Molecular Weight High Density Polyethylene Research Articles

Comparative study of dielectric characteristics and radio transparency of composite materials

Polymer composites with low dielectric properties and excellent radio transparency are essential for high signal transmission rates and high device integration. However, creating dielectric polymer composites with high radio transparency is a challenging task. This work presents comparative results of studying the dielectric loss, permittivity and radio transparency of composite materials, such as aramid-epoxy composite (AEC), glass fiber (GF), carbon fiber reinforced plastic (CFRP), and ultra-high molecular weight high-density polyethylene (UHMWPE). Based on the experimental results, the following transmission percentages were established for various materials: CFRP – 2.45%, AEC – 79.43%, UHMWPE – 80.16%, and GF – 78.88%. The dielectric loss tangent angle of carbon fiber is significantly higher compared to other polymer composites, UHMWPE exhibits the lowest tangent angle values, and AEC and GF have stable and low dielectric loss. The obtained polymer composites demonstrated low dielectric permittivity values: AEС 2.9, UHMWPE 2.56, and GF 3.64.

Cationic Tantalum Hydrides Catalyze Hydrogenolysis and Alkane Metathesis Reactions of Paraffins and Polyethylene.

Sulfated aluminum oxide (SAO), a high surface area material containing sulfate anions that behave like weakly coordinating anions, reacts with Ta(═CHtBu)(CH2tBu)3 to form [Ta(CH2tBu)2(O-)2][SAO] (1). Subsequent treatment with H2 forms Ta-H+ sites supported on SAO that are active in hydrogenolysis and alkane metathesis reactions. In both reactions Ta-H+ is more active than related neutral Ta-H sites supported on silica. This reaction chemistry extends to melts of high-density polyethylene (HDPE), where Ta-H+ converts 30% of a low molecular weight HDPE (Mn = 2.5 kg mol-1; Đ = 3.6) to low molecular weight paraffins under hydrogenolysis conditions. Under alkane metathesis conditions Ta-H+ converts this HDPE to a high MW fraction (Mn = 6.2 kDa; Đ = 2.3) and low molecular weight alkane products (C13-C32). These results show that incorporating charge as a design element in supported d0 metal hydrides is a viable strategy to increase the reaction rate in challenging reactions involving reorganization of C-C bonds in alkanes.

Theoretical and experimental assessment of UV resistance of high‐density polyethylene: Screening and optimization of hindered amine light stabilizers

AbstractHindered amine light stabilizers (HALS) are widely used chemical stabilizers for preventing polymer degradation. In this study, the effects of type of HALS and its content in high‐density polyethylene (HDPE) over the stabilization process are investigated employing density functional theory (DFT) and experimental approach. The electrophilicity index of four types of commercial HALS are compared by DFT to select the most effective one. Besides, the HDPE sample containing a phenolic antioxidant and different contents of the selected HALS are experimentally exposed to UV irradiation. The effect of HALS content on the changes in chain microstructure and molecular weight of HDPE are evaluated using gel permeation chromatography and frequency sweep rheometry. In the absence of HALS, HDPE suffers degradation and the phenolic antioxidant causes chain scission. However, by adding HALS, the chain branching and crosslinking mechanisms are predominant. The results of FTIR, DSC, and tensile analyses follow similar trends, revealing that the best stabilization performance corresponds to adding 600 ppm HALS to HDPE.

Applicability of the Cox-Merz Rule to High-Density Polyethylene Materials with Various Molecular Masses.

The Cox-Merz rule is an empirical relationship that is commonly used in science and industry to determine shear viscosity on the basis of an oscillatory rheometry test. However, it does not apply to all polymer melts. Rheological data are of major importance in the design and dimensioning of polymer-processing equipment. In this work, we investigated whether the Cox-Merz rule is suitable for determining the shear-rate-dependent viscosity of several commercially available high-density polyethylene (HDPE) pipe grades with various molecular masses. We compared the results of parallel-plate oscillatory shear rheometry using the Cox-Merz empirical relation with those of high-pressure capillary and extrusion rheometry. To assess the validity of these techniques, we used the shear viscosities obtained by these methods to numerically simulate the pressure drop of a pipe head and compared the results to experimental measurements. We found that, for the HDPE grades tested, the viscosity data based on capillary pressure flow of the high molecular weight HDPE describes the pressure drop inside the pipe head significantly better than do data based on parallel-plate rheometry applying the Cox-Merz rule. For the lower molecular weight HDPE, both measurement techniques are in good accordance. Hence, we conclude that, while the Cox-Merz relationship is applicable to lower-molecular HDPE grades, it does not apply to certain HDPE grades with high molecular weight.

