Pseudoplastic behaviour stress

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... 

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.

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 ... 

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. 

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... 

Many polymer melts exhibit pseudoplastic behaviour. At low shear rates they show an almost linear shear stress—shear rate behaviour with a constant viscosity, the so-called zero-shear-rate viscosity, t]o (Fig. 6.5). The normal stress differences approach the zero-shear rate values and... 

The optimization of the functional properties of a synthetic yarn (in particular from the point of view of its mechanical behaviour) requires maximum orientation of the macromolecules in the direction of flow. This objective can be difficult to achieve because of the generally pseudoplastic behaviour of such liquids. Tlie dies which the polymers cross typically have the shape of a funnel (see Rg. 2.6). When the macromolecules are brought into liquid state in the system, they will undergo stresses because of the irregularity of the die s diameter. Under the action of the pressures at the exit of the extm-sion device, the polymer will cross successively three zones in the die (see Fig. 2.6). The advance of the macromolecules from Zone I towards Zone II shows that the flow rate will increase because the section of the capillary... 

Colloidal dispersions often display non-Newtonian behaviour, where the proportionality in equation (02.6.2) does not hold. This is particularly important for concentrated dispersions, which tend to be used in practice. Equation (02.6.2) can be used to define an apparent viscosity, happ, at a given shear rate. If q pp decreases witli increasing shear rate, tire dispersion is called shear tliinning (pseudoplastic) if it increases, tliis is known as shear tliickening (dilatant). The latter behaviour is typical of concentrated suspensions. If a finite shear stress has to be applied before tire suspension begins to flow, tliis is known as tire yield stress. The apparent viscosity may also change as a function of time, upon application of a fixed shear rate, related to tire fonnation or breakup of particle networks. Thixotropic dispersions show a decrease in q, pp with time, whereas an increase witli time is called rheopexy. 

In a pseudoplastic mass the shear speed increases together with the shear stress and the viscosity decreases. This behaviour can be seen in solutions of guar gum and alginates. 

In some colloidal dispersions, the shear rate (flow) remains at zero until a threshold shear stress is reached, termed the yield stress (rY), and then Newtonian or pseudoplastic flow begins. A common cause of such behaviour is the existence of an interparticle or intermolecular network which initially acts like a solid and offers resistance to any positional changes of the volume elements. In this case flow only occurs when the applied stress exceeds the strength of the network and what was a solid becomes instead a fluid. 

Two of the most common empirical models used to describe the behaviour of pseudoplastic fluids with yield stresses are the Bingham plastic [377] model ... 

For practical purposes, casting slips are often characterized by so-called apparent viscosity. This characteristic is defined for any point on the rheological curve (Fig. 155) as a ratio of shear stress to the deformation rate at the given point. The inadequacy of this characteristic is demonstrated by Fig. 155 a dilatant and a pseudoplastic mix have the same apparent viscosity at a stress corresponding to the intersection of their rheological curves, despite their quite diverse rheological behaviours. However, determination of apparent viscosity may be useful, for example in routine qualitative inspection. ... 

Temporary viscosity loss All polymer solutions are non-Newtonian, that is, shear stress is not directly proportional to shear rate. There are several types of non-Newtonian behaviour, but VI improvers at high temperature exhibit only one pseudoplasticity, or shear-thinning. 

Figure 8.3 Basic types of rheological behaviour (a) Newtonian, (b)—(c) non-Newtonian /(b) shear thickening, (c) shear thinning, A) pseudoplastic. (e) plastic (Bingham plow), in which o0 is the yield stress and On is the Bingham yield stress/.

The most common type of time-independent non-Newtonian fluid behavioiu observed is pseudoplasticity or shear-thinning, characterised by an apparent viscosity which decreases with increasing shear rate. Both at very low and at very high shear rates, most shear-thinning polymer solutions and melts exhibit Newtonian behaviour, i.e. shear stress-shear rate plots become straight lines. 

A fluid with a linear flow curve for Ty > ro is called a Bingham plastic fluid and is characterised by a constant plastic viscosity (the slope of the shear stress versus shear rate curve) and a yield stress. On the other hand, a substance possessing a yield stress as well as a non-linear flow curve on linear coordinates (for Xyx > ro ), is called a yield-pseudoplastic material. Figure 1.8 illustrates viscoplastic behaviour as observed in a meat extract and in a polymer solution. 

Figure 1.8 Representative shear stress-shear rate data showing viscoplastic behaviour in a meat extract (Bingham Plastic) and in an aqueous carbopol polymer solution (yield-pseudoplastic)...

The slow reformation of the polymer structure observed with the decrease of the shear rate is expressed by a curve which describes in the same way (the low branch) this trend with repeating cycles. This behaviour of the PET melt suggests a pseudoplastic fluid, the tixotropic behaviour is evident only for the first cycle of stressing during the shearing of the PET melt through the capillary (Figure 3.249). 

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