Ultra-high Molecular Weight Polyethylene Powder Research Articles

Powder bed fusion additive manufacturing of ultra-high molecular weight polyethylene using a novel laser scanning strategy

Current processing routes (ram extrusion, compression molding, and sheet molding) for fabricating ultra-high molecular weight polyethylene (UHMWPE) greatly limit the geometries that can be processed. Due to these limitations, additive manufacturing (AM) of UHMWPE has long been a goal, as the ability to fabricate complex geometries from this polymer could open new applications not possible using these traditional processing routes. UHMWPE’s especially high wear properties also fill a gap in current AM materials. Powder bed fusion (PBF) processing of UHMWPE is challenging due to the high melt viscosity of UHMWPE and melt explosion, which occurs during melting of UHMWPE and causes powder particle expansion during scanning and leads to powder recoating failure. This work demonstrates a novel scan strategy for PBF of UHMWPE, in which layers are scanned using large (1.2 mm) hatch spacings. Scanning with a large hatch spacing reduces conduction between scanlines and distributes energy so as to encourage coalescence without melt explosion. UHMWPE powder is only partially melted during this scanning approach, which causes light coalescence between particles and avoids z-direction expansion that disrupts powder recoating. The lightly coalesced “green” parts are brittle and are densified by thermally post-processing them in the melt. Parts retain their shape during post-processing due to the high melt viscosity of UHMWPE. Using this combination of a novel scan strategy with large hatch spacing and a post-processing thermal treatment, a 3000 kDa UHMWPE was successfully printed, demonstrating the first reported multi-layer PBF of UHMWPE with complex geometries.

High-performance fibrous artificial muscle based on reversible shape memory UHMWPE

Shape memory polymer fibers (SMPFs) are an important branch of intelligent fibers. Due to their ability to deform under external stimuli, shape memory polymer fibers have been used in intelligent textiles, polymer fillers and medical fields, including surgical sutures, thrombectomy devices, compression stockings, etc. There have been many studies on SMPFs, but almost all reported SMPFs only exhibit a one-way shape memory effect (SME). The deformation process of the one-way SMPFs is irreversible, and the transition from temporary shape to permanent shape cannot be replicated by simply reversing the stimulus. To tackle this problem. We selected ultra-high molecular weight polyethylene (UHMWPE) as the polymer matrix to prepare external stress-free two-way SMPFs. UHMWPE powders are gel-spun and then hot drawn to yield raw UHMWPE fibers. After programming, the UHMWPE fibers possess high tensile strength (338.72 MPa) and Young's modulus (2287.34 MPa), as well as one-way SME, quasi two-way SME and external stress-free reversible two-way SME. The reversible strain of the prepared reversible shape memory UHMWPE fibers is 27.42%. Artificial muscles and actuators can be made from the programmed UHMWPE fibers. The programmed UHMWPE fibers can lift more than 250 times their own weight with ∼10% reversible strain. The programmed UHMWPE fibers are appropriate for industrial mass production since the preparation and programming processes are simple.

First example of cationic titanium (III) complexes with crown ether as catalysts for ethylene polymerization

Complex cations with a vacant coordination site are considered as the most plausible catalytically active centers of metallocene and post-metallocene polymerization catalysts. For the first time we demonstrated that cationic titanium (III) complexes stabilized with crown ether can be used as pre-catalysts for ethylene polymerization. In the presence of alkyl aluminum chlorides and MgBu2 as cocatalysts, these complexes catalyze ethylene polymerization to produce Ultra High Molecular Weight Polyethylene (UHMWPE) with productivity up to 4650 kgPE/mol Ti h⋅atm and molecular weight up to 1.8·106 Da. UHMWPE powders were processed by the solid-phase method with subsequent orientation drawing into high-strength (1.4–2.1 GPa) and high-modulus (91–118 GPa) films.

