Oriented crosslinked polyethylene pipes by a novel extrusion method

Crosslinking and stretching (2.5 times along the circumferential direction) of the molten polymer during extrusion produced pipes with dominantly circ umferential orientation and a lower degree of axial chain orientation. Diff erential scanning calorimetry (crystallinity and crystal thickness), densit y measurements (crystallinity), X-ray diffraction (c-axis orientation), inf rared dichroism measurements (crystalline and amorphous chain orientation) and contraction measurements (molecular draw ratio) assessed the microstruc ture of the pipe material. The mechanical properties of the oriented materi al were assessed by uniaxial tensile tests. The orientation was biaxial wit h the main orientation in the circumferential direction and a lesser orient ation in the axial direction. The maximum degree of circumferential orienta tion was obtained at the inner wall of the pipe. The lower degree of crossl inking of the core material allowed slippage of chains during the stretchin g of the molten polymer and it is suggested that this is the cause of the l ower degree of orientation of the core material. The oriented pipe material exhibited a 5-10% higher degree of crystallinity and higher crystal thickn ess than conventionally crosslinked material. The tensile modulus and the t ensile strength of the oriented, cross-linked material was greater along th e axial direction than along the circumferential direction. The circumferen tial and axial moduli for the oriented, crosslinked pipe were greater than the corresponding moduli of the non-oriented cross-linked pipe material. An other pipe based on crosslinked PE that were first circumferentially stretc hed 2.5 times and later axially stretched 10 times (in the molten state) sh owed, despite the fact that it exhibited pronounced axial orientation almos t a balanced tensile modulus (4.3+/-0.2 GPa) in the axial-circumferential p lane. Atomistic modelling showed that the orientational dependence of the d ensity of the amorphous phase is small.