Effect of CNT dispersion methods on the strength and fracture mechanism of interface in epoxy adhesive/Al joints

The tensile and shear strengths of adhesively bonded aluminum (Al) joints were inspected in the presence of amino functionalized multi-wall carbon nanotubes (MWCNTs). Tensile and shear tests were carried out using butt and lap-shear joints. The main goal was to compare the effects of dispersion methods of functionalized-CNT into epoxy on the mechanical performances and failure mechanisms of Al joints. Two different types of dispersion procedures, distributing CNT in the hardener (HH method) and distributing CNT in the resin (RR method), were applied. To identify the failure mechanisms, the morphology of fracture surfaces were analyzed using scanning electron microscopy (SEM). Comparing two dispersion methods against one another ascertained that following the RR method for dispersing CNTs in the adhesive displayed larger shear strength, while applying HH method offered fairly greater tensile strength. Moreover, dispersing CNTs in the resin induced more uniform dispersion of CNTs as compared to distributing nanofillers in the hardener. Following RR method, CNTs good dispersion as well as the presence of effective crack growth dissipating mechanisms, increased the shear strength of CNT reinforced adhesive joint. Incorporating CNTs using HH approach encouraged the plastic void formation of epoxy around the agglomerated CNTs, and as a consequence, promoted the plastic deformation under tension.     

Shear properties of isotropic conductive adhesive joints under different loading rates

Epoxy-based conductive adhesives have been widely used in the electronic field given the lead-free development of electronic packaging. The conductive adhesive joints must be subjected to shear loads during the service of electronic products considering the mismatch in mechanical properties between packaged chip and substrate. In this study, INSTRON 5544 universal material testing machine was used for tensile–shear tests of isotropic conductive adhesive joint specimens, which were prepared using pure copper plate adherend in the form of single-lap joints. Four loading rates, that is, 0.05, 0.5, 5, and 10 mm/min, were adopted. The relationship between shear load and displacement of two overlapping copper plates is deduced from a mechanical perspective. A mechanical model of the conductive adhesive shear specimen was developed by introducing dimensionless parameters, which are obtained from interfacial fracture energy and shear strength, to interpret the effect of loading rate on the shear properties of the conductive adhesive specimen considering the loading rate. Results show that this model can effectively reflect the relationship between shear load and displacement in the range of 0.05–10 mm/min.     

Influence of adhesive bond line thickness on joint strength

While the geometry of aerospace assemblies is carefully controlled, for many industrial applications such as marine structures bond line thickness can vary significantly. In this study epoxy adhesive joints of different thicknesses between aluminium substrates have been characterized using physico-chemical analyses (differential scanning calorimetry, DSC; dynamic mechanical analysis, DMA; spectroscopy), nano-indentation and mechanical testing. Thermal analyses indicated no influence of thickness on structure. Nano-indentation revealed no evidence of an interphase at the metal/epoxy interface, nor any change in modulus for different thicknesses, though Raman spectroscopy suggested there may be slight variations in composition close to the substrates. However, mechanical testing using the modified Arcan fixture indicated a significant drop in strength and failure strain under pure tension and a smaller reduction for tension/shear and pure shear loads as thickness increased. Examination of sections through joints did not indicate any physical reason for this, but numerical analysis of the stress state revealed larger stress concentration factors for tensile loading in thick joints, which may explain the thickness effect. It is recommended that joint thickness should be kept below 0.8 mm to avoid obtaining artificially low values with the Arcan test.     

Perfecting a polyurethane-type structural adhesive

A structural adhesive made from a woven glass material precoated with two-component polyurethane has been perfected. After the coating of the glass fabric, the adhesive is obtained by reacting a premix (dihydroxylated polymer) and a harder (free and blocked diisocyanates) in the presence of a tin catalysts. Values obtained for the strength of the assemblies (3 MPa on shearing) and for the working temperature (as low as 80°C) andt he allowed storage temperature (20°C) and time 91 year) of the adhesive are far better than those initially expected.     

