Microstructure Of Polyethylene Research Articles

The synthesis of asymmetrical anilinotropone nickel and palladium complexes for olefin polymerization

For the Coordination-insertion olefin polymerization, the development of high-performance transition-metal catalysts is always the key issue and extensive efforts have been made in this field. In this contribution, we designed several asymmetrical anilinotropone nickel and palladium catalysts. Compared to palladium complex, the nickel catalysts exhibited high activity in ethylene polymerization. The topology microstructure of polyethylene could be modulated by different polymerization temperature. The anilinotropone nickel catalysts Ni2 and Ni3 in this work demonstrated higher thermolability, generating polyethylene with higher molecular weight (up to 285.9 kg mol−1). Moreover, these nickel systems also promoted the copolymerization of ethylene with norbornene-type polar monomers (NB–COOMe, NB-OAc), which demonstrated great polar group tolerance in this system. The generated copolymer exhibited high molecular weight (up to 93.1 kg mol−1) and high polar monomer incorporation ratio (up to 6.9 mol%). The design of asymmetrical anilinotropone catalysts in this work presented a unique form in olefin polymerization and may attract wide application interests in metal catalysis areas.

Regulating the polyethylene microstructure by increasing steric crowding in naphthoxy imine-ligated Ni(ii) complexes

Here we report the syntheses of systematically sterically tailored naphthoxy imine-ligated nickel complexes and, their performance in ethylene polymerization. Sterically less bulky Ni1 and Ni2 produced highly branched (43–54) PE, while the bulkiest Ni3 suppressed branching (only 28).

Wear Resistance and Microstructure of Ultra High Molecular Weight Polyethylene During Uniaxial Tension

Wear Resistance and Microstructure of Ultra High Molecular Weight Polyethylene During Uniaxial Tension

Evolution of the microstructure of amorphous polyethylene under friction-induced plastic flows: A reactive molecular investigation.

The plastic flow of ultra-high molecular weight polyethylene (UHMWPE) at a frictional interface, which is critical to the wear behavior, was investigated by reactive molecular dynamics simulations. The UHMWPE substrate was found to experience various deformations during the friction process. First, some polyethylene (PE) chains could detach from the substrate because of their rapid movement. Second, the frequent motion of PE chains also resulted in the intermittent formation and breaking of cavities between intermolecular PE chains. These deformations were more obvious on a surface with a convex protrusion, where the plowing effect exacerbated the cavitation and elastic deformation of PE chains. Correspondingly, the plastic flow in turn reconstructed the convex protrusion by displacing the surface atoms on the Fe slab. The plastic flow of PE chains broke the C-C bonds, and the carbon moieties were then chemically bonded onto the metal surface. A rapid change of atomic charge, hence, happened when the bonds broke. Meanwhile, PE chains release short alkyl radicals gradually after bond breakage, indicating gradual wear of the substrate during friction. This work provides molecular insight into the evolution of interfacial microstructure under plastic flow on a UHMWPE substrate.

Critical insights into ethylene‐1‐alkene copolymers and long‐chain branched polyethylene microstructure on thermo‐rheological and sealing behavior of multicomponent systems

AbstractThe microstructure‐property relationships in multicomponent ethylene‐1‐alkene copolymers with different branching in the microstructures are demonstrated. The metallocene catalyzed linear low density polyethylene (mLLDPE), was miscible with both autoclave grade low density polyethylene (LDPE‐A) and/or tubular grade low density polyethylene (LDPE‐T). For these multicomponent systems, the rheological response was distinctly differentiating and sensitive to the microstructure of LDPE, at higher shear regimes. The thicker lamellae of LDPE‐T and/or LDPE‐A might co‐crystallize if there is a high density polyethylene‐like fraction present in the mLLDPE. Even though the macro parameters like density and melt index (MI) of the investigated multicomponent systems are comparable, the subtle differences in the microstructure manifested by type and distribution of comonomer and/or branching affected the sealing performance. Both high comonomer content and comonomer distribution in the mLLDPE matrix affording a higher fraction of material melting below 120°C were found to be critical for the heat sealing. The fraction of material melting at lower temperatures, attributed to the tertiary branches present in the hyper‐branched microstructure of LDPE‐A, participate in the sealing process, and lower the sealing temperature. It was evident that mLLDPE with asymmetric distribution of lamellae is more sensitive to the microstructure of the LDPE used.

