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Molecular Weight Polyethylene (molecular + weight_polyethylene)
Kinds of Molecular Weight Polyethylene Selected AbstractsEffect of long-term natural aging on the thermal, mechanical, and viscoelastic behavior of biomedical grade of ultra high molecular weight polyethyleneJOURNAL OF APPLIED POLYMER SCIENCE, Issue 1 2010H. Fouad Abstract In the total joint prostheses, Ultra High Molecular Weight Polyethylene (UHMWPE) may undergo an oxidative degradation in the long term. The overall properties of UHMWPE are expected to be altered due to the oxidative degradation. The goal of this study is to investigate the effects of natural aging up to 6 years in air on the thermal, mechanical, and viscoelastic properties of UHMWPE that was used in total joint replacement. The changes in UHMWPE properties due to aging are determined using Differential Scanning Calorimetry (DSC), uniaxial tensile tests, and Dynamic Mechanical Analysis (DMA). The DSC results show that the lamellar thickness and degree of crystallinity of UHMWPE specimens increase by 38% and 12% due to aging. A small shoulder region in the DSC thermograms is remarked for aged specimens, which is an indication of formation of new crystalline forms within their amorphous region. The tensile properties of aged and nonaged UHMWPE specimens show a significant decrease in the elastic modulus, yield, fracture stresses, and strain at break due to aging. The DM testing results indicate that the storage modulus and creep resistance of UHMWPE specimens decrease significantly due to aging. Also, it is remarked that the , relaxation peak for aged UHMWPE specimens occurs at lower temperature compared to nonaged ones. The significant reduction in the strength and creep resistance of UHMWPE specimens due to aging would affect the long-term clinical performance of the total joint replacement and should be taken into consideration during artificial joint design. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010 [source] Apatite Deposition on NaOH-Treated PEEK and UHMWPE Films for Sclera Materials in Artificial Cornea Implants,ADVANCED ENGINEERING MATERIALS, Issue 7 2010Monica Pino Abstract Cornea implants consist of a clear optic portion with a surrounding ring known as the skirt, which needs to integrate with the sclera. However, currently used skirt materials lead to poor tissue integration. Improvements in this respect may be achieved by using a bioactive skirt material that adapts to the metabolic activity of the cornea. Polyether etherketone (PEEK) and ultra-high molecular weight polyethylene (UHMWPE) might provide interesting alternatives, if they can be rendered bioactive. We, therefore, investigated the potential of surface-modifying PEEK and UHMWPE films through the use of a two-step treatment. This process involved a suitable chemical surface modification (via immersion in NaOH), with subsequent formation of apatite layers on the polymers' surfaces through exposure to supersaturated simulated body fluid (1.5 SBF). In the present work the effect of 5 and 10,M NaOH on formation of the apatite layer has been investigated with regard to wettability and topography features. In addition, the chemical stability of the apatite layer formed has been analyzed. Our data demonstrate that with an increase in NaOH concentration the wettability of the polymer increased, whilst some changes to the polymer film topography (increase/decrease in roughness) were observed. Most beneficially, the apatite layer that subsequently was grown on pre-treated PEEK and UHMWPE films through immersion in 1.5 SBF contained phosphate and carbonate ions, in similar ratios to those found in the apatite in dentine, thus, promising good in vivo bioactivity of these polymer films,a necessity if they are to be integrated into artificial cornea. [source] Global Techniques for Characterizing Phase Transformations , A Tutorial ReviewADVANCED ENGINEERING MATERIALS, Issue 6 2010Michel Perez To characterize phase transformations, it is necessary to get both local and global information. No experimental technique alone is capable of providing these two types of information. Local techniques are very useful to get information on morphology and chemistry but fail to deal with global information like phase fraction and size distribution since the analyzed volume is very limited. This is why, it is important to use, in parallel, global experimental techniques, that investigate the response of the whole sample to a stimulus (electrical, thermal, mechanical,). The aim of this paper is not to give an exhaustive list of all global experimental techniques, but to focus on a few examples of recent studies dealing with the characterization of phase transformations, namely (i) the measurement of the solubility limit of copper in iron, (ii) the tempering of martensite, (iii) the control of the crystallinity degree of a ultra high molecular weight polyethylene and (iii) a precipitation sequence in aluminum alloys. Along these examples, it will be emphasized that any global technique requires a calibration stage and some modeling to connect the measured signal with the investigated information. [source] PE-UHMW in Hip Implants: Properties of Conventional and Crosslinked Prosthetic ComponentsADVANCED ENGINEERING MATERIALS, Issue 10 2009Ruth Markut-Kohl Hip implants made of crosslinked ultra high molecular weight polyethylene,PE-UHMW,(different as-received conditions) are compared with a retrieval made from non-crosslinked PE-UHMW and a control PE-UHMW. Oxidation leads to recrystallization and the enhanced crystallinity corresponds to higher hardness values. These structure-property relations are discussed for conventional PE-UHMW and also for crosslinked material. [source] Modification of polypropylene by melt vibration blending with ultra high molecular weight polyethyleneADVANCES IN POLYMER TECHNOLOGY, Issue 3 2002Kejian Wang Abstract A novel vibration internal mixer was used to prepare polypropylene/ultra high molecular weight polyethylene PP/UHMWPE blends with two additional adjustable processing parameters (vibration frequency and vibration amplitude) as compared with those prepared in the steady mode. Microscopy, mechanical tests, and differential scanning calorimetry showed that vibration influenced the blend morphology and the product properties. The good phase homogeneity of the blends might be due to the variation of shear rate either spatially or temporally in blending. Additionally, the vibration internal mixer could be used to analyze the dependency of viscosity on the shear rate. Vibration enhanced the interpenetration of UHMWPE into PP and vice versa. Subsequently, the formed crystals of two components were connected, and there was epitaxy between PP and UHMWPE crystals. Moreover, the crystalline aggregates, with the amorphous UHMWPE, formed a complex network-like continuous structure, which improved the elongation ratio at the break and the yield strength. The higher the vibration frequency and/or the larger the vibration amplitude at a fixed average rotation speed of the mixer, the more significant these effects were. The larger amount of the connected crystals, especially of , form of PP in the bulk , form PP as well as with the continuous phase structure, led to a higher tensile properties of PP/UHMWPE vibration blended. © 2002 Wiley Periodicals, Inc. Adv Polym Techn 21: 164,176, 2002; Published online in Wiley Interscience (www.interscience.wiley.com). DOI 10.1002/adv.10020 [source] Effect of long-term natural aging on the thermal, mechanical, and viscoelastic behavior of biomedical grade of ultra high molecular weight polyethyleneJOURNAL OF APPLIED POLYMER SCIENCE, Issue 1 2010H. Fouad Abstract In the total joint prostheses, Ultra High Molecular Weight Polyethylene (UHMWPE) may undergo an oxidative degradation in the long term. The overall properties of UHMWPE are expected to be altered due to the oxidative degradation. The goal of this study is to investigate the effects of natural aging up to 6 years in air on the thermal, mechanical, and viscoelastic properties of UHMWPE that was used in total joint replacement. The changes in UHMWPE properties due to aging are determined using Differential Scanning Calorimetry (DSC), uniaxial tensile tests, and Dynamic Mechanical Analysis (DMA). The DSC results show that the lamellar thickness and degree of crystallinity of UHMWPE specimens increase by 38% and 12% due to aging. A small shoulder region in the DSC thermograms is remarked for aged specimens, which is an indication of formation of new crystalline forms within their amorphous region. The tensile properties of aged and nonaged UHMWPE specimens show a significant decrease in the elastic modulus, yield, fracture stresses, and strain at break due to aging. The DM testing results indicate that the storage modulus and creep resistance of UHMWPE specimens decrease significantly due to aging. Also, it is remarked that the , relaxation peak for aged UHMWPE specimens occurs at lower temperature compared to nonaged ones. The significant reduction in the strength and creep resistance of UHMWPE specimens due to aging would affect the long-term clinical performance of the total joint replacement and should be taken into consideration during artificial joint design. