«EUROPEAN COMMISSION Integrated Pollution Prevention and Control (IPPC) Reference Document on Best Available Techniques for the Textiles Industry July 2003 ...»
8.6.5 Acid donors The so-called acid donors represent a more sophisticated range of formulated products designed to create shifts in dye bath pH. They are hydrolisable acid esters which break down during dyeing, progressively lowering the pH. The shift of pH occurs as the acid is released by hydrolysis during heating or as one of the acid/base components is transferred to another phase such as the fibre or the air, e.g. ammonia release to air with ammonium sulphate.
They are widely used for wool and/or polyamide fibres to control the absorption of anionic dye onto the fibre. They are also employed for cotton and polyester blends when dyeing with disperse and reactive dyes in the one-bath one-step procedure.
Organic acid esters, fatty alcohol ethoxylates and aromatic sulphonates are commonly found in commercial products. They usually have good bio-eliminability.
8.6.6 Antifoaming agents Formulated products designed to suppress foam formation are used, which do not adversely influence the quality of the resultant dyeing. The majority are based on silicone derivatives.
8.6.7 Carriers Dyeing accelerants (so-called carriers) are used in batch dyeing of synthetic fibres (particularly polyester fibres) to promote the absorption and diffusion of disperse dyes into the fibre under low-temperature conditions. They are still important for dyeing blended fibres of wool and polyester, as wool cannot withstand dyeing under high temperature conditions (above 100 ºC).
Typical carrier formulations contain 60 – 80 % of active substance and 10 – 30 % of emulsifier and sometimes a small percentage of solvent.
Typical active substances for dyeing accelerants include:
· halogenated benzenes (1,2 dichlorobenzene; 1,2,4-trichlorobenzene; dichlorotoluene) · aromatic hydrocarbons such as alpha- and beta-methylnaphthalene, diphenyl, trimethyl benzene, etc.
· phenols such as o-phenylphenol, benzylphenol, etc.
· carboxylic acid and their esters such as methyl, butyl and benzyl benzoate, methylsalicylate, phthalic acid, dimethylphthalate, dibutylphthalate and diethylhexylphthalate · alkyl phthalimides such as N-butylphthalimide.
Most of the above-mentioned substances are toxic to humans, aquatic organisms and sewage sludge. Hydrophobic carriers exhaust at about 75 – 90 % onto the substrate, while hydrophilic types like phenols (e.g. o-phenylphenol), benzoate derivatives, N-alkylphthalimides and methylnaphthalene are mainly found in waste water. Exception made for benzoates (biodegradable) and N-alkylphthalimide derivatives (bioeliminable), the others are all poorly degradable and may pass undegraded through the waste water treatment system. On the other hand the carriers that remain on the textile material (hydrophobic types) are partially volatilised during the subsequent heat treatments (drying or fixing processes), thus producing air emissions.
Carboxylic acid esters and alkylphthamides derivatives are the substances that are most frequently used in Europe today as carriers. However, it is reported that carriers such as methyl naphthalene, mono-, di-, tri-chlorobenzene, biphenyl, orthophenyl phenol and benzyl alcohol are still found in use [61, L. Bettens, 1999].
8.7 Printing auxiliaries 8.7.1 Auxiliaries for dye printing Thickening agents Thickening agents are essential components of printing pastes. They are responsible for preventing capillary flow between the threads of the fabric, thus inhibiting dye diffusion.
Oil/water emulsions, used in the past as thickeners, have now been largely replaced by formulations similar to those used for warp sizes. Standard components of modern thickeners include natural polysaccharides, both unmodified and chemically modified (e.g. seed derivatives, starch degradation products, alginates) and fully synthetic polymers (mainly based on polyacrylic acid). Thanks to the improvements made in the characteristics of the starting materials, thickening agents are now supplied almost exclusively in cold-soluble form.
8.7.2 Auxiliaries for pigment printing Thickening agents Water-in-oil emulsions were widely used in the past as thickeners. They contained up to 70 % of white spirit (mixture of aliphatic hydrocarbons with C12-C50 chain length), which resulted in emissions of volatile organic carbon in the exhaust air from drying and curing ovens. Halfemulsion printing pastes (oil in water) are only occasionally employed today. Nevertheless, modern thickeners can still contain approximately 10 % of mineral oils, which are ultimately found in exhaust air. New generation thickeners have been developed which do not contain any volatile solvents. They are supplied in the form of non-dusting granules [64, BASF, 1994].
The pigment has no affinity for the fibre. Therefore, in order to coat the pigment and protect it from mechanical abrasion, a binder is added to the printing paste. Binders are in general selfcross-linking polymers which reticulate during the fixation step. They are supplied as aqueous polymer dispersions, based mainly on acrylates and less commonly on butadiene and vinyl acetate.
