Viscometric analysis of the hydration of a surface cross-linked EM-HEC polymer

Abstract

Hydrophilic polymers are used in many different products and processes, in which they function, for example, as rheology control agents, steric stabilizers, water retention agents, or flocculants. When the application requires dissolution of polymer powders in water, this occurs through hydration of the hydrophilic chains, and is accompanied by an increase in solution viscosity through a combination of chain entanglement, hydrophobic interactions and hydrogen bonding. On a practical level, however, the addition of high molecular weight polymer powder to water can result in the formation of a surface gel layer leading to partial hydration and particle agglomeration, creating operational and formulation problems. One way of overcoming this is by chemically crosslinking hydroxyl groups on the particle surfaces, for example, by reaction with dialdehydes, such as glyoxal. This retards the initial hydration rate, thereby allowing more time for effective powder dispersion. The present study is concerned with factors affecting the hydration of a commercial non-ionic glyoxal-crosslinked methyl ethyl hydroxyethylcellulose, in the concentration range 0.15–0.5 wt%. Hydration kinetics have been determined by monitoring the increase in solution viscosity with time using low shear rotational viscometry. Experimental variables include polymer concentration, temperature, pH, the presence of anionic, cationic and non-ionic surfactants, and ionic strength (sodium chloride concentrations in the range 0–5 mol/L). In deionised water, the hydration process is shown to involve two kinetic steps, assigned to breakage of hemiacetal crosslinks and polymer hydration. The kinetic studies enabled the various stages in the overall hydration process to be analysed, and mechanistic implications and activation parameters to be assigned.