Pseudoplastic behavior

Figure 2.5 shows some actual experimental data for versus 7, measured on a sample of polyethylene at 126°C. Note that the data are plotted on log-log coordinates. In spite of the different coordinates. Fig. 2.5 is clearly an example of pseudoplastic behavior as defined in Fig. 2.2. In this and the next several sections, we discuss shear-dependent viscosity. In this section the approach is strictly empirical, and its main application is in correcting viscosities measured...

The phenomenon under consideration is complicated and the theory developed in the last section is fairly simple-involved, but not really difficult. We have successfully discovered that the transition from Newtonian to pseudoplastic behavior is governed by the product 77, or the relative values of the shear rate and the rate of molecular response. 

Pseudozan is an exopolysacchaiide produced by a Pseudomonas species. It has high viscosities at low concentrations in formation brines, forms stable solutions over a wide pH range, and is relatively stable at temperatures up to 65° C. The polymer is not shear degradable and has pseudoplastic behavior. The polymer has been proposed for enhanced oil-recovery processes for mobility control [1075]. 

The typical viscous behavior for many non-Newtonian fluids (e.g., polymeric fluids, flocculated suspensions, colloids, foams, gels) is illustrated by the curves labeled structural in Figs. 3-5 and 3-6. These fluids exhibit Newtonian behavior at very low and very high shear rates, with shear thinning or pseudoplastic behavior at intermediate shear rates. In some materials this can be attributed to a reversible structure or network that forms in the rest or equilibrium state. When the material is sheared, the structure breaks down, resulting in a shear-dependent (shear thinning) behavior. Some real examples of this type of behavior are shown in Fig. 3-7. These show that structural viscosity behavior is exhibited by fluids as diverse as polymer solutions, blood, latex emulsions, and mud (sediment). Equations (i.e., models) that represent this type of behavior are described below. 

Rubber-based nanocomposites were also prepared from different nanofillers (other than nanoclays) like nanosilica etc. Bandyopadhyay et al. investigated the melt rheological behavior of ACM/silica and ENR/silica hybrid nanocomposites in a capillary rheometer [104]. TEOS was used as the precursor for silica. Both the rubbers were filled with 10, 30 and 50 wt% of tetraethoxysilane (TEOS). The shear viscosity showed marginal increment, even at higher nanosilica loading, for the rubber/silica nanocomposites. All the compositions displayed pseudoplastic behavior and obeyed the power law model within the experimental conditions. The... 

Undoubtedly other factors may promote pseudoplastic behavior. For example, a decrease in size of swollen, solvated, or even entangled molecules or particles may occur under the influence of shearing stresses, to give the same changes in fluid behavior as in particle alignment. 

Since the shear-stress-shear-rate properties of pseudoplastic materials are defined as independent of time of shear (at constant temperature), the alignment or decrease in particle size occurring when the shear rate is increased must be instantaneous. However, perfect instantaneousness is not always likely if the foregoing causes of pseudoplastic behavior are correct, as they are believed to be. Pseudoplastic fluids are therefore sometimes considered to be those materials for which the time dependency of properties is very small and may be neglected in most applications. 

The causes for thixotropic and rheopectic behavior are possibly very similar to those for pseudoplasticity and dilatancy, respectively. The proposed causes of pseudoplasticity, i.e., the alignment of asymmetrical molecules and particles or the breakdown of solvated masses, could not always be expected to be instantaneous with respect to time. Therefore it seems that pseudoplastic behavior may simply be that form of thixotropy which has too small a time element to be measurable on most instruments in current use. Exactly the same argument may be applied... 

The great practical importance of pseudoplastic behavior makes an understanding of these fluids of paramount importance. Their complexity is such, however, that an understanding of their behavior is frequently applicable to Newtonian, dilatant, and Bingham-plastic materials as well. 

Only in recent years has progress toward an engineering understanding of pseudoplastic behavior been substantial. Most developments in this field stem from relationships developed by Rabinowitsch (Rl) and exploited fully by Mooney (M15) in a classic paper that all engineers interested in non-Newtonian behavior must probably place at the top of their reading list. 

Measurements of the zero shear viscosity (20 °C) were made with a Bohlin VOR rheometer in the viscometry mode. If a Newtonian region was not found at the lowest measurable shear rates, the samples were characterized with a Bohlin-CS constant stress rheometer, with which it was possible to obtain extremely low shear rates. This approach was especially needed for highly viscous samples exhibiting pseudoplastic behavior on the VOR rheometer. Newtonian regions were found for each sample in this manner, yielding the zero shear viscosities reported. 

Figure 4.40 shows the shear-thinning behavior of an aqueous solution of ethyl hydroxyethylcellulose as a function of the concentration. The pseudoplastic behavior is observed at lower polymer concentrations as the molecular weight of the polymer increases. An aqueous solution of ethyl hydroxyethylcellulose becomes pseudoplastic at concentrations of less than 1%. Above the critical value of the shear stress the flow behavior is non-Newtonian, and viscosity decreases with the increasing shear stress. The critical stress is in the range of 0.1 N/m2 for the solution. 

Although not pointed out in the test, it may be observed from the data that there is some effect on mobility of velocity or total flow rate. Some shear thinning or pseudoplastic behavior has been observed under certain conditions. This is, of course, the more favorable of possible non-Newtonian behaviors for foam mobility control, since it would mean that less thickening occurred in the vicinity of the injection well, than further out in the formation. 

Because shear rate-shear stress data can be obtained relatively easily, a number of rheologists explored the possibility of determining the time constants from these data. Most shear-thiiming (pseudoplastic) fluids behave as Newtonian fluids at low shear rates and at a particular shear rate they begin to show their pseudoplastic behavior the reciprocal of the shear rate at which the transition from Newtonian to pseudoplastic behavior occurs is the characteristic time or the time constant. 

Polyacrylamide forms water-based gels at concentrations around 4%w/v, which exhibit pseudoplastic behavior. A PAAm ophthalmic gel containing pilocarpine was compared with other gel vehicles the ocular bioavailability for the PAAm gel was three times greater than that of the aqueous control solution. The kinetics of ibuprofen release for crosslinked PAAm gels was studied. A kinetic model was proposed for swelling induced loading of insulin into crosslinked PAAm gels. ... 

In PVC coating formulation fillers play a role. Filler choice mostly depends on the way the filler affect viscosity. The filler should not absorb the plasticizers nor interfere with the pseudoplastic behavior of the paste which is determined by the resin properties and by the choice of plasticizers. Fillers must be completely dispersed, since the gaps between the coated substrate and the knife are very small. There must be no lumps. Fillers should not interfere with deaeration which is... 

To further investigate the characteristics of the slags, plots of the logarithm of viscosity versus temperature were made. These indicated straight lines or two line segments. For slags with a sudden increase in viscosity at lower temperatures, two segments were observed. Shear rates were not varied and the various types of non-Newtonian behavior were not explored. A related study indicated pseudoplastic behavior for this type of material (9 ). 

Liquid fabric softeners are generally non-Newtonian and examples of the shear stress-shear rate relationship for two commercial products (A and B) is shown in Figure 4.25, determined at 22.5°C. In the shear rate range 0 to 250 sec-1, we note non-Newtonian pseudoplastic behavior. 

---