Rheological Behaviour of Nanoreinforced Epoxy Adhesives of Low Electrical Resistivity for Joining Carbon Fiber/Epoxy Laminates

Epoxy resins reinforced with carbon nanofibers (CNF) and nanotubes (CNT) were prepared and evaluated as adhesives of carbon fiber/epoxy laminates. Different percentages of nanofiller (0.1–3 wt%) have been tested. The viscosity of the non-cured nanoreinforced epoxy mixtures increased with the nanofiller content. On the other hand, the thermal treatment at high temperatures of the mixtures of amino-functionalized CNTs and epoxy monomer also caused an increase of their viscosity — this is likely due to the chemical reaction between the oxirane groups of the epoxy and the amine groups of the nanofiller. The joint strength of the carbon fiber/epoxy laminates bonded with nanoreinforced epoxy adhesives was analyzed by means of the single lap shear test. The shear strength of these joints was similar to that of the one made with unfilled epoxy resin. However, observation by Scanning Electron Microscopy of the fracture surfaces of the adhesive joints confirmed that the incorporation of carbon nanofillers caused the cohesive fractures inside the laminates (light-fiber tear failure). The electrical conductivity was drastically increased by the addition of nanofillers, especially CNTs.     

Effect of MWCNT filled epoxy adhesives on the quality of adhesively bonded joints

Most adhesively bonded joints exhibit adhesive or cohesive failure, i.e. failure at the adhesive/adherend interface or within the adhesive, respectively. The main objective of this study is to investigate the effect of surface modification of the metal substrate accompanied by modification of the adhesive properties on the strength and failure mechanism of bonded joints. A 5061 aluminium alloy has been used as the metal substrate onto which two types of surface treatments were applied; chemical surface modification and gritblasting. A standard epoxy resin was used as the adhesive medium, in which multi-wall carbon nanotubes (MWCNTs) were dispersed, with a range of weight fraction content (from 0.03% to 0.5%). The resin was fully characterised by mechanical testing in order to determine the optimum weight fraction to enhance its properties. Aluminium to aluminium and glass fibre reinforced polymer (GFRP) composite to aluminium single lap joints bonded with either pure epoxy resin or MWCNT reinforced epoxy resin were subsequently manufactured and tested. The tests show a moderate increase of the joint strength when MWCNTs are added into the adhesive with the failure mechanism changing from cohesive to adhesive. In addition, the comparison between different surface preparation methods shows that gritblasting results in considerably improved adhesive strength over chemical treatment.     

Mode-I adhesive fracture energy of carbon fibre composite joints with nanoreinforced epoxy adhesives

The effect of the addition of carbon nanoreinforcements to an epoxy adhesive on the strength and toughness of carbon fibre/epoxy composite joints was studied. The laminate surfaces, treated with peel ply, were characterised by profilometry, image analysis and wettability. The mechanical properties of the joints were determined by lap shear testing and double cantilever beam testing. The fracture mechanisms were studied by scanning electron microscopy.The addition of carbon nanofibres and carbon nanotubes caused an increase in the mode-I adhesive fracture energy, G_IC, of the joints while their lap shear strengths remained approximately constant. This improvement in the fracture behaviour was attributed to the occurrence of toughening mechanisms when carbon nanoreinforcements were added to the epoxy adhesive. Additionally, the use of carbon nanotubes improved the interfacial strength between the adhesive and the substrate, changing the crack growth behaviour and the macroscopic failure mode.     

Synergistic effect of graphene and carbon nanotube on lap shear strength and electrical conductivity of epoxy adhesives

In this study, synergy between graphene platelets (GnPs) and carbon nanotubes (CNTs) in improving lap shear strength and electrical conductivity of epoxy composite adhesives is demonstrated. Adding two-dimensional GnPs with one-dimensional CNTs into epoxy matrix helped to form global three-dimensional network of both GnPs and CNTs, which provide large contact surface area between the fillers and the matrix. This has been evidenced by comparing the mechanical properties and electrical conductivity of epoxy/GnP, epoxy/CNT, and epoxy/GnP-CNT composites. Scanning electron microscopic images of lap shear fracture surfaces of the composite adhesives showed that GnP-CNT hybrid nanofillers demonstrated better interaction to the epoxy matrix than individual GnP and CNT. The lap shear strength of epoxy/GnP-CNT composite adhesive was 89% higher than that of the neat epoxy adhesive, compared with only 44 and 30% increase in the case of epoxy/GnP and epoxy/CNT composite adhesives, respectively. Electrical percolation threshold of epoxy/GnP-CNT composite adhesive is recorded at 0.41 vol %, which is lower than epoxy/GnP composite adhesive (0.58 vol %) and epoxy/CNT composite adhesive (0.53 vol %), respectively. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 48056.     

