In-situ measurements of glass transition temperatures of epoxy adhesives by a joint creep method

A method is described for measuring the glass transition temperature (Tg) of epoxy joints bonding ceramic (SiC) substrates. This method is based on the measure of the strain of a single-lap joint subjected to a temperature variation. The resulting displacement (d) is observed as a function of the temperature (T) by means of a contact strain gauge extensometer and (d-T) curves are recorded on which Tg is determined. The influence of peculiar joint parameters (joint thickness and overlap length) and of other parameters such as the temperature scanning rate, the applied load and the surface state of substrates were studied for two epoxy adhesives showing different thermomechanical behaviour. The results show that it is possible, with appropriate experimental conditions, to measure Tg with this method and to find Tg values similar to those found by classical techniques such as the static thermomechanical analysis (TMA) and the differential scanning calorimetry (DSC). This method shows similarities with TMA, but it has the advantage to allow the thermomechanical analysis of adhesives without destructive sampling of joints.