Reactive dye,23.03.2016

DYEING OF COTTON WITH REACTIVE DYES
WORLD FIBRE CONSUMPTION (X1000 tonne)
Year Cotton Regen
erated
wool Synthetic Total
1990 18700
49%
2700 2000 14900
39%
39300
1996 20700
49%
2500 2000 17300
41%
42600
2000 23400
49%
2400 2200 20500
42%
48500
Dye consumption (Tonne)
Dye
class
1988 1995 2004
Sulphur 90 000 70 000 70 000
Direct 74 000 60 000 68 000
Vat 36 000 21 000 22 000
Indigo 12 000 12 000 12 000
Azoic 28 000 18 000 13 000
Reactive 60 000 109 000 178 000
Total 300
000
290 000 354 000
1. Basic Principles of Textile Coloration (Structures,
Hydrolysis)
-by Arthur D Broadbent
2. Cellulosics Dyeing (Reaction Mechanism)
-Edited by John Shore
3. Fundamentals and Practices in Colouration of
Textiles (Overall, Types, Examples, Effects)
-J.N. Chakraborty
Books Reference:

Reactive Dyes
• Discovered in 1956: British chemist rattee and stephen, ici now
zeneca colours
• After 100 years of discovery of first synthetic dye, perkins, british
chemist
• Chemical reaction between dye and fibre
• Covalent bond formation
• Named as reactive dyes
Dye manufacture
• Well reputed large scale manufacturer Like IDI, Clariant, Dyestar
• Large scale marketing: manufacture by small scale manufacturere,
marketed under reputed brand name
• Small scale manufacturer: Marketed by non-brand name
• Foreign reputed company : product manufactured in developing
country and marketed by foreign company
• Dye quality varies at the manufacturing stage.

Properties Of Reactive Dye
1. Reactive dyes are anionic dyes which are used for dyeing cellulosic and protein
(polyamide) fibres.
2. Reactive dyes are found in powder, liquid and print paste from.
3. During dyeing, the reactive group of this dye forms covalent bond with fibre
polymer and becomes an integral part of fibre.
4. Reactive dyes are soluble in water.
5. They have very good light fastness with rating about 6.
6. Reactive dyes give brighter shads and have moderate rubbing fastness.
7. Reactive dyes are comparatively cheap.
8. Reactive dyes have good perspiration fastness with rating 4-5.
9. Fixation occurs in alkaline condition

Chemistry behind reactive dyeing
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: W-D-Q-RG-X
1.Chromophoric grouping, contributing colour ====D CHROMOGEN
2.Reactive system, enabling dye to react with hydroxyl group in cellulose.=RG REACTIVE
GROUP
3.A bridging group that links reactive system to chromophore ==Q BRIDGING GROUP
4.One or more solubilising group, usually sulphuric acid substituent attached to
chromophoric group for their colour Most reactive dyes have 1 to 4 of these sulphonate
groups==W SOLUBILIZING GROUP
General form of reactive dye is as follows:

STRUCTURAL FEATURES
W-D-Q-RG-X
W SOLUBILIZING GROUP
D CHROMOGEN
Q BRIDGING GROUP
RG REACTIVE GROUP
X LEAVING GROUP
Leaving Group

REACTIVE DYES BASED ON TRIAZINE

Classification of Reactive Dyes
1) On the basis of reactive group:
a) Halogen (commonly chlorine) derivatives of nitrogen containing
heterocycle, like 3 types
i) Triazine derivatives: procion, cibacron.
Ii)Pyridimine derivatives: reactone
iii)Quinoxaline derivatives: levafix.
Quinoxaline/benzopyrazine
Pyrimidine

• b) Activated vinyl compound:
i) Vinyl sulphone: remazol
• Ii) Vinyl acrylamide: primazine
• Iii) Vinyl sulphonamide: levafix.
2) On the basis of reactivity:
a) Lower reactive dye: Medium reactive dye: here pH is
maintained 11-12 by using Na2CO3 in dye bath.
b) Higher reactive dye: here pH is maintained 10-11 by
using NaHCO3 in dye bath.

