Blends Of Ultrahigh Molecular Weight Polyethylene Research Articles

Computational nodal displacement analysis of acetabulum fossa for injection molded cemented polyethylene acetabular liner

The acetabular liner (AL) is one of the key components that determine the functionality and durability of the total hip joint replacement (THR) device. The performance of Ultra high molecular weight polyethylene (UHMWPE)-based AL depends critically on the manufacturing route and its properties, which are evaluated pre-clinically using a host of experimental and computational analyses. The conventional manufacturing of an AL involves multiple stages, including extrusion/compression molding followed by machining, which is time/cost intensive and leads to material loss. In such a scenario, injection molding is a promising alternative, yet its feasbility remains unexplored for the manufacturing of AL for THA applications. Against this backdrop, the two-fold objectives of this work are to report our recent efforts to establish the efficacy of the injection molding of new generation UHMWPE biomaterial; HU (60 wt% HDPE- 40 wt% UHMWPE blend) for manufacturing AL prototype and to present the key biomechanical response analysis of this prototype, in silico. A range of manufacturing relevant material properties, as well as customized mold design to manufacture HU-based AL with external design features, are discussed. Such guidelines are particularly relevant to mold polymeric parts with a higher thickness (>8 mm). As part of the pre-clinical validation of AL with new design features, a less explored in silico approach to assess biomechanical micro-strain in the acetabulum fossa is presented, and the results are analysed in accordance with the mechanostat theory. The outcomes revealed that for a 100 kg subject weight, average micro-strain in the remodelling region was 1132, while it was determined as 723 for a 55 kg subject weight. Such results highlight the influence of subject weight on micro-strain generation and distribution in the acetabulum fossa. The von Mises stress in AL also increased with subject weight from 17 MPa in a subject weight of 55 kg to 28 MPa in a subject weight of 100 kg. Taken together, this work demonstrates the feasibility and competence of this new generation biomaterial in terms of implant manufacturing via injection molding with a clinically desired biomechanical response.

Pelletizing ultra-high molecular weight polyethylene (UHMWPE) powders with a novel tapered die and addition of high density polyethylene (HDPE): Processing, morphology, and properties

The extremely high molecular weight and molecular entanglement have rendered ultra-high molecular weight polyethylene (UHMWPE) superior properties. However, poor inter-particle diffusion makes it difficult to pelletize UHMWPE powders for easy processing/handling. A novel tapered die with air cooling was proposed to pelletize UHMWPE and a UHMWPE blend with 5 wt% high-density polyethylene (HDPE) for compression molding and material characterization. The tensile strength of samples prepared with the tapered die outperformed those by the regular die or from virgin powders. The addition of HDPE further improved the tensile strength. Fourier transform infrared spectroscopy (FTIR) revealed little chain scission and the lowest amount of oxidation from the tapered die. Intrinsic viscosity (IV) measurements confirmed the negligible chain scission and showed an increment in IV for the UHMWPE/HDPE blend. Multi-angle light scattering with size-exclusion chromatography (SEC-MALS) indicated no change in the UHMWPE molecular weight distribution, but some crosslinking in the blend. Polarized optical microscopy (POM) showed that the HDPE and extrusion led to a finer UHMWPE domain size and better fusion between UHMWPE and HDPE. The combined effect of enhanced molecular chain diffusion, improved consolidation, and lower oxidation using the tapered die led to 40% improvement of tensile strength for the UHMWPE/HDPE blend.

Co-Processing of [Fe(NH2trz)3](2ns)2 and UHMWPE into Materials Combining Spin Crossover and High Mechanical Strength

