Research About Solid State Extrusion Of Ultra-High Molecular Weight Polyethylene

Extremely high molecular makes Ultra-high molecular weight polyethylene (UHMWPE) perform completely differently from ordinary thermoplastics, which really brings processing lots of difficulty. Solid state extrusion is a novel molding method operated under the melt temperature. Deformation of UHMWPE by solid state extrusion can avoid troubles caused by the melt property of UHMWPE, and also give the products merits such as high tensile strength, transparent appearance and low expansion ratio. Researches about solid state extrusion are not abundant at home these years and its application on UHMWPE is actually a blank.Researches about the solid state extrusion technique, products' properties and enhancement mechanics are presented in this paper. Solid state extrusion was implemented using a small ram extruder and axial stretch dies which were also explored in this research. The extrusion procedure was: first forming a billet by melting the UHMWPE powder in a closed die, then heating the billet to the extrusion temperature and push it out of a stretch die in a high pressure. The products appear to be smooth and transparent, and have a high tensile strength: The product prepared through a 6.3 draw ration die in 120℃gives a high tensile strength more than 110 MPa, which is 2.7-4.5 times of conventional extrusion products, and also a high modulus of 85 GPa, which is 40 times more of conventional ones. The enormously increase of mechanical property is notable. Micrograph of SEM shows a mass of microfibrillar structure are formed, which caused the great increase of tensile strength. DSC and WAXD analysis show that melt temperature, enthalpy and crystallinity become greater than those of the semi-finished billet. The heating process before extrusion and the crystallization induced by molecule chain orientation are supposed to be the cause.Effect of extrusion temperature and extrusion draw ratio is also discussed in this paper. Decrease of temperature and increase of draw ratio help to the reduction of the influence brought by pressure fluctuation. In addition, growth of temperature is propitious to the enhancement of products' crystallization. However, temperature also brings the increase of molecular chain flexibility, which induces the curve of melt temperature climbs first and fall afterwards along with the growth of extrusion temperature. The influence of extrusion draw ratio acts on thermal properties is complicate. The highest melt temperature and the enthalpy are not on the same point of draw ratio. It is presumed to be related to the complicated deformation process of solid state extrusion. At the first stage of extrusion, the action of tensile stress minishes the crystal size and increases the transparency of products. Cystallinity and enthalpy increases rapidly along with the extrusion draw ratio. But when extension goes to high ratio stage, they become stable.This paper proves that solid state extrusion is a possible and potential method to deform UHMWPE. The approaches extend the field of solid state extrusion.