Ultrahigh Molecular Weight Polyethylene Content Research Articles

All‐polyethylene compositions based on ultrahigh molecular weight polyethylene: Synthesis and properties

AbstractReactor polymer compositions (RPC) of ultrahigh molecular weight polyethylene (UHMWPE) with Mw = 1,000 kg/mol and low molecular weight high density polyethylene (LMWPE) with Mw = 160 kg/mol were synthesized in a two‐stage ethylene polymerization with a rac‐Me2Si(Ind)2ZrCl2/methylaluminoxane catalyst. The conditions of the stages that ensure the formation of UHMWPE and LMWPE fractions with the desired molecular weight and properties were selected based on the study of one‐stage ethylene polymerization over this catalyst in a wide temperature range. The order of fraction synthesis in RPC preparation was changed. Two series of compositions UHMWPE/LMWPE and LMWPE/UHMWPE with UHMWPE content from 20 to 90 wt% were obtained. The SEM method was used to study the morphology of the nascent polymer particles. It was shown that RPC physical and mechanical properties depend not only on the UHMWPE content, but also on the morphology of the particles of reactor powders, which is determined by the order of polymer fraction synthesis. Materials of LMWPE/UHMWPE series have higher tensile properties compared to UHMWPE/LMWPE of the same composition. The LMWPE/UHMWPE compositions containing ≤50 wt% of UHMWPE flow even at a load of 10.5 kg.

The Relation between Density and Flexural Strength of Geopolymer Based Ceramic with Addition of Ultra High Molecular Weight Polyethylene (UHMWPE) for Lightweight Ceramics

Geopolymer are being considered for a variety of application including formation of ceramics. The addition of Ultra High Molecular Weight Polyethylene as binder in geopolymer based ceramic is purposely to enhance the strength and toughness of the ceramic materials. This paper aims to study the relation of density and flexural strength of geopolymer based ceramic with addition of Ultra High Molecular Weight Polyethylene as binder. Geopolymer were synthesized by fixing the NaOH molarity, Na2SiO3/NaOH and solid to liquid ratio at 12 M, 0.24 and 1.0 respectively. Geopolymer in a powder form with addition of various content of UHMWPE (2 wt.%, 4 wt.%, 6 wt.%, 8 wt.% ) were pressed into pellets followed by sintering at 1200 °C. The highest flexural strength of geopolymer based ceramics was achieved at sintering temperature of 1200 °C which is 92.1 MPa with a lowest density of 1.88 g/cm3.

Self-Regulating Electrically Conductive Materials Based on Polyethylene Compositions with UHMWPE and Carbon Black

Electrically conductive composites based on high density polyethylene (HDPE) / ultrahigh molecular weight polyethylene (UHMWPE) blends filled with carbon black were studied. The work is a part of the research of electrically conductive materials for the manufacture of self-regulating polymer heaters. In this work, the authors investigated composites based on HDPE/UHMWPE (molecular mass of ~ 7 million) blends filled with carbon black (average particle size ~ 20 nm). The goal of the work was to obtain a self-regulating electrically conductive polymer material with optimal thermoelectric characteristics and high heat resistance. It was shown that the effect of adding UHMWPE to the HDPE/carbon black composites on the thermoelectric behavior of the resulting material was similar to cross-linking. This reduced the undesirable effect of the negative thermal coefficient (NTC) of the electrical resistance. In addition, the heat resistance of the material at elevated temperatures was increased. This makes it possible to exclude the radiation or chemical cross-linking in the manufacture of self-regulating polymer heating elements. The rheological, mechanical and thermoelectric properties of HDPE/carbon black composites modified with UHMWPE were also studied. It was found that a diffusion interphase layer with a reduced fluidity in the melt of HDPE/UHMWPE blends was formed. It was established that self-regulating composites containing 30–40% of UHMWPE had the best operational properties. We can recommend the methods of extrusion and injection molding for the processing of HDPE/carbon black composites mixed with 30% UHMWPE, and the pressing method in case of UHMWPE content of 40%. The ability of the composites to be molded is sharply reduced at higher contents of UHMWPE.

