Ultra-high Molecular Weight Polyethylene Reactor Research Articles

Phase Transformation in UHMWPE Reactor Powders Synthesized on Various Catalysts in Mechanical and Thermal Fields.

Nowadays, a solvent-free method for production of high performance fibers directly from ultrahigh-molecular-weight polyethylene (UHMWPE) reactor powder is being actively developed. It causes the interest in the morphology of the reactor particles and their behavior in thermal and mechanical fields. Changes in the phase composition in virgin particles of ultra-high molecular-weight polyethylene reactor powders and in particles of powders compressed at room temperature under different pressures were studied in real time using synchrotron radiation with heating in the range of 300-370 K. It was found that the content of the monoclinic phase in reactor powders depends on the type of catalyst used for synthesis and on the applied pressure. It is shown that there are monoclinic phases of different nature: a structurally stabilized monoclinic phase formed during synthesis, and a monoclinic phase resulting from plastic deformation during compaction at room temperature. The behavior of these phases in temperature and mechanical fields is compared.

Search for optimal conditions for monolithization of ultra-high-molecular-weight polyethylene reactor powder

In order to clarify the possibility of sintering reactor powders of ultra-high molecular weight polyethylene under pressure at a temperature higher than its equilibrium melting point at atmospheric pressure (T0m) without catastrophic changes in the internal structure of particles, a comparative WAXS analysis was carried out of the samples sintered below (T<T0m) and above (T>T0m) this temperature and cooled under different conditions. A structural analysis of the X-ray scattering curves recorded in the Bragg-Brentano mode on a 2D Phaser Bruker diffractometer from UHMWPE reactor powders sintered under different conditions was carried out, and the crystallite sizes were calculated. It was found that an increase in the sintering temperature above T0m does not significantly change the crystal structure of the polymer, and the precursors produced can be used for further orientational hardening. Keywords: ultrahigh molecular weight polyethylene reactor powders, sintering, WAXS.

Destruction Products from Gel Spinning of Ultrahigh-Molecular-Weight Polyethylene Fibers

Mechanical and thermal oxidative destruction was observed during gel spinning of ultrahigh-molecular-weight polyethylene (UHMWPE) high-modulus fibers. Destruction products (DPs) were observed in the solvent and on the surface of finished UHMWPE fibers using optical and scanning electron microscopy. IR spectra of the DPs were compared with those of UHMWPE reactor powder, solid paraffin, and the mineral oil used in the gel-spinning process to show that absorption bands corresponding to terminal aldehydes and acids and unsaturated hydrocarbons and ethers in the UHMWPE were strengthened by the destruction.

The Crystalline Structure of Nascent Ultra High Molecular Weight Single Particles and Its Change on Heating, as Revealed by in-situ Synchrotron Studies

A solvent-free route to high performance ultra-high molecular weight polyethylene (UHMWPE) film threads is currently under intensive development. It involves compaction/sintering of UHMWPE reactor powder at T < Tm followed by orientation hardening of the sintered film cut into narrow strips. However, not any kind of reactor powder can be transformed into the desired high-performance material. The presence of a monoclinic crystalline phase (MP) in the powder is considered as one of the key parameters indicating its applicability for solvent-free processing. Since the MP is stable only under stress, the assumption has been made that the observed MP is generated during tableting for X-ray analyses rather than during synthesis of the nascent powder. We show that comparative X-ray analysis of a tablet and a single “virgin” particle using synchrotron radiation indicates that the MP content in the virgin particle was far less than that in the compressed tablet. Only the (001) MP peak was resolved while the others were severely overlapped with the normal, orthorhombic reflections. Thus, it supports our idea that the widely observed MP phase is, for the most part, generated during the sample preparation for the X-ray analyses.

Evolution of Intercrystallite “Bridges” During Multi-Step Drawing of Gel-Spun Polyethylene Fibers