Study on the Biotribological Properties of Ti6Al7Nb and Ultra High Density Molecular Weight Polyethylene at Different Lubricant Temperature

Higher frictional heat will be produced during the service process of artificial hip joint prosthesis in vivo than natural joint, which would cause the rise of the temperature of synovial fluid and the change of the wear resistance of prosthetic materials. In this work, the biotribological properties of ZrO2/Ti6Al7Nb and ZrO2/UHMWPE pairs at different lubricant temperatures were investigated. The results show that there is a strong correlation between the wettability and average friction coefficient of Ti6Al7Nb, while there is also a strong correlation between the wettability and mass loss of UHMWPE. The wear loss of Ti6Al7Nb and UHMWPE decreases gradually and the friction coefficient increases gradually as the lubricant temperature rises from 20 ℃ to 46 ℃, which is mainly affected by the precipitation process of protein. As a protective layer, it can reduce the wear rate, but as a third body, it can participate in the friction process and increase the friction coefficient. The characteristics of lubricant change greatly at 60 ℃, but the increase of temperature is not the only reason for the precipitation of synovial protein. The synergistic effect with the friction process results in a significant change on the wear mechanism of Ti6Al7Nb and UHMWPE under this condition. The wear mechanism of Ti6Al7Nb is mainly abrasive wear and adhesion wear at different lubricant temperature. With the increase of lubricant temperature, the proportion of adhesion wear mechanism becomes larger. Due to the low thermal deformation temperature and thermal conductivity, the wear mechanism of UHMWPE changed obviously at 60 ℃, and the wear morphology is mainly plastic deformation.

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.

Realizing self-reinforcement of polyethylene via high-speed shear processing

As one of the most representative polyolefins, polyethylene (PE) constitutes the largest consumption of thermoplastics in the world. Thus, the high-performance issue of PE, such as effective self-reinforcement, represents highly expected demand in both of academic and industrial areas. However, it induces a materials engineering challenge due to the difficulties in preparations of multiphase materials and technological complexities of processing. Here, we report a readily accessible self-reinforcement behavior of high-density polyethylene (HDPE) realized via using a commercially available technique, high-speed shear extrusion. A common grade of HDPE can be enhanced significantly for nearly 60%, depending on the rotation speed. The chromatography indicates that the molecular weight of HDPE is nearly constant after experiencing high-shearing effect. While a quasi-solid-like character is detected in the HDPE melt, which could be attributed to the increment of entanglement causing by severe mixing during extrusion. Subsequently, the enhanced entanglement enables the appearances of high orientation structure and specific crystalline morphology, which are responsible for the mechanical reinforcement observed. Our present study offers a promising avenue to access the manufacture of structural parts using polyolefin resins.

Influence of annealing on the morphology and mechanical properties of iPP/HDPE blend with tailored oriented crystalline structures

In this article, two kinds of isotactic polypropylene (iPP)/high density molecular weight polyethylene (HDPE) blend samples were prepared by an accessible injection-molding method. These two samples contain iPP shish kebab matrix and HDPE dispersed phase (shish kebab and epitaxy crystal, respectively). The variation of microstructures upon annealing was characterized using scanning electron microscopy, differential scanning calorimetry and small-angle X-ray scattering. It was found that the variations of different crystalline structures were changed by varying annealing parameters. Only the rearrangement of partially melted chains into primary lamellae happened for the sample with epitaxy crystalline structure, which was distinguished from the sample with HDPE shish kebab structure. Moreover, the variation of the crystalline structure, crystallinity and melting point were discussed to account for the changes in mechanical properties. These results revealed the relationship among the morphology, annealing parameters and mechanical properties, which provide a practical method to industrial manufacture.

