Pad Batch Dyeing Pad Batch Dyeing is one of the widely used technique for semi-continuous dyeing process. It is mainly used in the dyeing of cellulosic fibre like cotton or viscose (knit and woven fabric) with reactive dyes. Pad batch dyeing is a textile dyeing process that offers some unique advantages in the form of versatility, simplicity, and flexibility and a substantial reduction in capital investment for equipment. It is primarily a cold method that is the reason why it is sometimes referred to as the cold pad batch dyeing. Working of a Cold Pad Dyeing Process The technique or process used in pad-batch dyeing starts with saturating first the prepared fabric with pre-mixed dye liquor. Then it is passed through rollers. The rollers, or padders, effectively forces the dyestuff into the fabric. In the process, excess dye solution is also removed. After removal of excess dye stuff the fabric is subsequently "batched". This batching is done by either storing it in rolls or in boxes. It takes a minimum of 4-12 hours. The batches are generally enclosed by plastic films. This prevents absorption of carbon dioxide and water evaporation. Finally as the reaction is complete the fabrics are washed. This is done by becks, beams, or any other washing devices. Special Features of Pad Batch Dyeing Process Significant cost and waste reduction as compared to other conventional dyeing processes. Total elimination of the need for salt and other specialty chemicals. For example there is no need for anti-migrants, leveling agents and fixatives that are necessary in conventional dyebaths. Optimum utilisation of dyes that eliminates specialty chemicals, cuts down chemical costs and waste loads in the effluent. All this results in a formidable reduction in wastewater treatment costs. Excellent wet fastness properties. Pad batch dyeing cuts energy and water consumption owing to low bath ratio (dye:water) required for the process. This is because unlike other dyeing processes it does not function at high temperatures. A uniform dye quality is achieved with even color absorbency and colour fastness. As compared to rope dyeing, Pad batch dyeing produces much lower defect levels. In pad batch dyeing, qualities like high shade reliability and repeatability are common. This is because of high reactivity dyes with rapid fixation rate and stability.
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
Transcript
Pad Batch Dyeing
Pad Batch Dyeing is one of the widely used technique for semi-continuous
dyeing process. It is mainly used in the dyeing of cellulosic fibre like cotton
or viscose (knit and woven fabric) with reactive dyes. Pad batch dyeing is
a textile dyeing process that offers some unique advantages in the form of
versatility, simplicity, and flexibility and a substantial reduction in capital
investment for equipment. It is primarily a cold method that is the reason
why it is sometimes referred to as the cold pad batch dyeing.
Working of a Cold Pad Dyeing Process
The technique or process used in pad-batch dyeing starts with saturating first the prepared fabric with pre-mixed
dye liquor. Then it is passed through rollers. The rollers, or padders, effectively forces the dyestuff into the fabric.
In the process, excess dye solution is also removed. After removal of excess dye stuff the fabric is subsequently
"batched". This batching is done by either storing it in rolls or in boxes. It takes a minimum of 4-12 hours. The
batches are generally enclosed by plastic films. This prevents absorption of carbon dioxide and water
evaporation. Finally as the reaction is complete the fabrics are washed. This is done by becks, beams, or any
other washing devices.
Special Features of Pad Batch Dyeing Process
Significant cost and waste reduction as compared to other conventional dyeing processes.
Total elimination of the need for salt and other specialty chemicals. For example there is no need for
anti-migrants, leveling agents and fixatives that are necessary in conventional dyebaths.
Optimum utilisation of dyes that eliminates specialty chemicals, cuts down chemical costs and waste
loads in the effluent. All this results in a formidable reduction in wastewater treatment costs.
Excellent wet fastness properties.
Pad batch dyeing cuts energy and water consumption owing to low bath ratio (dye:water) required for
the process. This is because unlike other dyeing processes it does not function at high temperatures.
A uniform dye quality is achieved with even color absorbency and colour fastness.
As compared to rope dyeing, Pad batch dyeing produces much lower defect levels.
In pad batch dyeing, qualities like high shade reliability and repeatability are common. This is because
of high reactivity dyes with rapid fixation rate and stability.
Lastly Pad batch dyeing can also improve product quality. The fabric undergoing the cold pad batch
dyeing process is able to retain an uniformly coloured appearance. It shows added luster and gives a
gentle feel. The fabric gives a brighter look in shades.
REACTIVE DYEING – Cold Pad Batch
Factors influencing Cold Pad Batch Dyeing
Substrate preparation and pH
Making up of Colour and Chemicals
Alkalie Proportionator
Fabric guiding system
The Pad box Dwell time in the pad box
Facilities to assist better application and pick of the colour.
Speed / Feeding rate and concentrationPad box pick up
Loading of the padding mangles
Selvedge thickness
Laboratory to Bulk reproducibility
Batch Rotation / reaction time
Washing off and Soaping.
Machine Cleaning
In Process Quality Control Check List
Reacatives -Cold Silicate Pad Batch
Exhaust method of dyeing and the related issues were discussed in the previous two articles
and the other important methods of dyeing of fabrics in open width with Reactives are:
Cold Pad batch
Pad – dry - Alkalie pad batch
Pad – dry or wet on wet Alkalie pad steam
Pad – dry - bake
Cold pad batch system offers the most economical and most convenient method of dyeing
Reactives. The energy and water consumptions are the lowest and salt addition is totally
made redundant thus rendering it more eco friendly. Dyestuffs with relatively lower affinity
and high reactivity make them most suited for cold pad batch techniques. Because of the high
reactivity, fixation of the component colours is fully ensured and consequently
reproducibility of shades is more assured. Primarily it was being applied to woven fabrics and
with specialized features in the pad box and the guiding systems this is extended to knit wares
also. With the least inputs in terms of capital outlay energy, water, manpower and Right First
Time (RFT) capabilities, this method is the most cost effective option for dyeing substrates
that are amenable to padding operation.
