Optimum Silane Treatment for the Adhesively Bonded Aluminum Adherends at the Cryogenic Temperature

Aluminum adherends for the adhesive joints at cryogenic application such as LNG (Liquefied Natural Gas) containment tanks were silane treated to improve bond strength of the aluminum joints. The bond strengths of the single-lap joints composed of aluminum adherends and epoxy adhesive were measured with respect to the condition of the silane solution and surface roughness obtained with grit blast. The chemical composition of the silane treated aluminum surface was analyzed by XPS (X-ray photoelectron spectroscopy). Also, contact angles of appropriate liquids on the silane treated aluminum surface were measured with respect to treatment conditions. From the experiments, the optimum treatment conditions for the aluminum adherends were obtained for the adhesive joints at the cryogenic temperature.     

Adhesive properties of ethyl 2-cyanoacrylate containing small amounts of acetic acid as adhesion promoter

The lap shear strengths of stell joints bonded with ethyl 2-cyanoacrylate adhesive containing various amounts of acetic acid have been determined, and an optimum adhesion promoting effect was found at an acetic acid content of 0.25% by weight. Further tests showed that adhesion promotion with acetic acid was dependent on the nature and surface treatment of various metal adherends, with selectivity being shown towards steel, stainless steel and duraluminium.     

The Effect of Contamination on Adhesive Strength: Wettability Characterization by the CSC Method

The correlation between the adhesive strength of joints of aluminum alloys and the wettability of their surfaces was studied. Two alloys, Al-1100 and Al-2024, were used, with two commercial epoxy adhesives, FM-73 and FM-300K, and the BR-127 primer. The wettability of the surfaces was modified by applying coatings of silicone oil and stearic acid at various concentrations. Wettability was characterized by the “complete spreading concentration” (CSC) method and by advancing contact angles. The CSC method was proven to be more reliable than contact angle measurements in detecting variations in the surface energy of the aluminum alloys studied. The adhesive strength was measured by the lap shear strength (LSS) and by the T-peel strength. The adhesive strength is only mildly sensitive to the concentration of silicone oil for the two adhesive systems. For FM-300K, the LSS decreases when the concentration of stearic acid in the coating increases. For FM-73, the LSS is only mildly sensitive to the concentration of stearic acid, but the T-peel strength shows appreciable sensitivity. The failure mode becomes more adhesive as the concentration of stearic acid and thereby the CSC increase. It is only mildly dependent on the concentration of silicone oil. The results indicate that silicone oil probably interacts with either the primer or an adhesive component in a way which counteracts the expected decrease in adhesive strength due to the reduction in surface energy and wettability of the adherends. However, the effect of stearic acid on the adhesive strength is associated with decreased wettability. All cases for which a pronounced decrease in the adhesive strength was measured are associated with contact angles larger than 90° and with either high CSC values or nonspreading situations.     

The Effect of Contamination on Adhesive Strength: Wettability Characterization by the CSC Method

The correlation between the adhesive strength of joints of aluminum alloys and the wettability of their surfaces was studied. Two alloys, Al-1100 and Al-2024, were used, with two commercial epoxy adhesives, FM-73 and FM-300K, and the BR-127 primer. The wettability of the surfaces was modified by applying coatings of silicone oil and stearic acid at various concentrations. Wettability was characterized by the “complete spreading concentration” (CSC) method and by advancing contact angles. The CSC method was proven to be more reliable than contact angle measurements in detecting variations in the surface energy of the aluminum alloys studied. The adhesive strength was measured by the lap shear strength (LSS) and by the T-peel strength. The adhesive strength is only mildly sensitive to the concentration of silicone oil for the two adhesive systems. For FM-300K, the LSS decreases when the concentration of stearic acid in the coating increases. For FM-73, the LSS is only mildly sensitive to the concentration of stearic acid, but the T-peel strength shows appreciable sensitivity. The failure mode becomes more adhesive as the concentration of stearic acid and thereby the CSC increase. It is only mildly dependent on the concentration of silicone oil. The results indicate that silicone oil probably interacts with either the primer or an adhesive component in a way which counteracts the expected decrease in adhesive strength due to the reduction in surface energy and wettability of the adherends. However, the effect of stearic acid on the adhesive strength is associated with decreased wettability. All cases for which a pronounced decrease in the adhesive strength was measured are associated with contact angles larger than 90° and with either high CSC values or nonspreading situations.     

