«EUROPEAN COMMISSION Integrated Pollution Prevention and Control (IPPC) Reference Document on Best Available Techniques for the Textiles Industry July 2003 ...»
Due to the predominantly batch nature of wool pretreatment operations for all types of makeups, the resulting emissions will be discontinuous and with concentration levels largely influenced by the liquor ratios used. An exception is represented by carpet yarn which can be scoured/ bleached and mothproofed on tape or "package to package" scouring machines (see Section 188.8.131.52.2) giving rise to continuous flows.
The pollutants that can be found in the waste water, originate in part from the impurities that are already present on the fibre when it enters the process sequence and in part from the chemicals and auxiliaries used in the process.
Pollution originating from impurities present on the raw material Residues of pesticides used to prevent the sheep becoming infested with external parasites can still be found on scoured wool in amounts which depend on the efficiency of the scouring process. These are mainly organophosphates (OPs) and synthetic pyrethroid (SPs) insecticides and insect growth regulators (IGRs), but detectable residues of organochlorine pesticides (OCs) can be observed. They partition between the fibre and the water according to their stronger or weaker lipophylic character and, as a consequence, traces of these compounds are released in the waste water. More information about ectoparasiticides can be found in Sections 184.108.40.206 and 220.127.116.11. The partition factors of the different classes of pesticides are discussed in more detail for the carpet sector in Sections 18.104.22.168 and 22.214.171.124.
Note that because of their steam volatility some pesticides (OPs) end up in the air emissions from open machines. This must be taken into account in input/output balances.
Spinning lubricants (see 8.2.3), knitting oils (see 8.2.5) and other preparation agents also represent an important issue in wool pretreatment. These substances are removed during the scouring process, contributing to the COD load and aquatic toxicity in the final effluent. The
main concerns are about:
· poorly refined mineral oils (content of aromatic hydrocarbons) · APEO (non-biodegradable and giving rise to toxic metabolites) · silicones (non-biodegradable and difficult to remove without scouring assistants) · biocides (toxic to aquatic life).
The dry spinning route in the carpet sector, described in Section 126.96.36.199, represents one exception because in this case spinning lubricants do not reach the water effluent.
Pollution originating from chemicals and auxiliaries used in the process Considerable amounts of surfactants are used in pretreatment as detergents, wetting agents, etc.
Surfactants with good biodegradability with acceptable performance are now available (see Section 4.3.3). Nevertheless, the use of alkylphenol ethoxylates is still common in some companies due to their low cost. Alkylphenol ethoxylates (APEOs) and in particular nonylphenol ethoxylates (NPEs) are under pressure due to the reported negative effects of their metabolites on the reproduction system of aquatic species. The environmental issues arising from surfactants in common use are discussed in Section 8.1.
Other pollutants of concern that may be found in water effluent from pretreatment activities are:
· reducing agents from bleaching treatments and chemical setting of carpet wool yarn (sodium metabisulphite): they contribute to oxygen demand in the waste water · poorly bio-eliminable complexing agents (e.g. EDTA, DTPA, phosphonates) from hydrogen peroxide stabilisers, etc.
· AOX from sodium hypochlorite bleaching · insect-resist agents in wool carpet yarn production.
More details regarding environmental issues associated with the above-mentioned substances are given elsewhere in this document, in particular in Section 188.8.131.52 and Section 8.5.
2.6.3 Pretreatment of silk The following information has been submitted by Italy [206, Italy, 2001]
Scouring To prepare a silk yarn for dyeing and silk fabrics for dyeing and printing, it is necessary to partially or completely remove sericin, as well as natural oils and organic impurities. Depending on the percentage of sericin removed during scouring (sericin is present in raw silk in a ratio between 20 % to 25 %), the end-product is defined as unscoured (used only for shirts and suits), ‘souple’ or degummed.
Scouring can be carried out either on the yarn or on the fabric. It is easier to remove sericin more evenly by performing the operation on the yarn itself. Nevertheless, the operation is generally carried out on the fabric, in order to exploit the protective action of this “natural size” against possible damage during weaving.
The scouring treatment can be carried out in a neutral, acid or alkaline solution, depending on the desired results. At the industrial level, treatment in alkaline conditions is by far the most common. It is extremely important to control the temperature.
Scouring baths present a high total organic charge; the concentration of nitrogen organic components in particular is high.