Cold Silicate Pad Batch:
This is the most commonly adopted method by most of the Process houses. Since the
Reactive colours are sensitive to alkalie to a lesser or greater extent and the general approach
is to apply the colour at neutral pH.and after the distrubution of the colour on the substrate,
the pH is raised for fixation. In an exhaust-dyeing situation we are able to provide the
required time dimension and Salt to facilitate exhaustion. In a pad situation, the exhaustion
phenomenon is replaced by the positive add on of the colour on the substrate and the
elaborate conditions of exhaustion and fixation are eliminated by padding the alkalie along
with the colour. The colour with the alkalie is mechanically squeezed between the mangles of
a padder and applied evenly on the substrate. Since the alkalie is added with the colour the
primary precaution is to ensure that the reactive dyestuff does not hydrolyze. High (with
respect to speed) reactive colours generally are not stable and would loose the colour yield
due to hydrolysis with water. Even more stable reactive colours have their limitations in that
the different clolours in the recipe will have relatively different rates of hydrolysis and thus
not likely to give reproductive results. This problem has been overcome by the development
of ingenious alkalie proportionator units that help in mixing the alkalie and the dye in the
required proportions just before feeding into the pad bath, thus avoiding the propensity to
hydrolyze. Due to the high degree of reactivity, the dye fixes on the substrate during the
reaction dwell time that is dictated by the alkalie (pH.) and the time taken (rate of reaction)
by the slowest of the component dyes to fully react with the substrate at that pH. Vinyl
Sulphones that are relatively slow reactive class of Colours are generally preferred for this
method of dyeing
The alkalie proportionator is an equipment that facilitates mixing of the colour and the
alkalie in the required quantities and proportions just before the liquor is delivered to the pad
box for application; otherwise they are stored as two different stock solutions. The dye and
alkalie are mixed at a convenient volume ratio (generally 4:1) and correspondingly they are
made up to that extent stronger so that in the final liquor, the dyes and chemicals are present
in the required recipe concentrations The principle is very convincing but the problems arise
in the execution.
Factors Influencing Cold Pad Barch Dyeing
Substrate preparation and pH
Substrate preparation has to be thorough as there is no scope for diffusion and migration
aspects of Exhaust dyeing and whatever applied through padding mangle should penetrate
uniformly across and along the fabric as the fabric speeds through the mangle at speed, where
the contact time could be just a few seconds. The preparation process is generally alkaline
and particularly after mercerization the fabric is rendered alkaline requiring effective
neutralization. This is best achieved with mineral acids preferably HCl; however the residual
mineral acid that would damage the cellulose needs to be neutralized again with Soda Ash or
Bicarbonate rendering the pH invariably 8.5 or more and therefore again neutralized with
Acetic Acid.
Secondly the neutralization is carried out in the wash compartments of the Mercerizer
washing range and invariably the caustic carry over from the recuperator which is a wet on
wet leaching process shall not be efficient compounded by uneven expression at the mangle.
Unlike add-on mangles like dye pad the washing unit mangles invariably are less maintained
resulting in variation in alkalie across the width and length of the fabric causing different
levels of left over caustic reaching the neutralization zone. With limited contact time in a
speeding fabric movement in the washing range the neutralization invariably remains
incomplete in core areas and even in the surface areas where the caustic carried over is not
uniform as pointed out earlier.
Such a fabric .shall measure different pH levels across the width and length of the fabric
causing hydrolysis/spontaneous fixation however small selectively on the surface of the
fabric resulting in delta E variations exceeding 0.5 in the pad batch/continuous dyeing
systems causing selvedge centre variations, shade variation across the width and length
apparently insignificant but when cut and stitched juxtaposition would show marked
deviation in shade. This problem is best addressed by the following procedure. It is
imperative that the caustic leached/extracted out at the recuperator of the Mercerizer should
be most efficient and the residual caustic carried over is minimal. A mineral acid passage
followed by neutralization of the mineral acid with Soda Ash and again a passage through
Acetic Acid to remove residual Soda Ash alkalinity does not guarantee uniform and or core
neutralization.
Doing away with Mineral acid and employment of specialty chemicals -Organic acids based
Citrates) and Sugar-Acrylic acid copolymers are in practice. The hydroxy Carboxylic Acids
and Sugar-Acrylic acid copolymers have the additional advantage of ready biodegradability.
Such Specialty chemicals like Invatex AC of Ciba or Sirrix 2UDI of Clariant or Neutracid
Organic (organic +Inorganic buffers) of CHT, - through a dozing system governed by a pH
control counter current closed circuit neutralization zone that give a steady uniform pH that is
faintly acidic (pH 6 to6.5) at the end of the range. With a higher dissociation constant (Ka)
than that of Acetic Acid the organic protonizer ionizes more and faster and therefore
neutralizes more efficiently and faster; at the same time it does not tender or damage
cellulose Secondly the residual organic acid/ salt forms a buffer in the subsequent dyeing
operations giving a stable and uniform pH to start with. Fabric can be controlled to a uniform
pH of 6 to 6.5 (by extraction method) confirming that the core portion is also neutralized.
Such processed fabric when dyed by Pad- Batch technique invariably gives delta E variations
less than 0.5 when checked any where on the fabric. The neutralization system can be
retrofitted in a Mercierizer washing range with very minimal modifications to include
dozing /control.
Residual peroxide after peroxide bleach needs to be removed by peroxide ‘killers’ preferably
by specialty chemicals of enzymatic sources
Making up of Colour and Chemicals
The colours have to be dissolved and made up with cold water (in tropical countries with ice
cooled water) . as per the directions of the manufacturer to obtain a temperature of the liquor
below 20 deg C. The required amount of Urea is to be added for breaking of the hydrogen
bonding and disaggregation of the colour for easy dissolution. Filtering and cooling with ice
are recommended while making up to volume. The made up colour should be tested for
complete dissolution by drop test on the filter paper. The Silicate should be from reliable
sources without contamination of heavy metals and Na2O : SiO2 ratio of 1 : 2 .1.The
chemical should be suitably diluted to obtain a 40˚ Tw solution and the recommended
quantity of Caustic Soda for the different depths of shades should be added and filtered
through a strainer. This would form the stock solution.
Alkalie Proportionator
The proportionator’s capability to pump the required quantity linearly and not at intervals has
a direct influence on the uniformity of the shade. Some of the simpler designs operate on the
level control principle and therefore the liquor flow would be as per the signals from the level
controller. The limits for this operation cannot be set very precisely such that there is no
perceptible delay in the intermittent flow of the liquor. Therefore there is bound to be certain
disruption in the continuity of the liquor flow. Where the substantiality of the dyestuff is
higher, there will be preferential absorption of the relevant colour from the bath resulting in
that much depletion in concentration of that colour. In ternary matchings where contribution
of all the colors is important for the shade like in a Grey or Khaki, variation in shade cannot
be ruled out.
Where the pad box volume is high this problem would be more pronounced. Colours with
moderate substantivity and high reactivity in terms of fixation and higher stability to
hydrolysis (in time dimension) would be more ideal.for cold silicate pad batch method of
dyeing. In the context of exact colour matching, the reproducibility is difficult unless care is
taken to eliminate the variables. Modern Pad batch systems provide the answers to the
problems with Proportioning units that synchronize with the speed of the machine and the
liquor off take - to continuously mix dye and alkalie in the required proportion just before
feeding the pad box in a linear fashion.
Fabric guiding System
The fabric guiding system should be able to feed the fabric in a fully open form without
selvedge curls, creases or distortion. In the case of warp knit ware the guiders should have the
capability to uncurl the selvedges. At the batching stage there should be expander system to
ensure the batch is wound without creases.