Unexpected Effect of Catalyst’s Structural Symmetry on the Branching Microstructure of Polyethylene in Late Transition Metal Polymerization Catalysis

Unexpected Effect of Catalyst’s Structural Symmetry on the Branching Microstructure of Polyethylene in Late Transition Metal Polymerization Catalysis

Laboratory performance of recycled polyethylene modified epoxy asphalt binders

ABSTRACT This paper deals with the impact of polymer loading on the microstructure, viscosity, thermal properties and mechanical performance of recycled polyethylene (rPE) modified epoxy asphalt binder. With this aim, rPE was used as a modifier to improve the toughness of a hot-mix epoxy asphalt binder. rPE particles were found to be highly swollen by the asphalt. Double phase separation was observed in rPE modified epoxy asphalt binders: asphalts dispersed in the continuous epoxy phase and rPE particles distributed in the discontinuous asphalt phase. The viscosity of epoxy asphalt binders increased with more rPE contents but the allowable construction time for pavement remained long. Results revealed that the thermal stability of epoxy asphalt binders was slightly improved with the addition of rPE. Furthermore, rPE lowered the glass transition temperature and increased the storage modulus at the rubbery state. All rPE modified epoxy asphalt binders performed better damping behaviours. In addition, at low rPE concentrations, the mechanical properties of epoxy asphalt binders were improved while at rPE loading higher than 2%, the results were opposite.

Exploring the Relationship between the Polyethylene Microstructure and Spatial Structure of α-Diimine Pd(II) Catalysts via a Hybrid Steric Strategy.

The branching density of polyethylene generated in the α-diimine Pd(II) system is usually very high, largely independent of simple ligand modifications with steric or electronic perturbations, or the polymerization conditions. In this study, we designed and synthesized a class of bulky hybrid α-diimine Pd(II) catalysts combining ortho-diarylmethyl and ortho-phenyl moieties to explore the relationship between the polyethylene microstructure and the spatial structure of catalysts. In ethylene polymerization, the hybrid α-diimine Pd(II) catalysts exhibited high activities (well above 105 g·mol-1·h-1) and yielded highly branched (90-110/1000C) polyethylenes with high molecular weights (up to 278.3 kg/mol). Compared with the two corresponding symmetrical ortho-diarylmethyl-based or ortho-phenyl-based Pd(II) catalysts, the hybrid catalysts generated polyethylene of significantly higher branching densities (92 vs 28-34/1000C) in marked higher activities. Similar phenomena are also observed in the copolymerization of ethylene with polar monomers. Moreover, the hybrid Pd(II) catalysts can more efficiently promote the copolymerization of ethylene with various polar monomers in comparison to the corresponding symmetrical catalysts. The more open spatial environment around the metal center by using a hybrid steric strategy was proposed to be responsible for above advantages.

Effect of Processing Techniques on the Microstructure and Mechanical Performance of High-Density Polyethylene.

The versatility of high-density polyethylene (HDPE) makes it one of the most used polymers for vast applications ranging from food packaging to human implants. However, there still is confusion regarding the proper selection of processing techniques to produce HDPE specimens for high-end applications. Herein, we compare the processing of HDPE by two relevant techniques: compression and injection molding. The fabricated samples were studied using uniaxial tensile testing to determine their mechanical performance. Furthermore, the microstructure of samples was analyzed using different characterization techniques. Compression-molded specimens recorded a higher degree of crystallinity (DC) using two different characterization techniques such as differential scanning calorimetry (DSC) and X-ray diffraction (XRD). With this information, critical processing factors were determined, and a general structure–property relationship was established. It was demonstrated that having a higher DC resulted in higher yield strength and Young’s modulus. Furthermore, premature failure was observed in the injection-molded specimens, resulting in lower mechanical performance. This premature failure was caused due to flow marks observed using scanning electron microscopy (SEM). Therefore, it is concluded that compression molding produces superior samples compared to injection molding.

Effects of a Twin-Screw Extruder Equipped with a Molten Resin Reservoir on the Mechanical Properties and Microstructure of Recycled Waste Plastic Polyethylene Pellet Moldings.

