Organic compounds -- part of the class 532-570 series – Organic compounds – Fatty compounds having an acid moiety which contains the...
Reexamination Certificate
2002-01-30
2004-04-13
Carr, Deborah D. (Department: 1621)
Organic compounds -- part of the class 532-570 series
Organic compounds
Fatty compounds having an acid moiety which contains the...
C554S207000
Reexamination Certificate
active
06720437
ABSTRACT:
FIELD OF THE INVENTION
This invention is in the field of recovery and recycling of fluorinated carboxylic acid used in fluoropolymerization in aqueous media.
BACKGROUND OF THE INVENTION
Polymerization of fluoroolefins to manufacture fluoropolymers is often performed in aqueous media. In one such process referred to as dispersion polymerization, fluorinated carboxylic acids are typically used as surfactants in the aqueous media at concentrations on the order of 0.1% by weight of water in the recipe. Examples of these fluorosurfactants include the perfluorinated alkanecarboxylic acids having 7 to 10 carbon atoms, in particular perfluorooctanoic acid (PFOA). These acids are generally used in the salt form, preferably as ammonium salts. Fluoropolymerization to make “granular fluoropolymer” is also done in aqueous media in a process sometimes referred to as suspension polymerization, though with less fluorosurfactant than is used in dispersion polymerization. For a discussion of the processes, see “Tetrafluoroethylene Polymers” in the Encyclopedia of Polymer Science and Engineering, John Wiley & Sons, New York, 1989, Vol. 16, p. 580.
When, after polymerization, the fluoropolymer is isolated from the aqueous medium, i.e., by coagulation in dispersion polymerization, a substantial portion of the fluorinated carboxylic acid typically remains in the fluoropolymer. When the fluoropolymer is heated for the purpose of drying, the fluorinated carboxylic acid is volatilized and carried away in the dryer or oven exhaust gas. The exhaust gas is scrubbed with water before being released to the environment. The fluorinated carboxylic acid accumulates in the scrubbing solution. In the interests of reducing emissions to the environment, and because of the considerable expense of the surfactant, it is desirable to recover the fluorinated carboxylic acid from the scrubbing solution and reuse it. Purification before reuse is necessary to avoid the introduction into the polymerization of impurities. Such impurities include fluoride ions as well as other materials, some of which cannot be identified because of their low concentrations, but that nevertheless affect polymer quality, for example molecular weight, or color.
Various methods for recovery and/or purification of surfactant are known. U.S. Pat. No. 5,990,330 discloses precipitation of perfluoroalkanecarboxylic acid with concentrated alkaline scrubbing solution. This requires strong caustic and the recovered fluorinated carboxylic acid needs further purification before it is suitable for reuse. U.S. Pat. Nos. 3,882,153 and 4,282,162 disclose adsorption on ion-exchange resin and subsequent desorption. Such treatment uses volumes of solutions both in operation and in regeneration of the ion-exchange resin. Methods of purification of fluorinated carboxylic acid include esterification of the surfactant, which sometimes requires preliminary acidification, distillation of the ester, and then conversion to the ammonium salt (U.S. Pat. No. 5,442,097). U.S. Pat. No. 4,609,497 discloses extraction of fluorinated carboxylic acid with chlorocarbon solvent, adsorption on alumina of the fluorinated carboxylic acid from the chlorocarbon solution, separation of alumina from the chlorocarbon solution, and desorption of the fluorinated carboxylic acid from alumina with ammonium hydroxide. These are multistep processes involving organics (alcohols in esterification, chlorocarbons in extraction).
Improved methods of fluorinated carboxylic acid recovery are needed that with a minimum of steps and reagents make the fluorinated carboxylic acid suitable for reuse.