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010 [source] Characteristic modeling of the wear particle formation process from a tribological testing of polyethylene with controlled surface asperitiesJOURNAL OF APPLIED POLYMER SCIENCE, Issue 1 2007Hsu-Wei Fang Abstract To study the ultra-high molecular weight polyethylene (UHMWPE) wear particles-induced osteolysis which leads to the failure of artificial joints, microfabricated surfaces with controlled asperities have been applied to generate narrowly distributed UHMWPE wear particles with various sizes and shapes. Our previous study further facilitated single wedge sliding tests to investigate the mechanism of the UHMWPE particle generation. In this study, the attempt was made to characterize the particle generation process into a mathematical model to predict particle volume with a given surface-texture dimensions and mechanical loading conditions. The particle-generation process is decomposed into two steps: (1) penetration of the cutting edge, and (2) lateral sliding of the cutting edge. By combining the indentation experimental data, the viscoelastic responses of UHMWPE was incorporated in the model. The effects of normal load, feature height, and cutting edge angle on the wear particle volume were illustrated from model predictions. Both experimental results and model predictions indicate the same trend of effects of surface-texture geometry and mechanical conditions on the volume of particles. The results of the model predictions are close to the experimental results of the particle generation. However, the particle volume predicted by the model is larger than the experimental results. It is believed that the reprocessing of the generated particles and viscoelastic recovery of UHMWPE in the experiments account for this difference. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 587,594, 2007 [source] Interlaboratory validation of oxidation-index measurement methods for UHMWPE after long-term shelf agingJOURNAL OF BIOMEDICAL MATERIALS RESEARCH, Issue 1 2002S. M. Kurtz Abstract An international oxidation index standard would greatly benefit the orthopedic community by providing a universal scale for reporting oxidation data of ultra-high molecular weight polyethylene (UHMWPE). We investigated whether severe oxidation associated with long-term shelf aging affects the repeatability and reproducibility of area-based oxidation index measurement techniques based on normalization with the use of 1370- or 2022-cm,1 infrared (IR) absorption reference peaks. Because an oxidation index is expected to be independent of sample thickness, subsurface oxidation was examined with the use of both 100- and 200-,m-thick sections from tibial components (compression-molded GUR 1120, gamma irradiated in air) that were shelf aged for up to 11.5 years. Eight institutions in the United States and Europe participated in the present study, which was administered in accordance with ASTM E691. On average, the 100-,m-thick samples were associated with significantly greater interlaboratory relative standard uncertainty (40.3%) when compared with the 200-,m samples (21.8%, p = 0.002). In contrast, the intralaboratory relative standard uncertainty was not significantly affected by the sample thickness (p = 0.21). The oxidation index method did not significantly influence either the interlaboratory or intralaboratory relative standard uncertainty (p = 0.32 or 0.75, respectively). Our interlaboratory data suggest that with the suitable choice of specimen thickness (e.g., 200 ,m) and either of the two optimal oxidation index methods, interlaboratory reproducibility of the most heavily oxidized regions in long-term shelf-aged components can be quantified with a relative standard uncertainty of 21% or less. Therefore, both the 1370-cm,1 and the 2022-cm,1 reference peaks appear equally suitable for use in defining a standard method for calculating an oxidation index for UHMWPE. © 2001 Wiley Periodicals, Inc. J Biomed Mater Res (Appl Biomater) 63: 15,23, 2002 [source] Influence of fibre position on the flexural properties and strain energy of a fibre-reinforced compositeJOURNAL OF ORAL REHABILITATION, Issue 7 2003A. Ellakwa summary, The introduction of laboratory-processed composite systems and fibre reinforcement techniques have increased the possibilities for the prosthetic replacement of missing tooth tissues. Laboratory fabrication variables may significantly influence the properties of the final prosthesis. During the construction of a fibre-reinforced bridge it is necessary to place the fibre at some distance from the fitting surface of the restoration in the pontic region. No guidelines are available for optimal fibre placement in this respect. The purpose of this study was to assess the influence of placing ultra high molecular weight polyethylene (UHMWPE) fibre at five different distances from the tensile side of test samples on flexural properties and the strain energy stored within