An investigation of chemical crosslinking effect on properties of high-density polyethylene

High-density polyethylene (HDPE) was chemically crosslinked with various amounts of di-tert butyl cumyl peroxide (BCUP). Crosslink density determined by rubber elasticity theory using hot set test showed an increase with increasing BCUP. Glass transition temperature (T_g), thermal stability, crystallization, melting behavior and tensile properties were studied. The results showed a new finding about decrease in T_g as a consequence of the ‘chemical crosslinking’ of HDPE. This was explained by observed reduction in crystallinity and expected increase in free volume as a result of restriction in chain packing. However, chemical crosslinking had no significant effect on the thermal stability. The stress at break, Young's modulus yield strength and elongation at break generally decreased with increase in BCUP. By increasing the temperature for slightly crosslinked HDPE, the elongation at break was increased but by increasing the crosslinking level an opposite effect was observed. Crosslinked HDPE showed an decrease in creep strain and an increase in creep modulus with increasing BCUP.     

高密度聚乙烯电子束辐射交联的研究

用高能电子束辐射技术研究了添加敏化剂季戊四醇三丙烯酸酯(PETA)和抗氧剂300的高密度聚乙烯(HDPE)体系的辐射交联效应.通过测定试样辐射后交联度、拉伸强度、直角撕裂强度等性能,考察了辐射剂量、w(PETA)和 w(300)对 HDPE辐射交联的影响;并用差示扫描量热法和热重分析研究了HDPE辐射后的结晶性和热稳定...     

The effect of cooling rate on the impact performance and dynamic mechanical properties of rotationally molded metallocene catalyzed linear low density polyethylene

This article examines changes to the morphology of rotationally molded metallocene catalyzed linear low density polyethylene brought about by varying the cooling rate during processing. These changes in morphology lead to variations in the impact performance, which is reflected in the dynamic mechanical characteristics of the materials. Various analytical techniques are used in an attempt to explain the differences in impact behavior. Slow cooling is shown to result in high crystallinity, and in the formation of large spherulites, which in turn is detrimental to the impact performance of the material, particularly at low temperatures. The high crystallinity corresponds with a shift in the β transition of the material to a higher temperature, and is shown to result in a higher brittle–ductile transition. A case study was also carried out on samples from a finished part provided by an industrial molder, one section of which failed in a brittle manner when impact tested while the other failed in a ductile manner. Microscopy results showed that the brittle material had large spherulites at the inside surface, while the ductile material showed incipient degradation at this surface, which has previously been shown to be of benefit to impact strength in rotationally molded parts. Dynamic mechanical studies again showed a β transition at a higher temperature in the brittle samples. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 101: 1963–1971, 2006     

Antioxidant presence in thick‐walled high‐density polyethylene materials after rotational molding

High‐density polyethylene (HDPE) sample tanks manufactured by using a rotational molding process were used in a study to determine the presence and characteristics of antioxidants at the inside and outside surfaces of the tanks. The sample tanks were manufactured by using three different processing times to create undercooked, ideal, and overcooked tanks. Differential scanning calorimetry (DSC) was used to perform oxidation induction time (OIT) studies by using specimens cut from the surfaces of the tank. The OIT portion of the analysis exposes the melted HDPE specimens to an oxygen environment at 200°C for over 2 h. The DSC monitors change in the energy transfer rate to or from the specimen because of chemical reactions. A numerical integration process was used to analyze the DSC‐OIT data and to obtain additional information about the energy levels measured during the analysis. Fourier transform infrared (FTIR) studies were also performed to determine the chemical characteristics of specimens cut from the processed tanks. Results showed increased degradation at the inside surfaces of the overcooked tanks because of a lack of antioxidants. The results also showed that metal ions from the mold wall could react with the outside surfaces of the tanks and influence the level of antioxidants at that surface. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 92: 3052–3066, 2004     

过氧化物交联HDPE材料的研究

研究了交联用高密度聚乙烯(HDPE)的种类、交联剂二叔丁基过氧化物(DTBP)、助交联剂T的用量对交联HDPE复合材料结构和性能的影响。选用辛烯为共聚单体和支化度(每1000C中的甲基数)为3.2的HDPE为原料树脂,主交联剂DTBP和助交联剂T的质量分数分别为0.15%,0.20%时,材料的各项性能符合交联HDPE管材的要求。     

Blend of high‐density polyethylene and a linear low‐density polyethylene with compositional‐invariant mechanical properties

