Reactive Dyes and their Application

Chapter 5: Reactive Dyes and Their Application
By J. R. ASPLAND, School of Textiles, Clemson University, Clemson, S.C.
rom theyear 1856,followingPerkin’s
F discovery of the first synthetic dye,
the growth of organicchemicalknowledge
paralleled that of the dye manufacturing
industry in particular, and the chemical
industry in general. It was explosive!
Perkin has even been referred to as “the
father of chemical engineering.”
By contrast, polymer and fiber chemis-
tries were late bloomers. Viscose rayon
fibers appeared ca. 1910 and secondary
celluloseacetate fibersca. 1920.Although
both eventually achieved a significant
market penetration, neither could ever be
called a dominant fiber. The emergence of
viscose rayon affected dye manufacturers
very little. On theotherhand, thearrivalof
secondary cellulose acetate created waves,
and was followed within two or three years
by the forerunners of the disperse dyes
used today. Although this added a little
ABSTRACT
This first part of Chapter 5 covers the
background to the development of
reactive dyes, explains the history which
led to their belated discovery and gives
the reasons for their present
importance. The nature of the dye-fiber
bond is covered as well as the processes
by which it is formed. The overall
procedure for reactive batch dyeing,
which involves the three steps of dyeing,
fixation and washingoff, is discussed in
terms of the chemical and physical
interactions involved. Some of the
potential problems are indicated, as are
possible means by which they might be
overcome.
KEY TERMS
Batch Dyeing
Covalent Bonds
Dye-Fiber Bonds
Dye Hydrolysis
Exhaustion
Fixation
Nucleophilic,Reactions
Reactive Dyes
Washingoff
Wetfastness
savor to dye research and development,
both continued to be directed primarily
towards cotton and, to a lesser extent,
wool. It was not until after the second
World War, in the early 195Os, that the
rapid growth of nylon and polyester began
in earnest, and it was decades later before
anyone thought it possible that King Cot-
ton might oneday be dethroned.
Duringthefirst50yearsfollowing 1856,
dyeresearch and development had already
discovered members of all but one of the
important cellulosic dye application cate-
gories known today. After 49 more years
the numbers of dyes for cellulose had been
extended, performance had been im-
proved, manufacturing and application
refinements had been put into practice, but
no important new dye application cate-
gory for cellulose had appeared.
Thus it was with justifiable pride that
IC1 introduced, in 1956, the first dyes for
cellulosics which would actually react
with the fiber molecules, to form covalent
dye-fiber bonds. The introduction of this
new dye application category coincided
with the centennial of Perkin’s discovery
of mauveine.
The General Nature of Reactive Dyes
At this stage, a number of questions may
spring to mind. For example: what is
special about covalent bonds? What are
these reactive dyes? Why did it take so
long to discover them? Are they impor-
tant? Why are they important? Do they
match up to the original expectations?
These and other such questions will all be
addressed, directly and indirectly, in the
following sections.
In all dyeing processes dissolved (sin-
gle) organic molecules or ions (dyes, dye
derivatives or precursors) are first ad-
sorbed at the surfaces of the fibers (sub-
strates) and then diffuse into them until
they aremore or lessuniformlydistributed
throughout. However, once the dyeing
process is over, it is necessary that the
colors now within the fibers have some
degree of resistance to removal by soi-
vents, such as the water which carried the
soluble dyes, dye derivatives or precursors
out of the dyebath onto and into the fibers
in the first place. This requirement may be
called wetfastness, and dyes for cellulosic
fibers can have a wide range of wetfastness
properties. Reactive dyes have generally
high wetfastness.
High Wetfastness
Achieving high wetfastnessis intrinsically
more difficult on the cellulosic fibers be-
cause of their extreme hydrophilicity.
They love water, and offer little protection
against water removal to any soluble
chemical species which has diffused inside
them. This contrasts with hydrophobic
fibers such as polyester. Polyester fibers
offerwater little opportunity to enter until
the temperature is almost up to the atmo-
spheric boil (see a later chapter on disperse
dyes).
Dyers and dye chemists have known for
a long time that there are only three ways
in which dyes can beretained byfibers, and
the first two have been used from time
immemorial.The three methods are:
0 Physical sorption: This relies on the
sameforceswhich attractedthe dyestothe
fibers initially being strong enough to hold
onto the dyes through subsequent wet
treatments-e.g, with direct dyeings on
cellulosic fibers-but also acid, basic and
disperse dyeings on other fibers. The wet-
fastness of direct dyes on cellulose is
distinctly limited.
0 Mechanical retention: This relies on
the formation of insoluble, pigmentary
materials out of the soluble chemicals
which first diffused into the fibers; e.g.,
with vat and sulfur dyeings, those of azoic
combinations, and also dyeings of mor-
dant and ingrain dyes. Noneof theseoffers
either a full color range or a very simple
application procedure.