Influence of dual-component microcapsules on self-healing efficiency and performance of metal-epoxy composite-lap joints

Dual-component microcapsules were synthesized by solvent evaporation technique using epoxy resin and hardener as core materials and polymethyl methacrylate (PMMA) as shell wall materials. Morphology, core content, and size distribution of microcapsules were monitored by controlling the various processing parameters such as agitation speed, core–shell weight ratio, and concentration of emulsifiers. The molecular structure, morphologies, and thermal characteristics of the microcapsules were examined under Fourier transform infrared spectroscopy (FTIR), field emission scanning electron microscopy (FESEM), and thermogravimetric analysis (TGA), respectively. Synthesized dual-component microcapsules were entrenched into the epoxy polymer to introduce the healing features in single lap shear epoxy adhesive joints. Healing efficiency as high as 89% was achieved when 10 wt% dual-component microcapsules were introduced in adhesives. Investigation of the fractured surfaces of the healing enabled adhesives reveals the presence of crack pinning and crack blunting sites represented by characteristic tails at the wake of microcapsules in cohesive zone. Such failure mechanisms responsibly influence the healing efficiency.     

Shear strength of epoxy adhesive joint between steel substrates with indented patterns

Indented patterns, about 40 μm wide and 30 μm deep, were created by pressing down a hardened steel file onto the surface of steel substrates using Instron test machine. We examined the penetration of epoxy into those micro-indented patterns and their interactions with Kevlar fibres (diameter of around 12 μm), and their influences on the shear strength of epoxy adhesive joints. To ensure complete wetting of the patterned substrate surface, resin pre-coating was applied first. Three different acetone/resin (without hardener) solutions were prepared for this purpose: they contained 1 wt%, 3 wt% and 5 wt% of resin in acetone, respectively. For the purpose of comparison, plain steel substrates polished by #120 sandpaper were also prepared. It had been found that the patterned substrate coated by the 3 wt% resin/acetone solution yielded the highest shear strength, showing about 20% improvement in comparison with that of polished steel substrates. Only limited Kevlar fibres were found within the indented patterns due to their relatively large spacing and shallowness, and, furthermore, due to the random fibre distribution of Kevlar fibres of around 14 mm in length. The limited Kevlar fibre bridging activities were reflected by only around 4% improvement in the shear strength.     

Effect of Temperature on the Joints Strength of an Automotive Polyurethane Adhesive

In this paper, the performance of an automotive polyurethane adhesive was studied through adhesive joints tests. Butt joints and single lap joints were fabricated and tested at seven temperature measuring points (TMPs). It is shown that both the tensile strength and lap shear strength decrease with the increasing of temperature. Quadratic polynomial expression obtained by the least square method can represent the tensile and lap shear strength as a function of temperature very well. ?40°C, 0°C, and 90°C were selected as the most ideal TMPs for this adhesive through the comparison of the residual sums of squares of 35 fitting curves with different combination of TMPs. Scarf joints with adhesive angles of 60° and 30° were fabricated and tested at ?40°C, 0°C, and 90°C. It also showed a decrease in joint strength with the increasing temperature. Joint strength as a function of adhesive angle is presented. It was found to closely follow a linear behaviour. A three-dimensional surface, consisting of temperature, adhesive angle, and joint strength, is presented finally to facilitate the design of automotive bonding structures.     

Processing study of a high temperature adhesive

An adhesive-bonding process cycle study was performed for a polyimidesulphone. The high molecular weight, linear aromatic system possesses properties which make it attractive as a processable, low-cost material for elevated temperature applications. The results of a study to better understand the parameters that affect the adhesive properties of the polymer for titanium alloy adherends are presented. These include the tape preparation, the use of a primer and press and simulated autoclave processing conditions. The polymer was characterized using Fourier transform infrared spectroscopy, glass transition temperature determination, flow measurements, and weight loss measurements. The lap shear strength of the adhesive was used to evaluate the effects of the bonding process variations.     

Lowering curing temperature in adhesive bonding of aluminum AA6061-T4

ABSTRACT

To minimize the part distortion and investment in the E-coat oven in adhesive bonding of metals for automotive applications, lowering the curing temperature of adhesive without apparent loss of the joint strength is desirable. The key to lower the curing temperature of adhesive bonding of lightweight materials is to accelerate the curing process of structural adhesives. In this study, curing agent (i.e., aliphatic polyamine) and curing accelerator (i.e., acetylacetone salt) were added into commercial Henkel 5089 adhesive and the effect of these curing additives on the curing temperature of Henkel 5089 in adhesive bonding of aluminum AA6061-T4 was studied. The test results showed that the addition of a curing agent and accelerator in Henkel 5089 lowered the curing temperature from 177°C to 130°C without sacrificing the strength of the adhesive-bonded aluminum AA6061-T4.     

