11

CHAPTER 2

LITERATURE REVIEW

2.1 DYES AND DYEING

(a) Dyeing

Archaeological evidence shows that, particularly in India and the

Middle East, dyeing has been carried out for over 5000 years. Dyeing is a

method which imparts beauty to the textile by applying various colours and

their shades on to a fabric. Dyeing can be done at any stage of the

manufacturing of textile fiber, yarn, fabric or a finished textile product

including garments and apparels. The property of colour fastness depends

upon two factors namely selection of proper dye according to the textile

material to be dyed and selection of the method for dyeing the fiber, yarn or

fabric.

(b) Dyes

A dye can generally be described as a coloured substance that has

an affinity to the substrate to which it is being applied. The dye is usually

used as an aqueous solution and may require a mordant to improve the

fastness of the dye on the fiber. Dyes are used for colouring the fabrics. Dyes

are molecules which absorb and reflect light at specific wavelengths to give

human eyes the sense of colour.

12

2.1 .1 Classification of Dyes

There are two major types of dyes - natural and synthetic dyes. The

natural dyes are extracted from natural substances such as plants, animals, or

minerals. Synthetic dyes are made in a laboratory. Chemicals are synthesized

for making synthetic dyes. Some of the synthetic dyes contain metals too.

2.1.1.1 Natural dyes

The natural dyes are obtained from animal, vegetable or mineral

origin with no or very little processing. The greatest source of dyes has been

the plant kingdom, notably roots, berries, barks, leaves and wood, but only a

few have ever been used on a commercial scale.

2.1.1.2 Synthetic dyes

The first man-made organic dye, mauveine, was discovered by

William Henry Perkin in 1856. Many thousands of dyes have since been

prepared and because of vastly improved properties imparted upon the dyed

materials quickly replaced the traditional natural dyes (Buchanan and Rita

1995).

A dye, whether it is from a natural or synthetic origin, is used, not

only to just colour the surface of fibers, but it must also become a part of the

fiber. After dyeing, the fabric should not be affected during the washing

process, dry cleaning with organic solvents etc and also the dye should give

fastness to light, heat and bleaching.

The global consumption of textiles is estimated at around 30 million

tonnes, which is expected to grow at the rate of 3% per annum. Moreover,

such a huge amount of required textile materials cannot be dyed with natural

13

dyes alone. Hence, the use of eco-safe synthetic dyes is also essential

(Samanta and Agarwal 2009).

The global market for textile dye stuffs is dominated by the

disperse (35%), and reactive (28%) dyestuff ranges, together with acid (12%),

direct (7%), Vat and indigo (8%), Sulphur (6%) and azoic and other dyestuffs

(4%) (Lan Holme 2008).

Synthetic dyes used in the textile industry are broadly classified as

follows:

1. Basic dyes

2. Direct dyes

3. Vat dyes

4. Reactive dyes

5. Azoic dyes

6. Sulphur dyes -

7. Mordant dyes

8. Acid dyes

9. Disperse dye

10. Oxidation dyes

11 Mineral and pigment dyes

1. Basic and modified basic dyes

Mauveine, the first to be discovered by Perkin, was a basic dye and

most of the dyes which followed, including magenta, malachite green and

crystal violet, were of the same type. Basic dyes dye wool and silk from a dye

bath containing acid, but they dye cotton fibers only in the presence of a

14

mordant usually a metallic salt that increases affinity of the fabric for the dye.

Basic dyes include the most brilliant of all the synthetic dyes known, but

unfortunately they have very poor light and wash fastness (Pellew 1998).

2. Direct dyes

These are soluble in water and have direct affinity for all cellulose

fibers. Some will also dye silk and wool. As these dyes, when dyed without

additives, do not exhaust well, an addition of salt is required to improve the

yield of the dye and to obtain deeper shades. Generally, the wash fastness of

these dyes is inferior, but there are a number of after treatments available to

improve the wash fastness of the dyeing. Direct dyes dye all cellulosic fibers,

including viscose rayon, and most of them also dye wool and silk (Buchanan

and Rita 1995).

3. Vat dyes

Indigo, probably the oldest dye known to man is one of the most

important members of this group (Barker 2004). These dyes, which are

insoluble in water, can be converted into alkali soluble leuco compounds

(colourless), when reduced with sodium hydrosulphite. After introducing into

the fabrics, the dye will be oxidized on exposure to air and will become

insoluble in water again (Shenai 1999). Previously, the reduction process of

the dye was carried out in wooden vats, hence the name vat dyes. These dyes

are used to colour cotton fibers. Indigo (plant source : Indigofera tinctoria) is

a good example for vat dye which is water insoluble and blue in colour when

reduced, convert into indigo white which is colourless and water soluble.

15

4. Reactive dyes

This is a new class of dye introduced in the market in 1956. They

react chemically with the fiber being dyed, and if correctly applied, it cannot

be removed by washing or boiling. The main feature of the dyestuff is its low

affinity to cellulose. Therefore, large amount of salt is required to force its

deposition on the fabric. After this has been achieved, addition of alkali

causes the deposited dyes to react with the fiber. Only a successfully

concluded reaction guarantees a fast dyeing. Basically there are two types of

reactive dyes: the cold dyeing and hot dyeing types (Shenai 1997).

5. Azoic dyes

The word 'Azoic' is the distinguishing name given to insoluble azo

dyes that are not applied directly as dyes, but are actually produced within the

fiber itself. This is done with impregnating the fiber with one component of

the dye, followed by treatment in another component, thus forming the dye

within the fiber (Shenai 1997). The formation of this insoluble dye within the

fabric makes it very fast to washing. The deposition of the free pigment on the

surface of the dyed fabric produces poor rub fastness, but once the loose

pigment is removed by boiling the fabric in soap, the dyeing becomes one of

the fastest available.

6. Sulphur dyes

The first Sulphur dye was discovered in France in 1873, and further

work done by Raymond Videl enabled the manufacture of ‘Videl black’. Its

outstanding fastness to light, washing and boiling far surpassed any cotton

black known at that time. The general disadvantage of the Sulphur dyes is that

they produce dull shades and lack red colour. The main advantage lays in

their cheapness, ease of application and good wash-fastness (Shenai1997). In

16

their normal state Sulphur dyes are insoluble in water, but they are readily

soluble in the solution of Sodium Sulphide. In this form they have high

affinity to all the cellulose fibers.

7. Mordant dyes

As the name suggests, these dyes require a mordant. This improves

the fastness of the dye on the fiber such as water, light and perspiration

fastness (Barker 2004). The choice of mordant is very important as different

mordants can change the final colour significantly. Most natural dyes are

mordant dyes and there is, therefore, a large literature base describing dyeing

techniques (Shenai 1997). The most important mordant dyes are the synthetic

mordant dyes (chrome dyes) used for wool. These comprise some 30% of

dyes used for wool and they are especially useful for black and navy shades.

The mordant potassium dichromate is applied as an after-treatment.

8. Acid dyes

Water soluble anionic dyes are applied to fibers such as silk, wool,

nylon and modified acrylic fibers from neutral to acid dye baths. Attachment

to the fiber is attributed, at least partly, to salt formation between anionic

groups in the dyes and cationic groups in the fiber. Acid dyes are not

substantive to cellulosic fibers (Barker 2004).

9. Disperse dyes

The introduction of a new regenerated cellulose acetate fiber in

1920 led to the necessity of developing an entirely new range of dyes. It was

found that cellulose acetate (or Celanese) fiber had hardly any affinity for

water-soluble dyes. So a new dyeing principle was introduced. Dyeing with

water dispersed coloured organic substances. These finely coloured particles

17

are applied in aqueous dispersion to the acetate material and actually

dissolved in the fibers (Buchanan and Rita 1995).

10. Oxidation dyes

These are not dyestuffs in the same sense as other soluble or

disperse dyes, but because of their exceptional fastness to light and washing

they are of great importance. The most important member of this group is

produced by oxidation of aniline and it is used much in the dyeing of fur and

leather goods (Buchanan and Rita 1995).

11. Mineral and pigment dyes

Although it is preferable to use water soluble dyes in textile dyeing

for two reasons, ease of application and greater softness of the fabric, there

are two processes where pigment colouration is used (Shenai 1997).

(a) Mineral khaki

Cotton army equipment, where it is used because of its cheapness

and because it also renders fabric resistant to rotting and attack by insects in

damp conditions.

(b) Synthetic resin printing

The introduction of heat setting synthetic resins has opened new

fields in textile printing. Mineral and organic pigments, as used in paint

manufacture, can now be applied to any fabric and rendered wash fast after

heat treatment.

18

2.1.2 Vat Dyes

Figure 2.1 Vat dyes

Vat dyes (Figure 2.1) are one of the important classes in the

synthetic dyes. These dyes are characterised by their insolubility in water.

They are applied to the fabric in a reduced, soluble form, which has affinity

for the substrate, and after the reduced dye has been absorbed, the fabric is

taken out from the dye bath and left in the air or immersed in solution of a

mild oxidising agent to reproduce the dyeing column. In general, vat dyes are

fast to washing, to light etc. Until early times of the present century, the only

vat dyes known were those related to indigo (Epp and Diane 1995). But due

to the constant research work in the field, a new class, anthraquinone dyes,

was found and it is of much prominence these days. Indigo was first a

naturally cultivated dye which was principally grown in our country.

2.1.2.1 History of vat dye

Indigo is thought to be the oldest dye. It was found in mummy

clothes over 5,000 years ago and in the garments of Tutankhamen. For

centuries, European dyers used the woad plant to achieve blues, and woad

growers fought the influx of this cheaper, deeper blue dye. In 1598, indigo

was prohibited in France and parts of Germany, and dyers had to swear, often

19

on the pain of death, that they would not use indigo. By the 17th century,

indigo was a chief trade article of both the Dutch and British East India

Companies. In 1744, indigo arrived in South Carolina, from where it travelled

to other states including New York (Balfour Paul 1999).

Baeyer's research was backed by BASF and Hoechst, who were in

competition to achieve artificial indigo. The laboratory work was assisted by

Baeyer’s constitutional formula of 1860 for indigo. The structure became

available in 1883, another triumph for Baeyer, then at the University of

Munich. In 1905, Baeyer won the Nobel Prize for his work with organic dyes.

Indigo dye can be purchased in both its natural and synthetic forms and it is

still popular in the dyer’s garden (Peter Morris and Anthony Travis 1992). Its

primary uses are in cosmetics as a laboratory indicator, and as the dye that

makes blue jeans blue. Fox and Pierce (1990) traced the history of indigo

from its origin to modern application techniques.

A brief history of the synthesis and use of vat dyes from BASF was

given (Nahr and Ruppert 1992), including modern developments such as

cottestren dyes, palanil T and indanthrene T dyes for dyeing cotton / polyester

blends. Reducing agents for dyeing with indanthrene dyes were discussed in

that work. Dyeing methods were indicated. Problems of environmental

protection and the pollutant content of dyeing effluents were considered. It

was claimed that vat dyes were safe, clean, and that they cause few effluent

treatment problems.

The history of indigo was traced (Schmidt 1997). Upto 1897 it was

derived from plants (Indigofera tinctoria), 4/5th

of which was grown in

Bengal. In 1897 BASF introduced the 1st synthetically manufactured indigo in

the market. In 1926 the company produced the Heumann-Pfleger process to

replace the others which had been employed hitherto. Developments and the

state of art were explained. Hyde (2005) presented in his paper that the indigo

20

has been used in dyeing textiles and colouring other articles for over 3000

years. It is unique in that it has retained its importance upto the present day,

despite the introduction of more sophisticated dyestuffs having higher

performance.

Bilgrami (2007) investigated that the indigo tradition has continued

to flourish in countries such as Japan. It has an impact on the social, economic

and cultural aspects of the country. A perfect balance between the grower, the

producer and the user has been achieved through indigo’s importance and

integration. One such designer is Jurgen Lehr, who exclusively uses natural

fiber and strongly believes in natural dyes.

The study of Chen et al (2008), 750g of Taiwan plant baphica

canthus cusia, indigofera tinctoria and polygonum tinctoria, with two

reducing agents - 60g of sodium hydrosulphite and 12g of thio urea dioxide

were used to dye the cotton fabric for 60-90 seconds, 1-15 times repeatedly.

As for washing fastness test when Indigofera tinctoria was reduced with

sodium hydrosulphite, the test was rated as grade 3-4. For other specimens,

they were just grade 3.

2.1.2.2 Indigo dye

Dyes may be natural or synthetic. Natural dyes come from animals,

minerals, and plants. Plants of the species Indigofera, Polygonum,

Lonchocarpus, Marsdenia, Strobilanthes, and Isatis contain indican, a

chemical that gives blue color. The leguminous Indigofera genus, with over

three hundred species, contains the most indican. Indigofera tinctoria

(Figure 2.2) (native of India and Asia) and Indigofera suffructiosa (native of

South and Central America) are the best known. From these plants the indigo

of commerce in the form of dark blue granular lumps with a characteristic

21

coppery lustre, was prepared by a comparatively simple process of

fermentation, extraction, and oxidation (Pellew 1998).

Human beings, in their evolutionary process, started to use clothes

to cover their bodies. Gradually, not satisfied with plain clothes, they started

using clothes in attractive colours and shades. Thus, the art of dyeing cloth

came up. It is believed to have been known in China, India and Egypt more

than even 4000 years ago. Indigo is one of the oldest natural dyes known in

India. The colouring principle in indigo is blue indigotin. In comparison to the

Japanese methods of indigo cultivation, our methods seem to be primitive as

the yields are comparatively low and of poor quality (Ganesh 2008)

Indigoid dyes are perhaps the oldest natural dyes used by man. It is

found as the glucoside indicant in the plant Indigofera tinctoria and has been

known in India for about 4000 years. The main dyeing component of this

plant is indigo (Samanta and Agarwal 2009). Some plant material like indigo

leaves, however, needs to be fermented, to release the glucosides of the dye.

The indigo plant is therefore, steeped in specially constructed water tanks,

called vats, churned, left to settle, the sediment collected and dried in the sun,

to get the indigo cake (Ganesh 2008).

Figure 2.2 Indigofera tinctoria (Indigo)

22

Indigo works by a chemical reaction called oxidation reduction.

Indigo does not dissolve in water. It must be reduced — i.e. the oxygen must

be removed— in the presence of alkali by a reducing agent such as thiourea

dioxide (thiox), sodium hydrosulfite, zinc or bacteria. Upon reduction, indigo

becomes colourless and water soluble. In this state, indigo has a high affinity

for cellulosic fibers and enters the open spaces of the fiber. The dyed fibers

are then exposed to air, which oxidizes the dye molecule back to its insoluble

form. The insoluble dye particles are trapped inside the fiber, colouring them

permanently blue. Unlike most dyes, indigo forms a mechanical, not chemical

bond (Sandberg and Gosta 1989).

2.1.2.3 Processing of Indigo / Vat dye

Indigo is a vat dye, so named because the traditional processing of

indigo included fermenting the leaves in a vat (vessel). The fermentation

process reduces indicant to its colourless, soluble form that fabric can absorb

(Sandberg and Gosta 1989). Vat dyes are so named because of their ability to

form a soluble alkali salt or vat from the insoluble dye. This is usually

accomplished by an alkaline reducing agent, and the affinity of the colourless

leuco salt for textile fibers, especially cellulosic fibers, is characteristic of the

group. To prevent premature oxidation, dyeing must take place in the absence

of air. This fact dictates the technology of indigo dyeing. If one brushes the

dye solution onto fabric, the brush might turn blue, but not the fabric as the

dye would become insoluble between the dye vat and the fabric. Traditional

patterning of indigo-dyed fabrics usually depends on 1) fabric structure —

mixing indigo yarns with other yarns 2) physical resists that prevent dye

penetration 3) chemical resists that prevent dye oxidation or 4) removal

(discharge) of colour after dyeing. A more recent option is the use of Inkodye,

23

a soluble vat dye that has been preprocessed into the reduced form for direct

application (Ziderman 1981).

