New and Simple Staining Method for Visualizing UHMWPE Lamellar Structure in TEM

Wear resistant UHMWPE with high toughness by high temperature melting and subsequent radiation cross-linking

Ultra high molecular weight polyethylene has been commonly used as a biomaterial for total joint replacements. This article concerns with the characterization of nano composites of ultra high molecular weight polyethylene (UHMWPE) with Mg0.15Ni0.15Zn0.70Fe2O3 using analytic techniques such as differential scanning calorimetry (DSC), and thermo gravimetric analysis (TGA). UHMWPE purchased from Sigma Aldrich was used for studies and preparation of nano composites with 1wt% and 2wt% Mg0.15Ni0.15Zn0.70Fe2O3 using the ball milling method. The thermal stability of composites with 1 % of Mg0.15Ni0.15Zn0.70Fe2O3 contents were less stable compared to pure UHMWPE and UHMWPE composites with 2wt% of nano scale Mg0.15Ni0.15Zn0.70Fe2O3. The degree of crystallinity obtained from DSC data was 41%, 40%, and 43% for UHMWPE, UHMWPE+1% of Mg0.15Ni0.15Zn0.70Fe2O3, and UHMWPE+2% of Mg0.15Ni0.15Zn0.70Fe2O3, respectively. The results of current study are useful while considering the nano composites of UHMWPE with Mg0.15Ni0.15Zn0.70Fe2O3 in order to enhance the mechanical strength of UHWMPE for industrial as well as medical applications such as orthopedic implants and bullet proof jackets etc.

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

A systematic approach was undertaken to increase strength, modulus, and toughness of low density polyethylene (LDPE) filaments through infusion of functionalized CNT and ultra high molecular weight polyethylene (UHMWPE). CNTs were functionalized with OH functional groups using chemical treatment. Functionalized CNTs and UHMWPE were first dry mixed with LDPE, and filaments were then drawn using a melt extrusion process. Loading of UHMWPE varied from 8–10 wt% while that of CNT was at 2–4 wt%. LDPE has been infused first with UHMWPE, and then with both UHMWPE and CNT, and filaments were extruded. Neat LDPE filaments were also extruded as control samples. Individual filaments from each category were tested under tension according to ASTM D3379-75. In addition, differential scanning calorimetry (DSC) and X-ray diffraction (XRD) studies were also conducted to measure changes in thermal and crystalline behavior. Filament tests have revealed that the tensile elongation of LDPE can be increased by about 200% with the addition of 10 wt% UHMWPE. This is however, is accompanied by a loss of about 50% ultimate tensile strength. In the next step, when 2 wt% CNTs and 8 wt% UHMWPE are added, tensile strength of the composite filament is restored to the level of neat LDPE (∼ 25 MPa) with an increase in modulus by 44% and in ultimate fracture strain by about 60% compared to that of neat LDPE. The source of improvement has been traced as formation of copolymer between LDPE and UHMWPE and strong interfacial interaction between the CNT and the polymers.

Characterization of surface and bulk properties of α-Tocopherol blended and γ irradiated UHMWPE sandwich nanocomposites

It is well known that osteolysis induced by polyethylene wear debris is the main cause of long-term failure of hip and knee prostheses. We developed a treatment of medical-grade ultra-high molecular-weight polyethylene (UHMWPE) in order to improve its tribologic properties and reduce its wear. Medical-grade UHMWPE was irradiated with a 200 kGy dose of radiation, thermally stabilized at a temperature close to the melting point, and then sterilized with ethylene oxide. The irradiation treatment was performed to crosslink the UHMWPE. The thermal stabilization treatment, contributing to the reaction between the free radicals generated by the irradiation process, was chosen to enhance crosslinking and to prevent oxidation and the shortening of chains. The non-invasive sterilization process with ethylene oxide was chosen to prevent the re-formation of free radicals. The wear performance of this material was compared to UHMWPE, untreated or treated with different sterilization techniques, using gamma and beta irradiation. Insoluble crosslinked constituents were measured with an extraction method. Wear was evaluated using a flat-on-ring wear test machine. While small differences were found among the different sterilization processes, 200 kGy-irradiated UHMWPE followed by thermal treatment and sterilization with ethylene oxide had the least wear and the greatest amount of crosslinking.

