The Formation and Evolution of the Hierarchical Structure of Polyethylene Pipe During Extrusion Processing

The formation and evolution of the hierarchical structure of polyethylene (PE) pipe during extrusion processing were investigated and its structure-performance correlation is discussed. The experimental results showed that during the PE pipe processing the axial stress by the traction machine oriented the molecular chains along the axial direction, and gave rise to some shish-kebab structures in the external layer which were parallel to the axis, so that the PE pipe easily cracked along the axial direction. Moreover, the temperature gradient generated by different cooling rates across the pipe wall caused the kebab crystals and the spherulites nucleated further from the outer wall to only grow toward the inner wall. As a result, the lamellae were oriented along the radial direction of the PE pipe. On the other hand, owing to the slow cooling of the inner wall, its tie molecules were of lowest density and hence its resistance to slow crack growth was poor. Therefore, the inner layer, which also suffered the highest hoop stress, would become the weak point in PE pipe applications; that is, the cracks extended from the inner wall of the pipe, through the wall, and then in a longitudinal direction.     

Study on Properties and Structure of Near Melt Point Extruded High‐Density Polyethylene

The effect of extrusion temperature on the mechanical properties of high‐density polyethylene (HDPE) was examined using solid‐state extrusion (SSE) and melt‐state extrusion (MSE) techniques. Differential scanning calorimetry (DSC) and scanning electron microscopy (SEM) investigations were employed to provide evidence for explaining the relationship between mechanical properties and morphology of extrusion moldings. Extruded from a convergence‐divergence die, compared with samples obtained by MSE, the yield strength of samples obtained by SSE was enhanced in both longitudinal and transverse directions with a ductile failure. The yield strength decreased sharply with increasing extrusion temperature. The maximum longitudinal yield strength of samples extruded at 112°C was 181 MPa with an 87% elongation at break; the corresponding values were 28 MPa and 800% for samples extruded at 140°C (MSE); in the transverse direction the yield strength was 27 MPa with a 101% elongation at break for samples extruded at 140°C, while the maximum yield strength was 51 MPa with a 45% of elongation at break for samples extruded at 116°C. Compared with sheets extruded at 140°C, DSC data shows a 5.3°C increase in melting point, a 9.5°C decrease in melt point width, and a 7.1% decrease in crystallinity for sheets extruded at 112°C. SEM indicates that spherulites predominate in MSE samples, while a preferred orientation of the lamellae along the extrusion direction were mainly produced by SSE.     

Microstructural Evolution and Its Effect on Mechanical Properties of Isotactic Polypropylene by Shear Plastic Deformation at Elevated Temperatures

Isotactic polypropylene (iPP) was plastically shear deformed by equal channel angular extrusion (ECAE) at extrusion temperatures varied from 45 to 125°C (25 mm/min). The evolutions of morphology and crystal orientation were studied by reflected optical microscopy (ROM), scanning electron microscopy (SEM), and X-ray diffraction. It was found that the original spherulites were deformed into nearly ellipsoids with their long axis tilted at an angle away from the flow direction. Azimuthal scanning results revealed that two preferred crystal orientations were formed after ECAE. The crystal plasticity was activated by increasing the extrusion temperature, followed by fast rotation of crystallites toward the shear direction. The thermal mechanical analysis (TMA) indicated that low extrusion temperature was favorable to fix the molecular orientation. The iPP samples processed at the investigated temperatures displayed a significant increase in the impact strength, especially for those extruded at 45°C and 65°C. The tensile results revealed a greater elongation at break in the samples deformed at low temperatures (45°C and 65°C) but not in those deformed at high temperatures (85°C or above).     

Flow patterns and heat-transfer characteristics of a top heat mode closed-loop oscillating heat pipe with check valves (THMCLOHP/CV)

The aim of this research was to investigate the flow patterns and heat transfer of a top heat mode closed-loop oscillating heat pipe with check valves (THMCLOHP/CV). In this study, the heat pipe was made of a high-quality glass capillary tube with an inner diameter of 2.4 mm bent into 10 meandering turns. The number of check valves was 2 and the tube was filled with R141b at a filling ratio of 50% of internal volume of the tube. The combined lengths of the evaporator, adiabatic and condenser sections were equal to 50 mm. The pipe was operated at the top heat mode, and the angles of inclination were 20°, 40°, 60°, 80°, and 90°. The heat applied at the evaporator section was controlled at 85°Cto 105°C, and 125°C. The results show that in the evaporator section, bubbles are produced and grow as a result of the continuous nucleate boiling. They coalesced and their volume expanded. Similarly, in the condenser section the vapor plug condensate caused the bubbles to collapse and accumulate as a liquid mass at the lower section of the U-bend tube. A new slug then developed and the bubbles coalesced in an upward flow. Heat flux increased when the evaporator temperature and inclination angle increased causing the average length of the vapor plug to decrease and the average velocity of vapor plug to increase. The maximum heat flux occurred at an evaporator temperature of 125°C and an inclination angle of minus 90°.     

