Introduction to pigments

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DYEING AND FINISHING PART I
Pigments: A pigment is any particular matter that is insoluble in, and essentially physically
and chemically unaffected by, the media into which it is dispersed.
According to Pratt, all insoluble dispersed and inert materials are not pigments. When all the
desirable factors like, particle size and shape, nature of surfaces, refractive indices, crystal
structure etc. of insoluble dispersible inert material are satisfactory, then only they can be
considered pigments.
Pigments are both natural and synthetic in origin, and organic and inorganic in composition.
Some desirable factors for a pigment is given below
a) Particle size: The covering power of a pigment is largely depended upon the
particle size. Smaller the particle size, higher the covering power. The particle size of
the commercial pigments varies from fine colloidal particle (0.01 µm) to relatively
coarse particle (100 µm). The particle size of organic pigments is smaller than the
inorganic pigment, but the covering powers of inorganic pigments is generally higher
than organic pigments due the factors like refractive indices.
b) Particle shape: The shape of pigments determines, to a great extent, the strengths
of bondage formed between the binder and the pigment. The bondage is strong when
the particles are irregular in shape and possess pointed sharp edges.
c) Nature of surfaces: The nature of surface of a pigment particle should be such that
it can easily reflect those light waves which represent the color of the pigment. When
the surface of a pigment particle become harsh, its brilliancy is largely reduced.
d) Refractive Indices: The covering power of a pigment also depends upon its optical
density which can be represented by its refractive index. Higher the refractive index,
higher is the covering power. Since the refractive indices of inorganic pigments are
very in high comparison to organic pigments, the covering power of inorganic
pigments are always superior than organic pigments.
e) Fastness: Pigment must be fast to light, heat, smoke, dust,
different gases present in the atmosphere, dilute acids and alkalis,
rain water and different common organic solvents. Fastness of a
pigment means its capacity to retain color and other properties
against all sort of environmental influences.
f) Compatibility: if the pigment particles are not chemically inert,
they may precipitate out many ingredients from the binder
solution into which the pigment is dispersed. A good pigment
should be compatible i.e., it should satisfactorily tolerate all the
materials used in paint preparation.
g) Flocculation: A good pigment powder should completely disperse when it is
sprinkled on stirring vehicle containing other necessary materials al slightly elevated
temperature. Pigments must have densities above a minimum limit to avoid
flocculation.
Among the inorganic
pigments, the materials
of high refractive indices
are considered as prime
pigments and color
pigments and the
materials of lower
refractive indices are
known as extenders and
fillers.

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h) Bleeding and Migration: Bleeding and migration of color are another two
important factors to be considered before selecting a pigment. When the color of
pigment diffuses into the vehicle, pigment is definitely a bleeding type. Bleeding
materials cannot therefore, be considered as pigment.
Sometimes no color diffuses into the vehicle but migrates into the pigment particle
itself. Such materials are also unsuitable to be considered as pigments.
i) Abrasion: The particles of a good pigment must show high degree of resistance to
abrasion. High resistance to abrasion for pigment particles are necessary to protect
them from internal friction during flexing of leather.
j) Crystallization: If a pigment crystallizes out when present in dilute paint emulsion,
it must be discarded as a pigment. The character of a crystal is quite different from its
ground particles, and so crystallization is a serious defect for pigments.
Classification of Pigments
Pigments are broadly classified into two groups- inorganic and organic pigments. Organic color
pigments are generally brighter, purer and rich in color than their corresponding inorganic
pigment counterparts.

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Inorganic Pigments
Inorganic pigments are two types-
• Earth Colors
• Prepared Pigments
Earth Colors: The naturally occurring insoluble colored materials come under this group.
Earth colors are mined in different parts of the world.
Yellow colored iron oxide called Limonite (Fe2O3.xH2O) is mined in different parts of
the world for yellow pigment preparation.
Red colored natural iron oxide Fe2O3 is called Hematite
Brown colored limonite is available in Cyprus and Turkey. The ore contains carbon
and MnO2
Prepared Pigments: The inorganic pigments which are manufactured in chemical factories
are called ‘prepared pigments’ or ‘synthetic inorganic pigments’.
These pigments are classified into three groups
a) White Prime or White hiding pigments
b) Extenders or Filler pigments
c) Color inorganic pigments
a. White Prime pigments: The inorganic pigments prepared from white materials like
Titanium dioxide, Zinc oxide, Zinc sulfide, Lithophone, Potassium titante, Antimony oxide
etc. come under the first group, white prime pigments. Since the refractive indices are high
(above 2.0), they themselves can produce prefect white pigments and so the name White
Prime pigments.
b. Extenders or Fillers: The extenders or fillers are inorganic white or colorless prepared
materials but they cannot produce white color to the paints as their refractive indices (near
about 1.5) are almost equal to the refractive indices of common oils and binders used in
paint preparations.
The density and hardness of extenders are generally lower than white prime and color
inorganic pigments. Precipitated Chalk and Silica, Kaolin, Talc, Asbestos, Mica Barium
Sulfate, Aluminum oxide etc. come under this group.
c. Color Inorganic pigments: These are synthetically prepared colored inorganic
compounds or high refractive indices (above 2.0), high density and hard pigments. Different
colored oxides of iron and chromium, Ultra-marines, Red lead, colored compounds of
different metals, Dust of different metals etc. come under this group. These materials
themselves can produce color to paints.

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Organic Pigments
Organic pigments generally are classified into three groups-
a) Pigment Dyestuffs
b) Toner pigments
c) Lakes
a. Pigments Dyestuffs: These are organic colorants or dyestuff that are already
insoluble.
The tinting power is very high but the different fastness properties are comparatively
poor, whereas bleeding is high.
Example: Toluidine Red, Hansa Yellow etc.
b. Toner pigments: Water soluble acid or basic dyes that are converted to insoluble
pigments by precipitation with metallic salts or acid respectively.
Example: Lithol Red, Acid red are example of acid dye and Triphenyl methane is
example of basic dyes.
c. Lakes: The precipitation conditions of lakes are essentially similar to those for toners
but in presence of an inorganic substrate or carrier. Most of the organic pigments,
available in the market are lakes because those are precipitated from dyes in presence of
inorganic carrier.
Example: Light hydrated alumina.
Difference between organic and inorganic pigments
Organic Pigments Inorganic Pigments
Good brilliancy Less Brilliancy
Less covering power Good covering power
Tendency to cause bleeding No bleeding
Less settling More settling due to high density
Less body Good body
Less light fastness Good light fastness
Available in the purest form Many contain impurities if made out of earth
color

Introduction to pigments

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