Environmental Ageing of Adhesively-Bonded Joints. II. Mechanical Studies

The effects of exposure to moisture on the mechanical properties of a series of adhesively-bonded structures are reported. Changes observed in the maximum load, shear modulus, strain at maximum load, fracture energy, fracture toughness and stress are discussed and correlated with variation of the dielectric parameters. An initial increase in fracture toughness observed in the joints correlates well with the uptake of moisture having led to a lowering of the glass-rubber transition temperature. Differences in the ultimate strength and energy to failure for different surface pretreatments are observed. Loss in mechanical properties observed over the period of the study are paralleled by changes in the dielectric properties of the joints.     

Influence of Thermal Spikes on the Ageing of Adhesively Bonded Structures - A Dielectric Study

Dielectric measurements are reported on epoxy bonded aluminium joints exposed to moisture at 75°C. The observed changes in the dielectric spectrum that occur with time of exposure are correlated with the variation in the mechanical strength. Certain joints were subjected to periodic cooling to -20°C to simulate the shock of aircraft flight. The effects of dehydration were examined for joints that had been exposed to moisture for a prolonged period of time. The data indicate that the initial hydration process lowers the glass transition temperature and the stress released creates micro-voids. Further ageing leads to hydroxide formation in the interfacial layer. Freezing of water during the thermal spike experiments induces cracking in the adhesive, and leads to greater water uptake. Plasticisation of the resin appears to be to a large extent reversible and dehydration allows some recovery of the bond strength. The shock cooling of the joints has only a relatively small effect on the ageing of the joints but does, however, produce differences in the dielectric data. This study illustrates the power of the dielectric technique for the assessment of ageing in adhesively bonded structures.     

Predicting the Fatigue Life of Adhesively-Bonded Joints

The fatigue behaviour of adhesively-bonded joints, which consisted of an epoxy-film adhesive bonding fibre-composite substrates, has been studied. Using a double-cantilever beam specimen, the rate of crack growth per cycle has been measured as a function of the maximum strain-energy release rate, G_max. These data have then been modelled, and used to predict the fatigue lifetime of bonded single-lap joints. The agreement between the theoretical predictions and experimental results for the fatigue behaviour of the single-lap joints was found to be excellent.     

Dielectric Response of Adhesive Joints to Water Absorption

Dielectric measurements have been used to infer the average moisture content of aluminum adhesive joints bonded with three commercial epoxy adhesives. Measurements were made on both aluminum-adhesive-aluminum joints and aluminum-adhesive specimens in order to assess the influence of moisture diffusion gradients. Similar experiments were also performed with foil electrodes embedded in the bondline. For average water concentrations less than approximately 2%, there existed a linear relationship between the permittivity and the average moisture level which was independent of the spatial nonuniformity produced by water diffusion. At higher moisture levels, the dielectric response varied considerably from one adhesive to another, but, in general, could be approximated as a linear dependence on the average moisture concentration. The dielectric response was recoverable upon drying.     

Ageing of Adhesive Bonds with Various Surface Treatments, Part 3: Aluminium-Dicyandiamide Cured Aluminium Filled Epoxy Joints

Ageing of adhesive-bonded aluminium-dicyandiamide cured epoxy alumina-filled joints prepared using a variety of different surface treatments and exposed to elevated temperatures and high humidity are reported. The uptake of moisture was followed using broadband dielectric spectroscopy, and attempts are made to correlate these changes with observations of variations in the mechanical properties and surface structure monitored by electron microscopy. It was found that the absorption of moisture, as indicated by the dielectric measurements, is similar for all the joints. Small differences observed may be ascribed to the influence of the pretreatment on the absorption behaviour. There was no evidence of changes in the oxide layer of the substrate. Detailed electron microscopic examination of the surfaces did indicate, after prolonged exposure, that change in the pretreatment is consistent with small differences in the dielectric data. Predominantly, the changes that were observed in the mechanical strength of the joints are consistent with the plasticization of adhesive in the joint rather than failure at the substrate interface. Ageing at 70°C did, however, indicate that there were changes in the interfacial layer and these can be correlated with the change in the failure mechanism. It was also observed that the titanium/zirconium (Ti/Zr) pretreatment showed signs of being less durable than the others used in this study.     

