«EUROPEAN COMMISSION Integrated Pollution Prevention and Control (IPPC) Reference Document on Best Available Techniques for the Textiles Industry July 2003 ...»
Spinning lubricants may be responsible for the emission not only of hard-to-biodegrade organic substances such as mineral oils, but also of hazardous compounds such as polyaromatic hydrocarbons, APEO and biocides.
The resulting pollution depends on the type and amount of lubricant applied to the fibre. In the wool spinning process, for example, a distinction must be made between fabric derived from carded yarn (woollen process) and fabric manufactured using combed yarn (worsted process).
There is a substantial difference, in fact, not only from a mechanical point of view, but also in the quantity of lubricants applied. This is approximately 5 % in the woollen process and is always less than 2 % in the worsted process (this consideration only holds for textiles, because in the woollen process for carpet yarn, the amount of lubricants ranges from 1 to 1.5 %).
As regards synthetic fibres (staple fibres), the amount of preparation agents applied at the yarn manufacturing stage is especially relevant in the case of elastomeric fibres where the final content of preparation agents (mainly silicone oils) can be in the order of 7 % of the weight of the fibre.
Additional information about the composition of spinning oils is given in Section 8.2. It is, however, difficult to analyse in detail the environmental aspects associated with the use of these substances because of the large variety of commercial products. Spinning mills usually buy lubricants from dealers, who buy chemical products from large petrochemical producers, and mix them according to the specific needs of each spinning mill. This makes even more difficult the identification of the chemical substances present on the yarn when this enters the finishing mill.
2.5 Cloth production Raw materials for cloth production are spun yarns and filament yarns. These raw materials can
then be converted into:
· woven textiles · knitted textiles · floor-coverings and non-woven fabrics.
The last item covers a number of different end-products which are typical of the carpet industry.
2.5.1 Woven textiles 22.214.171.124 Warping Before weaving, warp yarns are wound onto beams in a process called warping. During this operation there is no consumption of auxiliary agents that could have a negative influence on the emissions from the subsequent finishing processes.
In some cases (wool mills) the warp yarn is not sized, but treated with liquid paraffin.
126.96.36.199 Sizing In order to lubricate and protect the warp yarn during weaving, sizing agents (in the form of water solutions or water dispersions) are applied after warping. The main sizing agents can be
grouped into two classes:
Sizing agents based on native polysaccharides:
· starch · starch derivates such as carboximethyl starch or hydroxiethyl starch ether · cellulose derivates, especially carboximethyl cellulose (CMC) · galactomannans · protein derivates.
Fully synthetic polymers · polyvinyl alcohols (PVA) · polyacrylates · polyvinyl acetate · polyester The ratio of synthetic sizing agents to native sizing agents is variable (e.g. about 1:3 in Germany [179, UBA, 2001], 1:4 – 1:5 in Spain [293, Spain, 2002]).
It is important to bear in mind that:
· the type of sizing agent applied varies according to the fibres to be processed, the weaving technique adopted and the demands of any system used for recycling the sizing agents · sizing agent formulations are usually mixtures of the substances mentioned above.
With cotton, additional auxiliaries are present in the sizing mixtures. These are mainly [186,
· viscosity regulators: complex formation between borax and the hydroxyl groups of starch increases the viscosity of the paste, while urea reduces it. Important viscosity regulators include starch-degrading agents such as peroxodisulphates, peroxosulphates that act by oxidative cleavage of the macromolecules · sizing fats: are used to improve the weaving behaviour of the warp. Suitable materials include sulphated fats and oils and mixtures of fatty acid esters with non-ionic and anionic emulsifiers · antistatic agents (mainly based on polyglycol ethers) · wetting agents: fatty alcohols poly(glycol ethers) with a low degree of ethoxylation.
· de-foaming agents: their addition is often necessary when the sizing agents tend to produce foam (e.g. with PVA) or if wetting agents are added. Suitable products are based on paraffin oils, phosphoric esters, fatty acid esters or silicone oils · preservatives: for sizing liquors that are stored for long periods and contain degradable components such as starch and starch derivatives, fungicides and/or bacteriocides are added.
Typical preservatives include formaldehyde, phenol derivatives, heterocyclic compounds of the isothiazoline type.
Sizing agents used for synthetic fibres (e.g. polyacrylates, polyesters) do not contain these auxiliaries, except for the preservatives, which are always to prevent bacteria attack present when aqueous systems are used.
