Published September 2003
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 The polymerization medium together with the fluoropolymer and unreacted monomer are continuously withdrawn from the reactor.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.