«EUROPEAN COMMISSION Integrated Pollution Prevention and Control (IPPC) Reference Document on Best Available Techniques for the Textiles Industry July 2003 ...»
Some typical examples of reactive systems for cellulose and wool or polyamide fibres are reported in the following tables.
Table 9.8: Typical anchor systems for wool and polyamide fibres The reactive groups of the colourant react with the amino groups of the fibre in the case of protein and polyamide fibres, and with the hydroxyl groups in the case of cellulose.
In both cases, depending on the anchor system, two reaction mechanisms are possible: a nucleophilic substitution mechanism or a nucleophylic addition mechanism.
An important issue to consider when dealing with reactive dyes is the fact that two competing
reactions are always involved in the colouring process:
1. alcoholysis: dye + fibre → dye fixed on the fibre
2. hydrolysis: dye + water → hydrolysed dye washed away after dyeing (undesired reaction) This fact has important consequences, especially in the case of cellulose fibres. In fact, the alkaline conditions in which reactive dyes react with cellulose fibres, increase the rate of the hydrolysis reaction. The characteristics of the resulting hydrolysed dye are such that the dye is no longer a reactive substance and it is therefore discharged in the effluent.
Dyeing cellulose fibres with reactive dyes may imply the use of the following chemicals and
· alkali (sodium carbonate, bicarbonate and caustic soda) · salt (mainly sodium chloride and sulphate) · urea may be added to the padding liquor in continuous processes in the one-bath method (see also Section 4.6.13 for alternative techniques) · sodium silicate may be added in the cold pad-batch method (see also Section 4.6.9).
Reactive dyes are applied to wool or polyamide fibres under different conditions. In the case of wool and polyamide fibres, reactivity of the amino groups is considerably higher than that of hydroxyl groups in cellulose.
Levelling properties are often achieved with the use of speciality amphoteric levelling agents.
Reactive dyes are generally applied at pH values of between 4.5 and 7, depending on depth of shade, in the presence of ammonium sulphate and the specialised levelling agents mentioned above.
In cellulose printing, moderately reactive dyes are generally employed (mainly mochlorotriazine systems). Highly reactive sulphoethyl sulphones are also sometimes used.
Printing with reactive dyes requires the use of:
· thickening agents (mainly polyacrylates in combination with alginates) · urea · alkali (e.g. sodium carbonate and bicarbonate) · oxidising agents (mainly benzenesulphonic acid derivatives): they are used to prevent reduction of more sensitive dyes during steaming.
Poor dye fixation has been a long-standing problem with reactive dyes in particular in batch dyeing of cellulose fibres, where a significant amount of salt is normally added to improve dye exhaustion (and therefore also dye fixation). On the other hand, shade reproducibility and level dyeing were the major obstacle in “right-first-time” production using the most efficient dyes (high exhaustion and fixation rate).
Research and development has been faced with a number of objectives, all of which have been
or are in the process of being successfully achieved. These include [190, VITO, 2001]:
· increasing the robustness of individual dyes and dye combinations (trichromatic systems) · enhancing reproducibility of trichromatic combinations used in most commonly applied dyeing processes · reducing salt consumption and/or unused dye in the effluent · improving fastness properties (e.g. light fastness, fastness to repeated laundering).
With the use of sophisticated molecular engineering techniques it has been possible to design reactive dyes (e.g. bifunctional dyes and low-salt reactive dyes) with considerably higher performances than traditional reactive dyes. These recent developments are described in more detail in Sections 4.6.10, 4.6.11 and 4.6.13.
The environmental properties of reactive dyes are assessed in Table 9.9. Note that the table does not consider the environmental issues related to chemicals (e.g. salt) and auxiliaries employed in the dyeing process because these issues are dealt with in a specific annex.
Effluent contamination by dispersants and additives already in the dye
(1) [77, EURATEX, 2000] Fixation degree for:
- cotton batch dyeing: 55 - 80 %
- wool batch dyeing: 90 - 97 %
- printing (general): 60 % Table 9.9: Overview of the ecological properties of reactive dyes
9.9 Sulphur dyes Applicability Sulphur dyes are mainly used for cotton and viscose substrates. They may also be used for dyeing blends of cellulose and synthetic fibres, including polyamides and polyesters. They are occasionally used for dyeing silk. Apart from black shades, sulphur dyes play almost no part in textile printing.
