«EUROPEAN COMMISSION Integrated Pollution Prevention and Control (IPPC) Reference Document on Best Available Techniques for the Textiles Industry July 2003 ...»
Chemical characteristics and general application conditions Direct dyes (also called substantive dyes) can be azo compounds, stilbenes, oxazines, or phtalocyanines. They always contain solubilising groups (mainly sulphonic acid groups, but carboxylic and hydroxyl groups can also be found) that ionise in aqueous solution.
Direct dyes are characterised by long planar molecular structures that allow these molecules to align with the flat cellulose macromolecules, the dye molecules being held in place mainly through Van der Waals forces and hydrogen bonds.
Direct dyes may require the use of the following chemicals and auxiliaries for satisfactory
· electrolytes, usually sodium chloride or sodium sulphate. Their function is to favour the aggregation of dye ions on the fibre · wetting and dispersing agents: mixtures of non-ionic and anionic surfactants are used for this aim · aftertreatment agents: they are used to improve wet-fastness properties. So-called fixative cationic agents are the most commonly used. They are usually quaternary ammonium compounds with long hydrocarbon chains. Formaldehyde condensation products with amines, polynuclear aromatic phenols, cyanamide or dicyandiamide may also be used for this purpose.
Figure 9.3: Examples of typical direct dyes Environmental issues The environmental properties of direct dyes are assessed under the following parameters.
Note, however, that Table 9.2 does not consider the environmental issues related to chemicals and auxiliaries employed in the dyeing process because these issues are dealt with in a specific annex.
9.4 Disperse dyes Applicability Disperse dyes are used mainly for polyester, but also for cellulose (acetate and triacetate), polyamide and acrylic fibres.
Properties Fastness to light is generally quite good, while fastness to washing is highly dependent on the fibre. In particular, in polyamides and acrylics they are used mostly for pastel shades because in dark shades they have limited build-up properties and poor wash fastness.
Chemical characteristics and general application conditions Disperse dyes are characterised by the absence of solubilising groups and low molecular weight.
From a chemical point of view more than 50 % of disperse dyes are simple azo compounds, about 25 % are anthraquinones and the rest are methine, nitro and naphthoquinone dyes.
The dye-fibre affinity is the result of different types of interactions:
· hydrogen bonds · dipole-dipole interactions · Van der Waals forces.
Disperse dyes have hydrogen atoms in their molecule, which are capable of forming hydrogen bonds with oxygen and nitrogen atoms on the fibre.
Dipole-dipole interactions result from the asymmetrical structure of the dye molecules, which makes possible electrostatic interactions between dipoles on the dye molecules and polarised bonds on the fibre.
Van der Waals forces take effect when the molecules of the fibre and colourant are aligned and close to each other. These forces are very important in polyester fibres because they can take effect between the aromatic groups of the fibre and those of the colourant.
Disperse dyes are supplied as powder and liquid products. Powder dyes contain 40 – 60 % of dispersing agents, while in liquid formulations the content of these substances is in the range of 10 – 30 %. Formaldehyde condensation products and ligninsulphonates are widely used for this purpose.
Dyeing with disperse dyes may require the use of the following chemicals and auxiliaries:
· dispersants: although all disperse dyes already have a high content of dispersants, they are further added to the dyeing liquor and in the final washing step · carriers: for some fibres, dyeing with disperse dyes at temperatures below 100 ºC requires the use of carriers. This is the case with polyester, which needs the assistance of carriers to enable an even penetration of disperse dyes below boiling temperature. Because of environmental problems associated with the use of these substances, polyester is preferably dyed under pressure at temperature 100 ºC without carriers. However, carrier dyeing is still important for polyester-wool blends, as wool must not be submitted to wet treatment at temperatures significantly above 100 ºC · thickeners: polyacrylates or alginates are usually added to the dye liquor in padding processes. Their function is to prevent migration of the dye liquor on the surface during drying · reducing agents (mainly sodium hydrosulphite): they are added in solution with alkali in the final washing step.
Disperse dyes are widely used not only for dyeing, but also for printing synthetic fibres.
Environmental issues The environmental properties of disperse dyes are assessed under the following parameters.
Note, however, that Table 9.3 does not consider the environmental issues related to chemicals and auxiliaries employed in the dyeing process because these issues are dealt with in a specific annex.