The Effect of Temperature on the Strength of Adhesively-Bonded Composite-Aluminium Joints

Different materials have different coefficients of thermal expansion, which is a measure of the change in length for a given change in temperature. When different materials are combined structurally, as in a bonded joint, a temperature change leads to stresses being set up. These stresses are present even in an unloaded joint which has been cured at say 150°C and cooled to room temperature. Further stresses result from operations at even lower temperatures.

In addition to temperature-induced stresses, account also has to be taken of changes in adhesive properties. Low temperatures cause the adhesive to become more brittle (reduced strain to failure), while high temperatures cause the adhesive to become more ductile, but make it less strong and more liable to creep.

Theoretical predictions are made of the strength of a series of aluminium/CFRP joints using three different adhesives at 20°C and 55°C. Various failure criteria are used to show good correlation with experimental results.     

有机硅改性酚醛环氧树脂耐高温胶粘剂的研制

采用甲基苯基硅树脂对酚醛环氧树脂进行改性,硼酚醛树脂与自制固化促进剂作为固化剂,辅以纳米蒙脱土、绢云母粉作为填料,制备出一种能在300℃条件下长期使用的耐高温胶粘剂。在不同的配方及固化工艺条件下測定了有机硅改性酚醛环氧树脂体系的剪切强度,分析了甲基苯基硅树脂／酚醛环氧树脂比例关系、硼酚醛树脂与自制固化促进剂比例关系、固化剂用量、固化工艺条件、纳米蒙脱土的加入量对体系的影响。     

The Effect of Temperature on the Strength of Adhesively-Bonded Composite-Aluminium Joints

Different materials have different coefficients of thermal expansion, which is a measure of the change in length for a given change in temperature. When different materials are combined structurally, as in a bonded joint, a temperature change leads to stresses being set up. These stresses are present even in an unloaded joint which has been cured at say 150°C and cooled to room temperature. Further stresses result from operations at even lower temperatures.

In addition to temperature-induced stresses, account also has to be taken of changes in adhesive properties. Low temperatures cause the adhesive to become more brittle (reduced strain to failure), while high temperatures cause the adhesive to become more ductile, but make it less strong and more liable to creep.

Theoretical predictions are made of the strength of a series of aluminium/CFRP joints using three different adhesives at 20°C and 55°C. Various failure criteria are used to show good correlation with experimental results.     

Mode-l Fracture Behaviour of Adhesive Joints. Part I. Relationship Between Fracture Energy and Bond Thickness

The fracture properties of adhesive joints of aluminium were investigated using a rubber-modified tough epoxy resin system (G_IC = 2.76 kJ/m²) as adhesive material. Compact tension (CT) adhesive joints were manufactured for a wide range of bond thickness t (from 0.05mm to 10mm) and fracture tests conducted under static load. Scanning electron microscopy (SEM) was used to examine the fracture surface morphology. A large deformation elastic- plastic finite element model was developed to evaluate the J-integral value for different bond thickness. The fracture energy, J_c , was found to be highly dependent on the bond thickness and was lower than that of the bulk adhesive. As the bond thickness was increased J_c also increased, though not monotonically, towards the fracture energy of the bulk adhesive. This result was caused by the complicated interactions between the stress and strain fields, plastic deformation of the adhesive around the crack tip, constraint from the adherends and the failure path. It was shown that values of J_c as a function of bond thickness correlated well with the variation of plastic zone height. Scanning electron micrographs from the fracture surfaces of the CT adhesive joints illustrated that the failure path was mainly cohesive through the centre-plane of the adhesive layer. Brittle fracture mechanisms were observed for thin bonds (0.04mm < t< 0.5 mm) but tough fracture mechanisms were identified for thick bonds (t > 1 mm).     

