09/05/2012
Dyeing with Vat Dyes
Dr. Tanveer Hussain
Dean Faculty of Engineering & Technology
National Textile University Faisalabad.
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Dr. Tanveer Hussain
The name vat dyes
The name vat dyes is given from the
recipient/vessel in which dye reduction and
dissolving is performed
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General properties of vat dyes
Insoluble in water
Are converted into water-soluble leucoderivatives by reduction in alkaline solutions
Leuco-derivatives possess substantivity for
cellulose on which the colour is developed by
subsequent oxidation
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Classification of vat dyes - I
Based on chemistry:
Indigo derivatives
Anthraquinone derivatives
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Indigo derivatives
Include natural and synthetic indigo
The colour of the soluble leucoderivative is usually different
from the insoluble dye form and tends to be pale yellow
Alkalinity required for dye reduction is less as compared to
anthraquinone dyes
Dyes are soluble in pyridine
Many dyes may sublime when heated
Leucoderivatives are easily oxidizable by the oxygen in air
Have more brilliant colours than anthraquinone
Poor light and washing fastness than anthraquinone
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Anthraquinone derivatives
May be derivatives of anthraquinone,
indanthrone or benzanthrone
More alkalinity required during reduction than
indigo
Colour of the reduced form different from
insoluble dye form and tends to be different
for each dyes
Oxidize back by oxygen in air or oxidizing
agents
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Anthraquinone derivatives
Washing fastness generally better in dark shades
than light shades
Washing fastness better in case of flock dyeing
before spinning than fabric dyeing
Light fastness varies from dye to dye; from
excellent to poor
Light fastness of some dyes improves in
combination with other dyes
Some dyes may cause loss in resistance of
cellulose to light
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Classification of vat dyes - II
Based on application conditions
IN* or CI** dyes (Hot)
IW or CII dyes (Warm)
IK or CIII dyes (Cold)
IN special dyes
* Bayer classification
**Ciba classification
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Comparison of different vat dyes
IN dyes
IW dyes
IK dyes
IN special dyes
-Require high
temperature
-Temperature of
max. affinity 60C
-High amount of
sodium hydroxide
-Require medium
temperature
-Temperature of
max. affinity 45C
-Medium amount
of sodium
hydroxide
-Medium amount
of salt
-Medium levelling
-Require some
levelling agent
-Require low
temperature
-Temperature of
max. affinity 25C
-Low amount of
sodium hydroxide
-Require higher
temperature and
caustic soda than IN
dyes
-No salt
-Poor levelling
-Require more
levelling agent
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-High amount of
salt
-Good levelling
-Require no
levelling agent
Dr. Tanveer Hussain
Common phases in vat dyeing by exhaust method
Reduction
Conversion of insoluble vat pigment into soluble sodium leucovat anions
Diffusion
of sodium leuco-vat anions into cellulosic fibres
Rinsing
Removal of excess alkali and reducing agents by washing off
Oxidation
of the soluble dye into insoluble pigmentary form within the
cellulosic fibres, and
Soaping,
during which the isolated molecules of vat pigments are reorientated and associate into a different, more crystalline form
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Exhaust dyeing phase - 1
Reducing agent requirements
A level of reducing power sufficient to reduce
all commercial vat dyes to their water soluble
form, quickly and economically
Conversion of the vat dyes into products from
which the original pigment can be restored
(no over-reduction)
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Exhaust dyeing phase - 1
Commonly used reducing agent
Sodium hydrosulphite, commonly known as
hydros but more correctly known as sodium
dithionite, which has the chemical formula
Na2S2O4.
Although a part of the hydros is used up in the
reduction of vat dyes, a large part of it may be
destroyed by its reaction with oxygen in the
air (oxidation), particularly at higher
temperatures.