金属工件的表面处理及胶接工艺对胶接剪切强度的影响

采用环氧树脂-DDS固化体系作为胶接剂,探讨了各种金属表面处理方法、胶接时对胶接面施加的压力、胶层厚度等因素对胶接剪切强度的影响.结果表明,对不锈钢工件采用盐酸氧化法及草酸-硫酸氧化法进行表面处理,得到的胶接剪切强度最好;对铝合金工件采用硅酸磷酸钠法进行表面处理,得到的胶接剪切强度最高.胶接时对胶接面施加0.1～0.4Mpa的压力、胶层厚度在0.10～0.23 mm范围内时胶接效果最好.由于处理液对金属表面产生适度的化学腐蚀,改变了金属表面的物理化学性质,表现出更好的可浸润性和更强的表面吸附力,可以有效地提高胶接剪切强度.     

Fatigue Crack Growth in Epoxy/Aluminum and Epoxy/Steel Joints

The fatigue crack growth rate within epoxy/aluminum and epoxy/steel joints was evaluated as a function of a) type of surface pretreatment, b) water soak, c) fatigue cycle rate (Hz), d) adhesive thickness and e) type of epoxy adhesive.

For both adherends, aluminum and steel, a significant improvement in the fatigue behavior was obtained by use of a mercaptoester coupling agent. After an 8-day, 57°C water soak, the metal surfaces which were pretreated with coupling agent (CA) or by phosphoric acid anodization (PAA) still resulted in cohesive failure, while the controls had higher crack growth rate and showed greater scatter. The room-temperature cure matrix with CA-treated aluminum showed a less dramatic improvement, probably because of a known difference in the application procedure. For the steel joints and room-temperature adhesive the improvement in the fatigue behavior of CA-treated samples was maintained after the 8-day hot water soak. No significant change was found in the fatigue crack growth rate over a frequency range of 1 to 5 Hz, but a significant change was found as a function of the bondline thickness. The room temperature curing adhesive evaluated herein exhibited a much lower fatigue resistance than a heat-cured commercial structural adhesive FM-73.     

Fatigue Crack Growth in Epoxy/Aluminum and Epoxy/Steel Joints

The fatigue crack growth rate within epoxy/aluminum and epoxy/steel joints was evaluated as a function of a) type of surface pretreatment, b) water soak, c) fatigue cycle rate (Hz), d) adhesive thickness and e) type of epoxy adhesive.

For both adherends, aluminum and steel, a significant improvement in the fatigue behavior was obtained by use of a mercaptoester coupling agent. After an 8-day, 57°C water soak, the metal surfaces which were pretreated with coupling agent (CA) or by phosphoric acid anodization (PAA) still resulted in cohesive failure, while the controls had higher crack growth rate and showed greater scatter. The room-temperature cure matrix with CA-treated aluminum showed a less dramatic improvement, probably because of a known difference in the application procedure. For the steel joints and room-temperature adhesive the improvement in the fatigue behavior of CA-treated samples was maintained after the 8-day hot water soak. No significant change was found in the fatigue crack growth rate over a frequency range of 1 to 5 Hz, but a significant change was found as a function of the bondline thickness. The room temperature curing adhesive evaluated herein exhibited a much lower fatigue resistance than a heat-cured commercial structural adhesive FM-73.     