0 Fiber reaction: Here the dye mole-
cules or ions do not lose all their solubiliz-
ing groups after diffusion into the fibers,
but in the correct conditions theyreact and
attach themselves by covalent chemical
bonds to the much larger fiber molecules,
to form new colored derivatives of the
fibers. The small number of dye solubiliz-
ing groups is totally inadequate to cause
the large new dye-fiber molecules to dis-
solve in water.
Chemical Bonds
Attraction between atoms (bonding), to
give molecules or ions, and between the
sameor different molecules or ions toyield
different interaction or reaction products,
dependson the movement anddistribution
of electrical charges, the energy require-
ments of the process and the stabilityof the
products. The electrical charges represent
the probability of finding larger or smaller
May 1992 033 31

Reactive Dyes
localized excesses (-) or deficiencies (+)
in the distribution of electrons around
particular atoms.
For the chemist, the different types of
bonds involved in dyeing processes can be
discussed in chemical terms (1). Here the
treatment will be simple and descriptive
only.
Covalent Bonds: There aretwo possi-
ble extreme cases when considering bonds
between atoms. At one extreme lies the
covalent bond; e.g., the carbon-hydrogen
bonds of most organicchemicals, 3 C - H .
In this case, both atoms donate an electron
to the bond and the resulting pair of
electrons is shared between them. Such
bonds share a large amount of energy and
it requires even more energy to tear them
apart again. Bonds between reactive dyes
and cellulose areof this type.
Ionic Bonds: At the other extreme
from the covalent bond lies the ionic bond.
In the simplest case, two adjacent atoms
may both have an available electron to
contribute to a bond, but one atom is able
to gain stability by taking them both for
itself, thereby gaining an extra electron
(negativecharge).Thiswillleavetheother
atomminus oneelectron (positivecharge),
which makes it more stableas well. Such is
the case with common salt, sodium chlo-
ride, which does not exist as covalently
bonded molecules, Na-Cl, but as posi-
tively charged sodium ions and negatively
charged chloride ions, NafC1-. These
oppositely charged ions are attracted to
one another electrostatically but can exist
independentlysince nothingisshared (like
the opposite poles of two different mag-
nets). Such electrostatic attractions are
involved in dyeingwith acid and basic dyes
(see the appropriate chapter) where fiber
molecules can carry charges opposite to
those of the dye ions being applied, and
because of this, attract them.
Polarization and Reactivity: It is to
be expected that the fairness with which
electrons are shared should vary continu-
ously between the sharing of the idealized
covalent bond and the “winner takes all”
of the ionic bond. Some of the other atoms
in covalently bonded organic carbon com-
pounds will always want more than a fair
share of the available electrons-e.g.,
nitrogen, oxygen, fluorine, chlorine and
sulfur-whereby they themselves tend to
acquire an electronegativecharacter, and
their carbon neighbors, an electropositive
character. This tendency does not nor-
mally lead to ion formation. It is called
32
polarization, and makes individual atoms
within a molecule more susceptible to
electrostatic interaction with atoms of the
opposite characteristic whenever they are
in close proximity. Many reactions be-
tween different covalently bonded materi-
als are initiated by polarization, and may
be completed by formation of products
with quite different bonding (electron
distribution) than the original reactants.
The actual outcome of reactions often
depends on complex energy consider-
ations.
Dye-Fiber Reaction: Historical
Reactive dyes are those whose ions or
molecules contain groups which are reac-
tive with other groups present in fibers to
form covalent dye-fiber bonds. It has been
known for many years that wool contains
mercapto, amino and hydroxy groups,
-SH,-NH2 and -OH,respectively, listed in
decreasing order of reactivity, and that
cellulosicfibers containconsiderablenum-
bers of the latter. It takes two to tango. It is
not enough to have a reactive dye mole-
cule. One must also have a reactive group
in the fiber.
As early as 1885, Cross & Bevan made
yellow and red esters of cellulose by a
six-step synthesis. In these products, the
dye was covalently bonded to the cellulose
and the fastness to washing was excellent;
but the reaction conditions were too severe
and lengthy to be practical and the fibers
were badly degraded (2,3).
So, in 1885, the principle was clearly
established that chemical, covalent bond-
ing between dye and cellulose could result
in dyeings with outstanding wetfastness.
But the belief was also reinforced that the
hydroxy group of cellulose was of very low
reactivity and that reactive dyeing might
require to be driven by severe reaction
conditions. (Notice the subjectivity of
“very low” and “severe”: the meaning is in
the mind.)
Whether the reasons were conscious or
unconscious, for the next 70 years the
reaction to form covalent bonds between
the more reactive thiols and amino groups
in wool and reactive dye species received
more attention than that between cellulose
hydroxy groups and reactive dyes. The
emphasis was placed on reactive dyes for
wool despite the fact that improving the
already good wetfastness of the dyeings
which could be achieved on wool was not a
technologically important objective at
that time.