“Unscoured” silk The process to produce “unscoured” silk consists in removing from raw silk fabrics all residual substances from previous operations, with a minimal elimination of sericin (1 % - 2 %), so that the fabric keeps the characteristic stiff-handle. The operation is carried out in a slightly alkaline soap bath at low temperature.
“Souple” silk The process is carried out on weft yarns under acid conditions. The loss of weight is approximately 10 %.
Degummed silk This treatment is carried out on both yarns and fabrics and ensures a complete elimination of sericin, as well as substances added in previous operations, without modifying the fibroin.
Hydrolytic degradation of the sericin protein macromolecule can be obtained by simply using soaps, by using synthetic surfactants or mixtures of soaps and synthetic surfactants, by means of an enzymatic treatment, or by treating the silk in water at high temperature and under pressure.
In the degumming process with soap, yarn and fabric are plunged into two baths (degumming baths). Each bath contains green soap at different concentrations. The treatment is followed by washing with ammonia and rinsing. The process temperature varies between 95 °C and 98 °C.
The soap concentration in the degumming bath varies between 10 g/l and 15 g/l. On average the entire treatment lasts for 2 hours. It is possible to re-use exhausted baths after appropriate addition of soap.
Degumming with synthetic detergents implies the partial or total replacement of soap with synthetic non-ionic surfactants (e.g. ethoxylate fatty alcohol). It is also possible to combine a degumming treatment with an oxidizing or reducing bleaching and, in some cases, even with dyeing, thus improving water and energy saving. Generally, alkali and detergent mixtures are used at temperatures around 95 °C - 98 °C. Such a treatment is suited to continuous processing.
54 Textiles Industry
Chapter 2Degumming under pressure at high temperature is a specific treatment essentially used to degum yarns. It is necessary to prepare an aqueous bath without surfactants and the temperature should be between 110 °C and 140 °C. A post-treatment washing is required to eliminate substances used in previous processes.
Weighting The weighting operation is carried out mostly on yarns to promote recovery of the weight loss after the removal of the sericin. The treatment consists in the deposition of tin salts or in grafting polymer chains to the functional groups of the fibroin protein chain.
Weighting is defined as “equal” if the final substrate weight is the same as it was before degumming, and as “higher” if the weight is higher. Weighted silk is different in touch and in draping. The most frequently applied procedure is a “mix weighting” process.
Weighing with minerals
The silk yarn undergoes a treatment using tin tetrachloride in baths at different concentrations (strong or weak baths) in an acid medium. In a strong bath, silk can increase its weight by about 10 % simply by absorbing the salt. The procedure is followed by accurate washing cycles to eliminate unfixed salt and to hydrolyse the salt present on the fibre. Such operations can be repeated to further increase the silk weight. To ensure fixation of tin salts, weighting is completed with a treatment in a sodium phosphate bi-basic solution, followed by a second treatment in sodium silicate.
Disadvantages of this procedure are the long processing time and the high water and energy consumption. The high amount of tin in the waste water is an environmentally undesirable effect of this process.
Weighing with vinyl monomers Grafting vinyl monomers onto silk represents an alternative to the traditional mineral weighting.
Such a method not only allows the desired weight increase to be achieved, but also improves silk characteristics and performance. Co-polymerization with vinyl monomers is carried out using radical activation methods (redox systems, UV, γ rays and so on).
Methacrylamide (MAA) is one of the most frequently applied monomers at the industrial level.
MAA weighting is a simple application. Radical activation is obtained through ammonia or potassium persulphate. Other radical activators, which consist of redox systems described in the literature, are not currently applied at the industrial level.
Silk dyeing behaviour can be modified through the weighting treatment. Studies prove how the dyeing affinity of silk weighted with MAA towards the most widely used dyes changes according to the percentage of acquired weight. They also show that wet fastness is reduced in dyeing after weighting.
For the weighting of silk for ties, methacrylamide is the only technique used by industry; at present no MAA substitute is available on the market.
Mix weighting To achieve a hybrid of the final characteristics of compounds submitted to mineral or MAA weighting, a fairly widely used weighting process on the industrial level is mix weighting with tin/MAA.
2.6.4 Pretreatment of synthetic material 184.108.40.206 Principal manufacturing processes Typical operations before colouring are washing and thermofixing (heat-setting).
