Hydrated alumina surface treatment on a titanium dioxide pigment: changes at acidic and basic pH.

Amphoteric, inorganic particles used as pigments and extenders in dried coatings are not completely inert, particularly when exposed to acidity similar to atmospheric pollution, or when in a water-borne paint suspension at alkaline pH. Changes to these materials may harm the integrity of the dried coating and thus its performance during weathering or affect the colloidal stability during storage within liquid paint. The effect on aqueous titanium dioxide pigment suspensions was examined under various pH conditions. A substantial amount of aluminum was dissolved, but little silicon or titanium. Dissolution occurred at both acid and alkali pH leaving a different balance of hydrated alumina phases and overall surface composition. Changes in particle surface were apparent in topography, colloidal properties and by thermogravimetry. Such changes show that other, more complicated, changes may be possible, particularly in an aqueous paint medium where there are many other ingredients in solution or suspension.