The coordination polymer [Fe(NH2trz)3](2ns)2 (NH2trz = 4-amino-1,2,4-triazole and 2ns− = counterion 2-naphthalene sulfonate) exhibits the rare phenomenon of spin crossover in an attractive temperature range, i.e., somewhat above room temperature. Spin crossover in [Fe(NH2trz)3](2ns)2 is manifested by thermochromism, which is accompanied by a magnetic transition from diamagnetism to paramagnetism. However, [Fe(NH2trz)3](2ns)2 is brittle and difficult to process, which limits its use. In this study, we show that [Fe(NH2trz)3](2ns)2 can be co-processed with ultrahigh molecular weight polyethylene (UHMWPE), which possesses outstanding mechanical properties, particularly when tensile-drawn. Therefore, [Fe(NH2trz)3](2ns)2–UHMWPE blends were gel-processed by extrusion, employing a relatively poor solvent, which has recently been shown to offer advantages compared to good solvents. Uniform and flexible films, ribbons and fibers with [Fe(NH2trz)3](2ns)2 fractions as high as 33.3% m/m were obtained that could be readily drawn. Spin crossover in the coordination polymer is retained in these materials, as evident from their thermochromism. The tensile strength and Young’s modulus of the blends exceed those of typical commodity polymers. Thus, the films, ribbons and fibers constitute a special class of multifunctional materials that combine the flexibility and excellent mechanical properties of drawn UHMWPE with the spin crossover behavior of [Fe(NH2trz)3](2ns)2.

Co-Processing of [Fe(NH2trz)3](2ns)2 and UHMWPE into Materials Combining (Spin Crossover and High Mechanical Strength

The coordination polymer [Fe(NH2trz)3](2ns)2 exhibits the rare phenomenon of spin crossover in an attractive temperature range, i.e., somewhat above room temperature. Spin crossover in [Fe(NH2trz)3](2ns)2 is manifest by thermochromism, which is accompanied by a magnetic transition from diamagnetism to paramagnetism. However, [Fe(NH2trz)3](2ns)2 is brittle and difficult to process, which limits its use. In this study, we show that [Fe(NH2trz)3](2ns)2 can be co-processed with ultrahigh molecular weight polyethylene (UHMWPE), which possesses outstanding mechanical properties, particularly when tensile drawn. Therefore, [Fe(NH2trz)3](2ns)2–UHMWPE blends were gel-processed by extrusion, employing a relatively poor solvent, which has recently been shown to offer advantages compared to good solvents. Uniform and flexible films, ribbons and fibers with [Fe(NH2trz)3](2ns)2 fractions as high as 33.3% m/m were obtained that could be readily drawn. Spin crossover in the coordination polymer is retained in these materials, as evident from their thermochromism. The tensile strength and Young’s modulus of the blends exceed those of typical commodity polymers. Thus, the films, ribbons and fibers constitute a special class of multifunctional materials that combine the flexibility and excellent mechanical properties of drawn UHMWPE with the spin crossover behavior of [Fe(NH2trz)3](2ns)2.

Investigation of the rheological and mechanical behavior of Polypropylene/ultra-high molecular weight polyethylene compounds related to new online process control

Ultra-high molecular weight polyethylene (UHMWPE) is a high performance material that has excellent wear and impact strength compared to other polymers. Due to its chemical structure and molecular mass, UHMWPE is difficult to handle on standard extrusion systems. In this paper, the compounding, rheological, and mechanical behavior of different Polypropylene (PP)/UHMWPE blends were investigated. Raw materials were blended on a co-rotating twin screw extruder. The shear and extensional viscosity of polymer blends were investigated using an inline rheometer. Mechanical and rheological properties were presented for various UHMWPE loadings, and correlations between mechanical and rheological data were examined.

Rheological and mechanical properties of ultrahigh molecular weight polyethylene/high density polyethylene/polyethylene glycol blends

Abstract Ultra-high molecular weight polyethylene (UHMWPE) possesses many excellent properties, but poor processability. There are many ways to improve the processability of UHMWPE, but most of which will compromise its other outstanding properties. The effect of polyethylene glycol (PEG) and high-density polyethylene (HDPE) on the rheological and mechanical properties of UHMWPE were investigated in this paper, and the UHMWPE blends were prepared by twin screw extruder. The addition of PEG can remarkably improve the processability of UHMWPE, so the UHMWPE/PEG (100/4) blends can be steadily extruded from the capillary. In addition, the apparent shear viscosity, storage modulus, loss modulus and the complex viscosity decrease with the increasing amount of PEG. However, the mechanical properties of UHMWPE/PEG blends decrease as the PEG content increase. The incorporation of HDPE can further improve the processability of the UHMWPE/PEG blends and reduce its apparent shear viscosity, storage modulus, loss modulus and complex viscosity. Mechanical properties test reveals that the best ratio of UHMWPE/HDPE is 60/40. Compared with UHMWPE/PEG (100/4), the tensile strength, flexural strength, and flexural modulus of UHMWPE/HDPE/PEG (60/40/4) increase by 13.8%, 25.7%, and 32.5%, respectively.

