Vinylidene fluoride (VDF) is typically polymerized by a semi-batch emulsion
or suspension polymerization process using an aqueous medium. The polymerization
processes use a free-radical initiator, a fluorinated surfactant such as ammonium
perfluorodecanoate, and a chain transfer agent. After the polymer is produced,
significant energy is expended to isolate it from the aqueous medium. Large
quantities of wastewater are generated not only as a result of using water
as the polymerization medium, but also as a result of washing the polymer
prior to the final drying step to remove residual surfactants.
The use of supercritical or dense-phase CO2 as a polymerization medium to
reduce waste generation and decrease energy usage is a desirable alternative.
Carbon dioxide is relatively inexpensive, nontoxic, nonflammable and available
in large quantities at high purity. Carbon dioxide possesses relatively low,
easily accessible critical points, with a critical temperature of 31.1oC (88.0oF)
and critical pressure of 73.8 bar (1070 psi). In the CO2 process, the polymerization
reactor is operated under supercritical or dense-phase conditions. No surfactant
is used. The reaction mixture containing the fluoromonomer, CO2, and an initiator
is initially in one phase. When the growing oligomeric radicals reach a critical
molecular weight, they become insoluble in CO2. The polymerization medium
together with the fluoropolymer and unreacted monomer are continuously withdrawn
from the reactor. CO2 and unreacted monomer are continuously recycled back
to the reactor. Investment costs downstream of the polymerization reactors
are significantly reduced because separation of the CO2 medium from the polymer
can easily be achieved. Wastewater generation is significantly reduced. In
this report, we will review a process to produce polyvinylidene fluoride (PVDF)
resin by a continuous process using supercritical CO2 as the polymerization
medium. The results will be compared to a conventional emulsion polymerization
process to produce PVDF.
By Susan L. Bell
