Table of Contents

Definition of Dye..................................................................................................................................... 2
History of Dye ......................................................................................................................................... 2
Textile Fibers ........................................................................................................................................... 3
Classification of Dyes by Use or Application ........................................................................................... 5
Nomenclature of Dyes ............................................................................................................................ 8
Dyeing Technology.................................................................................................................................. 9
Printing ................................................................................................................................................. 12
Nontextile Use of Dye ........................................................................................................................... 13
Dye Intermediates ................................................................................................................................ 15
Dye Manufacturing Process .................................................................................................................. 16

1. Definition of Dye
Dyes are substance used to impart colour to textiles, paper, leather and other materials
such that the colouring is not readily altered by washing, heat, light or other factors to which the
material is likely to be exposed. Dyes differ from pigments, which are finely ground solids which
are dispersed in a liquid, such as paint or ink, or blended within other materials. Most dyes are
organic compounds whereas pigments may be inorganic.

2. History of Dyes
2.1. Natural Dyes
Until the 1850s virtually all dyes were obtained from natural sources, most commonly
from vegetables, such as plants, trees, lichens and from insects. Solid evidence that dyeing
methods are more than 4000 years old has been provided by dyed fabrics found in Egyptian
tombs. Ancient hieroglyphs describe extraction and application of natural dyes. Countless
attempts have been made to extract dyes from brightly coloured plants and flowers, yet only a
dozen or so natural dyes found widespread. Undoubtedly most attempts failed because most
natural dyes are not highly stable and occur as compensate of mixtures, the successful of
separation of which would be unlikely by the crude methods employed in ancient times.
Nevertheless, studies of these dyes in the 1800s provided a base for development of synthetic
dyes, which dominated the market by 1900.
2.2. Synthetic Dyes
In 1856, the first commercially successful synthetic dye, mauve, was serendipitously
discovered by the British chemist William H. Perkin, who recognized and quickly exploited its
commercial significance. The introduction of mauve in 1857 triggered the decline in dominance
of natural dyes in world markets. Mauve had a short commercial lifetime, but its success
catalyzed activities that quickly led to the discovery of better dyes.
The synthetic dye industry arose directly from studies of coal tar. By 1850, coal tar was
an industrial nuisance because only a fraction was utilized. It attracted the attention of chemists
as a source of new organic compounds, isolable by distillation. German chemist, August
Wilhelm von Hoffman directed the Royal College of Chemistry. He trained most of the students
in English dye industry, one of whom is Perkin. By trial and error, reactions of coal tar
compounds were found to yield useful dyes. By 1914 the synthetic dye industry was firmly
established in Germany, where 90 percent of the worlds dyes were produced.
A few new dye types were introduced in the 20th century, and major challenges were
posed by the introduction of synthetic fibers, which held a major share of the world market, and
by technological advances.

3. Textile Fibers
Textile fiber is the raw materials to produce various types of textile finished products. A
fiber that can be spun into yarn or processed into textile as such as a woven kit, knit, fabric, lace,
non-woven and others alike by means of an appropriate interlacing method.
3.1. Natural Fibers
3.1.1. Cotton
Cotton fibers are comprised mainly of cellulose, a long-chain polymer of
anhydroglucose units connected to ether linkages. Polymers can be classified into two,
the primary and secondary alcohol groups uniformly distributed throughout the length of
polymer chain. These hydroxyl groups impart high water absorption characteristics to the
fiber and can act as reactive sites. The morphology of cotton fiber is complex series of
reversing spiral fibrils. The fiber in total is convoluted collapsed tube with a high degree
of twist occurring along the length of the fiber.
3.1.2. Flax
Flax is also a cellulosic fiber but has a greater degree of crystallinity than cotton.
The morphology of flax is quite different from that of cotton. Flax fibers have a long
cylindrical shape with a hallow core. In recent years, its commercial importance as textile
fiber has decreased significantly.
3.1.3. Wool
Wool fibers are comprised mainly of proteins: the polypeptide polymers in wool
are produced from some 20 alpha-amino acids. The major chemical features of
polypeptide polymer chain and the cystine crosslinks, which occur in random spacing
between the polymer chains. The polymer contains many amine, carboxylic acid and
amide groups, which contribute in part to the water-absorbent nature of fiber.
The morphology of the wool is complex. There is an outer covering over the fiber,
the cortical. There are also overlapping scales having a ratchet configuration that causes
shrinkage and felting. The coefficient of friction in wool fibers is vastly different between
the tip and the root. Wool can be made washable by chemically abrading the scales or
coating the fibers with another polymer.
3.1.4. Silk
Silk, like wool, is a protein fiber but of much simpler chemical and morphological
make-up. It is comprised of six alpha-amino acids and is the only continuous-filament
natural fiber. Silk fiber is spun by the silkworm as a smooth double strand, each part
3

