TOPIC 10.

TEXTILE DYES (EXCLUDING REACTIVE DYES)

Textile fibres may be classified into three broad groups: natural, semi-synthetic and synthetic.
Unlike the commercial range of dyes and pigments, which are now almost entirely synthetic in origin,
natural fibres continue to play a prominent part in textile applications. The most important natural
fibres are either of animal origin, for example the protein fibres, wool and silk, or of vegetable origin,
such as cotton, which is a cellulosic fibre. Semi-synthetic fibres are derived from natural sources
although their production involves chemical processing. The most significant semi-synthetic fibres
used today are derived from cellulose as the starting material. Viscose, modal and lyocell are
regenerated cellulosic fibres. Viscose is a longestablished textile fibre that is manufactured by
reacting cellulose, e.g., from wood pulp, with carbon disulfide in alkali to give its watersoluble
xanthate derivative. This is followed by regeneration of the cellulose in fibrous form using sulfuric
acid. Lyocell is a more recent development, introduced in 1994 as a regenerated cellulosic fibre with
strong environmental credentials, commercialized under the trade name Tencel. Lyocell is
manufactured from wood pulp, from sustainably-farmed eucalyptus, dissolved in an organic solvent,
Nmethylmorpholine N-oxide (NMMO), which is reported to be nontoxic. The fibres are prepared from
the concentrated solution by spinning into a water bath, which regenerates the cellulose. The
manufacture involves a closed loop process from which the solvent is virtually completely recovered
and recycled. Cellulose acetate is a chemically modified cellulose derivative, manufactured by
acetylation and partial hydrolysis of cotton. The most important completely synthetic fibres are
polyester, polyamides (nylon), acrylic fibres and polypropylene. Textile fibres share the common
feature that they are made up of polymeric organic molecules. Most natural fibres exist as staple
fibres, roughly 2–50 cm long and 10–40 mm in diameter, and are converted by a spinning process
into yarns. In contrast, synthetic fibres are produced as continuous filament yarn, as is silk (by the
silkworm). Dye molecules are designed to ensure that they have a set of properties that are
appropriate to their particular applications. The most obvious requirement for a dye is that it must
possess the desired colour, in terms of hue, strength and brightness. The relationships between
colour and molecular constitution of dyes have been discussed before. A further feature of dye
molecules, which is of some practical importance, is their ability to dissolve in water. Since textile
dyes are almost always applied from an aqueous dyebath solution, they are required to be soluble in
water, or, alternatively, to be capable of conversion into a water-soluble form suitable for application.
Many dye application classes, including acid, mordant, premetallised, direct, reactive and cationic
dyes, are readily water-soluble. Disperse dyes for polyester are, in contrast, only sparingly soluble in
water, but they have sufficient solubility for their application at the high temperatures employed in
their application. A few groups of dyes, including vat and sulfur dyes for cellulosic fibres, are initially
insoluble in water and are thus essentially pigments. However, they may be converted chemically into
a water-soluble form and in this form they can be applied to the fibre, after which the process is
reversed and the insoluble form is regenerated in the fibre. Dyes must be firmly attached to the
textile fibres to which they are applied in order to resist removal, for example by washing.
This may be achieved in several ways. The molecules of many dye application classes are
designed to provide forces of attraction for the polymer molecules that constitute the fibre. In the case
of reactive dyeing, the dye molecules combine chemically with the polymer molecules, forming
covalent bonds. In other cases, a set of dye–fibre intermolecular forces operate, varying according to
the particular dye–fibre system. These interactions commonly involve a combination of ionic, dipolar,
van der Waals forces and hydrogen bonding. An additional feature of textile dyeing is that the dye
must distribute itself evenly throughout the material to give a uniform colour, referred to as a level
dyeing. Finally, the dye must provide an appropriate range of fastness properties, for example to
light, washing, heat, rubbing, etc. Large quantities of textile fabrics are also coloured by printing to
produce multicolour patterns and images. The most important technique used industrially in the
manufacture of printed textiles is screen printing, mostly by continuous rotary screen printing, which
is an especially economic method for long print runs. Screen printing is in effect a stencilling process
in which the image is produced on top of a woven polyester mesh using a photopolymerisation
process. The polymer forms the non-image areas which block the passage of ink while the image is
contained in the open areas. This image is transferred on to the fabric by pressing the ink through the
open areas. Textile printing may be considered essentially as localized dyeing. This feature means
that the dye classes appropriate to the fibre in question are used and that the principles of the dye
application and its interactions with the fibre are the same as in dyeing.