Table 9.3: Overview of the ecological properties of disperse dyes
9.5 Metal-complex dyes Applicability Metal-complex dyes (also called pre-metallised dyes) have great affinity for protein fibres.
Among metal-complex dyes, 1:2 metal-complex dyes are also suitable for polyamide fibres.
More than 65 % of wool is today dyed with chrome dyes (see next section) or metal-complex dyes and about 30 % of PA is dyed with 1:2 metal-complex dyes.
Properties Light-fastness is excellent, while washing fastness is not as good as with chrome dyes (particularly in darker shades).
Chemical characteristics and general application conditions Metal-complex dyes may be broadly divided into two classes, 1:1 metal-complexes, in which one dye molecule is co-ordinated with one metal atom and 1:2 metal complexes, in which one metal atom is co-ordinated with two dye molecules. The dye molecule will be typically a monoazo structure containing additional groups such as hydroxyl, carboxyl or amino groups, which are capable of forming strong co-ordination complexes with transition metal ions,
typically chromium, cobalt, nichel and copper. Note that phthalocyanine dyes cannot be classified as metal-complex dyes.
Typical examples of pre-metallised dyes are shown in Figure 9.4 and Figure 9.5.
Figure 9.4: Examples of molecular structures typical of 1.
1 metal-complex dyes Figure 9.5: Molecular structure typical of 1.
2 metal-complex dyes Metal-complex dyes do not represent a specific application dye class. Metal-complex dyes belong in fact to many application classes of dyes (i.e. they can be found, for example, among acid, direct and reactive dyes). When used in dyeing processes, metal-complex dyes are applied in pH conditions regulated by the user class and the fibre type (wool, polyamide, etc.). The pH levels for wool range from strongly acidic (1.8 - 4 for 1:1 metal-complex dyes) to moderately acidic neutral (4 - 7 for 1:2 metal-complex dyes). For polyamide fibres higher pH conditions are becoming more and more common.
1:1 metal-complex dyes exhibit excellent level dyeing and penetration characteristics and have the ability to cover irregularities in the substrate. Their light and wet fastness properties are good even in deep shades. They are particularly suitable for yarn and for piece dyeing of carbonised wool.
1:2 metal-complex dyes are used for both wool and polyamide. They form the most important
group in this class and may be divided into two sub-groups:
· weakly polar 1:2 complexes – solubilised by the inherent anionicity of the complex or containing non-ionic, hydrophilic substituents such as methylsulphone (-SO2CH3). These dyes exhibit excellent fastness to light and wet treatments and excellent penetration properties.
· strongly polar 1:2 complexes – solubilised by one or more sulphonic or carboxylic acid residues, these dyes possess lower levelling power than the weakly polar dyes mentioned above but superior wet fastness properties and are generally suitable for use in those applications where mordant dyes are used. This second group is also more suitable for dyeing polyamide fibres.
Textiles Industry 519 Annexes Dyeing with metal-complex dyes may require the use of the following chemicals and
· pH regulators: sulphuric, formic, acetic acid · electrolytes: sodium sulphate, ammonium acetate and sulphate · levelling agents: mixtures of anionic and non-ionic surfactants (these auxiliaries are not needed when using pH controlled adsorption dyeing techniques).
The environmental properties of metal-complex dyes are assessed under the following parameters. Note, however, that Table 9.4 does not consider the environmental issues related to chemicals and auxiliaries employed in the dyeing process because these issues are dealt with in a specific annex.
Table 9.4: Overview of the ecological properties of metal complex dyes
9.6 Mordant dyes (chrome dyes) Applicability Mordant dyestuffs are generally used for protein (wool and silk). They are practically no longer used for polyamide fibres or for printing.
Properties Thanks to their good levelling properties and very good wet fastness after chroming, chrome dyes are used principally to obtain dark shades (greens, blues and blacks) at moderate cost.
There are disadvantages, however, in their use: long dyeing times, difficulties with shading, the risk of chemical damage to the fibre during chroming and the potential release of chromium in waste water.
Chemical characteristics and general application conditions The Colour Index classifies these colourants as mordant dyes, but chromium has become the almost universally used mordant and the class is commonly referred to as chrome dyes.
From a chemical point of view they can be regarded as acid dyestuffs that contain suitable functional groups capable of forming metal complexes with chrome. They do not contain chrome in their molecule, which instead is added as dichromate, or chromate salt to allow dye fixation.
