6. Coating Principles

6.5 Pigments, Extenders and Additives


Pigments give the paint color and hiding power to cover the underlying surface. Of particular interest are the pigments which prevent or retard corrosion. Bioactive pigments are used to prevent biofouling. By definition, a pigment is defined as a substance that is insoluble in the application medium and is used as a colorant


They are substances that are insoluble in the application medium and add to the mechanical strength of the paint film and improve the application properties. Examples include chalk, talk, silica,etc.


They are added to achieve multipurpose objectives, increase shelf life, reduction of drying time and enhance desired properties.

Types of Pigments

Pigments can be classified in two categories

  1. Organic pigments

  2. Inorganic pigments

Examples are given below:


Inorganic Pigments

White pigment

Titanium dioxide is the most important pigment with a worldwide consumption of 2 x 106 tons. It has an excellent chemical resistance.

Iron oxide

The color range of these pigments includes red (g-Fe2O3,hematite), black (Fe3O4, magnetite), yellow (a, FeOOH, geothite) and orange (g-FeOOH, idocrocite)

Chromium oxide

They are composed of chromium oxide (Cr2O3), crylstallized in a corrundum lattice. They are known for color fastness, insolubility in solvents, acids and alkalies.

Complex inorganic pigments

They refer to the metal oxide pigments derived from spinel (MgAl2O4), rutile (TiO2), hematite (g-Fe2O3)

Ultra-marine pigments

These are sodium aluminum silicates of composition Na8Al6Si6O24.Sx . Blue, red, green and violet pigments can be obtained depending on the composition. They have a high heat resistance but a poor hiding power.

Transparent pigments

They are produced from colored inorganic pigments g-FeOOH (yellow), a-Fe2O3 (red) and g-CrOOH (green).

Metallic effect pigments

They are flakes of ferrous metals like copper and aluminum. They have a high decorative value. They are used widely in automobile topcoats. They can withstand hot weather and marine environments.



Organic Pigments

Blue pigments

Examples are Phthalocynine blue pigments. They have a high strength and excellent fasten to chemicals.

Green pigments

Example: halogental copper pthalocycanine

Yellow pigments Monoazo, benzimidazalone yellow pigment anddiazoyellow pigment.
Orange pigment o-dianisidine coupled with two moles of aceto-acetanalide

Functions of Pigments

The following are the major functions of pigments:

  1. Color
  2. Corrosion resistance
  3. Protection of binder
  4. Film reinforcement
  5. Increased surface coverage


The following are the desirable properties in extenders: