«EUROPEAN COMMISSION Integrated Pollution Prevention and Control (IPPC) Reference Document on Best Available Techniques for the Textiles Industry July 2003 ...»
· sulphates (e.g. alcohol ethoxysulphates, alkanolamides sulphates, sulphated vegetable oils) · sulphonates (e.g. alkylbenzene sulphonates, sulphonated vegetable oils, naphthalene sulphonates, ligninsulphonates) · alkyl ether phosphates · carboxylates (fatty acid condensation products, alkali salts of fatty acids).
The linear, more biodegradable compounds are the most commonly used (e.g. alkylbenzene sulphonates, fatty alkyl sulphates, etc.). Examples of recalcitrant anionic surfactants are the common lignin sulphonates and condensation products of naphthalene sulphonic acid with formaldehyde, which are widely used as dispersants for vat, sulphur and disperse dyes.
Anionic surfactants have several advantages: they are good oil emulsifiers and dye dispersants, they are excellent wetting agents and they are not expensive. In turn, they generate high levels of foam, and sulphate surfactants can be sensitive to calcium and magnesium [11, US EPA, 1995].
Cationic surfactants are relatively uncommon in textile processing. One example is quaternary ammonium compounds (salts) used as retarders for cationic dyes, which are water-soluble recalcitrant substances. Cationic surfactants have by far the highest toxicity of all classes of surfactants [179, UBA, 2001].
Amphoteric surfactants Amphoteric surfactants are not widely used in the textile industry. Their main advantage is the fact that they can be used in alkaline and acidic media and in combination with either cationic or anionic surfactants.
Quaternary ammonium compound derivatives are very rarely applied, while other low-toxicity
types are increasing in use. Examples are:
· betaine derivatives · imidazolines · modified fatty amino ethylates (they have very good emulsifying and dissolving capacity for removing oligomers in the reductive cleaning of polyester fibres).
Amphoteric surfactants are expensive and their use is required only in specialised situations where wide ranges of compatibility are needed.
8.2 Auxiliaries and finishing agents for fibre and yarn manufacturing Within this class are included those organic compounds that are applied to the fibre during its manufacturing and yarn formation processes. The names used for these auxiliaries in this section are taken from the TEGEWA nomenclature (“TEGEWA nomenclature, 1987”). They
are classified as follows:
· spinning solution additives, spinning additives and spinning bath additives · preparation agents for primary spinning · preparation agents for secondary spinning (conditioning agents and lubricants) · coning oils, warping oils and twisting oils.
Special attention is given to them because they are removed during textile pretreatment, in most cases contributing to a significant proportion of the emissions to water and to air at finishing mills.
The general chemical composition of yarn and fibre preparation agents is based on four main classes of components, the proportions depending on the function of the specific preparation
· lubricants · emulsifiers · wetting agents · antistatic agents · additives (e.g. biocides, antioxidants, agents for the compactness of the thread).
Typical applied lubricants are:
· mineral oils · ester oils · synthetic lubricants (which include synthetic esters, EO/PO adducts, silicones, etc.).
The term "mineral oils" is used to indicate lubricants derived from refined crude oil. They are mixtures of hydrocarbons with C12 – C50 chain length, having a range of boiling points between 220 ºC and 450 ºC. Due to the presence of undesirable and unstable impurities, mineral oils smoke and give rise to air emissions during high-temperature treatments.
Mineral oils are hardly biodegradable and can only be removed by absorption. However, as regards water pollution, the main concern is over the presence of polyaromatic hydrocarbons (these compounds are included in the list of priority hazardous substances to be controlled through EU-wide measures under the Water Framework Directive).
The amount of polyaromatic compounds in mineral oils varies according to the refining process adopted and decreases as the oils become less polydisperse (refined mineral oils are commonly known as white oils). Pharmaceutical-grade mineral oils contain less than 0.1 ppm of polyaromatic hydrocarbons, but they are about three times more expensive than conventional mineral oils.
The use of mineral oils is declining. Because of their low cost, however, they are still widely used in applications where cheap products are needed (mainly as coning oils and, to a lesser extent nowadays, as wool processing auxiliaries).
Esters oils are usually fatty acids esterified with fatty alcohols, alcohols or polyhydroxylic alcohols. They are normally obtained by saponification of natural fats or oils.
Ester oils are used as lubricants as an alternative to mineral oils. Compared to mineral oils, ester oils are more thermally stable, biodegradable and easier to emulsify. They are increasingly substituting mineral oils in primary spinning, but mineral oils still have the highest market share in secondary spinning.
