Smooth High-density Polyethylene Research Articles

Dynamic shear behavior of GMB/CCL interface under cyclic loading

The dynamic shear behavior of composite liner interface is of great importance for landfill seismic analysis. In this study, an experimental investigation of the shear behavior of the interface between smooth high density polyethylene (HDPE) geomembrane (GMB) and compacted clay liner (CCL) is presented. A series of displacement-controlled cyclic shear tests were conducted to investigate the effects of displacement amplitudes, normal stress levels and number of cycles on the GMB/CCL interface shear behavior. Cyclic loading with higher displacement amplitude will produce greater vertical contraction and lower interface initial shear stiffness. Also, significant shear strength degradation was observed within the first 5 shearing cycles, then followed by slight interface reinforcement in subsequent cycles. The dynamic shear modulus of GMB/CCL interface is dependent on both normal stress levels and displacement amplitudes, while the damping ratio is only affected by displacement amplitudes. Finally, a method considering the GMB/CCL composite liner as an equivalent soil layer was proposed, which is useful for landfill seismic analysis.

Shear behaviour of Canadian marine clay/geomembrane interface under freeze–thaw cycles

The influence of the number of freeze–thaw (F–T) cycles on the interface shear behaviour of a compacted marine clay (Leda clay)/smooth high-density polyethylene (HDPE) geomembrane is investigated in this paper. Shear tests are performed on samples subjected to various numbers of F–T cycles: zero, one, two, three, five and ten. The test results show that the interface shear strength of the Leda clay/smooth HDPE geomembrane is lower than that of Leda clay. The interface shear strength of the Leda clay/smooth HDPE geomembrane decreases with increasing number of F–T cycles. Furthermore, the interface shear friction angles of the Leda clay/smooth HDPE geomembrane are significantly reduced within the first three F–T cycles. The reduction in strength beyond the three F–T cycles is negligible. The total decrease in the shear strength of the samples that are not subjected to an F–T cycle compared to that of samples subjected to ten F–T cycles is 18·3, 17·6 and 13·6% under a vertical stress of 150, 250 and 350 kPa, respectively.

Shear behavior at interface between compacted clay liner–geomembrane under freeze-thaw cycles

Bentonite sand mixtures (BSMs) are commonly used in compacted clay liners (CCLs) in waste disposal facilities when the local soil is not suitable for use in the liners. Given that the temperature tends to vary drastically in many cold regions, it is important to have a good understanding of the interface shear behavior of composite liners that are affected by freeze-thaw (F-T) cycles. However, no studies on the shear behaviour at the interface between a compacted clay liner and high-density polyethylene (HDPE) geomembrane that are subjected to F-T cycles have been conducted in the literature. Therefore, an experimental investigation on the influence of F-T cycles on the shear behaviour at the interface between a BSM and smooth HDPE geomembrane is conducted in this study. The obtained results show that the shear strength at the interface is lower than that of the BSM itself. In addition, the shear strength decreases with increasing numbers of F-T cycles, and the total reduction in the interface shear strength of the samples without undergoing any F-T cycles is 23%, 15.2% and 11.4% under normal stresses of 150 kPa, 250 and 350 kPa, respectively (in comparison to the samples that are subjected to 10 F-T cycles).

Radiation dose and antioxidant depletion in a HDPE geomembrane

The impact of α and β radiation on antioxidant depletion in smooth high-density polyethylene (HDPE) geomembranes (GMs) is described. Smooth HDPE GMs having different thickness (0.04-mm, 0.1-mm, 0.2-mm) were created by mechanically pulverizing sections of 2-mm-thick smooth HDPE GM and extruding the polymer at different thicknesses using a film blowing machine. The 2-mm-thick smooth HDPE GM was also used in the experiments. HDPE GM specimens were exposed to sealed sources of 241Am and 99Tc for 1–50 h to simulate the impact of α and β radiation from U and 99Tc in low-level radioactive waste (LLW) leachate. Standard oxidative induction time (OIT) tests were conducted to determine antioxidant depletion. No change in OIT occurred in the 2-mm-thick HDPE GM after exposure to sealed sources of 241Am and 99Tc for 50 h. In much thinner GMs (e.g., 0.04 mm), however, significant antioxidant depletion occurred after exposure most likely due to penetration of α and β particles. Penetration depth of α and β particles and dose deposition in HDPE GMs were estimated with the GEometry ANd Tracking (GEANT4) program. Predictions from GEANT4 show that maximum dose deposition occurs at the surface of the HDPE GM and decreases with depth. A multilayer model is used to estimate antioxidant depletion in HDPE GMs for depth-dependent doses. These estimates suggest that radiation from LLW leachate has an insignificant effect on antioxidant depletion in HDPE GMs due to the low dose deposition (e.g., 2.42 Gy) expected over a 1000-yr service life, even if the level of activity in LLW leachate increases 10x to 100x the level typical of today.