Effect of blending small amount of high-density polyethylene on molecular entanglements during melt-drawing of ultrahigh-molecular-weight polyethylene

The effect of blending a small amount of high-density polyethylene (HDPE) on molecular entanglements during melt-drawing of ultrahigh-molecular-weight polyethylene (UHMW-PE) was investigated in this study. UHMW-PE powders with a small amount of HDPE added and a bimodal MW distribution were synthesized by reactor blending. In situ wide-angle X-ray diffraction and small-angle X-ray scattering measurements during melt-drawing of UHMW-PE and UHMW-PE/HDPE films were performed to evaluate the state of molecular entanglements on the basis of the behavior of oriented crystallization and extended-chain crystal formation. The addition of a small amount of HDPE reduced the amount of tight molecular entanglements originating from UHMW-PE chains, and further addition reduced the amount of molecular entanglements that transmitted the applied stress. These results demonstrated that the addition of a small amount of HDPE significantly affected the amount of molecular entanglements of UHMW-PE chains during melt-drawing.

Melting kinetics, ultra-drawability and microstructure of nascent ultra-high molecular weight polyethylene powder

For the production of high-performance polyethylene fibers and tapes by ultra drawing, using solid-state processing of disentangled nascent ultra-high molecular weight polyethylene (UHMWPE), the maximum draw ratio is an important design parameter, as it determines the maximum degree of chain alignment and, therewith, the properties of the final product. It would, therefore, be advantageous to have a fast scanning method to estimate the maximum drawability of reactor powders. In the present work, the melting behavior of nascent UHMWPE reactor powder was studied by differential scanning calorimetry, followed by isoconversional analysis to evaluate the apparent activation energy barrier of melting for the different UHMWPE grades of different level of disentanglement. Powder samples were solid-sate processed into tapes at elevated temperature. Ultra drawing of these tapes at the optimum drawing temperature allowed for the evaluation of the maximum draw ratio (λmax) that is determined by the entanglement density. The morphology of the lamellar structure of the nascent powder was studied by small-angle X-ray scattering and by scanning transmission electron microscopy analyses. A correlation between the melting kinetics, the microstructure and the maximum draw ratio was observed for all grades, suggesting that the apparent activation energy of melting can be used as a screening method to estimate the maximum draw ratio.

Optimization of Mechanical Activation Modes for UHMWPE Dry Mixed Powders and Nanomodificators in the Planetary Ball Mill Pulverisette 7

Introduction. Currently, in various technology areas, bronze, cast iron and other antifriction metals are replaced by polymer composites, which extend significantly service life of tribocoupling. An advanced antifriction polymer is ultra-high-molecular-weight polyethylene. The study deals with determining the optimal specific energy consumption for the mechanical activation of the polymer dry mixed powders and nanomodifiers in the planetary ball mill Pulverisette 7, which ensure the best complex of physico-mechanical and rheological properties of nanocomposites. Materials and Methods. In this work, we used GUR 4120 Ticona ultra-high-molecular-weight polyethylene with a molecular weight of 5 million, a Tuball Matrix Beta concentrate of activated carbon nanotubes at a concentration of 0.1%, calculated with reference to carbon nanotubes, and hydrophobic nanocrystalline silicon dioxide with a dispersion of 20 nm at the same concentration. Mechanical co-activation of polymer powders and nanomodifiers, when varying the specific energy consumption, was carried out in the planetary ball mill Pulverisette 7. The production of films from powders, for studying the elastic-strength and rheological characteristics of nanocomposites, was carried out with the use of the hydraulic press Gibitre. Tests were carried out respectively on the tensile testing machine UAI-7000 M and the rheometer Haake MARS III. Results. It has been established that the best physico-mechanical and rheological properties of nanocomposites are with specific energy consumption for mechanical activation of 3,000‒3,200 J/g that allows us to consider them optimal. The mechanical activation of ultra-high molecular weight polyethylene powder, reducing slightly the elasticity modulus and tensile strength of thermally pressed samples, does not affect the dynamic viscosity of melts at an energy consumption of 650‒4,550 J/g. Discussion and Conclusion. The use of carbon nanotubes and nanocrystalline silicon dioxide at a concentration of 0.1% can significantly improve the physical-mechanical and rheological properties of the polymer with energy costs of 3,000‒3,200 J/g for mechanical activation in planetary ball mills. Nanocrystalline silicon dioxide is a more effective modifier that can be explained by its better dispersion in the polymer matrix due to the lower tendency of nanoparticles to agglomerate.