According to the electronic theory of colour in dyes, the following

factors make a colour deeper: the length of the molecule, alternating regular

and double chemical bonds (conjugation), inclusion of atoms other than those

of carbon, hydrogen and oxygen. All this explains why the colour of the

Tyrian purple is deeper than that of indigo. Tyrian purple is derived from the

Mediterranean shell fish of the genera Purpura and Murex (Samanta and

Agarwal 2009) As seen, the only difference between the structures in Figures

2.3 and 2.4 is two bromine atoms.

Figure 2.3 The structure of indigo

Figure 2.4 The structure of the Tyrian purple

In the modern industry, this kind of colouring is called vat dyeing.

Its explanation is fairly simple. Many good stable dyes are insoluble in water

and they cannot be used directly for dyeing. However, it is possible to

perform the so-called reaction of reduction, which renders the dye colourless,

24

but soluble (Figure 2.5). The cloth is put into the vat (“Yora” in Hebrew,

which means just a kind of a big tub) to absorb the reduced dye and it is then

taken out to dry in the air. The oxygen transforms the dye back to its insoluble

but coloured form. The usual jeans are coloured this way by indigo. Indigo is

insoluble in water, but leuco-base is soluble in alkaline water (Prideaux and

Vivian 2004).

Figure 2.5 Vat dyeing process

Vat dyes are made up of any organic colouring matter (with the

exception of basic and sulphur colours), which is capable of undergoing a

reversible reduction-oxidation cycle without serious colour loss or change of

shade (Epp and Diane 1995). No general formula can be written for a vat dye.

It is almost always a coloured organic compound containing two or more keto

groups (i) which are alkali to give leuco compound (ii) which has affinity for

cellulosic fiber. In many cases this affinity extends also to other fibers such as

wool, nylon, and ‘Orlon’ acrylic fiber and even to cellulose acetate.

Vat dyes are insoluble in water, solubilised by treatment with

caustic soda and reducing agent, usually hyposulphite. The resulting leuco

compounds have affinity for textile fiber on exposure to air leuco compound

impregnated fiber reoxidises to the insoluble parent dye. Vat dyes mainly

belong to indigoid and anthra quinoid classes and they are characterised by

high fastness, especially anthraquinoids, most valuable for dyeing and

25

printing cotton, wool and silk. pH is kept below a point at which damage to

protein fiber may occur (Shenai 1999).

Some of the vat dyes are Vat Brown BR, Vat Corinth B, Vat

Bordeaux B, Vat Red-BT, Vat Pink-B, Vat Scarlet-R, Vat Violet 3B, Vat

Orange R, Vat Orange RB, Vat Yellow G, Brilliant IndigoBASF/4G,

Indanthrene Blue RS, Caledon Jade Green XBN, Indanthrene Olive Green B,

Indanthrene Black BB, Indanthrene Yellow G, Indanthrene Orange RR,

Indanthrene Scarlet GG, Navinon Jade green FFBU/C (Jade green XBN),

Indanthrene Grey BG etc (Shenai 1997).

The syntheses of a new angular azaphenoxazine and of two new

angular azaphenothiazine ring systems and the dyeing properties of their

derivatives were studied. Elemental analysis, infrared, ultraviolet, NMR and

mass spectroscopy agree with the assigned tetracyclic structures. Reduction

with Na2S2O4 and the ease of air oxidation of the reduced forms to the quinoid

coloured materials make them applicable as vat dyes. Fastness to washing,

light, acids and bases and also their toxicity in laboratory animals were

investigated (Charles et al 1987). The heterogeneous chemistry of vat dye

dissolution process, which involves the conversion of the water insoluble dye

into the water soluble leuco form by reduction, is the major factor in

determining the cost efficiency and environmental compatibility of dyeing

(Alan Bond et al 1997).

Sporadic R&D efforts have made in IIT Delhi, Forest Research

Institute, Dehradun, etc. But a more coordinated action in the following areas

is urgently necessary: identification of plant sources as raw materials and

developing high yielding varieties such as in the case of Indigo; designing

proper conditions and parameters for dyeing; establishing conditions for

standardizing of the colour shades of dyeing to the extent possible; and design

26

of modern units/ factories for production of vegetable dyes in the form of

powder or cake (Ganesh 2008).

2.1.2.4 Properties of vat dyes

Vat dyes have excellent fastness properties when properly selected

and they are often used for fabrics that will be subjected to severe washing

and bleaching conditions. The range of colours is wide, but shades are

generally dull (Epp and Diane 1995). Vat dyes are characterized by all-round

fastness properties. These are water insoluble dyes and they cover almost full

range of shades. Greens, yellows, blues etc., are most commonly dyed shades

with vat dyes. Vat dyes can be broadly grouped into two categories-

anthraquinone and indigoid. Anthraquinone class produces bright shades and

change their colour in water soluble form (Mittal 1985).

The article of Rosseboom (1997) gave information on vat dyeing,

particularly concerning shade build up and crock fastness properties in full

shades. It was illustrated and supported with examples of recipes for piece

and yarn dyeing.

A number of vat dyes and anthraquinone intermediates have been

synthesized and then pigmented using high energy bead milling. Results have

shown that these converted vat dyes and intermediates can give pigments with

excellent tinctorial properties as well as good light fastness, durability and

overspray fastness properties. However, none of the vat dyes or intermediates

has all of the necessary properties required of an automotive quality pigment

(Thetford and Chorlton 2004).

27

2.1.2.5 Chemical characteristics and general application conditions

From the chemical point of view, vat dyes can be classified into

two groups: indigoid vat dyes and anthraquinoid dyes. Indigo dyes are almost

exclusively used for dyeing warp yarn in the production of blue denim (Epp

and Diane 1995).

Like sulphur dyes, vat dyes are normally insoluble in water, but

they become water-soluble and substantive for the fiber after reduction in

alkaline conditions (vatting). They are then converted again to the original

insoluble form by oxidation and in this way they remain fixed into the fiber.

Vat dyes preparations basically consist of a vattable coloured

pigment and a dispersing agent (mainly formaldehyde condensation products

and lignin sulphonates). They are generally supplied in powder, granules and

paste form.

The chemicals and auxiliaries that may be found in vat dyeing

processes are reducing agents like sodium hydrosulphite, thiourea dioxide

etc., caustic soda, sodium sulphate, polyacrylates and alginates as anti-

migration agents in padding processes, formaldehyde condensation products

with naphthalene sulphonic acid and lignin sulphonates as dispersing agents

surfactants such as ethoxylated fatty amines etc (Shenai 1999).

Vat dyeing conditions can vary widely in terms of temperature and

the amount of salt and alkali required, depending on the nature of the dye

applied. Vat dyes are therefore divided into the following groups, according

to their affinity for the fibre and the amount of alkali required for dyeing

(Shenai 1999).

28

· IK dyes (I = Indanthrene, K = cold) have low affinity; they are

dyed at 20 - 30 °C (low vatting temperature) and they require a

little alkali and salt to increase dye absorption.

· IW dyes (W = Warm) have higher affinity; they are dyed at 40 -

45 °C (moderate vatting temperature) with more alkali and little

or no salt.

· IN dyes (N = Normal) are highly substantive and they are

applied at 60 °C (high vatting temperature) and they require

much alkali, but no addition of salt.

· IN (special) dyes, which include black dyes, is marked by very

high concentration of alkali, moderate to high temperature.

These dyes require individual treatment.

The review article by Baumgarte (1989) has 319 references and

covers developments in the dyes themselves, reducing agents, and vatting

procedures, fundamental work and production oriented work on their

application to cellulosic fibres, dyeing fibre blends, printed cellulosic fibres

and their blends and the use of vat leuco ester dyes.

2.1.2.6 Principles and applications of vat dye

A wide range of different techniques are used in colouring

processes with vat dyes (Shenai 1997, 1999). Nevertheless, all processes

involve the following three steps:

· Vatting, in which the commercial vat dye is reduced and

solublised (vatted) by using sodium hydrosulphite (conventional

reducing agent) or other reducing agents and solublised by

sodium hydroxide. Hence, the reduction followed by

29

solublisation is called the vatting of the dye. This process is

brought out in two steps: (1) reduction of the dye into weakly

acidic leuco vat form (2) salt formation by neutralizing these

acidic leuco vat dyes by sodium hydroxide to give a water

soluble sodium salt of the leuco vat dye.

· Oxidation, in which, after absorption by the fiber, the dye in its

soluble leuco form is converted to the original pigment by

oxidation. This process is carried out in the course of wet

treatment (washing) by the addition of oxidants such as

hydrogen peroxide, perborate or 3-nitrobenzenesulphonic acid

to the liquor.

· After treatment is the final step which consists of the material

in weakly alkaline liquor with a detergent at boiling

temperature. This soap treatment is not only aimed at removing

pigment particles, but also allows the crystallisation of

amorphous dye particles, which gives the material the final

shade and the fastness properties typical of vat dyes (Prideaux

and Vivian 2004).

2.2 COTTON FIBER

2.2.1 Introduction

Today cotton is the most used textile fiber in the world. Its current

market share is 56 percent for all fibers used for apparel and home furnishings

and it is sold in the United States. Another contribution is attributed to

nonwoven textiles and personal care items. It is generally recognized that

most consumers prefer cotton personal care items to those containing

synthetic fibers. World textile fiber consumption in 1998 was approximately

45 million tons. Of this total, cotton represented approximately 20 million

30

tons. (Lawrence Shaw 1998). The earliest evidence of using cotton is from

India and the date assigned to this fabric is 3000 B.C. There were also

excavations of cotton fabrics of comparable age in South America. Cotton

cultivation first spread from India to Egypt, China and the South Pacific.

Cotton plant (Figure 2.6) belongs to the natural order of the Malvaceac or the

Mallow family (Chakraborty et al 1998).

Cotton in its pure form and with blends is the principal textile fiber

of the universe and is one of the world’s most socially vital and economically

important agricultural cash crops. Millions of people depend on cotton

cultivation because it meets the basic necessity of mankind and is known for

its diversity, enormous utility, applicability, economic viability and

advantageous properties (Kulloli and Naik 2008).

Figure 2.6 Cotton

2.2.2 History of Cotton

Cotton was used for clothing in present-day Peru and Mexico

perhaps as long as 5,000 years ago. Also, cotton was grown, spun and woven

in ancient India, China, Egypt and Pakistan around 3000 B.C. The history of

cotton, which is also referred to as “White Gold” is given below.

31

Cotton belongs to the genus Gossypium. It is a native of the tropical

and subtropical regions and it has a large number of species found all over the

world now. The seeds of the cotton plant have a clump of soft fiber around

them that are referred to as bolls. The boll opens about 48 days after the bud

forms and it may be picked two days after it has cracked. The separation of

hairs from the seed is carried out by a process termed ‘ginning’ (Marsh 1951).

This fiber is spun into yarn that can then be used to spin cotton cloth, very

comfortable for warm tropical climates. From its humble beginnings in

ancient civilizations, cotton has spawned an industry that occupies an

important place in the world economy today.

Cotton is also called as fabric of India since it has played a very

important role in the lives of Indians. India holds the largest area of 8 m ha

under cotton cultivation and ranked third in world’s cotton production, next to

China & USA and second largest consumer of cotton. Majority of cotton

grown commercially in the world is white lint but in recent years the colour

linted cotton has gained popularity (Kulloli and Naik 2008).

Cotton fiber is distinguished for its comfort properties. Although in

the past this fiber was said to be common man’s fiber, today the situation is

much different. The cost of this fiber has increased tremendously and also the

relative cost of synthetic fibers, mainly polyester, has gone down. Due to the

increasing awareness of environmental damage and requirement of

ecofriendly processing, the new trend of producing “Organic Cotton”, is

setting in. Such cotton does not make use of synthetic organic fertilizers,

fungicides, herbicides and insecticides. Demand for such eco-cotton is

increasing in countries like Germany, Switzerland, U.S.A. and European

countries (Teli 1997).

Besides sustaining the country’s textile industry, it earns precious

foreign exchange by way of export both yarn as well as finished goods.

32

Further, more cultivation of naturally colour linted cotton on commercial

scale not only forms an income generating activity for cotton cultivators, but

also a source of livelihood for local spinners and weavers, thus positively

supports the socioeconomic status of the handloom weavers, the neglected

sector of the weavers community. Generally colour linted cotton is short

staple, with low fiber strength compared to white cotton. Hence, it may be

difficult to spin the colour cotton to finer counts to produce quality yarn with

absolute uniformity (Kulloli and Naik 2008).

2.2.3 Raw Cotton Components

The over-all contents are broken down into the following

components and they are shown in Table 2.1.

Table 2.1 Raw cotton components

80-90% Cellulose

6-8% Water

0.5 – 1% Waxes and fats

0 - 1.5% Proteins

4 - 6% Hemicelluloses and pectins

1 - 1.8% Ash

During scouring (treatment of the fiber with caustic soda), natural

waxes and fats in the fiber are saponified and pectins and other non-cellulose

materials are released so that the impurities can be removed by just rinsing.

After scouring, a bleaching solution (consisting of a stabilized oxidizing

agent) interacts with the fiber and the natural colour is removed. Bleaching

takes place at an elevated temperature for a fixed period of time.

Mercerization is another process of improving sorption properties of cotton.

Cotton fiber is immersed into 18- 25% solution of sodium hydroxide often

33

under tension (Duckett 1975). The fiber obtains better luster and sorption

during mercerization.

After scouring and bleaching, the fiber is 99% cellulose. Cellulose

is a polymer consisting of anhydroglucose units connected with 1, 4 oxygen

bridges in the beta position. The hydroxyl groups on the cellulose units enable

hydrogen bonding between two adjacent polymer chains. The degree of

polymerization of cotton is 9,000-15,000. Cellulose shows approximately

66% crystallinity, which can be determined by X-ray diffraction, infrared

spectroscopy and density methods.

Each crystal unit consists of five chains of anhydroglucose units,

parallel to the fibril axis. One chain is located at each of the corners of the cell

and one runs through the center of the cell. The dimensions of the cell are a =

0.835nm, b = 1.03 nm and c = 0.79 nm. The angle between ab and bc planes

is 84º for normal cellulose, i.e., Cellulose I (Kadolph and Langfold 1998).

2.2.4 Repeat Unit of Cellulose

The current consensus regarding cellulose crystallinity (X-ray

diffraction) is that fibers are essentially 100% crystalline, and that very small

crystalline units imperfectly packed together cause the observed disorder.

The density method used to determine cellulose crystallinity is

based on the density gradient column, where two solvents of different

densities are partially mixed. Degree of Crystallinity is then determined from

the density of the sample, while densities of crystalline and amorphous

cellulose forms are known (1.505 and 1.556 respectively). Orientation of

untreated cotton fiber is poor because the crystallites are contained in the

micro fibrils of the secondary wall oriented in the steep spiral (25-30o) to the

fiber axis.

34

The basic material of cotton is cellulose, having an empirical

formula (C6H10O5) n and the chemical structure is shown in Figure 2.7

(Chakraborty et al 1998).

Figure 2.7 Structure of cellulose

2.2.5 Structure and properties of cotton fibers

The botanical name of American Upland cotton is Gossypium

Hirsutum and it was developed from cottons of Central America. Upland

varieties represent approximately 97% of U.S. production (Kadolph and

Langfold 1998). The crystalline orientations in cotton fiber have been shown

in Figure 2.8. Each cotton fiber is composed of concentric layers. The cuticle

layer on the fiber itself is separable from the fiber and it consists of wax and

pectin materials. The primary wall, the most peripheral layer of the fiber, is

composed of cellulosic crystalline fibrils (Duckett 1975).

35

Figure 2.8 Crystalline orientations in cotton fiber

The secondary wall of the fiber consists of three distinct layers. All

three layers of the secondary wall include closely packed parallel fibrils with

spiral winding of 25-35o and they represent the majority of cellulose within

the fiber. The innermost part of cotton fiber- the lumen- is composed of the

remains of the cell contents. Before boll opening, the lumen is filled with

liquid containing the cell nucleus and protoplasm. The twists and

convolutions of the dried fiber are due to the removal of this liquid. The cross

section of the fiber is bean-shaped, swelling almost round when moisture

absorption takes place.

2.2.6 Common Properties of Cotton Fiber

· Cotton is non-allergic since it doesn’t irritate sensitive skin or

cause allergies.

