Viscosity-time measurements

Gla.ss Ca.pilla.ry Viscometers. The glass capillary viscometer is widely used to measure the viscosity of Newtonian fluids. The driving force is usually the hydrostatic head of the test Hquid. Kinematic viscosity is measured directly, and most of the viscometers are limited to low viscosity fluids, ca 0.4—16,000 mm /s. However, external pressure can be appHed to many glass viscometers to increase the range of measurement and enable the study of non-Newtonian behavior. Glass capillary viscometers are low shear stress instmments 1—15 Pa or 10—150 dyn/cm if operated by gravity only. The rate of shear can be as high as 20,000 based on a 200—800 s efflux time. 

From the Arrhenius form of Eq. (70) it is intuitively expected that the rate constant for chain scission kc should increase exponentially with the temperature as with any thermal activation process. It is practically impossible to change the experimental temperature without affecting at the same time the medium viscosity. The measured scission rate is necessarily the result of these two combined effects to single out the role of temperature, kc must be corrected for the variation in solvent viscosity according to some known relationship, established either empirically or theoretically. 

The measurement of viscosity is important for many food products as the flow properties of the material relate directly to how the product will perform or be perceived by the consumer. Measurements of fluid viscosity were based on a correlation between relaxation times and fluid viscosity. The dependence of relaxation times on fluid viscosity was predicted and demonstrated in the late 1940 s [29]. This type of correlation has been found to hold for a large number of simple fluid foods including molten hard candies, concentrated coffee and concentrated milk. Shown in Figure 4.7.6 are the relaxation times measured at 10 MHz for solutions of rehydrated instant coffee compared with measured Newtonian viscosities of the solution. The correlations and the measurement provide an accurate estimate of viscosity at a specific shear rate. 

If the density does not change significantly with concentration, the specific viscosity tjsp — t/tg — 1 can be approximated from the times measured for the solution (f) and solvent (f0). The dimension of the bore diameter of the capillary needs to be chosen carefully to minimize kinetic corrections. A detailed discussion of correction effects and other pitfalls can be found in the book by Van Wazer et al. [24]. 

To start the experiment all the tubes are placed in a rack at the same time and allowed to warm to room temperature finally they are placed in a thermostat at 50 °C.The tubes are removed at intervals of 1 h and immediately cooled in an acetone/dry ice bath.The samples that are still fluid are diluted with approximately 50 ml of chloroform and dropped into about 500 ml of stirred heptane or petroleum ether. For the very viscous or solid samples 1-2 g are dissolved in 50-100 ml of chloroform and the solution is added dropwise to 500-1000 ml of heptane or petroleum ether with stirring.The polymers are filtered off and dried to constant weight in vacuum at 50 C.The yield, the limiting viscosity number (measured in chloroform at 20 °C) and the degree of polymerization are plotted against reaction time. 

In all cases, intrinsic viscosities were measured at 25 C in constant temperature baths controlled to +0.1°C or better, using suspended level Ubbelohde dilution viscometers with solvent flow times of at least 100 sec.. No kinetic energy corrections were made. Solution flow times were measured at four concentrations for each sample, and intrinsic viscosities were obtained from the classical double extrapolation of hg /c vs. c and (In hj.)/c vs. c to a single intercept value. Concentration ranges were varied somewhat with the molecular weights of the samples, but were chosen such that both functions were straight lines in all cases. 

The same apparatus was used, but quantities of paste were removed to give an air space in the vessel. On rapid agitation the volume increased, dependent on the air content required. Paste viscosities were measured, using a Stormer viscometer, which is a type of concentric cylinder viscometer. Although it is possible to obtain results in absolute terms, for comparative purposes the times for 100 revolutions of the rotor under a fixed applied torque were recorded. 

Dynamic techniques are used to determine storage and loss moduli, G and G respectively, and the loss tangent, tan 6. Some instruments are sensitive enough for the study of liquids and can be used to measure the dynamic viscosity rj. Measurements are made as a function of temperature, time, or frequency, and results can be used to determine transitions and chemical reactions as well as the properties noted above. Dynamic mechanical techniques for solids can be grouped into three main areas free vibration, resonance-forced vibrations, and nonresonance-forced vibrations. Dynamic techniques have been described in detail (242,251,255,266,269—279). A number of instruments are listed in Table 8. Related ASTM standards are listed in Table 9. 

No mechanistic significance can be attached to the change of order being observed in the present experiments and not in the earlier ones. The kinetic experiments with n-butylmagnesium compounds were carried out at monomer concentrations 10 times higher than permissible in an NMR experiment. This not only favoured the limiting condition of Equation 3, but increasing viscosity restricted measurements to ca. 40 percent conversion. The deviation from zero order kinetics at 225 in the present work was not apparent until this conversion was exceeded. 

Shear Degradation. Aqueous solutions of Polyox were agitated vigorously with a 10-blade stirrer turning at 1080 r.p.m. The solution viscosities were measured at various times during the stirring period. 

Meyerhoff (4) and Goedhart and Opschoor (5) have measured the viscosity of each eluting GPC fraction by coupling an automatic capillary tube viscometer with the GPC syphon. The low polymer concentration in each fraction necessitated an extremely accurate efflux time measurement to 0.01 second since the flow time of each fraction containing polymer has flow times, th greater than that of pure solvent, t0, by at most 2.00 seconds. The specific viscosity sv. of the ith polymer fraction is calculated from the flow times of the pure solvent and the polymer fraction. 

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