Becausethe first commercial fiber reac-
tive dyes appeared so much more recently
than most other important dyeapplication
categories, it is worthwhile to point out
how completethe scientific and technolog-
ical documentation is. Not only that, but
many of those who worked with reactive
dyes since their commercial inception are
alive and kicking today, including several
of the authors whose work is cited in the
references. Even the authorof this chapter
was an undergraduate intern in the cotton
dyeing laboratories of IC1 during the
summer of 1956.
Properties of Reactive Dyeings
These are almost all that the early re-
searchers could have wished for. They
have a full range of bright shades across
the spectrum, good to excellent wetfast-
ness with minimal color loss and excellent
ratings for the staining of adjacent white
goods, with moderate to good lightfast-
ness. They offer all of these advantages,
along with great versatility in choice of
application methods.
On the other hand, they have moderate
tending to poor fastness to chlorine (2,4),
relatively long batch processing times and
high dye and chemical cost. The utiliza-
tion of color used to be relatively poor, and
the waste color going to drain can be easily
30 to 40%, but this deficiency is undergo-
ing serious recent improvements (see
later) to perhaps 10%or less in the future.
The salt content of the effluent has also
been very high, but is rapidly falling with
the use of lower liquor ratios in dyeing.
Importance of Reactive Dyes
In 1956,the discovery of reactive dyes was
considered so important and their poten-
tial utility was so great that all the major
dye manufacturersjumped onto the band-
wagon asfast as the patent situation would
allow. By the end of 1961, BASF, Bayer
(now Miles Laboratories in the US.),
Ciba and Geigy (now Ciba-Geigy), Ho-
echst (now Hoechst Celanese in the US.),
Sandoz and Sumitomo had joined IC1 in
the marketplace with no less than 12
different ranges of reactive dyes between
them, representing five different types of
reactive groups.
At least eight more ranges of reactive
dyes had been introduced by 1988 (3),and
now, 25 years after their first introduction,
the AATCC Buyers Guide (5)lists almost
200 different reactive dyes by Colour
Index name, representing more than 400
different commercialproducts.
Currently about one-third of the money
spent on dyes for cellulose in the U.S. is
spent on reactive dyes;ca. 30% more than
for any other class of dyes for any fiber.
Several factors might account for some of
this value dominance, including high con-
sumption of cotton, consumer demand for
bright colors, high wetfastness and most
importantly, the high relative cost per
pound of reactivedyes. But whichever way
you slice it, reactive dyes are very impor-
tant.
Reactive Dye Sub-Groups
The following will be restricted to these
reactive dyes which, when applied to
cellulosic fibers, react under mildly alka-
line conditions to give covalent bonds
between dye and fiber. The restriction is
03il Vol. 24,No. 5

not severe but does eliminate the obliga-
tion to discuss the few reactive dyes for
other fibers and a few more which were
fixed in other ways. This additional infor-
mation is not hard to find for those
interested (3).
There isno other class of dyeswhich has
been so fragmented into sub-groups.
These are based not only on the relative
reactivity of the different ranges of reac-
tive dyes but also on the normal subdivi-
sion of dyeranges into groups with similar
dyeing performance (as opposed to chemi-
cal reactivity) under a particular applica-
tion condition.
The differences in reactivity results, for
the most part, from the incorporation of
chemically different reactive groups in the
dye molecules. Historically, the most eco-
nomical reactive groups were inaccessible
to many of the different dye manufactur-
ers,and they needed to supply alternatives
withoutrunning intopatent problems.The
chemical nature of individual dye mole-
cules also plays a role in determining
reactivity, but it is not a dominant one.
The number and ranges of reactivity of
these sub-groups are the reasons this
chapter will not attempt to provide much
specific information on the type and con-
centrations of alkali and salt, or the
preferred application temperatures for
individual reactive dye sub-groups. They
are also the reasons dyehouses generally
stick to one supplier, and why it would be
foolhardyto mix reactivedyesfrom differ-
ent suppliers unless their compatibility
had been clearly demonstrated under the
intended conditions of use and, only then,
if the reactivegroupswere known to be the
same. However, since many of the early
patents have expired, it is now possible to
track down comparable competitive dyes.
The question is always whether any small
resultant savingsjustify thelarge potential
risks.
The ranges of available reactivities can
be illustrated graphically (3),but in prac-
tical terms it is better illustrated by the
results of a batch dyeing study, using
model compounds with the same dye
molecule substituted by different reactive
groups (2).The results indicate that very
reactive dyes (with dichlorotriazinyl
groups, see later) may be dyed at 30C
(85F) with amounts of salt and alkali
(sodium carbonate) from 25 to 55 and 2 to
15grams per liter respectively, depending
on the liquor ratio and depth of shade.
Corresponding figures for dyes of lowest
reactivity were 85C (185F), 30 to 100
grams per liter salt and 30 to 50grams per
litersodium carbonate, although a smaller
amount of a stronger alkali would almost
certainly be substituted in practice.
Reactive Groups in Cellulose
Chapter 1 (6) showed that the cellulose
polymer is abundantly supplied with hy-
droxy groups, -OH, which are themselves
relatively unreactive. There are actually
three for each repeat unit of the polymer
chain. Ignoring the fact that one-third of
these hydroxy groups are primary and
two-thirds are secondary alcohols, it is
convenientto write as a simple representa-
tion of cellulose: Cell-OH.
