A dye can generally be described as a colored substance that has an affinity to the substrate to which it is being applied. The dye is generally applied in an aqueous solution, and may require amordant to improve the fastness of the dye on the fiber. Both dyes and pigments appear to be colored because they absorb some wavelengths of light preferentially. In contrast with a dye, a pigment generally is insoluble, and has no affinity for the substrate. Some dyes can be precipitated with an inert salt to produce a lake pigment, and based on the salt used they could be aluminum lake, calcium lake or barium lake pigments. Dyed flax fibers have been found in the Republic of Georgia dated back in a prehistoric cave to 36,000 BP. [1][2] Archaeological evidence shows that, particularly in India and Phoenicia, dyeing has been extensively carried out for over 5000 years. The dyes were obtained from animal, vegetable or mineral origin, with no or very little processing. By far the greatest source of dyes has been from the plant kingdom, notably roots, berries, bark, leaves and wood, but only a few have ever been used on a commercial scale. Contents [hide] 1 Dye Types 2 Food dyes 3 Other important dyes 4 Chemical classification 5 See also 6 References 7 Further reading 8 External links [edit]Dye Types The first human-made (synthetic) organic dye, mauveine, was discovered by William Henry Perkin in 1856. Many thousands of synthetic dyes have since been prepared., [3][4] Synthetic dyes quickly replaced the traditional natural dyes. They cost less, they offered a vast range of new colors, and they imparted better properties to the dyed materials. [5] Dyes are now classified according to how they are used in the dyeing process. Acid dyes are water-soluble anionic dyes that are applied to fibers such as silk, wool, nylon and modified acrylic fibers using 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. Most synthetic food colors fall in this category.
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Transcript
A dye can generally be described as a colored substance that has an affinity to the substrate to which it is being
applied. The dye is generally applied in an aqueous solution, and may require amordant to improve the fastness of the
dye on the fiber.
Both dyes and pigments appear to be colored because they absorb some wavelengths of light preferentially. In contrast
with a dye, a pigment generally is insoluble, and has no affinity for the substrate. Some dyes can be precipitated with an
inert salt to produce a lake pigment, and based on the salt used they could be aluminum lake, calcium lake or barium lake
pigments.
Dyed flax fibers have been found in the Republic of Georgia dated back in a prehistoric cave to 36,000 BP.[1]
[2] Archaeological evidence shows that, particularly in India and Phoenicia, dyeing has been extensively carried out for over
5000 years. The dyes were obtained from animal, vegetable or mineral origin, with no or very little processing. By far the
greatest source of dyes has been from the plant kingdom, notably roots, berries, bark, leaves and wood, but only a few
have ever been used on a commercial scale.
Contents
[hide]
1 Dye Types
2 Food dyes
3 Other important dyes
4 Chemical classification
5 See also
6 References
7 Further reading
8 External links
[edit]Dye Types
The first human-made (synthetic) organic dye, mauveine, was discovered by William Henry Perkin in 1856. Many
thousands of synthetic dyes have since been prepared.,[3][4]
Synthetic dyes quickly replaced the traditional natural dyes. They cost less, they offered a vast range of new colors, and
they imparted better properties to the dyed materials.[5] Dyes are now classified according to how they are used in the
dyeing process.
Acid dyes are water-soluble anionic dyes that are applied to fibers such as silk, wool, nylon and modified acrylic
fibers using 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. Most synthetic food
colors fall in this category.
Basic dyes are water-soluble cationic dyes that are mainly applied to acrylic fibers, but find some use for wool and silk.
Usually acetic acidis added to the dyebath to help the uptake of the dye onto the fiber. Basic dyes are also used in the
coloration of paper.
Direct or substantive dyeing is normally carried out in a neutral or slightly alkaline dyebath, at or near boiling point, with
the addition of either sodium chloride (NaCl) or sodium sulfate (Na2SO4). Direct dyes are used on cotton, paper, leather,
wool, silk and nylon. They are also used as pH indicators and as biological stains.