having a trilobal cross-section. This configuration helps give silk its lustrous appearance.
The fiber is unwound from the cocoon the silkworm spins as it prepares its chrysalis.
Because of the labor-intensiveness of sericulture and subsequent preparation of the fiber,
silk remains a luxury fiber.
3.2 Regenerated Fibers
3.2.1. Rayon
Viscose rayon, like cotton, is comprised of cellulose. In the manufacturing
process, wood pulp is treated with alkali and carbon disulfide to form cellulose xanthate.
The reaction mass is forced through a spinneret and precipitated in an acid coagulation
bath as it is formed into a continuous filament. The fiber has a round striated crosssection. Rayon staple is made by breaking the continuous strands into staple-length of
fibers. Viscose rayon is conventionally produced in diameters varying from 9 to 43
microns.
3.2.2 Acetate
Triacetate and diacetate fibers are manufactured by the chemical treatment of
cellulose obtained from refined wood pulp or purified cotton lint. Most of the hydroxyl
groups are acetylated by treating the cellulose with acetic acid. Acetate is made by the
saponification of one of the acetylated groups. The conversion of hydroxyl groups causes
these fibers to be hydrophobic and changes the dyeing characteristic drastically from
those of the normal cellulosic fiber. Triacetate fibers are spun by mixing the isolated
reaction product with methylene chloride and alcohol. The spinning solution (dope) is
forced through the spinneret and dry-spun into continuous filaments. An alternate way of
wet spinning is also possible.
3.3 Synthetic Fibers
3.3.1. Nylon
Nylon is a polyamide fiber. There are two major types of polymer fiber that are
used in textiles. Type 6,6 is made by using hexamethylene glycol and adipic acid. Type 6
is made by polymerizing -caprolactam. Nylon fibers are made by melt-spinning the
molten polymer. The result is a continuous filament fiber of indeterminate fiber. The
cross-section is usually round, trilobal, or square with hallow channels when used as
carpet fiber.

3.3.2 Polyester
Polyester is made by the polymerization reaction of a diol and a diester. The main
commercial polymer is formed by a condensation reaction using ethylene glycol and
terepthalic acid. Fibers are made by melt-spinning. The fiber is usually spun with a round
cross-section. Polyester is the most-used synthetic fiber around the world.
3.3.3. Acrylics
Acrylics are made from the polymerization of acrylonitrile and other comonomers to allow dyeability. The fibers are produced by either solvent-spinning or wet
spinning. Acrylics have found a niche market as a substitute for wool or in wool blends,
in awnings or boat covers. Acrylic fibers are quick drying and wrinkle resistant.
3.3.4. Polyolefin
Polyolefin fibers are produced from the polymerization of ethylene or propylene
gas. The fibers made from these polymers are melt-spun. The cross-sections are round
and the fibers are smooth. They have extremely low dye affinity and moisture
absorbance.
3.3.4. Elastane
Elastane fibers are formed by dry-spinning or solvent-spinning. The cellulosic and
natural fibers are the most hydrophobic.
3.3.5. Microdenier Fibers
This fiber is less than one denier per filament. Yarns made from microdenier
filaments are able to give silk-like hand to fabrics.

