A Review of Certain Recent Work on the Durability of Aluminium Alloy Bonded with Epoxide and Phenolic Adhesives

Single lap joints of aluminium alloy, bonded with a number of structural adhesives, have been aged at 100% or 50% relative humidity (r.h.) at 50°C for up to 10000 hours. The adhesives used have included a simple epoxide and some modified phenolics and epoxides. Whilst joints are not significantly weakened on exposure at 50% r.h., significant weakening occurs at 100% rh. There is an initial fall in strength in the early stages of exposure, but after this period joints remain fairly stable, retaining approximately 40–60% of the strengths they had before exposure. Water diffusion coefficients in the adhesives have been obtained from experiments on the mass uptake of water by films of the adhesives. Water concentration profiles and overall levels of water in adhesive joints have been calculated from diffusion coefficients, and these show that the initial fall in strength is controlled by water diffusing through the adhesive layer. Joint strengths recover significantly when they are dried out. The behaviour of joints can be interpreted by there being ion-pairs at the interface. Water reduces the interionic force by raising the permittivity of the surroundings, and this is reversed when the water is removed.     

Bonding of Aluminium Alloy with some Phenolic Adhesives and a Modified Epoxide Adhesive, and Strength Changes on Exposure to Moist Air at 50°C

Single lap joints of aluminium alloy bonded with three phenol-based and one modified epoxide adhesive have been aged at 100% relative humidity (r.h) or at 50% r.h. for up to 10,000 hours at 50°C. Whilst joints are not significantly weakened on exposure at 50% r.h., at 100% r.h. strength falls over about 2000 h and then tends to remain steady. This fall in strength is controlled by the rate at which water enters the adhesive layer. Some joints were exposed firstly at 100% r.h. for 5000 h., and then at 50% r.h. for a further 5000 h whereupon some recovery of strength took place. The effect of moisture on joint strengths can be interpreted in terms of water, by virtue of its high permittivity, weakening ion-pairs at the interface.

X-ray photoelectron spectroscopy has been used to examine fracture surfaces at all stages, showing that visual inspection can lead to false conclusions about the mode of failure.     

Effect of carriers on the performance of aluminium alloy joints bonded with an epoxide-polyamide adhesive

The introduction of a carrier fabric into an epoxide nylon adhesive lowered the initial shear strengths of lap joints, the effect being more severe with a close knitted carrier. The close knitted carrier led to a large additional decrease in joint strength after exposure to warm humid conditions. Stressing at 20% of the dry lap shear strength sometimes led to complete failure of the joints; this effect was much more marked in the presence of the close knitted carrier. Freeze-thaw cycling after exposure to water did not cause further reduction in joint strength. Drying unsupported adhesive joints after exposure to high humidity resulted in recovery to 80% of the initial dry strength. The total amount of water in a lap joint, calculated from the diffusion coefficient, was found to be linearly related to joint strength.     

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.     

Effect of water on the strength of rubber bonding with adhesives based on reactive oligomers

The effect of water redistribution between SKEPT-40 rubbers and reactive adhesives based on SKN-18KTR mixtures with ED-20 on the strength of adhesive compositions was studied. Water vapor sorption isotherms were measured. It was shown that the water content of rubbers during their scheduled conditioning at a humidity of 65–70% leads to the spontaneous redistribution of water between the substrate and the adhesive. This process is accompanied by the retardation of the formation of the adhesive network structure and, as a consequence, by a fall in the strength of adhesive joints. The mechanism of formation and failure of SKEPT-40-(SKN + ED-20) adhesive joints was revealed. A procedure for calculating the amount of water capable of redistributing between the elements of adhesive joints is proposed. Rubber conditioning parameters that ensure the required quality of bonding the rubbers with SKN-18KTR-ED-20 compositions were determined.     

Preconditioning of epoxy film adhesives for bond strength improvement

The moisture level of two commercial overaged epoxy film adhesives has been controlled by drying under vacuum and/or exposure to humid atmosphere. Shear and peel bond strengths of the conditioned adhesives were evaluated. Predrying of the uncured adhesive under vacuum (3–5 mm Hg) at room temperature is shown to be very effective for bond strength enhancement. Additional humidifying/drying circles show the same effect but some irreversible degradation occurs and only partial improvement of adhesive bond strength is achieved.     