Improved crystallizability and processability of ultra high molecular weight polyethylene modified by poly(amido amine) dendrimers

Ultrahigh molecular weight polyethylene (UHMWPE) is usually difficult to be melt processed due to severe intermolecular entanglement in its melt. In this work, we reported the development of a poly(amido amine) (PAMAM) dendrimer (G1) and its derivatives with different terminal groups which showed promising feasibility for aiding UHMWPE melt processing. The rheological, crystalline, and mechanical properties of the blends were investigated comprehensively. The addition of the dendritic modifiers to the UHMWPE matrix resulted in a significant decrease in melt viscosity, thus an obvious enhancement of the processability. The dynamic analysis of the mechanical properties and rotation-rheological test indicated a disentanglement effect of the UHMWPE melt molecules was generated by the PAMAM dendrimer (G1) and its derivatives. Furthermore, the PAMAM dendrimer (G1), as an internal lubricant, was uniformly distributed in the UHMWPE matrix. The heteronucleation effect of the PAMAM dendrimer (G1) with its derivatives led to a higher degree of crystallinity, smaller crystallites in the matrix and enhanced tensile strength in the UHMWPE blends. POLYM. ENG. SCI., 57:153–160, 2017. © 2016 Society of Plastics Engineers

Tensile properties of blends of ultra-high molecular weight polyethylene with very low molecular weight polyethylenes

Tensile properties of blends of ultra-high molecular weight polyethylene with very low molecular weight polyethylenes

Melt miscibility of HDPE/UHMWPE, LDPE/UHMWPE, and LLDPE/UHMWPE blends detected by dynamic rheometer

The dynamic rheological behavior of high density polyethylene (HDPE)/ultrahigh molecular weight polyethylene (UHMWPE) blends, low density polyethylene (LDPE)/UHMWPE blends and linear low density polyethylene (LLDPE)/ UHMWPE blends was measured in parallel plate rheometer at 200°C. The analysis of log-additivity rule, Cole-Cole plots and Han curves of the three series blends indicated that the LDPE/UHMWPE blends were miscible in the melt, while the HDPE/UHMWPE blends and LLDPE/UHMWPE blends showed phase separation. The DSC results of LLDPE/UHMWPE blends and HDPE/UHMWPE blends were consistent with the rheological properties, while for the thermal properties of LDPE/UHMWPE blends, results revealed three endothermic peaks, which indicated a liquid-solid phase separation in LDPE/UHMWPE blends.

The effect of an additional phosphite stabilizer on the properties of radiation cross‐linked vitamin E blends of UHMWPE

Antioxidant stabilization of radiation cross-linked ultrahigh molecular weight polyethylene (UHMWPE) has been introduced to improve the oxidative stability of total joint implant bearing surfaces. Blending of an antioxidant with UHMWPE resin powder followed by consolidation and radiation cross-linking has been cleared by the FDA for use in both total hips and total knees for designs incorporating two antioxidants, namely vitamin E and Covernox™ (a medical grade version of Irganox™ 1010). The antioxidants in the polymer are expected to protect the polymer during consolidation, during radiation cross-linking, on the shelf before implantation, and in vivo after implantation. To maximize the protection of the polymer afforded by the antioxidant in vivo, a novel approach may be the use of multiple antioxidants, especially to protect the primary antioxidant for a longer period of time. We hypothesized that the addition of a phosphite stabilizer (Irgafos 168™) commonly used in conjunction with hindered phenolic antioxidants in polymer processing could improve the oxidative stability of radiation cross-linked blends of vitamin E. To test our hypothesis, we prepared UHMWPE blends with 0.05 wt% Irgafos and 0.05 wt% vitamin E and compared its cross-link density, wear resistance, tensile properties, and impact strength to control blends containing only vitamin E. Our hypothesis was not supported; the cross-link density of UHMWPE was significantly decreased by the additive without additional benefit to oxidative stability. To our knowledge, this was the first attempt at using multiple stabilizers in medical grade UHMWPE.

