Thermal Conductivity Of High-density Polyethylene Research Articles

Enhanced Thermal Conductivity of High-Density Polyethylene Composites with Hybrid Fillers of Flaky and Spherical Boron Nitride Particles.

The synergistic effect between different fillers plays a crucial role in determining the performance of composites. In this work, spherical boron nitride (BN) and flaky BN are used as hybrid fillers to improve the thermal conductivity (TC) of high-density polyethylene (HDPE) composites. A series of HDPE composites were prepared by adjusting the mass ratio (1:0, 4:1, 2:1, 1:1, 1:2, 1:4, and 0:1) of spherical BN and flaky BN. The SEM results indicate that the spherical BN (with a particle size of 3 μm) effectively filled the gaps between the flaky BN (with a particle size of 30 μm), leading to the formation of more continuous heat conduction paths with the composite. Remarkably, when the mass ratio of spherical BN to flaky BN was set to 1:4 (with a total BN filling amount of 30 wt%), the TC of the composite could reach up to 1.648 Wm-1K-1, which is obviously higher than that of the composite containing a single filler, realizing the synergistic effect of the hybrid fillers. In addition, the synergistic effect of fillers also affects the thermal stability and crystallization behavior of composites. This work is of great significance for optimizing the application of hybrid BN fillers in the field of thermal management.

Response-Surface-Methodology-Based Increasing of the Isotropic Thermal Conductivity of Polyethylene Composites Containing Multiple Fillers.

To optimize the thermal conductivity of high-density polyethylene, 15 hybrid filler composites containing either aluminum oxide, graphite, expanded graphite, carbon nanotubes or a combination of the former, have been studied using an extrusion-compression processing tandem. The experimental density of the cube-shaped specimens is substantially lower than the theoretical density calculated by the linear mixing rule, mainly for the composites with high filler contents. The morphology of the composites, as studied by scanning electron microscopy (SEM), highlighted a good dispersion quality and random orientation of the fillers in the test specimens but also revealed air inclusions in the composites, explaining the density results. It is shown that the addition of filler(s) increases both the melt viscosity (up to ca. 270%) and the thermal conductivity (up to ca. 1000%). Hence, a very strong increase of TC can be practically hampered by a too high viscosity to enable processing. Supported by ANOVA analysis, the application of response surface methodology (RSM), assuming a perfect compression, indicates that all fillers have a significant effect on the thermal conductivity and synergistic effects can be achieved. The regression model obtained can adequately predict the thermal conductivity of composites of various compositions, as already confirmed based on three validation experiments in the present work.

Thermally conductive microcapsule/high-density polyethylene composite for energy saving and storage

Phase change material (PCM) is useful for the storage and release of latent heat. However, its ability to conduct has hindered its engineering application. This study prepares a novel microencapsulated phase change material (MEPCM) by suspension polymerization. To improve the adhesion between the shell and the inorganic additive, triethoxyvinylsilane was incorporated copolymerizing with methyl methacrylate. Thermally conductive nanographite particle was added. This MEPCM was then incorporated into high-density polyethylene (HDPE) to form a series of thermally conductive PCM microcapsules that approached sphere shapes with diameters less than 2 [Formula: see text]m. Thermal analysis showed that the thermal stability and heat resistance of the microcapsule were improved. The thermal conductivity of HDPE was increased by 39% to 0.6358 W/m[Formula: see text]K, and the surface resistivity was lowered to [Formula: see text]/sq after the addition of MEPCM. The temperature on the top of the composite tested was lower than pristine HDPE. This was close to the onset melting temperature of the MEPCM (38.5[Formula: see text]C), [Formula: see text] lower than pure HDPE. The reduction is a significant improvement in temperature regulation. This enables MEPCMs to store and release heat much more effectively, and can thus be applied to medical construction materials to meet the temperature requirements of COVID-19 patients.