DYES FOR PROTEIN FIBRES

Protein fibres are natural fibres derived from animal hair sources. The most important protein fibre
used commercially is wool (from sheep), although luxury fibres such as silk (from the silkworm),
cashmere and mohair (both from goats) are important high value products. Human hair is also a
protein fibre. Both the physical and chemical structures of protein fibres are highly complex and there
is considerable variation depending on the source. The principal component of the fibres is the
protein keratin, the molecular structure of which is illustrated in outline in Figure 7.1. The protein
molecules consist of a long polypeptide chain constructed from the eighteen commonly encountered
amino acids that are found in most naturally-occurring proteins. The structures of these amino acids
are well documented in general chemical and biochemical textbooks and so they are not reproduced
here. As a result of the diverse chemical nature of these amino acids, the protein side-chains (R1,
R2, etc. in Figure 7.1) are of widely varying character, containing functionality that includes, for
example, amino and imino, hydroxy, carboxylic acid, thiol and alkyl groups and heterocyclic
functionality. At intervals, the polypeptide chains are linked together by disulfide (-S-S-) bridges
derived from the amino acid cystine. There are also ionic links between the protonated amino (–NH3
1) and carboxylate (–CO2_) groups, which are located on the amino acid side-groups and at the end
of the polypeptide chains.

