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

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:

                                                                                                                        (1)

                                                                                                                        (2)

There are some minor differences
in the definition of the shift factors, but the definitions are all similar. The
product a_ca_p 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
viscosity).

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].

References.

1. M. L. Williams, R. F. Landel and J. D. Ferry, Journal
of the American Chemical Society, 77 3701 (1955).

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).