Critical solutions in_the_dyeing_of_cotton_textile_materials-libre(1)

Abstract: Over the decades there have been several papers on the coloration of
cotton-based textiles. The number of articles dealing with the processing of cotton,
including preparation, dyeing, and finishing, may be in the thousands. An
investigation of the possible causes of problems occurring in the coloration of
textiles revealed that a comprehensive review of case studies and scientific
analysis would be a welcome addition to the already rich pool of knowledge in
this area.
Key words: Cotton, troubleshooting, pretreatment, dyeing, dyes, colorants.
1. INTRODUCTION
Cotton is the backbone of the world’s textile trade [1]. It has many qualities [2] and
countless end uses [3], which make it one of the most abundantly used textile fibres
in the world [4]. It is a seed hair of plant of genus Gossypium [5], the purest form of
cellulose found in nature. However, cotton is one of the most problematic fibres as far
as its general wet processing or dyeing is concerned. Quite frequently, the problems
in dyed cotton materials are not due to the actual dyeing process but due to some
latent defects introduced from previous production and processing stages. Often, the
root-cause(s) of a problem in the dyed material can be traced as far back as to the
cotton field. This monograph will address problems in the dyeing of cotton textile
materials in various forms. An overview of various textile operations for cotton will
be given in the beginning. Then, various key stages and factors involved in the
production of dyed cotton textile materials will be described in detail and problems
originating at each stage will be summarised.
1.1 Overview of Textile Operations for Cotton
The textile industry is comprised of a diverse, fragmented group of establishments
that receive and prepare fibres, transform fibres into yarn, convert the yarn into fabric
or related products, and dye and finish these materials at various stages of production.
Figure 1 shows some of the general steps involved in manufacturing cotton textiles.
Textiles generally go through three to four stages of production that may include
yarn formation, fabric formation, wet processing and textile fabrication [6]. Textile
fibres are converted into yarn by grouping and twisting operations used to bind them
together [7]. Although most textile fibres are processed using spinning operations,
the processes leading to spinning vary depending on whether the fibres are natural or
manmade. Figure 2 shows the different steps used in cotton yarn formation. Some of
CRITICAL SOLUTIONS IN THE DYEING
OF COTTON TEXTILE MATERIALS
R. Shamey and T. Hussein
doi:10.1533/tepr.2005.0001
© The Textile Institute

2 Textile Progress doi:10.1533/tepr.2005.0001
Fig. 1 General steps in manufacturing cotton textile goods.
Yarn
Formation
Fabric
Formation
Wet
Processing
Fabrication
Warping
Sizing
Weaving
Printing
Finished Goods Sewing
Cutting
Finishing
Dyeing
Preparation
Knitting
Spinning
Fibre Preparation
Raw Cotton
Fig. 2 General steps in yarn and fabric formation.
Raw Cotton
Cleaning
Blending
Carding
Combing
Drawing
Drafting
Spinning
Yarn
Knitting
(Weft or Warp)
Warping
Sizing
Weaving
Fabric

doi:10.1533/tepr.2005.0001 Critical Solutions in the Dyeing of Cotton 3
these steps may be optional, depending on the type of yarn and spinning equipment
used.
The major methods for fabric manufacture are weaving and knitting, although
recently nonwoven constructions have become more popular. Before weaving, warp
yarns are first wound on large spools, or cones, which are placed on a rack called a
creel. From the creel, warp yarns are wound on a beam wherefrom they are passed
through a process known as sizing or slashing. The size solution forms a coating that
protects the yarns against snagging or abrasion during weaving. Fabrics are formed
from weaving by interlacing one set of yarns with another set oriented crosswise. In
the weaving operation, the lengthwise yarns that form the basic structure of the fabric
are called the warp and the crosswise yarns are called the filling, also referred to as
the weft [8, 9]. Knitted fabrics may be constructed by using hooked needles to
interlock one or more sets of yarns through a set of loops. The loops may be either
loosely or closely constructed, depending on the purpose of the fabric. Knitting is
performed using either weft or warp knitting processes [10].
Woven and knitted fabrics cannot usually be processed into apparel and other
finished goods until the fabrics have passed through several water-intensive wet
processing stages. Wet processing enhances the appearance, durability and serviceability
of fabrics by converting undyed and unfinished goods, known as grey or greige
goods, into finished consumers’ goods. Various stages of wet processing, shown in
Fig. 3, involve treating greige goods with chemical baths and often additional washing,
rinsing and drying steps [11]. Some of these stages may be optional, depending on
the style of fabric being manufactured or whether the material being wet-processed
is a yarn, or a knitted or woven fabric.
Some of the key steps in the treatment of cotton material include singeing, desizing,
scouring, bleaching, mercerizing, as well as dyeing and finishing.
Fig. 3 General steps in wet processing.
Finished
Fabric
Mechanical
Finishing
Chemical
Finishing
PrintingDyeing
Mercerising
Bleaching
Scouring
Desizing
Singeing

Singeing is a dry process that removes fibres protruding from yarns or fabrics.
Desizing is a wet process that removes the sizing material applied to the warp yarns
before weaving. Scouring is a cleaning process that removes impurities from fibres,
yarns or cloth through washing, usually with alkaline solutions. Bleaching is a chemical
process that decolourizes coloured impurities that are not removed by scouring and
prepares the cloth for further finishing processes such as dyeing or printing.
Mercerization is a chemical process to increase dyeability, lustre and appearance.
Dyeing operations are used at various stages of production to add colour to textiles
and increase product value. Dyeing can be performed using batch or continuous
processes. Common methods of batch or exhaust dyeing include package, beam,
beck, winch, jet and jig processing. Continuous dyeing processes typically consist of
dye application, dye fixation with chemicals or heat, and washing. Dyeing processes
may take place at any of several stages of the manufacturing process (fibres, yarn,
piece-dyeing). Stock dyeing is used to dye fibres; yarn dyeing is used to dye yarn;
and piece/fabric dyeing is done after the yarn has been constructed into fabric. Printing
is a localized or patternised coloration of the fabrics. Fabrics are printed with colour
and patterns using a variety of techniques and machine types. Finishing encompasses
chemical or mechanical treatments performed on fibre, yarn or fabric to improve
appearance, texture, or performance.
2. PROBLEMS ORIGINATING FROM COTTON FIBRE
2.1 Problems Caused by Immature and/or Dead Cotton
Although it a common practice to use the terms ‘dead’and ‘immature’interchangeably,
it is useful to use these terms to indicate two different levels of maturity in cotton
fibres. The normal mature cotton fibre is bean-shaped in cross-section and has a thick
cell-wall. The other extreme, dead cotton, has virtually no cell-wall thickness. The
intermediate range between mature and dead is classified as immature. The immature
(sometimes called thin-walled) fibre does have some secondary wall thickening. The
thinner wall of the immature fibre lacks the rigidity of mature cotton. This increased
flexibility of immature or dead fibres makes them prone to be mechanically knotted
into a clump during ginning, lint cleaning and carding. These neps or clusters of
fibres may resist dye and appear as white specks in the dyed material [12–16].
The distinction between dead and immature fibres is very important. Both dye
lighter than fully mature fibres but only immature fibres respond to mercerization or
any other swelling treatment. In contrast, dead fibres lack the ability to accept some
dye even if pre-treated with a swelling agent.
The white or light-coloured specks caused by immature/dead fibres may be of one
of the following three types. The first type of the defect occurs when a surface knot
of entangled immature fibres is flattened during processing and takes on a glazed,
shiny appearance. The knot then becomes a small, reflective mirror on the surface of
the dyed material. Its greater reflectance makes the knot appear lighter at some
viewing angles than the surrounding area although it has actually been dyed to the
same depth. The second type occurs when the fabric is poorly penetrated during
dyeing. Since the clumps of immature fibres are often loosely attached to the material,
they can be moved or knocked loose during subsequent processes. If the clump, or

the yarn behind it, is not properly penetrated during dyeing, a light spot will be seen
when the clump changes its position. The third type is the classic case of the clump
of immature or dead fibres not dyeing to the same depth as the surrounding material.
The coverage of immature cotton depends upon the following factors:
Fibre preparation: There are several stages in the fibre preparation where an
attempt can be made to decrease the amount of neps of the immature and/or dead
fibres that are usually clumped together [17]. It is important to try to remove these
clumps prior to the carding process. Once past the main cylinder of the card, the
clumped fibres go into the subsequently formed yarn and the fabric.
Preparation sequence: The preparation sequence has little, if any, impact on the
coverage of immature cotton. Only pre-treatments that swell the cell wall, giving
it greater thickness, are effective in improving the dyeability of immature cotton.
Swelling pre-treatment: Treatment with swelling agents at optimum concentration
(e.g. caustic soda with a 14% or greater concentration) is effective in swelling the
secondary wall of immature cotton, and improving its dyeing affinity. On the other
hand, dead cotton lacks the necessary cell-wall thickness to be effectively treated
by any type of swelling pre-treatment system.
Dye selection: Dyes vary widely in their ability to effectively eliminate the white
or off-shade specks. It is recommended that dye suppliers be consulted for data on
the immature cotton coverage capabilities of specific dyes. Since caustic pre-
treatment is ineffective in eliminating white or off-shade specks caused by dead
cotton, dye selection is the best alternative in this case.Although the exact mechanisms
are unknown, one theory is that dyes that cover dead cotton are those which do not
penetrate into the cellulose of the fibre (the core) but are deposited mainly in the
outside layer. This gives the dead fibre a ‘coloured’ skin.
After-treatments: Swelling treatments such as mercerization or ammonia treatment
may be effective after dyeing, as well as before, if the problem is the presence of
reflective surfaces and not a genuine difference in dye uptake by the immature
cotton. However, such a procedure is justified only in extreme cases, as there is an
inevitable change of shade even when the fabric is dyed with dyes that are resistant
to strong alkalis.
2.2 Problems Caused by Dyeability Variation in Cotton
The results of research [18] confirm the dyeability variations in cotton obtained from
different sources. It has been suggested that the substrate should be obtained from a
single source, wherever possible, in order to keep the dyeability variations to a
minimum. Since some dyestuffs are more sensitive to dyeability variations than
others; those dyes should be selected for dyeing which are less sensitive to dyeability
variation.
2.3 Problems Caused by Contaminants in Cotton
While cotton fibre may be as much as 96 % cellulose, there are other components
present which must be removed in preparation for a successful dyeing. Table 1 gives
a summary of naturally occurring impurities in cotton [19].
The level of contamination in cotton is affected by: geology of cultivation area;
soil constitution; weather conditions during the maturing period; cultivation techniques;