4. Classification of Dyes by Use or Application

4.1. Reactive Dyes
These dyes form a covalent bond with the fiber, usually cotton, although they are used to
a small extent on wool and nylon. This class of dyes was first introduced commercially in 1956,
made it possible to achieve extremely high washfastness properties by relatively simple dyeing
methods. A marked advantage of reactive dyes over direct dyes is that their chemical structures
are much simpler; their absorption spectra show narrower absorption bands and the dyeing are
brighter. High-purity reactive dyes are used in the ink-jet printing of textiles, especially cotton.

4.2. Disperse Dyes

These are substantially water-insoluble nonionic dyes for application to hydrophobic
fibers from aqueous dispersion. They are used predominantly on polyester and to a lesser extent
on nylon, cellulose, cellulose acetate and acrylic fibers. Thermal transfer printing and dye
diffusion thermal transfer (D2T2) processes for electronic photography represent niche market
for selected members of this class.
4.3. Direct Dyes
These water-soluble anionic dyes, when dyed from aqueous solution in the presence of
electrolytes, are substantive to and have high affinity for cellulosic fibers. Their principal use is
the dyeing of cotton and regenerated cellulose, paper, leather and nylon. Most of the dyes in this
class are polyazo compounds. After treatments are frequently applied to the dyed material to
improve washfastness properties, include chelation with salts of metals, and treatment with
formaldehyde or a cationic dye-complexing resin.
4.4. Vat Dyes
These water-insoluble dyes are applied mainly to cellulosic fibers as soluble leuco salts
after reduction in an alkaline bath, usually with sodium hydrogensulfite. Following exhaustion
onto the fiber, the leuco forms are reoxidized to the insoluble keto forms and aftertreated to
redevelop the crystal structure. The principal chemical classes of vat dyes are anthraquinone and
indigoid.
4.5. Sulfur Dyes
These dyes are applied to cotton from an alkaline reducing bath with sodium sulfide as
the reducing agent. Numerically this is relatively small group of dyes. The low cost and
washfastness properties of the dyeing make this class important from an economic standpoint.
However, they are under pressure from an environmental viewpoint.
4.6. Cationic (Basic) Dyes
These water-soluble cationic dyes are applied to paper, polyacrylonitrile, modified nylons
and modified polyesters. Their original use was for silk, wool and tannin-mordanted cotton when
brightness of shade was more important than fastness to light and washing. Basic dyes are watersoluble and yield colored cations in solution. For this reason they are frequently referred to as
cationic dyes. The principal chemical processes are diazahemicyanine, triarylmethane, cyanine,
hemicyanine, thiazine, oxazine and acridine. Some basic dyes show biological activity and are
used in medicine as antiseptics.
4.7. Acid Dyes
These water-soluble anionic dyes are applied to nylon, wool, silk and modified acrylics.
6

They are also used to some extent for paper, leather, ink-jet printing, food and cosmetics.
4.8. Solvent Dyes
These water-insoluble but solvent-soluble dyes are devoid of polar solubilizing groups
such as sulfonic acid, carboxylic acid or quaternary ammonium. They are used for coloring
plastics, gasoline, oils and waxes. The dyes are predominantly azo and anthraquinone, but
phthalocyanine and triarylmethane dyes are also used.
Table 4.1 Usage Classification of Dyes
Class
Principal substrates
Acid

Method of
Application
Nylon, wool, silk, paper, Usually from neutral
inks and leather
to acidic dyebaths

Azoic components
and composition

Cotton, rayon, cellulose

acetate and polyester

Basic

Paper, polyacrylonitrile,
modified nylon,
polyester and inks

Direct

Cotton, rayon, paper,

leather and nylon

Disperse

Polyester, polyamide,
acetate, acrylic and
plastics

Fiber impregnated
with coupling
component and
treated with a solution
of stabilized
diazonium salt
Applied from acidic
dyebaths

