Slit rheometer

HVA 6 Automated CapiUary/Sht Viscometer MFM Melt Flow Rheometer Automatic Melt Flow Indexer AK Continuous Capillary Rheometer Han Slit Rheometer... 

Normally the coordinate system is chosen in such a way that T13 = T31 = T23 = T32 = 0 In general, use is made of normal stress differences, N1 and N2, because they do not include undetermined hydrostatic pressures that are always present but not affect the material properties (as long as they are not too high). In Table 15.1, also the possibilities to determine the normal stress differences or combinations are depicted. In the modem rheogoniometers also normal stress differences can be determined but. They follow from measurements of normal forces, Fn, or normal stresses, T22, as is also depicted in Table 15.1. For the measurements of the normal stresses T22 pressure gauges have to be mounted in the Couette cylinders, in the capillary of the capillary rheometer (in both cases quite difficult to mount) and in the plate of a cone and plate instrument at several distances from the axis (not that difficult). Sometimes use is made of a slit rheometer instead of a capillary rheometer, because pressure gauges are much easier to mount (Te Nijenhuis, 2007, Chap. 9.1.2). 

For slit rheometers of rectangular cross-section, with length L and sides W and H (L> W 3> H) (see Fig. 15.A1) the shear rate at the wall is given by... 

Figure 3-20 Slit Rheometer Geometry (Redrawn from Rao, 1995).

There are a vast number of rheometers available from a series of manufacturers, and Whorlow classified these into steady rotating instruments and tube and slit rheometers. Walters wrote a detailed account on the fundamentals of the design of rheometers, and Ferryf " reviewed the instrumental techniques available for the investigation of viscoelastic properties. Special requirements and techniques available for the industry were analyzed by Sherman. ... 

Han et al (1997) examined the chemorheology of a highly filled epoxy-resin moulding compound that is characterized by a modifed slit rheometer. Results show that a modified Cox-Merz rule relating dynamic and steady viscosities is established, >7(7 ) = (Tm )-Also the material was shown to exhibit a yield stress at low shear rates and power-law behaviour at higher shear rates. The temperature dependence of the viscosity is well predicted by a WLF model, and the cure effects are described by the Macosko relation. 

A number of experimental methods have been applied to measure the melt viscosity of polymers (53-55,65), but capillary extrusion techniques probably are generally preferred. Rotational methods are also used, and some permit the measurement of normal stress effects resulting from elasticity as well as of viscosity. Slit rheometers can also be used to measure normal stress (66). Oscillatory shear measurements are useful for measuring the elasticity of poljmier melts (57,58). Controlled stress methods have also been applied (59). Squeeze film flow has also been proposed as a geometry suitable for processibility testing of polymer melts... 

Han, C.D. (1971) Measurement of the rheological properties of polymer melts with slit rheometer. I. Homopolymer systems n. Blend systems ni. Two phase systems - higfi impact polystyrene and ABS resins, /. Appl Polym. Set., 15,2567-77,2579-89, 2591-6. 

Han, C. D., Measurement of the rheological properties of polynm melts with slit rheometer. I. Homopolymer Systems. II. Blend Systems. IB. TWo Phase Systems— High Impact Polystyrene and ABS Resins, J. Appl Pofym. ScL, IS, 2567-2577 (I), 2579-2589 (II), 2591-2596 (IB) (1971). 

Schematic of slit rheometer with flush-mounted pressure transducers /, .

The major advantage of the slit rheometer is also indicated in eq. 6.3.1, namely direct measurement of the pressure gradient dpjdx. This is possible because pressure transducers can be mounted on the wide flat sides of the slit geometry. Even if pressure holes are used, they are easier to construct and will not disturb the flow as much as in a small diameter capillary. If pressure transducers are not used, dp/dx can be evaluated from ptot as with the capillary (eq. 6.2.20). 

Comparison of shear and normal stress determined by parallel plates (solid symbols) and slit rheometer with hole pressure (open symbols) for a polyisobutylene solution at room temperature. From Lodge (1989). 

On-line slit rheometer with recycle. The conveying gear pump permits rapid sampling and flushing a melt stream from extruder b, gear pumps in front of and behind... 

Note that y appearing in Eq. (2.12) is constant for uniform shear flow and not constant for nonuniform shear flow. In Chapter 5 we present experimental methods for the determination of the rheological properties of polymeric liquids in the uniform shear flow field using a cone-and-plate rheometer and in the nonuniform shear flow field using a capillary or slit rheometer. 

In this section, we present experimental methods for measuring the rheological properties of polymeric fluids using a capillary rheometer and a slit rheometer. 

Another experimental method that is as important as continuous-flow capillary rheometry is slit rheometry. The basic idea of slit rheometry is the same as that of continuous-flow capillary rheometry insofar as the measurement of wall normal stress along the die axis is concerned. But, there is a significant theoretical difference between the two methods, as we will make clear, and also in the die design. The slit rheometer has some advantages over the continuous-flow capillary rheometer in the way that transducers can be mounted on the die wall, but there are also some disadvantages. 

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