chemicals, pesticides and fertilizers; as well as harvesting techniques [20]. For the
dyer, the elements that pose the greatest threat are alkaline earth and heavy metal
contaminants such as calcium, magnesium, manganese, and iron. Depending on its
origin, raw cotton can exhibit widely different contents of alkaline earth and heavy
metal ions. Table 2 gives an example of the metal content of cotton having different
origins [21].
Table 1 Typical Composition of Raw Cotton
Component Proportion (%)
Cellulose 88.0–96.0
Pectins 0.7–1.2
Wax 0.4–1.0
Proteins 1.1–1.9
Ash 0.7–1.6
Other organic compounds 0.5–1.0
Table 2 Metal Content of Cotton of Different Origins
Origin of Cotton
Metal Content (mg/kg)
Ca Mg Fe Cu Mn
Brazil Assai Piranha 3147 1156 680 6 30
Brazil Sao Paulo 845 555 46 6 11
Peru 700 440 13 < 1 < 1
USA Texas 810 365 75 < 1 < 1
USA California 600 540 40 < 1 < 1
Egypt Makko 640 452 11 < 1 < 1
Levels of fats, oils and waxes present in cotton can be reduced to acceptable limits by
the action of alkali and surface-active products. In extreme cases, the use of solvent
and surface active mixtures may be necessary [22]. Pectins and the related substances
can be rendered soluble by the action of alkali, usually caustic soda, which also acts
as a swelling agent. Amino acids are also rendered soluble in the presence of alkali
by producing the corresponding sodium salts. Metals, however, cannot be adequately
removed by conventional alkaline processes since, in an alkaline medium, sequestering
agents cannot quantitatively separate the minerals of a complex structure containing
heavy metals. Moreover, in the alkaline pH region, cellulose swells rapidly and
strongly, thus impairing the transport of crystalline minerals from the core to the
periphery of the fibre. Demineralisation with organic or inorganic acid is more effective
as compared to the alkaline treatment process. However, regardless of the efficacy of
an acid treatment, the use of organic or inorganic acids for the demineralisation of
cellulosic fibres involves a number of disadvantages such as corrosion of machine
parts, difficulties in handling, and risk of fibre damage with strong inorganic acids,
while organic acids give lower demineralisation and are more volatile.
Speciality products based upon strongly acidic sequestering agents or a mixture of
sequestering agents with organic buffer systems are recently being used for
demineralisation of cotton. These products offer numerous advantages over conventional

acids such as hydrochloric acid or sulphuric acid. Some of the advantages are given
as follows:
• No corrosion
• No steam volatility
• No unpleasant odour
• Prevention of dissolved metal ions from re-precipitating
• Synergy with surfactants, improving the washing effect, dispersion power and
soil suspension capacity
• Lower ash content
• Improved degree of whiteness
• No fibre damage
However, with such an intensive demineralisation treatment, care must be taken that
magnesium ions are added in subsequent peroxide bleaches, in order to avoid fibre
damage in the bleach owing to insufficient stabilisation of hydrogen peroxide [23].
2.4 Effect of Cotton Colour Grade on the Colour Yield of Dyed Goods
The difference in the colour yield of cotton of different original colour grades, when
dyed after scouring and bleaching, is so small as to be explicable by experimental
variation [24].
A summary of dyeing problems originating from cotton fibre is given in Appendix
A.
3. PROBLEMS ORIGINATING IN YARN FORMATION
As much as 25 percent of the faults responsible for downgrading cotton finished
garments may be attributed to yarn [25]. The key yarn parameters are as follows:
• Yarn count
• Twist per inch
• Twist direction
• Strength
• Type (open-end or ring-spun, combed or carded)
• Elongation at break
• Moisture content
• Hairiness/pilling characteristics
• Uniformity/variation
• Impurities/foreign matter
• Composition
• Single or ply
• Colour/shade
• Dyeability
• ‘Classimat’ majors [26]
Some common types of faults present in yarn are as follows:
• Neps
• Long thick places
• Short thick places

• Thin places
• Weak places
• Count variation
• Hairiness
• Dyeability variations [27–30]
The main causes of the dyeability variations in yarn are:
• Immature fibres
• Dead fibres
• Vegetable matter or other foreign matter
• Wrong twist
• Bad splice
• Neps
• Count variations
4. PROBLEMS ORIGINATING IN YARN WINDING FOR
PACKAGE DYEING
The success of package dyeing, in terms of both levelness and yarn quality, is greatly
influenced by the degree of care taken in the preparation of the yarn packages [31].
It is often said that ‘Well wound is half dyed’ [32]. The standard of winding affects
the quality of dyed yarn to a great extent. A well wound package not only increases
the chances of level dyeing but it also minimises the risk of many other dyeing
problems [33].
The most important winding parameters are as follows:
• Winding system or type of winding
• Winding angle or package traverse
• The dye tube
• Winding ratio, i.e. the ratio of the inside tube diameter to the outside package
diameter [34, 35]
• Package density [36–38]
• Package type or concentricity
There are three types of winding in common use: wild or random winding; precision
winding; and digital step winding. A comparison of the three different types is given
in Table 3. The winding angle or package traverse depends upon the type of winding
Table 3 Comparison of Different Winding Systems
Wild Random Winding Precision Cross Winding Digital or Step Winders
Stable package Fragile package—must be Stable package
handled with care
Constant winding density Density varies from Uniform homogeneous density
inside to out
Areas of ribboning are No ribboning No ribboning
possible
Liquor flow characteristics Good liquor flow Good liquor flow characteristics
are not optimum characteristics

system used. The winding angle remains the same in random winding. In precision
winding there is a decreasing winding angle, and in digital step winding each layer
has a slightly different angle from the previous one.
An important consideration in any package dyeing operation is the type of carrier
on which the yarn package is wound. A wide range of designs and materials has been
used as support media (dye tubes) for packages. Rockets, cones, springs, plastic
tubes and non-woven fabric centres have all found favour in certain regards. Each
system has its advantages and disadvantages. Ultimately, the decision lies with the
individual users based on the particular requirements of their businesses and the
circumstances in use [39].
The use of large diameter tubes is said to offer improved quality at no reduction
in productivity. Since the larger tube can hold an equivalent amount of yarn with less
yarn thickness, lower flow and reduced pressure create less yarn disturbance and
deliver a high quality product [40, 41].
Winding density is one of the most important package characteristics that affect
the quality of the dyed package [42–46]. Package density highly influences the flow
of dye liquor through the package and the exchange between dye liquor and the yarn.
As a result, density significantly affects the depth of shade and levelness of dyed
yarn. Uniform package density is essential to producing a perfect dyeing. Fluctuations
in winding density of ± 3% are regarded as very low, whereas differences of ± 5% to
8% are considered to be within the normal range [47]. If the package is too soft,
channelling of the dye liquor will result and ballooning may occur. Soft packages
also tend to have excessive yarn shifts when the dye liquor is forced through the
package, making subsequent operations, such as back-winding, more difficult because
the yarn tangles. If the package is too hard or dense, liquor circulation will be
restricted through the package and cause un-dyed spots where yarns cross over one
another. Higher winding densities within the area adjacent to the dyeing tube may
inhibit uniform dyeing conditions in all sectors of the yarn bobbin [48]. The higher
the compactness of the package, the lower is the liquor throughput [49]. The ideal
package is of uniform density throughout. It should be of sufficiently open construction
to permit dye liquor to flow freely, yet dense enough to prevent channelling of the
liquor through more accessible places.
In addition to levelness, package density also affects the shade depth. The inner
zone density influences the shade depth the most, and the outer zone the least.
Increasing the inner zone density decreases shade depth in all areas of the package.
Increasing the middle zone density increases shade depth in both the inner and the
middle zone, but decreases the outer zone shade depth. Increasing the outer zone
density increases the outer zone shade depth and decreases the inner zone shade
depth. Package density affects the inner zone shade depth the most and the outer zone
shade depth the least. To ensure the shade levelness among packages, the same
density profile should be used for all the packages. The influence of density profiles
on the levelness and the shade depth is eventually due to their effect on liquor flow
between and through the yarns. This indicates that the control of the dye liquor flow
is the most important factor in the success of package dyeing. The factors affecting
the density of the package, when surface winding, are different from those that
govern it in precision winding. The yarn supply and its position, speed of winding,

winding tension, and the pressure of the package on the winding drum all play an
important role in the build-up of the package, and various devices are available for
adjusting their effects in order to increase the possibility of producing packages that
are regular and even in density [50].
The shape of the package also has some influence on the pattern of the liquor flow.
Cheese-shaped packages of regular construction are shown to be ideally suited to
uniform liquor flow. Cones have certain disadvantages as compared to cylindrical
cheeses [51]. Parallel-sided packages are preferred on technical grounds, particularly
with regard to levelness [52]. In the case of cones, it has been found that at the centre
of the package the density is greater and more irregular than in the outer layers. In
contrast, the distribution of pressure in cheeses is more uniform. As the liquor flows
through the cones, an impact pressure builds up in the interior of the package, causing
the ends of the cones to bulge. The result is that the liquor cannot penetrate these
areas properly. Moreover, residual dyestuff is deposited in the area around the spacers,
as is sand and other suspended matter.
According to the maximum flow rate that can be achieved during the dyeing
process, there are three types of yarn package properties [53]: dyeable at low flow
rate, dyeable at medium flow rate and dyeable at high flow rate. Each type of package
has a particular flow-rate limit, above which it is not possible to work without
causing deformation, water channels and consequently all the associated defects.
Other factors that contribute to proper winding are as follows:
• Supply package quality
• Yarn delivery
• Tensioning device
• Winding speed
• Soft edges
• Package build
• Package holder pressure control
• Number of packages per spindle
A summary of problems caused by poor package winding is given in Appendix B.
5. PROBLEMS ORIGINATING IN FABRIC FORMATION
Woven fabrics are produced by interlacing a group of warp and weft threads. Defects
in woven fabrics can be broadly grouped as yarn defects and process defects. Process
defects originate from the processes involved. Based on the processes, the defects in
the woven fabrics may be attributable to spinning, winding, warping, sizing, drawing-
in, pirn winding, loom-setting and handling [54]. The identification [55], definitions
[56], and images of defects [57] in woven fabrics and methods for their numerical
designation [58] are given in the respective references. Major problems that become
more apparent after dyeing but may be attributable to weaving include:
• Variation in the warp density of the cloth (wrong draw, missing end, double end)
• Selvedges thicker than the centre of the fabric
• Variation in size application on warp yarns
• Variation in drying of warp yarn after sizing
• Variation in warp tension during weaving

