Titanium Dioxide Weather Resistance

The exterior durability of a pigment containing product, which is exposed to varying weather conditions, is often a very important property of that product. This so called weather resistance depends on many factors. Especially where paints are concerned, the type of pigment used may have a crucial impact. Because of this, there is great interest in the weather resistance and chalking properties of pigments. These properties are tested and evaluated by exposing sample coating specimens outdoors to weather changes. Both indoor and outdoor "accelerated weathering tests" have also been developed which yield results significantly faster.

Many factors contribute to the transformation of a paint film from a glossy smooth surface into a matte chalking one. The UV radiation in sunlight gives rise to photochemical and catalytic reactions in the paint film. The binder absorbs ultraviolet light that, in the presence of oxygen and moisture, initiates the break-down of large binder molecules. When a cross section of a paint film is examined by electron microscope, there is on the surface a thin layer of pure binder, or a thin "pigment deficient" layer containing little pigment. When the sun has eroded this so called clear layer, and the pigment surface has been exposed, the paint is said to have chalked. The term "chalk" refers to a loose pigment powder that reduces the thickness of the paint layer when it is removed. With chalking, also the absorption of dirt and moisture increase considerably, and this further accelerates erosion of the paint film.

The effect of TiO2 in a coating system may be mixed. On the one hand, TiO2 protects the binder from direct exposure by absorbing ultraviolet radiation. On the other hand, the ultraviolet radiation causes release of free electrons from the TiO2. These electrons may migrate to the surface of the titanium dioxide crystal where they may react with oxygen and moisture in the air to form hydroxy and perhydroxy radicals which can attack the binder system causing degradation of the paint film. The detrimental catalytic reactions of TiO2 in the paint film can mostly be prevented by suitable surface coatings on the pigment particles. Since not all pigment grades are surface treated to have excellent weather resistance or base crystals are not stabilized to a highest degree, the pigment must be carefully selected for outdoor applications. The catalytic effect of anatase is greater than that of the more stable rutile. Because of this, rutile pigments are normally used where exterior durability is desired. The currently available weather-resistant titanium dioxide pigments minimize chalking and thereby increase the durability of outdoor formulations.