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

Acid Dyes

What are acid dyes?
Acid dye class is a water soluble class of dyes with anionic properties. The textile acid dyes are effective for protein fibers such as silk, wool, nylon and modified acrylics. Acid dyes fix to the fibers by hydrogen bonding, vander waals forces and ionic linkages.

Chemical structure of acid dyes
These dyes are normally very complex in structure but have large aromatic molecules, having a sulphonyl or amino group which makes them soluble in water. Most of the acid dyes belongs to following three main structural molecules,
1.Anthraquinon type
2.Azo dye type
3.Triphenylmethane type.

Different types of acid dyes
The basic dyes are classified into several groups , based on the leveling properties, economy of the dyeing and fastness properties, however generally these are classified into these three classes,
1.Neutral acid dyes :-
These are supra milling or fast acid dyes, having medium to good wet fastness properties , some of the dyes have poor light fastness in pale shades . many of the dyes are used as self shades only. These are applied to the fiber in a weakly acid or neutral pH.
2.Weak acid dyesThese dyes belongs to the milling class of dyes. These dyes have good fastness properties but light fastness is moderate to poor.
3.Strong acid dyes
These dyes are applied in a strongly acidic medium and also called leveling dyes, however there wet fastness properties is a limitation. These dyes are very good to produce the combination shades.

Classification according to dyeing characteristics
Acid dyes are commonly classified according to their dyeing behaviour, especially in relation to the dyeing pH, their migration ability during dyeing and their washing fastness. The molecular weight and the degree of sulphonation of the dye molecule determine these dyeing characteristics. The original classification of this type, based on their behaviour in wool dyeing, is as follows:
(1) Level dyeing or equalising acid dyes;
(2) Fast acid dyes;
(3) Milling acid dyes;
(4) Super-milling acid dyes.
Milling is the process in which a woollen material is treated, in weakly alkaline solution, with considerable mechanical action to promote felting. Dyes of good fastness to milling are essential to avoid colour bleeding during the process.

Properties of acid dyes
The main properties of acid dyes are ,
Since these are sold as a sodium salt, there fore these form a large anion in the aqueous medium. 1.These dyes are anionic in nature.
2.These dyes are suitable for wool, silk, polyamide and modified acrylics.
3.These are applied from a strongly acidic to neutral pH bath.
4.These dyes have no affinity for cotton cellulose’s , hence not suitable for cellulosics.
5.These dyes combine with the fiber by hydrogen bonds , vander waals forces or through ionic linkages.
Mechanism of dyeing with acid dyesDissolution of dyes in aqueous solvent, produces a colored anion,

The protein and polyamide fibers produce cationic sites in water under acidic conditions, as the acidity of the solution is increased more cationic sites are produced under these strongly acidic conditions. These cationic sites are thus available for the acid dye anions to combine with through hydrogen bonding, vander waals forces or ionic bonding. These linkages are strong enough to break , and thus dyeing produced are fast .
Reaction between an acid dye and wool can be represented by following equation

ON WOOL
Electrolyte in the acid dye bath acts as a retarding agent because of chlorides ions attracted by the positive sites at the fiber and in the competition between. Addition of acid acts as a n exhausting agent , because strongly acidic conditions makes more cationic sites available and thus available dye anions got combined with these.

Dyeing temperature
The dyeing is generally carried out at boiling temperature for 30- 60 minutes depending upon the depth of the shade and dyestuffs used.
Dyeing leveling agents
In the case dyeing with acid dyes , mainly cationic agents such as ethoxylated fatty amines are used as leveling agents.
Heating rates
Heating rate is generally kept 1-30C/Min
Washing off process
A typical dyeing cycle of nylon filament dyeing with acid dyes is shown in the above chart,
Wool dyeing method with acid dyes

Method 1

At A set bath at 50° with:
4% Sulphuric Acid (96%)
5% Glaubers Salt anhydrous,
pH 2.5 to 3.5

At B add required amount to dyestuff.
Method 2

At A set bath at 50° with:
2% Formic Acid (85%)
5% Glaubers Salt anhydrous,
pH 3.5 to 4.5

At B add required amount of dye.

At C add 2% Sulphuric Acid (96%) or 2% Formic Acid (85%).

Thoroughly rinse after dyeing to remove loose colour.
A dyeing cycle for nylon filament dyeing



Fastness properties of acid dyes?
The wet and light fastness properties of the acid dyes varies from poor to excellent , depending upon the molecular structure of the dyes.
The fastness properties as per the category are as follows
Neutral acid dyes:-since these dyes have very good leveling and migration properties ,and have a low affinity for the fiber, therefore the wet fastness properties of this class are generally poor.
Weak acid dyes or half milling dyes :- These dyes have a medium to good affinity for the fiber and are generally applied in a weakly acidic bath, shows medium to good wet fastness properties. Strong acid dyes or super milling dyes :- These dyes have poor exhaustion properties, therefore applied under very strong acidic condition , exhibit good fastness properties.