• Variation in weft density (missing pick, double pick)
• Variation in warp or weft yarns with respect to twist, twist direction, count,
hairiness, colour, tensile properties, fibre composition and/or spinning batch
• Fly or foreign matter or fibre woven into the fabric
Knitting is a process of making cloth with a single yarn or set of yarns moving in only
one direction, instead of two sets of yarns crossing each other, as in weaving. There
are two basic categories of knitting: Warp knitting and weft knitting. Warp knitting
works with multiple yarns running vertically and parallel to each other. The fabric is
constructed by manipulating these warp yarns simultaneously into loops which are
interconnected, e.g. Tricot, Raschel, Milanese, etc. Weft knitting works with one yarn
at a time running in a horizontal direction. The fabric is constructed by manipulating
the needle to form loops in horizontal courses built on top of each other, e.g. Circular,
Flat, Hosiery, etc. The largest proportion of knitted fabrics used today is weft knits
[10]. The faults in knitted fabrics can be categorized into those caused by yarn, those
in the course or length direction and those due to, or apparently due to dyeing [59,
60]. Major problems that become more apparent after dyeing but may be attributable
to knitting include [61–65]:
• Variation in course length (a ‘course’ is a row of loops across the width of a
knitted fabric)
• Variation in yarn with respect to count, twist, twist direction, hairiness, colour,
tensile properties, fibre composition, lubrication and/or spinning batch
• Variation in wale density (a ‘wale’ is a column of loops along the length of a
knitted fabric; ‘wale density’ is the number of loops per unit length measured
along a course)
• Vertical lines of distorted loops, of tuck stitches, or of cut stitches
• Fly or foreign matter knitted into the fabric
6. PROBLEMS CAUSED BY POOR WATER QUALITY
The use of water in textile dyeing and finishing is ubiquitous, and the role of water
in such processes is manifold [66]. Although it is difficult to state definitive water
demand for various processes, the raw material used in the greatest quantity in
virtually every stage of textile wet processing is water [67]. The quality of textiles
produced by any manufacturing operation which employs wet processes, such as
preparation, dyeing and finishing, is profoundly affected by the water quality [68].
Various textile processes are influenced in different ways by the presence of impurities
in the water supply and there are several major water use categories to be considered
including water for processing, potable purposes, utilities, and laboratory use. Each
requires different water-quality parameters. Process water (for preparation, dyeing,
and finishing) is to be mainly used for making concentrated bulk chemical stock
solutions, substrate treatment solutions, and washing. Potable water is for drinking
and food preparation. Utility use includes non-contact uses such as boiler use, equipment
cleaning etc.
Water from almost all supply sources contains impurities to some extent. The type
and amount of impurities depend upon the type of water source. The most common
impurities that may be present in water are as follows:

• Calcium and magnesium (hardness)
• Heavy metals, such as iron, copper, and manganese
• Aluminium
• Chlorine
• Miscellaneous anions (sulphide, fluoride, etc.)
• Sediments, clay, suspended matter
• Acidity, alkalinity, and buffers
• Oil and grease
• Dissolved solids
Contaminants from the water source are not the only ones found in textile water
supplies. There are major internal contributions, too. Common sources of internal
contamination are as follows:
• Clear well (used for water storage)
• Greige goods or other substrate
• Plumbing, valves, etc.
• Machinery
• Prior processes in the case of water reuse
There are many quick qualitative tests for detection of trace quantities of ions and
elements in water. There are also quantitative tests for determining the exact
concentration of cations such as calcium, magnesium, iron, copper, and manganese
in water. A description of quick spot tests for commonly occurring contaminants is
given by Smith and Rucker [68]. Analytical methods for water testing are given by
Thompson [69].
Water contaminants, especially metals, can have a substantial effect on many
textile wet processes. The effects are not always adverse but even when a process is
enhanced by water impurities, it is not desirable to have variance in processes and
product quality due to water quality changes. Such variations in the quality of water
make process and machinery optimisation and control difficult [70].
6.1 Problems in the Textile Laboratory
It is a common practice in some mills to use potable water for the laboratory supply
while using non-potable water for production processing. Since potable water is
usually chlorinated, it can alter the shade of dyeings and contributes to poor lab-to-
bulk reproducibility. Moreover, most work in analytical laboratories is done with
distilled and/or deionized water. However, many situations arising in textile wet
processing laboratories will require the use of process water in order to correlate well
with production. The laboratory technician must be able to realize when to use
process water and when to use distilled or deionized water.
6.2 Problems in Preparation Processes
Metallic ions in water can have a dramatic effect by either enhancing or inhibiting the
action of many preparation processes.All of the wet preparation processes are affected
in some way by metallic ion contaminants in water.
In enzymatic desizing, the metallic ions may cause inactivation of the enzymes,
resulting in poor size removal.

In scouring processes, calcium and magnesium ions (water hardness) cause the
most problems. These ions will precipitate soaps, forming a sticky insoluble substance
which deposits on the substrate. Such deposits impair the fabric handle, cause resist
in dyeing, attract soil to the material and cause inconsistent absorbency in subsequent
processes.Although most synthetic detergents used in scouring today do not precipitate
in the presence of calcium and magnesium ions, the fatty acid hydrolysis products
formed by the saponification of natural waxes, fats, and oils in the fibres will precipitate.
The formation of complexes with alkaline and alkaline earth salts drastically reduces
the solubility and the rate of dissolution of surfactants, thus impairing the wash
removal ability of the surfactants [71]. It is, therefore, imperative to use soft water in
the scouring process.
Bleaching with hydrogen peroxide is greatly affected, even by trace quantities of
metal ions in the water. The transition metal ions such as iron, copper, manganese,
zinc, nickel, cobalt and chromium catalyze decomposition of hydrogen peroxide
[72]. The decomposition is so rapid that it frequently occurs before any significant
bleaching can occur. In addition, the decomposition products attack cotton fibres
leading to their degradation. Bleaching baths containing these ions will therefore
lead to reduction in whiteness and high loss in fibre strength, as well as an increase
in fluidity. The alkaline earth metal (magnesium), on the other hand, produces beneficial
effects when present in peroxide bleaching solutions. These ions increase the stability
of hydrogen peroxide under alkaline bleaching conditions, and as a result increased
whiteness and less fibre degradation is obtained. Electrolytes of other metals may
have a harmful effect [73].
6.3 Problems in Dyeing Processes
The most commonly observed dyeing problems caused by poor water quality include
inconsistent shade, blotchy dyeing, filtering, spots, resists, poor washing off, and
poor fastness [74]. Inconsistent shade can be caused by chlorine contamination of the
process water or iron, copper and other metals. The action of copper on the dyestuff
can be prevented by a suitable complexing agent but not the action of iron. For iron,
purification of water prior to dyeing is recommended. Chelating agents are frequently
used in an attempt to eliminate the undesirable effect of these metals in process water,
but in many cases, the chelate itself may cause unpredictable effects such as shade
changes. The best strategy is to remove the metal from water before using it in
processing.
The presence of calcium and magnesium ions in the process water can cause
inconsistent and uneven washing-off of unfixed dyes, leading to blotches, and/or
inconsistent shade. Hexametaphosphates are effective sequestering agents for removing
these ions and are generally safe in the sense that they do not cause other undesirable
effects such as shade variations.
Blotchy dyeing can result from acidity or alkalinity in the water, depending upon
the application class of dyes. Even when the pH is neutral, water (and substrate) may
contain substantial alkalinity. This can have effects on exhaustion, levelling and
fixation of dyes. Similar types of defects can result from the residual chemicals,
especially alum (aluminium) in water.
Filtering in package dyeing, resists and spots can result from sediments, alum or

other residual flocking agents left over from water treatment, from organic contaminants,
from metal hydroxides (copper and iron), or from fatty acid/hardness metal complexes.
Generally, the stiffness of textile material dried after rinsing is greater, the higher the
solids content of the rinsing water [75].
In order to avoid the problems outlined above, water for textile processing has to
meet fairly stringent demands [76, 77]. The main requirements are as follows:
• Freedom from suspended solids and from substances that can give staining in
processing
• No great excess of acid or alkali
• Freedom from substances affecting the textile processes, such as iron, manganese,
Calcium or magnesium salts, and heavy metals
• Non-corrosiveness to tanks and pipelines, and
• Freedom from substances that give rise to foaming or unpleasant odour
Table 4 gives a summary of the requirements that the processing water has to meet
[32].
Table 4 Dyehouse Water Standard
Characteristic Permissible Limit
Colour Colourless
Smell Odourless
pH value Neutral pH 7–8
Water hardness < 5 °dH (6.25°eH; 8.95°fH; 5.2 USA)
Dissolved solids < 1 mg/l
Solid deposits < 50 mg/l
Organic substances < 20 mg/l (KMnO4 consumption)
Inorganic salts < 500 mg/l
Iron (Fe) < 0.1 mg/l
Manganese (Mn) < 0.02 mg/l
Copper (Cu) < 0.005 mg/l
Nitrate ( NO3
1–
) < 50 mg/l
Nitrite ( NO2
1–
) < 5 mg/l
Table 5 gives the limits of impurities acceptable in water for steam boilers.
Table 5 Steam Boiler Feed Water Standard
Characteristic Acceptable Limit
Appearance Clear, without residues
Residual hardness < 0.05 °dH
Oxygen < 0.02 mg/l
Temporary CO2 0 mg/l
Permanent CO2 < 25 mg/l
Iron < 0.05 mg/l
Copper < 0.01 mg/l
pH (at 25 °C) > 9
Conductivity (at 25 °C) < 2500 µS/cm
Phosphate (PO4) 4–5 mg/l
Boiler feed water temperature > 90 °C

