COLOUR CHEMISTRY INTRODUCTION7. Solvent dyes 8. Sulfur dyes 9. Vat dyes 10. Mordant dyes Dye can...
Transcript of COLOUR CHEMISTRY INTRODUCTION7. Solvent dyes 8. Sulfur dyes 9. Vat dyes 10. Mordant dyes Dye can...
COLOUR CHEMISTRY
INTRODUCTION
Colour pervades all aspects of our lives, influencing our moods and emotions and generally
enhancing the way in which we enjoy our environment. In addition to its literal meaning, we
often use the term colour in more abstract ways, for example to describe aspects of music,
language and personality. Colours are all around us, in the earth, the sky, the sea, animals and
birds and in the vegetation, for example in the trees, leaves, grass and flowers. These colours
play important roles in the natural world, for example as sources of attraction and in defence
mechanisms associated with camouflage. Plant pigments, especially chlorophyll, the dominant
natural green pigment, play a vital role in photosynthesis in plants, and thus may be considered
as vital to our existence.
The natural world contains a variety of fantastic colours. Prehistoric humans used many naturally
occurring colours to colour their implements, clothing and dwellings. Colour is introduced into
materials all around us using substances known as dyes and pigments, or collectively as
colorants. The essential difference between these two colorant types is that dyes are soluble
coloured compounds which are applied mainly to textile materials from solution in water,
whereas pigments are insoluble compounds incorporated by a dispersion process into products
such as paints, printing inks and plastics. They could be natural or synthetic.
The dyes used to colour clothing were commonly extracted either from botanical sources,
including plants, trees, roots, seeds, nuts, fruit skins, berries and lichens, or from animal sources
such as crushed insects and molluscs. Pigments for paints were obtained from coloured minerals,
such as ochre and haematite which are mostly based on iron oxides, giving yellows, reds and
browns, dug from the earth, ground to a fine powder and mixed into a crude binder. Charcoal
from burnt wood provided the early forerunners of carbon black pigments. The durability of
these natural inorganic pigments, which contrasts with the more fugitive nature of natural dyes,
is demonstrated in the remarkably well-preserved Palaeolithic cave paintings found.
Synthetic colorants may also be described as having an ancient history, although this statement
applies only to a range of pigments produced from rudimentary applications of inorganic
chemistry. These very early synthetic inorganic pigments have been manufactured and used in
paints for thousands of years.
Sir Isaac Newton’s discovery that white light that is passed through a prism separates into a
spectrum of colours prove that light is the source of all colour; light is comprised in part of
various wavelengths of radiant energy.
The human eyes with its marvelous physiology of ones, interprets the wavelengths from 400-
700nm transforming the input into the realization of colours as seen in the visible region of the
electromagnetic spectrum.
Thus within this narrow portion of the total radiant energy of light lie all the colours that re
perceived.
Unlike most organic compounds, dyes possess colour because they 1) absorb light in the visible
spectrum (400–700 nm), 2) have at least one chromophore (colour-bearing group), 3) have a
conjugated system, i.e. a structure with alternating double and single bonds, and 4) exhibit
resonance of electrons, which is a stabilizing force in organic compounds. When any one of
these features is lacking from the molecular structure the colour is lost. In addition to
chromophores, most dyes also contain groups known as auxochromes (colour helpers), examples
of which are carboxylic acid, sulfonic acid, amino, and hydroxyl groups. While these are not
responsible for colour, their presence can shift the colour of a colourant and they are most often
used to influence dye solubility.
In 1856, a young English Chemist William Perkin discovered the first synthetic dye stuff while
trying to prepare quinine. The dye was “mauve” which dyed silk to a purple shade, this discovery
ushered in the modern era of organic dyes and pigments.
The important groupings are the azo, anthraquinone, indigoid, nito, nitro, oxazine,
phthalocyanine, quinolone, stilbene, thiazole, thiazine and triphenylmethane classes.
Each molecule of these different classes has a common conjugated electron system that responds
to different wavelengths of lights. That portion of the molecule is called the chromophore and to
a large extent determines the hue or shade of the dye.
Examples of chromophoric groups present in organic dyes
Dyes are classified into 10 classes or categories by their application to fiber or substrate.
