Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Polymers from only ethylenic monomers or processes of...
Reexamination Certificate
2002-12-10
2004-10-12
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...
C525S194000, C524S520000, C524S545000
Reexamination Certificate
active
06803437
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a process for producing a tetrafluoroethylene polymer, and it relates to a process for producing a tetrafluoroethylene polymer which is capable of providing a fine powder to be used e.g. as a material for a thread seal tape having high strength or as a binder for a fuel cell, and a molding powder or the like, of which a molded product has high strength.
BACKGROUND ART
Heretofore, a tetrafluoroethylene polymer (hereinafter referred to as PTFE) has been obtained by polymerizing tetrafluoroethylene (hereinafter referred to as TFE) alone, or by copolymerizing it with a modifying monomer, as the case requires, and it is used for various applications.
PTFE can be produced by dispersion polymerization or suspension polymerization of TFE. By the dispersion polymerization, it can be obtained in the form of a dispersion having polymer particles dispersed, or in the form of a fine powder obtained by coagulating the polymer dispersion, followed by drying. By the suspension polymerization, the obtained PTFE may be pulverized to obtain a fine powder, and further, this fine powder may be agglomerated to obtain a molding powder.
The dispersion of PTFE may be used in an application wherein it is impregnated to glass cloth to obtain a tent film. Further, a fine powder of PTFE is used as a starting material for a stretchable porous product such as a covering for electric wires, a tube, a thread seal tape, clothing, a filter, etc. Further, the molding powder of PTFE is used to obtain a molded product by e.g. compressing, sintering, followed by cutting.
Usually, in the polymerization of TFE, the amount of the polymerization initiator to be used, is extremely small as compared with polymerization for a usual hydrocarbon type polymer. Further, in order to maintain the mechanical properties of the resulting PTFE to be good, a very high molecular weight is required. Accordingly, the starting material to be used is required to have very high purity in order not to lower the polymerization rate by deactivating grown radicals or not to lower the molecular weight by chain transfer.
Namely, materials such as a dispersing agent and a dispersion stabilizer to be used for the polymerization are required to have very high purity. To meet such requirements, a method has, for example, been proposed as in JP-A-9-157310 in which impurities in paraffin wax are specified by the amount of an antioxidant contained in the paraffin wax. However, even with such paraffin wax, a substantial fluctuation may sometimes be observed in the polymerization rate or in the physical properties of the obtained polymer, and such can not be regarded as sufficient, as the specification for the impurities in the paraffin wax.
Further, very high purity is required also with respect to water to be used for the polymerization of TFE, and if even in a trace amount of impurities influential over the polymerization are contained therein, they may give a substantial influence over the polymerization rate or over the physical properties of the obtained polymer. Accordingly, it is usually required to employ water which has a very low content of e.g. hydrocarbon type organic substances or ions, which are influential over the polymerization.
However, it is very difficult to identify such impurities which adversely affect the polymerization of PTFE especially because their contents are very small. Heretofore, no adequate qualitative and quantitative analyses have been made, and the influence of impurities contained in raw materials, has not been apparent till actual polymerization. Especially, no case has been reported in which the purity of water to be used for the polymerization is prescribed, and no prescription has been made clear with respect to the purity of water to be used for the polymerization.
It is an object of the present invention to provide a process for producing PTFE, whereby a decrease or fluctuation in the polymerization rate during the polymerization of PTFE, will be minimal, the obtained PTFE will have a low standard specific gravity with minimal fluctuation, and a deterioration or fluctuation of the mechanical properties of the obtained PTFE will be minimal.
By present inventors considered that the impurities which adversely affect the polymerization of TFE are hydrocarbon type compounds capable of undergoing chain transfer during the polymerization, ionic compounds, and water-soluble compounds which readily enter into micelles in water as the reaction site during the polymerization, or organic colloids and extensively studied their influences. As a result, it has been found that when water purified to have a TOC and/or an electrical conductivity within specific ranges, is used as the water to be used for the polymerization, a decrease or fluctuation in the polymerization rate can be minimized, and a deterioration or fluctuation of the physical properties of the resulting PTFE can be minimized. The present invention has been accomplished on the basis of this discovery.
DISCLOSURE OF THE INVENTION
The present invention provides a process for producing PTFE, which comprises polymerizing TFE in an aqueous medium in the presence of a water-soluble initiator, wherein the water used for the polymerization has a TOC of 100 ppb or lower and/or an electrical conductivity of 1.0 &mgr;S/cm or lower.
Further, the present invention provides the above process for producing PTFE, wherein the polymerization is carried out in an aqueous medium in the presence of a dispersing agent and a dispersion stabilizer.
BEST MODE FOR CARRYING OUT THE INVENTION
The process for producing PTFE of the present invention is characterized in that water having a TOC and an electrical conductivity within the specified ranges, is used as the water to be used for the polymerization. The TOC is total organic carbon i.e. a total amount of organic carbon, which is used as an index for the content of organic carbon in water. In the present invention, the water to be used for the polymerization may be one which satisfies either one of conditions that the TOC is 100 ppb or lower, and the electrical conductivity is 1.0 &mgr;S/cm or lower, but one satisfying both conditions, is preferred.
In a case where the water to be used for the polymerization is defined by such TOC, the TOC is 100 ppb or lower, preferably 20 ppb or lower.
In a case where the water to be used for the polymerization is defined by the electrical conductivity, the electrical conductivity is 1.0 &mgr;S/cm or lower, preferably 0.1 &mgr;S/cm or lower. By using water having a TOC and/or an electrical conductivity within these ranges, the polymerization rate and the physical properties of the resulting PTFE will be improved, and a fluctuation in the physical properties will be small. Here, the electrical conductivity is in a relation of an inverse number to the resistivity. Accordingly, the range of the electrical conductivity of the present invention may be represented also by a range of the resistivity. The range of the resistivity is 100 &OHgr;·cm or higher, preferably 1000 &OHgr;·cm or higher.
In the dispersion polymerization of TFE, it is common to employ a method wherein in addition to a polymerization initiator, a dispersing agent made of a perfluoro compound or a dispersion stabilizer such as paraffin wax is added to the polymerization system. If the dispersing agent made of a perfluoro compound or paraffin wax is present in the polymerization system, the values of the TOC and the electrical conductivity of water in the polymerization system will be substantially higher than the values specified in the process of the present invention. However, when water having a purity as defined in the process of the present invention is used, and a commercially available dispersing agent made of a perfluoro compound having a high purity e.g. with a APHA color of at most 50, or a dispersion stabilizer such as paraffin wax having a high purity, e.g. where the content of the antioxidant is not more than 50 ppm, is used, the polymerization will proceed without any proble
Hirai Hiroyuki
Hoshikawa Jun
Kamiya Hiroki
Kato Kazuo
Kobayashi Shigeki
Asahi Glass Company Limited
Hu Henry S.
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Wu David W.
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