Performance of water source heat pump system using high-density polyethylene tube heat exchanger wound with square copper wire

Surface water source heat pump system is an energy-efficient heat pump system. Surface water heat exchanger is an important part of heat pump system that can affect the performance of the system. In order to enhance the performance of the system, the overall heat transfer coefficient (U value) of the water exchanger using a 32A square copper coiled high-density polyethylene tube was researched. Comparative experiments were conducted between the performance of the coiled high-density polyethylene tube and the 32A smooth high-density polyethylene tube. At the same time, the coefficient of performance of the heat pump was investigated. According to the result, the U value of the coiled tube was 18% higher than that of the smooth tube in natural convection and 19% higher in forced convection. The coefficient of performance of the heat pump with the coiled tube is higher than that with the smooth tube. The economic evaluation of the coiled tube was also investigated.

자외선 노출조건 하에서 가속시험에 의한 지오멤브레인의 분해거동 해석

본 연구에서는 UVB-313(자외선 파장 290-315 nm) 노출 하에 smooth HDPE 지오멤브레인과 flexible PP 지오멤브레인에 대한 자외선 노출의 영향을 평가하였다. 인장특성, 용융지수, 표준 및 고압 산화유도시간 및 FTIR/ATR 결과들을 고찰하였다. 노출된 지오멤브레인 시료의 인장특성은 변하지 않았지만, HDPE 지오멤브레인보다 flexible PP 지오멤브레인의 경우 산화방지제의 감소가 더 큼을 알 수 있었다. 대표적인 현장온도 <TEX>$20^{\circ}C$</TEX>에서의 산화방지제의 수명을 예측하기 위하여 외삽에 의한 Arrhenius 모델을 적용하였다. 자외선 노출 전후의 HDPE 지오멤브레인 시료는 용융지수의 큰 차이는 없었지만 flexible PP 지오멤브레인은 가교발생으로 인한 용융지수 감소가 발생하였다. FTIR 스펙트럼으로부터 자외선에 노출된 시료의 경우 카르보닐기와 관련된 <TEX>$1750\;cm^{-1}$</TEX> 부근에서 작은 피크가 발견되었으며, 이는 산화가 진행되었음을 예시하고 flexible PP 지오멤브레인의 경우 <TEX>$3100{\sim}3500\;cm^{-1}$</TEX>에서 하이드록시기 또는 하이드로퍼옥사이드기에 의한 새로운 피크를 확인할 수 있었다. In this paper the effect of UV (ultraviolet) exposure on HDPE (high density polyethylene)-smooth and f-PP (flexible polypropylene) geomembranes is evaluated under UVB-313 (ultraviolet wavelength 290-315 nm) exposure. Tensile property, melt flow index (MFI), oxidation induction time (OIT), both standard-OIT and high pressure-OIT and Fourier transform infrared spectroscopy/attenuated total reflectance (FTIR/ATR) results are discussed. Although tensile properties of the exposed geomembrane samples remained unchanged, the depletion of antioxidants was found higher for f-PP than for HDPE geomembrane. Arrhenius model by extrapolation was used on the data to predict the antioxidant lifetime to a typical site temperature of <TEX>$20^{\circ}C$</TEX>. There was no significant difference between the MFI value of the virgin and UV exposed HDPE geomembrane samples but a decrease in MFI was found in f-PP geomembrane that signifies that crosslinking has occurred. From FTIR spectra, the small peak (near <TEX>$1750\;cm^{-1}$</TEX>) observed in the spectrum of UV exposed sample corresponds to a carbonyl (C=O) linkage, which suggests that oxidation has occurred in the polymer structure, and another new band for f-PP between 3100 and <TEX>$3500\;cm^{-1}$</TEX> is attributed to a hydroxyl bond and/or hydroperoxide bond.