3) On the basis of dyeing temperature:
a) Cold brand:
These types of dyes contain reactive group of high reactivity. So dyeing can be
done in lower temperature i.e. 320-600C.
For example: PROCION M, LIVAFIX E.
b) Medium brand:
This type of dyes contains reactive groups of moderate reactivity. So dyeing is done
in higher temperature than that of cold brand dyes i.e. in between 600-710C
temperatures.
For example, Remazol, Livafix are medium brand dyes.
c) Hot brand:
This type of dye contains reactive groups of least reactivity. So high temperature is
required for dyeing i.e. 720-930 C temperature is required for dyeing.
For example PRICION H, CIBACRON are hot brand dyes.

Substution Mechanism
• Dye reacts with cellulose by nucleophilic substitution of Cl, F,
Methyl sulphone or nicotinyl leaving group activated by an
adjacent nitrogen atom in a heterocyclic ring. N-containing
heterocyclic rings bearing halogeno substituents undergo
nucleophilic substitution. Heteroatoms in aryl ring activate the
system for nucleophilic attack because of their
electrinegativity.

Addition Mechanism
• Reacts with cellulose by nucleophilic addition to a carbon-
carbon double bond, usually activated by a powerful electron
attracting sulphone group. Vinyl sulphone group is usually
generated in the dyebath by elimination of sulphate ion from
a 2-sulphatoethyl sulphone precursor group with alkali.
• This polarization confers a positive character on the terminal
carbon atom, favouring nucleophilic addition of cellulosate
ion.

Dyeing steps of reactive dye:
The dyeing mechanism of material with reactive dye takes place in 3 stages:-
i) Exhaustion of dye in presence of electrolyte or dye absorption.
ii) Fixation under the influence of alkali.
iii) wash-off the unfixed dye from material surface.
i) Dye absorption:
When fibre is immersed in dye liquor, an electrolyte is added to assist the exhaustion of dye.
Here NaCl is used as the electrolyte. This electrolyte neutralize absorption. So when the
textile material is introduces to dye liquor the dye is exhausted on to the fibre.
ii) Fixation:
Fixation of dye means the reaction of reactive group of dye with terminal –OH or-NH2 group
of fibre and thus forming strong covalent bond with the fibre and thus forming strong
covalent bond with the fibre. This is an important phase, which is controlled by maintaining
proper pH by adding alkali. The alkali used for this create proper pH in dye bath and do as the
dye-fixing agent. The reaction takes place in this stage is shown below: -
iii) Wash-off:
As the dyeing is completed, a good wash must be applied to the material to remove extra and
unfixed dyes from material surface. This is necessary for level dyeing and good wash-fastness.
It is done by a series of hot wash, cold wash and soap solution wash.

REACTIVE DYES BASED ON VINYL SULPHONE

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 can
not react with fiber. So hydrolysis causes 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

Side Reaction
•Hydrolysis of dichlorotriazine
1.Results in wasted dye
2.Economic and environmental concern
•Typically fixation of around 60% is obtained

SCHEMATIC REPRESENTATION OF TRIZINE REACTIVE DYES

Factors affecting dye uptake
1) Affinity and reactivity of dye
• Affinity of reactive dyes is directly proportional to their molecular
structure– dyes of lower molecular weight have lower affinity,
lower will be its hydrolysis and vice-versa.
• Removal of fully hydrolysed dye is relatively simple, but the same
for partially hydrolysed one is rather difficult.
• The larger the structure of dye the higher the energy requirement
for its removal as bond energy directly varies with structure of dye
• A dye with higher affinity results in higher dye uptake at shorter
time indeed but with an inferior fastness.
• Partially hydrolysed dyes are not completely removed during
soaping and washing, rather slow removal occurs during domestic
washing cycles causing poor wash fastness.
• Fixation of dye and rate of hydrolysis depend on presence of
reactive groups in dye molecule
• Higher the temperature, higher is the rate of diffusion for dye
molecules of lower size