Rheological investigation of cure kinetics and adhesive strength of polyurethane acrylate adhesive

In this work, we used rheological techniques to study both the cure characteristics and the degree of cure of polyurethane acrylate adhesive, a type of reactive adhesive used in hard disk component assembly. These results were then correlated with the tensile shear strengths of adhesives. Here, the cure characteristics of polyurethane adhesive were investigated at isothermal conditions ranging from 25 to 120°C. From the rheological results, the gelation time, the vitrification time, as well as the time required to reach the maximum degree of cure, decreased when increasing the curing temperature. The cure rates of adhesive increased with temperature in three temperature ranges, which were retardation zone, vitrification zone, and reaction‐controlled zone. The cure rates in these zones were controlled by slow diffusion, fast diffusion, and the rate of reaction, respectively. From the temperature sweep of fully‐cured adhesives, we found that the crosslinking level of adhesives increased with curing temperatures at different rates depending on the temperature zones as well. Moreover, the adhesive strength measured by tensile shear test was found to also increase correspondingly with the adhesives' T_g, indicating that the crosslinking level directly affected the adhesive strength. The strong dependence of adhesive strength with crosslinking level indicates that the crosslinking level was essential for high adhesive strength. The correlation of cure characteristics and adhesive strengths at various curing temperatures performed in this study can further provide useful information for planning appropriate curing schemes of polyurethane acrylate adhesives used in electronic and other industries. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Durability of Adhesive Bonds to Zinc-Coated Steels: Effects of Corrosive Environments on Lap Shear Strength

The effects of corrosive environments on adhesive bonds to electro-galvanized, zinc/aluminum alloy coated, coated electro-galvanized, and cold-rolled steels have been investigated. Bonds prepared using a rubber-modified dicyandiamide-cured epoxy adhesive, an epoxy-modified poly(vinyl chloride)-based adhesive, an acrylic-modified poly(vinyl chloride)-based adhesive a one-part urethane adhesive, and a two-component epoxy-modified acrylic adhesive were exposed under no-load conditions to constant high humidity or cyclic corrosion exposure for 50 days or 50 cycles (10 weeks) respectively.

Over the course of this study, exposure to constant high humidity had little effect on lap shear strength for any of the systems studied. Bond failures were initially cohesive, and with few exceptions remained so.

Bond strength retention under the cyclic corrosion exposure conditions employed was strongly dependent on adhesive composition and on substrate type. On galvanized substrates, lap shear strengths for the poly(vinyl chloride)-based adhesives were reduced by 90-100% during the course of the corrosion exposure, and a change in the mode of bond failure (from cohesive to interfacial) was observed. On the coated electro-galvanized steel substrate, the poly(vinyl chloride)-based adhesives showed about 50% retention in lap shear strength and a cohesive failure throughout most of the corrosion test. The dicyandiamide-cured epoxy adhesive used in this study generally showed the best lap shear strength retention to zinc-coated substrates; bonds to cold-rolled steel were severely degraded by corrosion exposure. The performance of the acrylic and urethane adhesives were intermediate to the dicyandiamide-cured epoxy and poly(vinyl chloride)-based adhesives in strength retention.     

Durability of Adhesive Bonds to Zinc-Coated Steels: Effects of Corrosive Environments on Lap Shear Strength

The effects of corrosive environments on adhesive bonds to electro-galvanized, zinc/aluminum alloy coated, coated electro-galvanized, and cold-rolled steels have been investigated. Bonds prepared using a rubber-modified dicyandiamide-cured epoxy adhesive, an epoxy-modified poly(vinyl chloride)-based adhesive, an acrylic-modified poly(vinyl chloride)-based adhesive a one-part urethane adhesive, and a two-component epoxy-modified acrylic adhesive were exposed under no-load conditions to constant high humidity or cyclic corrosion exposure for 50 days or 50 cycles (10 weeks) respectively.

Over the course of this study, exposure to constant high humidity had little effect on lap shear strength for any of the systems studied. Bond failures were initially cohesive, and with few exceptions remained so.

Bond strength retention under the cyclic corrosion exposure conditions employed was strongly dependent on adhesive composition and on substrate type. On galvanized substrates, lap shear strengths for the poly(vinyl chloride)-based adhesives were reduced by 90–100% during the course of the corrosion exposure, and a change in the mode of bond failure (from cohesive to interfacial) was observed. On the coated electro-galvanized steel substrate, the poly(vinyl chloride)-based adhesives showed about 50% retention in lap shear strength and a cohesive failure throughout most of the corrosion test. The dicyandiamide-cured epoxy adhesive used in this study generally showed the best lap shear strength retention to zinc-coated substrates; bonds to cold-rolled steel were severely degraded by corrosion exposure. The performance of the acrylic and urethane adhesives were intermediate to the dicyandiamide-cured epoxy and poly(vinyl chloride)-based adhesives in strength retention.     