Experimental studies on micro powder hot embossing for high-aspect-ratio microstructures with ultra-high molecular weight polyethylene powders

High-aspect-ratio microstructures have broad application prospects in many fields. Conventionally, lithography and etching processes are the primary methods for fabricating high-aspect-ratio microstructures on glass or silicon chips. However, as application scenarios have expanded, polymeric microstructures have received increasing attention, due to the low cost and wide ranges of special physical properties of polymers. Hot embossing is a high-efficiency method of processing large-area polymer films, but due to spring back, fabricating high-aspect-ratio microstructures remains a challenge. For this issue, micro powder hot embossing with the advantage of the good fluidity of the powder, is expected to achieve accurate microstructures with a high aspect ratio. In this paper, micro powder hot embossing was studied experimentally with the ultra-high molecular weight polyethylene (UHMWPE) powders which has excellent performances but poor formability. First, the processing window for preparing compact samples was investigated. Two indicators including filling depth and compactness were then proposed to characterize the forming quality of high-aspect-ratio microstructures. Afterwards, the effect of cavity size was also discussed and a ratio of cavity size to powder particle size was suggested to exceed 4 for high-aspect-ratio microstructures. Lastly, influences of process parameters (including temperature, pressure, and sintering duration) were further studied, which recommends a loading pressure greater than 10 MPa and a sintering temperature exceeding 140 °C for the micro powder micro embossing for UHMWPE. Microgrooves with an aspect ratio up to 8.0 were successfully prepared, demonstrating the feasibility of micro powder hot embossing in the fabrication of high-aspect-ratio microstructures with high precision and comprehensive properties.

Effect of Adhesion on Mechanical and Tribological Properties of Glass Fiber Composites, Based on Ultra-High Molecular Weight Polyethylene Powders with Various Initial Particle Sizes.

The aim of this study was to assess the effect of adhesion between the non-polar, ultra-high molecular weight polyethylene (UHMWPE) matrix and the glass fiber fillers of various lengths treated with the commercially available “KH-550” agent, on the mechanical and tribological properties of the UHMWPE-based composites. The motivation was to find the optimal compositions of the polymer composite, for the compression sintering manufacturing of lining plates for the protection of marine venders and construction vehicles, as well as transport equipment. It was shown that the initial powder size at equal molecular weight determined the distribution patterns of the glass fibers in the matrix, and, as a consequence, the mechanical and tribological properties of the composites. Based on the obtained experimental data and the results of the calculation by a developed computer algorithm, control parameters were determined to give practical recommendations (polymer powder size and glass fiber length), for the production of the UHMWPE-composites having specified mechanical and tribological characteristics. The “GUR4022 + 10% LGF” composite, loaded with the chopped 3 mm glass fibers treated with the “KH-550”, was recommended for severe operating conditions (high loads, including impact and abrasive wear). For mild operating conditions (including cases when the silane coupling agent could not be used), the “GUR2122 + 10% MGF” and “GUR2122 + 10% LGF” composites, based on the fine UHMWPE powder, were recommended. However, the cost and technological efficiency of the filler (flowability, dispersibility) and polymer powder processing should be taken into account, in addition to the specified mechanical and tribological properties.

Sintering the feasibility improvement and mechanical property of UHMWPE via selective laser sintering

ABSTRACTSelective laser sintering (SLS) is a branch of additive manufacturing based on powders. The research was carried out to demonstrate the sintering feasibility of ultra-high molecular weight polyethylene (UHMWPE) and the evaluation of the parts of the mechanical property prepared by SLS at various laser powers was also conducted. Meanwhile, the morphology of the parts was examined through scanning electron microscopy. Additionally, products with high three-dimensional stability were obtained via SLS with UHMWPE/SiO2. The effects of silica and laser power on the products’ relative density, dimensional accuracy and tensile property were then discussed. The incorporation of the silica reduces the difficulties in sintering the UHMWPE powder and eliminates the slice slides (layer slides between two closely packed layers) in the sintering process. However, the mechanical property decreased with the addition of the SiO2 into the powder. Meanwhile, the properties of parts show high relevance with the laser power.

