ABSTRACT:Cloud point data to 230°C and 2200 bar are presented for poly(acrylate)-ethylene mixtures. When the length of the alkyl tail is increased, the cloud point curve is shifted towards lower pressure, but this trend switches when going from poly(ethyl hexyl) to poly(octadecyl) acrylate. It is apparent that there is an optimum alkyl tail length that balances energetic acrylate-acrylate, ethylene-ethylene, and ethyleneacrylate interactions and free-volume, entropic effects. Both ethylene-poly(acrylate) and CO 2 -poly(acrylate) data are modeled by the Statistical Associating Fluid Theory (SAFT) equation of state. A pseudogroup contribution method is developed for the calculation of the following pure polymer characteristic parameters: m, the number of segments, and v 00 , the volume of a segment. This method cannot be applied for u 0 /k, the attractive energy of a segment, which changes in a nonlinear manner with changes in the structure of the acrylate repeat group. The energy parameter is then calculated from monomer data or fitted directly to one cloud point curve. The experimental data are represented well, even if little predictive power is obtained since a temperatureindependent interaction parameter k ij is needed.
Supercritical fluids are being increasingly used as media for fine
particles formation: the most
important techniques are the rapid expansion of a supercritical
solution process (RESS), the
particles from gas-saturated solutions process (PGSS), and the
supercritical antisolvent
recrystallization process (SAS). To verify the feasibility of such
processes, and to optimize the
choice of operative variables, it is important to understand the phase
behavior of the systems
involved. To perform a thermodynamic analysis, an equation of
state has to be used to take
into account the effects of pressure; moreover, a solid phase is
involved, and the heavy component
is usually a high molecular weight and poorly characterized compound.
In this work the Peng−Robinson equation of state with classical mixing rules and one or two
binary interaction
parameters are used. The fugacity of the heavy component in the
solid phase is calculated by
means of a subcooled liquid reference state: only heat of fusion and
melting temperature of the
heavy component are needed. The aim of this work is to develop a
thermodynamic model which
allows to calculate solid−liquid−vapor (S−L−V) equilibria of
binary (RESS and PGSS) and
ternary (SAS) systems. In regard to binary systems, the knowledge
of P
UCEP and T
UCEP
allows
the calculation of binary interaction parameters: then the
P
−
T trace of S−L−V
equilibrium
and the solubility of the heavy component in the light supercritical
fluid can be reasonably well
predicted. For ternary systems available S−L−V and
S−L1−L2−V equilibrium data are well
correlated, so that an analysis on the effect of operating variables
(P and T) on the SAS process
can be performed.
Cloud-point data are reported at temperatures to 245 °C and pressures to 2700 bar for poly(vinyl fluoride) (PVF) and poly(vinylidene fluoride) (PVDF) in CO 2 , CH 2 F 2 , dimethyl ether (DME), acetone, and ethanol and in mixtures of CO 2 with acetone, DME, and ethanol. PVF does not dissolve in CO 2 even at 245 °C and 2700 bar, but, PVF does dissolve in CH 2 F 2 at 180 °C and pressures in excess of 1500 bar. To dissolve PVF in DME, pressures in excess of 550 bar and temperatures in excess of 130 °C are needed although it only takes ∼100 bar to maintain a single phase to temperatures of ∼220 °C with ethanol and acetone. Compared to the conditions needed to dissolve PVF, it takes hundreds of bar less pressure to dissolve PVDF in CO 2 , CH 2 F 2 , and DME and ∼60 bar less pressure to dissolve it in acetone, but it does take ∼60 bar more pressure to dissolve it in ethanol. With CO 2 , ethanol is a better cosolvent than acetone for both fluoropolymers at high temperatures and at low ethanol concentrations. However, when the temperature is decreased or the ethanol concentration is increased, it acts as an antisolvent probably due to ethanol self-association. Compared to ethanol and acetone, DME is not as good a cosolvent more than likely as a result of its lower density and smaller dipole moment. For all three cosolvents, their impact on the reduction of the cloud-point pressure diminishes with increasing cosolvent concentration. It is also evident that CO 2 is an effective antisolvent since small amounts of it added to the polymer-cosolvent mixtures greatly increase the pressures needed to obtain a single phase.
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