Melt Elongation Properties of Linear Low-Density Polyethylene

The melt extensional properties of a linear low-density polyethylene (LLDPE) were measured using melt spinning techniques in a range of temperature varying from 150 to 200°C, and the entry flow method in the capillary extrusion at 170°C was used to investigate the effects of elongation strain rate, temperature, and extrusion velocity in the capillary on the melt elongation stress and viscosity. The melt stretching force at break decreased nonlinearly with a rise of temperature. A low melt elongation viscosity might be beneficial to improve the melt drawability. With the increase of elongation strain rate, the melt elongation stress increased while the melt elongation viscosity decreased nonlinearly. Both melt elongation stress and viscosity decreased with a rise of temperature. Under the experimental conditions, the melt elongation stress and viscosity decreased with an increase of extrusion velocity in the capillary. Moreover, the relationship between the elongation viscosity determined from the entry flow and strain rate was similar to that from the melt spinning flow.     

Study of Temperature and UV Wavelength Range Effects on Degradation of Photo-Irradiated Polyethylene Films Using DMA

Plastic bags mostly made of polyethylene (PE) cause pollution as solid waste due to their non-degradability nature. Initiation of a degradative process by enhanced photo-oxidation is a possible method for an accelerated degradation. This paper presents temperature treatment effects on PE films where photodegradation was initiated using ultraviolet (UV) irradiation in the ranges of 200–300 nm and 300–400 nm for 2 hr. Effects of temperature of 40°C and 55°C on non-UV-irradiated and UV-irradiated PE films processed by conventional methods were investigated and evaluated after 50 hr, 150 hr, and 350 hr of temperature exposure. The effects of UV wavelength range irradiation on the degradation were deduced. Measuring the dynamic moduli using a dynamic mechanical analyzer monitored the degradation. The decrease in average storage modulus was 62% with treatment at 55°C, higher than the 16% drop at 40°C for unirradiated samples after 350-hr exposure. Cross-linking in UV-exposed samples, characterized by an increase in dynamic modulus (stiffening), was observed followed by a reduction of storage modulus. Temperature treatment at 55°C together with 300–400-nm UV range irradiation resulted in the largest increase, i.e., 22% after 150 hr, followed by the largest reduction of storage modulus, i.e., 74.6% for a cumulative 350-hr exposure.     

Study of Metallocene-Catalyzed Linear Low-Density Polyethylene Solid-State Extrusion: Processing and Mechanical Properties

The effect of extrusion temperature and extrusion drawing ratio (EDR) on the die swell ratio (DSR) and mechanical properties of metallocene-catalyzed linear low-density polyethylene (m-LLDPE) was examined with the application of solid-state extrusion (SSE). Scanning electron microscopy (SEM) was employed to characterize the microstructure and morphology of the extrudates. Extruded from a convergence-divergence die, compared with samples obtained by melt-state extrusion (MSE), the DSR decreases for SSE samples prepared at low extrusion temperature and high EDR. Mechanically strong SSE samples were also obtained at low extrusion temperatures and high EDR. Mechanically strong SSE samples were also obtained at low extrusion temperatures and high EDR. SEM indicates that the microstructures of the MSE samples consist primarily of ring-banded spherulites; the microstructure of the SSE samples was microfibers oriented along the direction of extrusion. The highly oriented microfibers contribute to the improved mechanical strength of the SSE samples.     

The Effect of Cooling Mode on Slow Crack Growth Resistance of Polyethylene Pipe

High-density polyethylene (HDPE) pipes have been widely used as gas or water transport pipes owing to their comprehensive advantages. One of the principal failure modes determining pipe service lifetime is slow crack growth (SCG) with the crack occurring first at the inner surface due to the slow cooling rate of the pipe's inner wall during polyethylene (PE) pipe extrusion. In order to change the conventional cooling mode and increase the cooling rate in the inner wall of PE pipe during extrusion, a novel extrusion equipment was designed and manufactured by our research team. For this paper, compressed air as a cooling medium was introduced through the interior of the hot extruded pipe during its extrusion to realize the quick inner wall cooling, and the effects of the inner wall's cooling rate on the microstructure and mechanical properties of the PE pipe were investigated. The experimental results showed that simultaneously cooling of both the outer and inner walls could decrease the difference in the solidification rate across the pipe and reduce the residual internal stresses in PE pipe. The quick cooling of the inner wall of the extruded pipe could also decrease the PE crystal thickness, and increase the number of tie molecules in the inner wall, which is a key parameter determining the resistance to SCG. As a result, compared to the PE pipe produced by the conventional extrusion, the crack initiation time of the PE pipe manufactured by the novel method increased from 27 h to 45 h and the crack growth rate was slower.     

