Rheology complete rheological curve

The full rheological curve of such thixotropic system may be presented as a graph showing the effective viscosity, ry- = x/y, as a function of the shear stress, x, (Fig. IX-25). In this Figure r min corresponds to viscosity of the system with completely disintegrated structure. For the above mentioned bentonite... 

FIGURE 3.20 A complete rheological curve of structured disperse system tan 4> = qschw. tan c )ni = iIb-... 

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... 

In practice, complete rheological curves for concentrated emulsions often cannot be obtained. This is due to the instability of disperse systems with a high content of dispersed phase at high shear rates. Difficulties with the instrumental techniques employed for such measurements may also arise because the values of the viscosity and the shear stress at the transition from the imperturbed to a completely destroyed structure vary over a wide range. Under certain conditions, incomplete rheological curves must be used for analysis and prediction of the viscosity of emulsions. 

Tile models proposed for the description of the complete rheological curves are also based on a mathematical approximation (4). The power equations are characterized by their similarity to the second group of models which were used to describe the incomplete rheological cures and include the ... 

A more serious deficiency resides in reliance on MFI to characterize different polymers. No single rheological property can be expected to provide a complete prediction of the properties of a complex material like a thermoplastic polymer. Figure 11-27 shows log — log flow curves for polymers having the same melt index, at the intersection of the curves, but very differeni viscosities at higher shear stress where the materials are extruded or molded. This is the main reason why MFI is repeatedly condemned by purer practitioners of our profession. The parameter is locked into industrial practice, however, and is unlikely to be displaced. 

The two main rheological properties of a suspension are the yield stress and the viscosity. Yield stress determines when the system becomes a fluid state and when is in a solid state, whereas viscosity determines the ability to flow. In this section, we start with the viscosity measurement. Although one can extract the yield stress from the complete viscosity-shear rate curve, it is helpful to measure the yield stress directly as well. The dynamic and transient measurements are also important for concentrated suspensions. However, because these two types of measurements can be blended into the measurements of the two main rheological properties with some modifications to the measuring instrument, we refer to their measurements only briefly when it is relevant to the discussion. 

Consistency n. The property of a material or composition which is evidenced by its resistance to flow, represented by an undefined composite of properties, each measurable from the complete, force-rate flow curve as plastic viscosity, yield value and thixotropy. The term is applied to a variety of materials. For Newtonian liquids, consistency is simply viscosity. While consistency is an accepted rheological term, it has qualitative... 

In non-Newtonian fluids K a also depends on their physical and rheoiogical properties. The contribution of the latter has been normally expressed in terms of the apparent viscosity, and there is general agreement that this dependence is of the form Kj a 0(11 ) % where z can take values between 0.4 to 0.7. In the case of viscoelastic materials, inclusion of the fluid rheology is less straightforward. Several authors have tried to include the effect of elasticity via the Deborah number, which for stirred tanks is defined as the product of a characteristic time of the fluid and impeller speed. However, determination of the former is not an easy task because it is not always possible to characterize experimentally the viscoelastic properties of the fluid. Determination of the characteristic time of the fluid from experimental shear viscosity vs. shear rate curves [29] and from interpolation of published experimental data on viscoelastic properties [30] has been tried in the past. However, values thus obtained are not necessarily representative of the actual behavior of the liquid. At present, inclusion of the Deborah number in dimensional or dimensionless correlations has not been completely successful. 

It is a well-known fact that only a few polymer pairs are compatible. Moreover, the concept of compatibility is in itself difficult to define and quantify. Although, it is not completely correct, for our proposes, we can define the compatibihty on the basis of the property-composition cruwe, where properties are taken as macroscopic characteristics such as mechanical or rheological properties, etc. Figrme 5 plots the qualitative trends of property-composition curves for blends which... 

Rheological measurements were carried out with a computer-controlled Rheometer MCI (Paar Physica) rotational viscometer, with a Z4 concentric cylindrical measurement system at 25 0.1 °C. The complete flow and viscosity curves of the samples were plotted, and the yield value and the size of the thixotropic area were determined. 

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