2) Liquor ratio
• Hydrolysis of dye increases with increase in liquor in bath;
reactivity of dye, temperature, etc., also enhance hydrolysis
• too lower a ratio may result uneven dyeing due to higher
effective dye concentration in bath and higher strike ratio
3) Temperature
• Rise in temperature enhances hydrolysis
• should not go beyond 40°C for M-brands
• and 70°C for H-brands, with cold brand dyes, dyes of high
• molecular weight exhaust more at higher temperature, while
those of lower
• molecular weight at lower temperature

4) Electrolyte
• Higher the affinity, lesser the liquor ratio and lesser the shade depth,
lower is the dosing of salt.
• Cold and hot brand dyes are applied at a salt concentration of 50 and 75
g/l respectively.
• Quick exhaustion is very essential to resist loss of colour through
hydrolysis.
• A few reactive dyes require application of salt in excess due to lack in
affinity, e.g., yellow 6G, H5G, orange brown HG, scarlet HR, blue C4GP,
etc.
5) Alkalinity or pH of bath
• Dye bath should be free of alkali till exhaustion is completed to resist
premature fixation of dye and corresponding uneven dyeing.
• Rise in pH up to 11 increases exhaustion and reactivity, but beyond this,
exhaustion decreases. Excess alkali in bath promotes hydrolysis of dye.
• A pH around 10.5–11 with only Na2CO3 and 11–12.5 with combination of
NaOH and Na2CO3 are suitable for cold and hot brand dyes respectively.
• Due to higher reactivity of M dyes, a mild alkali alone is sufficient for
fixation, but a combination of NaOH and Na2CO3 (2 and 6 g/l
respectively) promotes fixation of H and remazol dyes through common
ion effect due to their lesser reactivity with cotton.

6) Nature of fibre
Dye uptake is also influenced by accessible free volume in fibre – higher the
volume, higher will be dye uptake. Scoured cotton has the least free volume,
bleached cotton has little higher and mercerized cotton has the highest free
volume due to removal of more impurity or immature fibre.
• Viscose has more free volume than mercerized cotton and so dye uptake will
be according to these free volume data.
7) Time of dyeing
Reactive dye solution must not be stored for long time; otherwise hydrolysis
will occur with loss of colour value. Time of dyeing must be reduced keeping
exhaustion of bath on higher side.
Dyeing time must be shorter, especially when the process is carried out at
higher temperature or with more alkali or more water to reduce hydrolysis.
• Time for both exhaustion and fixation are to be optimized as running
the process beyond calculated time is meaningless.
When no further exhaustion is possible or the reaction between dye and
fibre has ceased, dyeing for more time unnecessarily lengthens the process.

Dyeing Conditions
Time allowed for diffusion of dye into substrate
‰ Concentration of dye in fiber is up to 500 times greater than in solution
Acidity difference creates ~25-fold excess cellulose anion

Application by Exhaust method
=Dye is pasted with T R oil followed by dissolution in water along with little
urea. Cellulose to be dyed is wetted and treated in bath or loaded in jigger
or winch.
=Dyeing is carried out for 30 min with half dye solution in first turn and rest
half in second turn of jigger at required temperature for different classes
(30°c and 40–45°c for cold and hot brands respectively).
=Salt is added (30–50 g/l and 75–100 g/l for cold and hot brands respectively)
and dyeing is continued with slow heating of bath to required temperature
(40–45°c and 60–80°c for cold and hot brands respectively).
=Another 1–2 h after which alkali is added (5–10 g/l Na2CO3 for cold brands,
6–10 g/l Na2CO3 along with 2 g/l NaOH for hot brands).
=Fixation is carried out over a period of 45 min at this temperature. Dyebath
is discharged; goods are thoroughly washed, soaped at boil and hot
washed for removal of partially hydrolysed / unreacted / fully hydrolysed
dye.
=Addition of dye, if required for shade matching must be done in salting
stage and prior to addition of alkali, otherwise chances of formation of
unleveled shades exist.

