«EUROPEAN COMMISSION Integrated Pollution Prevention and Control (IPPC) Reference Document on Best Available Techniques for the Textiles Industry July 2003 ...»
Flame retardant (FR) agents function by different mechanisms depending on their chemical characteristics. The most commonly used FR agents in the textile sector belong to the following
· inorganic compounds · halogenated organic compounds · organo-phosphorus compounds.
Inorganic FR agents
Inorganic FR agents, used for cellulosic fibres, are water-soluble salts such as diammonium phosphate, aluminum sulphate, ammonium sulphate, etc. They are applied from aqueous solution by padding or spraying followed by drying. They are non-durable retardants, which means that they render the product flame retardant until it is laudered or otherwise exposed to water.
Other types of inorganic FR agents are used in the wool carpet sector. Although wool may be generally regarded as resistant to burning, the introduction of stringent flammability standards for floorcoverings fitted in aircraft and public service buildings necessitates the use of FR agents in some specific cases. Zirconium and titanium salts have been developed to meet the needs of this specialised market. Zirconium salts, commonly referred to as "Zirpro treatments", are the most widely used (potassium hexafluorozirconate). They do not give rise to significant water pollution. However, emissions of zirconium- and fluorine-containing compounds along with fairly high water consumption levels (four rinsing-baths are needed with the conventional IWS procedure) should be taken into account [281, Belgium, 2002].
502 Textiles Industry Annexes Aluminum hydroxide (Al2O3·3H2O) is another flame-retardant widely used in the carpet sector.
It is commonly added to the foam coating of the carpet, partially replacing CaCO3 (inactive filler). Aluminum hydroxide starts to break down at 180 to 200°C, the conversion to aluminium oxide taking place in an endothermic reaction. Aluminum hydroxide treatments do not pose significant environmental concerns.
Halogenated FR agents
Halogenated flame-retardants react in the gas phase by free-radical inhibition. The hydrogen and hydroxyl free radicals formed during the combustion process are high in energy and give rise to highly exothermic chain radical reactions (flame propagation). Halogenated flameretardants are capable of interrupting this radical reaction. The halogen deactivates the free
radical in the vapour phase according to the reaction (1):
(1) HX + OH* = H2O + X* (the X* radical formed is very low in energy) The effectiveness of halogen-containing flame-retardants increases in the order FClBrI.
However, only brominated and chlorinated compounds are used in practice. Fluorine and iodine based flame retardants are not used because neither of them interfere with the combustion process at the right point (the bond between the halogen atom and carbon is too strong for fluorine, and too loose for iodine).
Brominated compounds are the most effective ones. Bromine can be bound aliphatically or aromatically; the aromatic derivatives are widely used because of their high thermal stability.
Chlorinated flame-retardants include chlorinated aliphatic and cycloaliphatic compounds.
Chlorinated flame-retardants are less expensive than the brominated homologues, but higher amounts of active substance are required in order to achieve an equivalent performance.
Chlorinated compounds are thermally less stable and more corrosive to the equipment compared to the brominated forms.
Compounds in which antimony trioxide (Sb2O3) is used together with halogens represent another group of halogen-containing FR. Antimony trioxide is almost totally ineffective if used on its own. However, it shows a good synergistic effect with halogens, particularly chlorine and bromine. Antimony trioxide acts as a radical interceptor and with HBr forms a dense white smoke (SbBr3) that snuffs the flame by excluding oxygen from the front of the flame [303, Ullmann's, 2001]. Decabromodiphenyl ether, hexabromocyclodecane and chloroparaffins are typically used as synergistic agents.
Halogenated flame-retardants have come under intense environmental scrutiny in recent years.
Their properties and their effects on the environment vary depending on the different type of chemicals used.
Polybrominated flame-retardants include the following compounds:
· polybrominated diphenyl ethers (PBDE, sometimes also referred to as PBBE) § pentabromodiphenyl ether (penta-BDE) § octabromodiphenyl ether (octa-BDE) § decabromodiphenyl ether (deca-BDE) · polybromo biphenyls (PBB) § decabromobiphenyl · tetrabromobisphenol A (TBBA) Polybrominated FR used for textiles applications are almost mainly diphenyl ethers.
Commercially available, technical grade PBDE, are mixtures and contain molecules with different numbers of bromine atoms. For example, technical grade octabromodiphenyl ether contains penta-BDE in low concentrations and hepta-BDE.
Textiles Industry 503 Annexes Penta-BDE is a persistent substance liable to biocumulate. The risk assessment, which has been carried out under Council Regulation (EEC) 793/93 on the evaluation and control of the risks of existing substances, identified a need for specific measures for reducing risks of penta-BDE to the environment.
