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

8.5
Wool protection during dyeing by using
reactive dyes and/or anti-setting agents
Permanent setting is a contributory factor leading to loss in wool fibre strength
during dyeing. It is also the main cause of increased hygral expansion of
wool fabrics following piece dyeing, the reason for surface marks such as
‘crow’s-feet’ in piece dyeing and the source of reduced bulk or yarn leanness
following package dyeing.
58–60
 Apart from the above wholly negative effects,
permanent setting can sometimes be seen as beneficial: such instances include
the setting of wool yarn in hank form when using hank dyeing machines –
this gives extra bulk and resilience to yarns and explains why this dyeing
route remains popular for the production of wool carpets. It is necessary to
summarise the various chemistries involved in the production of permanent
set in wool dyeing.
Wool is a heterogeneous material made up of keratin proteins, a small
amount of non-keratinous proteins and even smaller amounts of lipid and
fatty acid materials. The keratinous or cystine disulphide crosslinked proteins
are responsible for most of wool’s physical properties and thus the reactivity
of the cystine disulphide residue is of paramount importance. The cystine
content of wool varies but an appropriate average value is 450
µmol g
–1
 and
for its reduced form, cysteine, 30
µmol g
–1
. The chemistry of these residues
bound to the protein chains through the amide linkage is illustrated in 
Fig. 8.12.
Both these amino acid residues can readily undergo a trans-1,2-
β-elimination
reaction to form the dehydroalanine residue which contains an activated
double bond capable of subsequent Michael addition with suitable nucleophiles.
These reactions occur in water; the extent and nature of the reaction depends
especially on the parameters pH, temperature and time.
61–63
 The reactions
are activated by the electron withdrawing character of the adjacent amide
carbonyl residue. In general the thiol form, cysteine, more readily undergoes
β-elimination than the disulphide form, cystine; even at pH 3, in boiling
aqueous dyebaths, there is some elimination of hydrosulphide ion from the
cysteine residue whereas the cystine disulphide only undergoes such reactions
above pH 7. 
Figure 8.13
 summarises these reactions.
 
  








C==O
CH—CH —S—S—CH —
NH
C==O
CH
NH
2
2
  




C==O
CH—CH SH
NH
2
Cystine
Cysteine
8.12
 Representation of bound cystine/cysteine in wool keratin.
© 2009 Woodhead Publishing Limited

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The coloration of wool
201
The hydrogen sulphide or hydrosulphide anion produced in the reaction is
capable of ready reaction with cystine disulphide residues to produce further
cysteine thiol residues which will undergo rapid 
β-elimination to
dehydroalanine and hydrogen sulphide in boiling dyebaths; this is clearly the
start of a runaway degradation reaction. The reactive entity, dehydroalanine,
will undergo Michael addition with amino nucleophiles present in histidine
and lysine residues and with thiol nucleophiles present in cysteine; in this
way, more stable crosslinks such as histidino-alanine, lysino-alanine and
lanthionine are formed.
64,65
 The extent and exact composition of these new
crosslinking amino acids vary greatly with pH and temperature of treatment,
since the nucleophilicities of amino and thiol residues increase with pH and
temperature increases. The chemistry of these crosslinking reactions is
exemplified, for lysine and cysteine residues, in 
Fig. 8.14.
 The crosslinks
will not undergo degradation or elimination reactions under conditions normally
encountered in wool dyeing and are thus likely to be of great importance in
explaining the phenomena of permanent setting during wool dyeing. Also of
some importance in setting is the so-called thiol-disulphide interchange
reaction.
66,67
It is clear from the above discussion that control of setting in dyeing can
be achieved by the addition of chemicals which scavenge hydrosulphide
anions as they are liberated or which rapidly modify free cysteine thiol
residues to prevent the elimination reaction; in practice this can be achieved
in two ways:
1.
Inclusion of oxidants in the dyebath.
2.
Inclusion of fibre-substantive electrophiles in the dyebath.
  








C==O
CH—CH —S—S—CH —
NH
C==O
CH
NH
2
2
  




C==O
C== CH
NH
2
  




C==O
HS—S—CH — CH
NH
2
+
Cystine
     Dehydroalanine
Perthiocysteine
  




C==O
HS—S—CH — CH
NH
2
  




C==O
C== CH
NH
2
(H
2
O) 
 H
2
S 
+
(H
2
O) 
+
  H
2
S
(OH
–
)
8.13
 Elimination reactions of cystine and cysteine.
  




