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8.2
The theoretical basis of wool dyeing
8.2.1
Anionic or so-called acid dyes in wool dyeing
Wool is a heterogeneous polymer, mainly made up of many different amphoteric
proteins; it may be coloured with a variety of water-soluble dyes but in
practice it is invariably dyed and printed with sulphonated dyes. Wool has
been, and still is to some extent, dyed with simple, low molecular weight
(circa 400–800 dalton), sulphonated chromophores, termed ‘acid dyes’; as
the dye-manufacturing and the dye-using industries grew more sophisticated
these acid dyes were modified, usually by increasing their molecular size, to
give better washing and light fastness properties. Thus acid-milling dyes,
chrome dyes, pre-metallised dyes and reactive dyes all find use in various
application sectors. The simplest way to write an acid dye structure is as
D( —SO
3
–
Na
+
)
n
where D is a chromophoric entity which may be substituted
azo, anthraquinone, phthalocyanine or others, and n is an integer, usually
1–3.
Since it is made up of many different amphoteric protein macromolecules,
© 2009 Woodhead Publishing Limited
The coloration of wool
185
according to the pH and temperature of the surrounding solution, wool can
show clear differences in charge; these differences are brought about by
protonation or deprotonation of carboxylic and amino side-chain functionalities.
Figure 8.1
schematic describes this situation. It is of course an approximation
and is governed by the acid dissociation constants of the
—NH
3
+
and
—COOH residues; for example at 25
°C lysine ε-amino has a p K
a
value of
about 10.5 and the glutamic acid terminal carboxylate is about 4.2.
Acid levelling dyes are generally monosulphonated dyes with little
substantivity for wool and nylon under neutral or weakly acidic conditions
but which exhaust well from boiling dyebaths under acidic conditions (pH 3-
4). The structure shown in
Fig. 8.2
is typical. The small molecular size of
acid dyes means that there is very significant dye migration during their boil
application, allowing good coverage of tippy wools and also for their application
in dyeing systems where there is limited interchange between liquor and
goods, e.g. piece dyeing in winches and hank dyeing of yarns.
Acid milling dyes are of greater molecular size and exhibit high ‘neutral’
(pH 5–7) substantivity for the wool fibre but can give unlevel dyeings due to
their poorer levelling properties; the dyeings produced with these dyes have
good fastness to water and mild washing treatments coupled with good light-
fastness. A typical example of this type of dye structure is shown in
Fig. 8.3.
Acid milling dyes, owing to their lower migration properties, find use in
dyeing machines where there is good liquor/fibre interchange and these
include package dyeing of wool yarn and soft-flow jet dyeing of wool piece
goods.
Pre-metallised dyes fall into two categories; the older 1:1 dye : metal complex
and the later 2:1 dye : metal complexes. The complexing metal cations are
NH
3
+
COOH
NH
3
+
COO–
NH2 COO
–
Wool fibre
Wool fibre
Wool fibre
+ve charge
zero charge
–ve charge
pH < 4
pH 4–8
pH > 8
8.1
Wool surface charge vs pH.
NH
2
SO
3
–
Na
+
NH
O
O
8.2
CI Acid Blue 25.
© 2009 Woodhead Publishing Limited
Advances in wool technology
186
either chromium
(3+)
or cobalt
(3+)
. The application conditions for these two
classes of dye are quite different – the former are dyed at pH 1–2 to achieve
maximum levelling, as required for piece dyeing, whereas the latter are dyed
at pH 6–7 since they have very high substantivity due to their large molecular
size and hydrophobicity. Comprehensive reviews are available.
4,5
8.2.2
Physical chemistry of wool dyeing
Classical wool dyeing theory attempts to model the physical chemistry of
wool dyeing according to electrostatic principles, the Gilbert/Rideal approach
or according to the Donnan membrane theory; it is not the intention to cover
these approaches in detail and therefore readers are referred to texts which
deal very fully with these analyses.
6,7
The assumptions made in applying the
above physical chemical equations to the wool dyeing system are too general
and do not take into account the bewildering complexity and heterogeneity
of the wool fibre proteins – let alone the fact that the fibre structure changes
physically and chemically during the boiling process itself.
The following is therefore an attempt to explain some of the more puzzling
aspects of acid dye absorption by wool. As mentioned above, the initial
driving force for dyeing to occur with a simple acid dye/polyamide fibre
combination is undoubtedly Coulombic, but, depending on dye structure,
non-polar interactions are also capable of playing an important role. In terms
of Coulombic interactions, an important factor which affects the rate of acid
dye uptake and its final saturation value on the fibre is the total number of
protonated amino sites in the fibre. In this context it is valuable to make a
comparison of wool, silk and nylon (Table 8.1).
C
12
H
25
N==N
OH
NHCOCH
3
Na
+ –
O
3
S
SO
–
3
Na
+
8.3
CI Acid Red 138.
Table 8.1 Amino groups in wool, silk and nylon
Wool
(lysine primary amino plus histidine
0.820 moles per kilogram
secondary/tertiary amino plus
arginine guanidino plus
α-terminal
amino)
Silk
(lysine, plus histidine plus
α-terminal
0.150 moles per kilogram
amino)
Nylon 6,6
(
α-terminal amino)
0.036 moles per kilogram
© 2009 Woodhead Publishing Limited
The coloration of wool
187
Thus one may expect the saturation values of acid dyes on these three
polyamide fibres to be in the same ratio as the above basic group contents,
i.e. wool : silk : nylon = 23:4:1. This effect was verified experimentally by
Skinner and Vickerstaff
8
who studied the equilibrium uptake of CI Acid Blue
45 (1,6-diamino-4,8-dihydroxy-3,7-disulpho-anthraquinone) on the above
three polyamide fibres at pH 1.6 and 85
°C; the corresponding ratio obtained
was 14:4:1.
The effect of varying the number of sulphonate groups in the dye molecules
on the substantivity of acid dyes for nylon, wool and human hair, has been
examined by numerous workers.
6–12
These workers clearly demonstrated
that dye substantivity for the polyamide substrates decreases in the following
order:
monosulphonate > disulphonate > trisulphonate > tetrasulphonate
In the light of the above observations it is thus important to discuss the
various types of molecular interactions which may occur in dye–fibre systems.
In terms of non-covalent interactions, the different forces between two
molecules, like or unlike, are usually divided into five categories:
13
van der
Waals forces, electrostatic interactions, induction forces, charge transfer
stabilisation effects and solvophobic interactions.
The three categories large enough to be of importance in terms of dyeing
processes are as follows:
1.
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