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.
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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.
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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)
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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).
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· 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
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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
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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).
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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
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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.
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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
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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
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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
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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.
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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.
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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)
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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
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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.
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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.
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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.
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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
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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
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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
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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).
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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