1. Acid dyes
2. Azoic dyes
3. Basic dyes
4. Direct dyes
5. Disperse dyes
6. Reactive dyes
7. Solvent dyes
8. Sulfur dyes
9. Vat dyes
10. Mordant dyes
Dye can also be classified using the colour index, in the colour index, dyes are classified
by usage, and hue and the chemical class usually is known, when known, the chemical
constituent of the dye is classified using a five digit constitution number.
The two major types of colourants produced today are dyestuff and pigments.
Dyes stuff is normally soluble in water i.e water-soluble or water dispersed compound
that is capable of being absorbed into the substrate, whereas a pigment is a water-
insoluble compound that requires a binder or is entrapped in the matrix of the substrate.
Majority of pigments are soluble in solvents and plastics, and both dyes and pigments
impart very high tinctorial values for the amount used to colour a product. The major end
use of dye is in the textile, leather and paper industries. Pigments find major use in the
paint, in and plastic industries.
The textile industries uses a large number of dyestuffs from each of the dye categories,
the choice depending on the shade, fiber used, dying process, end use of the textile
product, requirement for fastness and economic consideration. To provide an
understanding of the interrelationships that exist among the various dye classes and fiber
types, a brief survey of the major textile fibers follows.
Textile fibers
Textile fibres are hair-like substances with a high degree of fineness, outstanding
flexibility, reasonable strength, a minimum level of length and cohesiveness (ability to
hold to one another, when placed side by side). They may be short with a length at least
500 times (but commonly 1000 to 3000 times) their diameter or thickness or may be very
long with the length to diameter ratio being almost infinity. The short fibres are called
staple fibres while those with very long length are called filaments. However, this
distinction is generally not made and both short fibres and continuous filaments are called
fibres.
Commercially important textile fibers cam be grouped based on their origin, the fibres
may be classified as belonging to one of the following two categories: Natural and
Man-made.
Natural fibres can be further classified according to their origin into the following three
groups:
i) Vegetable Fibres: Most of these are cellulose fibres and include cotton, linen, jute,
flax, ramie, coir, sisal and hemp. Besides their use as textiles, cellulose fibres are also
used in the manufacture of paper and other useful products like ropes, cords, coir
mats, industrial fabrics, etc. ii) Animal Fibres: They are mostly protein fibres and
include wool and silk. iii) Mineral Fibres: Asbestos is the only naturally occurring
mineral fibre that was used extensively for making industrial products but is now
being gradually phased out due to its suspected carcinogenic effect.
Fibres in the second category, as the name implies, are made by man and are therefore
sometimes called artificial fibres or manufactured fibres. Like natural fibres they may
also be divided into the following three categories:
i) Derived from natural feedstock i.e regenerated: Most of the fibres in this
category are derived from cellulose which is obtained from bamboo, wood or
cotton linters. The most important fibre in this category is viscose rayon. For a
long time rayon was made by a complex route in which cellulose was first
converted to cellulose xanthate and then dissolved and made into a fibre
which was then regenerated into pure cellulose fibre called viscose rayon.
However, more recently solvents for cellulose have been found and the
cellulose fibres are made directly from a solution of cellulose —these are
available under the trade names Lyocell and Tencel. Small quantities of
chemically modified cellulose fibres are also made— they are cellulose
dilacerate and cellulose triacetate fibres. Rubber latex, which comes out from
rubber trees, is another natural feedstock from which rubber fibres are made
for use by the Textile and other industries.
ii) Derived from manufactured feedstock: The petrochemical industry is the main
source of fibres in this category with coal and natural gas also contributing a
bit. Low molecular weight chemicals are first produced and these are
converted into fibre forming polymers through polymerization. Synthetic
fibres like polyamides (Nylon 66, Nylon 6), polyesters, acrylics and
polypropylene are obtained through this route. Elastomeric fibres— Spandex
and Lycra are also similarly made.
iii) Miscellaneous fibres: Glass fibres obtained from silica and metallic fibres like
silver and gold are man-made fibres which are best put under this category.
