THE EFFECT OF ANNEALING TEMPERATURE ON THE FRACTURE PERFORMANCE OF ISOTACTIC POLYPROPYLENE

The influence of annealing temperature on the fracture behaviour of a commercial extrusion-grade isotactic polypropylene was studied. Fractu re mechanics analysis was carried out at room temperature and at a low crosshead speed under J-controlled conditions. Parameters characteriz ing fracture initiation, J(IC), and crack propagation, T-M, were deter mined. Some thermal treatments induced ''ductile instability'' after a certain amount of crack extension while others strongly enhanced the fracture toughness parameters and promoted completely stable behaviour . Aiming to correlate mechanical properties with the supermolecular st ructure, the different morphologies induced by thermal treatments were studied by differential thermal analysis. In addition, a qualitative fracture surface analysis was carried out by SEM. Craze formation appe ared to be the principal plastic deformation mechanism present. The de gree of crystallinity and the deg ree of interconnection related to th e thermal treatment the sample had undergone, are the main structural factors controlling fracture performance.