聚乙烯自增强复合材料损伤过程的声发射特征

复合材料在承受外载时, 声发射可产生于基体破裂、纤维-基体界面脱粘和纤维断裂等。测定了U HMWPE/ HDPE 复合材料在拉伸载荷作用下的声发射(AE) 振幅信号。对特殊试样, 即预测到断裂有明确方式, 如纤维-基体界面脱粘、基体破裂、纤维断裂和分层等的试样, 实施加载直至破坏。用扫描电子显微镜(SEM) 观测试样的断裂表面, 对产生于若干特殊损伤类型的AE 信号进行了鉴别。在相同加载条件下, 完成了不同种类的U HMWPE/ HDPE 准各向同性层合板声发射检测。结果在特殊试样损伤类型与声发射信号事件振幅之间建立了对应关系, 揭示了上述各种准各向同性层合板损伤扩展过程的AE 特征与损伤破坏机制。各种准各向同性层合板试样的声发射事件累计数对拉伸应力关系曲线相异, 其相同损伤类型发生时所对应的拉伸载荷水平不等, 表明它们的铺设角度和铺设顺序对损伤演变过程有显著的影响。结果证实了它们的最终破坏由严重层间分层造成。      

超高分子量聚乙烯冲击断裂形态分析EI

用辊压和模塑方法加工了超高分子量聚乙烯（ＵＨＭＷＰＥ）试样，用扫描电镜研究了它的带缺口断裂形态和断裂方式。结果表明，ＵＨＭＷＰＥ形成了一种类串晶形态，断裂是以两种方式进行的：晶间断裂和串晶自身断裂，而无定形部分呈云朵状，发白，同时断裂变形具有与金属非常相似的力学孪晶作用。     

Ion bombardment of polyethylene—influence of polymer structure

Polyethylenes of various macromolecular and supermolecular structures were studied from the point of view of their susceptibility to an ion beam treatment. An influence of molecular weight (M_w), molecular weight distribution (M_w/M_n) and the degree of branching were compared within the set of low-density polyethylenes (LDPE) studied. An influence of the length of branches was compared between LDPE, linear low-density (LLDPE) and high-density (HDPE) polyethylenes. An influence of the degree of crystallinity and the morphology of a crystalline phase were compared for HDPE samples solidified under various thermal conditions and ultra-high molecular weight polyethylene (UHMWPE). Plate polymer targets ∼2 mm were bombarded with 100 keV He⁺ or 130 keV Ar⁺ ions (dose of 10¹⁴-10¹⁶ ions/cm²; ion energy stream density <0.1 μA/cm²), micromechanical properties of their surface layer (hardness, mechanical modulus and elastic recovery) determined and compared to the virgin materials.Ar⁺ ion beam bombardment generally lowers micromechanical properties of the polyethylenes, whereas He⁺ ion beam treatment makes them higher. The effect is the stronger the higher the molecular weight of polyethylene. However, a long chain branching adversely affects the modification. The degree of crystallinity facilitates an ion beam bombardment from the point of view of micromechanical properties of the materials, however, also the morphology of a crystalline phase was found to play a role.     

Investigation of damage mechanisms in self-reinforced polyethylene composites by acoustic emission

The objective of this study is to investigate the damage mechanisms in self-reinforced polyethylene composite laminates (UHMWPE/HDPE) under monotonic tensile loading by the acoustic emission (AE) technique. Fracture surface examinations were conducted using a scanning electron microscope (SEM). Using model specimens exhibiting a dominant failure mechanism, correlations were established between the observed damage growth mechanisms and the AE results in terms of the events amplitude. These correlations can be used to monitor the damage growth process in the UHMWPE/HDPE composite laminates exhibiting multiple modes of damage. Results from this study revealed that the AE technique is a viable and effective tool for identifying damage mechanisms such as fiber–matrix debonding, matrix cracking, fiber pull-out, fiber breakage and delamination in the UHMWPE/HDPE composite materials.     