Mordant dyes require a mordant, which improves the fastness of the dye against water, light and perspiration. The choice
of mordant is very important as different mordants can change the final color significantly. Most natural dyes are mordant
dyes and there is therefore a large literature base describing dyeing techniques. The most important mordant dyes are the
synthetic mordant dyes, or chrome dyes, used for wool; these comprise some 30% of dyes used for wool, and are
especially useful for black and navy shades. The mordant, potassium dichromate, is applied as an after-treatment. It is
important to note that many mordants, particularly those in the heavy metal category, can be hazardous to health and
extreme care must be taken in using them.
Look up leuco
form inWiktionary, the free
dictionary.
Vat dyes are essentially insoluble in water and incapable of dyeing fibres directly. However, reduction in alkaline
liquor produces the water soluble alkali metal salt of the dye, which, in this leuco form, has an affinity for the textile fibre.
Subsequent oxidation reforms the original insoluble dye. The color of denim is due to indigo, the original vat dye.
Reactive dyes utilize a chromophore attached to a substituent that is capable of directly reacting with the fibre substrate.
The covalentbonds that attach reactive dye to natural fibers make them among the most permanent of dyes. "Cold"
reactive dyes, such as Procion MX,Cibacron F, and Drimarene K, are very easy to use because the dye can be applied at
room temperature. Reactive dyes are by far the best choice for dyeing cotton and other cellulose fibers at home or in the
art studio.
Disperse dyes were originally developed for the dyeing of cellulose acetate, and are water insoluble. The dyes are finely
ground in the presence of a dispersing agent and sold as a paste, or spray-dried and sold as a powder. Their main use is
to dye polyester but they can also be used to dye nylon, cellulose triacetate, and acrylic fibres. In some cases, a
dyeing temperature of 130 °C is required, and a pressurised dyebath is used. The very fine particle size gives a large
surface area that aids dissolution to allow uptake by the fibre. The dyeing rate can be significantly influenced by the choice
of dispersing agent used during the grinding.
Azoic dyeing is a technique in which an insoluble azo dye is produced directly onto or within the fibre. This is achieved by
treating a fibre with both diazoic and coupling components. With suitable adjustment of dyebath conditions the two
components react to produce the required insoluble azo dye. This technique of dyeing is unique, in that the final color is
controlled by the choice of the diazoic and coupling components.
Sulfur dyes are two part "developed" dyes used to dye cotton with dark colors. The initial bath imparts a yellow or
pale chartreuse color, This is aftertreated with a sulfur compound in place to produce the dark black we are familiar with in
socks for instance. Sulfur Black 1 is the largest selling dye by volume.
[edit]Food dyes
One other class which describes the role of dyes, rather than their mode of use, is the food dye. Because food dyes are
classed as food additives, they are manufactured to a higher standard than some industrial dyes. Food dyes can be direct,
mordant and vat dyes, and their use is strictly controlled by legislation. Many are azo dyes,
although anthraquinone and triphenylmethane compounds are used for colors such as green and blue. Some naturally-
occurring dyes are also used.
[edit]Other important dyes
A number of other classes have also been established, including:
Oxidation bases, for mainly hair and fur
Laser dyes: see, for example, rhodamine 6G and coumarin dyes.[6]
Leather dyes, for leather
Fluorescent brighteners, for textile fibres and paper
Solvent dyes, for wood staining and producing colored lacquers, solvent inks, coloring oils, waxes.
Carbene dyes, a recently developed method for coloring multiple substrates
Contrast dyes, injected for magnetic resonance imaging, are essentially the same as clothing dye except they are
coupled to an agent that has strong paramagnetic properties
I.II. DyesAcid Dye: An acid dye is a dye in which the coloring component is in the anion or negative charge in its chemistry. They are often applied from an acidic solution in order to intensify the staining.
Uses
[edit]Fibers
In the laboratory, the home or art studio, the acid used in the dyebath is often vinegar (acetic acid) or citric acid. The
uptake rate of the dye is controlled with the use of sodium chloride. In textiles, acid dyes are effective on protein fibers, i.e.
animal hair fibers like wool, alpaca andmohair. They are also effective on silk. They are effective in dyeing the synthetic
fiber nylon but of minimal interest in dyeing any othersynthetic fibers.
