Catalysis
Hideki Kurokawa; Shou Kikuchi; Hiroshi Miura
Abstract
MAO-free supported catalysts for ethylene polymerization were prepared by sequentially treating montmorillonites (MMTs) with R₃Al and then with zirconocene. MMTs possessing different physicochemical properties—such as surface area, acidity, and crystallinity—were employed for catalyst preparation. ...
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MAO-free supported catalysts for ethylene polymerization were prepared by sequentially treating montmorillonites (MMTs) with R₃Al and then with zirconocene. MMTs possessing different physicochemical properties—such as surface area, acidity, and crystallinity—were employed for catalyst preparation. Three zirconocenes, Cp₂ZrCl₂, (1,3-Me₂Cp)₂ZrCl₂, and Me₂Si(Cp)₂ZrCl₂, were used. Acid treatment of raw MMT (Na⁺-montmorillonite) increased the surface area while decreasing both the Al content and crystallinity. The maximum surface area (316 m² g⁻¹) was obtained when the residual Al content was 18%. The effects of triethylaluminum (TEA) and triisobutylaluminum (TIBA) on catalyst performance were investigated. Although treatment of surface OH groups was necessary to obtain the zirconocene-supported catalysts with high activity, no significant difference in catalytic activity was observed between TEA- and TIBA-treated MMTs. In contrast, the type of R₃Al used as a scavenger during polymerization strongly influenced catalytic activity: catalysts employing TEA exhibited much lower activity than those using TIBA. Since TEA is a stronger Lewis acid than TIBA, it likely coordinates more strongly to the active sites, thereby acting as an inhibitor. Therefore, TIBA was employed in all subsequent experiments. The catalytic activity, based on catalyst weight, increased with both surface area and the amount of supported zirconocene, reaching a maximum when the residual Al content was 54%. A linear correlation between the activity (per catalyst weight) and the amount of supported zirconocene was observed for two zirconocenes, Cp₂ZrCl₂ and (1,3-Me₂Cp)₂ZrCl₂, indicating that the supported zirconocenes functioned uniformly as active species for ethylene polymerization. However, 5–7 μmol g⁻¹-cat of zirconocene formed inactive species in both catalysts. Unusual behavior was observed for the supported Me₂Si(Cp)₂ZrCl₂ catalysts: at higher residual Al content, the correlation was similar to that of the other two catalysts, whereas at lower residual Al content, most of the supported zirconocene formed inactive species, and only a small fraction acted as active sites. We speculate that Me₂Si(Cp)₂ZrCl₂ is difficult to activate, and that only the species formed by reaction of strong acid sites with R₃Al can specifically activate this complex.
Characterization
Bahereh T. Marouf; Rasoul Lesan Khosh; Reza Bagheri; Yiu-Wing Mai
Abstract
Understanding the yielding of semicrystalline polymers such as isotactic polypropylene (iPP) remains challenging due to morphology-dependent deformation mechanisms and the sensitivity of yield stress to temperature, strain rate, and loading mode. Here, yield stress is measured across macro- (tensile, ...
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Understanding the yielding of semicrystalline polymers such as isotactic polypropylene (iPP) remains challenging due to morphology-dependent deformation mechanisms and the sensitivity of yield stress to temperature, strain rate, and loading mode. Here, yield stress is measured across macro- (tensile, compression) and micro-scales (nanoindentation) over a range of temperatures and strain rates. Nanoindentation-derived yield stresses obtained using the expanding cavity model agree closely with compression measurements, confirming the dominance of compressive fields, while Tabor-derived values correlate with tensile maximum stress. To rationalize the observed temperature and rate dependences, three theoretical frameworks—Eyring’s model, crystal plasticity, and the lamellar cluster model—are comparatively evaluated. The Eyring model captures the logarithmic strain-rate sensitivity and thermal softening but lacks structural specificity; the crystal plasticity model provides a slip-based interpretation with improved agreement at elevated temperatures; and the lamellar cluster model differentiates deformation modes, accounting for the convergence of compression and indentation yields. The combined experimental–modeling analysis demonstrates the utility of nanoindentation for localized yield assessment and highlights the model-dependent nature of structural interpretation in semicrystalline polymers.
Olefin polymerization and copolymerization
Teo Ming Ting; Ebrahim Abouzari Lotf; Mohamed Mahmoud Nasef; Wen Soong Lok; Thye Foo Choo; Nur Athilah Kamarudin; Ee Ling Aw; Noor Ashikin Mohamed
Abstract
Bicomponent polyethylene (PE)/polypropylene (PP) non-woven sheets made from sheath-core fibres were modified by radiation-induced grafting (RIG) of vinylbenzyl chloride (VBC) monomer in both emulsion- and solvent-mediated systems under varying conditions. The key grafting parameters, including reaction ...