Pad box
The pad box volume needs to be as low as possible. A narrow ‘U’ tube like pad box with a
dummy / spacer in the center would help achieve sufficient long dwell time and also reduce
the pad box volume. While this arrangement helps immediate application (consumption) of
the alkalie mixed colour on the substrate, it may not as much help in the penetration of the
colour in to fabric substrate particularly heavy fabrics. Therefore it would be helpful to have
built in lay on rollers in the pad box to facilitate penetration by additional squeeze passages.
Where sensitive /thin/delicate fabrics are involved the lay on rollers may be by-passed or can
be replaced by dummies to reduce the liquor volume. .
The dwell time once established for a given pad box features, the same speed should be
maintained for the relevant fabric sort. Therefore it would be prudent to establish two or three
speed categories for different sort groups and maintain these conditions every time. As the
speed also would influence the liquor pick up and the preferential pick up of the dye, it plays
an important part in the shade reproduction. This aspect also emphasizes the importance of
the fabric preparation that needs to be absolutely perfect with respect to absorbency, evenness
of the whiteness and dryness as discussed under fabric preparation.
It is also necessary to maintain the padding liquor tmperature at below 20 deg. C for which
jacketed pad box with cold (ice cold water in the tropical countries where the temperatues
can go vdry high) to avoid destabilisation of the bath by hydrolyzation of component colours
to different degrees depending on their Reactivity and consequent tailing with variation in
saturation and hue, particularly for colours with lower stability at higher temperatures..
Loading of the Mangles
The success of a pad batch system is in the capability of the pad box to uniformly apply the
colour on the fabric substrate. Different loading systems with ingenious designs to avoid
deflection of the padding mangles under load have been discussed at length in earlier issue
under padding. The choice of such systems will depend on fabric types in terms of their
construction, weight and width. Where there are frequent changes in the fabric
characteristics, care need to be exercised to provide for suitable facilities to accommodate
changes. Continuous running of narrow width fabric followed by a run on wide width fabric
could cause problem of center selvedge variation even with the modern mangles. Alternate
running of both wide and narrow width fabrics in frequent intervals would reduce this
problem. Also periodical buffing of the bowls would be advantageous.
Thick Selvedge
Fabrics with selvedges thicker than the body have always posed problems during winding
into a big batch after padding. The batch tends to develop a ridge at the selvedges as it builds
up and beyond certain size it becomes unmanageable where the fabric starts rolling over at
the selvedge giving crimps or short creases oblique to the selvedge. In a stenter batching
operation this problem is over come by selvedge shifting device and this may not be
successful in a pad operation. When such observations are made the best solution would be to
limit the batch to that size where the problem is not there. Some of the process houses resort
to insertion of flat paper at intervals at both the selvedges as the batch builds up and even out
the ridge. This works to a point. The best and permanent solution to this problem is to ensure
that the selvedge construction is taken care of at the weaving stage
Laboratory - Bulk Reproducibility
The padding mangle expression plays an important role as the colour picked up is directly
related to the expression. The absorbency of the substrate besides the additional features like
lay on rollers would also influence the pick up as already discussed. Therefore,
standardization of these parameters to meet certain norms should be established, monitored
and controlled every time.
While matching the shade in Laboratory, the bulk application parameters should be borne in
mind and the parameters for the lab Pad should be modified suitably such that the shade
produced in the lab pad is reproducible in the bulk. As the dyestuff is already mixed with
alkalie, the mobility of the dyestuff to migrate would be limited and the fixation phenomenon
would restrict such mobility, unlike in an exhaust-dyeing situation.
The pressure applied on the padding mangle, the dwell time (function of the speed of the
machine and the length of fabric immersed in the pad liquor - i.e. starting from entry in to the
liquor level in the pad box to the nip) should be manipulated and established to obtain the
shade that would reproduce in bulk with the same recipe. This can be established carrying out
a few trials. This exercise would be easier where the configuration of Bulk and Laboratory
pad boxes are similar.
Where the lab pad box does not provide the features available in the bulk, say as in the case
of an ordinary pad box without lay on rollers and the bulk padder having advanced features,
even under identical expressions, the laboratory matching would tend to give higher colour
yield than the bulk for the same recipe. The bulk would require increase in recipe
concentrations, particularly in heavier fabrics. In other words the estimates of cost based on
Lab recipe would be adverse.
In this instance, the explanation for this phenomenon is that the colour picked up does not
penetrate in to the fabric substrate as much as it does in the bulk model due to better facilities
and hence in the lab match ring dyeing type of application results giving an apparent colour
yield on the surface. In bulk, relatively more inner substrate cross section also gets dyed and
therefore requires that much extra colour for obtaining the same shade. If the laboratory could
simulate the same level of ‘efficiency’ of the dye penetration as in bulk the laboratory recipe
would be reproducible every time.
Process houses tend to believe that when expressions of the bulk and laboratory pad boxes
are made identical there is nothing further that can be done. They accept this as unavoidable
and provide a factor for conversion to bulk, which does not work every time as different
dyestuffs would penetrate to different extents and a single conversion factor would not be
valid. It would therefore be necessary to match any shade in Laboratory to the same
efficiency of penetration/diffusion across the cross section of the substrate in order to get
reproducible results as for as the padding mangle operation is concerned. In a real situation
quite a number of trials had to be taken to simulate bulk-dyeing results at the stage of
laboratory matching. Once such conditions and parameters are set, the Laboratory pad would
behave in the same fashion as the bulk and therefore each of the dyestuffs would tend to
behave similarly at the laboratory and bulk padding stages.Establishing laboratory padding
conditions and parameters that would correspond to bulk would solve most of the problems
related to Laboratory to bulk reproducibility.
The padded fabric may be checked for shade by drawing a sample and exposing the same
over a water bath in a micro oven that facilitates an accelerated fixation.
Batch Rotation / Reaction time
The dwell time for reaction (fixation) to complete would vary with the alkalie concentration
and the class of reactive colours used. There are shock develop (short time of 4 to 6 Hours)
and the long cycle times of 12 to 18hours systems -whichever the process, the dwell time
period should not be compromised. Care should be taken to ensure the batch is protected
from water drops or acid fumes during the period of fixation. Polythene covers that tightly
enclose the batch would serve the purpose. It is also necessary to rotate the batch during the
period of fixation, lest the alkaline liquor should collect at the lower portions of the batch due
to gravity that could result in intermittent variation in shade along the length of the fabric
Though these are elementary precautions, the operators tend to ignore, particularly when
there is a breakdown of the rotating motor. It would be prudent to have alternate banks for
rotating the batches to take care of such breakdowns.
Washing off and Soaping
Soaping is an important operation where the washing and soaping sequence has to be
followed meticulously to ensure complete removal of the silicate and the hydrolyzed colour.