One of our aims in developing ultrahigh molecular weight polyethylene (UHMWPE) joint implants is to improve the fatigue strength of cross‐linked UHMWPEs without sacrificing their wear resistance induced by cross‐linking. High pressure crystallization (HPC) of uncross‐linked UHMWPE, which results in the formation of less hindered extended chain crystals, increased fatigue strength. Vitamin E, also an antioxidant, acted synergistically with HPC and the resulting UHMWPE had increased wear and fatigue strength. In this study, we hypothesized that vitamin E‐blended, cross‐linked, and high pressure crystallized UHMWPE (X‐VEHPE) would have higher wear resistance and fatigue strength compared to radiation cross‐linked UHMWPE. To test this hypothesis, we irradiated 0.1 wt% vitamin E‐blended UHMWPE, subsequently radiation cross‐linked it to various doses and terminally high pressure crystallized it. We used bi‐directional pin‐on‐disc wear testing, tensile mechanical testing, and fatigue crack propagation resistance testing. We found that X‐VEHPE required less radiation dose and less cross‐linking to achieve the same wear rate when compared to irradiated UHMWPE and had improved fatigue resistance at the same wear rate. Therefore, this UHMWPE presented about 46 and 87% improvement in fatigue strength compared to irradiated and irradiated/melted UHMWPE and may be a feasible alternative for joint implants.

Exponential model for the tensile true stress-strain behavior of as-irradiated and oxidatively degraded ultra high molecular weight polyethylene.

In addition to confounding mass-based wear measurements in serum-lubricated hip simulator experiments, fluid absorption by the acetabular cups may simultaneously modify the wear resistance of the ultra-high molecular weight polyethylene (UHMWPE) from which they are composed. To decouple the fluid absorption and wear processes enabling clearer investigation of this effect, absorption was first imposed during an initial stage where UHMWPE was exposed to pressurized (10MPa) fluid. This was followed by a second stage, where resultant wear behavior was assessed by a multidirectional pin-on-flat technique that, though still providing a serum-lubricating environment, does not promote the simultaneous fluid absorption occurring in hip simulator testing. Both unirradiated and highly-crosslinked UHMWPE were investigated, each with both bovine calf serum and water soaking exposures of duration to 129 days. The pressurized soaking of a highly-crosslinked UHMWPE decreased its wear resistance, causing an increase in wear rate by approximately 50% during subsequent serum-lubricated multidirectional pin-on-flat sliding tests as compared to non-soaked material. The magnitude of this effect did not appear to depend on whether the soaking fluid was water or serum, nor did it appear to depend on soak time provided it was at least of a 14-day duration during which more rapid transient fluid absorption occurs. Such soaking did not produce as pronounced an effect on unirradiated UHMWPE, at its lack of wear resistance likely causes the absorption-affected surface region to the completed removed within the earliest stages of sliding contact.

Ultra High molecular weight polyethylene (UHMWPE) suffers wear degradation in total joint replacements and it needs to be improved. Thus, we enhanced wear resistance of UHMWPE with carbon nanofiller and paraffin oil and studied its tribological behavior in Simulated Synovial Fluid (SSF) for 60 days at 37 °C to reproduce the conditions of a real joint. Ageing in biological fluid accelerates the wear action but nanocomposite exhibited a higher wear resistance compared to UHMWPE because of its higher structural homogeneity. Carbon nanofiller closes the porosity of UHMWPE hindering SSF to penetrate inside. Wear resistance of the nanocomposite with 1.0 wt.% of CNF improved of 65% (before ageing) and of 70% (after 60 days in SSF) with respect to pure UHMWPE.