[edit]Medical
In staining for microscopic examination for diagnosis or research acid dyes are used to color basic tissue proteins in
contrast to basic dyes, which are used to stain cell nuclei and some other acidic components of tissues.
[edit]Description
Acid dyes are generally divided into three classes which depend on fastness requirements, level dyeing properties and
economy. The classes overlap and generally depend on type of fiber to be colored and also the process used.
Acid dyes are thought to fix to fibers by hydrogen bonding, Van der Waals forces and ionic bonding. They are normally
sold as the Sodium salt therefore they are in solution anionic. Animal protein fibers and synthetic Nylon fibers contain
many cationic sites therefore there is an attraction of anionic dye molecule to a cationic site on the fiber. The strength
(fastness) of this bond is related to the desire/ chemistry of the dye to remain dissolved in water over fixation to the fiber.
[edit]History of acid dye
[edit]Structures
The chemistry of acid dyes is quite complex. Dyes are normally very large aromatic molecules consisting of many linked
rings. Acid dyes usually have a sulfo or carboxy group on the molecule making them soluble in water. Water is the medium
in which dyeing takes place. Most acid dyes are related in basic structure to the following:
Number Available in Large number Comparatively lesser in number
Product resistance Lower as compared to pigments Very high
Lightfastness
Lower Dyes are very much vulnerable. Lights destroy colored objects by breaking open electronic bonding within the molecule
Traditionally pigments have been found to be more lightfast than dyes
Size
Dye molecules are comparatively smaller it's like comparing a football (pigment) to say a head of a pin (dye)
Pigment particles are about 1-2 microns in size. (1 micron =1/1000 meter). It means that the particles can be seen under a magnifying glass
Bonding
Taking the example of dyeing a wood surface, the dye and the substrate (wood) that is dyed are chemicals, that have certain features called functional groups. At the level of molecules these groups serve as open pockets of electrostatic charges (+ or -). The functional group in dyes, serve as a method for attaching the dye to the wood
For example taking the example of a wood surface Pigment requires the help of a binder for gluing. As it is an inert substance which is merely suspended in a carrier/binder
Structure during the application process
During application process there is a temporary alteration in the structure of the dyes
During application, pigments have the capacity to retain particulate or crystalline structure
Imparting of Colours
Dyes can only impart colour by selective absorption of the dyes
Pigments impart colours by either scattering of light or by selective absorption
Combustible properties
Taking the example of a Candle making process, if the candles are dyed it is easily combustible and can be applied throughout the candle
In the example of a candle making as pigments are colored particles, they tend to clog a wick when burned. This makes them undesirable for a candle if it is colored throughout and used for burning
Chemical Composition
Usually the dyes are organic (i.e. carbon-based) compounds
While pigments are normally inorganic compounds, often involving heavy toxic metals
Longevity factorThe dye based printing inks do not last as long as the pigment inks
In case of ink based printing prints made with pigments lasts longer
Printing on substrates
Compatible with almost all the substrates that needs to be dyed
Owing to the physical makeup of the pigment inks the range for suitable substrates are limited
Colour gamutTaking the case of printing inks, dye based inks offers a wide variety
As compared to dye-based inks, pigment inksets somewhat lags behind, on the same paper stock
Types of Pigments
There are today available a number of pigments. In fact after the advent of Synthetic Pigments there has evolved various classes of
pigments that are suited to particular types of Industries.
Inorganic Pigments- They are of the type mineral-earth but generally are metallic oxides or synthetics. Pigments that are of the
type Mineral-earth are very simple and naturally occurring colored substances. The preparation process is also simple and consists
of the steps of washing drying, pulverizing and mixing into a formulation. The following table shows the refractive index of some of
the very popular class of inorganic pigments.
Pigment Refractive Index
TiO2 (rutile) 2.71
TiO2 (Anatase) 2.55
Antimony Oxide 2.20
Zinc Oxide 2.01
Calcium Carbonate 1.65
Fumed Silica 1.45
Examples of inorganic pigments can be materials like lead oxide, cobalt blue, chromium oxide, cadmium yellow, molybdate orange,
and nickel titanate. As new environmental laws are very strict about toxicity a few of these heavy metal pigments are no longer in
use.