Effects of cooling process on the solid–liquid phase separation process in ultra-high-molecular-weight polyethylene/liquid paraffin blends

Ultra-high-molecular-weight polyethylene (UHMWPE) microporous membrane is generally prepared by thermally induced phase separation process. The phase separation process is closely related to the cooling process in practical production. In this paper, the phase separation temperature of UHMWPE/liquid paraffin blends was explored by the hot-stage-optical device and differential scanning calorimeter (DSC), and the result showed that the temperature was about 105–125 °C. The effects of cooling condition and crystallization ability of UHMWPE on the separation process of the blends were also investigated by DSC. The results showed that the phase separation was affected by the cooling rates rather than the initial cooling temperature. And the crystallization of UHMWPE was mainly limited by the nucleating at a low cooling rate, which could form larger porous structure. In a word, it should be inspirational for the actual production of the UHMWPE microporous membrane. The solid–liquid phase separation will occur in the cooling process of UHMWPE/LP blends, which is driven by the external temperature difference. It is significant to understand the phase separation process. In this paper, the effects of the melting properties and cooling condition on phase separation process were investigated, the S–L phase separation range is at 105–125 °C, and the speed of phase separation is mainly related to the external temperature difference and the UHMWPE content. It could be inspirational for actual production of the UHMWPE membrane with more uniform pore structure.

Short‐time fabrication of well‐mixed high‐density polyethylene/ultrahigh‐molecular‐weight polyethylene blends under elongational flow: morphology, mechanical properties and mechanism

AbstractAs linear polyethylenes, ultrahigh‐molecular‐weight polyethylene (UHMWPE) and high‐density polyethylene (HDPE) have the same molecular structure, but the large difference in viscosity between them makes it difficult to obtain well‐mixed blends. An innovative eccentric rotor extruder (ERE) generating an elongational flow was used to prepare HDPE/UHMWPE blends within short processing times. Compared with the obvious two‐phase morphology of a sample from a twin‐screw extruder observed with a scanning electron microscope, few small UHMWPE particles were observed in the HDPE matrix for a sample from the ERE, indicating the good mixing on a molecular level of HDPE/UHMWPE blends achieved by the ERE during short processing times. The morphological changes of blends prepared using the ERE evidenced the good integration of HDPE and UHMWPE even though the UHMWPE content is up to 50 wt% in the blends. Moreover, all blends retained most of the intrinsic molecular weight. The good mixing was further confirmed from the thermal, crystallization and rheological behaviors determined using differential scanning calorimetry and dynamic rheological measurements. Importantly, the 50/50 blend presented improved mechanical properties, especially super‐impact strength of 151.9 kJ m−2 with incomplete‐break fracture state. The strengthening and great toughening effects of UHMWPE on the blends were attributed to the addition of unwrapped UHMWPE long molecular chains. The effective disentanglement mechanism of UHMWPE chains under elongational flow was explained schematically by a non‐parallel three‐plate model. © 2019 Society of Chemical Industry

Effect of UHMWPE Incorporation on Microstructural and Mechanical Responses of HDPE Matrix

<span>Binary blends of High density Polyethylene (HDPE) / Ultra high molecular weight polyethylene (UHMWPE) were prepared using Twin Screw Extruder (TSE). The blends were then Injection molded and analyzed for their thermal, mechanical and morphological characteristics. Differential scanning calorimetry (DSC) analyses showed an increase in melting temperature, while other properties such as crystallization temperature and percentage crystallinity were unaffected. Investigated HDPE/UHMWPE blends showed improvement in tensile, impact and flexural properties with UHMWPE content. However, strain at break suffered a decrease. Scanning electron microscopy (SEM) of cryo-fractured surface (Samples fractured after dipping in liquid nitrogen) revealed distinct two-phase morphology for such blends. SEM analyses of tensile fractured surfaces facilitated in qualitatively understanding the underlying failure mechanisms. </span>

Bimodal Ultrahigh Molecular Weight Polyethylenes Produced from Supported Catalysts: The Challenge of Using a Combined Catalyst System

