These dyes are so named because they have some degree of fastness to milling process, which indicates a higher level of wet fastness than other acid dyes.
Two main subdivisions.
A. Monosulphonted dyes (mol.Wt 500-600), these have been described as half acid milling dyes since they migrate and cover well but are little inferior to traditional acid milling dyes in terms of wet fastness.
B. Disulphonated dyes of high mol. Wt (r.m.m) 600-900. These dyes diffuse much more slowly than typical leveling acid dyes and exhibit correspondingly higher wet fastness. However, migration and coverage properties are inferior and the addition of leveling agent is necessary.
Non-polar Vander Waals forces are involved between these dyes and wool. Resulting in relatively poor migration properties.
Hydrophobic interactions lead to reduced migration but increased wet fastness
Super milling acid dyes are similar to disulphonated milling acid dyes but contain higher alkyl substituents (e.g. butyl, octyl, dodecyl) to impart more hydrophobic character to the dye molecule.
These dyes show exceptionally good wet fastness.
They are used for bright colours on loose wool or slubbing where any batch to batch variations can be eliminated by blending
These dyes are applied with a leveling agent and dyeing method is carefully designed to ensure uniform uptake since the dyes do not migrate readily. .
This group comprises the dyes of high anion affinity which require minimum of acid. These are sometimes called neural dyeing acid dyes. The dyeing procedure is
The material is entered at 60 0 C into dyebath containing 2-5% of ammonium acetate The temp. is raised to boil in 45 min. It may be necessary to add 1-2% of acetic acid (30%) after boiling for 30 min. in order to exhaust the dye bath.
Chemically chrome mordant dyes are closely related to acid dyes but their molecules contain additional groups in O,O’ position ( such as OH OH, OH NH2, COOH COOH etc) which enables the dye to form stable co-ordination complex with chromium within the fibre.
Besides o o’dihydroxyazo compounds, the 1:1 metal complex dyes include o-amino-o’-hydroxy azo compounds and derivatives of salicylic acid. (COOH COOH groups).
These dyes are mostly monosulphonates of mol.Wt. 400-500.
This gives them dyeing properties somewhat similar to those of mono-sulphonated leveling acid dyes.
In spite of the decline in recent years in the use of 1:1 metal complex dyes, the dyes continue to be used in the dyeing of loose stock and yarn for floor coverings, hand knitting yarns and piece goods.
They exhibit excellent level dyeing and penetration characteristics.
The dyes have good light fastness and moderate wet fastness.
1:1 metal complex dyes are usually applied to wool from a strongly acidic (pH 2) dye bath ( hence dyes some times are referred to as acid dyeing metal complex dyes).
Under these conditions the dyes possess excellent migrating and leveling properties.
Since wool absorbs approximately 4% owf of sulphuric acid (96%), an excess of acid is required in order to maintain a suitably acidic dyebath.
Chelating agents for water softening should not be used owing to demetallization of some dyes.
Because prolonged boiling under such low pH conditions can cause fibre damage, either reduced amounts of sulphuric acid or other acids such as formic acid (8-10%) owf) or proprietory leveling agent can be used.
The dyes can also be applied at 80 0 C so as to reduce fibre damage.
BASF suggest the use of sulphamic acid in place of sulphuric acid.
The pH of the dye bath at the beginning is 1.8,
but as the temp. rises to boil the pH increases between 3 – 3.5 owing to hydrolysis of sulphamic acid leading to less fibre damage compared to sulphuric acid.
The high fixation ratio has been attributed to the high reactivity of partially hydrolyzed dye, since the reactivity of second fluorine -carbon is only slightly decreased after the first fluorine has reacted with wool.
These findings indicate that the dye molecule has two reactive centers, which can react independently with the nucleophiles in protein fibres.
There is also evidence that these dyes form cross-links both with wool and silk.
In silk tyrosine OH group also takes part in the reaction.
Levelling agents Drimagen F (S) and Avolan RE (BAY) are recommended to get uniform dyeing.
The novelty of this class of dyes lies in the controlled nature of the reaction with fibre.
The actual reactive group is formed only gradually during the exhaustion dyeing, so that in the early stages of dyeing these dyes behave more like an acid level dyes, offering a real chance of sufficient dye migration to obtain level dyeings.
These dyes are blocked vinyl sulphone derivatives which gradually activate to the reactive vinyl sulphone at elevated temperature.even under slightly acidic conditions.
The main advantage of such system is an improvement in dye levelness, due to suppression of dye-fibre covalent bonding at temp. below boil.
It is believed that Hostalan E brands are the most level dyeing N-methyltaurine adducts,
other Hostalans are -sulphatoethyl sulphone specially selected for their ready formation of the reactive vinyl sulphone form under the weakly acidic boiling conditions required for wool dyeing.