Color Measurement of Titanium dioxide (TiO2)
Titanium dioxide is the brightest, whitest pigment available. Due to its high refractive index (even higher than diamond) it effectively scatters the light and provides maximum opacity for a coating. Rutile is the most common natural form of TiO2
and preferred over anatase because of the lower photocatalytic activities and thus, better weather stability of the final coating.
The purity of TiO2
is process related. The chloride process makes up purer and brighter grades than the sulfate process. Additionally, impurities introduced by treatment chemicals or extraneous metal ions within the crystallites can degrade brightness. They usually discolor the pigment towards grey or yellow.
One way to perform a color measurement is to incorporate the TiO2
pigment into the coatings system of the final application. To ensure a smooth and homogeneous surface, the paint is applied to opacity drawdown charts with an automatic film applicator. The charts are made of black and white areas that are large enough to be measured with color instruments. An alternative is to take readings on dry pressed pucks. The pucks are made by applying high pressure to the dry TiO2
which is contained by a ring. The pressure is essential as it is the driving force to compact the pigment and hold it together for measurement. The face of the puck is then measured with a spectrophotometer.
The standardized CIE colorimetric values L* and b* are used to characterize brightness and undertone: the higher the L* value, the higher the brightness, the lower the b* value, the less yellow the appearance. In the table below, results for different grades of TiO2
are displayed. The spectro2guide can be used to measure CIELab color numbers. The instrument stores and directly transfers measurement data to smart-chart data analysis Software.
Besides brightness and undertone, optimum hiding power and tinting strength are properties that the TiO2
pigment has to meet.