Commercial Ultra-high Molecular Weight Polyethylene Research Articles

Identifying differences in the tribological performance of GUR 1020 and GUR 1050 UHMWPE resins associated to pressure × velocity conditions in linear reciprocating sliding tests

In this work, two commercial UHMWPE (ultra-high molecular weight polyethylene) resins used in orthopedics, GUR 1050 and GUR 1020, were evaluated through linear reciprocating dry friction tests. Average contact pressures (P) of 34 MPa and 50 MPa and sliding velocities (V) of 0.02 m/s and 0.10 m/s were selected to perform tests in four PV conditions. The friction coefficient (COF) with both resins was around 0.18 in average, without significant distinctions by PV; however, a distinction was seen in COF dispersion; it was in the range of 5%–19%, in dependence of the PV condition and resin type. COF with GUR 1020 was more disperse, and it was related to the vulnerability of the resin to undergoing dynamic changes in the intensity of adhesive (higher COF) or abrasive (lower COF) wear mechanisms. Both wear mechanisms are displayed simultaneously, but random changes in intensity may occur during the friction process. Such randomness was associated to the susceptibility to have the structure modified by friction, higher in GUR 1020 than GUR 1050. Concerning wear amount, contact pressure was the most influencing parameter on it. GUR 1020 performed more than 30% inferior than GUR 1050 under contact pressure higher than the yield strength of the material. Under pressures near the material strength, the wear level was in the range of surface roughness and both resins performed equal in average; however, in this case, the dispersion was systematically lower for GUR 1050, evidencing its better tribological stability. It was concluded that analyses on the dispersion of the tribological responses disclosed relevant information on stability related performance. Also, when procedural dependent properties, as such friction and wear, are considered as evaluation parameters, care must be taken to compare results from different tribosystems.

The Structural and Mechanical Properties of the UHMWPE Films Mixed with the PE-Wax.

Since obtaining a highly oriented structure based on a large-scale commercial ultra-high molecular weight polyethylene (UHMWPE) is considered very difficult due to its high molecular weight and melting index, modifying the structure of these cheap commercial UHMWPE brands into a supra-molecular structure with fiber-forming properties by adding a small amount of polyethylene wax (PE-wax) will provide the possibility to obtain highly oriented UHMWPE products with enhanced mechanical and tribological properties. In this work, highly oriented UHMWPE/PE-wax films were prepared. The PE-wax affected the UHMWPE as an intermolecular lubricant. The obtained lamellar structure of the UHMWPE/PE-wax composites had a better processability. The UHMWPE and UHMWPE/PE-wax structures for the xerogels and the films were studied by using differential scanning calorimetry and scanning electron microscopy. The PE-wax presence enhanced the mechanical properties of the UHMWPE/PE-wax films to a high degree. The highest average value of the tensile strength was 1320 MPa (an increase of 78%) obtained by adding a PE-wax content of 1.0 wt.%, and the highest average value of the Young’s modulus was 56.8 GPa (an increase of 71%) obtained by adding a PE-wax content of 2.0 wt.%. The addition of the PE-wax increased the work of fracture values of the UHMWPE/PE-wax films up to 233%. The formation of the cavities was observed in the virgin UHMWPE films more than in the UHMWPE/PE-wax films, and the whitening of the oriented films was related to the crystallization process more than to the cavitation phenomenon. The coefficient of friction of the oriented UHMWPE/PE-wax films improved by 33% in comparison with the isotropic UHMWPE, and by 7% in comparison with the oriented virgin UHMWPE films.

Hierarchical structure manipulation of UHMWPE/HDPE fibers through in-reactor blending with Cr/V bimetallic catalysts

A dramatically improved, uniform distribution of commercial ultrahigh molecular weight polyethylene (UHMWPE) with high-density polyethylene (HDPE) via solution blending, rather than that of melt blending, is often apparent in experimental investigation. Recently, we demonstrated that a UHMWPE/HDPE blend created by in-reactor polymerization with a Cr/V bimetallic catalyst supported on modified silica can obtain an even better molecular chain distribution while saving energy and solution costs, even more so than experiments using a solution blend. Furthermore, by gel-spinning processing, an abundance of hierarchical, interlocking shish-kebab crystals were generated in the in-reactor polymerized blend fibers (IRBFs), exhibiting better mechanical properties than those of solution blend fibers (SBFs). The tensile strength of the IRBFs increased by about 20% under the same draw ratio, since the interlocking shish-kebab structures contributed to homogenous deformation. The two kinds of fibers exhibit high crystallinity and have a highly oriented shish-kebab structure. Compared with the SBFs, the IRBFs inclined to form longer shish features and smaller kebab sizes, and the draw process was more conducive to the transformation of kebab to shish structures. Thus, the IRBFs can obtain an excellent hierarchal network structure, benefiting more from the hot-stretching process. Our work lays a solid foundation for the efficient fabrication of high-performance UHMWPE/HDPE fibers as well as the optimization of an original polyethylene catalyst system.

