Environmental stress cracking and morphology of polyethylene

Studies have been made of the fracture surfaces of high density polyethylene which failed by environmental stress cracking at very low stress. The failure appears to be almost entirely brittle, contrary to the evidence of plastic deformation and void formation reported by other workers. The failure occurred either in an interlamellar manner within a spherulite, or, when the lamellae matched poorly across an interspherulitic boundary, by cracking along that boundary. Because of the brittle nature of the failure, the fracture surface provides information on spherulite morphology in bulk crystallized material.     

Environmental stress cracking behavior of short-chain branch polyethylenes in Igepal solution under a constant load

An investigation of the influence of branch length and crystallinity on environmental stress cracking properties of short-chain branch polyethylenes (SBPEs) in Igepal solution is reported. The precise value of Igepal transition time (ITT) and difference between the failure process of SBPEs in air and in Igepal solution were determined by comparing their plots of notch opening displacements vs. time in air and in Igepal solution. Igepal transition time can only be found as the failure time (t_fI) is greater than the critical time Igepal required to “accelerate” the fracture of SBPEs in Igepal solution. Prior to ITT, time dependence of notch opening displacements and fracture surfaces of samples in Igepal solution were similar to those in air. In contrast, obvious voids appeared in the base of craze and crazelike structures containing clear voids were found on the fracture remnants of samples with t_fI longer than their ITTs. The value of ITT and t_fI were found to increase significantly for samples associated with higher crystallinity. However, no significant difference in value of ITT was found for samples with the same crystallinity, tie-molecule density, molecular weight, branch frequency, but different branch length. Finally, environmental stress cracking resistance (ESCR) and t_fI of SBPEs in Igepal solution increased dramatically as the short-chain branch length increased. This dramatic improvement in environmental stress cracking properties with short-chain branch length is attributed to the increasing sliding resistance of the polymer chains through the crystal and through the entanglement in amorphous region at time before and after ITT. © 1994 John Wiley & Sons, Inc.     

Environmental stress cracking resistance of blends of high-density polyethylene with other polyethylenes

A special modified tensile creep test was used to investigate the stress cracking behavior of various high-density polyethylenes (HDPE). Blends of HDPE with other HDPEs, with linear low-density polyethylene (LLDPE), and specifically with various slightly long-chain branched linear low-density polyethylenes (HBPE) were tested for their failure times. Whereas HDPE blends, including higher-molecular-weight HDPE components, yield only a minor improvement in stress cracking resistance, a considerable improvement was produced when an LLDPE weight fraction of 0.3 or more was used. Adding HBPE also improves the environmental stress cracking resistance. Environmental stress cracking resistance improves with increasing HBPE content, and, for a constant HBPE concentration, it increases with increasing octene content of the HBPE. Adding HBPE with a low octene content, however, results in reduced failure times of the blend compared with HDPE blends that exhibit relatively good environmental stress cracking behavior. The results are explained in terms of the tie-molecule density.     

Effect of high-energy radiation on the uniaxial tensile creep behaviour of ultra-high molecular weight linear polyethylene

The tensile creep (and other tensile) properties of ultra-high molecular weight polyethylene (UHMW PE) have been determined before and after electron beam irradiation and compared with similar results on normal molecular weight high-density polyethylene (NMW PE). In both polymers, irradiation increases the tensile modulus and the yield stress whilst reducing creep. The major effects occur over the first 20 MRad irradiation dose, though creep strain continues to diminish with dose in UHMW PE up to 64 MRad. Most of the effects can be attributed to crosslinking in the amorphous phase, though the rise in yield stress seems to require crosslinking in the crystalline phase, and the initial rise in modulus in UHMW PE seems to reflect a rise in crystallinity. Comparison with other polymers shows that the creep behaviour of UHMW PE remains relatively poor, even after irradiation. The improvements obtained may, however, be significant in applications where creep resistance is of secondary importance compared with, say, impact and wear resistance, in which UHMW PE excels.     

