Rheology thixotropy

The rheology (thixotropy, i.e. pseudo-plastic, or dilatant flow) must be considered and adjusted or the coating will not apply well (see later for fuller details). 

Appearance of rheological effects — yield stress, non-Newtonian viscosity, thixotropy... 

Grantham, C.K. and McLaurine, H.C. "Thixotropy Without Viscosity A New Approach to Rheology Control of Oil Muds," SPE paper 15415, 1986 SPE Annual Technical Conference and Exhibition of AIME, New Orleans, October 5 8. 

Since the rheology of many systems depends largely on the temperature, accurate and reproducible measurements require very careful temperature control. A 1°C temperature drop, for instance, increases the apparent viscosity / of an offset printing ink by approximately 15%. To demonstrate the correlation between thixotropy and temperature, Figs. 56 and 57 show the flow curves at different temperatures for two offset printing inks [134], Both materials clearly lose thixotropy-indicated by the area under the thixotropic loop-as the temperature increases. This effect is much more pronounced in the first case (Fig. 56), while the second ink exhibits a very slow decrease thixotropic behavior (Fig. 57). 

P.R.255 is recommended for high grade lead-free industrial finishes, especially automotive O.E.M. finishes. The commercially available grade tends to flocculate in many formulations. Additives to improve the fastness to flocculation are available on the market. The rheological behavior of finishes is characterized by an distinct thixotropy. This can cause problems at the production of mill pastes. P.R.255 is also of interest for the use in tinting systems for decorative paints. 

While rheological literature abounds with examples of thixotropic behavior, several leading authorities (L4, M12, W3) have shown that many of these examples are due to errors in experimental technique and not to the actual presence of thixotropy. In fact, the magnitude of the possible errors in most thixotropy studies to date as indicated by these references throws considerable doubt on the validity of most available quantitative conclusions concerning this phenomenon. 

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... 

Several rheological characteristics of milk fat and butter have practical significance. Setting, spreadability, hardness, work softening and thixotropy are affected by the rheology of milk fat. Setting refers to the continued increase in the firmness of newly manufactured butter. Increases in firmness... 

Sone, T. 1961. The rheological behavior and thixotropy of a fatty plastic body.. / Phys. Soc. Japan. 16, 961-971. 

This section draws heavily from two good books Colloidal Dispersions by Russel, Seville, and Schowalter [31] and Colloidal Hydrodynamics by Van de Ven [32] and a review paper by Jeffiey and Acrivos [33]. Concentrated suspensions exhibit rheological behavior which are time dependent. Time dependent rheological behavior is called thixotropy. This is because a particular shear rate creates a dynamic structure that is different than the structure of a suspension at rest. If a particular shear rate is imposed for a long period of time, a steady state stress can be measured, as shown in Figure 12.10 [34]. The time constant for structure reorganization is several times the shear rate, y, in flow reversal experiments [34] and depends on the volume fraction of solids. The viscosities discussed in Sections 12.42.2 to 12.42.9 are always the steady shear viscosity and not the transient ones. 

In some cases, an extrudable and injectable paste may consist of 65% vol. ceramic powder and 35% vol. polymeric binder. In others, an extrudable paste may consist of a highly loaded aqueous suspension of clay particles such that its rheology is plastic. Hie low shear (i.e., <100 sec ) viscosity of such a paste is between 2000 and 5000 poise at ambient temperature. Highly nonlinear stress strain curves are typical of ceramic pastes, as well as time dependent thixotropy. In many cases, pastes behave like visco-elastic fluids. This complex rheological behavior of ceramic pastes has made theoretical approadies to these problems difficult. For this reason, the discussion in this chapter is limited to Newtonian fluids where analytical solutions are possible, with obvious consequences as to accuracy of these equations for non-Newtonian ceramic pastes. 

Mild thixotropy has been observed in gelatiiized wheat starch dispersions characterized at low temperatures such as 25°C but not at 60°C and above (Doublier, 1981 Harrod, 1989 Bagley and Christianson, 1982). The time dependent rheological behavior shown by starch is attributed to structure formation by amylose leached into the suspending matrix on cooling to low gelling temperatures (Ellis et al., 1989). 

Thurston GB. Rheological parameters for the viscosity, viscoelasticity and thixotropy of blood. Biorheology 1979 16 149-162. 

Thixotropy is a rheological property that results in yield stress on standing. Thixotropic flow is defined as a reversible, time-dependent, isothermal gel-sol transition. Thixotropic systems exhibit easy flow at relatively high shear rates. However, when the shear stress is removed, the system is slowly reformed into a structured vehicle. The usual property of thixotropy results from the breakdown and buildup of floccules under stress. A small amount of particle settling takes place until the system develops a sufficiently high yield value. The primary advantage of thixotropic flow is that it confers pourability under shear stress and viscosity and sufficiently high yield stress when the shear stress is removed at rest. 

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. 

Thixotropy of casting slips is characterized by the time dependence of both viscosity and yield point. This phenomenon is caused by reversible sol-gel transition in the clay component, which can be affected by mechanical means (stirring, vibration). The effect is much more distinct with enamel slips than with ceramic casting slips. A more marked yield point is required with the former, while ceramic casting suspensions are usually characterized by a pseudoplastic rheological behaviour without a distinct yield point. 

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