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Exhaust dyeing phase - 1
Factors of rate of dye reduction
Dye-related:
Amount of dye
Particle size of the dye,
Auxiliary-related
Concentration of the reducing agent
Concentration of alkali/pH during reduction
Process related
Temperature
Time
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Exhaust dyeing phase - 1
Stability of alkaline solutions of reducing agent
May decrease with:
increased temperature,
greater exposure to air,
greater agitation and
lower concentration of the reducing agent
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Exhaust dyeing phase - 1
Over-reduction
Vat dyes of the Indanthrene type may produce
duller or greener shades at dyeing
temperatures higher than 60 C due to overreduction.
Over-reduction can be prevented by the use
of sodium nitrite if the reducing agent is
hydrosulphite
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Exhaust dyeing phase - 2
Factors affecting rate of dyeing
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Type of the substrate,
Temperature,
Liquor ratio,
Concentration of dye
Concentration of electrolyte
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Exhaust dyeing phase - 2
Effect of substrate
Mercerized cotton gives a higher rate of
dyeing as compared to un-mercerized
bleached cotton which in turn gives higher
rate than the grey material.
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Exhaust dyeing phase - 2
Effect of temperature
At low temperature, the rate of exhaustion is low
which might promote levelness but the rate of
diffusion is also low.
At high temperature, the rate of exhaustion is high
which might decrease levelness but the rate of
diffusion is high.
Maximum exhaustion, penetration and levelness can
be obtained by starting the dyeing at low temperatures
in the leuco stage and slowly raising the temperature.
Some dyes may not be stable to very high
temperatures, so the stability of dyes to temperature
must be taken into account.
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Exhaust dyeing phase - 2
Effect of liquor ratio
The higher the liquor ratio, the slower is the
rate of dyeing.
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Exhaust dyeing phase - 2
Effect of dye concentration
Most of the dyes exhaust more rapidly at low
concentrations, increasing the risk of unlevel
dyeing in light shades.
Some have the same rate of dyeing
irrespective of the concentration.
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Exhaust dyeing phase - 2
Effect of electrolyte
The higher the concentration of electrolyte,
the higher is the rate of dyeing.
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Exhaust dyeing phase - 3
Rinsing before oxidation
Purpose
To remove any loose dye, excess of reducing agent
and alkali to lower the pH and establish conditions
favourable for oxidation.
Parameters
The higher the temperature and/or pH of the rinsing
bath, the lower is the colour strength.
Very high pH and temperature during rinsing may also
result in the dulling of the shade.
The ideal is to do rinsing thoroughly at low
temperature at a rinsing bath pH value of 7.
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Exhaust dyeing phase - 4
Oxidation
Purpose
To convert the water-soluble leuco from of the vat
dye, back into the insoluble pigment form
Parameters
The type and concentration of oxidising agent,
The type of pH regulator and pH during oxidation,
Temperature during oxidation
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Exhaust dyeing phase - 4
Oxidation
Oxidizing agent
The oxidizing agent must provide a level of
oxidation potential sufficient to oxidize the
reduced vat dye into insoluble pigment, with no
over-oxidation i.e., beyond the oxidation state of
the original pigmentary form of the dye.
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Exhaust dyeing phase - 4
Oxidation
pH
Poor control of pH during oxidation may result in
uneven oxidation.
A pH below 7.5 should be avoided to prevent the
possible formation of acid leuco form of vat dyes.
The optimum pH for oxidation is 7.5-8.5.
The acid leuco form of vat dye is difficult to
oxidize, has little affinity for fibre and is easily
rinsed out.
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Exhaust dyeing phase - 4
Oxidation
Temperature
A lower temperature may result in slower
oxidation
The higher the temperature, the faster is the
oxidation, the optimum temperature being 5060C
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Exhaust dyeing phase - 5
Soaping
Purpose
To remove any dye that is not diffused into the fibre
To stabilize the final shade.
This results in improved fastness properties and resistance
to any shade change to a resin or other finish or to
consumer use.
Parameters
Time,
Temperature,
Type and concentration of soaping auxiliaries
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Vatting process
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Example: NOVASOL RED 6B
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Vat dye reducing agents
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Machine factors for consumption of
reducing agent
Factors
the sealing of the machine
the air volume in the machine
the liquor turbulence
When calculating the recipes, it is necessary to
incorporate:
2.0 liters caustic soda 36B (66Tw) and
1.7 kg hydrosulfite conc.
to eliminate the oxygen present in each cubic meter of air.
