Linear High-density Polyethylene Research Articles

Closed-Loop Recyclable and Nonpersistent Polyethylene-like Polyesters.

ConspectusAliphatic polyesters based on long-chain monomers were synthesized for the first time almost a century ago. In fact, Carothers' seminal observations that founded the entire field of synthetic polymer fibers were made on such a polyester sample. However, as materials, they have evolved only over the past decade. This is driven by the corresponding monomers becoming practically available from advanced catalytic conversions of plant oils, and future prospects comprise a possible generation from third-generation feedstocks, such as microalgae or waste. Long-chain polyesters such as polyester-18.18 can be considered to be polyethylene chains with a low density of potential breakpoints in the chain. These do not compromise the crystalline structure or the material properties, which resemble linear high-density polyethylene (HDPE), and the materials can also be melt processed by injection molding, film or fiber extrusion, and filament deposition in additive manufacturing. At the same time, they enable closed-loop chemical recycling via solvolysis, which is also possible in mixed waste streams containing polyolefins and even poly(ethylene terephthalate). Recovered monomers possess a quality that enables the generation of recycled polyesters with properties on par with those of the virgin material. The (bio)degradability varies enormously with the constituent monomers. Polyesters based on short-chain diols and long-chain dicarboxylates fully mineralize under industrial composting conditions, despite their HDPE-like crystallinity and hydrophobicity. Fundamental studies of the morphology and thermal behavior of these polymers revealed the location of the in-chain groups and their peculiar role in structure formation during crystallization as well as during melting. All of the concepts outlined were extended to, and elaborated on further, by analogous long-chain aliphatic polymers with other in-chain groups such as carbonates and acetals. The title materials are a potential solution for much needed circular closed-loop recyclable plastics that also as a backstop if lost to the environment will not be persistent for many decades.

Synthesis of Network Biobased Aliphatic Polyesters Exhibiting Better Tensile Properties than the Linear Polymers by ADMET Polymerization in the Presence of Glycerol Tris(undec-10-enoate).

Development of biobased aliphatic polyesters with better mechanical (tensile) properties in film has attracted considerable attention. This report presents the synthesis of soluble network biobased aliphatic polyesters by acyclic diene metathesis (ADMET) polymerization of bis(undec-10-enyl)isosorbide diester [M1, dianhydro-D-glucityl bis(undec-10-enoate)] in the presence of a tri-arm crosslinker [CL, glycerol tris(undec-10-enoate)] using a ruthenium-carbene catalyst, and subsequent olefin hydrogenation using RhCl(PPh3)3. The resultant polymers, after hydrogenation (expressed as HCP1) and prepared in the presence of 1.0 mol% CL, showed better tensile properties than the linear polymer (HP1) with similar molecular weight [tensile strength (elongation at break): 20.8 MPa (282%) in HP1 vs. 35.4 MPa (572%) in HCP1]. It turned out that the polymer films prepared by the addition of CL during the polymerization (expressed as a 2-step approach) showed better tensile properties. The resultant polymer film also shows better tensile properties than the conventional polyolefins such as linear high density polyethylene, polypropylene, and low density polyethylene.

Estimation Method of Molecular Weight of Polyethylene using Terahertz Spectroscopy

This paper uses terahertz (THz) spectroscopy for the first time as a simple method for estimating the molecular weight of a polymer across a very wide range. The viscosity average molecular weight (Mv) was measured and estimated as the molecular weight. Currently, to measure the Mv, a sample is dissolved in a solvent then measured its inherent viscosity. However, this procedure is complicated and time-consuming in practice. Additionally, as the molecular weight increases, the measurement accuracy decreases if Mv of a sample is above 3 million. Therefore, a new simple method is desired. A linear high-density polyethylene sample with an Mv range of 0.1 to 7.4 million was used. THz absorbance spectra of the samples were measured from 50 to 600 cm-1 (1.5 to 18 THz) then the multivariable analysis technique was applied. Consequently, a very high correlation coefficient (R2 ≈ 1) between the estimated and the measured Mv values were obtained across the range of the samples. This method is essential and suitable for industrial applications.