Influence of copolymer fraction composition in ultrahigh molecular weight polyethylene blends with ethylene/1‐hexene copolymers on material physical and tensile properties

AbstractThe reactor blends (RBs) with bimodal molecular weight distribution on the base of ultrahigh molecular weight polyethylene (UHMWPE) and low molecular weight random ethylene/1‐hexene copolymers (CEH) were synthesized by two‐step processes including ethylene polymerization followed by ethylene/1‐hexene copolymerization over rac‐(CH3)2Si(Ind)2ZrCl2/methylaluminoxane catalyst. The four series of blends differed in a composition of copolymer fraction that was varied in a wide range (from 3.0 to 37.0 mol % of 1‐hexene). The differential scanning calorimetric study shows the double melting behavior of the net semicrystalline CEHs, which can be attributed to intramolecular heterogeneity in chain branch distribution. The introduction of CEHs leads to the modification of nascent RB crystalline and amorphous phases. Physical and tensile properties as well as melting indexes of the materials depend not only on the percentage of copolymer fraction that varied from 6.9 to 35.8 wt % but also on its composition. The increase of copolymer fraction with high content of 1‐hexene (≥11.0 mol %) in the blends leads to the change of the character of stress–strain curves; the materials behave as elastomers. Controlled regulation of copolymer fraction characteristics in the synthesis yields RBs combining the enough high strength, good plastic properties with enhanced melting indexes as compared with the net UHMWPE. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014, 131, 40151.

Rheological, thermal, and morphological properties of low‐density polyethylene/ultra‐high‐molecular‐weight polyethylene and linear low‐density polyethylene/ultra‐high‐molecular‐weight polyethylene blends

AbstractThe dynamic rheological behavior of low‐density polyethylene (LDPE)/ultra‐high‐molecular‐weight polyethylene (UHMWPE) blends and linear low‐density polyethylene (LLDPE)/UHMWPE blends was measured in a parallel‐plate rheometer at 180, 190, and 200°C. Analysis of the log–additivity rule, Cole–Cole plots, Han curves, and Van Gurp curves of the LDPE/UHMWPE blends indicated that the blends were miscible in the melt. In contrast, the rheological properties of LLDPE/UHMWPE showed that the miscibility of the blends was decided by the composition of LLDPE. The differential scanning calorimetry results and scanning electron microscopy photos of the LLDPE/UHMWPE blends were consistent with the rheological properties, whereas with regard to the thermal and morphological properties of LDPE/UHMWPE blends, the results reveal three endothermic peaks and phase separation, which indicated a liquid–solid phase separation in the LDPE/UHMWPE blends. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013

Dissipative Particle Dynamics and Flory−Huggins Theories for Predicting the Rheological Behavior of Ultrahigh Molecular Weight Polyethylene Blends

This work aims to experimentally and theoretically study the effects of normal molecular weight polymers (NMWP) such as high-density polyethylene, low-density polyethylene, and polypropylene as processing aids on the morphology and rheological behavior of ultrahigh molecular weight polyethylene (UHMWPE). As is shown by scanning electron microscope observations, rheological measurements, and dissipative particle dynamics simulations, formation of a lubricating phase between the UHMWPE particles is responsible for the viscosity reduction of the UHMWPE. Besides, phase diagram studies on the UHMWPE/NMWP blend suggest that the optimum composition ratio of the blends lies in their composition-sensitive region when the parameter χ and molecular weight of each component are fixed at low shear rates. Meanwhile, the optimum range of parameter χ is above but close to the corresponding binodal curve at high shear rates.