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Bicomponent polyethylene (PE)/polypropylene (PP) non-woven sheets made from sheath-core fibres were modified by radiation-induced grafting (RIG) of vinylbenzyl chloride (VBC) monomer in both emulsion- and solvent-mediated systems under varying conditions. The key grafting parameters, including reaction medium, monomer concentration, absorbed dose, and temperature, were systematically investigated to control the grafting yield (GY%) and its distribution across fibres. The structural, morphological, and chemical properties of the resulting poly(VBC) grafted fibres were evaluated using Fourier transform infrared spectroscopy (FTIR), field emission scanning electron microscopy (FESEM) coupled with energy-dispersive X-ray spectroscopy (EDX), elemental analysis, X-ray diffraction (XRD), thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), and contact angle measurements. The results demonstrated that the reaction medium significantly influenced both the grafting yield (GY%) and graft distribution. Diffusion of poly(VBC) occurred similarly across the sheath and core; however, distinct differences in grafting rates and yields were observed between the emulsion and solvent systems. Under emulsion conditions, a higher density of poly(VBC) grafts was incorporated into the PE sheath than into the PP core compared to solvent-mediated grafting, whereas the side-chain grafts exhibited a generally homogeneous distribution throughout the bicomponent fibres in both systems. These findings demonstrate an effective approach for tuning the structure and morphology of PE/PP bicomponent fibres, favouring emulsion system, offering valuable insights for the design of advanced functional materials with promising applications in environmental remediation and electrochemical energy systems.
Polymer processing
Ahmed Hamdi
Abstract
The effect of hydrostatic pressure on the viscosity of polymer melts plays a critical role in injection molding processes, particularly under high-pressure conditions typical of thin-walled and micro-scale components. Despite its importance, pressure-dependent viscosity data are often incomplete or unavailable ...
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The effect of hydrostatic pressure on the viscosity of polymer melts plays a critical role in injection molding processes, particularly under high-pressure conditions typical of thin-walled and micro-scale components. Despite its importance, pressure-dependent viscosity data are often incomplete or unavailable in commercial simulation software, especially for semi-crystalline polymers such as polyolefins. In this study, the pressure dependence of viscosity for polypropylene (PP) and high-density polyethylene (HDPE) was investigated by analyzing rheological and volumetric parameters retrieved from the Moldflow material database. The coefficient describing the pressure effect on viscosity was evaluated using a thermodynamic approach based on polymer compressibility, thermal expansion, and the temperature dependence of zero-shear viscosity, and was compared with the corresponding parameter obtained from experimental rheological measurements embedded in the Cross–WLF viscosity model. A clear linear correlation was found between the estimated and experimentally derived pressure coefficients, with the former systematically exceeding the latter by a nearly constant proportionality factor. The proposed approach enables a simple determination of material parameters for injection molding simulations and contributes to improved prediction accuracy under high-pressure processing conditions
Composites and nanocomposites
Fatemeh Najarnia; Jalil Vahdati Khaki; Samaneh Sahebian; Ali Shajari
Abstract
As a novel class of lightweight conductive material, segregated polymer composites are widely regarded as their unique architecture enables the formation of electrical networks at exceptionally low filler loadings. In such configuration, the conductive phase is preferentially localized at the polymer ...
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As a novel class of lightweight conductive material, segregated polymer composites are widely regarded as their unique architecture enables the formation of electrical networks at exceptionally low filler loadings. In such configuration, the conductive phase is preferentially localized at the polymer microgranule boundaries, forming continuous channels for electron transport. This promotes superior performance, particularly in electromagnetic interference shielding. The influence of initial polyethylene particle size on electrical conductivity and electromagnetic interference shielding performance of polyethylene/carbon black segregated structured composites was studied in the present research. Composite samples were fabricated using polymer particles with varying size distributions (100–150, 150–250, 250–500, and mixed <500 μm). Characterization Results demonstrated that increase in initial polymer particle size enhances electrical conductivity; as an illustration, conductivity improved from 94.74 S/m at 105-150 μm to 163.31 S/m at 250-500 μm. In terms of shielding behavior, absorption ratio of 69-76%, confirms a dominant absorptive contribution in segregated structure regardless of initial particle size. The mixed-particle sample (<500 μm) exhibited the best overall performance, combining a high conductivity of 158.40 S/m with the highest total and absorption shielding effectiveness of 11.07 and 7.70 dB, respectively. The incorporation of finer particles not only improved interfacial adhesion among larger polymer particles, but also increased the number of “cages” that trap incident waves and prolong multiple scattering pathways.
Composites and nanocomposites
Guilherme Gama Simão; Clodoaldo Saron
Abstract
High-density polyethylene (HDPE) has been widely used in automotive industry for manufacture of fuel tanks due to their properties such as chemical stability, easy processability and low cost. However, the low barrier of the HDPE to evaporative emission of organic fuel is a deficiency. Thus, the fluoridation ...
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High-density polyethylene (HDPE) has been widely used in automotive industry for manufacture of fuel tanks due to their properties such as chemical stability, easy processability and low cost. However, the low barrier of the HDPE to evaporative emission of organic fuel is a deficiency. Thus, the fluoridation treatment or combination of the HDPE with layers of polar polymers of high barriers for hydrocarbon permeation have currently been the procedures usually employed. In present study, the aim was to evaluate the changes in the barrier property to fuel permeation of the HDPE through incorporation of organically modified montmorillonite clay Cloisite 30B, generating nanocomposites HDPE/Clay. It was verified that the barrier to the fuel permeation is significantly increased in the HDPE/Clay nanocomposites containing clay at 2 and 3 wt%, while thermal stability and mechanical properties of the material remain similar to the non-filled polymer. The properties performance of the HDPE/Clay nanocomposites allow its use to the manufacture of the fuel tanks, complying with current regulations for evaporative fuel emissions and proving advantages such as lower production cost and recyclability of the material when compared to the traditional methods using fluoridation treatment or multilayer with polar polymers.