Effect of media on degradability, physico-mechanical and optical properties of synthesized polyolefinic and PLA film in comparison with casted potato/corn starch biofilm

The present work focuses the degradation rate of compression moulded Low Density Polyethylene (LDPE), High Molecular Weight High Density Polyethylene (HMHDPE), Linear Low Density Polyethylene (LLDPE) and PLA (Poly lactic acid) films in comparison with solvent casted starch (corn and potato) films under different media. The control film (0 week) and degraded films were subjected to various physical and thermal test (density, crystallinity, tensile, melt flow index, Young’s modulus, elongation) and biodegradation quantification (mass loss, swelling index, haze, transmittance, gloss). The microstructure revealed that potato starch film containing lower amylose content (21%) and 2 ml (32 wt% of starch dry basis) glycerol (plasticizer) produced transparent, uniform, thin (117 μm) films with higher tensile (3.16 MPa) and good surface gloss at 45° (17.4%) whereas corn starch films (water solubility - 40.26%) has a higher biodegradability rate in all the media (94.23% in vegetable waste and 99.99% in 0.1 N HCl) can act as a better biofiller. The highest degradation was observed in soil in which LDPE showed the maximum weight loss (0.974%) and drop in tensile load at break (30.176 N to 14.93 N) and HMHDPE showed the minimum value of 0.868% and 44.67 N to 40.24 N respectively in one year.

Large-scale formation of shish-kebab in water-assisted injection-moulded high-density polyethylene with a high-molecular weight

To promote flow-induced orientation and reserve the formation of the shish-kebab structure, a modified injection-moulding technology named water-assisted injection moulding (WAIM), was applied to provide twice the shear flow and a rapid cooling rate on high-molecular weight high-density polyethylene (HDPE) melt in a mould cavity. Large-scale distribution of highly oriented shish-kebab was successfully achieved in a WAIM sample with a high-molecular weight. More importantly, the high-orientation degree of lamellae (larger than 0.4) in the thickness direction for a high-molecular weight WAIM sample is >60%, which is much more than a conventional injection-moulded sample (15%). This work provides a new insight to achieve injection-moulded products with enhanced oriented structures.

Interdiffusion versus crystallization at semicrystalline interfaces of sintered porous materials

Sintering process at temperature intervals close to the melting point of polymers is greatly important due to its role in synthesizing porous materials. During sintering, particles of polymeric materials coalesce throughout a process called interdiffusion. On the contrary, crystallization strongly affects the interdiffusion process at temperature intervals below and close to the melting point. Apparently, the outcome of the contention between these two factors would determine the interfacial width. Therefore, the current study presents a model, which takes both crystallization and interdiffusion into account, to predict sintering kinetic. Consequently, interfacial strength was assessed with respect to the following influencing mechanisms, “reentanglement” relying on mutual interpenetration distance and “cocrystallization” determined by interfacial lamellar thicknesses. Based on the results of the present study, by changing sintering temperature of high molecular weight high density polyethylene nascent powder from 125 to 129 °C, the mutual interpenetration distance changes from 4.8 to 52.9 nm and interfacial lamellae thicknesses also vary from L ≈ 0–35.4 nm. On the other hand, porosity measurements revealed the reverse dependency to the sintering temperature. Interfacial lamellae thicknesses were calculated by means of differential scanning calorimetry (DSC) and also scanning electron microscope (SEM) micrographs. Eventually, the results of the shear punch test clearly demonstrated the role of sintering temperature in interfacial strength. Accordingly, the maximum bearable load in the samples sintered at 125 and 129 °C, increases from 25 to 315 N, respectively, which was attributed to change in interfacial volume of two sintered particles in a simplified model. Resultantly, the present study indicates that even a degree centigrade temperature variation would significantly affect the interfacial strength and porosity of the samples due to its effect on crystallization; which in consequence might transform a porous material into a dense one.

Dynamic slip of polydisperse linear polymers using partitioned plate

The slip velocity of an industrial grade high molecular weight high-density polyethylene (HDPE) is studied in steady and dynamic shear experiments using a stress/strain controlled rotational rheometer equipped with a parallel partitioned plate geometry. Moreover, fluoroalkyl silane-based coating is used to understand the effect of surface energy on slip in steady and dynamic conditions. The multimode integral Kaye-Bernstein-Kearsley-Zapas constitutive model is applied to predict the transient shear response of the HDPE melt obtained from rotational rheometer. It is found that a dynamic slip model with a slip relaxation time is needed to adequately predict the experimental data at large shear deformations. Comparison of the results before and after coating shows that the slip velocity is largely affected by surface energy. Decreasing surface energy by coating increases slip velocity and decreases the slip relaxation time.

