Opacity and tinting strength

The primary control of opacity and brightness in white paint films depends on the scattering of light. TiO2 is unique because it efficiently scatters visible light, thereby imparting whiteness, brightness and opacity when incorporated into a coating.

Curves in the chart below derived from theoretical considerations in highly dilute systems show the relative scattering power of rutile TiO2 for blue, green and red light as a function of particle size. At about 0.2 microns, the sum of light scattered at all wavelengths is maximized. When the particle size is increased to between 0.25 and 0.30 microns, the scattering of blue light decreases rapidly, but the scattering of green and red is relatively unchanged; however, at 0.15 microns, the diameter corresponding to maximum scattering of blue light, light scattering in the red and green regions drops markedly.

Figure 1: Relative Scattering Power of Rutile TiO2 vs. Particle Size 

In an ideal white film that is pigmented to complete hiding, changing pigment particle size has no effect on color since all the light striking the film is completely scattered. When a pigment such as carbon black is added to the white formula, red light with the longer path length now has a greater chance to be absorbed. As a consequence, the reflected hue appears bluer. Thus, in a paint film containing some light-absorbing matter, decreasing TiO2 particle size will increase blueness. This phenomenon is called undertone.

Color and undertone are sometimes confused. It is possible for a paint containing blue undertone TiO2 to have a yellow color because of one of the undesirable effects listed below:

• Contamination - including abraded processing equipment, usually resulting from problems during the dispersion process.

• Colored products of reactions of TiO2 with other paint ingredients such as phenolics, strong reducing agents, etc. Blue, purple or gray discoloration in oxygen-impermeable films exposed to ultraviolet radiation.

• Excessive heat exposure - vehicle discoloration.

• Inadequate hiding - show-through of substrate.