High temperature melted,radiation cross-linked,vitamin E stabilized oxidation resistant UHMWPE with low wear and high impact strength

In a previous communication we showed improvement in the wear resistance and toughness of cross-linked ultrahigh molecular weight polyethylene (UHMWPE) for total joint implants by radiation cross-linking after high temperature melting (HTM). In this study, we hypothesized that introduction of vitamin E into UHMWPE before high temperature melting could improve the oxidative stability of these UHMWPEs with low wear and high toughness. Vitamin E was blended with UHMWPE powder at concentrations of 0.1 and 0.2 wt% and consolidated, followed by melting at 300 and 320 °C for 5 h, and subsequent irradiation with electron beam to 150 kGy. These vitamin E/UHMWPE blends showed improved tensile and impact toughness and good wear resistance in comparison with the radiation cross-linked vitamin E/UHMWPE blends. Aggressive accelerated aging with or without pro-oxidant lipids showed that vitamin E-blended, high temperature melted and subsequently irradiated UHMWPE had good oxidation resistance.     

Well-mixed blends of HDPE and ultrahigh molecular weight polyethylene with major improvements in impact strength achieved via solid-state shear pulverization

Compared with conventional polyolefins, ultrahigh molecular weight polyethylene (UHMWPE) possesses outstanding impact strength and crack resistance that make it desirable for a wide variety of applications. Unfortunately, UHMWPE has an ultrahigh viscosity that renders common, continuous melt-state processes ineffective for making UHMWPE products. Attempts to overcome this problem by blending UHMWPE with lower molecular weight high-density polyethylene (HDPE) by melt processing have typically led to poorly dispersed blends due to the vast viscosity mismatch between blend components. Here, we present solid-state shear pulverization (SSSP) as a mild, continuous, and simple approach for achieving effective and intimate mixing in UHMWPE/HDPE blends. These SSSP blends are easily processed by post-SSSP melt extrusion; for an SSSP blend with 50 wt% UHMWPE, we observe more than a factor of 1000 increase in viscosity at a shear rate of 0.01 s⁻¹ but less than a factor of 5 increase at 100 s⁻¹, the latter being more typical of melt-processing operations. Using extensional rheology, we confirm the strain hardening behavior of SSSP blends. Shear rheology and crystallization data show that the mixing between UHMWPE and HDPE can be improved with subsequent passes of SSSP and single-screw extrusion. Finally, we show that blending via SSSP leads to dramatic improvements in impact strength: as compared to literature results, injection-molded sample bars made from SSSP blends with 30–50 wt% UHMWPE exhibit very high values of notched Izod impact strength, 660–770 J/m (the impact strength of neat HDPE was 170 J/m).     

Development of UHMWPE modified PP/PET blends and their mechanical and abrasive wear behavior

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     

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

The goal of this study was to create wear resistant ultra high molecular weight polyethylene (UHMWPE) with improved strength and toughness. It was previously demonstrated that high temperature melting (HTM) of UHMWPE at 280-320 °C improved its toughness without detrimentally affecting its wear resistance. We hypothesized that radiation cross-linking after high temperature melting could further improve the wear resistance of UHMWPE, and the loss in toughness by radiation cross-linking could be compensated by the improved toughness achieved by the high temperature melting prior to irradiation. In this work, we demonstrated that irradiation after HTM generated UHMWPE with improved toughness compared to the irradiated UHMWPEs without HTM, partly due to the low cross-link density of irradiated HTM UHMWPE. At a given cross-link density, irradiated HTM UHMWPEs showed higher wear resistance than irradiated UHMWPE. Therefore, successive HTM and radiation cross-linking strategy is promising to create UHMWPE materials with low wear and improved mechanical properties for total joint implants.     

Shear-induced crystallization of isotactic polypropylene within the oriented scaffold of noncrystalline ultrahigh molecular weight polyethylene

Shear-induced crystallization of isotactic polypropylene (iPP) within the oriented scaffolds of noncrystalline ultrahigh molecular weight polyethylene (UHMWPE) was investigated by means of in situ synchrotron small-angle X-ray scattering (SAXS) and wide-angle X-ray diffraction (WAXD). The study was carried out using iPP/UHMWPE blends under isothermal crystallization at 145 °C (i.e., above the melting point of polyethylene) and step shear (shear rate=60 s⁻¹, duration=5 s) conditions. The oriented and isotropic iPP crystalline phases were extracted from the 2D WAXD pattern, and their kinetics data were evaluated with the Avrami equation. The dominant component in the oriented iPP phase was a kebab structure, whose nanostructure dimensions were determined by a novel SAXS analysis scheme. The minor non-crystalline but oriented UHMWPE component played a key role in the nucleation of iPP, which could be explained in terms of mutual diffusion at the interface, resulting in a significant increase in the relaxation time of iPP chains. As a result, after shear, the interfacial iPP chains could also retain their orientation and formed oriented nuclei to initiate the kebab growth.     

