Martin E.R. Shanahan

Diffusion of water into an epoxy adhesive: comparison between bulk behaviour and adhesive joints

Abstract The diffusion of water into a bulk epoxy adhesive at 70°C under ∼ 100% relative humidity has been studied by classic gravimetric analysis. As diffusion progresses, the elastic modulus decreases. This same effect has also been observed with torsional adhesive joints but the phenomenon seems to occur more rapidly. Since gravimetric analysis is not feasible with practical bonded joints, a ‘composite’ model has been developed in which water ingress may be estimated by changes in overall elastic behaviour of the polymer. Elastic moduli are due to a combination of rigidities resulting from the relative fractions of ‘wet’ and ‘dry’ polymer under load. The composite model gives values of the coefficient of diffusion, D, for the bulk adhesive in satisfactory agreement with those obtained by gravimetric analysis. However, the value of D for torsional joints is considerably greater. It is proposed that a phenomenon of ‘capillary diffusion’ exacerbates water ingress. Surface tension effects near the metal(oxides)-polymer interfacial region increase the effective driving force for water penetration.

Swelling of DGEBA/DDA epoxy resin during hygrothermal ageing

Abstract Understanding the swelling behaviour of polymers during hygrothermal ageing is potentially important for both composite materials and structural adhesives, since swelling may significantly influence the mechanical behaviour due to the increase of residual stresses at interfaces. In this article, the swelling of a DGEBA/DDA epoxy resin has been investigated. It has been found that the swelling of the polymer lags behind the behaviour predicted from the ideal mixing law. More importantly, our results show that swelling induced by hygrothermal ageing is not fully reversible, particularly at higher temperatures. Based on the assumption that water diffusion is governed by a classic diffusion controlled process and a swelling controlled process, a model is proposed to describe water uptake and swelling kinetics during hygrothermal ageing. The results show that the model is in good agreement with the experimental values.

Effects of evaporation on contact angles on polymer surfaces

Abstract The evolution of contact angles of water on three polymer surfaces has been studied both with and without an environment saturated in water vapour. It was observed that in a saturated atmosphere, evolution was slow and involved a slight reduction in drop height and contact angle at constant contact diameter. This effect is attributed to a slight diffusion of water into the polymer. In a dry atmosphere, evolution was far more rapid and involved three stages. Initially, the contact diameter remained constant whilst both drop height and contact angle decreased. When a small value of contact angle had been reached, both the drop height and diameter decreased concomitantly, thus maintaining an essentially constant value of contact angle. This stage did not occur on a rough substrate. Finally, height, diameter and contact angle all decreased somewhat sporadically as the drop volume tended to zero. This behaviour was probably related to anchoring effects of the triple line on heterogeneities. A simple model explaining the essentially linear decrease in drop height with time during the first stage is proposed.

Drop Motion on an Inclined Plane and Evaluation of Hydrophobia Treatments to Glass

Abstract Hydrophobic and anti-rain surface treatments are increasingly used to treat various glass articles such as windscreens, windows, headlamps, wing mirrors, optical lenses, sunglasses, etc. To evaluate the efficiency and durability of these treatments, we determine the smallest volume or critical sliding volume, V c , of a water drop able to slide down spontaneously under gravity after having been deposited on a vertical treated, glass surface. The property of water repellency is considered to be better when V c , is smaller. In this paper, a new simplified theory to describe the capillary force retaining the water drops on an inclined plane is proposed and verified practically. The experimental method allows us to compare the efficiency and durability of a commercial anti-rain and of a Corning Inc. proprietary hydrophobic surface treatment for glass, both being based on silicone derivative chemistry. As defined in this paper, the critical sliding volume appears to be a practical parameter which may...

A Novel Test for the Appraisal of Solid/Solid Interfacial Interactions

Abstract The Johnson, Kendall and Roberts (JKR) technique has been used with considerable success for assessing solid/solid interfacial interactions over the past 25 years or so. Nevertheless, the contact zone between the two spherical solids is often small and the energy of adhesion scales with the cube of the contact radius (at low load), thus potentially magnifying errors in adhesion assessment. The theoretical aspects of a novel technique are presented here, in which a hollow, slightly inflated, spherical membrane replaces a full sphere, and is placed in contact with a flat rigid solid. A judicious choice of experimental conditions should lead to increased contact radius and the energy of adhesion scales with its square (at low load), thus reducing possible errors. An added advantage is that the effective elasticity of the sphere depends on internal gas pressure. Thus surface and bulk effects are decoupled.