Bleach and wash fastness properties are very good, while light fastness varies from moderate to good. Although they encompass a broad shade range, sulphur dyes are mostly used for dark shades because lighter shades have poor resistance to light and laundering. Sulphur dyes tend to be dull compared with other dye classes.
Chemical characteristics and general application conditions Sulphur dyes are made up of high molecular weight compounds, obtained by reaction of sulphur or sulphides with amines and phenols. Many colourants exist that contain sulphur in their molecule, but only dyestuffs which become soluble in water after reaction with sodium sulphide under alkaline conditions can be called sulphur dyes.
The exact chemical structure is not always known because these are mixtures of molecules of a high level of complexity. Amino derivatives, nitrobenzenes, nitro and aminobiphenyls, substituded phenols, substituted naphhalenes, condensed aromatic compounds, indophenols, azines, oxazine, thiazol, azine and thiazine rings can be part of these compounds. Sulphur dyes contain sulphur both as an integral part of the chromophore and in polysulphide side chains.
As has already been mentioned, sulphur dyes are insoluble in water, but after reduction under alkaline conditions they are converted into the leuco form, which is water-soluble and has high affinity for the fibre. After absorption into the fibre they are oxidised and converted to the original insoluble state.
Sulphur dyes are available in various modifications, which are classified under the following
· sulphur dyes - available as amorphous powders or dispersible pigments. Amorphous powders are insoluble or partially soluble in water and are brought into solution by boiling with sodium sulphide and water. Dispersible pigments can be used in this form for pad dyeing in presence of a dispersing agent. They can contain a certain amount of reducing agent already in the formulation and in this case are called "partly reduced pigments".
· leuco-sulphur dyes (ready-for-use dyes) - available in liquid form and already contain the reducing agent required for dyeing. Therefore they must simply be diluted with water before application. Low-sulphide types are also available on the market · water-soluble sulphur dyes - available in the form of Bunte salts (Col-S-SO3Na) obtained by treating the dye in its insoluble form (Col-S-S-Col) with sodium hydrosulphite. They can be dissolved in hot water, but they do not have affinity for the fibre. The addition of alkali and reducing agent makes them substantive for the fibre.
Sodium sulphide and sodium hydrogensulphyde are generally employed as reducing agents to bring into solution the dye (unless ready-for-use sulphur dyes are applied). Binary systems made of glucose and sodium dithionite (hydrosulphite) or thiourea dioxide are also used as alternative reducing agents.
In all processes the dye is finally fixed on the substrate by oxidation. Nowadays, hydrogen peroxide or halogen-containing compounds such as bromate, iodate and chlorite are the most commonly used oxidising agents.
Apart from the above-mentioned reducing and oxidising agents, additional chemicals and
auxiliaries required when dyeing with sulphur dyes are:
· alkali (mainly caustic soda) · salt (sodium chloride and sulphate) · dispersing agents: usually they are naphthalenesulphonic acid-formaldehyde condensates, ligninsulphonates and sulphonated oils · complexing agents: EDTA and polyphosphates are used in some cases to prevent negative effects due to the presence of alkaline-earth ions.
Environmental issues The environmental properties of sulphur dyes are assessed under the following parameters.
Note, however, that Table 9.10 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 9.10: Overview of the ecological properties of sulphur dyes
9.10 Vat dyes Applicability Vat dyes are used most often in dyeing and printing of cotton and cellulose fibres. They can also be applied for dyeing polyamide and polyester blends with cellulose fibres.
Properties Vat dyes have excellent fastness properties when properly selected and are often used for fabrics that will be subjected to severe washing and bleaching conditions (towelling, industrial and military uniforms, etc.). The range of colours is wide, but shades are generally dull.
Chemical characteristics and general application conditions From a chemical point of view, vat dyes can be distinguished into two groups: indigoid vat dyes and anthraquinoid dyes. Indigo dyes are almost exclusively used for dyeing warp yarn in the production of blue denim.
Like sulphur dyes, vat dyes are normally insoluble in water, but they become water-soluble and substantive for the fibre after reduction in alkaline conditions (vatting). They are then converted again to the original insoluble form by oxidation and in this way they remain fixed into the fibre.
Figure 9.11: Examples of typical vat dyes Vat dyes are preparations that basically consist of a vattable coloured pigment and a dispersing agent (mainly formaldehyde condensation products and ligninsulphonates).
They are generally supplied in powder, granules and paste form.