Effect of molecular weight and branch content on the creep behavior of oriented polyethylene

Creep studies were carried out on a range of homopolymers and copolymers of polyethylene with well‐defined molecular weight and branch content. The creep data were analyzed in terms of two thermally activated processes acting in parallel and the effects of molecular weight and branch content are discussed. It is shown that increasing either the number‐average molecular weight or the weight‐average molecular weight gives improved creep behavior at all stress levels. The introduction of butyl branches leads to lower creep at low‐stress levels but can give rise to higher creep at high stress. Plots of the equilibrium log₁₀(strain rate) versus stress at fixed draw ratio (strain) can be used to define sections through a unique true stress/true strain/strain rate surface for each material. These creep results have an additional value in terms of the link between slow crack propagation (SCG) in polyethylene and fibril creep, confirming the proposal made elsewhere that SCG can be quantified in terms of creep to failure across the true stress/true strain/strain rate surface. © 2003 Wiley Periodicals, J Appl Polym Sci 89: 1663–1670, 2003     

Effect of rotation extrusion and molecular weight on ductile failure of polyethylene pipes under hydrostatic pressure

ABSTRACT

In this study, two kinds of polyethylene (PE) with different molecular weight were processed into the pipes via rotation extrusion and the failure behaviours under hydrostatic pressure as well as molecular relaxation were investigated. The experimental results showed that the high molecular weight PE exhibited slower relaxation behaviour with higher relaxation activation energy to facilitate the formation of shish-kebab under flow field, while for the low molecular weight one, the stretching molecules easily relaxed back coil state and spherulites were prone to form. Therefore, the low molecular weight PE pipes via convention and rotation extrusions had similar isotropic spherulite morphology and short failure times. In the case of high molecular weight PE, during the convention extrusion, polymer melts flowed along the axial direction to induce the alignment of shish-kebab accordingly, which went against to the hoop stress in hydrostatic pressure. With the rotation of mandrel and die, the direction deviated from the axis so that PE pipe exhibited better resistance to the hoop stress. The failure time was 182?h, 264% longer than the convention-extruded one. Accordingly, a new strategy to prepare high hydrostatic pressure PE pipe under cooperative effects of rotation extrusion and long molecular relaxation time was proposed.     

PE100管及其热熔接头的蠕变裂纹扩展行为

聚乙烯(PE)管性能优异,广泛应用于城市水及燃气供应系统。PE管的主要破坏形式是长期静压载荷下的慢速裂纹扩展失效。在蠕变条件下,采用光滑试样和裂纹圆棒试样对PE100管及其热熔接头进行了测试,得到了基于蠕变断裂参数C*的蠕变裂纹扩展动力学关系式。利用扫描电子显微镜(SEM)分析了裂纹圆棒试样的断面形貌,对比分析结果发现,蠕变裂纹扩展失效模式对应的最大应力为15.05 MPa,热熔接头熔合面分布有约11个/mm^2、直径范围为1~5μm的微气孔,热熔接头断裂微纤平均长度比母材约小20%~45%。当热熔对接时,熔合面存在的微气孔以及系带分子的浅渗透是导致PE100热熔接头蠕变裂纹扩展抗力降低的主要原因。     

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。     

Effect of rubber on the environmental stress-crack resistance of polyethylene

The property of environmental stress cracking is a critical performance factor in the polyethylene product areas of rigid and flexible containers, wire and cable insulation and pipe or conduit. It has been known for quite some time that the addition of an elastomeric material to polyethylene can improve its resistance to environmental stress cracking. However, the information reported in the literature on the subject is rather limited and as far as can be determined no attempt has been made to determine the rubber variables affecting the environmental stress cracking performance of polyethylene. In studying the effect of rubber on the environmental stress crack resistance (ESCR) of low and high density polyethylene, the choice of base resin is important. With some resins the addition of rubber doubles the ESCR while with others a fiftyfold improvement can be effected. Of the elastomers investigated in this study, Enjay Butyl 007 a copolymer of isobutylene and isoprene gave the greatest stress-crack improvement in both low and high density polyethylene. Evaluation of a series of polyisobutylenes indicate that the higher its molecular weight the more effective its performance as a stress crack additive. The effect of rubber on other properties of the polyethylenes such as melt index, stiffness, permeability, chemical resistance, brittle point temperature, and impact strength is also discussed.     

