Curing effects on viscosity and mechanical properties of a commercial epoxy resin adhesive

Before testing the ability of different pre-treatments to promote adhesion, a detailed description of the rheological and mechanical properties of the adhesive is needed. Change of the adhesive properties (due to incomplete curing) can indeed affect the results of the adhesion tests and screen out the effect of the pre-treatment. A two component room temperature curing adhesive paste (Araldite 2014) has been selected. The characterization of the adhesive properties was conducted with respect to the curing temperature and to the elapsed time between the gluing and the testing. It has been shown that any curing temperature increase leads to a more complete curing after a short period of time when the conversion factor reaches a plateau-like regime. In that regime the reaction rate becomes diffusion controlled and the reaction is drastically slowed down. Measurements of the mechanical properties of adhesive samples showed that 28 days are needed at room temperature to complete the curing process (only 4 h at 64°C) and that the properties of the adhesive during that period of time changed drastically. Ageing test (40°C in water vapour) has been conducted on adhesive samples and the adhesives properties recorded over a period of 36 days. It has been shown that the absorbed water works as a plasticizer leading to a softening of the adhesive. The E-modulus and the stress at break are lowered while the deformation at break and the plastic contribution to the total deformation are increased.