SUMMARY OF THE INVENTION
This invention provides a process for the recovery and recycle of fluorinated carboxylic acid contained in the exhaust gas produced from the drying of fluoropolymer polymerized in aqueous media containing fluorinated carboxylic acid. The process includes:
a) scrubbing the exhaust gas to produce a scrubber solution containing fluorinated carboxylic acid,
b) concentrating the scrubber solution to produce a concentrated scrubber solution,
c) contacting the concentrated scrubber solution with alumina to produce a recovered fluorinated carboxylic acid solution, and
d) directly reusing the recovered fluorinated carboxylic acid solution in a fluoropolymerization in aqueous media.
DETAILED DESCRIPTION
It has been discovered that fluorinated carboxylic acid in the solution from fluoropolymer dryer scrubbers can be concentrated to standard levels, the concentrated scrubber solution contacted with alumina, recovered therefrom by separation, and the recovered fluorinated carboxylic acid solution directly reused in the fluoropolymerization in aqueous media with no deleterious effect on the behavior of the polymerization or on the quality of the polymer produced. By “directly” is meant that the recovered fluorinated carboxylic acid solution is used in polymerization without further purification or treatment except for such incidental steps such as adjustment of concentration or pH, heating or cooling as called for by the polymerization recipe, or combination with other ingredients as might be required by the polymerization recipe. “Directly” has no implications as to the interval in time between the treatment of concentrated scrubber solution according to this invention, and the use of the recovered fluorinated carboxylic acid solution in polymerization.
Fluoropolymers are defined herein as polymers produced in polymerization in which at least some of the constituent monomers are fluoromonomers. Fluoropolymers are further defined as containing at least about 10% fluorine by weight, preferably at least about 35% fluorine by weight. Preferred fluoromonomers are fluoroolefins. Most preferred are fluoroolefins in which at least one of the substituents on the doubly bonded carbon atoms is a fluorine atom. Aqueous polymerization is well-known in the fluoropolymer art. Examples are disclosed in U.S. Pat. Nos. 5,399,643, 4,391,940, 4,189,551, and 3,855,191. Tetrafluoroethylene (TFE) hompolymers and copolymers may be made this way. Important comonomers used with TFE are hexafluoropropylene (HFP), perfluoro(alkyl vinyl ether) (PAVE), and ethylene (E). Perfluoro(alkyl vinyl ethers) include perfluoro(propyl vinyl ether), perfluoro(ethyl vinyl ether), and perfluoro(methyl vinyl ether), as well as others in which the alkyl group is substituted with groups that can give rise to functionality, such as find use in ion-exchange polymers and as cross-linking sites in elastomers. Chlorotrifluoroethylene, trifluoroethylene, vinylidene fluoride and vinyl fluoride may also be homo- or copolymerized in aqueous media using fluorinated carboxylic acid surfactant. Typical fluoropolymers made by polymerization in aqueous media include polytetrafloroethylene (PTFE), TFE/HFP (FEP), TFE/PAVE (PFA), and E/TFE (sold by DuPont under the trademark Tefzel®).
The fluorinated carboxylic acid that is the subject of this invention is a highly fluorinated alkanecarboxylic acid or the salt thereof. By “highly fluorinated” is meant that at least about half of the substituents on the alkyl chain of the alkanecarboxylic acid are fluorine atoms. Preferably the highly fluorinated alkanecarboxylic acid is a perfluoroalkanecarboxylic acid or salt thereof, more preferably six- to twelve-carbon perfluoroalkanecarboxylic acids or salts thereof, and most preferably perfluorooctanoic acid (PFOA) or its salts, preferably the ammonium salt. PFOA in the ammonium salt form is available commercially from the 3M Company, St. Paul, Minn., USA. PFOA preparation is disclosed in U.S. Pat. No. 5,945,562. The term “fluorinated carboxylic acid” as used herein is understood to include the fluorinated carboxylic acid per se as well as the salts of the fluorinated carboxylic acid such as the ammonium salt.
The fluorinated carboxylic acid used in fluoropolymerization in aqueous media must be substantially free of impurities that can affect the polymerization or the quality of the polymer made. By “substantially free” is meant that the concentration of such impuritie
Fu Ta-Wei
Jones Clay Woodward
Carr Deborah D.
E. I. du Pont de Nemours and Company
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