Impact of Dose-Rate on Rheology,Structure and Wear of Irradiated UHMWPE

Ultrahigh molecular weight polyethylene (UHMWPE) is used as a bearing material in total joint replacements (TJR). UHMWPE for TJR is usually modified by irradiation and thermal treatment to increase wear resistance. We modified UHMWPE in three ways, differing in radiation dose-rate and/or atmosphere during irradiation. Rheological properties before and after irradiation were determined by means of oscillatory shear measurements. Structural changes were followed by X-ray diffraction, infrared spectroscopy, electron spin resonance, and solubility measurements. Wear resistance of selected samples was obtained by the pin-on-disk method. Rheological properties changed sensitively with modification conditions including radiation dose rate. Moreover, rheological results correlated well with both crosslinking extent and wear resistance. Finally, it was demonstrated that the optimal radiation dose, i.e. the dose leading to maximum crosslinking density and wear resistance, was different for each of the three modification procedures.     

Powder compaction,sintering, and rolling of ultra high molecular weight polyethylene and its composites

The applicability of powder compaction and sintering techniques to the processing of ultra high molecular weight polyethylene (UHMWPE) powder is demonstrated. With proper processing procedure and type of UHMWPE powder, the mechanical properties obtained are nearly equivalent to those obtained by conventional melt processes. The properties were optimized by selection of a sintering temperature just above the melting point and by close control of particle size and distribution. The processability of UHMWPE is dependent on the morphology of the powder. Only those powders with a fibrous morphology provided good mechanical properties after sintering. The mechanical properties of powder compacts can be improved by several techniques. Liquid sintering with added normal molecular weight polyethylene, with close control of particle size and distribution and amount of the second component, yielded improved properties. Composites of UHMWPE, with short glass and graphite fiber reinforcement, processed by powder compaction and sintering resulted in increased modulus. The properties of these composites depended upon the amount of fibers, fiber length, fiber-matrix bonding, and fiber orientation. Rolling the powder-processed UHMWPE oriented the structure and improved the mechanical properties, although it decreased the mechanical properties of the glass and graphite fiber composites because of debonding between fiber and matrix. The properties of carbon black—UHMWPE mixtures were improved by rolling because of a more uniform distribution of carbon black.     

FTIR study of the surface properties and tribological behaviors of plasma-modified UHMWPE and zirconia

Zirconia and ultra-high molecular weight polyethylene (UHMWPE) are common materials for artificial joints. However, the failure of artificial joints is mainly caused by the wear of UHMWPE. Therefore, the development of effective measures to enhance the service life of UHMWPE is one of the most important facets of research on total joint replacement. The purpose of this study is to use an atmospheric-pressure plasma system to modify zirconia and UHMPWE surfaces, which is expected to increase the adsorption of joint-lubricating fluids, thus reducing the abrasion and wear of UHMWPE (73% improvement). Surface modification experiments were carried out using an atmospheric-pressure plasma system, while fourier transform infrared spectroscopy was used to characterize the surfaces treated with atmospheric-pressure plasma. The results of water contact angle tests indicated that the plasma-treated material surfaces exhibited excellent hydrophilicity (70% improvement). In addition, the treatment of materials with atmospheric plasma was confirmed to increase the adsorption of lubricating fluid and reduce wear, thus extending the service life of UHMWPE.     

Effects of Addition of Ultra-High Molecular Weight Polyethylene on Tie-Molecule and Crystallization Behavior of Unimodal PE-100 Pipe Materials

With the fast-growing global market demand for high-grade plastic pipe materials, high-density polyethylene (HDPE) products, such as PE-100, are playing a more and more important role. On the other hand, lack of basic understanding about these materials hinders the further development of this field. To investigate the effects of addition of an ultra-high molecular weight polyethylene (UHMWPE) on tie-molecules, crystallization kinetics and long-term properties of a unimodal HDPE pipe material (UMPE-100) made from a Cr-based catalyst, the blends of UMPE-100/UHMWPE were prepared through a twin-screw extruder. The probability of tie-molecules was calculated by a statistical approach, which has been proposed by Huang and Brown (Huang, Y.L.; Brown, N.J. Polym. Sci. B. 1991, 29, 129–137). It showed that as UHMWPE was added, the probability of tie-molecules increased due to increased molecular weight. The crystallization kinetics of the blends was investigated by an isothermal crystallization method using differential scanning calorimetry. Addition of small amount of UHMWPE improved the crystallization rate greatly. The natural draw ratio of blends decreased with improvement of tie-molecule probability and crystallization rate, indicating improvement in long-term properties.     

