Detection of moisture in adhesive bonds using electrochemical impedance and dielectric spectroscopies

The ingress of moisture into an adhesive bonded structure can lead to a variety of effects. Moisture has the ability to plasticize the adhesive but may also segregate into microvoids. The water in the voids will have less of an effect as it does not plasticise the matrix and is unable to influence the interface between the resin and the substrate. Only if the Tg of the matrix drops below ambient will it allow expansion of the voids and then there will be consequential lowering of the fatigue characteristics of the resin. Water also has the ability to interact with metallic surfaces leading to changes in the nature of the interaction between adhesive and adherend. Water has a large dipole signature and is easily studied in terms of its effect on the electrical properties of bonded joint structures. In this paper, applications of electrochemical impedance and dielectric spectroscopies are described. Both techniques measure the electrical response of the joint, however the nature of the measurement performed allows slightly different, but complementary, approaches to be adopted. Application of the techniques to similar joint structures illustrates the similarities and differences of the two approaches. The article demonstrates the value of using electrical methods to probe the changes that occur in joints on ageing and emphasizes the value of these methods in developing and understanding of the phenomena that are responsible for loss of bond integrity