Effects of UHMWPE Filler on the Tribological and Mechanical Properties of Biocompatible Epoxies

The effects of ultra-high-molecular-weight polyethylene (UHMWPE) as a filler on the tribological and mechanical properties of two biocompatible epoxies were studied. SU-8 and Structalit 8801 (a trade name) were the two biocompatible epoxies selected for this investigation. UHMWPE powder (40–48 μm particle size) was added to the selected epoxies to tailor their mechanical and tribological properties. In dry sliding tests, after adding 25 wt% of UHMWPE particles, the coefficient of friction (COF) was reduced by 79 and 67% for SU-8 and Structalit, respectively, and the specific wear rate was reduced by 92 and 58% for SU-8 and Structalit, respectively. Fourier transform infrared (FTIR) analysis conducted on the samples indicated that there was no chemical bonding between UHMWPE fillers and the epoxy matrices. A biocompatibility test conducted on SU-8 cured with the hardener confirmed that the samples were not cytotoxic. In addition, it was noted that the ductility ratio of SU-8 and Structalit increased by 71 and 40%, respectively, with the addition of UHMWPE. These novel composites can find applications as biomaterials for tribologically demanding applications such as hip–knee joint implants, dental implant, etc.

Preparation of ultrahigh-molecular-weight polyethylene fibers by combination of melt-spinning and melt-drawing

Ultrahigh-molecular-weight polyethylene (UHMW-PE) fibers were prepared by a combination of melt-spinning and melt-drawing. An as-spun fiber without melt fracture was continuously obtained by melt-spinning from UHMW-PE powder. The as-spun fiber was melt-drawn at 145 and 150 °C. The fiber diameter was reduced by the melt-drawing process. The melt-drawn fiber consisted of extended- and folded-chain crystals. The chain orientation with the development of the extended-chain crystals was accelerated by the melt-drawing at 145 °C under higher strain rates, resulting in higher tensile strength. Consequently, a UHMW-PE fiber with a diameter of approximately 150 μm and a high tensile strength of approximately 1.1 GPa was prepared by the combination of melt-spinning and melt-drawing.

Crystal morphology and corresponding physical properties of nascent ultra-high molecular weight polyethylene powder with short-branched chains

Due to the linear and ultra-long molecular chains, ultra-high molecular weight polyethylene (UHMWPE) has been playing a vital role in many fields. Based on more and more demands for special UHMWPE resins, the relationship between microstructure of nascent powder and its corresponding physical properties was investigated in this work, and one special powder with high degree of short-chain branch was fully contrasted with one general-purpose UHMWPE resin. Based on series characterization results, it revealed that short-branched chains led to a decrease on the entanglement density and brought a significant increase in the processing performance and mechanical properties. In addition, the short-branched chains also made a huge difference on crystal morphology of nascent UHMWPE powders. By correlating high-resolution scanning electron microscopy, DSC curves and Raman spectroscopy characteristics, the slurry polymerization mechanism of UHMWPE was deduced, and the corresponding performance of nascent UHMWPE powders was fully explained.

Processing of ultra-high molecular weight polyethylene/graphite composites by ultrasonic injection moulding: Taguchi optimization

Ultrasonic injection moulding was confirmed as an efficient processing technique for manufacturing ultra-high molecular weight polyethylene (UHMWPE)/graphite composites. Graphite contents of 1 wt%, 5 wt%, and 7 wt% were mechanically pre-mixed with UHMWPE powder, and each mixture was pressed at 135 °C. A precise quantity of the pre-composites mixtures cut into irregularly shaped small pieces were subjected to ultrasonic injection moulding to fabricate small tensile specimens. The Taguchi method was applied to achieve the optimal level of ultrasonic moulding parameters and to maximize the tensile strength of the composites; the results showed that mould temperature was the most significant parameter, followed by the graphite content and the plunger profile. The observed improvement in tensile strength in the specimen with 1 wt% graphite was of 8.8% and all composites showed an increase in the tensile modulus. Even though the presence of graphite produced a decrease in the crystallinity of all the samples, their thermal stability was considerably higher than that of pure UHMWPE. X-ray diffraction and scanning electron microscopy confirmed the exfoliation and dispersion of the graphite as a function of the ultrasonic processing. Fourier transform infrared spectra showed that the addition of graphite did not influence the molecular structure of the polymer matrix. Further, the ultrasonic energy led oxidative degradation and chain scission in the polymer.