Crystallization and Phase Separation of Branched Low Molecular Weight Polyethylene/Ultrahigh Molecular Weight Polyethylene Blend under a Controlled Temperature Gradient

Crystallization and phase separation of polyethylene blend were first studied under a controlled temperature gradient (T-gradient) condition. Branched low molecular weight polyethylene (B-LMWPE) and ultrahigh molecular weight polyethylene (UHMWPE) blend films were prepared by casting the blend solutions on the T-gradient stage and then evaporating the solvent. The morphological evolution of blend films was investigated in detail. The blend films showed a continuous gradients surface. With increasing temperature, the degrees of crystallinity for both of the B-LMWPE and UHMWPE decreased in the blend films. The results indicated that phase separation behavior of B-LMWPE and UHMWPE blend with composition 10/1 or 5/1 occurred in both of solution and gel, and the phase separation was prior to crystallization. It is suggested that phase separation should induce molecular ordering and crystallization in the B-LMWPE/UHMWPE blend in the initial stage, and the phase separation and crystallization compete with each other in the later period. Another fact is that high viscosity of blend solution plays a more important role in phase separation rather than in crystallization, and it will be the main disincentive in inhibiting the Marangoni phenomenon when the composition of UHMWPE increases in the blend.

Effects of carbon nanofibers on crystalline structures and properties of ultrahigh molecular weight polyethylene blend fabricated using twin‐screw extrusion

AbstractMelt mixing in an extruder with polymers is an effective approach for forming nanocomposites, allowing mass production applications. The intent of this study is to investigate carbon nanofiber composites with ultrahigh molecular weight polyethylene (UHMWPE) matrix using the twin‐screw extruder. To decrease the high viscosity of UHMWPE, a low density polyethylene (LDPE) was added into the UHMWPE. The effects of carbon nanofibers (CNFs) on the crystalline structures and properties of the nanocomposites were analyzed. The differential scanning calorimetry (DSC) and X‐ray diffraction (XRD) measurements showed the addition of CNFs decreases the degree of crystallinity, but does not impart significant effects on the crystalline structure of the UHMWPE/LDPE blend. Tensile test results showed that the nanocomposite with loading of 3 wt % CNFs had an increase of 38% in tensile strength and 15% in modulus. The thermal stability and thermal conductivity of UHMWPE/LDPE blends were also enhanced by the addition of CNFs. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008

Aromatic Thermosetting Copolyester (ATSP)/UHMWPE Blends with Improved Wear Properties and Biocompatibility

ABSTRACTDespite the recognized success and worldwide acceptance of ultrahigh molecular weight polyethylene (UHMWPE) for total joint arthroplasty, the generation of UHMWPE wear debris from the articulating surface is of major concern and limits the longevity of the artificial joint device. Blends of UHMWPE with an aromatic thermosetting copolyester (ATSP) at 50:50 volume ratio using poly(ethylene-co-acrylic acid) (PEA) as a compatibilizer were developed to address this problem. The amount of PEA was 0, 5, 10, 15 and 20 weight percent of the weight of ATSP, respectively. The wear properties of these blends were evaluated by a pin-on-disk wear test in which an ASTM F-75 cobalt-chromium alloy disk was chosen as a counterface and sterile filtered bovine serum diluted in deionized water to 75% (volume) was used as a lubricant. The wear tests from 0.2 to 3 million cycles showed that UHMWPE/ATSP blend with 10 wt% PEA had average weight loss smaller than UHMWPE and comparable to crosslinked UHMWPE. The wear debris study indicated that UHMWPE/ATSP blend with 10 wt% PEA created a smaller size percentage of the wear debris particles in the biologically active range than UHMWPE and crosslinked UHMWPE. The direct-contact cytotoxicity tests showed that ATSP and UHMWPE/ATSP blend with 10 wt% PEA were not toxic.

Wear properties of UHMWPE/aromatic thermosetting copolyester blends in unlubricated sliding

To improve the wear resistance of ultrahigh molecular weight polyethylene (UHMWPE), blends of UHMWPE, and an aromatic thermosetting copolyester (ATSP) (50/50, v/v) were developed, taking advantage of the crosslinked structure and good wear resistance of ATSP. As a compatibilizer, poly(ethylene-co-acrylic acid) (PEA) was added into the blends with its contents changing from 0 to 20% (w/w). Dynamometer wear tests (sliding against stainless steel surface with contact pressure ranging from 600 to 2500 kPa) showed that the UHMWPE/ATSP blend with 10% PEA had lower wear rate than the UHMWPE sample. The improved wear resistance resulted from the change of the wear mechanism. Scanning electron microscopy (SEM) images of the worn surfaces revealed that the presence of ATSP and PEA would prevent the lamellar alignment in the UHMWPE phase and adding PEA effectively enhanced the interaction between UHMWPE and ATSP.

