Photoageing of rigid PVC—I. Films containing Ca/Zn thermal stabiliser

The photo-oxidation (λ > 300 nm, T = 40, 55 and 70 °C) of ˜80μm films of PVC stabilised by Ca and Zn stearates was studied using gravimetry, IR and UV-visible spectrophotometry. The stabilisers disappear in the early days of exposure according to a pseudo first-order process, presumably by HC1 scavenging, which gives an idea of the rate of HC1 release during this period. The concentration of polyenes absorbing at λ = 500 nm begins to increase rapidly when the stabiliser is totally consumed. At the same time, weight loss takes place, showing that there are more HC1 elimination events than oxidation events. These results are consistent with the following hypotheses: (a) oxidation is diffusion controlled; (b) long polyene formation is HC1 catalyzed; (c) conjugated sequences—(CH₂ = CH₂)_n—with n = 10–12 result essentially from a thermal process; (d) the rate of carbonyl and hydroxyl build-up is nearly not influenced by the increase in polyene concentration or rate of HCl evolution which can also be explained, at least partially, in terms of O₂ diffusion control of oxidation kinetics.