Rheometer types

Fora farther discussion of a frequency sweep test, see UNITH3.1. The range of frequencies used here and in step 5 will depend on both the sample and the rheometer type. 

A paper entitled Obtaining Flow Properties Directly from an Extruder [47] details a technique for measuring rheometer-type data using extruders on the plant floor. It describes how extrudate samples, collected from an extruder in a matter of minutes, are used to measure throughput. These measurements are then used along with processing values such as screw speed and head pressure to calculate the polymer s viscosity and power law index. 

Rheological experiments were carried out on a stress-controlled rheometer type CVO-120 (Bohlin, Germany) with cone-plate geometry, cone-angle a = 1 , cone radius R = 20 mm. 

Currently, two general rotational rheometer types [29] are available commercially, that can be used for LAOS experiments controlled strain rheometers (also known as separated motor-transducer, SMT) and controlled stress (or combined motor-transducer, CMT) rheometers. Apart from the rotational rheometers, custom built instruments, such as the sliding plate rheometer by Giacomin et al. [13] are also suitable, but will not be discussed here, as they are not available commercially. 

Another type of rotational viscometer is the hehcal-screw rheometer (176). This iastmment is basically a screw-type metering pump that does not pump. The measure of force is the pressure difference resulting from the rotational motion. It is possible to use a bank of pressure transducers of different sensitivities to measure viscosity over a wide range. The iastmment can be used for high temperature rheometry and to foUow polymerkation, shear and heat degradation, and other developments. 

A capillary rheometer is another type of instmment, in which the uncured mbber is extmded through a small orifice and the change in dimensions of the extmdate is measured with a laser [2]. This instmment generates high shear rates, compared to Mooney rheometer. The capillary rheometer can thus represent flow of compounds on mbber processing machinery, such as injection molds. 

Table 5. Results from shear degradation measurements in two different rheometers (capillary, Couette-type) compared with model predictions of narrowly distributed polystyrene samples in toluene... 

A particular type of rheometer developed by Monsanto, now known as Alpha Technologies. The mechanical portion of the rheometer consists of a biconical disc embedded in an elastomeric specimen contained in a specially designed... 

Rheological Measurements Three types of rheological measurements have been carried out. In the first type, transient (creep) measurements were performed on a 20% w/w dispersion of latex A, as a function of coverage by PVA. These experiments were carried out using a "Deer" rheometer (PDR 81, Integrated Petronic Systems, London) fitted with a stainless steel concentric cylinder. The procedures used have been described in detail before (21,22). 

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). 

Figure 3.6 Solution capillary rheometers a) Ubbelohde-type rheometer (courtesy of Cannon Instrument Company, USA), and b) a schematic of an Ostwald viscometer...

The rheometer most often used to measure viscosity at low shear rates is the cone and plate viscometer. A schematic of a cone and plate rheometer is found in Fig. 3.24. The device is constructed with a moving cone on the top surface and a stationary plate for the lower surface. The polymer sample is positioned between the surfaces. Two types of experiments can be performed the cone can be rotated at a constant angular velocity, or it can be rotated in a sinusoidal function. The motion of the cone creates a stress on the polymer between the cone and the plate. The stress transferred to the plate provides a torque that is measured using a sensor. The torque is used to determine the stress. The constant angle of the cone to the plate provides an experimental regime such that the shear rate is a constant at all radii in the device. That is, the shear rate is independent of the radial position on the cone, and thus the shear stress is also independent of the position on the cone. 

The mechanical properties of actin filament networks depend on the manner in which actin monomer is prepared and stored, as well as how they are polymerized conditions. Differences in mechanical properties are not the consequence of using two different types of forced oscillatory rheometers. Xu et aid found that filaments assembled in EGTA and Mg from fresh, gel-filtered ATP-actin monomer (1 mg/mL) have an elastic storage... 

At this point it should be noted that the conclusion drawn from flow birefringence measurements, viz. that p22 — p33 of polymer systems is very small compared with pn — pn is not always supported by other types of measurement. With the aid of pressure measurements in the walls of various rheometers (e.g. cone-and-plate apparatus) results have been obtained by a number of authors (refs. 26, 43, 44), showing that p23 — p33 should be positive and can have values up to 20 per cent of Pn Pta- 1-7 suggests for the investigated polyisobutylene solution... 

In the second type of rheometer, the control-led-stress device, the motor is optimized to... 

Basic Protocol 2 is for time-dependent non-Newtonian fluids. This type of test is typically only compatible with rheometers that have steady-state conditions built into the control software. This test is known as an equilibrium flow test and may be performed as a function of shear rate or shear stress. If controlled shear stress is used, the zero-shear viscosity may be seen as a clear plateau in the data. If controlled shear rate is used, this zone may not be clearly delineated. Logarithmic plots of viscosity versus shear rate are typically presented, and the Cross or Carreau-Yasuda models are used to fit the data. If a partial flow curve is generated, then subset models such as the Williamson, Sisko, or Power Law models are used (unithi.i). 

This section describes common steps designed to measure the viscosity of non-Newtonian materials using rotational rheometers. The rheometer fixture that holds the sample is referred to as a geometry. The geometries of shear are the cone and plate, parallel plate, or concentric cylinders (Figure HI. 1.1). The viscosity may be measured as a function of shear stress or shear rate depending upon the type of rheometer used. 

The first second of a stress relaxation step can also show this type of ringing, but it is generally caused by the transducer itself. Thus, the first part of the data may be electronically filtered to remove the transducer ringing by setting a filter cutoff frequency of -40% of the value for the resonant frequency of the transducer and geometry. Some rheometers allow for the measurement of transducer resonant frequency when measuring the geometry inertia. 

It was of interest to compare the results obtained with the FRAP technique with those obtained with classical surface rheological techniques. Our detailed knowledge of properties of solutions of /3-lg containing Tween 20 made this an ideal system on which to compare the methods. Firstly, surface shear viscosity measurements were performed on the Tween 20//3-lg system [47] using a Couette-type torsion-wire surface rheometer as described previously [3,48]. All the experiments were carried out at a macroscopic n-tetradecane-water interface at a fixed protein concentration of O.Olmg/ml. In the absence of Tween 20, the surface shear... 

The flow properties by extrusion rheometer MCR type (11) were determined at 180° and 200°C in the shear rate range of 1.76 to 1760 sec"1. 

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