«EUROPEAN COMMISSION Integrated Pollution Prevention and Control (IPPC) Reference Document on Best Available Techniques for the Textiles Industry July 2003 ...»
· chlorinated aromatic compounds (mono-chlorobenzene, trichlorobenzenes etc.) · o-phenylphenol · biphenyl and other aromatic hydrocarbons (trimethyl benzene, 1-methyl naphtalene etc.) · phthalates (diethylhexylphthalate, dibutylphthalate, dimethylphthalate) Human and aquatic toxicity, high volatility and high odour intensity are the main concerns associated with the use of the above-mentioned substances (see also Section 18.104.22.168). Not only do water and air become contaminated by the emissions, but it is increasingly suspected that consumer health problems can be caused by remobilisation of halogenated carriers (e.g. 1,2,4trichlorobenzene) in the treated textiles [18, VITO, 1998].
The application of HT-dyeing processes avoids the use of carriers. This technique is currently widely applied when dyeing pure polyester and wool-free PES blends.
However, due to the sensitivity of the wool substrate to high temperatures, it is still necessary to use carriers when dyeing polyester blends and, in particular, polyester/wool blends. In these cases, hazardous carriers can be replaced by chlorine-free substances with improved
toxicological and environmental characteristics. New carriers are based on:
· benzylbenzoate · and N-alkylphthalimide.
Main achieved environmental benefits In the case of HT-dyeing processes, waste water and off-gas are carrier-free. The quantity of environmentally problematic substances is reduced.
PES/wool blends may be dyed with benzylbenzoate and N-alkylphthalimide based carriers;
benzylbenzoate is a readily biodegradable substance (the degree of mineralisation for the benzylbenzoate is 79 % [179, UBA, 2001]), while N-alkylphthalimide is bioeliminable (the (BOD30/COD)x100 is 50 – 100 %) with a fish toxicity between 10 and 100 mg/l. Moreover, because of their low volatility, odour nuisance (especially in the workplace) is negligible.
Both substances show great affinity for the aqueous medium, which makes them easy to prepare (without need for emulsifying and dispersing agents) and easy to remove at the end of the dyeing process (with lower water consumption).
Operational data In the case of benzylbenzoate carriers, applied concentrations range between 2.0 and 5.0 g/l (dyeing at boiling temperature; average liquor ratio) [179, UBA, 2001].
N-alkylphthalimide carriers are applied in the range of 2 % (liquor ratio 1:10) to 1 % (liquor ratio 1:20) for dyeing of light shades. For dark shades the amount of carrier varies between 6 % (L.R. 1:10) and 3 % (L.R. 1:20) [179, UBA, 2001].
Cross-media effects When dyeing in HT-conditions a higher quantity of oligomers tends to migrate to the surface of the fibre.
Dyeing at high temperature also requires higher energy consumption. The balance between the different effects involved (the effects of the hazardous carriers on the environment and the effects of higher energy consumption) is, however, still largely in favour of the application of this technique [179, UBA, 2001].
Benzylbenzoate and N-alkylphthalimide based carriers are less effective than conventional carriers. They have a reduced penetrating and swelling effect. As a result, a longer residence time and higher amounts (about three times as much) are needed to reach the same effect.
Applicability Carrier-free dyeing at high temperature can be applied to all PES qualities, provided that HTdyeing equipment is used. Application to PES blends depends on the sensitivity to high temperature of the fibres in the blend, being particularly critical for PES/WO blends.
Dyeing with optimised carriers is applicable to all PES blends.
Economics Optimised carriers described in this section cost approximately the same as common carriers [179, UBA, 2001].
Driving force for implementation Limit values enforced by environmental legislation on workplace safety have been one of the main driving forces in the process of elimination/substitution of halogenated and other hazardous carriers.
In 1994, OSPAR recommended cessation of use of organohalogen carriers ([51, OSPAR, 1994]). Moreover, a number of eco-label schemes for textile products include, among their requirements, prescriptions regarding dye carriers. The European Ecolabel, for example, requires that halogenated carriers should not be used. GuT-label requirements (for carpets) state that dye carriers must not be used in manufacture or be detectable in the product.
Reference plants Carrier-free HT-dyeing processes and the above-mentioned optimised carriers are applied across Europe and the world.
Reference literature [179, UBA, 2001], [18, VITO, 1998], [61, L. Bettens, 1999], [52, European Commission, 1999], [59, L. Bettens, 2000].
4.6.2 Use of non-carrier dyeable PES fibres Description The polymer industry has long shown an interest in the entire series of aromatic polyester polymers made from the homologous series of n-methylene glycols. Among these polymers, PET based polyester fibres are the most important ones in the textile industry. They show excellent mechanical properties and resistance to heat, but their high degree of crystallinity makes them non-dyeable below 100 °C unless dye carriers are applied.