Various measures and treatments may be employed in order to remove impurities
from water and to avoid problems in textile processing [76, 78], such as follows:
• Sedimentation and filtration treatments
• Softening treatments [such as cold lime-soda-softening or Zeolite softening]
• Reverse osmosis [79]
• The use of sequestering agents [80–83]
A summary of problems caused by poor water quality is given in Appendix C.
7. PROBLEMS IN SINGEING
Textiles are singed in order to improve their surface appearance and wearing properties
[84]. The burning-off of protruding fibre-ends which are not firmly bound in the
yarn, results in a clean surface which allows the structure of the fabric to be clearly
seen. Unsinged fabrics soil more easily than singed fabrics. The risk of cloudy dyeings
(a defect consisting of random, faintly defined uneven dyeing) with singed piece-
dyed articles in dark shades is considerably reduced, as randomly protruding fibres
cause a diffused reflection of light. Although cotton textile materials can be singed in
yarn [85], and knitted [86–88] as well as woven forms [84], singeing of woven
fabrics is much more common as compared to other forms. Two main methods of
singeing are direct flame singeing and indirect flame singeing [89].
There are singeing faults that are optically demonstrable and are quite easily
remedied during the actual working process. On the other hand there are singeing
faults that are not visible until after dyeing and that can no longer be repaired once
they have occurred.
A summary of problems in the singeing of woven fabrics is given in Appendix D.
8. PROBLEMS IN DESIZING
Sizing has been considered as an ‘invention of the devil’ by some dyers and finishers
because it is the main source of many processing problems [90, 91]. Warp yarns are
coated with sizing agents prior to weaving in order to reduce their frictional properties,
decrease yarn breakages on the loom and improve weaving productivity by increasing
weft insertion speeds. The sizing agents are macromolecular, film-forming and fibre
bonding substances, which can be divided into two main types [92]: natural sizing
agents which include native and degraded starch and starch derivatives, cellulose
derivatives and protein sizes; and synthetic sizes which include polyvinyl alcohols,
polyacrylates and styrene–maleic acid copolymers. Starch-based sizing agents are
most commonly used for cotton yarns because of being economical and capable of
giving satisfactory weaving performance. Other products are also used, either alone
or in combination with starch sizes, when the higher cost can be off-set by improved
weaving efficiency. Some auxiliaries are also used in sizing for various functions and
include softening agents, lubricating agents, wetting agents, moistening agents, size
degrading agents, and fungicides. The desizing procedure depends on the type of
size. It is therefore necessary to know what type of size is on the fabric before
desizing. This can easily be determined by appropriate spot tests [93].
The sizing material present on warp yarns can act as a resist towards dyes and
chemicals in textile wet processing. It must therefore be removed before any subsequent

wet processing of the fabric. The factors on which the efficiency of size removal
depends are as follows:
• Viscosity of the size in solution
• Ease of dissolution of the size film on the yarn
• Amount of size applied
• Nature and the amount of the plasticizers
• Fabric construction
• Method of desizing
• Method of washing-off
Different methods of desizing are [94, 95]:
• Enzymatic desizing
• Oxidative desizing
• Acid steeping
• Rot steeping (use of bacteria)
• Desizing with hot caustic soda treatment
• Hot washing with detergents
The most commonly used methods for cotton are enzymatic desizing [96–98] and
oxidative desizing [99–101]. Acid steeping is a risky process and may result in the
degradation of cotton cellulose while rot steeping, hot caustic soda treatment and hot
washing with detergents are less efficient for the removal of the starch sizes.
Enzymatic desizing consists of three main steps: application of the enzyme, digestion
of the starch and removal of the digestion products. The common components of an
enzymatic desizing bath are as follows:
• Amylase enzyme
• pH stabiliser
• Chelating agent
• Salt
• Surfactant
• Optical brightener
The enzymes are only active within a specific range of pH, which must be maintained
by a suitable pH stabiliser. Chelating agents used to sequester calcium or combine
heavy metals may be injurious to the enzymes and must be tested before use. Certain
salts may be used to enhance the temperature stability of enzymes. Surfactants may
be used to improve the wettability of the fabric and improve the size removal. Generally,
non-ionic surfactants are suitable but it is always recommended to test the compatibility
of surfactants before use. Some brighteners may also be incorporated in the desizing
bath which may be carried through the end of the pre-treatment, resulting in improved
brightness but again, their compatibility must be ascertained before use. Enzymatic
desizing offers the following advantages [102]:
• No damage to the fibre
• No usage of aggressive chemicals
• Wide variety of application processes
• High biodegradability

Some disadvantages of enzymatic desizing include lower additional cleaning effect
towards other impurities, no effect on certain starches (e.g. tapioca starch) and possible
loss of effectiveness through enzyme poisons.
Oxidative desizing [103] can be effected by hydrogen peroxide [104, 105], chlorites,
hypochlorites, bromites, perborates or persulphates. Two important oxidative desizing
processes are [106]: the cold pad-batch process based on hydrogen peroxide with or
without the addition of persulphate; and the oxidative pad-steam alkaline cracking
process with hydrogen peroxide or persulphate. The advantages offered by oxidative
desizing are supplementary cleaning effect, effectiveness for tapioca starches and no
loss in effectiveness due to enzyme poisons. Some disadvantages include the possibility
of fibre attack, use of aggressive chemicals and less variety of application methods.
After desizing, the fabric should be systematically analyzed to determine the
uniformity and thoroughness of the treatment. It is first weighed to determine the
percent size removed. The results are compared with a sample known to have been
desized well in the lab. If the size is not adequately removed then either the treatment
or washing have not been thorough. Iodine spot tests are then conducted on the fabric
[107–109]. The fabric is not spotted randomly but from side-centre-side at different
points along the length of the fabric. The results of this evaluation give some idea of
the causes of any inadequate treatment.
Some of the most common problems in enzymatic desizing and their possible
causes are given in Appendix E.
9. PROBLEMS IN SCOURING
Various aspects of cotton fabric preparation have been presented by Rosch [110–118]
and Sebb [119–124]. An important, if not the most important, operation in the pre-
treatment of cotton is the scouring or alkaline boil-off process. The purpose of alkaline
boil-off and the ensuing washing stage is to perform extensive fibre-cleaning by
ensuring a high degree of extraction of pectins, lignins, waxes and grease, proteins,
alkaline earth metals (Ca and Mg), heavy metals (iron, manganese and copper), low
molecular weight cellulose fragments, dirt and dust; and softening of husks. The
result is an increased responsiveness of cotton to subsequent processing [125]. The
process removes water insoluble materials such as oils, fats, and waxes from the
textile material. These impurities coat fibres and inhibit rapid wetting, absorbency
and absorption of dyes and chemical solutions. Oils and fats are removed by
saponification with hot sodium hydroxide solution. The process breaks the compounds
down into water-soluble glycerols and soaps. Unsaponifiable material such as waxes
and dirt are removed by emulsification. This requires the use of surfactants to disperse
the water-insoluble material into fine droplets or particles in the aqueous medium.
Both of these processes (saponification and emulsification) take place in a typical
scouring process. In addition, the scouring process softens and swells the motes to
facilitate their destruction during bleaching. Depending on the amount of impurities and
the reaction and wash conditions, the loss in weight of the raw cotton material due to
boil-off can reach up to seven percent or even higher in case of high-impurity cotton.
The important parameters of the scouring process are as follows:
• Concentration of caustic soda
• Type and concentration of auxiliaries

• Treatment temperature
• Reaction time
The higher the caustic soda concentration, the shorter can be the dwell time. In other
words, the shorter the dwell time, the higher the concentration required. The caustic
soda concentration normally employed neither affects the ash content nor the average
degree of polymerisation [DP] of cotton. Too high a concentration (e.g. > 8% o.w.f)
may result in a reduction in DP as well as yellowing of the cotton fibre. The higher
the concentration, the greater will be the fat removal. Due to the high degree of fat
removal, the absorbency will also increase but there may be harshness in the handle
of the material.
Two important auxiliaries used in scouring are chelating agents and surfactants.
Other auxiliaries that may sometimes be employed include antifoaming and anti-
creasing agents. Chelating agents are used to eliminate water hardness and heavy
metals, such as iron and copper which can affect the scouring process. These agents
bind polyvalent cations such as calcium and magnesium in water and in fibres, thus
preventing the precipitation of soaps. If polyvalent ions are present, insoluble soaps
may form, settle on the fabric and produce resist spots. There are four major types of
sequestering agents to choose from: inorganic polyphosphates, aminocarboxylic acids,
organophosphonic acids, and hydroxycarboxylic acids. The inorganic polyphosphates
such as sodium tripolyphosphate and sodium hexametaphosphate are probably the
best overall in that in addition to sequestering most metals they also aid in cleansing
the fibres. They may, however, hydrolyze at high temperature and loose their
effectiveness.
The aminocarboxylic acid types such as ethylenediaminetetraacetic acid (EDTA)
are very good in that they sequester most metal ions and are very stable under
alkaline conditions. They are the most used types. The organophosphonic acid types
such as ethylenediaminetetra (methylene phosphonic acid) are also very effective but
comparatively expensive. Oxalates and hydroxycarboxylic acids (citrates, etc.) are
excellent for sequestering iron but not effective for calcium and magnesium.
In order to quickly and effectively bring the chemicals to the textile material, i.e.
to improve their wettability and to ensure that the fibrous impurities will be removed
as far as possible, it is necessary to add surfactants with good wetting and washing/
emulsifying properties. A surfactant of optimal versatility to be used for preparation,
and in particular for the scouring and bleaching processes, ought to meet the following
requirements:
• It should have an excellent wetting ability within a wide temperature range
• It should permit a good washing effect and have a high emulsifying power for
natural fats, waxes and oils
• It should be resistant to oxidants and reducing agents
• It should be resistant to water-hardening substances
• It should be highly stable to alkalinity
• It should be biodegradable and non-toxic
Care should be taken in selecting the surfactants because of the inverse effect of
temperature on the solubility of non-ionic surfactants. If the process temperature is
above the cloud point of the surfactant, the surfactant may be ineffective and may

