Metal-complex dyes
Metal-complex dyes (Pre-metallised dyes) which have good affinity
for protein fibres are classified into two classes:
1) 1:1 metal complex
2) 1:2 metal complex
The dye molecule will be typically a mono-azo structure containing
additional groups such as
• Hydroxyl
• Carboxyl or
• Amino groups
which are capable of forming strong coordination complexes with
transition metal ions. Typically, below metals are used
• Chromium
• Cobalt
• Nickel
• Copper
Bio-eliminability of dyes can be <50%. Some dyes contain organic
halogens (absorbable organic halogen – AOX) in wastewater,
which depends on the bio-eliminability of the dyes concerned.
Metals can be found in the effluent due to unfixed dye. However, Cr
III and the other transition metals used in metal-complex dyes are
an integral part of the chromophore
Applications in Textiles
The most prevalent metal complex dyes for textile and related
applications are metal complex azo dyes. They may be
1:1 dye: metal complex or
2:1 complex and contain mainly one (mono-azo) or two (dis-azo)
azo groups
Use of Chromium in Dyes
The most important 2:1 metal complex azo dyes are the 2:1
Cr3+dyes.
These may be symmetrical dyes, such as
CI Reactive Brown 10
CI Acid Violet 121
General properties
• Metal-complex dyes have low water solubility. The solubility
decreases from 1:1 metal-complex type to 1:2 complex type
• Due to the better stability of 1:1 complex in strong acidic
conditions (pH < 4.0), these dyes are preferably applied to
textiles at a pH lower than 4.0.
• 1:2 complexes, on the other hand, are more stable in weak
acidic conditions, and are therefore applied in a weak acidic or
neutral bath (~pH 5.0–7.0).
• Preparation of a 1:1 complex is possible at lower
temperatures, but 1:2 complexes are synthesised only at an
elevated temperature, thus limiting production by
manufacturers.
• Metals often used to form a complex are chromium, copper,
cobalt, and nickel. Chromium complexed dyes are exclusively
used on substrates requiring high light and wash fastness
qualities