Viscosity curve

The U.S. Bureau of Mines has employed glass for forming ceramic materials at high temperatures (75). The viscosity curve for a soda—lime—siUca glass in Figure 19 indicates the high viscosity available at hot forming temperatures. 

The viscosity of sulfonation and sulfation reaction mixtures increases with conversion, often producing extremely high viscosities. Figure 1 provides temperature—viscosity curves for oleum and SO -detived products. Sulfonation process design must accommodate such viscosities. 

From this relatively simple test, therefore, it is possible to obtain complete flow data on the material as shown in Fig. 5.3. Note that shear rates similar to those experienced in processing equipment can be achieved. Variations in melt temperature and hypostatic pressure also have an effect on the shear and tensile viscosities of the melt. An increase in temperature causes a decrease in viscosity and an increase in hydrostatic pressure causes an increase in viscosity. Topically, for low density polyethlyene an increase in temperature of 40°C causes a vertical shift of the viscosity curve by a factor of about 3. Since the plastic will be subjected to a temperature rise when it is forced through the die, it is usually worthwhile to check (by means of Equation 5.64) whether or not this is signiflcant. Fig. 5.2 shows the effect of temperature on the viscosity of polypropylene. 

Fig. S.17 Viscosity Curves for a Range of Plastics (230°C unless otherwise shown)...

As mentioned previously, and as expected, the mixtures containing a TLCP component exhibited shear viscosities lower than those of their original components, whether the TLCP weight fraction was 10% or 30%. The higher the TLCP weight fraction, the lower the blend viscosity. The viscosity curves of the two pure components crossed each other at a shear rate about 25 s , i.e., at this point the viscosity ratio was 1. 

Figure 4 Experimental viscosity curves of PES, TLCP, and their blends.

Figure 5 Comparison of theoretical viscosity curves with experimental value points of PES/TLCP blends for fixed shear stress t.

Figure 11 Viscosity curves of PC, TLCP, and their blends with respect to the test temperatures ( from WLF equation).

From the y(jc) functions and the two melt temperatures used, and by using the viscosity curves from rheological examinations (Fig. 11), viscosity distributions T](jc) of the two pure components were easily determined, as shown in Figs. 15a and 15b. Subsequently, the viscosity ratio functions 6(jc) were also calculated (Fig. 16). All four curves fall slightly from the core to the outside. 

The Ml test equipment is easy to operate, provides repeatable results, and low cost to operate. It is widely used for quality control and for distinguishing between members of a single family of plastics. Specifically, this MI makes a single-point test that provides information on resistance to flow at only a single shear rate. Because variations in branching or MWD can alter the shape of the viscosity curve, the MI may give a false ranking of plastics in terms of their shear rate resistance to flow. To overcome this problem, extrusion rates are sometimes measured for two loads, or other modifications are made. 

The measurements are carried out at preselected shear rates. The resulting curves are plotted in form of flow-curves t (D) or viscosity-curves ti (D) and give information about the viscosity of a substance at certain shear rates and their rheological character dividing the substances in Newtonian and Non-Newtonian fluids. 

Pectins in aqueous solutions show pseudoplastic non-thixotropic behaviour, independent of their degree of methoxylation. Figure 1 shows the viscosity curve of a 2,5 % pectin solution, sheared the preselected shear rate-time function. The viscosity curves for the increasing and decreasing shear rate are superimposed. The pseudoplasticity of pectin solutions decreases with decreasing concentration. 

The value of critical concentration depends strongly on the pectin being used. Figure 2 gives the viscosity curves of two different pectins at the same concentration of 2.5 % w/w. The different production parameters, that have been used for these pectins, have strongly influenced their flow behaviour. The enzymatic reduction of the molecular weight down to... 

Figure 2. Viscosity curves of pectins with different molecular weight (2.5 % sol)...