Continuous Method
• Due to overall low affinity of reactive dyes for fibre, production
of levelled shades is comparatively easy.
• Cold brand dyes are applied through a pad– dry or pad–dry–
steam technique, the latter is suitable and popular for all brands
of reactive dyes.
• In pad–dry method, fabric is padded with dye, NaHCO3 (15–20
g/l), urea (50–100 g/l) and wetting agent (2–5 g/l), followed by
batching for 2–12 h or drying at 100–110°C when water
evaporates and transports dye at the interior of fibre for fixation.
• NaHCO3 turns to Na2CO3 at higher temperature in presence of
moisture raising dye bath pH to facilitate fixation.
• Urea acts as potential solvent, activates moisture to retain
soluble form of dye and is essential for deeper shades.

• Pad-dry-steam method gives excellent result, steaming may
be done at 105°C for 5 min.
• To avoid oxidation during steaming, resist salt L, e.g. ludigol
(5–7 g/l) may be incorporated in padding liquor. Reactive
dyes, being water soluble, migrate during drying throughout
cotton from a point at lower temperature to a point at higher
one as evaporation of water from latter point is fast causing
movement of water molecules along with dye.
• Drying must be uniform at each point of fabric to avoid
development of unlevelled shades.
• Continuous dyeing may be best done by padding with a liquor
consisting of dye, urea (50 g/l), NaHCO3 (20 g/l), resist salt L (5
g/l) followed by drying, steaming, soaping and washing.

Pad Batch Dyeing
Merits
• Modest investment layout
• Suitable for small and fairly large batches
• Very simple working conditions, Limited manpower required
• Low energy consumption
• Lower water consumption than exhaust dyeing
• Good penetration and level dyeing, Good reproducibility
• Suitable for dyeing knit goods
Demerits
• Batch process
• Higher dye consumption than pad‐dry‐pad‐steam
• Moderate coverage of dead and immature cotton

Continuous method( Pad – steam)
Continuous method mainly used for dyeing fabrics with high liquor ret
ention,
such corduroy, because no intermediated drying is required
Merits
• No migration problems
• Reduced energy costs
• Good appearance of the dyedfabrics
• No detrimental influence onfastness
Demerits
• Higher amounts of dye are required to produce deep shades
compared to the pad‐batch or pad‐dry‐pad‐steam processes
• Worthwhile for dyeing deep shades when the higher dye costs are
at least balanced by savings in energy and gains in productivity

Pad Dry- Pad Steam Process
Merits
• Economical process for large production runs
• Still economical for fairly small runs (>5000m) on modern
equipment
• High colour yield
• Very good appearance of the dyed fabric
• Good reproducibility
• No detrimental influence on light and/or chlorine fastness
Demerits
• Shade changes are time consuming
• Less suitable for dyeing fabrics prone to
migration problems or difficult to dry(pile fabrics)

Pad Thermofix Process
Merits
• Good colour yield on fabric and coverage of dead fabric
• Very good lab to bulk reproducibility
• Good batch to batch reproducibility
• Moderate soiling of machinery
• No need for a chemical pad liquor
Demerits
• Not recommended for dyeing regenerated cellulose
• Possible specky appearance of the dyed fabric
• A negative influence on the fabric handle is possible
• Danger of yellowing of the substrate
• Lower light / chlorine fastness level
• The process requires urea

Dyeing with Bi-fuctional reactive (H-E) dye
These dyes offer unique dyeing profiles including controlled rate of
primary exhaustion in neutral alkali, high migration and diffusion
properties and controlled rate of secondary exhaustion after addition of
alkali, and are the preferred choice for package dyeing (Khatri, 2004);
levelness in shades and reproducibility are excellent even in little
variation of dyeing parameters (Anon, 1982).
These dyes differ from M and H brand dyes in respect to the location of
reactive groups in dye structure – when one or two reactive sites are
available on the same nucleus of H and M dyes, the reactive groups in H-
E dyes are oriented throughout the whole structure of dye molecule
(Betrabet et al., 1977); the advantage is that if one reactive group is
hydrolysed, another is left for reaction with cellulose from another
location (Peters, 1975). Their tendency for hydrolysis lies in between that
of H and M dyes.