Effect of thermal cycling on tensile properties of degraded FML to metal hybrid joints exposed to sea water

In the present paper, the mechanical properties of hybrid bonded bolted joints between Fiber metal laminate (FML) and stainless steel adherends are investigated using experimental tensile tests. Three and five layered FMLs were fabricated using 430 stainless steel sheets and fiberglass prepreg layers. The adherends were bonded by AD-314 resin mixed with HA-34 hardener as adhesive and steel bolt was used for the mechanical fastening. The specimens were immersed into the sea water for 30 days and degradation of the mechanical strength of the joints was studied. Thermal cycles including heating (40 °C to100 °C) and cryogenic (−100 °C to −40 °C) cycles were applied in order to study their effects on the strength of the degraded joints. The failure mode for the adhesive bond was mixed failure and that of the bolted joint was the net-tension failure. The results showed 52% strength recovery in hybrid joints subjected to heating cycles. Cryogenic cycles also caused a 50% improvement in the tensile strength of the hybrid joints. In addition, the joint stiffness and absorbed energy of the specimens were improved significantly for both heating and cryogenic cycles. Moreover, the effect of FML stacking sequence on the results was also investigated. The results revealed that the mechanical fastening failure load for 5 layered FML joint is more affected by thermal cycles in comparison with 3 layered FML joint.     

Bonding characteristics and corrosion resistance of silane–cerium-treated aluminum adherend

A silane–cerium treatment was applied on an aluminum adherend to simultaneously improve the bonding performance and corrosion resistance of the adhesively bonded aluminum joint in cryogenic applications, such as with liquefied natural gas containment tanks. The lap shear strengths and corrosion performances of the adhesively bonded joints composed of treated aluminum adherends were measured with respect to the silane–cerium treatment and the surface pretreatment on the aluminum adherend. The bonding characteristics of the aluminum adherend were investigated by measuring the water contact angle and conducting the potentiodynamic polarization test after the aluminum adherends with different surface treatments of silane–cerium were immersed in a 0.5?M NaCl solution. In addition, the surfaces were analyzed with scanning electron microscopy, X-ray photoelectron spectroscopy, and Fourier transform infrared spectroscopy to characterize the chemical compositions of the silane–cerium-treated aluminum adherend. The experimental results show that an appropriate silane–cerium treatment on the aluminum adherend produces an effective corrosion-resistant layer and that it has a highly reliable bonding characteristic for the adhesive joint at a cryogenic temperature of ?150?°C.     

Ultrasonic testing for bond efficiency of carbon steel to stainless steel adhesive joints

Structural adhesive joints between carbon steel and stainless steel were made and evaluated. Various combinations of a two-component epoxy adhesive were used and diverse polymerization cycles were employed to obtain joints of different mechanical strengths. Two ultrasonic procedures were used: ultrasonic scanning with a focused beam to ascertain the quality and the uniformity of the joints; and ultrasonic spectrum analysis to evaluate bond strength. The influence on adhesion of surface calamine of the carbon steel was also investigated: bond strengths were comparable with those attained with ground carbon steel adherends. Adding lubricating oil to the adhesive was found to increase the mechanical properties of the joints.     

Dynamic strength of single lap joints with similar and dissimilar adherends

The increased use of adhesives for joining structural parts demands a thorough understanding of their load carrying capacity. The strength of the adhesive joints depends on several factors such as the joint geometry, adhesive type, adherend properties and also on the loading conditions. Particularly polymer based adhesives exhibit sensitivity to loading rate and therefore it is important to understand their behavior under impact like situations. The effect of similar versus dissimilar adherends on the dynamic strength of adhesive lap joints is addressed in this study. The dynamic strength is evaluated using the split-cylinder lap joint geometry in a split Hopkinson pressure bar setup. The commercial adhesive Araldite 2014 is used for preparing the joints. The adherend materials considered included steel and aluminum. The results of the study indicated that the dynamic strength of the lap joint is influenced by the adherend material and also by the adherent combination. Even in the case of joints with similar adherends, the strength was affected by the adherend type. The strength of steel–steel joints was higher than that for aluminum–aluminum joints. In the case of dissimilar adherends, the strength was lower than that of the case of similar adherends. The results of this study indicate that the combination of adherend material should also be accounted for while designing lap joints.     