Heterogeneous distribution of chain mobility in nascent UHMWPE in the less entangled state

The structure of chain entanglements in the solid state is important for revealing the relationship of structure and properties of polyolefin. In this work, low-field solid-state 1H NMR is used to study the chain dynamics of ultra-high molecular weight polyethylene (UHMWPE) in the solid state. It has been found that the relaxation distribution, analyzed by a multi-exponential inversion program, is an effective method to characterize the heterogeneous chain mobility. It is evidenced from the results that the UHMWPE in a less entangled state presents an obviously heterogeneous distribution of chain mobility in the non-crystalline phase, corresponding to its heterogeneous distribution of entangled points. In comparison, the commercial UHMWPE with a large number of entanglements shows a much more uniform mobility of the non-crystalline components. This heterogeneous distribution of chain mobility becomes even more critical after annealing the samples below the melting point, especially for the less entangled UHMWPE.

Effect of entangled state of nascent UHMWPE on structural and mechanical properties of HDPE/UHMWPE blends

ABSTRACTIn this study, a synthesized ultra‐high molecular weight polyethylene (UHMWPE) with a less entangled state and a commercial UHMWPE with a highly entangled state were blended with high‐density polyethylene (HDPE) by melt blending, respectively. Rheology, 2D small‐angle X‐ray scattering (2D‐SAXS), differential scanning calorimetry (DSC), and tensile test were used to study the relationship between the microstructure and the mechanical properties of blends. It was demonstrated that the UHMWPE with the less entangled state was easy to be oriented at a given flow. More mechanical networks were achieved among the HDPE matrix and the UHMWPE chains due to the fewer entanglements of synthesized UHMWPE, improving the melting recovery of blends. Furthermore, notably oriented structures (shish‐kebabs) with increased long‐periods were made in the blends of weakly entangled UHMWPE and HDPE. The tensile strength of this blend was thus enhanced, i.e., the tensile strength raised for neat HDPE from 45.7 to 83.1 MPa for HDPE/UHMWPE blends containing 10 wt % of less entangled UHMWPE. However, the phase separation of blends was characterized when more weakly entangled UHMWPE was incorporated. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017, 134, 44728.

Melt processing and structural manipulation of highly linear disentangled ultrahigh molecular weight polyethylene

Melt processing of commercial ultrahigh molecular weight polyethylene (UHMWPE, Mη>106g/mol) is an insurmountable bottleneck due to the extremely high melt viscosity arising from its numerous chain entanglements. In the current work, we demonstrated that a UHMWPE (Mη=∼3.4×106g/mol) with a highly linear structure and low entanglements synthesized by a single–active–site Ziegler–Natta catalyst can be melt injection molded without the aid of any additives and, more strikingly, structurally manipulated by means of an intensive shear flow during the packing stage of injection molding. Therewith, large amounts of self-reinforced superstructures, i.e. shish-kebabs and oriented lamellae, were generated in bulk UHMWPE. Appealing interlocked shish-kebabs appeared due to the overstocked shishes that made the epitaxial kebabs penetrate into each other. The self-reinforced superstructures, together with the eliminated structural defects and the increased crystallinity, brought about considerable mechanical enhancement. In particular, the yield strength and ultimate tensile strength of structured linear disentangled UHMWPE were increased by 75% and 71%, from 23.3±0.2 and 40.4±0.6MPa for compression-molded counterpart to 40.8±1.3 and 69.0±0.8MPa, respectively. Our current effort makes a pivotal breakthrough in efficient fabrication of high-performance UHMWPE parts, holding a great prospect towards the application in severe conditions and the instructive effects on synthesis in return.

Unique Rheological Response of Ultrahigh Molecular Weight Polyethylenes in the Presence of Reduced Graphene Oxide

The paper addresses the difference in electrical conductivities and rheological properties between two nanocomposites of reduced graphene oxide nanosheets (rGON) with commercial ultrahigh molecular weight polyethylene (C_PE) and a low-entanglement-density UHMWPE synthesized under controlled conditions (Dis_PE). It has been found that composites made with Dis_PE can reach conductivities at least 100 times higher than those made with C_PE on doing thermal treatment at lower temperatures. However, the difference in the electrical conductivity diminishes when both sets of samples are given a high temperature treatment. This phenomenon is attributed to the difference in morphology of the polymer matrices, for example, grain boundaries between the nascent particles. Furthermore, rheological analyses of the two sets of UHMWPE/rGON nanocomposites conclusively demonstrate differences in the interaction between polyethylene chain segments of the disentangled UHMWPE and rGON, compared to the entangled commercial UHM...