Silicate on fabric needs to be washed off in the first two compartments with warm over
flowing water and then followed by soaping at near boil (need to establish temperature charts
for different dyestuff combinations) with good anionic soaping agents with small additions of
polyphosphate in the wash baths.
While soaping, where Vinyl Sulphone based dyestuffs are involved, it is necessary to have
luke warm / preferably cold water over flow in the initial soaping baths to remove unfixed
colour followed by acidification to bring the pH to 5- 6 or neutral before raising the
temperature to boil to avoid the possible dye fibre bond cleavage, whereas this precaution is
not required in the case of Chloro Triazine based dyes.
Once the silicate is eased out soaping operation is rendered more efficient. Compartment nos.
3, 4 and 5 are with soaping chemicals. Compartment 6- washing off; 7 and 8 wash/rinse at
lower temperatures Nos.3 to 6.can be counter current An eight-compartment soaper with an
average of 20 meters capacity in each compartment should serve the purpose for a good
soaping for fabric weights up to 200 grms of plain weave at a speed of 50 meters/min. The
soaping compartments need to be totally enclosed to maintain temperature parameters. The
guide rollers should be absolutely true and smooth on their ball bearings with tension
adjustments to ensure crease free passage of the fabric through the soaping range.
As the fabric weight increases either the speed need to be reduced or number of
compartments should increase. Wash boxes with advanced designs provide accessories to
facilitate good agitation and therefore are efficdient over a range of speeds. Also certain
levels of over flow in the soaping zone (compartments Nos. 3 to 5) may be necessary
particularly for heavy shades. The last two compartments need to be lukewarm or cold.
For heavier weight fabrics neutralization of core alkalie to satisfy the extraction method,
addition of specialty chemicals like Invatex AC by itself or mixed with Acetic Acid (to save
costs) may be helpful instead of only Acetic Acid in the seventh compartment with a dozing
system like what is mentioned under neutralization in the Mercerizer. Compartments 7 and 8
can be counter current.
It is pertinent to caution that Bi-carbonate hardness in water is generally neglected. The water
may apparently show neutral pH in cold but the fabric rinsed with this water in the last
compartment will show alkalinity after drying. Where the water is bought from different
sources and if this aspect is not taken into consideration one can get different results despite
other stringent controls. Check for Bicarbonate hardness and include removal of bicarbonate
hardness sequence in the process water treatment. Otherwise provision to neutralize
bicarbonate if any is to be built in the dozing and control system in the neutralization
compartment
Cleaning of Pad Box /Soaper
Cleaning of the Pad box, feed lines stock tanks and the pump is an important function after
every shade change. Likewise the cloth guiders and both the fixed and rotating tension bars
are to be meticulously cleaned. It is a general principle to plan discretely.the sequence of
change of shades from light to dark or from dark to light depending on the day’s programme
for dyeing Sensitive shades like yellows and blues and pastel shades that show up
contamination of colour glaringly need to be handled separately after thorough
cleaning.Similarly at the washing off and soaping stages also the precautions need to be taken
to empty and clean the different compartment bath after shade runs that could cause
contamination in the next shade.Washing of the Padding mangle and unloading and lifting
/separating the bowls after the run before stopping for the day is mandatory as the silicate is
difficult to wash off once dried up and the Mangle will be rendered unfit for carrying out
padding of colour.
Cold Silicate Pad Batch Quality Control Check List
Fabric ready for Dyeing
Parameters Method of
Checking
Frequency Norms/Limits Action
White/Yellownes
Index
Whiteness Yellowness
AATCC Test
Method110 of
1979
Every lot
Every lot
Left Cen Right
--- 80 (not < 75) --
0.07 to 0.08
Reject,re-
Process or
segregate
before use
for dyeing
Absorbency Spot test Every lot < 2 Secs.
pH. Indicator Every lot Left Cen Right
---------- 6 –6.5
-----------
Drying Feel at
different
places
Every lot Dry / room
temperature
Defects Check lot card
remarks
Every lot No compromise
*Caustic Soda addition to ˚40 Tw Silicate in the following table .
Concentration Caustic (38˚ Be /72˚ Tw
/ 32.5% w/w or 450g p
l)
Light <30gpl 6.5 g p l
Medium 30 to 50 g p l 11.5 g p l
Dark 50 to 60 g p l 16.5 g p l
Very dark >70 g p l 21.5 g p l
The Process house can decide on the alkalie additions based on their Silicate quality and
working results.
Dye Make up
ParametersMethod of
CheckingFrequency
Norms/Limits
Recipe Against Lab/Std. Every Recipe
Make up tank
cleaningVisual
Before every
make up
Sequence of colour
/chemical addition
Against std.
sequence displayed
No tolerance
Silicate strength ˚ Tw.
Every time
silcate is made
up.
No tolerance
Addition of Caustic As per table*Every shade
group.
No tolerance
Make up of colour to
volume
Metering device or
by dip rod
No tolerance
DissolutionSpot test on filter
paperEvery make up
Clear circular
spreading; No
sediments.
Fixation / Batch Rotation
Parameters Method of Checking
Dwell Time Tag the batch Indicate time of start and end
Covering of the Batch Polythene cover fully enclosed and secured
Precautions Keep away from steam, water spray or acid fumes
Rotation Ensure uninterrupted rotation
Dye Pad
Parameters Method of checking Frequency Norms/Limits Action
Cleanliness Visual /Manual Start of every
shade
Feeding line Visual / Manual Start of every
padding
Pad box
liquor level
Visual At the start of
every
padding
Mangle
Pressure
Read on dial Start of
Padding
Asper setting L
M R
Speed Speedometer As per table
Batching Visual Continuously
Even batch
without creases/
other defects*
Look for
ridges and
take action
The cloth
guiders
Should be fully
functional
If fluff or thread attaches to the bowl repeat spots may occur, the bowl should be cleared of
the contamination
Washing and Soaping
Compartment 1 2 3 4 5 6 7 8
Temperature Display norm temp for each compartment. Check
and record – every 15 min. The dial thermometers
should be functional.
Additions of
soaping
chemicals
Display starting, feeding quantities and frequency of
addition for each bath, Monitor additions.
Alternatively dozing can be arranged.
The efficiency of soaping may be checked - take a window sample at the delivery end nip
and sandwich between bleached poplin (without optical brightener); contact dry on a hot
steam cylinder and check any staining of the white. There should be no staining.