Organic Pigments- Organic Pigments are not usually found in nature. That is the reason that a majority of these pigments are
chemically synthesized. They contain carbon and comes with relatively low levels of toxicity, not providing any major environmental
concern. Raw materials can include coal tar and petroleum distillates that are transformed into insoluble precipitates. Traditionally
organic pigments are used as mass colourants. They are popular in plastics, synthetic fibres and as surface coatings-paints and
inks. In recent years the organic pigments are used for hi-tech applications that includes photo-reprographics, opto-electronic
displays and optical data storage.
Categories of Organic Pigments
Organic pigments are generally categorized into six types :
Monoazo Pigments
Diazo Pigments
Acid and base dye Pigments
Phthalocyanine Pigments
Quinacridone Pigments
Other polycyclic Pigments
Chemical structure of an Azo Pigment
Key features and characteristics of Organic Pigments
Very good stability to solvents, light, heat, and weathering
Good tinctorial strength
Cost effectiveness
Consistency and unique shades
Completely non-toxic
Very bright, pure, rich colors
Organic pigments shows good color strength
Metallic Pigments
As these days Metallic Pigments are a very popular category of Pigments, a further classification can be done for the metallic
pigments.
Metallic pigments, can be of two types aluminum and zinc.
Aluminium Pigments: Aluminium pigments are further divided into two categories namely leafing grade and non-leafing grade.
The aluminum pigments are produced from aluminium that has purity in the range of 99.3-99.97%. The particle has lamellar shape
with 0.1-2 um in thickness and diameter of 0.5-200 um. These pigments founds use in automotive topcoats.
Some of the preferred applications of Aluminium Pigments are the following:
As Corrosion protection coats
As Reflective paints
In Marine paints (covering coats)
Roof coatings
In Heat-proof and highly heat-resistance paints
Chrome effect paints
Aerosols
Zinc Pigments: Zinc Pigments come in two forms of powder and dust. Usually the zinc dust is finer as compared to powder and is
spherical in shape. The dust also has a light coating of zinc oxide.
The following table cites some popular Zinc Pigments along with their applications.
Zinc Pigments Application
Zinc dust Chemical applications Metallurgical applications
Zinc phosphate Active ingredient in domestic cleaning products
Organic Pigments Organic vs Inorganic Pigments Mixed Metal Oxide Pigment Phosphorescent Pigment Intermediates Synthetic Iron Oxide Pigments Titanium Dioxide Natural Iron Oxide Pigments
Refractive index and Classification of pigmentsThe whole idea with pigment is to give colour and protect the substrate. Now, to provide color to a material the pigment must create an opacity.
Can the pigment impart opacity depends on the characteristic property of refractive index. As we are aware that there is a particular limit to the
concentration of pigment particles in a coating determined by the pigment volume concentration (PVC). This is the reason for having pigment
particles with a high refractive index.
The image here tries to show if the particles do not have high refractive index, there will be insufficient hiding.
Based on such refractive index, pigments can be divided into two categories of hiding and extender pigments.
Hiding pigments:
These pigments possess generally refractive index values that are greater than 1.5. Examples are titanium dioxide, zinc oxide, lithophone etc.
Extender pigments:
These pigments have refractive index values that are close to 1.5. Examples include calcium carbonate, silicas, alkali and alkaline earth metal
silicates etc.
Selection of Pigment and Binder StabilityIf you are considering pigmentation of a system, that is inherently unstable, for example an unstable binder that is to be pigmented with titania, the
selection of the pigment is then for both coloration and function. While formulating a colored coating, the pigment's role should be as an UV
absorber or reflector. Thus blocking UV transmission through the binder, where degradation takes place.
Performance Criteria for commercial organic pigments
The commercial performance of organic pigments are guided by the following capabilities:
Coloristic performance
Rheological behaviour
Durability
Ecological compatibility
Tinctorial strength
Opacity
Resistance to heat
High performance Organic PigmentsA comparatively recent addition they are finding use in speciality applications. They display all round fastness properties in terms of light weather
and heat.