Synthetic lubricants (so-called synthetic oils) are synthetic base fluids especially tailored to the lubrication function. Because synthetic oils are composed of molecules that are uniform in weight and structure, they can withstand temperatures exceeding 200 °C, which also results in higher oxidative and thermal stability than mineral oils. For these reasons they outperform mineral oil-based products in many respects, allowing for higher operating temperatures, less lubricant loss and improved flexibility in a wide range of operating conditions.
Synthetic oils are free of all metals, sulphur, phosphorus and wax. Certain lubricants are highly biodegradable, thereby having reduced negative impact on the environment.
There are several major classes of synthetic lubricants:
· synthesised hydrocarbons, such as polyalphaolefins (PAO) and dialkylated benzenes, which are the most common type · synthetic esters, such as dibasic acid and polyol esters · polyglycols · silicones.
Synthetic esters are synthesised from relatively pure and simple starting materials to produce predetermined molecular structures designed specifically for high performance lubrication.
Compared to ester oils obtained from natural fats and oils, these molecules are more uniform in size, which means that they are more thermally and oxidatively stable.
EO/PO copolymers are used for texturised chemical fibres because they do not interfere with the process as mineral oils do.
The chemical structure of these synthetic lubricants can be schematised as follows:
S-(EO)x-(PO)y-B S = starting component which can be short-chain alcohols (e.g. C4-), polyols, organic acids or primary amines;
B = block component which can be ethers (OR), esters (COOR), acetales CH(OR)2 or OH The high molecular EO/PO-adducts (sum of EO and PO units is more than 15 moles and the sequence of PO units is higher than 5) are non- or hardly biodegradable.
Silicones are used as lubricants in several areas, including the manufacturing of fibres such as elastane and polyamide. They are chemically inert, non-toxic, fire resistant and water repellent.
They are of great value in applications involving extreme temperatures, where high oxidative and thermal stability is required.
Silicones show the highest level of COD of all lubricants and they are hardly biodegradable, but they are bioeliminable and not dangerous to aquatic life. The main disadvantage is that they are difficult to emulsify and remove from the fibre. APEO are usually used to remove them and quite a high percentage (approximately 40 %) remains on the fibre after washing, giving rise to air emissions in the subsequent high-temperature treatments.
Emulsifiers In order to apply the preparation agent as an aqueous system when the lubricant is not soluble in water, an emulsifier is normally present in the formulation. Anionic and non-ionic surfactants
are used as emulsifiers. The main surfactants employed are:
Anionic surfactants: - sulphonated and sulphated vegetable oils
Antistatic agents The anionic surfactants also have anti-electrostatic properties. Mono and diesters of phosphorus pentoxides (mainly their potassium salts) are in use as special anti-electrostatic agents as well as amphoteric surfactants such as sarcosides, amine oxides and sulpho succinates.
Aqueous systems can be attacked by bacteria and so must incorporate a bactericide. Biocides such as formaldehyde-containing compounds are applied as preservatives with a load of about 50 mg/kg fibre. Heterocyclic compounds (imidazolinone and isothiazolinone derivatives) with a load of about 2 mg/kg fibre are also encountered.
When the preparation agents are applied as neat oils or sufficiently stable solutions, instead of aqueous emulsions, the addition of biocides can be avoided, unless they are needed to protect the yarn during storage.
The amounts of active substances added to the fibres and the composition of the applied formulations may vary widely with fibre type and end-use. A rough overview is given in Table 8.1, but a more detailed description of typical formulations used and the load applied on the substrate is given in the following sections (based on an updated version of [7, UBA, 1994]). In Table 8.1 no distinction has been made between the different types of fibres; only elastomeric fibres and fibres destined for the manufacturing of knitted fabric have been considered separately because of the higher amount applied to the substrate in these cases. Furthermore, the load indicated under the column "Yarn manufacturing" refers to the overall amount of preparation agents applied to the fibre after the production of the fibre itself (including coning oils, twisting oils, oils applied to the filament after the texturising process, etc.).
8.2.1 Spinning solution additives, spinning additives and spinning bath additives Within this group of auxiliaries only those that are washed off during pretreatment operations are mentioned. In this respect, the so-called “modifiers” used for viscose are most relevant. The applied load varies between 5 mg/kg fibres to a few grams per kg of fibre depending on the application field. They mainly consist of ethoxylated fatty amines or polyethylene glycol ethers with molecular weights of about 1500. During pretreatment, more than 90 % of these substances are washed off.
