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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 fibers 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
Zinc oxide Agricultural applications
Rubber industry
Brick kilns
Ceramics

Industrial Pigments


Refractive index and Classification of pigments

The 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.

Refractive index and Classification of pigments

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 Stability

If 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.

Selection of Pigment and Binder Stability



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 Pigments

A 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.

High performance Organic Pigments

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