the dental composite. The results of this investigation showed that whilst moving the fibre reinforcement away from the tensile side by up to 1·5 mm led to a significant reduction in flexural strength, there was no significant decline in the increase in strain energy stored within the tested composite until this distance was exceeded. [source] The influence of fibre placement and position on the efficiency of reinforcement of fibre reinforced composite bridgeworkJOURNAL OF ORAL REHABILITATION, Issue 8 2001A. E. Ellakwa The effect of placement of ultra-high molecular weight polyethylene (UHMWPE) fibres on the flexural properties and fracture resistance of a direct dental composite was investigated. The UHMWPE fibres are increasingly being used for the reinforcement of laboratory fabricated resin composite crown and bridgework. The aim of this study was to assess the effect of a commonly used laboratory fabrication variable on the in vitro strength of beam shaped specimen simulating a three-unit fixed bridge. Four groups (10 specimens per group) of Herculite XRV were prepared for flexural modulus and strength testing after reinforcement with UHMWPE fibres. Two groups of control specimens were prepared without any fibre reinforcement. Half the specimen groups were stored in distilled water and the other groups were stored dry, both at 37 °C for 2 weeks before testing. The results of this study showed that placement of fibre at or slightly away from the tensile side improved the flexural properties of the composite in comparison with the unreinforced control specimen groups whilst the mode of failure differed according to fibre position. Scanning electron microscope (SEM) investigation revealed that placement of the fibre slightly away from the tensile side favoured crack development and propagation within the resin bridging the interfibre spaces in addition to debonding parallel to the direction of fibre placement. Laboratory fabrication variables may effect the strength of fibre reinforced bridgework significantly. [source] Effect of crosslinking, remelting, and aging on UHMWPE damage in a linear experimental wear modelJOURNAL OF ORTHOPAEDIC RESEARCH, Issue 7 2007Suzanne A. Maher Abstract The objective of this study was to establish the effect of postirradiation melting as a function of irradiation dose on the wear behavior and material characteristics of ultrahigh molecular weight polyethylene. Our hypothesis was that a low dose of irradiation followed by melting would have the same improved wear performance as is found with higher doses of irradiation, but without the disadvantages associated with reduced fracture toughness. The hypothesis was tested by measuring the wear performance (wear track area, incidence of pitting and delamination) in a linear doubly curved-on-flat cyclic test, material behavior (elastic modulus, fracture toughness), and aging response (density changes through the thickness) of the following materials: elevated crosslinked groups,radiated at 25, 65, and 120 kGy, melted, sterilized and aged; a melted group,melted, sterilized, and aged; and a control group,sterilized and aged. Our findings suggest that postirradiation melting, not the irradiation dose, dominates the material property changes and wear response. Melting ensured reduced modulus and therefore decreased contact stresses, superior wear performance, and good resistance to aging, even after low levels of irradiation (25 kGy). The low modulus of the 25 kGy elevated crosslinked group, coupled with increased fracture toughness compared to samples irradiated at higher doses and a resistance to aging not found in the melted group, support our hypothesis. A low dose of irradiation followed by heat treatment has the same beneficial effects in terms of improved wear performance, but without the disadvantages of reduced fracture toughness found with higher doses of irradiation. © 2007 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 25:849,857, 2007 [source] Polyethylene cross-linking by two different methods reduces acetabular liner wear in a hip joint wear simulatorJOURNAL OF ORTHOPAEDIC RESEARCH, Issue 5 2003Darryl D. D'Lima Advances in cross-linking have led to the development of wear resistant ultrahigh molecular weight polyethylene for total joint replacement. This study compared wear reduction by two different cross-linking methods as measured in a hip wear simulator. One highly cross-linked polyethylene was treated with 7.5 Mrad gamma irradiation with post-irradiation annealing and a sterilization dose of 2.5 Mrad (10 Gamma), while the other used 9.5 Mrad warm irradiation with 10 MeV electron-beam (9.5 EB). Liners of the same design, made from nominally cross-linked (gamma sterilized) polyethylene were also tested. Gravimetric