Opacity

Hiding power is the ability of a pigmented coating to obliterate the surface. It is dependent on the ability of the film to absorb and scatter light. Naturally, the thickness of the film and the concentration of the pigment play a fundamental role. The color is also important.

Dark, saturated colors, such as blacks and deep blues, absorb most light falling upon them, whereas yellows do not. However, carbon black and most organic blue pigments are fairly transparent because they do not scatter the light that falls on them. In contrast, titanium dioxide absorbs almost no light, yet its capacity to scatter light ensures that at a sufficiently high concentration it will cover the substrate being coated. It is common practice to use a combination of pigments to achieve the best results.

A key factor in the opacity of a pigment is its refractive index (RI), which measures the ability of a substance to bend light. The opacifying effect is proportional to the difference between the refractive index of the pigment and that of the medium in which it is dispersed. This is one of the main reasons why titanium dioxide is now almost universally used as the white pigment in paint. (see Table)

Medium	RI
Air	1.0
Water	1.33
Film Formers	1.4-1.6
Pigment / Filler	RI
Calcium carbonate	1.58
China clay (aluminium silicate)	1.56
Talc (magnesium silicate)	1.55
Barytes (barium sulphate)	1.64
Lithopone 30% (zinc sulphide/barium sulphate)	1.84
Zinc oxide	2.01
Zinc sulphide	2.37
Titanium dioxide: Anatase Rutile	2.55 2.76

Inorganic pigments have a high refractive index and organic pigments have much lower values. Consequently, most inorganic pigments are opaque, whereas organic pigments are transparent.

The particle size distribution of the pigment is another factor that also plays an important role in opacity. As the particle size increases, the ability of the particle to scatter light increases, up to a maximum (see figure 6). It then starts to decrease. This ability to scatter light increases the hiding power of the pigment, and therefore the hiding power also reaches a maximum and then decreases as the particle size increases.

Whereas the refractive index of a compound cannot be altered, the pigment manufacturer can influence the particle size of pigments; consequently particle size selection has become one of the principal developments in pigment technology in recent years.

Measurement of opacity

The coating is applied in a wedge shape over a contrast chart. The film thickness is built up over the length of the chart, which is attached to a metal panel. The point at which complete obliteration is observed is noted and the film thickness at that point measured.