Each year, increasing amounts of plastic waste are generated, causing environmental pollution and resource loss. Recycling is a solution, but recycled plastics often have inferior mechanical properties to virgin plastics. However, studies have shown that holding polymers in the melt state before extrusion can restore the mechanical properties; thus, we propose a twin-screw extruder with a molten resin reservoir (MSR), a cavity between the screw zone and twin-screw extruder discharge, which retains molten polymer after mixing in the twin-screw zone, thus influencing the polymer properties. Re-extruded recycled polyethylene (RPE) pellets were produced, and the tensile properties and microstructure of virgin polyethylene (PE), unextruded RPE, and re-extruded RPE moldings prepared with and without the MSR were evaluated. Crucially, the elongation at break of the MSR-extruded RPE molding was seven times higher than that of the original RPE molding, and the Young’s modulus of the MSR-extruded RPE molding was comparable to that of the virgin PE molding. Both the MSR-extruded RPE and virgin PE moldings contained similar striped lamellae. Thus, MSR re-extrusion improved the mechanical performance of recycled polymers by optimizing the microstructure. The use of MSRs will facilitate the reuse of waste plastics as value-added materials having a wide range of industrial applications.

Thermal Effect of Short-term Overload on High-voltage Cross-linked Polyethylene (XLPE) Cable Insulation

This paper describes changes on micro-structure and aggregation structure of cross-linked polyethylene (XLPE) with different thermal histories after different times of overheating treatment to analogize the short-term fault or emergency overload in actual operating cables. 5, 10 and 20 times overheating treatment were conducted on a spare cable and a retired cable with service years of 32, whose insulation were analyzed by the diagnostic measurements of Fourier transform infrared spectroscopy (FTIR), differential scanning calorimetry (DSC) and thermogravimetry (TG). The results show that the spare cable has presented a trend of improvement on the insulation properties after overheating treatment below 20 times, but slight deterioration occurs after 20 times overheating treatment; the retired cable with service years of 32 has presented a trend of degradation on the insulation properties at the first 5 times overheating treatment, and a severe deterioration occurs after 20 times overheating treatment.

Amorphous to high crystalline PE made by mono and dinuclear Fe-based catalysts

We designed a series of dinuclear structures (F1–F5) bearing different linkers (rigid to flexible) between the Fe centers to investigate that how they could control the catalyst behaviour and polymer properties. The dinculear structures were used for polymerization of ethylene in presence of MMAO and TiBA. Based on initial results, catalyst F2 containing methyl-substituted phenyl bridge owned the highest activity (4.9 × 106 g PE/mol Fe.h) through dinuclearity and optimum bulkiness among the structures studied. This performance was along with the greatest crystallinity (χc) of PE made by the catalyst. Polymerization at higher temperature and monomer pressure exhibited high thermal stability and performance of catalyst F2, respectively. For further structures, decreasing of effective electronic and steric features led to lower activity. In addition, not only catalyst F4 bearing the short ethylene bridge exhibited the lowest productivity, but also produced the PE containing high level of short chain branches (37.2 SCB/1000C) affording a branched microstructure of polyethylene in presence of TiBA. It could attribute to low steric and electronic effects and short distance between the centers (ethylene linker) that made it suitable and active for SCB formation. Regarding to it, the electrophilicity index (ω) of F4 also was greater that led to high capacity or propensity of the specie to accept the (macro) monomer. For this sample, virtually no χc observed in DSC and SSA thermograms. Moreover, MMAO acted as an effective cocatalyst in comparison to TiBA, according to the kinetic profiles of ethylene polymerization. The observations were in respect of strength and affinity of the cocatalysts in deactivation and reactivation of the centers at prolonged time.

Effect of Doping Microcapsules on Typical Electrical Performances of Self-Healing Polyethylene Insulating Composite