Applied from neutral

or slightly alkaline
baths containing
additional electrolyte
Fine aqueous
dispersions often
applied by high
temperature/pressure
or lower temperature
carrier methods; dye
may be padded on
cloth and baked on or
thermofixed

Chemical Types
Azo(including
prematellized),
anthraquinone,
triphenylmethane,
azine, xanthene, nitro
and nitroso,
azo

Cyanine,
hemicyanine,
diazahemicyanine,
diphenylmethane,
triarylmethane, azo,
azine, xanthene,
acridine, oxazine and
anthraquinone
Azo, phthalocyanine,
stilbene and oxazine

Azo, anthraquinone,
styryl, nitro and
benzodifuranone

Fluorescent
Brightners
Food, drug
cosmetic

Soaps and detergents, all From

solution,
fibers, oils, paints and dispersion
or
plastics
suspension in a mass
and Foods,
drugs
and
cosmetics

Mordant

Wool,
leather
and
anodized aluminum

Oxidation Bases

Hair, fur and cotton

Reactive

Cotton, wool, silk and

nylon

Solvent

Sulfur

Plastic,
gasoline,
varnishes,
lacquers,
stains, inks, fats, oils
and waxes
Cotton and rayon

Vat

Cotton, rayon and wool

Stilbene, pyrazoles,
coumarin,
and
naphthalinides
Azo, anthraquinone,
carotenoid,
and
triarylmethane
Applied
in Azo
and
conjunction with Cr anthraquinone
salts
Aromatic amines, and Aniline black and
phenols oxidized on indeterminate
the substrate
structures
Reactive site on dye Azo, anthraquinone,
reacts with functional phthalocyanine,
group on fiber to bind formazan,
oxazine
dye covalently under and basic
influence of heat and
pH
Dissolution in the Azo,
substrate
triphenylmethane,
anthraquinone
and
phthalocyanine
Aromatic
substrate Indeterminate
vatted with sodium structure
sulfide and reoxidized
to insoluble sulfurcontaining products
on fiber
Water-insoluble dyes Anthraquinone
solubilized
by (including polycyclic
reducing with sodium quinones)
and
hydrogensulfite, then indigoids
exhausted on fiber
and reoxidized

5. Nomenclature of Dyes
Dyes are named by either by their commercial trade name or by their Colour Index (C.I)
name. The commercial names of dyes are usually made up of three parts. The first is a trademark
used by the particular manufacturer to designate both the manufacturer and the class of dye, the
second is the color and the third is a series of letters and numbers used as a code by the
manufacturer to define more precisely the hue and also to indicate important properties of the
dye.

The CI name for a dye is derived from the application class to which the dye belongs, the
color or the hue of the dye and a sequential number. A five digit CI number is assigned to a dye
when its chemical structure has been disclosed by the manufacturer. The following example
illustrates these points:
Chemical Structure:

Molecular Formula: C33H20O4

Chemical Abstract Name: 16,17-dimethoxydinaphthol[1,2,3-cd:3`,2`,1`-lm]perylene-5,10-dione
Trivial Name: Jade Green
CI Name: C.I. Vat Green 1
C.I. Number: C.I. 59825
Application Class: vat

6. Dyeing Technology
The goal of every dyeing is a colored textile in the desired shade, homogeneous in hue
and depth of shade, produced by an economic process and which exhibits satisfactory fastness
properties in the finished state.
Although modern automation techniques have been introduced for color measurement,
metering of dyes and auxiliaries, and automatic control of the dyeing process much human
intervention is still required. Fibers can only be standardized to a limited extent, due to biological
and environmental factors, in growing cotton or raising sheep. To remain flexible with regard to
fashion and fastness properties, dyeing is carried out at the end of the production process
whenever possible.
The textile material needs a pretreatment before dyeing. Wool must be washed to remove
wax and dirt and is sometimes bleached; cotton must be boiled and bleached to remove pectins
and cotton seeds then it will undergo mercerization. Sizes and spinning oil must be eliminated.
9

