Study on Reactive Dyeing

STUDY ON
REACTIVE DYEING
Habibur Rahman
BSc. In Textile (SARSTEC)
Executive (R&D)
Lantabur Group
Phone: +8801849104653

DISCUSSIONS
Reactive Dyeing.
Reactive dyes & their functions.
Reactive dyeing mechanism.
Auxiliaries and their functions used is Reactive dyeing.
Chemistry behind Reactive dyeing.
Process parameter of Reactive dyeing.
Effects of different parameters used in reactive dyeing.
Hydrolysis of reactive dye.
Stripping of reactive dye.

REACTIVE DYEING
A dye which is capable of reacting chemically with a substrate to from a covalent dye-substrate
linkage is known as a reactive dye.
Here the dye contains a reactive group and this reactive group makes covalent bond with the
fibre polymer and act as an integral part of fibre .This covalent bond is formed between the dye
molecules and the terminal –OH (hydroxyl) group cellulosic fibre or between the dye molecules and the
terminal amino (-NH 2) group of polyamide fibres.
The general formula of reactive dye can be
written as following
D-X-Y
Here,
D=Dye part( color producing part).
X=Bridge.
Y=Functional group.
D-X-Y+ Fibre = Fibre covalent bond.
This is shown in reactions below-
D-SO2-CH2-CH2-SO3Na+HO-Cell=
D-SO2-CH2-CH2-O-Cell+NaHSO3
D-SO2-CH2-CH2-SO3Na+H2N-Wool=
D-SO2-CH2-CH2-NH-Wool+ NaHSO3
Here,
D=dye part
Wool=Wool polymer.
Cell=Cellulose polymer

DYES AND THEIR PROPERTIES
A dye is a colored organic compound that absorbs light strongly in the visible region
and can firmly attach to the fiber by virtue of chemical and physical bonding between group of
the dye and group on the fiber. To be of commercial importance a dye should be fast to light,
rubbing and water. Color and dye have always played an important role in the life of man from
time immemorial.
Properties of reactive dye:
 Reactive dyes are anaionic dyes which are used for dyeing cellulosic protein polyamide fibres.
 Reactive dyes are found in powder, liquid and print paste from.
 During dyeing, the reactive group of this dye forms covalent bond with fibre polymer and
becomes an integral part of fibre.
 Reactive dyes are soluble in water.
 They have very good light fastness with rating about 6.
 The dyes have very stable electron arrangement and the degrading effect ultraviolet ray.
 Reactive dyes give brighter shads and have moderate rubbing fastness.
 Reactive dyes are comparatively cheap.
 Reactive dyes have good perspiration fastness with rating 4-5.
 Fixation occurs in alkaline condition.

REACTIVE DYEING MECHANISM
Dyeing of cellulosic fibers with reactive dyes consists of two phases:
Firstly, exhaustion phase, where dye is absorbed by material in neutral medium,
Secondly fixation phases, where reaction between dye & fibre takes place.
Cellulose in its reaction with reactive dyes is considered as alcohol. Electro negativity of oxygen
atoms governs tendency of hydroxyl group to ionize. Cellulose is consequently ionized under
alkaline conditions & can act as nucleophilic reagent & shows subsequent reactions with acid
halides (nucleophilic substitution). Mechanism of nucleophilic substitution is as below:
Most of reactive dyes require alkaline catalyst for fixation on fibre. During dyeing with vinyl
sulphone dyes, vinyl sulphone group is formed from parent dye under alkaline conditions, may
be represented by:
Mechanism of nucleophilic addition reaction is as below:
When alkali is added to vinyl sulfone dye, it is converted to

AUXILIARIES AND THEIR FUNCTIONS
Salt:
As a salt, NaCI is used widely.
The salt end the following
things-
Salt are used to increase the
affinity of dye to fibre.
It decreases the hydrolysis
rate of dyes.
It neutralize the electro
negativity of fibre surface
when immersed in solution.
It puts extra energy to push
dye the fibre polymer ie
increase absorption.
The amount of salt used
depend upon the shade to be
produced-
For light shade -10-20
gm/litre salt is used.
For medium shade-30-50
gm/litre is used.
For deep shade-60-100
gm/litre is used.
Alkali:
Alkali is used for the
following purpose-
Alkali is used to maintain
proper pH in dye bath & thus
to create alkaline condition.
Alkali is used as a dye fixing
agent.
With out alkali no dyeing will
take place.
The strength of alkali used
depend on the reactivity of
dyes.
As strong alkali caustic
(NaOH) is used to create pH
12-12.5 when the dye is of
lower reactivity.
As medium alkali sods
ash(Na2co3) is used to create
pH 11-12. when the dye is of
medium reactivity.
As weak alkali (NaHCO3) is
Urea:
Urea is used in continuous
method of dyeing .It helps to
get required shade of dye. To
get dark shade more urea is
used and for light shade less
amount of urea used.
Urea:
Urea is used in continuous
method of dyeing .It helps to
get required shade of dye. To
get dark shade more urea is
used and for light shade less
amount of urea used.

