Acid dyes with improved light fastness have become important particularly in connection with the usage of acid dyes in information recording systems. The inferior light fastness may be due to several reasons. Auto oxidation reaction of dyes is generally considered to occur on exposure to ultraviolet (UV) radiation and prevented by the addition of UV absorbers or antioxidants such as hindered phenols or naphthylamines. In recent years as an approach to the photostabilisation of dyes attempts have been made to prepare dyes with built-in photostabilising moiety.
Acid dyes, named for their application under acid conditions, are reasonably easy to apply, have a wide range of colours and, depending on dye selection, can have good colour fastness properties. The dyes are divided into three categories according to their levelling and fastness properties, namely levelling, milling and super milling dyes.
Levelling, or equalising, acid dyes have good levelling properties and are applied from a bath containing sulphuric acid to achieve exhaustion. Because of the ease of migration of dye molecules into and out of the fibre, equalising acid dyes have poor fastness to washing, and are normally used for pale, bright shades where fastness is not paramount.
Milling acid dyes have a greater substantivity for the fibre than levelling dyes, and therefore have poorer levelling properties. These dyes have better fastness properties than levelling acid dyes, and have reasonable wet fastness, particularly if alkaline milling is to take place in a subsequent process.
Super milling acid, or neutral dyeing, dyes are applied in a similar way to milling acid dyes, except that greater control over the strike rate of the dye is exercised. Super milling dyes give very good fastness and, with an appropriate after-treatment, can satisfy requirements for shades of medium depth, especially where reasonable brightness is needed.
Thus there are considerablef differences in the properties and application methods within the whole range of acid dyes. The dyer must take care to ensure that the dyes chosen in combination are from the same group and have very similar properties.
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 or naphthoquinone dyes. Disperse dyes are used mainly for polyester, but also for cellulose acetate and triacetate, polyamide and acrylic fibres. 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 lignin sulphonates are widely used for this purpose. The following chemicals and auxiliaries are used for dyeing with disperse dyes;
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 polyester fibre, dyeing with disperse dyes at temperatures up to 100°C requires the use of carriers. Because of environmental problems associated with the use of carriers, polyester is preferably dyed under pressure at temperature >100°C without carriers. However, carrier dyeing is still important for polyester-wool blends.
Thickeners: polyacrylates or alginates are usually added to the dye liquor in padding processes.
Reducing agents (mainly sodium hydrosulphite) are added in solution with alkali in the final washing step for the removal of unfixed surface dye.
Owing to their low water solubility, disperse dyes are largely eliminated by adsorption on activated sludge in waste water treatment plants. Some disperse dyes contain organic halogen, but they are not expected to be found in the effluent after waste water treatment because of their adsorption on activated sludge.
Reactive dye introduced on 1956 and for the first time dyeing became possible by direct chemical linkage between dye and fiber (Shenai, 1993). But all classes of reactive dye do not react in the same manner. So the group of dyes used for a ternary shade should have compatibility among themselves. Importantly, reactive dyes in a mixture should all exhaust and react with the fiber at about the same rate so that the shade builds up accurately. Dyes which are from different ranges, with different reactive groups, should not be used together because of their different dyeing character and reactivity.
Compatible dyeing performance requires careful control of the dyeing parameters such as temperature, salt and alkali concentrations, the dyeing time and the liquor ratio. There is often a doubt about the particular reactive group presents in a reactive dye. For that reason in most of the cases selection of dyes depends on the maker’s recommendations (Broadbent, 2001).
Shenai (1997) discussed in detail about the chemistry of vinyl sulphone dyes like Remazol class. Common salt and alkali plays the vital role in exhaustion and fixation of these dyes and addition of salt to the dye bath before adding the alkali is also essential. In reactive dyeing, though water is the competitor for reaction with the dye, cellulose fiber takes part in the reaction in majority. Because the substantivity of reactive dye to the fiber is greater than that to water (Chinta and Vijaykumar 2013).
But factually all the reactive dyes do not have the same range of substantivity and reactivity, and intermediates are usually used. Reactivity is compulsory for these dyes but higher reactivity of a dye can spoil the dyeing due to hydrolysis. So the compatibility of the dyes used for ternary shades should be analyzed carefully to make the maximum utilization of each dyestuff especially when the reactive groups in them are different.