Importance of Particle Size Control
Effect of Particle Size on Opacity
A large, pure, single crystal of titanium dioxide or titania is colorless, and the closely related zirconia is often used for imitation 'diamond' jewelry. In both cases, the crystals have a high refractive index, diffracting and refracting light which gives the 'sparkle' required for jewelry. In this form, they have no value as pigments because although the crystals scatter light, they have only a few light scattering surfaces and therefore little opacity. See Figure 5.1 for a comparison of single crystal and pigmentary particle light scattering.
For pigmentary purposes, the particle size of TiO2 must be controlled to approximately half the wavelength of visible light. This gives maximum light scattering and the highest possible opacity in any application. Figure 5.2 shows a comparison of TiO2 opacity at different particle sizes.
The target primary mean size is generally between 0.20 – 0.25 microns, depending on the required undertone of the pigment.
Of course, no manufacturing process produces every particle to exactly the same size but yields a distribution of sizes around a target mean. The width of the distribution may be measured as the Geometric Standard Deviation (S.D.), with the higher the value, the wider the distribution. But the narrower the distribution around the target mean, the more particles are available to scatter incident light and the higher the opacity. See Figure 5.3 for particle size distribution graphs.
The broad relationship between particle size distribution as measured by SD and opacity can be clearly shown. The correlation is clear and the spread of values around the correlation line can be almost completely explained by differences in surface treatment levels (more treatment, applied for specific performance attributes means less actual TiO2 and lower potential opacity), and differences in flocculation state.
Effect of Particle Size on Dispersion & Gloss
It is also important that particles over 0.5 microns are minimized since these affect dispersion, the gloss of a paint film and also its durability.
Whereas opacity and color are usually assessed by observing a surface at right angles for diffusely scattered light, specular gloss is a function of the proportion of incident light observed when the light source and the observer are at equal angles in relation to the surface. As gloss is a surface phenomenon, a pigment only has an effect at or very close to the observed surface, but any surface roughness caused by oversized pigment particles or poor dispersion is a pigment related problem.
Thus pigment particles with diameters in the region 0.5 to 5.0 micrometers are too small to appear as poor dispersion and too large to generate effective opacity but produce a matting effect at the surface of a film. Figure 5.5 shows the effect of particle size on gloss.
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