Synthesis and characterisation of high performance flocculants and superabsorbents from chemically modified starch and glycerol

Abstract

Superabsorbent polymers from chemically modified starch and glycerol have been prepared by acryloylation of starch followed by grafting with acrylic acid (AA) using Fenton’s initiation system (Fe2+/ H2O2). Fourier-transform infrared spectroscopy (FTIR) analyses provided evidence of starch ester formation and grafting of AA onto its backbone. Further characterisation of the product was carried out using X-ray diffraction (XRD), scanning electron microscopy (SEM) and thermogravimetric (TGA) techniques. The number of acryloyl groups per starch molecule and degree of neutralisation determine the superabsorbent behaviour of the samples. Under all the experimental conditions studied, polymer samples with improved grafting percentage, ratio, efficiency and low amount of homopolymer with excellent water retention ability and remarkable absorbency under load were obtained. Furthermore, glycerol acrylate (GA) was synthesised by acryloylation reaction with acryloyl chloride. The ester was used as cross-linking agent at varying proportions in the synthesis of poly(acrylic acid) (PAA) and acryloylated starch-g-poly(acrylic acid). The amount of cross-linking density in the products and the degree of neutralisation determine the absorbency of the polymer samples. The use of the cross linker enhances the absorbency of the samples up to a level when excessive cross-linking produces a rigid and a tightly-framed structure that limits the absorption of water within the polymer network. Moreover, the thermal behaviour of the samples was affected by the chemical processes involved. Alternatively, starch grafted with poly(acrylic acid) (starch-g-PAA) was synthesised via free radical polymerisation using a new radical initiator. Oxy-catalyst, which is a ˙OH generating catalyst from H2O2, was used for the first time as the initiator with aluminium triflate as co-catalyst. The percentage add-on (% add-on) and the grafting efficiency (GE %) were dependent to a degree on the amount of co-catalyst, temperature, starch to monomer ratio and time of the reaction. Starch and glycerol xanthates were also synthesised and used for metal scavenging activities. Xanthates from both glycerol and insoluble starch are synthesised and effectively used in the removal of Pb, Cd and Cu from aqueous solutions. The insoluble metal complex formed between the sulphur atoms in the xanthates and the heavy metals easily separated. Moreover, use of glycerol xanthate requires no pH adjustment to give a 100 % heavy metal removal within the range of the detection limit. Butyl xanthate was also synthesised to allow a good comparison with the glycerol and insoluble starch xanthate. The latter was proven to be more effective in metal scavenging activities. FTIR was used to prove evidence of xanthation. In addition, 1H and 13C NMR were used to characterise the glycerol xanthate. The chemical modification of the two sustainable resources find application in other areas such as capping agents of nanoparticles and as sulphur donor species in complex reactions for the synthesis of nanoparticles