PHOTOAGEING OF RIGID PVC - III - INFLUENCE OF EXPOSURE CONDITIONS ON THE THICKNESS DISTRIBUTION OF PHOTOPRODUCTS

Unfilled, non photostabilized PVC bulk samples were exposed to acceler ated ageing in various photochemical reactors equipped with fluorescen t or filtered high pressure mercury lamps in which the UV intensity, d etermined by polycarbonate actinometry, varied between about 7x10(18) and 7x10(19) photons per cm h(-1). IR (carbonyls) and UV-visible (conj ugated polyenes) spectra were recorded on 20 mu m thin slices cut with a microtome parallel to the irradiated surface. The carbonyl thicknes s profiles have a pseudo exponential shape with a maximum in the super ficial layer, whereas the polyene profiles display a subcutaneous maxi mum. Despite the fact that a steady-state regime is rapidly establishe d, the carbonyl profiles display no horizontal plateau in their superf icial part, which can be interpreted tin mechanistic terms) by the fac t that alkyl radicals play a noticeable role in terminations, even in superficial layers where the oxygen concentration is close to its equi librium value. The thickness of the oxidized layer (X-co) determined f rom the carbonyl profile, displays a tendency to increase with the tem perature. It is not surprising in the case of photooxidation where the activation energy of initiation is practically zero, so that Xco is e ssentially governed by the change of oxygen diffusivity with temperatu re. In contrast, the changes of Xco with UV light intensity I seem to be within experimental error despite the very large variations of I. T he kinetic and mechanistic consequences of this surprising result are discussed. The depth of the layer corresponding to the maximum polyene concentration seems to be a decreasing function of the UV light inten sity and an increasing function of the temperature. A kinetic model is tentatively proposed to explain these results. (C) 1998 Elsevier Scie nce Limited. All rights reserved.