Solid-state 1H-NMR relaxation behavior for ultra-high-molecular-weight polyethylene reactor powders with different morphologies

The relaxation behaviors of several commercial ultra-high-molecular-weight polyethylene (UHMW-PE) reactor powders were compared. The surface and internal morphologies of these powders were characterized by scanning and transmission electron microscopy observations. On the basis of these morphological analyses, the reactor powder consisted of particles and fibrils between them, the relative amounts of which depended on the powder preparation conditions. Molecular motions were detected by solid-state proton nuclear magnetic resonance (1H-NMR) techniques. The temperature dependence of the molecular motion characterizes the structural differences between these powders. The intermediate region between crystal/amorphous phases first relaxes during gradual heating from room temperature for all reactor powders examined in this study. Such a release of constraint for boundary chains induces structural reorganization beyond the critical temperature before melting. This trend was also confirmed by an increase in crystallinity beyond the critical temperature. In contrast, the results of differential calorimetry (DSC) analysis could not distinguish these powder characteristics because of the remarkable reorganization of these reactor powders during heating scans. A relationship between the morphological and 1H-NMR relaxation characteristics was interpreted for several reactor powders. The relaxation behaviors for several commercial ultra-high molecular weight polyethylene reactor powders were compared. On the basis of the morphological analyses, the reactor powder consisted of particles and fibrils between them, the relative amounts of which depended on the powder preparation conditions. The results of differential scanning calorimetry analysis could not distinguish these powder characteristics because of the remarkable reorganization of these reactor powders during heating scans. In contrast, molecular motions detected by solid-state proton nuclear magnetic resonance techniques characterize the structural differences between these powders.

The Effect of Post Hot Compaction on Crystallinity and Thermal Behavior of Ultra-High Molecular Weight Polyethylene Fiber Laminates

Thermal compacting of previously prepared ultra-high molecular weight polyethylene (UHMWPE) fiber laminates can raise its melting temperature and crystallinity. In this article, thermal shrinkage and the effect of post hot compaction on a commercial UHMWPE fiber laminate at various temperatures was investigated. The temperature range of post hot compaction was between 115 and 145°C, while other processing parameters like pressure, time, and cooling rate were kept constant during compaction. The shrinkage of the fiber laminates increased slowly up to 138°C; as soon as the temperature passed 140°C, the shrinkage increased rapidly and reached its maximum value very quickly. The crystallinity of the fiber laminates increased with rising temperature up to 135°C, then decreased at 145°C.

Nitric oxide and prostaglandin E2 production in response to ultra-high molecular weight polyethylene particles depends on osteoblast maturation state.

Recent studies have shown that osteoblast-like cells respond directly to ultra-high molecular weight polyethylene particles in culture, suggesting that they may be involved in aseptic loosening of endoprostheses. We tested the hypothesis that the state of cell maturation plays a role in the response of osteogenic cells to ultra-high molecular weight polyethylene particles. MG63 cells (immature osteoblast-like cells), OCT-1 cells (mature secretory osteoblast-like cells), and MLO-Y4 cells (osteocyte-like cells) were treated for twenty-four hours with commercial ultra-high molecular weight polyethylene particles with an average diameter of 1 mm. The effect of particle treatment on cell proliferation was assessed by measuring the number of cells, whereas the effects on differentiation and local factor production were assessed by measuring the production of osteocalcin, prostaglandin E2, and nitric oxide. The effect of particles on apoptosis was also evaluated. The addition of ultra-high molecular weight polyethylene particles increased the number of MG63 cells, did not affect the number of OCT-1 cells, and led to a decrease in the number of MLO-Y4 cells. The observed changes in cell number were not due to programmed cell death, as no more than 3% of the cells in cultures treated with the highest concentration of particles were undergoing apoptosis. Osteocalcin production was not affected by the addition of particles. Prostaglandin E2 production was increased in all three types of cultures, but the effect was greatest in OCT-1 cell cultures, as was the absolute amount of prostaglandin E2 produced. Nitric oxide production was unaffected in MG63 cell cultures, but it was stimulated in OCT-1 and MLO-Y4 cell cultures. The results of the present study support the hypothesis that osteoblast cell maturation state plays an important role in the response to ultra-high molecular weight polyethylene particles and that the terminally differentiated osteocyte may be involved in the bone response to wear debris in vivo.