Preparation of a weakly basic ion exchanger by crosslinking starch with epichlorohydrin in the presence of NH4OH

Crosslinking of starch with epichlorohydrin in the presence of NH4OH, NaOH and water was studied. The reaction was optimized and the products analysed by solid-state C-13-NMR in the presence of water, acid-base titration and dye-binding measurements. The presence of different types of hydroxypropyl groups was confirmed by NMR, but their linkage to polysaccharide could not be confirmed because of their minor abundance relative to all other CH2 groups, and because the motion of the sidechains is much greater than the starch component, which remains fairly constant despite the presence of water. Yield of crosslinked, water-insoluble product is increased as a greater amount of NH4OH and a lower amount of water is used in the reaction. Product yield using water-insoluble starch is higher than with a water-soluble starch. Closed-vessel (pressurized) reaction conditions increases product exchange capacity. Potentiometric titration of products in Cl--form or in non-ionized form indicated smaller exchange capacity values than expected from product nitrogen contents, reflecting the difficulty in fully protonating the products. Dye binding by the crosslinked-starch exchangers is pH-dependent, with an increased quantity of dye bound at lower pH, which is consistent with the weak-amine nature of the products. Hydrolyzed Remazol Brilliant Red F3B, a trivalent reactive dye, binds less effectively under acidic conditions, on a molar basis, to the starch-based exchangers than does Alizarin Red S, a monovalent mordant dye. Dye-binding capacities are somewhat lower than the total ion-exchange capacities determined by potentiometric titration, suggesting that a portion of the exchange sites are unavailable to the dyes.