A Chart showing a more detailed classification
Natural fibers
Cotton: cotton fibers are comprised mainly of cellulose, a long- chain polymer in which
the repeating unit is a glucose anhydride molecule connected by ether links. The polymer
has primary and secondary alcohol groups uniformly interspersed throughout the length
of the polymer chain. These hydroxyl units impart high water absorption characteristics
to the fiber and can act as reactive sites.
Flax: flax is like cotton chemically, is a cellulosic fiber that has a higher degree of
crystallinity than cotton.
Wool: wool fibers are composed mainly of proteins; the polypeptide polymers in wool
are produced from some 20 alpha amino acids. The major chemical features of the
polypeptide polymer are the amide links which occur between the amino acids along the
polymer chain and the cystine (sulfur-to-sulfur) cross links which occur in a random
spacing between thee polymer chains. The polymer contains many amine, carboxylic acid
and amide group, which contribute to the water absorbent nature of the fiber.
Silk: Silk like wool is a protein fiber, but of a much simpler chemical and morphological
make up. It is comprised of six alpha amino acids and is the only continuous filament
natural fiber.
Regenerated fibers
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.
Subsequently, the reaction mass is forced through a spinnerette and precipitated in an
acid coagulation bath as it is formed into a continuous filament.
Acetate triacetate and diacetate fibers: these are manufactured by the chemical
treatment of cellulose obtained from refined wood pulp or purified cotton fluff. Most of
the hydroxyl groups are acetylated (esterifid) by treating the cellulose with acetic acid.
Synthetic Fibers
Nylons: There are two major types of nylon polymers used in textiles: type 6,6 which is
made from condensation reaction of hexamethylene diamine and adipic acid (moomers) and type
6, which is made by polymerizing caprolactam. Nylon is made when the appropriate monomers
(the chemical building blocks which make up polymers) are combined to form a long chain via a
condensation polymerisation reaction.
The nylon molecules are very flexible with only weak forces, such as hydrogen bonds, between
the polymer chains, which tend to tangle randomly. The polymer has to be warmed and drawn
out to form strong fibers.
Polyester: is made by the polymerization reaction of ethylene glycol and terephthalic acid.
Other types of synthetic fibers are acrylics, spandex and polyolefins.
Dye classification
This classification looks into each classes of dyes including a definition of the class, the types of
fibers best suited for each class, and the mechanism by which the dye is retained in the substrate.
Regardless of the dye class or the fiber being dyed, dyeing proceeds according to the following
sequence:
1. Movement of the dye to the substrate
2. Absorption of the dye into the fiber
3. Diffusion of the dye onto the fiber polymer
4. Retention of the dye by one of the following mechanisms:
Ionic bonding
Covalent bonding
Entrapment
Hydrogen bonding
Solid solution
Acid dyes
Acid dyes contain one or more sulphonic acid substituents or other acidic groups. The term
applies to the application class rather than a chemical class, as other classes of dyes also contain
acid substituents. Because of the presence of the acidic substituents in the dye molecule, acid
dyes are anionic and water-soluble. Example is the acid yellow 36 ( Metanil yellow)
N
N
S
O
OOH
NH
Structure of acid yellow 36
Acid dyes are applied from acidic dye baths and are used to dye wool and nylon. The anionic,
negatively charged portion of the dye molecule in the solution aligns with a cationic positively
charged site on the fiber. The ionic bonding that occurs is a result of the amino groups on the
fibers forming ion pairs with, typically sulfonic acid groups present as part of the dye molecule
(fiber-NH3+:-SO3-dye).
Traditionally, acid dyes have been categorized by dyers into four distinctive groups based largely
on their unique properties and dying characteristics: leveling dyes, milling dyes, supermilling
dyes, and metal complex dyes.
i. a. Leveling dyes for wool: dyestuffs have low molecular weights and are applied from
strongly acid dye baths.
b. leveling dyes for Nylon: dyestuffs have higher molecular weights and are applied
from neutral or weakly acidic dye baths.
Attributes: Even dyeing with overall good light fastness but poor wash fastness
properties.
ii. Milling dyes: dyestuffs have high molecular weights and are applied from weak
acidic dye baths.
