Particle Size Distribution and TiO2 Opacity
TiO2 particle size sets opacity — too small and particles don't scatter light effectively, too large and you waste material.
The opacity of titanium dioxide in coatings and plastics depends critically on particle size — and not just the median size, but the entire particle size distribution (PSD).
The physics: Light scattering by particles is most efficient when particle size is approximately λ/2, where λ is the wavelength of light. For visible light (400–700 nm), this means optimal particle size is roughly 200–350 nm (0.2–0.35 μm).
For TiO2 specifically, with its very high refractive index, the optimum is in the 0.20–0.30 μm range. Most commercial coating-grade TiO2 targets D50 (median diameter) of 0.25–0.28 μm.
Below optimum (too small): Particles smaller than 200 nm scatter light less efficiently per particle. In nano-TiO2 (15–30 nm), the particles are too small to scatter visible light at all — the powder appears transparent on skin (which is the design intent for sunscreen) but provides no opacity.
Above optimum (too large): Particles larger than 400 nm also scatter less efficiently. They also feel rough in coatings and cause anilox blinding in fine-screen printing.
PSD width matters too: - Narrow PSD (tight standard deviation around the median) maximizes the fraction of particles in the optimal scattering range - Broad PSD includes both too-small (wasted material that doesn't contribute) and too-large (also wasted) - Chloride-process TiO2 has inherently narrower PSD than sulfate-process
Practical implications for formulators: - Two TiO2 grades with identical D50 but different PSD will give different hiding power - Chloride rutile typically delivers 5–10% better hiding-per-kg than sulfate equivalents because of narrower PSD - Bead-mill grinding can reduce D50 but also broadens PSD if over-milled — there's a quality optimum, not just "finer is better"
Measuring PSD: - Laser diffraction (Malvern Mastersizer, Beckman LS) is standard for production QC - Reports D10, D50, D90, plus mean, standard deviation, and surface area equivalent diameter - For TiO2 coating-grade, typical specs: D10 0.15 μm, D50 0.25–0.28 μm, D90 0.40–0.50 μm
For grade selection: Higher tinting strength (Reynolds method) is often a useful proxy for narrower PSD around the optimum. Grades reported at 1900+ Reynolds tend to have tighter PSD than grades at 1700–1800.
PSD also affects rheology in coatings and dispersion behavior in plastics. A grade with bimodal PSD (peaks at both 0.2 μm and 0.8 μm) will behave very differently in mill dispersion than a tight unimodal grade at 0.25 μm — even though the D50 may be similar.