«EUROPEAN COMMISSION Integrated Pollution Prevention and Control (IPPC) Reference Document on Best Available Techniques for the Textiles Industry July 2003 ...»
Singeing has no effect on the effluents because only cooling water is necessary. During singeing relatively strong odours and emissions of dust and organic compounds are observed [209, Germany, 2001]. Odorous substances can be destroyed using catalytic oxidation techniques (see Section 4.10.9).
Desizing Desizing is used for removing from woven fabric sizing compounds previously applied to warp (see Section 188.8.131.52) and is usually the first wet finishing operation performed on woven fabric.
Desizing techniques are different depending on the kind of sizing agent to be removed.
Currently applied techniques can be categorised as follows:
1. techniques for the removal of starch-based sizing agents (water-insoluble sizes)
2. techniques for the removal of water-soluble sizes
3. techniques for the removal of water soluble and insoluble sizes.
1. Techniques for the removal of starch-based sizes
Starch-based sizes are difficult to remove and require either catalytic action of an enzyme (catalytic degradation) or other chemical treatment in order to be converted into a washable form. This chemical degradation is mainly achieved by either enzymatic or oxidative desizing.
Enzymatic desizing is the most widely used method for the removal of starch, amylases being particularly suitable. The advantage in the use of enzymes is that starches are decomposed without damaging cellulose fibre.
In order to reduce the number of steps in the pretreatment process, it is common practice to combine desizing with cold bleaching in a single step. In this case the process is also called "oxidative desizing". The fabric is impregnated in a bath containing hydrogen peroxide and caustic soda, together with hydrogen peroxide stabilisers and complexing agents. Persulphate is also usually added to the solution.
Due to the action of NaOH, this treatment, beside a desizing/bleaching effect, also serves as a pre-scouring treatment. Furthermore, oxidative desizing is particularly useful when the textile contains enzyme poisons (fungicides) or when sizes are present that are difficult to degrade.
However, because starch scarcely differs from cellulose in the cross-linking of the cellulose rings, chemical oxidation, if not well controlled, can damage the fibre.
Desizing is usually carried out in pad-batch, but discontinuous (e.g. jigger) and continuous (padsteam) processes can also be applied. In the case of enzymatic desizing, pad-steam is applied only for big lots and with enzymes that are stable under steaming conditions. After the reaction time, the fabric is thoroughly washed in hot water (95 ºC).
2. Techniques for the removal of water soluble sizes The removal of water-soluble sizes such as PVA, CMC and polyacrylates, theoretically only requires washing with hot water and sodium carbonate. However, the washing efficiency can be
· adding suitable auxiliaries (wetting agents) to the desizing liquor (with some restrictions in case of size recovery) · allowing adequate time for immersion in the desizing liquor (this ensures maximum liquor pick-up and adequate time for the size to swell) · washing thoroughly with hot water in order to remove the solubilised size.
In this case the process is carried out in normal washing machines. Continuous washers are often used, but sometimes the treatment time may be too short to allow complete desizing. Padbatch and pad-steam or discontinuous processes for prolonging the residence time are therefore also in use.
3. Techniques for the removal of water soluble and insoluble sizes
The “oxidative desizing” technique mentioned above is applicable not only for water insoluble sizing agents, but also for water soluble ones. This technique is particularly useful for textile finishers dealing with many different types of fabrics and therefore sizing agents (see also Section 4.5.2).
Mercerising is carried out in order to improve tensile strength, dimensional stability and lustre of cotton. Moreover an improvement in dye uptake is obtained (a reduction of 30 – 50 % of dyestuff consumption can be achieved thanks to the increased level of exhaustion).
Mercerising can be carried out on yarn in hanks, woven and knitted fabric through one of the
following different treatments:
· mercerising with tension · caustification (without tension) · ammonia mercerising.
Mercerising with tension Caustic soda mercerising is the most commonly applied mercerising technique (it is not applied to flax). Cotton is treated under tension in a solution of concentrated caustic soda (270 - 300g NaOH/l, which means also 170 – 350 g NaOH/kg) for approximately 40 - 50 seconds.
During mercerising the temperature is adjusted at low values (5 - 18 ºC) when the lustre is the priority and at slightly higher levels when the improvement of the other characteristics is preferred. Because the reaction between caustic soda and cellulose is exothermic, cooling systems are applied to keep down the temperature of the bath.
Beside the conventional cold treatment, a hot mercerising process is also now increasingly applied (for hanks and fabric). The material is soaked in a solution of caustic soda close to boiling point. After hot stretching, the fabric is cooled down to ambient temperature and washed under tension.
In order to ensure a homogeneous penetration of the liquor, especially when operating at ambient temperature, wetting agents are employed. Sulphonates mixed with non-ionic surfactants and phosphoric esters are the most commonly used.
