COLOUR CHEMISTRY INTRODUCTION7. 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

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.