Effect of polycrystalline structure of TiO2 particles on the light scattering efficiency

The opacifying power of synthesized polycrystalline TiO2 particles in a cellulose matrix was found experimentally and theoretically to be superior to that of a commercial rutile pigment, depending on crystal structure of the synthesized particles. The crystal structure of the particles was varied by calcination of amorphous titania nanoparticles at different temperatures and was characterized using SEM, TEM, and XRD. Polycrystalline anatase pigments had less opacifying power than commercial rutile, while polycrystalline pigments containing a one-to-one mixture of anatase and rutile had similar opacifying power as the commercial pigment if they have a similar overall particle size. The polycrystalline rutile pigments composed of a linear linkage of several individual rutile crystals gave 6% more opacity than the commercial rutile pigment. Theoretical light scattering calculations using the T-matrix method showed the light scattering efficiency of linearly arranged polycrystalline rutile particles to depend on number and size of crystals composing the particle. It is suggested that the efficiency of rutile pigments can be increased dramatically by controlling both the primary crystal size and the overall particle size. It is believed that the greater than expected light scattering efficiency of the biphasic pigment results from reflection and refraction of light at the grain boundaries between crystals of different phase, which have different refractive indices.