Continuous dyeing Faults

Continuous Dyeing Faults
This is probably the most common fault to occur in continuous dyeing. Sometimes
these are retained visible folds formed during preparation or the dyeing process itself,
but more often they are temporary folds that have appeared and vanished again in a
preparatory process, their presence only being revealed later by differential dye
uptake.

The source of such problems may be difficult to identify with certainty. Excessive,
insufficient or variable tension in running fabrics can be important causes of
crease formation.

Tight selvedges, differential shrinkage, the development of bowed or skewed
weft , incorrectly bowed expanders and worn or badly rotating rollers can all
produce rippled or irregular patterns of creasing.

The build-up of lint, loose threads or other insoluble debris as hard deposits on
roller surfaces is another potential source of variability in fabrics running over
them.
Creasing

Shade matching in continuous dyeing is a challenge quite different in scale from
that of automated batchwise dyeing, where a mis-matched dyelot can often be
given a minor correction without removing the dyed batch from the dyeing vessel.

If a continuous dyeing arrives at the delivery end of the range slightly off-shade,
some form of reprocessing is usually inevitable.

Even if the dyeing range is equipped with correctly positioned on-line colorimetric
control , substantial amounts of off-shade material are produced whilst a colour
correction step is being implemented.


Slightly off-shade but otherwise commercially acceptable fabric can often be
disposed of by negotiation with the original customer or sale elsewhere as
seconds quality.

More serious divergencies from the target shade are usually corrected in
batchwise open-width equipment rather than attempting to apply a correction in a
second run on the continuous range. However, it may be technically and
commercially preferable to cover the faulty goods by re-dyeing to navy or black,
because these deep shades are normally in demand and usually readily disposable
as seconds.
Shade Matching Faults

Level bulk-scale dyeings that unexpectedly fail to match the relevant laboratory
dyeing for shade are often attributable to human error in formulating the
dyebath, either in bulk or in the lab. Such errors can be cross-checked by
repeating the lab dyeing and testing the bulk-scale liquor in a lab-scale dyeing.

If the same shade has shown satisfactory lab-to-bulk reproducibility in a previous
dyelot, the off-shade result may have a different cause, such as a change in
substrate dyeability, liquor ratio, water supply, non-standard preparation, or a
failure in the bulk-scale control of dyebath pH, temperature or chemical
additions.

Modern dyeing control systems are so reliable that substrate variability is far
more likely to be involved.

Consistency of substrate dyeability can be readily monitored in a vertical
organisation with a limited number of fabric suppliers to the dyehouse, but the
wide variety of fabric qualities dealt with in a typical commission dyehouse
demands constant vigilance to keep the frequency of off- shade faults down to an

acceptable level.
Lab-to-Bulk Reproducibility
Two Sidedness

This occurs when one face of the fabric is subjected to a higher temperature than
the other during the pre-drying stage.

Dye migration takes place preferentially towards the hotter surface to give a
slightly deeper shade than on the cooler one.

If the component water-soluble dyes in a trichromatic combination differ in
substantivity, however, the least substantive will tend to migrate more readily
than the most substantive component and the two sides of the dyed fabric will
show differences in hue as well as depth.
Two Sidedness
Listing
Listing is the term used to describe weftway differences in hue or depth across the
fabric width, normally a gradual shading from one selvedge to the other, or a
difference between the centre of the fabric and both selvedges. Possible causes of
listing include weftway variation in:
1. nip loading in a preparation treatment
2. residual size content after preparation
3. heat setting before dyeing
4. moisture regain after padding
5. nip loading at the padding stage
6. temperature or moisture content during drying
7. temperature during thermofixation.
Listing
Ending
Ending is the term used to describe warpway variations in hue or depth along the

fabric length of a dyelot. Possible causes of ending include random variations of:
1. desizing, scouring or bleaching conditions
2. heat setting before dyeing
3. moisture regain before padding
4. nip loading at the padding stage
5. migration during pre-drying after padding
6. temperature in drying, thermofixation or steaming treatment
7. dwell time during fixation or aftertreatment.
Ending
Tailing
This term refers to the depletion of dye concentration in the pad liquor that takes
place gradually during continuous running. The higher the dye substantivity and the
lower the applied depth, the more pronounced is the depletion or tailing effect.

If the component dyes in a trichromatic combination differ significantly in
substantivity, tailing may be more obvious because it manifests itself as a
gradual change in hue. This fault can be minimised by rapid recirculation of
the pad liquor from the trough back into the stock feed tank.

However, it is also essential to consider carefully the relationship between
the laboratory pad, stock tank and pad liquor formulations.

A lab-scale padding gives a similar shade to that of the first few metres
dyed in bulk, whereas the equilibrium shade reached after several minutes
of bulk-scale running may be significantly paler or off-shade relative to the
lab result.

Quantification of these differences can be used to calculate allowance
factors, so that the stock feed and lab-scale formulations can be adjusted to
ensure that the pad liquor at equilibrium yields the target shade on the

finished goods.
Tailing
Chemical Pad Bleeding
Another substantivity-dependent fault of a similar nature can arise in the pad- dry-
thermofix-reducing pad-steam process for vat or sulphur dyes and the pad- dry-
thermofix-alkaline pad-steam application of reactive dyes to polyester/cotton blends.

Although the electrolyte concentration of these chemical pad liquors is invariably
high, there may be significant desorption of unfixed dyes from the dried goods
during immersion.

