Rheology of Aluminum Dilaurate in Toluene

Abstract

The stress-optical properties of aluminum dilaurate-toluene solutions were investigated over a wide range in shear rates, D. Flow curves of 1, 2, and 3 wt-% of the aluminum soap dispersed in toluene displaced the type of behavior previously reported by Weber and Bauer: i.e., Newtonian behavior at low shear rates, pronounced shear thinning at D greater than 10−2 sec−1, and dependence of apparent shear stress, τ, on length-to-radius ratio, L/R, of the capillary tubes used at high shear rates. Flow birefringence behavior was similar. For example, at low shear rates the degree of birefringence, Δn, increased in proportion to D and the extinction angle, χ, was nearly 45°. Above a critical shear rate, Δn, increased overproportionally, finally leveling out at a high value. Simultaneously χ decreased equally rapidly to about 1°. Despite these dramatic changes in dependence of τ, Δn and χ on D, the stress-optical coefficient remained constant over the entire range of variables. Elastic recoverable shear strains, or simply “recoverable shears,” s, were calculated from the variance of τ with L/R. These were found to be in good agreement with data obtained from normal stress measurements in a Weissenberg Rheogoniometer. At low shear stress s increased linearly with τ. Above a critical shear stress the recoverable shear increased rapidly with small increases in τ finally leveling off at a value of approximately 300 shear units and remaining constant with additional increases in shear stress. This behavior is similar to that observed with polyethylene in the region where melt fracture occurs. In these experiments, however, the transition takes place in a rotational instrument under continuous shear in which no “entrance effects” can be postulated as causing melt fracture. During the progress of this investigation it was ascertained that the rotational viscometer could be used to determine elasticity in aluminum soap solutions under dynamic conditions. Quantitative evaluation of the results produced data which correlated with dynamic test data obtained by a variety of techniques. The authors believe that this is a unique and novel method for measuring the dynamic rheological properties of viscoelastic materials.