J. Comyn

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

Abstract 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.

The effect of humidity on the durability of aluminium-epoxide joints

Abstract Single lap joints of sandblasted aluminium alloy have been prepared with anepoxide adhesive based on dgeba and 1, 3-diaminobenzene. They have been exposed to air at 50°C and to a range of humidities for up to 10080 h, and then tested. After 10080 h ageing the results give some evidence for a critical relative humidity at 65%, such that joints weaken to a greater extent if this value is exceeded. Measurement of mass uptake of films of the adhesive suspended in air at various humidities has provided the water absorption isotherm and diffusion coefficients. The isotherm is gently curved and of BET type IV; there are no sharp changes that might account for a critical relative humidity. The presence of salt hydrates at the interface is a feasible explanation for this.

The Application of Inelastic Electron Tunnelling Spectroscopy to Epoxide Adhesives

Abstract The theory of inelastic electron tunnelling spectroscopy (IETS) and the suitability of this technique for examining adhesive-aluminium oxide interfaces are discussed. IET spectra are presented of an epoxide resin (the diglycidyl ether of bisphenol A), two aliphatic amine hardeners (di-[1 -aminopropyl-3-ethoxy] ether and triethylene tetramine), and mixtures of resin and hardener before and after subjection to the usual heat curing schedules. These show that the curing reaction does not take place within an IETS junction; a possible reason for this is the epoxide resin is physically adsorbed on to the aluminium oxide surface whilst the hardeners may be chemically adsorbed through the amine groups.

The effect of moisture and temperature on the properties of an epoxide-polyamide adhesive in relation to its performance in single lap joints

Abstract A template cutting technique has been used to prepare dumb-bell shaped specimens of the epoxide-polyamide adhesive FM 1000, and their mechanical properties have been examined as a function of water exposure, testing speed, and temperature. The glass transition temperature of the wet adhesive has also been examined. Strengths of single lap joints bonded with the adhesive have been measured after exposure to warm moist air for up to 2500 hours. Strengths of wet and dry joints have been compared over a wide temperature range. The results show that the weakening effect which water has is due to plasticization of the adhesive, and that the rate of weakening depends on water diffusion within the adhesive layer.

An inelastic electron tunnelling spectroscopy (IETS) study of poly(vinylacetate) poly(methyl methacrylate) and poly(vinylalcohol) adsorbed on aluminium oxide

Abstract IETS is used to investigate the adsorption of poly(vinylacetate) (PVA), poly(methylmethacrylate) (PMMA), and poly(vinylalcohol) (PVOH) on aluminium oxide. These polymers are of interest in the field of adhesion science, and until now synthetic macromolecules have not been studied in this way. Both commercially available polymers and those synthesized in our laboratory have been used. On the basis of IET spectra presented here, and existing i.r. spectra it is believed that PMMA and PVA undergo ester cleavage at the oxide surface leading to their subsequent adsorption. For PMMA this is thought to be via carboxylate anions generated on the polymer side groups, while PVA is expected to be adsorbed as PVOH. Bonding of PVOH to the oxide is not fully understood, but may occur by the formation of an AlOC bridge. Another possibility for the above polymers, that of intermolecular hydrogen bonding between polar polymer side groups, and adsorbed hydroxyl species present on the oxide surface, cannot be ruled out.

Surface treatments of carbon fibres studied by X-ray photoelectron spectroscopy

Abstract X-ray photoelectron spectroscopy (XPS) was used to determine the levels and nature of oxidation in carbon fibres before and after various pre-treatments. All the pretreatments caused large increases in the oxidation of the carbon fibres but there was no large change in the mechanical properties of composites formed with these fibres. XPS showed that the surface treatment for the composite removed most, but not all, of the mould release agent used. Surface treatment of the fibres changed the mode of failure of composites bonded with an epoxide adhesive.

Diffusion of water in some modified phenolic adhesives

Abstract The uptake of water at 25°C and 50°C has been measured for some modified phenolic adhesives. In all cases water uptake is Fickian, and for two adhesives there is evidence of leaching at 50°C.

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

Abstract 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

Abstract 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.

Surface treatment and analysis for adhesive bonding

Abstract Improper or inadequate surface treatment is one of the commonest causes of failure in adhesive bonding. On the other hand, the selection of a good surface treatment can bring marked improvements in the wet durability of adhesive bonds to metals and glasses, and can permit the bonding of otherwise difficult-to-bond materials such as polytetrafluoroethylene (PTFE) and the polyolefins. The reasons why untreated surfaces may be unsatisfactory for adhesive bonding are that they may be contaminated, they may lack polar chemical groups or the interface they make with an adhesive may be susceptible to hydrolysis.