· Cotton’s softness makes it a preferred fabric for underwear and

other garments worn close to the skin.

· Cotton’s adaptability allows it to blend easily with most other

fibers including synthetics such as polyester and lycra.

36

· Cotton is one of the easiest fabrics to dye, making it very

popular with fashion and home ware designers.

· The three basic cotton weaves are: Plain (gingham, percales,

chambray, batistes and many other fabrics), Twill (denim,

gabardine, herringbone and ticking) and Satin (cotton sateen).

· Cotton can be given a coating or a finish. For example, cotton

used in fire fighting uniforms is coated and finished with

Proban®, a flame-retardant chemical treatment.

· Durable press (sometimes called permanent press) is a finishing

treatment used in cotton garments to eliminate creasing and this

reduces the need to iron. It retains specific contours such as

creases and pleats to be resistant to normal usage, washing or

dry cleaning.

· Cotton has a high absorbency (Meenaxi Tiwari et al 2009) rate

and holds up to 27 times its own weight in water.

· Cotton also becomes stronger when wet.

· Cotton’s strength and absorbency levels make it an ideal fabric

for medical and personal hygiene products such as bandages and

swabs.

· Terry cloth is a cotton fabric used to make common items such

as towels. It can be safely washed in very hot water and with

strong bleach and/or detergent.

· Cotton keeps the body cool in summer and warm in winter

because it is a good conductor of heat.

· Cotton is often used in the manufacture of curtains, tents and

tarpaulins as it is not easily damaged by sunlight.

37

· Cotton breathes easily as a result of its unique fibre

structure. This attribute makes cotton more comfortable to wear

than artificial fibers which are unable to provide similar

ventilation.

· Unlike synthetic fibres, cotton is a natural product and contains

no chemicals (Duckett 1975).

2.2.7 Chemical Properties of Cotton Fiber

Cotton swells in a high humidity environment, in water and in

concentrated solutions of certain acids, salts and bases. The swelling effect is

usually attributed to the sorption of highly hydrated ions. The moisture regain

for cotton is about 7.1~8.5% and the moisture absorption is 7~8% (Brandrup

and Immergut 1989).

Cotton is attacked by hot dilute or cold concentrated acid solutions.

Acid hydrolysis of cellulose produces hydro-celluloses. Cold weak acids do

not affect it. The fibers show excellent resistance to alkalis. There are a few

other solvents that will dissolve cotton completely. One of them is a copper

complex of cupramonium hydroxide and cupriethylene diamine (Schweitzer's

reagent).

Cotton degradation is usually attributed to oxidation, hydrolysis or

both. Oxidation of cellulose can lead to two types of so-called oxy-cellulose,

depending on the environment in which the oxidation takes place.

The effects of desizing, scouring, kiering and bleaching on the

removal of impurities from cotton fabrics and on fabric properties especially

whiteness and hydrophilicity were discussed (Mehie and Soljacic 1989).

38

An X-ray diffractometer was used (Parikh et al 2007) to study the

crystalline structure of cotton fibers after bleaching, cross linking and a

combination of bleaching and cross linking treatments. Wet cross linking was

accomplished with formaldehyde (Form W) and dry cross linking was carried

out with either dimethyloldihydroxyethyleneurea (DMDHEU) or citric acid

(CA). The results indicated that cross linking of bleached cotton did not

change the crystalline nature of cotton (i.e. it was Cellulose I), but it did

increase its degree of crystallinity when cross linked with either DMDHEU or

CA; cross linked formaldehyde (Form W) was relatively less crystalline.

Majority of natural dyes need a chemical in the form of metal salt

to create an affinity between the fiber and the pigment. These chemicals are

known as mordants. Some of the commonly used mordants are alum,

potassium dichromate, ferrous sulphate, copper sulphate, stannous chloride

and stannic chloride. Treatment of cotton with tannin, either natural or

synthetic, introduces additional hydroxyl and carboxyl groups on the fiber

matrix. This tannin-metal complex creates affinity on cotton for the natural

dye (Mahangade et al 2009)

2.2.8 Uses of Cotton

Apparel is of a wide range such as wearing apparel blouses, shirts,

dresses, children’s wear, active wear, swimwear, suits, jackets, skirts, pants,

sweaters, hosiery, neckwear and home fashion like curtains, draperies,

bedspreads, comforters, throws, sheets, towels, table cloth, table mats and

napkins.

In addition to the textile industry, cotton is used in fishnets, coffee

filters, tents and in book binding. The first Chinese paper was made of cotton

fiber as is the modern US dollar bill and federal stationery. Fire hoses were

39

once made of cotton Denim, a type of durable cloth, which is made mostly of

cotton, as are T-shirts.

The cotton seed which remains after the cotton is ginned is used to

produce cottonseed oil, which after refining can be consumed by humans like

any other vegetable oil. The cottonseed meal that is left is generally fed to

livestock.

2.2.9 Significance of Cotton Fabric

Cotton is one of the most versatile of all fibers, being strong,

comfortable and light. Cotton can be dyed and printed readily in a wide range

of colours and designs (Shukla 2000). Its fibers are highly porous, making

cotton clothing light and breathable, and it can be woven into any desired

density. This quality also enables cotton fabric to be dyed easily, making it a

natural choice for designers. Cotton fabric is especially soft and pleasing to

the touch; and, since it is derived naturally, those with sensitive skin are able

to wear the cotton fiber without any adverse reactions. Cotton is also a fabric

that responds well to sewing: it has a slight give, and it is not difficult to

handle like spandex or lycra. Its drape conforms well to the curves of the

body, which makes it a brilliant choice for women’s garments.

Cotton fabric is the most widely used textiles in the world,

accounting for more than 50% of total consumption, which is mainly made up

of cellulose. Several classes of dyes can be used to dye cellulosic fibers,

namely vat, direct, reactive, sulphur colorants, the different classes varying in

terms of factors such as their cost, ease of application, fastness properties etc.

(Yu and Zhang 2009).

Cotton fabric is also a hugely popular choice for undergarments: it

naturally wicks away moisture while retaining breathability. Cotton does not

40

require the maintenance of silk or other fabrics: it does not need to be dry-

cleaned and will not be ruined in a rainstorm. However, because the cotton

fibers are so porous, shrinkage of the material is a possibility (Chakraborty et

al 1998). Cotton fiber fabric has good market in India and abroad. The

processing, especially dyeing, is the critical part in textile mills. Attempts are

being made to adopt new approaches which can help to reduce load on

effluents at an optimum cost (Thakare et al 2006).

2.3 APPLICATION OF DYESTUFFS IN COTTON DYEING

Cotton has become one of the versatile textile fibers of the world

and now it ranks alongside wool and silk as the three great sources of clothing

material to meet the needs of the human race. At the earlier stage, dyeing of

cotton was employed with the vegetable dyes. Basic colours and acid colours

were adopted only to wool and silk, and they were found less application to

cotton. When the benzidine or direct colours were introduced, a new field in

cotton dyeing was opened up and the widespread use of dyed materials was

much stimulated. Because of the poor fastness property, introduction of

aniline black as a specialized feature in cotton dyeing greatly helped to extend

the use of dyed cotton materials by providing an extremely fast colour. The

later introduction of various sulphur dyes also stimulated the use of cotton

material by providing a number of fast shades (Chakraborty et al 1998).

With the advent of the so-called vat dyes, which permitted the

production of cotton of a wide range of beautiful shades of the highest

possible qualities of fastness, cotton fabrics were lifted out of their previous

rather low-grade class and it was elevated to the rank of fabric aristocracy. At

the present time, therefore, it may be said that cotton materials are used for

high grade fabrics, meeting the demand of high grade colours. Vat dyes are

special class of dyes that work with a special chemistry, especially on

cellulosic fibers (Meenaxi Tiwari et al 2009).

41

In the application of dyestuffs to cotton, several factors are

considered as of prime importance. In the first place, the form in which the

cotton is dyed will have much influence in the selection of the dyestuff.

Dyestuffs that are suitable for raw stock dyeing may not be suitable for

dyeing woven cloth or knit fabrics and vice versa. Cotton warp dyeing

requires special consideration (Chakraborty et al 1998).

Another consideration that is important in selecting cotton dyes is

the kind of material into which the fabric will be manufactured and the

eventual use to which it will be put. This will determine the qualities of

fastness of the dyestuff to be employed. Cotton goods go into all kinds of

materials at the present time. These goods are subjected to repeated washing

and laundering, and they must also stand exposed to light and perspiration. So

the colours must be fast to these agencies and a high class of dyestuff. The vat

dyes were being largely used for these goods (Jesse fields 1979).

Next, cotton denims are used extensively for over-all and

similar garments. Though this class of fabrics is perhaps not so much before

the eyes of the general public as some others, it is one of the great staples of

the cotton business and very large amounts of dye stuffs are used in them. The

principal colour used is blue, the fancy shades being negligible in amount and

the chief dyestuff used is indigo. In fact, this is where the great bulk of indigo

is used. The colour has to withstand very severe usage and repeated washings.

Logwood can be used to approximate the shade, but the fastness is much

inferior. Sulphur blues can be used with good advantage, and there are some

who may be inclined to think that sulphur dyes are as satisfactory for this

work as indigo. Hydron blue may also be considered as superior to indigo.

But the trade has long been accustomed to indigo and it will probably stick to

it for a long time to come ( Matthews 1918).

42

Over the decades there have been several papers on the colouration

of cotton based textiles. The number of articles dealing with the processing of

cotton, including preparation, dyeing and finishing, may be in thousands. An

investigation into the possible causes of problems occurring in the colouration

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 (Shamey and Hussein 2005).

In order to match the reflectance profile of the greenish leaf at NIR

region, four commercially available vat dyes were used to dye cotton fabrics.

The reflectance of dyed fabric and the transmittance of dye liquor in alcohol

solution were measured by using U-4100 spectrometer with an integrating

sphere. The effects of the combination dyeing, dyeing concentration (% owf)

and fabric weaves on the reflectance were also studied (Zhang and Zhang

2008). The results show that the reflectance of the combination dyeing is

determined by one of the dyes whose reflectance curve emerges at longer

wavelengths. Fabric weave has little effect on the reflectance for the same

dyeing program.

In the present scenario of environmental consciousness, the new

quality requirements not only emphasize on the intrinsic functionality and

long service life of the product but also a production process that is

environment- friendly. A comprehensive review on natural product based on

bioactive agents such as chitosan, natural dyes, neem extract and other herbal

products for antimicrobial finishing of textile substrates. The major challenges

and future potential of application of natural products on textiles have also

been critically reviewed (Joshi et al 2009)

43

2.3.1 Application of Vat Dye on Cotton Fabric

2.3.1.1 Introduction

Vat dye is the most popular among dye classes used for colouration

of cotton, particularly, when high fastness standards are required to light,

washing and chlorine bleaching (Roessler and Jin 2003). When outstanding

fastness is required in dyed cotton yarn, anthraquinone vat dyes remain the

dyer’s choice. No other dye class for cotton can match the fastness properties

of vat dyes. For this reason, these dyes are particularly valuable in the

colouration of yarns used in the manufacture of textiles having plaids, stripes

or other decorative patterns (Etters 1998b). The term ‘vat’ dye is derived from

the method of application of these colours rather than from any chemical

family (Crayton Black 1940). Vat dyes are practically insoluble in water, but

it can be converted into water soluble form called leuco dye by reduction with

a strong reducing agent like sodium hydrosulphite and solubilising agent

sodium hydroxide. The reduced dyestuff penetrates into the fiber and it is

reoxidised on the fiber back to the insoluble form, which remains fixed in the

fabric (Roessler and Jin 2003). The use of sodium hydrosulphite is being

criticized for the formation of non-environment friendly decomposition

products such as sulphite, sulphate, thiosulphate and toxic sulphur (Aspland

1992b). Therefore, many attempts are being made to replace the sodium

hydrosulphite by ecologically more attractive alternatives.

2.3.1.2 Conventional reducing method for vat Dye – Hydrose as

reducing agent

Reducing agents are compounds which donate hydrogen to,

subtract oxygen from or add electrons (negative charges) to other chemicals.

The affected chemicals are said to be reduced. During the reduction process,

the reducing agent itself is changed (oxidized), often irreversibly (Aspland

44

1992a). Several options were explored earlier such as fermentation method,

ferrous sulphate–lime method, zinc-lime method, bisulphite zinc-lime

method, thiourea dioxide, sodium borohydride etc. However, due to one

problem or the other these reducing agents were not found to be commercially

successful (Chavan and Patil 2004). The most commonly employed reducing

agent in vat dyeing is sodium hydrosulphite (Na2S2O4), commonly known as

hydrose. This compound is not stable in strong alkaline conditions in the

absence of air. Alkaline solution of hydrose has a certain degree of reduction

potential and thus, it can reduce all commercial vat dyes to their water soluble

forms, economically and quickly, without any chance of over reduction. As

the leuco sodium salt of the vat dye results salt of strong alkali and weak acid,

the sodium salt has the natural tendency to exist in enol form. To ensure

complete conversion, sodium hydroxide is added prior to hydrose addition.

Since hydrose reacts with aqueous as well as atmospheric oxygen, it leads to

the formation of a number of acidic compounds which have to be neutralized

by sodium hydroxide (Roessler and Jin 2003).

This is another reason why sodium hydroxide has to be added in

excess to avoid the dye bath to becoming acidic due to the precipitation of

leuco acid compound.

To avoid oxidation of the leuco vat, a large excess of sodium

hydrosulphite is used in conventional vat dyeing. The excess amount is at

least five times the theoretical amount, which might lead to the problems like

over reduction, hydrolysis and crystallization of vat dyes, especially at higher

temperatures.

45

2.3.1.3 Alternate reducing methods

Vat dyes are costly, and they require costly chemicals for dyeing

and they are also a source of pollution. Effort should be to develop non

polluting reducing agent and also to monitor and control the consumption of

sodium hydrosulphite (Gulrajani 1996). Until now, in most industrial vat

dyeing processes, vat dyes are reduced mainly using sodium dithionite. This

process produces large amounts of sodium sulphate and sulphite as

byproducts, which increase the costs for waste water treatment. Hence, many

attempts are being made to replace the environmentally unfavourable sodium

dithionite by ecologically more attractive alternatives such as organic

reducing agents or catalytic hydrogenation.

In recent investigations to improve the biocompatibility of the

vatting process even further, various electrochemical reducing methods have

been described such as indirect electrochemical reduction employing a redox

mediator, direct electrochemical reduction of indigo via the indigo radical,

electro catalytic hydrogenation and direct electrochemical reduction of indigo

itself on graphite (Roessler and Pandalai 2004). These methods offer

tremendous environmental benefits, since they minimize the consumption of

chemicals as well as effluent load. However, most of these electrochemical

processes are still in the developmental stage. Roessler et al (2003a) gave an

overview of the processes most commonly used and the state of development

of recent electrochemical innovations. Various developments that have taken

place in the eco-friendly dyeing of vat dyes have been briefly discussed in

Teli et al (2001) paper.

Entwicklungin der et al (1991) investigated that the two electrodes

were immersed in the dye solution. The cathode potential was kept below the

voltage at which hydrogen was evolved. A reducing agent was then used, the

redox potential of which increased by charge transfer. Over voltage required

46

for the reduction of the oxidized form of reducing agent to the reduced form

at the cathode was less than the cathode potential.

Zavrsnik (2002) presented about vat dyeing process in the past and

at present with focus on the upto date electrochemical reduction process. The

possibilities and new prospects of vat dyes, provided that all expectations

promised by this process come true are indicated. Introduction, history,

developments and standard dyeing process, principle and description of

properties, significance and applicability of these dyes from economical and

ecological view points and also described the dyeing trials in pilot devices in

detail. The article concludes mentioning electrochemical dyeing and

indicating the expected benefits promised by this new process.

It is the need of the day to substitute hazardous chemicals by

environmentally benign methods. The electrochemical dyeing using the

mediator technique claims technical, ecological and economic benefits, with

shorter and more reliable dyeing process, improved reproducibility, lower

effluent costs and better quality. This method is suitable for both vat and

sulphur dyes. This is a promising field, which needs to be explored

extensively (Shukla and Roshan Pai 2004). Chakraborty and Chavan (2004b)

presented the review paper related to the proper application of indigo on

denim as well as various aspects relating to this application.

