Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Polymers from only ethylenic monomers or processes of...
Reexamination Certificate
2001-12-20
2003-11-11
Wu, David W. (Department: 1713)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Polymers from only ethylenic monomers or processes of...
C526S250000, C526S255000, C524S700000
Reexamination Certificate
active
06646078
ABSTRACT:
The present invention relates to a method of producing a paraffin wax used in aqueous dispersion polymerization of tetrafluoroethylene and a method of producing a polytetrafluoroethylene using the resulting paraffin wax. The resulting polytetrafluoroethylene is used for thread seal tape, a binder for fuel cells and so on.
Polytetrafluoroethylene (hereinafter referred to as PTFE) has been obtained by polymerizing tetrafluoroethylene (hereinafter referred to as TFE) alone or, if necessary, with comonomers and is used for various products.
PTFE is obtainable by aqueous dispersion polymerization and available in the form of a dispersion of polymer particles or in the form of fine powder after coagulation and drying.
PTFE dispersions are used for glass cloth impregnation to make canvas. PTFE fine powder is used as a raw material for wire coatings, tubes, thread seal tape, clothing and stretched porous films such as filters.
It is known that in aqueous dispersion polymerization of TFE, formation of a small amount of coagulum makes coagulation of the dispersion proceed acceleratingly and finally the dispersion coagulates entirely. Therefore, to prevent entire coagulation by removing a small amount of coagulum from the micells where TFE is polymerizing, a paraffin wax is usually added to the polymerization system as a dispersion stabilizer.
However, when used in aqueous dispersion polymerization of TFE, usual commercially available paraffin waxes show different influences on the polymerization behavior and the physical properties of the resulting PTFE depending on the grade or the production lot used, and this is a big problem for consistent production of PTFE by aqueous dispersion polymerization. As the influences on the polymerization behavior, reduction of the polymerization rate and hindrance of the polymerization may be mentioned. As the influences on the physical properties of the resulting PTFE, it may be mentioned that the primary particle diameters and the paste extrusion pressure are off the required standard range for molding.
Further, paraffin waxes are usually disposed as waste after they are used. It is used in maximum amounts of at least 10% in relation to water, and reduction of waste paraffin waxes is one of the major issues.
The object of the present invention is to provide a method of producing a paraffin wax for aqueous dispersion polymerization of TFE which allows high speed aqueous dispersion polymerization of TFE with excellent polymerization behavior such as excellent dispersion stability and consistent production of PTFE having excellent physical properties, and a method of producing PTFE using the resulting paraffin wax.
The present inventors have conducted extensive research on aqueous dispersion polymerization of TFE and found that the influences of the quality of the paraffin wax on the polymerization behavior of TFE and the physical properties of the resulting PTFE can be reduced when the paraffin wax is used in ordinary aqueous dispersion polymerization of TFE in the presence of a polymerization initiator and an emulsifier at least once and then recovered and used. The present invention has been accomplished on the basis of this discovery.
Namely, the present invention provides a method of producing a paraffin wax for aqueous dispersion polymerization of TFE, which comprises conducting aqueous dispersion polymerization of tetrafluoroethylene in the presence of a paraffin wax, a polymerization initiator and an emulsifier at least once and then recovering the paraffin wax.
The present invention also provides the above-mentioned method of producing a paraffin wax wherein paraffin wax is washed with water after it is recovered.
The present invention provides the above-mentioned method of producing a paraffin wax wherein the washing with water is carried by stirring the recovered paraffin wax in from 10 to 80 parts of water in relation to 100 parts of the paraffin wax at a temperature of from the melting point of the recovered paraffin wax to 95° C.
The present invention further provides a method of producing PTFE, conducting aqueous dispersion polymerization of tetrafluoroethylene by using the recovered paraffin produced by any of the above-mentioned methods in the presence of a polymerization initiator and an emulsifier.
The paraffin wax used in the method of producing a paraffin wax of the present invention is not particularly restricted and may be a commercially available paraffin wax.
When used in aqueous dispersion polymerization of TFE, commercially available paraffin waxes show different influences on the polymerization behavior and the physical properties of the resulting PTFE depending on the grade or the production lot used, and this is a big hindrance to consistent production of PTFE by aqueous dispersion polymerization.
A paraffin wax having a melting point from 40 to 65° C., particularly from 50 to 65° C., is preferable. In the aqueous dispersion polymerization, the paraffin wax is preferably used in an amount of from 0.1 to 12% (by mass, the same applies hereinafter), particularly from 0.1 to 8%, based on the mass of the water.
As the paraffin wax used in the method of producing a paraffin wax of the present invention, a fresh paraffin wax may be used alone or in combination with a recovered paraffin wax already produced by the method of producing a paraffin wax of the present invention. In the case of combination use, the ratio of a fresh paraffin wax/a recovered paraffin wax is preferably at most 80/20, particularly at most 40/60. Use of a large amount of a recovered paraffin wax and a smaller ratio of the fresh paraffin wax reduces the influences of the fresh paraffin wax on the polymerization behavior of the aqueous dispersion polymerization of TFE and the physical properties of the resulting PTFE. Therefore, even if a fresh paraffin wax is used in combination, the polymerization rate and the physical properties can be brought within given ranges. Further, the method of producing a paraffin wax of the present invention may be applied to an ordinary PTFE production process to more effectively produce a recovered paraffin wax.
The aqueous dispersion polymerization of TFE in the method of producing a paraffin wax of the present invention is conducted in the presence of a paraffin wax, a polymerization initiator and an emulsifier.
The polymerization of TFE in the method of producing a paraffin wax of the present invention may be homopolymerization of TFE or copolymerization of TFE with comonomers such as fluorine-containing monomers other than TFE. As comonomers, fluorine-containing monomers such as hexafluoropropylene, perfluorobutene-1, perfluorohexene-1, perfluorononene-1, perfluoro(methyl vinyl ether), perfluoro(ethyl vinyl ether), perfluoro(propyl vinyl ether), perfluoro(heptyl vinyl ether), (perfluoromethyl)ethylene, (perfluorobutyl)ethylene and chlorotrifluoroethylene may be mentioned. These fluorine-containing monomers may be used alone or in combination of at least two of them. The ratio of a comonomer to be copolymerized is usually at most 1%, preferably at most 0.5%.
The polymerization temperature is preferably within the range of from 5 to 120° C., particularly within the range of from 60 to 100° C. The polymerization pressure is selected appropriately and preferably within the range of from 0.5 to 4.0 MPa, particularly within the range of from 1.0 to 2.5 MPa.
As the polymerization initiator, a water-soluble radical polymerization initiator or a water-soluble redox type polymerization initiator is preferred. Preferred water-soluble radical polymerization initiators are persulfates such as ammonium persulfate and potassium persulfate and water-soluble organic peroxides such as disuccinic acid peroxide, diglutaric acid peroxides and tert-butyl hydroperoxide. They may be used alone or in combination or at least two. The amount of the polymerization initiator is preferably from 5 to 200 ppm, particularly from 15 to 120 ppm, based on the mass of the water. If the amount of the polymerization initiator is too small, the recovered paraffi
Hirai Hiroyuki
Hoshikawa Jun
Kamiya Hiroki
Kato Kazuo
Kobayashi Shigeki
Asahi Glass Company Limited
Hu Henry S.
Wu David W.
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