Molecular precatalysts complexes (nBuCp)2ZrCl2 (Zr) and (η1:η5‐Me2NCH2CH2C5Me4)CrCl2 (Cr) have been successfully supported on silica nanoparticles, via a single support (SS) or a double support (DS) strategy. These catalyst systems have been successfully used to produce bimodal polyethylenes with an ultrahigh molecular weight polyethylene content in a single reactor. The SS and DS catalyst systems have been fully evaluated under an identical polymerization condition to assess the challenges in tailoring the molecular weight distribution. The results show that a detrimental interaction exists between Zr and Cr catalysts, part of the Cr catalyst species is deactivated during polymerization in the both DS and SS systems. The detrimental interaction in the DS system is reduced because the catalysts are supported on separate nanoparticles. But, surprisingly the two catalysts in the DS system are still able to “communicate” to each other via cocatalyst‐induced catalyst leaching or other potential reasons. image

Tailoring the crystalline morphology and mechanical property of olefin block copolymer via blending with a small amount of UHMWPE

In this study, the crystalline morphology and mechanical property of Olefin Block Copolymer (OBC) were tailored by adding a small amount (up to 10 wt%) of ultrahigh molecular weight polyethylene (UHMWPE). To do this, OBC and UHMWPE was solution blended first and then subjected to compression molding or injection molding, respectively. It was found that a small quantity of UHMWPE (0.5 wt%, 1 wt%) can be fully mixed with OBC, while phase separation occurs when UHMWPE content is higher than 2.5 wt%, as suggested by SEM and DSC results. For compression molded samples, an obvious increase of Young's modulus from 12 MPa to 22 MPa while maintaining the good elasticity of OBC was observed as the content of UHMWPE is less than 2.5 wt% (in the phase miscible range). With further increasing of UHMWPE content to 10 wt%, only slightly increased Young's modulus is seen. Thus the miscibility between OBC and UHMWPE plays an important role in determining the mechanical property for compression molded samples. However, a continuous increase of Young's modulus was observed with the increase of UHMWPE content for samples obtained via injection molding, with a slight increase (from 23 MPa to 40 MPa) as the content of UHMWPE is less than 2.5 wt%, while with a large increase (from 40 MPa to 156 MPa) as the content of UHMWPE increases from 2.5 wt% to 10 wt%, accompanied with a greatly decrease of elongation (from 1100% to 50%). Even elastomer-to-plastic transition takes place when the content of UHMWPE is large (5 wt%, 10 wt%). Structural analysis of injection-molded samples shows that shish-kebab-like structures can be successfully induced as addition of UHMWPE along with gradually increased crystal orientation. It was interesting to find that small-addition of UHMWPE favors increased tie-effect due to the miscibility between OBC and UHMWPE, which is helpful to the elasticity retaining as well as strength enhancement, while large-addition of UHMWPE gives rise to phase separation and mixed shish-kebabs (OBC/UHMWPE hybrid shish-kebabs and UHMWPE homo-shish-kebabs), leading to elastomer-to-plastic transition.

Effects of ultrahigh molecular weight polyethylene and mould temperature on morphological evolution of isotactic polypropylene at micro-injection moulding condition

The effects of ultrahigh molecular weight polyethylene (UHMWPE) and mould temperature (Tmould) on an isotactic polypropylene (iPP) matrix moulded via micro-injection were investigated via polarized light microscopy, scanning electron microscopy, differential scanning calorimetry, wide-angle X-ray diffraction and small-angle X-ray scattering. Results showed that the complex viscosity of the system increased significantly when the UHMWPE content was more than 5%; however, this viscosity decreased when the UHMWPE content was less than 5%. In addition, the addition of UHMWPE increased the onset of crystallisation temperature and the relative crystallinity of the β-form crystals in micro-injection moulded specimens. Moreover, the UHMWPE phase induced the formation of fan-shaped β crystals in iPP/UHMWPE blends. When mould temperature was 50 °C, the degree of orientation of microparts increased and the crystalline structures were highly compact. However, the relative crystallinity of the β-phase form (Kβ) was lower than those prepared at 130 °C Tmould. Most importantly, well-oriented, bundle-like β crystals have been discovered for the first time in 5 wt.% UHMWPE/iPP blends obtained at 130 °C Tmould owing to the “orientation-maintenance” and “shear-amplification” effects of UHMWPE.