What isgenerallynot appreciated (even
by many polymer chemists) is that, in the
presence of even dilute alkalis, cellulose
behaves as a very weak acid and will ionize
according to the normal basic dissociation
equation:
Cell-OH +OH-=
Cell-0- +HzO Eq. 1
where
[Cell-OH] X [OH-]f
i [Cell-0-1 = KB Eq. 2
Eq. 1 is an equilibrium (two-way ar-
rows) in which celluloseplus hydroxide ion
(alkali) reacts to give cellulosate anions,
Cell-0-, and water.
Eq. 2 indicates that in such a reaction
the fraction (cellulose concentration X al-
kali concentration in the fiber + cellulo-
sate ion concentration), in an excess of
water, isa constant, Ks, known asthe basic
dissociation constant of cellulose.
In work touched on later in this chapter,
Sumner (7) has been able to calculate
the concentrations of cellulosate anions
[Cell-0-1 and hydroxide ions [OH-jf
within cellulosic fibers (thesquared brack-
ets indicate concentration, subscript f
indicates in the fiber and the cellulosate
ion could be nowhere else) for any given
concentration of chloride ion (from com-
mon salt) and hydroxideion (from alkali)
which might be added to the dyebath.
Sumner’s work, using the Donnan
model of the fiber (see later), made it
possible to confirm that the chemical
species which reacts with dyes to give
dye-fiber bonds is the cellulosate anion,
Cell-0-, because the rates of reaction are
related tothecellulosateion concentration
and not to the concentration of cellulose.
By the sametoken, it can be shown that the
concentrations of hydroxide ions within
the fiber [OH-]f and in the dyebath
[OH-], (where subscript s means in the
solution or dyebath) are also responsible
for the rate at which the reactivity of the
dye for the fiber is lost, due to the nonfiber
reaction of the dye. This undersirable side
reaction is called hydrolysis.
The big surprise to chemists was that it
does not require extreme alkalinity to
generate cellulosate ions, and that these
are many times more reactive than cellu-
lose itself.
Since both the cellulosate ions and
hydroxide ions are negatively charged, it
makes sense that they would be “on the
look-out’’ for areas within molecules
which are electron deficient-i.e., posi-
tively polarized-with which to react.
These include carbon atoms adjacent to
nitrogen in the aromatic heterocyclicrings
which make up many reactive groups (see
later). Such negataively charged species
are called nucleophiles; hydroxide and
cellulosate ions are nucleophilic reagents.
General Dye Features
Reactive dyes have been represented as
having as many as five different interlock-
ing features which may be assigned differ-
ent letters according to the native lan-
guage or the personal preference of the
assignor. The features are: the basic part
of the color molecule (C); solubilizing
groups (S); bridging groups linking the
reactivepart of the molecule to the soluble
color portion (B); reactive groups (R) and
leaving groups (X). This means reactive
dyes may be written: S-C-B-R-X. It can be
useful to split up the chemical entities
primarily responsible for fiber reaction (B,
R, X) in this way when it is necessary to
emphasizehow they individually affect the
reactivity (3).
Every group, including the bridging,
reactive and even the leaving groups, has
an effect on the physical properties of the
total dye molecule. These include color,
molecular size, ability to diffuse into fi-
bers, solubility, substantivity,saltsensitiv-
ity,lightfastness,and soon.
However, for general purposes, the au-
thor prefers to think of reactive dyes in
simpler terms: as water soluble dye mole-
cules with functional substituents; the
function being to react with cellulosate
ions. For simplicity and convenience in
talking about their dyeing reactions, let us
identify reactive dye molecules as Dye-X,
where the word Dye includes all the
physical, structural componentsof a water
soluble cellulose reactive dye, except the
leaving group X. It will be necessary to
deal with the bridging, reactive and leav-
ing groups for the purpose of structure
recognition in Chapter 5,Part 2.
All fiberreactivedyes react with nucleo-
philic reagents, and the overall reactions,
regardless of their mechanisms, can be
May1992 CCO 33

Reactive Dyes
represented by the formula below, where
Nu- isanynucleophilic anion, and X isthe
leaving group.
Dye-X +Nu- -Dye-Nu +X-
Eq.3
Yes, the reactionsaremorecomplicated
than this, but the equation represents the
beginning and endingstate of affairsquite
reliably. Chemists should refer to
Zollinger (3)for more completedetails.
Batch Dyeing Mechanism
Now that the general introduction to the
unique, reactive dye application category
has been made, the actual batch dyeing
mechanisms can be discussed. The nature
of the processes involved, both chemical
and physical, will be related to the three
steps of conventional reactive batch dye-
ing (exhaustion, fixation and washing-
off), and the possibility of future technol-
ogy refinements will be explored.