6.1. Principle of Dyeing

Basically there are three methods of dyeing textile: Mass dyeing, dyeing of synthetic
polymer before fiber formation; Pigment dyeing, affixing an insoluble colorant on the fiber
surface with a binder; Exhaustion dyeing from an aqueous bath with dyes that have an affinity
for the fiber. Exhaustion dyeing will be discussed more in detail since it is the most used process
in the industry.
In exhaustion dyeing, the dye is transported to the fiber surface by motion of the dye
liquor or the textile. It is then adsorbed on the fiber surface and diffuses into the fiber. Finally, it
is fixed chemically or physically. The dye can be applied to the textile discontinuously or
continuously by immersing the textile in a concentrated bath and squeezing off excess liquor,
followed by separate steps for diffusion and fixation in the fiber.
The speed of exhaustion of individual dyes can vary widely, depending on their chemical
and physical properties, the kind of textile used could also affect this. The dyeing factor depends
temperature, liquor ratio, dye concentration, and the chemicals and auxiliary products in the dye
bath. High dyeing rates bear the danger of unlevel dyeings. Dyes have to be carefully selected
when used together in one recipe.
The end of the dyeing process is characterized by the equilibrium phase. Under standard
conditions, the distribution coefficient of the dye between liquor and fiber is constant; in other
words, the rate of desorption and adsorption are equal. When the dyeing is carried out
continuously, it is important that the dye application must be homogeneous and avoid migration
during subsequent steps. Leveling out a dyeing after fixation of the dye is tedious and timeconsuming.
6.2. Bath Dyeing Technology
6.2.1. Circulating Machines

10

The goods are packed loose and the liquor is pumped through the goods. The
pump characteristics and the density of the material determine the circulating speed of the
liquor and the necessary dyeing time.
6.2.2. Circulating-Goods Machine

Traditional dyeing equipment belongs to this group. Fabric is moved as a rope or

in open width by mechanical means or liquor jet produced by a circulating pump.
6.2.3. Process Control in Bath Dyeing
Bath dyeing runs discontinuously, automatic process control must work in cycles.
A completely automated dyeing process is almost impossible to achieve, because there
are a lot of variables determine the result of dyeing and a wide range of operating factors
interact with each other during dyeing. A precondition for automatically controlling the
dyeing process is detailed knowledge of the characteristics of the fiber to be dyed, the
dyes and auxiliary to be used and the equipment available. To assure level dyeing from
the beginning of the exhaustion curves for the dyes combined in one recipe must be
controlled. This requires constant color measurement of dye concentration.
6.3. Continuous and Semi-Continuous Dyeing
Continuous dyeing means treating fabric in a process unit in which application of the dye
to the fabric and fixations are carried out continuously. Continuously working units are
assembled into lines of consecutive processing steps, sometimes including pretreatment of the
fabric. Fabric will be treated in an open-width, any unevenness in the equipment across the width
of the goods to unlevel dyeing. The width of the goods and longitudinal tension influence each
other. The running speed determines the dwelling time in the treatment unit. Any interruption in
the process will lead to the spoilage of the fabric.

11

7. Printing
A wide variety of techniques exist for applying dyes by printing. Four kinds of printing
have long been recognized: direct, dyed, discharge and resist.
Direct printing is the application of a painting paste containing dye, thickeners and
auxiliaries directly to the fabric by rollerprinting. The dominant technique is screen printing.
Discharge printing is the application of dischargeable dye and then printed with a
discharge paste in the desired pattern. The discharge dye may contain a discharge-resistant dye.
Printing is most often done with rotary screens etched in the design to be printed. Printing
paste is fed constantly to the center of the rotating screens from a nearby supply and a squeegee
pushes the colored paste through the holes in the screen, leaving the dye paste only in the
intended areas, a separate screen is required for each color in the pattern.