wear analysis was performed every 500,000 cycles for 5,000,000 cycles. After correcting for weight gain due to water absorption, the nominally cross-linked liners demonstrated mean wear rates of 15.7 (±1.7) and 12.5 (±1.0) mg/million cycles. Both highly cross-linked polyethylene liners demonstrated significantly less wear than their respective controls (with mean wear rates of 1.5 (±1.2) and ,1.4 (±1.5) mg/million cycles). The 9.5 EB liners gained weight presumably due to increased fluid absorption, in addition to that measured in loaded,soaked control implants. Any wear occurring was therefore assumed to have been more than offset by weight gain. Highly cross-linked polyethylene was significantly more wear resistant than non- or nominally cross-linked polyethylene. The differences in wear rates between the two highly cross-linked polyethylene designs (9.5 EB or 10 Gamma) are probably too small to be clinically significant. © 2003 Orthopaedic Research Society. Published by Elsevier Science Ltd. All rights reserved. [source] Wear Simulation of Alumina-on-Alumina Prosthetic Hip Joints Using a Multidirectional Motion Pin-on-Disk DeviceJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 11 2002Vesa Saikko The wear of a state-of-the-art implant alumina against itself was studied with a circularly translating pin-on-disk (CTPOD) device, a wear simulator for prosthetic hip joint materials. The direction of sliding changed continually relative to the pin, preventing erroneous uniaxial grooving typical of ordinary pin-on-disk devices. The dominating wear mechanism was mild abrasion manifested as a relieflike surface, which agreed with clinical findings. The wear factor ranged from 1 × 10,8 to 6 × 10,8 mm3/(N·m). The CTPOD device, validated earlier for ultrahigh molecular weight polyethylene, was shown to be the first simple wear test device to produce wear similar to that known to occur clinically in alumina-on-alumina total hip prostheses. [source] Experimental study of the friction and wear behaviour of a polymer disc/primer coating combination used in ball-joints by means of large-scale testingLUBRICATION SCIENCE, Issue 2 2006P. Samyn Abstract The surfaces of a heavily loaded ball-joint were initially covered with a sliding spray, and suffer wear. A solution is found by incorporating ultra high molecular weight polyethylene (UHMWPE) discs with a carbon fibre/epoxy reinforced ring as sliding material into the chairs of the structure. The ball side is covered with a zinc phosphate primer coating. For design purposes the local static and dynamic behaviour of the hybrid UHMWPE discs in contact with steel or Zn-coated counterfaces should be large-scale tested in terms of their loading capacity, low friction and wear resistance. Also the influence of creep and wear on friction is examined. After the large-scale verification tests in laboratory, a good correlation is found with a test in the field. Copyright © 2006 John Wiley & Sons, Ltd. [source] Chiroptical Properties of Terthiophene Chromophores Dispersed in Oriented and Unoriented Polyethylene FilmsMACROMOLECULAR CHEMISTRY AND PHYSICS, Issue 6 2004Andrea Pucci Abstract Summary: Two new chiral terthiophene chromophores II and III were prepared with 99% enantiomeric excess. Chiroptical properties of these dyes dispersed in ultra high molecular weight polyethylene (UHMWPE) films were determined and compared with the same properties in solution. In the solid state, the optical activity strongly depends on the interaction mechanisms within small crystalline aggregates of chromophores. The film morphology and chromophore dispersion were also investigated by scanning electron microscopy (SEM) and differential scanning calorimetry (DSC). The good correlation between chromophore aggregation and chiroptical activity of the binary films promotes circular dichroism (CD) as an effective technique for monitoring the phase dispersion behaviour of dichroic dyes into polymer matrices. By mechanical stretching of the film a linearly dichroic orientation of the chromophores is obtained which results in a high degree of linear dichroism. The influence of the uniaxial orientation of terthiophene molecules along the drawing direction of UHMWPE on the chiroptical properties of the films, and the possible application of the oriented devices as linear polarizers are discussed. Absorption and CD spectra of unoriented UHMWPEII film at different rotation angles ,. [source] Development of UHMWPE modified PP/PET blends and their mechanical and abrasive wear behaviorPOLYMER COMPOSITES, Issue 2 2007Navin Chand In this study, polypropylene and polyethylene terephthalate blend were modified by incorporating different percentages of ultrahigh molecular weight polyethylene (UHMWPE) ranging from 1 