actually be deposited on the substrate. The surfactant used should have a cloud point
temperature just above the operating temperature, to be most effective. The cloud
point of non-ionic surfactants decreases in the presence of alkalis and electrolytes
and the degree to which it is lowered increases with concentration. The cloud point
should therefore be checked under application conditions to ensure that the surfactant
is effective under those conditions. The adverse effect of temperature on non-ionic
surfactants can be reduced by the addition of an anionic surfactant. Crypto-non-ionic
surfactants do not exhibit a cloud point. These are non-ionic surfactants that are
capped with an ionic group and they exhibit the excellent emulsifying properties of
non-ionics along with the good solubility properties of anionics.
Higher scouring temperatures will reduce treatment times and vice versa. At high
temperature, however, there will be complete removal of fats and waxes, which will
promote harsh handle of the material. Moreover, the cloud point of the surfactant also
has to be taken into account while applying high temperature.
In the case of pad-steam scouring, a typical process consists of the following
steps: Saturating the fabric with a solution of sodium hydroxide, surfactant and
sequestering agent; steaming; and thorough washing. After scouring, the material is
checked for thoroughness and uniformity of scouring as well as other scouring faults.
Appendix F gives most common problems in scouring, their possible causes, and
countermeasures.
10. PROBLEMS IN BLEACHING
Cotton, like all natural fibres, has some natural colouring matter, which confers a
yellowish brown colour to the fibre. The purpose of bleaching is to remove this
colouring material and to confer a white appearance to the fibre. In addition to an
increase in whiteness, bleaching results in an increase in absorbency, levelness of
pre-treatment, and complete removal of seed husks and trash [126]. In the case of the
production of full white finished materials, the degree of whiteness is the main
requirement of bleaching. The amount of residual soil is also taken into consideration
because of the possibility of later yellowing of the material. In the case of pre-
treatment for dyeing, the degree of whiteness is not as important as, for example, the
cleanliness of the material, especially the metal content. Similar demands refer to the
production of medical articles. In this case, too, the metal content as well as the ash
content are important factors [127].
If whiteness is of primary importance, it requires a relatively large amount of
bleaching agent as well as a high operating temperature and a long dwell time.
Accurate regulation of the bleaching bath is a further obligatory requirement. Where
the destruction of trash, removal of seed husks and an increase in absorbency is a
prime necessity (e.g. for dyed goods), a high degree of alkalinity is all important. It
is, however, not the alkali alone that is responsible for these effects. The levelness of
pre-treatment can only be guaranteed if cotton of the same or equal origin is processed
in each bath. If this is not the case, suitable pre-treatment will have to be undertaken
to obtain, as closely as possible, the required uniformity. A pre-treatment with acid
and/or a chelating agent will even out (better still eliminate) varying quantities of
catalytic metallic compounds.
Although there are different bleaching agents that can be used for bleaching cotton,

hydrogen peroxide is, by far, the most commonly used bleaching agent today [128].
It is used to bleach at least 90% of all cotton and cotton blends, because of its
advantages over other bleaching agents. The nature of the cotton colour, its mechanism
of removal with hydrogen peroxide [129] and the basic rules for formulation of
bleaching liquors have been presented in detail elsewhere [120]. The mere formulation
of the correct initial bath concentration is not sufficient to ensure a controlled bleaching
process. Of equal importance are regular checks of the bath composition during the
operation. Such checks do not only contribute to an economic bleaching operation
but also allow an early tracing of the defects and failures of the system [122]. The
important parameters for bleaching with hydrogen peroxide are as follows:
• Concentration of hydrogen peroxide
• Concentration of alkali
• pH
• Temperature
• Time
• Nature and quality of the goods
• Water hardness and other impurities
• Types and concentration of auxiliaries
• Desired bleaching effect
• Available equipment, and stabilizer system employed [130, 131]
Most of these factors are inter-related, and all have a direct bearing on the production
rate, the cost and the bleaching quality. Though they operate collectively, it is better
to review them individually for the sake of clarity.
There are two concentrations to be considered: that based on the weight of the
goods and that based on the weight of the solution. All other factors being equal, the
concentration on the weight of the goods determines the final degree of whiteness. In
order to get adequate bleach there must be enough peroxide present from the start. On
the other hand, the peroxide concentration based on the weight of the solution will
determine the bleaching rate — the greater the solution concentration, the faster the
bleaching. No peroxide bleaching system ever uses up its entire peroxide charge for
active bleaching, as some is always ‘lost’ during normal process.
The alkalinity in the system is primarily responsible for producing the desired
scour properties and maintaining a reasonably constant pH at the desired level throughout
the bleaching cycle. The quantity of the alkali to be added depends above all on the
character of the goods, the finish required and the kind and quality of the other
ingredients in the liquor. The alkalinity is defined as the ‘amount’ of alkali in the
system and should be distinguished from the pH, which is a measure of the hydrogen
ion concentration in the solution. The pH value in peroxide bleaching is of extreme
importance because it influences bleaching effectiveness, fibre degradation and peroxide
stability in bleaching cotton fibres, as shown in Table 6.
With increasing pH, whiteness index increases to a maximum at a pH of 11.0 and
then decreases. Fibre degradation is at minimum at a pH of 9.0 but that which occurs
at a pH of 10.0 is well within acceptable values. Above a pH of 11.0, fibre degradation
is unacceptably severe. A pH range of 10.2–10.7 is considered optimum for bleaching
cotton with hydrogen peroxide. Lower pH values can lead to decreasing solubility of

sodium silicate stabiliser (see below) as well as lower whiteness due to less activation
of the peroxide [132].
By increasing the temperature, the degree of whiteness as well as its uniformity
increases. However, at too high a temperature, there is a possibility of a decrease in
the degree of polymerisation of the cotton. Moreover, due to good fat removal at high
temperatures such as 110 °C, the handle of the material can become harsh and the
sewability of woven cotton fabrics may also decrease. Time, temperature and
concentration of peroxide are all inter-related factors. At lower temperatures, longer
times and higher concentrations are required.As the temperature of bleaching increases,
shorter times and lower peroxide concentrations can be employed.
The amount of peroxide decomposed is greatly reduced with increasing weight of
cotton fibre in the bleach liquor. The raw fibre almost completely suppresses
decomposition, while the scoured fibre is somewhat less effective. The demineralised
fibre is the least effective stabiliser [133]. While impurities such as magnesium and
calcium may have a good stabilising effect when present in appropriate amounts,
other impurities such as iron, copper and manganese can have very harmful effect,
resulting in catalytic decomposition of hydrogen peroxide leading to fibre damage [134].
A good stabilising system is indispensable in bleaching cotton with hydrogen
peroxide. While sodium silicate is one of the most commonly used stabilisers, its use
may result in a harsh handle of the fabric as well as resist spots leading to spotty
dyeing. The best alternatives to sodium silicate are organic stabilisers or a combination
of silicate and organic stabilisers.
In addition to the most important ingredients of the bleaching recipe, namely
hydrogen peroxide, caustic soda and the stabilizer, auxiliaries are used sometimes to
aid the bleaching process. These may include surfactants and chelating agents. The
type and concentration of these auxiliaries also plays an important role in the bleach
effect obtained. The desired bleaching effect does not need necessarily be optimal
white. For goods-to-be-dyed, the main concern will normally be achieving good and
uniform absorbency.
The available equipment plays a role in determining which process criteria must
be taken into account such as: cold, hot or HT bleaching; dry-wet or wet-on-wet
impregnation; discontinuous or continuous processing; process control.
The most common problems in bleaching cotton with hydrogen peroxide are as
follows:
• Inadequate mote removal
• Low degree of whiteness
Table 6 Effect of pH on Bleaching Effectiveness, Fibre Degradation, and Peroxide Stability in
Bleaching Cotton Fibres
Initial pH Final pH Whiteness CUEN % Peroxide
Index Fluidity Remaining
8.0 4.4 66.8 5.48 72.5
9.0 8.7 67.3 1.44 71.6
10.1 9.9 71.3 2.44 63.3
11.0 11.7 72.2 7.29 7.0
12.0 12.4 69.5 17.8 2.0

• Uneven whiteness (or bleaching)
• Pinholes, tears, broken yarns, catalytic damage, loss in strength [135, 136]
• Resist marks
• Formation of oxycellulose
A summary of the possible causes of these problems and their countermeasures is
given in Appendix G.
It is not always possible to find the cause of these problems without detailed
analyses [72]. The most useful tests that can be carried out to check the effectiveness
of the bleaching process are for whiteness, absorbency and tensile strength. Checks
and measures are required also to assure level dyeing properties. After bleaching, for
example, the pH of the goods should be adjusted in the last rinse. Control of residual
moisture content (e.g. 7% for cotton) is part of the standard pre-treatment, which
should be uniform throughout the material [126].
11. PROBLEMS IN MERCERIZATION
Mercerization is the treatment of cotton with a strong sodium hydroxide solution.
This process improves many properties of cotton fibres and may actually reduce or
eliminate some dyeing problems. Some of the properties of cotton fibres that are
improved by this process include [137, 138]:
• Increase in dye affinity
• Increase in chemical reactivity
• Increase in dimensional stability
• Increase in tensile strength
• Increase in lustre
• Increase in fabric smoothness
• Improvement in the handle
• Improvement in the appearance
There are many possible variations in the mercerization process.A review of technical
researchandcommercialdevelopmentsinmercerisationhasbeengivenbyGreenwood
[139]. Mercerization of cotton can be carried out on raw fibre [140], yarn, and knitted
[141–147]orwovenfabric,andatanystageduringpreparation.Fabricmaybe mercerised
in greige form, after desizing, after scouring or after bleaching. The choice depends
upon the type of goods, the particular plant set-up, and the requirements of the final
mercerizedfabric.Fabricscanbemercerizedwithouttensiontoeffectmainlyanincrease
instrengthanddyeaffinity,orundertensiontoeffectmainlyanincreaseinthelustre [148].
The treatment may be wet-on-dry, wet-on-wet or add-on [149–151] at cold or hot tem-
peratures [152]. A comparison of cold and hot mercerization is given in Table 7 [153].
The most common of the various mercerization processes is that of treating the
fabric in the cold after bleaching with or without tension. The conventional method
of mercerization generally consists of the following steps:
• Padding the fabric through a strong sodium hydroxide solution
• Allowing time for the alkali to penetrate and swell the cotton fibres
• Framing to provide the tension required for lustre development
• Thorough rinsing to remove the alkali