The change in the structure and properties of ultrahigh molecular weight polyethylene (UHMWPE) due to gamma irradiation has been studied in air and in a nitrogen atmosphere at different doses up to 400 kGy. Thermal analysis using differential scanning calorimetry (DSC) and swelling were used to monitor changes in crystallinity and cross-linking. New crystallites with different lamellae thicknesses were formed in samples irradiated in air. The samples irradiated in nitrogen did not show new crystallites unless irradiated at more than 200 kGy. Gel percentage and the Charlesby-Pinner equation were used to confirm the increase in cross-linking in the absence of oxygen. Characterization by all the techniques used indicated that irradiation in air causes the production of new crystallites due to oxidation, by which shorter molecular chains are formed and lamellar thickness increases. A lower degree of oxidation and scission occur in nitrogen, as confirmed by scanning electron microscopy (SEM) analysis, which allowed the determination of fracture shape and morphology. Fewer micro-voids and less brittleness were noticed in an oxygen-free environment. To gain more cross-linking, less production of crystalline regions, and more ductility, it is recommended that UHMWPE be irradiated in the absence of oxygen.

Correlation of residual radical’s with three phase morphology of UHMWPE: Analysis for the dependence on heat involved during vitamin E diffusion

ABSTRACTHigh‐temperature melting (HTM) of ultrahigh molecular weight polyethylene (UHMWPE) was shown to improve its elongation and toughness. This was believed to be due to increased scissioning and increased diffusion of polymer chains. It was hypothesized here that the toughness of previously radiation cross‐linked UHMWPEs could also be improved by HTM. To test this hypothesis, the wear resistance, tensile mechanical properties, and Izod impact strength of radiation cross‐linked virgin (no additive) and antioxidant‐blended (with vitamin E) UHMWPEs were tested. The results suggested that although the impact strength of cross‐linked UHMWPEs could be improved significantly by HTM, the wear resistance was decreased. Thus, this procedure can be optimized to be especially suited in high‐stress applications, such as total knee replacements with lessened wear concerns. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015, 132, 42735.

Radiation and chemical crosslinking promote strain hardening behavior and molecular alignment in ultra high molecular weight polyethylene during multi-axial loading conditions.

For many years, ultra-high molecular weight polyethylene (UHMWPE) components of joint replacement prostheses have been sterilized with a 25 to 37 kilograys dose of gamma radiation in an air environment. Gamma radiation is known to cause chemical and morphological changes in polyethylene that continue to occur with both in vivo and shelf-life age. Generation of free radicals leading to predominantly crosslinking and chain scission (to a lesser extent) are immediate chemical effects. Slow oxidation of the free radicals contributes to further chain scission, thereby lowering the overall molecular weight of UHMWPE components with aging. Since lower molecular weight polyethylenes have a lower resistance to wear, oxidation-induced chain scission is believed to lower the wear resistance of UHMWPE components used in joint replacement prostheses. We have studied mechanisms of morphological alterations in UHMWPE components (Montell 1900) that accompany radiation-induced chemical changes with shelf-life aging. Differential scanning calorimetry on aged and unirradiated Montell 1900 UHMWPE revealed that there was an increase in crystallinity in components of post-irradiation aged Montell 1900 UHMWPE, in agreement with previous studies. Small-angle X-ray scattering measurements on unirradiated Montell 1900 UHMWPE revealed that there was a low degree of morphological order. One-dimensional correlation functions obtained by Fourier transformation of the scattering curves were employed to characterize the morphology of Montell 1900. The long period or the most probable center-to-center distance between neighboring crystalline lamellae, was 940 ± 15 and 550 ± 15Å. In the case of post-irradiation aged Montell 1900 UHMWPE, a new long period of 480 ± 15 Å was present in addition to the original long period. The emergence of a new long period along with an increase in crystallinity showed that recrystallization with post-irradiation aging occurred by growth of new crystallites in the amorphous domains in-between crystallites rather than by thickening of original crystallites. Growth of new crystallites was attributed to oxidation in the amorphous domains that releases constraints on interlamellar tie molecules by chain scission, thereby allowing the chains to form new crystallites.