BIAXIAL ORIENTATION OF LINEAR POLYETHYLENES USING THE COMPRESSIVE DEFORMATION PROCESS

The tensile properties of three grades of linear polyethylene were enh anced by a factor of as much as 15 using a melt/solid phase compressiv e deformation process that produced equi-biaxial planar orientation in the sheet. Ultra high molecular weight polyethylene wi th planar isot ropy and an in plane modulus of 10 GPa and a tensile strength of 330 M Pa was produced using this method. It was found that the molecular wei ght had a significant influence on the optimum processing temperature, the ultimate biaxial deformation ratio and hence the ultimate tensile properties. High density polyethylene processed under ideal condition s had a tensile modulus of 2.3 GPa and a tensile strength of 250 MPa. The tensile strength increased linearly with increasing biaxial deform ation ratio and the tensile modulus increased non-linearly with increa sing biaxial deformation ratio. The deformation rate and the dwell tim e did not have a significant effect on the tensile properties. Shrinka ge tests showed that biaxial deformation was less effective than uniax ial deformation in inducing orientation of the polymer chains, however differential scanning calorimetry results were consistent with the pr esence of extended chain crystals in very highly oriented ultra high m olecular weight polyethylene sheets.