In 1978, Gibson et al. [1] observed a paradoxical x-ray scattering pattern in oriented ultra-high-molecularweight polyethylene (UHMWPE) that indicated that the crystallite thickness in the fiber exceeded significantly the long period. The researchers proposed a bridging model that assumed the presence of a large number of straightened connecting molecules (SCM) joining like bridges successive crystallites along the fiber axis in order to explain this obvious dimensional disparity. Subsequently, this model was confirmed in other x-ray investigations [2-5] and independently from results of low-frequency Raman spectroscopy [6-8]. The bridging model was used successfully to interpret several mechanical properties of highly oriented UHMWPE fibers, in particular, to explain the behavior of Young’s modulus during various sample-preparation procedures and various test conditions [9 and references therein]. The model turned out to be useful also for understanding the solid-state extrusion process [9]. Nevertheless, little reliable information about the formation of intercrystallite bridges in drawn fibers is available in terms of a general concept of the oriented state. The goal of the present work was to study the formation of SCM-bridges in gel-spun UHMWPE fibers. Hot drawing of fibers from a spinning solution is usually carried out in several stages in industry. Each stage has a different special structural organization of drawn fibers that can be characterized using instrumental methods. Herein we present results from a study of the formation of intercrystallite bridges in fibers drawn at the pilot plant (SRISF, Tver´). Fibers that were obtained at various stages of oriented drawing were studied using low-frequency Raman spectroscopy and wide-angle x-ray scattering. Samples were prepared from UHMWPE reactor powder with crystallinity degree 64% (according to differential scanning calorimetry) and molecular weight 2.7"10 6 g/mol (according to viscosity measurements). The reactor powder and drawing temperature—time regime were chosen based on previous results [10, 11]. The drawing process included the following successive stages. Reactor powder was dissolved with stirring in mineral oil at 172°C. The resulting homogeneous solution of concentration 3 mass% was forced at the same temperature by a dispensing six-channel pump through six spinnerettes with 48 openings of diameter 0.5 mm each. The ratio of capillary length to diameter was 5. The solution from the spinnerette head was fed into a cooling bath filled with mineral oil at 43.0 ± 1.5°C. The fluid lost its flowability and converted into a gel as a result of phase separation during such rapid cooling. The spun gel-threads were continuously transferred from the cooling bath to a mineral-oil bath at 70°C. Then, the threads were freely placed in a container where they were shrunk to the equilibrium state.

Preparation of UHMWPE/MMT Nanocomposites via Mechanical Alloying&lt;sup&gt;&lt;/sup&gt;

The ultra high molecular weight polyethylene (UHMWPE) has a molecular mass of the order of a millions of grams per mole. So, UHMWPE presents prominent properties, for instance, abrasion and impact resistance. However, due to its very high viscosity in the melt state, the preparation of composites by conventional extrusion and injection molding is not possible. Therefore, in this work we studied the possibility of incorporating montmorillonite (MMT) into the matrix of the UHMWPE in the solid state, via high energy milling. The formulations were prepared in Attritor mill by milling UHMWPE reactor powder and MMT. The samples were characterized by bulk density, XRD, AFM and SEM. Results show that this route of processing was effective to incorporate MMT into the matrix of UHMWPE.

Solid-state 1H-NMR relaxation behavior for ultra-high-molecular-weight polyethylene reactor powders with different morphologies

The relaxation behaviors of several commercial ultra-high-molecular-weight polyethylene (UHMW-PE) reactor powders were compared. The surface and internal morphologies of these powders were characterized by scanning and transmission electron microscopy observations. On the basis of these morphological analyses, the reactor powder consisted of particles and fibrils between them, the relative amounts of which depended on the powder preparation conditions. Molecular motions were detected by solid-state proton nuclear magnetic resonance (1H-NMR) techniques. The temperature dependence of the molecular motion characterizes the structural differences between these powders. The intermediate region between crystal/amorphous phases first relaxes during gradual heating from room temperature for all reactor powders examined in this study. Such a release of constraint for boundary chains induces structural reorganization beyond the critical temperature before melting. This trend was also confirmed by an increase in crystallinity beyond the critical temperature. In contrast, the results of differential calorimetry (DSC) analysis could not distinguish these powder characteristics because of the remarkable reorganization of these reactor powders during heating scans. A relationship between the morphological and 1H-NMR relaxation characteristics was interpreted for several reactor powders. The relaxation behaviors for several commercial ultra-high molecular weight polyethylene reactor powders were compared. On the basis of the morphological analyses, the reactor powder consisted of particles and fibrils between them, the relative amounts of which depended on the powder preparation conditions. The results of differential scanning calorimetry analysis could not distinguish these powder characteristics because of the remarkable reorganization of these reactor powders during heating scans. In contrast, molecular motions detected by solid-state proton nuclear magnetic resonance techniques characterize the structural differences between these powders.