Some of the important high performance organic pigments along with their molecular structures is given here.
While talking about Global Dyestuff industry, it has seen an impressive growth over the years. The Global Dyestuff Industry
primarily constitutes of three sub-segments. These are Dyes, Pigments and Intermediates. The dye intermediates are essentially
derivatives of petroleum products which after further processing gets transformed into finished dyes and pigments. The dye and
dye intermediaries industry is now an integral part of a huge number of industries. Be it chemical, of which it has a substantial
stake, along with that it also provides inputs to a large number of other major industries like textiles, leather, plastics, paints, paper
and printing inks, pharmaceuticals to name a few. The following diagram highlights how the Dyestuff industry's interconnection with
other facets of the industrial setup.
In 2005, the global market size for dyes, pigments and
intermediaries was estimated at around $23 billion. If
we see the total volume, then global dyestuff production
is estimated to be somewhere around 34 million tonnes.
The annual global sales of textile dyestuff alone is
estimated approximately around $ 6 billion. One of the
major factors that had emerged within the last few years
is that the major production centres for dyestuff has
shifted from the west to the east. The global dye
manufacturing industry originally dominated by
suppliers from Europe namely UK, Switzerland
Germany, has shifted to Asia over the past 20 years or
so. This is primarily because of two reasons. First, due
to much lower costs of production in the Asia region.
Secondly Asia's growing prominence as the hub for
global textile industry.
China's share in the world market is estimated to be around 25%. Along with China, Taiwan, India, Japan and Pakistan are among
the major dyestuff producing countries in the industry. But in terms of the sheer volume of market share, Europe is the leading. This
is due because of its allegiance towards specialty products. The global market share of Indian dyes industry is between 5 - 7%, and
it is expected to increase to almost 10% by 2010.
The markets for the dyes are predominantly dominated by reactive and disperse dyes. In fact the demand for these two dyes is
expected to grow in future also. Nations like China, South Korea and Taiwan are strong players in the field of disperse dyes.
Interestingly, India has taken lead in production of reactive dyes because of the availability of an intermediate called vinyl sulphone
in the country. The following diagram illustrates the expected trends of some of the prominent dyes.
Major challenges facing the Global Dyestuff Industry
Though there has been impressive growth over the years but there are some serious challenges facing the Global dyestuff industry
today. They are the following:
Problem of over capacity but falling margins- It is a fact that China and India now have high potential as regards production
capacity is concerned. This is due to a shift in the manufacturing bases from Europe and some other industrialise nations. But there
is varying demand across these regions and that is the cause for volatility in the market. Thus affecting prices.
Fierce competition- Again, because of the shift of companies from West to East has resulted in concentration of all the
companies in the Asian region which has created intense competition in the global market.
Research & Development- Though there is a a constant spending on R&D across all the manufacturers between 1-3%, but the
market demands a higher spending for innovation in products like natural dyes.
Environmental Considerations- The industry can only prosper if the dyes are not hazardous and environment friendly. A very
good example is the banning of the Azo dyes in Europe along with closure of the units. Similarly in many countries there are certain
dyes that are now banned. The thrust is now towards the environment friendly natural dyes.
Support of the Government and Trade Association- Any industry that moves ahead needs the back-end support of the
government as well as the trade associations. This helps to promote that industry in other countries, allocation of investments and
giving other supports. These associations should actively promote technology institutes that does research activities.
Product quality vis-a-vis competitive prices- Increasingly the manufacturers have to focus on the quality of products along with
competitive prices for retaining market. The production share of the developed countries in the market has gone down from 65% to
around 50%. This is further expected to reduce in future.
Availability of World Class Infrastructure- As ports and roads are the primary sources of transport, the Governments must
emphasis to improve the clearance of the goods at a quicker rate to facilitate trade.
Classification of products and services- A decline in the growth for products has prompted the manufacturers to move to
specialty products.
High cost of energy and interest- The exorbitant cost of interest can lower the investment in R&D which is at the core of product
and service innovations. Also, the high energy cost has also adversely impacted the manufacturing units.