Fatigue behavior of medium-density polyethylene pipes for gas distribution

This paper illustrates the factors that control brittle failure under fatigue loading for test specimens cut from medium-density polyethylene pipes for gas distribution. A square bar specimen cut from a pipe with a notch was made and a fatigue test was conducted to cause a brittle failure. To obtain the correlation among stress range, frequency, temperature, and cycles to failure in this fatigue test, Coffin-Manson's frequency-modified fatigue life equation was adopted and the material constants were determined. By gradually lowering the frequency, the resistance to creep can be estimated because cycles to failure—indicating the fatigue damage—decreased, and the actual loading time—indicating the creep damage—increased.     

Degradation of unstabilized medium-density polyethylene pipes in hot-water applications

Pipes of an unstabilized medium-density polyethylene have been pressure tested with internal stagnant water and moderately circulating air as the external medium at temperatures ranging from 70 to 105°C and changes in molecular structure and crystallinity have been studied. The stage III (fracture induced by thermal oxidation) life of the unstabilized polyethylene pipes was less than 12% of the life of the corresponding stabilized polyethylene pipes. Infrared spectroscopy and size exclusion chromatography showed an earlier and more extensive increase in the quantities of oxidation end-products and a more pronounced decrease in molar mass of the outer-wall-material than of the inner-wall-material of the pipe. Mass crystallinity, measured by differential scanning calorimetry, increased on an average by a quantity corresponding to 45 methylene groups per chain scission event. The life of the unstabilized pipe was divided into an induction period during which no detectable thermal oxidation occurred and a subsequent polymer degradation period. The induction period exhibited an Arrhenius-temperature-dependence with an activation energy of 75 kJ mol^?1.     

Structure and crack growth in gas pipes of medium-density and high-density polyethylene

The microstructure and resistance to slow crack growth of two commercial polyethylene pipe materials were studied. Differential scanning calorimetry, small-angle X-ray scattering, and transmission electron microscopy were used to reveal the crystallite thickness and width distributions and the size of the lamellar stacks. The resistance to slow crack growth was assessed by uniaxial constant loading of notched specimens and by hydrostatic pressure testing of notched and unnotched pipes. The high-density material contained roof-lamellae, suggestive of a segregation of low molar mass species. Notched uniaxial testing revealed large differences in slow crack growth-resistance between the two PEs despite the fact that the average tie-chain concentration was similar. Hence, low-molar mass segregation, which was found to be higher for the high-density material, definitely decreases the resistance to slow crack growth. Notched uniaxial testing was a sensitive method for ranking these PEs according to their resistance to slow crack growth, and 15 times faster than that achieved in conventional unnotched pressure testing. Failure time extrapolations from higher temperatures to 20°C were made, using a multiple linear regression method (SEM-Q1), the Arrhenius equation, and universal shift-functions to investigate their applicability. The extrapolations resulted in longer life times compared with experimental data, regardless of the method used. The SEM-Q1 method (lower-confidence-limit data) gave the best fit to the 20°C experimental data followed by the Arrhenius equation.     

Molecular and lamellar structure of an extrusion-grade medium-density polyethylene for gas distribution

An extrusion-grade medium-density polyethylene was fractionated by size exclusion chromatography and selective solvent extraction. The fractions were studied by infrared spectroscopy, differential scanning calorimetry, and transmission electron microscopy. Infrared spectroscopy showed that chain branching was concentrated to the low molar mass material. Transmission electron microscopy showed the presence of roof-shaped crystals in the low molar mass samples and curved S-or C-shaped lamellae in the high molar mass samples. These features could be used for identification of different molar mass species in the unfractionated sample.     

The effect of carbon black on the oxidative induction time of medium-density polyethylene

An extensive series of: (i) oxidative induction time (OIT) measurements derived from differential scanning calorimetry experiments, and (ii) stabilizer analyses by high performance liquid chromatography were conducted on medium-density polyethylene (MDPE) stabilized with various levels of Irganox 1010 (penta-erythrityl-tetrakis-(3,5-di-tert-butyl-4-hydroxyphenyl propionate), Irgafos 168 (tris-(2,4-di-tert-butyl-phenyl)-phosphite), and carbon black (CB). The OIT data were analyzed using a novel interpretation method, which is described and applied. It was found that: (i) addition of CB to base-stabilized MDPE increases stability and reduces OIT variability; (ii) the optimum effective concentrations of Irganox 1010, Irgafos 168 and CB are 1690 ppm, 1580 ppm and 2.5% w/w, respectively; and (iii) interactions between CB and the base co-stabilizers may occur at high loadings of the latter but CB does not affect the extraction of the base co-stabilizers which are extracted with efficiencies of ca. 80% and 100% for Irganox 1010 and Irgafos 168, respectively.     