Effect of multi-walled carbon nanotubes and silicon carbide nanoparticles on the deleterious influence of water absorption in adhesively bonded joints

In this study, the deleterious influence of hot deionized water on adhesively bonded joints was reduced with silicon carbide (SiC) nanoparticles and multi-walled carbon nanotubes (MWCNTs). A gravimetric method was used to study the kinetics of water ingress into the neat and nanocomposite epoxy adhesives. Then, joints were manufactured using the same neat and nanocomposite adhesives and aged for different periods according to the results obtained from the bulk sample tests and finite element modeling. The results showed that the reinforcing effect of nanofillers on the strength was about three times higher for the wet epoxy adhesive compared to the dry one. Moreover, it was found out that introducing 4.4 wt% of SiCs or 0.52 wt% of MWCNTs to the adhesive can compensate the degrading influence of aging under near-saturated condition. Furthermore, the scanning electron microscope (SEM) fractography was used to assess the fracture surfaces of the neat and reinforced samples.     

Mechanically robust,electrically and thermally conductive graphene-based epoxy adhesives

This study develops a facile approach to fabricate adhesives consists of epoxy and cost-effective graphene platelets (GnPs). Morphology, mechanical properties, electrical and thermal conductivity, and adhesive toughness of epoxy/GnP nanocomposite were investigated. Significant improvements in mechanical properties of epoxy/GnP nanocomposites were achieved at low GnP loading of merely 0.5?vol%; for example, Young’s modulus, fracture toughness (K_1C) and energy release rate (G_1C) increased by 71%, 133% and 190%, respectively compared to neat epoxy. Percolation threshold of electrical conductivity is recorded at 0.58?vol% and thermal conductivity of 2.13?W m^?1 K^?1 at 6?vol% showing 4 folds enhancements. The lap shear strength of epoxy/GnP nanocomposite adhesive improved from 10.7?MPa for neat epoxy to 13.57?MPa at 0.375?vol% GnPs. The concluded results are superior to other composites or adhesives at similar fractions of fillers such as single-walled carbon nanotubes, multi-walled carbon nanotubes or graphene oxide. The study promises that GnPs are ideal candidate to achieve multifunctional epoxy adhesives.     

Comparison of fracture behavior between acrylic and epoxy adhesives

An experimental study was conducted on the strength of adhesively bonded steel joints, prepared epoxy and acrylic adhesives. At first, to obtain strength characteristics of these adhesives under uniform stress distributions in the adhesive layer, tensile tests for butt, scarf and torsional test for butt joints with thin-wall tube were conducted. Based on the above strength data, the fracture envelope in the normal stress-shear stress plane for the acrylic adhesive was compared with that for the epoxy adhesive. Furthermore, for the epoxy and acrylic adhesives, the effect of stress triaxiality parameter on the failure stress was also investigated. From those comparison, it was found that the effect of stress tri-axiality in the adhesive layer on the joint strength with the epoxy adhesive differed from that with the acrylic adhesive. Fracture toughness tests were then conducted under mode l loading using double cantilever beam (DCB) specimens with the epoxy and acrylic adhesives. The results of the fracture toughness tests revealed continuous crack propagation for the acrylic adhesive, whereas stick-slip type propagation for the epoxy one. Finally, lap shear tests were conducted using lap joints bonded by the epoxy and acrylic adhesives with several lap lengths. The results of the lap shear tests indicated that the shear strength with the epoxy adhesive rapidly decreases with increasing lap length, whereas the shear strength with the acrylic adhesive decreases gently with increasing the lap length.     

Failure behavior of adhesively bonded joints with a rubber modified epoxy adhesive under tensile-shear combined cyclic loading

Rubber-modified epoxy adhesives are used widely as structural adhesive owing to their properties of high fracture toughness. In many cases, these adhesively bonded joints are exposed to cyclic loading. Generally, the rubber modification decreases the static and fatigue strength of bulk adhesive without flaw. Hence, it is necessary to investigate the effect of rubber-modification on the fatigue strength of adhesively bonded joints, where industrial adhesively bonded joints usually have combined stress condition of normal and shear stresses in the adhesive layer. Therefore, it is necessary to investigate the effect of rubber-modification on the fatigue strength under combined cyclic stress conditions. Adhesively bonded butt and scarf joints provide considerably uniform normal and shear stresses in the adhesive layer except in the vicinity of the free end, where normal to shear stress ratio of these joints can cover the stress combination ratio in the adhesive layers of most adhesively bonded joints in industrial applications.