Effect of ultrasonic oscillations on the fluidity of heavy oil products at low temperatures

We develop a laboratory setup to estimate the force of rotation of a metal branch pipe in a viscoelastic medium. We show that 2-min action of shearing ultrasonic oscillations (frequency, 32.5 kHz; specific power, no more than 0.008 W/cm²) reduces by 17% the static limit of fluidity brought to an initial temperature of ì-100 fuel oil cooled to −15°C in the wall layer of a rotating branch pipe. We obtain a linear regression dependence between the ratio of the threshold force of the onset of branch pipe motion to the consumption current of the ultrasonic transducer and the fuel temperature.     

Simple and fast preparation of graphene oxide@ melamine terephthaldehyde and its PVC nanocomposite via ultrasonic irradiation: Chemical and thermal resistance study

Melamine terephthaldehyde modified graphene oxide (MTR-GO) with optimum content was easily prepared via ultrasonication method and used as anti-corrosion additive for Poly (vinyl chloride) (PVC). The effects of ultrasonicated MTR-GO on the mechanical, chemical and thermal resistance of the PVC were thoroughly studied. Change percentage of tensile strength and weight change percentage of PVC (P) and PVC/MTR-GO nanocomposite (PN) in acetone and sodium hypochlorite (NaClO) media at two different exposure temperature (20 °C and 50 °C) were examined. The PN sample showed lower change loss percentage of tensile strength in acetone uptake as compared with P sample at 20 °C. In higher temperature (50 °C), P sample was decomposed while PN still showed tensile data. The change loss percentage in tensile strength of PN sample showed 13% change at 50 °C in sodium hypochlorite while P sample showed 63% change for the parameter. Protective behavior of MTR-GO nanofiller on PVC matrix against thermal HCl releasing was investigated by Congo red tests. The results showed that the nanocomposite release less amount of HCl as compare to the neat PVC.     

Comparison of the properties of Nextel 720/SiC composites at room temperature and 1300°C

Continuous Nextel 720 fibers reinforced SiC composites with PyC interface are fabricated by LPCVI at 1000°C for 200 h using SiCH₃Cl₃ as precursor. The mechanical properties at RT and 1300°C are measured by three-point bending. The microstructures of the interface are characterized by TEM. The results indicate the composites have the metal-like behavior of fracture, whether they are at RT or high temperature. The RT and 1300°C strengths are 310 MPa and 140 MPa, respectively. The RT and 1300°C strains are 0.32% and 0.12%, respectively. The loss of flexural strength and strain of the Nextel 720/SiC composites at high temperature result from stronger residual thermal stress caused by the mismatch of CTE between fibers and matrix. A gap appears between fibers and PyC interface after the 1300°C test, which could be resulted from 7.7% compressive strain of PyC interface caused by the residual thermal stress and 0.1% sintering shrinkage of Nextel 720 fiber.     

应用光纤布喇格光栅传感器监测地下管道腐蚀的新方法研究

地下管线工作环境恶劣,腐蚀是引起它破坏和失效的主要原因之一.本文提出了一种应用光纤光栅传感器监测管道腐蚀的新方法.输送石油和天然气等一些物质的管道内部存在的压力会使管道发生膨胀,进而导致管道外壁产生环向变形.假定管道为无限长,管道内部压力导致的管道外壁轴向应变可忽略不计,当管线发生腐蚀时,会导致管壁变薄,管道内部的压力又可以看作基本不变,这样就会增大管道外壁的环向应变.当管道内部压力保持恒定时,管道壁厚与应变之间存在一定的反比例关系,本文方法就是将封装好的光纤光栅传感器粘贴在管道外壁,通过监测外壁环向变形来判断管道腐蚀的情况和程度.通过理论值、模拟值和试验值三者的对比分析来说明了这种新方法是可行的,并且值得被广泛应用,为埋地管道的防灾减灾提供了一种对策.     