Dyeing of Reactives by Exhaust Method
DYEING OF REACTIVE DYES BY EXHAUST METHOD
REACTIVE DYES
EXHAUSTION PHASE
Primary Exhaustion Phase
AdsorptionDiffusionSubstantivity
REACTIVE DYES AND DIRECT COTTON DYES
Direct Cotton Dye Reactive Dyes Role of ElectrolytePartition /Distribution Coefficient and Degree of ExhaustionLiquor RatioTemperatureInfluence of pH.Influence of Substantivity
Migration phase
Secondary Exhaustion
Hydrolysis of Reactive dyes
Typical Examples
REACTIVE DYES
Choice of Reactive class of Dyes has become indispensable for application of colours on the cellulosics to provide bright range of shades with reasonably good fastness features. No other class of colours can boast of the versatile range of shades with unmatched brilliance, yet economically viable and cost effective that this class of dyes can offer. Even as Reactive dyes are most popular for dyeing solid shades it is equally sought after for various resist and discharge printing styles, thanks to its suitability to be resisted or discharged readily and effectively
The reaction mechanism is apparently simple in that on just altering the pH after exhaustion, formation of covalent bonds between the reactive group of the dye and the OH of cellulose proceeds. For the same reason of ready reactivity
with Cell OH groups, it reacts with Water also to get hydrolyzed in which state the dye behaves no better than a direct cotton dye. The management of the various factors/variables that govern the transport of dye uniformly from an aqueous bath to the cellulose substrate and its preferential reactivity to the fibre than to water is far more complex and critical to perform to obtain a satisfactory dyeing. As the shades invariably are tertiary matchings, the behaviour of individual dyes with different exhaustion and reactivity characteristics, all the more compounds the complexity of the problems of differential shade build up, variations, uneven dyeings, reproducibility, fastness etc multifold.
Though there are other methods of dyeing ‘Reactives’ like pad batch, pad –dry-cure or pad-dry-steam etc exhaust dyeing is practiced widely because of its flexibility to process fabrics in rope form and in the case of yarn and other packages, exhaust dyeing is the only alternative as on date. Tubular knit-ware, by its very physical form is more amenable to exhaust dyeing in ‘rope‘s form; however, advanced machineries obtainable in recent years claim satisfactory open width dyeing by Pad Batch technique.
The exhaust method of dyeing would include the following phases
1. Primary exhaustion phase /Migration 2. Secondary exhaustion phase, 3. Fixation (Reaction) phase -Secondary exhaustion and Fixation can run concurrently/over lapping. 4. Washing off phase.
Top
EXHAUSTION PHASE
Primary Exhaustion Phase
Exhaustion of dye from the dye bath to the cellulose during Primary Exhaustion phase is governed by the following three physical processes and the phenomenon of substantivity
It would be relevant to briefly look at cellulose structure with respect to its Hydrogen bonding behaviour at the surface layers and in the interiors of the cellulose micro fibrils The interior layers contain both forms - 1Alpha and 1 Beta of Cellulose molecular chains that are packed compactly and there are intra molecular Hydrogen bonding parallel to the 1.4 Beta Glucoside link (OH of #2 to #6 of the succeeding glucose unit and #3 OH with the ring O of the preceding Glucose Unit) that stabilize the cellulose chain.
The other four hydroxyl groups are fully free for Hydrogen bonding. At the surface layers of cellulose even the O-3 (OH) and 2-6 Hydrogen bondings are reported to be absent and therefore all the six Hydroxyl groups in the Cellobiose repeat units at the surface are free to attract Hydrogen bonding with the water molecules.
Adsorption in an exhaust dyeing process is fundamentally the inter-phase phenomenon of a dye (solute) in its solution in water coming in to surface contact with the substrate and forming a surface layer/ coating. That is the starting phase for the rest of the diffusion and absorption phenomenon. In the case of Cellulose exposed to a dye solution in water at slightly acidic pH there is no ionization of cellulose. However, with abundance of ‘free’ OH groups available at the surface
(six numbers in each of the repeat Cellobiose unit), water molecules are drawn in clusters around the cellulose molecules to form hydrogen bonds causing an overall charge separation. Resultant surface thus carries a negative charge known as the zeta potential
This surface negative charge would repel the advances of the negatively charged ionized dyestuff anions. The zeta potential is partially overcome due to the presence of large amount of dye anions, some of which are forced across the electron cloud through increase in energy (raise in temperature) or through mechanical agitation to come within the effective distance for the inter molecular forces like Wander Vaal’s forces/secondary valence forces to facilitate the dye anion to get adsorbed on the surface of cellulose. Presence of electrolyte also helps in providing the positive charge that can effectively neutralize the zeta potential and improve the adsorption. (Discussed under ‘Role of Electrolyte’)
Diffusion phenomenon takes over followed by the absorption and migration of dyestuff across the cellulose membrane. Diffusion is influenced by the concentration gradient across the interface of cellulose surface and dye bath, the surface area of the cotton substrate in contact with the dye bath, temperature and time and the physical characteristics of the substrate. This is termed as the primary exhaustion phase. The term exhaustion would include the collective phenomenon of adsorption, absorption diffusion and migration in that order.
Top
Diffusion
Diffusion process is explained by the relationship (Ficks Law of Diffusion in its simplest form.) F = -D (C1-C2) / L And D = Do e -E/RT WhereF = Mass flow of dye gms/cm2 secD = Diffusion coefficient of the dye m2/secD0 = Diffusion Coefficient at Infinite TemperatureC1 = Concentration of dye in the dye bath g/cm3C2 = Concentration of dye on surface of the fiber g/cm3
L = Thickness of the layer cme, E, R = Constants (E activation Energy; e exponential; R Universal Gas Constant)T = Temperature Kelvin
Applying the above relationship the following dynamics may be inferred during the diffusion / exhaustion stages of the dye to the cotton substrate.
F is the dyestuff sorbed across Unit area of the fiber surface in unit time (Rate)
Greater the surface area of the fiber in contact with the dye bath greater is the dyestuff sorbed.
(C1-C2) concentration gradient during the process of diffusion.
The concentration gradient at the initial stages would be higher and therefore the rate of dyestuff transport to the fibre phase will be correspondingly higher tending towards zero at equilibrium.
D Diffusion coefficient Higher the Diffusion coefficient, lesser the time taken to reach the equilibrium. Time taken for dyeing 50% of the equilibrium depth of shade is an index of the speed
Temperature Increase in Temperature increases Diffusion coefficient.
Since surface area is a factor, the characteristics of the fiber and construction would influence the diffusion. Nature of cotton from different sources would have different shape, cross section, micronaire, fineness, impurities, etc and different packing densities of the cellulose molecular chains thus altering the surface area characteristics. The corollary is that thinner the fibre/count and lower the density factor greater is the surface area available and better would be the diffusion.