Sizing agents are introduced by the weaving firm, but have to be removed by the finisher (during the operation called desizing). This desizing process results in high waste water loads.
In the case of woven fabric, sizing agents can represent 30 – 70 % of the total COD load in waste water. The lower percentage is for finishing of woven fabric mainly consisting of flat filament yarns and the higher for staple fibres, especially for cotton and in case of native sizing agents. Therefore it is important to know the COD of these substances and their characteristics in terms of biodegradability and bioeliminability. Note that additives present in the formulations (e.g. the preservatives) also influence the aquatic toxicity and biodegradability of the resulting emissions (toxicity and biodegradability cannot be discriminated by using only COD measurements).
Section 8.3 gives more details about:
· characteristics of the chemical substances commonly used as sizing agents and specific COD and BOD5 values · typical amounts applied on the various fibres.
188.8.131.52 Weaving Weaving is the process by which yarns are assembled together on a loom and a woven fabric is obtained. The process only requires electricity. Lubricants and oils are used to lubricate the loom, but in particular cases they may contaminate the fabric.
2.5.2 Knitted textiles 184.108.40.206 Waxing The yarn specially made for the knitting industry is lubricated or waxed (generally with paraffin wax) to allow knitting at higher speed and protect the yarn from mechanical stresses. Waxing may be done while re-winding the yarn onto bobbins, in which case the process is normally called “preparation for knitting”.
Like weaving, knitting is a mechanical process and involves knotting yarn together with a series of needles. Mineral oils are widely used to lubricate the needles and other parts of the knitting machinery. The quantity of oils used depends on the technology of the machine and on the speed of the needles. The value ranges between 4 and 8 % of the weight of the fabric (when mineral oils are used the amount may rise to 10 %).
The oil and the wax that remain on the final fabric will be washed out during the finishing treatments. Their contribution to the total pollution load coming from finishing mills may be significant.
Additional information about knitting oils is reported in Section 8.2.5.
2.5.3 Textile floor-coverings Textile floor-covering is a collective noun for articles having a usable surface of textile material.
This particular class of cloths can be schematically described as composite substrates made up
of the following layers:
· a carrier layer, which is mainly made of polypropylene strips of fabric (75 %), PP or PES webs (16 % and 8 %, respectively) and less commonly of jute fabric (1 %) [18, VITO, 1998] · a pile yarn (or more generically a face fibre), which can equally well be made from filament or staple fibre yarns (mainly polypropylene, polyamide, polyester, wool, acrylic fibres).
· a pre-coating layer, which is typical of tufted carpet and whose function is to anchor the pile onto the carrier layer. This coating layer is made from synthetic rubber or an artificial dispersion based on carboxylated styrene-butadiene rubber latex.
· a coating layer, which is an additional layer applied to the bottom side of the carpet. It is possible to distinguish between foam coating, textile back coating and heavy coating methods. The purposes of this final layer are various. Mainly, it is intended to strengthen the attachement of the pile, improve dimensional stability of the carpet and provide the carpet with properties such as anti-slip, heat insulation, stepping elasticity, or even flame retardancy (carpet coating techniques are described in more detail under the finishing treatments in Section 2.11).
This is, however, a very general definition. In fact, the characteristics of the end-product and the applied manufacturing techniques can vary significantly. The principal methods of manufacturing carpet/floor-coverings are tufting, weaving and needling (other methods are knotting, bonding, knitting, etc.). The best way to describe these techniques is probably to describe the different types of carpets.
220.127.116.11 Tufted carpet
As the figure below shows, tufted carpets are made up of the following different components:
· the face yarn (pile), which can be made from either staple fibres (PA, PP, PES, PAC, wool and cotton) or synthetic filaments · the primary backing (carrier layer) · the pre-coating layer · the back-coating layer (which can consist of a textile back-coating or of a foam coating layer).
Figure 2.6: Cross-section of a tufted carpet[63, GuT/ ECA, 2000]
In the tufting process, pile yarns (face) are inserted by needles into a woven or non-woven carrier material (primary backing), made of polypropylene, polyester or jute, across the entire width of the substrate (up to 5m). During subsequent manufacturing processes the base of the pile yarn is anchored in the back of the carpet by means of an applied coating.
Figure 2.7 is a simplified representation of a tufting plant.
Figure 2.7: Simplified representation of a tufting plant [63, GuT/ ECA, 2000] Through variations of the tufting technique, different three-dimensional pile structure can be produced as well as patterned carpets (e.
g. level loop pile, multi-level loop pile, cut and loop, velvet and velour, saxony).
