«EUROPEAN COMMISSION Integrated Pollution Prevention and Control (IPPC) Reference Document on Best Available Techniques for the Textiles Industry July 2003 ...»
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 pretreatment, the wool surface becomes anionic.
The polymer may be, in some case, sufficiently effective on its own to make pretreatment unnecessary. However, the combination of subtractive and additive processes has the greatest technical effect.
Combined treatments: Hercosett process The oldest combination process is the so called Hercosett process (by C.S.I.R.O), which consists in chlorine pretreatment 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.
The continuous process consists of the following steps (see Figure 2.27):
1. chlorine treatment in acid medium (using chlorine gas or sodium hypochlorite)
2. reduction of chlorine using sulphite in the same bath
4. neutralisation with sodium carbonate
6. resin application
7. softener application
8. drying and polymerisation.
Figure 2.27: Schematic representation of the Hercosett process[7, UBA, 1994]
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.
2.9.3 Environmental issues Among textile finishing processes, the chemical ones are those that are more significant from the point of view of the emissions generated. As in dyeing, the emissions are quite different between continuous and discontinuous processes. Therefore this distinction will be used in the Textiles Industry 103 Chapter 2 discussion of the main environmental issues associated with finishing. Anti-felt treatments represent a peculiar type of finishing both in terms of applied techniques and emissions. The environmental issues related to this process are therefore discussed in Section 126.96.36.199 together with the description of the process itself.
Environmental issues associated with continuous finishing processes
With some exceptions (e.g. application of phosphor-organic flame-retardant), continuous finishing processes do not require washing operations after curing. This means that the possible emissions of water pollution relevance are restricted to the system losses and to the water used to clean all the equipment. In a conventional foulard, potential system losses at the end of each
· the residual liquor in the chassis · the residual liquor in the pipes · the leftovers in the batch storage container from which the finishing formulation is fed to the chassis.
Normally these losses are in the range of 1 – 5 %, based on the total amount of liquor consumed; it is also in the finisher's 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.
This depends on the application system (e.g. size of foulard chassis) and the size of the lots to be finished. In this respect, with application techniques such as spraying, foam and slop-padding (to a lower extent due to high residues in the system) system-losses are much lower in terms of volume (although more concentrated in terms of active substance).
Residues of concentrated liquors are re-used, if the finishing auxiliaries applied show sufficient stability, or otherwise disposed of separately as waste destined to incineration. However, too often these liquors are drained and mixed with other effluents.
Although the volumes involved are quite small when compared with the overall waste water volume produced by a textile mill, the concentration levels are very high, 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 on short batches, the system losses can make up a considerable amount of the overall organic load. In addition, many substances are difficult to biodegrade or are not biodegradable at all and sometimes they are also toxic (e.g. biocides have a very low COD, but are highly toxic).
The range of pollutants that can be found in the waste water varies widely depending on the type of finish applied. The typical pollutants and the environmental concerns associated with the use of the most common finishing agents are discussed in Section 8.8. In particular, the release
of the following substances in the environment gives rise to significant concerns:
· ethylene urea and melamine derivatives in their “not cross-linked form” (cross-linking agents in easy-care finishes) · organo-phosphorous and polybrominated organic compounds (flame retardant agents) · polysiloxanes and derivatives (softening agents) · alkyphosphates and alkyletherphosphates (antistatic agents) · fluorochemical repellents.
In the drying and curing operation air emissions are produced due to the volatility of the active substances themselves as well as that of their constituents (e.g. monomers, oligomers, impurities and decomposition by-products). Furthermore air emissions (sometimes accompanied by odours) are associated with the residues of preparations and fabric carry-over from upstream processes (for example, polychlorinated dioxins/furans may arise from the thermal treatment of textiles that have been previously treated with chlorinated carriers or perchloroethylene).
104 Textiles Industry
Chapter 2The emission loads depend on the drying or curing temperature, the quantity of volatile substances in the finishing liquor, the substrate and the potential reagents in the formulation.
The range of pollutants is very wide and depends on the active substances present in the formulation and again on the curing and drying parameters. In most cases, however, the emissions produced by the single components of the finishing recipes are additive. As a result, the total amount of organic emissions in the exhaust air (total organic carbon and specific problematic compounds such as carcinogenic and toxic substances) can easily be calculated by means of emission factors given for the finishing recipes by manufacturers (see also Section 4.3.2). Note however, that Germany is the only Member State where there is a fully developed system in which the manufacturers provide the finisher with such information on the products supplied.
Another important factor to consider regarding air emissions is that the directly heated (methane, propane, butane) stenters themselves may produce relevant emissions (noncombusted organic compounds, CO, NOx, formaldehyde). Emissions, for example, of formaldehyde up to 300 g/h (2 - 60 mg/m3) have been observed in some cases, which were attributable to inefficient combustion of the 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 of formaldehyde-free finishing recipes is totally lost if the burners in the stenter frames are poorly adjusted and produce high formaldehyde emissions.
The active substances in the most common finishing agents and the possible associated air emissions are discussed in Section 8.8. Moreover a more comprehensive list of pollutants that can be found in the exhaust air from heat treatments in general, is reported in Section 12.
Environmental issues associated with discontinuous processes
The application of functional finishes in long liquor by means of batch processes is used mainly in yarn finishing and in the wool carpet yarn industry in particular. Since the functional finishes are generally applied either in the dye baths or in the rinsing baths after dyeing, this operation does not entail additional water consumption with respect to dyeing. For the resulting water emissions, as with batch dyeing, the efficiency of the transfer of the active substance from the liquor to the fibre is the key factor which influences the emission loads. The efficiency depends on the liquor ratio and on many other parameters such as pH, temperature and the type of emulsion (micro- or macro-emulsion). Maximising the efficiency is particularly important when biocides are applied in mothproofing finishing. As mothproofing agents are not water-soluble they are applied from emulsions. The degree of emulsification and the pH are critical in the application of mothproofing agents (i.e. the efficiency of the process is higher when the active substance is applied from micro-emulsions and at acidic pH). Note here that the finishing agents are dosed based on the weight of the fibre and not on the amount of bath (in g/litre).
