Pseudoplastic behaviour

In common with other thermoplastic melts polystyrene exhibits pseudoplastic behaviour. At shearing stresses below 60/M,J, MPa (where = average molecular weight), the ratio of shear stress to shear rate is almost constant and the melt is substantially Newtonian. Above this shear stress non-Newtonian... 

The flow behaviour of aqueous coating dispersions, because of their high pigment and binder content, is often complex. They have viscosities which are not independent of the shear rate and are therefore non-Newtonian. Shear thickening (when the viscosity of the dispersion increases with shear rate) and shear thinning or pseudoplastic behaviour (when the viscosity decreases with shear rate), may... 

If we compare the shear stress against shear rate it can be practically difficult to distinguish between plastic and pseudoplastic behaviour. However when the same data is represented on a log-log plot clear differences emerge. The low shear viscosity begins to reduce the stress... 

Fluid polymers mostly show pseudoplastic behaviour the rate of flow, 7, increases more than proportionally with increasing shear stress t. A unique viscosity value can, therefore, not be defined when, for instance, with a certain value of 7, a value of t is measured, then... 

Generally large yield stress effects were dominant in the nematic melts, but they were strongly pre-history dependent. A three region flow curve for 15 mol % modified poly(pheny1-1,4-phenylene terephthalate) was probably due to a not completely molten system. Dynamic viscosity measurements showed strong pseudoplastic behaviour. Strain and time dependence phenomena were not observed. 

Fig. 6.20. Flow curves (stress-strain rate) for concentrated suspensions. In curve (1) pseudoplastic behaviour without a yield value is shown. Only an extrapolated so-called Bingham yield value can be seen (tb). Curve (2) shows non-linear plastic behaviour. An apparent yield value Xy is present. Curve (3) shows the almost Newtonian behaviour of a stable concentrated suspension.

Polymers exhibit a decrease in viscosity with increasing shear rate, This behaviour is known as pseudoplastic behaviour, and the result is that equation (11.10) does not yield a constant. Figure 11,15 shows the different shape of the viscosity curves. 

The pseudoplastic behaviour of polymers can be explained by the fact that in response to shear, the thread-like molecules that are initially present in coils become uncoiled and an orientation takes place which results in a reduction in friction and hence in viscosity. 

Figure 13.2 Generalized flow curve for non-Newtonian behaviour. (A) shear-thinning (pseudoplastic) behaviour with low shear Newtonian plateau (B) shear-thinniiig behaviour with high shear Newtonian plateau i oo (C) dilatant (shear thidceniiig) behaviour...

Such systems are termed pseudoplastic or shear thinning. For such dispersions, as the viscosity decreases, the faster material is pumped through a pipe, sprayed out of a nozzle or mixed with a mixer. As Figure 6.12 shows, pseudoplastic behaviour in dispersions can be caused by alignment, stretching, deformation or... 

The entire flow curve for pseudoplastic behaviour can often be fit using the Reiner-Philippoff equation ... 

The pseudoplastic fluids do not show yield stress value. Their apparent viscosity decreases with the shear rate. The flow curve reveals linear character at very high shear rate. The logarithmic plot of shear rate as a function of shear stress of these fluids is often a straight line with the slope between 0 and 1. For the pseudoplastic behaviour description is hence frequently used the power-law equation ... 

One can readily see that for n > 1, equation (1.13) predicts increasing viscosity with increasing shear rate. The dilatant behaviour may be observed in moderately concentrated suspensions at high shear rates, and yet, the same suspension may exhibit pseudoplastic behaviour at lower shear rates, as shown in Figure 1.9 it is not yet possible to ascertain whether these materials also display limiting apparent viscosities. 

If a stress above the yield stress is applied, the gelled structure is broken into smaller units (floes), which can then move past each other. If floe attrition is affected by the strength of the hydrodynamic and attractive forces, pseudoplastic behaviour is observed and the viscosity decreases with shear rate. The strong shear forces at high shear cause flow units to be smaller and flow is facilitated. The destruction of floes releases constrained solvent, which results in a decrease in the effective volume-fraction of the floes. This phenomenon may create thixotropic behaviour in the system if the breakup and formation of floes is reversible. 

Using a rotational-torsional surface viscosimeter, the surface shear viscosity of the C15-C20 straight-chain fatty acids have been determined (Moo-Young et aL, 1981). The even fatty acids were found to show a surface-Newtonian behaviour, with s-values of 1.5, 2 and 3 mN s/m for the Ci, Cis and C20 members respectively. The odd adds, however, showed a surface-pseudoplastic behaviour. Thus C17 gave decreasing surface shear viscosity up to a sharp rate of 8 s , and beyond that value the viscosity remained constant at about 0.6 mN s/m. These data indicate different monolayer structures of the even and odd members. 

The pseudoplastic behaviour of polymer melts is particularly useful in processing. Flow is easier, and less energy required, when shear rates are increased. The flow index depends on the architecture of the polymer chain. It is higher for chains with long branches, implying faster reduction in 77 as 7 rises. 

The pseudoplastic behaviour also appears at the length of the capillary of 20 mm and 40 mm. 

The following example shows the practical implication of this pseudoplastic behaviour. 

Both in melt-mixed and solution-cast systems, the viscosity decreased with increase of shear stress, indicating pseudoplastic behaviour (Figure 18.8). In both cases, negative and positive deviations in viscosity could be seen at a high and low shear rate. However, as compared to solution-casted blends, in melt-mixed ones, degradation of NR and PS due to high temperature and shear was... 

It was explained that at closer to zero shear, the molecules were randomly oriented and highly entangled and therefore exhibited high viscosity. Under the application of shearing force, the polymer chains oriented, resulting in the reduction of shear viscosity and thus exhibited pseudoplastic behaviour. The reduction in viscosity of the blends at higher shear rate was also due to the decrease in the particle size of the dispersed domains. Solution-cast blends showed a higher viscosity as compared to melt-mixed blends. This is associated... 

As from Figure 18.45, the value of n reflected the deviation of the flow profiles from uniform parabolic flow patterns i.e. n= for Newtonian flow) to plug-like flow. The n values of the TPVs with various vulcanizing systems were similar and very low i.e. ra 0.24). Furthermore, the neat NR-g-PMMA exhibited the lowest n value i.e. ra = 0.15), while the neat PMMA showed the highest value i.e. n = 0.41). Therefore, the melt flow of the TPVs and the neat NR-g-PMMA exhibited highly pseudoplastic behaviour and the greater shearthinning behaviour. 

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