Synthesis And Properties Of Sweet Potato-based Superabsorbent

Starch is a well-known abundant natural polymer with good biodegradability, biocompatibility and reproducibility, and at least 10 megaton of sweet potato starch are estimated to be synthesized every year in China. Graft copolymerization of starch is more attractive to expand the applications as functional materials. In this dissertation, a few of superabsorbent polymer(SAP) based on sweet potato starch were synthesized via aqueous solution polymerization method,such as starch-graft-polyacrylamide SAP, starch-g-poly(sodium acrylate) SAP. The structures and properties of these sweet potato starch graft copolymers were investigated, and also some absorption mechanisms and swelling theory.1. Grafting of acrylamide onto sweet potato starch initiated by KMnO₄ was studied.Two orthogonal experiments were carried out to achieve the best graft and saponification conditions.The best graft conditions are that the quantity of starch,monomer,water and the concentration of catalyst,initiator are 1g, 7.5g,100ml, 1.54×10^-2mol·L^-1,1.5mmol·L^-1 respectively. The best saponification conditions are that the quantity of NaOH, the saponifying temperature, the saponifying time are 26ml/(g monomer),100℃,3h respectively, the absorbency of deionized water can reach 1127g/g.2. A simplified method to synthesis SAP was put forward. Sodium acrylate was grafted onto sweet potato starch without N₂ atmosphere and starch was gelatinizated by NaOH in normal temperature. The result showed that the absorbency of deionized water of the SAP synthesised by this method was 980g/g. The porous superabsorbent using sodium bicarbonate,acetone and sucrose as porosity generators were also synthesised. In comparison with a superabsorbent prepared under porous-free condition, porosity generators can improve absorbency rate effectively, Morphological studies using SEM showed that the superabsorbent has loose and porous structure.3. The influence of terpolymerization and starch-g-poly(sodium acrylate)/Al₂(SO₄)₃ composites, starch-g-poly(sodium acrylate)/EDTA composites on water absorbency and 0.9% NaCl solution absorbency were investigated. The results showed that starch-g-poly(sodium acrylate/acrylamide) ternary copolymer and starch-g-poly(sodium acrylate)/EDTA composites can improve the salt resistant obviously. The hydrogel strength of different SAP and its repeated absorbency were also studied, the results showed that the water retentiveness of starch-g-poly(sodium acrylate/acrylamide) reached more than 80% after absorbing eight times.4. The influential parameters that affect water absorption such as ionic type, concentration of external solution are investigated. It is proved that water absorbency depends largely on the cationic type and concentration of the solutions in terms of ionic strength, irrelative to anionic valence and type. The process of water absorption is consistent with Donnan equation, which confirms that the swelling behavior is related to the osmotic pressureÏ€of the hydrogel, it can explain the absorption phenomena of SAP very well.5. The influence of the composition and size of SAP and the concentration of solution on the swelling behavior were analyzed in terms of the Berens-Hopfenberg non-Fickian equation, the kinetics of the swelling process of starch-g-acrylamide SAP in distilled water were interpreted by the diffusion- relaxation model and offer quantitative information, it shows that diffusion contribution is the main cause at the beginning of absorbing process , while the relaxation contribution became the main cause later. The results have also beeen confirmed within experimental error.6. The biodegradation of the superabsorbent in different microorganism environment was investigated via agar plate cultivation, various superabsorbents with different degree of grafting all have good biodegradability, the effect of the biodegradation varies with different microorganisms. The fungi have better effect of biodegradation than bacteria. The result shows that inherent biodegradability does not depend very significantly the degree of grafting of the polymer.