Flow behavior, thixotropy and dynamical viscoelasticity of sodium alginate aqueous solutions

Abstract

Understanding of the rheological behavior of sodium alginate (SA) is fundamental either to design optimized products or to ensure stable flows. Of the SA samples tested that characterized by a M/G ratio of 0.38 and a weight–average molecular mass (Mw) of 1.180 × 105 g/mol, steady–shear flow, thixotropy and dynamical viscoelasticity tests were carried out to characterize rheological behavior of G–rich SA aqueous solutions as influenced by concentration (1.0–3.0%, w/v) and temperature (5–35 °C). These measurements were carried out by using an Anton Paar–Physica MCR 301 Rheometer. It was found that, above a critical shear rate, all G–rich SA solutions (1.0–3.0%, w/v) exhibited non–Newtonian shear–thinning behavior and the flow curves could be well described by the Cross model. The temperature–dependent behavior of 2.5% (w/v) SA had higher correlation to the Cross model and the Arrhenius relationship, while the flow activation energy tended to decrease with the increase of shear rate. The upward–downward rheograms showed that all the systems evaluated in this paper had a hysteresis loop which indicating strong thixotropic behavior, the higher the SA concentration, the stronger the thixotropic behavior. The dynamical viscoelastic properties characterized by oscillatory frequency sweep under small–deformation conditions showed a fluid–like viscoelastic behavior. The behavior of SA aqueous solutions was predominantly more viscous than elastic. Multiwave temperature ramp test for 2.5% SA aqueous solutions was consistent with the results from oscillatory frequency sweep test.