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Preparing dye dispersions
Strew dye into 5-8 times its weight of soft water
at 20-25C, while stirring at a high speed.
Alternatively, mix dye to a smooth paste with the
gradual addition of about 5 times its weight of
soft water at 20-25C and then dilute as required.
Dispersions should not be made up with hot
water or boiled up with live steam as this can
cause dye agglomeration.
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Novasol Exhaust Vat dyeing methods
1.
2.
3.
4.
5.
6.
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Semi-pigmentation method
Hot pigmentation method
HT method
Leuco method
Pigment dispensing
Special dyeing process for black
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Semi-pigmentation dyeing method yarn package dyeing
A -Dye
-Anionic wetting
agent
-Anionic
Sequesterant
-Anionic dispersing
agent
B -Caustic Soda 36Be
-Sodium dithionite
-Non-ionic Leveller
C Salt for CII dyes
only
D Re-sharpen with
Hydros if necessary
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Semi-pigmentation dyeing method piece jet dyeing
A -Dye
-Anionic wetting
agent
-Anionic
Sequesterant
-Anionic dispersing
agent
B - 2/3Caustic Soda
36Be
C - 1/3Caustic Soda
36Be
-Sodium dithionite
-Non-ionic Leveller
D Salt in case of CII
E Re-sharpen with
Hydros if necessary
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Semi-pigmentation dyeing method yarn package dyeing
Merits
With most dyes, slow vatting
means leveling through slower
uptake of the sodium leuco
compound
.All-in method is readily
integrated into automation
concepts
Levelness is further improved
by raising the dyeing
temperature above 60C
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Demerits
Low package permeability
in the initial phase of dyeing
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Hot pigmentation method yarn package dyeing
A -Dye
-Anionic wetting
agent
-Anionic
Sequesterant
-Anionic dispersing
agent
B -Caustic Soda 36Be
-Sodium dithionite
-Non-ionic Leveller
C Salt for CII dyes
only
D Re-sharpen with
Hydros if necessary
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Hot pigmentation method piece jet dyeing
A -Dye
-Anionic wetting
agent
-Anionic
Sequesterant
-Anionic dispersing
agent
B - 2/3Caustic Soda
36Be
C - 1/3Caustic Soda
36Be
-Sodium dithionite
-Non-ionic Leveller
D Salt in case of CII
E Re-sharpen with
Hydros if necessary
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Hot pigmentation method yarn package dyeing
Merits
High package permeability
at high initial temperature
in neutral bath
Suitable for all shade depths
(except black)
Problems solved in critical
cases
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Demerits
High energy cost
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HT dyeing method (90105C) yarn package dyeing
A -Dye
-Anionic wetting
agent
-Anionic
Sequesterant
-Anionic dispersing
agent
-Caustic Soda 36Be
-Rongal HT (BASF)
-Non-ionic Leveller
B Re-sharpen with
Hydros if necessary
C Salt for CII dyes
only
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HT dyeing method (90105C) yarn package dyeing
A -Dye
-Anionic wetting
agent
-Anionic
Sequesterant
-Anionic dispersing
agent
-Caustic Soda 36Be
-Rongal HT (BASF)
-Non-ionic Leveller
B Re-sharpen with
Hydros if necessary
C Salt for CII dyes
only
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HT dyeing method (90105C) yarn package dyeing
Merits
Perfect levelness
Almost an all-in method
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Demerits
Slightly limited dye
selection
Less suitable for deep
shades (rubbing fastness)
Special reducing agent
necessary (Rongal HT, BASF)
DP decrease possible
Time-consuming
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Leuco dyeing method
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-Anionic wetting agent
-Anionic Sequesterant
-Anionic dispersing agent
-Caustic Soda 36Be
-Rongal HT (BASF)
-Non-ionic Leveller
Stock Vat
Salt for CII dyes only
Resharpen Hydro if needed
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Leuco dyeing method
Merits
Preferred method for dark
shades, including black
Better leveling with
graduated temperature
method (for example
2080C)
Suitable for all vat dyes
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Demerits
Pre-vatting is laborious
Success depends on the
leveling power of the
individual dyes
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Pigment dispensing method
A -Anionic wetting agent
-Anionic Sequesterant
-Anionic dispersing
agent
-Caustic Soda 36Be
-Rongal HT (BASF)
-Non-ionic Leveller
B
Dye
Resharpen Hydro if
needed
D Salt for CII dyes only
Merits
.. Novel, reliable way to solve levelling problems with critical shades
.. Suitable even for incompatible combinations
Demerits
.. Dispensing equipment necessary
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Special method for blacks
A -Anionic wetting agent
-Anionic Sequesterant
-Anionic dispersing
agent
-Caustic Soda 36Be
-Rongal HT (BASF)
-Non-ionic Leveller
Dye
B
Resharpen Hydro if
needed
NOVASOL black dyes are vatted only in long liquors. The dye is dispensed, then added to a bath
which already contains the full amount of caustic soda 36B (66Tw) and sodium hydrosulfite.
