The cohesive mechanical properties of a toughened epoxy adhesive as a function of cure level

The cohesive performance of an epoxy adhesive as a function of cure applied is evaluated through the use of mechanical testing, thermal analysis and electron microscopy techniques. These material characterisation techniques allow a complete analysis of the reference material in terms of mechanical behaviour, surface topography and thermally induced events during cure. The effect of cure level within the adhesive was explored by examining all the material properties as a function of post cure duration and temperature. Post cure temperature and duration were both found to be directly proportional to the amount of curing reactions taking place within the system, with increases in post cure duration found to lower the temperature required to cure to a specific level. The mechanical properties of the adhesive varied as a function of post cure harshness with a number of mechanisms contributing to the strength during the post cure. The elastic modulus of the material was found to decrease with increases in post cure temperature, more so than with increases in post cure duration. Large increases in ductility were witnessed at certain post cure levels for each mode of loading. As curing levels are increased the glass transition temperature increases, until a point where the energy required to cause the transition then falls and the Tg is lowered. The morphology of the toughening phase has also been characterised with spherical elements ranging from approximately 0.06 to 0.75 μm, and inter-phase spacing also noted along with the distribution of the elements. Basic mechanical testing of the adhesives was carried out in the form of lap shear and T-peel testing upon mild steel substrates, the correlation with glass transition temperature being stronger in these modes.