Molecular Structure of Polymer/Metal Interphases

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Abstract
The molecular structure of interphases in aluminum/epoxy and steel/epoxy adhesive joints was characterized using infrared spectroscopy. In one series of experiments, adhesive joints were prepared by curing beams of epoxy against aluminum or steel substrates. When the joints were cooled to room temperature, the residual stresses were sufficient for crack propagation along the interface. The adhesive and substrate failure surfaces were then analyzed with reflection-absorption infrared spectroscopy (RAIR), attenuated total reflection infrared spectroscopy (ATR) and X-ray photo-electron spectroscopy (XPS). When an epoxy/anhydride adhesive was cured against aluminum substrates primed with an aminosilane coupling agent, amide and imide groups were formed in the interphase. Chemical reaction between the primary amine of the primer and the anhydride of the curing agent precluded chemical bridge formation between the primer and adhesive. Metal cations from the 2024 aluminum substrate reacted with the anhydride to form carboxylate salts on the surface. When an epoxy/tertiary amine adhesive was cured against steel substrates, evidence of oxidation of the primary amine to imine was observed in the interphase.