Investigation of microstructural changes in M250 grade maraging steel using positron annihilation

Microstructural evolution of M250 grade maraging steel subjected to various thermal-aging treatments has been investigated using positron annihilation, X-ray diffraction, hardness and electron microscopy studies. Isochronal aging treatment in the range of 600–900 K, in steps of 100 K, was carried out and positron lifetime, austenite volume fraction and hardness values have been measured. The stages corresponding to point defect dynamics and formation of intermetallics have been clearly identified. Based on these results, measurements were performed on specimens isothermally aged at 755 K from 0.1 to 100 h to elucidate the time evolution of microstructural changes, and the results are discussed.     

Preparation of Ultrahigh Molecular Weight Polyethylene/WS2 Composites for Bulletproof Materials and Study on Their Bulletproof Mechanism

Ultrahigh molecular weight polyethylene (UHMWPE)/WS₂ nanoparticle fibers were prepared by adding inorganic fullerene-like (IF) WS₂ nanoparticles treated by a coupling agent to the precursor solution of UHMWPE. The influence of WS₂ nanoparticles on the microstructure and properties of UHMWPE fibers were characterized by the scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), and mechanical property measurements. The bulletproof performance of the UHMWPE/WS₂ composite was tested by a bullet-shock test, and the bulletproof mechanism of the UHMWPE/WS₂ composite was preliminarily studied. The results showed that WS₂ nanoparticles could be uniformly dispersed in the UHMWPE fiber. After incorporating of WS₂ nanoparticles, the UHMWPE fibers became stiffer and tougher than the pristine ones. In particular, the modulus of the fibers increased from 1203 to 1326 cN/dtex. The introduction of IF-WS₂ nanoparticles led to significantly improved bulletproof performance of UHMWPE fibers.     

Steady-State and Frequency-Domain Lifetime Measurements of an Activated Molecular Imprinted Polymer Imprinted to Dipicolinic Acid

We recently demonstrated the synthesis and fluorescence activity associated with an optical detector incorporating a molecular imprinted polymer (MIP). Steady-state and time-resolved (lifetime) fluorescence measurements were used to characterize the binding activity associated with MIP microparticles imprinted to dipicolinic acid (DPA). DPA is a unique biomarker associated with the sporulation phase of endospore-forming bacteria. Vinylic monomers were polymerized in a dimethylformamide solution containing DPA as a template. The resulting MIP was then pulverized and sorted into small microscale particles. Tests were conducted on replicate samples of biologically active cultures representing both vegetative stationary phase and sporulation phase of Bacillus subtilis in standard media. Samplers were adapted incorporating the MIP particles within a dialyzer cartridge (500 MW). The permeability of the dialyzer membrane permitted diffusion of lighter molecular weight constituents from microbial media effluents to enter the dialyzer chamber and come in contact with the MIP. Results showed dramatic (10-fold over background) steady-state fluorescence changes (as a function of excitation, emission and intensity) for samples associated with high endospore biomass (DPA), and a frequency-domain lifetime of 5.3 ns for the MIP-DPA complex.     

Raman characterisation of conventional and cross‐linked polyethylene in acetabular cups run on a hip joint simulator

Five sets of differently sterilised conventional ultra‐high molecular weight polyethylene (UHMWPE) and cross‐linked polyethylene (XLPE) acetabular cups were run for 5 million cycles on a hip joint simulator in order to evaluate their wear behaviour in relation to material properties (PE grade, conventional or cross‐linked) and sterilisation method (ethylene oxide (EtO) treatment or γ‐irradiation). Gravimetric measurements revealed that conventional UHMWPE wore significantly more than XLPE. The differences in wear behaviour could be partly related to the orthorhombic contents obtained by Raman spectroscopy in the unworn areas of the cups: XLPE cups showed a significantly higher crystallinity degree than the UHMWPE specimens. Raman analysis showed that wear testing did not significantly modify the orthorhombic content of any of the tested acetabular cups. However, the set of cups that showed the highest weight loss, i.e. γ‐sterilised PE GUR1020, appeared the most homogeneously polished upon wear testing; from a molecular point of view, only this set of cups showed a significant increase of the I₁₁₃₀/I₁₀₆₀ intensity ratio, suggesting the occurrence of chain orientation. On the other hand, XLPE cups, despite the lowest weight loss undergone, showed a decrease in the amorphous content upon wear testing as well as a limited orthorhombic → monoclinic transformation, which did not appear detrimental. Copyright © 2011 John Wiley & Sons, Ltd.     

