Reversible discoloration effects in the photoaging of poly(vinyl chloride)

It is often observed that the photochemical degradation of titanium dioxide pigmented PVC leads to a latent discoloration that is revealed only during a further period of storage of the aged material in the dark. This effect is reversible, and photobleaching can be provoked by a new irradiation of the polymer. This behavior can be attributed to the formation of polyenic sequences with a short conjugation length, which present an absorption below 400 nm. The screen effect of the pigment protects these polyenes against photooxidation, which permits these polyenes to accumulate in the degraded polymer. In the absence of light, these polyenes can be thermally isomerized, so leading to isomer forms absorbing at longer wavelength above 400 nm. This absorption is responsible for a yellowing of the polymer. The isomerization is perfectly reversible and a further irradiation provokes the conversion to the isomer form absorbing below 400 nm, leading then to a photobleaching of the sample. These experiments can be repeated many times before any distortion occurs.