Bentonite suspension rheology viscosity

The influence of addition of sodium bentonite (a commonly used antisettling system) on the rheological behaviour of a pesticide suspension concentrate (250 g dm ) has been investigated. Steady state shear stress-shear rate curves were carried out to obtain the yield value and viscosity as a function of shear rate. The shear modulus was also measured using a pulse shearometer, and the residual viscosity was obtained in afew cases from creep measurements. The rheological parameters Tg (Bingham yield value),... 

Gg (instantaneous modulus), Hg (residual viscosity) and G (shear modulus) all showed a rapid increase above 30g dm bentonite. This was attributed to the formation of a gel network structure in the continuous medium and the strength of such a gel increased with increase in bentonite concentration. The results could be qualitatively described in terms of the elastic floe model of Hunter and co-workers. Moreover, the settling characteristics of the structured suspensions were found to be consistent with the predictions from the rheological measurements. This demonstrates the value of rheological studies in predicting the longterm physical stability of suspension concentrates. 

The sedimentation results obtained with the structured suspensions, are consistent with the predictions from rheological investigations. In the absence of any bentonite clay, the pesticidal suspension exhibits Newtonian behaviour with unmeasurable yield value, modulus or residual viscosity. In this case the particles are free to settle individually under gravity forming a dilatant sediment or clay. On the other hand, at bentonite concentrations above the gel point (> 30 g dm the non-Newtonian behaviour of the suspensions and in particular their viscoelastic behaviour results from the formation of a "three-dimensional" network, which elastically supports the total mass. After 21 weeks standing in 100 ml measuring cylinders, no separation was observed when the bentonite concentration was >37.5 g dm corresponding to a modulus > 60 Nm. Clearly the modulus value required to support the mass of the suspension depends on the density difference between particle and medium. 

As seen in Figure 3.21, a complete rheological curve contains four characteristic regions. Region I corresponds to low stresses under which the system may demonstrate a solid-like behavior with high viscosity (Kelvin model). This case is characteristic of the already mentioned bentonite clays. The studies of relaxation structures in moderately concentrated suspensions of bentonite clays indicated the appearance of elastic aftereffect at low shear stresses. This effect has an entropic nature, as it is associated with the... 

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