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The role of oxidants to prevent setting in dyeing
One of the most useful anti-setting systems for use in wool dyeing was
developed by workers at CSIRO, IWS and BASF.
74
This system, offering
improved fibre physical properties, was based on a mixture of hydrogen
peroxide and a special auxiliary Basolan AS; the latter auxiliary performed
two functions:
• Inhibition of the degradative effect of hydrogen peroxide on some wool
dyes.
• Enhancement of the stability of the oxidant in the boiling dyebath.
The BASF/IWS/CSIRO process
74
recommended dyeing wool in the presence
of hydrogen peroxide (35%) at a level of 1 mL/L (minimum 2% on o.m.f.)
and Basolan AS at 0.5 g/L (minimum 1% o.m.f.).
Kim and Lewis
75
studied the effect of hydrogen peroxide concentration
on set after boiling wool fabric for 1 hour at pH values 3, 5 and 7 in the
presence of citric acid/phosphate buffers (McIlvaine buffers); their results
are reproduced in Fig. 8.18.
Hydrogen peroxide (30%) concentrations as low as 2 cm
3
L
–1
gave set
values of 21% at pH 3 (control without oxidant gave 56% set), set values of
27% at pH 5 (control 59%) and set values of 43% at pH 7 (control 67%). The
effect of pH on set parallels the effect of pH on cystine degradation; if the
pH 3
pH 5
pH 7
80
70
60
50
40
30
20
10
0
0
2
4
6
8
10
12
14
16
18
20
Amount of hydrogen peroxide
(30 wt% solution in water) (cm
3
L
–1
)
Set
(%)
8.18
Percentage set after boiling wool, at pH values 3, 5 and 7, with
H
2
O
2
.
© 2009 Woodhead Publishing Limited
Advances in wool technology
206
mechanism is ß-elimination of cysteine as shown in
Fig. 8.14,
then a higher
concentration of oxidant should be more effective at controlling set at pH 7
where elimination is more rapid –
Fig. 8.18
confirms this hypothesis since 4
cm
3
dm
–3
hydrogen peroxide gives the best control of set. The oxidant clearly
functions by rapidly oxidising cysteine back to cystine or cysteic acid; of
possible equal importance is the oxidation of hydrogen sulphide to bisulphate
anion thus removing this reactive reducing nucleophile from the system.
Kim and Lewis
75
used Fourier transform infrared (FTIR) second
derivative spectroscopy to follow the production of cysteic acid when treating
wool at the boil with hydrogen peroxide (0-10 cm
3
dm
–3
); by measuring the
sulphonate band intensity, attributed to cysteic acid, at 1040–1044 cm
–1
it
was shown that over-oxidation to cysteic acid was most significant in those
treatments carried out at pH 7. Other oxidants shown to have anti-setting
properties include sodium bromate and sodium tetrathionate.
76
It is not possible
to use oxidants such as hydrogen peroxide when dyeing with reactive dyes
since the perhydroxy anion is a potent nucleophile which reacts at the
electrophilic site in the reactive dye to rapidly produce the hydrolysed, inactive
dye.
The role of electrophilic reagents to control setting in dyeing
At the same time as the hydrogen peroxide-based Basolan AS-A system was
launched, a system based on application of an electrophile Basolan AS-B
was also marketed but the structure of the latter reagent remains undisclosed.
Kim and Lewis
77
showed that sodium maleate (MAS) was a very effective
anti-setting agent when included in boiling pH 3 wool dyebaths. Their results
are reproduced in
Fig. 8.19,
which clearly demonstrates that the maleate
anion is most effective at controlling set at pH 3; it shows only modest effect
on set at pH 5 and has no effect on set at pH 7. The reason for this strong pH
dependance lies in the modest substantivity of the maleate ion for wool
under acidic conditions and its almost total lack of substantivity at pH 7 and
above. It was estimated,
77
from capillary electrophoretic analysis of the
treatment baths, that when using 2 g/L sodium maleate for the 1 hour boiling
treatment, maleate anion uptake was 22% at pH 3, 15% at pH 5 and 4% at
pH 7. The mechanism whereby maleate anions reduce the extent of wool
setting in boiling aqueous treatments is described in
Fig. 8.20.
Liao and Brady
78
synthesised N-naphthylmaleimide and showed it inhibited
wool setting and also protected the wool component of a wool/polyester
blend from serious damage during the high-temperature dyeing step required
to adequately dye the polyester component with a disperse dye. These authors
and others
74
attribute the main cause of setting, hence wool damage, in hot
aqueous wool treatments to the thiol-disulphide interchange reaction without
implicating cysteine residue ß-elimination reactions.
© 2009 Woodhead Publishing Limited
The coloration of wool
207
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