In-line rheometer

An alternative approach is to measure two stresses. Lodge (1988) has shown how the pressure hole method can be used as an in-line rheometer. N is measured in slit flow by two opposed pressure transducers as shown earlier (Figure 6.3.5). A third, recessed transducer downstream is used to get the shear stress. Together this gives N (r), which is very sensitive to polymer concentration and molecular weight. Padmanabhan and Bhattacharya (1993) have proposed a combination of entrance pressure drop and wall shear stress. 

In the first experiments, an in-line rheometer die with a defined slit geometry was used, which enables the measurement of the melt viscosity during the tests. The pressure level in the in-line rheometer die can be varied by means of an adjustable throttle at the end of the die. A photograph of the extrusion set-up with the in-line rheometer die is shown in Figure 2. 

Figure 3. Micrograph of a sample produced with the in-line rheometer die...

To reduce the time needed for fluid evaluation, a new testing method and an associated Instrument have been developed. This method saves time by utilizing an automated, continuous, variable temperature, single-pass, pipe rheometer capable of adding additives In-line. This eliminates both the need for the large number of Individually prepared samples and a large part of the time spent using the conventional cylindrical rheometer. 

Figure 10.8 The density of network junctions as a function of the volume fraction of paraffinic oil in EPDM/oil vulcanisates [74], The solid line represents the result of a linear regression analysis of the dependence (intercept = 453 5 mmol/kg slope = -6.2 0.0.3 mmol/kg the correlation coefficient = 0.996). Maximum torque in the rheometer curve for the vulcanisates is shown on the right ordinate...

For mobile liquids, the use of this kind of controllable instruments is practically impossible. For these liquids, the non-controllable measurement techniques are available only and in general an apparent transient viscosity will be obtained. Nevertheless these measurements are still of great value, because in many cases they approximate industrial process conditions. Mostly used is the spinning line rheometer, where an elastic liquid is pressed through a spinneret and the liquid is pulled from the die by winding the filament around a rotating drum or by sucking the tread into a capillary tube. This is schematically shown in Fig. 15.25. A serious problem is the translation of the obtained data to the extensional viscosity. Many other non-controllable devices are discussed by,... 

We will finish this Section with some interesting experimental results on dilute polymer solutions. In Fig. 16.23 extensional viscosity data are shown, obtained from a spin-line rheometer on a solution of polybutadiene in decalin (Hudson and Ferguson, 1976). Here rie not only shows normal behaviour, starting with an increase, followed by a substantial decrease, but also at very high extensional rates a substantial increase. It is not clear yet whether this ultimate tension thickening also occurs for stiff polymeric systems. Concentration dependent extensional viscosities are presented in Fig. 16.24 for 0.0125-0.75%... 

FIG. 16.23 Extensional viscosity curve for a 6.44% solution of polybutadiene in decalin determined with a commercial spin-line rheometer. Analogous to Hudson and Ferguson (1976). 

An in-line measurement is performed in a process line an on-line measurement is performed in a bypass loop from the main process line and the food may be returned to the main process line after measurement is performed. A near-line measurement is performed on a sample taken from a process line which is often discarded after measurement. Because foods are complex materials (e.g., suspensions, emulsions, gels), stmctural changes may take place during sampling (e.g., flow through a valve) for on-line and near-line measurements (Roberts, 2003). Nevertheless, in principle, the previously described capillary flow, and rotational concentric cylinder, plate-cone, and mixer viscometers may be used for in-line, on-line, and near-line measurements. In this respect, Tamura et al. (1989) proposed a helical screw rheometer as an on-line viscometer. The empirical measurement methods described previously are used primarily in near-line measurements. 

GBC Scientific Equipment offers a Micro Fourier Rheometer, MFR 2100. The rheometer applies a squeezing motion to the sample, performing analyses on sample volumes less than 100 p.1. An automated sample injection system is included in the instrument design. The rheometer is capable of handling low-viscosity fluids, 1 mPas, with storage modulus measurement down to 10-4 Pa. Using a different measurement technique, both benchtop and in-line, the real-time ultrasonic rheometer and fluid characterization device uses spatially resolved ultrasonic... 