Damage monitor and life prediction of carbon fiber-reinforced ceramic-matrix composites at room and elevated temperatures using hysteresis dissipated energy-based damage parameter

In this paper, the damage monitor and life prediction of carbon fiber-reinforced ceramic-matrix composites (C/SiC CMCs) have been investigated using the hysteresis dissipated energy-based damage parameter. The evolution of the interface shear stress, hysteresis dissipated energy, hysteresis dissipated energy-based damage parameter and the broken fibers fraction vs. cycle number, the fatigue life S?N curves of unidirectional, cross-ply and 2.5D C/SiC composites at room temperature and 800 °C in air atmosphere have been analyzed. For unidirectional C/SiC, the hysteresis dissipated energy and hysteresis dissipated energy-based damage parameter first increase and then decrease with cycle number, and the fatigue limit stress decreases from 88% tensile strength at room temperature to 20% of the tensile strength at 800 °C in air atmosphere; for cross-ply C/SiC, the hysteresis dissipated energy and hysteresis dissipated energy-based damage parameter decrease with increasing applied cycles, and the fatigue limit stress decreases from 85% tensile strength at room temperature to 22% tensile strength at 800 °C in air; and for 2.5D C/SiC, the hysteresis dissipated energy and hysteresis dissipated energy-based damage parameter increases with cycle number, and the fatigue limit stress decreases from 70% tensile strength at room temperature to 25% tensile strength at 800 °C in air.     

Gas volume fraction and velocity profiles

Upward inclined gas-liquid flows are frequently encountered in the oil industry and data relating to the local gas volume fraction distribution and the local gas velocity distribution is important, for example, in pressure gradient prediction and in modeling oil well ‘blowouts’. In this paper measurements are presented of the local gas volume fraction distribution and the local axial gas velocity distribution which were taken in bubbly air-water flows in an 80 mm diameter pipe which was inclined at angles of 0°, 15° and 30° to the vertical. Qualitative arguments are presented to explain the influence of the liquid superficial velocity on the local gas volume fraction distribution in inclined flow and also to explain the very high axial gas velocities observed towards the upper side of the inclined pipe.     

A COMPARISON OF THE HOT-COMPACTION BEHAVIOR OF ORIENTED,HIGH-MODULUS,POLYETHYLENE FIBERS AND TAPES*

The purpose of this article is twofold. First, there is an account of the hot-compaction behavior of a new, highly oriented, high-modulus polyethylene (PE) tape with the trade name of Tensylon® (manufactured by Synthetic Industries, USA). This tape, produced by a melt spinning route, has mechanical properties comparable to those of commercially available gel-spun fibers. Unidirectional samples were produced for a range of compaction temperatures to determine the optimum compaction conditions to obtain the best mechanical properties of the resulting compacted sheets. Second, the mechanical properties of the best Tensylon sample, manufactured at a compaction temperature of 153°C, was compared with three other hot-compacted, highly oriented PE materials, based on Certran®, Dyneema®, and Spectra® commercial PE fibers. The results showed that the optimum compaction temperature was in most cases about 1°C below the point at which substantial crystalline melting occurred. At this optimum temperature, differential scanning calorimetry (DSC) melting studies showed that approximately 30% of the original oriented phase had been lost to bond the structure together. In the case of Dyneema, the properties of the fiber were not translated into the properties of a compacted sheet, and morphological studies showed that this was because melting did not occur on the fiber surfaces, but rather in the interior of the fiber due to a skin structure. The properties of the compacted Tensylon tapes were found to be exceptional, combining very high modulus and strength with interlayer bonding and good creep resistance. Moreover, the optimum temperature appeared to be about 2°C below the point at which complete melting occurred, giving a wider processing window for this material.

     

Oriented Crystallization of Polyethylene in Compatible Blends with Polyamide 11

The oriented crystallization of polyethylene (PE) in uniaxially oriented films of compatible blends with polyamide 11 (PA11) was studied. The PE sample used was a random copolymer of PE with methacrylic acid (MAA), poly(ethylene-co-methacrylic acid) (PEMAA), with 4wt% MAA units. Oriented films of PA11/PEMAA blends were prepared by uniaxial drawing of the melt-mixed blends. The drawn films with fixed lengths were heat-treated at 120°C for 3min to melt the PE component, followed by cooling the sample to room temperature at a rate of 2°C/min to recrystallize the PE (designated slowly crystallized sample). The PE component crystallized in elongated domains of PEMAA with diameters of 0.5–2 μ m and lengths of 5–10 μ m for the PA11/PEMAA = 80/20 blend, resulting in the oriented crystallization of PE crystals. The crystal b-axis of PE was highly oriented in the direction perpendicular to drawing, while the crystal a-axis was weakly oriented in the drawing direction in the slowly crystallized sample of the PA11/PEMAA = 80/20 blend. The a-axis orientation of PE crystals in the PA11/PEMAA = 80/20 blend contributes to the improvement of mechanical properties in the direction perpendicular to drawing.     

