Evolution of tensile properties of UHMWPE after short residence times in the melt state followed by high pressure molding

Abstract

Event Abstract Back to Event Evolution of tensile properties of UHMWPE after short residence times in the melt state followed by high pressure molding Beatrice Yeung1, Pierangiola Bracco2, Thomas S. Thornhill1 and Anuj Bellare1 1 Brigham & Women's Hospital, Harvard Medical School, Department of Orthopedic Surgery, United States 2 University of Turin, Department of Chemistry, Italy Introduction: Consolidation of ultra-high molecular weight polyethylene (UHMWPE) powder during molding has received considerable attention due to damage associated with incomplete UHMWPE fusion in total joint replacement prostheses components [1],[2]. In this study, we studied the early stage fusion of UHMWPE powder in the melt state using a unique high pressure process that enables the polymer to be maintained in the melt state for very short times. Figure 1 outlines the phase diagram of polyethylene and the path taken to enable UHMWPE to reside in the melt state for specific time intervals. This is possible since the polymer enters the melt state isothermally via depressurization at elevated temperature compared to conventional compression molding wherein melting occurs via increase in temperature at moderate pressures, which is a slow process due to the low thermal conductivity of UHMWPE. We measured the tensile properties of the resulting UHMWPE to evaluate fusion of the consolidated polymer. Materials and Methods: GUR 1020 UHMWPE powder (Celanese, Oberhausen, Germany) was compression molded at 180˚C and 10 MPa pressure for 2 hours, and served as the control. A high pressure cell was used to apply 300 MPa pressure to the UHMWPE powder, heated to 180˚C, depressurized and maintained in the melt state for 5, 30 and 60 minutes respectively (HP-5min, HP-30min and HP-60min, respectively), re-pressurized to 300 MPa, cooled and depressurized. ASTM 638 type V specimens were prepared (n=5) and subjected to tensile testing at 10mm/min crosshead speed. The tensile modulus, yield stress, maximum strain and tensile strength were measured. A Differential scanning calorimeter (DSC) was used to measure the degree of crystallinity using a heat of fusion of 293 J/g (n=3). Results: DSC revealed a crystallinity of 53.1 ± 0.9%, 66.4 ± 0.3%, 71.1 ± 3.3%, 71.5 ± 2.5% and 70.0 ± 1.8% for Control UHMWPE, UHMWPE powder, HP-5min, HP-30min and HP-60min, respectively. There was no statistically significant difference in crystallinity for the HP UHMWPEs (p>0.05, ANOVA). The tensile modulus of the Control UHMWPE was approximately three times lower than those of the HP group (see Table 1). The HP UHMWPEs also had a higher yield stress and UTS compared to the Control but there was no statistically significant difference among the HP UHMWPEs. The maximum strain of HPE-5min and HP-10min was significantly lower than that of the Control but this was not the case for HP-60min. Discussion: This study showed that it took 60 minutes of melt fusion for the maximum strain of UHMWPE to attain a value close to that of the control, in which the polymer resided in the melt state for 2 hours. Such a rapid evolution of tensile properties indicates that co-crystallization of polymer chains across powder boundaries must play a significant role in consolidation since diffusion of chains across powder boundaries would likely take several hours to days. The study also shows that high pressure molding provided a high modulus UHMWPE with high yield stress, tensile strength and comparable maximum strain at 60 minutes of residence time in the melt state. Conclusions: The fusion of UHMWPE resin powder in the melt state results in rapid evolution of tensile properties during high pressure molding providing a UHMWPE with high tensile properties compared to conventional molding. Figure 1. Phase diagram of polyethylene indicating processing route. Table 1. Tensile properties of UHMWPEs