Orientation-insensitive spectra for Raman microspectroscopy

Separating effects due to molecular conformation from those due to orientation in the spectra of oriented samples obtained by Raman microspectroscopy is a complex issue. To solve this problem, we propose a procedure to calculate an orientation-insensitive spectrum (so-called isotropic spectrum) from polarized spectra obtained by Raman microspectroscopy that is valid for systems that exhibit a uniaxial symmetry. The method has first been tested on highly oriented samples of high-density polyethylene (HDPE). Polarized and isotropic spectra of a highly oriented HDPE cylindrical rod and an isotropic HDPE sample have been compared. The differences in the relative intensities, which occur in the polarized spectra and are due to orientation of the polyethylene chains, are nearly cancelled in the isotropic spectra, showing that the orientation-insensitive spectrum adequately represents the molecular conformation without contributions of orientation. Second, spectra of silk fibroins have been compared in the amide I region for Bombyx mori cocoon silk fibers and methanol-treated regenerated fibroin films. The similarity of the shape of the amide I band of the isotropic spectra indicates that the secondary structure of the fibroins is very close in both samples. These experimental results support the conclusion that the molecular conformation can be efficiently characterized from the intensity and the shape of Raman bands in the orientation-insensitive spectrum.     

超高分子量聚乙烯的冲蚀磨损

用气流喷砂到冲依试验装置测试了超高分子量聚乙烯（UltraHighMolecularWeightPolyethylen）的冲蚀磨损佳能．考察了冲蚀粒子的入射角、速度、粒子的硬度对冲蚀磨损的影响．用扫描电子显微镜观察冲蚀后报表面形统指出：在高角冲蚀时，摩根机理主要为塑性变形和显微裂纹；在低角冲蚀时，磨损机理主要为显微切削和显微犁耕．冲蚀磨损机理与冲蚀粒子有关．     

Radiation enhanced modification of HDPE for medical applications

Comparison of gamma irradiation induced change in properties in terms of thermal and mechanical properties between two grades of HDPE and UHMWPE was carried out. It was found that the responses to irradiation of two grades of HDPE investigated were close whereas a difference in response was found between HDPE and UHMWPE. The irradiation dose that caused the lowest wear and highest hardness for UHMWPE was 500 kGy. When irradiation dose was above 500 kGy, no significant changes in wear and hardness properties were observed. The irradiation dose for HDPE, both 2208J and 7000F, that caused the wear resistance and hardness comparable to irradiated UHMWPE at 500 kGy was 1000 kGy. In addition, the dose of 750 kGy was needed for HDPE to achieve the similar level of wear resistance as non-irradiated UHMWPE. ©2003 Kluwer Acadamic Publishers     

Enhancement in micro-fatigue resistance of UHMWPE and HDPE processed by SCORIM

The tribological performance of UHMWPE and HDPE polymers, processed by a novel polymer process technology SCORIM (Shear-Controlled ORientation TEChnology) in injection moulding, shear controlled orientation in injection moulding (SCORIM) was evaluated against a through-hardened steel under different test conditions and compared with those processed by conventional injection moulding. Results indicate a significant improvement in the wear resistance using the SCORIM technology as a result of an increase in the mechanical properties. SEM microscopy shows a change of wear mechanisms dominated by micro-fatigue when using conventional injection moulding, compared to mild abrasive wear when using the SCORIM technology under the same test conditions. Examination of microstructure reveals the random aggregation of polymer molecules in the samples processed by a conventional injection moulding. Using the SCORIM technology, the molecules were sheared and orientated and a fibril microstructure formed as an in situ fibre reinforced composite. DSC analysis shows an increase in a second phase shish kebab structure in the samples processed by SCORIM technology. The fibril microstructure with an increase in shish kebab structure results in a significant improvement in wear resistance. Using the surface normal to the direction of the orientated molecular fibril microstructure as a contact surface, the possibility of the initiation and development of micro-cracks was reduced, especially the micro-cracks parallel to the contact surfaces.     