In this study, to investigate the effect of rubber modification on fatigue strength with various combined stress conditions in the adhesive layers, fatigue tests were conducted for adhesively bonded butt and scarf joints bonded with rubber modified and unmodified epoxy adhesives, wherein damage evolution in the adhesive layer was evaluated by monitoring strain the adhesive layer and the stress triaxiality parameter was used for evaluating combined stress conditions in the adhesive layer. The main experimental results are as follows: S–N characteristics of these joints showed that the maximum principal stress at the endurance limit indicated nearly constant values independent of combined stress conditions, furthermore the maximum principal stress at the endurance limit for the unmodified adhesive were nearly equal to that for the rubber modified adhesive. From the damage evolution behavior, it was observed that the initiation of the damage evolution shifted to early stage of the fatigue life with decreasing stress triaxiality in the adhesive layer, and the rubber modification accelerated the damage evolution under low stress triaxiality conditions in the adhesive layer.     

Bond behavior of epoxy resin–polydicyclopentadiene phase separated interpenetrating networks for adhering carbon fiber reinforced polymer to steel

The performance of bonded joints of carbon fiber reinforced polymer (CFRP) and steel relies on the mechanical properties of the adhesive used. Despite the high strength and modulus of epoxy adhesives, their brittleness limits their application to defect-sensitive structures. The development of interpenetrating polymer networks (IPNs), either homogeneous or phase separated, provides a route to toughen the epoxy while maintaining its high strength and modulus. Microphase separated IPNs consisting of a diglycidyl ether of bisphenol A-based epoxy resin and a thermoset with high toughness, polydicyclopentadiene (PDCPD), has been previously shown to demonstrate superior combinations of strength and toughness. This work investigates the most critical adhesive properties that affect bond strength by characterizing CFRP-steel double-lap shear joints containing the epoxy resin–PDCPD blend as the adhesive, using a wet lay-up manufacturing technique. The epoxy resin–PDCPD blend adhesives realized much higher bond strengths compared to either neat epoxy or neat PDCPD. Correlations between the bond strength and the bulk material properties are presented. Theoretical calculation of the bond strength indicates that the higher bond strength that can be achieved by using the epoxy resin–PDCPD blend adhesive is due to the increased shear toughness of the new formulations. POLYM. ENG. SCI., 60:104–112, 2020. © 2019 Society of Plastics Engineers     

镀铜碳纳米管导电胶的研究

通过化学镀方法制备了镀铜碳纳米管,用透射电镜(TEM)对其表面形貌进行表征;并以镀铜碳纳米管为填料制备了环氧树脂导电胶。结果表明:镀铜碳纳米管上的镀层的质量、填料在树脂中的分散性以及填料的质量分数对于导电胶电性能和黏结性能起着决定因素。     

Effect of silicon carbide nanoparticles on the adhesion strength of steel–epoxy composite joints bonded with acrylic adhesives

This paper reports a study on the effect of silicon carbide nanoparticles on the adhesion strength of steel–glass/epoxy composite joints bonded with two-part structural acrylic adhesives. The introduction of nanosilicon carbide in the two-part acrylic adhesive led to a remarkable enhancement in the shear and tensile strength of the composite joints. The shear and tensile strengths of the adhesive joints increased with adding the filler content up to 1.5?wt%, after which decreased with adding more filler content. Also, addition of nanoparticles caused a reduction in the peel strength of the joints. DSC analysis revealed that Tg values of the adhesives rose with increase in the nanofiller content. The equilibrium water contact angle was decreased for adhesives containing nanoparticles. SEM micrographs revealed that addition of nanoparticles altered the fracture morphology from smooth to rough fracture surfaces.     