The Long-Term Mitigating Effect of Horizontal Ground-Source Heat Exchangers on Permafrost Thaw Settlement

This study investigated the long-term effect of horizontal Ground-Source Heat Exchangers (GSHEs) on mitigating permafrost thaw settlement. In the conceptual system, a fan coil was used to chill the recirculating fluid in the linear High-Density Polyethylene (HDPE) ground loop system. A fully coupled thermo-hydro-mechanical finite element framework was employed to analyze multiphysics processes involved in the thaw settlement phenomenon. To investigate the sustainability of the system, a period of 50 years was simulated. Two operational modes were defined: one without and the other with HDPE. Different heat carrier velocities and inlet temperatures, and heat exchanger depths were examined to explore their effects on the thaw settlement rate. It was concluded that the proposed system can effectively alleviate the predicted permafrost thaw settlement over the study period. Moreover, the heat carrier temperature was found to have a prominent impact on the thaw settlement rate amongst other parameters.

Influence of weak Fe–N bond on iron catalyzed ethylene polymerization

The weak bond cooperativity effect in metal-ligand interactions plays an important role on the polymerization properties in late-transition metal based polyolefin catalysts. In this contribution, a class of iron (II) chloride complexes bearing pendant N-donor modified α-diimine ligands were synthesized and used as precatalysts for ethylene polymerization. The weak Fe–N bond cooperativity effect plays a critical role in α-diimine Fe-catalyzed ethylene polymerization. Upon activation with methylaluminoxane (MAO) or modified methylaluminoxane (MMAO) with the low [Al]/[Fe] molar ratio of 100, ortho-N-piperidyl substituted precatalysts resulted in moderate catalytic activities and highly linear HDPE (high-density polyethylene) with high molecular weights (Mw up to 1.8 × 105 g mol−1) and high Tm (up to 139 °C).

Pursuit of the correlation between yield strength and crystallinity in sintering-molded UHMWPE

Crystallinity (Xc) is a crucial crystallographic factor that related closely to mechanical properties in linear polyethylene or high density polyethylene (HDPE), whose molecular weight (MW) is at a 105 level. However, as a typically intractable-processing polymer, the structure-property relationship concerned Xc is still ambiguous for ultrahigh molecular weight polyethylene (UHMWPE) with MW﹥1.5 × 106. The reason is that high entanglement density hinders chain mobility in the course of UHMWPE crystallization, thus crystalline degree in UHMWPE molding parts is difficult to be artificially modulated within a wide scope. In this study, low chain-entanglement status of UHMWPE melt was achieved by (1) utilizing lowly entangled nascent UHMWPE synthesized via the single-site Ziegler-Natta catalyst, and (2) decreasing the sintered temperature. It is impressive that partial disentanglement reserved in the melt is favorable for obtaining higher crystallinity. Meanwhile, an isothermal crystallization treatment was additionally adopted to further improve the capacity of regulating crystallization after the traditional two-step sintering procedure. By a combination of varying the initial entanglement in nascent powders, sintering temperature, isothermal crystallization duration, and molecular weight of UHMWPE, the Xc of the compression-sintered products has been successfully adjusted in the relatively broad range of 29%≦ Xc ≦ 65%, which offered an opportunity to validly survey the correlation between mechanical property and crystallinity. Particular attention was paid to the yield strength. A linear master straight-line can be plotted as yield strength vs. Xc for all of the samples inspected, indicating crystallinity also plays a dominant role on mechanical performances of UHMWPE sintering parts, as same in the case of HDPE. However, the mechanical reinforcement induced by increment of Xc in UHMWPE is more efficient than in HDPE. The yield strength of UHMWPE is much higher than that of HDPE at the same Xc, indicating the amorphous phase or entanglements in UHMWPE is crucial to determine the mechanical property. These findings are of significance for guiding the manufacturing of high-performance UHMWPE materials via crystallization manipulation.

Effects of Liquid Paraffin on Dynamic Mechanical Properties of Linear High-Density Polyethylene

We found that the dynamic mechanical spectra of linear high-density polyethylene (HDPE) can be controlled by swelling using liquid paraffin (LP) at 100 and 110 °C. The relaxation strength of α rela...

