«EUROPEAN COMMISSION Integrated Pollution Prevention and Control (IPPC) Reference Document on Best Available Techniques for the Textiles Industry July 2003 ...»
Depending on the degree of completeness of the cross-linking reaction and on the temperature employed in heat treatments, different levels of formaldehyde and aliphatic alcohols are found in the exhaust air. The presence of paraffin wax contributes to increased levels of volatile organic carbon in the emissions.
Silicone repellents These products are generally supplied as aqueous emulsions consisting of polysiloxane-active substances (dimethylpolysiloxane and modified derivatives), emulsifiers, hydrotropic agents (glycols) and water.
In the case of modified polysiloxanes with reactive functional groups, and depending on the drying and curing conditions, cyclic dimethylsiloxanes can be released in the exhaust air.
Fluorochemical repellents The success of these agents, despite their high cost compared to other types of water repellents, is especially due to the fact that they are permanent and they provide both oil and water repellency.
Commercial fluorochemical repellents are mostly copolymers of fluoroalkyl acrylates and methacrylates.
Marketed formulations contain the active agent together with emulsifiers (ethoxylated fatty alcohols and acids, but also fatty amines and alkylphenols) and other byproducts which are often solvents such as:
· acetic acid esters (e.g. butyl/ethyl acetate) · ketones (e.g. methylethyl keton and methylisobutyl ketone) · diols (e.g. ethandiol, propandiol) · glycolethers (e.g. dipropylenglycol).
Fluorochemical repellents are usually applied in combination with other finishing auxiliaries by a pad-dry-cure process. In many cases they are applied with “extenders” which can be other repellents themselves (e.g. melamine resins repellents or polyisocyanates). The use of these “extenders” allows a reduction in the required amount of fluorochemical, with a corresponding reduction in costs for this treatment.
Finishing treatments with fluorochemical repellents produce emissions of volatile organic
compounds in exhaust air. These emissions are attributable to:
· the solvents contained in the formulations (as regards ketons, esters, alcohols, diols) · the “extenders”, which under high-temperature conditions give rise to cracked by-products such as alcohols and ketones, but also oximes and in particular butanoxime (which is carcinogenic) · the organo-fluoro components which also release cracked fluor-organic by-products.
As regards water pollution, it has to be taken into account that polysiloxanes, melamine and fluorocarbon resins are all characterised by poor biodegradability and bio-eliminability.
8.8.6 Softeners This group of chemicals is designed for hand modification of fabric. Softeners reduce the fibre/fibre friction, an effect which hand-feel describes as “soft or smooth”.
Quite often softeners are used together with resins and/or optical brighteners in sometimes complex finishing recipes.
Fabric softeners are water-based emulsions or dispersions of water-insoluble active materials
· non-ionic surfactants · cationic surfactants · paraffin and polyethylene waxes · organo-modified silicones.
Note that phthalates are plasticisers for e.g. PVC, but never textile softeners [195, Germany, 2001].
The formulation of the above-mentioned ingredients often requires additives such as emulsifiers and compatibilisers (e.g. glycols). Problematic APEO emulsifiers are no longer used by European producers.
As for surfactant-type softeners the trend is towards mainly non-ionic and cationic compounds.
Non-ionic softeners do not have substantivity for the fibres and are as wash-fast as the cationics.
In spite of this, their usage is increasing as the volume of textiles with more permanence and increased wrinkle resistence is growing. Non-ionic surfactants such as fatty acids, fatty esters and fatty amides belong to this group.
Because of their substantivity, cationic softeners produce a more permanent softening effect than non-ionic compounds. Furthermore, they are more effective at much lower concentrations.
Their substantitvity for synthetic hydrophobic fibres is limited, increasing in the order:
polyester, polyamide, acetate, cotton, viscose and wool. Some disadvantages of cationic agents are their lack of compatibility with anionic compounds typically employed as detergents and soaps, etc. As such, cationic softeners are applied after the complete removal of anionic detergents from the fabric [298, Dyechem Pharma, 2001].
Cationics used as softeners are [298, Dyechem Pharma, 2001]:
· quaternary ammonium compounds such as stearyl or distearyl dimethyl ammonium chloride · amido amines formed by reaction of a fatty acid or a glyceride and a substituted or unsubstituted short chain polyamine (e.g diethylene triamine, N, N-diethyl ethylenediamine). The amide thus formed is quaternised with acetic acid or hydrochloric acid to give the cationic softener (especially used for chlorinated wool) · imidazolines which can be acetylated or reacted with ethylene oxide.