Additional fixing agents are sometimes necessary to enhance the level of wet-fastness, especially with smooth fibres such as PES. Melamine-formaldehyde condensates are used for this purpose. In order to reduce the consequent formaldehyde emissions, modified compounds of the same chemical type, but with a low formaldehyde content, are now common.
Plasticisers Plasticisers are mainly silicones or fatty acid esters, which are used to improve the dry rubbing fastness and give a smooth dry handle to the fabric.
Emulsifiers In high- and low solvent pigment printing pastes, the emulsifiers serve to stabilise the solvent (white spirit). In solvent-free pigment printing they are used to prevent agglomeration of the pigment, screen blocking and separation of components of the print paste [186, Ullmann's, 2000]. Non-ionic surfactants such as aryl- and alkyl polyglycol ethers are the most commonly used for this purpose.
8.8 Finishing auxiliaries 8.8.1 Easy-care finishing agents Easy-care finishing agents are chemical finishes which are applied to woven and knitted fabrics composed of cotton, other cellulosic fibres and their blends with synthetic fibres. Their function is to reduce the propensity of cellulose-containing fabrics for wrinkling when treated under wet and dry conditions and to stabilise them against progressive shrinkage during laundering. Their action is based on the introduction of cross-links between the cellulose molecules of cotton and viscose, thus reducing swellability and extensibility of the fibre (the fabric tends to return to the state in which it was when the cross-links were introduced).
Other names also commonly used to name these finishing agents are: resin-finishing, wash-andwear finishing, no-iron, wrinkle resistant, etc.
Recipes for resin-finishing liquors are in general aqueous solutions or dispersions which consist of a cross-linking agent, a catalyst, a wetting agent/emulsifier (mainly a non-ionic surfactant) and a number of additives.
Among the additives a distinction is drawn between those that are used to improve the handle and to compensate the adverse effects of the cross-linking agent and some others that serve to impart additional functional properties to the final textile good (e.g. water-repellents, hydrophilising agents, etc.). Only the first group of additives is discussed in this section while the others, being less specific to this finishing treatment, are described in other parts of this Annex.
Cross-linking agents The cross-linking agents play an essential role in this finishing treatment: they are also the major components in the formulation (they represent approximately 60 to 70 % of the total preparation).
From a chemical point of view there are three distinct groups:
· cross-linking agents based on melamine and formaldehyde · cross-linking agents based on urea and formaldehyde · heterocyclic linking agents based on urea, formaldehyde and various other substances such as diamines and, in particular, glyoxal.
All these products may potentially produce emissions of free formaldehyde and methanol. In particular, formaldehyde is suspected of carcinogenity and its presence in these finishing agents represents a potential risk not only for waste water and exhausted air, but also for the workplace and the final user of the textile good.
Manufacturers have put much effort into producing cross-linking agents with a low to very low content of free formaldehyde and one compound is also available on the market, which is formaldehyde-free.
Existing cross-linking agents are therefore better classified as:
· formaldehyde-rich · formaldehyde-poor or very poor · formaldehyde-free.
The first group includes compounds (so-called “self-crosslinking” agents) like (HOCH2-NH-CO-NH-CH2OH) hydroxymethyl urea and (CH3OCH2-NH-CO-NH-CH2OCH3) (bis(methoxymethyl) urea or the correspondent melamine derivatives (hydroxymethyl melamine and bis(methoxymethyl) melamine).
Textiles Industry 499 Annexes Low to very low levels of free formaldehyde are achievable with heterocyclic compounds (socalled “reactant cross-linking” agents) which are mainly based on the derivatives of the molecule better known as bis(hydroxymethyl)-dihydroxyethylene urea (see Figure below).
Modified derivatives of this compound with methanol and diethyleneglycol are among those reported to have a very low free-formaldehyde content in the product (0.5 %, but also 0.1 % for some products is now achievable).
Among the formaldehyde-free cross-linking agents, only the compounds based on dimethyl urea and glyoxal (see Figure below) have gained a small share of the market [36, BASF, 2000]. In the absence of other sources of formaldehyde such as fixing agents and preservatives, the finishes obtained using these cross-linking agents are formaldehyde-free.
The emissions of formaldehyde, methanol and other volatile organic substances in the exhaust air (from curing and drying operations) and in the workplace (especially with melamine resins) represent the main environmental issue in the application of these agents.
Water pollution is also an issue since these active substances (ethylene urea and melamine derivatives), when in their not-cross-linked form, are water-soluble and in general hardly biodegradable. This problem is, however, restricted to residual liquors, which as a fundamental rule should not be discharged together with the other effluents in order to avoid contamination with these concentrated liquors.