HDPE/UHMWPE共混物的动态流变性能

将少量超高分子量聚乙烯(UHMWPE)引入高密度聚乙烯(HDPE)构成共混体系,考察了共混物的动态流变行为随组成、温度、频率的变化规律。研究结果表明,体系储能模量、损耗模量、复数黏度随UHMPWE含量增加而增大;Han曲线斜率均小于2且不存在温度依赖性;Cole-Cole曲线均呈半弧形未表现出相分离特征,说明共混体系在熔体状态下并未发生分相过程;所有试样的时温叠加(TTS)主曲线在低频区并未出现特殊的粘弹松弛行为,而在高频区随着UHMWPE含量增加,时温叠加原理失效现象更加显著。     

Effects of crystallization rates on interface layer thickness of epitaxial HDPE on oriented iPP

The effect of crystallization rate on the epitaxial interface layer thickness of high-density polyethylene (HDPE) in the epitaxial system with oriented isotactic polypropylene (iPP) has been investigated by electron microscopy. The results of bright-field electron microscopy and electron diffraction indicate that the epitaxial growth of HDPE on iPP takes place only on the interface between them. The thickness of the epitaxial layer of HDPE is markedly affected by the crystallization rate of HDPE. The critical layer thickness of epitaxial HDPE is about 100 nm under slow crystallization conditions, e.g. isothermal crystallization at 124°C. When the crystallization rate is higher (quenching into air at room temperature), the epitaxial layer thickness of HDPE increases up to 250 nm.     

Treatment of functionalized graphitic nanofibers (GNFs) and the adhesion of GNFs-reinforced-epoxy with ultra high molecular weight polyethylene fiber

Though ultra high molecular weight polyethylene (UHMWPE) fiber made of carbon and hydrogen has superior mechanical properties and effective cosmic shielding properties, it shows weak composite properties due to poor interfacial adhesion between UHMWPE fibers and polymer matrix. In this study, functionalized graphitic nanofibers (GNFs) were treated further using the sonication method. High-level sonication with a series of conditions was employed for the treatment of functionalized GNFs. submicron particle size analyzer and transmission electron microscope (TEM) were used to study effects on the length and morphology of treated nanofibers by sonication conditions. The sonication conditions were optimized for preparation of a nano-epoxy matrix containing well-dispersed, reactive, functionalized graphitic nanofibers. The adhesion ability of the nano-epoxy to UHMWPE fiber was investigated. Bundle fiber pullout specimens with single and double-ends were designed and prepared for study of the adhesion property of the nano-matrix with UHMWPE fiber. Test results showed that the nano-epoxy matrix could effectively improve interfacial adhesion property with UHMWPE fiber.     

三元层状陶瓷Ti_3AlC_2/超高分子量聚乙烯复合材料的制备及性能

为了增强超高分子量聚乙烯(UHMWPE)的性能,研究采用表面改性的Ti_3AlC_2填充UHMWPE,通过热压成型制备了Ti_3AlC_2/UHMWPE复合材料。采用SEM观察复合材料的微观结构,表明Ti_3AlC_2均匀分散在UHMWPE基体中,表面处理后的填料与基体界面熔合较好;热分析结果表明,Ti_3AlC_2的添加降低了UHMWPE的结晶度和结晶热焓,同时提高了聚合物的热传导性;DMA分析结果表明,添加Ti_3AlC_2有效地提高了Ti_3AlC_2/UHMWPE复合材料的抗蠕变性能,得益于无机粒子改善了复合材料的硬度和刚性,提高了复合材料抗外界应力变形能力;摩擦学性能分析表明,适量的Ti_3AlC_2(质量分数≤15wt%)填充UHMWPE能有效提高复合材料的减磨抗摩性能,同时磨痕表面形貌分析结果表明,Ti_3AlC_2/UHMWPE复合材料的摩擦磨损机制由粘着磨损向磨粒磨损转变。     

Wear of crosslinked polyethylene under different tribological conditions

Ultra high molecular weight polyethylene (UHMWPE) wear debris has been shown to be a major cause of long term failure of total joint replacements. Recently, crosslinking has been extensively introduced to reduce the wear of UHMWPE. In this study the wear of non-crosslinked and crosslinked UHMWPE were compared under a range of conditions. The materials examined were UHMWPE GUR 1050, non-crosslinked, moderately crosslinked—5MRad, and highly crosslinked—10MRad. The wear was examined on a multidirectional pin on plate rig. The effect of counterface roughness on wear under different kinematics was examined. The results from the different counterface conditions showed that highly crosslinked UHMWPE had significantly lower wear against both smooth and scratched counterfaces. However the reduction in wear for crosslinked polyethylene was less for scratched counterfaces. The second part of the study showed that all the UHMWPE's produced lower wear rates under lower multidirectionality because of reduced cross shear frictional forces and work. These findings are relevant to the consideration of the use of crosslinked polyethylene in the knee, where the kinematics have lower levels of cross shear and in the hip and knee against roughened metallic counterfaces.     