The term substantivity is primarily a measure of the amount of the molecular dye chromophore that can penetrate/diffuse into the interstices of cellulose micro fibrils assisted by physical forces from an aqueous dye bath. This is influenced by the salt concentration in the dye bath, the liquor ratio, the temperature and the fibre surface area characteristics, besides the chemistry of the dye chromophore. Substantivity ratio is the unit concentration of dye on the fibre to the unit concentration of dye in the bath at the equilibrium state (both expressed in the same units)
The process of primary exhaustion proceeds to its limiting values dictated by the substantivity beyond which it ceases. In the absence of salt, the dye uptake by substantivity phenomenon as stated above is around 20 to 40% of the starting bath concentration or lower, a figure far too low to have any significant economically feasible colour yield. Therefore, as a general rule, without salt additions, substantvity by primary exhaustion of Reactive dye to cellulose cannot be improved or maximized, at the present status of Colouration technology.
[Efforts are on for reduced salt /salt-less systems based on changes in the chemistry of the dyes to exhibit reduced anionic behaviour, fibre substrate modification/sensitization to display cationic behavior to induce exhaustion with less/no salt, while retaining the reactive system for the ultimate fixation. Such developments are still in the R&D Labs and not presently available for bulk]
Top
REACTIVE DYES AND DIRECT COTTON DYES
Reactive and Direct Cotton dyes sport similar dye chromophoric structures but for the Reactive groups present in the Reactive dyes as opposed to Direct cotton dyes. The Reactive dyes are smaller sized more akin to Acid class of dyes (not necessarily as a general rule) with Reactive groups.
Direct Cotton Dye
Direct Cotton Dyes molecules are engineered to include some or all of the important features listed below 1. More number of hydrogen bonding groups, groups that would facilitate inter molecular attraction / diminish repelling forces and groups that can chelate with hydroxyl groups of the Cellulose 2. Molecules of sufficiently large enough size and shape that on aggregation could get trapped in the interstices of the Cellulose molecular chains thus difficult to be removed/washed off.. 3. Optimized number of solubilizing groups (invariably ‘-SO3Na’), just enough for the dye to go in to aqueous solution. Dyeing is invariably carried out at boil, to provide the heat energy to facilitate diffusion and migration. Higher temperatures can also cause de-aggregation and consequent de-sorption Since the dyes have good substantivity due to affinity caused by physical forces like Hydrogen bonding, metal chelation etc. there is less propensity to desorb and higher temperatures facilitates migration within the substrate forming the same physical
bonding at new sites (High substantivity always causes an initial ‘strike’ – aggregation of colour in most favourable loosely packed sites and migration to other sites to increase uniformity in dyeing is facilitated only by imparting energy.) Fastness characteristics are just adequate even for the most satisfactory dyes of its class due to bonding only by physical forces that are relatively week to the more powerful covalent bonds.
Top
Reactive Dyes
Reactive Dyes are capable of forming chemical covalent bonds with the Hydroxyl groups of cellulose fibre and therefore, better anchored to the substrate and not depend on the relatively weak physical forces to give better levels of fastness. All of the features that are desirable for a reasonably ‘fast to wash’ Direct Cotton dyes are not essential for Reactive class of dyes (because of the more strong covalent bond), though cannot be totally discarded as undesirable. Some of them could be counter productive. For example, Reactive Dyes with features listed under I and 2 of the Direct cotton dyes would exhibit problems of low migration and or difficulty to wash off the hydrolyzed dye. Certain quantity of Hydrolyzed dye is inevitable after the fixation stage and non removal of such unfixed dye would entail bleeding/staining of white during washing. Migration is facilitated by increase in temperature; but higher temperatures induce hydrolysis of Reactive dye during the fixation phase and therefore it would be necessary to bring down the temperature to the most favourble temperature for the reaction between dye stuff and substrate before alkalie addition can be made. There fore, in the case of Reactive dyes the following aspects are most important 1. Degree of Exhaustion of the dye bath on to the fibre (both primary and secondary) that is directly related to the substantivity should be maximized /optimized (assisted more by salt addition than by the physical forces). 2. The migration of the dye within the substrate during the primary exhaustion phase should be maximized. 3. Efficiency of reaction of the exhausted dye to the fibre should be maximized during fixation phase. 4. The kinetics of reactivity has the final influence on the success of dyeing, irrespective of high levels of success achieved in the exhaustion stages, though
exhaustion is an important (primary and or secondary) pre-requisite... 5. The above four aspects need to be performed within a reasonable span of time. 6. The corollary here is that the extent of hydrolysis of the dye during exhaustion and fixation stages needs to be minimized.
Top
Role of Electrolyte
Addition of electrolyte induces exhaustion both its rate and extent. Where the substantivity is lower the prime driving mechanism for diffusion /exhaustion of dye into the fibre is the concentration gradient across fibre/liquor interface and presence of common ion- i.e. electrolyte (Salt). The electrolyte, say, Sodium Chloride dissociates in water into Na+ and Cl - and Na+ has higher propensity to travel to the fibre /water interface and neutralize the negative charge thus facilitating the free transport of dye anion to be adsorbed onto the surface of the fibre and the subsequent diffusion/ absorption (exhaustion) to take place. Secondly, the dissociated NaCl ions are more associated with water than with the large molecular dye Chromophore with a few SO3Na or other solubilizing groups and thus occupy the limited available sites in the water effectively displacing the dye Chromophore.The distribution coefficient of dye therefore shifts towards fibre. It is not the quantity of the salt but its concentration that influences the degree of exhaustion. The degree of exhaustion increases with increasing concentrations of Salt to a limiting concentration. Higher concentrations of Salt result in aggregation of the dye in the dye bath itself and hence ‘it is salted out’ much in the same manner as in the manufacture of the dyestuff and less and less monomolecular dyes are available for reaching the fibre phase The optimal quantity of Salt in terms of concentration depends on the chemistry of the dye, its molecular size, its solubilizing groups, quality of water and the fibre substrate etc. Secondly, dyes displaying higher substantivity in the absence of salt would need lesser salt concentrations.
Top
Partition /Distribution Coefficient and Degree of Exhaustion
At a given liquor ratio and bath concentration of dyestuff and salt, the exhaustion of the dye proceeds from the liquor phase to the solid phase (cellulose) until it reaches an equilibrium. This state would be different for different solutes (dyestuffs) and the factors that contribute to this variability are their molecular size, ionic character, extent of hydrogen bonding groups, inter molecular forces, temperature etc. Such equilibrium, where the number of molecules absorbed is equal to the number of molecules desorbed at the cellulose/dye liquor interface, can safely be assumed to have been reached in a time span of infinity, i.e. at the end of Exhaustion phase or Partition of the dye from the liquor phase to the solid phase at a notional infinite time It is desirable that the exhaustion proceeds at a satisfactory rate to achieve close to equilibrium exhaustion within a manageable /practicable time span a condition that is influenced by diffusion coefficient. Higher the diffusion coefficient faster the exhaustion as discussed earlier under diffusion...