Graphene-Supported Dual-Site Catalysts for Preparing Self-Reinforcing Polyethylene Reactor Blends Containing UHMWPE Nanoplatelets and in Situ UHMWPE Shish-Kebab Nanofibers

The catalytic ethylene polymerization on dual-site catalysts, supported on functionalized graphene, enables nanostructure formation in polyethylene reactor blends by in situ formation of uniformly dispersed ultrahigh molecular weight polyethylene (UHMWPE) nanoplatelets and in situ formed aligned UHMWPE shish-kebab nanofibers. For tailoring bimodal molar mass distributions, the quinolylsilylcyclopentadienylchromium(III) complex (Cr-1), producing UHMWPE with Mw > 3 × 106 g mol–1, is blended together with bisiminopyridine complexes of either chromium(III) (CrBIP), producing polyethylene (PE) wax (2 × 103 g mol–1), or iron(II) (FeBIP), producing PE with Mw = 2.0 × 105 g mol–1. Hence, the FeBIP/Cr-1 and CrBIP/Cr-1 molar ratios govern the PE/UHMWPE weight ratio without affecting the average molar mass of the individual PE fractions. In sharp contrast to conventional UHMWPE/PE reactor blends, the UHMWPE content is substantially increased up to 17 wt % without impairing melt processing. In the case of graphene-supported FeBIP/Cr-1, SEM and TEM analysis reveal that UHMWPE nanoplatelets are formed during polymerization. This is attributed to graphene-mediated mesoscopic shape replication. During injection molding, the UHMWPE nanoplatelets are transformed into aligned UHMWPE shish-kebab nanofibers, thus enabling efficient polyethylene matrix reinforcement.

Unusual hierarchical structures of mini-injection molded isotactic polypropylene/ultrahigh molecular weight polyethylene blends

The introduction of ultrahigh molecular weight polyethylene (UHMWPE) leads to very rich and unusual hierarchical crystalline morphologies in mini-injection molded isotactic polypropylene (iPP) based materials and the role of UHMWPE in the formation of the hierarchical structures has been studied. The hierarchical structures, including the shish-kebab-like structure, β-cylindrite, β-spherulite and α-spherulite, in the injection molded samples were characterized by wide-angle X-ray diffraction, scanning electron microscopy and differential scanning calorimetry. Results indicated that the content of crystal modifications and the distribution of the crystal morphologies formed are closely related to the density and arrangement of the row nuclei formed, which can be tuned by the content of UHMWPE and its dispersion in the matrix. The density and arrangement of the row nuclei are determined by the hydrodynamic interaction, melt adsorption, and interdiffusion between iPP and UHMWPE. With the distance between the row nuclei increasing, the corresponding dominating crystals are shish-kebab-like structure, β-cylindrite, β-spherulite and α-spherulite in sequence.

Dynamic Rheological Behavior of HDPE/UHMWPE Blends

The rheological behaviors of high-density polyethylene (HDPE)/ultra-high molecular weight polyethylene (UHMWPE) blends prepared by melt blending and solution blending were studied. The results showed that the rheological parameters (G′, G ″, and η*) of both types of blends increased gradually with increasing fraction of UHMWPE, while the tanδ decreased correspondingly. Comparing blends with the same UHMWPE content, all G′, G ″, and η* values of solution blends were higher, and the tanδ of the solution blends were remarkably lower than those of the melt blends. Combined with the scanning electron microscopy (SEM) observations, it was proved that, because of its very high melt viscosity, the UHMWPE chain is difficult to diffuse and be distributed well in the HDPE matrix by melt blending, resulting in a two-phase-like morphology. On the other hand, the blends prepared by the solution blending showed a homogeneous distribution of UHMWPE in the HDPE matrix. In addition, the state of aggregation of the UHMWPE in the HDPE matrix can be distinguished well by time–temperature superposition (TTS) curves; i.e., the two-phase-like morphology in the melt blends can be detected by the failure of TTS in the high-frequency range, which cannot be reflected by Cole–Cole plots and Han curves.

