Local dynamics of adhesives in aggressive environment in the pre-damage stage

Molecular aspects of chemical and physical changes in adhesive joints caused by absorbed moisture were investigated. The focus was on the pre-damage stage that precedes the formation of voids and microcracks. A model and a commercial epoxyamine formulation were studied. Local dynamics were monitored by broad-band dielectric relaxation spectroscopy (DRS). One portion of the absorbed water does not form hydrogen bonds with the network and gives rise to a fast relaxation process (termed γ) with activation energy of 28 kJ/mol. The local β dynamics are slowed down by the interactions between water and various sites on the network that include the ether oxygen, the hydroxyl group and the tertiary amine nitrogen. One particularly significant finding is that the average relaxation time for the β process above 20°C is of the order of nanoseconds or less and, hence, the detection and monitoring of this process hinges upon the ability to perform high-precision DRS at frequencies above 1 MHz. This is an important consideration in the ongoing efforts aimed at the implementation of DRS as nondestructive inspection (NDI) tool for adhesive joints.