Miscibility and crystallization of metallocene polyethylene blends with polypropylene

The crystallization and morphology of very‐low‐density polyethylene (VLDPE) and ultra‐low‐density polyethylene (ULDPE) blends with isotactic polypropylene (PP) were studied by differential scanning calorimetry (DSC) and hot‐stage optical microscopy (HSOM) with polarized light. In particular, the isothermal crystallization of PP in molten PE was investigated. A polypropylene homopolymer was melt‐blended with six types of VLDPEs and ULDPEs, with variations in branch content and length and in molecular weight. All the blends contained 20% PP by mass. It was found that the crystallization temperatures of PP and PE changed in the blends, and the crystallization of PP was affected by branch length and content and by the molecular weight of the PE, indicating a certain degree of miscibility between PP and PE. The isothermal crystallization rate of PP decreased in the blends; in particular, the crystallization rate of PP was slower in the ULDPE with lower MFI, suggesting that crystallization of PP was hindered by PE and that its rate was regulated by the viscosity of ULDPE. HSOM images showed that a portion of the PP crystallized from molten PE, although phase separation was obvious, providing additional information on the miscible behavior between PP and VLDPEs (or ULDPEs). Furthermore, the miscible level between the PP and the ULDPEs was higher than that between the PP and the VLDPEs because the degree of change in the crystallization behavior of the PP and PE was greater in the PP–ULDPE blends. This was possibly a result of the higher branch content in the ULDPE. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 87: 1179–1189, 2003     

Embrittlement and stress cracking of polyethylene by fuming nitric acid

Oxidative embrittlement and stress cracking of high-density polyethylene has been studied by using fuming nitric acid at 60°C as the oxidative agent. Oxidative stress cracking was found to be insensitive to changes in molecular weight in contrast to environmental stress cracking in a surface-active medium. The oxidative attack was found to be influenced by the surface crystalline texture of the polyethylene. Oriented polyethylene showed an increased resistance to oxidative embrittlement and stress cracking, the mode of failure being dependent upon the prior annealing treatment.     

双峰PE80级管材专用树脂的开发

介绍了高密度聚乙烯管材专用树脂6366M的结构特性、物理性能及其加工应用试验的情况。结果表明，6366M具有相对高的相对分子质量，其相对分子质量分布结构特性呈双峰分布。该材料具有优异的力学性能和耐环境应力开裂性能，而且加工性能较好，实现了刚性和韧性的平衡。成品管材的性能指标可以满足用户的需求。     

石墨烯/聚乙烯复合改性沥青胶结料的流变性能研究

为提高沥青胶结料的综合路用性能,尤其是高温性能,本文采用高速剪切机将质优价廉的聚乙烯(PE)与石墨烯纳米片(GNPs)复合制备新型沥青胶结料,同时使用温度扫描(TeS)、多重应力蠕变恢复(MSCR)、线性振幅扫描(LAS)和傅里叶变换红外光谱(FTIR)研究了石墨烯/聚乙烯复合改性沥青胶结料的流变性能和作用机理。结果表明:GNPs和PE能够协同改善沥青胶结料的高温性能,提高路面的高温车辙抗性;预混的PE/GNPs母粒具有良好的中温疲劳和低温开裂抗性。同时复合改性沥青的FTIR光谱中未出现新的吸收峰,表明石墨烯和聚乙烯在沥青基体中以物理改性为主。     

Microstructural aspects of failure in semicrystalline polymers

Microstructural features of semicrystalline polymers are reviewed, as are the roles of chain properties and thermal history in their development. Experimental results relating failure properties to microstructural detail are described. General models to explain inter- and intra-spherulitic failure behavior are proposed. Important are (a) the role of spherulite size in boundary incompatibility, (b) competition between spherulite boundary cracking and intraspherulitic yielding, (c) competition between localized (interspherulitic) fracture and dispersed (intraspherulitic) fracture.     