(I) suitable for exhaust dyeing with higher exhaustion rate,
(ii) higher substantivity,
(iii) higher tinctorial value,
(iv) possess excellent compatibility within group, even with H dyes,
(v) good build-up of shade,
(vi) insensitivity to liquor ratio,
vii) economy in use because of high tinctorial yield,
(viii) consistently high fixation,
(ix) reliability– excellent reproducibility owing to insensitivity to probable
variations in Liquor ratio, dyeing time, salt concentration, temperature
etc.,
(X) complete shade range,
(xi) higher molecular weight to provide good light and excellent wash
fastness,
(xii) resistant to acid and alkaline hydrolysis and to attack by detergents
containing mild oxidising agents, and
(xiv) too low wastewater load (atul ltd., 2009).
Features of H-E dyes

Reactive Red-HE3B (C I Reactive Red 120, C I 25810)
Reactive Orange-HER (C I Reactive Orange 84)
Reactive Green-HE4BD (C I Reactive Green 19)
Reactive Blue-HERD (C I Reactive Blue
160) is the 1:1 copper complex of
above structure

Method 1: Addition of salt by parts
This method is suitable for all depths of shade but only where circulation of liquor is
absent and electrolyte is manually added to control rate of exhaustion.
For better levelling and penetration, temperature may be raised to 95°C for 20 min
during salt treatment and is to be lowered down to 80°C before alkali is added. In
dyeing with turquoise H-EG, Glauber’s salt (Na2SO4.10H2O) is
used.
Dyeing is started at 50°C along with sequestering agents. Pre-dissolved dye is added
over 20 min at 50°C, temperature is raised to 95°C and electrolyte is added in portions
of 10%, 30% and 60% with 15 min intervals. Dyeing is continued at 95°C for further 30
min, followed by cooling down to 80°C.
Alkali is added (Na2CO3 + NaOH: 5 g/l + 0.2 g/l, respectively) and dyeing is continued
for further 60 min at 80°C (Fig. 7.1).

Method 2: Salt at start
Dyeing is started along with salt and is suitable for machines with liquor
circulation and for dyeing of all medium to heavy shades. In dyeing pale
shades (< 0.5%) on mercerised yarns or with high density packages,
temperature may be raised to 95°C and maintained for 20 min, then cooled
down to 80°C and circulated at 80°C for 5 min before addition of alkali (Fig.
7.2).

Method 3: All in
In this case, alkali and salt are added at the start of dyeing and is suitable for
machines with liquor circulation. Combined alkali system is preferred. Green
H-E4BD, green H-E3B and navy blue H-ER should not be used by this ‘all-in’
method (Fig. 7.3).

Method 4: Migration technique
Useful for machines with microprocessor controlled addition systems for dyeing
pale shades (< 0.5%) and for all shades on difficult substrates such as
mercerised cotton and viscose packages (Fig. 7.4).

Method 5: Isothermal technique
This is for machines with microprocessor controlled addition systems for medium to heavy
depths (> 0.5%) on unmercerised cotton where exposed selvedge is protected from being
cooled down by:
(i) using closed jigger,
(ii) controlled batching to avoid slippage during dyeing and using all pieces of fabric having equal
width,
(iii) controlling dye bath temperature at 90°C during salting stage; better penetration of difficult
fabrics is achieved by raising the bath temperature to 100°C during this stage,
(iv) adjustment of dyebath temperature to 85–90°C to ensure that the fabric is maintained at a
minimum of 80°C during the whole period of dye fixation (the alkaline stage) and
(v) dyeing time given was adequate to fix dye on the selvedges (Fig. 7.5).

