Most of the whiteness annotations are compared with the standard sample or expressed with L, a and b. Almost all application fileds require good whiteness. The impurity of titanium dioxide and particle dialogue degree have significant influence on tio2 whitness. The international lighting associationCIE, short for commission internationals DE eclairage has set out L*, a* and b* values to measure the color value. This method is called CIELAB. L* stands for lightness, from brightness (L*=100) to darkness (L*=0).A * stands color from green (A *) to red (+ A *), and b * stands color from yellow (+ b *) to blue (b *).
Main factors affecting the titanium dioxide whiteness:
(1) the types and contents of harmful impurities in titanium dioxide: the content of harmful impurities, such as iron, chromium, cobalt, cerium, copper, manganese, vanadium, lead, etc. will have obvious affect on the whiteness. This is because of the color effect of impurities themselves, and because of the presence of impurity ions, especially heavy metal ions, the distortion of the lattice curves of titanium white powder has lost its symmetry.
Rutile type titanium pigment is perticularly sensitive to the influence of impurities. For example if the iron oxide content is more than 0.003% in the rutile type titanium pigment, it will present the color, but the anatase tio2 wil appear colors only whenit reaches to more than 0.0099%.
(2) shape and size distribution of titanium dioxide particles
: the particle shape of titanium dioxide requires smooth contour, without edges and corners, because the surface of edges and corners will reduce the reflection of light. The particle size should be controlled between 0.2 and 0.4 microns, that's about half the wavelength of visible light, so that you can get high light scattering, making it even whiter.When the particle size is less than 0.1 micron, the crystal is transparent;When the particle size is too large, that is more than 0.5 micron, it can reduce the scattering ability of particles to light, which may have a very harmful effect on pigment performance. For particle size distribution, uniform and narrow distribution is required.