Metal Complex Dyes

Metal complex dyes
Metal complex dyes are premetalised dyes , in which one or two dye molecules are co-ordinated with a metal ion. The dye molecule is typically a monoazo structure containing additional groups such as hydroxyl, carboxyl or amino, which are capable of forming a strong co-ordination complexes with transition metal ions such as chromium, cobalt , nickel and copper.
Types of metal complex dyesThe classification of metal complex dyes is dyes is based on the basis of number of dye molecules which are complexed with one metal ion , these are
1. 1:1 metal complex dyes :-in 1:1 metal complex dyes one metal ion is complexed with one dye molecule.
2. 1:2 metal complex dyes :-in 1:2 metal complex dyes there are two dye molecules which are complexed with one metal atom.
Properties of the metal complex dyes
Fastness:-The metal complex dyes exhibit good fastness towards exposure to light , however the wet fastness properties are moderate , particularly in darker shades. However the fastness is also depends upon the fiber type and dye type.
Solubility:-These are water soluble dyes.
Application:-These dyes are applied from neutral , weakly acidic to a very strongly bath depending upon the dye sub class.
Applicability:-These dyes are suitable for dyeing of wool, silk, polyamides and leather.
Application of the metal complex dyes
a.1:1 metal complex dyes
These dyes have good leveling and penetration properties ,and are particularly suitable for application on carbonized wool. These dyes are applied under a strongly acidic bath at a pH of 1.8 –2.5 with sulfuric acid or at a pH of 3-4 with formic acid, there fore these are not suitable for the blends having cotton component. Glauber salt is used as exhausting agent and organic leveling agents are used to assist the level dyeing.
A typical dyeing cycle for wool dyeing is as given below,

b. 1:2 metal complex dyes
These dyes are subdivided into two subgroups based on the solubilising groups present in the dye molecule, these dyes shows moderate migration properties on nylon but very good overall fastness properties , because metal complex dyes not only attatch to nylon with ionic linkages , but also with co-ordinate bonds.
1.weakly polar dyes :-These dyes exhibit excellent light and wet fastness properties with excellent penetrating power. These dyes are solubilised by the inherent anionicity of the complex or containing non ionic, hydrophilic substitutes such as methylsulphone. Weakly polar dyes are applied in the range of pH 4-7 adjusted with acetic acid in the presence of ammonium acetate.
2.Strongly polar dyes:-These dyes have lower to moderate leveling properties , but have excellent fastness properties. These are solubilised by one or more sulphonic acid or carboxylic acid residues. These dyes are mainly used for the dyeing of polyamide , wool and their blends with cotton. These dyes are applied to the substrate in the range of pH 5-6 adjusted with the help of acetic acid in the presence of ammonium sulphate.
Polyamides(nylon)
On polyamides dyes containing sulphonic groups are preferred because they are more water soluble and produce better wet fastness .As the degree of sulfonation increases , lower pH are required for complete bath exhaustion and at the same time critical temperature is higher and at the same time with lower degree of sulfonation the exhaustion is better. To improve the absorption of low affinity dyes dyeing is carried out in weakly acidic bath, high affinity dyes are applied in neutral to slight alkaline bath in the presence of non ionic or amphoteric leveling agents.
Here A= Leveling agent + pH adjustment
B= pre dissolved dye