Mechanism of cement paste reinforced by ultra-high molecular weight polyethylene powder and thermotropic liquid crystalline copolyester fiber with enhanced mechanical properties

In this study, a series of cementitious composites with high toughness and flexural strength was obtained by melt-dispersing ultra-high molecular weight polyethylene (UHMWPE) into a cement matrix followed by water immersion. The structure and chemical composition of the composites were characterized by scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). Three point bending tests showed that the flexural strengths of the composites were improved from 5.5 MPa to 18.2 MPa with the presence of 25 wt% UHMWPE, and could be further enhanced to 28.1 MPa with the addition of only 0.1 vol% oriented thermotropic liquid crystalline copolyester (TLCP) fibers. An adhesive test revealed that the interfacial binding force between polymer and fiber was much stronger than that between cement and fiber. Our findings provide a simple way for utilizing polymer to improve the interface between the fibers and cement matrix, consequently achieving a dramatic increase in the flexural strength and toughness.

Porous UHMWPE Membranes and Composites Filled with Carbon Nanotubes: Permeability, Mechanical, and Electrical Properties

Application of electric fields as a mechanism against membrane fouling is an emerging cleaning process for membranes. Sintering of ultrahigh molecular weight polyethylene powder particles decorated with multiwalled carbon nanotubes (MWCNT) is performed to produce electrically conductive porous filtration membranes. The contact of the molten particle surfaces leads to their fusion through neck formation during sintering. As the MWCNT only cover the surface of the powder particles, they form a conductive network with a high number of inter‐MWCNT contacts in pathways through the polymer matrix. Membranes with an electrical conductivity of 0.9 S m−1 at a MWCNT concentration of 5.0 wt% are prepared. Additionally, a comparison of sintered and compression molded samples shows that after the heating interval, crystallization during cooling strongly influences the formation of the MWCNT network. The study reveals that electrically conductive, porous polymer membranes for microfiltration applications can be prepared using a solvent‐free process. image

Highly hydrophilic ultra-high molecular weight polyethylene powder and film prepared by radiation grafting of acrylic acid

The surface properties of ultra-high molecular weight polyethylene (UHMWPE) are very important for its use in engineering or composites. In this work, hydrophilic UHMWPE powder and film were prepared by γ-ray pre-irradiation grafting of acrylic acid (AA) and further neutralization with sodium hydroxide solution. Variations in the chemical structure, grafting yield and hydrophilicity were investigated and compared. FT-IR and XPS analysis results showed that AA was successfully grafted onto UHMWPE powder and film; the powder was more suitable for the grafting reaction in 1wt% AA solution than the film. Given a dose of 300kGy, the grafting yield of AA was ∼5.7% for the powder but ∼0.8% for the film under identical conditions. Radiation grafting of a small amount of AA significantly improved the hydrophilicity of UHMWPE. The water contact angle of the UHMWPE-g-PAA powder with a grafting yield of AA at ∼5.7% decreased from 110.2° to 68.2°. Moreover, the grafting powder (UHMWPE-g-PAA) exhibited good dispersion ability in water.

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

Spectroscopic and chromatographic quantification of an antioxidant-stabilized ultrahigh-molecular-weight polyethylene.

