Commonly known or commercially employed fluoropolymers include polytetrafluoroethylene (PTFE), copolymers of tetrafluoroethylene (TFE) and hexafluoropropylene (HFP) (FEP polymers), perfluoroalkoxy copolymers (PFA), ethylene-tetrafluoroethylene (ETFE) copolymers, terpolymers of tetrafluoroethylene, hexafluoropropylene and vinylidene fluoride (THV) and polyvinylidene fluoride polymers (PVDF). Commercially employed fluoropolymers also include fluoroelastomers and thermoplastic fluoropolymers. Fluoropolymers and their applications are further described in “Modern Fluoropolymers”, edited by John Scheirs, John Wiley & Sons Ltd., 1997. The making of such fluoropolymers generally involves the polymerization of gaseous monomers, i.e. monomers that under ambient conditions of temperature and pressure are present as a gas. Several methods are known to produce the fluoropolymers. Such methods include suspension polymerization as disclosed in e.g. U.S. Pat. No. 3,855,191, U.S. Pat. No. 4,439,385 and EP 649863; aqueous emulsion polymerization as disclosed in e.g. U.S. Pat. No. 3,635,926 and U.S. Pat. No. 4,262,101; solution polymerization as disclosed in U.S. Pat. No. 3,642,742, U.S. Pat. No. 4,588,796 and U.S. Pat. No. 5,663,255; polymerization using supercritical CO2 as disclosed in JP 46011031 and EP 964009 and polymerization in the gas phase as disclosed in U.S. Pat. No. 4,861,845.
Currently, the most commonly employed polymerization methods include suspension polymerization and especially aqueous emulsion polymerization. The aqueous emulsion polymerization involves the polymerization in the presence of a fluorinated surfactant, which is generally used to ensure the stabilization of the polymer particles formed. Accordingly, the emulsion polymerization process differs from suspension polymerization in which generally no surfactant is used which generally results in substantially larger polymer particles than in case of the aqueous emulsion polymerization.
In the aqueous emulsion polymerization, the polymerization speed, yield of polymer, comonomer incorporated, particle size and number are dependent on each other as well as on the emulsifier content. In order to optimize polymer yield, polymerization time and particle size, one can modify the pressure, temperature as well as the amount of fluorinated surfactant. For example, EP 612770 discloses the use of high amounts of fluorinated surfactant to increase the polymerization speed. However, increasing the amount of fluorinated surfactant has the disadvantage of increasing the cost and furthermore is environmentally undesirable.
U.S. Pat. No. 4,864,006 describes an aqueous emulsion polymerization process wherein an aqueous micro-emulsion of a perfluoropolyether is added to the polymerization reaction. The microemulsion in this US-patent is characterized as a monophase solution which is stable in the long run without providing “dispersion energy”. It thus seems that the micro-emulsion discussed in this patent form upon simple mixing of the components without the need to apply “dispersion energy” through the use of special emulsifying equipment. Accordingly, this limits the type of surfactants that can be used and further may complicate the polymerization process. For example, as shown in the examples of this patent, the microemulsion of the perfluoropolyether is formed using a perfluoropolyether that has a carboxylic acid group as a surfactant. Such surfactants are expensive. Further, as shown in the examples of U.S. Pat. No. 4,864,006, the microemulsion forms at elevated temperatures which further complicates the process relative to one in which such a heating is not required and the surfactant can simply be added to the polymerization vessel at room temperature.
EP 969027 discloses a similar process as in U.S. Pat. No. 4,864,006 for making PTFE dispersions. Also in this patent application, a microemulsion of a perfluoropolyether obtained through emulsification with a perfluoropolyether surfactant is used to prepare the PTFE dispersion. Although small polymer particles are achieved in this process, it appears that large amounts of surfactant are used which is disadvantageous in that it makes the process more costly and puts an environmental burden on the process.
U.S. Pat. No. 5,895,799 discloses a microemulsion polymerization of TFE to produce very small particle dispersions of PTFE in water. Thus, in accordance with the teaching of this US patent, the polymerization of TFE proceeds in a microemulsion of liquid perfluorinated hydrocarbons. This microemulsion is formed by adding the perfluorinated hydrocarbon liquid and a fluorinated surfactant to water at a temperature and in proportions that result in a microemulsion. As is well known, microemulsions only form when the surfactant is used above its critical micelle concentration (CMC) (P. Lovell, “Emulsion Polymerization and Emulsion Polymers”, J. Wiley, 1997, pages 700–739). As a result, the process taught in this US-patent will generally require large amounts of surfactant, typically much higher than the amounts employed in the conventional aqueous emulsion polymerization process which uses the surfactant in amounts below the CMC of the surfactant and therefore does not involve the use of a microemulsion during the polymerization process. The use of a microemulsion polymerization has the disadvantages already mentioned above and moreover may result in foam building during processing.
Because of the environmental burden that fluorinated surfactants may impose, it is generally desired to produce polymer dispersions that have substantially no fluorinated surfactant. Further, the presence of such fluorinated surfactants in the polymer product may be undesired in some applications of the fluoropolymers. Further, the fluorinated surfactants are generally expensive and it is therefore desired to minimize the amount necessary for the polymerization and/or to recover and recycle the fluorinated surfactants. Several methods are known to recover and recycle the fluorinated surfactants used in the aqueous emulsion polymerization. Such methods are disclosed in e.g. EP 524585, EP 566974, EP 632009, EP 731081, WO 99/62858, WO 99/62830 and DE 19932771. It will be appreciated by one skilled in the art that these recovery processes will be more efficient and less costly if the amount of fluorinated surfactant employed in the polymerization is minimized. For example, the amount of waste water produced in these recycling and recovery processes will generally be less if less fluorinated surfactant is used in the polymerization.
It would now be desirable to improve the aqueous emulsion polymerization of fluorinated monomers to produce fluoropolymers whereby low amounts of fluorinated surfactant can be employed, for example not more than 1% by weight based on the weight of the aqueous phase of the aqueous emulsion polymerization. It is further desirable that the process can be practiced with commonly employed fluorinated surfactants that can be easily and effectively recovered and/or removed from the fluoropolymer dispersions produced. It is in particular desirable to reduce the polymerization time, improve the yield of polymer solids obtained and/or to obtain fluoropolymer dispersions having a small particle size.