Rheology yield point

PS-7 gum solutions have a rheological yield point. The shear stress, shear rate curve is shown in Figure 12. 

A third definition of surface mobility is essentially a rheological one it represents the extension to films of the criteria we use for bulk phases and, of course, it is the basis for distinguishing states of films on liquid substrates. Thus as discussed in Chapter IV, solid films should be ordered and should show elastic and yield point behavior liquid films should be coherent and show viscous flow gaseous films should be in rapid equilibrium with all parts of the surface. 

Rheology Ideal requirements low viscosity with the yield point and geis necessary for hole cleaning and solids suspension. Practical recommendation A compromise should be found to minimize pressure losses and bring sand or cutting to the surface at reasonable circulating rate. 

The rheological characteristics of AB cements are complex. Mostly, the unset cement paste behaves as a plastic or plastoelastic body, rather than as a Newtonian or viscoelastic substance. In other words, it does not flow unless the applied stress exceeds a certain value known as the yield point. Below the yield point a plastoelastic body behaves as an elastic solid and above the yield point it behaves as a viscoelastic one (Andrade, 1947). This makes a mathematical treatment complicated, and although the theories of viscoelasticity are well developed, as are those of an ideal plastic (Bingham body), plastoelasticity has received much less attention. In many AB cements, yield stress appears to be more important than viscosity in determining the stiffness of a paste. 

Of even more economic importance is the rheological impact of the addition of KC1 to conventional water-base drilling fluids. KC1 causes undesirable increases in both yield point and gel strength that can only be eliminated by chemical disperants or by dilution with fresh water. Dilution in turn requires more KC1 for clay inhibition, and the cycle continues with mud costs escalating exponentially. 

The results of these tests and the fluid formulations axe shown in Table IV. The adverse effect on fluid rheology of adding potassium, KC1, Test B, and TKPP, Test C, is evident. Both salts cause increases in yield point, YP, and initial gel strength. TKPP produced only about half as much increase as did KC1 and can be added in much higher concentrations before the rheological properties of the drilling mud become unacceptable. 

The results of the latest research into helical flow of viscoplastic fluids (media characterized by ultimate stress or yield point ) have been systematized and reported most comprehensively in a recent preprint by Z. P. Schulman, V. N. Zad-vornyh, A. I. Litvinov 15). The authors have obtained a closed system of equations independent of a specific type of rheological model of the viscoplastic medium. The equations are represented in a criterion form and permit the calculation of the required characteristics of the helical flow of a specific fluid. For example, calculations have been performed with respect to generalized Schulman s model16) which represents adequately the behavior of various paint compoditions, drilling fluids, pulps, food masses, cement and clay suspensions and a number of other non-Newtonian media characterized by both pseudoplastic and dilatant properties. 

Consider a suspension of identical, spherical (radii a), force- and couple-free particles. Upon neglect of both inertia and Brownian movement, the proper rheological starting point is Eq. (2.33) for the particle stress. As in Eq. (4.1), the contribution arising when particle-particle interactions are absent is known. Its explicit inclusion in yields... 

Another well documented, but less widely used method for formulation development is the measurement of electrokinetic properties [14]. These tests require more disdnc-tive/elaborate sample preparation and are mosdy restricted to use during development. Also widely used and relied on are rheological measurements. The scope of rheological measurements ranges from viscosity measurements to the determination of yield points or oscillatory properties, such as the G and G -modules [15]. Since suspoemulsions are not ideal viscous but mostly viscoelastic or dilatant, a wide range of characterization techniques exists. Instrumentation required for this are simple rotary viscosimeters (Brookfield) or more sophisticated stress or shear controlled rotational viscosimeters. 

In ceramics, the term plastic mass or paste denotes an easily workable mixture which keeps its shape after forming. Up to a certain stress, such a mixture behaves as a solid and exhibits approximately elastic behaviour. It is irreversibly deformed only beyond a critical stress called the yield point. Beyond the yield point its behaviour can be characterized by the respective deformation rate from which the so-called plastic viscosity can be determined. The rheological behaviour involved is essentially of the... 

When attempting to describe more accurately the rheological behaviour of ceramic plastic mixes, one should also take into account the elastic behaviour above the yield point. If a plastic body is abruptly stressed by a constant load, there first occurs rapid clastic deformation followed by delayed elastic deformation and irreversible flow. Similarly, instant as well as delayed relaxation take place after stress relief. If a formed product has only a limited possibility to relax, it retains some interna stress w hich may be the cause of drying defects. 

The yield point is lowered by gradual increase in water content in the raw material mix, until the range of liquid suspension is reached. If the system contains so much liquid that the particles of solids are mutualy separated by thick liquid layers, these suspensions behave like Newtonian liquids. Deviations from this simple rheological behaviour arise when the particles come into mutual contact. The yield point will arise or various types of non-linear dependence of deformation rate on stress will occur. 

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. 

Rheological properties of casting slips are controlled by small additions of suitable electrolytes which promote deflocculation of aggregates and thus cause a decrease in yield point and increase in fluidity, which is desirable with respect to reduction of the required water content (refer to the section on slip casting). The stability of the suspension is also improved, i.e. the sedimentation of the solids is suppressed. 

The adhesives have to show adjnsted rheological behavior. Depending on the application, materials with both high and low viscosity, with and without thixotropy, and with and without yield point are needed. 

In terms of rheology ceramic bodies hold a special position between ideal elastic and ideal plastic bodies, as they exhibit Bingham behaviour. Plotted on a shear stress/shearing speed graph, ceramic plastic bodies start to deform only after having reached a certain shear stress tq, the so-called yield point. 

Firstly we investigated the rheology of Wacker HDK N20 and H18 in pure styrene. Figure 3a shows the relaxation test experiment. Fig. 3b the deformation sweep, and Fig. 3c the determination of the yield point. 

---