Vatting time is about 15 min at 60C.
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Example amounts of Aux. for CI dyes
Wetting agent 0.5-1 g/l
Dispersing agent 1-3 g/l
Sequestering agent 0.5-2 g/l
Leveling agent 1-2 g/l
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Oxidation of Novasol vat dyes
Anionic dispersant
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Oxidation of Novasol vat dyes
Anionic dispersant
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Soaping off Novasol vat dyes
Non-ionic Detergent
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Soaping off Novasol vat dyes
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Jigger dyeing
Pre-pigmentation
Dispersing agent 2 g/l
Wetting agent 0.5-1 g/l
Sequestering agent 1 g/l
Dye 1/2 , 1 end; 1 end
Development
Caustic soda y g/l
Sodium dithionite z g/l
Leveling agent 2 g/l
Dyeing
amount 2 ends, amount 2 ends
Temp. depends on type of dyes
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Rinsing
Oxidation
Soaping
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Pad dyeing vat dyes
Pad-dry-pad steam
Pad-jig
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Pad-dry-pad-steam
Dye padder
Sequestering agent 1-2 g/l
Wetting agent 1-2 g/l
Dispersing agent 1-2 g/l
Anti-migrant 5-10 g/l
Dye x g/l
Padding temp. 20-30C
Pick-up 60-80%
Drying
120C
Cooling
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Pad-dry-pad-steam
Chemical padder
Sequestering agent 1-2 g/l
Wetting agent 1-2 g/l
Dispersing agent 1-2 g/l
Caustic soda y ml/l
Hydros z g/l
Glucose 2 g/l (for dyes prone to over-reduction)
Dye padding liquor 20-40 ml/l
Temp. 20-30C
Pick-up 80-100%
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Pad-dry-pad-steam
Steaming
40-60 seconds
102C saturated steam
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Pad-dry-pad-steam
Rinsing
Oxidation
Soaping
Rinsing
Drying
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Pad-jig method
Dye padder
Sequestering agent 1-2 g/l
Wetting agent 1-2 g/l
Dispersing agent 1-2 g/l
Anti-migrant 5-10 g/l (only for intermed. Drying)
Dye x g/l
Padding temp. 20-30C
Pick-up 60-80%
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Pad-jig method
Jigger development
Sequestering agent 1-2 g/l
Wetting agent 1-2 g/l
Dispersing agent 1-2 g/l
Caustic soda y ml/l
Hydros z g/l
Leveling agent
Dye padding liquor 20-40 ml/l
Salt for low affinity dyes
Dyeing
50-70C
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Pad-jig method
Rinsing
Light to medium shades
rinse with cold, soft water with overflow during one end
Dark shades
rinse with a cold, reductive rinsing bath with 2 ml/l caustic
soda solution 32.5 % (38 B), 2 g/l sodium hydrosulphite
and 1 g/l dispersing agent
Oxidation
5 g/l oxidising agent, 60C, 2 ends
Soaping
2 g/l detergent, boiling, 2 ends
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