The important mercerization parameters are as follows:
• Moisture content in the substrate for mercerization
• Concentration of caustic soda
• Penetration of caustic soda
• Temperature of caustic soda
• Wet pick-up
• Time of contact of the fabric with caustic soda
• Post-framing/tension on the material
• Washing/neutralization
If the fabric to be mercerized has a high moisture content, there may be a dilution of
the caustic soda concentration and the reaction between caustic and water generates
heat which may increase the bath temperature. The optimum concentration of sodium
hydroxide concentration is between 25 and 30% (48–54°Tw). Lower concentrations
will result in a lower degree of mercerization and less lustre. Higher concentrations
have no beneficial effect. A good wetting agent is necessary to improve penetration
of the caustic soda. The wetting agent must be stable and effective at the high alkaline
concentrations used [154], so only those wetting agents designed specifically for
mercerization should be used. The temperature of the bath can affect the degree of
mercerization. Swelling of the cotton and thus mercerization decreases with increasing
temperature [155]. The optimum temperature is 70–100 °F [21–38 °C]. Lower
temperatures do not affect the process adversely if the sodium hydroxide concentration
is in the proper range. At lower concentrations, the degree of mercerization increases
as the temperature decreases. Lower degrees of mercerization are obtained at
temperatures above l00 °F.
Wet pick-up in padding can affect mercerization in several ways. Less swelling
may occur at low wet pick-up, leading to incomplete mercerisation. The caustic
solution also plasticises the fabric so that it is easily stretched. At low wet pick-up
values, less plasticisation occurs and the fabric may tear during stretching on the
frame. Wet pick-up should be about 100%. The optimum time after padding is at least
30 seconds, to allow for the caustic to swell the cotton fibres before tension is applied
on the frame. Shorter times will result in incomplete mercerization.
As cotton fibres are swollen by the alkali, the fabric shrinks [156]. To obtain lustre
Table 7 Comparison of Conventional (Cold) and Hot Mercerization
Conventional Mercerization (10–20 °C) Hot Mercerization (70 °C)
Strong fibre swelling Less fibre swelling
Slower swelling Rapid swelling
Slower ‘relaxation’ Rapid ‘relaxation’
Incomplete ‘relaxation’ Good ‘relaxation’
Higher residual shrinkage Lower residual shrinkage
Surface swelling Complete swelling
Unevenness Evenness
Harder hand Softer hand
NaOH diffusion inhibited Uninhibited NaOH diffusion
Less lustre Optimised lustre

and shrinkage control, the fabric must be stretched on a frame. It should be stretched
in the width direction to its greige width or slightly more. No stretching in the length
direction is required unless extreme lustre is desired. If lengthways stretching is
needed, the frame speed should not exceed the padder speed by more than five
percent.
Removal of caustic soda from the fabric is very crucial for the development of
lustre and shrinkage control. The caustic soda solution concentration in the fabric
(not the rinse solution) should be reduced to less than 5% with the fabric still on the
frame. If not, low lustre and shrinkage of the fabric will occur. If the fabric shrinks
as it comes off the frame, the caustic concentration in the fabric has not been reduced
sufficiently. After the fabric comes off the frame, the remaining caustic should be
thoroughly rinsed out. It is difficult to remove residual amounts of caustic soda from
the fabric by rinsing alone, so they are usually neutralized with a dilute acid solution.
Care must be taken in using acetic acid for neutralization as some of the sodium
acetate formed may remain in the fabric and alter the pH in the subsequent wet
processes.
After mercerization, an analysis is carried out to determine the degree of
mercerization, which is specified by the Barium Number [157–160]. The Barium
Number obtained should be at least 130 and preferably 150. Low numbers result
from incomplete swelling of cotton fibres. A quick test for determination of the
degree of mercerization is to dye samples of the mercerized fabric along with a
sample known to be properly mercerized, using a direct dye such as C.I. Direct Blue
80. Any differences in the depth of the dyeings are indicative of different degrees of
mercerization.A red or blue dye should be used, since it is easier to observe differences
in depths of these colours visually. There is no standard test for analysis of the lustre
of mercerized fabric. It must be judged visually.
A summary of common problems in mercerization is given in Appendix H.
12. PROBLEMS IN DYEING WITH REACTIVE DYES
Reactive dyes are one of the most commonly used application class of dyes for cotton
materials, Two important aspects of reactive dyeing, namely dye variables and system
variables, are discussed in this section, along with important characteristics of
reactive dyeing such as exhaustion, migration and levelling, fixation and colour
yield, and washing-off and fastness. A significant portion of this section also deals
with the problem of the reproducibility and difficulties in obtaining right-first-time
dyeing.
12.1 Dye Variables in Reactive Dyeing
The major dye variables that affect reactive dyeing are dye chemistry, substantivity,
reactivity, diffusion coefficient and solubility. Each of these will be briefly discussed
below.
Dye chemistry: Reactive dyes have a wide variety in terms of their chemical structure
[161]. The two most important components of a reactive dye are the chromophore
and the reactive group.
The characteristics governed by the chromophore are colour gamut, light fastness,

chlorine/bleach fastness, solubility, affinity, and diffusion [162]. The chromophores
of most of the reactive dyes are azo, anthraquinone, or phthalocyanine [163]. Azo
dyes are dischargeable. Disazo dyes have the disadvantage of being much more
sensitive to reduction and many of them are difficult to wash-off. Anthraquinone
dyes exhibit relatively low substantivity and are easier to wash-off. Most of them
possess excellent fastness to light and to crease-resistant finishes, but they are not
dischargeable. Phthalocyanine dyes diffuse slowly and are difficult to wash-off [164].
Metal complex dyes containing copper possess rather dull hues, but show a high
degree of fastness to light and to crease-resistant finishes. Their substantivity is fairly
high; 1:2 complexes diffuse relatively slowly, so a longer time is needed to wash-out
unfixed dye completely.
The dye characteristics governed by the reactive group are reactivity, dye–fibre
bond stability, efficiency of reaction with the fibre, and affinity. Dyeing conditions,
especially the alkali requirement and temperature as well as the use of salt also
depend on the type of reactive group [165]. Dyes based on s-triazine do not have
good wet fastness properties in acidic media and, due to their high substantivity, have
poor wash-off properties. Similarly, dyes having a vinyl sulphone reactive system
have poor alkaline fastness. The chemical bond between the vinyl sulphone and the
cellulosic fibre is very stable to acid hydrolysis. The substantivity of hydrolysed by-
products of vinyl sulphone is low, so washing off is easy. Monochlorotriazines have
good fastness to light, perspiration and chlorine. The turquoise reactive dye shows an
optimum dyeing temperature that is generally about 20 °C higher than that of other
dyes with the same reactive group [166]. The fluorotriazine groups form linkages
with cellulose that are stable to alkaline media. Reactive dyes of dichloroquinoxaline,
monochlorotriazine and monofluorotriazine types show a tendency for lower resistance
to peroxide washing and dye–fibre bond stability [167]. A lower sensitivity to changes
in dyeing conditions (particularly temperature) is the most important characteristic
feature of the monochlorotriazine-vinyl sulphone heterobifunctional dyes. Dyeing
properties of some important reactive groups have been discussed in detail by various
authors [168–173].
Substantivity: Substantivity is more dependent on the chromophore as compared to
the reactive system. A higher dye substantivity may result in a lower dye solubility
[174], a higher primary exhaustion [175], a higher reaction rate for a given reactivity
[176], a higher efficiency of fixation [177], a lower diffusion coefficient, less sensitivity
of dye to the variation in processing conditions such as temperature and pH [178],
less diffusion, migration and levelness [179, 180], a higher risk of unlevel dyeing,
and more difficult removal of unfixed dye. Substantivity is the best measure of the
ability of a dye to cover dead or immature fibres. Covering power is best when the
substantivity is either high or very low [181]. An increase in the dye substantivity
may be effected by lower concentration of the dye, higher concentration of electrolyte
[182], lower temperature, higher pH (up to 11) and lower liquor to goods ratio [183].
Reactivity: A high dye reactivity entails a lower dyeing time and a lower efficiency
of fixation. (To improve the efficiency of fixation by reducing dye reactivity requires
a longer dyeing time and is, therefore, less effective than an increase in substantivity.)

Also there is a wider range of temperature and pH over which the dye can be applied.
Reactivity of a dye can be modified by altering the pH or temperature, or both. By a
suitable adjustment of pH and temperature, two dyes of intrinsically different reactivity
may be made to react at a similar rate.
Diffusion coefficient: Dyes with higher diffusion-coefficients usually result in better
levelling and more rapid dyeing. Diffusion is hindered by the dye that has reacted
with the fibre and the absorption of active dye is restrained by the presence of
hydrolysed dye. Different types of dyes have different diffusion characteristics. For
example, the order of decreasing diffusion is: unmetallised dyes, 1:1 metal-complex
dyes, 1:2 metal complex dyes; phthalocyanine dyes. An increase in the diffusion is
affected by increasing temperature, decreasing electrolyte concentration, adding urea
in the bath [184] and using dyes of low substantivity.
Solubility: Dyes of better solubility can diffuse easily and rapidly into the fibres,
resulting in better migration and levelling. An increase in dye solubility may be
effected by increasing the temperature, adding urea and decreasing the use of electrolytes.
12.2 System Variables in Reactive Dyeing
Temperature: A higher temperature in dyeing with reactive dyes results in a higher
rate of dyeing [185], lower colour yield [186], better dye penetration, rapid diffusion,
better levelling, easier shading, a higher risk of dye hydrolysis, and lower substantivity.
Raising the temperature appears to result in an opening-up of the cellulose structure,
increasing the accessibility of cellulose hydroxyls, enhancing the mobility as well as
the reactivity of dye molecules and overcoming the activation energy barrier of the
dyeing process, thereby increasing the level of molecular activity of the dye–fibre
system as well as dye–fibre interaction [187]. A comparison of hot and cold reactive
dyes has been given in [188, 189] along with some technical advantages of hot
reactive dyes over cold reactive dyes.
pH: The initial pH of the dyebath will be lower at the end of the dyeing by one half
to a whole unit, indicating that some alkali has been used up during dyeing. The
cellulosic fibre is responsible for some of this reduction, while a smaller part is used
by the dyestuff as it hydrolyses [190]. In discussing the effect of pH, account must be
taken of the internal pH of the fibre as well as the external pH of the solution. The
internal pH is always lower than the external pH of the solution. As the electrolyte
content of the bath is increased, the internal pH tends to equal the external pH. Since
the decomposition reaction is entirely in the external solution, the higher external pH
favours decomposition of the dye rather than reaction with the fibre. pH influences
primarily the concentration of the cellusate sites on the fibre. It also influences the
hydroxyl ion concentration in the bath and in the fibre. Raising the pH value by 1 unit
corresponds to a temperature rise of 20 °C. The dyeing rate is best improved by
raising the dyeing temperature once a pH of 11–12 is reached. Further increase in pH
will reduce the reaction rate as well as the efficiency of fixation. Different types of
alkalis, such as caustic soda, soda ash, sodium silicate or a combination of these