Non-isothermal extrudate swell

The non-isothermal extrudate swell of a high molecular weight high-density polyethylene (HDPE) in long capillary and slit dies is studied numerically (ANSYS POLYFLOW®) using an integral K-BKZ constitutive model including crystallization kinetics, determined experimentally. The Nakamura model is used for crystallization of the HDPE, where the crystallization rate parameter is evaluated by using the well-known Ziabicki equation. This non-isothermal extrudate swell phenomenon is simulated using the pseudo-time integral K-BKZ model with the Wagner damping function along with the differential form of the Nakamura model to account for the crystallization of the extrudate. The swell measurements were carried out under non-isothermal conditions by extruding the polymer melt at 200 °C through long capillary and slit dies to ambient air at 25 °C, 110 °C, and 200 °C. The numerical results are found to be in excellent agreement with experimental observations.

The effect of damping function on extrudate swell

The extrudate swell of an industrial grade high molecular weight high-density polyethylene (HDPE) in capillary dies is studied using the integral K-BKZ constitutive model. The non-linear viscoelastic flow properties of the polymer resin are studied for a broad range of large step shear strains and high shear rates using the cone partitioned plate (CPP) geometry of the stress/strain controlled rotational rheometer. This allowed the determination of the rheological parameters accurately, in particular the damping function, which is proven to be the most important in simulating transient flows such as extrudate swell. A series of simulations performed using the integral K-BKZ Wagner model with different values of the Wagner exponent n, ranging from n=0.15–0.5, demonstrates that the extrudate swell predictions are extremely sensitive to the Wagner damping function exponent. Using the correct n-value resulted in extrudate swell predictions that are in excellent agreement with experimental measurements.

Twisted lamellae in water-assisted injection molded high density polyethylene

A distinct skin-core-water-channel structure was formed in water-assisted injection molded low molecular weight high density polyethylene. A large number of well-developed banded spherulites were observed in core region. Two types of shish-kebabs, i.e., shish-kebabs without and with twisted lamellae, were appeared in both skin and water-channel region. The results imply that the formation mechanisms of twisted lamellae in banded spherulites and shish-kebabs are different, but both depend on the level of applied shear stress.

The coupling relation between chain architectures and secondary flow field determined by an unusual dependence of shish-kebabs on molecular weight of high-density polyethylene

Selecting high molecular weight has become a quite popular approach for effective tuning of more shish-kebabs in semi-crystalline polymers. However, here, an unusual dependence of shish-kebabs on molecular weight of high-density polyethylene (HDPE) is found under the injection molding with a secondary flow. Characterization with electron microscope and X-ray scattering of the crystal structures reveals that the richest shish-kebabs develop in the HDPE with medium high molecular weight (HMW) chains rather than in the HDPE with more HMW chains. Both macroscopic scales (fluid behaviors related to the intensity of flow) and molecular scales (rheological properties related to orientation and relaxation) need to be overall considered and a physical model has been proposed to explain how the coupling between the chain architectures and the secondary flow field contributes to the unusual phenomenon. The factors, such as molecular parameters, interfaces between dispersed phase, and matrix as well as the properties of filler, profoundly influence both the two scales, which can be employed to tune the morphology related to physical properties. These significant results provide a simple but effective morphology control technique under a secondary flow field.

Extrudate swell of a high-density polyethylene melt: II. Modeling using integral and differential constitutive equations

The extrudate swell phenomenon of a high-molecular-weight HDPE is modeled using the following viscoelastic constitutive equations: the multi-mode Kaye-Bernstein–Kearsley–Zapas (K-BKZ) integral model with the Wagner and the Papanastasiou–Scriven–Macosko (PSM) damping functions, and multi-mode Phan-Thien–Tanner (PTT), Giesekus and Double Convected Pom-Pom (DCPP) differential models. The high-molecular-weight high density polyethylene (HDPE) is rheologically characterized and the experimental data have been used to fit the parameters of the constitutive models. Experimental results on extrudate swell are compared with the numerical predictions. The simulation results showed that the integral K-BKZ model highly overpredicts and the differential PTT, Giesekus and DCPP models slightly under-predict the experimental measurements. When the capillary reservoir is excluded from the simulations, the predictions of the integral models are significantly reduced close to the experimental data, while those of the differential models remain practically unaffected. Although both integral and differential models used represent the rheological data equally well, an explanation of the dramatically different predictions of extrudate swell by the integral and differential models remains elusive.