Chemical Processes
The chemical equations which follow are
absolutely essential for understanding re-
active dyeing:
Cell-OH +OH- Cell-0- +H20
Eq. 4
Cell-0- +DyeX k_RDye-O-Cell+X-
Eq. 5
OH- +DyeX k_trDye-OH +X-
Eq. 6
CompareEqs. 4,5, and 6 to Eqs. 1 and 3.
The reaction represented by Eq. 4 is a
reversible equilibrium reaction between
cellulose and alkali, to generate the fiber
nucleophile with which the dye can react,
Cell-0-. This equilibrium is established
almost immediately when the alkali is
added to the system.
The reaction represented by Eq. 5 is the
key to the formation of the covalent
dye-fiber bond and is the reaction, within
the fiber, of the dye with the fiber nucleo-
phile, Cell-0-. This fixation reaction pro-
ceeds at a rate determined by the product
of the concentrations of the reactants and
their intrinsic reactivity, expressed by the
rate constant kR. Squared brackets indi-
cate concentrations; subscripts f and s
mean fiber and solution respectively:
Fixation Rate =
[Dye-X]f X [Cell-0-1 X k R Eq. 7
The reaction represented by Eq. 6 is the
34
ITable 1. Solution and Fiber Phases
of Reactive Dyes
Solution (3) Fiber (f)
CI- or SOT CI- or SO4
X-=
OH-
Dye-X Dye-Xb
Dye-OH Dye-OHb
Na+ Na+c
Cell-OH
Cell-0-
-Notes:FH-+ the dye leaving groups,Cell-O-Dyebmay also be
CI- or SO;.bDyescontain anionic SOggroups, so
these species are anionic. Dye-X is short lived.
CSodiumions from all sources; dyes, salts and al-
kali.
hydrolysis reaction, by which reactive
dyes are wasted, to form by-products
which will no longer react with the fiber.
This reaction takes place both in the fiber
and in the solution (dyebath) with the
same rate constant of hydrolysis, kH, but
at different rates, because the concentra-
tions of the reactants are different in the
two locations:
In-Fiber Hydrolysis Rate =
[Dye-X]f X [OH-If X kH Eq. 8
In-Dyebath HydrolysisRate =
[Dye-XI, X [OH-], X kH Eq. 9
The total rate of reaction of the dye, or the
rate at which the dye loses its reactivity, is
given by the sumof the three rates given in
Eqs. 7 through 9.Clearlythis islarger than
just the rate of fixation (see upcoming
section, Dyeing, Step 2:Fixation).
Total Rate of Reaction =
Fixation Rate (Eq. 7) +
In-Fiber Hydrolysis Rate (Eq. 8) +
In-Dyebath Hydrolysis Rate (Eq. 9)
Eq. IO
Physical Processes
Exhaust dyeing reactive dyes on cellulose
involves inorganic and dye ions in two
phases, liquid and solid, dyebath and fiber.
Chemical reactions can take place in the
bath or in the fiber, but the physical
processes often involve those ions which
can move reversibly from dyebath to fiber
and back again. Other ions and molecules,
particularlycellulose and cellulose deriva-
tives, are not mobile. The situation can be
represented in a simple diagram, Table I,
which indicates solution and fiber phases,
those mobile ions which attempt to equili-
brate between dyebath and fiber (double
arrows) and the immobilespecies.
It is no easy matter to scientifically
study all these ion movementsand equilib-
ria while studying chemical reaction rates
as well. The next sections outlinea method
which is very helpful in such an attempt.
The same general approach has proved
useful for studying the acid dyeing of
nylon and the basic dyeing of acrylics (7).
However, the next three short sections
may be of little interest to dyers.
The Donnan Model:
A Research Tool
In science, all sorts of model systems are
used. They do not need to be completely
realistic, although it helps; however, they
need to be useful! The Donnan model of
fibers is linked with the name of a pioneer
in theareaof semi-permeable membranes,
and it can be used to examine how the
mobile and immobile, reactive and inert,
ionic species will equilibrate in dyebath-
fiber systems. It is particularly useful for
studying the behavior of alkalis, salts and
anionic dyes in cellulose-dyebath systems
so that the concentration of the different
ions in the fiber may be calculated from
experimental measurements on the solu-
tions.
The model has been based on a number
of assumptions,and it has been used along
with a number of constants some of which
might be labeled empirical.However,they
all have a realistic basis, which means that
their use leads to conclusions which make
good scientific and practical sense. The
assumptions are given below, as are the
nature of the constants used to study
cellulose. Their values are given else-
where (7).
Donnan Assumptions
The following are Donnan assumptions:
Cellulose in (textile) wet processes
can behave as if the external dyebath or
solution phase and the internal or fiber
phase are both aqueous liquids, separated
by a membrane through which ions but not
cellulose (fiber)derivatives may pass; i.e.,
as if the properties of diffusible ions within
the fibers at equilibriumcan be confined to
a special reservoir of water intimately
associated with those outer and inner fiber
surfacesaccessible to water.