Washing is necessary to remove from the yarn the preparation agents that have been applied to its surface in the previous treatments (usually 2 – 3 %, but can be up to 4 % of the weight of the fibre). Most preparation agents (about 95 %) are removed at this stage (ethoxylated fatty alcohols are commonly used as emulsifying agents). Elastomeric fibres (elastan) are an exception because they contain preparation agents mainly made up of silicone oils in the order of 6 - 7 %. Silicones are more difficult to remove and partly still remain on the fibre (40 % of the initial add-on) after washing. To improve their removal it is common practice to use ethoxylated nonylphenols.
When pretreating woven fabric the removal of sizing agents is a crucial step. The extraction of
these substances is achieved thanks to the synergistic action of:
· surfactants (non-ionic or mixtures of non-ionic and anionic): they act as wetting and emulsifying agents and promote the solubilisation of the size · complexing agents (e.g. phosphonates): they are used when there is a risk of re-precipitation of the components of the sizing agents. An increase in hardness level can occur, particularly, in continuous lines when processing synthetic blends with cellulose fibres (characterised by the presence of Ca, Fe and Mg salts as natural impurities) · alkali (caustic soda or sodium carbonate): the alkali is chosen according to the sizing agent employed (for example for the removal of sizing agents based on polyacrylates ammonium salts the use of caustic soda is obligatory, while a polyester sizing agent would precipitate in the same pH conditions).
Thermofixation is also another important operation in synthetic fibres pretreatment. Its position within the process can be different, depending on the make-up and the fibre. As a result the
following possible sequences are possible:
1. thermofixation – washing – dyeing
2. washing – thermofixation – dyeing
3. washing - dyeing - thermofixation.
If white fabrics are to be produced, bleaching of the fabric may be necessary after thermofixation.
220.127.116.11 Environmental issues Potentially harmful impurities and additives are already present on synthetic fibres before they are processed at the finishing mill and they account for a large fraction of the pollution load coming from pretreatment.
Some of these impurities are produced during the manufacture of the fibre. They are polymer synthesis by products such as unreacted monomers (for example caprolactame, in the production of PA 6) low-molecular-weight oligomers, and residual catalysts and they are emitted to air during thermal treatments.
Other substances are intentionally added to the fibre to improve subsequent processing. These are the preparation agents used in fibre and yarn manufacturing and the sizing agents.
The average amount of preparation agents applied on man-made fibres (except for elastomeric, where the load can be much higher) ranges between 2 and 4 % of the weight of the fibre (see also Section 8.2).
When the textile is washed, about 80 % of these substances are released to the waste water and the remaining 20 % can be emitted to exhaust air in the subsequent high temperature treatments (drying and thermofixation). Conversely, when thermofixation is carried out on the grey material before washing (which is the case when fine woven and knitted fabric are processed), the main percentage of pollution load is found in the exhaust air.
During high-temperature treatments the lower molecular weight components of the preparation agents (basically lubricants and surfactants) either decompose — resulting in smaller, more volatile molecules — or they react with each other, forming tar. Volatility and tar are undesirable effects because they lead to air emissions and damage to the yarn.
However, thanks to considerable technological improvements achieved by the technology in the field of synthetic fibres, the fibres/filaments leaving the primary spinning process (fibre production) no longer cause fuming. The remaining problems today are connected with those cases where, after the fibre/filament production, lubricants such as coning oils are added to the fibre at a rate of over 2 or even 3 % of the yarn weight [48, VITO, 2001].
The main concerns, as regards waste water, arise from the discharge of poorly or nonbiodegradable substances such as mineral oils, EO/PO adducts, silicone oils, hard surfactants, etc. Furthermore, biocides, which are normally contained in the aqueous formulations, contribute to aquatic toxicity of the waste water.
Likewise for air emissions, as for water emissions, the main charging load in waste water again comes from the preparation agents (coning oils, overspray, etc.) that are applied to the fibre/filament after the primary spinning stage. These substances (secondary spinning agents) are applied in significantly higher amounts than primary spinning agents. Moreover, they usually have low affinity with water and are therefore difficult to remove.
For woven fabric, sizing agents also have to be considered. Sizing agents do not give rise to air emissions during HT treatments, but they may be responsible for poorly biodegradable substances and aquatic toxicity.
Information about alternative preparation agents for man-made fibres is given in Section 4.2.1.
2.7 Dyeing In the following sections the general principles of dyeing and the most commonly used dyeing techniques for the different fibres are described. For practical reasons, detailed information concerning dyeing auxiliaries, dyestuffs and dyeing equipment is given in specific annexes (namely, Sections 8.6, 9 and 10).
2.7.1 General principles of dyeing