Ni-Based Complexes in Selective Ethylene Oligomerization Processes

Linear alpha-olefins are widely used in the petrochemical industry and the world demand for these compounds increases annually. At present, the main method for producing linear alpha-olefins is the homogeneous catalytic ethylene oligomerization. This review presents modern nickel catalysts for this process, mainly systems for obtaining of one of the most demanded oligomer—1-butene—which is used for the production of linear low density polyethylene (LLDPE) and high density polyethylene (HDPE). The dependence of the catalytic performance on the composition and the structure of the used activated complexes, the electronic and coordination states of the nickel center was considered.

Effects of Modifiers and Binder Properties on the Performance of Asphalt Mixtures in the Hamburg Wheel-Tracking Device Test

The Hamburg Wheel Tracking Device (HWTD) test has been widely used in practice and reported to be successful in identifying hot mix asphalt (HMA) mixes that are prone to rutting and/or susceptible to moisture damage. This paper presents a comprehensive study aiming to offer informative references for pavement engineers to select modified asphalt materials with good moisture and rutting resistance. First, the impacts of these modifiers on the HWTD test results were investigated. Based on the degree of their improvement in the HWTD results, additives were classified into the following three grades: (1) the first grade including branched styrene–butadiene–styrene (SBS) and Gilsonite; (2) the second grade including linear SBS, high-density polyethylene, and polyphosphoric acid; and (3) the third grade including asphalt rubber (AR) and terminal blend (TB) asphalt rubber. In addition, the effects of modifier content on the Hamburg performance of the asphalt mixes were studied. It was found that higher modifier dosages do not necessarily result in the improvement of Hamburg performance. The results show that an optimal content existed for most additives, whereas a poor-performance dosage range (10.0 % - 18.0 %) existed for crumb rubber content in the AR. Finally, based on the results of the various test materials, the roles of different properties of modified asphalt binders on the Hamburg performance were also investigated. The healing, adhesive, viscosity, and elastic properties (HAVE) of modified binders were found to play different roles: the healing property of modified asphalt binder is a necessary factor; the adhesive property is a fundamental factor; the viscosity has a maximum limitation (3.5 Pa∙s at 135°C); and the elastic property due to the modification is the determining factor in achieving good Hamburg performance of modified asphalt mixtures.

Studying the effect of biosilver nanoparticles on polyethylene degradation

The current study is focussed on the silver nanoparticle (AgNP)-based degradation of polyethylene. Two kinds of polyethylenes were used—linear high-density polyethylene and branched low-density polyethylene. Owing to the demerits of chemically synthesized nanoparticles, the biological synthesis of AgNPs from Aspergillus oryzae was carried out. The AgNPs produced were used along with the culture broth for the degradation studies. This nanoparticle-based method for bioremediating the polyethylene proved to be successful since it could degrade 64.5% of low-density polyethylene (LDPE) and 44.4% of high-density polyethylene (HDPE) in 5 weeks. The action of nanoparticles on polyethylene wax emulsion caused the thinning of the fluid. The HDPE samples when subjected to FTIR exhibited bending and stretching of the C–H bonds which form the backbone of the linear polymer. The degraded LDPE showed formation of phenols, alcohols, ketones and other smaller compounds, indicating the breakdown of the branched plastic. The GC–MS studies of the nanoparticle-treated polyethylene revealed the liberation of esters, alcohols and alkenes in HDPE, and aldehydes, alkenes, cyano compounds, esters and alkanes in LDPE. The 100% concentration of the AgNPs-treated degradation products of polyethylene was found to be 34.36% toxic in Allium cepa when compared with 97.35% phytotoxicity of the polyethylene wax emulsion.

