Preparation of biodegradable superabsorbent of cassava starch-acrylamide/potassium acrylate

Abstract

Preparation of a new type of biodegradable superabsorbent polymer by graft copolymerization of acrylic acid/acrylamide onto cassava starch via redox initiator system of hydrogen peroxide-ascorbic acid was investigated. Several important parameters were studied: starch-to-monomer ratio, acrylic acid-to-acrylamide ratio, concentrations of H2O2-ascorbic acid initiating system, and crosslinking agent of N,N'-MBA to produce the highest water absorption of the product. By-products of the reaction, the free polymers of the homopolymer and the copolymer, were removed by water extraction. Subsequently, the purified graft copolymer was saponified with a 5% aqueous solution of potassium hydroxide at room temperature for 1 hour to change the carboxyl group into the carboxylate group, and thereafter the product was measured for the water absorption capacity. The graft copolymer was characterized by infrared spectroscopy technique for grafting evidences. Comparisons of the graft copolymers between the preneutralized and postneutralized systems (via neutralization) were conducted. Biodegradation of the polymers was carried out using alpha-amylase to degrade the grafted copolymer. Viscosity measurement, iodine and Benedict's solution tests for the biodegraded grafted copolymers were conducted. The saponified cassava starch-g-polyacrylamide (in nitric acid additive) gave the higher water absorbency (270 g/g dry weight) than that of the saponified cassava starch-g-poly (acrylic acid) (142 g/g dry weight). However, the saponified cassava starch-g-poly (acrylic acid-co-acrylamide) provided the increased water absorbency, especially at the ratio of acrylic acid-to-acrylamide of 1:1, gave the highest water absorbency due to osmotic pressure. The highest water absorbency (312 g/g dry weight) grafted copolymer was produced by the starch-to-monomer ratio of 3:4, acrylic acid-to-acrylamide ratio of 1:1, 5% wt of hydrogen peroxide, 0.5% wt of ascorbic acid, and 0.5% wt N,N'-MBA based on the total of monomers. The postneutralized system provided the grafted copolymer having the higher water absorbency than those of the preneutralized system because of the different dissociation of the monomers. After the alpha-amylase hydrolysis of the graft copolymers, the viscosity decreased because the graft copolymers were hydrolyzed into the glucose units. The hydrolyzed solution gave a negative test with the iodine and a positive test by the Benedict's solution, an indication of the existence of glucose units. The water absorptions of the product in salt and buffered (pH) solutions were measured. Moreover, the surface morphologies of the products were revealed by SEM technique, which indicated that highly porous structure was found in the graft copolymers with the higher water absorption. This research explains effects and phenomena of the reaction parameters on grafting parameters and water absorption.