to 5 phr. Modified blends were prepared by melt mixing the PP/PET blend and UHMWPE. Ultimate tensile strength of UHMWPE filled blend was determined at 10, 20, 50, and 100 mm/min cross head speeds of testing. It was found that increase of cross head speed from 10 to 100 mm/min increases the tensile strength of PP/PET/UHMWPE blends. Maximum ultimate tensile strength is exhibited by the blend containing 2 phr UHMWPE. Breaking strain of the UHMWPE modified and unmodified PP/PET blend increased with the increase of cross head speed due to the highly entangled chain structure of UHMWPE. Shore A hardness of the filled blends also increased from 341 to 356, which is highest for 2 phr UHMWPE. High stress abrasive wear of UHMWPE modified blend was determined by using Suga abrasion tester, model NUS-1 Japan. Wear rate of the PP/PET(90/10) blends having 1, 2, and 5 phr of UHMWPE was determined at different loads such as 1, 3, 5, and 7 N and sliding distances from 6.4 m to 25.6 m. Wear rate values show that UHMWPE has prominent effect on abrasive wear of PP/PET blends. Addition of 2 and 5 phr UHMWPE improved the wear resistance of PP/PET blends at different loads, which has been explained on the basis of improved bonding as compared with pure PP/PET blend and increased hardness. Maximum abrasive wear rate reduction was achieved by adding 2 phr UHMWPE in PP/PET(90/10) blend. POLYM. COMPOS. 28:267,272, 2007. © 2007 Society of Plastics Engineers [source] A novel fabrication route for auxetic polyethylene, part 2: Mechanical propertiesPOLYMER ENGINEERING & SCIENCE, Issue 7 2008R.S. Webber Auxetic ultra high molecular weight polyethylene (UHMWPE) has been fabricated by omitting the extrusion stage usually required to form the characteristic nodule-fibril microstructure of this material. This new route consists of compaction followed by multiple sintering treatments, with the best results using two successive sintering treatments. This article examines the mechanical properties of cylindrical compacts subjected to between one and four successive sintering treatments. The indentation resistance of the double-sintered material was found to be 2.5 times that of conventionally processed UHMWPE and similar to that found in the extruded form of auxetic polyethylene. The flexural strength and strain to failure increase dramatically with the first sintering treatment and then remain almost constant. This processing route has potential for the production of more complex, useful shapes than can currently be produced since it removes the limitations imposed by the geometry of the barrel required for the extrusion stage. POLYM. ENG. SCI., 2008. © 2008 Society of Plastics Engineers [source] Processing of ultra-high molecular weight polyethylene by hot isostatic pressing, and the effect of processing parameters on its microstructurePOLYMER ENGINEERING & SCIENCE, Issue 10 2004Rizwan M. Gul The processing techniques available for ultra-high molecular weight polyethylene (UHMWPE) do not lead to a homogeneous material. The processed material exhibits particle boundaries (fusion defects) reflecting the powder flakes of the virgin resin. Hot isostatic pressing has been used in this study as a possible technique to produce a homogeneous, defect-free material, and to study the effect of processing parameters on consolidation and other properties. The extent of consolidation was determined by optical microscopy using thin sections, and by scanning electron microscopy using cryofractured and solvent-etched specimens. Processing temperature is the most important variable affecting consolidation; fusion defects can be eliminated by increasing the processing temperature. This is sometimes accompanied by the appearance of spherulties; however, the degree of crystallinity, melting point, and density remain constant. DMA results show that the molecular weight between entanglements increases and the entanglement structure changes with an increase in processing temperature. Polym. Eng. Sci. 44:1848,1857, 2004. © 2004 Society of Plastics Engineers. [source] Drawing in high pressure CO2,a new route to high performance fibers in memory of late Roger PorterPOLYMER ENGINEERING & SCIENCE, Issue 2 2001Terry Hobbs In this paper, we introduce a new draw technique for polymer orientation and apply it to different polymer fibers: poly(ethylene terephthalate) or PET, nylon 6,6, and ultra-high molecular polyethylene (UHMWPE). In this technique, a polymer is drawn uniaxially in supercritical CO2 using a custom high-pressure apparatus. This technique can be used in replacement of a traditional drawing process or as a post-treatment process. With PET, the technique is not effective at temperatures at or below 130°. In contrast, the process is highly effective for nylon 6,6 where