«EUROPEAN COMMISSION Integrated Pollution Prevention and Control (IPPC) Reference Document on Best Available Techniques for the Textiles Industry July 2003 ...»
The load of coning oils depends on the further processing and the volume of the yarn. It varies for PES from 5 to 30 g/kg (15 g/kg average). For common PA the load is 5 g/kg and in the case of highly elastic PA it is up to 15 - 20 g/kg. It is reported that imported fabric can have loads of coning oils above 50 g/kg.
The load for warping oils and twisting oils is about 5 g/kg.
8.2.5 Knitting oils The needles of knitting machines have to be lubricated with so-called knitting machine oils. Due to machine losses, these oils remain on the knit fabric (about 4 – 8 % of the weight of the fibre).
They should now be based on synthetic oils, but in practice mineral oil-based formulations are
still common. This also means that surfactant aids need to be added in the scouring bath in order to emulsify them.
8.3 Sizing agents
In this section the following information is presented:
· characteristics of the most common sizing agents · typical amounts of sizing agents applied on the different substrates (Table 8.6) · specific COD and BOD5 values for the most common sizing agents (Table 8.7).
Starch is the most common natural size and can be derived from various substances, especially corn and potatoes. It is not always suitable for high performance weaving mills. When stored at lower temperatures, it tends to solidify to pulpy mass and loose the hydratation water (retrogradation). This retrogradation leads to poor storage properties, skin formation, formation of deposits on the rollers and reduced adhesive strength. As a result natural starch is commonly used in formulation with other sizes [186, Ullmann's, 2000].
Starch is completely biologically removed, but it is only sparingly soluble in water; the action of animal or vegetable enzymes is needed. These enzymes decompose the starch into water-soluble sugars, which are then removed by washing before scouring. As a result starch cannot be recycled and is responsible for most of the COD load in finishing effluents. Moreover, in comparison with other sizing agents, starch must be applied at about twice the rate of carboxymethyl cellulose, because of the poorer sizing effect, thereby leading to a higher COD in the waste water.
Starch derivatives Starch derivatives include dextrins, starch esters and starch ethers. They are increasingly replacing natural starches, as their tendency to retrograde is considerably reduced.
The starch esters most commonly used are the esters with phosphoric acid (phosphate starches) and acetic acid (acetyl starches).
The three most important starch ethers are the hydroxyethyl, hydroxypropyl and carboxymethyl starches.
Starch derivatives based on esters can, to a great extent be biologically reduced, while starches based on ethers are more difficult to biodegrade. However, the great variety of possible chemical modifications makes it possible to produce good slashing agents, which can be virtually completely biodegraded.
Like natural starch, starch derivatives must be also applied at about twice the rate of CMC or synthetic agents, thereby leading to higher COD in the effluents.
Cellulose derivatives (carboxymethyl cellulose) Sodium carboxymethyl cellulose (CMC) is the only water-soluble cellulose derivative used as a sizing agent. It is produced by reacting cellulose with sodium hydroxide and sodium chloroacetate, whereby the cellulose polymer is also depolymerised.
CMC is the preferred additive for increasing the adhesion of starch sizes for cotton.
Its chain length and degree of substitution influence the level of biological reduction. However, carboxymethylcellulose (CMC) must be classified as being very difficult to degrade.
According to UBA, only 20 % of the initial amount is eliminated after 7 days. However, it is also reported that CMC can be reduced after long periods of adaptation (4 weeks) and favourable conditions (especially higher temperatures) [7, UBA, 1994]. Other sources mentioned by UBA state that a biological reduction rate of 60 % is attained after 7 days.
Hydroxypropyl and carboxymethyl galactomannan derivatives are water-soluble and easy to remove by washing. In general they can reach high levels of biological reduction (greater than 90 %). However, biodegradability as well as other properties can vary significantly with molecular weight and the mannose/ galactose ratio.
Polyvinyl alcohol (PVA) Since the monomeric vinyl alcohol does not exist, poly(vinyl alcohol) is produced by the hydrolysis of the poly(vinyl acetate).
The parameters that determine sizing properties are the degree of polymerisation and the degree of hydrolysis. There are two grades: the partly hydrolised grade with a degree of hydrolysis of ca. 88 % and the fully hydrolysed grade with a degree of hydrolysis of ca. 98 %.
Partially hydrolysed PVA grades are preferred for use as sizing agents because they show maximum water solubility and they adhere strongly to non-polar fibres such as PES. In addition to these conventional poly(vinyl alcohol) grades, copolymers with methyl methacrylate or with co-monomers having carboxyl groups are also used as sizing agents.