The pollutants that may be encountered in waste water vary depending on the finishing agents applied; Section 8.8 gives more details. The main issues worth mentioning are the application of mothproofing agents (emissions of biocides) and the low level of exhaustion of softeners (emissions of poorly biodegradable substances).
2.10 Coating and laminating 2.10.1 Coating and laminating processes Usually, coated and laminated textiles consist of a textile substrate - typically a woven, knitted, or non-woven textile fabric - combined with a thin, flexible film of natural or synthetic polymeric substances.
A coated fabric usually consists of a textile substrate on which the polymer is applied directly as a viscous liquid. The thickness of the film is controlled by applying it via a blade or similar aperture.
A laminated fabric usually consists of one or more textile substrates, which are combined with a pre-prepared polymer film or membrane by adhesives or heat and pressure.
The basic techniques for coating/laminating fabrics require the following conditions:
· the fabric to be coated/laminated is supplied full width on a roll · the fabric is fed under careful tension control to a coating or laminating heat zone · after application of the coating auxiliaries, the fabric is passed through an oven to cure the composite and remove volatile solvents before cooling and rolling up.
In the textile industry the flame lamination of foams is a widely used technique: a pre-prepared thin, thermoplastic foam sheet is exposed to a wide slot flame burner located before the laminating rolls. No drying or curing oven is required in this process. Air emissions produced during this treatment are highly irritant and may trigger allergic reaction in susceptible persons.
The typical coating compounds and auxiliaries used are described in Section 8.9 2.10.2 Environmental issues The main environmental concerns in coating/laminating operations relate to air emissions arising from solvents, additives and by-products contained in the formulations of the coating compounds. A distinction must therefore be made between the various products available (the following information is taken from [179, UBA, 2001]).
The emission potential of coating powders is in most cases negligible with the exception of polyamide 6 and its copolymers (the residual monomer epsilon-caprolactam is released at standard process temperatures). In some cases softeners (often phthalates) can be found in the emissions.
The emissions from the coating pastes result mainly from the additives (except in the case of PA 6, which is mentioned above).
These are mainly:
· fatty alcohols, fatty acids, fatty amines from surfactants · glycols from emulsifiers · alkylphenoles from dispersants · glycol, aliphatic hydrocarbons, N-methylpyrrolidone from hydrotropic agents · aliphatic hydrocarbons, fatty acids/salts, ammonia from foaming agents · phthalates, sulphonamides/esters ex softeners/plasticisers · acrylic acid, acrylates, ammonia, aliphatic hydrocarbons from thickeners
Polymer dispersions (aqueous formulations)
The emission potential of polymer dispersions is quite low compared to coating pastes.
Components that are responsible for air emissions are the dispersing agents, residual compounds from the polymerisation (especially t-butanol used as catalyst in radically initialised polymerisation reactions) and monomers arising from incomplete reaction during polymerisation. The latter are particularly relevant to the workplace atmosphere and odour
nuisances. They include:
· acrylates as acrylic acid, butylacrylate, ethylacrylate, methylacrylate, ethylhexylacrylate and vinylacetate · cancinogenic monomers like acrylonitrile, vinylchloride, acrylamide, 1,3-butadiene and vinylcyclohexene.
Vinylcyclohexene is not often identified in the exhaust air. However it is always formed (2 + 2 cycloaddition-product) if 1,3-butadiene is used.
Acrylamide in the exhaust air is often related to formaldehyde emissions (reaction products of methylolacrylamide).
Melamine resins Melamine resins are widely applied. Melamine resins are produced by the reaction of melamine and formaldehyde and subsequent etherification mostly with methanol in aqueous medium. The products can contain considerable amounts of free formaldehyde and methanol. During their application the cross-linking reaction of the resin with itself or with the fabric (e.g. cotton) is initiated by an acid catalyst and/or temperature, releasing stoichiometric amounts of methanol and formaldehyde.
Polymer dispersions (organic solvent-based formulations) Solvent coating is not very common in the textile finishing industry. When this technique is applied, exhaust air cleaning equipment based on thermal incineration or adsorption on active carbon is normally installed.
2.11 Carpet back-coating The backing process is an important production step which is applied to improve the stability of textile floor-coverings. Moreover, backing may have a positive influence on properties such as sound-proofing, stepping elasticity and heat insulation.
One can distinguish the following types of coatings:
· pre-coating · foam coating · textile back-coating · heavy coating · reinforcement · back finish.
Pre-coating A common feature of tufted carpets is that they are pre-coated after tufting to permanently anchor the needled pile loops in the carrier layer (Figure 2.28). The pre-coating material used
· x-SBR latex, which is a dispersion containing a copolymer produced from styrene, butadiene and carbonic acid · fillers · water · additives (e.g. thickeners, anti-foam, foam-stabilisers, etc.).
Figure 2.28: Pre-coated tufted carpet [63, GuT/ ECA, 2000]
The pre-coating can be applied:
· unfoamed, by means of slop-padding (Figure 2.29) · foamed, by means of the doctor-blade technique (Figure 2.30).
Figure 2.29: Pre-coating application by slop-padding [63, GuT/ ECA, 2000] Figure 2.
30: Pre-coating application by doctor-blade technique [63, GuT/ ECA, 2000] During the subsequent drying stage, thanks to the formation of hydrogen bonds, the polymer chains are netted into a three-dimensional web and an elastic plastic layer is produced.