Influence of the crystalline structure on the fragmentation of weathered polyolefines

Abstract

Blown-extruded low density polyethylene (LDPE) and polypropylene (PP) films were submitted to accelerated weathering in water in order to identify their fragmentation mechanism and to compare the fragments number, size and shapes generated for long irradiation time. The study of the chemical modification during weathering was performed by following the progressive oxidation (FTIR, contact angle), increase of crystallinity (DSC) and surface rigidity (AFM) for the two polymers. It was demonstrated that although the kinetics of degradation is faster for PP than for LDPE films, the same fracture mechanism namely slow crack propagation is observed for both polymers. The analysis of the cracks structures on the surface of the films by AFM indicates that the crack initiation and propagation strongly depends on the crystalline morphology of the polymer. Indeed, for both material cracks propagation mainly occurs in the direction perpendicular to the extrusion direction explaining the elongated shape observed for the LDPE fragments. However, for PP film, the presence of spherulite structures induced the initiation of cracks in other directions (between the lamellae) leading to the formation of an increased number of fragments significantly smaller and with non-elongated shape compared to LDPE. To our knowledge this study is the first to directly link the size, shape distributions of plastics fragments to the crystalline morphology of the weathered polymers. These results point out the importance of taking into account the crystalline morphology to predict plastic fate in the environment.