Shear-induced crystallization in isotactic polypropylene containing ultra-high molecular weight polyethylene oriented precursor domains

Melt blends of short ultra-high molecular weight polyethylene (UHMWPE) fibers and isotactic polypropylene (iPP) were subjected to shear at 145 °C, above the melting point of polyethylene (PE). Structural evolution and final morphology were examined by in situ synchrotron X-ray scattering/diffraction as well as ex situ microbeam X-ray diffraction and high resolution scanning electron microscopy, respectively. Results indicate that the presence of oriented UHMWPE molten domains significantly facilitated the crystallization of iPP and enhanced the initial ‘shish-kebab’ structure leading to the final cylindritic morphology. It is argued that shear flow aligns the fibrillar UHMWPE domains, where the interfacial frictions between PE and iPP effectively retards the relaxation of iPP chains, allowing the aligned iPP chains to create a shish-like structure. Nucleation on the iPP shish initiates the folded chain lamellae (kebabs), which grow perpendicularly to the iPP/PE interface.     

Ultra high molecular weight polyethylene with improved plasticity and toughness by high temperature melting

Our goal was to improve the strength and toughness of ultra high molecular weight polyethylene (UHWMPE), which is the preferred polymeric bearing material in total joint implants. Based on accelerated diffusion of UHMWPE chains at high temperatures, our hypothesis was that high temperature melting could minimize the structural defects and thus improve the toughness of consolidated UHMWPE. Melting of consolidated medical-grade UHMWPE at 280, 300, and 320 °C in inert atmosphere improved the elongation at break, work-to-failure and impact strength, presumably due to chain scissioning and structural defect elimination through self-diffusion. An important finding of this study was that the gain in plasticity and toughness did not sacrifice the wear resistance under optimized melting conditions, which may be promising for next generation high performance UHMWPE materials for joint implant bearing surfaces.     

Processing–Nanostructure–Property Relationships of All‐Polyethylene Composites Reinforced by Flow‐Induced Oriented Crystallization of UHMWPE

All‐polyethylene composites exhibiting substantially improved toughness/stiffness balance are readily produced during conventional injection molding of high density polyethylene (HDPE) in the presence of bimodal polyethylene reactor blends (RB40) containing 40 wt% ultrahigh molar mass polyethylene (UHMWPE) dispersed in HDPE wax. Scanning electron microscopy (SEM) and differential scanning calorimetry (DSC) analyses shows that flow‐induced crystallization affords extended‐chain UHMWPE nanofibers forming shish which nucleates HDPE crystallization producing shish‐kebab structures as reinforcing phases. This is unparalleled by melt compounding micron‐sized UHMWPE. Injection molding of HDPE with 30 wt% RB40 at 165 °C affords thermoplastic all‐PE composites (12 wt% UHMWPE), improved Young's modulus of 3400 MPa, tensile strength of 140 MPa, and impact resistance of 22.0 kJ/m². According to fracture surface analysis, the formation of skin‐intermediate‐core structures accounts for significantly improved impact resistance. At constant RB40 content both morphology and mechanical properties strongly depend upon processing temperature. Upon increasing processing temperature from 165 °C to 250 °C the average shish‐kebab diameter increases from the nanometer to micron range, paralleled by massive loss of self‐reinforcement above 200 °C. The absence of shish‐kebab structure at 250 °C is attributed to relaxation of polymer chains and stretch‐coil transition impairing shish formation.     

Densification of copper oxide doped alumina toughened zirconia by conventional sintering

The effect of copper oxide doping (0.05–1 wt%) on the densification, microstructure evolution and mechanical characteristics of alumina toughened zirconia (ATZ: 80 wt% Y-TZP + 20 wt% Al₂O₃) ceramic composites was investigated. Green samples were pressureless sintered using a short hold time of 12 min at temperatures varying from 1250 °C to 1500 °C. The incorporation of up to 0.2 wt% copper oxide was beneficial in promoting densification at low sintering temperature and improving the mechanical properties of ATZ without affecting the tetragonal phase stability. It was found that 0.2 wt% copper oxide addition was most efficacious, and the samples could attain a relative density of approximately 92% at 1250 °C, approximately 97% dense at 1350 °C and above 99% dense at 1450–1500 °C. This approach was also accompanied by an improvement in the Vickers hardness (12.7 GPa) and fracture toughness (6.94 MPam^1/2) when consolidated at 1450 °C/12 min. In comparison, the undoped composite exhibited relative densities of approximately 80% at 1250 °C, 87% at 1350 °C and approximately 97%–98% at 1450 °C-1500 °C. However, the study also found that higher dopant levels (0.5 wt% and 1 wt%) was not beneficial because the tetragonal zirconia phase was disrupted upon cooling from sintering, resulting in the monoclinic phase formation. In addition, low densification and poor mechanical properties were obtained.     

