Thickness distribution of degradation products during photochemical aging of rigid PVC

The photooxidation of bulk (1 mm) and thin (similar to 80 mu m) samples of unfilled PVC free of photostabilizers was studied in various photoreactors in the 40-70 degrees C temperature range. In bulk samples, the thickness di stributions of photoproducts (carbonyls, conjugated polyenes, chain scissio n, crosslinks) and thermal stabilizer (Zn/Ca stearate) were established usi ng various analytical methods from measurements made on 20 mu m microtome s lices parallel to the irradiated surface. The experimental results clearly reveal the existence of the following thre e distinct zones across the sample thickness, starting from the irradiated surface: i) A thin (<100 mu m) superficial layer rich in oxidation products and in which chain scissions predominated. ii) A subcutaneous layer extend ing up to 400 mu m depth, containing essentially polyenic species and in wh ich crosslinking predominated. iii) A non degraded core zone beyond about 4 00 mu m depth. During UV exposure, the subcutaneous maximum of polyene concentration moves towards the surface whereas the superficial oxidized layer keeps an almost constant thickness, practically independent of irradiation conditions. These features, and the kinetic data obtained on thin films, are consistent with a mechanistic scheme close to standard one, but in which polyenes (an d crosslinks) are generated from alkyl (P degrees) radicals. These latter c an "survive" at a given distance from the sample surface, because oxidation is diffusion controlled. This behavior is further complicated by the fact that after a certain exposure time the screen effect by the polyene layer t akes the control of the thickness distribution of the reactive species.