Wedge test of carbon‐nanotube‐reinforced epoxy adhesive joints

Epoxy adhesives reinforced with carbon nanotubes (CNTs) were developed. The distribution of the CNTs in the epoxy matrix was observed with transmission electron microscopy. Joints were formed by unclad 2024‐T3 aluminum adherents bonded with the CNT‐filled epoxy adhesives. The durability of the joints was studied with a wedge test under water at 60°C. The addition of CNTs to the epoxy greatly improved the adhesive joint durability. The initial crack length of the joint with 1 wt % CNTs, which was obtained before the wedge specimen was put into water, was only about 7% of that with neat epoxy. After immersion of the specimens in 60°C water, the joint with neat epoxy failed after 3 h, but all of the joints adhered with different fractions of CNTs were still bound together after the experimental time of 90 h. The significant enhancement by CNTs of the adhesive joint durability was mainly attributed to the high mechanical properties of the CNTs and their ability to resist water. Nevertheless, the experimental results also reveal that the durability of the joints showed an optimum value at approximately 1 wt % CNTs, beyond which a decrease in the property was observed. In addition, the failure mechanism of the joints was also investigated in terms of interfacial failure and cohesive failure. Cohesive dominated failure was found for the joint bonded with 1 wt % CNT‐filled epoxy. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

水性环氧树脂包覆有机蒙脱土制备木材胶粘剂

摘要：水性环氧树脂通常含有水溶性分子或分子链，导致在高温和潮湿条件下作为木材胶粘剂时耐水性及力学性能较差。采用有机改性的纳米蒙脱土改性水性环氧树脂增强水性环氧树脂胶粘剂的耐水性及力学性能。并通过乳液包覆蒙脱土的方法与直接共混的方法对比，研究了不同添加量有机蒙脱土（0%，3%，6%，9%）对胶粘剂性能的影响。胶粘剂的耐水性及力学性能通过测量胶粘剂在干燥及潮湿条件下的剪切强度来表示。通过TGA、SEM、TEM、DSC研究了复合胶粘剂的热稳定性和结构。结果表明，在水性环氧树脂中添加有机改性的纳米蒙脱土，可以有效地提高胶粘剂的粘结强度，此外，采用乳液包有机覆蒙脱土的方法比直接共混的方法制备得到胶粘剂，有机蒙脱土在胶粘剂中分布更均匀，具有更优异的力学性能，说明有机蒙脱土在复合材料中的分散质量是影响复合胶粘剂性能的主要原因。     

Hotmelts with improved properties by integration of carbon nanotubes

With the aim of the development of conductive and mechanically improved adhesives, carbon nanotubes (CNTs) were dispersed by melt mixing into a non-reactive polyolefine based hotmelt adhesive. The composite materials, containing 0.5 to 5.0 wt% multi-walled CNTs (MWNTs), showed electrical percolation at about 0.75 wt%. Investigations of the mechanical properties using tensile tests resulted in a significant enhancement of Young's modulus up to 372% and nearly doubling of tensile strength at 5.0 wt%. Even if the hotmelt material is highly elastic compared to typical thermoplastic matrices, the melt mixing resulted in suitable CNT dispersion. The melt viscosity increased with CNT loading, however near the observed electrical percolation threshold the processability was not notably reduced. Most important, next to conductivity at low CNT loadings, also a significant enhancement in the shear strength of bonded joints of AlMg3 up to values of 250% of the pure hotmelt could be obtained. The property profile can be tailored with CNT concentration, indicating the suitability of CNT addition into these hotmelt adhesives.     

Improving electrical and mechanical properties of a conductive nano adhesive

In this experimental study, lap shear strength and electrical conductivity of nanohybrid adhesives containing multi-walled carbon nanotubes (MWCNT) and silver (Ag) nanoparticles were investigated. Ag nanoparticles were produced via arc-discharge method in liquid nitrogen. For characterizing the Ag nanoparticles, X-ray diffraction analysis, transmission electron microscopy, and scanning electron microscopy (SEM) were performed. Tensile lap shear properties were determined in accordance with ASTM D 1002-10 standard. Mechanical and the electrical properties of nanohybrid adhesives were compared with neat epoxy adhesive. The best electrical conductivity of nanohybrid adhesive was obtained for the 1% wt MWCNT-2% wt Ag-contained sample. However, the samples which contain 0.5% wt. MWCNT–0.5% wt. Ag nanoparticles reached the highest lap shear strength. The results showed that Ag nanoparticles enhance the conductivity in the presence of MWCNT. It is concluded that the MWCNT act as conductivity bridges among epoxy adhesive and facilitate the electron transfer. As seen in the tensile test results, the ductility of the adhesive was improved by adding the nanoparticles in to the epoxy resin.     