CO2 drawn fibers show significantly higher crystallinity and orientation along with improved mechanical properties. While the fibers are plasticized, the drawability of the fibers is only slightly dependent on temperature. High pressure CO2 drawing of ultrahigh molecular weight polyethylene (UHMWPE) fibers is equally effective. Commercial high performance fibers can be drawn up to a ratio of 1.9 in asecond stage, resulting in large increases in tensile modulus and small improvements in tensile strength. [source] Conductive composites of UHMWPE and ceramics based on the segregated network conceptPOLYMER ENGINEERING & SCIENCE, Issue 1 2000J. Bouchet The manufacturing of composites of ultra high molecular weight polyethylene and ceramics with conductive properties has been investigated. Attention has been focused on the lowering of the amount of filler necessary to achieve low resistivity. Using segregated networks, mixing large polymer particles and submicron metal or conductive ceramic particles may be an interesting route, provided that the processing method enables to generate the desired structures. Because sintering avoids the intimate blending of the components, it is a suitable technique for this aim. The combined effects of temperature, pressure and sintering time have been investigated. The influence of the blending of the solids on the covering of the polyethylene particles before the sintering has also been pointed out. The typical features related to the concept of a segregated network are discussed in connection with the morphologies of the polymer and ceramic particles. The successful application of the reduction of the percolation threshold by a segregated network in conductive composites of polymer and metallic particles is described. [source] Towards advanced circuit board materials: adhesion of copper foil to ultra-high molecular weight polyethylene compositePOLYMERS FOR ADVANCED TECHNOLOGIES, Issue 6 2002Dmitry Abstract Polyethylene based composites are attractive materials for advanced circuit board applications because of their unique combination of properties: low dielectric constant and loss factor, light weight, high flexural modulus and low thermal expansion coefficient controlled in all spatial directions. This investigation describes a process to consolidate chopped fibers of ultra-high molecular weight polyethylene concurrently with its bonding to a copper foil. Bonding is affected by a thin sheet of low-density polyethylene, incorporating a crosslinking agent with a concentration gradient across the sheets thickness. In this single step process, the composite material is formed and bonded to the metal foil, achieving good adhesion without the use of extraneous glue. Copyright © 2002 John Wiley & Sons, Ltd. [source] Wear of the artificial hip joint material under lubricationASIA-PACIFIC JOURNAL OF CHEMICAL ENGINEERING, Issue 1 2009E. P. J. Waters Abstract A study of wear properties of hip-replacement materials, namely high-nitrogen stainless-steel femoral heads and ultra-high molecular weight polyethylene (UHMWPE), was conducted in a non-conforming apparatus using various liquid lubricants. The liquids used were normal saline solution, sodium azide solution, pure ethanol, aqueous hyaluronic acid and aqueous hyaluronic acid/cholesterol and cholesterol palmitate liquid crystal lubricant. Saline solution proved to be unsuitable as a lubricant while sodium azide that was used as a bactericide provided some evidence of mixed lubrication. A bactericide was included to overcome degradation. The aqueous hyaluronic acid exhibited cushion form lubrication as evidenced by retention of the original polymer surface features within the wear indent. Cholesterol addition showed little improvement on the wear properties but massively increased the bacterial activity. Again, inclusion of a bactericide was necessary. Liquid crystal lubricant significantly reduced wear and the atomic force microscope (AFM) showed that the liquid crystal formed protective layers on the counter face surfaces. The sub-surface of the polymer possessed plastic creep under load but low adhesive wear was present. There also was an absence of sub-micron polymer debris. It was concluded that a dramatic reduction in wear could be achieved by incorporation of liquid crystal lubricant in hip-replacement elements. Copyright © 2009 Curtin University of Technology and John Wiley & Sons, Ltd. [source] Triisobutylaluminum as cocatalyst for zirconocenes.JOURNAL OF POLYMER SCIENCE (IN TWO SECTIONS), Issue 11 2001Abstract An investigation of the catalytic behavior of the dimethylated zirconocenes Me2SiCp*NtBuZrMe2 [Cp* = C5(CH3)4; 1Me], Me2SiCp2ZrMe2 (2Me), Cp2ZrMe2 (3Me), Ind2ZrMe2 (4Me), Me2SiInd2ZrMe2 (5Me), Et(2-MeInd)2ZrMe2 (6Me), and