After the French authorities have proposed to classify TiO2 as classify it as Carcinogenic Category 1B, there has been much discussion of alternatives. Akzo Nobel Decorative Paints has studied the formulation of white opaque coatings with alternative substances.
Hans de Jong and Jitte Flapper of Akzo Nobel Decorative Paints carried out a theoretical study to estimate the potential of other opacifying materials to replace TiO2 as a white pigment. The maximum theoretical hiding power of other non-absorbing (i.e. white) opacifying substances was assessed. In order to do this, the Scattering Coefficient was calculated as a function of particle diameter. Calculations were then performed to determine how thick a coating layer should be, compared to one containing TiO2, to achieve equal hiding.
Substances evaluated in the study
Inorganic pigments were selected from the international Colour Index. The best pigment without any associated adverse labelling is zirconium dioxide, with a refractive index of 2.13. Widely used filler materials such as calcium carbonate (CaCO3) and barium sulfate (BaSO4) were also studied, but their refractive indices (1.4-1.7 and 1.6 respectively) are very close to those of theresins used in paints and consequently they will not be effective opacifiers.
Even the best alternatives show great limitations
The best result for a material with no associated adverse labelling was obtained for zirconium dioxide. Using this material, a four-fold increase in layer thickness would be required to achieve similar hiding compared to a titanium dioxide pigmented coating. Frequently used filler materials such as calcium carbonate or barium sulfate would require a layer more than 100 times thicker.
This also means that four times more raw materials would have to be used (binders and solvents etc.). All of this would mean a considerable increase in (for example) labour, raw material depletion, VOC emissions and end-user costs.
Table 1: Calculated optimal Scattering Coefficient and relative layer thickness:
Substances show no sufficient hiding power
The theoretical calculations carried out in this project conclude that titanium dioxide is by far the most efficient opacifier for use in coatings and that is no other material or other way of opacifying available that would give sufficient hiding power to a coating. The only other way to achieve hiding would be to increase layer thickness. Even in the best-case scenario, a layer four times thicker would be required. But this would bring along unacceptable disadvantages, both from a practical point of view (e.g. costs, labour) as well as from environmental point of view (e.g. raw material use, VOC emission). Hans de Jong and Jitte Flapper therefore conclude that the coating industry cannot create opaque white coatings without the use of titanium dioxide.
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