0 Electrical neutrality is achieved in
both the external (dyebath) and internal
(fiber)phases. This means there are equal
numbers of positive and negative ionic
charges in each individual phase. But the
two phases may have different numbers of
ions.
0 The conditions of Donnan distribu-
tion of solutes (such as alkali, salt and
dyes)across the membrane areapplicable.
The most useful condition can be simply
stated. At equilibrium, the product of the
concentrations of each of the ions of a
solute are equal in both the external and
internal phases. The only caution needed
here is that solutes producing more than
two ions, even if two or more are the same,
must be treated as if each ion were
different. For example, sodium sulfate,
Na2S04, ionizes to give three ions, two
sodium ions and one sulfate. The product
of the concentrationsof these three ions is:
[Na'] X [Na'] X [SOT] or [Na+12
X [SO%].The same considerations apply
Vol. 24,No. 5

4
to the sodium salts of anionic dyes which
can have four or more sodium ions.
Cellulose Constants
For cellulosic fibers (7) these include the
ionic product for water Kw; either the
acidic dissociation constant for cellulose,
KA,or the basic dissociation constant,KB,
where KB= KA+Kw;the availability of
Cell-OH (actually glucose residues) for
provision of internal surfaces (41% for
cotton, 74% for viscose rayon), which
percentages are a function of fiber mor-
phology, crystallinity and amorphous
character; the concentration of Cell-OH
residues; the concentration of acidic, car-
boxylate groups in cotton and viscose
rayon and the volumeof the surface phase
in liters per kilogram of fiber (for cotton
and viscose rayon: 0.22 and 0.45). These
lattervaluesareknown asvolumeterms.
Dyeing
Wearenow ready to moveon todiscuss the
threeprinciple stepsof all reactivedyeing:
exhaustion, fixation or reaction and wash-
ing-off. A section will be devoted to ex-
plaining the significance of graphical rep-
resentations of the process.
0 Step 1: Exhaustion: In conventional
processes therequired amount of dissolved
dye is added to the dyebath at ambient
temperature and circulated through the
goods. The temperature is gradually
raised to the recommended dyeing and
fixation temperature and the required
amount of salt is added in portions, to
exhaust the bath.
This parallels direct dyeing, except in
one respect: the substantivity of the reac-
tive dyes is generally quite low, which
would require the amount of salt to be
high, sometimes as high as 200% owg, or,
at a 20:l liquor ratio, 100grams per liter.
The exhaustion might be quite low, re-
gardless;maybeaslowas 10%.
In the next few sections we will see why
low exhaustion and high salt concentra-
tions aretolerated. Certainly reactivedyes
could be derivatives of selected direct dye
molecules with high substantivity (al-
though they are not outstandingly bright
incolor). Why is this usually not the case?
0 Step 2: Fixation: The required (or
recommended) amount of alkali is added
to the goods with good circulation, and
fixation (reaction) proceeds. The hydrox-
ide ion, being a very small ion, enters the
cellulose readily and the equilibriumreac-
tion, Eq. 4, the ionization of cellulose, is
quickly established. This produces a sys-
tem with three possibilities for nucleo-
philic reaction with the dye: two in the
fiber (internal) phase, with the cellulosate
and hydroxide ions, one in the dyebath,
with hydroxide ions.
The dye is distributed between the
dyebath and the fiber after Step 1, and
reaction proceeds in both phases at rates
which aregiven in Eqs. 7 and 8.
Reaction proceeds until no further in-
crease of color depth takes place on the
fiber, at which time there is essentially no
free reactive dye left. At the end of this
stage, Step 2, there might be a total of
anywhere from 75-90% of the color, either
exhaustedor fixed,on thefiber;i.e., thereis
10-25% of hydrolyzed color left in the
bath. Of the color on the fiber, 15-25% is
not fixed (reactedwiththefiber) but isheld
by substantivity; Le., only 50-75% of the
reactive dye added to the bath initially is
covalently bonded to the fiber. The re-
maining 25-50% of hydrolyzed color will
probably end up, alongwith a high concen-
tration of salt, in thedrain.
This state of affairs has satisfied no-
body, andin many developed countriesit is
becoming an environmentalliability.
Exhaustion and Fixation Graphs
Steps 1 and 2 can be illustrated graphi-
cally; e.g., Fig. 1. This shows the changes
in dyebath exhaustion and dye fixation on
the fiber, with time, in a conventional
dyeing process lasting a nominal two
hours, with the alkali addition after 30
minutes. The curves represent the behav-
ior of two commercial dyes from the same
sub-group, but with no suggestion that
they are suitable for use together. The use
of the word coloron they axis obviates the
need to distinguishclearlybetween Dye-X
and Dye-OH, or reactive and hydrolyzed
dye.
After 30 minutes, just before alkali
addition, Dye I, curve EI, is 46% ex-
hausted, and the majority of the dye at this
stage will be in its original reactive form,
Dye-X. The shape of the curve seems to
indicate that the dye is moderately sub-
stantiveandcould haveexhausted toabout
60% if the alkali had not been added. The
equivalentcurvefor Dye 11,E 11,indicates
that the original reactive dye, Dye-X, has
very little substantivity under the given
conditions and has achieved its full ex-
haustion of about 10%before the addition
of alkali.