alkalis, are used in order to attain the required dyeing pH. The choice of alkali
usually depends upon the dye used, the dyeing method as well as other economic and
technical factors.
Electrolyte: The addition of electrolyte results in an increase in the rate and extent of
exhaustion, increase in dye aggregation and a decrease in diffusion. The electrolyte
efficiency increases in the order: KCl < Na2SO4 < NaCl [191]. There may be impurities
present in the salt to be used, such as calcium sulphate, magnesium sulphate, iron,
copper and alkalinity, that can be a source of many dyeing problems [192].
Liquor ratio: At lower liquor ratios, there is a higher exhaustion [193] and higher
colour strength. An increase in colour strength may be attributed to greater availability
of dye active species in the vicinity of the cellulose macromolecules, at lower liquor
ratio.
Surfactants and other auxiliaries: It is possible to enhance dye uptake on cellulosic
fibres with the aid of suitable surfactants. Amongst all the systems, the highest dye
uptake is obtained with anionic surfactants [194]. Non-ionic surfactants may result in
a decrease in dye exhaustion and colour yield, and a change in shade. Some non-ionic
surfactants may slow down the dye hydrolysis [195]. Triethanolamine (TEA) is known
to enhance colour strength by enhancing the swellability and accessibility of the
cellulose structure. It may also modify the state of the dye, thereby enhancing its
reactivity and increasing the extent of covalent dye fixation.
12.3 Important Characteristics of Reactive Dyeings
The best guide to the dyeing performance of a reactive dye can be obtained from two
sources of information: the SERF profile and migration properties under application
conditions. The SERF profile is constructed by the determination of substantivity
factor, exhaustion factor, fixation percentage and rate of fixation. The performance of
a reactive dye can also be defined by the Reactive Dye Compatibility Matrix (RCM)
[196, 197]. The critical measures of performance are the substantivity equilibrium
(S), the migration index (MI), the level dyeing factor (LDF) and an index of the
reactivity of the dye (T50). Evaluation of these four measures of performance provides
a measure of the compatibility of the dye to provide right-first-time production. Right
first-time production is maximised if these fundamental measures of performance
within the RCM are set at:
Substantivity 70–80%
Migration index >90
LDF >70%
T50 a minimum of 10 minutes
In the following, some important characteristics of reactive dyeings, namely exhaustion,
migration, levelness, fixation and colour yield, washing-off, dye-fibre bond stability,
and fastness properties will be discussed.
Exhaustion: There are two types of exhaustion that relate to the application of reactive
dyes: primary exhaustion and secondary exhaustion. Primary exhaustion occurs before

the addition of the alkali, while secondary exhaustion takes place after the addition
of the alkali. Both the rate of exhaustion and the extent or degree of exhaustion are
important. The rate of exhaustion can be increased by selecting dyes of high substantivity,
increasing the temperature and increasing the electrolyte concentration. The degree
of exhaustion can be increased by selecting dyes of high substantivity, lowering the
temperature and increasing the electrolyte concentration.
Migration: The intrinsic properties of a reactive dye that affect migration are
substantivity, molecular structure, physical chemistry and stereochemistry. The higher
the dye substantivity, the lower is the migration. The external factors that affect
migration are: concentration of the dye, temperature, time, liquor ratio, liquor circulation
and the form of the textile material.
Levelness: Levelness of dyeing may be inhibited by high substantivity, lower dye
migration [198], too much salt in the dyebath [199], too high rate of exhaustion, too
high concentration of alkali [200], a rapid shift of dyebath pH, too high rate of
fixation, too high rate of rise of temperature [201] and poor liquor agitation. Levelling
is difficult to obtain in light shades and easier to obtain in dark shades. Addition of
salt in portions is recommended for light shades while for deep shades, salt can be
added all at one step.
Levelness can be achieved in two ways [202]: either by controlling the rate of
absorption so that a controlled absorption is obtained, or by using the migration
properties of the dyes to compensate for the unlevelness that has occurred during the
early stages of the process. Controlled absorption can be obtained by salt dosing,
alkali dosing, and/or controlling the rate of heating. During the primary exhaustion,
the dye is free to migrate. During the secondary exhaustion stage, dye migration is
poor. For pale dyeing shades (less than 1 % o.w.f.) the degree of primary exhaustion
is over 80% and the degree of secondary exhaustion is very small. Therefore control
of the primary exhaustion stage is very important if level dyeing is to be obtained.
The rate of primary exhaustion is dependent on the amount of electrolyte used.
Dosing or split addition of salt is recommended to obtain level dyeing. For medium
shades, both primary and secondary exhaustion steps are important for obtaining
level dyeing. Both controlled salt and alkali addition are important in this case. In the
case of deep shades, the all-in salt addition may be possible, but during the secondary
exhaustion, alkali dosing is important [203]. Dyes with high substantivity, low secondary
exhaustion, and low MI (Migration Index) values require controlled addition of
electrolyte after the addition of the dye. In contrast, dyes with low substantivity, high
secondary exhaustion, and medium to high migration index values require precise
control of liquor ratio, concentration of electrolyte, and addition profile of the fixation
alkali [204]. Table 8 gives a comparison of two different approaches to achieve level
dyeing.
Fixation and colour yield: The fixation and the colour yield depend upon the following
factors [205]:
• Fibre cross-section
• Porosity of the substrate

• Dye structure with respect to substantivity ratio, dye diffusion, reactivity, etc.
• Degree of fibre preparation
• Liquor ratio
• Concentration of salt and alkali
• Use of reaction catalyst
• Use of dye–fibre cross-linking agents
• Introduction of other chemical groups in the fibre
• Use of film-forming agents
• Chemical modification of cellulose
• After treatments
There are various ways to increase fixation and colour yield which include:
• Use of fixation accelerators
• Use of shorter liquor ratio
• Dyeing at low temperature (with decreasing temperature the substantivity for
fibre increases, causing increased exhaustion)
• Modification of chromophore and reactive group
• Use of dyes with high substantivity and high reactivity
• Treating cellulosic fibres with swelling agents
• Modification in appearance techniques
• Changing the morphology of fibre by chemical modification.
A uniform rise in rate of fixation can be obtained by: controlling the temperature of
the dyeing process suitably (possible for hot dyeing dyes only); adding alkali in
stages (it is virtually impossible, however, to prevent a sharp rise in fixation rate
whenever alkali is added); starting with a weaker alkali such as soda ash, and following
this with a stronger alkali, but only after a higher degree of fixation has been achieved;
progressive metering of alkali (such as the Remazol automet process); and adding salt
in stages (suitable for high substantivity dyes).
Washing-off of reactive dyes: The removal of unfixed dye takes place in three phases
[206]: dilution of dye and chemicals in solution and on the surface of the cellulose;
diffusion of the deeply-penetrated, unfixed, hydrolysed dye to the fibre surface; and
dilution and removal of the diffused-out dye. Goods are rinsed cold twice to remove
electrolyte, then rinsed hot to desorb some hydrolysed dye from the fibre prior to a
‘soaping process’ at or near the boil. A subsequent cold rinse completes the task of
Table 8 Ways to Obtain Level Dyeing
Control of Levelling Based on Migration Control of Levelling Based on Controlled
Absorption
A relatively low level of control may be A very good level of control is necessary to
sufficient to get level dyeing get level dyeing
Poor reproducibility Better reproducibility
Poor colour yield Better colour yield
Dye additions or corrections may have Less need of additions and corrections
to be made

removing un-reacted and hydrolysed dye [207]. The factors which affect the washing
off of hydrolysed reactive dyes from the dyed material are as follows [208–212]:
• Dye substantivity
• Diffusion behaviour
• Reactive group
• Liquor ratio
• Washing temperature
• Electrolyte concentration
• pH
• Presence of calcium and magnesium ions in the ‘boiling soap’/hardness of water
• Liquor carry-over of the substrate
• Amount of unfixed dye
• Washing time
• Number of washing cycles/washing baths [213]
• Washing auxiliary employed
• Mechanical action
• Filling and draining
• Heating and cooling rates
Dye–fibre bond stability: Dye–fibre bond stability primarily depends upon the reactive
system. Dyes that react by a nuceophilic displacement mechanism show good stability
to alkali and, to different degrees, less stability to acid. Dyes that react by nucleophilic
addition give dye–fibre bonds with good stability to acid, but are less stable to alkali.
One of the most stable dye–fibre bonds is achieved with pyrimidinyl-based systems.
The triazine–cellulose bond is generally resistant to oxidative breakdown in the
presence of perborate, whereas this is a serious defect of some of the pyrimidine-
based systems. Dye–fibre bonds formed by monochlorotriazine dyes are less fast to
alkali (particularly at high temperature) than those formed between dichlorotriazinyl
dyes and cellulose. Vinyl sulphone dyes possess the same deficiency, but their higher
reactivity enables the problem to be avoided by the use of milder fixation conditions.
In case of pyrimidine dyes, the dye–fibre bond is more stable than in either of the
above two cases [214].
Fastness of reactive dyes: The factors that affect the fastness of reactive dyes are: the
chromophoric group, the stability of the dye–fibre bond and the completeness of the
removal of the unfixed dye. To maximise wet fastness, particularly in deep shades, it
is advisable to apply cationic after-treatments.
A summary of problems in dyeing with reactive dyes is given in Appendix I.
13. PROBLEMS IN DYEING WITH DIRECT DYES
Direct dyes represent an extensive range of colorants that are easy to apply and also
are very economical [215–217]. There are three common ways to classify direct
dyes, namely, according to their chemical structure [218], according to their dyeing
properties, and according to their fastness properties. Of these three possible ways of
classifying direct dyes, the first is of least importance to the dyer, although of
considerable importance to those interested in dye chemistry [219]. According to the