Me2Si(2-MeInd)2ZrMe2 (7Me) with the combined activator triisobutylaluminum (TIBA)/CPh3B(C6F5)4 (Al/Zr = 250; B/Zr = 1) in ethylene polymerizations at increased monomer pressures (5,11 bar, 30 °C) was carried out. Sterically opened zirconocenes in ternary catalysts gave rise to active species effective in the formation of low molecular weight polyethylenes (PEs). These active species tended to increase the PE molecular weight [1Me (2100) < 2Me (20,000) < 5Me (89,000) < 3Me (94,500)] under similar conditions. PE obtained with 4Me showed a bimodal gel permeation chromatography curve with a 64% peak area [weight-average molecular weight (Mw) = 43,000] and a 36% peak area (Mw = 255,000). The increase in sterical demands from the zirconocenes was also demonstrated by the reduction of the chain transfer to monomer, the reinsertion of vinyl-ended PE chains, and their ability for isomerization. These reactions were most pronounced for the zirconocenes 1Me and 2Me. The active species responsible for the formation of low molecular weight PEs deactivated quickly. The zirconocenes 6Me, 7Me, and (2-PhInd)2ZrMe2 (8Me) bearing substituent at the 2-position of the indenyl ring was activated with TIBA alone, yielding active species effective in ethylene and propylene polymerizations. PEs formed with 6Me,8Me complexes activated with TIBA had high molecular weights. An increase in the Al/Zr ratio in the catalytic system 8Me/TIBA from 50 to 300 led to an enhancement of the molecular weight of polypropylene (PP) samples from oligomeric products to an viscosity-average molecular weight of 220,000. The increase in the molecular weights of PPs with an increase in the propylene concentration was also observed. An analysis of the catalytic performance of the 8Me/TIBA system showed first-order dependency of the initial polymerization rates on the TIBA concentration and close to second-order dependency on propylene. The second-order dependency on the monomer concentration is explained in terms of the monomer participation in the initiation step of the polymerization reaction. © 2001 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 39: 1915,1930, 2001 [source] FI Catalysts: new olefin polymerization catalysts for the creation of value-added polymersTHE CHEMICAL RECORD, Issue 3 2004Makoto Mitani Abstract This contribution reports the discovery and application of phenoxy,imine-based catalysts for olefin polymerization. Ligand-oriented catalyst design research has led to the discovery of remarkably active ethylene polymerization catalysts (FI Catalysts), which are based on electronically flexible phenoxy,imine chelate ligands combined with early transition metals. Upon activation with appropriate cocatalysts, FI Catalysts can exhibit unique polymerization catalysis (e.g., precise control of product molecular weights, highly isospecific and syndiospecific propylene polymerization, regio-irregular polymerization of higher ,-olefins, highly controlled living polymerization of both ethylene and propylene at elevated temperatures, and precise control over polymer morphology) and thus provide extraordinary opportunities for the syntheses of value-added polymers with distinctive architectural characteristics. Many of the polymers that are available via the use of FI Catalysts were previously inaccessible through other means of polymerization. For example, FI Catalysts can form vinyl-terminated low molecular weight polyethylenes, ultra-high molecular weight amorphous ethylene,propylene copolymers and atactic polypropylenes, highly isotactic and syndiotactic polypropylenes with exceptionally high peak melting temperatures, well-defined and controlled multimodal polyethylenes, and high molecular weight regio-irregular poly(higher ,-olefin)s. In addition, FI Catalysts combined with MgCl2 -based compounds can produce polymers that exhibit desirable morphological features (e.g., very high bulk density polyethylenes and highly controlled particle-size polyethylenes) that are difficult to obtain with conventionally supported catalysts. In addition, FI Catalysts are capable of creating a large variety of living-polymerization-based polymers, including terminally functionalized polymers and block copolymers from ethylene, propylene, and higher ,-olefins. Furthermore, some of the FI Catalysts can furnish living-polymerization-based polymers catalytically by combination with appropriate chain transfer agents. Therefore, the development of FI Catalysts has enabled some crucial advances in the fields of polymerization catalysis and polymer syntheses. © 2004 The Japan Chemical Journal Forum and Wiley Periodicals, Inc. Chem Rec 4: 137,158; 2004: Published online in Wiley InterScience (www.interscience.wiley.com) DOI 10.1002/tcr.20010 [source] | |||||||||||||