Extensional rheometer

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As rheometer be rheometer indicated that liquid or melt-shaped samples on stretching claim and so the extensional viscosity measured. Due to experimental difficulties, this characterization method - unlike the solid state - does not play a major role in comparison to shear : e.g. For example, there are around 35,000  publications dealing - at least partially - with shear rheology , while only around 9,300 publications deal with extensional rheology (as of Feb. 2008, according to SciFinder (R)).

There are three basic types of extensional rheometers, which differ in the type of stretching:

  • uniaxial elongation
  • planar stretch
  • equibiaxial stretch.

In practice there are almost only extensional rheometers for uniaxial expansion, although v. a. biaxial stretching plays a technically important role in plastics processing . The reason for this is the incomparably more difficult realization of the other two modes.

construction

A extensional rheometer consists of three components:

  • a motor that normally stretches the sample at an exponentially increasing rate of expansion, the so-called Hencky expansion rate .
  • a force measuring device
  • a temperature control device, e.g. B. an oil bath or a hot gas stream that heats the sample to the desired temperature and also usually has the task of compensating for gravity .

The latter is important because the typical samples that are measured with an extensional rheometer - plastic melts and dispersions - are much less rigid than normal solids and would therefore deform under their own weight ( sagging ) if no suitable countermeasures should be taken. Density-adapted silicone oils ( polydimethylsiloxane ), gas cushions or suitable geometries with high moments of inertia are used as countermeasures . Nevertheless, this phenomenon is one of the greatest obstacles to extensional rheology.

Measuring principle

Constant length

In this type of extensional rheometer, the sample length (with uniaxial expansion) remains unchanged. For this purpose, the sample is stretched between one or two winding devices and then stretched by winding. The effective sample volume becomes smaller, but the original length is retained.

This type of rheometer is the dominant one today.

Constant volume

This is essentially a tensile testing machine , which, however, is designed for much smaller forces and greater elongations. The sample is clamped at the ends and one end is pulled at an exponentially increasing speed .

This type of rheometer is more precise in practice, but also technically more complex than the “constant length” type and is therefore only used by a few specialists.

Low viscosity fluids

The so-called Capillary Breakup Extensional Rheometer (CaBER ) is particularly suitable for the extensional rheological characterization of low-viscosity fluids (such as polymer solutions) , as it enables practical total expansions of ≈ 10 ... 14. Here, a small volume of liquid is jerkily stretched and the tapering of the resulting liquid thread is analyzed. Characteristic parameters such as the elongation relaxation time or the transient elongation viscosity can be derived from the development of the thread diameter over time.

Bibliography

  • Introduction of Henckydehnung: Hencky H (1928) About the form of the law of elasticity in ideally elastic materials. Journal for Technical Physics 9 215–220.
  • First descriptions of today's common forms of uniaxial extensional rheometers:
    • Constant sample length: Meissner J (1969) Rheometer for the study of mechanical properties of deformation of plastic melts under definite tensile stress. Rheologica Acta 8 (1): 78-88.
    • Constant sample volume: Münstedt H (1979) New Universal Extensional Rheometer for Polymer Melts. Measurements on a Polystyrene Sample. Journal of Rheology 23 (4): 421-436.
  • Introduction of the Trouton ratio: Trouton FT (1906) On the viscous traction and its relation to that of viscosity. Proc Roy Soc 77 426