A paint coating or an element made of plastic pigmented with titanium dioxide show changes of its appearance due to time of exposure, closely depending on operation conditions. It is particularly visible in case of their exposure to action of an aggressive environment.

Not only concentrated, caustic chemicals are to be understood as aggressive environment but also:

  • UV radiation
  • high or low temperatures
  • high humidity
  • impurities (e.g. dusts)

Action of these factors causes not only loss of mechanical properties of coatings but also changes in their appearance. An essential influence on these changes – apart from the type of binder – has got also the kind of applied titanium dioxide pigment.
Titanium dioxide pigments fulfil twofold role in the mechanism of coating degradation:

  • as a very good absorber of ultraviolet light, the titanium dioxide particles (especially of rutile) protect polymer against destructive, direct action of such radiation;
  • as a catalyst activated with ultraviolet radiation, titanium dioxide particles (especially of anatase) catalyse reactions leading to oxidation of polymeric chains and to destruction of a coating.

To sum up, the function that is fulfilled by titanium dioxide pigment in the degradation process is the resultant of these two reciprocally opposite roles. The aim of actions of manufacturers of titanium dioxide pigments is to increase the efficiency of absorption of UV radiation with simultaneous reduction of photocatalytic activity of pigment. The actions in the first direction aim at supplying of very easily dispersible pigment of optimum distribution of particles enabling obtaining a uniform pigmentation level that will allow for maximum protection of polymeric matrix, and the actions in the second direction lead to stabilisation of crystallographic structure and to surface treatment of pigment particles.

Also the type of used binder, presence of other components of recipe (e.g. other pigments strongly absorbing UV radiation), volume concentration of pigment, etc., influence of course, the improvement of coating resistance. And so, through increase of volume concentration of pigment – on one hand – photocatalytic degradation increases but – on the other hand – direct degradation decreases as resin is better protected as the result of absorption of UV radiation by pigment.

Tests of resistance of coatings to the atmospheric factors are carried out in natural conditions at exposure sites specifically designed for such purpose or in accelerated conditions in special devices simulating atmospheric conditions.