After treatment
•In the dyeing of deep shades or inefficient washing
equipment there may be incomplete removal of
unfixed dye.
•After treatment with cationic dye fixing agent.
•Insolubilizes the unfixed dye.
•Improves wash fastness.
•Treated with 5-10 g/L cationic dye fixing agent at
50-60º C for 10-30 mins.
•Treatment with dye fixing agent is not substitute of
wash off process.

Recipes for exhaustion dyeing
• The optimum dyeing temperatures and dyeing conditions for
reactive dyes differ according to the type of reactive group
involved, so the first thing that must be decided is the type of
dye that will be used.
• The substantivity to cellulose of reactive dyes is lower that
that of direct dyes,
• but the addition of inorganic salts to the dye bath can raise its
substantivity. Usually, Glauber's salt/common salt is used at a
conc. of 50g/L,
• but this conc. should be increased in the dyeing of deep
shades,
• and can be reduced for low salt reactive dyes. Inorganic salts
can be added incrementally to dye baths for level dyeing,

• Alkali is employed as the catalyst in the reaction between
the dye and the fiber and
• the important point here is not the type of alkali used but
• the need for the pH of the bath after addition of the alkali to
be around 11.5.
• Soda ash is easily used as the alkali because the addition of
20g/L yields a pH of around 11.5.
• Caustic soda is also convenient, but because it is a strong
alkali, adjusting the pH to around 11.5 is extremely difficult,
and

• Because the dyeing rate of reactive dyes depends on a
combination of exhaustion through substantivity and fixing
through reaction, quantitative adjustments through the control
of the dyeing rate by adjusting the rate of temperature
increase, as are used for other classes of dyes, are not effective,
• but adjustments with the incremental addition of inorganic
salts or alkalis (pH) are effective.
• Consequently, isothermal (at constant temperature) dyeing
through the incremental addition of alkali or alkali dosing is
used. The specifics of incremental addition and dosing are
decided by the agitation efficiency of the dyeing apparatus.
• if hard water is used in the dyeing process, a sequestering agent
can be added.

The role of alkali in reactive dyeing
• In the dyeing of cellulose with reactive dyes, alkali is necessary
because it acts as a catalyst in the reaction between the
dyestuff and the fiber. The important point is not the type or
amount of alkali but rather the pH of the dyebath, which must
be closely supervised.
• The most suitable pH for dyeing varies with the temperature,
being approx. 11.5 for common warm dyeing (dyeing at
approx. 60), 10-11 for hot dyeing (80) and 12.5 for cold dyeing
(40). The relationship between temperature and the optimum
pH is shown in the following graph.

•

• Soda ash is often used because 20g/l usually produces a pH of
around 11.5.
• However, it can be used with sodium phosphate or caustic
soda when a higher pH is required, or
• with sodium bicarbonate (baking soda) when a lower pH is
required.
• Caustic soda is not often used because it is a strong alkali and
achieving a pH in the range of 10-12 is difficult.

The role of sequestering agents in reactive dyeing
• When the water of the dyebath contains metallic ions,
• there is the danger that uneven dyeing, such as specking,
• or reduction of the concentration of the dye will be caused by the dyestuff's
coagulation and reduced solubility.
• metallic ions can also be introduced by impurities in Glauber's salt or
common salt,
• so even when soft water is used in the dyeing, the presence of metallic ions
can lead to problems.
• the use of sequestering agents is very important.
• In the past, traditional thinking has been that "if the chromophore of a
dyestuff is a metallic complex salt, then sequestering agents should not be
used.
• " Recently, however, it has been ascertained that sequestering agents added
to the dyebath have practically no effect on the metal in the chromophore of
the dye.
• Sequestering agents effective in neutral to alkaline conditions are preferable
• The use of sodium hexa meta phosphate is common

