Could Al2O3 replace TiO2 as paint pigment if it was economically available in the 300 to 800 nanometer range?  TiO2 is generally coated with Al2O3 because of TiO2 photochemical reactions.  Is TiO2 only used because it is easy to make the particle size range economically?  Since lead, zinc, and other metal oxides have been used in the past it seems that titanium has no intrinsic properties other than its particle size.  I have a way to make spherical sub-micron Al2O3 particles at low cost, so this is an obvious use.

I've heard it said that TiO2 has better UV reflectance (presumably relative to other oxides) and thus less pigment is required per unit area to achieve good coverage/color.

I thought TiO2 was "whiter" than alumina

TTFN

TiO2 has an exceptionally high index of refraction. Thus particles in a transparent liquid or polymer have maximum light scattering effect (hiding power). Particles that have the same index as the liquid they are in become invisible and don't affect transparency. It is whiter than alumina.

Often times, "pure" white isn't desired in paint.
In fact, many times, white paint bases are knocked off slightly with some "B" or "LB" black colorants in order to increase the "hide" of the coating.  It's basically a cheat that everyone in the industry uses.

The ratio of Ti02 to binder/solvent in a paint is not linear when used to increase hide.  There is a point that adding more Ti02 does nothing more than increase cost.


As far as a Ti02 alternative, if Al2O3 grinds in the same manner as Ti02, performs the same under exposure to UV, is compatable with glycol based colorants, has the same tint strength, doesn't chalk, etc, etc, etc, then it might just have a shot at offsetting Ti02.

Right now demand on Ti02 is so tight, you probably could make a small fortune for yourself developing an offset.