Epoxy-Metal Interphases: Chemical and Mechanical Aging

Aging of epoxy (EP)-metal-joints is studied by FTIR microspectroscopy, tapping mode scanning force microscopy and nanoindentation on sample cuts prepared with low angle microtomy. The influences of temperature and water are separated by exposure to dried or moist air (90% rel. humidity) at 60 degrees C and dried air at 120 degrees C for up to a few hundred days. In the course of aging, diffusion-controlled chemical reactions start at the interface to the metal and extend as a gradient up to 50m into the EP. Water is more detrimental to the epoxy than elevated temperature. The chemical aging proceeds slower than at the contact to ambient air, and, contrary to that surface zone, oxidation of amines dominates autoxidation of CH2 at other sites in the network chains. Thus, intensity and rate of chemical aging at epoxy-metal contacts cannot be deduced simply from the aging at the air-bulk contact. Chemical aging is accompanied by the formation of gradients of mechanical properties, at least in moist conditions.