A type of acid dye applied in a pre-formed metal (e.g. chromium) complex form. The Colour Index classifies it under Acid Dye, but the source treats it separately from acid mordant dyes and ordinary acid dyes.
Claims and evidence (5)
The limitations noted for some sources are recorded in Japanese only — see the Japanese edition.
Metal-complex acid dye ・typeStates that two classes of dye bond to polyamide fibre in the form of a dye-metal complex: acid mordant dyes and metal-complex acid dyes. Acid mordant dyes were developed first, but because they form a chromium complex through a chemical reaction on the wool during dyeing, reproducibility was poor and colour matching difficult, especially for pale-to-medium shades; metal-complex acid dyes, pre-complexed with metal before dyeing, were developed to address this.contextual
Scope: Classification of metal-complex acid dyes
Describes two classes of dye that bond to polyamide fibre as a metal-dye complex — acid mordant dyes and metal-complex acid dyes — noting acid mordant dyes were developed first but, because they form a chromium complex through an on-fibre reaction on wool, left a colour-matching problem especially for pale-to-medium shades, which metal-complex acid dyes (pre-complexed before dyeing) were developed to solve.
Metal-complex acid dye ・bondingStates that metal-complex acid dyes form a coordination bond between the metal and the oxygen atoms flanking the azo group, which shields the azo group and gives better fastness (e.g. lightfastness) than ordinary azo acid dyes without such complexation; also states the metal, bonding with fibre amino and carboxyl groups, gives firmer dyeing than ordinary acid dyes. There are two types: 1:1 (one metal atom, one dye molecule) and 1:2 (one metal atom, two dye molecules); the 1:1 type needed a strongly acidic bath, a drawback later addressed by the 1:2 type, dyeable under weakly acidic to neutral conditions.contextual
Scope: Bonding of metal-complex acid dyes to wool/nylon
States that in metal-complex acid dyes, the metal coordinates with the oxygen atoms flanking the azo group, shielding it and giving better fastness than uncomplexed azo acid dyes, and that the metal also bonds with fibre amino/carboxyl groups for firmer dyeing than ordinary acid dyes; describes the 1:1 type (one metal atom, one dye molecule) versus the improved 1:2 type (one metal atom, two dye molecules), the former needing a strongly acidic bath and the latter dyeable under weakly acidic to neutral conditions.
States there are two types of premetallised dye: a 1:1 type (one metal combined with one dye molecule) applied from a strongly acid bath, and a second type (two dye molecules combined with one metal ion) applied from a neutral to weakly acid bath. Table 14.1 itself labels these '1:1 Metal complex' and '1:2 Metal complex'.
Metal-complex acid dye differs from acid mordant dyecontextual
Scope: Metal-complex acid dyes vs acid mordant dyes (Table 1)
Table 1 contrasts metal-complex acid dyes and acid mordant dyes on colour tone, fastness, optimum pH, initial uptake, migration, levelling, dyeing time, fibre-strength loss, wastewater chromium load and price. Verified against a 400dpi page image.
Metal-complex acid dye ・propertyThe same Table 1 describes metal-complex acid dyes as having a structure in which one metal atom is coordinated to 1-2 dye molecules, with 0-3 sulfonic acid groups, containing chromium or cobalt, giving good lightfastness but a duller hue.contextual
Scope: A comparison of dyes for polyamide fibres (Imada part 6, Table 1)
The "Metal-complex acid dye" row of Table 1 gives chemical structural characteristics as "one metal atom coordinated to 1-2 dye molecules", sulfonic acid groups "0-3", metal "Cr, Co", and characteristics/use as "good lightfastness / dull hue".
Metal-complex acid dye ・requiresFor the neutral-dyeing 1:2 premetallised dyes, states the fabric is pretreated at 40°C with ammonium sulphate or acetate, the dye is then added and the bath raised to the boil at 1-2°C/minute, held at the boil for half to one hour. Notes that both 1:1 and 1:2 premetallised dyes are slow to dye, with good levelling but poor migration.contextual
Scope: 1:2 premetallised (metal-complex) dyes on wool.
As stated in the claim above.
Open questions (2)
Is the coordination bond between a metal-complex acid dye's metal and the fibre's functional groups the same mechanism as the coordination bond between a mordant metal ion and a natural dye?
Imada (part 7) states that in metal-complex acid dyes, the metal forms a coordination bond with the fibre's amino/carboxyl groups, giving more durable dyeing. Our records already have a similar claim for natural-dye mordanting (coordination between a mordant metal ion and the dye), itself flagged as unconfirmed. Here the dye's own metal bonds to the fibre's functional groups, which looks like the reverse direction from natural-dye mordanting (where the mordant metal bonds to the dye, not the fibre) -- are these chemically the same phenomenon?
Does Tanaka Nao's Delkus 'metal-complex type' correspond to the 1:1/1:2 metal-complex acid dyes described in Imada's paper?
Tanaka Nao's Delkus product page states only that the dye comes in an acid type and a metal-complex ('含金') type, without specifying the metal or whether it is a 1:1 or 1:2 complex. This could not be confirmed from the sources checked.
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