The Partition/Distribution coefficient of a solute between two phases is calculated as the ratio of the concentration of the solute in one phase to the concentration of the solute in the other phase under equilibrium conditions
Interestingly, at the equilibrium state of exhaustion where the concentrations of dye on fibre and in the final bath tend to become steady and constant, it is an established fact that as the dye bath concentration is increased, the concentration in fiber phase at equilibrium though increases, does not do so linearly but progressively diminishes giving relatively lower distribution coefficient values.
Degree of exhaustion is the ratio of the total amount of dye present in the cellulose at the end of exhaustion to the amount of dye present in the original bath before the start of the exhaustion process.
Degree of Exhaustion in terms of distribution coefficient and liquor ratio is given by the relationship
Where E Degree of Exhaustion K Partition coefficientL Material Liquor Ratio
Top
Liquor Ratio
Recipe of x% owf (on weight of fabric) in terms of absolute quantity would be present in the starting dye bath but its concentration in the dye bath would vary depending on the liquor ratio
The recipe equivalent dye % on the fabric after the completion of dyeing would not be x% but would tend towards x% - depending on the efficiency of dyeing/the substantivity /reactivity of the dye. In an ionic kind of reactions like Acid dyes on wool the degree of exhaustion would proceed to almost to .100% subject to the dye present in the dye bath does not exceed the saturation capacity of the reacting sites present in the substrate.- the limiting degree of exhaustion in this case.
In a model scenario where the liquor ratio is changed to a higher one: Amount of dyestuff expressed owf, when present in the higher liquor ratio would register proportionately a lower concentration of the dye in the starting bath and consequently lower concentration gradient at the fibre liquor interface resulting in lesser rate of diffusion of the dye from liquor phase to fiber phase
Only 50% of the dye molecules are available at the interface for adsorption and diffusion in case 2.and therefore the rate of diffusion will be lowered and it would take relatively far longer time to reach the equilibrium state.In case 1 starting from 1:10 going to 1:5, the increased concentration of dye in the bath would increase the rate of diffusion (increased concentration gradient) and take shorter time for exhaustion.
The relationship E= K/ (K+L) as discussed under Distribution coefficient (K); any increase in L would diminish the E –the degree of exhaustion. Such a situation would entail higher starting concentration of the dye and or increase in concentration of Salt to ‘occupy the available sites in water’ (as explained earlier under salt concentration) in a larger volume of water to displace the dye anion to shift the distribution coefficient to the fiber phase. But increased salt addition cannot always fully compensate for the adverse exhaustion behaviour but only to a point (as discussed under Role of Electrolyte) Therefore, not only increase in concentration of the dye, but also that of salt will be necessary (barring certain marginal cases) - quantitative aspects governed by the substatnivity characteristics of the dyestuff.Such a situation would be more pronounced in the case of low/poor substantive dyes compared to the dyes with better substantivity. There are ready reckoners for recipe correction available for changes in liquor ratios from the dyestuff manufacturers but they are only for guidance. As individual dyes would behave differently, an intelligent understanding and application of the given information only can give meaningful results.
The corollary is that a change in liquor ratio would affect the least in dyes with high substantivity and most in those with poor substantivity
Top
Temperature
Temperature of the bath is another factor influencing exhaustion As explained earlier presence of salt increases the substantivity facilitating aggregation of the dye in the fiber phase Increase in temperature in the case of high substantive dyes as in the case of direct cotton dyes help in the migration of the dye within the substrate but in the case of dyes that are less substantive increase in temperatures could be counterproductive Temperature up to 50 deg C contributes to de-aggregation of the molecules of dye, both in fibre and water phases; but relatively less in fiber phase and more in the water phase. Therefore the net effect is that there are more de-aggregated monomolecular dye free to move towards the fiber phase than that is desorbed from the fibre and therefore the exhaustion proceeds. There is a maxima in the exhaustion curves of dyes of low substantivity at temperature around 40 to 50 deg C. beyond which increase in temperatures results in decreasing degrees of exhaustion explained by the higher degree of de-aggregation of the dye in the fiber phase and lesser physical forces to resist desorption, unlike in the case of substantive direct cotton dyes; annulling the influence of salt..
Top
Influence of pH.
The pH is relevant to the Reactivity aspect and not considered as a factor in the exhaustion process. Also, the dye bath pH during the exhaustion phase is maintained at 5.5 to 6. As long as the pH of the bath is slightly acidic, no reaction can take place and therefore primary exhaustion and bringing the temperature close to the reaction temperatures can be carried out conveniently.
Influence of Substantivity
High substantivity facilitates exhaustion process; also requiring less concentration of salt for exhaustion but for the same reason migration of the dye would be restricted resulting in unlevel dyeing. However dyes with medium+ substantivity engineered to provide the balance in the molecular structure to promote migration and good reactivity
that matches the exhaustion curve (primary and secondary) would give the best results both in terms of dye yield and washing efficiency. Poor substantive dyes that are also not sensitive to electrolyte additions are poor builders and therefore will give poor yields.
High substantivity .dyes with low reactivity (Fixation) falling below the exhaustion levels would result in high levels of unfixed and hydrolyzed dye to be washed off and the dye and its hydrolyzed version also being highly substantive, the washing efforts also will be high requiring more water, energy and mechanical efforts
Top
Migration phase
Since fiber surface area is a factor in diffusion process, the exhaustion would proceed to locations where relatively more surface area is presented like in the amorphous areas and less densely packed crystalline areas in that order in the cellulose and therefore the dye concentration within the cellulose substrate would not be uniform/even. Such a situation would result in uneven build up of the dye both in hue and intensity. In a trichromatic mixture the situation could be worse. The process of Migration of the exhausted dye depends on the molecular size of the dye its spatial profile (Steric) and the solubilizing groups present. The other external factors would relate to temperature, machinery used and the package profiles and densities (in case of package dyeings). Raising the temperature would provide the required thermal energy; but cannot be increased arbitrarily due to limitations discussed under ‘Temperature’. Both exhaustion and migrations can be maximized /improved by better mechanical agitations that would facilitate intimate surface area contact of the cellulose with dye liquor and by improved flow designs that facilitate better liquor exchange at the fiber liquor inter-phase.
Migration phase should precede the fixation phase as once the reactive dye forms a covalent bond with Cell O- it is anchored strongly and cannot be shifted.