Physical and Chemical Effects in An Epoxy Resin Exposed to Water Vapour

A commercial epoxy adhesive based on DGEBA cured with dicyandiamide (DDA) and containing fillers has been aged in water vapour (ca. 100% RH) at various elevated temperatures. Viscoelastic characterisation was effected after ageing for various times up to saturation and after subsequent drying. Youngs modulus in the glassy state is reduced after water sorption but returns virtually to its original value after drying, suggesting physical weakening due to plasticisation whereas, in the rubbery state, the observed diminution is only slightly restored on drying. An analysis based on the theory of rubber elasticity leads to the conclusion that chemical degradation is occurring by hydrolysis and chain scission. Gravimetric measurements effected on the polymer exposed to vapour for a fixed time and then dried to (virtual) equilibrium show an increase in weight followed by a decrease and finally a weight loss as exposure time is increased. A theory is proposed to explain the phenomena by initial chemical combination of water followed by leaching of severed chain segments. Calculations of average inter-crosslink molecular weight using both viscoelastic and gravimetric data are in good agreement. It is concluded that long-term exposure of the epoxy adhesive to water leads to both reversible (physical) and irreversible (chemical) degradation of the material.

Effects of plasma treatment on the adhesion of an epoxy composite

Abstract Improvements to adhesion between an epoxy composite material substrate and an epoxy structural adhesive can be obtained by plasma treatment of the former. However, quantitative evaluation of the strength increase is often difficult due to the rigidity of the systems. In this study, we consider a composite material (epoxy/carbon fibre), both untreated and treated by oxygen and nitrogen plasmas, adhering to an epoxy gel. The latter was chosen as a compromise since it has chemical similarities with structural epoxy resins yet inherent flexibility, allowing peel tests to be performed. After studying effects due to variation of contact time during cure (total cure time remained constant to ensure comparable mechanical properties), it was found that plasma-treated composite gave bettter adhesion under similar test conditions. A model has been tentatively suggested to explain the improvement by the formation of chemical bonds at the composite-epoxy gel interface. It is concluded that even if only 1% of atomic sites develop chemical bonds, adhesion is considerably increased.

Fibre Surface Energy Characterization

Abstract A new technique has been developed to study the surface energy characteristics of small diameter fibres. This method is tensiometric and based on wetting properties in a two phase liquid system. Two types of carbon fibre have been investigated under both static and dynamic conditions in immersion and emersion. The high strength fibre, which was found to have a polymeric coating, gave reproducible results consistent with typical values for a polymer. However, the high modulus fibre could not be characterized due to considerable scatter in the experimental results. It is suggested that the source of this phenomenon is the existence of surface energy gradients which leads to hysteresis effects.

Effect of surface cavities on static and dynamic adhesion to an elastomer

Abstract The analysis of contact between two spherical surfaces introduced in the 19th century by Hertz was modified some 30 years ago by Johnson, Kendall and Roberts (JKR) to allow for adhesion between the two solids. Since then, the technique has been much used with various systems employing sphere/sphere and sphere/flat solid (sphere of infinite radius) geometry. We consider here the geometry in which one solid has a negative radius of curvature: a spherical solid contacts the second solid in a shallow spherical cavity. It is thus shown that the contact area of a rigid sphere on the smooth surface of an elastomer depends markedly on the flatness of the latter. Any neglect of cavities of large radius of curvature leads to an overestimate of the value of the intrinsic adhesion of Dupre, W0, and falsifies interpretation of separation kinetics under (variable) applied load. By allowing for the negaive radius of curvature of the cavities in the rubber, correction can be made leading to coherent values of W0 and debonding kinetics. The analysis may be of use for the assessment of flatness of surfaces and the increase in contact radius may prove beneficial for improving the precision of static adhesion tests.

Water absorption and leaching effects in cellulose diacetate

Abstract Degradation of polymers caused by water ingress is a recognised problem for the long term strength of structural materials but may be considered too slow for some packaging materials! Following an earlier study of water degradation in an epoxy resin, we here consider effects in cellulose diacetate—a biodegradable material. As before, initial weight increase following exposure to water is followed by weight decrease such that, at sufficiently long times, there is a net weight loss. The overall behaviour has been wholly or partially observed at six temperatures ranging from 10°C to 90°C. A model based conceptually on the initial attachment of water to the polymeric structure (causing weight increase) followed by chain segment separation and subsequent leaching (leading to weight decrease) was developed and explains satisfactorily the overall behaviour.