Mechanical Properties of Thermoplastic Rods by Hot Extrusion in Solid State under Constrained Strain Recovery

Abstract

Improvement of mechanical properties in thermoplastics is experimentally investigated by solid-state extrusion at elevated temperatures. Hot extruded rods of thermoplastics such as polypropylene (PP), polyoxymethylene (POM), high-density polyethylene (HDPE), polycarbonate (PC) and polymethyl methacrylate (PMMA) are cooled rapidly under constrained strain recovery. Rods with 6 to 24mm in diameter can be extruded at elevated temperatures below the melting temperature and the glass transition temperature. A high degree of molecular orientation is observed for semicrystalline polymers extruded below the melting temperature. Uniaxial tensile tests, compressive tests and shear tests are made at room temperature. The stress-strain curves are affected by extrusion processing conditions, such as extrusion ratio, extrusion temperature and constrained strain recovery. Young's modulus and tensile strength of amorphous polymers in the extrusion direction is improved more than those in solid-state extruded rods cooled under free strain recovery. The maximum values can become approximately 6 to 7 times larger than those of virgin materials. The mechanical properties are improved with increasing extrusion ratio for crystalline and amorphous polymers, and decreasing extrusion temperature for amorphous polymers. The tensile modulus of extruded polymers is greater than the compressive modulus and the difference comes to be greater with increasing extrusion ratio. Shear strength of extruded rods is also higher than that of virgin materials.