Afterthe addition of alkali, two new dye
species begin to form. Curves FI and FII
represent the increase from zero of-the
covalently bound dyes I and I1 respec-
tively; Dye-0-Cell. The continuation of
curves E1 and E11 (beyond the 30 minute
alkali addition) represent the sum total of
all the colored species associated with the
fiber. At 30minutes this color isalmost all
Dye-X, but Dye-0-Cell and Dye-OH ap-
pear gradually, until at t = 120minutes, it
is probably all Dye-0-Cell and Dye-OH.
It is not possible to distinguish from the
curves alone what percentages of which
species of fiber color E1and E11represent.
The figures at the right hand side of the
Fig. 1indicate:
(i) the percentage of the dyes I and I1
actuallyfixed: (68%, 66%);
(ii) the percentage of hydrolyzed dyes
dropped with the dyebath: (16%, 25%);
(iii) the percentages of hydrolyzed dye
exhausted ontothe fiber: (16%, 9%);
Together, ii +iii indicate the percent-
agesof wasted color: (32%, 34%).
It is apparent that the increased sub-
stantivity of Dye I over that of Dye I1has
hardly affected the overall yield of fixed
dye, but has produced 7%(16% minus 9%)
more of a product on the fiber which is
moderately substantive and will have to be
removed by washing. This is a distinct
drawback.
But, how does 66% of Dye I1 end up
fixed, when its substantivity is so low
before the additionof alkali? Certainlythe
added alkali (being an electrolyte, like
Key: E1 = Exhaustion of Dye I i16%
E11 = Exhaustion of Dye I1
FI = Fixation of Dye I
FII = Fixation of Dye I1 1E1
% Color
on Fiber
68%
1 t. mins. (nominal)
Fig. 1. Exhaustionand fixation vs.time for two reactive dyes.
Mav 1992 CCO 35

Reactive Dyes
salt) could drive more dye onto the fiber,
but surely not seven times as much! The
answer lies in the diffusion equilibria. Dye
in the bath and dyein thefiber aretryingto
establish an equilibrium. But every time a
dye molecule reacts with thefiber, its place
has to be taken by a molecule of dye being
transferred out of the bath and into the
fiber, in an attempt to re-establish the
equilibrium. Even if equilibrium is never
reached, there is always a driving force to
attract dye out of solution and into the
fiber as long as dyecontinuesto react.
Step 3: Washing Off The washing
off process is lengthy but unavoidable. To
achieve the high wetfastness expected
from a fiber reactivedyeing, all the hydro-
lyzed, unfixed dye has to be removed. The
more substantive the hydrolyzed dye, the
more difficult it will be to remove, and it
could continuetobleed slowly over time-
c.f. direct dyes-giving unexpectedly poor
wash- or wetfastness results.
A batch washing-off sequence might
include: acoldwater wash at 25 to 60C(75
to 140F); a hot water wash at approxi-
mately 60 to 80C (140 to 175F);scouring
with anionic surfactant at 80 to 90C (175
to 200F); and all this followed by hot and
then cold water washing. The sequence
can be critical and manufacturers’ recom-
mendations should be followed. Some
dyes, notablyvinylsulfone derivatives, can
have some of the reactive dye-fiber bonds
hydrolyzed if the first process iscarried out
too hot, with consequent color loss. All
dye-fiber bonds are capable of being hy-
drolyzed under severe enough conditions
of temperature and at too high or too low
pH’s. Vinyl sulfone derivatives are more
sensitive on the alkaline side; heterocyclic
carbon-nitrogen derivatives are more sen-
sitive on the acid side. For the structures
of those reactive species, see Chapter 5,
Part 2.
Hydrolysis and washing have been dis-
cussed at length (2,3),and the danger of
too severe washing producing acidic inter-
nal fiber pH’s and dye hydrolysis is real
(7). The economics of washing have also
been examined (8).
A good test for whether all the hydro-
lyzed dye has been removed, or, for the
effectiveness of the washing procedure, is
the so-called Hot-Wet Press test. A sam-
ple of the dyed goods is sandwiched be-
tween layers of bleached cotton fabric and
heated with a hand iron or a hot head press
for about 15 seconds. Any loose color in
water migrates towards the heating sur-
facesand is transferred to the white cotton
fabric. A stain generally indicatesunsatis-
factory washing-off, although for some
heavy shades no process will be adequate.
If the goods are not to be resin finished,
there may be no alternative other than to
resort to cationic fixatives. This may lead
to lowered lightfastnessand is not prefera-
ble to a good washing-off.
Process Improvement
What can be done to reduce the amount of
hydrolyzed dyeand increase the amount of
dye covalently bonded to the fiber? This
question can be expressed in chemical
terms by examination of Eqs. 7,8 and 9.