The damage zone ahead of the arrested crack in polyethylene resins

Formation and growth of the crack tip damage zone during slow stepwise crack propagation in polyethylene resins was studied experimentally. The study focused on the differences between the damage zone in high density polyethylene (HDPE), that represented traditional single-craze morphology, and the damage zone in more fracture resistant ethylene copolymers (MDPE) under plain strain conditions. It was shown that improved fracture resistance correlated with development of an epsilon-shaped damage zone that consisted of the central craze and an accompanying pair of hinge shear zones of comparable length. The shear zones emanated from the crack tip immediately above and below the central craze where highly stretched material formed a membrane that separated the crack tip from the cavitated material in the craze. The remarkable observation that the shear zones underwent crazing despite the presumably unfavorable stress-strain conditions was attributed to a dilatational stress component resulting from partial re-distribution of the load as the main craze opened. Microscopic analysis revealed differences in the crazed material between the single-craze (HDPE) and the epsilon-shaped (MDPE) morphology. An array of cellular cavities separated by walls of biaxially oriented material in the MDPE craze contrasted with the traditional structure of uniaxially stretched fibrils in the HDPE craze. The stepwise development and fracture of the damage zone was monitored in time, and the differences in kinetics of these processes between the two types of morphologies were characterized.     

Effects of screen-grid bias voltage on the microstructure and properties of the ultrahigh molecular weight polyethylene (UHMWPE) modified by oxygen plasma

The specimens of ultrahigh molecular weight polyethylene (UHMWPE) were treated by oxygen plasma using a pulse-biased screen-grid technique under different negative bias conditions. The screen-grid was used to provide an electric field to accelerate the oxygen ions towards the UHMWPE substrate during the plasma treatment process. The effects of the screen-grid voltage on the surface microstructure, wettability, mechanical properties and wear resistance of UHMWPE were investigated. It was found that the degree of crosslinking, oxidation, wettability and surface roughness of UHMWPE can be increased with the increasing of the screen-grid voltage. Owing to the increase of the degree of crosslinking, the hardened layer formed on the surface of the UHMWPE samples was also strengthened greatly with the increase of the grid voltage. However, the wear results indicated that the UHMWPE sample modified at higher bias voltage exhibits poor wear performance, which could be mainly related to the embrittlement resulted from the aggravation of oxidation.     

Friction and wear of hydroxyapatite reinforced high density polyethylene against the stainless steel counterface

Hydroxyapatite (HA) reinforced high density polyethylene (HDPE) was invented as a biomaterial for skeletal applications. In this investigation, tribological properties (e.g. wear rate and coefficient of friction) of unfilled HDPE and HA/HDPE composites were evaluated against the duplex stainless steel in dry and lubricated conditions, with distilled water or aqueous solutions of proteins (egg albumen or glucose) being lubricants. Wear tests were conducted in a custom-built test rig for HDPE and HA/HDPE containing up to 40 vol % of HA. It was found that HA/HDPE composites had lower coefficients of friction than unfilled HDPE under certain conditions. HA/HDPE also exhibited less severe fatigue failure marks than HDPE. The degradation and fatigue failure of HDPE due to the presence of proteins were severe for low speed wear testing (100 rpm) as compared to high speed wear testing (200 rpm). This was due possibly to the high shear rate at the contact which could remove any degraded film instantaneously at high sliding speed, while with a low sliding speed the build-up of a degraded layer of protein could occur. The degraded protein layer would stay at the contact for a longer time and mechanical activation would induce adverse reactions, weakening the surface layer of HDPE. Both egg albumen and glucose were found to be corrosive to steel and adversely reactive for HDPE and HA/HDPE composites. The wear modes observed were similar to that of ultra-high molecular weight polyethylene. Specimens tested with egg albumen also displayed higher wear rates, which was again attributed to corrosion accelerated wear.     

