O o d h e a d p u b L i s h I n g L i m I t e d

8.3
Environmental impact of wool coloration
processes
8.3.1
Chrome dyeing processes
In order to achieve very black shades on wool, chrome dyes have found
widespread usage since they give the desired shade of black, coupled with
outstanding wash- and light-fastness. It is interesting to note that the blackest
blacks produced for the Japanese tuxedo market are invariably produced by
dyeing chrome dyes on chlorinated wool. It has been estimated that chrome
dyes still share some 20% of the total wool dye market, predominantly for
dyeing black and navy blue shades. Invariably the dyeing is after-treated
with sodium dichromate to produce the metal complex dye in the fibre. It is
the toxicity of Cr(VI)
 
which should give rise for concern both from the point
of view of its effect on worker’s health and its effect in the environment
following discharge of exhausted dyebath. Chromium(III) is relatively safe,
it is naturally present in soils and is even considered an essential human
dietary component. If Cr(III) could be used then chrome dyeing would be a
much more environmentally friendly procedure.
The Cr(III) cation cannot be applied directly as the chroming agent as
under the acidic conditions used in dyeing it has low substantivity for the
positively charged, protonated wool fibre. However, the trivalent chromium
cation can be readily converted to an anionic complex by reaction with
suitable ligands; such complexes have substantivity for wool under hot acidic
conditions. The complexes must not be too stable since the carrier ligands
have to readily exchange the chromium with the ligand groups in the dye.
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Advances in wool technology
192
Suitable complexing compounds include 
α-hydroxycarboxylic acids such as
lactic acid and salicylic acids, and polycarboxylic acids such as maleic acid.
34–
41
 Much of the published work has focused on lactic acid
34–38
 and 5-sulpho-
salicylic acid.
39,40
An important factor to be taken into consideration when replacing Cr(VI)
with anionic Cr(III) complexes is the oxidising effect of the dichromate
anion. During after-chroming dyed wool with dichromate, absorbed Cr(VI)
is reduced by the wool fibre to a Cr(III) species which complexes with the
dyestuff to give full rich shades of excellent wet-fastness. The exact nature
of the oxidising effect of dichromate is not fully understood, but it has been
shown that in order to produce equivalent dyeings with the Cr(III) complex
system a substantial amount of an oxidising agent (usually hydrogen peroxide)
must be added during the chroming stage to effect full shade development
and to produce dyeings of the expected wet-fastness. The amount of hydrogen
peroxide required is influenced by the dye used; in some cases the dye
molecule itself should be oxidised to give the expected chrome-dyed shade
as is the case when dyeing with CI Mordant Black 9 (Diamond Black PV –
DyStar).
38
 This oxidation process is shown in 
Fig. 8.6.
Xing and Pailthorpe have described the use of the anionic complex prepared
from 5-sulpho-salicylic acid and chromic sulphate
39
 for after-chrome dyeing
of wool and have recently further developed the method for the industrial
processing of cashmere fibres.
40
 To achieve the correct shade and wet-fastness
properties these authors proposed the addition of a small amount (0.6% on
mass of fibre, o.m.f.) of Cr(VI) to the after-chrome process 5 minutes after
the addition of the Cr(III) complex. The structure of this latter complex was
not revealed, simply being coded SCA-Cr; this addition of Cr(VI) can be
interpreted as an oxidation step. Lewis, et al.
41
 have described an after-
chrome dyeing system using sodium maleate as the preferred complexing
agent for Cr(III) cations and oxidation at the end of the after-chroming
process with hydrogen peroxide.
The maleate complex was readily prepared by dissolving maleic anhydride
(18.7 g) in water (100 mL) and the solution boiled for 5 minutes, then cooled;
this solution was neutralised using solid sodium carbonate (20 g); chrome
alum (48.4 g) was dissolved in water (200 mL), this solution was added to
the sodium maleate solution and the volume was made up to 500 mL with
water; boiling for 5 minutes ensured complex formation (ligand : chromium
= 2.1). When dyeing 1 kg of wool, 500 mL of this solution would be added
to the dyebath at the after-chrome stage giving a Cr factor of 0.35. The
nature of the complex is shown in 
Fig. 8.7.