Polyethylene cables, as important transmission equipment of modern power grid, would inevitably be slightly damaged, which seriously threatens the safety of the power supply. This paper has pioneered the preparation and typical performances of a self-healing polyethylene insulating composite. The self-healing performance to structural damage was verified by tests of electrical and mechanical damage. The effect mechanism of doping microcapsules on the electrical performance of polyethylene was emphatically analyzed. The results show that in appropriate conditions (such as 60 °C/30 min), the composite can not only repair the electrical tree and scratches, but also restore the insulation strength of damaged area. The effect of doping microcapsules on the electrical performances of polyethylene, such as breakdown strength, volumetric resistivity, dielectric properties, and space charge characteristics, are mainly related to impurity and the interface of microcapsule. Polarization and ionization of impurities can reduce the electrical performance of polyethylene. The interface not only improves the microstructure of polyethylene (such as how the heterogeneous nucleation effect increases the number of crystal regions, and the anchoring effect enhances the stability of amorphous regions), but also increases the charge traps. Moreover, the microstructure and charge trap can affect the characteristics of carrier transport, material polarization, and space charge accumulation, thus improving the electrical performance of polyethylene. In addition, the important electrical performance of the composite can meet the basic application requirements of polyethylene insulating material, which has good application prospects.

Plasticizing effect of oxidized biodiesel on polyethylene observed by nondestructive method

This paper explores the compatibility of biodiesel with different grades of polyethylene, specifically examining the extent of plasticization in order to gain a better understanding of the biofuel’s compatibility with this common polymer. Its bulk influences on polyethylene were investigated by gravimetric and mechanical testing, and by the application of a newly developed nondestructive ultrasonic testing method. Diffusion rates and the extent of plasticization by biodiesel were compared to results obtained with toluene, a known plasticizer for polyethylene. Mechanical and gravimetric analysis showed that biodiesel exhibited bulk attributes of a plasticizer for the tested polyethylenes with reduced moduli proportional to the amount of fuel uptake and that uptake was inversely proportional to crystalline content of the polymer. Based on uptake amounts, the efficiency of biodiesel as a plasticizer towards polyethylene was found to be more than double that of toluene. However, spectral analysis by ultrasonics showed that absorbed toluene and biodiesel influenced the microstructure of polyethylene differently. Notable differences in internal stresses were noted between the two fluids for the same amount absorbed. A subsequent study analyzed the impact that biodiesel degradation had on plasticization. Although the trend showed a slight change in diffusion rate with increasing oxidation of the medium, the mechanical and ultrasonic results did not show significant differences between fresh and degraded biodiesel within the 45 days span of the test. Combining the evidence observed in this study, a mechanism is proposed for biodiesel plasticization that can help with failure prevention and material selection.

Relation between life time, failure stress and craze microstructure in polyethylene as evidenced by fracture surface texture analysis after an accelerated Full-Notch Creep Test

A set of five polyethylenes, of which four have 1-butene as co-monomer and one has 1-hexene as co-monomer was evaluated to determine their respective strain hardening values at 80 °C and to determine their failure times applying an accelerated Full-Notch Creep Test at 90 °C with loads varying between 1 and 6 MPa using Lauramine Oxide as detergent in an aqueous solution. The resulting failure times, fracture surface analysis and strain hardening values were described using the Kramer-Berger theory for craze propagation and the Paris-Erdogan phenomenological description of crack growth. From this exercise it is clearly demonstrated that the experimental findings are in line with the Kramer-Berger model. Hence, the viability and validity of these tests for probing the slow crack growth performance of polyethylene grades was shown.

Design and testing of high‐density polyethylene nanocomposites filled with lead oxide micro‐ and nano‐particles: Mechanical, thermal, and morphological properties

ABSTRACTThis research discusses the mechanical behavior and the microstructure of high‐density polyethylene (HDPE)‐based composites, manufactured using the melt‐mixing and thermal‐pressing techniques, where HDPE is mixed with various percentages of either bulk lead monoxide (bulk PbO) or PbO nanoparticles (PbO‐NPs) acting as fillers. The scanning electron microscope and the field emission transmission electron microscope were utilized to identify the morphology of polymeric composites. Both showed the proper dispersion of PbO in the HDPE matrix without substantial agglomerations. The effect of PbO on the thermal behavior of the HDPE was studied using the thermogravimetric analysis. Tensile tests were implemented to find out how the mechanical characteristics of the composites were affected. Yield stress, % elongation at break, stiffness, tensile energy (toughness), ultimate tensile strength, and ultimate tensile strain were elucidated in this work. The values of stiffness, ultimate tensile strength, and yield stress increased by increasing either the bulk PbO or PbO‐NPs' loading up to 40.0 wt % with reference to the hosting matrix. The values of ultimate tensile strain, tensile energy, and % elongation at break of the assembled composites diminished dramatically by increasing the filler's content from 10.0 to 50.0 wt %. Besides, composites with PbO‐NPs as a filler were identified as having higher mechanical characteristics than those with bulk PbO for the same wt %. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci.2019,136, 47812.