Suspension stability and sintering influence on yttria-stabilized zirconia fabricated by colloidal processing

The stability of nano-zirconia 3YSZ powder in suspension was extensively studied by the colloidal method, and the optimum sintering temperature of the green sample fabricated through slip casting was determined. Zirconia suspensions with 10 vol% powder loading were prepared with distilled water, and HNO₃ was used to adjust the pH of the suspension to pH 1–6. All of the suspensions were subjected to sedimentation test, and the results showed that the suspensions adjusted to pH 2 had the lowest sediment volume. This finding indicates that a suspension with pH 2 produces higher packing density. Viscosity test was carried out for the suspensions added with dispersant ranging from 0.3 wt% to 0.7 wt% polyethyleneimine (PEI) with and without pH adjustment. The suspension containing 0.5 wt% PEI with pH 2 adjustment produced the lowest viscosity because of interparticle bond breakage in the aggregates, thus forming colloidally stable suspensions. The zirconia suspension containing 0.5 wt% PEI and whose pH was adjusted to pH 2 was chosen to be slip casted into cylindrical shape. Green samples were sintered at various sintering temperatures that ranged from 1100 °C to 1500 °C through a two-step sintering method. The sample sintered at 1500 °C was found to be porosite-free, and its highest relative density was 99.6% of the theoretical density. Morphological studies detected pores in the microstructure of the samples sintered at low sintering temperatures (1100 and 1200 °C). By contrast, the samples sintered at 1400 and 1500 °C were fully densified. However, the grain size of the sample sintered at 1500 °C was 230 nm, which indicated excessive grain growth. The Vickers hardness of the sample sintered at 1400 °C was found to be highest (12.9 GPa) and comparable to results found in the literature.     

Ultradrawing of blend films of ethylene-dimethyl-aminoethyl methacrylate copolymer and ultra-high molecular weight polyethylene prepared by gelation/crystallization from solutions

Copolymer ethylene-dimethyl-aminoethyl methacrylate (EDAM) with 3.9% DAM side groups and ultra-high molecular weight polyethylene (UHMWPE) were blended in decalin solvent. The hot homogenized solution was poured into an aluminum tray to form gels and the decalin was allowed to evaporate from the resultant gels under ambient condition. Surprisingly, the resultant dry blend films could be elongated to more than 200-fold (λ=200) even for the blend film with 90% EDAM content (9/1 composition), although the maximum draw ratio of EDAM homopolymer films was 1.6-fold (λ=1.6). The mechanism of the great drawability was dependent upon the content of EDAM. The drawability for the 9/1 composite films was attributed to large crystal lamellae of UHMWPE ensuring crystal transition from a folded to a fibrous type. Accordingly, EDAM chains were independent of ultradrawing of UHMWPE and kept a random orientation under ultra-drawing process. The storage (Young's) modulus was 10 GPa at 20 °C. In contrast, EDAM chains within the 1/1 composite films were oriented drastically together with UHMWPE crystallites. The modulus of the 1/1 composition at 20 °C reached 68 GPa, which was higher than the value (40 GPa) of polypropylene films with λ=100. Such considerable difference of modulus due to EDAM content was analyzed in relation to the gelation/crystallization from solutions.     

Time resolved WAXS/SAXS observations of crystallisation in oriented melts of ultra high molecular weight polyethylene

Ultra high molecular weight polyethylene (UHMWPE) has been drawn in the melt state at 140, 145 and 150 °C at extension rates ∼1 s⁻¹ while simultaneously recording two dimensional SAXS and WAXS with a time resolution of 0.1 s. The first observable crystallisation is mainly in the orthorhombic form at a level of about ∼1 wt%. At higher draw ratios additional crystallisation is in the hexagonal form up to ∼10 wt%. The crystallisation is accompanied by strong SAXS equatorial scatter with maxima at ∼25 nm period; in some cases meridional maxima are also visible at ∼120 nm. Substantial crystallisation occurs on subsequent cooling to 130 °C, accompanied by strong meridional maxima of narrow lateral width. The observed crystal forms are consistent with a temperature-strain phase diagram, favouring hexagonal at higher strains. There are indications that the thermodynamic orthorhombic to hexagonal transition T_tr is above 150 °C so that all the observable hexagonal structures are metastable. The initial orthorhombic crystals are associated with the high molecular weight tail and provide the strain hardening to enable the formation of subsequent hexagonal crystals. The equatorial SAXS lobes are interpreted in terms of lateral density fluctuations that are associated with an arrangement of columns of oriented chains comprising both orthorhombic and hexagonal structures. The columns are embryonic shish structures that on cooling nucleate kebab overgrowths.     