Setting the dyeing temperature
• optimum dyeing temperatures can be divided into the following three
groups:
• Cold dyeing (30-40): eg. dichlorotriazine, difluorochloropyrimidine dyestuffs
• Warm dyeing (50-60): eg. vinyl sulfone, monofluorotriazine dyestuffs
• Hot dyeing (80-90): eg. monochlorotriazine, trichloropyrimidine dyestuffs
• Traditional reactive dyestuffs use a chromophore of relatively low molecular
weight similar to that of acid dyestuffs. Thus while the dyeing temperature
for most dyestuffs can still be decided according to the reactant as outlined
above,
• some recently developed dyestuffs have higher molecular weights
approaching those of direct dyestuffs and increased affinity which results in a
higher degree of fixation.
• For these dyes, the temperature as traditionally determined according to the
type of reactive group may not necessarily be the most suitable.
• higher dyeing temp is recommended for bifuntional reactive dyes

Liquor ratio and dyeing properties
• Reactive dyestuffs tend to have a low substantivity and are thus readily
influenced by the liquor ratio.
• rise in the liquor ratio, will lower the degree of exhaustion.
• Reactive dyestuffs have a lower substantivity and thus dyeing with these
dyes tends to be more readily influenced by the liquor ratio.
• In winch and jet dyeing, the liquor ratio must be decided in view of the fact
that any reductions in the liquor ratio increase the risk of uneven dyeing.
• Since cotton knits are held at conditions of low tension during the jet dyeing
process, the weight of dye solution held by the substrate is twice the weight
of the substrate itself.
• This means that dyeing cannot be conducted at liquor ratios lower than 1:2.
• A volume of dye solution equal to that necessary to fill the pipes, pumps and
heat exchanger is also necessary.
• Furthermore, in order to enhance the efficacy of the dye circulation to
achieve an evenly dyed surface, the volume of dye solution must be
sufficient for it to move freely over the surface of the fabric.

Liquor ratio and dyeing properties
• Some atmospheric low liquor ratio jet dyeing machines are marketed as
being capable of dyeing at liquor ratios of 1:4,
• but in reality dyeing at a liquor ratio of 1:4 is likely to lead to many problems
including that of uneven dyeing.
• Within the limits set by modern technology, a ratio of at least 1:6 is necessary
for achieving high-quality dyeing.
• It is important to note that problems can easily arise if pretreatment is
conducted in the same low liquor ratio dyeing equipment.
• When the substrate is wound in a compact form as in cheese dyeing, the
substrate holds less dye solution and efforts to reduce the internal volume of
the dyeing equipment can make dyeing at a liquor ratio of around 1:3
possible.
• In this case, lowering of the liquor ratio leads to an increased number of
circulations which allows level dyeing to be achieved

Reactive dye on silk
• Reactive dyes develop bright shades with good wash, light and
perspiration fastness properties due to its reaction with –NH2 group
of silk. Silk should be thoroughly degummed before dyeing to
remove sericin completely which may otherwise react with dye to
show poor wash fastness. Silk can be dyed in either of alkaline or
acid methods.
• In alkaline method, dye bath is made up with dissolved cold brand
dye and NaCl (10%). Dyeing is carried out at room temperature for
30 min after which temperature is slowly raised to 50°C and dyeing
is continued for further 30 min; fixation is done by adding Na2CO3
(6–8 g/l) at 50°C for 40 min. Shading, if required, can be affected by
addition of dye to the alkaline bath. A final wash-off with soap or a
synthetic detergent at 90– 95°C completes the process. Hot brand
dyes can also be applied in the same way as with cold brand dyes,
only difference is in temperature of fixation which is 70°C. Building-
up of shade is not satisfactory.

• Acid method is preferred when silk / cotton blend is dyed
to produce reserved shades in acidic pH when reactive dyes
do not react with cotton.
• Dye bath is prepared with dye and formic acid (0.5%),
temperature is raised to 40°C and silk is added to bath.
Dyebath is further slowly heated up to 85°C, when further
addition of formic acid (3.5%) is made. Dyeing is continued
for further 30 min followed by rinsing and finally washing-
off.
• All cold brand dyes can be used in this method though
alkali method is easier to control. Hot brand dyes are not
used in acid dyeing of silk due to poor build up of shade.

