RHEOLOGY OF GALACTOMANNAN SOLUTIONS: COMPARATIVE STUDY OF GUAR GUM AND LOCUST BEAN GUM

The rheological properties of several samples of guar gum and locust bean gum, characterized by light scattering and intrinsic viscosity measurements, were studied. The flow curves of concentrated solutions were obtained with a cone‐plate viscometer and their viscoelastic properties were measured with a concentric hemispheres viscoelastometer. The two flow‐units Ree‐Eyring model and the Cross equation gave a good fit to the experimental flow curves. The zero shear rate viscosity, and the relaxation time, of the Ree‐Eyring model depend mainly on the reduced concentration c[η]. This result, as well as the viscoelastic properties, indicate that entanglements play a dominant role in concentrated solutions. From a practical viewpoint, it is possible to predict the flow curve of a galactomannan solution at any concentration if the intrinsic viscosity of the sample is known. However, there are secondary differences in the flow parameters, mainly in the relaxation time, which are not ascribed to [η]. It is hypothesized that these differences are due to the mean branching degree rather than to the botanical origin. As, in solution, the smoothest galactomannan macromolecules are more probably aggregated than isolated, further work should aim at establishing the relationships between aggregate formation and degree of branching.