COD andAOX.The formation of AOX is attributable not only to the oxidant,but also to the resin. In fact, the typical resin applied in theHercosett process is a cationic polyamide whosemanufacturing process involves the use of epichloridrine,which is another source of the chlorinated hydrocarbons inthe effluent.Alternative resins have been developed, based onpolyethers, cationic aminopolysiloxanes synergic mixtures ofpolyurethanes and polydimethylsiloxanes, but they all havesome limitations concerning their applicability. Newprocesses have also been developed, but so far the resultsachieved with the Hercosett process cannot be fully matchedby any alternative, which is why it is still the preferredprocess particularly for treatments such as the anti-feltfinishing of combed tops.Environmental issues in Finishing2 9 3 Environmental issuesAmong textile finishing processes, the chemical ones arethose that are more significant from the point of view of theemissions generated. As in dyeing, the emissions are quitedifferent between continuous and discontinuous processes.Therefore this distinction will be used in the104
  • 24. discussion of the main environmental issues associatedwith finishing. Anti-felt treatments represent a peculiar typeof finishing both in terms of applied techniques andemissions. The environmental issues related to this processare therefore discussed in Section 2.9.2.8 together with tiledescription of the process itself.Environmental issues associated with continuousFinishing processesWith some exceptions (e.g. application of phosphor-organicflame retardants) finishing processes do not requirewashing operations after curing. This means that thepossible emissions of water pollution relevance are restrictedto the system losses and to the water used to clean all theequipment.In a conventional foulard, potential system losses at the endof each batch are:• the residual liquor in the chassis• the residual liquor in the pipes• the leftovers in the batch storage container from which thefinishing formulation is fed to the chassis.Normally these losses are in the range of 1-5%, based onthe total amount of liquor consumed: it is also in thefinishers interest not to pour away expensive auxiliaries.However, in some cases, within small commission finishers,losses up to 35 or even 50 % may be observed. Thisdepends on the application system (e.g. size of foulardchassis) and the size of the lots to he finishedIn this respect, with application techniques such as spraying.foam and slop-padding (to a lower extent due to highresidues in the system) system-losses are much lower interms of volume (although more concentrated in terms ofactive substance). Residues of concentrated liquors are re-used, if the finishing auxiliaries applied show sufficient
  • 25. stability, or otherwise disposed of separately as wastedestined to incineration.However, too often these liquors are drained and mixed withother effluents. Although the volumes involved are quitesmall when compared with the overall waste water volumeproduced by a textile mill, the concentration levels are veryhigh, with active substances contents in the range of 5-25%and COD of 10 to 200 g/litre.In the case of commission finishing mills working mainly onshort batches, the system losses can make up aconsiderable amount of the overall organic load.In addition, many substances are difficult to biodegrade orare not biodegradable at all and sometimes they are alsotoxic (e.g. biocides have a very low COD, but are highlytoxic).The range of pollutants that can be found in the waste watervaries widely depending on the type of finish applied.The typical pollutants and the environmental concernsassociated with the use of the most common finishingagents are discussed in Section 8.8. IIn particular, the release of the following, substances in theenvironment gives rise to significant concerns:• ethylene urea and melamine derivatives in their "notcross-linked form" (cross-linking agents in easy-carefinishes)• organo-phosphorolis and polybrominated organiccompounds (flame retardant agents)• polysiloxanes and derivatives (softening agents)• alkyl phosphates and alkylelherphosphates (antistaticagents)• fluorochemical repellents.
  • 26. In the drying and curing operation air emissions areproduced due to the volatility of the active substancesthemselves as well as that of their constituents (e.g.monomers, oligomers, impurities and decomposition by-products).Furthermore air emissions (sometimes accompanied byodours) are associated with the residues of preparations andfabric carry-over from upstream processes (for example,polychlorinated dioxins/furans may arise from the thermaltreatment of textiles that have been previously treated withchlorinated carriers or perchloroethylene).104105 The emission loads depend on the drying or curingtemperature, the quantity of volatile substances in theFinishing liquor, the substrate and the potential reagents inthe formulation.The range of pollutants is very wide and depends on theactive substances present in the formulation and again onthe curing and drying parameters.In most cases, however, the emissions produced by thesingle components of the finishing recipes are additive. As aresult. the total amount of organic emissions in the exhaustair (total organic carbon and specific problematic compoundssuch as carcinogenic and toxic substances) can easily becalculated by means of emission factors given for thefinishing recipes by manufacturers (see also Section 4.3.2).Note however, that Germany is the only Member Statewhere there is a fully developed system in which themanufacturers provide the finisher with such information onthe "products supplied. Another important factor to considerregarding air emissions is that the directly heated (methane,propane, butane) stenters themselves may produce relevantemissions (non- combusted organic compounds, CO, NO,,
  • 27. formaldehyde). Emissions, for example, of formaldehyde upto 300 g/h (2 - 60 mg/m3) have been observed in somecases, which were attributable to inefficient combustion ofthe gas in the stenter frame [179, UBA, 2001].It is therefore obvious - when speaking about air emissions -that the environmental benefit obtained with the use offormaldehyde-free finishing recipes is totally lost if theburners in the stenter frames are poorly adjusted andproduce high formaldehyde emissions. The activesubstances in the most common finishing agents and thepossible associated air emissions are discussed in Section8.8. Moreover a more comprehensive list of pollutants thatcan be found in the exhaust air from heat treatments ingeneral, is reported in Section 12.Environmental issues associated with discontinuousprocesses The application of functional finishes in longliquor by means of batch processes is used mainly in varnfinishing and in the wool carpet yam industry in particular.Since the functional finishes are generally applied either inthe dye baths or in the rinsing baths after dyeing,, thisoperation does not entail additional water consumption withrespect to dyeing.For the resulting water emissions, as with batch dyeing, theefficiency of the transfer of the active substance from theliquor to the fibre is the key factor which influences theemission loads. The efficiency depends on the liquor ratioznd on many parameters such as pH, temperature and thetype of emulsion (micro- or macro-emulsion).Maximising the efficiency is particularly important whenbiocides are applied in mothproofing finishing. Asmothproofing agents are not water-soluble they are appliedfrom emulsions. The degree of emulsification and the pH arecritical in the application of mothproofing agents (i.e. theefficiency of the process is higher when the active substance
  • 28. is applied from micro-emulsions and at acidic pH). Note herethat the finishing agents are dosed based on the weight ofthe fibre and not on the amount of bath (in g/litre). Thepollutants that may be encountered in waste water varydepending on the finishing agents applied: Section 8.8 givesmore details. The main issues worth mentioning are theapplication of mothproofing agents (emissions of biocides)and the low level of exhaustion of softeners (emissions ofpoorly biodegradable substances).