Bonding to aged surfaces : a thermally-aged epoxy

It is common experience that aged surfaces are often difficult to bond to. An examination of bonding to thermally-aged epoxy surfaces is reported, using as the adhesive the same epoxy as that of the aged surface. The cured and postcured epoxy was aged at 200 deg C, with the ageing time varying from 2-8 h. The fracture energy of the bond line was measured by mode I cleavage under conditions of relatively slow crack growth. The bondline fracture energy was found to decrease logarithmically with ageing time. The fracture energies for bonds to surfaces aged for 2, 4, and 8 h at 200 deg C were 0.077, 0.059, and 0.050 kJm exp --2 , respectively. These compare to 0.13 kJm exp --2 for a bond to an unaged surface and 0.21 kJm exp --2 for bulk fracture. Fracture surfaces resulting from both slow and rapid fracture were examined by optical and scanning electron microscopy. Fracture features different from those arising from bulk fracture were found. Areas with good adhesion occurred amidst fields of featureless fracture surface; the frequency and size of these areas decreased with increased ageing time. Evidence of plastic deformation was found, always occurring on the new side of the bond: ridges parallel with crack propagation at high crack speeds and subsurface undulations perpendicular to crack propagation at low speeds. The bond has the effect of channelling the crack along the bondline, but fracture does not always remain exactly at the interface. Fracture often occurred a relatively constant distance away from the interface, suggesting that the presence of the interface was left for some distance. Photomicrographs. 15 ref.--AA