Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Mixing of two or more solid polymers; mixing of solid...
Reexamination Certificate
2002-02-04
2003-12-16
Wilson, Donald R. (Department: 1713)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Mixing of two or more solid polymers; mixing of solid...
C525S326400, C525S356000
Reexamination Certificate
active
06664337
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a method for stabilizing a fluorine-containing polymer. In particular, the present invention relates to a method for stabilizing a fluorine-containing polymer by treating a fluorine-containing polymer which has unstable chain ends and/or unstable bonds in the backbones under specific conditions.
PRIOR ART
In the case of, for example, emulsion copolymers of tetrafluoroethylene and hexafluoropropylene, bubbles or voids may be formed from volatile materials in products produced by melt processing. The volatile materials are generated from the unstable chain ends and unstable backbones of the polymers, when heat or shear force is applied to the polymers.
The kinds of unstable chain end groups vary with polymerization methods, and the kinds of polymerization initiators and chain transfer agents. For example, carboxylic acid terminal groups are formed, when a conventional persulfate salt (e.g. ammonium persulfate, potassium persulfate, etc.) is used as a polymerization initiator in emulsion polymerization. It is known that such carboxylic acid terminal groups are the sources for volatile materials in the melt processing. Depending on the conditions in the melt processing, groups such as olefinic groups (—CF═CF
2
), acid fluoride groups (—COF) and the like are formed at the chain ends. These end groups may cause bubbles or voids in the final products of the polymers.
Backbones which may generate volatile materials may be bonds between comonomers other than tetrafluoroethylene (TFE), as U.S. Pat. No. 4,626,587 describes. In the case of tetrafluoroethylene-hexafluoropropylene copolymers (FEP), the unstable bonds in the backbones are bonds between hexafluoropropylene monomers (HFP). This is confirmed form the fact that, when a gas generated by heating and melting FEP around 400° C. is analyzed, a molar ratio of HFP to TFE in the generated gas is about two times larger than that in the polymers.
U.S. Pat. No. 4,626,587 proposes the removal of unstable chain end groups and unstable bonds in the backbones, which may be the cause of bubbles or voids found in the final products of fluorine-containing polymers, by the application of a shear force with a twin-screw extruder. However, the use of a twin-screw extruder can remove the unstable bonds in the backbones because of the large shear force of the extruder, but hardly stabilizes the unstable end groups because of the too short residence time. In addition, it is very difficult to remove coloring which appears because of the severe melting conditions, and the residues of polymerization initiators or contamination. Thus, additional stabilization treatment such as fluorination with other equipment is necessary after the treatment with the twin-screw extruder. Furthermore, molded articles should be treated at a temperature lower than the melting point of the polymer, when the unstable end groups are stabilized after melt molding, since the shapes of the molded articles should be maintained.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a method for effectively removing unstable end groups and unstable bonds in backbones from fluorine-containing polymers and also coloring, in the melt kneading step.
The above object can be achieved by a method for improving the thermal stability of a fluorine-containing polymer comprising melt kneading a melt-processable fluorine-containing polymer with a kneader which has a residence time of at least 10 minutes, a usable volume ratio (usable space in a container/space in a container) of larger than 0.3, and a power factor K of less than 8000, the power factor K being represented by the formula:
K=Pv/&mgr;
2
wherein Pv is a power requirement per unit volume (W/m
3
), &mgr; is a melt viscosity (Pa.s), and n is a rotation speed (rps).
DETAILED DESCRIPTION OF THE INVENTION
A kneader used in the method of the present invention is distinguished from the above twin-screw extruder in that the kneader has a longer residence time than the extruder, that is, the residence time is usually at least 10 minutes, preferably between 10 and 120 minutes, and that the structures (e.g. usable volume ratios, etc.) and the power factors are different between them.