The observations and inferences in the above deliberations related to primary exhaustion in a Reactive exhaust dyeing process are incomplete without the final fixation. When Alkali is added, the cellulose ionizes to form Cell-O- and H+ (Cell O– Na+) and starts forming covalent bonds with the reactive functional groups of the dye Chromophore. When more and more of dye anions are covalently bond, the distribution coefficient shifts to fiber phase effecting further exhaustion due to deficiency of dye anions in the cellulose phase and dye bath concentration starts depleting further. The degree of alkalinity in terms of pH plays a major role in shifting the fixation of dye to its hydrolysis reacting with water. Any exhaustion during this stage if it is hydrolyzed dye it would be far more undesirable In a reactive dye system therefore, primary exhaustion alone does not govern the efficiency of dyeing. The degree of secondary exhaustion also would influence the efficiency. During the secondary exhaustion when alkalie is added, there is a second reaction that also sets in motion in parallel ( i.e. the hydrolysis of the Reactive dye with water) in competition to the fixation of the dye that is the primary aim. The dye anion is equally facilitated to react with OH of water to form the hydrolyzed dye in which state the dye is as good as a direct dye with all its ‘undesirable’ characteristics. It is the reactive group in the dye, pH and temperature that influence the hydrolysis of dye in preference to reacting with cellulose. It becomes critical that the hydrolysis is curbed to maximize efficiency. The relationship between temperature and reactivity is that higher temperatures require lower alkalinity; to optimize on hydrolysis. They can be broadly grouped under ‘High’ ‘Medium’ and ‘Low’ categories requiring 40º C. 60 º C and 80º.C respectively - levels of pH 12.5 for High (cold dyeing), 11.5 for Medium (Warm) and 10 - 11.0 for Low (Hot Dyeing) for the reaction to proceed more favorably towards the substrate. The term more reactive is used in the sense that it requires lesser levels of alkalinity and lower temperatures (and not the reaction itself. Given the right temperatures, alkalinity and time the reaction proceeds to completion in all cases.)
The most critical part of the Reactive dyeing is the actual fixation where the covalent bond takes place between the Cellulose O - and the Reactive group of the Dye Chromophore.
The electron attracting Sulphone group causes electron deficiency on the terminal carbon atom enabling neucleophylic attack to take place. . (Addition reaction)
Where [-O-R1] is [-O Cellulose] or [-OH] of water, etc.The liberated acid in both the two reactions is continuously neutralized by alkalie for the forward reaction to proceed during the fixation process..Efficiency of Reactive dyeing (Rate of Fixation /Rate of Hydrolysis) for a given exhaust dyeing process has been expressed in mathematical terms making use of the competing First order /pseudo first order rate constants of the reaction of the dye with the cellulose and the dye hydrolysis with water , the equilibrium concentration of the dye on fabric and concentration of dye in the aqueous phase (For details please refer Chapter 4 of ‘The Dyeing of Cellulosic Fibres’ by Maurice R Fox and Harry H Sumner Edited by Clifford Preston 1986 - SDC Publication}
It has also been emphasized that the expression is too ideal and relates to certain assumptions and conditions that are
not practically achievable in the real situation. However, the broad principles are applicable and the direction of the reactions proceeds towards the ideal. To whatever extent the variables can be controlled and maintained, the results achieved could be optimized and also reproduced maintaining the same conditions and controls every time.
Top
Typical Examples
Reviewing the critical variables that govern the dyeing Efficiency in a Reactive dyeing process, the following few examples will highlight the pros and cons of the factors discussed. 1. Low Primary exhaustion (P) and high Reactivity (R) (incidentally higher secondary exhaustion) Where P is low and Fixation is high
Initial exhaustion phase will not be critical as less amount of dye is transported. On addition of alkalie the reaction starts and the secondary exhaustion proceeds as more and more of the dye takes part in the reaction. During this phase the
competing reaction - hydrolyzation of the un-exhausted dye close to the substrate phase and in the dye bath also starts and it would become critical to control minimize this aspect of the reaction. The direction and rate of reaction towards covalent bonding with substrate have to be controlled by careful manipulation of pH and temperature. That would require precision instruments /plc controls. Secondly since the exhaustion is low and better part of the dye exhaustion takes place in the secondary phase, migration would be affected and the dyeing would be non uniform.
Where P, Substantivity and R are high
Primary exhaustion would be high and whatever exhausted would be fixed. In this case it would be critical during exhaustion phase as the substantivity is high and migration could be a problem.
Higher temperatures need to be resorted to for migration and that would not be in favour with Dyes of the low reaction temperatures in view of its high reactivity. Such a situation would warrant graduated salt additions to avoid initial strike – linear or step wise in order to facilitate phased migration. It would require cooling if higher temperatures were to be adopted. Because of the high reactivity pH control to maintain low and constant alkaline pH through out the reaction/fixation phase would be critical. Depending on the hot or cold class of colours the temperature maintenance will be critical. In the above example where the substantivity before salt addition is relatively lower but enhanced by salt addition, migration would be better facilitated. It could be possible to standardize on an isothermal dyeing sequence starting with salt bath
The desirable features of the dyestuff would be to posess reasonbly good substantivity and migration capability, a good exhaustion percntage including the seondary exhustion that are achievabe within a paracticable time dimension and reactivity that matches the degree of exhaustion so that all the exhausated dye is fixed.
This would mean ideally that the curves S and F should super impose at the concluding stages of the dyeing process. Such a dyeing would require least effort for soaping, However such an ideal system
is not practicable but efforts should be to move towards the ideal system Dyes with similar substantivity that are moderate and having good primary exhaustion (assisted by salt addition) and migration potentials and also a relatively lower secondary exhaustion with reactivity reaching close to equilibrium exhaustion would be the most suitable choice where auto dozing and sophisticated control systems are not available.
Top
Evaluation of Substantivity
A very useful and simple practical method to assess
substantivity of the Reactive dyestuffs in the lab based on chromatographic principles is given in the article “Effects of Dye Substantivity in the Dyeing of Cotton with Reactive Dyes” a prize winning article By Canadian Association of Textile Colourists and Chemists in TCC Nov 91).
The individual process house labs can conveniently assess substantivity of the dyes and group them for using in their recipe mixtures. The dyestuff manufacturers themselves recommend colours that have similar substantivity features; however it would be safe to assess in ones own lab unless supplied by propriety manufacturers.
Evaluation of Migration Index
Ref.material Practical method to evaluate migration Index “Reactive Dye Selection and Process Development for Exhaust Dyeing of Cellulose”BY M.J. Bradbury, P. S. Collishaw and S. Moorhouse, ZENECA Colours, Blackley, England. August1995, Vol. 27, No. 8
Dyeing of Reactives - Further Options
Pad colour – dry - Alkalie pad batch
Pad Colour - Dry - Chemical Pad - Steam
Pad Bake System
The Problems and Precautions
Pad colour – dry - Alkalie pad batch
Selected Mono-chloro Triazine based dyes are mostly suited for this method of dyeing. Di-
coloro Triazine may also be used but as this class of dyestuffs can be dyed more easily at
lesser costs by other methods may not be an attractive proposition. Mono - Chloro Triazines
require higher temperatures for dyeing and therefore the cost of additional drying would not
be a constraint. Also, when the fabric is hot flue dried without alkalie in the system the colour