Lab-scale dip tests may give qualitative confirmation, but as with tailing problems
they cannot reproduce the equilibrium state reached on prolonged running.

Nevertheless, quantitative measurements of colour differences do enable
allowance factors to be determined for defining the relationship between dye
padding formulation, chemical pad composition and target shade on the finished
goods.
Chemical Pad Bleeding
Staining Faults
These are of almost infinite variety and tend to occur randomly, but careful analysis
of the processing history of the fabric batch in question often pinpoints the source of
the problem .

The fault may appear in a repeat pattern along the fabric length and it is
important to record the exact circumference of all rollers and other cylindrical
components that come into contact with the running fabric.

Staining faults may be roughly categorised as random staining, resist marks,
spotting and foam marks.

Staining Faults
Random Staining
This may be variously described as mealiness, swealing, patchiness or
blotchiness. When these effects are encountered on continuously dyed fabric the
source of the fault is frequently migration during pre-drying after padding. The
migration of water-soluble dyes at this stage is inversely related to substantivity and
can be minimised by careful incorporation of electrolytes and migration inhibitors .
The degree of reactivity of reactive dyes is also significant, because highly
reactive dyes become partly fixed during pre-drying and this effect competes with
migration. Thus high-reactivity dyes with high substantivity are the least prone to
migration problems .
The migration of an individual disperse dye can be restricted by other dyes present in
combination that may have a larger particle size or a tendency to flocculate. However,
the higher the concentration of migration inhibitor present, the less apparent are
these differences in the migration behaviour of individual dyes . The reduced liquor
retention attainable by vacuum impregnation with disperse dyes greatly suppresses
the extent of dye migration at the subsequent pre-drying stage . In the absence of a
migration inhibitor, vacuum impregnation is more effective than infrared treatment as
a means of inhibiting migration but it is important to recycle the extracted dye liquor
back to the pad trough .
Random Staining
Resist Marks
Most of these faults arise from inadequate preparation but it can be quite difficult to
deduce precisely where or how the fault originated.
Occasionally, chemical analysis may confirm that the poor dye uptake was caused by
the presence of residual cotton wax, size polymer, oil or other contaminant before
dyeing but far more often the dyeing and washing-off processes extract the offending
impurities and such tests are then negative.
Localised acidic or alkaline resists are normally caused by soluble contaminants that
are soon extracted or neutralised at the dyeing stage.

Resist Marks
Light Spots
Repeated white or pale-coloured spots may be attributable to localised deposits on
roller surfaces that interfere with the application of uniform pressure to the fabric
containing absorbed dye liquor.
Random light spots can arise as a result of water droplets falling onto the moving
fabric after condensation has occurred on roof surfaces within or above hot and wet
processing equipment.
These wet spots dilute the unfixed dyes and chemicals locally, thus inhibiting full
fixation in the spotted region.
Light Spots
Dark Spots
These spots or specks are usually local concentrations of deposited dye
attributable to unsatisfactory dissolution or dispersion of the dyes and inadequate
sieving before feeding to the pad trough.
Incompatibility between dyes and auxiliaries, or between different classes of dyes
when dyeing blends, pH fluctuations, variations in the water supply and desorption of
impurities from the fabric have all occasionally been found to contribute to dye
spotting problems.
Airborne dye particles released when weighing, dissolving or dispersing must not be
allowed to contaminate fabric or machinery in the vicinity; this is taken care of in
modern dispensary facilities.
Low-energy disperse dyes may volatilise and contaminate the interior surfaces of
thermofixation equipment, so thorough cleaning between dyelots can be critical.
Dark Spots
Foam Marks
These faults often arise when a scum or foam on the surface of a dye liquor contains
undissolved dye particles that can become airborne or collapse as a random deposit
on the fabric surface.
The presence of excess migration inhibitor, wetting agent or other surfactant,

accompanied by turbulence of the dye liquor when operating at high speeds, may
contribute to such problems.
Similar faults can arise during the reoxidation of vat or sulphur dyes when leuco
compounds desorbed from the fabric surface become reoxidised in particulate form if
foam is building up at the liquor surface.
Foam Marks
Typical semi- and fully-continuous dyeing of woven fabrics
Process Type Example
Pad-batch Semi-continuous Reactive on cellulosics
Pigment pad-jig develop Semi-continuous Vat, sulphur on cellulosics
Pad-batch-beam Semi-continuous Disperse/reactive on
polyester/cellulosics
Pad (coupler) – jig develop (diazo) Semi-continuous Azoics on cellulosics
Pad (leuco ester and oxidant) – jig develop
(acid)
Semi-continuous Vat leuco ester on
cellulosics
Pad-dry-wash Continuous High-reactivity dyes on
cellulosics
Pad-dry-bake Continuous Reactive on cellulosics
Pad-dry-thermofix Continuous Disperse on polyester
Pad-dry-steam Continuous Reactive, vat, sulphur on
cellulosics
Dye pad-dry-alkaline pad-steam Continuous Reactive on cellulosics
Pigment pad-dry-reducing pad- steam Continuous Vat, sulphur on cellulosics
Pad-dry-thermofix-alkaline pad- steam Continuous Disperse/reactive on
polyester/cellulosics
Pad-dry-thermofix-reducing pad- steam Continuous Disperse/vat or /sulphur
on polyester/cellulosics