Adhesion Science and Surface Analysis. Typical Examples

A number of surface modification methods using mechanical, chemical, electrochemical or physical processes have commonly been employed to treat adherend surfaces and improve adhesion in various bi- or multi-layered systems which play a key role in many advanced technologies. Results reported in this review paper deal more particularly with components of polymer-metal systems and provide typical examples of surface analysis obtained by x-ray photoelectron spectrometry (XPS), ion scattering spectrometry (ISS), low-energy electron induced x-ray spectrometry (LEEIXS), optically stimulated electron emission (OSEE) and Fourier transform infra-red spectrometry (FTIR). These examples concern, on the one hand, metallic adherends (aluminum, stainless steel, zinc-coated steel, gold) the surfaces of which were modified by cleaning, etching, oxidation, conversion or chemical grafting and, on the other hand, bonded joints (polyester-steel and epoxy-aluminum systems) which were delaminated using a peel test and a three-point flexure test, respectively.     

Cleavage Strength of Steel/Composite Joints

This paper presents the experimental results of an on-going study to examine cleavage strength, particularly at the interface regions of epoxy adhesive with steel and glass reinforced epoxy (GRE) composite. The adhesion is characterised by mechanical testing of cleavage specimens. A standard specimen was modified to allow testing of hybrid joints. The effects of adhesive thickness and various surface conditions of both adherends were examined. Among key conclusions, the study found that cleavage strength is not strongly dependent upon adhesive thickness and that polished composite gives better adhesion compared with polished steel. Test results were analysed and compared with aspects of numerical analyses. The study has also established a new methodology to test hybrid adhesive cleavage joints.     

Molecular Structure of Polymer/Metal Interphases

The molecular structure of interphases in aluminum/epoxy and steel/epoxy adhesive joints was characterized using infrared spectroscopy. In one series of experiments, adhesive joints were prepared by curing beams of epoxy against aluminum or steel substrates. When the joints were cooled to room temperature, the residual stresses were sufficient for crack propagation along the interface. The adhesive and substrate failure surfaces were then analyzed with reflection-absorption infrared spectroscopy (RAIR), attenuated total reflection infrared spectroscopy (ATR) and X-ray photo-electron spectroscopy (XPS). When an epoxy/anhydride adhesive was cured against aluminum substrates primed with an aminosilane coupling agent, amide and imide groups were formed in the interphase. Chemical reaction between the primary amine of the primer and the anhydride of the curing agent precluded chemical bridge formation between the primer and adhesive. Metal cations from the 2024 aluminum substrate reacted with the anhydride to form carboxylate salts on the surface. When an epoxy/tertiary amine adhesive was cured against steel substrates, evidence of oxidation of the primary amine to imine was observed in the interphase.     

Cleavage Strength of Steel/Composite Joints

This paper presents the experimental results of an on-going study to examine cleavage strength, particularly at the interface regions of epoxy adhesive with steel and glass reinforced epoxy (GRE) composite. The adhesion is characterised by mechanical testing of cleavage specimens. A standard specimen was modified to allow testing of hybrid joints. The effects of adhesive thickness and various surface conditions of both adherends were examined. Among key conclusions, the study found that cleavage strength is not strongly dependent upon adhesive thickness and that polished composite gives better adhesion compared with polished steel. Test results were analysed and compared with aspects of numerical analyses. The study has also established a new methodology to test hybrid adhesive cleavage joints.     

Molecular Structure of Polymer/Metal Interphases

The molecular structure of interphases in aluminum/epoxy and steel/epoxy adhesive joints was characterized using infrared spectroscopy. In one series of experiments, adhesive joints were prepared by curing beams of epoxy against aluminum or steel substrates. When the joints were cooled to room temperature, the residual stresses were sufficient for crack propagation along the interface. The adhesive and substrate failure surfaces were then analyzed with reflection-absorption infrared spectroscopy (RAIR), attenuated total reflection infrared spectroscopy (ATR) and X-ray photo-electron spectroscopy (XPS). When an epoxy/anhydride adhesive was cured against aluminum substrates primed with an aminosilane coupling agent, amide and imide groups were formed in the interphase. Chemical reaction between the primary amine of the primer and the anhydride of the curing agent precluded chemical bridge formation between the primer and adhesive. Metal cations from the 2024 aluminum substrate reacted with the anhydride to form carboxylate salts on the surface. When an epoxy/tertiary amine adhesive was cured against steel substrates, evidence of oxidation of the primary amine to imine was observed in the interphase.     

