Degradation of polymer blends: Part VII. Photo-oxidation in natural weathering conditions of polyethylene containing styrene-butadiene or styrene-isoprene copolymers

Abstract

The photo-oxidation of a polyethylene (PE) matrix can be initiated by a dispersed unsaturated phase. Photo-initiation of PE degradation is related to the diffusion of small fragments of the easily degraded dispersed phase into the continuous phase; the rate of chain scission of the dispersed phase is thus rate determining. As a consequence, unsaturated polymers which crosslink are very poor photoinitiators of PE degradation. In contrast, polyisoprene blocks which do not crosslink but degrade with high efficiency are very good photo-initiators. Diffusion of these small fragments implies that the continuous phase of the phase separated copolymer is rubbery. Copolymers with a high styrene content are thus disadvantageous even if chain scission of the unsaturated part is efficient. The concentration of antioxidant in the unsaturated phase must also be minimal. Indeed, the anti-oxidants inhibit the formation of hydroperoxides in the PE phase during processing even if they have a low molecular weight and rapidly diffuse out of the exposed films. The type of fragmentation of the film, which occurs at high dose for some of them, is governed by the morphology of the blend. The heavily degraded dispersed phase has then completely lost its mechanical properties. If elongated co-continuous phase or long fibrils are present, they can be considered as elongated voids where fracture is initiated and propagated giving long narrow ribbons. Isotropic fragmentation can only be obtained if the photo-initiating dispersed polymer is present as small spheres and if the previously cited conditions are present.