Titanium Dioxide in Coatings, Durability

Coatings durability can be measured by a number of properties depending upon the end use of the product. It can even be used as a measure of interior end uses using such properties as dirt collection, washability etc. However, durability of coatings is usually assessed on exterior exposure with emphasis placed on

• Chalk Resistance
• Gloss Retention

Figure 1 shows the magnified surface of a long oil alkyd film after 30 weeks exposure at 45^o orientation. Note the loose pigment on the surface of the film. This is the chalk. This chalk, plus the pits in the film, cause a reduction in gloss and thus influence gloss retention.

The titanium dioxide pigment used in the production of a pigmented coating can significantly influence its durability as defined above and in some cases its corrosion barrier function as well.

• Influence of the Pigment Type on Durability
• Surface Treatments For Durable Pigments
• Titanium Dioxide and Corrosion Resistance

Titanium dioxide absorbs much of the ultra-violet (UV) radiation that may otherwise degrade the binder. Thus it protects the coating photochemically. However, untreated titanium dioxide is photocatalytic and accelerates the breakdown of the surrounding binder by the formation of free radicals that cause degradation of the coating. The level of photocatalytic activity can be greatly reduced by correct surface treatment of the pigment with suitable inorganic compounds.

Titanium Dioxide pigments can be categorized into several groups from low durable to superdurable. Multipurpose and superdurable pigments are the most, suitable for coatings. Figure 2  and Figure 3  compare superdurable and multipurpose pigments in a medium durability system, a Long Oil Alkyd Gloss Paint, and a moderately high durability system, a Thermosetting Acrylic Gloss Paint. Note that in both systems, the superdurable pigment gives better chalk resistance than the multipurpose pigment. However, in the lower durable system, where the polymer is directly attacked by UV light, the gloss retention can be better with the multipurpose pigment. This is due to direct polymer degradation by UV radiation, which cannot be controlled as effectively by a superdurable pigment. In the durable system, where the polymer is not directly attacked by UV radiation, the gloss retention with the superdurable pigment is better than that with the multipurpose. Thus it is essential that the durability characteristics of the polymer are known, particularly its resistance to UV light, before selecting the Titanium Dioxide. Superdurable pigments are only fully effective when used with polymers that are resistant. In either system the untreated pigment will be inferior to either, as shown in Figure 3.

Titanium Dioxide and Durability Top

There are two main ways of producing very durable (superdurable) TiO₂ pigments. The first involves encapsulating the TiO₂ particle with a dense layer of silica and alumina, thus placing a physical barrier between the TiO₂ and the binder. However, resultant TiO₂ levels can be as low as 88% with consequent probable adverse influences on the opacity and gloss potential of the coating. Nowadays, silica encapsulated with high TiO₂ contents are available from the Tiona TiO₂ range, so overcoming this potential problem.

Alternatively a surface treatment of zirconia and alumina can be applied. The presence of the zirconia quenches the formation of free radicals and reduces the photo-catalytic activity of the pigment. This approach creates pigments with excellent opacity and gloss potential although chalk resistance may be lower than that of the encapsulated pigments.

Millennium Inorganic Chemicals' utilizes both approaches and is able to offer a wide range of products allowing the formulator to optimize the durability/opacity balance of his coating.

Titanium Dioxide and Durability Top

Titanium Dioxide and Durability Top