Solid-State 1H NMR Relaxation Analysis of Ultrahigh Molecular Weight Polyethylene Reactor Powder

Solid-echo 1H NMR relaxation was analyzed for ultrahigh molecular weight polyethylene reactor powders. Their high crystallinities reveal the characteristics of crystalline free induction decay (FID). We propose here a very simple but appropriate fitting function for the crystalline component: combination of Weibullian and sine functions. The applicability of this function was tested for the series of samples having much higher crystallinity of over 90%, which was achieved by fuming nitric acid etching at room temperature over a period of 1 year. The typical beat at 20−30 μs on the observed FID was further emphasized by prolongation of etching time, which corresponds to “crystalline” relaxation. An introduction of the simple Weibullian/sine function for FID fitting reproduces well this beat profile. On the other hand, the FID relaxation of the amorphous chains could be represented by the single Weibullian function. However, they were classified into two categories: intermediate and mobile, depending on their relaxation characteristics. Among these three components (crystalline, intermediate, and mobile ones) exhibiting different chain motions, the intermediate amorphous relaxation was an effective guide for characterization of phase arrangements in our powder samples. The component percentage of this phase decreased with prolongation of the etching time, which was coincident with the crystallinity development detected by thermal analysis. Also, the spin−spin relaxation time of intermediate amorphous chains gradually decreased with the progress of the etching treatment. These results indicate that the molecular motion of the surviving intermediate amorphous phase is restricted. A comparison of morphological changes and these FID characteristic transformations obtained during etching suggested that such an intermediate amorphous phase is located in the region between crystalline domains within the powder globules.

Structural Defects Generated upon Two-Stage Solid-State Coextrusion Drawing of Ultrahigh Molecular Weight Polyethylene Reactor Powder.

A film of ultrahigh molecular weight polyethylene reactor powder, prepared by compaction below the melting point, was drawn by two-stage coextrusion at 110-130°C to a total extrusion draw ratio (EDRt) of 24. EDRt is a product of the EDRs of the two stages. Two kinds of flaws were observed; kink bands running at an angle of 30-60° to the fiber axis and periodic variations in the sample thickness along the fiber axis. The effects of extrusion variables on the formation of these flaws were studied by polarized light microscopy, atomic force microscopy (AFM) and wide-angle X-ray diffraction recorded with normal beam (diameter; φ = 0.5mm) and micro-beam (φ = 10 or 100μm). The number of flaws of both types increased with decreasing coextrusion temperature and/or decreasing die EDR used for the second-stage coextrusion. It was found that the kink bands were generated by the spring back associated with the die swell. In contrast, the thickness variation along the draw axis was found to form within an extrusion die during the second-stage draw. The effect of each of the two kinds of flaws on the stress/strain behavior is also discussed.

Ultradrawing of ultrahigh molecular weight polyethylene reactor powders prepared by highly active catalyst system

Drawability of ultrahigh molecular weight polyethylene (UHMW-PE) reactor powders, prepared by using a highly active Ziegler type catalyst, has been studied as functions of catalyst activity, polymerization temperature, monomer pressure and sample molecular weight. Compacted powder films of UHMW-PE, prepared in a 1000-fold range of polymerization rates, were drawn by a two-stage technique; i.e. the initial solid-state coextrusion at 110°C followed by tensile drawing at 135°C, some 5°C below the melting point. It was found that an appropriate range of molecular weights (Mv=1.6–3.0×106) existed for achieving a high draw. The UHMW-PE prepared even at a high polymerization rate of 43,600 (g of PE)/(mmol of transition metal,h) could be drawn to draw ratios (DR) over 50, producing tapes with tensile modulus and strength over 100 and 1.0GPa, respectively. This level of catalyst activity is sufficient for the modern industrial production of UHMW-PE. The reduction of polymerization rate, by lowering monomer pressure and polymerization temperature, produced UHMW-PE with improved ductility. Furthermore, at the same polymerization rate, the ductility of reactor powder was improved more significantly by lowering the former parameter than the latter, reflecting the characteristic effect of each of these polymerization variables on the morphology formation during polymerization. Thus, the reactor powders prepared at 40–60°C and monomer pressure of 1atm could be drawn to DR's of 100–125. Such highly drawn products exhibited tensile moduli and strength up to 144 and 1.6GPa, respectively.