This article presents the tensile properties and morphological characteristics of binary blends of the high‐density polyethylene (HDPE) and a linear low‐density polyethylene (LLDPE). Two constituents were melt blended in a single‐screw extruder. Injection‐molded specimens were evaluated for their mechanical properties by employing a Universal tensile tester and the morphological characteristics evaluated by using a differential scanning calorimeter and X‐ray diffractometer. It is interesting to observe that the mechanical properties remained invariant in the 10–90% LLDPE content. More specifically, the yield and breaking stresses of these blends are around 80% of the corresponding values of HDPE. The yield elongation and elongation‐at‐break are around 65% to corresponding values of HDPE and the modulus is 50% away. Furthermore, the melting endotherms and the crystallization exotherms of these blends are singlet in nature. They cluster around the corresponding thermal traces of HDPE. This singlet characteristic in thermal traces entails cocrystallization between these two constituting components. The clustering of thermal traces of blends near HDPE meant HDPE‐type of crystallites were formed. Being nearly similar crystallites of blends to that of HDPE indicates nearness in mechanical properties are observed. The X‐ray diffraction data also corroborate these observations. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 83: 2604–2608, 2002     

辐射交联聚乙烯泡沫的研究

通过辐射交联技术制成聚乙烯泡沫 ,研究了辐照剂量、凝胶含量、发泡剂用量、助剂用量、发泡温度、发泡时间对发泡倍数的影响。结果表明 ,辐照剂量 30kGy ,发泡温度 180℃ ,发泡时间 2min可制得发泡倍数 30倍的聚乙烯泡沫。     

High density polyethylene/ultra high molecular weight polyethylene blend. II. Effect of hydroxyapatite on processing,thermal, and mechanical properties

Hydroxyapatite (HA) is part of bone mineral composition. Several attempts have been made to incorporate HA into high density polyethylene (HDPE) to produce bone replacement biomaterials since neat HDPE is not suitable as bone replacement. The blending of HDPE with ultra high molecular weight polyethylene (UHMWPE) up to 50% by weight was performed with the aim of improving the toughness of composites. Reinforcement of blend with HA of up to 50% by weight was carried out. Methods of characterizing the composites included density, differential scanning calorimetry, thermal gravimetric analysis, ash content, and morphological examination using scanning electron microscope. For the mechanical properties of the composites, tensile, flexural, and impact tests were carried out. Incorporation of HA into HDPE has resulted in the brittleness of the composites. Blending of HDPE with UHMWPE in the presence of HA was found to improve the mechanical properties and promote a ductile failure of the resulting composites. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 3931–3942, 2006     

Preparation of granular crosslinkable medium‐density polyethylene

Granular crosslinkable medium‐density polyethylene (XLPE) without scorch inhibitor was prepared adding organic peroxide [2, 5‐dimethyl‐ 2, 5‐ di‐ (t‐butyl‐peroxy) hexyne‐3] through extrution process, in industrial scale. Twin screw extruder was used to mix the polyethylene and the peroxide. The temperature zones of the extruder were controlled very carefully to prevent unwanted crosslinking during extrusion. Compression molding, rotational molding, and injection molding of XLPE at 155°C made no crosslinking in PE, and then they were exposed to higher temperatures at which the organic peroxide decomposed to provide free radicals which led to the crosslinking of the MDPE. The thermal properties (using dynamic mechanical analysis, DMA, differential scanning calorimetric analysis, DSC, and thermogravimetric analysis, TGA, techniques) and mechanical properties (including strain at break and stress at break) of virgin PE, crosslinkable PE, and crosslinked PE have been compared. The crossliked PE and virgin PE were also studied by X‐ray diffraction (XRD) technique. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 104: 1873–1879, 2007     

我国高密度聚乙烯进口情况分析

从高密度聚乙烯(HDPE)的进口量、贸易方式、进口来源和价格等方面分析了我国近几年HDPE的进口情况和变化趋势。我国HDPE的进口量呈快速增长态势。在不同贸易方式中,以一般贸易方式进口HDPE为主,且每月进口HDPE的量波动很大;从进口来源上看,中东国家对我国的出口量增长很快。我国进口HDPE品种全且牌号多,涉及到HDPE应用的各个方面;从中东和东南亚等低成本地区主要进口通用型牌号,而从欧洲、日本、韩国和美国等主要进口高附加值产品。最后,对我国HDPE生产企业提出了相关建议。     

Structure development and property changes in high density polyethylene during pan‐milling