Interaction with the fibre is established through ionic bonds formed between the anionic groups of the colourant and ammonium cations available on the fibre. In addition chromium acts as a link between dye and fibre. This gives rise to a very strong bond, which is reflected in the excellent fastness obtained. Figure 9.6 shows the ionic and coordination bonds in the case of wool.
Figure 9.6: Representation of possible ionic and coordination bonds between wool and chrome dyes [69, Corbani, 1994] The use of chrome dyes in dyeing processes requires the use of the following chemicals and
· potassium and dichromate or chromate · formic or acetic acid as pH regulators · other organic acids such as tartaric and lactic acid. They are used to enhance the degree of conversion of Cr VI to Cr III · sodium or ammonium sulphate.
The environmental properties of chrome dyes are assessed under the following parameters.
Note, however, that Table 9.5 does not consider the environmental issues related to chemicals and auxiliaries employed in the dyeing process because these issues are dealt with in a specific annex.
Table 9.5: Overview of the ecological properties of chrome dyes
9.7 Naphthol dyes (azoic dyes developed on the fibre) Applicability Azoic dyes, also known as naphthol dyes, are used for cellulosic fibres (particularly cotton), but may also be applied to viscose, cellulose acetate, linen and sometimes polyester.
Properties Azoic dyes have excellent wet fastness properties as well as good light, chlorine and alkali fastness, while rubbing fastness is poor.
Chemical characteristics and general application conditions From a chemical point of view naphtol dyes are very similar to azo dyes, the main difference being the absence of sulphonic solubilising groups.
They are made up of two chemically reactive compounds that are applied to the fabric in a twostage process. The insoluble dye is synthetised directly in the fibre as the result of the coupling reaction between a diazotised base (developing agent) and a coupling component.
The coupling components are usually derivatives of the anilides of the 2-hydroxy-3-naphthoic acid (also called naphthol AS). These naphthols are available in powder form or in liquid form (in this case the solution also contains caustic soda, the naphthol concentration ranges between 30 % and 60 %).
Figure 9.7: Examples of typical coupling components for naphthol dyes Developing agents can be derivatives of aniline, toluidine, orto and meta anisidine, diphenyl
amine. They are available as:
· free bases (fast colour bases) · liquid bases (these formulations are aqueous dispersions of the aromatic amines, they are safer and simpler to apply than solid bases) · fast colour salts (these are already diazotised diazonium compounds that are marketed in stabilised forms and do not need to be diazotised before use in dyeing: some examples are given in the figure below).
Application of azoic colourants involves a number of steps:
· preparation of the naphtolate solution: naphthol is converted to the naphtolate form to be able to couple with the diazonium salt · application of the naphtholate to the fibre · preparation of the diazotised base: in order to make the coupling reaction possible, the base must first be diazotised in the cold, using sodium nitrite and hydrochloric acid (this step can be avoided when using fast colour salts) · formation of the azoic dye into the fibre.
The environmental properties of naphtol dyes are assessed under the following parameters.
Note, however, that Table 9.6 does not consider the environmental issues related to chemicals and auxiliaries employed in the dyeing process because these issues are dealt with in a specific annex.
Table 9.6: Overview of the ecological properties of naphthol dyes
9.8 Reactive dyes Applicability Reactive dyes are mainly used for dyeing cellulose fibres such as cotton and viscose, but they are also increasingly gaining importance for wool and polyamide.
Properties They provide high wet fastness (better than the less expensive direct dyes), but their use is not always viable because of the difficulty in obtaining level dyeing. Chlorine fastness is slightly poorer than that of vat dyes, as is light fastness under severe conditions.
The range of available reactive dyes is wide and enables a large number of dyeing techniques to be used.
Chemical characteristics Reactive dyes are unique in that they contain specific chemical groups capable of forming covalent links with the textile substrate.
The energy required to break this bond is similar to that required to degrade the substrate itself, thus accounting for the high wet fastness of these dyes.
The structure of Reactive Black 5, one of the most important reactive dyestuffs in terms of volumes consumed, is illustrated in Figure 9.10.
Figure 9.10: Reactive Black 5
Chemical structure of reactive dyes can be schematically represented by the following formula:
· Col is the chromophore that is in general constituted by monoazoic, anthraquinone, phthalocyanine and metal-complex compounds · B is the linking group between the chromophore and the reactive group · R represents the reactive group (anchor system with the leaving group). The anchor systems are characterised by their reactivity. Based on this, they are classified as hot, warm or cold dyers.