Polyethylene wax emulsions are widely used for towelling, where a good “bunch” hand is required, rather than in applications such as dress apparel. Among the advantages, it is worth mentioning their compatibility with cationic, non-ionic and anionic softeners [298, Dyechem Pharma, 2001].
Silicone softeners, used as emulsions or additives to other softeners, are increasing in importance. They have good effectiveness and besides softening they impart to the fabric additional properties such as water repellency.
Softeners are mostly applied by forced application (padding, spraying) from relatively concentrated solutions, which transfers all of the liquor onto the fabric [195, Germany, 2001].
In batch processing softeners are often applied by exhaustion from diluted baths on machines such as jet, overflow or winch. Here the exhaustion rate is relevant to ecological considerations of waste water loads. Machine technology with extremely short liquor ratios and skilled formulation of products help to minimise losses of active material [195, Germany, 2001].
If softeners enter the waste water, their behaviour in biological waste water treatment has to be taken into account.
Fatty derivatives generally are highly biodegradable. Cationic softeners are known to be toxic to aquatic life. Silicones and waxes are partially removed from the waste water by adsorption onto the sludge, after the stabilising emulsifiers have been degraded.
As the active ingredients of softener formulations are chemicals with high molecular weight (even polymers), the volatility is low. Volatile by-products of silicones (cyclics) are stripped before the production of the softener. Some waxes or fatty ingredients, however, may have some sensitivity towards cracking, if stenter temperatures are too high [195, Germany, 2001].
8.9 Coating compounds and auxiliaries According to their chemical composition, coating agents can be classified as follows [179, UBA, 2001].
Coating powders They can be based on polyolefins (especially polyethylene), polyamide 6, polyamide 6.6, copolyamides, polyester, polyurethane, polyvinylchloride, polytetrafluoroethylene.
They are based on the chemicals mentioned above, but they also contain additives such as:
· dispersing agents (surfactants, often alkylphenolethoxylates) · solubilising agents (glycols, N-methylpyrrolidone, hydrocarbons) · foaming agents (mineral oils, fatty acids, fatty acid ammonia salts) · softeners (especially phthalates, sulphonamides) · thickeners (polyacrylates) · ammonia.
Polymer dispersions (aqueous formulations)
They contain approximately 50 % water and are based on:
· poly(meth)acrylate (butyl, ethyl, methyl etc.) · polyacrylic acid · polyacrylonitrile · polyacryloamide · 1,3-polybutadiene · polystyrene · polyurethane · polyvinylchloride · polyvinylacetate · and copolymeres of the above-mentioned polymers.
Additives are also present, as they are in coating pastes.
Melamine resins They are produced by reaction of melamine and formaldehyde and subsequent etherification mainly with methanol in aqueous medium (water content 50 – 70 %).
Polymers dispersions (organic solvent-based formulations) They are based on polyurethane and silicones dispersed in organic solvent.
9 ANNEX II DYES AND PIGMENTSTextile dyes can be classified according to their chemical composition (azo, antrachinone, sulphur, triphenilmethane, indigoid, phtalocyanine, etc.) or according to their application class.
At the industrial level the second method is preferred.
9.1 Acid dyes Applicability Acid dyes are mainly applied to polyamide (70 – 75 %) and wool (25 – 30 %). They are also used for silk and some modified acrylic fibres. Acid dyes exhibit little affinity for cellulose and polyester fibre.
Properties Colours are generally bright and fastness to light and washing range from poor to excellent, depending on the chemical structure of the dyestuff.
Chemical characteristics and general application conditions Acid dyes are azo (the largest group), anthraquinone, triphenylmethane, Cu phthalocyanine chromophoric systems which are made water-soluble by the introduction in the molecule of up to four sulphonate groups.
Figure 9.1: Examples of acid dyes
Their interaction with the fibre is based partly on ionic bonds between sulphonate anions and the ammonium groups of the fibre, as shown below for wool and for polyamyde, at different pH conditions.
Moreover, the fibre/dye interaction is based on secondary bonds such as Van der Waals forces.
Secondary bonds are established in particular in the case of higher molecular weight dyes, which form aggregates with high affinity for the fibre.
In use, acid dyes are classified by their dyeing behaviour and wet fastness properties, rather than chemical composition, hence the generic term acid dyes includes several individual dye classes.
The arbitrary classification normally adopted, in order of increasing fastness is:
· level-dyeing or equalising acid dyes · fast acid, half-milling or perspiration-fast dyes · acid milling dyes · supermilling dyes.