Comparative Degradation of LDPE, HDPE and HMHDPE under Different Soil Conditions

The Present work includes, degradation of polyethylene under different environmental conditions to know the effect of physical, chemical and biological factors prevailing in those conditions on degradation of polyethylene. Plastic films viz., Low density polyethylene (LDPE), High density polyethylene (HDPE) and High molecular weight high density polyethylene (HMHDPE) each were incubated in three different conditions viz., black soil, sandy soil and red soil for a period of 3 months. The changes in the properties of plastic films after incubation was studied by change in the weight of the plastic film and mechanical parameters like tensile strength, breaking load and percentage of elongation. Among all the plastic films HDPE was found to be highly susceptible with 33% weight loss and 40% reduction in percentage of elongation compared to LDPE with 26% weight loss and 34 % reduction in percentage of elongation in black soil. Whereas HMHDPE found to be highly resistant in all the soils with no significant weight loss and percentage of elongation (15%). None of the plastic films had shown degradation in sandy soil even after incubation for 3 months. FTIR spectroscopy results showed that HDPE film incubated in black soil had undergone extensive degradation when compared with un incubated HDPE film.

Well-mixed blends of HDPE and ultrahigh molecular weight polyethylene with major improvements in impact strength achieved via solid-state shear pulverization

Compared with conventional polyolefins, ultrahigh molecular weight polyethylene (UHMWPE) possesses outstanding impact strength and crack resistance that make it desirable for a wide variety of applications. Unfortunately, UHMWPE has an ultrahigh viscosity that renders common, continuous melt-state processes ineffective for making UHMWPE products. Attempts to overcome this problem by blending UHMWPE with lower molecular weight high-density polyethylene (HDPE) by melt processing have typically led to poorly dispersed blends due to the vast viscosity mismatch between blend components. Here, we present solid-state shear pulverization (SSSP) as a mild, continuous, and simple approach for achieving effective and intimate mixing in UHMWPE/HDPE blends. These SSSP blends are easily processed by post-SSSP melt extrusion; for an SSSP blend with 50 wt% UHMWPE, we observe more than a factor of 1000 increase in viscosity at a shear rate of 0.01 s−1 but less than a factor of 5 increase at 100 s−1, the latter being more typical of melt-processing operations. Using extensional rheology, we confirm the strain hardening behavior of SSSP blends. Shear rheology and crystallization data show that the mixing between UHMWPE and HDPE can be improved with subsequent passes of SSSP and single-screw extrusion. Finally, we show that blending via SSSP leads to dramatic improvements in impact strength: as compared to literature results, injection-molded sample bars made from SSSP blends with 30–50 wt% UHMWPE exhibit very high values of notched Izod impact strength, 660–770 J/m (the impact strength of neat HDPE was 170 J/m).

The relationships between crystallization characteristics and heat sealing properties of high-density polyethylene films

The relationships between crystallization characteristics and heat sealing properties of high-density polyethylene (HDPE) films were determined. HDPE with a 1.1 g/10 min melt flow index (MFI) (190℃, 2.16 kg) was melt mixed in a twin-screw extruder with a 0.04 g/10 min MFI HDPE. The high molecular weight HDPE (HMW HDPE) concentration was varied between 1 and 20 wt%. The HDPE blends were cast into a 50 µm thick film. The films were heat sealed at 125℃ and 128℃. The seal bar pressure was kept constant at 0.13 MPa. The heat seal time was varied from 0.5 to 2.0 s. Heat-sealed strength was measured by T-peel test. The molecular structure development during the heat seal process was evaluated by differential scanning calorimeter and wide-angle X-ray diffraction. The HMW HDPE decreased the film heat seal strength when sealed at 125℃. However, at 128℃ the presence of up to 10 wt% reduced the heat sealing time.