A review of the common photodegradation processes in polymeric coatings is presented, and the photoactivity of titanium dioxide and its consequences are discussed. Experimental work which explores the durability implications of the use of titanium dioxide in coatings is presented. This includes the effects of pigment flocculation, film thickness and photostability of the pigment. The importance of the correct selection of light source for accelerated weathering is stressed. Finally, results are presented from a recent study of the influence of geographical location on the weathering of titanium dioxide pigmented paint films.

It must be remembered that the exercise described in part 1 was carried out in a carbon arc weathering machine to provide a stable environment. Referring to Fig 7, which shows the absorption spectra of the resins used in this section, it can be seen that the acrylic resin only absorbs to any great extent in the UVB region, and if reference is made to Fig 3, where the spectra of carbon arc radiation and UK summer sunlight are compared, it is seen that the carbon arc is deficient in this area. The protective aspect of titanium dioxide will thus not operate in this weathering environment. If the same exercise was repeated in natural sunlight, then the pigmented paints would show an even greater protective tendency, since all resins absorb to a greater extent as wavelength decreases, but the extent of photocatalysis remains constant. In Fig 7, the two alkyd/MF systems have been represented by a single line because of their chemical similarity.

No abstract

No abstract