There are notable exceptions, however, such as the CEP Lodge Stressmeter . This rheometer is unique in that it measures both viscosity and first normal stress difference, Nl. Both laboratory and in-line configurations are available. The instrument is shown in Figure 4.13. The Lodge Stressmeter is applicable to low-viscosity systems and available commercially from Chemical ElectroPhysics Corp. (Delaware, NJ). 

Rheometry is the most widely used on-line monitoring technique. Rheometers of different types have been developed for use in closed loop process control. Commercially available instm-ments all use steady state shear deformation. One commercial instrument attempted to operate in the dynamic mode, Rheometrics On-line Rheometer (ROR) but is not currently commercialized. In dynamic mode, the frequency dependence... 

A capillary rheometer (Fig. 6.8) is similar to a melt indexer. The main difference is that instead of by gravity, the piston is driven by a variable speed motor. Also, a load cell in-line with the piston measures ram force in real time. This configuration allows tests to be run at a controlled shear rate, and even to vary the rate over a large range during a single test run. The piston force and the orifice geometry provide the data necessary... 

Another current development is the use of in-line capillary rheometers. Two main types available are those that can be used by polymer or compound producers and those for use by processors such as in extrusion of polymers (Figure 9.10). Granular or powder-fed types can automatically sample materials separated from the main production stream. Response time can be very similar to that of an extruder-mounted, melt-fed rheometer, and unlike an... 

Schematic diagram of the Carri-med Spin Line Rheometer (Jones et al., 1987). Air pressure, A forces test liquid the reservoir B through tube C to the spinning nozzle D, where it is drawn down by the rotating drum E. Liquid is scraped from the drum by F and falls into the beaker G. The feed tube C also acts as a force spring, whose deflection is measured by LVDT, H. Fiber diameter is reccxrded with a video camera near J. The whole apparatus is contained in an environmental chamber.

There are two basic types of process line rheometer those that can operate directly in the process and those that pull a side stream off the process for analysis. The side stream may be either dumped or recycled back to the process. 

The Haake ProFlow on-line rheometer, which was designed for measuring the melt viscosity and flow index, was side-mounted at the end of a single screw extruder with a diameter of 20 mm and the melt shear viscosity of each resin was measured online at 250°C. The values of the shear rates inside the channel of conventional screw element and in the barrier clearance of the fluted mixing element used in this work, whieh are approximated by the Couette shear rate, are calculated to be about 10 s and 100 s, respectively. The viscosity ratios of the dispersed phase to the continuous phase for used PP/PA6 blend at these two shear rates are about 2.6 and 4.2, respectively. 

The decomposition generates amine and MBTS. SmaH quantities of MBT and benzothiazyl (Bt) polysulftdes form just before the onset of cross-linking. Sulfur disappears and large quantities of MBT are formed during the main cross-linking reaction. The development of cross-links is shown as the rheometer line (curve C) in Figure 3. 

Chocolate does not behave as a tme Hquid owing to the presence of cocoa particles and the viscosity control of chocolate is quite compHcated. This non-Newtonian behavior has been described (28). When the square root of the rate of shear is plotted against the square root of shear stress for chocolate, a straight line is produced. With this Casson relationship method (29) two values are obtained, Casson viscosity and Casson yield value, which describe the flow of chocolate. The chocolate industry was slow in adopting the Casson relationship but this method now prevails over the simpler MacMichael viscometer. Instmments such as the Carri-Med Rheometer and the Brookfield and Haake Viscometers are now replacing the MacMichael. 

An Ostwald viscometer is similar to an Ubbelohde-type rheometer except that it is simpler in design and is less expensive. A schematic of an Ostwald viscometer is shown in Fig 3.6(b). It is characterized by a lower bulb that acts as a solution reservoir. A solution of known polymer concentration is placed in the lower bulb. A single capillary tube in which the measurement is taken is connected to the bottom of the bulb and to two small bulbs at the top of the capillary. Fluid is forced from the lower bulb through the capillary into the two small bulbs attached to the top of the capillary. There is a line between the two bulbs and at the exit of the lower bulb. The fluid is then allowed to drain back into the lower bulb through the capillary, and the time for the fluid to travel between the two lines is recorded. The time, if there were no end effects, is proportional to the kinematic viscosity (/j/p). 

The data from step-change tests is modeled mathematically to fit one or more exponential time constants depending upon the curvature of the line. Each exponential time constant is related to a relaxation time in the sample response. Most rheometers have software that automatically fits the data to these models. 

---