Sodium bisulphite is used to partially replace the costlier sodium

hydrosulphite in vat dyeing of cotton yarn in package or cheese dyeing (Nair

2005). The dye bath for vat dyeing of yarn packages in the forms of cheese,

cop and cone is an aqueous solution of caustic soda and hydrosulphite, to

which the leuco vat, prepared externally, is added normally in two equal

portions under proper adjustments for liquor circulation in a package dyeing

machines. Sodium bisulphite exhibits better oxidation stability than

hydrosulphite, but it fails to reduce vat dyes when used alone. Sodium

47

bisulphite is found to be effective in package vat dyeing, where it can replace

25% of hydrosulphite normally used. At the mill where this method was

introduced, more than 45 regular shades have been successfully dyed with

savings.

The article (Bozic and Kokol 2008) gives a summary of the most

commonly used ecologically unfriendly processes for the reduction and

oxidation of vat and sulphur dyes. It also describes the new alternatives that

are in the developmental stage and could be important in the near future.

Sodium dithionite as the dominant reducing agent produces large amounts of

sodium sulphate, and also toxic sulphite and thiosulphate as by-products.

Consequently, high amounts of hydrogen peroxide and alkali are required for

the treatment of effluents, which adds to the cost of the process. Attempts

have been made to use organic biodegradable reducing agents, enzymes,

catalytic hydrogenation, and also indirect or direct, electrochemical reductive

methods that employ a redox mediator (electron-carrier). The reduction was

also carried out via the dye radical molecule or, in the case of indigo, by

direct electrochemical reduction using graphite as the electrode material.

Physical techniques, for example using ultrasound, magnetic fields

or UV, have been shown to be effective only when used to accelerate methods

using classical reduction and oxidation processes. Although these methods

offer some environmental benefits, there is still no satisfactory alternative

reducing and /or oxidizing agent available today. In Lester Boyd (1992)

paper, common problems with continuous dyeing of cotton and cotton /

polyester with vat dyes and with combinations of vat and disperse dyes were

reviewed. Recommendations were given for preferred equipment and dyeing

procedures.

48

(a) Vat process using reducing chemicals

In the conventional process of dyeing cotton, vat dyes and

especially indigo are traditionally reduced by sodium dithionite in the

presence of sodium hydroxide as alkali. This technique presents various

disadvantages (ecological problems, problems of storage, difficulty of control

of the process, colour variation of the dyed fabrics etc). The following papers

reported, give an idea about a novel promising eco-friendly technique of

cotton dyeing.

A suitable redox titration procedure was standardized and the effect

of sodium borohydride in various vat dye baths was studied (Nair and Shah

1970). An analysis of the redox curve gives valuable data on the stability of

the reducing system, leuco potential of the dye, and over-all stability of the

dye baths. It is shown that sodium borohydride does not improve the stability

of the dye bath against oxidation. In certain systems, it shows even adverse

effects. Results indicated that sodium borohydride cannot be a total or partial

substitute for sodium hydrosulfite in the current vat dyeing practice. This

finding was verified by hank-dyeing. In vatting and dyeing, the specificity of

a reducing agent appears to be more important than its reduction potential.

Previous investigations were focused on the replacement of sodium

hydrosulphite by organic reducing agents like a-hydroxy ketones (Marte

1989), which meet the requirements in terms of reductive efficiency and

biodegradability. However, such compounds are expensive and their use is

restricted to closed systems due to the formation of strong smelling of

condensation products in an alkaline solution. Some other reducing

compounds such as hydroxyalkyl sulphonate, thiourea, were also

recommended (Makarov2002). The relatively low sulphur content and lower

equivalent mass than hydrosulphite lead to lower amounts of sulphur based

49

salt load in the waste water. However, in these cases too, it is not possible to

dispense with sulphur based problems totally. Ciba-Geigy et al (1990)

patented a method for dyeing and printing with vat dyes using organic water-

miscible solvents as reducing agent.

Lunyaka and Logacheva (1988) investigated the possibility of

producing reduced forms of vat dyes in bifunctional organic solvents. The vat

dyes were almost completely reduced in mono ethanolamine and ethanol

hydrazine at 100◦-130

◦C. The dyeing of viscose rayon, polyester and polyester

/ viscose rayon fabrics with these leuco vat dyes was discussed. Marte E

(1989) investigated that hydroxy acetone as the reducing agent in refine

dyeing process. The advantages of this vatting process include reduced

chemical consumption, higher dye yield, deeper dyeings and reduced

pollutant load in the dyeing effluent, which can be degraded biologically. The

use of this process for continuous dyeing was outlined.

Ferrous hydroxide being a strong reducing agent in alkaline

medium, was also been explored for reducing organic dye stuffs. The

reducing effect of ferrous hydroxide increases with increase in pH. However,

ferrous hydroxide is poorly soluble in an alkaline solution and gets

precipitated. It has to be complexed in order to hold it in solution (Somet

1995).

A stable complex with good reducing power is obtained with

weaker ligand such as gluconic acid. Regarding eco-friendliness, gluconic

acid can be eliminated in the sewage tank through neutralization with alkali.

Free ferrous hydroxide can be aerated and converted to ferric hydroxide

which acts as a flocculent and brings down waste water load. Tartaric acid

was also tried as a ligand by Chakraborty (Chavan and Chakraborty 2000) for

complexing ferrous hydroxide in the presence of sodium hydroxide for

reduction and dyeing of cotton with indigo and other vat dyes at room

50

temperature. Efforts were made to complex ferrous hydroxide with single and

double ligand systems using tartaric acid, citric acid and gluconic acid, which

have shown encouraging results.

Redox potential measurements at various concentrations and

temperatures were carried out for a number of reducing agents selected

randomly to include the classical laboratory agents. The aim was to determine

(Adesanya Ibidapo 1992) their possible application in vat dyeing and to

obtain a comparison with the conventional reducing agent Na2S2O4. From the

redox titration, the leuco potentials of the dyes were determined, firstly with

Na2S2O4 and then with other reducing agents to assess their suitability. Using

the appropriate dyeing process, the dyes were applied to pretreated cotton

materials, which were then subjected to spectrophotometric absorption

measurements. Georgieva and Pishev (1999) studied the exhaustion kinetics

of leuco vat dyes on cotton yarns in the presence of triethanolamine. It was

shown that the addition of triethanolamine improves the dyeing uniformity.

Thomas and Edward (1995) patented a process comprising the

reduction of textile dyestuffs in an aqueous alkaline medium by means of a

reducing compound, which is a complex of an organic complexing agent and

an iron (II)-salt. Semet et al (1995) also investigated the iron (II) salt

complexes as alternatives to hydrosulphite in vat dyeing. Iron-gluconic acid

permits environmentally gentle dyeing without major changes of dyeing time

and apparatus.

The use of gluconic acid as a ligand for complexing iron (II) salts

and for vat dyeing of cotton was reported previously. This paper (Chavan and

Chakraborty 2000) reported the observations on the use of iron (II) salts

complexed with such ligands as tartaric acid and citric acid for the reduction

of indigo at room temperature and subsequent cotton dyeing (Samanta and

Agarwal 2009). This study includes the measurement of reduction potentials

51

of various iron complexes, their effect on dye uptake and the deposition of

iron on dyed fabric (Chavan and Chakraborty 2004). In the work of

Chakraborty and Chavan (2005) Fe (II) co-ordination complexes with suitable

ligands was reported.

Chakraborty and Chavan (2005) found that if two selective ligands,

one of which was essentially triethanolamine and the second either tartaric or

citric acid, were engaged to coordinate with the electrons in the -3d orbital of

Fe(II) at suitable molar ratios, a liquor was produced with very high reduction

potential, which reduced all vat dyes instantly at room temperature. In this

work, continuous and semicontinuous dyeing of vat dyes on cotton fabric at

room temperature was explored using two ligand based iron (II) salt

complexes and the results were compared with those obtained from the

existing hydrosulphite method. Iron (II) salt –two ligand based reducing

systems imparts better stability to reduced dye to cause better dye uptake

compared to the hydrosulphite system. Ligands must be applied prior to the

addition of NaOH for complete complexion of Fe (II) though the structure and

nature of these ligands control the stability of coordination linkage. Efficiency

of reduction baths was found to be maximum around room temperature

(Chakraborty et al 2005).

Rekha and Taroporewala (2002) investigated the use of eco-

friendly reducing agents such as glucose and dextranil for vat colours as a

substitute for conventionally used hydrose, which can also reduce pollution

load during dyeing. Bhattacharya (2005) in his paper explores that natural and

synthetic indigo dyes are reduced with natural ingredients, namely fenugreek,

jaggery, lime and soda ash. For comparison, cotton yarns were also dyed at

different concentrations of the indigo dyes using natural reducing system and

dye uptake was determined. The results revealed that dye uptake with natural

reducing system are about 25-30% less as compared to hydrosulphite system.

52

Complete substitution of hydrosulphite by natural reducing system is not

possible. The efficacy of the reducing bath for both the reducing system was

checked by the measurement of redox potential of the blank baths.

Anne Vuorema (2008) proved in her doctoral dissertation that

glucose can serve as a reducing agent of natural indigo. This finding is

significant for devising more ecological dyeing practices for the textile

industry. Indigo is a vat dye and it needs to be reduced to its water-soluble

leuco-form before dyeing. This allows the actual dye to pass on to textile

fibres. Glucose is known to be a good reducing agent, and Vuorema’s work

demonstrates that it also works with natural indigo.

The kinetics of heterogeneous reduction of red-brown Zh vat dye

with formaldehyde sodium sulfoxylate (rongalite) was studied. The dye was

reduced in the form of nonporous disk, which eliminates the effect of mass

transfer of the reaction rate. The stages of the reduction mechanism and the

reaction rate equations were proposed. (Polenov et al 2001).

Carver and David (1994) invented a process for reducing vat dyes

such as indigo, into their leuco form by placing a metal such as aluminum, in

water in the presence of a reduction facilitator to form a first solution, after

which a vat dye is mixed with the first solution to form a dye solution where

substantially all of the vat dye is reduced to its soluble leuco form. The

process includes dyeing fabric in the dye solution.

A process was investigated for reducing dyestuffs of the group

consisting of sulfur dyestuffs and vat dyestuffs, wherein the reduction was

carried out in an alkaline medium with isomaltulose or an isomaltulose-

containing mixture as the reducing agent. Reducing agent contains in

addition, isomaltose, saccharose, glucose, fructose or carbohydrate oligomers.

The aqueous alkaline medium has a pH>11, the reduction is carried out at

53

least 50оC and also the reduction is carried out under the influence of

ultrasound. A process for dyeing or imprinting cellulose-containing textile

materials with dyestuffs of the group composed of sulfur dyestuffs or vat

dyestuffs, wherein the initially water-insoluble dyestuff is reduced with a

process, then applied to the material, and oxidised thereafter (Dietmar Grull et

al 2000).

Mairal and Patel (2001), in their work replaced the conventional

reducing agent sodium hydrosulphite by sodium bisulphite and white dextrin

and sodium hydroxide by sodium carbonate and ethylene diamine nearly

100% replacement of sodium hydroxide and 25% sodium hydrosulphite is

possible with sodium carbonate and ethylene diamine and sodium bisulphite

and white dextrin respectively by leuco vat process and gives higher k/s

values which in turn depends on the particular class of dye.

Meksi et al (2007) in their paper presented a new technique of

reduction of indigo by sodium borohydride in the presence of a catalyst and

without the addition of alkali. This reduction was carried out at a temperature

range of 40–70 °C. In order to determine the leuco-indigo concentration in the

bath, a potentiometric titration procedure was established. The effect of some

main reaction parameters was studied: the temperature and the amount of

catalyser. This study indicated that the maximum yield of indigo reduction

took place at 55 °C with an amount of 1% of catalyser. The dyeing study of a

cotton fabric was carried out at the same temperature of the reduction

reaction. A “6-dip–6-nip” technique was used to study the performance of this

dyeing. The dyeing results were evaluated by measuring the colour yield (k/s)

at 660 nm. The best results were obtained at a temperature of 40 °C and with

an amount of 1% of catalyser.

In this study, the effect of the sodium borohydride amount on

indigo reduction yield was investigated (Meksi Nizar et al 2008). It appears

54

that the reduction yield increased with increasing of the amount of reducing

agent until 100% of sodium borohydride. After this value, the reduction yield

remained quite constant. The dyeing quality of cotton fabrics was also studied

in these conditions. It seems that generally these colorimetric parameters

depended closely on the reduction yield.

Two different methods for the direct application of indigo dye were

developed in England in the eighteenth century and they remained in use well

into the nineteenth century. One of these, known as china blue, involved iron

(II) sulfate. After printing an insoluble form of indigo onto the fabric, the

indigo was reduced to leuco-indigo in a sequence of baths of ferrous sulfate

(with reoxidation to indigo in air between immersions) (Egon wildermuth et

al 2005).

The ingredients of the zinc powder vat are zinc powder, lime and

indigo; in the presence of the lime and indigo the zinc takes up oxygen from

the water, liberating the hydrogen necessary to reduce the indigo to indigo

white, which dissolves in the excess of lime present (Michael Byrne 2007).

(b) Vat process with an Ultrasonic reactor

Optimal concentrations and reaction conditions in the vatting

reactor can be achieved by controlling the concentration of dye, reducing

agents and sodium hydroxide by appropriate modern analytical methods

(Goavaert 1999a, Etters 1995, Etters 1999). In contrast, it could be

demonstrated that cent percent vatting can be accomplished in a few minutes

in a completely oxygen free atmosphere and with the use of ultrasound (Marte

et al 1990). Ultrasound enhances the vatting rate by disintegrating the

dispersed water insoluble dye aggregates into smaller particles. Owing to the

increase of the dye surface, in addition to the simultaneous shortening of the

diffusion interface, the probability for collisions between molecules of the

55

reducing agent and the indigo molecules increases, and finally the reaction

rate will be faster. An apparatus for the above process comprises a mixing

vessel and atleast one reactor with ultrasonic resonators.

Since the degradation products of the excess dithionite can be

acidic and have to be neutralized by alkali, the hydroxide in the vatting

mixture is also consumed during the process. Thus, the process and the

apparatus have the great advantage, that is a high colour yield is achieved and

less environmental pollution is produced. However, further lowering of

electrolyte concentration which would increase the concentration of dissolved

leuco dye can be effected only with electrochemical vatting or by

hydrogenation.

The influence of ultrasound on the vatting rate of indigo RN with

a-hydroxy acetone was determined (Poulakis et al 1996). In the absence of

ultrasound, the reduction reaction is not sensitive to any variation of the test

parameters as long as the hydroxy acetone concentration remains within the

limits of 12.5 ml/l and 5ml/l, and the NaOH concentration is between 12.5 g/l

and 5 g/l. With the use of ultrasound, the vatting rate in all cases investigated

was increased by a factor of approximately 4-5. The higher vatting rate was

connected with the disintegration of the dispersed water insoluble dyestuff

aggregates caused by the action of ultrasound. Owing to the resulting

enlargement of the dye surface, in addition to the simultaneous reduction of

the diffusion interface, a greater probability of collision of the reducing agent

with indigo molecules and ultimately a higher reaction rate of vatting was

produced.

Thakore et al (1990) reviewed the application of ultrasound to

textile wet processing. The use of ultrasound in textile wet process offers

benefits in terms of saving in process time, energy, chemicals and

56

improvement in product quality. Ultrasound has great potential for

commercial applications. Knowledge of previous systematic studies would

allow an increase in productivity, easier process control and reduction in

water pollution.

(c) Catalytic hydrogenation method

A long time ago (1917) the catalytic hydrogenation of vat dyes was

suggested by Brochet. In particular, by reducing an alkaline indigo paste

using Raney nickel as catalyst at a hydrogen pressure generally between 2 and

4 bars and at a temperature between 60°C and 90°C. However, it is

impossible to use this technique on-site in a dye house due to high explosion

and fire risk. Thus, prereduced indigo shipped as a 40% aqueous solution can

be used directly in a dye bath, which has only to be stabilized by reducing

agents. This offers the advantage that in the dyeing process, a considerable

proportion of the chemical reducing agent, dithionite can be dispensed with.

