Viscosity-temperature measurements

Currently, the dependence of t on temperature is deduced from viscosity-temperature measurements. At T < T, the temperature dependence of T obeys an Arrhenius law, but this dependence is much more complex at T > T. In the latter case it is referred to an empirical Vogel-Tamman-Fulcher (VTF) law (Vogel, 1921 Tamman and Hesse, 1926 Fulcher, 1925). 

Table 2. Comparison of the energy contributions obtained in thermoelastic (f /f) and thermomechanical (AU/W)v,x measurements and values of din O /dT calculated from the energy contributions and values of viscosity-temperature measurements on isolated chains...

Thermoelasticity results are also used to test some of the assumptions used in the development of the molecular theories. The results [72] indicate that the ratio f /f is essentially independent of the degree of swelling of the network, and this supports the postulate made in Section 1.1.4 that intermolecular interactions do not contribute significantly to the elastic force. The assumption is further supported by results [72] showing that the values of the temperature coefficients of the unperturbed dimensions obtained from thermoelasticity experiments are in good agreement with those obtained from viscosity-temperature measurements on the isolated chains in dilute solution. 

Fig. 12. Viscosity at different temperatures measured by a capillary viscometer injection-molding grade of poly(methyl methacrylate) (43). To convert N/m to psi, multiply by 145 to convert (N-s)/m to (dyn-s)/cm (P), multiply by 10.

The Ferranti-Shidey viscometer was the first commercial general-purpose cone—plate viscometer many of the instmments stiU remain in use in the 1990s. Viscosities of 20 to 3 x 10 mPa-s can be measured over a shear rate range of 1.8-18, 000 and at up to 200°C with special ceramic cones. Its features include accurate temperature measurement and good temperature control (thermocouples are embedded in the water-jacketed plate), electrical sensing of cone—plate contact, and a means of adjusting and locking the position of the cone and the plate in such a way that these two just touch. Many of the instmments have been interfaced with computers or microprocessors. 

Viscosity. The measurement procedure for API funnel viscosity is the same as for water-base muds. Since temperature affects the viscosity, API procedure recommends that the mud temperature should always be recorded along with the viscosity. 

Viscosity is normally measured at two different temperatures typically 100°F (38°C) and 210°F (99°C). For many FCC feeds, the sample is too thick to flow at 100°F and the sample is heated to about 130°F. The viscosity data at two temperatures are plotted on a viscosity-temperature chart (see Appendix 1), which shows viscosity over a wide temperature range [4]. Viscosity is not a linear function of temperature and the scales on these charts are adjusted to make the relationship linear. 

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. 

There s another reason why the computed solution average temperature had decreasing accuracies in Tests 1, 2 and 3 respectively. The reason is that we started with increasingly viscous solutions, which caused the response time of the temperature measurement to increase rapidly. This response time becomes even more significant because as the solution viscosity increases there are significant rises in the reaction rates and temperatures. 

A similar experiment was conducted using N-299 carbon black. In this case the premastication was limited to 3 min of mixing time. The average batch temperature measured after this mixing operation was 309°F. Each experiment was performed in duplicate the average of two mixes is shown in Figure 16.6. The viscosity of the final control compound was similar to that of the premasticated mbber. 

Sensor-based methods. Whilst many methods use sensors, the simplest being temperature measurement, this terminology is often used to cover viscosity, pH, oxygen and humidity determination, etc. These are true in-line techniques and offer rapid, inexpensive real-time analysis. Humidity determination in drying ovens is a common example. 

Rheological measurements. Routine viscosity measurements were made with a Wells-Brookfield micro-cone and plate viscometer, or a Brookfield LVT(D) viscometer with UL adapter. Viscosity-temperature profiles were obtained using the latter coupled via an insulated heating jacket to a Haake F3C circulator and PG100 temperature programmer or microcomputer and suitable interface. Signals from the viscometer and a suitably placed thermocouple were recorded on an X-Y recorder, or captured directly by an HP laboratory data system. 

The latter point is illustrated by the surface shear viscosities of the homochiral and heterochiral films at surface pressures below the monolayer stability limits. Table 7 gives the surface shear viscosities at surface pressures of 2.5 and 5 dyn cm -1 in the temperature range given in Fig. 19 (20-40°C). Neither enantiomeric nor racemic films flow under these conditions at the lower temperature extreme, while at 30°C the racemic system is the more fluid, Newtonian film. However, in the 35-40°C temperature range, the racemic and enantiomeric film systems are both Newtonian in flow, and have surface shear viscosities that are independent of stereochemistry. These results are not surprising when one considers that (i) when the monolayer stability limit is below the surface pressure at which shear viscosity is measured, the film system does not flow, or flows in a non-Newtonian manner (ii) when the monolayer stability limit is above the surface pressure... 

Suitable conversion tables are available (ASTM D341), and each table or chart is constructed such that for any given petroleum or petroleum product, the viscosity-temperature points result in a straight line over the applicable temperature range. Thus, only two viscosity measurements need be made at temperatures far enough apart to determine a line on the appropriate chart from which the approximate viscosity at any other temperature can be read. 

Variable temperature measurements were carried out on a Varian A-60 spectrometer with a Varian-4060 probe heater attachment. Probe temperatures were calibrated with a standard ethylene glycol sample. Samples were prepared by dissolving up to 25 per cent by weight of polymer in tetrachloroethylene. Even with such concentrated solutions no viscosity problem was encountered above 60°. 

This study addresses two questions 1) Is polymer aggregation in solutions directly related to solvent quality 2) If not, does solvent quality exert an effect on the viscosity of semidilute solutions separate from the effect of aggregation The copolymer poly(vinylbutyral) (PVB) was chosen for this investigation. PVB is known to aggregate in several solvents (IS). Light scattering and intrinsic viscosity measurements were used to assess solvent quality. Viscosities were measured at one concentration in three solvents and temperatures from 25 to 55 C. 

Fiq. 33. Vari tlon of solvent viscoiity with changing temperature and composition of water-methanol mixtures. The composition is expressed as percent methanol (v/v) at 20.S°C and the viscosity is in centipoise. The temperature at which the viscosity was measured is indicated below each curve. The data are taken riuin Colin er ul, 142). 

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. 

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. 

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