Preparation of a Poly(methyl methacrylate)/Ultrahigh Molecular Weight Polyethylene Blend Using Supercritical Carbon Dioxide and the Identification of a Three-Phase Structure: An Atomic Force Microscopy Study

Supercritical carbon dioxide (scCO2) has been used as the processing medium to facilitate effective impregnation and subsequent polymerization of methyl methacrylate monomer in ultrahigh molecular weight polyethylene (UHMWPE), providing a route to new nanocomposite materials. In this paper, the morphological structures of UHMWPE, scCO2-treated UHMWPE, and a blend of (53/47 w/w) UHMWPE/poly(methyl methacrylate) (PMMA) prepared in scCO2 are investigated using tapping mode atomic force microscopy (TMAFM). Under moderate tapping conditions, phase imaging offers a good contrast of the nanostructures of the semicrystalline polymers due to a large difference in stiffness between the crystalline and amorphous phases. This is demonstrated by a TMAFM force mode characterization method through amplitude−phase−distance (APD) measurements on the nanoscale crystalline and amorphous domains. The use of this method has also identified that the UHMWPE/PMMA blend consists of three phases: UHMWPE crystalline, UHMWPE amorph...

Ultra-drawing of low molecular weight polyethylene — ultra-high molecular weight polyethylene blend films prepared by gelation/crystallization from semi-dilute solutions

Greatest drawability was studied for blend films with branched low molecular weight polyethylene (B-LMWPE) and ultra-high molecular weight polyethylene (UHMWPE) prepared by gelation/crystallization from solutions. The morphology of B-LMWPE–UHMWPE dry gel film and its deformation mechanism were mainly estimated by using differential scanning calorimeter, small-angle X-ray scattering, wide-angle X-ray diffraction and solid-state 13C NMR. The detailed analysis was carried out for the blend films including large amounts of B-LMWPE. As a result, it was found that the UHMWPE and B-LMWPE were crystallized separately from the mixed solution. The greatest drawability was attributed to a suitable number of entanglements between UHMWPE crystal lamellae that are highly oriented with their large flat faces parallel to the film surface. These entanglements play an important role to transmit the drawing force as intermolecular cross-links and ensured smooth crystal transition of UHMWPE from a folded to a fibrous. In this process, large amount of B-LMWPE is thought to be almost independent of the ultra-drawing behavior of UHMWPE, since the domains of UHMWPE and B-LMWPE within the blend have no interaction and B-LMWPE crystallites take almost a random orientation.

Kinetics of nonisothermal crystallization and melting of normal high density and ultra-high molecular weight polyethylene blends

The kinetics of nonisothermal crystallization and melting of blends of ultra-high molecular weight polyethylene (UHMWPE) and polyethylene high density with normal molecular weight (NMWPE) are investigated by means of differential scanning calorimetry (DSC). Mixing the components at a temperature below the flow temperature of UHMWPE (215 °C) results in increased crystallization/melting rates of the individual components in the blends above the corresponding additive values. The morphological observations of the blends, carried out by means of polarization microscopy, show that a strong boundary of both types of structures (UHMWPE non-flowing aggregates and NMWPE spherulite structures) does not exist. The NMWPE spherulites' dimensions decrease on increasing the UHMWPE concentration in the blends, but their number increases. The facilitation of the crystallization/melting of the components in the blends is explained in terms of mutual influence exhibited by the components with respect to each other. It is due to the inner stresses in nonflowing UHMWPE characterized with a lot of entangled tie molecules and to the partial co-crystallization of NMWPE molecules with the flowing part of UHMWPE. At mixing temperatures above 215 °C the melting/crystallization integral kinetic curves have only one linear part in contrast to these of the same blend (1∶1 ratio of components), prepared at 190 °C. The rates of melting/crystallization remain almost constant with the increase of the mixing temperatures.