Adhesive joints water uptake

Fig. 4.21. Theoretical water concentrations within a lap joint bondline quadrant, after immersion in water at 20 °C. Adhesive aliphatic amine cured epoxy at 20 °C. M, = mass o.f water absorbed at time t. = equilibrium mass water uptake at time t = . Coefficient of diffusion D = 6.6 X 10- m /s (H2O, 20 °C).

Water uptake by polymers is accommodated largely by swelling. For uptakes of only a few mass per cent, volumetric swelling would be of a similar or lower order(98, 99), and barely measurable. Moire fringe interferometry has been used to quantify the swelling stresses developed in a layer of adhesive upon exposure to water(lOO), and Comyn(90) describes some other work related to calculations of the stresses induced in bonded joints by water sorption. 

For cold-curing epoxides wide variations in adhesive material properties are possible, with different combinations of resin, hardener, filler, and the multitude of modifiers. Products which cure at ambient temperature cannot achieve the same performance as is obtained by curing at elevated temperature. For products cured at room temperature their TgS, at 40-50 °C initially, are relatively low and may be lowered even further by absorbed water, in liquid or vapour form. This may also be accompanied by a reduction in strength and modulus. Thus the use of materials with a slow and small water uptake is to be preferred, which implies a fairly highly cross-linked formulation. Such considerations do of course depend upon the performance and durability expectations in service. Whilst the environmental durability of joints can often be improved enormously by the surface pretreatment methods employed (see Chapters 3 and 4), the adhesive must be selected carefully to ensure long term durability in consideration of the modes and duration of loading, and the environmental conditions. Ideally the adhesive should be fairly tolerant of poor surface pretreatment procedures. 

The last severe environment on our list is external stress. External stresses (14,33) especially affect the water uptake or saltwater penetration of adhesive bonds. However, some adhesives may appear insensitive to stresses. Presumably, the critical stress level (34) is so high that no environmental attacks can take place below that level. Thus, it is difficult to discuss the effect of external stresses without considering the fracture mechanics of adhesive joints. In general, external stresses accelerate bond degradation when the joint is exposed to severe environments. 

In the case of structural joints, cohesive failure of the adhesive is often observed. Indeed, in many instances, the requirement will be to engineer failure within the bulk of the adhesive layer. Modelling of the adhesive response as a function of water uptake with temperature is carried out to represent the influence of environmental exposure. With fatigued structural joints, by combining the use of fracture mechanics and appropriate modelling tools, the resultant crack propagation rates and consequently durability levels can be predicted as a function of various environmental and mechanical test parameters, such as frequency. Fatigue threshold values can be determined, which are used to predict durability performance. " ... 

Perhaps, the earlier materials found to have a useful capacity for adhesive bonding underwater depended upon the use of a stoichiometric excess of water-scavenging polyamide hardener in an epoxide-based adhesive. This approach can lead to the production of effective joints in the short term, but formulations of this type, which are hydrophilic in the uncured state, are also likely to absorb significant amounts of water in the cured condition. It is a widely accepted view that the extent of joint weakening in susceptible joints, quite apart from the consequences of plasticization, is a function of the water-uptake characteristics of the adhesive (see Glass transition temperature). The consequence is therefore likely to be that such joints will show poor durability in the presence of water, when rapid uptake of water may lead to equally rapid degradation of both cohesive and interfacial properties (see Durability fundamentals). 

Brewis et al have studied the effect of exposure to warm moist air on aluminum bonded with a DGEBA/di(l-aminopropyl-3-ethoxy)ether adhes-ive. They observed a linear relationship between the loss in joint strength and the fractional water content of the joints, calculated under the assumption that the water entered the joint by diffusion through the adhesive. The excellent correlation observed strongly supports the view that the loss of strength is due to plasticization of the adhesive caused by water uptake. 

There are many examples to show that adhesive joints exposed to typical laboratory conditions, or temperate climates, are often unaffected, even after long periods of time. The relative humidity of such environments can be as much as 50%, although the temperature does not normally deviate very much from 20°C. Thus adhesive joints under some conditions are able to tolerate a certain level of water uptake, without apparently any detrimental effects on the strength (see Section H.C.3.). 

