Saminathan Ratnapandian

Acid dyes
These are dyes primarily used to colour nylon, wool and silk under acidic conditions. The low number of reaction sites in nylon results in a poor colour yield. This is rectified by the formation of metal-complexes by mordanting, which is the application of metallic salts in a separate step or by using pre-metallized dyes. In the case of pre- metallized dyes, the metal is incorporated in the dye molecule during manufacture.
Depending on the number of dye molecules attached to a metal atom, they are classed
as 1:1 or 1:2 metal complex dyes. The dyes exhibit high saturation values, compatibility in mixed shades and good light and wet fastness properties [23].


Disperse dyes
These non-ionic, insoluble or sparingly soluble dyes are applied to hydrophobic fibres, generally from an aqueous dispersion. They are used primarily to dye polyester, although they have been used for other fibres as well [24]. Dyeing is usually followed by a reduction-clear to prevent staining from the unfixed dye. Some unique methods are used for the application of disperse dyes.


Basic dyes
These dyes are special in that they form coloured cations in solution. They are used particularly to dye acrylic and paper. Their variable migration properties necessitate the use of special auxiliaries during dyeing [11].


Pigments
These are colouring matter that is insoluble in water with minimal or zero affinity to textile materials. Special techniques are used to print pigments onto textiles [25].

2. Natural dyes

The name natural dye applies to all colouring matter derived from natural sources, such as plants, animals and minerals. The use of natural dyes has been recorded across the globe since ancient times [10, 26]. They were the only dyes available and used until 1856 when Sir W. H. Perkin discovered the first synthetic dye “mauviene”, a basic dye [27].

The extraction process for natural dyes, although time-consuming and laborious, is fairly straightforward and involves no applied chemistry. The dyes are mostly derived from flora and fauna. This organic origin makes them readily susceptible to climatic changes and other natural variations. The result is either a) the textile acquires a unique shade for every batch or b) consistent and repeatable shades can be produced with difficulty only by a skilled and knowledgeable dyer. Similar to the multitude of colours present in nature, a wide variety of colours can be obtained using these dyes. Dye classification, which began on simple alphabetical order, is now in line with the

directives of the Colour Index and is based on the parameters of structure, origin, application method and colour [10, 21, 28].
Natural dyes are broadly classified as adjective and substantive dyes based on their inherent affinity for textiles. Substantive dyes possess a high affinity for textiles. They may be classified as belonging to vat, acid, direct and pigment classes [21, 29].
However, a large number of natural dyes have low affinity and require the use of a mordant to fix the dye to textiles and hence are known as mordant or adjective dyes [30]. The word ‘mordant’ is derived from Latin mordere meaning ‘to bite’ because mordants were believed to eat away the surface or open up the pores in a fibre and thus facilitate dye absorption [31]. Mordants are now identified as chemicals that react with fibre and/or dye and thereby are able to fix the dye [28, 31]. The three broad classes of mordants are single or bi-metallic salts, tannins and oil mordants [29].

Most natural dyes form metal complexes with mordants and, according to the metal involved, each complex has a distinct shade [32-34]. This gives the monogenetic and polygenetic classification of natural dyes. As the names suggest, the first type of dyes yields only one shade while the second yields different colours according to the mordant employed [21]. Naturally occurring aluminium, copper, potassium and iron salts were the traditionally used mordants and are employed even today. Their quality, in terms of purity, affects the richness and durability of the shades obtained [28]. Purer salts and other metals such as tin and chromium were introduced with advancements in chemistry [35]. Health and environmental hazards posed by incorrect handling of some of these salts, chromium salts in particular have been identified. Hence, at present the most widely used metal salt mordants are aluminium sulfate, potash-aluminium sulfate, stannous (tin) chloride, copper (II) sulfate and iron (II) sulfate. At times, tannin-metal complexes and oil-metal complexes serve as mordants for other natural dyes [36]. In general, the presence of metal increases the fastness property of the dyed material by mechanisms such as dye aggregation (lake formation) and absorption of free-radicals [37].

The process of applying mordants or mordanting is classified into the following three types [30, 31]:

1.