Improving Polarity of UHMWPE Through γ-Ray Irradiation and Its Application

Gamma-ray irradiation treatment in air has been used to induce some oxygen-containing groups onto the molecular chains of ultra-high molecular weight polyethylene (UHMWPE) to improve its polarity. The effect of γ-ray irradiation on the structure and polarity of UHMWPE was studied by the Fourier transform infrared spectroscopy, gel content measurement, and static contact angle measurement. Gamma-ray-irradiated UHMWPE was then blended with two commonly used engineering plastics, polyamide (PA) and polycarbonate (PC), and their mechanical and frictional properties were studied. The experimental results show that the content of oxygen-containing groups increased with irradiation dose, improving the polarity of UHMWPE. Both the friction coefficient and the yield strength of the blends with PA and PC decreased with increasing UHMWPE content. However, compared with the nonirradiated blends of the same UHMWPE content, the yield strength and notched impact strength of the irradiated blends were improved, while the friction coefficient decreased.

Formation and Stability of Shear-Induced Shish-Kebab Structure in Highly Entangled Melts of UHMWPE/HDPE Blends

The formation and stability of a shear-induced shish-kebab structure was investigated by in situ rheo-SAXS (small-angle X-ray scattering) and -WAXD (wide-angle X-ray diffraction) measurements of highly entangled polyethylene melts based on two polymer blends, containing small fractions (2 and 5 wt %) of ultra-high molecular weight polyethylene (UHMWPE) and high-density polyethylene (HDPE). Immediately after shear, the combined SAXS and WAXD results at 142 °C confirmed the sole formation of shish without kebabs, indicating the interplay between the topological deformation of highly entangled UHMWPE chains and the extended-chain crystallization of stretched segments without the participation of coiled segments. The presence of HDPE chains influenced the entanglement of UHMWPE but they were not involved in the shish-kebab formation at the initial stage of crystallization. The final shish lengths in both blends were nearly identical at the same strain (ε = 500), even though the UHMWPE concentration was different. When the temperature was cooled to 134 °C, both sheared blends exhibited the kebab formation, following the diffusion-controlled growth process. Although the total kebab nucleation was higher in the 5/95 wt % UHMWPE/HDPE blend, the kebab density per shish was higher in the 2/98 wt % UHMWPE/HDPE blend. The thermal stability of the shish-kebab structure was also investigated by constrained melting. Both blends exhibited identical melting behavior of kebabs but different melting behavior of shish that is governed by the entanglement restraints of the stretched-chain network.

Preparation of HDPE/UHMWPE/MMWPE blends by two-step processing way and properties of blown films

Blends of high density polyethylene (HDPE) and ultra high molecular weight polyethylene (UHMWPE) were prepared by two-step processing way. Middle molecular weight polyethylene (MMWPE) as a fluidity modifier and compatilizer was added into UHMWPE in the first step, and then modified UHMWPE and HDPE were blending extruded to prepare the HDPE/UHMWPE/MMWPE blends used for blown films. The mechanical test of the blown films revealed that when the content of MMWPE in modified UHMWPE was 40wt% and the content of UHMWPE in the blends was 20 wt%, the film had the optimal mechanical properties. The tensile strength and tear strength of the film increased by 50% and 21%, respectively, compared with those of pure HDPE film. Rheological curves indicated that the melt torque and the apparent viscosity of the HDPE/UHMWPE/MMWPE blends made by two-step processing way both greatly reduced than other blends. The results from DSC suggested that the blends by two-step processing way may form more stable and perfect co-crystallization. PLM (polarized light microscopy) and SEM micrographs revealed that two-step processing way can improve the surface morphology of the films and make the dispersion of UHMWPE particles in HDPE increase.