Surface Modification of Ultrahigh-molecular-weight Polyethylene Fibers with Coupling Agent during Extraction Process

Ultrahigh molecular weight polyethylene (UHMWPE) fibers were treated with a coupling agent following the extraction of gel fibers, resulting in modified fibers after subsequent ultra-drawing. The structure and morphology of the modified UHMWPE fibers were characterized and their surface wetting, interfacial adhesion, and mechanical properties were investigated. It was found that the coupling agent was absorbed into the UHMWPE fiber and trapped on the fiber surface. Compared with unmodified UHMWPE fibers, the modified fibers had smaller contact angle, higher crystallinity, and smaller crystal size. The interfacial adhesion and mechanical properties of UHMWPE fibers were significantly improved with increasing coupling agent concentration and gradually reached a plateau value. After treatment with 1.5 wt% solution of a silane coupling agent (γ -aminopropyl triethoxysilane, SCA-KH-550), the interfacial shear strength of the UHMWPE-fiber/epoxy composites was increased by 108% and the tensile strength and modulus of modified UHMWPE fibers were increased by 11% and 37% respectively.     

A novel sample preparation method to avoid influence of embedding medium during nano-indentation

The effect of the embedding medium on the nano-indentation measurements of lignocellulosic materials was investigated experimentally using nano-indentation. Both the reduced elastic modulus and the hardness of non-embedded cell walls were found to be lower than those of the embedded samples, proving that the embedding medium used for specimen preparation on cellulosic material during nano-indentation can modify cell-wall properties. This leads to structural and chemical changes in the cell-wall constituents, changes that may significantly alter the material properties. Further investigation was carried out to detect the influence of different vacuum times on the cell-wall mechanical properties during the embedding procedure. Interpretation of the statistical analysis revealed no linear relationships between vacuum time and the mechanical properties of cell walls. The quantitative measurements confirm that low-viscosity resin has a rapid penetration rate early in the curing process. Finally, a novel sample preparation method aimed at preventing resin diffusion into lignocellulosic cell walls was developed using a plastic film to wrap the sample before embedding. This method proved to be accessible and straightforward for many kinds of lignocellulosic material, but is especially suitable for small, soft samples.     

Magnetic and mechanical properties of Cu-strengthened aged HSLA-100 steel

Magnetic and mechanical properties were evaluated for various heat-treated Cu-strengthened HSLA-100 steels. After austenitizing at 1183?K for one hour and then water quenching, the material was aged at different temperatures ranging between 623?K and 973?K for one hour. Scanning and transmission electron microscopes with an energy dispersive X-ray attachment were used for microstructural analysis. Nanosize coherent copper precipitates caused an increase in hardness with increase in ageing temperature. The steel exhibited maximum hardness at 773?K followed by decrease in hardness with over-ageing. Copper precipitates coarsened and lost coherency during over-ageing. Ageing behaviour again exhibited an increase in hardness at 973?K due to the formation of new martensite islands. Magnetic hysteresis loop and magnetic Barkhausen emissions techniques were used to characterize the aged materials. No correlation was observed between the magnetic parameters and hardness. The results were explained by the fact that nanosize Cu precipitates, the size of which is much smaller than the domain wall width, did not influence the magnetic domain dynamics.     

Characterization of elastomeric materials by NMR-microscopy

This review reflects a long experience with the application of NMR-imaging methods to elastomeric materials. The experimental techniques, used to obtain parameter selective NMR-images (T₁,T₂, T_1ρ-images), are described in detail and the methods required for the data analysis are explained. A special emphasis is put on the analysis of experimental errors within the framework of NMR-imaging.

