Titanium dioxide as a pigment used in latex paints is subjected to a surface treatment, both in the production in the chloride and sulphate process. The surface treatment is used in the rutile form and the anatase form is devoid of surface treatment. The type of surface treatment used has a great influence on the dispersibility, gloss or weathering resistance, which in turn depends on the durability of the treatment itself and the lack of its migration during the paint production process (e.g. during grinding).
The three most common compounds used for inorganic surface treatments are silica, alumina and zirconium oxide. They are deposited on the titanium white particles in the form of an aqueous solution. Titanium dioxide achieves its properties to a large extent due to the quality of the surface treatment (packing density, tightness), as well as the amount of:
• 0.5 to 4% silica – provides gloss retention and resistance to chalking, graying and yellowing
• 6 to 11% silica – provides resistance to atmospheric agents and increases the dispersibility (especially in latex paint applications)
Alumina treatment has a loose spongy structure – which promotes weather resistance
The zirconium oxide treatment has a positive effect on the integration of the surface treatment layers. Titanium white after inorganic surface treatment has a specific surface area of 7 to 20 m2/g (due to the spongy structure of precipitated metal oxides)
Untreated titanium white has a specific surface area of 4 to 9 m2/g.
The second step in the treatment of many types of titanium dioxide is organic surface treatment with TMP (trimethylolpropane) and dispersing additives used as dispersants for water-based paints.
The surface treatment of titanium dioxide pigments is carried out after wet grinding and before filtration and drying, which are the last operations of the production process, both in the chloride and sulphate processes.
Surface treatment fulfills its role only when it is sufficiently durable and it does not rinse and migrate in the form of salt, which reduces the performance of the pigment both in terms of stability in the liquid paint (viscosity disturbances, pH drift) and changes in the properties of the coatings. In order to determine the leaching properties of titanium dioxide pigments, analysis is performed according to ASTM D2448 “Standard Test Method for Water-Soluble Salts in Pigments by Measuring the Specific Resistance of the Leachate of the Pigment”. The principle of the method is weighed quantity of pigment is added to water and boiled. For hydrophobic pigments methyl alcohol is used to facilitate wetting. After filtration, the specific resistance of the filtrate is determined using a conductivity bridge. The water-soluble salts content is a function of the specific resistance of the solution formed by extracting the pigment with water.
The test is carried out by boiling the pigment in demineralized water and, after filtering, the resistivity of the extract is determined. It is very important to feed the filter by washing it with reagent water in the funnel until resistance of the filtrate is greater than 200,000 Ω · cm. The ASTM D2448 control method allows the quick determination of the quality of titanium dioxide in terms of durability of surface treatment and migration of water-soluble salts. This is especially important when you want to change the supplier of titanium dioxide in existing formulations.
See what this test looks like in our laboratory on the video from our YouTube channel:
Titanium dioxide is an expensive raw material which, due to market turbulences, is also periodically difficult to obtain. Then it is especially important to properly check the samples received for testing from new suppliers. Of course, this test is not a substitute for the preparation of paint samples and further tests and tests in the laboratory, but should be the first to be carried out to quickly check the durability of the surface treatment, compared to the pigment used so far. On the other hand, from the point of view of the titanium dioxide producer, it is a parameter worth declaring in the technical data sheet, especially as it is required by the ASTM D476 classification for titanium dioxide pigments used in paints.
On the importance of the correct selection of titanium dioxide, also in the context of surface treatment durability, I recommend my article published in Polymers Paint Color Journal (December 2021 issue), in which I described many case studies on titanium dioxide grinding, as well as comparisons of rutile and anatase dioxide in the context of the impact on performance properties in latex paints.
A link to an article in PPCJ e-magazine is HERE.