Antioxidant diffusion in polyethylene hot-water pipes

This paper presents oxidation induction time (OIT) data for samples taken from a range of polyethylene hot-water pipes before and after hydrostatic pressure testing with internal stagnant water and external air. Linear relationships between OIT and antioxidant concentration were established for the antioxidants/polymers used. A model assuming Fickian diffusion of the antioxidant to the surrounding media and chemical consumption of the antioxidant (Regime B model) was applied to the OIT-profile data. The Regime B model was successfully applied to OIT-profile data except for in the case of a pipe with a substantial scatter in the OIT data, indicative of compositional heterogeneity. The choice of initial conditions for the modeling was critical. The use of initial conditions based on insufficiently exposed pipes led to an overestimate of the diffusivities caused by the combined action of Regimes A (internal precipitation) and B loss mechanisms. The time period associated with Regime A constituted up to 25% of the lifetime for the pipes studied. Antioxidants with sterically accessible polar groups showed a higher melting point and greater interaction with dissolved water and carbon black. Obtained radial dependences of the antioxidant diffusivities (D) indicate that the water concentration in the polymer influenced D primarily through cluster formation involving water and antioxidant molecules and by competition between water and antioxidant molecules about adsorption sites on carbon black particles and to a much lesser extent by plasticization. Antioxidant concentration profiles calculated for the failure time interval in pressure testing appeared in the same concentration range, and fracture initiation occurs in the regions in the pipe wall first reaching depletion of the antioxidant system.     

聚乙烯塑料管的设计选择

概略介绍了聚乙烯塑料管的材料性能规格、作为压力管线的设计选择,以及管子安装的要求。     

The joining of biaxially oriented polyethylene pipes

Die-drawing techniques recently developed at Leeds University make it possible to produce oriented polymer tubes with both axial draw and hoopwise expansion. These products have increased axial stiffness, improved barrier properties, and excellent resistance to chemical reagents. Normally mechanical methods are used to join such tubes in order to preserve their orientation: however we show that electrofusion techniques produce joints of strengths such that in tensile tests, failure always occurs in the fittings, not at the joint interfaces. Optical and electron microscopy reveal different zones in the welds and indicate that only 20 percent of the wall thickness is affected by the electrofusion process. The pipe studied was biaxially drawn medium density polyethylene of outside diameter 63 mm with draw ratios of 4 in the axial direction and 2, inner hoop. Optimum welding conditions were determined using socket and saddle electrofusion fittings. The joints did not fail in a standard crush test. Careful control of welding parameters is essential in butt fusion welding when the maximum weld strength exceeds that of the undrawn polymer.     

交联聚乙烯管国内外发展概况

介绍了交联聚乙烯(PE-X)及其管材的交联机理和产品特点.比较了过氧化物PE-X管、硅烷PE-X管、辐照PE-X管的生产工艺和产品优缺点.综述了PE-X管国内外近几年的发展、市场需求、生产以及专用料的开发应用情况,针对地暖管领域目前市场使用的PE-X管、耐热聚乙烯管、无规共聚聚丙烯管、聚1-丁烯管进行性能比较,阐述了P...     

耐热聚乙烯管材专用树脂的性能

研究了耐热聚乙烯管材专用树脂QHM22F的常规物性、长期热稳定性、加工性能、静液压强度等,并在管材生产厂家进行了不同口径的管材加工应用试验。结果表明:QHM22F的简支梁缺口冲击强度在70 kJ/m2以上;200℃氧化诱导期大于120 min;熔体强度在0.24 N以上;加工性能良好,可满足高速牵引的加工需要。采用QHM22F生产的管材内外表面光滑、壁厚均匀,性能满足用户要求。     

Fatigue crack growth in polyethylene

Fatigue crack propagation (FCP) rates are studied in 6 mm thick specimens of high density polyethylene (HDPE) containing razor notches, Centrally-notched plates and single-edg notched bars are subjected to sinusoidal tension-compressio or tension-zero cycling at 0.5 or 2.0 Hz under load control a room temperature; crack growth is monitored using a travelling microscope. After many thousands of cycles with no observable damage at the tip of the razor notch, a craze like zone begins to form. This zone grows slowly until it reaches the length characteristic of a mature crack at the same ΔK. Crack growth proper then begins. The number of cycles to initiate crack growth falls linearly with increasing ΔK at the razor notch Subsequent crack growth is determined both by the current value of ΔK and by loading history. When ΔK is increasing, FCP rates follow a standard Paris law curve. However, reduced, FCP rates are observed following an overload.     