8.2.2 Preparation agents for primary spinning
These preparation agents are applied (mainly as aqueous solutions) during the manufacture of chemical fibres, directly after primary spinning (see Section 2.2). They enable subsequent processes such as drawing, twisting, warping, texturising and further spinning (secondary spinning, in the case of staple fibres).
The preparation agents give the chemical fibres the necessary properties (optimal friction, avoidance of electrostatic charging and cohesion in the case of multifilament yarns) not only between the fibres, but also between the fibres and the guide elements of the machines.
In general the substances applied have high affinity with water, either because the emulsifiers are already contained in the formulations or because the lubricant molecules themselves are self-emulsifying systems.
The application loads and the characteristics of the formulations applied are given for:
· non-texturised filament fibres (Table 8.2) · texturised filament fibres (Table 8.3) · staple fibres (Table 8.4).
Source: [179, UBA, 2001]
The reported loads relate to the quantity of active substance, not to the quantity of applied aqueous emulsion Table 8.2: Load of preparation agents on non-texturised filament yarns (flat yarns)
8.2.3 Preparation agents for secondary spinning (conditioning agents and spinning lubricants) For these agents there is no clear definition. In the following, preparation agents for secondary spinning of synthetic staple fibres and cotton are referred to as "conditioning agents", while preparation agents for wool will be referred to as "spinning lubricants".
Conditioning agents are also required during secondary spinning of synthetic fibres, when the fibres have been previously submitted to bleaching or dyeing processes. The amount initially applied is in fact lost during these processes.
The chemical composition of conditioning agents for synthetic fibres is similar to that of the preparation agents used for primary spinning of staple fibres (see Table 8.4). The load ranges between 1 and 10 g/kg fibres.
Spinning lubricants are applied to wool fibres to assist efficient mechanical processing during yarn manufacturing (spinning). They are generally applied as aqueous emulsions and for this reason they also contain an emulsifier and a biocide to prevent bio-attack. In most cases the emulsification system is based on APEO [66, CRIT, 1999], although according to other sources ([32, ENco, 2001]) the major suppliers have sought to eliminate alkylphenol ethoxylates by substituting them with linear alcohol ethoxylates.
When the spinning lubricant is to be applied to wool blends with synthetic fibres, an antistatic agent is also needed.
As regards wool and blends of wool with synthetic fibres, four basic types of spinning
lubricants can be identified [32, ENco, 2001]:
· emulsifiable mineral wool oils based on a mixture of refined mineral oil (1 % polyaromatic content) and an emulsification system · water-dispersible lubricants (sometimes referred to as "Super Mineral Oils" or "Semi synthetics") based on a mixture of refined mineral oil, saponifiable fatty oils and an emulsification system. These formulations generally contain a higher proportion of emulsifiers than emulsifiable wool oils · synthetic water-soluble lubricants based on polyethylene- polypropylene glycols, which are particularly useful when the yarn is to be scoured in the dye bath, but may also be used on yarns that will be scoured in a separate operation · dry spinning lubricants (only for carpet yarn – see Section 188.8.131.52), which differ from the above in that they are applied at lower levels and they remain on the yarn following conversion to carpet. These materials generally contain a higher proportion of antistatic agents.
The loads for cotton, viscose staple fibres and wool are reported in Table 8.5
Table 8.5: Load of conditioning agents and lubricants applied on cotton, viscose (staple fibres) and wool If further processing of dyed yarns or flocs is planned, an additional amount of conditioning agents (3 - 5 g/kg) is applied normally in discontinuous by bath exhaustion at the textile finishing mill.
Thereby the exhaustion rates for PES and PA can be very low (10 – 30 %).
Conversely, they are relatively high for CO and PAC (80 %).
8.2.4 Coning oils, warping and twisting oils For processes such as coning, twisting and warping of flat and texturised filament yarns as well as of staple fibre yarns, chemicals have to be applied in order to enhance smoothness, lubrication and antistatic properties.
Oils for coning, warping, twisting and those applied to the filament after the texturing process (sometimes called overlay oils) consist of 70 – 95 % white oils and of 5 – 30 % non-ionic surfactants, especially fatty alcohols and fatty acids ethoxylates. Twisting oils often consist of ester oils which are more biodegradable than white oils. Ester oils are used especially if evaporation has to be avoided or minimised, especially during heat-setting.