Unfortunately, the eco-efficiency of this process is negatively affected by the

high water content of the product (60%). Some dye houses are already using

the product eventhough they have to completely rely on the dye supplier,

because at present only a very few companies are offering this technology

(Roessler and Jin 2003).

The application of electro catalytic hydrogenation of vat dyes at

electrodes comprising a thin grid coated with a layer of nickel in which fine

particles of Raney nickel are dispersed was investigated by

spectrophotometric and voltametric experiments in laboratory cells.

Experiments show the feasibility of this new route, which offers tremendous

environmental benefits and has a vast potential in textile dyeing processes,

because it does not require any reducing agent (Roessler et al 2002b).

57

The industrial feasibility of the electro catalytic hydrogenation of

indigo on Raney Ni electrodes as a suspension in an aqueous medium was

studied in a divided flow cell. An attempt was made to establish optimized

conditions in the system, and a scale-up in indigo concentration to 10 g/L was

achieved. Increasing pH can enhance the reduction rate and a maximum

conversion was found by optimizing current density and temperature. The

reaction rate is clearly enhanced in the presence of ultrasonic waves and

organic solvents. Methyl alcohol is among the best co-solvent. With Raney Ni

electrodes an optimized current efficiency of 13% could be reached at 95%

conversion. This value was enhanced to 19% by using electrodes doped with

Pt particles (Roessler et al 2003b).

Cosmisso and Mengoli (2004) showed that the reduction can

alternatively be performed in an alkaline bath at room temperature by using

H2 activated by a Pd catalyst. Sulphur dyes are thus directly reduced, whereas

vat dyes require an electron transfer mediator. This alternative reaction can

also be driven using H2 at sub-atmospheric pressures. It further avoids all

drawbacks raised by electrolytic reductions, so far suggested as alternative

green processes.

The goal of Ragunathan’s (2005), project was to test whether or

not electrolysis can reduce vat dyes without the assistance of sodium

hydrosulphite, which is the commercial toxic reducing agent used by vat

dyers. Results show that an electrolytic method for reducing vat dyes is

indeed a time consuming process , but it is definitely a safe alternate method.

The indirect cathodic reduction of the vat dye indigo (C.I. Vat Blue

1) by cathodically reduced Lawsone (2-hydroxy-1, 4-naphthoquinone; C.I.

Natural Orange 6) was studied in aqueous solution at different pH values.

Cyclic voltammetry and spectroelectrochemistry were used to investigate the

electrochemical behavior of 2-hydroxy-1, 4-naphthoquinone at a hanging

58

mercury drop electrode. Spectrochemical experiments were used to prove

the indirect cathodic reaction of dispersed vat dyes by 2-hydroxy-1,

4-naphthoquinone (Komboonchoo et al 2009).

Marte et al (2003) patented a method and apparatus for electro

catalytic hydrogenation of vat dyes and sulphur dyes. The inventive method is

suitable for batch operation and continuous operation. It works entirely

without any reducing agents and provides subsequently salt free dye

concentrations of upto 200g/l.

(d) Electrochemical techniques

Increasing eco-efficiency of textile wet processes became an

important topic in the research group (Roessler et al 2003a). Reducing agents

required for the application of vat and sulfur dyes cannot be recycled, and

they lead to problematic waste products. Therefore, modern aspects of

economical and ecological requirements are not fulfilled. The application of

direct electrochemical reduction of indigo as a novel route was investigated

by spectrophotometric and voltametric experiments in the laboratory cells.

Experiments yield information about the reaction mechanism and the kinetics,

and they show the possibility of this new route for the production of water -

soluble indigo, which offers tremendous environmental benefits.

Bechtold et al (1997a) described the application of indirect

electrolysis as a reduction technique in indigo dyeing. The new process offers

environmental benefits and others offers the prospects of improved process

stability, because the reduction state in the dye bath can be readily monitored

by measuring reduction potential. This particular technology entails that

solutions to dyeing problems need no longer be found solely by plant

engineering and process optimisation methods, but also by appropriate

selection of the mediator systems used. The application of electrochemical

59

process technology in textile dyeing offers a wide range of advantages

(Bechtold et al 2000).

The process of electrochemical dyeing developed and patented by

Dystar, and currently being tested in a pilot study under production conditions

by the Austrian firm Getzner textil (2002) was described. The process was

designed for the vat dyeing of yarn in packages. The new process uses

electrical current to reduce the dyes instead of reducing agent, with the help of

a regenerable Fe2+

/ Fe3+

reducing system. This will reduce the number of

chemical processes required and thus lead to less pollution.

Grotthus (1807) was the first person to discover that electric current

itself is capable of reducing indigo. Electrochemical reduction can be

achieved by direct and indirect electrochemical reductions. In, direct

electrochemical reduction chemical reducing agents are replaced by electrons

from electric current, and effluent contaminating substances can be dispensed

with altogether (Schrott et al 2000). Thus they offer tremendous

environmental and ecological benefits providing a vast potential in textile

dyeing processes. Schrott (2004) discussed the various aspects of direct and

indirect electrochemical dyeing.

Frede et al (1996) patented a process for the electrochemical

reduction of vat dyes in aqueous solution in the presence of a mediator

system, in which carbon or graphite felt is used as the cathode material. From

5% to 60% by weight, aqueous alkaline solution of reduced indigoid dyes is

prepared by reducing the indigoid dye electrochemically in the presence of a

mediator (Bechtold et al 2001 and 1999). Chavan and Patil (2004)

investigated the use of electrochemical dyeing with different reducing agents

for vat and sulphur dyes which produced good colours on the dyed products.

Though Sodium dithionite is universal reducing agent, it is very unstable and

gets decomposed oxidatively and thermally to several byproducts.

60

Ayoub Haj Said et al (2008) in their research conducted an

experimental parameters optimization study to establish more suitable

conditions for industrial use. They tested the electrochemical reduction of vat

Blue1 (indigoid) in lucid, average and dark shades. Furthermore, they tested

potentiometry at imposed low current as a new control means of the dyeing

bath. The colour and fastness evaluations of the obtained samples indicated

that the performances of the dyeing operation were similar to those obtained

by the conventional method. This result offers new prospects for the

electrochemical reduction of indigo.

Electrochemical methods are being used increasingly as an

alternative treatment process for the remediation of textile waste waters. This

study focused mainly on the colour removal and chemical oxygen demand

(COD) reduction of vat textile dye (CI Vat Blue 1: indigo) from its aqueous

solution by electrochemical oxidation. This process was carried out in a

batch-type divided electrolytic cell under constant potential, using a Pt cage as

anode and Pt foil as cathode. Operating variables such as supporting

electrolyte, pH, ultrasonification and treatment time were investigated to

probe into their effects on the efficiency of the electrochemical treatment. The

experimental results indicate that this electrochemical method could

effectively be used as a pretreatment stage before conventional treatment

(Dogan and Turkdemir 2005).

Electrochemical determination of redox active dye species was

demonstrated in indigo samples contaminated with high levels of organic and

inorganic impurities. In this work (Vuorema et al 2008) the indigo content of

a complex plant-derived indigo sample (dye content typically 30%) was

determined after indigo was reduced by the addition of glucose in aqueous 0.2

M NaOH. The soluble leuco-indigo was measured by its oxidation response at

a vibrating electrode. Determinations of the indigo content of 25 different

61

samples of plant-derived indigo were compared with those obtained by

conventional spectrophotometry. This comparison suggests a significant

improvement by the electrochemical method, which appears to be less

sensitive to impurities (Vuorema et al 2008).

Marte et al 2003 patented a method for electrochemical reduction

of vat and Sulphur dyes in aqueous solutions in steady state conditions of

reaction and a cycle which is largely free of reducing agents. The invention

also related to the apparatus for carrying out the said method. The steady state

conditions of reaction were obtained by means of a start reaction. The

substances used for this reaction and the products resulting therefrom were

extracted from the cycle. To maintain the cycle only dyes, an alkali and

possibly small quantities of additional substances such as surface active

agents, need to be added. No other chemicals which are active in the

oxidation – reduction process are used.

Roessler et al (2002b) in their paper presented a novel route for the

environmentally friendly production of water soluble indigo, which is also

based on electrochemical reduction (Marte 2000, Roessler et al 2003a). A

conventional two compartment glass H-cell (each with a volume of 200ml)

separated by Nafion-34 membrane (DuPont) was used both for voltammetry

and small scale preparative electrolysis. For spectrophotometric measurement

two glass fiber sensors were connected to a diode array spectrophotometer.

Cell potential, cathode potential and current were measured with multimeters.

In the case of voltametric studies a computer controlled potentiostat was used

together with the radiometer rotating disc electrode model.

Direct electrochemical reduction of indigo via the indigo

radical: This method is based on a reaction mechanism, in which a radical

anion is formed by a comproportionation reaction between the dye and the

62

leuco dye, followed by the electrochemical reduction of this radical. The

leuco dye acted as an electron-shuttle between the electrode and the surface of

the dye. Pigment has to be generated first in a small quantity to initiate the

reduction. Further electrochemical reduction is self-sustaining. From the

electrochemical investigations it was concluded that the reaction product

(leuco indigo) is stable under the conditions used (Rossler et al 2002a). The

effect of several parameters such as current density, pH and temperature, on

electrochemical kinetics was analysed and that was the basis for further work

on scale-up and optimization (Rossler et al 2002a). Until now, however,

reactor performance even for bath stabilization is still too low. Further

investigations focusing more on enhancing the radical concentration (by

surfactants) is necessary.

Direct electrochemical reduction of indigo on graphite

electrodes: Carbon and graphite are extensively used in electrochemistry

because of its high surface area. Chaumat used a specially prepared cathode of

finely divided indigo and graphite powder in a solution of Sodium Carbonate

(Chaumat 1907). It was shown that graphite granules can act as electrode

material for the direct electrochemical reduction of indigo in aqueous

suspension. Optimized conditions were sought and a scale-up in indigo

concentration to 10g/l was achieved (Rossler et al 2002a). Due to high

hydrogen over-voltage on graphite under the applied conditions, no

chemisorption or very weak chemisorption of hydrogen is possible (Kinoshite

1998). Unfortunately, the reduction rate was very low. Therefore, a great deal

of work was focused on the acceleration of the process (Rossler et al 2002b).

Thus special pretreatment of the graphite (say soaking with

hydrogen peroxide or pre anodization) was investigated to enhance the

reduction rate (Rossler et al 2002b). Another approach to enhance the electro

63

catalytic properties was based on the covalent bonding of quinonoid

molecules onto the graphite surface. In the particular case of indigo reduction,

anthraquinone was also used as redox-active molecules. These substances are

already well known from the mediator process. Till now, it is possible to

reduce an extensive range of vat dye (indanthrene dyes) and indigo

suspensions upto 100g/l without blocking the reactor. The results are

obviously the basis for further development of cheap, continuously and

ecologically working cell for the direct electrochemical reduction of dispersed

vat dyes.

In the work of Roessler and Crettenand (2004) the application of

electrochemical reduction of several vat dyes and even mixtures of them on a

fixed bed cathode consisting of graphite granules was investigated by

spectrophotometric experiments. These experiments yield information about

the kinetics and show the possibility and versatility of these mixtures for the

production of water soluble leuco indigo, which offers environmental benefits

(Roessler et al 2003a). Regarding the effect of different molecular structures

on the kinetics and a preliminary understanding of the reaction mechanism,

relationships between computed molecular parameters and the reduction rate

were presented. The discovery seems to be of future interest, both from an

economical and ecological point of view for the industrial application of an

electrochemical vatting process. However, these results are a basis for further

development of cheap, continuously and ecologically working cell for the

direct electrochemical reduction of dispersed vat dyes (Roessler and Pandalai

2004).

In the application of vat dyes, the substitution of non-regenerable

reducing agents such as Na2S2O4 by cathodically regenerable reducing agents

offers great ecological advantages. In a demonstration project (Bechtold and

Turcanu 2009) a multi-cathode electrolyser was successfully coupled to a

64

dyeing apparatus for package dyeing with a capacity of 1 – 10 kg material.

Iron-complexes with triethanolamine and Na-D-gluconate were used as

cathodically regenerable reducing agents. Dye bath regeneration was

performed by ultra-filtration. Substantial reduction in chemical consumption

and reduction in waste water volume, released from the dyeing process could

be demonstrated. The quality of dyeing was assessed with regard to levelness

and iron content analysed in the dyed samples.

Indirect electrochemical reduction or mediator enhanced

electrochemical reduction: The dyeing process of indigo can be greatly

improved by means of regenerable reducing systems (Blatt et al 1999). For

this purpose suitable electrochemical mediator systems such as iron (II) –

triethanolamine (TEA) were identified. With regard to the needed electrochemical

equipment, an electrochemical cell, especially a cathode construction as a

solution to achieve a large cathode area with sufficient cell current, was

described.

In order to meet these requirements in an economical way a multi-

cathode cell was suggested. Bechtold et al (1991) investigated

electrochemical process techniques for reducing species produced insitu in

textile dyeing processes. A redox potential corresponding to conventional

reducing agents can be achieved electrochemically in dye liquors indirect

electrolysis (Bechtold et al 1997b).

In order to colour cellulose fibers with vat dyes, large amounts of

non-regenerable reducing agents are usually required. Bechtold et al (1999)

presented the use of indirect electrochemical reduction, thus offering

economic and ecological advantages. Multi-cathode cells were used to

overcome low cell current, and a further increase in cell current was achieved

by using three-dimensional flow-through electrodes. In indirect

electrochemical reduction technique, the reduction of dye is achieved through

65

a redox mediator system. This process is mediated by an electron carrier

whereby reduction takes place between separated surfaces of the electrode

and the dye pigment instead of by direct contact between both surfaces.

An attempt was made with complex mixtures of triethanolamine

and D-gluconate as mediator (Rossler and Jin 2003). They are of particular

interest for the mediator process because it is possible to combine the

advantages of both ligands (Mohr and Bechtold 2001). These mediators,

however, are expensive and are not entirely harmless from the toxicological

point of view. A great advantage of this technique is the direct information

about the state of reduction in the dye bath, which is available by redox

potential measurement, and thereby control by adjustment of the cell current

is possible. The reducing agents normally used cannot be monitored in a

comparable manner. Thus often a surplus of reducing agents has to be applied

to guarantee stable dye bath conditions.

Among the various mediator systems suggested in the literature,

iron triethanolamine complex (iron-TEA) seems to be promising (Bechtold et

al 1999). In order to meet all the requirements in an economical way, a multi-

cathode cell with a large number of cathodes, electrically connected with one

or two anodes, was suggested. This configuration allows the operation of the

cell with maximum area cathode and minimum area anode. Recently,

Bechtold et al (2000) demonstrated that mediator system obtainable by

mixing one or more salts of a metal capable of forming a plurality of valence

states with atleast one amino containing complexing agent and a hydroxyl

containing, but amino devoid complexing agent in an alkaline medium, has

the improved capacity to reduce vat dyes. Both the electrochemical reduction

techniques have not yet been commercialized and the research and

developmental efforts are in progress in this direction. Here the most

challenging engineering task is to achieve a dye reduction rate and a current

66

efficiency, which are high enough to make electrochemical reaction

industrially feasible.

Anbukulandainathan et al (2007) presented the study of indirect

electrochemical reduction of selected vat dyes using iron-triethanolamine

complex as a reducing agent. Essential requirements for the design of

electrochemical cell were suggested. Iron-TEA-NaOH molar ratio was

standardized to get the dyeing of cotton by indirect electrochemical reduction

technique. Colour yields were compared with those of conventional sodium

dithionite method. Repeated use of dye bath after dye separation was also

explored. Kulandainathan et al 2007, in their paper overviews the recent

progress made in both direct and indirect electrochemical dyeing processes

and the parameters that control the dyeing process.

Continuous regeneration of reducing agent is possible by cathodic

reduction. However, colour depth appears to be poor with the implemented

electrochemical system as compared to the conventional vatting technique

using sodium dithionite as reducing agent. Better results may be possible with

sophisticated electrochemical systems and optimization of mediator system.

