EFFECT OF SUBSTRATES ON THE STRUCTURE OF POLYMER INTERPHASES .2. EPOXY ANHYDRIDE ADHESIVES CURED AGAINST ALUMINUM AND COPPER

The molecular structure of the interphases formed when a model epoxy anhydride adhesive was cured against 2024 aluminum and copper was determined using X-ray photoelectron spectroscopy (XPS) and reflection-absorption infrared spectroscopy (RAIR). Adhesive joints were prepared by curing beams of epoxy against polished substrates at elevated temperatures. When the joints were immersed in liquid nitrogen, the residual stresses were sufficient to enable cracks to be propagated very near the interface, exposing the substrate and adhesive failure surfaces for analysis. Results obtained from XPS and RAIR showed that the anhydride curing agent reacted with the cuprous oxide (Cu2O) on the copper substrates. Cuprous ions were removed from the oxide and oxidized, resulting in the formation of a layer of cupric carboxylate salt in the interphase. The anhydride adsorbed onto the surface oxide of the 2024 aluminum substrates, which was mostly Al2O3, to form a surface carboxylate. However, no metal ions were removed from the oxide and no layer of metal carboxylate was formed. Copper in the 2024 aluminum was mostly in the form of an aluminide below the surface oxide and did not react with the anhydride. Similar results were obtained when films of the anhydride were applied to copper and 2024 aluminum substrates, heated to elevated temperatures, and rinsed. Thick films of cupric carboxylates were formed on the copper substrates but only surface carboxylates were formed on the 2024 aluminum. © 1990.