In order to make parameter selective images generally useful their information should be correlated to material properties, so that images of the material properties can be obtained. This is demonstrated for the case of crosslink density, which is certainly one of the molecular properties in rubber materials, exhibiting the main influence on mechanical and other material properties.

Sulfur cured and carbon black filled technical rubbers with different degree of crosslink density and oxidative aging were investigated using parameter selective imaging techniques. The image data were analyzed by means of gaussian and multiexponential fitting procedures, revealing spatially resolved NMR relaxation parameters. The further interpretation of these parameters was based on physical models describing molecular motions in crosslinked polymers.     

Acoustical properties of double porosity granular materials

Granular materials have been conventionally used for acoustic treatment due to their sound absorptive and sound insulating properties. An emerging field is the study of the acoustical properties of multiscale porous materials. An example of these is a granular material in which the particles are porous. In this paper, analytical and hybrid analytical-numerical models describing the acoustical properties of these materials are introduced. Image processing techniques have been employed to estimate characteristic dimensions of the materials. The model predictions are compared with measurements on expanded perlite and activated carbon showing satisfactory agreement. It is concluded that a double porosity granular material exhibits greater low-frequency sound absorption at reduced weight compared to a solid-grain granular material with similar mesoscopic characteristics.     

The structure, surface topography and mechanical properties of Si-C-N films fabricated by RF and DC magnetron sputtering

Silicon carbon nitride thin films were deposited on Co-Cr alloy under varying deposition conditions such as sputtering power and the partial pressure ratio of N₂ to Ar by radio frequency and direct current magnetron sputtering techniques. The chemical bonding configurations, surface topography and hardness were characterized by means of X-ray photoelectron spectroscopy, atomic force microscopy and nano-indentation technique. The sputtering power exhibited important influence on the film composition, chemical bonding configurations and surface topography, the electro-negativity had primary effects on chemical bonding configurations at low sputtering power. A progressive densification of the film microstructure occurring with the carbon fraction was increased. The films prepared by RF magnetron sputtering, the relative content of the Si-N bond in the films increased with the sputtering power increased, and Si-C and Si-Si were easily detachable, and C-O, N-N and N-O on the film volatile by ion bombardment which takes place very frequently during the film formation process. With the increase of sputtering power, the films became smoother and with finer particle growth. The hardness varied between 6 GPa and 11.23 GPa depending on the partial pressure ratio of N₂ to Ar. The tribological characterization of Co-Cr alloy with Si-C-N coating sliding against UHMWPE counter-surface in fetal bovine serum, shows that the wear resistance of the Si-C-N coated Co-Cr alloy/UHMWPE sliding pair show much favourable improvement over that of uncoated Co-Cr alloy/UHMWPE sliding pair. This study is important for the development of advanced coatings with tailored mechanical and tribological properties.     

INFLUENCE OF PROCESSING CONDITIONS ON THE STRUCTURAL AND FRACTURE BEHAVIOR OF PP PIPE GRADES

Polypropylene (PP) is being used increasingly as a material for pressure pipes for distribution of hot and cold water. In these applications, the long-term performance and reliability are critical, key properties. The lifetime of PP is influenced by a large number of structural and morphological parameters. The structural changes due to processing conditions were studied by “rapid” SIS/DSC method (stepwise isothermal segregation). The full notch creep test (FNCT) was used to estimate the pipe lifetime. The application of both structural and fracture methods allows to describe these changes. A correlation between time to failure and structural parameters (MFR—melt mass-flow rate, drift molecular parameter τ, and kinetic parameter k) was found.     

Effect of High-Density Polyethylene Nanocomposite Compatibilizer Type on the Interfacial Adhesion and Mechanical Properties of Polyethylene Nano-Homocomposites

In this paper, the effect of nanocomposite compatibilizer type on the interfacial adhesion and mechanical properties of new class of polyethylene (PE) homocomposites, comprising PE/clay nanocomposites as matrix and ultra high molecular weight polyethylene (UHMWPE) fibers as reinforcement, was investigated. These were manufactured by a combination of powder impregnation and film stacking methods, introduced in previous research. Three types of high-density polyethylene (HDPE) Nanocomposites were prepared based on the various compatibilizers used: (i) nanocomposites containing HDPE-grafted maleic anhydride (HDPE-g-MA) as compatibilizer of clay and HDPE matrix, (ii) linear low-density polyethylene-grafted maleic anhydride (LLDPE-g-MA) used as compatibilizer, and (iii) nanocomposites without any compatibilizer. The effects of the presence and compatibilizer type on the quality of clay dispersion, and also the interface features of HDPE-nanocomposite and UHMWPE fibers were investigated and compared with each other. The results demonstrated that the kind of compatibilizer was an important factor determining the dispersion state of clay platelets, and influenced the UHMWPE fiber–PE matrix interface adhesion and the mechanical properties of the PE nano-homocomposites.     