Caustification In the caustification process, the material is treated at 20 – 30 ºC with caustic soda at lower concentration (145 – 190 g/l) without applying tension. The material is allowed to shrink, thus improving the dye absorption.
Ammonia mercerising Cotton yarn and fabric can be treated with anhydrous liquid ammonia as an alternative to caustic soda. Effects similar to mercerising are obtained, although the lustre grade is inferior to caustic soda mercerising. Traces of ammonia have to be removed, preferably with dry heat treatment followed by steaming.
This method is not widely used. Only a few plants are reported to use ammonia mercerising in Europe.
Two examples of mercerising machines respectively for woven fabric (Figure 2.11) and for knitted fabric in tubular form (Figure 2.12) are reported below.
Figure 2.11: Example of Mercerising equipment for woven fabric [69, Corbani, 1994] Figure 2.
12: Example of Mercerising equipment for knitted fabric in tubular form [318, Sperotto Rimar, 2002] Scouring Scouring (also known as boiling-off or kier boiling) is aimed at the extraction of impurities
present on the raw fibre or picked up at a later stage such as:
· pectins · fat and waxes · proteins · inorganic substances, such as alkali metal salts, calcium and magnesium phosphates, aluminium and iron oxides · sizes (when scouring is carried out on woven fabric before desizing) · residual sizes and sizing degradation products (when scouring is carried out on woven fabric after desizing).
Scouring can be carried out as a separate step of the process or in combination with other treatments (usually bleaching or desizing) on all kind of substrates: woven fabric (sized or desized), knitted fabric and yarn.
For yarn and knitted fabric, scouring is usually a batch process which is carried out in the same equipment that will subsequently be used for dyeing (mainly autoclaves or hank dyeing machines for yarn and overflows, jets, etc. for knitted fabric). Woven fabric is scoured in continuous mode using the pad-steam process.
The action of scouring is performed by the alkali (sodium hydroxide or sodium carbonate)
together with auxiliaries that include:
· non-ionic (alcohol ethoxylates, alkyl phenol ethoxylates) and anionic (alkyl sulphonates, phosphates, carboxylates) surfactants · NTA, EDTA, DTPA, gluconic acid, phosphonic acids as complexing agents, which are used to remove metal ions (and, in particular, iron oxides, which catalyse the degradation reaction of cellulose when bleaching with hydrogen peroxide) · polyacrylates and phosphonates as special surfactant-free dispersing agents · sulphite and hydrosulphite as reducing agents (to avoid the risk of formation of oxycellulose when bleaching with hydrogen peroxide).
Bleaching After scouring, cotton becomes more hydrophilic. However, the original colour stays unchanged due to coloured matter that cannot be completely removed by washing and alkaline extraction.
When the material has to be dyed in dark colours it can be directly dyed without need of bleaching. On the contrary, bleaching is an obligatory step when the fibre has to be dyed in pastel colours or when it will need to be subsequently printed. In some cases, even with dark colours a pre-bleaching step may be needed, but this is not a full bleaching treatment.
Bleaching can be performed on all kinds of make-ups (yarn, woven and knitted fabric).
The most frequently used for cellulosic fibres are oxidative bleaches, namely:
· hydrogen peroxide (H2O2) · sodium hypochlorite (NaClO) · sodium chlorite (NaClO2).
Apart from these, peracetic acid is also applicable [7, UBA, 1994]. Also optical brightening agents are commonly used to obtain a whitening effect.
Bleaching with hydrogen peroxide Bleaching can be carried out as a single treatment or in combination with other treatments (e.g.
bleaching/scouring or bleaching/scouring/desizing can be carried out as single operations).
The textile is treated in a solution containing hydrogen peroxide, caustic soda and hydrogen peroxide stabilisers at pH 10.5 – 12 (the optimal pH for scavenging of the OH* radical is 11.2, as described in Section 4.5.6). Recent investigations ([210, L. Bettens, 1995]) have shown that the bleaching agent of peroxide is not the anion HOO-, but the dioxide radical anion OO*- (also known as superoxide). In competition with the formation of the bleaching agent, the OH* radical is formed, which is responsible for attacking and depolymerising the cellulose fibre. The formation of the OH* radical is catalysed by metals such as iron, manganese and copper. The prevention of catalytic damage as a consequence of uncontrolled formation of OH* is mostly taken care of by using complex formers that inactivate the catalyst (stabilisers). Sodium silicate together with Mg salts (MgCl2 or MgSO4) and sequestering/complexing agents (EDTA, DTPA, NTA, gluconates, phosphonates and polyacrylates) are commonly used as stabilisers (see also
Section 8.5 for more information about stabilisers and Sections 4.
3.4 and 4.5.6 for alternative techniques).