A plot of Tvs. G yields a rheogram or a flow curve. Flow curves are usually plotted on a log-log scale to include the many decades of shear rate and the measured shear stress or viscosity. The higher the viscosity of a liquid, the greater the shearing stress required to produce a certain rate of shear. Dividing the shear stress by the shear rate at each point results in a viscosity curve (or a viscosity profile), which describes the relationship between the viscosity and shear rate. The... 

For pH<7, the hydrolysis is very fast ( see figure 2) and a limiting value of viscosity can be reached after 1 month but the polymer is slightly ionized the product ar passes through a maximum which also induces a maximum in the viscosity curves. 

Shear rate and viscosity curves were obtained at 25°C using Contraves low shear viscometer (Model LS-30). 

Viscosity Behavior. The polymeric nature of triorganotin fluorides dissolved in nonpolar solvents is outlined in the introduction. As a result of the transient polymer formation, these solutions exhibit nonlinear concentration vs. viscosity curves. 

The plot of viscosity versus shear rate is shown in Fig. 3-3b, in which the line represents Eq. (3-24), with n = 0.77 and m = 1.01 dyn s /cm2 (or poise ). In this case the power law model represents the data quite well over the entire range of shear rate, so that n = n is the same for each data point. If this were not the case, the local slope of log T versus log rpm would determine a different value of n for each data point, and the power law model would not give the best fit over the entire range of shear rate. The shear rate and viscosity would still be determined as above (using the local value of n for each data point), but the viscosity curve could probably be best fit by some other model, depending upon the trend of the data (see Section III). 

Consider each of the fluids for which the viscosity is shown in Fig. 3-7, all of which exhibit a structural viscosity characteristic. Explain how the structure of each of these fluids influences the nature of the viscosity curve for that fluid. 

ARe>s is the Reynolds number based on the solvent properties, /zs is the solvent viscosity, D is the pipe diameter, F is the velocity in the pipe, and A is the fluid time constant (from the Carreau model fit of the viscosity curve). 

Note that err = y (crr)a3/k Tand recall that in a concentrated dispersion the Peclet number is Pe = 67ry (crr)a3/k T. The use of the suspension viscosity implies that the particle diffusion can be estimated from an effective medium approach. Both Krieger and Cross gave the power law indices (n and m) as 1 for monodisperse spherical particles. In this formulation, the subscript c indicates the characteristic value of the reduced stress or Peclet number at the mid-point of the viscosity curve. The expected value of Pec is 1, as this is the point at which diffusional and convective timescales are equal. This will give a value of ac 5 x 10 2. Figure 3.15 shows a plot of Equation (3.57a) with this value and n = 1... 

Polyelectrolytes provide excellent stabilisation of colloidal dispersions when attached to particle surfaces as there is both a steric and electrostatic contribution, i.e. the particles are electrosterically stabilised. In addition the origin of the electrostatic interactions is displaced away from the particle surface and the origin of the van der Waals attraction, reinforcing the stability. Kaolinite stabilised by poly(acrylic acid) is a combination that would be typical of a paper-coating clay system. Acrylic acid or methacrylic acid is often copolymerised into the latex particles used in cement sytems giving particles which swell considerably in water. Figure 3.23 illustrates a viscosity curve for a copoly(styrene-... 

If we extrapolate this slope toward low rates, where the tangent equates with the low shear viscosity and assume the Peclet number here is unity we eventually obtain a value of b = 2.55. This defines the Peclet number as unity, at a stress somewhere just after the curvature of the viscosity curve deviates from the low shear limit. This seems quite an appropriate reference system. By setting the Peclet number to the appropriate value of b we can determine the variation in packing fraction with stress between the high and low shear limits to the viscosity ... 

As expected, when the degree of neutralization of PAA is 0%, the specific viscosity falls rapidly with addition of polybase. For degrees of neutralization 3 and 6% we also get viscosity curves characteristic of a compact complex formation. For the values of a higher than 10% the linear variation is due to the addition of PEO without complex formation. These results agree with the potentiome-tric ones for instance in both cases no point characteristic of a fixed stoichiometry is observed. 

---