Viscosity plateau

The disadvantage of the power law model is that it cannot predict the viscosity in the zero-shear viscosity plateau. When the zero-shear viscosity plateau is included, a nonlinear model must be specified with additional fitting parameters. A convenient model that includes the zero-shear viscosity and utilizes an additional parameter is the Cross model [30] ... 

The viscosity, plateau modulus, limiting compliance and maximum (terminal) relaxation time derived firom the basic D-E model are power laws of the molecular weight M ... 

Prud homme are about a factor of three larger than the predictions of Eq. (9-55), if Y() and C (Newtonian viscosity plateau at low shear rates, while Eq. (9-55) predicts yield behavior at low shear rates, with a power-law viscosity-shear rate slope of—1. The emulsions of Otsubo and Prud homme are evidently affected to some extent by Brownian motion, which is not accounted for in Eq. (9-55). Further experimental and theoretical work on emulsion rheology will be required to establish general scaling rules for these complex emulsions. 

Results given in Table VII show that the viscosity versus shear rate variation of these three Santoprene grades fits the power law over the entire range from 10 to 5200 s . Both the viscosity and the extrudate swell at constant shear rate increase with decreasing rubber particle content (or increasing hardness). Thus, these olefinic thermoplastic vulcanizates essentially behave like highly filled fluids in flow. When compared with unvulcanised rubber (see previous sections) or polypropylene - EPDM blends (32), the extrudate swell appears low and there is no Newtonian viscosity plateau. 

Polymer melts are in most cases viscoelastic, meaning the viscosity is dependent oti measurement conditions, like frequency or shear rate. Most polymer melts show a viscosity plateau at low shear rates, called Newtonian plateau and the viscosity drops with increasing shear rate. 

It was found that complete solvation of both polymers was not obtained when the apparent viscosity plateau was used as the sole criterion for solvation. A more definitive standard for determining polymer solvation should also include tests that would assure that partially dissolved polymer agglomerations could not be removed from the fluid during its application. In the work described here, viscosity measurements were supplemented by filtration testing and particle settling experiments. 

When solutions of fully water-soluble Type 1 polymers are investigated using a flow curve experiment, it is possible to see a classic double plateau profile in the viscosity versus shear rate profile. An upper Newtonian viscosity plateau exists at very low shear rates. This is... 

In a creep experiment, a small constant stress is applied to the sample and the resultant displacement and/or deformation is monitored versus time. Due to the low stress being applied, the resultant shear rate can be in the region of the upper Newtonian viscosity plateau and simple viscous flow will be observed. In other systems, an instantaneous elastic response can be followed by viscoelastic behaviour and, eventually, Newtonian flow. 

The answer to this problem is to tighten the level of scrutiny with which the instrument decides if steady state is achieved, and then the normal viscosity plateau is seen. 

Dutta [27] has shown that for most thermoplastic melts lossessing a low-shear Newtonian viscosity plateau), a reasonably good estimate of zero-shear viscosity can be obtained from the knowledge of MFI using Eq. (4.12). 

A Newtonian shear behavior for the PVP solutions can be obtained at low shear rates, while higher concentrated PVP K30 solutions, for example, 45 m%, show shear thinning behavior at shear rate higher than 1000 s . The extensional viscosities of K30 are depicted in Fig. 19.1. The plateau value for the elongational viscosity at higher strain rates is characterized by the Trouton factor for Newtonian fluids where the extensional viscosity (plateau value) scales by a constant factor concerning the average shear viscosity. The theoretical values about three are represented by dashed lines. 

Notice in Fig. 11.21 that as the dimensionless strain rate e increases above unity, the viscosity plateau decreases. This is because the stress has reached a saturation value set by the backbone tension required to pull the arms into the backbone tube. Thus, as the strain rate is increased beyond this point, since the stress is constant, the steady-state viscosity decreases, since the viscosity is stress divided by strain rate. A local maximum in the extensional viscosity as a function of strain rate is indeed commonly observed for long-chain-branched polyethylenes see Fig. 11.23. 

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