Active site selectivity of 2,3-Bis[(2,6-diisopropylphenylimino)butane] nickel/MAO/ZnEt2 system toward ethylene polymerization for modulating polyethylene microstructure

2,3-Bis[(2,6-diisopropylphenylimino)butane] nickel/MAO system toward ethylene polymerization displays two active sites with ultraviolet absorption bands at 456 nm and 492 nm to produce branched polyethylenes, respectively. It is surprising that adding ZnEt2 results in selective catalyst active site to modulate polyethylene microstructure. 200 equiv. of ZnEt2 causes a more than 30% decrease of branching degree from 140 to 97. The UV,EPR and CV results suggest that the active site at 456 nm is easily reduced by ZnEt2. However, the active site at 492 nm still exhibits high catalytic activity, producing polyethylenes with a relatively low branching degree.

Effects of thermo-oxidative aging on structure and low temperature impact performance of rotationally molded products

In this work, the effects of thermo-oxidative aging on the microstructure and impact properties of polyethylene during the rotational molding process were studied. FTIR and Raman analysis showed that the degradation of polyethylene was responsible for the low-temperature impact strength (LT-IS) and the brittle fracture behavior of samples those subjected to prolonged heating time during rotational molding process. The DSC and polarizing microscope analysis showed that the degradation process was delayed by the addition of antioxidants. Through use and optimization of antioxidants, the thermo-oxidative aging of polyethylene can be modulated, a broader processing window can be achieved, and the products may exhibit better mechanical properties with a wider heating time range.

Monitoring deformation mechanism of foam cells in polyethylene foams via optical microscopy: Effect of density and microstructure

The effect of cellular microstructure and density of low-density polyethylene (LDPE) foam during the compressive deformation were evaluated. Progressive foam cell structure deformation at crosshead speed of 10 mm/min was closely monitored and captured using a portable digital microscope with the assistance of a proposed coating technique to enhance contrast of the foam cells. Results revealed that higher density foam, LDPE1.0, experienced higher compressive stress than the LDPE0.5 foam. It was believed that thicker cell walls in the LDPE1.0 foam contributed to the foam stiffness and higher stress was required to impose cell bending, buckling, and collapse. It was also discovered that the existence of larger cells within a group or cluster of small cells in the LDPE1.0 foam acted as stress concentration points and initiated the formation of shear bands in the foam cells deformation mechanism. The extent of compressive strain on the foam cells were also investigated, and results showed that cells undergo lower compressive strain compared to the actual deformation level given directly by the machine crosshead. Cells located in the middle section of the foam recorded a strain of about 13% when the bulk foam was deformed at 20% strain. The success in capturing the actual cell strain using the proposed coating technique proved that with the right improvisation, the micromechanical deformation of optically challenged white polyethylene foam samples can be monitored effectively using optical microscopy.

Correlating polyethylene microstructure to stress cracking; correlations to post yield tensile tests

From experimental size exclusion chromatography/infrared detection and differential scanning calorimetry data coupled with random walk statistics, the primary structure parameter (PSP3*) and the average number of crystals per chain (ANCC) were estimated and used to correlate microstructure effects of various high density polyethylenes to their measured natural draw ratio (NDR) and strain hardening modulus (SHM). Results suggest that while both the NDR and SHM reflect the extent of connectivity between crystalline lamella, the SHM reflects the strength of those bridging entanglements and tie molecules. Based on this view and supporting stress strain curves, different but related mechanisms for SHM were suggested for homopolymers versus copolymers. Moreover, a delineation between the effects of branch type was suggested. A “crystal tie mesh (CTM)” model that views the system to be a type of network formed by “crosslinking” multiple bridging and entangled tie chains through common crystalline lamella was proposed. In this semi-empirical model, rather than a rigorous development, basic scaling arguments using the PSP3* and ANCC parameters along with branch length factors (fB) were used. A linear correlation (R2 = 0.9615) was found between the SHM and the structural relationship, fB (PSP3*3/ANCC), for samples used in this study which included monomodal and bimodal resins made using various catalyst types.