Reactive dye on wool
• Dye–fibre attachment resembles the same with that of acid dye on wool,
i.e. chlorine of dye forms ionic bond with amino group of fibre with
relatively higher bond energy. Reactive dyes produce very bright shades
on wool with light fastness 5–7 and wash fastness 4–5.
• Affinity of reactive dye for wool is very high posing problem on production
of level shades. Non-uniform initial absorption of dye on fibre must be
controlled with non ionic surfactants. Dye-fixing agents are only applied
during last wash to avoid staining and to improve wash fastness.
• Bath is prepared with dye, 30% CH3COOH (2–5%) and non ionic dispersing
agent (2%). CH3COOH may be replaced with CH3COONH4 or (NH4)2SO4
(1–3%) when producing light shades. Wool is entered in bath and
temperature is raised to 90–95°C over a period of 45 min. Dye fixing
agents may be applied in the last rinsing bath. The process gives excellent
result on woollen yarn in package form as well as knitwears.

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
70-100o 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 (NaOCl) 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 (Room Temp x 10min)
Bleaching powder ---------1g/L

Stages of Dyeing
First stage
(Dissolving of the dye):
In this first stage, the dye, in
solid form, is equilibrated
according to the dye
dissolved in molecular form
or in micellar form
(aggregates of many
molecules with good
solubility), or in form of
dispersed micropowder
(microcrystals of dye
molecules poorly soluble)

Second stage (Adsorption):
During this stage, by the effect of the dye-fibre
affinity, the dye is adsorbed at the surface of the
fibre, thus forming chemical bonds with it.
Affinity, temperature, (sometimes pH and/or
auxiliaries) affect the thermodynamic
interactions:
a) The balance of the reactions, thus determining
the exhaustion degree of the dyeing liquor.
b) The affinity between the dye and the fibre is
the ability of both dye and fibre to form a
permanent bond. The greater the affinity, the
stronger and higher are the fibre-dye bonds and
the smaller is the dye for the solvent (water).
Generally it is also directly proportional to the
molecular weight (molecular size) of the dye.
Affinity is therefore a condition strictly related to
the chemical composition of the dye and the
fibre.

A quick adsorption of the dye on the surface of the fabric
reduces the dye concentration near the fibre, thus reducing
the adsorption speed. A correct speed of the liquor change in
contact with the fibre allows the maximum concentration of
the dyeing solution near the fibre, and consequently the
correct speed.
At the same time, the liquor flow in contact with the material
is spread homogeneously and allows a good distribution of
the dye in all the areas of the textile surface; this enhances
the dye consistency with the same operating times.
The adsorption reaction is usually sufficiently quick not to
affect the dyeing speed, and often it must be slowed down or
adjusted (T°, pH, and auxiliaries) on optimum values to
avoid an irregular distribution of the dye.

Third stage (Diffusion):
During this stage the dye,
adsorbed in molecular form
by the surface, by breaking
and restoring the bonds
many times tends to
penetrate into the bulk of the
fibre through amorphous
areas, to spread
homogeneously and fix
steadily.
Figure: Dye concentration in the liquor near
the fibre depending on the hydrokinetic
condition

Fundamental factors are:
• Crystallinity of the fibre: the dyes penetrate the
fibers through amorphous areas and therefore the
higher the crystallinity, the lower the diffusion
speed.
• Molecular size of the dye: the bigger the dimensions
of the dye molecules, the more difficult the
diffusion through amorphous areas.
• Strength or dye-fibre bonds (affinity): the stronger
the bond, the more difficult the diffusion.
• Fibre makes the diffusion quicker but
simultaneously reduces the affinity and therefore
the exhaust. The presence of auxiliaries, facilitating
the fibre swelling or increasing the concentration of
dye near the fibre (swelling agents), tends to
increase the diffusion speed.
The operating time must be adequate to allow a
good penetration of the dyes, since this is a
prerequisite for developing the maximum fastness.

Reactive dye,23.03.2016

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Reactive dye,23.03.2016