Level-dyeing or equalising dyes are subdivided into two classes, monosulphonated (mainly for PA) and disulphonated (mainly for wool). Due to their poor affinity for the fibre, they all have very good levelling properties. Their wet fastness is, however, sometimes poor, limiting their use to pale/medium shades.
Fast acid dyes (also known as half-milling dyes or perspiration-fast dyes) are only used for PA.
They are generally monosulphonated and exhibit superior fastness properties to level-dyeing acid dyes, while retaining some of the migration properties. The shade range available in this class is not as wide as that of the levelling or milling dyes and they therefore tend only to be used when alternatives would have poorer fastness properties.
Acid milling dyes are so named because they have a degree of fastness to the wet treatments employed when milling (mild felting) woollen fabrics. The class is further subdivided to include supermilling dyes, which have good wet fastness properties, arising from long alkyl side-chains attached to the chromophore. Due to their high molecular weight, milling dyes have a good affinity for the fibre and do not migrate well at the boil. Milling dyes are used mainly for wool for those applications where good wet fastness is required, for example in the dyeing of loose fibre which will receive a further wet treatment during hank scouring.
Depending on the class they belong to, acid dyes are applied under pH conditions that vary from strongly acidic to more neutral ones (3 – 7.5). For low-affinity dyes it is necessary to increase the level of cationisation of the fibre (by acidification) in order to improve dye uptake.
Conversely, dyes with higher molecular weight and high affinity would adsorb too rapidly on the fibre if applied under such strongly acidic conditions.
512 Textiles Industry Annexes
The most common chemicals and auxiliaries applied when dyeing with acid dyes are:
· sodium sulphate (for level-dyeing and fast acid dyes), sodium acetate and ammonium sulphate (for acid milling dyes) · pH regulators: acetic, formic and sulphuric acid, but also (typically for PA in the carpet sector) NaOH, NH3 salts, phosphoric acid salts and higher (hydroxy)carboxylates · levelling agents, mainly cationic compounds such as ethoxylated fatty amines.
The most common chemicals and auxiliaries applied when printing with acid dyes are:
· thickening agents · solubilising agents such as urea, thiourea, thiodiglycol, glycerine · acid donors: ammonium sulphate, tartrate or oxalate · defoamers (e.g. silicone oils, organic and inorganic esters) and “printing oils” (mainly mineral oils) · aftertreatment agents such as formaldehyde condensates with aromatic sulphonic acids.
Environmental issues The environmental properties of acid dyes are assessed under the following parameters. Note, however, that the following table does not consider the environmental issues related to chemicals and auxiliaries employed in the dyeing process.
Effluent contamination by additives in the dye formulation Table 9.1: Overview of the ecological properties of acid dyes
9.2 Basic (cationic) dyes Applicability Basic dyes were initially used to dye silk and wool (using a mordant), but they exhibited poor fastness properties. Nowadays these dyestuffs are almost exclusively used on acrylic fibres, modified polyamide fibres, and blends.
Properties On acrylic fibres fastness performances are excellent.
Chemical characteristics and general application conditions Cationic dyes contain a quaternary amino group which can be an integral part (more common) or not of the conjugated system. Sometimes a positively-charged atom of oxygen or sulphur can be found instead of nitrogen.
Ionic bonds are formed between the cation in the dye and the anionic site on the fibre.
Figure 9.2: Examples of typical basic dyes Cationic dyes are slightly soluble in water, while they show higher solubility in acetic acid, ethanol, ether and other organic solvents.
In dyeing processes, they are applied in weak acid conditions. Basic dyes are strongly bound to the fibre and do not migrate easily. In order to achieve level dyeing, specific levelling auxiliaries, (also called retarders) are normally employed (unless a pH controlled absorption process is used). The most important group of retarders is represented by quaternary ammonium compounds with long alkyl side-chains (cationic retarders). Electrolytes and anionic condensation products between formaldehyde and naphthalenesulphonic acid may also be found.
Environmental issues Many basic dyes exhibit high aquatic toxicity but, when applied properly, they show fixation degrees close to 100 %. Problems are more often attributable to improper handling procedures, spill clean-up and other upsets [11, US EPA, 1995].
The following dyestuffs have been classified as toxic by ETAD:
· Basic Blue 3, 7, 81 · Basic Red 12 · Basic Violet 16 · Basic Yellow 21.
9.3 Direct (substantive) dyes Applicability Direct dyes are used for dyeing cotton, rayon, linen, jute, silk and polyamide fibres.
Properties Colours are bright and deep, but light-fastness can vary greatly depending on the dyestuff.
Wash-fastness properties are also limited unless the textile is after-treated. Only occasionally are direct dyes used in direct printing processes.