Society of Dyers and Colourists’ classification, which is essentially based upon the
compatibility of different groups of direct dyes with one another under certain conditions
of batch dyeing, there are three classes of direct dyes: A, B and C. Class A consists
of self-levelling direct dyes. Dyes in this group have good levelling characteristics
and are capable of dyeing uniformly even when the electrolyte is added at the beginning
of the dyeing operation. They may require relatively large amounts of salt to exhaust
well. Class B consists of salt-controllable dyes. These dyes have relatively poor
levelling or migration characteristics. They can be batch dyed uniformly by controlled
addition of electrolyte, usually after the dyebath has reached the dyeing temperature.
Class C consists of salt- and temperature-controllable dyes. These dyes show relatively
poor levelling or migration and their substantivity increases rapidly with increasing
temperature. Their rate of dyeing is controlled by controlling the rate of rise of
temperature, as well as controlling the salt addition.
Important dyebath variables that influence the dyeing behaviour of direct dyes
include temperature, time of dyeing, liquor ratio, dye solubility, and presence of
electrolyte [220] and other auxiliaries.
Direct dyes can be applied by batch dyeing methods (on jigs, jet or package
dyeing machines), by semi-continuous methods (such as pad-batch or pad-roll) and
by continuous methods (such as pad-steam). Many direct dyes are suitable for application
by combined scouring and dyeing. In this process the usual practice is to employ soda
ash and non-ionic detergent. However, dyes containing amide groups are avoided
because of the risk of alkaline hydrolysis.
Direct dyes vary widely in their fastness properties, and staining effects on various
fibres. Most direct dyes, however, have limited wet fastness in medium to full shades
unless they are after-treated. The fastness of selected direct dyes can be improved in
several ways [221–224], such as the following:
• Treatment with cationic fixing agents
• Treatment with formaldehyde
• Treatment with copper salts such as copper sulphate
• Treatment with cationic agents and copper sulphate in combination
• Diazotisation and development
• Treatment with crosslinking agents or resins
An important consideration in dyeing with direct dyes is the ability of the dyes to
cover the immature cotton fibre neps, which has been explained, in most cases, in
terms of both the molecular weight and hydrogen bond formation capacity of the dye
molecules [225–227]. Given a similar capacity to form hydrogen bonds, dyes having
lower molecular weight show proportionately better nep coverage than those having
higher molecular weight. Table 9 gives Colour Index number of dyes with better
coverage of immature fibres [228].
A summary of common problems in the dyeing of cotton with direct dyes is given
in Appendix I.
14. PROBLEMS IN DYEING WITH SULPHUR DYES
Despite their environmental concerns, which are constantly being addressed [229–
234], sulphur dyes occupy an important place for dyeing of inexpensive black, blue,

brown and green shades in medium to heavy depths on cellulosic fibres [235, 236].
The history, development and application of sulphur dyes have been widely reviewed
by various authors [237–248]. Sulphur dyes have been classified into four main
groups [249]: CI Sulphur dyes; CI Leuco Sulphur dyes; CI Solublised Sulphur dyes;
and CI Condensed Sulphur dyes. CI Sulphur dyes are water-insoluble, containing
sulphur both as an integral part of the chromophore and in attached polysulphide
chains. They are normally applied in the alkaline reduced (leuco) form from a sodium
sulphide solution and subsequently oxidised to the insoluble form on the fibre. Sulphur
dyes differ from the vat dyes in being easier to reduce but more difficult to re-oxidise,
different oxidants producing variations in hue and fastness properties. A leuco sulphur
dye has the same CI constitution number as the parent sulphur dye but exists as the
soluble leuco form of the parent dye together with a reducing agent in sufficient
quantity to make it suitable for application either directly or with only a small addition
of extra reducing agent. A solublised sulphur dye has a different constitution number
because it is a chemical derivative of the parent dye, non-substantive to cellulose but
converted to the substantive form during dyeing. Condensed sulphur dyes, although
containing sulphur, bear little resemblance to traditional sulphur dyes in their constitution
and method of manufacture. Sulphur dyes are available in various commercial forms
such as powders, pre-reduced powders, grains, dispersed powders, dispersed pastes,
liquids, and water soluble-brands.
The various steps in the application of sulphur dyes depend very much on their
type and commercial form. The main steps in the application of water-insoluble
sulphur dyes are as follows:
• Reduction, whereby the water-insoluble dye is converted into water-soluble form
• Application, whereby the solubilised dye is applied onto the substrate by a suitable
exhaust or continuous method
• Rinsing, whereby all loose colour is removed before the oxidation stage
• Oxidation, whereby the dye absorbed by the substrate is oxidised back into
water-insoluble form, and
• Soaping, which results in an increase in brightness as well as improved fastness
of the final shade
Various application methods for sulphur dyes, along with suggested recipes, have
been discussed in [243, 245, 246, 249, 250].
Table 9 Colour Index Number of Dyes with Better Coverage
of Immature Fibres (Numbers in Brackets Have
Lower Overall Coverage than Others)
Colour Colour Index Number
Yellow 7, 11, 27
Orange (1, 15, 37, 102)
Red 32 (20, 24, 76)
Violet 9, 22, 66
Blue 8, 26, 27, 98
Green (1, 26)
Brown 25, 29
Black 3, 22, 39

The auxiliaries used in sulphur dyeing are: reducing agents, antioxidants, sequestering
agents, wetting agents, oxidising agents and fixation additives. The two most important
reducing agents for sulphur dyes are sodium sulphide [Na2S] and sodium hydrosulphide
[NaHS]. Caustic soda/sodium dithionite are conventional chemicals for vat dye reduction
but this system is difficult to control in the application of sulphur dyes and tends to
give inconsistent results except with certain sulphur vat dyes. A sodium carbonate/
sodium dithionite mixture is too weakly alkaline for the water-insoluble type sulphur
dyes and requires careful control if over-reduction and consequent low colour yield
are to be avoided. Glucose in the presence of alkali, usually caustic soda or a caustic
soda/soda ash mixture, has been used as another possible sulphur dye reducing agent,
but it is a weak reducing agent as compared to sodium sulphide or sodium hydrosulphide.
Other reducing agents such as thioglycol, hydroxyacetone and thiourea dioxide, have
had limited success. Sodium polysulphide and sodium borohydride can be used as
antioxidants to inhibit premature oxidation, promote better dyebath stability and
lessen the risk of bronzing, poor rubbing fastness and dark selvedges. Sequestering
agents are used where water quality is poor or variable, to avoid poor rubbing fastness
or unlevelness in the presence of multivalent ions in the dye liquor or in the substrate.
Wetting agents may be used to improve the wettability of the substrate. Although the
majority of sulphur dyes are unaffected by most wetting agents, some non-ionic
wetting agents may inhibit the dye uptake in exhaust dyeing or precipitate the dye as
a tarry leuco product.
Traditionally, the most preferred oxidising system has been sodium dichromate/
acetic acid because of its ability to rapidly and completely oxidise all reduced sulphur
dyes, resulting in good colour yield and fastness properties. Nevertheless, it has been
criticised increasingly on environmental grounds, and for its effects on handle and
sewability, especially with sulphur blacks. The addition of 1 g/l copper sulphate to
batchwise oxidation baths of sodium dichromate/acetic acid improves the light fastness
but may result in dulling of the shades, as well as harsher handle. It is not recommended
with sulphur blacks, where the presence of copper promotes acid tendering. Other
oxidising agents that have been tried as alternatives to sodium dichromate/acetic,
with various degrees of success, include [251, 252]: potassium iodate/acetic acid;
sodium bromate; hydrogen peroxide and peroxy compounds; and sodium chlorite.
Fixation additives, such as alkylating agents based on epichlorohydrin, give dyeings
of markedly improved washing fastness but often at the risk of some decrease in light
fastness. Moreover, in the event of the dyeing needing subsequent correction, alkylated
sulphur dyeings are difficult to strip and attempted removal will often entail destruction
of the dye chromogen.
Two special problems in dyeing with sulphur dyes are acid tendering and bronziness.
In severe conditions of heat and humidity, some sulphur dyeings, notably black, can
generate a small amount of sulphuric acid within the cellulosic fibres, leading to
tendering. AATCC Test Method 26-1994 (Ageing of sulphur dyed textiles) can be
used to determine whether the sulphur dyed textile material will deteriorate under
normal storage conditions [253]. Bronziness and other problems in sulphur dyeing
and their possible causes are summarised in Appendix J.

15. PROBLEMS IN DYEING WITH VAT DYES
Vat dyes remain the primary choice where the highest fastness to industrial laundering,
weathering and light are required [254]. Several primers [255–257] and reviews have
been published on progress in their development [258–265], and their application by
batch [266–270] as well as by continuous processes [271, 272]. This section gives
briefly some fundamentals of vat dyeing and reviews various problems in the dyeing
of cotton with vat dyes in an endeavour to consolidate the previous work done in this
regard [273–276].
Vat dyes are insoluble pigments, available in different forms [277]. Based on the
temperature and the amount of caustic soda, hydrosulphite and salt used in dyeing,
vat dyes can be classified into four main groups [278]: IN dyes require high temperature
and a large amount of caustic soda and sodium hydrosulphite; IW dyes require medium
temperature and a medium amount of caustic soda and sodium hydrosulphite with
salt added; IK dyes require low temperature and a small amount of caustic soda and
sodium hydrosulphite with salt added; and IN Special dyes require more caustic soda
and higher temperature than IN dyes. Generally, vat dyes have a very rapid strike, a
good degree of exhaustion and a very low rate of diffusion within the fibre. Vat dyes
of different chemical structure may differ in the solubility of their sodium leuco-vat,
stability towards over-reduction, stability towards over-oxidation, substantivity and
rate of diffusion. Commercial competitive dyes have fairly equal particle sizes. Large
particle sizes give dispersions of poor stability. For some vat dyes, colour yield
decreases with increasing particle size. The effect is generally dye-specific [279].
The main stages in the dyeing of cotton with vat dyes are as follows:
• Conversion of insoluble vat pigment into soluble sodium leuco-vat anions
[reduction]
• Diffusion of sodium leuco-vat anions into cellulosic fibres
• Removal of excess alkali and reducing agents by washing off
• Oxidation of the soluble dye into insoluble pigmentary form within the cellulosic
fibres
• Soaping, during which the isolated molecules of vat pigments are re-orientated
and associate into a different, more crystalline form
Important requirements of vat dye reducing agents are a level of reducing power
(reduction potential) sufficient to reduce all commercial vat dyes to their water-
soluble form quickly and economically, and conversion of the vat dyes into products
from which the original pigment can be restored (no over-reduction).Various reducing
systems for vat dyes have been proposed and used [280–282]. The most common
type of reducing agent used for dyeing with vat dyes is 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. The rate of reduction of vat dyes
depends upon various factors, such as the particle size of the dye, the temperature,
time and pH during reduction and access of the reducing agent. The stability of
alkaline solutions of reducing agents may decrease with increased temperature, greater
exposure to air, greater agitation and lower concentration of the reducing agent. Vat

Critical solutions in_the_dyeing_of_cotton_textile_materials-libre(1)

Critical solutions in_the_dyeing_of_cotton_textile_materials-libre(1)

Recommended

Recommended

More Related Content

What's hot

What's hot (20)

Viewers also liked

Viewers also liked (7)

Similar to Critical solutions in_the_dyeing_of_cotton_textile_materials-libre(1)

Similar to Critical solutions in_the_dyeing_of_cotton_textile_materials-libre(1) (20)

Recently uploaded

Recently uploaded (20)

Critical solutions in_the_dyeing_of_cotton_textile_materials-libre(1)