The degree of polymerisation and the degree of hydrolysis lead to significant differences in terms of biodegradability between the individual types of polyvinyl alcohols. PVA is not acutely toxic to micro-organisms and does not inhibit nitrification. It can be quantitatively eliminated by biological degradation after adaptation of the sludge. The conditions for a 90 % reduction are: relatively high concentrations (20 mg/l), temperature above 18 ºC and a regular load over a long period of time (in wintertime and in the holiday period problems can occur) [7, UBA, 1994]. Given such adaptation, PVA can be classified as having good bioeliminability.
Poly(meth)acrylates show considerable variation in chemical structure (the building blocks can be: acrylic acid, acrylic esters, acrylamide, acrylonitrile, methacrylic acid) and therefore have a very wide range of applications. Hydrophilic monomers like methacrylic acid and its salts and acrylamide give good adhesion to polar fibres and ease of removal by washing. Hydrophobic monomers such as the acrylic esters increase the elasticity of the sizing film and improve the ease of removal from waste water.
Poly(methacrylates) cannot be reduced biologically, not even after a period of adaptation.
Hydrophilic products cannot even be eliminated from the aqueous phase, because they behave as hydrophilic polyelectrolytes.
An improvement is represented by poly(methacrylates) based on esters. In this case, thanks to the presence of hydrophobic groups, the poly(methacrylates) can be 90 % bio-eliminated by adsorption onto sludge after a residence time of 6 – 9 hours (which in practice is secured many times over) [18, VITO, 1998]. However, it is important to point out that the bio-elimination curve gives rise to a number of questions about the elimination mechanisms (the elimination mechanisms are not clear).
Polyesters Polyesters are generally condensates of aromatic dicarboxylic acids with diols (e.g. ethylene glycol, diethylene glycol) and sulphonated aromatic dicarboxylic acids (the latter provide solubility or dispersability in water, they are mainly added to improve dyeability).
Their biodegradability is bad, but they show a certain degree of bioelimination [77, EURATEX, 2000]. They are mainly used for sizing flat filament yarns. They are also found as cocomponents in sizing agents for staple fibres made of polyester blends.
Source: [179, UBA, 2001]
(1) the figures refer to woven fabrics with 60 % of warp yarn (2) as wax Table 8.6: Amounts of sizing agents applied on different types of substrates
Table 8.7: Specific COD and BOD5 values for the most common sizing agents
8.4 Detergents/ wetting agents These auxiliaries are mainly used in pretreatment operations (scouring, mercerising, bleaching)
in order to allow:
· thorough wetting of the textile material · emulsification of lipophilic impurities · dispersion of insoluble matter and degradation products.
Non-ionic and anionic surfactants are the compounds more frequently used for this purpose (see also Section 8.1). Some examples of products available on the market are listed in Table 8.8.
Class Examples of products available on the Bio-degradability Bio-eliminability market (1) (2) Alcohol and fatty alcohols ethoxylates 90 % 80 – 85 %
(1) OECD-test 301 E (2) OECD-test 302 B Source: [77, EURATEX, 2000], [218, Comm., 2000] Table 8.8: Typical compounds used as detergents/ wetting agents
8.5 Auxiliaries containing sequestering agents The presence of ions of alkaline earth metals (calcium and magnesium) and/or other metals (especially iron) may have important negative effects on various wet processes not only in pretreatment, but also in dyeing. Purified and softened water is used in textile finishing mills, but often this is not enough and specific auxiliary formulations containing complexing agents need to be added to the baths.
Auxiliaries with this effect are often classified according to the specific process for which they are applied (extractants, sequestering agents/ dispersants, etc.). Commonly used complexing agents are EDTA, NTA, DTPA, phosphonic acid and gluconic acid derivatives.
The hydrogen peroxide stabilisers represent another important category of auxiliaries containing complexing agents. The uncontrolled decomposition of hydrogen peroxide gives rise to OH* radicals. These radicals attack the cellulose fibre starting with oxidation of the hydroxyl groups and eventually ending with the cleavage of the cellulose molecules, decreasing the degree of polymerisation. This reaction is catalysed by heavy metals such as iron, manganese, copper and cobalt.
In order to inhibit these reactions, bleach stabilisers, containing sequestering agents, are usually applied. EDTA, DTPA, NTA, gluconates, phosphonates and polyacrylates are typical stabilisers.