The environmental concerns raised by the use of these hazardous substances and the proposed techniques to counteract this problem are described in the previous section. An additional option to consider is the use of non-carrier dyeable PES-fibres, such as polytrimethylene terephthalate (PTT) polyester fibres (see also Section 22.214.171.124).
Polytrimethylene terephthalate remained an obscure polymer for a long time not because it lacked good physical and chemical properties and potential applications, but because the high cost of synthesis of the starting monomer (1,3-propanediol) has prevented the resulting fibre from being brought to market. PTT polymers are of renewed interest today thanks to Shell’s recent breakthrough in a lower-cost synthesis process.
Fibres made from polytrimethylene terephthalate can be dyed or printed using standard disperse dyestuffs without the need for any special processing steps or carriers to accelerate the dyeing process.
Main achieved environmental benefits The following environmental benefits are achieved in the dyeing process compared to standard
polyester fibres (PET type):
· emissions of carriers in the workplace and in the environment are completely avoided · a lower amount of energy is consumed compared with PET dyed under high-pressure-high – temperature (HT) conditions (PTT is dyed atmospherically at 100 °C with excellent dye exhaustion and colourfastness).
Operational data Disperse dyes are the recommended dye class for PTT, especially for dark shades. Basic dyes may also be used, but only for light shades.
Dyeing equilibrium for a medium shade is obtained in 30 to 60 minutes, depending on the dyes selected. To achieve a good dye utilisation without compromising productivity, 30 to 45 minutes holding time at 100 °C is recommended [178, VITO, 2001].
The recommended dyeing conditions are pH 7 and 100 °C (PTT at 100 °C was dyed with the same or slightly deeper shade than PET at 130 °C) [178, VITO, 2001].
Cross-media effects None believed likely.
Applicability PTT fibres are not only easy to process, but also easy to manufacture. They can be extruded on all commonly used machinery with minor modifications.
Thanks to their performance, they may have extensive applications in carpeting, textiles and apparel, engineering thermoplastics, non-wovens, films and mono-filaments. According to manufacturers, fibres made from CORTERRA® Polymers demonstrated performance equal to and, in many cases, better than other materials such as polypropylene, nylon and PET. However, due to the difference in structure, their physical and mechanical properties obviously differ from standard PES fibres (PET type). As a result, they do not cover exactly the same product market and they cannot be regarded to as “substitutes” for PET fibres.
Economics The new synthesis route pursued by Shell for the manufacturing of the poly(trimethylene terephthalate) has made PTT fibres competitive on the market with standard PES fibres.
Low dyeing temperature and the broad dyeing pH allowance help to decrease the cost of dyeing.
Moreover, the environmental costs associated with the presence of carriers are avoided.
Driving force for implementation Limit values and restrictions on dye carriers that are currently required by environmental legislation and the leading voluntary Eco-label schemes are the main driving forces for implementation.
In the carpet industry, the possibility of dyeing PES carpets in piece at atmospheric conditions without the use of carriers is particularly convenient (due to the high cost of pressurised machinery for dyeing in piece under HT conditions).
Reference plants Shell Chemical Company announced the commercial introduction of PTT in 1996 under the trade name of CORTERRA® Polymers.
Reference literature [178, VITO, 2001] 4.6.3 Dispersing agents with higher bioeliminability in dye formulations Description Dispersing agents (see Section 8.6.3) are present in disperse, vat and sulphur dye formulations (and they are further added in the subsequent steps) to ensure uniform dispersion throughout the dyeing and printing processes. Disperse dyes in powder or granulated form contain 40 – 60 % (in some cases up to 70 %) of dispersing agents, whereas liquid formulations contain 10 – 30 % (see also Table 2.15, Section 2.7.8). Usually, in the case of dark shades, no additional dosage of dispersing agents is required, whereas this may be necessary for pale shades. The quantities in vat and sulphur dyestuffs may be similar but precise information is not available.
The dispersing agents do not have affinity for the fibres and they are therefore found in the final effluent. Due to the significant amounts applied and to their often-low biodegradability/ bioeliminability they contribute to most of the recalcitrant organic load originating from dyeing and printing processes.
In particular, the lignosulphonates and the condensation products of naphthalene sulphonic acid with formaldehyde, which are widely applied as dispersing agents, show COD levels as high as 1200 mg/g (lignosulphonates) and 650 mg/g (naphthalenesulphonic acid condensation products). Elimination in biological waste water treatment is insufficient for both products.
Thus, they contribute to residual (recalcitrant) COD in treated waste water [179, UBA, 2001].