Soy proteins as plywood adhesives: formulation and characterization

Soybean proteins have great potential as bio-based adhesives. The objectives of our study were to develop and characterize formaldehyde-free soybean wood adhesives with improved water resistance. Second-order response surface regression models were used to determine the effects of soy protein isolate concentration, sodium chloride, and pH on adhesive performance. All three variables affected both dry and wet strengths of bonded wood specimens. The optimum operation zone for preparing adhesives with improved water resistance is at a protein concentration of 28% and pH 5.5. Sodium chloride had negative effects on adhesive performance. Soy adhesives modified with 0.5% sodium chloride had dry strength, wet strength, and boiling strength of bonded specimens comparable to nonmodified soy adhesives. Rheological study indicated that soy adhesives exhibited shear thinning behavior. Adhesives modified with sodium chloride showed significantly lower viscosity and yield stress. Sodium chloride-modified soy adhesives formed small aggregates and had low storage moduli, suggesting reduced protein–protein interactions. These formaldehyde-free soy adhesives showed strong potential as alternatives to commercial formaldehyde-based wood adhesives.     

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.     

Synthesis and properties of allyloxyethyl 2-cyanoacrylate adhesive

Allyloxyethyl 2-cyanoacrylate monomer was synthesized and characterized for the first time. It was found that this monomer retains the typical properties of cyanoacrylate adhesives such as fast setting time at room temperature, adhesion to most materials, and high strength of bonded joints. Because of its long ester group and the reactive allyl group, this cyanoacrylate monomer produces adhesive bonds which have improved elasticity and heat resistance. IR and DSC studies showed crosslinking of the adhesive layer when subjected to elevated temperature, which explains the increased tensile shear strength of steel bonded joints. It was found that allyloxyethyl 2-cyanoacrylate can also be used as a crosslinking component for cyanoacrylate adhesives, based on ethyl 2-cyanoacrylate. Less than 10% of allyloxyethyl 2-cyanoacrylate in the mixture is needed for increasing, over three times, the tensile shear strength of the adhesive joint after ageing at 100°C.     

Effects of the aluminum filler content on moisture diffusion into epoxy adhesives in distilled water and sea water

Epoxies are the most common of high‐performance structural adhesives, especially in automotive and aircraft manufacturing. In a variety of industrial applications, epoxy adhesives are required to have enhanced thermal conductivity. The normal method of changing this property is to add to the epoxy a filler of higher conductivity than the continuous phase. Although the improvement in the thermal properties of adhesives by the addition of metal fillers is obvious, their influence on water sorption characteristics of adhesives is not clear. It was the objective of this study to shed light on these aspects, which are lacking in the literature. The emphasis was placed on determining the moisture sorption behavior of aluminum‐powder‐filled epoxy adhesives under complete immersion in distilled water and sea water. Moisture diffusion tests show that the addition of aluminum filler into epoxy decreases the total amount of water intake at saturation in both fluids. However, there appears to be no significant effect of the aluminum filler content on the moisture diffusivity in epoxy adhesive specimens in either distilled water or seawater. It has also been determined that the adhesives adsorb a larger amount of water upon exposure to distilled water than when exposed to seawater, whereas the moisture diffusion rate in the adhesive immersed in seawater is higher than that in distilled water. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 98: 1165–1171, 2005     

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。     

Hygrothermal Properties of Adhesively Bonded Joints and Their Correlation with Bulk Adhesive Properties

The combined effects of heat (50[ddot]C) and humidity (95% R.H.) on the lap shear and T-peel strengths of 120[ddot]C, 150[ddot]C and 215[ddot]C service epoxy film adhesives have been characterized. Experimental results have indicated that effects of hygrothermal conditioning on lap shear and peel properties vary with exposure time and final testing temperatures and type of adhesive tested. In the cases where cohesive failure was observed in the shear and peel specimens, a correlation could be established between the bulk properties of the adhesives (tensile strength and elongation) and their adhesively bonded joint properties (shear and peel). When testing was carried out at room temperature, a general correlation between the tensile elongation and T-peel or shear could be obtained. At below freezing temperatures, lap shear strength seemed to be correlated with bulk tensile strength while peel correlated with bulk tensile elongation. At elevated temperatures, the relative contributions of bulk strength and elongation were the decisive factors as far as shear and peel strengths are concerned.     