Chain Mobility in Polymer Systems: On the Borderline between Solid and Melt. 3. Phase Transformations in Nascent Ultrahigh Molecular Weight Polyethylene Reactor Powder at Elevated Pressure As Revealed by in Situ Raman Spectroscopy

The phase transformations under elevated pressure have been followed in situ by X-ray and Raman spectroscopy for ultrahigh-molecular-weight polyethylene (UHMW−PE) reactor powder. Using the in situ X-ray as a reference for the Raman work, it has been shown that Raman spectroscopy is a convenient method to follow phase transformations in polyethylene under pressure. The distinction has been made between different solid phases in polyethylene (monoclinic, orthorhombic, and hexagonal phases) and the melt by studying the conformational changes occurring within the skeletal mode (1000−1200 cm-1) upon the phase transformations. The crystal field splitting (1415/1440 cm-1) in the −CH2− bending mode of the Raman spectrum of polyethylene, related to the orthorhombic unit cell, was confirmed to be sensitive to transformations between different crystal structures. By following the changes occurring in the methylene bending mode together with the changes within the C−C skeletal mode, it has been shown that the crystal...

Flaws Generated upon Two-Stage Solid-State Coextrusion of Ultrahigh Molecular Weight Polyethylene Reactor Powder.

Flaws Generated upon Two-Stage Solid-State Coextrusion of Ultrahigh Molecular Weight Polyethylene Reactor Powder.

Real-Time X-ray Diffraction Study on Two-Stage Drawing of Ultra-High Molecular Weight Polyethylene Reactor Powder above the Static Melting Temperature

The polyethylene for the study was ultra-high molecular weight polyethylene (UHMW-PE) reactor powder extrusion drawn to a low draw ratio (DR) of ∼6 (first-stage draw). The structural change during the second-stage tensile draw of the extrudate has been studied above the static melting point by real-time wide-angle X-ray diffraction (WAXD). Information on the phase structure was also obtained by WAXD as a function of sample DR, temperature (Td), and applied tensile load and by retractive stress measurements. Upon heating the extrudate with the fixed ends in an X-ray high-temperature chamber, part of the initially orthorhombic crystals transformed into the hexagonal crystalline and amorphous phases at ≥150 °C, indicating that the retractive stress was not homogeneously distributed within a drawn sample. Such a transformation temperature increased with sample DR. When draw was initiated at a constant Td of 150−155 °C, the chain orientation in both the hexagonal and the amorphous phases rapidly increased, and...

Rolling and roll‐drawing of ultrahigh molecular weight polyethylene reactor powders

AbstractUltrahigh molecular weight polyethylene (UHMWPE) reactor powders have been found to be processable in the solid state by the techniques of rolling and roll‐drawing. Plates of compacted UHMWPE reactor powder were prepared below their melting points. These plates were then rolled at 124°C. The maximum uniaxial draw ratio (DR) obtained by multiple rolling was about 10. In additional experiments, rolled plates of a DR of 6 were further drawn by tensile stretching at a temperature of 135°C. The specimens prepared by rolling and by the two‐stage draw were characterized by tensile measurements, differential scanning calorimetry (DSC), and X‐ray diffraction. Results show that, on rolling alone, the tensile modulus and tensile strength achieved were 3 GPa and 42 MPa, respectively, at a DR of 9.6. The rolled plates were effectively drawn further to a total DR of 86. Such highly drawn films exhibited tensile moduli and tensile strength up to 81 and 1.3 GPa, respectively. A high crystallinity and high crystal orientation were also obtained by the two‐stage draw.

Two-stage drawing of ultra-high molecular weight polyethylene reactor powder

A two-stage drawing technique has been successfully applied for drawing of ultra-high molecular weight polyethylene reactor powder. Compacted powder films prepared below the ambient melting point of the initial powder was solid-state coextruded to a low extrusion draw ratio of 6 followed by tensile drawing at elevated temperatures of 120–135°C. The drawability for the second-stage tensile drawing and the uniformity of the resultant ultradrawn films were significantly affected by the drawing temperature and rate. The initial powder and drawn films were characterized by the tensile measurements, differential scanning calorimetry (d.s.c.) and X-ray diffraction. The results show the characteristic morphology of the reactor powder. Thus, the compacted powder films were effectively drawn up to a draw ratio of 77. Such highly drawn films exhibited tensile moduli up to 107 GPa. As the samples fractured by a repetitive partial rupture at the edge of cramp, the observed tensile strength at break were fairly low and in the range 1.0–1.4 GPa at a draw ratio range of 35 to 77.