Surface free energy of ultra-high molecular weight polyethylene modified by electron and gamma irradiation

Surface free energy of biocompatible polymers is important factor which affects the surface properties such as wetting, adhesion and biocompatibility. In the present work, the change in the surface free energy of ultra-high molecular weight polyethylene (UHMWPE) samples, which is produced by electron beam and gamma ray irradiation were, investigated. Mechanism of the changes in surface free energy induced by irradiations of doses ranging from 25 to 500 kGy was studied. FTIR technique was applied for sample analysis. Contact angle measurements showed that wettability and surface free energy of samples have increased with increasing the irradiation dose, where the values of droplet contact angle of the samples decrease gradually with increasing the radiation dose. The increase in the wettability and surface free energy of the irradiated samples are attributed to formation of hydrophilic groups on the polymer surface by the oxidation, which apparently occurs by exposure of irradiated samples to the air.     

Mechanical,Thermal, and Surface Properties of Ultrahigh Molecular Weight Polyethylene/Polypropylene Blends

Various compositions of ultrahigh molecular weight polyethylene/polypropylene (UHMWPE/PP) blends were prepared in decalin, with the rheological, mechanical, thermal, and surface properties of the blends being determined using the solution cast film. Viscosity and mechanical properties of the blends decreased below the additivity value with increasing PP content implying that PP molecules disturb the entanglement of UHMWPE. Contact angle of the blend films with a water drop increased with increasing content of PP. The atomic force microscope (AFM) images showed that the surface of cast UHMWPE was very smooth whereas that of cast PP was very uneven. For blends, the surface became rough and uneven with increasing content of PP. The melting temperature of PP (T _mP) decreased in the blends with increasing UHMWPE content while that of UHMWPE (T _mU) remained almost constant in blends.     

Experimental study of transcrystallinity in UHMWPE/LLDPE composites

This study deals with the effect of a transcrystalline LLDPE (linear low-density polyethylene) layer grown on Spectra 1000 UHMWPE (ultrahigh molecular weight polyethylene) fibres. Chemical similarity between the fibre and the surrounding melt does not promote transcrystallinity as no transcrystalline microstructure appears from the surface of as-received Spectra 1000 UHMWPE fibres. However, oxygen plasma treatment of the UHMWPE fibres yields a degree of surface roughness that appears to promote easy nucleation and growth of LLDPE transcrystallinity. The kinetics of transcrystalline growth were investigated quantitatively. The growth rate increased by a factor of about 12 for a 10°C increase in supercooling, and at 105°C the maximum observed thickness of the transcrystalline layer was about one fibre diameter. The induction time was found to decrease as the crystallization isotherm increased. We discuss the possibility of using surface energy parameters to define a better criterion for the nucleation of transcrystallinity from the UHMWPE fibre substrate. Preliminary data were generated for the interfacial mechanical shear strength by means of the microbond test. It is conjectured that the combined effects of a thermal treatment and the presence/absence of a transcrystalline layer might produce significant changes in the interfacial shear strength, as illustrated here by a 43% increase observed with specimens subjected to different thermal treatments.     

Mild degradation processes in ZnO-based varistors: the role of Zn vacancies

The effects of a degradation process on the structural and electrical properties of ZnO-based varistors induced by the application of dc bias voltage were analysed. Capacitance and resistance measurements were carried out to electrically characterize the polycrystalline semiconductor before and after different degrees of mild degradation. Vacancies’ changes in the varistors were studied with positron annihilation lifetime spectroscopy. Variations on the potential barrier height and effective doping concentration were determined by fitting the experimental data from impedance spectroscopy measurements. These results indicate two different stages in the degradation process consistent with vacancy-like concentration changes.