Accelerated-curing epoxy structural film adhesive for bonding lightweight honeycomb sandwich structures

Accelerating the curing of epoxy/aromatic amine adhesives and improving their toughness are challenges in heat-resistant epoxy structural adhesives. Herein, we report an epoxy/aromatic amine adhesive accelerated curing system with an oxo-centered trinuclear (chromium III) complex, which is toughened using a thermoplastic block copolymer (TPBC). The reaction characteristics, heat resistance, microstructure, and bonding properties of the accelerated epoxy adhesives were analyzed. The reaction peak temperature of the epoxy with 3% catalyst was 113.1°C, which was 113.6°C lower than that of epoxy without catalyst, and the modified epoxy resin demonstrated a potential for rapid curing at medium temperature. The glass transition temperature of the TPBC-toughened epoxy adhesive was 125°C after curing, indicating excellent thermal stability after medium temperature curing. The introduction of the TPBC increased the single-lap shear strength of the epoxy adhesive without reducing its heat resistance. The shear strength at room temperature and 120°C of the modified epoxy adhesive with 50 phr of TPBC was 25.2 and 10.9 MPa, respectively. Moreover, the epoxy film adhesive exhibited outstanding bonding properties when used in the bonding of lightweight honeycomb sandwich structures.     

Strength of epoxy adhesive-bonded stainless-steel joints

The strength of stainless-steel joints bonded with two epoxy adhesives was investigated. The experimental programme included tests on single-lap and butt joints, as well as thick-adherend and napkin ring shear tests. Results suggested that the tensile and shear strengths of the epoxy adhesives were quite similar. However, finite element (FE) analyses raised doubts on the true adhesive strengths, due to the complex stress state in joint tests and pressure-dependent adhesive behaviour. In spite of some uncertainties, FE analyses showed that failure could be fairly well predicted by a maximum shear strain criterion.     

An investigation of fatigue failure prediction of adhesively bonded metal/metal joints

A fracture mechanics-based model for fatigue failure prediction of adhesive joints has been applied in this work. The model is based on the integration of the kinetic law of evolution of defects originated at stress concentrations within the joint. Final failure can be either brittle (fracture toughness-driven) or ductile (tensile/shear strength-driven) depending on the adhesive. The model has been validated against experiments conducted on single-lap shear joints bonded with a structural adhesive. Three different kinds of adhesives, namely a modified methacrylate, a one-part epoxy and a two-part epoxy supplied by Henkel, have been considered and three different overlap lengths have been tested. Fracture toughness and fatigue crack growth properties of the adhesives have been determined with mode I tests. The number of cycles to failure has been successfully predicted in several cases. It is interesting to notice that in the case of joints loaded at the same average shear stress, the shorter the joint, the longer the duration. This fact is also captured by the model.     

New Developments in Structural Adhesives for the Automotive Industry

Recently developed epoxy paste adhesives, reactive hot melts, adhesive film tape and polyurethane adhesives are presented for structural bonding in the automotive industry. Paste adhesives usually require a precure stage to obtain handling strength of the joints and to guarantee wash-out resistance of the adhesive in the paint baths. This step can be omitted with reactive hot melts and adhesive film tape, which are solid before and after their application. In addition they allow an improved working hygiene. Some mechanical properties of the adhesives are shown such as lap shear strength and peel strength as well as lap shear strength as a function of the bondline thickness. Results of the excellent durability of epoxy one-component pastes, reactive hot melts and adhesive film tape are given from cyclic environmental and salt spray tests.     

Selected aspects of epoxy adhesive compositions curing process

The paper focuses on selected parameters of curing process – temperature and time. The tests aimed at evaluating the impact of short-term thermal recuring on 1050A and 2017A aluminium alloy sheet adhesive joints strength. Joints were formed with two different adhesives, the main component of which was in both cases epoxy resin Epidian 53 and two different cure agents – poliamineamide C (PAC) and triethylenetetraamine (PF) curing agents. Curing conditions – first curing time, recuring time and recuring temperature – were modified for each of the four tests conducted. For the sake of comparative analysis, adhesive joints were subjected to a single-stage cure cycle at ambient temperature. A two-stage cure cycle of both Epidian 53 compositions at 80?°C for 1 and 2?h produces a material of different mechanical properties than the same material which submits a single-stage cure cycle at ambient temperature, as well as at 60?°C for 30?min. Simultaneously, Epidian 53/PF/100:50 composition proves to produce higher joint strength after recuring than Epidian 53/PAC/100:80; the strength of a joint formed with the former composition increases up to 50% when compared with joints subjected to a single-stage cure cycle. Moreover, tests show that recuring of the adhesive joint formed with both compositions at 60?°C for 30?min does not have a considerable influence on either 1050A or 2017A aluminium adhesive joint strength.