Viscosity shear rate effects

Although shear rate effects are more pronounced in good solvents, the intrinsic viscosity decreases with shear rate even in 0-solvents, where excluded volume is zero (317,318). The Zimm model employs the hydrodynamic interaction coefficients in the mean equilibrium configuration for all shear rates, despite the fact that the mean segment spacings change with coil deformation. Fixman has allowed the interaction matrix to vary in an appropriate way with coil deformation (334). The initial departure from [ ]0 was calculated by a perturbation scheme, and a decrease with increasing shear rate in 0-systems was predicted to take place in the vicinity of / = 1. 

The rheological properties of plastisols generally could not be characterized on the basis of viscosity at some fixed shear rate (effective viscosity). Nevertheless, for practical purposes plastisols are conventionally classified into low-viscous (1-3 Pa s), medium-viscous (10-15 Pa s), and high-viscous (100-1000 Pa s) plastisols. Viscosities are measured at low shear rate y = 1 s l when plastisol flow is Newtonian. 

The shear-rate effect on the viscosity of thermosetting resins, r]Jff), is also essential to the determination of the chemoviscosity, r]. The exact relationship will depend on the type of... 

The effect of overall molecular weight or the number of blocks on rheological properties for the samples from the second fractionation can be illustrated as a plot of reduced viscosity vs. a function proportional to the principal molecular relaxation time (Figure 2). This function includes the variables of zero shear viscosity, shear rate, y, and absolute temperature, T, in addition to molecular weight, and allows the data to be expressed as a single master curve (10). All but one of the fractions from the copolymer containing 50% polystyrene fall on this... 

Melt viscosity increases as the molecular weight of the polystyrene blocks is increased, but the effect tends to diminish as the rate of shear is increased. The influence of block size is expressed as a family of converging viscosity—shear rate curves for three copolymers of differing block size, (Figure 3). These curves also illustrate the non-Newtonian character of the polystyrene-polydimethylsiloxane block copolymers. The effect of changing block size cannot be expressed as a single master curve as in the case of overall molecular weight. Such master curves must be based on polymers of constant block size. 

The melt viscosity data were obtained in a conventional manner using an Instron capillary rheometer over the temperature range of 180-240°C. The capillary had a 90° entry angle, and it was 5 cm long and 0.125 cm in diameter. The well known equations were used to calculate apparent viscosity. Shear rates at the wall, calculated assuming a Newtonian fluid, were corrected for nonparabolic velocity profile using the Rabinowitch equation. No correction was made for entrance effect because of the length-to-diameter ratio of the capillary used. 

Unlike hard, noninteractive particles, however, shear rate affects viscosity as a result of bond breakage of the large fractal aggregates during viscosity measurement [18]. Hence, the viscosity values of this derivation should be considered the true viscosity of solution with negligible shear rate effects. 

Figure 7. Effect of sodium chloride on the viscosity-shear rate relationship.

Figure 19. Effect of Neodol 25-3S on the viscosity-shear rate reiationship.

Adding a non-ionic species (Triton X-100) had no significant effect on the viscosity-shear rate relationship. However, adding an ionic species (sodium chloride, calcium chloride, or an anionic surfactant) reduced the hydrodynamic size of the polymer molecule (physical change), changing the viscosity-shear rate relationship. 

The main purpose of a capillary rheometer is to generate a viscosity-shear rate curve over as wide a range of shear rates as possible. The effect of temperature on the viscosity can also be determined by running curves across a range of temperatures. The selection of specific dies and pressure transducers tailor the shear rate range to match a desired process window. This information can be used in many ways. 

Flow properties of epoxide oligomers were reported by several workers. Aleman [2,3] discussed shear-rate effects on the shear viscosity measured by an Instron rheometer and found that each epoxide oligomer studied had Newtonian behavior up to the shear rate of 2000 sec Ghijsels et al. [4] reported that the temperature dependence of the zero-shear viscosity was... 

The flow domain of interest has to be subdivided into a number of volumes, or cells, to form the computational grid. The conditions at all the boimdaries of the grid must then be specified—f/ze boundary conditions, e.g. in-flow, outflow, no-slip, velocity or pressure. The next step is to specify the properties of the liquid flowing through the domain—this is called the constitutive equation, which might be as simple as a power-law description of viscosity/shear-rate or might be as complicated any of the most advanced models that account for viscoelasticity and time effects such as thixotropy, see below. The completion of these three steps makes it possible to solve the flow of the liquid throughout the domain. 

This unusual effect is a property of only a few select water-soluble polymers, among which are the extensive family of acrylamide polymers and copofymers. The exact flow mechanism which causes this resistance factor has not been established but it does appear to be a complex combination of several factors. Although polymer solutions are generally highly non-Newtonian, this is not the only factor. The resistance factor is substantially constant over normal field fluid advance rates as shown in Fig. 2, which also shows the slope line and range of the viscosity-shear rate data from 4.8 to 960 sec determined with a Fann viscometer. Since the shear... 

---