Rheometers, extensional

A sliding plate rheometer (simple shear) can be used to study the response of polymeric Hquids to extension-like deformations involving larger strains and strain rates than can be employed in most uniaxial extensional measurements (56,200—204). The technique requires knowledge of both shear stress and the first normal stress difference, N- (7), but has considerable potential for characteri2ing extensional behavior under conditions closely related to those in industrial processes. 

Capillary Breakup Extensional Rheometer (CaBER), 21 740 Capillary columns, 6 377, 408 band broadening, 6 412 instrumentation, 6 424 speciality appbcations, 6 427t Capillary condensation, 1 585, 591 ... 

To reach steady state, the residence time of the fluid in a constant stretch rate needs to be sufficiently long. For some polymer melts, this has been attained however, for polymer solutions this has proved to be a real challenge. It was not until the results of a world wide round robin test using the same polymer solution, code named Ml, became available that the difficulties in attaining steady state in most extensional rheometers became clearer. The fluid Ml consisted of a 0.244% polyisobutylene in a mixed solvent consisting of 7% kerosene in polybutene. The viscosity varied over a couple of decades on a logarithmic scale depending on the instrument used. The data analysis showed the cause to be different residence times in the extensional flow field... 

At the time of writing this article, there are only two instruments in the market in the U.S.A. - the Rheometrics Extensional Rheometer, the RFX, and the Rheometrics Elongation Melt Rheometer, based on Meissner s rotary clamp method. The RFX is based on the opposing jet method. 

The semisolid consistency and the occurrence of slip effects during flow could limit the use of the rotary clamp method, the conveigent flow method or even Sridhar s extensional rheometer for a large number of food products such as mayonnaises and salad dressings. A novel rheometer (15) based on the concept of filament stretching has been developed by Plucinski et al (Fig. 10) to measure the extensional properties... 

These are early days in the development of extensional rheometers for foods and in our understanding of the extensional rheology of foodstuffs. The lack of commercial availability of extensional rheometers could be stalling the application of extensional rheology in characterizing products and processing operations in the food industry. For industrially important food products, product specific rheometers may need to be developed. 

Thus, in order to create a steady simple uniaxial extensional flow, the rheometer must cause the thin filament length to increase exponentially in time. 

Fig. 12.25 The Universal testing platform fitted on a rotational Rheometrics RDA II rheometer host station. The two counterrotating cylinders where the film is mounted cause the application of the extensional strain. [Reprinted by permission from E. G. Muliawan, S. G. Hatzikiriakos, and M. Sentmanat, Melt Fracture of Linear Polyethylene, Int. Polym. Process., 20, 60 (2005).]...

FIG. 15.23 Extensional rheometer, designed by Miinstedt (1979). A servo control system is used to maintain a specified extensional rate of strain or a specified tensile stress. Courtesy Society of Rheology. For a modern version, see Miinstedt et al., 1998. 

FIG. 15.24 Schematic representation of the four roller extensional rheometer, designed by Meissner (1972) to attain high Hencky strains. Two sets of rotary clamps are individually driven by two motors at constant rotation rates. The force in the sample is measured by a transducer F mounted on a leaf spring. From Barnes, Hutton and Walters (Gen Ref 1993). Courtesy Elsevier Science Publishers. 

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). 

FIG. 16.24 Extensional viscosity data obtained from a spin-line rheometer for very dilute aqueous poly (acryl amide) (1175 grade) solutions, as specified. From Walters and Jones (1988,1989). Courtesy Elsevier Science Publishers. 

FIG. 16.25 (A) Extensional viscosity data obtained from a spin-line rheometer for dilute aqueous poly... 

The maximum strain rate (e < Is1) for either extensional rheometer is often very slow compared with those of fabrication. Fortunately, time-temperature superposition approaches work well for SAN copolymers, and permit the elevation of the reduced strain rates kaj to those comparable to fabrication. Typical extensional rheology data for a SAN copolymer (h>an = 0.264, Mw = 7 kg/mol,Mw/Mn = 2.8) are illustrated in Figure 13.5 after time-temperature superposition to a reference temperature of 170°C [63]. The tensile stress growth coefficient rj (k, t) was measured at discrete times t during the startup of uniaxial extensional flow. Data points are marked with individual symbols (o) and terminate at the tensile break point at longest time t. Isothermal data points are connected by solid curves. Data were collected at selected k between 0.0167 and 0.0840 s-1 and at temperatures between 130 and 180 °C. Also illustrated in Figure 13.5 (dashed line) is a shear flow curve from a dynamic experiment displayed in a special format (3 versus or1) as suggested by Trouton [64]. The superposition of the low-strain rate data from two types (shear and extensional flow) of rheometers is an important validation of the reliability of both data sets. 

A few rheometers are available for measurement of equi-biaxial and planar extensional properties polymer melts [62,65,66]. The additional experimental challenges associated with these more complicated flows often preclude their use. In practice, these melt rheological properties are often first estimated from decomposing a shear flow curve into a relaxation spectrum and predicting the properties with a constitutive model appropriate for the extensional flow [54-57]. Predictions may be improved at higher strains with damping factors estimated from either a simple shear or uniaxial extensional flow. The limiting tensile strain or stress at the melt break point are not well predicted by this simple approach. 

Birefringence setups can be designed to characterize molten materials undergoing isothermal homogeneous flow. The ranges of strains and strain rates also often coincide with those of rheometers, and consequently may be limited relative to those used in fabrication. Similarly, time-temperature superposition approaches may be used to expand the rate window. State-of-the-art setups suitable for rapid screening of new materials with research-scale quantities (5-20 g) are available for shear flow [72] and startup of uniaxial extensional flow [73,74]. 

Miinstedt H (1979) New universal extensional rheometer for polymer melts. Measurements on a polystyrene sample. J Rheol 23 421-36. 

Figure 2. Schematic drawing of extensional rheometer, (a) motors, (b) screws, (c) transducer, (d) sample, (e) oil bath, (f) laser, (g) lower position of bath.

Figure 3-25 A Spin-Line or Fiber-Spin Rheometer for Extensional Viscosity Measurement (Redrawn from Bames et al., 1989).

Figure 3.81. Extensional-flow rheometer geometries. Adapted from Figure 7.1.2 (Macosko, 1994). Copyright (1994). Reprinted with permission of John Wiley and Sons, Inc.

WOO 169231, Method of Fluid Rheology Characterization and Apparatus U.S. 6,378,357, Method of Fluid Rheology Characterization and Apparatus U.S. 5,456,105, Rheometer for Determining Extensional Elasticity U.S. 5,532,289, Apparatus and Method for the Study of Liquid-Liquid Interfacial... 

---