Surface Treatment
'Pure' TiO2 base pigments, whether anatase or rutile, provide high opacity and excellent color but have a relatively reactive surface. The majority of commercial pigments are therefore coated with an inorganic treatment or coating, and often an organic treatment.
Reasons for surface treating titanium dioxide pigment
Nearly all commercial TiO2 pigments have a surface treatment or coating to:
- Improve wetting and dispersion in different media (water, solvent or polymer)
- Improve compatibility with the binder and dispersion stability
- Improve color stability
- Improve durability
- Increase air in the film in flat latex paints
A diagrammatic representation of a surface treated TiO2 pigment is shows in Figure 8.1.
Depending on the end application, the surface treatment is usually a combination of alumina, silica and/or zirconia, deposited onto the core TiO2 particles from either the chloride or sulfate base pigment process, by a wet precipitation process. A final coating of polyol or other organic chosen to enhance the dispersion of the pigment in a resin system and/or dry bulk handling characteristics is then added.
Following surface treatment, the pigment is dried, intensively milled and packed into bags. It may also be supplied in semi and full bulk deliveries or, especially in North America, converted to high concentration slurry for direct use in water based paint systems.
The coatings on TiO2 pigments are deposited under very closely controlled conditions of time, temperature and pH to develop the required characteristics. The final analysis of a surface coating shows only part of the expertise in making high performance TiO2 pigments. Order of addition and precipitation as well as the processes parameters are critical to making the optimum product.
Electron Micrographs
Although the surface treatment is quite thin, and often 5% or less of the pigment weight, it has a major effect on the performance of a pigment. The surface treatment can be seen using an electron microscope: in the attached photographs, the treatment layer has been artificially colored to increase visibility.
Figure 8.2 is a micrograph of typical rutile base particle prior to surface treatment.
Figure 8.3 shows a leading multi-purpose pigment, treated with alumina and polyol. This type of surface treatment is designed to produce easy dispersion in water and solvent based systems, powder coatings and some plastics applications, with excellent color, high gloss where required, and a high level of durability.
Figure 8.4 shows a pigment specifically designed for flat latex paints. These systems, formulated above Critical Pigment Volume Concentration, provide maximum opacity by the inclusion of air in the dried paint film, and this is enhanced by the pigment surface treatment design which has an open silica structure.
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