Experiments with shorter material to liquor ratios have shown better colour

depths. Fastness properties appear to be equivalent with the conventional

dyeing with good light, washing and rubbing fastness.

9, 10-Anthraquinone-2-sulfonicacidsodium-salt (AQS2), 9, 10-

anthraquinone-1, 5-disulfonic Na-salt (AQDS1, 5) and 1, 4-dihydroxy-9, 10-

anthraquinone (DHAQ1, 4) were investigated as to their capacity to act as

mediators for indirect electrochemical reduction of dispersed organic dyestuff

(Bechtold et al 1999).

The indirect cathodic reduction of dispersed vat dyes CI Vat

Yellow 1 and CI Vat Blue 5 was investigated by cyclic voltammetry and with

67

batch electrolysis experiments. Fe3+-d-gluconate and Ca2+-Fe3+-d-gluconate

complexes as mediators for indirect cathodic reduction of vat dyes

experiments. The batch reduction process was followed experimentally by

photometry and redox potential measurement in the catholyte (Bechtold and

Turcanu 2004).

The indirect cathodic reduction of dispersed vat dyes CI Vat

Yellow 1 and CI Vat Blue 5 was investigated (Bechtold and Turcanu Aurora

2006) by cyclic voltammetry and spectroelectrochemistry. N, N-bis (2-

hydroxyethyl)-3-amino-2-hydroxy-propane-sulfonic acid and 2, 2-bis-

(hydroxyethyl)-(iminotris)-(hydroxymethyl)-methane were used as ligands to

form electrochemically active Fe-complexes in alkaline solution.

Bechtold (2006) eventually developed a new electrochemical

dyeing process (EUROENVIRON ECDVAT) become a future standard

technique in vat dyeing. The reducing agent added in the conventional method

was replaced by a cathodic electron transfer with a reversible redox system

(mediator). The reduction oxidation process was performed in an

electrochemical cell. The cell was coupled to the dyeing apparatus and the dye

bath is reduced electrochemically. The oxidised dyestuff was added to the dye

bath and got reduced therein by means of a mediator. The redox potential

required for proper dyestuff reduction is formed by cathodic reduction, which

was a completely new technique in vat dyeing.

Yet in 2003, the necessary equipment remained too expensive from

the commercial point of view. Since then, the research group has concentrated

on making the machinery cheaper to broaden the uses of the high quality vat

dyeing technology, in other words concentrated on making it more attractive,

competitive and profitable. This innovative electrochemical dyeing with vat

dye process has ushered in a new era in the technological textile dyeing

industry with further applications and markets to be explored.

68

Dispersed vat dyestuffs can be electrochemically reduced by

indirect electrolysis using iron–triethanolamine complex as a reducing agent.

The application and mechanism of indirect electrolysis as a reduction

technique have been described in detail in this paper (Kulandainathan et al

2007). Electrochemically reduced vat dye was tested on a laboratory scale in

dyeing experiments, and the results of different reduction conditions were

discussed. The influence of the concentration of the complex-system on the

build-up of colour depth, shade and fastness was discussed and compared

with samples of the standard dyeing procedure using sodium dithionite as the

reducing agent. The new process offers environmental benefits as well as

prospects for improved process stability, because the state of reduction in the

dye-bath can be readily monitored by measuring the reduction potential

(Kulandainathan et al 2007).

Vuorema (2008) also investigated indirect electrochemical

reduction. She discovered that 1, 8 - dihydroxyanthraquinone was an efficient

catalyst for glucose-induced reduction. Electrochemical reduction can be

introduced only by major companies as it requires an investment in special

equipment.

2.4 OTHER DIFFERENT TECHNOLOGIES FOR VAT DYEING

PROCESS

Environmental legislation and its enforcement have undoubtedly

forced the textile industry to be rather cautious in selecting the appropriate

processes and equipment. The most efficient, economic and minimal

environmental pollution processing methods was increasingly demanded

throughout 1990s. However, the majority of the textile industry consists of

small and medium enterprises, where the lack of expertise on the use of best

available techniques leads to the levels of operation far away from the

optimal. Implementation of new technological advances in indigo dyeing can

69

result in improved quality control in both dyeing of denim yarn and

laundering of denim garments, lower dyeing and laundering costs and reduced

pollution (Etters 1995). Kulikova et al (1987) investigated the use of

ultrasonic radiation for developing leuco ester dyes on cotton fabrics. The

results of laboratory tests and mill trials were presented.

The ever-buoyant market for indigo denim was assessed (Holme

1992). The indigo recovery system which uses ultra-filtration system from

osmonics was described, and the latest research by the American Association

of Chemists and Colorists indicates that indigo dyeing should be carried out

for the best results related. Boyd (1992) reviewed the common problems with

continuous dyeing of cotton and cotton / polyester with vat dyes and with

combinations of vat and disperse dyes. Recommendations were given for

preferred equipment and dyeing procedures.

A new method of dyeing with vat dyes in jet and package dyeing

machines involves Rongal 5242 (BASF) and hydrosulphite with control of the

redox potential (Latham and Kramrisch 1991). Operations at the world’s

newest indigo dyeing plant at Granite ville (SC) were described (Anon 1995).

The plant incorporates two new Morison indigo rope dyeing ranges. Granite

ville intends to become a major player in the indigo denim market; several

other indigo denim dyeing operations were also described. Hoechet et al

(1988) developed a leuco vat ester padded with a vanadate by padding

through oxalic acid or a peroxy disulphate.

Industrial trials were conducted (Pobedinski and Ryzhakov 1996)

with a view to develop a photochemical method for dyeing cotton-containing

fabrics with kobozol leuco vat dyes. The use of equipment developed for

treating the fabrics shows that it can be used for the quality dyeing of cotton

and cotton / polyester fabrics with kobozol dyes in light and medium shades.

70

The chemistry of indigo dyeing was reviewed (Paul et al 1996).

Indigo is a vat dye which oxidizes and reduces very easily. This makes it

possible to apply relatively dark shades to cotton by repetitive dipping in the

dye bath, but is also a source of shade depth variations and levelness and

fastness problems. The requirements for successful indigo dyeing and

solutions for problems which can arise have been outlined.

Foam was used (Magda et al 1997) in a pigmented vat dye bath to

attain a more homogeneous distribution and efficient sedimentation of the vat

dye particles as well as a lower wet pick up. However, another chemical pad

was still required to develop colour on the fabric. Three anthraquinone vat

dyes were used. The variables that may be changed with foam dyeing were

studied and finally the fastness properties of the dyed cotton samples were

correlated to those of normal dyeing.

Frey (1997) described a brief outline of the technology and market

for indigo dyeing and the new Ben-indigo technology developed by

Benninger Co.Ltd., Switzerland. The benefits of the technology were claimed

to be better dyestuff fixation, the need for only three steeping baths, savings

in reducing agents, improved consistency in dyeing results and automatic

process. Horne (1995) presented a review of vat dyeing on cotton yarns. Vat

dyeing on cotton covered the preparation, dye classification, reduction,

exhaust rate, migration, oxidation and soaping procedures . Problems in vat

dyeing are reviewed and problems with certain vat dyes were noted.

Rosseboom (1996) presented an elucidation of the technique of vat dyeing

with residual colour baths under modern conditions while closely combining

economic requirements with ecological requirements.

Schrott et al 2000 illustrated the most important fields of

application for vat dyeing according to product divisions and requirements.

The latest process developments for more simple, reliable and cost effective

71

vat dyeing were also presented. The electrochemical dyeing process, in which

chemical reducing agents were replaced by electric current, dispenses

altogether with effluent. Patton and Hall (1998) patented a process for

recovering a vat dye from textile scrap using heated organic solvent. This

process is particularly advantageous for recovering indigo dye from denim

scrap.

Cotton, polyester and polyester/cotton blended fabrics were dyed

with 12 anthraquinone vat and vat disperse dyes. Details of the substrate,

thickening agents and dye stuff were noted (Ali and Nassar 2003). The dyeing

process was discussed, the effect of temperature of steaming and the effects of

different thickeners used were then analysed, followed by a discussion of the

fastness properties of the printed fabrics, most of which were found to be very

good.

Chavan et al (2002) investigated that organic alkalis showed a

higher colour yield of indigo on cotton, compared to conventionally used

alkali. The alkalinity of organic alkali was much less compared to NaOH

alkalinity, and therefore it was necessary to use higher concentrations of these

alkalis to bring out the reduction of indigo in the presence Na2S2O4.

Bleached cotton fabric samples were dyed with two leuco vat ester

dyes and the colour wass developed using ultraviolet light. Mathematical

functions were selected which most accurately describe the relationship

between colour and the dye concentration in the dye bath. A universal method

of modeling was developed using spline approximation (Pobedinski and

Telegin 2002).

Photo stability of nine (seven anthraquinone and two thioindigoid)

vat dyes on cellulosic films was examined (Toshio et al 2002). The ease (k0)

with which the dyes were photo-oxidized was estimated by the relative fading

72

of vat dyes on cellophane immersed in aqueous Rose Bengal solution on

exposure, a condition resulting in only the photo-oxidative fading. The

photosensitivity (f) was estimated by the relative fading of an

aminopyrazolinyl azo vinylsulfonyl dye on cellophane dyed in admixture with

vat dyes examined under the same conditions as above.

Photo degradation of indigo was studied with emphasis on the

degradation products. Ultraviolet-Visible spectrometry, a high temperature

resolved mass spectrometry, was employed for characterization of aged

samples (Novotna et al 2003). The degradation of synthetic and natural indigo

was compared and an explanation for the higher rate of degradation of natural

indigo was proposed.

Indigo dye blue denim is more popular than any other items of

apparel because it maintains its brightness and hue on repeated washings.

Indigo belongs to a class of vat dyes and contain about 96% indigotin,

requiring only about 2% dye to produce the navy blue. Dyeing method

comprises reduction of the dye (indigo) with alkali (sodium hydroxide) in the

presence of a reducing agent (hydrosulphite) followed by treatment of the

fabric in the dye bath. Major disadvantages of this method are variation in

concentration of reactants in the dye bath affecting the quality of dyed fabric,

frequent replenishment of reactants and lesser affinity which requires repeated

dipping and nipping techniques. In the case of cotton fabric, it has other

disadvantages such as instability of reducing agent, poor dye uptake, shade

depth fluctuation, poor and non-uniform colour yield, no affinity for dye, etc.

From this point of view, particularly for dyeing cotton fabric with indigo, a

dyeing process with the use of an organic alkali (alternative alkali) was

developed. This process is economical, time saving and consumes less

amount of indigo dye (Chavan 2006).

73

A process for the simultaneous caustic treatment and dyeing of

cotton goods with vat dyes by the pad steam process, wherein the goods, after

being padded with a dye liquor and dried, was subjected to a caustic treatment

step and subsequently further treated in conventional manner (Horn and Peter

1989).

In the study of Perkins (1997), the reduction of solid indigo in

buffered and non-buffered aqueous media was investigated. Data were

compared to those obtained from a Volta metric study of indigo dissolved in

dimethyl sulfoxide (DMSO), N, N dimethyl formamide (DMF) and in

pyridine and the interpretation of results was facilitated by in situ UV–VIS

spectroscopy and atomic force microscopy (AFM) studies. For the reduction

of solid indigo two distinct types of reduction processes, ‘surface-type’ and

‘bulk-type’, were observed. Alan Bond et al (1997) in their study applied a

new technique for an investigation of the complex reductive dissolution

process of indigo in aqueous media. Importantly, it was proposed that the

parameters readily obtained from this type of solid state voltammetry may

enable the rate and mechanism of reductive dissolution of vat dyes to be

rationalized.

Westbrook et al (2003) described a sensor system for continuous

on-line and in-line measurement of indigo and sodium dithionite

concentrations during textile dyeing with indigo. The system was based on an

electrochemical method, more specifically, multistep chronoamperometry. A

repetitive sequence of potentials was applied at the surface of a platinum

electrode, resulting in the oxidation of indigo and dithionite. The electrons

released in these reactions were measured as an electrical current, and this

signal is proportional with the concentrations of indigo and dithionite.

Kostas Metaxiotis (2004) presented an expert system, and designed

and implemented it in four stages: (1). formal description of the key factors

74

that affect the dyeing process in the textile industry, (2).development of

models for the representation of knowledge, (3).integration of the above

mentioned models in a unified information system (4).supports the decision-

making process in the management of textile enterprises.

A process was suggested for using reduced vat dyes in a continuous

dyeing process for the production of dyed yarns and fabrics of different

colours. In the process, dye composition was introduced to a treatment unit

for reduction to desired dye composition (Arioglu et al 2007). The dye

concentration in the treatment unit was lower than feeding dye concentration

so that dye precipitation does not occur, but it is significantly higher than the

circulating dye concentration so that the dye is reduced efficiently. Although

the preferred location for the treatment unit is before the circulation line, it

may be at any location before the dip-dye tank.

A Gram-positive, anaerobic, moderate thermopile, strain WvGT,

capable of reducing indigo dye was isolated from a fermenting woad vat

prepared essentially as in medieval Europe. Based on the results of the

phylogenetic analysis and phenotypic criteria, it was proposed that the

unknown moderate thermophile should be classified as Clostridium isatidis

sp. nov., a new species of the genus Clostridium (Nikki Padden et al 1999). It

was concluded that Clostridium isatidis was the organism responsible for

indigo reduction in the woad vats of the past.

The reduction of water-insoluble indigo by the recently isolated

moderate thermophile, Clostridium isatidis, was studied with the aim of

developing a sustainable technology for industrial indigo reduction. Addition

of madder powder, anthraquinone-2, 6-disulfonic acid, and humic acid all

stimulated indigo reduction by C. isatidis. Redox potentials of cultures of C.

isatidis were about 100 mV more negative than those of C. aurantibutyricum,

C. celatum and C. papyrosolvens, and reached –600 mV versus the SCE in the

75

presence of indigo, but the potentials were not consistently affected by the

addition of the quinone compounds, which probably act by modifying the

surface of the bacteria or indigo particles. It was concluded that C. isatidis can

reduce indigo because (1) it produces an extra cellular factor that decreases

indigo particle size, and (2) it generates a sufficiently reducing potential

(Nicholson and John 2005) .

Out of the different sections of textile processing, ultrasonic

applications to dyeing appears to be most promising (Patra and Das 2006). In

other areas the application was also beneficial. The high energy ultrasonic

waves in general accelerate the rate of textile wet processes. There is a scope

for saving in time, energy and chemicals by using ultrasound in processing.

Moreover, effluent load also decreases in many dyeing operations. However,

no large scale industrial installations have yet undertaken ultrasonic assisted

processing. The possible reasons could be non-availability of such large

equipment, the absence of techno-economic data of production scale, lack of

knowledge of predictability of ultrasonics with production size equipment and

lack of clarity of process control parameters and the possibility of intensity

variation with large production equipment. Till date all trials are more or less

in laboratory scale only. This means that to commercialise the process, more

studies from the actual application point of view and parameter control are

called for.

The evolution of pad–steam processes for fixing printed vat dyes on

cellulosic fabrics was traced. The behaviour of a wide range of vat dyes under

laboratory "flash-ageing" conditions with sodium dithionite (hydrosulphite)–

caustic soda as the reducing system indicated the most satisfactory thickening

agent, dyes, and padding and steaming conditions, and this led to the design

of a bulk-scale steamer operating at about 7 yd. /min. with a steaming time of

about 20 sec. A modified process is being worked out with thiourea dioxide as

76

reducing agent in the printing paste, the development taking place by padding

with caustic soda before steaming. This is particularly suitable for designs of

low coverage, or where it is essential to process other types of dyes alongside

vat dyes. Preliminary experiments indicate that emulsion thickenings are

unlikely to show much technological advantage over conventional thickenings

in the flash-ageing process (Fern and Liquorice 2008).

Indigo has been used for thousands of years to colour and paint

different substrates, including medicines, food, cosmetics and textiles. Its

commercial importance led to the development of a number of quantitative

determination procedures based on titrimetric, spectrophotometric, and

chromatographic methods. In the work of Eugenio Reyes Salas et al (2005),

the electrochemical behavior of indigo in dimethyl sulfoxide (DMSO) was

studied to establish a simple, direct determination method and to apply it to

the quantification of a natural indigo sample.

