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The degree of degradation that occurs in adhesive structures when in service is linked directly with the amount of moisture absorbed. The moisture absorption kinetics of polymer systems will differ widely and also change with physical ageing. The approach often adopted, is to define a constant relative humidity environment that will produce a moisture level that is representative of a bonded structure that has been exposed to a real life environment at a particular geographical location.
The variability of a natural environment, that is daily, monthly or seasonal changes in temperature and humidity are known to be a major factor in determining both the final moisture equilibrium level in the material and the distribution of moisture in the outer surface layers. The military consider the worst worldwide environment to be represented by a relative humidity of 85% and a temperature of 70°C.
The natural process of moisture absorption in adhesive structures is normally very slow, and this makes it very difficult to reach an adequate degree of degradation in a structural test element in practical timescales. It has been found necessary, therefore, to speed up the moisture diffusion process by employing an accelerated conditioning technique that can ensure a representative level of degradation in a significantly reduced time.
The usual approach used to accelerate moisture uptake is to increase the diffusivity of the adhesive by elevating the temperature of the conditioning environment. However, there is a temperature limit to which most polymeric materials can be raised without affecting a change in the mechanism by which moisture is absorbed. Above this critical temperature, there is a strong possibility that the degradation mechanism is altered.
An alternative approach to attempting to reach an equilibrium condition, involves altering the acceptance criteria to a given percentage of the chosen equilibrium condition. Conditioning the material to 95% of the full equilibrium state takes a relatively shorter time to reach than the full (100%) equilibrium condition. The time required to obtain the last 5% can take longer than the time taken to reach the 95% level. Clearly a very large saving in time is possible if a 95% of equilibrium can be justified in terms of a non-significant change in the structural strength and stiffness properties.
Further acceleration can be obtained by increasing the relative humidity to the maximum that can be readily maintained (i.e. 96%). It is required that the acceleration method takes full advantage of the high rates of diffusion given by temperature and humidity and still achieve a reasonably realistic moisture absorption. It is not satisfactory to precondition at 96% RH, so that the equilibrium level in the outer surface layers exceeds the bulk equilibrium level obtained due to exposure in service.
A rapid method for measuring corrosion-dominated degradation in adhesive joints is to store continuously expose the adhesive joint to 5% NaCl salt spray solution at 95% relative humidity. Significant changes can be expected to occur within 6 weeks (~1,000 hours).
Note: Accelerated ageing is considered a conservative approach as it yields greater degradation of properties compared with slower tests that closely simulate actual service conditions. Although accelerated ageing is widely used, a full understanding of the effects is not yet available and there is no agreement within industry as to a satisfactory accelerated test.
The majority of thermoset resins are susceptible to oxidation by numerous oxidizing agents, from exposure to elevated temperatures (i.e. thermo-oxidation), hydrogen peroxide and bleaches (e.g. hypochlorite). It is often other constituents, such as fillers, catalysts, hardeners, pigments or fire retardants, rather than the adhesive that are more reactive to these chemicals. Formulators and users, although usually aware of the potential durability problems associated with these additives, often overlook minor constituents such as catalysts, hardeners, pigments and processing aids. It is worth noting that many structural adhesive systems are far more chemical resistant to strong acids, salt solutions and oxidative agents than stainless steel or aluminium alloys. However, exposure to secondary solvents, such as paint strippers, can lead to irreversible material damage. Methylene chloride, a constituent of many paint strippers, is a known degradant of epoxy resins and other polymers. The effect of these highly destructive processes is often evidenced by a degraded surface appearance (i.e. discolouration, loss of surface reflectivity, increased surface roughness and exposure of underlying substrate).
Materials used in the automotive and aerospace industry may come in contact with aircraft fuel, gasoline, oil, hydraulic, brake and transmission fluids, lubricants, coolants and de-icing and antifreeze compounds. Many of these are known to have adverse effects on material performance and will often have a synergistic effect. The combination of JP-4 fuel and water is more aggressive than the two media acting alone.
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