The Formation and Evolution of the Hierarchical Structure of Polyethylene Pipe During Extrusion Processing

The formation and evolution of the hierarchical structure of polyethylene (PE) pipe during extrusion processing were investigated and its structure-performance correlation is discussed. The experimental results showed that during the PE pipe processing the axial stress by the traction machine oriented the molecular chains along the axial direction, and gave rise to some shish-kebab structures in the external layer which were parallel to the axis, so that the PE pipe easily cracked along the axial direction. Moreover, the temperature gradient generated by different cooling rates across the pipe wall caused the kebab crystals and the spherulites nucleated further from the outer wall to only grow toward the inner wall. As a result, the lamellae were oriented along the radial direction of the PE pipe. On the other hand, owing to the slow cooling of the inner wall, its tie molecules were of lowest density and hence its resistance to slow crack growth was poor. Therefore, the inner layer, which also suffered the highest hoop stress, would become the weak point in PE pipe applications; that is, the cracks extended from the inner wall of the pipe, through the wall, and then in a longitudinal direction.