Extensional Viscometers

Extensional viscometers are useful to measure tensile viscosity in processes such as stretch orientation. 

Thermal Electron Corp. technical brief, HaakeCaBER 1 extensional viscometer, 2004. 

Polymer melt spinning is one of the major processes for the manufacture of synthetic fibers. Typically, a polymer melt is extruded vertically downward through a spinneret or die, which may have hundreds of openings. The resulting filaments are simultaneously cooled by crossflow air and are stretched by the action of rollers. On solidification, the yam is wound onto a bobbin. Different aspects of this operation have been described in the literature.(, 59-63) When the fiber-spinning process is used as an extensional viscometer, the setup is simplified and only a single filament of circular cross section is employed. This is shown schematically in Fig 6. Also, the operation is run in an isothermal manner by spinning into an isothermal oven. 

As we see, these procedures are quite difficult, and thus it is not surprising that few commercial instruments are available to measure extensional viscosity. The present choice is quite small, being limited to Rheometric Scientific s RME extensional viscometer for polymer melts. Anyone interested in measuring extensional viscosity of other systems will have to resort to home-made instruments. 

Even though eommereial instruments are available for making extensional viscosity measurements on polymer melts, this is not a routine measurement. The stretch-rate range of these extensional viscometers is such that the maximum stretch rate that can be achieved is of the order of 1 sec in polymer processing operations, a stretch rate of 100 sec is commonplace. Also, not every polymer stretches uniformly, and, even when it does, steady-state stress levels are not always attained. For all of these reasons, extensional viscometry is an area of current research. Additional details regarding extensional and other viscometers may be found in the book by Dealy [16]. 

Anklam et al. [91] have attempted to measure the extensional rheological properties of w/o emulsions and HIPEs, using a nozzle-type viscometer. However, the results showed a dependence on the nozzle size used, and long relaxation times. Experiments on other non-Newtonian fluids indicated that it was not possible to obtain reliable results with this kind of instrument. 

There are several direct methods of measurement of yield stress. The constant stress rheometer is most frequently used to determine value in shear. Dzuy and Boger [1983, 1985] used a rotational vane viscometer. Yield stresses in compression can be calculated from the unrelaxed stress values in parallel plate geometry. Its value in elongation has been directly measured as the critical stress value below which no sample deformation was observed during 30 minutes of straining in an extensional rheometer. 

Various instruments are available to measure the viscosity of polymer melts and solutions, and more generally their rheological behavior, which include capillary and rotational viscometers. The former can be used to measure parameters such as shear viscosity, melt fracture, and extensional viscosity, which are important for many polymer processes. The latter type of device can be used in either steady or oscillatory mode, thus providing a measure of the viscosity as well as viscoelasticity (G", G, and tan 5) as a function of frequency and temperature. 

At low shear rates, polymeric liquid properties are characterized by two constitutive parameters zero shear rate viscosity t]o and recoverable shear compliance Jq, which indicates fluid elasticity. At higher shear strain rates, rheological behavior is measured with a viscometer. Extensional strain viscosity, associated with extensional flow, occurs with film extrusion. 

Flow entering an orifice from a larger tube produces both shear and extensional deformation of the fluid (259). Although both shear and extensional effects are present, the pressure across the orifice is often dominated by extensional effects. This has been used as the basis for an extensional viscosity attachment for the Vilastic VE tube viscometer (236,260). 

The major advantage of a constant-stress rheometer over a constant-strain-rate rheometer is that, for a given polymer, a steady state is frequently achieved in the former mode but not in the latter one.(28) Even when a steady state is obtained with the use of both instruments, the total strain needed to achieve a steady state is lower for the constant stress viscometer. This extends the range of strain rates at which the extensional viscosity can be determined for an apparatus of a given size. Finally, it has been observed(4 20) that the stress tends to decrease slightly in a constant stretch rate experiment even after a plateau appears to have been reached. The physical significance of this last observation is not entirely clear. (29)... 

The rotational viscometers and the capillary rheometers described in sections 3.1 and 3.2 are those applicable for shear flows. However, there are processing operations that involve extensional flows. These flows have to be treated differently for making mecisurements of extensional viscosity. The extensional viscosity of a material is a measure of its resistance to flow when stress is applied to extend it. In general, measurement of steady-state extensional viscosity has proven to be extremely difficult. Steady extensional rate would be achieved by pulling Ihe ends of the sample apart such that I = Zq exp(ef) or in other words, at a rate that increases exponentially with time. Steady-state is reached when the force is constant. However, often d e sample breaks before steady-state is achieved or the limits of the equipment are exceeded or at the other extreme, die forces become too small for the transducer to differentiate between noise etnd response signal. Nevertheless, there have been various methods attempted for the measurement of extensional viscosity. 

Although most viscometric and rheological studies are carried out in simple shear flows such as rotational viscometers, real flows experienced by real liquids are very often extensional (stretching or elongational) in nahire, and for some liquids there can be a very large difference between their shear and extensional viscosities. 

Viscometers may be separated into three main types capillary, rotational, and moving body. There are other kinds, usually designed for special applications. For example, it has been reported that the viscosity of volumes of liquid in the nanoliter range can be measured by monitoring the shape of a droplet while it is suspended in a medium with an extensional or shear field (184). For any given... 

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