Manufacturing method of a highly conductive...

Details Manufacturing method of a highly conductive... Manufacturing method of a highly conductive...

1998-09-02

2001-08-07

Ortiz, Angela (Department: 1732)

Plastic and nonmetallic article shaping or treating: processes

Forming electrical articles by shaping electroconductive...

C264S118000, C264S119000, C264S127000, C264S146000, C264S175000, C264S150000

Reexamination Certificate

active

06270707

ABSTRACT:

FIELD OF INDUSTRIAL APPLICATION
This invention relates to a manufacturing method of a highly conductive polytetrafluoroethylene sheet (hereinafter called PTFE sheet) which is suitable for a raw material of sheet-type heating elements and electrodes of primary batteries, secondary batteries and fuel cells, especially highly conductive PTFE sheet which is suitable for a raw material of highly conductive PTFE sheet having a wide-width and a long-length. This invention also relates to a highly conductive PTFE sheet which conductivity in the longitudinal direction is nearly uniform in the cross direction.
PRIOR ART
Such a highly conductive PTFE sheet has been conventionally manufactured by the following method. PTFE prepared by the suspension polymerization method is ground into powder with a particle size of 20-40 &mgr;m. This powder is mixed with conductive carbon (5-15 weight percent),
The resulting mixture is placed in a certain metal mold for compression preforming at 400-600 kgf/cm
2
. The molded block is sintered at 380° C. for 5-10 hours. Thus obtained block is shaved into thin sheets.
These sheets have volume resistivity of 10
2
-10
4
&OHgr;-cm which is low conductive. The surface of this low conductive sheets lack smoothness, because scratches are formed on the surface during the shaving process.
PTFE sheet can be also produced by another method. Aqueous dispersion containing PTFE with a particle size of 0.15-0.35 &mgr;m is obtained by the emulsion polymerization method.
A nonionic surface-active agent, water and carbon are kneaded in a ball mill to prepare the conductive paste. The aqueous dispersion and the paste are mixed to prepare a coating material. This coating material is then coated onto a plate. After drying, the plate is sintered at 380° C. for approximately 15 minutes. The coated film separated from the plate can be obtained as a conductive sheet.
The volume resistivity of the sheet produced by this method ranges from approximately 10
0
&OHgr;-cm to 10
1
&OHgr;-cm if conductive carbon is added at 8-30weight percent. The volume resistivity depends on the amount of conductive carbon added, but the conductivity of this sheet is riot yet sufficient. And, producing a sheet having a thickness exceeding 60 &mgr;m and without crack by this technique is extremely difficult.
Therefore, highly conductive PTFE sheet is conventionally produced by the following method. A paste of PTFE powder admixed with conductive substance and extrusion lubricant is extruded through dies as shown in FIG.
5
(
a
) and FIG.
5
(
b
). These extruded materials are then calendered and sintered to the final products.
According to FIG.
5
(
a
) and FIG.
5
(
b
), said paste
10
is extruded through cylindrical die
1
(
a
) or oval body type die
2
(
b
) into rod-like unsintered extruded material
3
or
4
. These extruded materials are calendered.
The highly conductive PTPE sheet having a wide-width and a long-length is obtained by the following method. As shown in FIG.
5
(
c
), the paste is extruded through T die
5
. This extruded sheet
6
is calendered. Although the technique is not illustrated in figure, extruded sheet
6
can be also calendered by a multistage calendering roll (d). In
FIGS. 5
,
11
,
12
and
13
show respectively a ram, a fixing ring and a heater.
However, the above-mentioned producing techniques (a) and (b) can provide sheets as commercial products having the maximum width of 160 mm because the width of such obtained products are restricted. When the width of the products are larger, their conductivity in the longitudinal direction decreases and loses uniformity in the cross direction. The conductivity in the longitudinal direction of the above-mentioned techniques (c) and (d) is low and a multistage calendering roll or a roll with large diameter is needed to produce these sheets.
[A roll with large diameter (e.g 500 mm&phgr;) or the like is preferably used to produce PTFE sheets with higher conductivity.] These techniques require complicated and large equipment, thus increasing the cost.
OBJECT OF THE INVENTION
The objects of this invention are to provide highly conductive PTFE sheet which is useful for a raw material of sheet-type beating elements and electrodes of primary batteries, secondary batteries and fuel cells, and to provide a producing method thereof. This highly conductive PTFE sheet requires the following conditions:
(1) Wide width
(2) High conductivity in the longitudinal direction
(3) Conductivity in the longitudinal direction is nearly uniform in the cross direction.
CONSTRUCTION OF THE INVENTION
This invention concerns a method to manufacture highly conductive PTPE sheets. According to this method, A paste of PTFE powder admixed with conductive substance and extrusion lubricant is extruded to prepare an unsintered tube-like material. At least one place on the circumference of this unsintered tube-like extruded material, the material is cut open longitudinally and the resulting sheet is further calendered if necessary. This invention also concerns a method to manufacture highly conductive PTFE sheets by sinterinig the sheets produced by the above-mentioned method.
(These methods are regarded as the invented manufacturing methods.)
These invented manufacturing methods can be applied to the manufacturing of highly conductive PTFE sheets being not less than 170 mm in width and also those being less than 170 mm in width. These invented manufacturing methods can be applied to the manufacturing of highly conductive PTFE sheet having a wide-width and a long-length (the invented sheet) which is not less than 170 mm in width and whose variance of volume resistivity (conductivity) in the longitudinal direction is 10% or less in the cross directions, preferably 7% or less.
(The definition of the variance is described later.)
According to these invented manufacturing methods, at least one place on the circumference of the tube-like extruded material, the material is out open longitudinally to a sheet. This sheet is calendered if necessary so that PTFE particles are sufficiently oriented in the longitudinal direction (on the internal and external surfaces of the extruded material or in the whole extruded material). This orientation is carried out during the calendering process. Therefore, the mixed conductive substances such as conductive carbon are arranged lengthwise in nearly straight lines. This parallel arrangement of conductive substances is indispensable for producing highly conductive PTFE sheet with conductivity in the longitudinal direction is nearly uniform in the cross direction.
The PTFE sheets with following excellent properties of the invention can be obtained with high reproducibility by the invented manufacturing methods. Those with uniform exothermic properties are used for sheet-type heating elements, while those with water repellent properties are used for the electrodes of primary batteries, secondary batteries and fuel cells.
These invented sheets are highly conductive which are not less than 170 mm in width and whose variance of volume resistivity (conductivity) in the longitudinal direction is 10% or less, preferably 7% or less.
PTFE, which is used for a raw material of the invented method and sheet, can be preferably obtained usually from PTFE aqueous dispersion prepared by emulsion polymerization of tetrafluoroethylene (hereinafter called TFE).
PTFE includes not only PTFE itself but also modified PTFE which are modified by PTFE modifying monomers.
PTFE modifying monomers include fluorine-containing unsaturated monomers except for TFE, such as fluoroalkyl vinyl ethers expressed by the following chemical formulas:
X(CF
2
)
n
OCF═CF
2
where,
“X” indicates hydrogen, fluorine or chlorine and “n” indicates an integer ranging from 1 to 6,
or,
C
3
F
7
(OCF
2
CF
2
CF
2
)
m
(OCF(CF
3
)CF
3
)
l
OCF═CF
2
where,
“m” and “l” are integers ranging from 0 to 4, that are never zero simultaneously.
CF
3
—CF═CF
2
, CF
2
═CFH, CF
2
═CFC
l
, CF
2
═CH
2
or RfCY═CH
2
where,
“Rf” indicates straight or branched

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