Photoageing of rigid PVC—II. Degradation thickness profiles

Bulk samples of non-photostabilized PVC were exposed in a photochemical reactor equipped with a fluorescent lamp (λ ≥ 300 nm) at 70 °C. After 500 h of exposure, thin (20 μm) slices parallel to the surface were cut with a microtome and analyzed by FTIR and UV spectrophotometry and steric exclusion chromatography. The results reveal the existence of at least three distinct zones: (a) The superficial zone of thickness lower than 100 μm, which is characterized by the predominance of oxidation products and chain scissions; (b) The subcutaneous zone, in the 100–400 μm depth interval characterized by the predominance of the products of HC1 zip elimination (conjugated polyenes) and crosslinking (increase of M_w; and (c) The undegraded core zone beyond 400 μm depth. These characteristics can be partially explained in terms of diffusion controlled oxidation kinetics, but with some peculiarities linked to conjugated polyenes. Among these are the consequences of their photoreactivity and screen effect, and the fact that they are concentrated in fractions of high molecular weight.