A new self‐designed mechanochemical reactor, inlaid pan‐mill, was used in studying high density polyethylene (HDPE). The effects of pan‐milling stress on the structure and properties of HDPE were investigated. Gel permeation chromatography, melt indexer, Fourier transformed infrared spectroscopy, electron spectroscopy for chemical analysis, differential scanning calorimetry, X‐ray diffraction, capillary rheometer, and Instron material testing system were used to characterize the structures and evaluate the properties of HDPE. The results showed that mechanochemical degradation of HDPE occurred under the stress fields of pan‐mill, the molecular weight of HDPE was reduced, and HDPE with higher initial molecular weights were easier to degrade under the stress fields. Oxygen‐containing groups such as COOH, C=O, and C—O were introduced to HDPE chains as a result of degradation during milling. Crystallinity of HDPE first decreased slightly followed by gradual increases with increasing milling times; monoclinic crystals appeared after four cycles of milling and increased markedly with increasing milling times. Pressure oscillation in capillary flow occurred at significantly higher shear stress and shear rate for milled HDPE than unmilled HDPE. After milling, mechanical properties were improved. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 78: 2016–2024, 2000     

Thermal conductivity of high density polyethylene filled with graphite

The effect of the grade, the content, and the particle diameter on the thermal conductivity of high‐density polyethylene (HDPE) filled with graphite were studied. The results show an increase of thermal conductivity of the HDPE/graphite composite with increase of graphite content. The thermal conductivity of the HDPE filled with the expanded graphite was larger than that of the HDPE filled with the colloid graphite system. At the same volume content (7%), the thermal conductivity of the former was twice that of the latter one. The particle diameter of the graphite also affected the thermal conductivity of HDPE composites. With increase of the particle diameter of the colloid graphite, the thermal conductivity of the HDPE/graphite increased. However, when the particle diameter of colloid graphite was larger than 15 μm, the increase of thermal conductivity of HDPE/graphite changed by inches. Some models proposed to predict thermal conductivity of a composite in a two‐phase system could not be applied to HDPE filled graphite powder composites, such as Maxwell‐Eucken, Cheng and Vachon, Zieblend, Lewis and Nielsen, Agari and Uno equations. But, according to the increase of thermal conductivity of HDPE composites filled with the colloid graphite, we find that Ziebland equation is suitable except of some constant. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 101: 3806–3810, 2006     

Structural explanation on natural draw ratio of metallocene-catalyzed high density polyethylene

The natural draw ratio of metallocen catalyzed high density polyethylenes was investigated with different crosshead speeds, molecular weights, and the cross-section shapes of sample specimens where the elastic components included in the conventional natural draw ratio such as residual strain and elastic aftereffect were eliminated. The perfect plastic deformation took place below a critical crosshead speed, whereas void formation occurred above the critical one. The natural draw ratio without elastic components and void formation was found to be dependent on the molecular weight of samples and the dimension of their specimens. On the basis of the SEM images, we proposed a simple structural model for explaining the necking formation in addition to the molecular weight and the cross-sectional shape dependences of the natural draw ratio.     

反相气相色谱法表征HDPE的表面性质

采用反相气相色谱技术,分别测定不同分子探针在不同温度下通过高密度聚乙烯(H DPE)的保留时间,计算HDPE表面色散自由能与表面Lewis酸碱常数.HDPE表面色散自由能随温度的升高呈线性降低,且为弱碱性的Lewis两性聚合物材料.     

Self‐reinforcement of high‐density polyethylene/low‐density polyethylene prepared by oscillating packing injection molding under low pressure

This article reports the toughness improvement of high‐density polyethylene (HDPE) by low‐density polyethylene (LDPE) in oscillating packing injection molding, whereas tensile strength and modulus are greatly enhanced by oscillating packing at the same time. Compared with self‐reinforced pure HDPE, the tensile strength of HDPE/LDPE (80/20 wt %) keeps at the same level, and toughness increases. Multilayer structure on the fracture surface of self‐reinforced HDPE/LDPE specimens can be observed by scanning electron microscope. The central layer of the fracture surface breaks in a ductile manner, whereas the break of shear layer is somewhat brittle. The strength and modulus increase is due to the high orientation of macromolecules along the flow direction, refined crystallization, and shish‐kebab crystals. Differential scanning calorimetry and wide‐angle X‐ray diffraction find cocrystallization occurs between HDPE and LDPE. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 71: 799–804, 1999     

CPE专用高密度聚乙烯树脂的开发

通过工艺条件的优化和催化剂的选型,在淤浆法聚乙烯装置上开发了氯化聚乙烯(CPE)专用高密度聚乙烯(HDPE)树脂。该CPE专用HDPE树脂与同类树脂的相对分子质量及其分布相当,密度介于国产树脂A和进口树脂B之间;结晶温度接近进口树脂A,但熔融温度及热焓较进口树脂A略高;熔体流动速率和堆密度与进口树脂A相差不大;大颗粒及细粉含量都少于进口树脂,且粒径分布更集中;孔径、孔容以及比表面积与进口树脂A接近。采用该CPE专用HDPE树脂生产的制品各项性能均达到了指标要求。     

