Metal Complex Dyes: Properties, Classification, Uses, Advantages and Disadvantages
Nikhil Yogesh Upadhye
Department of Textiles (Textile Chemistry)
DKTE’S Textile and Engineering Institute, Ichalkaranji, India
Intern at Textile Learner
Pre-metallized acid dyes are another name for metal complex dyes. To make these dyes, specific proportions of copper, chromium, cobalt, or nickel are used. One metal ion reacts with one dye molecule to produce 1:1 metal-complex dyes, whereas one metal ion reacts with two dye molecules to produce 1:2 metal-complex dyes. Because all of a metal’s valences aren’t coordinated with just one dye molecule, the 1:1 complex has unsaturation. It’s possible that this will result in the formation of 1:2 unsymmetrical complexes containing two different dye molecules. This is due to the incorporation of a metal, usually chromium, into the dye molecule during production. The presence of metal in the dye molecule allows the dye to be applied more easily to the fiber, resulting in a faster dyeing process. Despite the fact that most transition metals can form complexes with the dye, chromium complex dyes are the most commonly synthesized and marketed.
Properties of Metal Complex Dyes:
- Premetallized dyes are another name for these dyes.
- Premetallisable acid dyes were used to make earlier members of this class of dyes.
- Therefore these dyes are also classified as acid dyes in the color Index.
- Though most of the transition metals can form complex with the dye, commercially, chromium complex dyes are mostly synthesized and marketed.
Classification of Metal Complex Dyes:
The dyes can be classified in two groups depending on their constituents.
- 1:1 metal complex dye
- 1:2 metal complex dye
These dyes are further classified into two groups.
- Unsulphonated 1:2 metal complex dye (conventional)
- Sulphonated 1:2 metal complex dye
All the above dyes are held in the fiber by co-ordinate and electrovalent bonds.
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All types of metal complex dyes are described below:
1:1 Metal complex dyes
1:1 metal complex dyes are also known as acid dyeing pre-mezallised dyes i.e. one metal atom, usually chromium, combined with one molecule of an acid dye. The 1:1 metal complex dyes are obtained from azo dyes in which -SO, H groups are present in the molecule (thus making it water soluble) and there are two hydroxyl groups in the two ortho positions relative to the azo groups, e.g. o, o’ dihydroxyazo compounds, o-amino-o’-hydroxyazo compounds and derivatives of salicylic acid. As shown in Figure 1.
Certain azo dyes having hydroxyl groups in each of the ortho positions related to the azo group are capable of forming stable complexes with chromium. In complex formation, the stable outer shell of the chromium atom consists of 12 electrons i.e. its coordination number is 6. A simpler complex with one molecule of dihydroxy azo dye can thus be formulated. The two original -OH groups are salt forming, changing Cr³+ to Cr, the lone pair of nitrogen atoms of the azo links donating two electrons, the other 6 electrons coming from the lone pairs of oxygen atoms of three water molecules. Such donor atoms or groups are called “ligands”.
1:2 Metal complex dyes
1:2 metal complex dyes are also known as neutral dyeing pre-metallised dyes, where one metal atom complexes with two molecules of an azo dye. These dyes are dyed frem a neutral or slight acidic bath. The 1:2 metal-complex dyes are mostly chromium complexes and differ from the 1:1 metal-complex dye in that the metal atom chromium is at the center of an anion. This is brought about since the four hydroxyl or other acidic groups alter the original Cr³+ to Cr, the two azo groups acting as ligands to complete the twelve-electron system, as shown in Figure 2.
Sulphonated 1:2 metal complex dyes
These dyes are made by adding sulphonic acid groups to 1:2 metal complex dye. They are applied to wool at pH 4.5 to 5, near its iso-electric point, but the application procedure is similar to that for the 1:2 metal complex dye.
Mechanism of dyeing protein fiber with metal complex dye:
The various kinds of bonds that link metal complex dye to a protein fiber are depicted in Figure 3. Note the presence of ionic bonds or salt linkages, coordinate bonds and hydrogen bonds.
Silk and Nylon mechanism
The 1:1 metal complex dyes are hardly used for silk because the highly acidic dyeing conditions would certainly have a detrimental effect on the fiber properties.
Nylon may be dyed as described above, but by using milder acidic pH. The various kinds of bonds that are formed between a 1:1 metal complex dye and nylon 6 fiber is shown in Figure 4. Note the coordinate and ionic bonds.
Advantages and Disadvantages of Metal Complex Dyes:
1:1 metal complex acid dye:
- Dyeing method is simple.
- Uniform and level dyeing.
- Suitable for wool fiber, tops, hanks and fabric.
- Shading and brightening is possible.
- Excellent fastness properties such as washing, seawater, light, dry cleaning, crocking, etc.
- Excellent reproducibility.
- Excellent dyeing behavior in the combination process.
- Good migration property.
- Poor fastness compared to chrome dyes.
- Addition of sulphuric acid must be done in 2-3 time-bound to avoid fiber degradation.
1:2 metal complex acid dye:
- Excellent fastness properties.
- Better level dyeing.
- Better reproducibility.
- Less damage on wool.
- Shorter dyeing cycle.
- More exhaustion of dyes.
Uses of Metal Complex Dyes:
- Due to their ease of application and fastness properties, they are used for dyeing high class dress material, hosiery, ladies wear.
- Metal complex dye is using for a variety of applications like wood stains, leather finishing, stationery printing inks, inks, coloring for metals, plastic etc.
- These are suitable for wool, silk, polyamides.
- The metal used are copper, chromium and cobalt and nickel.
Metal complex dyes are also known as pre-metallized acid dyes. The dye molecule will typically be a monoazo structure with additional groups such as hydroxyl, carboxyl, or amino groups capable of forming strong coordination complexes with transition metal ions in order to make these dyes. Chromium, cobalt, nickel, and copper are commonly used. Protein fibers respond well to metal-complex dye. One of the most important aspects of successful textile trading is the dyeing process.
- Handbook of textile and industrial dyeing (Volume 1) by M. Clark Woodhead Publication
- Handbook of textile and industrial dyeing (Volume 2) by M. Clark Woodhead Publication
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