Stock material or miscellaneous articles – Web or sheet containing structurally defined element or... – Composite having voids in a component
Utility Patent
1998-06-05
2001-01-02
Speer, Timothy M. (Department: 1775)
Stock material or miscellaneous articles
Web or sheet containing structurally defined element or...
Composite having voids in a component
C428S913000, C429S062000, C429S145000
Utility Patent
active
06168858
ABSTRACT:
TECHNICAL FIELD
This invention relates to a polyethylene microporous film, a separator composed thereof and a battery in which the same is used as a battery separator.
BACKGROUND ART
Polyethylene microporous films are used in micro filtration membranes, battery separators, condenser separators and the like. Among these uses, the use as a battery separator, particularly a separator for lithium ion cell, requires that the polyethylene microporous film exerts, in addition to general physical properties such as mechanical strength and film permeability, a so-called “fuse effect” such that when the inner part of the battery is overheated, the separator fuses to form a film, which covers the electrode, thereby cutting off the electric current to secure the safety of battery.
In the case of a polyethylene microporous film, it is known that the fuse temperature, namely the temperature at which the fuse effect is exerted, is about 130 to 150° C. Even when the inner part of a battery is overheated by any cause, the microporous film fuses upon reaching the fuse temperature to form a film with which the electrode is covered, so that the passage of ions is cut off and the cell reaction is stopped. However, when the temperature elevation is very rapid, the temperature in the inner part of battery is further elevated even after the polyethylene microporous film has fused, and consequently the film is broken, which causes a short in the battery in some cases, so that the development of a polyethylene microporous film having a lower fuse temperature has become a problem.
For example, JP-A-5-25,305 and JP-A-2-21,559 disclose a method for lowering the fuse temperature of a film by blending into an ultrahigh molecular weight polyethylene (UHMWPE) a branched, low-density polyethylene (LDPE) or a linear, low-density polyethylene (LLDPE). According to this method, the fuse temperature of the film can be expected to be lowered to some extent. On the other hand, however, the melt index (MI) is 0.1 to 100 which is very high as compared with UHMWPE and HDPE, so that there have been problems such as the mechanical strength and permeability of film are deteriorated, and when the amount of LDPE and LLDPE added is increased the film is not made porous. Moreover, the above method has had a problem in film productivity, such as at least one hour being required for dissolving the polymer by heating because UHMWPE having a low dissolvability is used as the essential component.
The object of this invention is to solve the above-mentioned problems and provide a polyethylene microporous film which is excellent in mechanical strength, permeability and productivity and has a low fuse temperature.
The present inventors have examined a linear copolymeric polyethylene having a much higher molecular weight than LLDPE which has been used in the prior art. As a result, it has been found that a linear copolymeric polyethylene can be made porous even when used alone and a polyethylene microporous film produced therefrom has a high strength and a low fuse temperature, which compares quite well with the prior art. However, such a film has not had a sufficient permeability for using as a separator as compared with a film produced from HDPE having a high molecular weight.
Therefore, a further examination has been made on the kind of a comonomer to be introduced as a monomeric unit into the linear copolymeric polyethylene, and the present inventors have surprisingly found that a microporous film composed of a linear copolymeric polyethylene in which propylene is used as the comonomer (referred to hereinafter as the C3 copolymer in some cases) is smaller in shrinkage than a microporous film composed of a copolymer in which butene-1 is used as the comonomer (referred to hereinafter as the C4 copolymer in some cases) and a microporous film composed of a copolymer in which other &agr;-olefins are used, and has a sufficient permeability for using a separator.
The reason why the C3 copolymer specifically imparts a high porosity to the film has not been clarified, but it is considered that the methyl group which is the side chain of the C3 copolymer is easily incorporated into the crystals of the polymer as compared with an ethyl group and butyl group, so that it could be possible that the C3 copolymer has a crystalline structure close to HDPE in spite of being a copolymer, and exhibits a higher permeability than copolymers in which other comonomers are used such as the C4 copolymer.
Further, the present inventors have made extensive research to achieve a higher permeability and have consequently found that when the C3 copolymer is not used alone but used in admixture with a high density polyethylene, a higher permeability is obtained simultaneously with the same fuse temperature, and based on this knowledge, they have succeeded in producing a battery having more improved discharge characteristics and safety.
DISCLOSURE OF INVENTION
The first mode of this invention is a polyethylene microporous film which is composed of a linear copolymeric polyethylene having a melt index of less than 0.1 and a propylene unit content of 0.1 to 4 mol % and which has a fuse temperature of less than 136° C.
The second mode of this invention is a polyethylene microporous film which is composed of a mixture of a linear copolymeric polyethylene having a melt index of less than 0.1 and a propylene unit content of 0.1 to 4 mol % and a high density polyethylene having a comonomer unit content of less than 0.1 mol %, the above mixture having a weight average molecular weight of 250,000 to 700,000 and a propylene unit content of 0.1 to 4 mol % and which has a fuse temperature of less than 136° C.
The third mode of this invention is a separator composed of the above-mentioned polyethylene microporous film.
The fourth mode of this invention is a battery in which the above-mentioned separator is used as a battery separator.
BEST MODE FOR CARRYING OUT THE INVENTION
The melt indexes (MI) of the C3 copolymer and the mixture of the C3 copolymer and a high density polyethylene are less than 0.1, preferably less than 0.07 and more preferably less than 0.05. When the MI is 0.1 or more, it is difficult to make the film porous. As the C3 copolymer having such an MI and the above mixture, those having a weight average molecular weight of 200,000 to 4,000,000, preferably 250,000 to 700,000, and more preferably 250,000 to 500,000 are mentioned.
When the C3 copolymer is used alone, the weight average molecular weight thereof is 200,000 to 700,000, preferably 250,000 to 600,00, and more preferably 250,000 to 500,000.
The propylene unit content of the C3 copolymer is 0.1 to 4 mol %, preferably 0.2 to 3 mol %, and more preferably 0.5 to 2 mol %, based on the ethylene unit. When the propylene unit content is less than 0.1 mol % based on the ethylene unit, the lowering of the fuse temperature is insufficient, and when it exceeds 4 mol %, the crystallinity is lowered too much and it becomes difficult to make the film porous.
The density of the C3 copolymer is 0.85 to 0.97, preferably 0.90 to 0.96, and more preferably 0.92 to 0.95.
The C3 copolymer used in this invention can be produced by various known methods. For example, it can be produced by polymerization using a chromium compound-supported catalyst or magnesium compound-containing Ziegler catalyst as disclosed in JP-B-1-12,777.
Furthermore, in this invention, by mixing the above C3 copolymer with a high density polyethylene having a comonomer unit content of less than 0.1 mol % (referred to hereinafter as HDPE in some cases), the relation between permeability and fuse temperature can be improved more than when the C3 copolymer is used alone. The reason therefor has not be clarified; however, it is considered that when a mixture having a lowered comonomer concentration is used, which is prepared by uniformly dispersing the C3 copolymer having a high comonomer concentration in a high density polyethylene having a high porousness to dilute it, the relation between porousness and fuse effect can be better improved than when a C3 c
Hasegawa Takuya
Kondo Takahiko
Asahi Kasei Kogyo Kabushiki Kaisha
Birch & Stewart Kolasch & Birch, LLP
Savage Jason
Speer Timothy M.
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