Thixotropy: test methods

For the evaluation of the rheology of the silica dispersions, different test methods were applied (a) a shear rate-controlled relaxation experiment at = 0.5 s (conditioning), 500 s (shear thinning), and 0.5 s (relaxation) to evaluate the apparent viscosity, the relaxation behavior, and thixotropy (b) shear yield-stress measurements using a vane technique introduced by Nguyen and Boger [5] (c) low deformation dynamic tests at a constant frequency of 1.6 s in a stress range of ca. 0.5 - 100 Pa. All samples contained 3 wt% of fumed silica. 

Frequently used single-point viscosity tests in the starch plant are orifice pipettes,56 orifice funnels,57 the Hot Scott viscometer, and various methods to determine alkaline fluidity.58 For absolute measurements of the rheological properties, rotating viscometers with coaxial cylinders are used.59 The paper industry uses mainly the Brookfield viscometer and the Hercules viscometer for determining shear-dependent viscosity, pseudoplasticity, and thixotropy. Oscillatory and capillary viscometers are used for more detailed viscosity characterization, such as yield value, elastic properties, and viscoelasticity.60... 

Generally speaking, two methods can be applied to study thixotropy in a suspension. The first, and the most commonly used, procedure is the loop test, whereby the shear rate is increased continuously and linearly in time from zero to some maximum value and then decreased to zero in the same way this is illustrated in Figure 20.4. 

An alternative method for studying thixotropy is to apply a step change test, whereby the suspension is suddenly subjected to a constant high shear rate and... 

Whorlow (1992) published a book on rheological techniques that inctudes dynamic tests and wave propagation tests. In the appendix, he listed a number of rheologicat inves-tigahon equipment manufacturers. Some of the techniques appiy more to polymers and are not relevant to our discussion. Dynamic vibration tests have been extended to fresh concrete (Teixera et at., 1998). Concord and Tassin (1998) described a method to use rheo-ophcs for the study of thixotropy in synthetic clay suspensions. A rheometer optical analyzer was used on laponite, a synthetic hectorite clay. Laponite was mixed with water and tests were conducted at various intervals for up to 100 days. Rheo-ophcs seems to be... 

Generally speaking, two methods can be applied to study thixotropy in a paint formulation. The first and the most commonly used procedure is the loop test whereby the shear rate is increased continuously and linearly in time Irom zero to some maximum value and then decreased to zero in the same way [43]. This is illustrated in Fig. 4.17. The main problem with the above procedure is the difficulty of interpreting the results. The nonlinear approach used is not ideal for developing loops because by decoupling the relaxation process from the strain one does not allow the recovery of the material. However, the loop test gives a qualitative behavior of the paint thixotropy. An... 

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