Саlculation of heat transfer processes at canlendering of polyethylene sheets

Introduction. The technological limitations of the calendering process include the condition of leaving the sheet from the calender in the presence of a middle layer of melt, or a sufficient value of the average temperature of the sheet, which determines the flatness of the sheet with subsequent cooling and glazing in a flat state on the roller conveyor. The process of vitrification of the polymer melt has a wide temperature range, which greatly complicates the calculations.Materials and methods. Processing of high density polyethylene (HDPE) on a calender with three rolls is considered. A series of numerical experiments was performed.Results and discussion. Boundary conditions for the mathematical model of sheet cooling and conditions for numerical integration of the equation of nonstationary thermal conductivity taking into account the heat of crystallization are formulated, the equation is presented in finite differences, using a grid with a step along the sheet length.The letter from HDPE in the physical model is conditionally expanded, the following assumptions are made:1. the thickness of the sheet is much smaller than the diameter of the shafts of the calender, so the orthogonal coordinate system is used;2. the transition of the melt from the initial thickness of the workpiece to the final occurs at the same linear velocity (the melt does not slip on the shaft);3. the thermal conductivity of HDPE is much less than the thermal conductivity of the shaft material, so the temperature of the sheet at the point of contact with the shaft is equal to the temperature of the shaft;4. reduction of sheet thickness in the second gap does not occur;5. energy dissipation in the gaps does not affect the temperature fields in the melt;6. heat transfer along the sheet due to thermal conductivity is absent;7. the width of the sheet is much larger than the thickness, so the model of the cooling process is simplified to one-dimensional, without heat transfer across the sheet.For the surface of the sheet, which is cooled by air by free convection, the method of control volume (heat balance method) was used for the boundary layer with a thickness of half a step of the grid along the thickness of the sheet.Numerical experiments were performed for the following conditions: HDPE performance 500 kg / h; initial melt temperature 235°C; diameter of calender shafts 0,25 m; sheet thickness 1.0; 2.0; 3.0; 5.0 mm. Sheet width 1.0 m. The temperature of the first shaft 125°С, shaft temperatures №2 and №3 were selected under the condition of the average melt temperature at the outlet of the calender 125°С, ambient air temperature 20°С, heat transfer coefficient from the sheet and the shaft to air 25 W / (m2K).An example of the results of numerical integration by the Runge – Kutta method (temperature fields and average melt temperature) is shown in the figures.The theoretical analysis of heat exchange processes during calendering of sheets from high density polyethylene melt is carried out, the technological parameters of calendering and their influence on the main quality parameters are analyzed.For the conditions of the numerical experiment, it was found that when calendering sheets with HDPE with a thickness of 1.0 to 2.0 mm will have to choose a high-temperature coolant, which greatly complicates the system of thermal stabilization of the shafts. A 1 mm thick sheet will not have gloss due to the high temperature of the shafts, and the temperature inhomogeneity of the shaft surface due to the high heat flux density can lead to the formation of a central strip on the surface of the sheet without gloss. Calendering of sheets with HDPE with a thickness of 5 mm or more for the conditions of numerical experiment causes difficulties due to the possible "blasting" of the sheet up (behind the hot shaft).Conclusions. It is established that when calendering sheets of smaller thickness, the heat load on the middle shaft increases, and it may be necessary to use high-temperature coolants. The optimal temperature of the calender shafts is in a narrow range, so when designing new equipment, thermal calculation is important.

Thermally Conductive Polyethylene/Expanded Graphite Composites as Heat Transfer Surface: Mechanical, Thermo-Physical and Surface Behavior.

Composites of high-density polyethylene (HDPE) and expanded graphite (EG) are prepared for heat exchangers in multi-effect distillation (MED) desalination. At 50 wt.% EG loading, the thermal conductivity of HDPE was increased by 372%. Moreover, the surface wettability of the HDPE/EG composite was enhanced by corona and RF plasma treatment as demonstrated by the increase in surface free energy from 28.5 mJ/m2 for untreated HDPE/EG to 55.5 and 54.5 mJ/m2 for HDPE/EG treated by corona and RF plasma, respectively. This enhanced surface wettability was retained over a long time with only a 9% and 18% decrease in RF and corona plasma-treated samples’ surface energy after two months. The viscoelastic moduli and the complex viscosity profiles indicated that EG content dictates the optimum processing technique. At loading below 30 wt.%, the extrusion process is preferred, while above 30 wt.% loading, injection molding is preferred. The plasma treatment also improved the HDPE/EG composite overall heat transfer coefficient with an overall heat transfer coefficient of the composite reaching about 98% that of stainless steel. Moreover, the plasma-treated composite exhibited superior resistance to crystallization fouling in both CaSO4 solution and artificial seawater compared to untreated composites and stainless-steel surfaces.