Environmental problems associated with sequestering agents arise from the same properties for which they are used, which is their ability to form stable complexes with heavy metals. EDTA, NTA, and DTPA in particular form very stable metal complexes. EDTA and DTPA are also poorly eliminable compounds. There is therefore there is the risk they can pass undegraded through the common waste water treatment system and then eventually release the metals into the receiving effluent or that they may remobilise heavy metals in aquatic sediments.
NTA is biodegradable (when treated in waste water treatment plants under nitrifying conditions) and recent studies have shown that it only plays a minor role, if any, in the remobilisation of heavy metals in aquatic sediments [280, Germany, 2002].
Concerning the other organic substances used as complexing agents, gluconates are biodegradable; phosphonates are not biodegradable, but they are bioeliminable (photocatalytic degradation is also observed [77, EURATEX, 2000]).
8.6 Dyeing auxiliaries 8.6.1 General characteristics and environmental aspects Auxiliaries are essential components in dyeing processes. This section describes the ones most commonly used to assist the dyeing process. They are presented based on their function in the
process as follows:
· wetting, penetrating agents · dispersing agents · levelling agents · acid donors · antifoaming · carriers.
Other auxiliaries such as, for example, the complexing agents that are also commonly used in dyeing processes but that can also be encountered in other operations, are described in other parts of this annex.
Commercial products are in general preparations containing several components. These are mainly surfactants, but non-surface-active substances such as water-soluble polymers and oligomers and polymer dispersions are also frequently employed.
With a few exceptions dyeing auxiliaries are released to waste water. Some of them are biodegradable (e.g. fatty alcohol ethoxylates, linear alkylbenzene sulphonates), while others have poor biodegradability, but they are sparingly soluble in water and are therefore largely eliminated by absorption on the activated sludge in waste water treatment plants. There is, however, a list of substances that are soluble in water and poorly eliminable in common waste water systems. Compounds that belong to this category and that are frequently encountered in
dyeing auxiliaries are listed as follows:
8.6.2 Wetting, penetrating and de-aerating agents This group of products is perhaps the most difficult to define in terms of technical function.
Wetting and de-aerating agents often perform the same function: that of expelling air from the textile assembly contained in the dye bath. The use of penetrating agents is invariably associated with the dyeing of yarns with a high twist factor, where they enhance transport of the dye into the yarn assembly. In this respect it could be argued that they are in fact a levelling agent. All the products in this class are invariably powerful surfactants.
Commonly used commercial products are mainly readily bio-eliminable compounds such as alcohol polyglycol ethers and esters (also in blends with alkane sulphone), but poorly degradable products such as ethoxylated amines can also be encountered.
8.6.3 Dispersing agents Vat, disperse and sulphur dyes already have a high content of dispersing agents in their formulation, which allows the application of these colourants in the form of aqueous dispersions. Additional amounts of dispersants are usually added (also for other classes of dyes) in the subsequent steps of the dyeing process to maintain the stability of the dispersion throughout the dyeing (or printing) process.
Textiles Industry 495 Annexes Substances commonly used as dispersing agents are condensation products of naphthalene sulphonic acid with formaldehyde, lignosulphonates. Anionic and non-ionic surfactants (e.g.
ethoxylated alcohols, phosphated alcohols and naphthalene sulphonates) are also applied.
Environmental aspects of these substances are reported in Section 8.6.1, whereas more general information about surfactants can also be found in Section 8.1. Dispersing agents with improved bioeliminability are now available for some dye formulations (see Section 4.6.3).
8.6.4 Levelling agents
Levelling agents are used in batch dyeing processes to improve the uniform distribution of the dye in the fibre. They are probably the most important class of dyeing auxiliaries, as a grossly unlevel dyeing is of no commercial value and is difficult to correct. They are employed for different types of fibres, therefore the substances employed can be different. Nevertheless two main groups of levelling agents can be identified: products which have an affinity for the fibre and products which have an affinity for the dye. Products which have an affinity for the fibre compete with the dye for dye-sites on the fibre. In this way they reduce the rate of absorption of the dye and improve their migration. To the second group belong substances that form loosely bound complexes with the dye, reducing its mobility and in some cases neutralising the electrostatic attraction between the dye and the fibre.
The most common substances used as levelling agents are reported below, divided on the basis of the fibre to which they are applied and dyestuff.
Consideration of the environmental aspects associated with the use of these compounds is reported in Section 8.6.1 and more general information about surfactants can be found in Section 8.1.