Towards enhancement of grafting performance and water absorption of cassava starch graft copolymer by gamma radiation

Abstract

The enhancement of gamma radiation grafting of acrylonitrile onto gelatinized cassava starch was investigated via various parameters of importance: the addition of metal sheet wrapping the inner wall of the irradiated vessel with the aluminium foil, copper sheet, lead and zinc plate, respectively, nitric acid, methyl ether hydroquinone inhibitor, and styrene comonomer. A homopolymer of polycrylonitrile (PAN) was a by-product of graft copolymerization which was latter removed by extraction with dimethylformamide. The purified grafted copolymer was subsequently saponified with an 8.5% aqueous solution of potassium hydroxide at 100℃ to convert the nitrile groups into a mixture of the acrylamide and carboxylate groups, which were responsible for water absorbency. Infrared spectrometer was used to follow up the chemical changes of grafting and saponification. The saponified starch-g-PAN (HSPAN) was then characterized in terms of grafting parameters for a guideline to judge an optimum total dose (kGy) and the quantity of acrylonitrile (ml) at the fixed dose rate of 0.25 kGy/min. A thin aluminium foil for covering the inner wall of the reaction vessel was found to be far more effective than any other metal sheet in enhancements of the grafting reaction and the water absorption. The presence of nitric acid in medium increases the grafting yield and the water absorption. Methyl ether hydroquinone inhibitor was evaluated to increase homopolymerization and decrease graft reaction. The styrene comonomer hampered the grafting of acrylonitrile onto starch backbone. The water absorption capacity was improved by using a freeze dryer for drying HSPAN. The treatment of the HSPAN with aluminium trichloride hexahydrate was found to enhance the degree of wicking but decrease water absorbency. Water absorptions of the HSPAN in deionized distilled water, simulated urine and saline solutions was carried out. Water absorption capacity in saline solutions and simulated urine decreased dramatically with increasing the salt concentrations and the type of cations. Water retention in sand by mixing it with the HSPAN showed a linear relationship of water increase with increasing amount of absorbent added. Discussions of the effects of the parameters on the reaction and water absorption were also given in this research.