Thickness distribution of degradation products during photochemical aging of rigid PVC

The photooxidation of bulk (1 mm) and thin (∼80 μm) samples of unfilled PVC free of photostabilizers was studied in various photoreactors in the 40-70°C temperature range. In bulk samples, the thickness distributions of photoproducts (carbonyls, conjugated polyenes, chain scission, crosslinks) and thermal stabilizer (Zn/Ca stearate) were established using various analytical methods from measurements made on 20 μm microtome slices parallel to the irradiated surface. The experimental results clearly reveal the existence of the following three distinct zones across the sample thickness, starting from the irradiated surface: i) A thin (<100 μm) superficial layer rich in oxidation products and in which chain scissions predominated. ii) A subcutaneous layer extending up to 400 μm depth, containing essentially polyenic species and in which crosslinking predominated. iii) A non degraded core zone beyond about 400 μm 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 (and crosslinks) are generated from alkyl (P°) radicals. These latter can 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 takes the control of the thickness distribution of the reactive species.