A study on durability of joints bonded with pressure-sensitive adhesives

The use of adhesive bonding as fastener presents very high usage potential, especially for different environments. However, the designers do not have enough data yet to address the changes in the mechanical properties of adhesives in the course of time. In this study, the ageing of film-type adhesives, Structural Bonding Tapes (SBT) 9244 and 9245, which are different in thickness and possess pressure-sensitive and visco-elastic properties, was investigated. First, the bulk adhesive specimens were prepared and exposed to two different environmental conditions until saturated. In the same time, single lap joints with two different adhesive layer thicknesses, consisting of AA2024-T3 as the adherend, were exposed to the same environmental conditions for exposure times of 90 days. The immersion environments were 100% relative humidity (RH) and 3.5% NaCl solution. At the end of exposure time, the failure surfaces were examined by Scanning Electron Microscopy (SEM) after the strength of joints was determined with the lap shear test. Both SBT adhesives absorb more water from 100% RH than from 3.5% NaCl solution and, therefore, they deteriorate rapidly in 100% RH. In addition, as the thickness of adhesive layer increases, the loss in the strength increased.     

Flexibilized Copolyimide Adhesives

Two copolyimides, LARC-STPI and STPI-LARC-2, with flexible backbones were prepared and characterized as adhesives. The processability and adhesive properties were compared to those of a commercially available form of LARC-TPI.

Lap shear specimens were fabricated using adhesive tape prepared from each of the three polymers. Lap shear tests were performed at room temperature, 177°C, and 204°C before and after exposure to water-boil and to thermal aging at 204°C for up to 1000 hours.

The three adhesive systems possess exceptional lap shear strengths at room temperature and elevated temperatures both before and after thermal exposure. LARC-STPI, because of its high glass transition temperature provided high lap shear strengths up to 260°C. After water-boil, LARC-TPI exhibited the highest lap shear strengths at room temperature and 177°C, whereas the LARC-STPI retained a higher percentage of its original strength when tested at 204°C [68% versus 50% (STPI-LARC-2) and 40% (LARC-TPI)].

These flexible thermoplastic copolyimides show considerable potential as adhesives based on this study and because of the ease of preparation with low cost, commercially available materials.     

Structural adhesive performance in marine environments

Durability of adhesively bonded aluminium joints was investigated for both stressed and unstressed joints exposed to environments of air, distilled water, 0.5% sodium chloride solution and 5% sodium chloride solution. Testing involved the use of marine grade aluminium, two acrylic adhesives and two epoxy resin adhesives. Surface preparation consisted of a solvent wipe degreasing procedure. The acrylic adhesives used in this study displayed greater bond durability than cpoxy resins. Results also reaffirmed that the combination of stress and environmental exposure is more detrimental to bond durability than environmental exposure alone. The combined effects of stress and dilute salt solution were found to be more detrimental to adhesive bond durability than stress combined with distilled water, or concentrated salt solution. Two possible causes of bond deterioration considered were corrosion of the aluminium adherend and instability of the interface. Either of these explanations could account for the observed results; however, more work is required to verify these theories.     

Adhesive Bonding to Galvanized Steel: I. Lap Shear Strengths and Environmental Durability

Initial (i.e., unaged) adhesion, as well as adhesion after seven day, 60°C water immersion and six week scab corrosion accelerated environmental exposures, has been assessed for five different one and two-part epoxy adhesives, bonded to three different types of galvanized steel substrates. We have shown that adhesion, as measured by lap shear strength, is specific to the galvanized substrate type. In general, for a given adhesive, adhesion to “hot-dipped” galvanized substrates is harder to achieve and maintain under accelerated environmental exposure than is adhesion to “electroplated” galvanized. Also, for a given type of galvanized steel, the one-part epoxies evaluated generally showed higher initial strengths, as well as better strength retention under environmental exposure than did the two-part epoxies.     

Adhesive Bonding to Galvanized Steel: I. Lap Shear Strengths and Environmental Durability

Initial (i.e., unaged) adhesion, as well as adhesion after seven day, 60°C water immersion and six week scab corrosion accelerated environmental exposures, has been assessed for five different one and two-part epoxy adhesives, bonded to three different types of galvanized steel substrates. We have shown that adhesion, as measured by lap shear strength, is specific to the galvanized substrate type. In general, for a given adhesive, adhesion to “hot-dipped” galvanized substrates is harder to achieve and maintain under accelerated environmental exposure than is adhesion to “electroplated” galvanized. Also, for a given type of galvanized steel, the one-part epoxies evaluated generally showed higher initial strengths, as well as better strength retention under environmental exposure than did the two-part epoxies.