Attributes: good wash fastness ; dyeings lack brightness
iii. Supermilling dyes: dyestuffs have high molecular weights; are applied from neutral
dye baths, usually with an auxiliary to help ensure level dyeing.
Attributes: Excellent wash and light fastness properties.
iv. Metal complex dyes (cobalt or chromium)
a. 1:1 Metal Complex: dyestuffs have one equivalent of metal for each equivalent of
dye with sulfonic groups; they are applied from strongly acidic dye baths.
b. 2:1 Metal Complex: Dyestuffs have one equivalent of metal to two equivalents of
dye, but have no free sulfonic groups; are applied from a neutral or weakly acidic
dye bath, usually used with an auxiliary to promote level dyeing.
Attributes: Excellent wash and light fastness; dyeings are dull.
Basic or Cationic Dyes
Basic dyes have a positive charge on the dye molecule and as such are ammonium, sulfonium or
oxonium salts. The positive charge can be pendant or delocalized where the charge resonates
between the heteroatom connected by a conjugated carbon chain.
Basic dyes are applied from weakly acidic dye bathes and are used to dyes acrylic and cationic
dyeable polyester. The cationic positively charged portion of the dye molecule in solution aligns
with an anionic, negative charged site on the fiber that the fiber manufactures include in the
polymer. The ionic bonding that occurs is the result of the anionic dyesite forming ion pairs with
the quaternary amine group present as part of the dye molecule (Fiber-R-:+NH3-Dye).
Basic dyes were the first dyes made synthetically; “mauve” was a basic dye. The basic dye first
used to dye silk and wool, but they had poor fastness properties. Today basic dyes are used for
acrylics or cationic dyeable polyester have high tinctorial value; they are the brightest dyes
available and have unlimited colour range and good fastness properties.
Cationic dyes are divided into three classes
1. Basic Brown 1 (Bismark Brown) I an amino-containing dye that is readily protonated
under the pH 2 to 5 conditions of dyeing:
Basic Brown 1
2. Basic violet 3 (crystal violet) is an example of ‘classical’ cationic dye in which the
positive charge is delocalized by resonance and may be present at any one of the basic
centers at any time. These resonance forms of almost equivalent energy are one of the
reasons why Crystal Violet is among the strongest known dyes. This high colour value
(tinctorial strength) has important commercial interest in the hectograph copying system.
In this system, the Crystal Violet in a wax base is transferred to the back of a type written
copy sheet by using paper moistened with alcohol, more than 200 good copies can be
made from the master.
Basic Violet 3
Developed from Fisher’s base (2,3-dihydro-1,3,3-trimethly-2-methylene-1H-indole) and
Fisher’s aldehyde ([1,3-dihydro-1,3,3-trimethyl-2H-indol-2-xylidene]-acetaldehyde). It
later found use in dyeing acrylic fibers. They provide bright yellow to violet dyeing with
acceptable lightfastness. Other examples with a delocalized charge are Basic Red 14 and
Basic Yellow 11.
Further improvements in lightfast dyeing of acrylic fibers were provided by azocyanines,
which contain a delocalized charge. They give tinctorially strong bright shades with good
light fastness, and are exemplified by Basic Blue 54.
Basic Blue 54
3. Examples of the third type of cationic dye, with a localized pendant charge are Basic
Red 18 and Basic Blue 21:
Basic Red 18
Direct Dyes
Direct dyes are anionic dyes that are substantive to cellulose when applied from an aqueous
dye bath containing an electrolyte.
Although anionic, they are not classified as acid dyes because the acidic substituent is not the
means of attachment to the fiber.
Direct dyes provide the simplest means of colouring cellulosic fibers, normally being applied
from neutral or slightly alkaline dye baths, with the addition of sodium chloride or sodium
sulphate salts. The purpose of the salt is to counteract the slight negative charge that cellulose
have in aqueous conditions because it would repel an anionic dye.
For a direct dye to be substantive to cellulose, the following criteria must apply:
1. The molecule should be capable of assuming a linear configuration.
2. The molecule nuclei should be capable of assuming a coplanar arrangement.
3. The molecule should contain groups capable of forming hydrogen bonding.
4. The hydrogen bonding groups should be widely spaced along the molecule.
5. There should be a minimum of solubilizing groups to impart solubility, and these groups
to impart solubility, and these groups preferably should lie along one side of the
molecule.