CHEMISTRY BEHIND REACTIVE DYEING
Reactive dyes differ from other coloring matters in that they enter in to chemical
reaction with fibre during dyeing & so become a part of fibre substances. A reactive dye is
represented as R-B-X, where, R-Chromogen, B-Bridging group X-Reactive system. When it
reacts with fibre, F, it forms R-B-X-F. Wet fastness of dyed material produced, depends on
stability of true covalent bond X-F.
Dyeing principle is based on fibre reactivity & involves reaction of a functional group
of dyestuff with a site on fibre to form a covalent link between dye molecule & substance. 4
structural feature of typical reactive dyes molecule are:
1. Chromophoric grouping, contributing color.
2. Reactive system, enabling dye to react with hydroxy group in cellulose.
3. A bridging group that links reactive system to chromophore,
4. One or more solubilising group, usually sulphuric acid substituent attached to chromophoric
group for their color, although Azo chromophore –N=N- is by itself the most important.
All reactive dyes contain sodium sulphonate group for solubility & dissolve in water
to give colored sulphonate anions & sodium cations. Most reactive dyes have 1 to 4 of these
sulphonate groups; General form of reactive dye is as follows: S R----B----X
Where,
S = Water solubility group
R = Chromophore
X = Reactive System
B = Bond between reactive system & Chromophore

PROCESS PARAMETER
Dye Bath pH:
For most of dyes optimum pH is 10.8 to 11.0 at 20-25°C soda ash is the
best alkali for dyeing at 30°C for cotton, mercerized cotton & linen.
Increased fixation (due to higher temperature) & increased dye bath
stability & better reproducibility are advantages of soda ash as fixing
agent.
Dyeing Temperature:
As increase in temperature affects rate of physical & chemicals processes
involved in dyeing, it is important in reactive dyeing also. Affinity of dye
for fibre decreases with increases in temperature (dyeing is an
exothermal reaction), & at same time rate of dye hydrolysis increases,
adversely affects color yield fixation. However, rate of diffusion of dye in
fibre increases with increased temperature. At temperatures lower than
20°C, rate of fixation is very low. Hence for most of dyes a temperature,
while for some others' dyeing at 50-60°C with sodium bicarbonate as
alkali gives maximum color value.
Process parameters:
Internal fabric pH
Working liquor ratio on the
machine
Effective salt concentration
(actual)
Effective alkali concentration
(actual)
Rate of heating
Rate of cooling
Fixation temperature
Electrolyte Concentration:
Since reactive dyes have low affinity for cellulose, exhausting dye bath can increase fixation, by adding
common salt or Glauber’s salt prior to fixation. Amount of salts required to produce adequate
exhaustion decreases with decreasing liquor ratio.
Dyeing Cycle Time:
Generally, dye may be added in two portions. Salt may also be added in two lots. Exhaustion takes place
in 20-30 mins. Alkali is then added in 2 lots (also in Progressive dosing® system developed by Hoechst)
& dyeing is continued for 30-90 mins. Shade depth & dye reactivity decides dyeing time. For deeper
shades, longer times are required.
Liquor Ratio:
With increased liquor ratio, both exhaustion & fixation takes place to increased extent. However, rate of
fixation of most of dyes is not significantly affected. As liquor ratio is decreased, effectiveness of
increasing salt addition also decreases. Hence lower amounts of salts are sufficient to get optimum
exhaustion.

