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

Chromium content of residual dyebaths
Analysis by atomic absorption spectroscopy indicated that the effluent obtained
from the chrome(III) complex dyeings contained considerably higher
concentrations of residual chromium than the dichromate standards. However,
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The coloration of wool
195
a significant reduction in the concentration of residual chromium may be
achieved by a reduction of the Cr factor when using the chrome(III)/maleate
system (Table 8.2). Trials have shown that the Cr factor may be reduced to
0.20 without affecting the shade or fastness to potting. Reduction of the Cr
Wool—SH
Wool—S—S—Wool
[O]
Cr
3+
Wool—S
Wool—S
S—Wool
S—Wool
L
L
Cr
Wool—S—S—Wool + Cr
3+
[O]
Diamond Black PV
L = ligand group
O
O
O
O
N
N
NaO
3
S
O
O
O
N
N
Cr
O
NaO
3
S
8.8 
Proposed mechanism for the action of hydrogen peroxide in the
Cr (III)/maleate complex chroming system.
Table 8.2 Residual chromium levels in effluent from 4% o.m.f. Diamond
Black PV (200%) dyeings with varying Cr factors
Cr factor and method
Chromium level detected
in effluent (ppm)
0.35 Dichromate-thiosulphate standard
12.0
0.35 Cr(III) complex
104.0
0.25 Cr(III) complex
53.6
0.20 Cr(III) complex
32.8
0.175 Cr(III) complex
24.6
0.15 Cr(III) complex
12.3
0.12 Cr(III) complex
11.5
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Advances in wool technology
196
factor to lower than 0.175 leads to poor chroming and inadequate shade
development.
Further reduction in the chrome content of the effluent was achieved by
fresh bath chroming. However, King and Brady
43
 have already noted this
effect when dyeing with dichromate and attributed it to soluble protein reacting
with dye and chromium in the after-chrome bath. Rinsing prior to after-
chroming will remove residues of dye and soluble protein which could compete
in the bath for the Cr(III) complex. Amino acid residues such as aspartic
acid, glutamic acid, cysteine and histidine will form strong complexes with
Cr(III).
The reflectance spectra of 4% o.m.f. dyeings of Diamond Black PV chromed
by the standard and the new Cr
3+
 maleate method were taken and K/S values
calculated, where K is the coefficient of absorption and S is the coefficient
of scalter. 
Figure 8.9
 reproduces these spectra and it is clear that the colour
yields of the two processes are very similar.
8.3.2
Absorbable organo halogens in wool dyeing and
finishing effluents
Special attention is required when discharging compounds containing AOX.
The concept of AOX has arisen from a German drinking water directive
(DIN 38409414 – 1985). Consent limits as low as 2 mg/L have been applied.
Products used by the dyer and printer which fall into this category are the
insect-resist agents applied to wool in dyeing, some types of carriers for
disperse dyeing polyester, certain chromophores themselves and some halogen-
containing classes of reactive dyes. This type of foreseeable legislation will
therefore direct the colour chemist to develop new systems which will reduce
400 420 440 460 480 500 520 540 560 580 600 620 640 660 680 700
Wavelength (nm)
•  Dichromate
+  Cr(III) complex
60
50
40
30
20
10
0
K/S
8.9
 K/S spectra for Diamond Black PV, using standard dichromate
and the chrome maleate complex (fresh bath chroming methods).
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The coloration of wool
197
these discharges to the allowable limits. These implications are indeed far-
reaching. For example, in the case of reactive dyeing cellulosic fibres, the
demand for achieving virtually 100% dye uptake and covalent bonding is
urgent. Research is therefore required whether to develop innovative reactive
systems or to develop fibre pre-treatments which solve the problem of existing
reactive dyes.
It is worthy of note that dyeings of chlorinated wool leads to effluents
containing on average more than 1000 mg/L AOX. So far special dispensation,
to discharge this concentration of AOX, has been given to dyers of machine-
washable wool produced by the chlorine-Hercosett (TM Hercules Powder
Corporation) continuous top process. An important research activity is the
production of machine washable wool using non-chlorine procedures. In
fact there is renewed urgency in this area as expected EU directives are
likely to force processors to abandon the chlorination route.
Absorbable organo-halogen (AOX) free shrink-resist finishing and its
relation to dyeing
Currently the most popular method to produce machine-washable wool tops
is the so-called chlorine-Hercosett process. The chlorination pre-treatment
removes much of the covalently bound lipid material from the wool surface
and also oxidises surface cystine disulphides to cysteic acid; these effects
cause the fibre surface to become both hydrophilic and anionic and highly
receptive to the cationic polymer, Hercosett, which ‘exhausts’ and spreads
onto the surface of each individual fibre.
Alternative systems for preparation of the fibre surface for shrink-resist
treatments based on peroxide chemistries have been explored but to date
have not realised the same commercial success as chlorine-Hercosett. Most
studies have in fact explored the potential of Caro’s salt (KHSO
4
·KHSO
5
) as
a direct chlorine replacement
44
 but in this case it is vital to include a sodium
sulphite treatment not only to remove excess oxidant but to generate sufficient
shrink resistance; undoubtedly this system produces anionic polypeptides at
the fibre surface by a process of oxidative sulphitolysis
45
 to give cysteine-S-
sulphonate (wool Bunte salt) as the hydrophilic anionic residues. In continuous
processing this technique has not allowed the application of Hercosett polymer,
presumably because removal of covalently bonded lipid is inefficient compared
to chlorination processes. Other workers
46
 have trialled permonosulphuric
acid/sulphite followed by silicon polymer application in an attempt to re-
dress the above problem; unfortunately silicon polymers lead to subsequent
problems in spinning and dyeing.
It is something of a hindrance, and an extra cost to be borne by the
processor, that a separate process has to be carried out to shrink-proof wool,
usually prior to dyeing. The author has reported on a possible system
47
 to
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Advances in wool technology
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achieve these objectives; this system involved the co-application, in a pad–
batch fabric dyeing process, of a tri-functional Bunte salt-acetoxy-polyol,
with reactive halo-s-triazine reactive dyes. Providing that urea and sodium
bisulphite were also present in the pad-liquor, excellent dye fixation and
level dyeings were obtained – the fabrics were also machine washable and
possessed a good soft handle. The Bunte salt ‘capped’ polyol is water soluble
and surface active; reaction with wool thiol groups converts it to a crosslinked
poly-disulphide-polyol which is water insoluble and imparts the shrink-proofing
effect via fibre–fibre spot welding.