Effect of reinforcements at different scales on mechanical properties of epoxy adhesives and adhesive joints: a review

Improvement of the mechanical properties of adhesives and adhesive joints has been a subject of great interest in recent years. Up to now, several methods have been presented such as modifying substrate shapes, adding microparticles (MPs) and nanoparticles (NPs), and embedding micro and macrofibers in the adhesive layer. This review aims to investigate how these reinforcements of different scales in the adhesive layer influence the mechanical properties of adhesive joints and adhesives. Characteristics and applications of reinforcements are introduced in the first part. In the second part, the effects of several parameters commonly investigated by researchers on the strength, stiffness and fracture toughness improvement of polymeric materials are reviewed for reinforcements of different scales. Finally, damage mechanisms involved in increasing or decreasing the mechanical properties are reviewed and discussed.     

耐高温聚酯碳硼烷聚氨酯胶粘剂研制

制备了一种聚酯碳硼烷胶粘剂,用IR、H-NMR和C-NMR对其结构做了分析。胶粘剂100℃固化物,二碳癸硼烷链段-C B H-没有变化;450℃/1 h固化物已分解产生硼氧化物、芳环化物和碳化物。进行了TG和2 10 10DSC分析,胶粘剂固化物氮气中500℃保温30 min残留率为76%,1 000℃/30 min残留率为67%,有完好的外形。对于钛合金、石墨、不锈钢和碳耐高温材料,粘接件在500~800℃有2 MPa左右的拉剪强度。铝合金/镀铜聚酰亚胺薄膜粘接件和复合材料在150~200℃共固化时,不会脱粘,拉剪强度较高。胶粘剂耐温性起源于B-C、B-O键的高键能,碳硼烷的电子和立体结构,高温分解生成B-O键产物,高温加热进一步交联。胶粘剂可以用作耐高低温材料。     

Numerical assessment of the Double-Cantilever Beam and Tapered Double-Cantilever Beam tests for the GIC determination of adhesive layers

ABSTRACT

Fracture mechanics-based techniques have become very popular in the failure prediction of adhesive joints. The most commonly used is cohesive zone modeling (CZM). For both conventional fracture mechanics and CZM, the most important parameters are the tensile and shear critical strain energy release rates (G_IC and G_IIC, respectively). The most common tests to estimate G_IC are the Double-Cantilever Beam (DCB) and the Tapered Double-Cantilever Beam (TDCB) tests. The main objective of this work is to compare the DCB and TDCB tests to obtain the G_IC of adhesive joints. Three adhesives with varying ductilities were used to verify their influence on the precision of the typical methods of data reduction. For both tests, methods that do not need the measurement of crack length (a) were tested. A CZM analysis was considered to reproduce the experimental load–displacement (P-δ) curves and obtain the tensile CZM laws of each tested adhesive, to test the suitability of the data reduction methods, and to study the effect of the CZM parameters on the outcome of the simulations. The CZM models accurately reproduced the experimental tests and confirmed that the data reduction methods for the TDCB test tend to underestimate G_IC for ductile adhesives.     

Effect of different surface preparations on the tensile strength of adhesively bonded metal joints

In this in vitro study, the effects of different surface preparations and resins on the strength and durability of adhesively bonded joints were evaluated. Disk-shaped cobalt-chromium substrate samples of the first group were treated by the Silicoater MD® system. Samples of the two subgroups were bonded with two different bisphenol-A glycidyl methacrylate (Bis-GMA) adhesives. Samples of the second group were treated by the Rocatec®) system and bonded with a Bis-GMA adhesive. Alumina-blasted samples of the third group were bonded with two different types of Bis-GMA adhesive modified with a phosphate monomer. Samples were stored in water for 3 days, or thermocycled and stored in water for 6 months. The joint samples were then tested for tensile bond strength. When the alumina-blasted samples were bonded with Panavia Ex® or Panavia 21® adhesive the highest bond strength was obtained, regardless of the storage conditions. The Silicoater MD method in combination with the Bis-GMA adhesive yielded high initial bond strengths comparable to those obtained with the Panavia systems, but also recorded the highest drops in bond strengths with both types of adhesive after thermal stressing and water storage. The Rocatec system in combination with Nimetic Grip adhesive produced a low but stable bond strength even after thermocycling and water storage.     

