Rheological Behavior Of Ultra High Molecular Weight Polyethylene And Its Application In Molding Processing

Polymer processability depends on its rheological behavior. To improve the processability of ultrahigh molecular weight (UHMWPE), the rheological behaviors of UHMWPE, including dilute solution, concentrated solution, gel, and melt, were studied. The relationships of the entanglement state of UHMWPE long chains, viscosity, shear rate, temperature, polymer concentrations were obtained. The suitable processing parameters for ram extrusion of UHMWPE rod and twin screw extrusion of UHMWPE/octacalcium phosphate composite were explored using the theoretical results. A metastable phenomenon was observed during the ram extrusion of UHMWPE, and a low temperature and low pressure processing method was developed to solve the difficulties in processing UHMWPE. A bioactive UHMWPE/OCP composite was processed using twin screw extrusion of the UHMWPE gel. A heat resistant and high strength composite of UHMWPE and calcium carbonate was processed by in situ synthesis.The relationship between the critical entanglement concentration (Ccr) of UHMWPE dilute solution in paraffin oil and the degree of polymerization (N) was obtained by measuring the viscosity, . UHMWPE gel in the paraffin oil exhibits a single chain with polycrystalline forms. The UHMWPE lamellar crystal size becomes smaller with the decrease of the gel concentration. The UHMWPE/paraffin oil gel point (TC cr? N1/2 =94gel) increases with the mass concentration of UHMWPE (Cm), Tg el = 0.32C m+ 110.4(0.8%≤≤8%), (10%≤≤40%), CmTg el = 31.9 Cm+ 115.1C mTg el = 24.5 Cm+ 113.2( Cm≥45%).UHMWPE/paraffin oil gel is a pseudoplastic gel. When the gel melts sufficiently at 165℃, UHMWPE chains in the melt state exhibit as Gaussian distributed coils with higher storage modulus and lower loss modulus. At temperature above 165℃, a plateau appears in the low shear rate region of the viscosity curve, and the plateau decreases with the increase of shear rate. Higher temperature broadens the viscosity plateau. The swelling effects of paraffin oil can reduce the viscosity of UHMWPE, and the viscosity of gels with different UHMWPE concentration has a power law relation with the shear rate. The gel viscosity is more sensitive to temperature and UHMWPE concentration than to shear rate.The viscosity (η) of UHMWPE melt has a power law relation with the shear rate (γ),η= kγα. In the molecular weight range of 2.5 million to 6.0 million (g/mol), molecular has little effect on the rheological behavior of UHMWPE, but the shear thinning behavior and the temperature sensitivity become more severe with the increase of molecular weight. When the temperature rises to 185℃, the intramolecular crystal structure disappears completely and the density of entanglement network is maximum, and the molecules exhibit a random coil conformation. When the temperature rises to 225℃, the entangling and disentangling of chains reach a dynamic equilibrium, so the viscosity keeps constant.A metastable phenomenon was observed during the rheological behavior study of UHMWPE (viscosity average molecular weight 3.5 million g/mol) in a"temperature window"of 154-157℃using a capillary rheometer. The extrusion pressure decreased abruptly and then became stable, and the rods surface was smooth without melt fracture. A low temperature and low pressure extrusion method was developed to process UHMWPE using a traditional ram extruder, which can significantly decrease the energy consumption.UHMWPE gel with a UHMWPE concentration of 50% in paraffin oil was successfully extruded using a twin-screw extruder at 165℃and screw speed of 40 N/min. When octacalcium phosphate (OCP) fiber weight 25%, the UHMWPE/OCP composite prepared by swell mix extrude through common twin-screw extruder have more excellent mechanical performance, and this composite can induce bone-like apatite layer generate at its surface. There will be many polar functional groups on the surface of UHMWPE powder after acrylic acid modification, which attracted Ca2+ and CO32- ions to grow nano-CaCO3 particles in-situ for prepare UHMWPE/CaCO3 composite by in-situ chemical synthesis. The as-prepared composite exhibited an improved tensile strength and heat distortion temperature compared to mechanical blended composite. When the load rate of CaCO3 is 9.5%, the tensile strength and heat distortion temperature of composite prepared by in-situ chemical synthesis are 43.8MPa and 106℃respectively.