Exploring interfacial interactions,dielectric, ferroelectric and piezoelectric properties of ultrahigh molecular weight polyethylene/graphene oxide nanocomposites

Graphene oxide (GO) incorporated ultra-high molecular weight polyethylene (UHMWPE) nanocomposites were prepared by encapsulating GO by UHMWPE in an aqueous media via high-shear mixing, which were subsequently dried and compression molded. Morphological characterizations via scanning electron microscopy revealed the intercalation of UHMWPE chains in the graphitic stacks corresponding to GO. Further, dielectric permittivity of UHMWPE/GO nanocomposite of 1 wt% GO showed a drastic increase (~61) as compared to pure UHMWPE (~2) due to an enhanced interfacial polarization. A significantly higher value of remnant polarization (~10 nC/cm²) and coercive field (~3 kV/cm) was observed in UHMWPE/GO nanocomposite of 1 wt% GO, which showed a strong hysteresis loop of polarization versus electric field plot as compared to pure UHMWPE, which displayed a very weak hysteresis loop. The piezoelectric coefficient (d₃₃) of ~9.5 pm/V was estimated in UHMWPE/GO nanocomposite of 1 wt% GO via piezoresponse force microscopy. Nanocomposite sensor devices were also fabricated and piezoelectric output voltage of ~6 V was recorded in UHMWPE/GO nanocomposite of 1 wt% of GO. We report here for the first time the unique ferroelectric and piezoelectric properties displayed by UHMWPE/GO nanocomposites.     

多级拉伸中超高相对分子质量聚乙烯纤维的结构与性能研究

采用凝胶纺丝-超拉伸技术纺制了超高相对分子质量聚乙烯(UHMWPE)纤维。采用双折射法、DSC热分析法对不同拉伸级数的纤维进行了结构分析并测试了其力学性能和抗蠕变性能。研究结果表明:随着拉伸倍数的增加,纤维的取向度、结晶度及热性能得到了提高,力学性能和抗蠕变性得到改善,可得到纤维强度大于30cN/dtex,模量超过1200cN/dtex的高性能纤维。     

Ultrahigh molecular weight polyethylene with improved crosslink density,oxidation stability,and microbial inhibition by chemical crosslinking and tea polyphenols for total joint replacements

Highly crosslinked ultrahigh molecular weight polyethylene (UHMWPE) stabilized by vitamin E (VE) is widely applied in artificial joints as the bearings. Despite the approval, there is a discord that VE lowers the crosslinking efficiency, limiting its use at high concentration. In this work, we aim to obtain highly crosslinked and oxidation resistant UHMWPE through the conjunction of tea polyphenol and chemical crosslinking. We hypothesized that highly incorporated tea polyphenol with multiple reactive sites can ameliorate crosslinking efficiency of chemical crosslinked UHMWPE in comparison to VE. Epigallocatechin gallate (EGCG) as representative tea polyphenol was incorporated into UHMWPE at high concentration (2–8 wt%), followed by chemical crosslinking with 2 wt% organic peroxide. Unlike VE/UHMWPE blends as the control, chemical crosslinking achieved an increasing trend in crosslink density of EGCG/UHMWPE blends with increasing antioxidant concentration. High concentration of EGCG also enhanced the oxidation stability of UHMWPE. Intriguingly, EGCG endowed UHMWPE with an excellent antimicrobial property, which was inefficient in VE/UHMWPE. Cell viability was hardly affected by the high loaded antioxidant and peroxide. The chemically crosslinked UHMWPE blended with EGCG is proved to be a reasonable, cost effective and realistic alternative for use in artificial joints.     

An in-situ X-ray scattering study during uniaxial stretching of ionic liquid/ultra-high molecular weight polyethylene blends

An ionic liquid (IL) 1-docosanyl-3-methylimidazolium bromide was incorporated into ultra-high molecular weight polyethylene (UHMWPE) and formed IL/UHMWPE blends by solution mixing. The structure evolution of these blends during uniaxial stretching was followed by in-situ synchrotron wide-angle X-ray diffraction (WAXD) and small-angle X-ray scattering (SAXS) techniques. During deformation at room temperature, deformation-induced phase transformation from orthorhombic to monoclinic phase was observed in both IL/UHMWPE blends and neat UHMWPE. The elongation-to-break ratios of IL/UHMWPE blends were found to increase by 2-3 times compared with that of pure UHMWPE, while the tensile strength remained about the same. In contrast, during deformation at high temperature (120 °C), no phase transformation was observed. However, the blend samples showed much better toughness, higher crystal orientation and higher tilting extent of lamellar structure at high strains.     