Other auxiliaries used in hydrogen peroxide bleaching are surfactants with emulsifying, dispersing and wetting properties. Employed surfactants are usually mixtures of anionic compounds (alkyl sulphonates and alkyl aryl sulphonates) with non-ionic compounds such as alkylphenol ethoxylates or the biologically degradable fatty alcohol ethoxylates [186, Ullmann's, 2000].
Operating temperatures can vary over a wide range from ambient to high temperature.
Nonetheless, a good bleaching action occurs when operating at around 60 – 90 ºC.
Bleaching with hydrogen peroxide in neutral conditions (pH range of 6.5 – 8) is also possible in some cases (e.g. when treating cotton in blends with alkali-sensitive fibres such as wool). At these pH conditions activators are required to give bleaching activity. Note that below pH 6.5 H2O2 decomposes into H2O and O2 by HOO*- / O2* disproportionation. Under these conditions hydrogen peroxide is wasted (production of inactive O2 gas).
A wide range of bleaching processes can be used, including cold pad-batch, bleaching under steaming conditions and bleaching processes in long bath.
Because the bleaching agent of peroxide is anionic in nature (hydrophilic behaviour), it is not possible with this bleaching method to destroy selectively the coloured hydrophobic material present on natural fibres without attacking the polymer itself.
Bleaching with sodium hypochlorite The high reactivity of this bleaching agent imposes softer operative conditions than hydrogen peroxide (pH 9 - 11 and temperatures not above 30 ºC). Otherwise there is a risk of damage to the cellulose fibre.
The bleaching stage is followed by an anti-chlorine treatment in order to eliminate completely the hypochlorite and decompose the chloroamines generated during bleaching.
Bleaching with sodium hypochlorite can be carried out in batch (e.g. overflow, jet, jigger, winch beck), semi-continuous (pad-batch) or continuous mode. A two-stage process is also in use in which hypochlorite and hydrogen peroxide are used.
The use of hypochlorite as bleaching agent is in decline for ecological reasons. It is still applied for yarn and knitted fabric when a high degree of whiteness is required, for articles that remain white (e.g. linen), or require a white background or in processes where the ground-dye is discharged with a bleach treatment (see also Section 4.5.5 for alternative techniques).
Bleaching with sodium chlorite/ chlorate Chlorite/chlorate bleaching, although in decline, is still applied for synthetic fibres, cotton, flax and other cellulosic fibres, often in combination with hydrogen peroxide.
The bleaching agent is the chlorine dioxide gas (ClO2), which follows a completely different working mechanism compared to hydrogen peroxide. Whereas the superoxide radical ion in hydrogen peroxide is hydrophilic and therefore works preferentially in the hydrophilic region of the fibre (attack of the fibre polymer), ClO2 absorbs preferentially on the hydrophobic associated material, such as the woody part of bast fibres. For this reason it is an excellent bleaching agent (ensuring a high degree of whiteness and no risk of damage of the fibre) especially for synthetic fibres and for bast fibres such as flax where, compared to cotton, there is a higher percentage of hydrophobic impurities.
46 Textiles Industry
Chapter 2Because chlorine dioxide is unstable as a gas and can only be stored as a solution of approximately 1 % in water, it must be generated on-site as an aqueous solution. There are two ClO2 precursor chemicals in present general industrial use, namely sodium chlorite and sodium chlorate. Although sodium chlorate is considerably less expensive than sodium chlorite, it is more difficult and expensive to convert to ClO2, which explains why it is less commonly used.
Both sodium chlorite and sodium chlorate are used in strong acid conditions (pH 3.5 – 4 by formic or acetic acid). Chlorine dioxide solutions have a great corrosive action on construction materials including stainless steel. Sodium nitrate is used as a corrosion inhibitor to protect the stainless steel parts of equipment. It is also necessary to select detergent/wetting agents that can resist acid conditions. On the other hand, sequestering agents are not necessary because the oxalic acid used for acidification also serves for sequestering metals. The order of introduction of the different auxiliaries has to be controlled to avoid direct contact between the concentrated sodium chlorite/ chlorate solution and acids.
The textile material is bleached by padding or in long bath processes. The temperature is normally kept at 95 ºC, but cold procedures have also been developed to diminish toxicity and corrosion problems, using formaldehyde as an activator for sodium chlorite.
The advantages of chlorine dioxide bleaching are the high degree of whiteness and the fact that there is no risk of damage to the fibre. The main disadvantages are the high stresses to which the equipment is subjected and the chlorine residues that may be left on the fibre, depending on the way chlorite (or chlorate) is produced and activated. Recent technologies using hydrogen peroxide as the reducing agent of sodium chlorate are now available to produce ClO2 without generation of AOX (see also Section 4.5.5).
Bleaching with peracetic acid