As a consequence of this risk assessment there is already EU agreement for a ban on pentaBDE, which is confirmed by the inclusion of this chemical under the “Priority Hazardous Substances” targeted for priority regulatory action under the Water Framework Directive 2000/60/EC.
Penta-BDE is not reported as being used in the textiles sector. There are suspicions that decaBDE, the major PBDE for textile applications, and octa-BDE could break down to penta-BDE and tetra-BDE after release into the environment. This theory is the subject of the EU risk assessment and OSPAR Working Groups. A ban covering deca-BDE and octa-BDE is being considered following the conclusions of the official EU risk assessments for these compounds.
Deca-BDE should be prohibited from 1 January [299, Environment Daily 1054, 2001]. At Member States level, countries such as Sweden, the Netherlands and Norway are already taking actions to implement wide-ranging marketing restrictions covering the octa and deca forms of BDE, as an application of the precautionary principle.
As for chlorinated flame-retardants, short chain- (SCCP C10-13) and medium chain chlorinated paraffins (MCCP C14-17) have been the object of a risk assessment under the Council Regulation 793/93/EEC. SCCP and MCCP are acutely toxic for aquatic life. For SCCP longterm toxicity is observed in algae, fish and mussels. Medium-chain chlorinated paraffins are toxic to Daphnia, whereas no toxicity has been observed in the available experiments with fish, other invertebrates or algae. For both classes of compounds the hormonal effects seen in animals are considered unlikely to be relevant to humans [301, CIA, 2002]. No studies have been carried out on long-chain chlorinated paraffins.
Short-chain chloroparaffins (C10-13) have been identified as “Priority Hazardous Substances” targeted for priority regulatory action under the Water Framework Directive 2000/60/EC.
Moreover, both SCCP and MCCP are included on the List of Substances for Priority Action set under the OSPAR Convention.
Discharges of halogenated FR into waste water from textile finishing operations may come from excess liquor dumps, end-of-run bath drops and draining of washing water.
Deca-BDE is poorly water-soluble and should be largely retained by the sludge in the waste water treatment system. Chlorinated paraffins are also potentially bioeliminable by adsorption to the sludge (93 % removal from water during waste water treatment has been reported) [301, CIA, 2002]. However, since the amount/load of active substance applied on the fabric is typically in the order of 20 – 30 % w/w, the amount of FR not retained by the sludge and therefore potentially released into the environment may be significant. Process design and operation should avoid the discharge of concentrated liquors to waste water, minimise losses to the effluent, and ensure that adsorption to the sludge is effective in the waste water treatment plant.
Furthermore, special care should be taken for the disposal of the sludge and solid waste containing these halogenated compounds. All halogenated FR (less for aliphatic derivatives), are involved in the formation of dioxins and furans when submitted to high temperature treatments. Dioxins and furans can be formed in small amounts during the synthesis of these compounds and as a side reaction when they are subjected to combustion/ burnt for disposal [302, VITO, 2002]. Incineration should therefore only be carried out in properly constituted incinerators, running at consistently optimal conditions.
For the widely used antimony-organo-halogen FR systems, in addition to the considerations reported above for brominated and chlorinated compounds, dust emissions of Sb2O3
(carcinogenic) from dried pastes and mechanical treatment (cutting, etc.) on finished fabrics also need to be taken into account.
Phosphor-organic FR agents Phosphorus-based flame retardants can be active in the vapour phase or in the condensed phase.
Phosphine oxides and phosphate esters are thought to act in the vapour phase through the formation of PO* radicals, which terminates the highly active flame propagating radicals (OH* and H*). The condensed phase mechanism arises as a consequence of the thermal generation of phosphoric acids from the flame retardant, e.g. phosphoric acid or polyphosphoric acid. These acids act as dehydrating agents on the polymer (they decompose to form water vapour and phosphorus oxides which then react with the polymer matrix and dehydrate it, reforming phosphoric acids). The fire retardancy effect is produced via the alteration of the thermal degradation of the polymer and the formation of a very high melting point char at the interface of the polymer and the heat source [303, Ullmann's, 2001].
Organo-phosphorus compounds used in textile applications, particularly for cotton, are available as reactive (durable) and non-reactive (non-durable) systems.
There are two principle chemical types of reactive phosphor-organic FR agents. Both of them are halogen-free formulations.
One type (fibre-reactive systems) is widely commercialised under trade names as Pyrovatex® and Spolapret®. The phosphor-organic compound is represented by the molecule: phosphonic acid, (2-((hydroxymethyl)carbamyl)ethyl)-dimethyl ester.