Many of the functional groups on the wool fibre play some part in the forces of attraction involved
when dyes are applied to the fibres. Protein fibres may be dyed using a number of application
classes of dyes, the most important of which are acid, mordant and premetallised dyes, the structural
features of which are discussed in the rest of this section, and reactive dyes which are considered
separately before. Protein fibres may be degraded chemically under aqueous alkaline conditions, but
they are relatively stable to acidic conditions. Thus, most protein dyeing processes are carried out by
applying the dyes under mildly acidic to neutral aqueous conditions, usually at elevated
temperatures. Acid dyes derive their name historically from the fact that they are applied to protein
fibres such as wool under acidic conditions. A characteristic feature of acid dyes for protein and
polyamide fibres is the presence of one or more sulfonate (–SO3 _) groups, usually present as the
sodium (Na1) salt. These groups have a dual role. Firstly, they provide solubility in water, the medium
from which the dyes are applied to the fibre. Secondly, they ensure that the dyes carry a negative
charge (i.e., they are anionic). When acid conditions are used in the dyeing process, the protein
molecules acquire a positive charge. This is due mainly to the fact that, under acidic conditions the
amino (-NH2) and imino (јNH) groups on the amino acid sidechains are protonated as -NH3 + and -
NH2+ groups, respectively, while ionisation of the carboxylic acid groups is suppressed. The positive
charge on the polymer attracts the acid dye anions by ionic forces, and these displace the
counteranions within the fibre by an ion exchange process. As well as these ionic forces of attraction,
van der Waals forces, dipolar forces and hydrogen-bonding between appropriate functionality of the
dye and fibre molecules also play a part in the affinity of acid dyes for protein fibres. The molecular
size and shape is very commonly a critical feature in the design of dyes for application to specific
substrates. In this context, acid levelling dyes may be described as a small to medium-sized planar
molecules. Most acid dyes, especially yellow, oranges and reds, belong to the azo chemical class
while blues and greens are often provided by carbonyl dyes, especially anthraquinones, and to a
certain extent by arylcarbonium ion types.These are some typical acid dye structures.
A notable aspect of the structure of dyes 7.2–7.5 is the strong intramolecular hydrogen-bonding that
exists contained in sixmembered rings, a feature that enhances the stability of the compounds and, in
particular, confers good lightfastness properties. One explanation that has been proposed is that the
hydrogen bonding leads to a reduction in electron density at the chromophore, and that this in turn
reduces the sensitivity of the dye towards photochemically- induced oxidation. It has also been
suggested that intramolecular proton transfer within the excited state of the dye molecules can lead
to enhanced photostability. A comparison between the two isomeric monoazo acid dyes CI Acid
Orange 20, 7.1 and CI Acid Orange 7, 7.2, illustrates the effect of intramolecular hydrogen bonding.
Dye 7.2 shows significantly improved fastness to alkaline washing and lightfastness compared with
dye 7.1 in which intramolecular hydrogen bonding is not possible. A comparison of the structurally-
related monoazo dyes 7.3a (CI Acid Red 1) and 7.3b (CI Acid Red 138), and of the anthraquinone
acid dyes 7.5a (CI Acid Blue 25) and 7.5b (CI Acid Blue 138), illustrates the distinction between acid
levelling and acid milling dyes. Dyes 7.3b and 7.5b show excellen resistance to washing as a result
of the presence of the long alkyl chain substituent (C12H25), which is attracted to hydrophobic or
nonpolar parts of the protein fibre molecules by van der Waals forces. Because of the extremely
strong dye/fibre affinity, dyes of this type are of the acid supermilling type. Dyes 7.3a, 7.5a, CI Acid
Black 1, 7.4, a typical disazo acid dye, and CI Acid Blue 1, 7.6, an example of a triphenylmethine acid
dye, are acid-levelling dyes. In the case of dye 7.6, it is important to note that while the nitrogen
atoms carry a formal single delocalised positive charge, the presence of two sulfonate groups
ensures that the dye is overll anionic. The ability of transition metal ions, and especially chromium
(asCr31), to form highly stable metal complexes may be used to produce dyeings on protein fibres
with superior fastness properties, especially towards washing and light. Chrome mordant dyes
generally have the characteristics of acid dyes but with the ability in addition to form a stable complex
with chromium in its Cr(III) oxidation state. Most commonly, this feature takes the form of two
hydroxyl groups on either side of (ortho, ortho-to) the azo group of a monoazo dye, as illustrated for
the case of CI Mordant Black 1, 7.7. In the most important method for application of mordant dyes,
the so-called afterchrome process, the dye is applied to the fibre as an acid dye and then the dyed
fibres are treated with a source of chromium, most frequently sodium dichromate (Na2Cr2O7) in
which the chromium exists in oxidation state Cr(VI). During the process, the chromium(VI) undergoes
reduction by functional groups (which are consequently oxidized) on the wool fibre, for example the
cysteine thiol groups, and a chromium(III) complex of the dye is formed within the fibre by a process
such as that illustrated in Figure 7.3.

A dye of this type acts as a tridentate ligand, the chromium bonding with two oxygen atoms derived
from the hydroxyl groups and with one nitrogen atom of the azo group. Complexes of Cr(III) are
invariably six-coordinate with octahedral geometry. It has not been established with certainty how the
remaining three valencies of the chromium are satisfied in the mordant dyeing of protein fibres. There
are several possibilities, which include bonding with water molecules, with coordinating groups (-OH,
-SH, -NH2, -CO2H, etc.) on the amino acid side chains on the fibre, or with another dye molecule.
Chrome mordanting has been traditionally of particular importance in the dyeing of loose wool in very
dark (especially black) shades to provide very high levels of fastness. However, there are serious
ecological problems associated with the use of chrome mordant dyes, associated with the severe
toxicity of Cr(VI) and its presence in dyehouse effluent. Consequently, the process is little used
nowadays and continues to decrease in importance. Premetallised dyes, as the name implies, are
pre-formed metal complex dyes.22,23 They are usually six-coordinate complexes of chromium(III)
with octahedral geometry, as exemplified for example by CI Acid Violet 78, 7.8, although some
complexes of cobalt(III) are also used. Most premetallised dyes are azo dyes, with one nitrogen atom
of the azo group playing a part in complexing with the central metal ion. Since in this case there are
two azo dye molecules coordinated with one chromium atom, compound 7.8 is referred to as a 2 : 1
complex. The purpose of the sulfone group in dye 7.8 is to enhance the hydrophilic character of the
molecule and hence its water solubility, without increasing the charge on the dye anion. Protein fibres
may also be dyed with certain chemical types of reactive Dye as will be discussed in nect topics.