Effect of arrangement of nano and micro barium titanate (BaTiO3) particles on the enhanced dielectric constant of high‐density polyethylene (HDPE)/BaTiO3

ABSTRACTThe aim of this study is to improve the dielectric and mechanical properties of HDPE/BaTiO3 composites by binary BaTiO3 particles, when the volume fraction of BaTiO3 is constant. In this study, it was found that the pack density of binary BaTiO3 particles in HDPE/BaTiO3 composite relies on particle ratio and volume fraction of small particles. It is found that the addition of 50 vol % 1600 nm BaTiO3 particles can boost the dielectric constant of HDPE control from 2 to 30 (14 times higher) at 40 Hz and 19 (8.5 times higher) at 40 MHz, respectively. When the particle ratio was 4, the substitution of 10 vol % 1600 nm BaTiO3 particles by 10 vol % 400 nm BaTiO3 particles can further enhance the dielectric constant of HDPE/L‐BT (10/10) from 30 to 50 (67% increase) at 40 Hz and from 19 to 42 (121% increase) at 40 MHz, respectively, without greatly influencing the volume resistivity of HDPE composites. In addition, the thermal conductivity of HDPE with binary BaTiO3 particles were all above 2.0 W/(m•K). © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2018, 56, 1101–1108

Aluminum oxide particles/silicon carbide whiskers’ synergistic effect on thermal conductivity of high-density polyethylene composites

Aluminum oxide (Al2O3) particles and silicon carbide (SiC) whiskers improved the thermal conductivity of high-density polyethylene (HDPE). To improve the dispersion of inorganic fillers in the matrix, 5 wt% of maleic anhydride-modified polyethylene was added into HDPE as a compatibilizer, and the hybrid matrix was denoted as mHDPE. The thermal conductivity, heat resistance, and tensile properties of resulting HDPE composites were characterized. The results showed that the thermal conductivity reached its maximum value of 0.8876 W/(m K) at 1/4 weight ratio of Al2O3/SiC, which was 110.3, 54.8, and 8.8% higher than that of pure HDPE, mHDPE/Al2O3, and mHDPE/SiC composites, in the order given, indicating that hybrid fillers have synergistic effect on the thermal conductivity of HDPE composites. Moreover, they also have a synergistic effect on the heat resistance and Young’s modulus. As the SiC content increases, the heat resistance of the composites increases at first and then falls, and the maximum VST is reached at an Al2O3/SiC weight ratio of 3/2, which is 5.4 °C higher than that of HDPE. The maximum Young’s modulus of the composites (1160 MPa) is obtained at an Al2O3/SiC weight ratio of 1/4, and the yield strength increases gradually as the SiC whiskers’ content increases.

Thermal conduction behaviors of chemically cross-linked high-density polyethylenes

• The crystal structure and crystallinity of HDPE were controlled through the chemical crosslinking reaction. • Spherulite and crystallite size decreased with an increase in the crosslinking density. • Thermal conductivity synchronously decreased with a decrease in the crystallinity. The effect of chemical crosslinking on the thermal conductivity of high-density polyethylene (HDPE) was investigated. The crosslinking density of HDPE was controlled by the content of the crosslinking agent of dicumyl peroxide (DCP) in molten state. As the content of DCP increased, the gel content of HDPE increased, and crystal structure such as spherulite size and crystallite size, and crystallinity decreased, as network formation suppressed the crystallization of polymer molecules. The thermal conductivity of crosslinked HDPE was proportional to the crystallinity and reciprocal to the gel content.

Effect of Annealing on Low-Temperature Thermal Conductivity of High-Density Polyethylene

Thermal conductivity of high-density polyethylene was measured by the steady-flow method at temperatures from 4.2 to 20°K. Results were analysed assuming two scattering processes of phonons: grain boundary scattering with a mean free path ΛG which is independent of phonon wave number k and intragrain scattering with a mean free path ΛD=A/ak2. It was found that ΛG=0.48 μ and A/a=0.51×1010 cm−1 for the as-received sample, and both increase with annealing. The radius of spherulite which was kept constant with annealing ranged 5 to 10 μ. ΛG is attributed to the extension of lamellar crystallite in the ab-plane. ΛD is assigned to intralamellar sub-boundaries. According to results in literature, the ratio of thermal conductivity K to specific heat C is almost constant from 1 to 3°K, where the grain boundary scattering is predominant. Since at 1°K the contribution of two-dimensional lattice waves (Lamb waves in lamellae) is more appreciable than three-dimensional ones, the above result implies that ΛG is the same for the two types of waves, which is consistent with our conclusion for ΛG.