Mold replication in injection molding of high density polyethylene

In order to deepen the mechanisms at the basis of mold surface replication onto the molded plastic surface, a novel experimental approach is proposed. Up to 20 different mold surface textures were made by machining with repetitive patterns of peaks and valleys. Mold replication tests were performed by over-molding of high density polyethylene (HDPE) on steel inserts. The surface morphology of inserts and injection molded parts was acquired by surface analyzer, and all the main roughness parameters were extracted and compared as well as the geometrical profiles. Surface morphology was also measured on molded samples after thermal relaxation at 100°C. As expected, a strong correlation was found between the roughness of mold insert and molded part over the full experimented range. Profiles on the molded surface have the same repetitive pattern of the corresponding insert surface but with lower peaks, higher valleys, and a horizontal shrinkage. Comparing molded HDPE surface profiles before and after thermal relaxation, it was observed a similar change to the one highlighted between mold insert and molded part. This occurrence suggests that the final surface appearance of the molded part is also a function of the relaxation mechanism during or immediately after injection molding.     

Crystallization behavior and molecular orientation of high density polyethylene parts prepared by gas‐assisted injection molding

The dependence of hierarchy in crystalline structures and molecular orientations of high density polyethylene parts with different molecular weights molded by gas‐assisted injection molding (GAIM) was intensively examined by scanning electron microscopy, two‐dimensional wide‐angle X‐ray scattering as well as dynamic rheological measurements. The non‐isothermal crystallization kinetics of the samples were also analyzed with a differential scanning calorimeter at various scanning rates. It was found that oriented lamellar structure, shish‐kebab and common spherulites were formed in different regions of the GAIM samples. The scanning electron microscope observations were consistent with the two‐dimensional wide‐angle X‐ray scattering results and showed that the molecular chains near the mold wall had strong orientation behavior, revealing the distribution of the shear rate of the GAIM process. The differences in crystal morphologies can be attributed to molecular weight differences as well as their responses to the external fields during the GAIM process. The formation mechanism of the shish‐kebab structure under the flow field of GAIM was also explored. Copyright © 2012 Society of Chemical Industry     

Effects of annealing on the hierarchical crystalline structures and mechanical properties of injection‐molded bars of high‐density polyethylene

The effect of annealing on the microstructural evolution and mechanical properties of high‐density polyethylene parts molded via gas‐assisted injection molding was investigated using scanning electron microscopy, differential scanning calorimetry, two‐dimensional wide‐angle X‐ray diffraction and tensile testing. The results indicated that a variety of annealing temperatures could induce considerable variations in the hierarchical structures, crystallinity, lamellar thickness and yield stress of the molded bars. According to these results, the annealing temperatures could be divided into three regions. In the low‐temperature region of annealing at 80 °C, the spatial variation of the superstructure developed along the thickness direction and mechanical properties of the annealed sample were mainly unchanged and similar to those of the original specimen. At 100 and 120 °C, the intermediate temperature region of annealing, the thickness of the crystals, degree of orientation and yield stress of annealed samples were greatly improved. Finally, at 127 °C, the degree of orientation decreased and yield stress slightly improved, an indication of the high‐temperature annealing region being characterized by increasing melting/recrystallization and causing relaxation of oriented molecular chains. A model is proposed to interpret the mechanism of the annealing treatment of the samples at various temperatures. © 2013 Society of Chemical Industry     

Origin of the molecular structure and properties of low density polyethylene

It is generally accepted that the origin of the structural features of low density polyethylene (branching, unsaturation, molar mass and its distribution) can be explained by various isomerisation and decomposition reactions of the macroradicals and by various chain transfer processes. On the other hand, it is known that ethylene molecules under high pressure are organised in various supermolecular particles. There are several phases in compressed ethylene (α, β and γ) depending on the pressure and temperature. The purpose of the present work was to determine the effects of the phase state of ethylene on the structure and properties of polyethylene. The authors have compiled published data about the effects of the synthesis pressure and temperature on the structure and properties of polyethylene. The entropy of ethylene under sythesis conditions for those published experiments was determined from the available information on pressure and temperature. The effects of ethylene entropy on the number and type of short chain branches, long chain branches, unsaturated bonds, molar mass and its distribution, chain flexibility, density and the melting point of polyethylene are demonstrated. It was found that under given ethylene entropy conditions, the same structure and properties of polyethylene are obtained.