Porous film, separator for cell, and cell

Details Porous film, separator for cell, and cell Porous film, separator for cell, and cell

2001-10-23

2003-10-28

Foelak, Morton (Department: 1711)

Synthetic resins or natural rubbers -- part of the class 520 ser

Synthetic resins

Cellular products or processes of preparing a cellular...

C210S500270, C210S500340, C210S500420, C264S288800, C264S289600, C264S290200, C264S235800

Reexamination Certificate

active

06638988

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a porous thermoplastic resin film suitable for use as a separator of a cell, and more particularly to a porous thermoplastic resin film suitably used as a separator of an electrolytic capacitor, a lithium cell, a fuel cell, a battery, or the like.
BACKGROUND OF THE INVENTION
Hitherto, as a porous thermoplastic resin film having air permeability, there is known, for example, a porous film obtained by stretching a thermoplastic resin film containing a filler. Such a porous film having air permeability has a good moisture permeability or airy property, and is used widely as a hygienic material such as a disposable diaper.
For example, Japanese Unexamined Patent Publication No. 09-176352 discloses a porous film obtained from a polypropylene composition composed of 100 parts by weight of polypropylene, 10 to 120 parts by weight of resin particles having an average particle diameter of 0.01 to 10 &mgr;m, and 0.01 to 3 parts by weight of a &bgr;-crystal type nucleating agent. The porous film disclosed above has a gurley value, which shows air permeability, of 10 to 30000 sec/100 cc, a porosity of 10 to 70%, and the maximum pore diameter of 0.1 to 9 &mgr;m. However, according to the studies made by the inventors of the present invention, the above porous film, when used as a separator in a cell, particularly a lithium cell, increases the internal resistance of the cell and is not sufficient as the separator.
An object of the present invention is to provide a porous thermoplastic resin film suitable for a separator of a cell.
SUMMARY OF THE INVENTION
The inventors of the present invention have made eager studies in order to develop a porous thermoplastic resin film that gives a low internal resistance when the film is used as a cell separator, and have found that a porous film comprises a thermoplastic resin and a filler and satisfies a particular relationship among the thickness, the gurley value, and the average pore diameter thereof functions well when used as a cell separator. Thus, the present invention has been completed.
The present invention provides a porous film comprising a thermoplastic resin and a filler, wherein X
R
defined by the following formula:
X
R
=25×
T
GUR
×d
2
/Y
is smaller than 5, where Y (&mgr;m), T
GUR
(sec/100 cc), and d (&mgr;m) represent the thickness, the gurley value, and the average pore diameter of the film, respectively.
Further, the present invention provides a separator for a cell which separator is made of the said porous film, and a cell having the said separator.
According to the studies made by the inventors of the present invention, the porous polypropylene film disclosed in Japanese Unexamined Patent Publication No. 09-176352, for example, has the parameter X
R
as defined above of about 10 to 800; and the porous film increases the internal resistance of the cell when used as a separator of a cell. Thus the cell obtained from above porous film does not provide a sufficient performance. In contrast, the porous film provided by the present invention has a value X
R
of less than 5 and reduces the internal resistance of the cell, when the porous film provided by the present invention is used as a separator of a cell. Thus the cell containing the porous film of the present invention shows a high performance as a cell.
DETAILED DESCRIPTION OF THE INVENTION
In the porous film of the present invention, the value X
R
defined by the following formula:
X
R
=25×
T
GUR
×d
2
/Y
is less than 5, preferably not more than 3, more preferably not more than 2, where Y (&mgr;m), T
GUR
(sec/100 cc), and d (&mgr;m) represent the thickness, the gurley value, and the average pore diameter of the porous film, respectively. A porous film having a value X
R
of 5 or more, when used as a separator, increases the internal resistance of the cell, and does not provide a cell functioning well.
The gurley value is represented by the period of time that is needed for a predetermined amount of air (typically 100 cc) to permeate through a predetermined area (typically 645.16 mm
2
) of the film, and is measured according to JIS (Japanese Industrial Standard) P8117, as will be described later.
The average pore diameter d is typically measured by the bubble point method. The bubble point method is the method which includes a step of filling micro pores of a film with liquid and a step of squeezing the liquid from the micro pores by the force prevailing the surface tension of the liquid filling the micro pores. The average pore diameter d can be obtained according to ASTM F316-86 as will be described later.
Here, the gurley value T
GUR
and the average pore diameter d of the porous film of the present invention are not particularly limited as long as they are combined to give a parameter X
R
of smaller than 5; however, the gurley value T
GUR
is preferably within the range from 40 to 3000 sec/100 cc, more preferably within the range from 60 to 1000 sec/100 cc, and the average pore diameter d is preferably within the range from 0.04 to 0.4 &mgr;m, more preferably within the range from 0.04 to 0.2 &mgr;m.
The film thickness Y of the porous film of the present invention is typically from 1 to 200 &mgr;m, preferably from 5 to 50 &mgr;m, more preferably from 5 to 30 &mgr;m.
A combination giving a value X
R
of less than 5 may be obtained in the following manner. Typically, a predetermined film thickness Y &mgr;m is first set, and then the value T
GUR
×d
2
is determined in accordance with the value of Y so that the value X
R
will be less than 5. Since T
GUR
is generally correlated to the pore diameter d and the number of pores, it is sufficient to determine the relationship between d and T
GUR
experimentally. Since the pore diameter and the number of pores are correlated to the average particle size and the filling amount of the filler as described later, respectively, it is sufficient to set the particle size and the filling amount of the filler so that the value X
R
will be less than 5. Alternatively, for example, T
GUR
×d
2
may be determined in advance by experimental measurement or the like, and the value of Y may then be set so that the value X
R
will be smaller than 5.
The thermoplastic resin used in the porous film of the present invention may be, for example, a polyolefin resin such as a homopolymer of olefin such as ethylene, propylene, butene, or hexene, or a copolymer of two or more kinds of these olefins, or a copolymer of one or more kinds of these olefins and one or more kinds of monomers polymerizable with the olefins; an acrylic resin such as polymethyl acrylate, polymethyl methacrylate, or ethylene-ethyl acrylate copolymer; a styrenic resin such as butadiene-styrene copolymer, acrylonitrile-styrene copolymer, polystyrene, styrene-butadiene-styrene copolymer, styrene-isoprene-styrene copolymer, or styrene-acrylic acid copolymer; a vinyl chloride resin; a vinyl fluoride resin such as polyvinyl fluoride or polyvinylidene fluoride; an amide resin such as 6-nylon, 6,6-nylon, or 12-nylon; a saturated ester resin such as polyethylene terephthalate or polybutylene terephthalate; polycarbonate; polyphenylene oxide; polyacetal; polyphenylene sulfide; a silicone resin; a thermoplastic urethane resin; polyetheretherketone; polyether imide; thermoplastic elastomers of various kinds; cross-linked products of these; or the like.
The porous film of the present invention may contain one or more kinds of thermoplastic resins.
Among the aforesaid thermoplastic resins, a porous film made of a polyolefinic resin is excellent in solvent resistance, and is melted to close pores thereby at a temperature law enough to restrain the abnormal reaction of the cell. Thus, the porous film made of a polyolefin resin may be preferable for using as a separator for a lithium cell.
The olefins used in the present invention may be, for example, ethylene, propylene, butene, hexene, or the like. Specific examples of polyolefins include polyethylenic resins such as low-density polyethylene, linea

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