Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – At least one aryl ring which is part of a fused or bridged...
Reexamination Certificate
2001-11-14
2003-05-20
Wu, David W. (Department: 1713)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
At least one aryl ring which is part of a fused or bridged...
C524S546000, C524S545000, C524S543000, C524S544000, C526S247000, C428S402000
Reexamination Certificate
active
06566440
ABSTRACT:
The present invention relates to a tetrafluoroethylene agglomerated product which is useful as a molding material for e.g. sliding members.
Polytetrafluoroethylene (hereinafter referred to as PTFE) produced by suspension polymerization is used for molding by compression molding or ram extrusion molding. By suspension polymerization, PTFE particles having an average particle diameter of from 2 to 5 mm are usually obtained, and such particles are pulverized and used for various applications in the form of a PTFE powder having an average particle diameter of from 25 to 65 &mgr;m. Particularly, a PTFE agglomerated product obtained by uniformly mixing a PTFE powder having an average particle diameter of from 25 to 65 &mgr;m with various fillers to obtain a composition, followed by agglomeration, is used as a powder for molding.
A molded product obtained by molding such a PTFE agglomerated product has physical properties such as abrasion resistance and hardness improved over a molded product of PTFE containing no filler, and is widely used as various sliding members for sealing. However, in a case of producing a thin molded product or a very small molded product, such a PTFE agglomerated product has had no adequate property such as powder flow properties required for automated filling into a mold for molding.
JP-A-7-11088 discloses a PTFE agglomerated product obtained from a PTFE powder having an average particle diameter of at most 20 &mgr;m and a filler. It is disclosed that a PTFE agglomerated product obtained by using a glass fiber powder, a graphite powder, a bronze powder, a carbon fiber powder, etc., as a filler, has an excellent powder flow properties and a high bulk density and is useful for automation of the molding process by a mold or for improvement of the productivity. However, with respect to use of a coke powder as a filler, there is no disclosure.
Recently, a sliding member such as a so-called non-cut seal ring to be fit on a slide shaft while elongating the seal ring by means of a jig, is used in various apparatus. Accordingly, a sliding member having high elongation, which is useful for application to such a seal ring, is required, and the demand for a PTFE molded product having high elongation is increasing. Particularly, a sliding member having a high hardness excellent in abrasion resistance in addition to the high elongation is required for a seal ring on a sliding surface of an apparatus made of a material of high hardness such as iron or stainless steel.
JP-B-8-30135 discloses a composition comprising a graphite powder and a PTFE powder. A molded product obtained by molding this composition has high elongation, but the hardness is low. Further, such a composition is inadequate in the powder flow properties at the time of molding. Further, with respect to a PTFE agglomerated product, there is no disclosure.
JP-A-11-21407 discloses a composition comprising PTFE and a carbon filler having a specific surface area of at most 3 m
2
/g. A molded product obtained by molding such a composition has high elongation, but such a composition is inadequate in the powder flow properties. Further, with respect to a PTFE agglomerated product, there is no disclosure.
It is an object of the present invention to provide a PTFE agglomerated product having excellent powder flow properties and a high bulk density, and a molded product having high elongation and high hardness, obtained by molding such a PTFE agglomerated product.
The present invention provides a polytetrafluoroethylene agglomerated product obtained from a polytetrafluoroethylene powder and a coke powder, wherein the polytetrafluoroethylene powder contains at least 8.3 mass % of a polytetrafluoroethylene fine powder having a particle diameter of from 0.1 to 9.5 &mgr;m, and the mass ratio of the polytetrafluoroethylene powder/the coke powder is 95-60/5-40.
Further, the present invention provides a molded product obtained by molding such a PTFE agglomerated product.
Now, the present invention will be described in detail with reference to the preferred embodiments.
The PTFE powder in the present invention contains at least 8.3 mass % of a PTFE fine powder having a particle diameter of from 0.1 to 9.5 &mgr;m (hereinafter referred to simply as the PTFE fine powder). If the content of the PTFE fine powder (hereinafter referred to as the PTFE fine powder content) in the PTFE powder is less than 8.3 mass %, the agglomerative tendency at the time of the production of the agglomerated product tends to be low, the powder flow properties of the PTFE agglomerated product thereby obtained, tends to be poor, and the elongation of the molded product tends to be low. The PTFE fine powder content in the PTFE powder is preferably at least 10 mass %, more preferably at least 12 mass %.
Further, the smaller the particle diameter of the PTFE fine powder, the better the agglomerative tendency. However, if the particle diameter is too small, production of such a PTFE fine powder tends to be difficult. On the other hand, with a PTFE fine powder having a particle diameter exceeding 9.5 &mgr;m, the effect for improvement of the agglomerative tendency is not large enough. If PTFE particles having too large article diameters are contained in the PTFE powder, the agglomerative tendency tends to be low, the powder flow properties of the agglomerated product tends to be low, and a molded product having a high elongation tends to be hardly obtainable. The maximum particle diameter of the PTFE powder is preferably at most 750 &mgr;m, more preferably at most 600 &mgr;m, most preferably at most 450 &mgr;m.
As a method for producing the PTFE powder in the present invention, it is preferred to pulverize PTFE particles having an average particle diameter of from 2 to 5 mm obtained by suspension polymerization. Particularly, a PTFE powder having an average particle diameter of at most 20 &mgr;m is preferred. For example, a PTFE powder having an average particle diameter of 20 &mgr;m obtained by pulverization contains 15 mass % of a PTFE fine powder.
Further, as a method for producing the PTFE powder in the present invention, it is also preferred to mix the PTFE powder having an average particle diameter of at most 20 &mgr;m, obtained by the pulverization, with a PTFE powder having an average particle diameter of from 25 to 65 &mgr;m, which is readily available. For example, when a PTFE powder having an average particle diameter of 5.7 &mgr;m and having a PTFE fine powder content of 90 mass % and a PTFE powder having an average particle diameter of 35.1 &mgr;m and having a PTFE fine powder content of 8 mass % are mixed by means of a Henschel mixer in a mass ratio of 20/80, a PTFE powder having an average particle diameter of 32.9 &mgr;m and having a PTFE fine powder content of 11.7 mass %, can be obtained.
As PTFE, a homopolymer of tetrafluoroethylene (hereinafter referred to as TFE) is employed, but PTFE containing polymerized units based on a comonomer copolymerizable with TFE in a proportion of at most 0.5 mol %, which will not impart melt flow properties, may also be employed.
Specific examples of such a comonomer may be fluoroolefins other than TFE, such as hexafluoropropylene, chlorotrifluoroethylene and trifluoroethylene, fluorovinyl ethers such as a perfluoro(alkyl vinyl ether), a perfluoro(alkoxy alkyl vinyl ether) and a (perfluoroalkyl) methyl trifluorovinyl ether, and olefins such as a (perfluoroalkyl)ethylene, ethylene and propylene. Particularly preferred are hexafluoropropylene, perfluoro(n-propyl vinyl ether) and (perfluoro-n-butyl)ethylene.
In the present invention, the average particle diameter of the coke powder is preferably from 1 to 50 &mgr;m. If the average particle diameter of the coke powder is too small, elongation of the PTFE molded product will be low, and if it is too large, the abrasion resistance of the PTFE molded product will be low. More preferably, the average particle diameter of the coke powder is from 10 to 35 &mgr;m.
In the present invention, the mass ratio of the PTFE powder/the coke powder is 95-60/5-40. If the PTFE
Chiba Tadao
Ishikawa Takamichi
Kuroki Satoru
Nomura Masumi
Asahi Glass Company Limited
Hu Henry
Wu David W.
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