(583e) Rotational Rheometry of Polymers under High Pressure Carbon Dioxide | AIChE

(583e) Rotational Rheometry of Polymers under High Pressure Carbon Dioxide


Wingert, M. J. - Presenter, The Ohio State University
Lee, L. J., the Ohio State University
Tomasko, D. L., The Ohio State University
Koelling, K. W., The Ohio State University

Polymer based foams can be used
in many applications, such as packaging, insulation, and even scaffolds for
tissue engineering. Carbon dioxide (CO2) is a new physical blowing
agent for foams. To expedite the introduction of CO2 to industrial
foaming applications, it is important to understand its effects on viscosity,
since viscosity is so important to foam extrusion.

Several studies have measured the
viscosity of polymer melts under high pressure, using a variety of techniques.
However, few studies assist in foaming design because most do not contain
predictive scaling. Here scaling factors are used. The pressure and
concentration shift factors are defined, respectively, as follows:




There are some minor differences
in the definition of the shift factors, but the definitions are all similar. The
product acap should be conceptually the same for all
studies. These shift factors are completely analogous to WLF time-temperature
superposition [1].

Of the studies that report predictive
scaling, none contain measurements in the low shear rate region. The reason is
that these studies use extrusion slit dies and capillary rheometers to measure
CO2-impregnated viscosity. Accurate characterization of the entire
viscosity curve is needed for die design in foam extruders. If the viscosity at
low shear rates is off by a factor of two (very likely based on the noise of
previous experiments), simulations predict significant deviations in
properties, such as pressure drop and viscous heating.

In this study, a high pressure
couette rheometer is used to measure the viscosity of polymer melts under a
carbon dioxide atmosphere. Rotational rheometry overcomes the capillary/slit
problem of averaging pressures since the polymer reaches equilibrium with its
headspace (static pressure). Furthermore, it has the ability to measure low
shear rates that often correspond to the Newtonian regime (zero-shear

Preliminary results were obtained using polystyrene (PS). The
zero-shear viscosity of PS-CO2 was directly measured for a variety
of CO2 concentrations and temperatures. The viscosity depression due
to carbon dioxide is much more significant than those in the literature using
high shear rate rheometers [2-4].


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2. C. Kwag, C. W. Manke and E. Gulari, J. Polym. Sci. B:
Polym. Phys.
, 37 2771 (1999).

3. J. R. Royer, Y. J. Gay, J. M. Desimone and S. A. Khan, J.
Polym. Sci. B: Polym. Phys.
, 38 3168 (2000).

4. R. Gendron and M. F. Champagne, SPE ANTEC Proceedings,
1747 (2003).