A wide range of different techniques are used in colouring processes with vat dyes.
Nevertheless, all processes involve three steps:
· vatting · oxidation · aftertreatment.
The step in which the reduction of the dyestuff into its leuco-form takes place is called vatting.
Vat dyes are generally more difficult to reduce than sulphur dyes. Various reducing agents are used. Sodium dithionite (hydrosulphite) is still the most widely employed although it has some limits. Sodium dithionite is consumed by reduction of the dye and also by reaction with atmospheric oxygen, therefore an excess of reducing agent has to be used and various techniques have been proposed to reduce these losses (see also Section 4.6.6). In addition sodium dithionite cannot be used for high temperature or pad-steam dyeing processes because over-reduction can occur with sensitive dyes. In these application conditions and also for printing, sulphoxylic acid derivatives are normally preferred.
532 Textiles Industry Annexes Thiourea dioxide is also sometimes used as a reducing agent, but a risk of over-reduction exists as its reduction potential is much higher than that of hydrosulphite. Furthermore the oxidation products of thiourea dioxide contribute to nitrogen and sulphur contamination of waste water.
Following increasing environmental pressures, biodegradable sulphur-free organic reducing agents such as hydroxyacetone are now available. Their reducing effect, however, is weaker than that of hydrosulphite, so they cannot replace it in all applications. Nevertheless hydroxyacetone can be used in combination with hydrosulphite, thus reducing to a certain extent the sulphite load in the effluent.
After absorption by the fibre, the dye in its soluble leuco form is converted to the original pigment by oxidation. This process is carried out in the course of wet treatment (washing) by addition of oxidants such as hydrogen peroxide, perborate or 3-nitrobenzenesulphonic acid to the liquor.
The final step consists in after-treating the material in weakly alkaline liquor with a detergent at boiling temperature. This soap treatment is not only aimed at removing pigment particles, but also allows the crystallisation of amorphous dye particles, which gives the material the final shade and the fastness properties typical of vat dyes.
Vat dyeing conditions can vary widely in terms of temperature and the amount of salt and alkali required, depending on the nature of the dye applied. Vat dyes are therefore divided into the following groups, according to their affinity for the fibre and the amount of alkali required for
· IK dyes (I = Indanthren, K = cold) have low affinity, they are dyed at 20 – 30 ºC and require little alkali and salt to increase dye absorption · IW dyes (W = warm) have higher affinity, they are dyed at 40 – 45 ºC with more alkali and little or no salt · IN dyes (N = normal) are highly substantive and applied at 60 ºC and require much alkali, but no addition of salt.
The following chemicals and auxiliaries may be found in dyeing processes:
· sodium dithionite, thiourea dioxide and sulphoxilic acid derivatives as reducing agents · caustic soda · sodium sulphate · polyacrylates and alginates as anti-migration agents in padding processes · formaldehyde condensation products with naphthalenesulphonic acid and ligninsulphonates as dispersing agents · surfactants (including ethoxylated fatty amines) and other components such as betaines, polyalkylenamines, polyvinylpyrrolidone as levelling agents · hydrogen peroxide, perborate, 3-nitrobenzenesulphonic acid as oxidants · soap.
The following chemicals and auxiliaries may be found in printing processes:
· thickening agents (starch esters with seed flour derivatives) · reducing agents: various chemicals are used depending on the printing method (all-in or two-phase process), dye selected and steaming conditions. Sulphoxylic acid derivatives are the most common, but hydrosulphite can also be used (in the two-phase process when very short steaming time is required) · alkali: potassium carbonate, sodium carbonate, sodium hydroxide · oxidising agents (the same used for dyeing) · soap.
Environmental issues The environmental properties of vat dyes are assessed under the following parameters. Note, however, that Table 9.11 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 9.11: Overview of the ecological properties of vat dyes
9.11 Pigments Pigments are widely used in printing processes (pigment printing).
Pigments are insoluble in water and organic solvents. Organic pigments are for a large part derived from benzoids. Inorganic pigments are derivatives of metals such as titanium, zinc, barium, lead, iron, molybdenum, antimony, zirconium, calcium, aluminium, magnesium, cadmium or chromium.
10.1 Loose fibre 10.1.1 Autoclave Various types of machines are used for processing fibres in loose form. These include conical pan machines, pear-shaped machines and radial flow machines. They are used for all wet operations, that is, pretreatment, dyeing, application of finishing agents and washing.