How can one: (a) increase the ratio
k R / k H , i.e., make the fixation (reaction)
rate constant higher relative to the hydrol-
ysis rate constant, Eqs. 7, 8 and 9; (b)
increase the fraction [Cell-O-If f
[OH-],-, also with a view of making the
fixation reaction proceed faster relative to
hydrolysis, Eqs. 7and 8;and (c) reduce the
value of [Dye-XI, X [OH-],, Le., reduce
the exposure of dye molecules in solution
to hydroxide ions in solution, Eq. 9, by
reducing either or both?
Viewed from these three angles the
problem looks very difficult.
0 First, the ratio of kR/kH is a property
of individual dye molecules and is out of
the hands of the dyer.
0 Second, provided the pH is kept
below 11, increasingthe saltconcentration
can raise the value of (b) [Cell-O-]~+
[OH-]? However, the amount of salt
suggested by the manufacturer is usually
the optimum for the particular procedure.
Sothere is not much help here either.
0 Finally, the traditional batch dyeing
method, of salting of the dyeand only then
adding alkali, should have taken care of
the hydrolysis of dye in solution, Eq. 9.
Certainly the dye in solution could be
reduced to very little before the alkali
addition if substantive direct dye struc-
tures were used. But the problems of
wash-off would be a serious drawback.
Alternatively, increasingthesubstantivity
of existing dyes by increasing the recom-
mended salt additions or lowering the
recommended temperature might be ef-
fectivewere it not for the adversepossibili-
ties. Salt addition may decrease the rate of
dye diffusion and increase the possibility
of dye aggregation, even induce precipita-
tion, while adding to the effluent problem.
Lowering the temperature would slow the
rate of dyeing.
There are some benefits from lowering
the liquor ratios, which would increase
substantivity, although the salt may have
to be reduced to avoid aggregation or
solubility problems.
Surely lowering the liquor ratio is not
the only possibility? This question will be
addressed in the section on Dyestuff Re-
search.
Review
Some aspects of reactive dyeing should
be repeatedly emphasized:
0 Whenever several dyes are used to-
gether to match a particular shade they
should have been selected with similar
dyeing and fixation characteristics (com-
patibility) in mind. In the conditions of
application they should not only be ad-
sorbed at similar rates and to similar
extents, but they should also react at
similar rates. In general, this means that
they should come from a set of dyes with
the same fiber reactivegroup(s).
0 Once an appropriate dyeing recipe
has been determined, the processvariables
of time, temperature, salt, alkali and
liquor ratio should all be controlled as
carefully as possible to ensure the opti-
mum shade reproducibility and dye fixa-
tion.
It is safe to say that unless these
conditions are fulfilled, the results will not
always be up toexpectations.
The traditional independence of the
dyer, to be flexible with formulations and
procedures, is seriously challenged by the
reactive dyes. Many of the tools needed to
fine tune the dyeing properties and proce-
dures for the different groups of reactive
dyes depend on chemistry, and are in the
hands of the dye manufacturers. The
market demand for bright colors of high
wetfastness has put reactive dyes in a
prominent position which seems likely to
be reinforced in the future-so this situa-
tion is unlikely to goaway.
There is no virtue in fighting what
cannot be changed, so the dyer must learn
to accept reactive dyes for what they can
do for him, not fight against what they will
not permit him to do. But first he should
carefully select the supplier, or suppliers,
most likely to be responsive to his present
and future needs.
Many reactive dyes have drawbacks
which will demand the attention of chem-
ists. The overall percentage fixation of the
early reactive dyes is low compared with
that expected from other dye categories.
The result is that the effluent is high on
both waste (hydrolyzed) dye and salt.
Furthermore, the color yield of many
reactive dyes can be quite sensitive to
relatively small changes in the application
conditions. These problems, lying between
dye manufacture and the dyed goods
market, arewaiting to be solved.
Research groups of some dye manufac-
turers are pursuing these problems with
References
vigor. m
(1) Giles, C. H., The Theory of Coloration of
Textiles,Edited by A. Johnson, Society of Dyers and
Colourists, 1989, Chapter 2.
(2) Fox, M.R. and H.H. Sumner, The Dyeing of
CellulosicFibers,Edited by C. Preston, Chapter 4.
(3) Zollinger, H., ref. 1,Chapter 6.
(4) Dolby,P.J., TextileChemist and Colorist,Vol.
12.No.9, September 1980,p74.
( 5 ) AATCC Buyer’s Guide /or the Textile Wet
Processing Industry, AATCC, published annually as
theJuly issueof TextileChemist and Colorist.
( 6 ) Aspland, J. R., Textile Chemist and Colorist,
Vol.23,No. 10,October1991,p14.
(7) Sumner,H. H.,ref. l,Chapter4.
(8) Quebec Section, Canadian Association of Tex-
tile Colourists and Chemists, Textile Chemist and
Colorist,Vol. 23,No. I I, November 1991, p2 I.

Reactive Dyes and their Application

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