Durability

Durability, in the context of titanium dioxide, relates to the weathering performance of coatings and plastics, and also the light stability of paper laminate systems when pigmented with appropriate TiO₂ pigments. Poor durability can mean loss of surface gloss, 'chalking' (physical loss of pigment as the surface is degraded), color change or mechanical breakdown.

In a system pigmented with TiO₂, parameters affecting the overall durability of a system include:

• the binder system
• the TiO₂ crystal form with rutile pigments being more durable than anatase forms.
• the particle size distribution of the dispersed pigment
• the surface treatment

The particle size distribution of the pigment can directly affect both the initial gloss of a film and also the gloss retention as the film is weathered. Figure 9.1 shows the initial surface profile of a paint or plastic film and its roughness after two stages of weathering. Clearly, large particles in a wide particle size distribution disrupt the surface more than a narrower distribution, reducing gloss as the film weathers. As the larger particles are lost from the film as a white powdery layer, the film is said to be 'chalking'.

Similarly, the dispersion state of the pigment can also directly affect initial gloss and gloss retention as the film is weathered and surface treatment must therefore be carefully designed to ensure good dispersion and dispersion stability as well intrinsic durability. Figure 9.2 diagrammatically shows the surface profile of pigmented films, with different types of pigment dispersion, as they weather. Again, loss of pigment particles from the film causes ‘chalking’.

Titanium dioxide inherently improves the durability of an organically bound system (paint or plastic) by absorbing UV radiation which can directly affect the polymer by photochemical degradation. However, interaction between UV radiation and a pure TiO₂ surface produces free radicals which in turn degrade organic binders. Figure 9.3 shows how free radicals are formed at a Ti surface, which then oxidize the binder by photocatalytic degradation. (Note that both UV radiation and moisture are critical components of the weathering ‘reaction’).

Formation of Ti³⁺, which is a blue-gray color, can also occur when paper laminates are exposed to the UV radiation in daylight, due to reduction of Ti⁴⁺ by residual chemical species in the cured laminate. Because laminate is relatively impervious to air/oxygen, the Ti³⁺ may not be re-oxidized to Ti⁴⁺ and the discoloration may persist.

The effect of free radicals, and reduction of Ti⁴⁺ to Ti³⁺, can be significantly reduced by the application of a carefully designed surface treatments or coatings. At its simplest, the coating physically separates the TiO₂ surface from the surrounding organic binder, but specific, durable and 'superdurable' pigments go further by using either a dense silica shell to encapsulate the TiO₂ core or by incorporation of a free radical scavenger such as zirconia into the coating.