Studies on nonisothermal crystallization of ultra–high molecular weight polyethylene in liquid paraffin

AbstractThe nonisothermal crystallization kinetics for ultra–high molecular weight polyethylene (UHMWPE) in liquid paraffin (LP) systems was investigated through differential scanning calorimetry (DSC) measurement. The influence of UHMWPE concentration and cooling rate on crystallization mechanism and spherulitc structure as implied by the modified Avrami equation and Mo's analysis was determined, whereas the Ozawa's approach fails to describe the crystallization behaviors of these UHMWPE‐diluent systems. As a result, in the modified Avrami analysis, it was found that the Avrami exponent is constant around five at various concentrations of UHMWPE and cooling rates. Further, the value of F(T) in the Mo's approach increases with the increasing of relative crystallinity and UHMWPE content in the blends. The nonisothermal crystallization kinetics presented here are the first for UHMWPE‐diluent systems. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci, 2006

A polyethylene-reinforced polymer composite abraded by bulk solids

The abrasive wear characteristics of polymer matrix composite materials reinforced with particles of ultra-high molecular weight polyethylene (UHMWPE) have been investigated. Granular ignimbrite (a hard mineral) was used as the abrasive. The effect of varying the volume fraction of UHMWPE reinforcement upon wear rate has been measured and scanning electron microscopy (SEM) used to study the wear mechanisms that operate. The results demonstrate that there is a distinct transition that occurs in the dominant wear mechanism as the volume fraction of UHMWPE reinforcement is increased. In particular, as the UHMWPE concentration in the resin is increased, friction at the surface decreases and the wear mechanism appears to change from predominantly three- to two-body wear, with a corresponding decrease in wear rate. This transition in mechanism appears to occur when the wall friction drops below the internal friction of the granular bed. At higher values of wall friction, the plane of shear failure is within the granular material so that the particles in contact with the surface have a rolling component across the surface. At lower values of wall friction, the plane of shear failure occurs at the surface.

An investigation on rheological and impact behaviour of high density and ultra high molecular weight polyethylene mixtures

Blends of high density polyethylene (HDPE) and ultra high molecular weight polyethylene (UHMWPE), with a content of UHMWPE up to 20% wt, were prepared using two different types of processing apparatus, a single screw extruder and an internal mixer, as a function of mixing time. A detailed morphological investigation revealed that UHMWPE is only partially dissolved, even under the most intensive and extended mixing conditions. Evaluation of melt viscosity of the blends showed a double effect of UHMWPE due to its slight dissolution and to its presence as a filler. A further analysis of the experimental data, confirmed that UHMWPE, with respect to its initial content, dissolved up to about 3% only. The dissolution effect is also validated by both impact strength behaviour, increasing with the amount of dissolved UHMWPE, and a crystallization kinetics study by differential scanning calorimetry (DSC).

Blends of normal high density and ultra-high molecular weight polyethylene, ? irradiated at a low dose

The kinetics of a nonisothermal crystallization and melting ofγ irradiated with dose of 6 Mrad blends of an ultra-high molecular-weight polyethylene (UHMWPE) and a high-density polyethylene with normal molecular weight (NMWPE) is investigated by means of DSC. The blends have been prepared at temperature below the flow temperature of UHMWPE: The enthalpies of melting of the polyethylenes increase, while those of their blends decrease after irradiation. The enthalpies of crystallization of the pure polyethylenes are higher, while those of their blends almost do not change or are a bit higher after irradiation. The rates of a nonisothermal crystallization and melting of the polyethylenes increase, while those of the polyethylenes in the blends decrease after irradiation. Thermomechanical measurements under constant load in wide-temperature interval of irradiated polyethylenes and their blends have been made. A high-elastic plateau in viscous-liquid state is established on the thermomechanical curves of UHMWPE, and the blends with high content of UHMWPE. On the basis of results obtained assumptions have been made about the processes taking place in the blends under the action of irradiation, as well as about the character of the mutual influence between the components in the process of irradiation.

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

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