Bonding of highly oriented polypropylene sheets by epitaxial crystallization of polyethylene

The bonding of highly oriented polypropylene (PP) by means of a thin epitaxial polyethylene (PE) interlayer, was investigated. Epitaxy occurred in HDPE over a range of molecular weight, including ultra-high molecular weights. It was also observed for LDPE, although the epitaxy was less well-developed than for HDPE. Laminates of highly oriented PP thinly coated with PE and treated to induce cross-hatchng of PE showed much improved adhesion, compared with uncoated PP treated in a similar fashion. The PE epitaxy is believed to be responsible for this behaviour. The adhesive performance was also shown to be a function of the molecular weight and its distribution of PE, as well as of the annealing treatment of the PE coat.     

Epitaxial crystallization of linear low-density polyethylene on high-density polyethylene

The oriented crystallization of linear low-density polyethylene (LLDPE) on high-density polyethylene (HDPE) has been investigated by transmission electron microscopy. From morphology and electron diffraction, it is confirmed that epitactic growth of LLDPE lamellae on the HDPE crystals takes place with an adoption of the HDPE crystal thickness at the interface and a continuous thinning of the LLDPE lamellae in the interface. This revised version was published online in November 2006 with corrections to the Cover Date.     

Investigation of the drawing mechanism of UHMWPE fibers

The influence of draw ratios (DR) of gel-spun ultrahigh molecular weight polyethylene (UHMWPE) fibers on resultant morphologies, tensile, degrees of orientation, and crystal phase transition properties were investigated using wide-angle X-ray diffraction (WAXD), differential scanning calorimetry (DSC), and scanning electron microscopy (SEM). The anisotropic crystalline structure with full concentric circular rings originally shown on the WAXD patterns of the as-prepared and drawn UHMWPE fibers gradually transform into oriented fibers with azimuthal spots on the equator as their DR values increase from 1 to 20, in which their orthorhombic crystals, percentage crystallinity, crystalline orientation, and the birefringence values increase significantly. As evidenced by SEM and WAXD analysis, the chain-folded molecules originally present in kebab crystals of the as-prepared UHMWPE fiber specimens gradually transformed into shish-like crystals with relatively high orientation as their DR values increase from 1 to 20. In contrast, the crystallinity and crystal orientation values of the drawn UHMWPE specimens increase only slightly, as their DR values increase from 20 to 40, wherein both crystallinity values of orthorhombic and monoclinic crystals increase slightly. In fact, barely any oriented kebab but only shish crystals were observed on the surfaces of drawn UHMWPE fiber specimens with DR values higher than 20. The birefringence values increase only slightly with further increasing DR values, while crystallinity and crystal orientation values of the drawn UHMWPE fiber specimens remained relatively unchanged as their DR values increase from 40 to 150. In the meantime, the monoclinic crystals gradually grow at the expense of the orthorhombic form crystals as the DR values of drawn UHMWPE fiber specimens increase from 40 to 150. Possible reasons accounting for these interesting properties found for the drawn UHMWPE fibers with varying draw ratios are proposed in this study.     

Influence of biological lubricant on the morphology of UHMWPE wear particles generated with microfabricated surface textures

Immunological response induced by wear particles of ultra-high molecular weight polyethylene (UHMWPE) has been recognized as the major factor causing the failure of total joint replacements. A previous study has applied surface texturing techniques to generate UHMWPE wear particles with specific size and shape to study the effects of particle size and shape on osteolysis.In the present study, the effects of biological lubricants on the morphology of UHMWPE wear particles generated with microfabricated surface textures were investigated. It was observed that UHMWPE wear particles generated in bovine serum are smaller and thinner than the particles generated in water. The reason may be due to the reduction of the friction between UHMWPE and the surface feature under serum lubricating condition. It means a smaller material resistant force to overcome during the surface-feature sliding process and leads to a smaller lateral displacement of a micro-cutting process. Thus a larger aspect ratio (or a smaller particle width) was observed for the particles generated in serum. Compared to original UHMWPE, highly cross-linked UHMWPE has better wear resistance and generates smaller wear particles under the articulation with microfabricated surface textures in a biological environment. The potential application is to apply surface textures on the articulating surface of joint implant in order to control the size and shape of UHMWPE wear particles. While maintaining a low wear rate of UHMWPE parts, further reduction of the most “toxic” particles released into human body shall prevent particles induced osteolysis.