Barrier permeation properties of EVOH thin-film membranes under aqueous and non-aqueous conditions

Ethylene vinyl alcohol (EVOH) copolymers can provide a superior barrier to hydrocarbons and are increasingly being used in co-extruded geomembranes for geoenvironmental applications. These thin-films behave differently under different humidity conditions. This study investigated the permeation properties of toluene through two EVOH thin-films (32 mol% ethylene and 44 mol% ethylene) for both non-aqueous and aqueous solutions. The results of this study are used to gain a better understanding of the behaviour of the EVOH layer used in co-extruded geomembranes. The thin-film results are compared with published values for co-extruded linear low density polyethylene (LLDPE) and high density polyethylene (HDPE) geomembranes with an EVOH core. Permeation coefficients are presented over a range of contaminant concentrations from 25 ppm to 99% toluene based on almost five years of continuous testing and the effect of moisture is discussed. A number of EVOH thin-films were affected by humidity (i.e., where moisture diffused into the film) prior to diffusion testing under non-aqueous conditions. This observation led to an investigation of the effect of moisture uptake on the permeation of toluene under non-aqueous testing. In these cases, the 44 mol% thin-film had lower toluene permeation coefficient values than the 32 mol% thin-film. These values were similar to toluene permeation coefficient values from tests with aqueous solutions. When relative humidity was less than 60%, the 32 mol% had slightly lower permeability values than 44 mol% thin-film. However, even when affected by humidity, the permeability of both thin-films were considerably (two to three orders of magnitude) lower than previously observed in a water-saturated solution. Permeation of toluene from a 1/1 toluene/hexane solution was also examined for the 32 mol% EVOH thin-film at temperatures of 23–50 °C and results fit well with a conventional Arrhenius relationship of increasing Pg values with increasing temperature.

Novel insulation materials for high voltage cable systems

Cross-linked polyethylene (XLPE) has replaced oil-paper insulated systems as the primary solution for medium and high voltage AC cables decades ago, since they enable marginally higher operating temperatures and can be produced with high throughput and well-controlled extrusion technology [1]. The base for this insulation is polyethylene (PE), which can be crosslinked either with peroxide cure (involving thermal decomposition), or by grafting silane onto the polymer chains, and the use of moisture-based cure [2]. Cross-linking is deemed necessary, since commercially available, branched low-density PE (LDPE) has more significant melting at temperatures around 100°C and the material loses all of its mechanical stability. In contrast, linear high-density PE (HDPE) has a higher melting point and can achieve higher operating temperatures. But, although HDPE found some success in medium voltage cables, it has not managed to establish a foothold in the high voltage (HV) cable sector [3]. After decades of research, conventional XLPE is at the limit of its capabilities, as outlined below, and further development is bound to have diminishing returns. Further, the costs of large extrusion and catenary cross-linking manufacturing facilities and the costs and time of degassing larger cross-section HV and extrahigh voltage cables present significant sustainability issues for cable manufacturers.

Study on Blending Modification of Metallocene Linear Low Density Polyethylene

In recent years, with the rapid development of China's wire and cable industry, optical cable demand gradually increased, but the wire and cable water resistance is not good, and vulnerable to corrosion caused by the distortion of the optical signal, so the cable aluminum-plastic composite belt should be born. In this study, The aluminum-plastic composite tapes were hot-pressed with low density polyethylene (LDPE) or metallocene linear low density polyethylene (MLLDPE) and high density polyethylene (HDPE) to test their heat sealing strength and mechanical properties. The rheological study shows that the HDPE/MLLDPE blends and HDPE/LDPE blends have a linearity in the lgG'-lgω curve in the low frequency region, and the heterogeneous system is in the molten state. The tanδ value of the blends gradually increased with the increase of the MLLDPE content, which indicated that the viscosity of the blends became higher, which improved the process ability of blends. The TGA blend shows that HDPE has better heat resistance. The melting point of HDPE in HDPE/MLLDPE blends decreased with the increase of MLLDPE content, which indicated that the two had mechanical compatibility. In the HDPE/ LDPE blends, two melting peaks were found, which indicated that the HDPE and LDPE had different crystal structure compatibility, and they were more likely to crystallize separately.

Rheological properties of HDPE and LDPE at the low-frequency range under supercritical CO2

The effects of temperature, pressure (using helium as the pressurizing medium) and dissolved CO2 concentration on the rheological properties of linear high-density polyethylene (HDPE) and branched-chain low-density polyethylene (LDPE) were carefully investigated using a high-pressure rheometer. The results showed that at low pressure, the rheology of the PEs was sensitive to pressure increase. As the pressure increased to higher values, the rheology leveled off for HDPE and increased for LDPE. Under supercritical CO2, the plasticization of the dissolved gas contributed primarily to the thickening effect of pressure. Furthermore, the loss factors decreased with increasing pressure and increased with increasing CO2 pressure. Lastly, the effect of gas concentration was described by the Fujita-Kishimoto model on the assumption that the coupled effect of pressure and CO2 concentration could be separated.

