Environmental effects on epoxy adhesive and joints

The use of adhesive joining in mechanical structures, especially in industrial usage, is rapidly increasing. However, to be confident in their use it is essential to evaluate the performance of the joints, especially when their strength tends to decrease upon exposure to warm and humid conditions that cause degradation. Since degradation is a long time process and evaluating the degradation of adhesive joints in normal condition needs several months or years tests, accelerated test under controlled environment is unavoidable. Water can affect both the physical and mechanical properties of the adhesive itself and also the nature of the interface or interphase that exists between it and the substrate. The aim of this thesis is considering both aspects, including also other factors that can affect the durability of bonded joints. To obtain in short time experimental data on the degradation of adhesives exposed to moisture or immersed in water, a valuable technique is represented by the open-face configuration. Apart from the acceleration of moisture uptake which is obtained due to the larger area exposed, a further advantage is the uniformity of degradation. A further acceleration can be obtained by adding a hygroscopic contaminant at the adhesive/adherend interface, which speeds up moisture uptake and accentuates the interfacial nature of the failure. The decay of the mechanical strength in absence or presence of a contaminating agent was evaluated. The effects of a humid and warm environment and also immersion in water on tensile strength, Young’s modulus and Poisson’s ratio of a twocomponent epoxy adhesive, Henkel Hysol 3425, were evaluated using bulk specimens,. To relate the exposure time to the moisture uptake, measurements were performed for both bulk adhesive specimens and bonded open face joint, in periods of time between 1 and 3 months depending on the type of test, exposure to warm moisture or immersion in water. In a further part of the work, the performance of different surface pretreatment techniques of aluminum open-faced single lap joints was evaluated. Four different surface treatment processes were tested, namely: Degreasing and Roughening, treating with Picklex 20, Phosphoric Acid Anodizing (PAA) without and with contamination. Finally, the effect of warm moisture on the fracture energy of an epoxy-based adhesives was measured. To evaluate the effect on fracture energy and other related parameters, open-faced double cantilever beam (DCB) specimens of epoxy-steel adhesive system were exposed to warm moisture, at 100 % relative humidity and 50 °C. The methodology introduced in this thesis leads to further investigation on the durability of the considered adhesive (in terms of joint strength, water uptake measurements, fracture energy, etc) under different environmental conditions. The methods adopted to accelerate the degradation process (open face and contamination) revealed an interesting reduction in duration of degradation process. Surface preparation methods to protect aluminum bonded joints against severe environmental condition were evaluated and the most suitable one (PAA process) was identified.