OXIDIZED POTATO-STARCH FILMS AS PRIMER COATINGS OF ALUMINUM

Potato-starch (PS) films for use as primer coatings of aluminium subst rates were prepared in two steps, chemical-thermal-catalysed oxidation routes. The PS was modified with cerium (IV) ammonium nitrate (CAN) a s a chemical oxidizer, followed by thermal oxidation at 150 degrees 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 bo nds in glycosidic rings, thereby resulting in the ring openings. Incre asing oxidation by raising the temperature to 200 and 250 degrees C pr omoted the conversion of carbonyl into carboxylate derivatives, while facilitating the breakage of C-O-C linkages in the open rings. The lat ter phenomenon reflected the formation of another carboxylate. The int ermediate carboxylate derivatives favourably reacted with Ce4+ ions re leased from CAN to form cerium-bridged carboxylate complexes. Cerium-c omplexed carboxylate films used as primer coatings not only afforded s ome protection of aluminium substrates against corrosion, but also dis played excellent adhesion to both the polyurethane (PU) top-coating an d aluminium sites. The latter demonstrated that the loss of adhesion a t PU/primer/aluminium joints occurs in the PU layers, representing the mode of cohesive failure.