Morphology in thermally oxidized high density polyethylene pipes

In a continuation of previous work (1), the melting and crystallization behavior of the layered oxidized skin on thermally oxidized inner wall surfaces of different high density polyethylene (HDPE) pipes was studied. By hot stage polarized light microscopy on 5 μm thick cross-sections of the skin, melting was shown to proceed successively as a front moving inwards as temperature was raised. Analogously, crystallization of the skin layers proceeded with the front advancing outwards towards the skin surface at decreasing temperature. The kinetics were followed and the data was compared with previous DSC thermograms (1) on similar samples. The high temperature melting peak of the oxidized skin reported in earlier DSC-work (1) is shown to be associated with material with normal spherulitic texture.     

耐热聚乙烯管材料的稳定性

研究了3种耐热聚乙烯(PE-RT)管材料的氧化诱导期、加工稳定性、长期热稳定性等。结果表明:3种管材料的氧化诱导期(210℃)均在60 min以上,其中,PE-RT QHM22F的加工稳定性和长期热稳定性最优,5次造粒后熔体质量流动速率变化率小于3%,4 800 h热水老化后断裂标称应变大于700%。     

The anisotropy of slow crack growth in polyethylene pipes

Slow crack growth was measured in the perpendicular and parallel directions relative to the extrusion direction of the pipe. For five pipes from different manufacturers, the anisotropy factor, the lifetime to fracture in the perpendicular direction divided by lifetime in the parallel direction, varied from 1.2 to 4.7 for complete fracture and 1.4 to 4.0 for crack initiation. The degree of molecular orientation was determined by measuring the shrinkage that occured when a pipe specimen was heated near its melting point. The amount of shrinkage correlated with the anisotropy factor for slow crack growth. The shape change after shrinkage was related to the flow pattern of the resin during extrusion and the cooling rate after extrusion.     

管材专用高密度聚乙烯的研究进展

综述了国内高密度聚乙烯(HDPE)的生产装置及工艺。采用双峰聚合工艺使短支链更多地分布在高相对分子质量部分是HDPE管材从PE80级升至PE100级的主要原因。管材的耐环境应力开裂性能随HDPE相对分子质量减小而下降,提高短支链含量可提高管材的耐环境应力开裂性能。HDPE的相对分子质量越高,管材抗裂纹扩展性能越好,将短支链分布在高相对分子质量端可提高抗裂纹扩展性能。     

Development of glass fibre reinforced polyethylene pipes for pressure applications

Abstract

Thermoplastic filament winding with in line melt impregnation has been investigated for the manufacture of continuous glass fibre reinforced thermoplastic pipes. With polyethylene matrixes it was found that the high melt viscosity hindered full melt impregnation and resulted in high pull forces in the case of pipe grade polyethylene. Using a lower viscosity polyethylene it was possible to operate the melt impregnation process, but the product obtained exhibited a high void content. Surprisingly, it was found that filament winding resulted in a considerable decrease in void content, to an acceptable level. It was found possible to wind good quality pipes and achieve high failure pressures that fully reflected the strength of the reinforcement.

The non-linear strain response of glass–polyethylene pipes can be modelled using laminate theory modified to take account of the non-linearity of the matrix and the change in fibre angle that occurs as the pipe deforms.     

管材专用高密度聚乙烯的热分级研究

利用差示扫描量热法和连续自成核退火分级(SSA)法研究了PE100级管材专用高密度聚乙烯(HDPE)树脂的结晶熔融行为,三个牌号HDPE树脂的1-己烯共聚单体含量按试样1、试样2、试样3逐渐降低。结果表明:两种方法得到的结晶度和熔融温度均按试样1、试样2、试样3递增,且SSA处理后试样的结晶度和熔融温度较高;短链支化度按试样1、试样2、试样3逐渐降低;试样3表现出最佳的刚性和较低的韧性,而试样1则相反。     

Structure and morphology of thermally oxidized high density polyethylene pipes

The effects of thermal oxidation on the- molecular structure and on the morphology of a series of high-density polyethylene pipes have been studied “as received” or after service. The existence of a 40 to 170 μm thick oxidation skin was established, the structure and morphology of which were characterized by gloss measurements, scanning electron microscopy, polarized microscopy, infrared spectroscopy, gel measurements, and differential scanning calorimetry. The skin surface frequent contained craterlike structures, 25-50 μm in diameter, whereas small-scale structures occurred much less frequently than in non-oxidized surfaces, and the gloss increased significantly. The presence of a carbonyl concentration gradient through the skin was established, and the gel measurements also indicated a crosslink gradient. The skin was found to be composed of a top layer with a mainly non-spherulitic structure of very low crystallinity and melting point and a large-scale spherulitic layer with a crystallinity and melting point slightly below the corresponding bulk values. The formation of this brittle surface layer and its effect on the long-term fracture performance of the pipe are discussed.