DYES FOR CELLULOSIC FIBRES

Cellulosic fibres provide the most important natural fibres and arederived from plant sources. The
most important cellulosic fibre used in textiles is cotton, but there are many others, including linen,
jute, hemp and flax, and there is also interest in cellulosic fibres derived from bamboo and nettles.
The principal component of the cotton fibre is cellulose, the structure of which is shown in Figure 7.4.
 Cotton is in fact almost pure cellulose (up to 95%). Cellulose is a polysaccharide. It is a high
molecular weight polymer consisting of long chains of repeating glucose units, with up to around
1300 such units in each molecule. Cellulose has a fairly open structure, which allows large dye
molecules to penetrate relatively easily into the fibre. Each glucose unit contains three hydroxyl
groups, two of which are secondary and one primary, and these give the cellulose molecule a
considerable degree of polar character. The presence of the hydroxyl groups is of considerable
importance in the dyeing of cotton. The hydrophilicity that they confer means that the fibre is capable
of absorbing significant quantities of water, the medium from which the dyes are applied. For
example, the ability of the hydroxyl groups to form intermolecular hydrogen bonds is thought to be of
some importance in direct dyeing, while reactive dyeing involves a chemical reaction of the hydroxyl
groups with the dye to form dye–fibre covalent bonds. The tendency of the hydroxyl groups to ionise
to a certain extent (to –O_) means that the fibres may carry a small negative charge. There are a
larger number of application classes of dyes that may be used to dye cellulosic fibres such as cotton
than for any other fibre. These dye application classes include direct, vat, sulfur, azoic and reactive
dyes. Viscose, modal and lyocell are regenerated cellulosic fibres that may be dyed in essentially the
same way, and with the same dye classes, as natural cellulosic fibres. Paper is also derived from
cellulose.
Cellulosic materials are generally sensitive to degradation under acidic conditions but are quite
resistant to alkaline conditions. This contrasts with the behaviour of protein fibres where the opposite
is the case. Cellulosic fibres are thus commonly wet-processed under alkaline conditions and this
includes dyeing. Direct dyes are a long-established class of dyes for cellulosic Fibres. They derive
their name historically from the fact that they were the first application class to be developed that
could be applied directly to these fibres without the need for a fixation process such as mordanting.
For this reason, they are also commonly referred to as substantive dyes. In some ways, direct dye
molecules are structurally similar to acid dye molecules used for protein fibres. Arguably the most
important features of direct dye molecules that influence their application properties are associated
with their size and shape. They are, in general, large molecules and in shape they are long, narrow
and planar. Direct dyes show affinity for cellulose by a combination of van der Waals, dipolar and
hydrogen-bonding intermolecular forces. Individually, these forces are rather weak. The long, thin
and flat molecular geometry allows the dye molecules to align with the long polymeric cellulose fibre
molecules and hence maximise the overall effect of the combined set of intermolecular forces.
Chemically, direct dyes, of which compound 7.9 (CI Direct Orange 25) is a typical example, are
almost invariably azo dyes, commonly containing two or more azo groups. The long, flat, linear shape
of compound 7.9 allows groups such as the -OH, -NHCO (amide), and –N=N– groups in principle to
form hydrogen bonds with OH groups on cellulose as it lines up with the cellulose molecule. There
are only two sulfonate groups in compound 7.9 and these are well separated from one another. This
is sufficient to give adequate water-solubility for their application. In addition, it may be argued that
the sulfonate groups are on the opposite side of the molecule from groups that may be participating
in hydrogenbonding with the fibre and this means that they will be oriented away from the cellulose
molecule, thus minimising any negative charge repulsion effects.
DYES FOR SYNTHETIC FIBRES

The three most important types of synthetic fibres used commonly as textiles are polyester,
polyamides (nylon) and acrylic fibres.

