Plastic and nonmetallic article shaping or treating: processes – Forming articles by uniting randomly associated particles – Autogenously or by activation of dry coated particles
Reexamination Certificate
1999-09-17
2001-09-11
Pezzuto, Helen L. (Department: 1713)
Plastic and nonmetallic article shaping or treating: processes
Forming articles by uniting randomly associated particles
Autogenously or by activation of dry coated particles
C264S310000, C264S331140, C428S421000, C428S422000, C526S242000, C526S250000, C526S255000
Reexamination Certificate
active
06287497
ABSTRACT:
FIELD OF INVENTION
The present invention relates to a molded article made of polytetrafluoroethylene (hereinafter also referred to as “PTFE”) and a method of producing the same. According to the present invention, a large PTFE molded article block having less distortion and small residual stress can be produced.
More particularly, the present invention relates to a PTFE block-shaped molded article (popularly referred to as a large-sized block), which has less distortion and height (i.e. length) of 800 mm or more. The present invention also relates to a film and sheet, which are obtained by cutting from the block and have less distortion and residual stress.
RELATED ART
PTFE has a very high melt viscosity of about 10
11
poises at 380° C. and, therefore, molding methods such as extrusion and injection used for general thermoplastic resins (having a melt viscosity of 10
3
to 10
4
poises on molding) can not be applied.
For this reason, as the molding method of PTFE, the following compression molding method is most popular and block-shaped molded articles can be molded by this method.
(a) A mold is filled uniformly with raw material powder, and the mold is compressed by using a press at a pressure of 100 to 1,000 kg/cm
2
.
(b) The resulting comparatively brittle preform is put in a furnace and heated to a baking temperature of 360 to 380° C., and then maintained at the same temperature until the sintering has uniformly completed.
(c) The furnace temperature is lowered to room temperature as such to obtain a block-shaped molded article.
The block thus obtained is cut to obtain a film having a thickness of about 25 &mgr;m. The resulting PTFE film can be used in heat-resistant electric wires, heat-resistant insulating tapes for vehicle motor and generator and the like.
However, the following problems occur when producing particularly large block-shaped molded articles by using this method.
(1) The internal stress because of raw material powder's own weight remains in the block-shaped molded article after baking.
(2) When particularly using modified PTFE as the raw material powder, the block-shaped molded article is deformed during the baking.
Cutting of such block-shaped molded article causes large loss in expensive PTFE material and curl of the resulting film.
SUMMARY OF INVENTION
An object of the present invention is to produce a large molded article block free from distortion and residual stress.
The present invention provides a method of producing a polytetrafluoroethylene molded article, which comprises compression-molding polytetrafluoroethylene powder to form a preform and then baking the resulting preform with rotating.
The present invention also provides a polytetrafluoroethylene molded article wherein loss in weight until a stable film or sheet can be cut:
[(weight of whole molded article)−(weight at minimum outer diameter)]÷(weight of whole molded article)×100
is not more than 0.7% by weight and a height of the molded article is not less than 800 mm.
The present invention further provides a film or sheet having less distortion, which is obtained by cutting from the polytetrafluoroethylene molded article.
REFERENCES:
patent: 4-185426A (1992-07-01), None
patent: 6-15663A (1994-01-01), None
patent: 615663 (1994-01-01), None
patent: 4185426 (1997-07-01), None
Honji Kunihiko
Kawachi Shoji
Shirasaki Osamu
Uchida Tatsuro
Yamada Masahiko
Birch & Stewart Kolasch & Birch, LLP
Daikin Industries Ltd.
Pezzuto Helen L.
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