The conventional twin-screw has a usable volume ratio (usable space in a container/space in a container) of 0.3 or less, while a kneader which is preferably used in the present invention, that is, a so-called “surface renewal type kneader” has a usable volume ratio of larger than 0.3, preferable at least 0.5. Herein, a usable space in a container means the space volume of a container in which paddles, a shaft, and the like are equipped, while a space in a container means a space volume of a container not having paddles, a shaft, or the like.
The twin-screw extruder has a power factor K, which is defined by the above formula, in the range between 8,000 and 12,000, while the surface renewal type kneader has a power factor of less than 8,000, often 7,000 or less. The surface renewal type kneader has self-cleaning properties, and high piston flow properties in continuous operation.
Typical examples of the surface renewal type kneaders include HVR, SCR and NEW-SCR (all manufactured by Mitsubishi Heavy Industries, Ltd.), BIBOLACK (manufactured by Sumitomo Heavy Machinery and Industries, Ltd.), HITACHI EYEGLASS-PADDLE POLYIMERIZER and HITACHI GATE-PADDLE POLYMERIZER (manufactured by Hitachi Ltd.), AP-MACHINE and NEW AP-MACHINE (manufactured by LIST), and the like.
Examples of the fluorine-containing polymers which are stabilized by the method of the present invention include melt-processable copolymers comprising at least two monomers selected from the group consisting of tetrafluoroethylene, hexafluoropropylene, perfluoroalkyl vinyl ethers, ethylene, vinylidene fluoride and chlorotrifluoroethylene; vinylidene fluoride homopolymer; chlorotrifluoroethylene homopolymer; and the like.
The perfluoroalkyl vinyl ethers include a vinyl ether of the formula:
CF
2
═CFO (CF
2
)
m
F
wherein m is an integer of 1 to 6, and a vinyl ether of the formula:
CF
2
═CF(O—CF
2
CF(CF
3
))
n
OC
3
F
7
wherein n is an integer of 1 to 4.
In particular, when a fluorine-containing polymer, which is treated by the method of the present invention, is a tetrafluoroethylene-hexafluoropropylene copolymer (FEP), it preferably comprises 72 to 96 wt. % of tetrafluoroethylene and 4 to 28 wt. % of hexafluoropropylene. When a fluorine-containing polymer is a tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer (PFA), it preferably comprises 92 to 99 wt. % of tetrafluoroethylene and 1 to 8 wt. % of a perfluoroalkyl vinyl ether. When a fluorine-containing polymer is a tetrafluoroethylene-ethylene copolymer (ETFE), it preferably comprises 74.5 to 89.3 wt. % of tetrafluoroethylene and 10.7 to 25.5 wt. % of ethylene.
These polymers may comprise other monomers in an amount such that the inherent properties of each copolymer do not deteriorate. Examples of the other monomers include hexafluoropropylene (when the fluorine-containing polymer does not comprise hexafluoropropylene), perfluoroalkyl vinyl ethers (when the fluorine-containing polymer does not comprise a perfluoroalkyl vinyl ether), ethylene (when the fluorine-containing polymer does not comprise ethylene), vinylidene fluoride (when the fluorine-containing polymer does not comprise vinylidene fluoride), and chlorotrifluoroethylene (when the fluorine-containing polymer does not comprise chlorotrifluoroethylene).
The melt processable fluorine-containing polymers are preferably prepared by emulsion or suspension polymerization. When the polymers are FEP, PFA, and copolymers of tetrafluoroethylene, hexafluoropropylene and a perfluoroalkyl vinyl ether, they have a melt viscosity in the range between 0.1 and 100 kPa.s at 372° C.
The method of the present invention is carried out preferably at a temperature in the range between 200 and 450° C.
The method of the present invention requires a residence time of at least 10 minutes to achieve the desired effects. Wh
Hiraga Yoshiyuki
Imanishi Hiroyuki
Komatsu Satoshi
Noda Tomohisa
Daikin Industries Ltd.
Wilson Donald R.
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