A standard dyeing apparatus for yarn on X-cones was coupled to a

multicathode electrolyser for indirect reduction of dispersed oxidized vat dyes.

An alkaline solution containing 0.01 M iron-triethanolamine complex was

used as mediator solution. During the electrochemical dyeing process, the cell

must carry out different functions: reduction of dissolved oxygen, indirect

reduction of dispersed oxidized vat dye, buildup of a certain reduction

capacity against air oxygen and control of redox potential in the dyeing

apparatus by adjustment of cell current. The concentrations of dissolved

oxygen and reduced form of the mediator during electrolysis can be described

by a simple mathematical model, which permits efficient optimization of

important technical parameters for process design and scale-up (Bechtold and

Turcanu 2002).

Indigo dye has only moderate substantivity for cotton and a lot of

dye remains in the dye vats, creating major effluent problems. Deo and

77

Roshan Paul (2004) investigated a new standing bath technique for effluent

free denim dyeing. The repeated reuse of an indigo vat of higher

concentration could provide a wide range of lighter shades, with considerable

cost saving. This could also probably provide a solution to the effluent

problems faced by indigo dyeing units.

Colloidal indigo is reduced to an aqueous solution of leuco-indigo

in a mediated two-electron process converting the water-insoluble dye into the

water-soluble leuco form. The colloidal dye does not interact directly with the

electrode surface, and to employ an electrochemical process for this

reduction, the redox mediator 1, 8-dihydroxyanthraquinone (1, 8-DHAQ) is

used to transfer electrons from the electrode to the dye. The mediated

reduction process was investigated at a (500-kHz ultrasound-assisted) rotating

disc electrode, and the quantitative analysis of voltametric data was attempted

employing the Digisim numerical simulation software package. The average

particle size and the number of molecules per particles were estimated from

the apparent bimolecular rate constant and they were confirmed by scanning

electron microscopy (Vuorema et al 2006).

Indigo is extensively used for the dyeing of denim. Chakraborty

and Chavan (2004a) gave an overview of the proper application of indigo on

denim as well as various aspects relating to this application. In the paper

(pektive and boja 2004) the specific characteristics of conventional way of

dyeing with chemical reducing agents were examined, with wider review on

the process of reduction and oxidation. Innovative methods of dyeing were

also examined by which the dyeing with vat dyes was technologically simple

and at the same time the emission of chemical in the waste water was used as

a donor of the electrons for the reduction.

Optimization of dyeing poly (lactic acid) fibers with vat dyes was

investigated. Conventional method for dyeing cellulose fibers with vat dyes

was able to be applied for dyeing poly (lactic acid) fibers. It has become

78

obvious that higher dyeing temperature and concentration of auxiliaries have

negative effects on the dyeability of dyes on poly (lactic acid) fibers.

Determination of optimal dyeing condition was also investigated. Sawada and

Ueda (2007) found that optimal concentrations of dyes and auxiliaries could

be estimated through simple linear experimental equation.

The Indanthrene Olive Green B (C.I. Vat Green 3; C.I. 69500),

VG3 dye, a vat dye bearing an anthraquinonoid group and a ketonic group,

can be detected by differential pulse voltammetry in alkaline solution using

glassy carbon electrode. An analytical method was proposed for determining

the vat dye using modified glassy carbon electrode by electrochemical

activation in alkaline medium (Maria Valnice et al 2006).

The reduction of indigo (dispersed in water) to leuco-indigo

(dissolved in water) is an important industrial process and it was investigated

here for the case of glucose as an environmentally benign reducing agent. In

order to quantitatively follow the formation of leuco-indigo, two approaches

based on (i) rotating disk voltammetry and (ii) sonovoltammetry, were

developed. Leuco-indigo, once formed in alkaline solution, can be readily

monitored at a glassy carbon electrode in the mass transport limit employing

hydrodynamic voltammetry. The presence of power ultrasound further

improves the leuco-indigo determination due to additional agitation and

homogenization effects. The redox mediator 1,8-dihydroxyanthraquinone was

found to significantly enhance the reaction rate by catalysing the electron

transfer between glucose and solid indigo particles ( Vuorema et al 2008).

Haworth and Kilby (1998) outlined the trend in the development of

the application of vat dyes to piece goods. While a survey of the whole field

was made, so far as is possible from the published literature, particular

attention was paid to the difficulties encountered in developing the stand fast

molten-metal machine as a bulk production unit.

79

Anthraquinone immobilized onto the surface of indigo micro

crystals enhances the reductive dissolution of indigo to leuco-indigo. Indigo

reduction is driven by glucose in aqueous NaOH and a vibrating gold disc

electrode was employed to monitor the increasing leuco-indigo concentration

with time. Anthraquinone introduces a strong catalytic effect which was

explained by invoking a molecular wedge effect (Figure 2.9) during co-

interaction of Na+ and anthraquinone into the layered indigo crystal structure.

The glucose-driven indigo reduction, which is ineffective in 0.1 M NaOH at

65 °C, becomes facile and goes to completion in the presence of

anthraquinone catalyst. Electron microscopy of indigo crystals before and

after reductive dissolution confirms a delamination mechanism initiated at the

edges of the plate-like indigo crystals. Catalysis occurs when the

anthraquinone–indigo mixture reaches a molar ratio of 1 : 400 (at 65 °C;

corresponding to 3 M anthraquinone) with excess of anthraquinone having

virtually no effect. A strong temperature effect (with a composite EA 120 kJ

mol-1

) is observed for the reductive dissolution in the presence of

anthraquinone. The molar ratio and temperature effects are both consistent

with the heterogeneous nature of the anthraquinone catalysis in the aqueous

reaction mixture (Vuorema et al 2009).

Figure 2.9 Wedge effect

80

A comparative study of enzyme-mediated indigo reduction (Bozic

et al 2009a) was presented as an environmentally-friendly alternative to

alkaline sodium dithionite reduction. The effect of the mediator 1,8-

dihydroxy-9,10-anthraquinone in enzymatic reduction was studied by means

of voltammetry, both in the presence and absence of different textile materials

(polyamide 6, polyamide 6,6 and cotton), and compared to chemically

reduced indigo. It was observed that bio-catalytic formation of leuco indigo

and its exhaustion on substrates is inversely proportional to the pH within the

range of 7–11. Additionally, substrate colouration was strongly influenced by

the mediator, resulting in situ formation of leuco indigo. The wash,

perspiration, and light fastness properties of all textile materials dyed with

enzymatically-reduced indigo were comparable or even better than those

obtained with chemically reduced indigo. The use of enzyme-mediated

reduction of indigo combined with potential reuse of the reduction bath

represents a cost effective and environmentally-friendly dyeing process that

can be applied for the dyeing of natural cellulosic and synthetic polyamide

fibers (Bozic et al 2009b).

In the work of Dhaouadi and Henni (2009), sewage sludge was

used as a textile dye adsorbent. A sample of crude dehydrated sewage sludge

issued from an urban wastewater treatment plant (high-rate aeration, activated

sludge process) was utilized for vat dye retention. The main objective of this

work was to evaluate the “efficiency” of the crude material on vat dye

sorption.

2.5 EFFECT OF VARIOUS PARAMETERS ON DYEABILITY

IN VAT DYEING OF COTTON

The amount of unfixed indigo (Etters 1993) that was removed from

denim yarn in the wash boxes on continuous dyeing ranges was influenced

strongly by the application method used during dyeing. Traditional methods

81

that employ needlessly high concentrations of unfixed dye were removed

from the yarn in the wash boxes. On the other hand, a new dyeing technique

permits the utilization of much less indigo to achieve a given sample results in

much lower concentrations of indigo in the effluent. Etters (1998b) presented

the trouble shooting in package dyeing with vat dyes. Many of the problems

encountered are not solvable; hence hands on experience are needed.

Depending on the dye bath pH, reduced indigo can exist in three

forms: as the nonionic acid leuco, the mono phenolate ion or the bi phenolate

ion. Shade depth for a given amount of fixed dye, i.e. colour yield, was shown

to be highly correlated with the fractional amount of indigo that exists as a

mono phenolate ion in a dye bath. The correlation was explained (South

eastern section, AATCC 1989) in terms of the increased apparent affinity of

the mono anion for cotton overpossessed by other ionic forms of indigo. As

the affinity increases, the strike rate of the dye for the yarn surface increases

leading to a more ring dyed yarn.

The effect of pH, quantity and granulation of the absorbent on the

adsorption of indanthrene dyes by activated carbon from cotton dyeing

effluent was investigated (Dosen et al 1990) in order to optimize the process.

Lunyaka and Ozenova (1988) examined the composition of surface active

agents in relation to shade variation during the dyeing of regenerated cellulose

films with vat dyes. Nikol’skaya et al (1989) discussed the dyeing of cotton

fabrics with vat dyes from suspensions in liquid ammonia. It was found that

ethylene glycol in the concentration range 35-45 g/l improved the quality of

dyeing. High levels of colour fastness were attained. Krichevskii and

Bulusheva (1986) presented the use of catalysts and accelerators for the

reduction stage in vat dyeing and printing.

The use of polar solvent type system, in particular ethanol / water

mixtures for the shade development of 21 commercially available vat dyes on

82

cotton fabrics was investigated (Bram hall 1985). After dyeing, the shade was

measured in its oxidized state developed by various methods and then

remeasured. The results showed that the shade development for some dyes

was in the same direction as conventional soaping when using polar solvents,

but not to the same degree. The fastness properties were generally unchanged.

Apparent equilibrium sorption of indigo on cotton denim yarn was

determined at dye bath pH ranges of 13.1-13.3 and 11.1-11.3 under infinite

bath conditions at 25◦C. The equilibrium concentration of indigo on the cotton

fiber is much higher for dyeing at the lower of the two pH ranges. The higher

apparent affinity of the dye at the lower pH range is correlated with the mono-

ionic form of indigo, and sorption was effectively described by the empirical

Freundlich isotherm (Etters and Hou 1991). Etters (1990) in his paper

presented that the higher the pH, the better was the penetration of the yarn

bundle, and the worse are the resulting colour yield and wash down

characteristics. Also it was possible that the new knowledge with regard to the

use of buffered alkalies in indigo dyeing may open up new areas of

application for indigo that were previously unattainable when the

conventional alkali, sodium hydroxide, was used.

A comparison was made of dyes which differ only in their

chromophore groups, but which possess a third group, said to be mainly

responsible for their affinity. Starting from an aromatic diamine, the

tetrazotization, coupling or condensations to the test dyes were described and

the dyeing results were discussed (Riesz 1990). The mechanism of poor wet

rubbing fastness of fabrics dyed in deep shade was explored and the ways of

dealing with the problem were discussed (Herlinger and Schulz 1990).

The effect of particle size on colour yield, wash fastness and

frosting in continuous vat dyeing of 100% cotton was investigated (Palmetto

section 1991). The findings showed that colour yield for Vat Blue 6 was

83

independent of particle size. The colour strength for Vat Red 13 decreased

with increasing particle size. Vat Brown 1 showed an irregular dyeing

behaviour. Two possible reasons for the behaviour of vat Red 13 –migration

and incomplete reduction were investigated. It was found that the particle size

have no effect on wash fastness or flat abrasion.

Lunyaka et al (1990) investigated the mechanism of dyeing kiered

and bleached cotton and viscose rayon fabrics with vat dyes. The mechanism

consists of forcing out the capillary water.

Vashurina et al (1991) investigated the use of ethylene glycol in

liquid ammonia as an aggregation promoter in the dyeing of cotton fabrics

with vat dyes. Lunyaka (1991) investigated the mechanisms involved in the

dyeing of cellulosic fabrics with leuco vat dyes. The negative effect of dyeing

temperature on dye aggregation was studied. Nikol’skaya et al (1989)

discussed a procedure for combining vat dyeing with the caustic soda

mercerizing of cotton fabrics. Reagent concentrations were optimized and the

saving which can be achieved was examined. Gartseva et al (1992) presented

a technology for developing leuco vat dyes on cellulosic fibers using ozone

treated water. Optimum ozone concentrations in the process of water were

specified. Dyeability was improved by 30-100%, depending on the dye type.

Levelness, brightness and colour fastness were also improved.

Due to the steady trend toward smaller batches, the jig has regained

importance. When dyeing with vat dyes in the jig, caustic soda and

hydrosulphite play an important role. They must be constantly present in a

minimum concentration. The concept of a metering process for solving

typical problems in jig dyeing was presented (Schnitzer 1991) and illustrated

with an example from actual practice. The article concluded with a discussion

of the advantages of metering chemicals.

84

The colour yield of indigo dyeing was affected by a number of

process parameters such as dye concentration, immersion time and oxidation

time, number of dippings, temperature and pH (Chong et al 1995). Best

colour yield can only be achieved by dyeing under optimum conditions.

However, other than the temperature, pH and perhaps the redox potential,

most of the parameters are not controllable. Pretreatment of the material with

suitable cationic active compounds could increase the colour yield achieved

and possibly reduce the number of necessary dippings. The pretreated

material also showed a lower dependency on the pH of the dipping bath. On

the other hand, rubbing fastness of the indigo colour yield on the pretreated

material was also improved.

The modification introduced in the mordanting technique has

shown an improvement in colour strength in the range 140-300% with

excellent fastness to washing, rubbing, perspiration and light. Mahangade et

al (2009), has identified four sustainable leaf sources and also provides an

improved method of mordanting to obtain enhanced strength with excellent

fastness properties of the dyed cotton.

The k/s values increased with increase in concentration of

mordants. However, beyond 2% concentration of each of the mordant, the

increase in k/s values and fastness properties on fabric was not significant

enough. It was also evident that a wide variety of shades ranging from

yellowish brown to golden brown, grey to dark brown and pink to red could

be obtained from the dyes when mordanted with different mordants (Bhuyan

et al 2004).

High Performance Liquid Chromatography (HPLC) analysis of

indigo, showing that with the increase in the dyeing time, the structural

changes of indigo component are attributed to decrease in colour strength

(Samanta and Agarwal 2009). Fastness properties of resin dye-fixatives on

85

cotton fabrics are not only related to their structure characteristics, but also

related to their molecular weight bigness and distribution. Addition of

functional groups which can be reacted with dyes or cellulose into these dye-

fixatives would improve their fastness. The molecular weight of these dye-

fixatives should be also controlled (Yu and Zhang 2009).

Twenty anthraquinone based compounds with different substituents

were prepared (Shakra and Ali 1995), recrystallised from the appropriate

solvent and used as vat, disperse and vat/disperse dyes. Good light fastness

was obtained. Also good sublimation fastness will be obtained if the used dye

has high molecular weight.

Wash fastness and light fastness can be increased by the use of

metal salts or tannic acid on cotton fabrics. Cotton yarns, when treated with

Acalypha dye after pre-mordanting with potash, alum, potassium dichromate,

copper sulphate and ferrous sulphate showed excellent colour fastness

properties. Indigo showed light fastness rating of 3-4 or 5-6 depending on the

mordant used. In the ISO II test, the fastness of the indigo and logwood was

superior to that of the native natural dyeing, such as presian berries and water-

lilly root respectively (Samanta and Agarwal 2009).

Mairal et al (1996) made an attempt to study the effect of

developing bath concentration in dyeing of vat dyes to mercerized and

unmercerized cotton fabrics in the presence and absence of wetting agent by

pigment padding technique. It was concluded that, the dye uptake increases

and shows maximum value at higher concentrations of caustic soda and

sodium hydrosulphite in the developing bath.

Etters (1996) observed that dye uptake by cotton fiber and the

extent of penetration of indigo dye into the denim yarn cross section are

influenced strongly by the pH controlled ionic species of both dye and cotton

86

in dye bath. High dye bath pH results in a level of ionization of both indigo

dye and cotton cellulose, resulting in decreased substantivity and increased

penetration of the yarn fiber bundle by the dye, with associated lower colour

yield. On the other hand, moderate dye bath pH promotes lower ionization of

dye and fiber, resulting in increased ring-dyed yarn and greater colour yield.

