Low-molecular weight polytetrafluoroethylene (PTFE) powder with a molecular weight of 600,000 or less (also called PTFE micropowder) not only has excellent chemical stability and extremely low surface energy, but particle fibrillation is also unlikely to occur therein. As a result, PTFE micropowder is used as an additive to enhance the gliding properties and texture of coated surfaces in plastics, inks, cosmetics, paint, grease, and the like (e.g., see Patent Document 1).
Examples of known methods for producing low-molecular weight PTFE include a method in which high-molecular weight PTFE is brought into contact with a specific fluoride and reacted under high temperature conditions to thermally degrade the same (e.g., see Patent Document 2) and a method in which a high-molecular weight PTFE powder or formed body is irradiated with an ionizing radiation (e.g., see Patent Document 3).
However, methods involving thermal degradation of high-molecular weight PTFE and methods involving the exposure of high-molecular weight PTFE to radiation are not always advantageous from the standpoint of equipment costs and convenience.
A process involving direct polymerization of the TFE monomer in the presence of a chain transfer agent is also known as a method for producing low-molecular weight PTFE. For example, Patent Document 4 proposes performing polymerization using a C1-3 fluoroalkane or chlorofluoroalkane as a chain transfer agent (telogen).
As in the case of the method of Patent Document 4, it is not always clear which process is to be used as the industrial polymerization method, but such industrial methods can be roughly divided into suspension polymerization and emulsion polymerization.
In suspension polymerization a polymerization initiator is dispersed in an aqueous medium in the presence of a chain transfer agent using little or no surfactant, TFE is polymerized either with TFE or a monomer copolymerizable therewith, and a low-molecular weight PTFE granular powder is directly isolated from the reaction (e.g., see Patent Documents 5 and 6). In suspension polymerization the initially formed polymer is solidified at an early stage of polymerization by high-shear stirring, and polymerization continues to occur on the solid particles in a gas-solid reaction wherein the water functions mainly as a thermal conduction medium (e.g., see Patent Document 7). Little or no surfactant is used in suspension polymerization, and low-molecular weight PTFE powder can be obtained directly, but the particle size is difficult to control.
In contrast, in emulsion polymerization a fluorine-containing surfactant that functions as a polymerization initiator and emulsifying agent is dispersed in an aqueous medium in the presence of a chain transfer agent, and TFE is polymerized either with TFE or a monomer copolymerizable therewith to obtain low-molecular weight PTFE. Unlike suspension polymerization, because of the presence of the fluorine-containing surfactant, with emulsion polymerization an aqueous dispersion containing emulsified particles (also called micelles or primary particles) of 1 μm or smaller are obtained (e.g., see Patent Document 8). The resulting aqueous dispersion can either be used as is, or can be enriched and used in a water-based coating material, etc.
When the low-molecular weight PTFE obtained by emulsion polymerization is to be used as a powder, it can be coagulated from the above aqueous dispersion and made into powdered particles (micropowder).
Low-molecular weight PTFE powdered, particles obtained by emulsion polymerization have the following characteristics: the specific surface area is greater than those obtained by suspension polymerization at 7 to 20 m2/g (rarely, 5 to 20 m2/g), and because the particles are soft, they are very effective for surface modification, e.g., improving the texture of a coated surface. Moreover, they have greater oil absorption, and a stable dispersion in a matrix material can be obtained thereby. In addition, low-molecular weight PTFE powdered particles obtained by emulsion polymerization are preferred because it is possible to control particle size by adjusting the conditions of the coagulation process noted above.
However, the cost is higher with the emulsion polymerization described above because a fluorine-containing surfactant or other expensive material must be used as an emulsifier. In addition, residual surfactant can cause discoloration, etc., of the PTFE particles.
As a result, a process for producing PTFE wherein polymerization is performed without the addition of these fluorine-containing surfactants is needed.
A TFE suspension polymerization process is known wherein the reaction is performed in an aqueous medium using TFE and a water-soluble peroxide (e.g., see Non-Patent Document 1) as a method of carrying out polymerization without the addition of a fluorine-containing surfactant. Patent Document 9 also discloses that TFE polymerization was performed in an aqueous medium without the addition of a surfactant by using disuccinic acid peroxide as the polymerization initiator, and an aqueous dispersion of PTFE was obtained thereby.
However, the aqueous dispersions disclosed in the examples of Patent Document 9 have a very dilute concentration of polymer solids at only 6.5 wt %, which is entirely impractical in terms of productivity. In addition, Patent Document 9 does not mention the addition of a chain transfer agent, and it discloses no data indicating the size and molecular weight of the resulting emulsified polymer particles.
Furthermore, a process has been disclosed wherein emulsion polymerization is carried out between TFE and either TFE or a monomer copolymerizable therewith in an aqueous medium using a chain transfer agent and a water-soluble peroxide as a method for performing polymerization without the addition of a fluorine-containing surfactant (e.g., see Patent Document 10).    Patent Document 1: Japanese Patent Application Laid-open No. H10-147617    Patent Document 2: Japanese Patent Application Laid-open    Patent Document 3: Japanese Patent Application Laid-open No. S48-78252    Patent Document 4: Japanese Patent Application Laid-open No. S51-41085    Patent Document 5: WO 2004/050727    Patent Document 6: Japanese Patent Application Laid-open No. 2005-2322    Patent Document 7: Japanese Translation of PCT Application No. 2006-509072    Patent Document 8: Japanese Patent Application Laid-open No. H7-165828    Patent Document 9: U.S. Pat. No. 2,534,058    Patent Document 10: Japanese Patent Application Laid-open No. 2009-1745    Non-Patent Document 1: Takaomi Satogawa “Handbook of Fluoropolymers” The Nikkan Kogyo Shimbun, Ltd. (1990), page 27.
There was room for improvement, however, with polymers obtained by the kind of emulsion polymerization disclosed in Patent Documents 8 and 10 in properties such as dispersibility, viscosity, and the like when used as an additive in a coating material, etc. Moreover, there was room for improvement with polymers obtained by the kind of suspension polymerization disclosed in Patent Documents 5 to 7 in properties such as the appearance, transparency, texture, and the like of a coating film obtained thereby.