Crystallization Behavior of Amorphous Poly(l-Lactide)

Amorphous poly(l-lactide) (PLLA) was annealed in two different ways: amorphous samples were heated at a given temperature to induce crystallization (one-step annealing); and amorphous samples were first crystallized at a low temperature and subsequently annealed at a higher temperature than the crystallization temperature. Samples thus prepared were measured by DSC. The original amorphous sample exhibited an exothermic peak at about 100°C (exothermic peak I), an exothermic peak just below the melting point (exothermic peak II), and an endothermic peak when it was melted. Exothermic peak I was caused by cold crystallization. When the melting points of PLLA samples, heat-treated in various ways, were plotted as a function of annealing temperature, there was discontinuity at about 120°C. From analyses of wide-angle X-ray diffraction patterns, it was found that when amorphous PLLA was crystallized at a temperature below 120°C, crystallites of the β-form formed, and when annealed at a temperature above 120°C, crystallites of the α-form grew. Thus, exothermic peak I was attributed to cold crystallization of the β-form, and peak II was caused by the phase transition of the β-form to a more stable form.     

Production of biologically inert Teflon thin layers on the surface of allergenic metal objects by pulsed laser deposition technology

Allergic-type diseases are current nowadays, and they are frequently caused by certain metals. We demonstrated that the metal objects can be covered by Teflon protective thin layers using a pulsed laser deposition procedure. An ArF excimer laser beam was focused onto the surface of pressed PTFE powder pellets; the applied fluences were 7.5–7.7 J/cm². Teflon films were deposited on fourteen-carat gold, silver and titanium plates. The number of ablating pulses was 10000. Post-annealing of the films was carried out in atmospheric air at oven temperatures between 320 and 500 °C. The thickness of the thin layers was around 5 μm. The prepared films were granular without heat treatment or after annealing at a temperature below 340 °C. At 360 °C a crystalline, contiguous, smooth, very compact and pinhole-free thin layer was produced; a melted and re-solidified morphology was observed above 420 °C. The adhesion strength between the Teflon films and the metal substrates was determined. This could exceed 1–4 MPa depending on the treatment temperature. It was proved that the prepared Teflon layers can be suitable for prevention of contact between the human body and allergen metals and so for avoidance of metal allergy. Received: 12 June 2002 / Accepted: 13 June 2002 / Published online: 4 November 2002 RID="*" ID="*"Corresponding author. E-mail: bhopp@physx.u-szeged.hu     

Spherical shape BaO-ZnO-B<Subscript>2</Subscript>O<Subscript>3</Subscript>-SiO<Subscript>2</Subscript> glass powders prepared by spray pyrolysis

Fine-sized BaO-ZnO-B₂O₃-SiO₂ (BZBS) glass powders were directly prepared by high temperature spray pyrolysis. The hollow glass powders prepared at low preparation temperature of 1000 °C had a low density of 2.65 g/cm³. However, the densities of the BZBS powders obtained at preparation temperatures of 1200 and 1400 °C were each 3.92 and 4.13 g/cm³. The mean size of the BZBS glass powders prepared by spray pyrolysis at preparation temperature of 1400 °C was 0.98 μm. The glass transition temperature (T_g) of the prepared BZBS glass powders was 518.9 °C. The dielectric layers formed from the prepared BZBS glass powders with a dense structure had a clean surface and a dense inner structure without voids at the firing temperature of 580 °C. The transparencies of the dielectric layers formed from the prepared BZBS glass powders were higher than 90% within the visible range. PACS 42.70.Ce; 85.60.Pg; 71.55.Jv     

Elevated temperature directional recrystallization of high-purity nickel

Electron backscattered diffraction has been used to characterise the three different kinds of boundaries that occur in grains that are generated by secondary recrystallization during directional annealing of high-purity nickel. Boundaries between columnar grains (CC boundaries) can be twin boundaries, low-angle boundaries or high-angle grain boundaries. The frequency of low-angle CC boundaries dropped from 25% to 0% while the frequency of the high-angle CC boundaries increased from 19% to 67% when the annealing temperature was increased from 1000°C to 1200°C. The misorientation angles of boundaries between columnar grains and small equiaxed grains ahead of them (CE boundaries) was random at 1200°C but had a 40° rotation relationship about ?111? at 1000°C. It was found out that the character of the CC boundaries is determined by relative mobility of the CE boundaries, which is determined by the processing temperature rather than the energy of the CC boundaries themselves. The character of the island grain boundaries sometimes found with columnar grains was not affected by the annealing temperature or the drawing velocity.