Improved dispersing agents are now available that can substitute conventional dispersing agents
in the dye formulations up to a maximum of 70 %. Two options are possible [179, UBA, 2001]:
Option A (to date only applicable to liquid formulations of disperse dyestuffs): it consists in the partial substitution of conventional dispersing agents with optimised products based on fatty acid esters. A mixture of dispersing agents is used, where fatty acid esters represent the main components. The dispersing effect of the product is improved compared to conventional dispersing agents, which means that the amount of dispersing agent in the formulation can be significantly reduced. The tinctorial strength of the dye is also improved (from 100 % to 200 %) due to the correspondingly higher concentration of the dyestuff in the formulation (“Grütze, 2000”).
Option B (applicable to common dispersing agents in powder and granulate formulations): it consists in applying dispersing agents based on mixtures of the sodium salts of aromatic sulphonic acids (“Kilburg, 1997”). It is reported that these products are modified compounds of the common condensation products of naphthalene sulphonic acid with formaldehyde. This chemical modification leads to higher bioelimination rates because of increased adsorption rate to the biomass. However, they remain non-biodegradable compounds.
Main achieved environmental benefits
Option A) According to the Modified Zahn-Wellens-Test (OECD 302 B), bioelimination rates are between 90 and 93 %. A comparison between conventional disperse dyes and optimised formulations (average values considering the whole dye range) is given in Figure 4.13.
Differences in tinctorial strength are taken into account.
[%] Figure 4.13: Comparison between the composition of conventional and new liquid formulations of liquid disperse dyes, before and after biological treatment (the (%) in the y axis indicates the percentage of dispersing agents related to the overall formulation) [179, UBA, 2001] Option B) Figure 4.14 compares the bioelimination rates of conventional condensation products of naphthalene sulphonic acid with formaldehyde and the modified optimised ones. The degree of bioelimination of the modified dispersing agent is about 70 % (test method according to OECD 302 B) compared to 20 – 30 % for the conventional one.
Bioelimination in [% COD reduction], 100 % = 400 mg COD/l Figure 4.14: Comparison of the bioelimination rates of conventional and modified dispersing agents, both based on condensation products of naphthalenesulphonic acid with formaldehyde [179, UBA, 2001] Operational data The application of environmentally optimised dispersing agents proposed does not imply changes in the process compared to the application of conventional products.
Cross-media effects None believed likely.
Applicability Option A) These dispersing agents can only be used for liquid formulations of disperse dyes;
there is no restriction regarding application, but the dyestuff palette is currently limited.
Option B) These dispersing agents can be used both for disperse and vat dyes (solid and liquid formulations).
Economics Dye formulations containing dispersing agents with improved bioeliminability are in general more expensive than conventional ones [179, UBA, 2001].
Driving force for implementation The improvement of the environmental performance is the main driving force encouraging finishing mills to select dyes containing dispersing agents with a better degree of bioelimination.
Reference plants Many plants in Europe.
Reference literature [179, UBA, 2001] with reference to “BASF Aktiengesellschaft, Technical Information TI/T 7063 d, 1998”.
4.6.4 One-step continuous vat dyeing in pastel to pale shades Description The conventional pad steam process with vat dyes (see Section 2.7.3) includes the following
· padding of dyestuff pigments · intermediate drying · padding of chemicals/auxiliaries (reducing agents) · steaming · oxidising · washing (several washing and rinsing steps).
In some cases the process can be carried out without steaming and subsequent washing,
according to the following simplified sequence (similar to the dyeing process with pigments):
· padding of dyestuffs and chemicals/auxiliaries in one step · drying · fixation.
Special selected vat dyes with a low tendency to migration need to be used. Moreover, auxiliaries based on polyglycols and acrylic polymers are necessary, which improve pad liquor stability and provide a high fastness level.
Main achieved environmental benefits A number of steps, in particular the washing operations, are avoided. As a result, only the residual padding liquors have to be disposed of at the end of the process and water consumption is minimised to approximately 0.5 l/kg of textile [179, UBA, 2001].
Savings in chemicals and energy are also obtained.
A typical recipe for the padding liquor includes [179, UBA, 2001]:
· binder: 30 - 40 g/l · sodium sulphate: 5 - 10 g/l · antimigrant: 10 - 20 g/l · dyestuff: up to 2.5 g/kg Among typical process parameters, the pick-up should be as low as possible (50 – 65 %) and the liquor temperature should be kept below 35 °C. Intermediate drying is carried out at 100 - 140 °C, while thermofixation conditions are typically 30 s at 170 °C for cellulose and 30 s at 190 °C for polyester/cellulose blends [179, UBA, 2001].
Cross-media effects None believed likely.