Rheometer rotational-type

Most current commercial viscometers worthy of the description rheometer are of the rotational type, and many are sophisticated and versatile instruments. Axial flow instruments are often user-designed and built. 

Chapter 3 deals with rheometry which is the method of measurement of the various rheological parameters described in Chapter 2. The rheometers may be of Ae rotational type or the capillary type for shear flows and the shear free t)rpe for extensional flows. 

Molten polymers are viscoelastic materials, and so study of their behaviour can be complex. Polymers are also non-ideal in behaviour, i.e. they do not follow the Newtonian liquid relationship of simple liquids like water, where shear-stress is proportional to shear strain rate. Unlike Newtonian liquids, polymers show viscosity changes with shear rate, mainly in a pseudoplastic manner. As shear rate increases there is a reduction in melt viscosity. This is true of both heat-softened plastics and rubbers. Other time-dependent effects will also arise with polymer compounds to complicate the rheological process behaviour. These may be viscosity reductions due to molecular-mass breakdown or physical effects due to thixotropic behaviour, or viscosity increases due to crosslinking/branching reactions or degradation. Generally these effects will be studied in rotational-type rheometers and the extrusion-type capillary rheometer. 

Rheometers used for determining the material functions of thermoplastic melts can be divided into two broad categories (1) rotational type and (2) capillary type. Furfiier, subdivisions are possible and Gese are shown in Table 3.2. In what follows, only those rheometers which are ptqmlarly used for riieological... 

All materials were dried at 120 °C in vacuo for at least 24 h before use, to minimize the effects of moisture. The PEN/CNT nanocomposites were prepared by a melt blending process in a Haake rheometer (Haake Technik GmbH, Germany) equipped with a twin-screw (nonintermeshing co-rotating type). The temperature of the heating zone, from the hopper to the die was set to 280, 290, 295, and 285 °C, and the screw speed was fixed at 20 rpm. For the fabrication of PEN/CNT nanocomposites, PEN was melt blended with the addition of various CNT content, specified as 0.1, 0.5,1.0, and 2.0 wt% in the polymer matrix, respectively. 

Figure 3.2 Plots of log versus logy and loglVj versus logy for a low-density polyethylene at various temperatures ( C) (O, ) 180, (A, A) 200, and ( , ) 220. The data with open symbols were obtained using the cone-and-plate fixture of a rotational-type rheometer, and the data with filled symbols were obtained using a continuous-flow capillary rheometer. Refer to Chapter 5 for details of the experimental methods employed to obtain the data. (Reprinted from Han et ah. Journal of Applied Polymer Science 28 3435. Copyright 1983, with permission from John Wiley Sons.)...

Devices used for flow-induced crystallization (FIC) experiments are aU types of rheometers rotational plate/plate, cone/plate, Couette, sliding plates, capillary rheometers, including the multipass rheometer (MPR) [24-26], and shear devices in-house built [27], Linkam shear cell [28,29], fiber pull-out [30-34], FS, and complex flows and contraction/expansion and cross-slot [35-38]. 

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. 

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. 

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 Couette rheometer. Another rheometer commonly used in industry is the concentric cylinder or Couette flow rheometer schematically depicted in Fig. 2.48. The torque, T, and rotational speed, 0, can easily be measured. The torque is related to the shear stress that acts on the inner cylinder wall and the rate of deformation in that region is related to the rotational speed. The type of flow present in a Couette device is analyzed in detail in Chapter 5. 

Measurement of the flow properties of non-Newtonian fluids is typically accomplished via rotational techniques. The rotational methods fall into two basic types, concentric cylinder and cone and plate rheometers. In a concentric cylinder rheometer, a bob is placed inside a cylinder so that the fluid to be studied may be placed into the gap between the cylinders. This arrangement helps approximate a uniform shear rate throughout a sample by shearing only a thin film of sample fluid between... 

In the cone and plate rheometer, a cone-shaped bob is placed against a flat plate so that the fluid to be studied may be placed into the gap between the lower face of the cone and the upper face of the plate. Again, in the Searle method, the cone is rotated while in the Couette method the plate turns. In each case, the torque on the cone is measured. Figure 6.5 shows a Searle-type cone and plate arrangement. For this arrangement the shear stress is given by ... 

The most common dynamic method is oscillatory testing, in which the sample is subjected to a sinusoidal oscillatory strain, and the resulting oscillatory stress measured. The more sophisticated rotational viscometers have the additional capability of dynamically testing liquid-like materials using small angle oscillatory shear. A parallel disc viscometer can be set up for testing solid-like materials (e.g., butter), in oscillatory shear. Some UTM-type solids rheometers, in which the moving crosshead can be made to reciprocate sinusoidally, can be used to test solid-like materials in oscillatory deformation in compression, tension or shear. 

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). 

A number of Fann or Baroid rheometers have been used for the rig site measurement of fluid rheology (62, 63). The design of the rheometers is similar and the various viscometer types differ largely in the control of shear rate. Early models of the concentric cylinder rheometer were limited to two shear rate measurements made at rotation speeds of r = 300 rpm and r = 600 rpm. Later models have extended the number of rotational speeds (shear rates) at which the torque can be measured, enabling a more complete rheogram to be constructed. 

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 cone-and-plate and parallel-plate rheometers are rotational devices used to characterize the viscosity of molten polymers. The type of information obtained from these two types of rheometers is very similar. Both types of rheometers can be used to evaluate the shear rate-viscosity behavior at relatively low vales of shear rate therefore, allowing the experimental determination of the first region of the curve shown in Figure 22.6 and thus the determination of the zero-shear-rate viscosity. The rheological behavior observed in this region of the shear rate-viscosity curve cannot be described by the power-law model. On the other hand, besides describing the polymer viscosity at low shear rates, the cone-and-plate and parallel-plate rheometers are also useful as dynamic rheometers and they can yield more information about the stmcture/flow behavior of liquid polymeric materials, especially molten polymers. 

Viscosity with stational flow. A cone-plate type rotating rheometer (Shimadzu, RM-1, equipped with a reduction gear, RDG-1) was employed. The rate of shear available ranged from 7.48x10" to 74.8/sec. The apparent viscosity at a given rate of shear was calculated from the rate of shear and the observed shear stress. Samples were dissolved in the buffer solution mentioned before at 2 or 4% concentration and measured at room temperature (22+l C). 

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. 

Fresh concrete analyses were conducted by help of a mobile concrete rheometer of the type ConTec Rheometer-4SCC. By measuring the torque in mA at varied rotational speeds and appliance of a Bingham approach, this equipment allows drawing qualitative conclusions on 5deld stress and viscosity of the mixes. It does not provide real physical measurands, and the results in this paper, which can only provide qualitative information on the properties, are thus referred to as G-yield and H-viscosity, respectively. 

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