The current machines are very successful at furnishing one of a kind and for use in rapid
prototyping.

12

7.1. Pigment Dyeing and Printing

Pigment dyeing and printing are processes that compete with the more conventional
means of dyeing and printing. These processes use water-insoluble dyes or pigments that are
bound to the surfaces of pigments that are bound to the surfaces of fabrics with resins. A paste or
an emulsion, containing pigment and resin or a resin-former, is applied to the fabric. The goods
then are dried and cured by heat to produce the finished dyeing or print. During the heating or
curing, fabric, resin and pigment become firmly bonded together. This method of color
application is economical and produces good results. It should be noted that the pigment is
confined to the surface of the fabric and can be selected without regard for fiber affinity.

8. Nontextile Uses of Dye

Colorants for nontextile use have been developed mainly for use in hair dyeing,
photography, biomedical applications and electronics and reprographics. In several nontextile
applications, dyes are not used for their ability to deliver color. Instead, they are used because of
their potential electrical properties, ability to absorb IR radiation.
8.1. Liquid Crystal Dyes
Dyes for liquid crystalline media typically have nonionic structure, high purity, solubility
and compatibility with the medium, a transition dipole that is parallel with the alignment axis of
the molecular structure, and good alignment with the liquid crystal molecule. Example includes
the disazo and anthraquinone dyes, which are shown below.

8.2. Ink-jet Dyes

Inkjet dyes are higky concentrated colorants specifically designed for todays inkjet
markets. These ultra pure dyes are low in chlorides and prepared to meet all the standard criteria
for the inkjet industry. Inkjet printing is meticulously produced using comprehensive purification
and filtration processes. The quality of an inkjet printing is very much influenced by the physicchemical properties of printing ink.
Dye inks are prepared by dissolving of the liquid colored dyes into a fluid carrier. This
makes the dyes easy to apply. When it is applied to a paper, the dyes are absorbed very
13

uniformly so they reflect light very evenly. As the printing is a high precision job the inkjet dyes
need to have superior quality in terms of colors, physical properties and stability. Generally
direct, reactive and acid dyes are used as dyes for inkjet ink.
8.3. Thermal and Pressure-Sensitive Printing
In pressure-sensitive printing technology the color former is dissolved in a solvent and
encapsulated. The use of pressure ruptures microcapsules containing the color former, which
generates color upon contacting a developer. Black prints are usually obtained from fluorans or
from color-former mixtures.
8.4. Organic Photoconductors and Toners
Photoconductors and toners are used in photocopiers and laser printers to produce
images. Organic photoconductors are consists of a charge-generating layer and a chargetransporting layer. The former is comprised of pigments and the latter is comprised of electronrich organic compounds that are usually colorless. Suitable organic pigments for charge
generation include azo pigments, tetracarboxydiimides, polycyclic quinones, phthalocyanine,
perylenes and squarylium compounds.
Colorants are used in toners to provide color and control the electrostatic charge on toner
particles. Diarylides and monoarylides have been used as the yellow pigments in colored tones.
The magenta pigments are often quinacridones and the cyan pigments are copper
phthalocyanines.
8.5. Infrared Absorbing Dyes
Infrared dyes include indolenincyanines and azulenium compounds, both of which are
used in optical reading materials.
8.6. Laser Dyes
Lasers are which dyes comprise the active medium have become of the most widely used
types. The key virtue of these systems is their ability to cover virtually the entire fluorescence
spectral region. Accordingly, the most commonly used dyes are highly fuorescent.
8.7. Biomedical Dyes
Dyes can be used clinically in bioanalysis and medical diagnostics and in the treatment of
certain diseases.
8.8. Hair Dyes
About 80% of the dyes used in hair coloring are known as oxidation hair dyes. The
remaining 20% of the available hair dyes are mainly synthetic dyes that have affinity for protein
substrates. Oxidation dyes are produced directly on hair by oxiding diamines with suitable
oxiding agent. In this regard, the diamines have been referred as primary intermediates and the
oxidizing agent as the developer.