Bulk and Interphase Effects in Aged Structural Joints

Although structural adhesives are becoming widespread in numerous applications, one important limitation at present is the long term behaviour of bonded assemblies under conditions of high humidity, especially at elevated temperatures. This study presents a comparison between bulk properties of a structural epoxy resin and its behaviour in a torsional joint consisting of a hollowed-out cylinder bonded to a plate—both substrates being in stainless steel. Exposure to ca. 100% relative humidity at 70°C leads to modification of the bulk properties of the polymer, notably reduction of its elastic modulus. Although this may explain some differences in behaviour of the torsional joint, premature failure is attributed to weaknesses in the interphase zone.

Auger Electron Spectroscopy (AES) has been employed to investigate both unbonded steel surfaces and fracture zones. Although prolonged exposure to water leads to a more extensive degree of (apparently) adhesive failure at the interface polymer/metal, AES has shown the presence of non-negligible quantities of carbon, attributed to residual polymer. Failure would seem to occur, at least partly, in a weak interphase of the polymer, near, but not at, the interface.

Various possible causes are evoked. For dry failure, residual polymer may be due to the topography of the metal surface and/or local modification of the adhesive during cure. In the case of aged joints, in addition there are potential effects due to swelling and differential stresses, secondary bond failure and molecular chain scission within the polymer, all provoked by the presence of water.     

ZnO包覆Al_2O_3填充环氧树脂的导热与介电性能

采用溶胶-凝胶法制备出ZnO包覆Al_2O_3的复合粉体,并将其添加入环氧树脂中,研究其热导率和介电常数。结果表明,发现经550℃煅烧2h后,产物的包覆层从水合草酸锌被煅烧成为ZnO;ZnO峰形尖锐,结晶良好,ZnO较好地包覆在Al_2O_3周围。ZnO包覆Al_2O_3的复合粉体添加到环氧树脂后,其热导率得到明显改善,且介电常数仍维持在较低水平(小于5)。     

环保型耐高温集成电路封装用导电银胶的研究

采用E-51型环氧树脂做基体,甲基纳迪克酸酐做固化剂,1~3μm片状银粉做导电填料,2-乙基-4甲基咪唑做促进剂,按照一定的质量配比,采用混合工艺制备出了一款电学性能、力学性能、耐高温性能都很好的绿色环保型导电银胶。经过性能测试,该产品的体积电阻率为72 mΩ·m,剪切强度为16.1 MPa,满足集成电路封装使用。     

耐高温单组分环氧胶粘剂的研制

以马来酸酐(MA)为封端剂,以2,2-双(3-氨基-4-羟基苯基)六氟丙烷(BAHPFP)、2,2-双[4-(4-氨基苯氧基)苯基]丙烷(BAPOPP)、2,2-双[4-(3,4-二羧基苯氧基)苯基]丙烷二酐(BPADA)为主原料合成了含酚羟基的聚醚酰亚胺树脂(HPEI);以HPEI为耐高温增韧剂,与N,N,N,N-四缩水甘油基-4,4′-二氨基二苯甲烷(TGDDM)、氢化双酚A环氧树脂(HBPAE)、潜伏性固化剂等配制了综合性能优异的耐高温单组分环氧胶粘剂。     

The Effect of Ageing and Environment on the Static and Fatigue Strength of Adhesive Joints

This paper describes the results of a durability programme designed to test the effects of ageing and environment on the performance of adhesive joints. Specimens were kept under a variety of loading and environmental conditions and the paper reports results of static and fatigue tests after 8-9 years storage. Some adhesive joints showed excellent durability performance, while others were adversely affected by the environment, particularly high humidity and natural exposure. It was found that the effect of ageing on static and fatigue performance is not necessarily the same.     

