Fluoropolymners are primarily produced via heterogeneous polymerization reactions, including suspension, emulsion and microemulsion systems. Generally, each of these reactions requires at least one monomer and a radical initiator in a suitable reaction medium. In addition, emulsion polymerizations of halogen-containing monomers generally require a surfactant capable of emulsifying both the reactants and the reaction products for the duration of the polymerization reaction. The surfactant of choice in the synthesis of fluoropolymers is generally a perfluoroalkyl surfactant. The most common perfluoroalkyl surfactant in the production of halogenated polymers is ammonium perfluorooctanoate (AFPO). Non-fluorinated surfactants typically are not used, because they have the disadvantages of inhibiting the reaction, and affecting the molecular weight and other properties of the product, such as color.
The emulsion polymerization of vinylidene fluoride (VF2) using a fluorinated surfactant and isopropyl peroxydicarbonate (IPP) as a free-radical initiator, is taught in U.S. Pat. No. 3,475,396. This polymerization process has since been refined to provide polymers having various improved properties. See, U.S. Pat. Nos. 3,857,827, 4,076,929, 4,360,652, 4,569,978, 6,187,885. The emulsion polymerization of VF2 using a partially fluorinated surfactant and an inorganic peroxide initiator is taught in U.S. Pat. No. 4,025,709.
The use of perfluorinated surfactants has recently become the focus of increasing scrutiny due to safety and environmental concerns. A primary property of perfluoroalkyl surfactants that underlies their utility in polymerization reactions is their stability to the reaction conditions. Because of their resistance to chemical degradation, fluoroalkyl surfactants have the potential to accumulate in the environment and in organisms.
Several different approaches have attempted to reduce or eliminate the use of perfluoroalkyl surfactants in the polymerization of halogen-containing monomers.
Some emulsion polymerization processes have been demonstrated which employ partially fluorinated surfactants instead of perfluorinated surfactants. See, U.S. Pat. Nos. 4,524,197, 5,763,552. Another attempt to reduce the amount of perfluoroalkyl surfactant in heterogeneous polymerization involved a protocol wherein a conventional fluorinated surfactant was added in combination with a non-fluorinated hydrocarbon surfactant. However, this modification served to substantially lower the rate of the reaction. See, WO 95-08598A, the entire disclosure of which is incorporated herein by reference.
In another modification, the perfluoroalkyl surfactant was eliminated by employing a surfactant-free polymerization process, but the process described therein often relied on the use of metal salts as promoters, which metal salts can contaminate the product. See, WO 97 17381 and J. Appl. Polym. Sci., 2211, 70, 1998.
New polymerization processes are needed that utilize non-perfluoroalkyl surfactants or reduced amounts of perfluoroalkyl surfactants.