14

8.9. Photographic Dyes

Color photography is still one of the most important and interesting nontextile uses for
synthetic dyes. The chemistry employed is the same to that of oxidation of hair dyes, in that an
oxidizable substrate is combined with a coupler to produce the target colorant. In this case the
diamine is referred to as the developer and it is oxidized by silver halide in the photographic
film. The oxidized developer then reacts with the coupler to form the dye.

9. Dye Intermediates
The dye intermediates are generally found as petroleum downstream products. For
application they are further processed. On processing they are transformed to finished dyes and
pigments. The dye intermediates are vital inputs for a number of major industries. Some of the
major industries they serve are textiles, plastics, paints, printing inks and paper. Further, dye
intermediates also serve as an important raw material for the acid, reactive and direct dyes. A
major application of dye intermediates are found in hair dyes.
Most dye intermediates are prepared by reaction involving electrophilic or nucleophilic
substitution processes. The electrophilic processes include nitration, sulfonation, and
halogentation reactions, and the nucleophilic processes include hydroxylation and amination
reactions.
Other key dye intermediates are prepared by oxidation and reduction processes. The most
common dye intermediates are shown below.

15

10. Dye Manufacturing Process

Each chemical and physical step in the process can generate process wastewater, solid
waste and air emissions. The anthraquinone-based vat dyes require more synthetic steps than the
acid, basic, direct, disperse and reactive dye classes. The multiple chemical reactions increase
the consumption of raw materials, resulting in vat dyes having the highest ratio of raw materials
to finished dye as compared to the other dye classes. Wastewater from the manufacture of vat
dyes is on the order of 8,000 liters per kg of product compared to a maximum of 700 liters per kg
for the other dye classes.
The most significant material losses in dye production come from incomplete chemical
reactions. The yield of the various reactions discussed in section III B ranges from 39 to 98
percent, with an average of only 79 percent of theory. Some of the vat dyes require five or more
synthetic steps. If each step averages a 79 percent yield, the overall yield of a five step process is
only 31 percent of theory. If seven steps are required, which is the case for Vat Brown 1, the
overall yield is only 19 percent of theory.
Most of the raw materials used in the manufacture of vat dyes are hazardous since they
are ignitable, corrosive, or toxic. The low yields result in hazardous chemicals in the wastewater
and in solid wastes such as solvent still bottoms and filtration clarification sludges.
The wastewater from vat dye synthesis will contain unreacted raw materials and
byproducts which are soluble, in addition to inorganic salts formed by neutralization. The heavy
metal catalysts and reagents used in key intermediate steps, such as mercury, arsenic, copper and
chromium, are primarily found in the wastewater as soluble salts, and can contaminate soil and
groundwater if improperly treated or disposed of.