Lewis et al.
41
 developed the following chrome dye application process:
dyeing was carried out in a 1 kg Obermaier sample package dyeing machine.
The bath, containing 1 kg of wool yarn wound on a package, was set at 50
°C
at a liquor to goods ratio of 16:1, and at pH 4.0; chrome dye was added, the
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The coloration of wool
193
bath raised to the boil over 40 minutes and maintained at the boil for 1 hour.
The temperature was then lowered to 80
°C and the appropriate amount of
Cr(III)/maleate complex added; the pH was adjusted to 3.5–4.0 with formic
acid and the bath returned to the boil. After 15 minutes hydrogen peroxide
(65 mL of 35% w/w material) was added and after another 15 minutes a
further 65 ml H
2
O
2
 (35%) added; boiling was then continued for a further 25
minutes. The dyed package was washed-off for 20 minutes at 50
°C (pH 8.0
NaO
3
S
OH
OH
N    N
OH
C.I. Mordant Black 9
OH
OH
N    N
NaO
3
S
[O]
O
O
NaO
3
S
OH
O
O
O
N    N
H
8.6 
Oxidation of Diamond Black PV (CI Mordant Black 9).
H
H
O
O
O
O
H
H
O
O
O
O
Cr
H
2
O
H
2
O
Na
+
8.7
 Maleic acid/chromium (III) complex with Na
+
 counter-ion.
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Advances in wool technology
194
with ammonium hydroxide, 1 g/L 0.880 NH
3
); final rinsing with dilute acetic
acid (pH 5) at 40
°C completed the process.
For comparative purposes the following standard chrome dyeing process
was carried out. In this case the first dyeing stage was as described but the
after-chrome process was performed by cooling the dyebath to 80 
°C and
adding the requisite amount of dichromate (for 4% o.m.f. Diamond Black
PV (200%) a Bayer G Cr factor of 0.35 was used); the bath was returned to
the boil and sodium thiosulphate (Na
2
S
2
O
3
·5H
2
O) added (1.7 
× mass of
K
2
Cr
2
O
7
 used). Boiling was continued for a further 30 minutes.
In the case of those dyes that do not change structure on oxidation, for
example, Diamond Fast Bark Blue RRN (CI Mordant Blue 9) and Diamond
Fast Bordeaux BL (CI Mordant Red 30), the total amount of peroxide required
for a 1 kg package dyeing must be reduced to 120 ml of 35% H
2
O
2
. The
requirement to add peroxide is necessary to produce dyeings having a wet-
fastness level comparable to those dyeings produced with the usual dichromate
process. In particular, potting fastness tests critically distinguish the non-
peroxide after-treated samples.
It was proposed that the necessity to use oxidising agents to achieve the
maximum wet-fastness properties from the Cr(III)/maleate system is due to
the strong binding of Cr(III) to thiol ligands. During dyeing of wool at the
boil significant setting occurs due to thiol–disulphide interchange reactions;
42
undoubtedly the small amount of cysteine-free thiol (circa 39 
µmoles per kg
of wool) undergoes 
β-elimination to produce dehydroalanine and H
2
S. The
latter attacks intact disulphides to produce further free thiols, initiating a
chain reaction which produces significant permanent set. There is thus a
significant concentration of free thiols in the wool at the end of the dyeing
cycle which, being highly nucleophilic, selectively binds the chromium when
the Cr(III) maleate complex is introduced. An oxidant is thus necessary to
free the chromium from this thiol ligand binding in order to make it available
for complex formation with the dye. 
Figure 8.8
 summarises this mechanism.
If oxidation in the after-chrome cycle re-forms toxic Cr(VI) then these
research results would not be too useful practically. Thus the sensitive diphenyl-
1-carbazide colour test for Cr(VI) formation was carried out on residual
dyebaths from dyeings produced by the new process; no positive results
were seen. However when peroxide oxidation of the Cr(III) complexes was
carried out under alkaline conditions, clear evidence for Cr(VI) formation
was seen.