18680002 Critical Solutions in the Dyeing of Cotton Textile Materials

8/9/2019 18680002 Critical Solutions in the Dyeing of Cotton Textile Materials

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Abstract: Over the decades there have been several papers on the coloration ofcotton-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. INTRODUCTIONCotton is the backbone of the worlds 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 MATERIALSR. Shamey and T. Hussein

doi:10.1533/tepr.2005.0001

The Textile Institute


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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


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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.

FinishedFabric

Mechanical

Finishing

ChemicalFinishing

PrintingDyeing

Mercerising

Bleaching

Scouring

Desizing

Singeing


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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. Printingis 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 [1216].

The distinction between dead and immature fibres is very important. Both dyelighter 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


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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 anattempt 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 theoutside 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 givesa 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;


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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 88.096.0Pectins 0.71.2

Wax 0.41.0Proteins 1.11.9Ash 0.71.6Other organic compounds 0.51.0

Table 2 Metal Content of Cotton of Different Origins

Origin of CottonMetal Content (mg/kg)

Ca Mg Fe Cu Mn

Brazil Assai Piranha 3147 1156 680 6 30Brazil Sao Paulo 845 555 46 6 11Peru 700 440 13 < 1 < 1

USA Texas 810 365 75 < 1 < 1USA California 600 540 40 < 1 < 1Egypt 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, sequesteringagents 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


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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 FORMATIONAs 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


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Thin places

Weak places

Count variation

Hairiness

Dyeability variations [2730]

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 DYEINGThe 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 [3638]

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 givenin 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 packagemust be Stable packagehandled with care

Constant winding density Density varies from Uniform homogeneous densityinside to out

Areas of ribboning are No ribboning No ribboningpossibleLiquor flow characteristics Good liquor flow Good liquor flow characteristicsare not optimum characteristics


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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 oflargediametertubes 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 affectthe quality of the dyed package [4246]. 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,


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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. Defectsin 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


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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 onlyone 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 [6165]:

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 QUALITYThe 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 aspreparation, 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:


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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 affectedin 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.


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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 significantbleaching 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 inprocessing.

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


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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 ColourlessSmell OdourlesspH value Neutral pH 78Water hardness < 5 dH (6.25eH; 8.95fH; 5.2 USA)Dissolved solids < 1 mg/lSolid deposits < 50 mg/lOrganic substances < 20 mg/l (KMnO4 consumption)Inorganic salts < 500 mg/lIron (Fe) < 0.1 mg/lManganese (Mn) < 0.02 mg/lCopper (Cu) < 0.005 mg/lNitrate (NO

3

1 ) < 50 mg/lNitrite ( NO

2

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 residuesResidual hardness < 0.05 dHOxygen < 0.02 mg/lTemporary CO2 0 mg/lPermanent CO2 < 25 mg/lIron < 0.05 mg/lCopper < 0.01 mg/l

pH (at 25 C) > 9Conductivity (at 25 C) < 2500 S/cmPhosphate (PO4) 45 mg/lBoiler feed water temperature > 90 C


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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 [8083]

A summary of problems caused by poor water quality is given in Appendix C.

7. PROBLEMS IN SINGEINGTextiles 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 [8688] 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 DESIZINGSizing 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, cellulosederivatives and protein sizes; and synthetic sizes which include polyvinyl alcohols,

polyacrylates and styrenemaleic 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


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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 [9698] and

oxidative desizing [99101]. 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 combineheavy 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


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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 treatmentor washing have not been thorough. Iodine spot tests are then conducted on the fabric

[107109]. 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 SCOURINGVarious aspects of cotton fabric preparation have been presented by Rosch [110118]

and Sebb [119124]. 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 bysaponification 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


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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 averagedegree 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


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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 followingsteps: 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 BLEACHINGCotton, 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,


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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 throughoutthe 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 10.210.7 is considered optimum for bleaching

cotton with hydrogen peroxide. Lower pH values can lead to decreasing solubility of


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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 hightemperatures 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 andcalcium 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 toaid 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 asfollows:

Inadequate mote removal

Low degree of whiteness

Table 6 Effect of pH on Bleaching Effectiveness, Fibre Degradation, and Peroxide Stability inBleaching Cotton Fibres

Initial pH Final pH Whiteness CUEN % PeroxideIndex Fluidity Remaining

8.0 4.4 66.8 5.48 72.59.0 8.7 67.3 1.44 71.6

10.1 9.9 71.3 2.44 63.311.0 11.7 72.2 7.29 7.012.0 12.4 69.5 17.8 2.0


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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 isgiven 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 MERCERIZATIONMercerization 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

research and commercial developments in mercerisation has been given by Greenwood

[139]. Mercerization of cotton can be carried out on raw fibre [140], yarn, and knitted

[141147] or woven fabric, and at any stage during preparation. Fabric may be 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

mercerized fabric. Fabrics can be mercerized without tension to effect mainly an increase

in strength and dye affinity, or under tension to effect mainly an increase in the lustre [148].

The treatment may be wet-on-dry, wet-on-wet or add-on [149151] 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


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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% (4854Tw). 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 70100 F [2138 C]. Lowertemperatures 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 (1020 C) Hot Mercerization (70 C)

Strong fibre swelling Less fibre swellingSlower swelling Rapid swelling

Slower relaxation Rapid relaxationIncomplete relaxation Good relaxationHigher residual shrinkage Lower residual shrinkageSurface swelling Complete swellingUnevenness EvennessHarder hand Softer handNaOH diffusion inhibited Uninhibited NaOH diffusionLess lustre Optimised lustre


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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 wetprocesses.

After mercerization, an analysis is carried out to determine the degree of

mercerization, which is specified by the Barium Number [157160]. 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 DYESReactive 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 dealswith 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,


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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, dyefibre

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 havegood 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 dyefibre 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 [168173].

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.)


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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 dyefibre

system as well as dyefibre 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 usedby 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 1112 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


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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 DyeingsThe 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. Rightfirst-time production is maximised if these fundamental measures of performance

within the RCM are set at:

Substantivity 7080%

Migration index >90

LDF >70%

T50 a minimumof 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


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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, toohigh 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

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18680002 Critical Solutions in the Dyeing of Cotton Textile Materials

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