The adsorption of starch derivatives onto kaolin

Modified starches are widely used as binders and thickening agents in paper coating formulations. Frequently, kaolin is used as the coating pigment. In this work, the adsorption of modified starches onto kaolin has been studied. Adsorption behaviour was found to depend both upon the type of chemical modification and botanical origin. With a phosphate ester of potato starch, adsorption was reduced in the presence of sodium polyacrylate used as a deflocculant on the clay. No fractionation of the starch components was observed during adsorption. These results imply an electrostatic mechanism of interaction with the edge aluminium sites. In contrast, a hydroxyethyl ether of corn (maize) starch showed preferential adsorption of amylose, which is shown to be a result of preferential etherification of this component with ethylene oxide. With this non-ionic derivative, adsorption was not reduced by the presence of polyacrylate. A mechanism of adsorption involving hydrogen bonding of hydroxyethyl groups to silanol or aluminol groups on the kaolin surface is proposed in this case. A hydroxyethyl ether of potato starch showed preferential adsorption of amylopectin, together with some sensitivity to polyacrylate. With potato starch, the adsorption behaviour is dominated by the presence of naturally occurring phosphate ester groups, which are associated with the amylopectin. (C) 1998 Elsevier Science B.V.