Shear-Induced Changes in Smooth HDPE Geomembrane Surface Topography

The shear strength of particulate material-“smooth” high density polyethylene (HDPE) geomembrane interfaces results predominantly from plowing and/or sliding of the particles at the interface. The relative contribution of particle sliding and plowing for a given “smooth” geomembrane surface is principally a function of the relative material hardness, the particle size and angularity, the normal stress, and the soil density. When plowing occurs, the geomembrane surface wears, resulting in altered surface roughness and changed interface strength. This paper summarizes the results of a study that quantified changes in surface roughness as a function of shearing displacement, normal stress, and soil particle shape. Increased particle angularity significantly increased both the geomembrane surface wear and the interface strength. Change in surface topography occurred relatively early in the shearing process with continued displacement inducing additional wear, but not substantially altering the surface roughness further. This study provides quantitative insight into the wear mechanisms of smooth geomembranes during interface shearing.

Expanded Clay and Shale Aggregates for Leachate Collection Systems

In regions dominated by high-carbonate aggregates, landfill owners are faced with using alternative materials for leachate collection systems. Expanded clay and shale, known as lightweight aggregates, contain essentially no carbonates and may provide a suitable alternative. We performed an engineering evaluation of expanded clay and shale aggregates to assess their suitability for leachate collection systems. The hydraulic conductivities of the aggregates under 350 kPa normal stress exceeded the often-specified minimum regulatory value (0.01 cm/s) by one or two orders of magnitude. The aggregate showed no deterioration after being immersed in municipal solid waste leachate for more than 60 days. Interface friction angles between the dry aggregates and smooth high-density polyethylene geomembrane at 7 kPa normal stress ranged from 23° to 27°. Based on the results of our evaluation, it is our conclusion that expanded clay and expanded shale aggregates are viable and competent materials for use in leachate collection systems in municipal solid-waste landfills.

Interface Shear‐Strength Properties of Roughened HDPE

It is generally perceived that roughened high‐density polyethylene (HDPE) has superior interface frictional properties, when in contact with any type of granular material, compared to smooth HDPE. Large‐scale direct shear interface test results are presented that indicate that this is not always the case and that the shear strength of finer‐grained materials may be better mobilized by the currently available roughened HDPE than by coarser‐grained materials. Based upon a limited number of tests, there appears to be a direct relation between soil composition, depth of texturing, and the amount of shear strength mobilized during shearing. This study indicates that the ratio of D80 of the granular material to the height of texturing (htext) of the roughened HDPE may be used to determine how much of a materials shear strength may be mobilized at the interface. The results of this testing suggest the importance of testing all interfaces when performing analysis and design for geosynthetic liners and covers.

Mathematical model of geomembrane stress-strain curves with a yield peak

A simple mathematical model is proposed to describe the stress-strain curves of geomembranes, such as high-density polyethylene (HDPE) geomembranes, that exhibit a yield peak in a uniaxial tensile test. This model is useful for the design of geomembrane applications because it provides an analytical relationship between stresses and strains that can be used in design equations and computer programs. The model consists of an n-order parabola, with the exponent, n, constant or variable as a function of the strain. It has been calibrated with results of uniaxial tensile tests conducted with smooth HDPE geomembranes. If n is a constant (typically n = 4 ), the discrepancy between experimental stress-strain curves and the n-order parabola with a constant exponent is less than 10–15%, which is acceptable in many applications. When more precision is needed, an n-order parabola with a variable exponent can be used. Calculations are then more complex, but the discrepancy between experimental and theoretical stress-strain curves is then less than 3%. This study provides interesting information on the geomembrane modulus. Tests show that the initial modulus of HDPE geomembranes is approximately four times the secant modulus between the origin of the stress-strain curve and the yield peak. Both the tangent and the secant moduli decrease significantly as strain increases, making it very inaccurate to use the initial modulus for any strain other than zero. The proposed mathematical model (with n constant or variable) provides an efficient means of obtaining the tangent and secant moduli at any point of the stress-strain curve between the origin and the yield peak.