氧化铝/环氧树脂/聚氨酯导热复合材料的制备及表征

采用硅烷偶联剂KH550对氧化铝表面进行改性,并以改性氧化铝为导热填料,以环氧树脂为基体树脂,自制的聚氨酯预聚体为柔性改性剂,制备了氧化铝/环氧树脂/聚氨酯导热复合材料。采用红外光谱对KH550改性氧化铝的结构进行了表征,探讨了影响复合材料热导率的主要因素,研究了改性氧化铝用量对复合材料力学性能的影响,并利用扫描电镜对复合材料的微观结构进行了观察。结果表明,KH550已通过化学键接枝在氧化铝表面。随着KH550改性氧化铝用量的增加,复合材料的拉伸强度逐渐增大,而导热率和断裂伸长率呈现先上升后下降的趋势。当改性氧化铝的用量为150 phr时,复合材料的导热率达到最大值0.66 W/(m·K),拉伸强度和断裂伸长率分别为37.2 MPa和1.62%。随着m(PUA)/m(EP)的增大,复合材料的导热率相应下降,适宜的m(PUA)/m(EP)为15/85。     

胶粘剂在航天工业中的应用

介绍了国内外航天工业用胶粘剂的主要种类、特点、研究现状及其使用情况,并对航天工业用胶粘剂的发展进行了展望。     

酚醛胺/环氧树脂胶粘剂耐化学介质性能研究

以自制酚醛胺作为环氧树脂(EP)的固化剂,研究了酚醛胺/EP胶粘剂的耐化学介质性能和耐久性。结果表明:酚醛胺/EP胶粘剂在水、10%NaOH和2%NaCl溶液中浸泡48 h时,其剪切强度分别为6.61、4.66、5.12 MPa,其强度保留率分别为87.90%、61.97%、68.09%;酚醛胺/EP胶粘剂既具有良好的耐水性、耐碱性和耐盐性,又具有较好的碱、盐耐久性,可作为特种胶粘剂——水下胶粘剂广泛用于房屋、水利、地下建筑、医疗、仿生和养殖等领域,并具有良好的经济效益和应用前景。     

建筑结构胶耐湿热老化性能测试方法研究

GB50367《混凝土结构加固设计规范》中关于耐湿热老化性能测试方法由于试验周期较长,而给建筑结构胶的工程进场复检带来困难。以环氧树脂类建筑结构胶的配方特点为基础,研究对比了适用于工程进场检验的湿热老化性能快速测试方法,并和GB50367进行对比．考查了增韧剂、填料、固化剂、助剂等各种因素以及各种因素的“叠加”效应对2种测试方法结果的影响。研究表明,800C恒温水浴／7d条件下的快速测试方法和GB50367结果基本一致,2者的偏差绝对值不超过15％。快速测试方法能快速地检验出建筑结构胶的耐湿热老化性能．     

常温快速固化粘合剂的合成与应用

本文以改性环氧树脂、丁腈—40混炼胶液，钡酚醛树脂三组份按一定比例共混成甲组份，以多元胺类、酚类、醛类合成的一种固化剂为乙组仿，组成常温快固双组份粘合剂。文中介绍了胶的组份、制备方法、性能、粘接工艺等．探讨了固化机理。     

Silyl-Crosslinked Urethane Elastomer Modifying Epoxy Resin: I. Morphological Studies

Silyl-crosslinked urethane elastomer modifying epoxy resin has drawn much interest. Here triethoxysilyl-terminated polycaprolactone elastomer (PCL-TESi) modifying diglycidylether of bisphenol A epoxy resins (DGEBA) system was chosen; then the effect of the type of curing agent on the phase structure of the studied system was investigated. The modified systems with different phase structures were obtained by varying the formulations of the curing agents. It was experimentally shown that the crosslinked density was greatly increased by the addition of an aminosilane curing agent (KBE-9103). And the SEM and TEM analysis of the cured system showed that the addition of KBE-9103 increased the compatibility between the PCL-TESi and DGEBA, but decreased the ductility of the systems. The TEM results indicated that the addition of too much of the KBE-9103 made the resulting silicone particles coagulate with each other. The state of phase separation from the TEM in the cured system was explained theoretically. These considerations would serve the deeper studies of the mechanism of silyl-crosslinked urethane elastomer modifying epoxy resin in the future.