Perkins (1997) dealt with exhaust dyeing process developed by

cotton incorporated and BASF. The advantages of this process are complete

dye penetration, better colour yield, eco-friendliness due to dye bath reuse,

carrying out finishing and washing procedures in the same vessel, Conducting

dyeing at constant temperature with out garment pretreatment.

Thumm (1997) investigated the effect of softeners to minimise the

fading of indigo dye by photochemical smog. Some softeners accelerate the

dye destruction. Optimized softeners reduced the rate of the process. Counter

measures were presented for minimizing complaints due to yellowing

problems.

Bechtold et al (1997a) found out that the dispersed indigo dyestuff

can be reduced by indirect electrolysis using iron (II) - triethanolamine

complex. The iron (III) form of complex can be transformed to the iron (II)

form by cathodic reduction, thus leading to a regenerable reducing agent.

Electrochemically reduced indigo was tested in laboratory scale dyeing

experiments, and the results of the different reduction conditions in the dye

bath were discussed. The influence of the concentration of the complex -

system on the build up of colour depth and shade with increasing number of

dips was discussed and compared with the samples of the standard dyeing

procedure using sodium dithionite as reducing agent. The results of the latter

conventional process show that the dyestuff in the dye bath behaves in a

manner similar to that when a regenerable iron (II) complex was used as the

reducing agent.

87

The danger of trying to obtain the benefits of pH controlled indigo

dyeing by the exclusive use of caustic dosing in the dye bath was emphasized

(Etters 1998a). It was revealed that when the beneficial influence of a dye

bath pH of 11.0 was sought by the use of exceeding small concentrations of

NaOH in an unbuffered bath, small fluctuations in dye bath pH resulted. Such

dye bath pH instability was a primary source of quality problems in denim

garments. Indigo dye baths stabilized by proven buffering systems were much

to be preferred for achieving the optimum pH, since those buffered systems

maintain the dye bath alkalinity at a consistently safe level.

Simulated garment dyeing of both cotton and white denim and

cotton knit fabric with indigo revealed (Etters 1999) useful information with

regard to the effect of dye bath pH, concentration of NaCl and Concentration

of PVC, Poly Vinyl Pyrolidine on dye uptake resulting shade depth and

fastness.

Linear sweep and cyclic voltametric studies of indigo, indanthrene

dyes and sodium dithionite in alkaline solutions were reported (Govaert et al

1999b). The results were applied to develop amperometric determination

methods for these vat dyes and reducing agent, on different electrode

materials (Au, glossy carbon, Pd, Pt). The reduced dyes gave an anodic

voltametric signal. The reactions referring to the oxidation of dyestuff were

concentration, rotation and potential scan rate dependent. Multipulse

amperometry allowed to measure in a continuous way of voltametric signal

that was proportional to the dye concentration. The sodium dithionite

concentration can also be monitored continuously using chronoamperometry,

but only in the absence of dyestuff due to the mentioned adsorption.

A novel regenerated cellulosic fiber called enVix was prepared

from cellulose diacetate fiber using environment-friendly manufacturing

process. Vat dyeing properties of the enVix were investigated (Lee et al 2005)

88

and compared with those of regular viscose rayon. The enVix exhibited better

dyeability than viscose rayon. For the economical and ecological demands of

today, Benninger developed and successfully introduced the dyeing of indigo

in closed nitrogen filled troughs. The main features and advantages of this

technology are the concerns of this article (Coutsicos and Benninger 2006).

Chavan and Chakraborty (2000) investigated the principle of

cationization of cotton to improve colour yield which was exploited in a

unique and simplified way. Cotton fabric was pretreated with different water

soluble metal salts followed by padding with indigo. Colour yield for each

metal salt treated fabric was measured in terms of k/s values. Among the

various metal salts, FeSO4 and cobalt sulphate show higher increase in colour

yield. Dry methods are effective at lower concentration of FeSO4, whereas

wet methods for higher concentration of FeSO4. Wash and light fastness

properties of FeSO4 pretreated and indigo padded cotton are equivalent to

those obtained in conventional method, but rubbing fastness, especially wet

staining, was severely affected for FeSO4 pretreated and indigo padded

samples.

The dyeing behaviour of hydrogenated indigo in electrochemically

reduced dye baths was studied (Bechtold et al 2008) on cotton yarn with

particular focus on dark shades. Indigo concentration in the dye bath was

studied from 0.9 g l−1

indigo to 7 g l−1

to achieve indigo fixation near the level

of dyestuff saturation. The number of repetitive cycles of immersion into the

dye bath and air oxidation was increased up to eight dips. The highest k/s

value at 660 nm was measured with k/s 31 at samples dyed with eight dips in

a dye bath containing 1.8 g l−1

indigo and at a bath pH level of 11.7–12.1.

When comparable amounts of indigo had been fixed on the yarn from baths

containing 3.5 or 7 g l−1

indigo, k/s values decreased. An analysis of the

reflectance curves between 400 and 700 nm proved that the visually perceived

89

increase in darkness was caused by lowered reflectance in the wavelength

range of 400–525 nm.

A brief survey of practical vat pigment padding procedures (Fox

and Mawson 2008) was supplemented with data on the physical properties of

suitable dyes. The effect of particle size on the rate of reduction was discussed

in relation to its practical implications as well as factors such as colour yield,

dispersion stability and migration effects during drying. The use of a new

organic polyelectrolyte as a migration inhibitor was described.

Shim et al (2006) investigated the dyeing and fastness properties of

regular viscose rayon with three vat dyes according to a different amount of

reducing agent. It was found that smaller particle size of leuco vat dyes has a

greater specific surface area than large particles, and it would be expected to

react more rapidly unless overreduced by adding excessive reducing agent.

Also, a sample with a larger proportion of small particles would be expected

to reduce more rapidly than a sample with predominantly large particles. The

colour yield of the vat dyes on regular viscose rayon was dependent on the

amount of concentration of the dye bath auxiliaries, especially reducing agent

to convert leuco vat dye.

Haga and Ariuchi (2008) studied the physical properties of cotton

fabric, which was studied with natural indigo reduced by fermentation.

Natural indigo dyeing gave cotton fabric smaller air permeability, smaller

heat conductivity and smaller initial maximum value of great flux than

synthetic indigo dyeing with an increase in dyeing times. The cotton fabric

dyed with natural indigo exhibited larger mechanical properties except for

abrasion resistance than those with synthetic indigo reduced by hydrosulphite

at dye uptake of about 45 mg/g. It was suggested that natural indigo dyeing

changed the structure of cotton fiber in association with high order structure

of fiber.

90

Light fastness is the degree where a colourants resists fading caused

by light exposure. All dyes have different degrees of resistance to fading by

light, and colourants have some susceptibility to light damage. The resistance

to degradation of fabric dyes and prints caused by light was a necessary

requirement of a garment. Higher colour fastness was now being demanded

for apparel that will be worn predominantly outdoors (Nimkar and Bhajekar

2007). There are two methods widely accepted by most customers in testing

the criteria of light fastness, which are American Test Method (AATCC 16,

option 3) and ISO Test Methods (ISO 105/B02). These methods recommend

the use of artificial light source, Xenon Arc lamp exposure, because it is

representative of natural day light. Among the factors that affect colour

fastness to light are dye stuff, dye shade, fabric surface and finishing

chemicals.

The only Indian dye stuff manufacturer, Atul, has developed a new

range of micro-disperse vat dyes – Novatic MD, a full range covering entire

shade gamut exclusively suited for dyeing of cotton and blends in woven

fabric form on continuous dyeing range. The Novatic MD dyes featured

super-fine micro molecular particle size, stable dye dispersion properties,

speck-free performance, non-foaming, ease of reduction and many others

(Athalye et al 2008).

Centigrade introduced a new indigo-dyeing technology which

makes it possible to piece out the deepest indigo shades using a conventional

pad / steam range (Alpert 2008). When stone washed these dyeings show a

yarn dyed aspect similar to conventional yarn dyed denims. The specially

prepared indigo enables exhaust dyeing systems such as garment and jet

dyeing systems into deep shades. The indigo is applied to the fabrics as a

pigment, which is held to the fabric surface by an ionic bond.

91

Deo and Paul (2004) carried out tests on two denim sample using

both natural and synthetic indigo dyes. The attributes and methods employed,

the dyeing procedures, results and discussions of those results have been

recorded. This experiment shows that repeated re-use of indigo dye vats can

produce a range of lighter colours (than standard dark indigo colours) that are

acceptable in the market, while still representing an environmentally friendly

process through dye re-use rather than disposal as effluent.

Loginov et al (2003) investigated the using of ozone treated water

in washing off speeds up oxidation of leuco ester compounds as compared to

some oxidizing agents. Brook et al (2003) described a sensor system for

continuous online and inline measurement of indigo and sodium dithionite

concentration during textile dyeing. The system is based on electrochemical

method, more specifically, multistep chrono amperometry.

Kawahito et al (2003) investigated the effect of washing on colour

unevenness in cotton cloth dyed with natural and synthetic indigo.

Colorimeter was used to elucidate the colour difference between cloth dyed

with natural dye and synthetic dye. Colour unevenness is related to dye

absorption and dye aggregation. The characteristics of running of colour were

quantitatively analysed in order to elucidate the difference between natural

indigo dyeing and conventional dyeing. Synthetic indigo reduction method

(sodium hydrosulphite) was greater than the natural indigo reduction method

(fermentation). The running of colour was concluded to be due to the mode of

indigo penetration into fiber, which depends on dye bath preparation. A

critical difference in dyeing process was the speed of reduction of indigo

which could not be attributed to a small amount of impurity contained in

natural indigo.

92

Vashurina et al (2004) assessed the effectiveness of the traditional

dispersant with eco-friendly natural humic acids. It was shown that the

technique considerably increased the dye yield and reduced the losses which

occur during alkali / hydrosulphite impregnation. A mechanism was proposed

for the intensifying action of the proposed additive.

Shah and Patel (2008) investigated that the application of vat dyes

on cotton fabric was greatly influenced by the total dissolved solids present in

water. The k/s strength of dyes, which required levelling agent in dye bath

decreases, while for dyes which required exhausting agent, k/s strength

increases. The tone of final dyed sample varies with amount and type of

dissolved solids present in water. The fastness properties of vat dyed samples,

except wet rubbing, were not affected by the quality of water.

2.6 TREATMENT OF VAT DYED WASTE WATER

Nowadays environmental aspects are gaining more importance and

are becoming a part of every living organism. Textile industry is said to be

one of the major polluters of water, and creates effluent problems by releasing

waste dyes and chemicals into the running stream. Textile wet processors are

left with two options in order to be environmentally- friendly either to reduce

the effluent at the source or to treat it before final discharge. The recovery and

reuse has the potential for saving dyes, chemicals, water, steam and reducing

waste water treatment cost. Recycling system can be paid for in maximum

two-year period. Usually, the recovery and reuse process is fully automated.

Each process should be carefully analyzed and the future needs should be

considered before a selection is made (Teli et al 2000).

Processing units discharge thousand tones of pollutants especially

in the form of BOD (Biological Oxygen Demand), COD (Chemical Oxygen

Demand) and TDS (Total Dissolved Solids). As a result, the running costs

93

relating to the washing medium (water supply and effluent purification) are

very high. Successful waste water treatment of textile effluent requires

thorough investigation of waste water characteristics, source reduction and the

application of cleaner production principles, such as chemical process of the

final effluent treatment as simple as possible (Wasif et al 2008).

Compared to the other classes of dyes, vat dyes have high degree of

fixation and therefore they normally give no problems with regard to

colouration of the waste water. There can be problems, however, with the

mostly relatively high COD values that are due to the dispersing agents

present in the dye preparations and the dyeing auxiliaries added to the dye

bath. The salt content of the dye baths caused by the reducing agent continues

to be regarded as a problem, but it is lower than with other classes of dyes

(Schluter 1995). Of the six common dye classes namely acid, basic, direct,

disperse, fiber reactive and vat, all contain metals including chromium,

arsenic, cadmium, mercury, copper, lead and zinc, all of which are now

targeted by the new criteria (Karen and Michael 1994). The average metal

content of vat dye in ppm is arsenic less than 1, cadmium less than 1,

chromium 83, cobalt less than 1, copper 110, lead 6, mercury 1 and zinc 4.

These metal contents are controlled by the new water quality criteria (Smith

1989).

In the study of Manu (2007), colour removal and chemical oxygen

demand removal from indigo dye wastewater was evaluated using various

coagulants/flocculants. Laboratory-scale experiments were conducted using

chemical coagulants–flocculants on synthetic as well as actual indigo dye

waste water and under ambient liquid temperature. Up to 99% colour removal

was observed during the treatment of synthetic indigo dye effluent with

FeSO4, alum, polyelectrolyte and lime. Up to 95%, 94% and 87% colour

removal efficiencies were observed when denim plant (dye bath) waste water

94

was treated with alum, lime and FeSO4 at dosages of 225, 1000 and 225 mg/l,

respectively.

Uddin et al (2003) reported the use of Malaysian peat in the form of

coagulant for the removal of textile dyes. Tropical peat soil was chemically

modified to introduce positively charged functional group to work as a

coagulant. This was chemically modified to introduce positively charged

functional group to work as a coagulant. This chemically modified peat was

found effective for the removal of colloidal particles from aqueous solution.

The aim of this research work is to study the effectiveness of this peat

coagulant to remove reactive dyes, vat dyes and disperse dyes from their

aqueous solution and the removal of colour from textile waste water. This

study was focused on the effect of pH, dosage and comparison with

conventional coagulants, namely alum and poly aluminium chloride.

Karthikeyan and Alexander (2008) presented about the various

waste minimization methods in textile industry. The textile industry emits a

wide variety of pollutants at all stages in the processing of fibers and fabrics.

These include liquid effluent, solid waste, hazardous waste, emissions to air

and noise pollution. The consumption of energy must also be taken into

account, as the fuel used to provide this energy contributes to the pollution

load. In general, effluents that are high in COD are most effectively treated by

biological methods, either aerobic or anaerobic. There are a number of

methods for removing colour from effluents, depending on the class of dye

used, but the most effective over the range of dyes is oxidation methods (such

as Fenton’s reagent) or membrane treatment using reverse osmosis. Effluents

that are high in BOD and Suspended Solids are best removed through

coagulation and flocculation methods followed either by settling or dissolved

air floatation. Those effluent streams containing alkaline (mercerizing and

95

bleaching) can be treated by membranes (ultra filtration) or evaporation and

reused in the same process.

The same is true of synthetic sizes, where they can be recycled after

filtration. There is no single treatment technology that can effectively treat the

final effluent from the textile industry, so a combination of the available

methods is necessary in order to achieve the required discharge standards. It is

important to investigate all aspects of reducing wastes and emissions from the

textile industry, since this will result not only in improved environmental

performance, but also substantial saving for the individual companies

(Karthikeyan and Alexander 2008).

In the face of growing popularity of environmental protection

around the world, textile dye manufacturers are quickly responding to develop

new environment-friendly dye products for the textile and apparel supply

chain (Rongqi 2007). In order to catch up with eco-friendly trend, dyestuff

manufacturers in the world have been developing green dyestuff products

continuously. It showed that new technologies have brought a bright future to

the industry (Chen 2007).

In cotton textile industry, possible pollutants in waste water from

dyeing process are various dyes, mordants and reducing agents like sulphides,

hydro sulphides, acetic acid, soap etc. The volume of waste water is large, i.e.

50 l/ kg cloth approximately. The nature of waste water is highly coloured,

fairly high BOD (Pathe et al 1998). If the pollution load is less in the raw

effluent, the treatment becomes easier. The parameters of the treated effluent

are below the norms set by Pollution Control Board. Due to reduction in

effluent load, the treatment cost of effluent is also reduced sufficiently (Wasif

et al 2008).

96

Important characteristics of waste water from dyeing process at

cotton textile industries have been shown in Table 2.2.

Table 2.2 Important characteristics of waste water from dyeing process

at cotton textile industries

S.No Effluent ParametersWaste water from dyeing

process (mg/l except pH)

1. pH 9.2-11.0

2. Total dissolved solids 3230-6180

3. Total Alkalinity 1250-3160

4. Sulphates 70-600

5. BOD 130-820

6. COD 465-1400