Figure 2.8 shows two examples of carpet styles.
Figure 2.8: A: Level loop pile; B: Cut and loop pile [63, GuT/ ECA, 2000] 2.
5.3.2 Needle-felt carpet Figure 2.9 shows the manufacturing process for needle felt carpets. The fibres lie criss-cross to each other and are compressed with needles. This compression is fixed with binding agents [18, VITO, 1998]. Needle-pile carpets can consist of one or several layers, with or without a carrier layer. They may be equipped with various types of backings (textile backing, foam, heavy coating). For lasting binding of the fibres, the needled substrate is subsequently subjected to further thermal or chemical treatment (chemical reinforcement).
Figure 2.9: Manufacture of needle-felt carpet [63, GuT/ ECA, 2000] Almost all fibres may be used for the production of needle felts; however, man-made fibres are mostly used (PP, PA, PES, PAC, wool, jute/sisal, viscose).
18.104.22.168 Woven carpet The manufacturing process for woven carpets is similar to the manufacturing of any other woven good. Since coarse types of yarns are used for woven carpets, the warp is normally not sized. A scheme of the production process for woven carpets is reported in Figure 2.10.
Figure 2.10: Representation of the production process for woven carpets [63, GuT/ ECA, 2000] As a rule, woven carpets are provided with a stabilising back-coating.
Only in rare cases is an additional heavy coating (e.g. PVC or bitumen) applied.
2.5.4 Non-woven textiles Textiles manufactured without an intermediate yarn-processing step are called non-wovens. The application of these textiles is quite broad ranging from backings for coatings, filters, geotextiles and other technical textiles to dishcloths, etc.
In the fabrication of non-woven textiles the only significant environmental impact is normally associated with the off-gases emitted during the thermal and chemical bonding steps. The volatile compounds mainly originate from fibre inherent monomers (especially caprolactame), monomers in the bonding polymers, etc.
40 Textiles Industry
Pretreatment processes should ensure:
· the removal of foreign materials from the fibres in order to improve their uniformity, hydrophilic characteristics and affinity for dyestuffs and finishing treatments · the improvement of the ability to absorb dyes uniformly (which is the case in mercerising) · the relaxation of tensions in synthetic fibres (without this relaxation of tension, unevenness and dimension instabilities can occur).
The position of pretreatment within the production scheme is closely related to the position of dyeing in the sequence. The point is that pretreatment comes immediately before dyeing (and printing).
Pretreatment processes and techniques depend:
· on the kind of fibre to be treated: for raw goods made of natural fibres such as cotton, wool, flax and silk the technical task is more difficult than for those made of synthetic and artificial fibres. Natural fibres in fact are accompanied by a higher amount of substances that can interfere with later processing. Man-made fibres, in turn, usually contain only preparation agents, water-soluble synthetic size and soil · on the form of the fibre (flock, yarn, woven or knitted fabrics) · on the amount of material to be treated (for example, continuous methods are more efficient, but are economically viable only for large production capacities).
Pretreatment operations are often carried out in the same type of equipment used for dyeing (in batch processing, in particular, the material is most often pretreated in the same machine in which it is subsequently dyed). For ease of reading of this part of the document, machines that are not specific to a given treatment are described in a separate annex (see Section 10).
2.6.1 Pretreatment of cotton & cellulose fibres 22.214.171.124 Principal manufacturing processes
Cotton pretreatment includes various wet operations, namely:
· singeing · desizing · scouring · mercerising (and caustification) · bleaching.
Some of these treatments are obligatory steps only for certain make-ups (e.g. desizing is carried out only on woven fabric).
Moreover some of these treatments are often combined together in one single step in order to respond to the need to reduce production time and space as much as possible. Nevertheless, for practical reasons they will be described as separate treatments, leaving the discussion about possible process sequences for specific make-ups to the following sections.
Singeing Singeing can be carried out both on yarns and woven fabrics, but it is more common on fabrics, especially on cotton, cotton/PES and cotton /PA substrates.
Protruding fibre ends at the fabric surface disturb the surface appearance and produce an effect known as "frosting" when dyed. It is therefore necessary to remove the surface fibres by passing the fabric through a gas flame. The fabric is passed over a row of gas flames and then immediately into a quench bath to extinguish the sparks and cool the fabric. The quench bath
often contains a desizing solution, in which case the final step in singeing becomes a combined singeing and desizing operation.
Before singeing, the fabric is combed under aspiration to eliminate remaining dust and fibres.