Laser sealing of dissimilar polymers for manufacturing packaging products

Laser sealing of thin/dissimilar polymers has the potential to become a superior technique that could be applied to a range of general packaging products. Upon successfully applying laser sealing to such products could result to faster throughputs, shorter lead-times, lower energy consumption, and in turn, a competitive advantage for packaging product manufacturers. The work herein is focused on the physical laser-material interaction resulting from laser sealing a 42 μm high density linear polyethylene (HDLPE) film to a 1 mm polypropylene (PP) substrate. The paper also details a comparison of laser sealing with the conventional pressure/heat sealing process. The results showed that a 10.6 μm wavelength laser, the required parameters to produce an acceptable seal were: 60 W of laser power, 1.67 mm spot diameter, and a traverse speed of 300 mm/min using a double pass. Microstructural study showed sufficient bonding of the laser sealed region with an amorphous zone. Defects were also present in various areas when comparing the results of laser sealing to the conventional sealing process. Nonetheless, the peel strength tests showed laser sealing required up to 30% higher force than the conventionally sealed HDLPE film to PP substrate. A steady-state finite element model of the laser sealing process showed distribution of heat at the film/seal activation temperature. This confirmed the sealing temperature at the laser-polymer interaction zone to gain further understanding of the new process as a whole.

Processing of a LLDPE/HDPE pressure vessel liner by rotomolding

Biaxial rotational molding is a low-pressure process that uses a polymer powder to produce large and/or complex-shaped hollow thermoplastic goods using low cost molds, being very suitable for prototype production. In the present work, liner prototypes (Φ 78 × 280 mm) made of linear low density (LLDPE) and high density polyethylene (HDPE) blends were produced aiming to later use them as compressed natural gas type IV cylinders. The LLDPE/HDPE blend composition and the process parameters (temperature/time cycle, and rotation speed) were varied to evaluate their effect on blend miscibility and liner final properties, such as density and crystallization behavior. Blend composition and process parameters were found vital for the production of a homogeneous wall and for the liner to reach adequate characteristics for the intended product.

Solvent and co-catalyst dependent pyrazolylpyridinamine and pyrazolylpyrroleamine nickel(II) catalyzed oligomerization and polymerization of ethylene

The pyrazolylamine ligands, 2-(3,5-dimethyl-1H-pyrazol-1-yl)-N-(pyridin-2-ylmethyl)ethan-amine (L1a), 2-(3,5-di-tert-butyl-1H-pyrazol-1-yl)-N-(pyridin-2-ylmethyl)ethan-amine (L2a), 2-(3,5-dimethyl-pyrazol-1-yl)-ethyl-(1H-pyrrol-2-ylmethyl)-amine (L3a), 2-(3,5-di-tert-butyl-pyrazol-1-yl)-ethyl-(1H-pyrrol-2-ylmethyl)-amine (L4a), were prepared by reducing the corresponding imine compounds L1–L4. L1a–L4a were then reacted with [NiBr2(DME)] to form the nickel complexes, [NiBr2(L1a)] (1), [NiBr2(L2a)] (2), [NiBr2(L3a)] (3) and [NiBr2(L4a)] (4) in high yields.When activated with aluminum co-catalysts, all four nickel complexes were able to catalyze the oligomerization or polymerization of ethylene; but the products of these reactions were co-catalyst and solvent dependent. With EtAlCl2 as co-catalyst and in toluene, butene and hexene were formed, which subsequently alkylated the solvent selectively to butyl-, dibutyl- and tri-butyltoluene. The same catalysts in chlorobenzene produced butene, hexene and highly branched polyethylene. Changing the co-catalysts to MAO and with toluene as solvent, complexes 2 and 3 catalyzed the polymerization of ethylene to high molecular weight (447,300Da) linear high density polyethylene (ca Tm=135.5°C); with polydispersity indices ranging from 1.63 to 2.51.