Studying the effects of chain extenders chemical structures on the adhesion and mechanical properties of a polyurethane adhesive

Chain extenders including diethanol amine (DEA) and 2-cyano-N,N-bis-hydroxyethyl acetamide (CNBA) were utilized to enhance the adhesion and mechanical properties of a polyurethane (PU) adhesive. The adhesion and mechanical properties of the adhesives were studied by peel test, tensile test and dynamic thermal analysis (DMTA) respectively. Results revealed that both chain extenders (DEA and CNBA) increased peel test modulus. The cross-linking density and toughness of the adhesive were significantly increased using chain extenders. Results showed that chain extender without side chain (DEA) could improve the adhesion and mechanical properties of the adhesive greater than chain extender with aliphatic side group (CNBA).     

室温固化环氧树脂结构胶粘剂的研究

制备了室温快速固化综合性能较好的聚氨酯改性环氧树脂结构胶粘剂,探讨了增韧剂、固化剂等因素对胶粘剂力学性能的影响,通过扫描电镜分析了胶粘剂断裂面的形貌特征,研究了胶粘剂形貌特征与力学性能的关系。结果表明:聚氨酯增韧环氧树脂既有物理交联产生“强迫互溶”和“协同效应”,又与环氧树脂发生了化学反应,产生的化学键进一步加强了聚合物交联密度和聚合物的强度,当PU预聚体用量为环氧树脂的40%时,聚氨酯与环氧树脂形成的互穿网络聚合物互穿程度最大,制备的胶粘剂力学性能最优,室温剪切强度达到20.16MPa。     

Effects of a silane coupling agent on the tensile adhesive strength between resin and titanium

Adhesion of a cured dental opaquer, which masks metallic appearance and color, to sandblasted Ti aided by 3‐methacryloyloxypropyltrimethoxysilane (MPTS) is investigated. The tensile bond strengths of Ti and the opaquer are 43.3 and 45.9 MPa, respectively, when MPTS dissolved in ethanol (6 wt %) and premixed with the opaquer (3 wt %) are applied to the sandblasted Ti surface. Mirror finishing alone and sandblasting alone result in the bond strengths of 1.8 and 21.3 MPa, respectively. After thermal cycling (4 and 60°C for 5000 cycles), the bond strengths of the Ti with sandblasting and the MPTS treatment remain higher than that of the Ti with sandblasting only. Sandblasting and the MPTS treatment effectively promote the adhesion of the opaquer to Ti that rapidly forms an oxide surface layer. The silane‐aided adhesion of the opaquer to the sandblasted Ni? Cr, Co? Cr, and Au? Ag? Pd alloys attain adhesive strengths of 33.2, 31.9, and 31.6 MPa, respectively. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

醇溶性聚氨酯胶粘剂的合成与性能研究

醇溶性聚氨酯(APU)胶粘剂是一种新型、环保的胶粘剂,它既克服了溶剂型聚氨酯(PU)胶粘剂毒性大的缺点,又克服了水性PU水挥发慢、影响生产效率和能耗大的缺点,是目前综合性能优良的新品种胶粘剂。采用聚醚二醇、甲苯二异氰酸酯、二羟甲基丙酸和三羟甲基丙烷为基本原料,用丙酮为溶剂合成了聚醚型阴离子APU胶粘剂。讨论了预聚反应-NCO/-OH的配比、交联体系、硅烷偶联剂KH-550的加料方式和扩链反应温度等因素对APU胶粘剂性能的影响。当初聚n(-NCO)/n(-OH)比值为6,扩链剂反应温度控制在(60±2)℃时,合成的聚醚型阴离子APU具有较好的力学性能和耐水性,而且贮存稳定性较好。     

Effects of aging on the adhesive properties of poly(lactic acid) by atmospheric air plasma treatment

The aim of this study was to analyze the durability of a plasma treatment on the surface of poly(lactic acid) (PLA). We used atmospheric‐plasma treatment with air to improve the wettability of PLA by evaluating the aging effect under controlled conditions of relative humidity (RH) and temperature (25% RH and 25°C). We studied the durability of the atmospheric‐plasma treatment by measuring the contact angle, calculating the surface energy, and observing changes in the resistance of the PLA–PLA adhesive bonds. These techniques allowed us to evaluate the hydrophobic recovery phenomenon that the PLA surface suffered as a consequence of the aging process. The results provide the maximum storage time of PLA treated with atmospheric plasma at which the sample retained its good adhesion properties; this time was lower than 3 days under normal atmospheric conditions. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43040.