Polyester and the semi-synthetic fibre cellulose acetate are dyed almost exclusively using disperse
dyes. Polyamide fibres may be coloured using either acid dyes, the principles of which have been
discussed in the section on protein fibres or with disperse dyes. Acrylic fibres are dyed mainly with
basic (cationic) dyes. Polyesters are polymers whose monomeric units are joined through ester (-
COO-) linkages. The most important polyester fibre by far is poly(ethylene terephthalate), PET,
whose production far outstrips that of any other synthetic fibre. PET, commonly referred to simply as
polyester, has the chemical structure shown in Figure 7.8. Polyester is relatively hydrophobic (non-
polar) in character, certainly in comparison to the natural protein and cellulosic fibres, largely as a
result of the prominence of the benzene rings and the –CH2CH2- groups.
Figure 7.9 illustrates the chemical structures of some typical disperse dyes and demonstrates that a
wider range of chemical types are encountered in the range of commercial disperse dyes than is the
case with any other dye application class.29,30 Numerically, azo dyes form by far the most important
chemical class of disperse dyes. Azo disperse dyes may be classified into four broad groupings. The
most numerous of these are the aminoazobenzenes, which provide important orange, red, violet and
blue disperse dyes. They are exemplified by CI Disperse Orange 25, 7.13, CI Disperse Red 90, 7.14,
and CI Disperse Blue 165, 7.15. A comparison of these three aminoazobenzene dyes provides an
illustration of the bathochromic shift provided by increasing the number of electron-accepting and
electron-donating groups in appropriate parts of the molecules, in accordance with the principles of
the relationship between colour and molecular structure as discussed in Chapter 2. There are two
further groups of disperse dyes that are heterocyclic analogues of the aminoazobenzenes.
Derivatives based on heterocyclic diazo components provide bright intense colours and are
bathochromically shifted so that they serve the purpose of extending the range of blue azo disperse
dyes available. An example of such a product is CI Disperse Blue 339, 7.16. Derivatives based on
heterocyclic coupling components are useful for their ability to provide bright intense yellow azo
disperse dyes. An example is CI Disperse Yellow 119, 7.17, which, as illustrated in Figure 7.9, exists
as the ketohydrazone tautomer. The fourth group are disazo dyes of relatively simple structures, for
example CI Disperse Yellow 23, 7.18. Carbonyl disperse dyes, especially anthraquinones, are next in
importance to the azo dyes and there are also a few products belonging to the nitro and polymethine
chemical classes. CI Disperse Red 60, 7.19, and CI Disperse Green 5, 7.20 are examples of typical
anthraquinone disperse dyes, while compound 7.21 is an example of the more recently-introduced
benzodifuranone carbonyl type. CI Disperse Yellow 42, 7.22 and CI Disperse Blue 354, 7.23,
respectively, provide commercially relevant examples of the nitro and polymethine chemical classes.
Polyamides are polymers whose monomeric units are joined through secondary amide (-NHCO-)
linkages. The two most important polyamide fibres are based on nylon 6.6, 7.27, and nylon 6, 7.28,
whose structures, both completely aliphatic in character, are illustrated in Figure 7.13.

Because the molecular structures of polyamide fibres are relatively hydrophobic, similar in this
respect to polyester, they may be dyed using the range of disperse dyes by a mechanism analogous
to the dyeing of polyester discussed earlier in this chapter.