Effect of stearic acid and sodium stearate on cast cornstarch films

Stearic acid and sodium stearate were used as lipids to form inclusion complexes with cornstarch through a cooking process, respectively. The complexing index and the surface tension of the cooked pastes were measured. The films of the pastes were formed by casting, paper-sizing, and paper-coating methods. X-ray diffraction and scanning electron microscopy were used to characterize the crystal and surface structures. Such properties of the complex films as the moisture absorption, friction coefficient, and tensile properties were tested. The results reveal that the two lipids could form an inclusion complex with amylose in the cornstarch more or less as a result of forming V-type crystals that restrained the crystallite size of the B-type crystal in the starch film. The addition of lipids reduced the sensitivity of the complex films to environmental moisture, decreased the friction coefficient of the films, and visibly increased the smoothness of the surfaces of the films. When the lipids were added in the starch at a weight content of 1%, the elongation at break of the complex film increased and the strength decreased slightly, and above a 1% content of the lipids, the tensile properties of the starch film were greatly deteriorated. (C) 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012