By measuring water uptake, the diffusion coefficient and equilibrium concentration of water for the bulk adhesive were obtained at different temperatures. A value of 37 kJ/mol was also calculated for the activation energy of diffusion. A value for the plane-strain stress intensity factor, Kic, for the bulk adhesive was obtained using compact tension specimens. Tensile butt joints were prepared from mild steel blocks bonded with the epoxy adhesive and the fracture stress determined as a function of time of exposure to water at the different temperatures. An activation energy of 32 kJ/mol was calculated for joint failure, in close agreement with that obtained for the diffusion of water. This supports the view that the processes responsible for loss of joint strength are controlled by water diffusion. It was found that joints exposed to 20°C/55% RH showed no reduction in strength, even... 

The durability of epoxy-aluminium joints that used a homopolymerised epoxy resin was studied by researchers based in Spain [15], and the effects of relative humidity, temperature, and salt concentration analysed. The homopolymerised epoxy resin absorbed little water (1.5 wt%) because of its non-polar network structure. Increasing relative humidity and temperature enhanced water uptake, but the joint strength remained constant because of epoxy plasticisation. A saline environment was damaging to the adhesive joints because of metal corrosion, but was not significantly harmful to the epoxy resin because of the lower diffusion coefficient of salt water. The decrease in glass transition temperature of the epoxy adhesive due to water absorption was dependent upon only the amount of absorbed water and was independent of hydrothermal ageing conditions. The durability of epoxy adhesive joints made underwater has been studied [16]. 

As commented earlier, the rate of loss of strength of a joint under environmental attack will usually be faster if a tensile or shear stress is present 145,126-130], albeit an externally applied stress or internal stresses induced by adhesive shrinkage (incurred during cure) or by adhesive swelling (due to water uptake) [131,132]. Such stresses render primary and secondary molecular bonds more susceptible to environmental attack and also probably increase the rate of diffusion and the solubility in the adhesive of the diffusing medium [133-135]. 

The alternative approach involves the determination of the water uptake by the adhesive, checking at the same time that the difihision leading to saturation obeys Pick s two laws, deducing the diffusion constant from the rate of uptake of water and thence calculating the distribution of water in the joint. These steps are outlined by Comyn (1981) together with data on a number of epoxide-curative combinations. The epoxide resins were aU based on the commonly used bis phenol A with various amine curing agents. 

As a guide to the entry of moisture into a typical lap-shear joint Althof (1981) quotes an epoxy adhesive joint, 10 cm wide and 3 cm overlap in which, after 60 days exposure to 95% relative humidity (RH) at 70°C showed the equilibrium water content of 4% at the boundaries with virtually zero in the centre of the overlap. Moloney et al. (1981) performed a similar exercise with a lap-shear 2-5 cm wide and 1-25 cm overlap bonded with FM1000 and exposed to saturated water vapour at 50°C for 1000 h (approx. 40 days). The overall uptake of moisture of the joint was 69% of the true equilibrium value at... 

All adhesives absorb water, and water uptake data for a number of structural adhesives are collected in Table 31.3. Such data are obtained by measuring the weight of water absorbed by an immersed film, and include the diffusion coefficient D and the weight absorbed at equilibrium M. This means that adhesive layers in joints will absorb water and its vapor, and transmit it to the interface. This cannot be prevented by sealing the edges with a paint or lacquer, as these also absorb water. The data can be used to calculate the rate at which water will enter joints, and water concentration profiles within them (Comyn 1983). This is discussed in more detail in Sect. 31.5.2. 

Comparison of joint strength (experimental points) with calculated water uptake (line) by joints bonded with DGEBA-DAPEE epoxide adhesive. After Brewis, Comyn and Tredwell (1980b). Crown copyright... 

There also appears to exist a critical water concentration within the adhesive below which water-induced damage of the joint will not occur. This also infers that there is a critical humidity for deterioration. For an epoxy system, it is estimated that the critical water concentration is about 1.35 to 1.45 percent and that the critical humidity is 50 to 65 percent.39,40 Any loss in joint strength by the absorbed water can be restored upon drying if the equilibrium moisture uptake is below the critical water concentration. 

Available experimental evidence on the uptake of water vapour by structural adhesives is that the isotherms are straight lines or gentle curves [38]. The consequence of this is that at 50% r.h. the adhesive layers in metal joints would be expected to absorb significant amounts of water. The point to be taken from this information is that water absorption isotherms of epoxide adhesives do not show any sharp changes which might be the cause of the critical r.h. 

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