The hydrogen bonding that occurs is a result of the dye molecule aligning itself with the
hydroxyl groups that are present along the length of the cellulosic chains.
Hydrogen bonding between cellulose and a direct dye.
Direct dyes in and among themselves are categorized according to their dyeing
characteristics:
Class A- good migrating and level dyeing
Class B- poor leveling, control with salt additions
Class C- poor leveling, control with temperarure.
Direct dyes belong to the dis-, tris and polyazo classes. Other miscellaneous dyes belong to
oxazine, thiazole, phthalocyanine, and stilbene classes. Direct orange 26 and Direct black 22
are typical direct dyes.
Direct Orange 26
Direct Black 22
In addition to their use of cellulosic fibers, direct dyes are important for colouring paper.
The first commercial product of this group is exemplified by the blue dye based on copper
phthacyanine
.
copper phthacyanine
Disperse Dyes
Disperse dyes are nonionic dyes with infinitely low water solubility, used to dye polyester,
acetate, triacetate, and nylon fibers from aqueous dye baths where the dye particles are
suspended by means of a protective colloid. The dye particles must be dispersed in a fine and
uniform manner with a dispersing agent to distribute the dye evenly throughout the dye bath,
to prevent filtration of the dye by the fiber being dyed, and to present a large surface area of
dye particles.
Drawn from a large number of chemical groups, disperse dyes are referred to as high energy,
medium energy and low energy products.
1. High energy disperse dyes are intended for polyester fibers and are applied in pressure
dyeing equipment at temperatures of 260 to 270 ºF or by continuous methods, that is
thermasol or thermaflix dyeing methods at temperatures of 375 to 410 ºF. These dyes
possess good sublimation fastness characteristics. Examples of high energy disperse dyes
are Disperse Blue 79 and Disperse Red 177.
Disperse Dye 79
Disperse Red 177
2. Medium energy disperse dye energy disperse dyes cover a wide latitude of end uses and
application methods, and are usually applied to polyester by atmospheric dyeing using a
carrier. These dyes usually have fair sublimation characteristics examples of medium
energy dyes are Disperse Orange 25 and Disperse Blue 27
Disperse Orange 25
Disperse Blue 27
3. Low energy disperse dyes are intended for the dyeing of acetate, triacetate, and nylon,
and represent the only practical way to dye acetate and triacetate. Also low energy dyes,
generally with molecular weights between 250 and 350, are used in transfer printing.
These dyes have poor sublimation characteristics. Examples of low energy disperse dyes
are Disperse Yellow 3 and Disperse Red 4
Disperse Yellow 3
Disperse Red 4
Reactive Dyes
Reactive dyes posse a group(s) that will combine with the substrate, usually cellulose and
thus become part of the fiber. Reactive dyes generally contain labile halogen atoms that can
undergo nucleophilic displacement by the hydroxyl of cellulose. It is necessary to add alkali,
such as soda ash, trisodium phosphate or caustic at some stage in the dyeing cycle to
facilitate the condensation of the reactive dye with the fiber to form an ether linkage.
Dye—X + Cell-OH+ NaOH Dye—Ocell + NaX + H2O
Some reactive moieties include the following
N
N
N
NH NH
Cl
R
R
N
N
N
NH Cl
Cl
R
N
NNH Cl
Cl
R
N
NNH Cl
Cl
R
Cl
N
NNH F
F
R
Cl
Dichloropyrimidine trichloropyrimidinemonochlorotriazine
dichlorotriazine chlorodifloropyrimidine
N
N
Cl
Cl
NH
R
2,3-dichloroquinoxaline
Another important reactive dye is the sulfatoethyl sulfone (‘vinly sulfone’), which involves
an activated vinly sulfone grouping that, can react with a cellulose hydroxy in the presence of
a base, as follows
Dye SO2CH2CH2OSO3Na Dye SO2CH=CH2 + Cell-OH Dye SO2CH2CH2OCell
The newest development of reactive dye chemistry involves the combination of different reactive
dyes such as monochlorotriazine and vinyl sulfone.