Effective dissolution of UHMWPE in HDPE improved by high temperature melting and subsequent shear

A simple and effective way was expected to improve the blending of ultra‐high‐molecular‐weight polyethylene (UHMWPE) in high‐density polyethylene (HDPE) matrix. HDPE/UHMWPE blends were subjected to high temperature melting (HTM) at 280°C for up to 10 h, followed by shear at 175°C. These results were examined by scanning electron microscopy, polarized optical microscopy, and melt rheological behavior. UHMWPE particle was swelled partially during HTM, and this swollen region could be peeled from the particle by the subsequent shear, which resulted in more “dissolution” of UHMWPE in HDPE matrix. These results were also validated by the rheological behavior. POLYM. ENG. SCI., 55:270–276, 2015. © 2014 Society of Plastics Engineers     

Pentadecane functionalized graphite oxide sheets as a tool for the preparation of electrical conductive polyethylene/graphite oxide composites

Covalent functionalization of pentadecane-decorated thermally reduced graphite oxide (GO) sheets has been studied as a tool for the preparation of polyethylene/GO composites exhibiting rheological and electrical percolation thresholds. It was accomplished through pentadecane based radical addition onto unsaturated bonds located on the GO sheets' surface using dicumyl peroxide as hydrogen abstractor. This chemical functionalization influences the affinity of the formed pentadecane grafted GO sheets for various solvents. Then, the compounding of the composites pentadecane grafted GO/PE was performed at a processing temperature of 140 °C with 25, 20, 15, 10, 8 and 5 wt% loadings. Rheological and electrical percolation thresholds were found between 10 and 15 wt% for polyethylene/pentadecane functionalized graphene oxide composites while the composite graphite/PE at the same loading percentage did not reach any percolation threshold.     

Formation and characterization of polyethylene blends for autoclave‐based expanded‐bead foams

Blends of linear‐low‐density polyethylene (LLDPE), low‐density polyethylene (LDPE), and high‐ density polyethylene (HDPE) were foamed and characterized in this research. The goal was to generate clear dual peaks from the expanded polyethylene (EPE) foam beads made from these blends in autoclave processing. Three blends were prepared in a twin‐screw mixing extruder at two rotational speeds of 5 and 50 rpm: Blend1 (LLDPE with 20 wt% HDPE), Blend 2 (LLDPE with 20 wt% LDPE), and Blend 3 (LLDPE with 10 wt% HDPE and 10 wt% LDPE). The differential scanning calorimetric (DSC) measurement was taken at two cooling rates: 5 and 50°C/min. Although no dual peaks were present, the results showed that blending with HDPE has a more noticeable effect on the DSC curve of LLDPE than blending with LDPE. Also, the rotational speed and cooling rate affected the shape of the DSC curves and the percentage area below the onset point. The DSC characterization of the batch foamed blends revealed multiple peaks at certain temperatures, which may be mainly due to the annealing effect during the gas saturation process. POLYM. ENG. SCI., 2010. © 2009 Society of Plastics Engineers     

Porous mullite honeycomb by hydrothermal treatment of fired kaolin bodies in NaOH

A conventional hydrothermal treatment with various concentrations of NaOH was used at 150° and 190°C to dissolve excess silica glass and thus make porous mullite ceramics from a fired New Zealand kaolin body. The effect of hydrothermal treatment time on the dissolution of the glass was examined. At 150°C, the dissolution of glass was almost complete after treatment for 8 hrs in 5N-NaOH solution and about 40–43 wt% of the glass was removed from the fired kaolin body leading to porous mullite. However, when the fired kaolin body was treated for more than 5 hrs in 5N-NaOH at 190°C, a composite of mullite and a nonporous crystalline phase of unknown symmetry resulted. These crystals formed from the dissolution and recrystallization of the glass. After the dissolution of glass in 2N-NaOH solution at 190°C for 5 hrs, a porous mullite body of 52.8% porosity with an average pore diameter of 0.57 m could be obtained, and this was only composed of mullite whiskers. Growth of unidentified nonporous crystals in the body which was treated in 5N-NaOH solution at 190°C led to a decrease in specific surface area and therefore, these crystals should be avoided.