Oxidized potato-starch films as primer coatings of aluminium

Abstract

Potato-starch (PS) films for use as primer coatings of aluminium substrates were prepared in two steps, chemical-thermal-catalysed oxidation routes. The PS was modified with cerium (IV) ammonium nitrate (CAN) as a chemical oxidizer, followed by thermal oxidation at 150°C in the presence of atmospheric oxygen; this led to the formation of a functional carbonyl derivative caused by cleavage of the glycol C-C bonds in glycosidic rings, thereby resulting in the ring openings. Increasing oxidation by raising the temperature to 200 and 250°C promoted the conversion of carbonyl into carboxylate derivatives, while facilitating the breakage of C-O-C linkages in the open rings. The latter phenomenon reflected the formation of another carboxylate. The intermediate carboxylate derivatives favourably reacted with Ce4+ ions released from CAN to form cerium-bridged carboxylate complexes. Cerium-complexed carboxylate films used as primer coatings not only afforded some protection of aluminium substrates against corrosion, but also displayed excellent adhesion to both the polyurethane (PU) top-coating and aluminium sites. The latter demonstrated that the loss of adhesion at PU/primer/aluminium joints occurs in the PU layers, representing the mode of cohesive failure.