Bottom-up self-organizing dissipative polymer nanostructures

Based on scaling concepts a methodology of research of non-equilibrium polymer systems has been elaborated. Polymers with flexible chains (melt-crystallized linear high density polyethylene is chosen as an example) are solutions in melt as well as in solid state, the ends of a chain serving as a solvent for it. At critical polymerization degree all phases (melt, solid isotropic or oriented state) are identical. The square of the neck draw ratio is equal to the product of the square of the draw ratio at break and the chain ends collision probability. This probability in its turn is proportional to the average thickness of amorphous layers in the isotropic material. Depending on the type of polymer statistics (Gauss or Lévy–Khinchin) and the number of components of an ordering field for the second case, the melt viscosity and the self-diffusion coefficient vs. the molecular weight of linear flexible-chain polymer follow the power laws with the 3.50, 3.41 or 3.33 and −2.50, −2.05 or −1.65 exponents, respectively, within the reptation model near the critical point. Vibrational–rotational Brownian motions of chain ends about the polymer melt flow direction were taken into account to find better agreement with the experiment. The recent experimental results of dynamic mechanic and dielectric spectroscopy show the value 3.5±0.1 for the viscosity exponent of long chains, while NMR data result in −2.3±0.1 for the self-diffusion coefficient exponent of short chains. Possible reasons are discussed.

Post-irradiation effects in polyethylenes irradiated under various atmospheres

If a large amount of polymer free radicals remain trapped after irradiation of polymers, the post-irradiation effects may result in a significant alteration of physical properties during long-term shelf storage and use. In the case of polyethylenes (PEs) some failures are attributed to the post-irradiation oxidative degradation initiated by the reaction of residual free radicals (mainly trapped in crystal phase) with oxygen. Oxidation products such as carbonyl groups act as deep traps and introduce changes in carrier mobility and significant deterioration in the PEs electrical insulating properties. The post-irradiation behaviour of three different PEs, low density polyethylene (LDPE), linear low density polyethylene (LLDPE) and high density polyethylene (HDPE) was studied; previously, the post-irradiation behaviour of the PEs was investigated after the irradiation in air (Suljovrujic, 2010). In this paper, in order to investigate the influence of different irradiation media on the post-irradiation behaviour, the samples were irradiated in air and nitrogen gas, to an absorbed dose of 300kGy. The annealing treatment of irradiated PEs, which can substantially reduce the concentration of free radicals, is used in this study, too. Dielectric relaxation behaviour is related to the difference in the initial structure of PEs (such as branching, crystallinity etc.), to the changes induced by irradiation in different media and to the post-irradiation changes induced by storage of the samples in air. Electron spin resonance (ESR), differential scanning calorimetry (DSC), infra-red (IR) spectroscopy and gel measurements were used to determine the changes in the free radical concentration, crystal fraction, oxidation and degree of network formation, respectively.

Effect of hot-air ageing on properties of ethylene–propylene–diene monomer (EPDM) rubber/samarium borate/polyolefin and EPDM/antimony-doped tin oxide/polyolefin composites

Ethylene–propylene–diene monomer (EPDM) rubber/samarium borate (SmBO3)/polyolefin and EPDM/antimony-doped tin oxide (ATO)/polyolefin composites are aged at 150°C for different intervals. The addition of low-density polyethylene (LDPE), linear low-density polyethylene (LLDPE) and high-density polyethylene (HDPE) can delay the decrease in elongation at the break of EPDM/filler composites in the first 7 days of ageing. Moreover, the risen branch content of polyolefin can cause more crystals melt during ageing. It is found that acidic ATO particles would promote the formation of oxidative products during ageing. The substitution of SmBO3 by acidic ATO can boost the decrease amplitude of elongation at break of EPDM/filler/LDPE, EPDM/filler/LLDPE and EPDM/filler/HDPE in the first 7 days of ageing. The color change is correlated well with the variation in carbonyl index. The crystallinity is mainly influenced by the chemical cross-link points during ageing. The tendency of tensile strength is well consistent with that of swelling ratios, and the electric properties are correlated with increased polar groups and crystallinity.