N
N
N
Dye NH
Cl
S
O
O
O
S OO
Na
These bifunctional dyes which also are termed multi-anchor dyes, are distinguished by their
ability to achieve a good yield over a wide temperature range, by low salt demand, by a high
degree of exhaustion and fixation and by good chlorine resistance.
Reactive dyes are used primarily to dye cellulosics, cotton and rayon but can also be used to dye
nylon and wool. The reaction in the latter case is with the amino group instead of hydroxyl. Fiber
reactive dyes as a class are noted for their bright shades and excellent wetfastness properties.
Sulfur Dyes
Sulfur dyes are insoluble dyes that must be reduced with sodium sulfide before use. In the
reduced form they are soluble and exhibit affinity for cellulose. They dye by adsorption, as do
the direct dyes, but upon exposure to air they are oxidized to reform the original insoluble dye
inside the fiber. Thus unlike the direct dyes, they become very resistant to removal by washing
The exact constitution of most sulfur dyes are unknown although the conditions required to
reproduce given types are very well established. They are fairly cheap and give dyeings of good
fastness to washing as noted above. Their brightness and fastness to beaching are often inferior.
Vat Dyes
Vat dyes are water insoluble organic pigments that become water-soluble when mixed with
powerful reducing agents in the dyeing process. The water-soluble species subsequently is
oxidized, and the pigment is re-precipitated in situ. The reducing operation formerly was carried
out in wooden vats, giving rise to the name vat dye. Vat dyes are used to colour cotton and rayon
fibers.
Vat dyes are classified into two major groups:
i. Anthraquinoids: attributes are superior wash and light fastness
ii. Indigonoids: attributes are brilliant shades, excellent wash and bleach fastness.
The application of vats can be accomplished by continues methods or by exhaust dyeing
procedures. In either case the sequence of event is as follows:
Reduce the vat pigment to the “leuco” form
Apply the “leuco” form evenly to the substrate
Oxidize the “leuco” form re-precipitate the pigment
Soap off to remove un-trapped pigment and develop the shade.
Thus a vat dye is retained as an insoluble pigment, entrapped insitu in the interstices of
the fibers
An example of a vat dye is Vat Blue 4 (Indanthrone)
Vat Blue 4 (Indanthrone)
Vat dyes are quite expensive and must be applied with care. They offer excellent fastness
when properly selected and are the dyes most often used on cotton fabrics that are to be
subjected to severe conditions of washing and bleaching.
At times materials find it impossible to tolerate the strongly alkaline conditions used to
reduce vat dyes, for example when dyeing fibers that are sensitive to alkali. For this
reason and for added convenience, some manufacturers offer soluble vat dyes, which
usually are the sodium or potassium salts of the sulfuric esters of reduced vat dyes. When
applied to the fiber and subjected to an acid treatment in the presence of an oxidizing
agent, they hydrolyze, reverting to the original form of the dye.
Below is a summary of the relationships that exist between the various fibers and dyes
classes.
Origin Fiber Functional group(s) Dye class
Natural Cotton
Wool/Silk
OH
NH2,COOH,CONH
Direct/Reactive/Vat
Acid/ Reactive
Regenerated Rayon
Acetate
Triacetate
OH
OH, OCOCH3
OCOCH3
Direct/ Reactive/Vat/Sulfur
Disperse
Disperse
Synthetic Acrylic
Nylon
Polyester
Spandex
COOH, SO3H, OSO3H
NH2, COOH,CONH
OH, COOH,COOR
Basic
Acid/ Disperse/ Reactive
Disperse
Acid/ Disperse
Pigments
These are water insoluble colourants which can be introduced into the fiber during manufacture
by mixing them with the fiber-forming substances prior to extrusion to form the filaments. This
coloration method is called “mass-pigmentation” or “mass-colouration” and is only possible with
man-made fibers.
Textiles can also be coloured with pigments with the aid of a binder (resin), which polymerizes
during baking to form a transparent film around the fibers in which the coloured pigments
particles are embedded. Both natural and man-made fibers can be coloured by this technique, to
give dyeings a very high fastness to wet treatments.