聚乙烯通讯电缆护套料DFH2076的开发

介绍了在全密度聚乙烯（PE）装置上生产PE通讯电缆护套料DFH2076基料的生产技术和经验，解决了生产过程中出现的反应器静电、结块以及产品熔体流动指数控制的难点。研制的PE电缆通讯护套料产品具有分子量分布较宽、氧化诱导期达到130min、耐环境应力开裂性超过600h、加工性能良好等特点，经过国家检测中心的评定，各项性能指标达到或超过了国家标准。在用户实际使用中，产品加工性能良好，挤出的电缆稳定光滑，完全满足了用户的要求。     

A DSC investigation of the crystallization kinetics of polyoxymethylene

The large spherulite size, heterogeneous nucleation and constant spherulite growth rate of POM suggests a straightforward two dimensional Avrami analysis, with two adjustable parameters, t_O(T) and K(T), to be appropriate to thin samples in the DSC. For T<147°C, spherulite densities inferred from results for the rate parameter K(T) and microscopical measurements of spherulite growth rates are consistent with observed spherulite sizes. The K(T) values may also be used to model non-isothermal crystallization in the same temperature range (dT/dt<10 K/minute). Whilst K(T) shows little dependence on molecular weight, samples containing 30 wt% TPU crystallize more slowly owing to a reduced nucleation density.     

影响聚乙烯管材耐环境应力开裂的因素及对策

讨论聚乙烯(PE)分子量及其分布、结晶度、分子链结构、组分和成型工艺对PE管材耐环境应力开裂(ESCR)性能的影响,并提出改善PE管材ESCR性能的措施     

Short-term mechanical and structural approaches for the evaluation of polyethylene stress crack resistance

Short-term mechanical behavior together with structural habits at various scale levels of polyethylene are studied in relation to molecular architecture in order to contribute to the understanding of the environmental stress crack resistance (ESCR). Three series of homopolyethylene and ethylene-hexene copolymers issued from 1st and 2nd generation chromium oxide catalysis as well as tandem-reactor Ziegler-Natta catalysis are investigated. Properties are discussed via the incidence on the chain topology of the co-unit concentration and distribution as well as the molar weight distribution. The creep compliance is mainly governed by density, i.e. stiffness. However, at similar density, the three polymers series displays a clear compliance drop that follows the increasing ESCR hierarchy. The natural draw ratio displays monotonous decrease with decreasing density that perfectly parallels the ESCR increase. This property also holds for materials of similar density. The creep compliance and the natural draw ratio are indicators of the macromolecular network strength via the intercrystalline tie molecules. Stepwise isothermal segregation (SIS) allows indirect assessment of the chain fraction that generates a high rate of tie molecules. Correlation is made between SIS and ESCR via the tie molecules that provide strength to the macromolecular network. SIS also enables discriminating polymers of similar density but different molecular architecture. Small-angle X-ray scattering affords quantitative evaluation of the stacking disorder of the semi-crystalline microstructure for which a correlation is established with the crystallization kinetics and the resulting chain topology.     

Influence of powder particle size on microstructure and performance of calcium oxide based ceramic core

A calcium oxide (CaO) based ceramic core used for titanium alloy casting was prepared by injection moulding technology. Through quantitative characterisation and statistical analysis of the microstructure, the influence of powder particle size on the microstructure and properties of the ceramic core was investigated. The results show that, by increasing powder particle size, the size of pores and porosities after sintering were increased with decreased of grain boundary density. Meanwhile, the flexural strength of ceramic core at room temperature was found to decrease with increasing core porosity, and the creep resistance of ceramic core increased with decreasing grain boundary density. The increase in powder particle size of ceramic core is beneficial to reduce linear shrinkage and improve high temperature creep resistance.