Refractive Index and Hiding Power
TiO2's hiding power comes from its exceptionally high refractive index — the highest of any white pigment. Here's the optics.
The hiding power of a white pigment in a coating or plastic depends on how much light it scatters at the pigment-binder interface. Light scattering, in turn, depends on the refractive index difference between the pigment and the surrounding medium.
Refractive index reference values: - Air: 1.00 - Water: 1.33 - Typical paint binder (acrylic, alkyd, PU): 1.45–1.55 - TiO2 rutile: 2.75 - TiO2 anatase: 2.50 - Zinc oxide: 2.00 - Calcium carbonate: 1.59 - Aluminum oxide: 1.77 - Zinc sulfide: 2.37
The relative scattering efficiency depends on the ratio of pigment RI to binder RI. For TiO2 rutile in a typical paint binder, RI(TiO2)/RI(binder) ≈ 2.75/1.50 = 1.83 — exceptionally high, which is why TiO2 outperforms every other white pigment for hiding power.
Hiding power per kg comparison (relative to TiO2 rutile = 100): - TiO2 rutile: 100 - TiO2 anatase: ~75 (due to lower RI: 2.50 vs 2.75) - ZnO: ~60 - Lithopone (BaSO4 + ZnS): ~40 - Calcium carbonate: ~5 (essentially zero hiding from extender)
This is the fundamental reason TiO2 dominates white pigment markets despite costing 5–10x more per kg than calcium carbonate. The TiO2 fraction of formulation hides; the extender fraction doesn't.
Why anatase has less hiding power: The 0.25-unit RI advantage of rutile (2.75 vs anatase 2.50) translates to roughly 30% better hiding power. For coatings where hiding-at-low-loading matters, this is the dominant economic factor in choosing rutile over anatase.
The hiding power formula (simplified Mie theory): Scattering efficiency Q_scat depends on particle size, refractive index ratio, and wavelength. For TiO2 rutile in typical paint at 0.25 μm particle size: - Peak scattering at ~0.28 μm particle size - Drops off sharply for particles < 0.15 μm (rapidly approaching transparency) - Drops off gradually for particles > 0.4 μm
Practical implications: 1. You can't substitute TiO2 with extender pigments at equal loading — extenders don't hide 2. You can extend TiO2 by replacing some of the binder fraction with TiO2 + extender mix, but the hiding-per-kg-of-formulation declines proportionally to the TiO2 fraction 3. The "TiO2 demand" in your paint formula is set by hiding power target; lowering TiO2 below the optimum requires more film build (thicker coats) or accepting lower opacity 4. Premium hydrophilic-tuned TiO2 (like SEMITI 706W) achieves marginally better hiding-per-kg by keeping particles better-spaced in the dried film — the same RI, but better optical spacing efficiency