16

The acid, basic, direct, disperse, and reactive dye classes are generally manufactured in
aqueous media. Vat dyes, however, require high boiling solvents in many of the intermediate
steps, since temperatures over 200 deg. C are necessary to drive the reactions. The most
common solvents are nitrobenzene, naphthalene and the chlorinated solvents chlorobenzene, 1,
2-dichlorobenzene (o-dichlorobenzene), and 1, 2, 4-trichlorobenzene (trichlorobenzene). All of
these solvents are hazardous chemicals with the potential of severe environmental contamination.
In the vat dye industry, the solvents can be recovered by collecting the mother liquor
from the filtration step in a distillation vessel equipped with a condenser and receiver. However,
it is more common to use a venuleth (paddle) dryer. This is a horizontal rotary vacuum dryer
used to obtain dry powder from wet cake or solutions and to recover the solvent at the same
time. Steam is supplied to an exterior jacket and to a hollow shaft and paddles within the unit.
Solvent recovery generates still bottoms that must be removed from the equipment
between batches in order to facilitate heat transfer. The tarry residue is scraped from the interior
of the equipment and usually packed in drums for disposal. The spent solvent still bottoms from
vat dye manufacture are listed as hazardous wastes. The still bottoms may also contain unreacted
raw materials and reaction byproducts.
Filtration operations also result in solid waste when off-specification intermediates or
dyes are purified by recrystallization in solvents. Diatomaceous earth and activated carbon are
typically added to the solution to adsorb the unreacted raw material or other impurities and to
prevent blinding of the filter media. The filtration clarification sludge from vat dye operations
will likely contain RCRA-listed hazardous wastes including organic chemicals and heavy metals.
Empty raw material containers represent another source of potentially hazardous solid
waste disposed of by dye manufacturers. The chemicals can stick to the walls of the container or
to the paper or plastic liners.
It was common practice in the dye industry to pack the spent still bottoms and filtration
sludge wastes in steel drums. Many of these disposal locations became Superfund sites or state
hazardous waste sites due to the serious contamination of soil and groundwater from the
drummed wastes.

11. Azo Dyes

Azoic Dyes are classified either according to the fibers for which these can be used
economically or the methods by which these dyes are applied. These dyes cannot be applied
directly on the fibers as dyes. Actually, these dyes are produced within the fibers itself. For this
production, first the fiber is impregnated with one component of these dyes and then the fiber is
treated in another component of these dyes. In this way the AZO dyes are formed. This specialty
makes
these
dyes
very
fast
to
washing
within
the
fabric
market.
When these dyes are used upon the cellulose fabric then initially this fabric starts to suffer from
poor rub fastness. This is due to the deposition of the free pigments on the surface of the fabric.
This problem can be rectified by boiling the fabric in soap.

17

AZO Dyeing Process is such a process in which the insoluble azoic dye is produced on
the or within the fiber. By treating a fiber with diazoic and coupling components, this process
can be achieved. After adjusting the dye bath conditions appropriately, the two above mentioned
components react. From this reaction the required insoluble AZO dye is produced. This is a
unique technique. The required color can be changed by altering of the diazoic and coupling
components.

12. Triphenylmethane Dyes

It is any member of a group of extremely brilliant and intensely coloured synthetic
organic dyes having molecular structures based upon that of the hydrocarbon triphenylmethane.
They have poor resistance to light and to chemical bleaches and are used chiefly in copying
papers, in hectograph and printing inks, and in textile applications for which lightfastness is not
an important requirement.
The triphenylmethane derivatives are among the oldest man-made dyes, a practical
process for the manufacture of fuchsine having been developed in 1859. Several other members
of the class were discovered before their chemical constitutions were fully understood. Crystal
violet, the most important of the group, was introduced in 1883.
The range of colours is not complete but includes reds, violets, blues, and greens. They
are applied by various techniques, but most belong to the basic class, which are adsorbed from
solution by silk or wool, but have little affinity for cotton unless it has been treated with
a mordant such as tannin.

13. Xanthene Dyes

Xanthene is a yellow organic heterocyclic compound. It is soluble in diethyl ether.
Xanthene is used as a fungicide and it is also a useful intermediate inorganic synthesis.
Derivatives of xanthene are commonly referred to collectively as xanthenes, and among
other uses are the basis of a class of dyes which includes fluoroscein, eosins, and rhodamines.
Xanthene dyes tend to be fluoroscent, yellow to pink to bluish red, brilliant dyes. Many xanthene
dyes can be prepared by condensation of derivates of phthalic anhydrous with derivates of or 3ominophenol.

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References
Kent, James A., Kent and Riegels Handbook of Industrial Chemistry and Biotechnology,
11 edition, Springer Science+Business Media LLC, 2007
th

Hunger, Klaus, Industrial Dyes, WILEY-VCH Verlag GmbH and Co KgaA, 2003
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