Viscosity instrumentation rotational rheometer

Rheometer Any instrument designed for the measurement of non-Newtonian as well as Newtonian viscosities. The principal class of rheometer consists of the rotational instruments in which shear stresses are measured, and a test fluid is sheared between rotating cylinders, plates, or cones. Various types of rotational rheometers are concentric cylinder, cone-cone, cone—plate, double cone—plate, plate—plate, and disc (16). 

Rotational rheometer n. An instrument for measuring the viscosity of molten polymers (any many other fluid types) in which the sample is held at a controlled temperature between a stator and a rotor. From the torque on either element and the relative rotational speed, the viscosity can be inferred. The most satisfactory type for melts is the cone-and-plate geometry, in which the vertex of the cone almost touches the plate and the specimen is situated between the two elements. This provides a uniform shear rate throughout the specimen. It may be operated in steady rotation or in an oscillatory mode. 

In another elongational rheometer (264), the specimen is suspended on the end of a flexible tape, which is wound onto a wheel turned by a servo-controlled torque motor. This design is the basis for the Gottfert Rheostrain instrument. One device for melts and solids is an add-on fixture for rotational rheometers. The SER testing platform (267) incorporates dual wind-up drums and can be used for tensile, tear, peel, and friction testing as well as for elongational viscosity measurements. 

As the chapter quotation indicates, almost the first thing Cou-ette did after he built his famous rotational rheometer was to compare its results to those from Poiseuille s capillary instrument. If we do our measurements right and make the appropriate corrections, all the instruments shown in Figure 5.1.2 should give the same value of the viscosity. The major theme of Chapters 5-9 is determining what it takes to get absolute material function data. We will see how well this can be done by comparing results (for G, t],, etc.) by the different shear methods at the end of Chapter 6. 

Extensional viscosity was measured at 170°C on a Sentmanat Extensional Rheometer (SER) fixture (Xpansion Instruments).[1] The SER is based on a dual drum system. It is designed as a fixture of a standard rotational rheometer which consists of a master and slave wind-up dmms coupled via intermeshing gears. A constant Hencky strain rate is obtained simply by setting a constant winding speed. The SER fits inside the environmental chamber of an Advanced Rheometric Expansion System (ARES) rheometer. Tests were carried out on strips cut out of a 0.5 mm thick compression molded sheet. Constant Hencky strain rates (1 and 10 s ) were applied and the time-dependent stress was determined from the measured torque and the sample time-dependent cross-section. The extensional viscosity, tie, was obtained by dividing the stress by the Hencky strain rate. 

The viscosity of a liquid can also be determined by measuring the torque needed to rotate a cylinder in the liquid. Brookfield viscometers and rheometers fall into this class of instrument (Fig. 3.7). The viscometer measures the torque produced when a spindle is rotated at constant velocity in a liquid. The Rheometer produces a constant torque... 

A number of instruments are based on the extmsion principle, including slit flow and normal capillary flow (Table 6). These instruments are useful when large numbers of quality control or other melt viscosity test measurements are needed for batches of a single material or similar materials. When melt viscosities of a wide range of materials must be measured, rotational viscometers are preferable. Extmsion rheometers have been applied to other materials with some success with adhesives and coatings (10,161). 

Another type of rotational viscometer is the helical-screw rheometer (176). This instrument 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 instrument can be used for high temperature rheometry and to follow polymerization, shear and heat degradation, and other developments. 

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,... 

Other errors, which could influence the results obtained, are, for example, wall effects ( slipping ), the dissipation of heat, and the increase in temperature due to shear. In a tube, the viscosity of a flowing medium is less near the tube walls compared to the center. This is due to the occurrence of shear stress and wall friction and has to be minimized by the correct choice of the tube diameter. In most cases, an increase in tube diameter reduces the influence of wall slip on the flow rate measured, but for Newtonian materials of low viscosity, a large tube diameter could be the cause of turbulent flow. ° When investigating suspensions with tube viscometers, constrictions can lead to inhomogeneous particle distributions and blockage. Due to the influence of temperature on viscosity (see Section Influence Factors on the Viscosity ), heat dissipated must be removed instantaneously, and temperature increase due to shear must be prevented under all circumstances. This is mainly a constructional problem of rheometers. Technically, the problem is easier to control in tube rheometers than in rotating instruments, in particular, the concentric cylinder viscometers. ... 

The Brookfield DV-II+-series rotational viscometers and DV-IIl-series rheometers have a built-in Time-to-Torque feature. The selected instrument is run in stand-alone mode, at one constant speed with one spindle. The motor rotation automatically stops at a user-selected torque reading that is a percent of full-scale range or "FSR." The elapsed time and the setpoint torque are then shown on the instrument s display. This system can therefore be used as a type of gel timer - the instrument monitors the torque increase to 90 % of full-scale range, for example, as the sample s viscosity increases during gelation. However, it is a "one-point" test - that is, only one data point is acquired. If the instrument s "continuous printing" mode is selected and a... 

The effect of temperature on the mechanical properties of a liquid can be investigated using a special type of dynamic mechanical analyser called an oscillatory rheometer. In this instrument the sample is contained as a thin film between two parallel plates. One of the plates is fixed while the other rotates back and forth so as to subject the liquid to a shearing motion. It is possible to calculate the shear modulus from the amplitude of the rotation and the resistance of the sample to deformation. Because the test is performed in oscillation, it is possible to separate the shear modulus (G) into storage (G ) and loss modulus (G") by measuring the phase lag between the applied strain and measured stress. Other geometries such as concentric cylinders or cone and plate are often used depending on the viscosity of the sample. 

Perhaps the most important factor to a process engineer in predicting extrusion or molding behavior is melt viscosity. Several methods are used to obtain the viscosity of polymer solutions and melts experimentally as a function of shear rate [19]. Instruments for making such measurements must necessarily accomplish two things (1) the fluid must be sheared at measurable rates, and (2) the stress developed must be known. Two kinds of instruments having simple geometry and wide use a rotational viscometer and capillary or extrusion rheometer. 

If one needs to investigate the dependence of r] on shear rate, y, one must have access to a rheometer, an instrument that can characterize the dependence of viscosity on shear rate, thus enabling an extrapolation to the Newtonian limit. Typically, such measurements are conducted in Couette (concentric cylinder) or cone and plate geometry. In the Newtonian limit, for Couette geometry, when the inner cylinder is rotated, and provided that the gap between the inner and outer cylinder is small (i.e., RilRo < 0.99, where Ri and Ro are the radii of the inner and outer cylinders), the shear stress on the wall of the outer (resting) cylinder is... 

Viscosity is measured with rheometers or viscometers. The methods used include rotational deformation, squeezing deformation, extrusion (capillary) flows, and free surface stretching. For rotational instruments, there are two modes of operation controlled strain and controlled stress. Rotational measurements can be further subdivided into different measuring methods (flow, oscillatory, stress relaxation, and creep) and different measurement devices (spindle, cone-and-plate, parallel plate, concentric cyhnder). As for a capillary rheometer, there are two modes of operation controlled flow (strain) and controlled pressure (stress). 

The setup of all experiments is illustrated in Fig. 20.13. The main device is the rheometer DHR-1 of TA instruments. An air bearing and a magnetic bearing are used to minimize the friction of the rotation of the geometry. The low viscosity of... 

---