The oxidative stability of various antioxidant-containing ultrahigh-molecular-weight polyethylene (UHMWPE) formulations has been widely reported. Depending on which specific antioxidant is used, the process by which it is incorporated into UHMWPE, and the amount of the antioxidant incorporated, there could be substantial differences in the material and toxicological properties of the UHMWPE formulation. Pentaerythritol tetrakis (3-[3,5-di tertiary butyl-4-hydroxyphenyl] propionate) (PBHP) has been extensively used as an efficient antioxidant in various applications. However, it has not thus far been used to stabilize UHMWPE in orthopaedic implants. It is therefore important to characterize and verify the concentration and homogeneity of distribution of PBHP in the composition, the chemical consequence of exposure of the antioxidant to gamma irradiation, and to assess the toxicological risk of use by the identification and quantification of leachables before the use of PBHP-containing UHMWPE in implantable devices. (1) Can the concentration and uniformity of distribution of the antioxidant PBHP in UHMWPE powder and in the consolidated, preirradiated formulation be verified? (2) Can the leachable compounds in the gamma radiation crosslinked PBHP/UHMWPE formulation be identified and quantified? PBHP in GUR 1020 UHMWPE was quantified by Fourier transform infrared (FTIR) and ultraviolet-visible (UV-Vis) spectroscopy. The chemical byproducts generated by gamma irradiation of PBHP were identified using gas chromatography in conjunction with mass spectrometry followed by a second-stage mass spectrometry (GC-MS/MS). When GC-MS/MS was coupled with Stir Bar Sorptive extraction, leachable components in the UHMWPE formulation were identified and quantified. The percent concentration of PBHP in UHMWPE powder was confirmed by UV-Vis spectroscopy and the concentration and uniform distribution of PBHP in UHMWPE after consolidation and before radiation crosslinking was verified through FTIR spectroscopy. GC-MS/MS analysis enabled the identification and quantification of 16 gamma irradiation byproducts of PBHP. These 16 compounds were verified as potentially leachable compounds in PBHP-stabilized UHMWPE and were found to be well below the safety threshold concern of 150 ng/device in orthopaedic knee inserts made from PBHP-stabilized UHMWPE. Spectroscopic analysis has been successfully used to demonstrate the ability to reliably quantify the amount as well as the distribution of PBHP in UHMWPE in orthopaedic bearings. State-of-the-art chemical extraction and analytical techniques have enabled the identification of the gamma radiation-induced byproducts of PBHP and the quantification of these components as leachables from the PBHP-stabilized UHMWPE formulation. Antioxidant-stabilized UHMWPE materials being considered for orthopaedic bearings must be fully characterized for composition before use because it is apparent that exposure to high doses of gamma radiation would cause the formation of new chemical entities. It is important to verify the identities and quantities of chemical species that could leach out of implanted devices in the long term to enable their toxicological risk assessment.

Guanidine functionalized radiation induced grafted anion-exchange membranes for solid alkaline fuel cells

Alkaline anion-exchange membranes (AAEMs) for solid alkaline fuel cells (SAFC) application were successfully prepared by radiation induced grafting of Vinyl benzyl chloride onto ultra-high molecular weight polyethylene powder (UHMWPE), followed by film fabrication by melt pressing and quaternization with a Guanidine derivative, 1,1,3,3-tetramethyl-2-n-butylguanidine (TMBG). The chemical structures of the resulting AAEMs were examined by Fourier transform infrared, which showed that the grafted membranes were successfully functionalized by modified guanidine. The performance of the AEMs, including ion exchange capacity, water uptake, in-plane swelling, methanol uptake, methanol permeability, and hydroxide ion conductivity were investigated. Thermal analysis showed that the Guanidine-based AAEMs comprises better thermal stability. The AAEMs membrane exhibited a maximum ionic conductivity of 27.7 mS cm−1 at 90 °C. Methanol permeability is found to be in the order of 10−9 cm2 s−1, which is significantly lower than that of Nafion®. The membranes have useful properties as an anion exchange membranes suitable for alkaline fuel cells.

Electrostatic adsorption method for preparing electrically conducting ultrahigh molecular weight polyethylene/graphene nanosheets composites with a segregated network

Electrically conductive ultrahigh molecular weight polyethylene (UHMWPE)/graphene nanosheets composites with a segregated structure were prepared by electrostatic adsorption method. It was found that UHMWPE powders could generate static electricity after high-speed mechanical friction which was beneficial to adsorb fluffy graphene on the surface of polymer powders. UHMWPE/graphene nanosheets composites with a segregated network produced by hot-pressing exhibited a dramatic enhancement in electrical conductivity with the percolation threshold of 0.1 vol.%.