C==O
HS—CH — CH
NH
2
© 2009 Woodhead Publishing Limited

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Advances in wool technology
202
  








C==O
CH—CH — NH—(CH ) —
NH
C==O
CH
NH
2
2 4
It is thus important to measure set following dyeing and most of the published
research in this area uses Køpke’s crease angle method to achieve this;
68
typically blank dyeings of wool fabric in pH 5 buffer for 1 hour at the boil,
without anti-setting agent present, give set values of about 70%, whereas
including an effective anti-setting agent gives a set value of about 30%.
Reactive dyes, dyed in moderate to heavy depths of shade, actively prevent
damage in wool dyeing
69
 especially those dyes which contain activated carbon–
carbon double bonds and which thus react with fibre nucleophiles via a
Michael addition mechanism (these dyes include acrylamido dyes and
vinylsulphone dyes). The magnitude of this effect increases with increasing
amounts of reactive dye applied being optimum at circa 3% dye o.m.f. The
importance of this effect when dyeing wool fabric at pH 4 with the 
α-
bromoacrylamido reactive dye, Lanasol Red 6G – 4% o.m.f., is demonstrated
in 
Fig. 8.15.
It is interesting to reflect why reactive dyes based on reactive halogenated
heterocycles, which react with wool fibre nucleophiles by a nucleophilic
substitution reaction, are less effective in controlling wool damage in dyeing
than are the activated carbon–carbon double bond type of reactive dye. From
the above discussions it is clear that successful control of damage and set go
hand-in-hand and it is thus necessary to look carefully at the reactivity/
stability of the reactive dye-cysteinyl residue covalent bond. Thioether
derivatives of triazine or pyrimidine heterocycles will react further with
amines to form bonds of greater stability; the leaving group in this reaction
being the substituted thiol.
70
 The thioether formed from reaction with an
activated carbon–carbon double bond is, however, resistant to nucleophilic
Lanthionine
  




C==O
C== CH
NH
2
  




C==O
H N—(CH ) — CH
NH
2
2 4
Lysine
Lysino-alanine
  




C==O
C== CH
NH
2
  




C==O
HS—CH — CH
NH
2
  








C==O
CH—CH —S—CH —
NH
C==O
CH
NH
2
2
+
+
8.14
 Formation of lysino-alanine and lanthionine crosslinks.
Cysteine
© 2009 Woodhead Publishing Limited

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The coloration of wool
203
attack or 
β-elimination under the mildly acidic conditions pertaining in wool
dyeing.
71
When set was measured from dyeings on wool fabric produced at pH 5
from the activated halogenated heterocycle type of reactive dye, Drimalan
Red F-2G – Clariant (3% o.m.f.), a value of 74% was obtained; in contrast
when the above dye was replaced with a reactive dye containing an activated
carbon–carbon double bond, Lanasol Red 6G – CIBA (3%omf), a set value
of 41% was obtained.
72
 The reactions responsible for these differences are
summarised in 
Fig. 8.16.
The importance of hydrogen sulphide as a catalyst to promote setting/
wool damage under dyeing conditions was proven by Lewis and Smith
73
who demonstrated that a bis-(dye-sulphonylethyl)-thioether dye was present
in the bath after dyeing with a model vinylsulphone dye. This dye arises
from the reaction of the vinylsulphone with free hydrogen sulphide, according
to the mechanism shown in 
Fig. 8.17.
100
90
80
70
60
50
40
0
0.5
1.0
1.5
2.0
Dyeing time, h
% strength retained
100
°C
110
°C
With reactive dye
105
°C
120
°C
Without reactive dye
8.15 
Effect of dyeing time and temperature on wool fabric strength.
© 2009 Woodhead Publishing Limited

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Advances in wool technology
204
D-NH
N
N
F + 2 Wool-SH
Cl
F
D-NH
D-NH
Cl
Cl
N
N
N
N
S-Wool
S-Wool
Wool-NH
2
NH-Wool
NH-Wool
+ 2 Wool-SH
Activated carbon–carbon double bond dye:
D-NH—CO—C(Br)==CH
2
      + 2 Wool-SH
→
D-NH—CO—CH(S-Wool)—CH
2
—S-Wool
→
Wool-NH
2
No reaction
8.16
 Cysteinyl reactions with active heterocyclic and double bond
types of reactive dye.
D—SO
2
—CH==CH
2
+ H
2
S 
→ D—SO
2
—CH
2
—CH
2
—SH
(D—SO
2
—CH==CH
2
) 
↓
D—SO
2
—CH
2
—CH
2
—S—CH
2
—CH
2
—SO
2
—D
Thioether dye
8.17
 Thioether dye formation in wool dyebaths.
Fluoro-chloropyrimidine dye:
© 2009 Woodhead Publishing Limited

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The coloration of wool
205