Adhesion characteristics of carbon/epoxy composites treated with low- and atmospheric pressure plasmas

Although an adhesive joint can distribute load over a larger area than a mechanical joint, requires no holes, adds very little weight to structures and has superior fatigue resistance, it requires careful surface preparation of adherends for reliable joining and low susceptibility to service environments. The load transmission capability of adhesive joints can be improved by increasing the surface free energy of the adherends with suitable surface treatments. In this study, two types of surface treatment, namely the low pressure and the atmospheric pressure plasma treatment, were performed to enhance the mechanical load transmission capabilities of carbon/epoxy composite adhesive joints. The suitable surface treatment conditions for carbon/epoxy composite adhesive joints for both low and atmospheric pressure plasma systems were experimentally investigated with respect to chamber pressure, power intensity and surface treatment time by measuring the surface free energies of the specimens. The change in surface topography of carbon/epoxy composites was measured with AFM (Atomic Force Microscopy) and quantitative surface atomic concentrations were determined with XPS (X-ray Photoelectron Spectroscopy) to investigate the failure modes of composite adhesive joints with respect to surface treatment time. From the XPS investigation of carbon/epoxy composites, it was found that the ratio of oxygen concentration to carbon concentration for both low and atmospheric pressure plasma-treated carbon/epoxy composite surfaces was maximum after about 30 s treatment time, which corresponded with the maximum load transmission capability of the composite adhesive joint.     

Strength prediction of epoxy adhesively bonded scarf joints of dissimilar adherends

In this study, strength of epoxy adhesively bonded scarf joints of dissimilar adherends, namely SUS304 stainless steel and YH75 aluminum alloy is examined on several scarf angles and various bond thicknesses under uniaxial tensile loading. Scarf angle, θ=45°, 60° and 75° are employed. The bond thickness, t between the dissimilar adherends is controlled to be ranged between 0.1 and 1.2 mm. Finite element (FE) analysis is also executed to investigate the stress distributions in the adhesive layer of scarf joints by ANSYS 11 code. As a result, the apparent Young's modulus of adhesive layer in scarf joints is found to be 1.5-5 times higher than those of bulk epoxy adhesive, which has been obtained from tensile tests. For scarf joint strength prediction, the existing failure criteria (i.e. maximum principal stress and Mises equivalent stress) cannot satisfactorily estimate the present experimental results. Though the measured stress multiaxiality of scarf joints proportionally increases as the scarf angle increases, the experimental results do not agree with the theoretical values. From analytical solutions, stress singularity exists most pronouncedly at the steel/adhesive interface corner of joint having 45-75° scarf angle. The failure surface observations confirm that the failure has always initiated at this apex. This is also in agreement with stress-y distribution obtained within FE analysis. Finally, the strength of scarf joints bonded with brittle adhesive can be best predicted by interface corner toughness, Hc parameter.     

Adhesion of glow discharge polymers to metals and polymers

Adhesion of glow discharge polymers to metals and polymers in an adhesive joint was measured by lap-shear test and immersion in hot water of 70°C °C for an extended time. A glow discharge polymer was deposited onto polymers [polyethylene and poly(tetrafluoroethylene)] and metals (aluminum and stainless steel) prior to when the polymer and metal were joined. It is found that the lap-shear strength is enhanced by coating the surfaces of these substrates with plasma film produced from methane, ethylene, and acetylene, and that deterioration of the adhesive bonding part, when immersed in hot water of 70°C, is strongly dependent on the gas used as well as operational conditions where a polymer film is formed. The adhesion of a polymer produced from methane on the polymer and metal is strong enough to apply for durable, adhesive joints.     

The durability of some epoxide adhesive-bonded joints on exposure to moist warm air

Single lap joints bonded with an epoxide adhesive have been exposed to warm moist air for periods up to one year, and then subjected to strength measurement. Surface treatment of the aluminum alloy adherends had an effect upon the ability of joints to resist exposure, the order of surface treatment efficiency in this respect being chromic acid anodize > chromic acid etch > sandblast > degrease. A linear relationship has been observed between joint strength and the water content of joints, leading to the conclusion that water enters a joint by diffusion through the adhesive, rather than by passage along the interface.