The FR is applied to cotton via the pad-dry-bake technique in combination with a melamine resin, a fabric softener and phosphoric acid. After padding, the fabric is dried and cured thermally to achieve fixation. No ammonia is used in curing. Because of the presence of melamine resins as cross-linking agents, formaldehyde and methanol are evolved as off-gases (emissions are normally abated via scrubbers). Following the curing process, the fabric is washed off, resulting in some unreacted P-containing reagents being discharged to the waste water. These compounds are non-readily biodegradable and water-soluble (they are not bioeliminated by adsorption on the sludge). According to one source, this product is not toxic or harmful to aquatic organisms and shows no potential to bioaccumulate [301, CIA, 2002].
Another source concludes that too little is known about the toxicology of the compound for a health risk assessment to be made. The same source states that no summaries of the environmental toxicity and fate have been identified [304, Danish EPA, Lokke et al., 1999].
Residual finishing liquors and rinse water containing phosphor-organic flame retardant of this type should be collected and not mixed with the other effluent in the waste water treatment system [200, Sweden, 2001].
With the other type of reactive phosphor-organic FR (self-reactive systems), the fabric is impregnated with phosphonium salt and urea precondensates. The subsequent drying process step does not require complete drying. Processing temperatures are therefore low (between 60 and 100°C). After drying, the fabric is treated with ammonia to produce an insoluble polymer within the fibres. The fabric is subsequently oxidised with hydrogen peroxide and washed. In this process there is no curing treatment other than the treatment with ammonia.
The levels of formaldehyde evolved during drying are reported to be within the OEL limits for worker exposure over an eight hour period and maximum concentration limits over a fifteen minute reference period [301, CIA, 2002]. According to the same source, limits set for atmospheric emission of formaldehyde (20 mg/m3) are achieved at the majority of finishing sites without the need to install a scrubber.
No methanol is present in the emissions and no melamine resins or cross-linking agents are used in the process.
Phosphonium salt and urea precondensates have been shown to have levels of fixation of 95 % or higher [301, CIA, 2002]. However, since washing is necessary with these flame-retardants to remove unreacted agents and by-products, some residual phosphorous organic compounds end up in the waste water treatment plant. These compounds are non-readily biodegradable and because of their water-solubility they may pass undegraded through the waste water treatment system.
Concentrated padding liquors and rinse water containing phosphor-organic flame retardants of this type should be collected and not discharged with the other effluents in the waste water treatment [200, Sweden, 2001].
According to FR manufacturers, the phosphorous compounds from these treatments do not have the capability to bioaccumulate. It is also stated that the effluent can be converted to an inorganic phosphorous effluent [301, CIA, 2002]. In this way phosphorous can be removed as phosphate, which would prevent the release of organo-phosphorous compounds into the environment.
Non-durable phosphor-organic flame-retardants do not react with the fibre. It has been reported that some of them release organic volatile compounds like glycols, alcohols, glycolether or parts of the active substances [77, EURATEX, 2000]. This information is contradicted by EFRA and CIA FR manufacturers who state that non-durable flame-retardants of this chemical classification produced by EFRA and CIA member companies do not release any of the abovementioned compounds [301, CIA, 2002].
As articles treated with non-durable phosphor-organic flame-retardants are not washed after the finishing treatment (and also as the final product is rarely washed), this results in a minimisation of any release of P-containing reagents to waste water [301, CIA, 2002].
8.8.5 Hydrophobic/ Oleophobic agents
The most commonly applied commercial formulations fall under the following categories:
· wax-based repellents (paraffin -metal salt formulations) · resin-based repellents (fatty modified melamine resins) · silicone repellents · fluorochemical repellents.
These formulations consist of ca. 25 % of a paraffin and 5 – 10 % of zirconium-, aluminiumbased salts. They are usually applied to natural and synthetic fibres by padding and drying without curing. The discharge of residual liquors leads to emissions of metals. Concentrations can be high in some cases. However, from a global point of view the amounts discharged can be considered negligible compared to emissions of metals from dyeing and printing.
Moreover metals like Zr and Al should not be confused with more hazardous metals such as Cu, Ni, Co, Cr used in dyeing processes (note that Zr is also used in the “Zirpo process” in carpets – see Section 8.8.4) [281, Belgium, 2002].
Concerning exhaust air emissions, the presence of paraffin waxes may produce fumes and high levels of volatile organic carbon during heat treatments.
Resin-based repellents Resin-based repellents (mainly applied as “extenders”) are produced by condensing fatty compounds (acids, alcohols or amines) with methylolated melamines. Formulations often also contain paraffin wax. They are applied by the pad-dry-cure process, often together with crosslinking agents in the presence of a catalyst.