Catalyst The function of the catalyst in the resin-finishing recipe is to allow the curing reaction to be carried out at the usually employed curing temperatures (130 – 180 ºC) and times.
The most commonly used catalyst is magnesium chloride. In many cases liquid mixtures are used, based on magnesium chloride and organic acids (e.g. alpha-hydroxycarboxylic acids) or inorganic Lewis acids.
Ammonium salts (e.g. chloride, sulphate and nitrate) commonly used in the past, particularly with cross-linkers based on urea and formaldehyde or melamine, are in decline today, following the corresponding decline in importance of these cross-linking agents.
Additives (softeners, stiffening agents, etc.) As mentioned above, easy-care recipes contain various different additives. Attention is given
here only to:
· products that are added to easy-care recipes to offset the reduction in softness, tear strength
and abrasion resistance that is produced on the fibre as a secondary effect of the crosslinking reaction (their content in easy-care recipes is in general in the range of 10 – 40 %):
these can be dispersions of polyacrylates, polysiloxanes, polyethylene waxes (partially oxidised polyethylenes), polyurethanes · products that are added as softeners to improve the handle of the fabric: fatty acid condensation products (cationic softners) represent the most important group. They can also be supplied as emulsified mixtures with other products (e.g. polyethylene wax) · products that are used as stiffeners, hand builders: these can be dispersions of polyvinyl acetate, polyacrylate derivatives.
These substances are responsible for emissions of VOCs in the exhaust air from curing and drying operations.
The poor biodegradability of the above-mentioned polymer dispersions has also to be taken into account when considering water pollution.
8.8.2 Biocides Some commonly used biocides are those that are applied in the carpet sector to impart wool fibre lifetime protection against a range of textile pests. These auxiliaries are usually known as mothproofing agents.
Currently used mothproofing agents are formulated from the following active ingredients:
· permethrin (synthetic pyrethroid) · cyfluthrin (synthetic pyrethroid) · sulcofuron (halogenated diphenylurea derivative).
Permethrin-based formulations account for approximately 90 % of the market. Cyfluthrin based products are only available in the UK, while sulcofuron-based products are currently not used to any significant degree.
Permethrin and synthetic pyrethroids in general are reported to have low toxicity to humans but high aquatic toxicity. Dyphenylurea derivatives may exhibit less aquatic toxicity but, in some cases, are less biodegradable [11, US EPA, 1995].
Other types of biocides are used in the textile industry, mainly as antimicrobial finishes, (e.g.
hospital textile material), or as odour suppressants for socks and hosiery. The active substances
used for this purpose are most commonly [77, EURATEX, 2000]:
· zinc organic compounds · tin organic compounds · dichlorophenyl(ester) compounds · benzimidazole derivatives · triclosane · isothiazolinones (the most commonly used today).
All biocides give rise to environmental concern when they are discharged in waste water, because of their toxicity to aquatic life.
8.8.3 Antistatic agents This section focuses on the antistatic agents that are applied as functional finishes to selected textile materials for use in static-sensitive environments. They are commonly used in the carpet sector.
From a chemical point of view, formulated products are mostly based on:
· quaternary ammonium compounds · phosphoric acid ester derivatives.
Quaternary ammonium compounds are highly toxic to aquatic life. Furthermore, the discharge of these substances in waste water can give rise to high levels of AOX due to incomplete quaternisation (epichlorhidrine and chloromethane are often used as initial reagents).
Alkylphosphates and alkyetherphosphates used as antistatic agents are water-soluble, hard-totreat substances which may pass undegraded through common waste water systems.
Nevertheless the use of phosphoric acid esters in antistatic finishing treatment leads to emissions loads that are negligible if compared with other potential sources of this pollutant.
According to [77, EURATEX, 2000], in fact, the pollution load released during pretreatment and attributable to antistatics from preparation agents is more than 100 times higher compared to the amount of antistatics discharged in waste water from antistatic finishing treatments.
On the other hand, for air emissions, the use of phosphoric esters-based antistatics may lead to the release of non-reacted alcohols (mostly n-butanol, which is very odour intensive) and phosphoric acid (corrosive).
8.8.4 Flame-retardants When speaking about flame-retardant finishing it is useful to differentiate between durable and non-durable treatments. Durable flame-retardants react with the fibre, thereby providing permanent fire retardancy properties to the treated product. This is not the case with non-durable retardants, which although effective, are removed by laundering and are thus suitable only for fabrics which are seldom or never laundered or which can be re-treated whenever laundering is carried out.