EFFECT OF DIFFERENT PARAMETERS
Time:
Time is very important in dyeing. The fixation of
dye depends on time. With the increase of time
the fixation of dye will increase. For example if a
fabric is dyed in a dyeing bath for five minute the
amount of dye it will absorb is less than the
amount of dye will absorb by a same fabric in ten
minute. As we have worked with reactive dye so
the optimum time for fixation of this dye is sixty
minute. If a fabric is dyed for sixty minute with
reactive dyes it will show all required properties,
i.e. wash fastness, rubbing fastness, light
fastness, perspiration fastness etc.
pH:
In the case of most popular fiber reactive dyes, a
high pH actually activates the cellulose (cotton)
fiber, forming a cellulosate anion, which can then
attack the dye molecule, leading to a reaction
that produces a strong, permanent covalent
bond. Without a high pH, the dye will not fix
permanently to the cellulose fiber. In dyeing
cotton and other cellulose fibers with popular
fiber reactive dyes such Synozol yellow K-HL,
Synozol red K3BS150%,Synozol black B 150%,
sodium carbonate is used for no other reason
than to increase the pH of the dye reaction, so
that the fiber will react with the dye.Temperature:
Higher temperature causes the hydrolysis of the dye with water to become more frequent. Experiment
showed that high temperature caused the cotton material to have a very poor ability to be dyed.
Higher temperature cause cotton material to absorb dye more because of the molecules having more
kinetic energy, thus more collision would occur between dye molecule and fibre, increasing the chance of
a reaction taking place and bond formation.
Higher the temperature in general increases the rate of any reaction. The same thing occurs with dye-but
also with the water that the dye is dissolved in. The dyes can react with either the cellulose fibre or with
the water, the latter reaction being known as hydrolysis. The effect of the added energy is much greater
on the dye reaction rate than on the ability of the dye to soak into the fiber. Increasing temperature too
much cause the dye to react with the water before it ever gets into the fibre. Infact its better to let the dye
soak into the fibre for some time before beginning dye reaction by adding soda ash (or other ph increaser)
and any heat.
The cellulose molecules in cotton material can dye well at high temperature (80°c), if the dye is already

EFFECT OF DIFFERENT PARAMETERS
Color:
Color is the sensation which occurs when light
enters the Eyes. It is rising from the activity of
the retina of the eye and its attached nervous
mechanisms. This activity is being , in nearly
every case in the normal individual a specific
response to radiant energy of certain
wavelength and intensity.
The hue refers to the actual color sensation
(red, blue, yellow), the sensation or
chroma(depth of color) to the degree of
differentiation from grey(dull of vivid), and
lightness to the amount of light reflected from
the object (light or dark). In the Munsell color
system, these attributes are assigned alphabetic
and numerical levels.
Wavelength(nm) color
380-400 Violet
400-435 Indigo
435-480 Blue
480-490 Greenish blue
490-500 Bluish green
500-560 Green
560-580 Yellow green
580-595 Yellow
595-605 Orange
605-740 Red
Light:
That aspect of radiant energy of which a human
observer is aware through visual sensations
arising from stimulation of the retina by the
radiant energy. The wavelength of perceived
colors of visible spectral light are between 380
to 740 (as shown in table one)

HYDROLYSIS OF REACTIVE DYE
Hydrolysis of reactive dye:
Under alkaline reactive dye reacts with the terminal hydroxyl group of cellulose. But if the
solution of the dye kept for long time its concentration drops. Then the dye react with the
hydroxyl (OH) group of water. This reaction of dye with water is known as hydrolysis of reactive
dye. After hydrolysis dye con not react with fiber. So hydrolysis increases the loss of dyes.
Hydrolysis of halogen containing reactive dyes:
D-R-Cl + H-OH ---------------- DR-OH + HCl
Hydrolysis of activated vinyl compound containing group:
D-F-CH2-CH2-OSO3H + H-OH ------------- D-F-CH2-CH2- OH + H2SO4

STRIPPING OF REACTIVE DYE
Stripping of reactive dye:
Partial stripping: Partial stripping is obtained by treating the dyed fabric with dilute acetic acid
or formic acid. Here temperature is raised to 70-100 C and treatment is continued until the
shade is removed by desired amount.
Acetic acid -------------- 0.5 - 10 g/L
Temperature ----------- 70 – 100 C
Full stripping: For complete stripping the goods are first treated with sodium hydro sulphite
(hydrose) at boil and then washed off and bleached with 1 g/L sodium hypochlorite (NaCl) or
bleaching powder at room temperature. This is carried out as following steps-
Wetting agent ------ 0.5 – 1.0 g/L
NaOH -----------------3-6 g/L (Temp100-105 x 60-30min)
Hydrose ---------------7-10g/L
Then,
Wetting agent --------------1g/L (RoomTemp x 10min)
Bleaching powder ---------1g/L

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