Tailoring closely packed gel–particle systems for use as thickening agents

Abstract

AbstractWhen used as a thickening agent in aqueous suspension, hydrolyzed starch–polyacrylonitrile graft copolymer (H‐SPAN) has broadly variable rheological properties, such as viscosity, storage modulus, and stress overshoot characteristics. A series of H‐SPAN preparations with variable swelling ratio Q were made by pretreating portions of stock material. As 1% suspensions in water, they had viscosities η that were non‐Newtonian. At a constant shear rate, η of the suspensions depended on Q and had a sharp maximum in the midrange of Q. In terms of reduced concentration cQ, where c is weight fraction of polymer, the highest η occurred when cQ was approximately 2. With isoionic dilution, each suspension had a constant reduced viscosity function, η/cQ, provided cQ > 2. The value of η/cQ fell rapidly when cQ approached 1. All thickeners of the closely packed gel–particle type so far examined have this relation of the reduced variables. The shear modulus calculated from measurements of primary normal force and corrected for solvent swelling, according to theory for rubber for elasticity, was essentially constant for each suspension measured during isoionic dilution to cQ = 2. The density of crosslinks calculated from the modulus was extremely low for all samples. The variable rheological properties of the preparations resulted primarily from differences in their low effective crosslink densities.