U.S. patent application number 11/303144 was filed with the patent office on 2006-06-29 for polyethylene microporous film for a rechargeable battery separator and a method of preparing the same.
Invention is credited to Chang Ho Suh.
Application Number | 20060141351 11/303144 |
Document ID | / |
Family ID | 36612022 |
Filed Date | 2006-06-29 |
United States Patent
Application |
20060141351 |
Kind Code |
A1 |
Suh; Chang Ho |
June 29, 2006 |
Polyethylene microporous film for a rechargeable battery separator
and a method of preparing the same
Abstract
Disclosed are a polyethylene microporous film and a method of
preparing the same. The polyethylene microporous film, which has a
laminated structure comprising B layer/A layer/B layer, prepared by
melt-mixing a polyethylene and an aliphatic hydrocarbon solvent
together at controlled mixing ratios to separately form an A layer
and a B layer having different porosities, and then coextruding the
A and B layers, thus exhibiting excellent mechanical properties,
such as strength and elongation, and high-temperature stability.
Therefore, the polyethylene microporous film is suitable for use in
a rechargeable battery separator.
Inventors: |
Suh; Chang Ho;
(Daegu-gwangukshi, KR) |
Correspondence
Address: |
BELL, BOYD & LLOYD, LLC
PO BOX 1135
CHICAGO
IL
60690-1135
US
|
Family ID: |
36612022 |
Appl. No.: |
11/303144 |
Filed: |
December 16, 2005 |
Current U.S.
Class: |
429/144 ;
264/235.8; 429/254 |
Current CPC
Class: |
H01M 50/403 20210101;
Y02E 60/10 20130101; H01M 50/449 20210101; H01M 50/411
20210101 |
Class at
Publication: |
429/144 ;
429/254; 264/235.8 |
International
Class: |
H01M 2/16 20060101
H01M002/16; B29C 71/00 20060101 B29C071/00 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 23, 2004 |
KR |
2004-110928 |
Jan 12, 2005 |
KR |
2005-02932 |
Claims
1. A polyethylene microporous film for a rechargeable battery
separator, comprising: an A layer formed by melt-mixing 20 to about
40 wt % of a polyethylene with 60 to about 80 wt % of an aliphatic
hydrocarbon solvent; B layers formed by melt-mixing 45 to about 65
wt % of a polyethylene with 35 to about 55 wt % of an aliphatic
hydrocarbon solvent, the B layers are laminated on both surfaces of
the A layer; and the A layer and B layers are coextruded to form a
laminated structure comprising B layer/A layer/B layer.
2. A polyethylene microporous film for a rechargeable battery
separator, comprising: an A layer formed by melt-mixing 12.8 to
about 64.9 wt % of a polyethylene, 0.1 to about 7.2 wt % of a
thermoplastic resin incompatible with the polyethylene, and 35 to
about 80 wt % of an aliphatic hydrocarbon; B layers formed by
melt-mixing 20 to about 65 wt % of a polyethylene with 35 to about
80 wt % of an aliphatic hydrocarbon solvent, the B layers are
laminated on both surfaces of the A layer; and the A layer and B
layers are coextruded to form a laminated structure comprising B
layer/A layer/B layer.
3. The polyethylene microporous film as set forth in claim 1,
wherein the polyethylene is selected from the group consisting of
high density polyethylene having an average weight molecular weight
ranging from 100,000 to 500,000, ultrahigh molecular weight
polyethylene having an average weight molecular weight ranging from
1,000,000 to 5,000,000, and a mixture of thereof.
4. The polyethylene microporous film as set forth in claim 3,
wherein the mixture comprises 60 to about 80 wt % of high density
polyethylene having an average weight molecular weight ranging from
100,000 to 500,000, and 20 to about 40 wt % of ultrahigh molecular
weight polyethylene having an average weight molecular weight
ranging from 1,000,000 to 5,000,000.
5. The polyethylene microporous film as set forth in claim 1,
wherein the polyethylene has a melt index of 1 g/10 min or
less.
6. The polyethylene microporous film as set forth in claim 2,
wherein the thermoplastic resin incompatible with the polyethylene
is selected from the group consisting of copolyester and nylon
6.
7. The polyethylene microporous film as set forth in claim 2,
wherein the polyethylene microporous film has a first melting
temperature ranging from 125 to 145.degree. C., and a second
melting temperature ranging from 175 to 235.degree. C.
8. The polyethylene microporous film as set forth in claim 1,
wherein the aliphatic hydrocarbon solvent is selected from the
group consisting of nonane, decane, undecane, dodecane, and liquid
paraffin oil.
9. The polyethylene microporous film as set forth in claim 1,
wherein the polyethylene microporous film is 3 to about 50 .mu.m
thick.
10. The polyethylene microporous film as set forth in claim 1,
wherein the A layer is 1 to about 20 .mu.m thick.
11. The polyethylene microporous film as set forth in claim 1,
wherein the B layer is 1 to about 10 .mu.m thick.
12. A method of preparing a polyethylene microporous film for a
rechargeable battery separator comprising the steps of: melt-mixing
20 to about 40 wt % of a polyethylene with 60 to about 80 wt % of
an aliphatic hydrocarbon solvent to form an A layer, melt-mixing 45
to about 65 wt % of a polyethylene with 35 to about 55 wt % of an
aliphatic hydrocarbon solvent to form B layers, coextruding the A
layer and B layers to laminate B layers on both surfaces of the A
layer, and then cooling the coextruded layers, to form a gel
composition having a laminated structure comprising B layer/A
layer/B layer; biaxially stretching the gel composition, to prepare
a film; extracting the aliphatic hydrocarbon solvent from the A
layer and B layers of the film using an organic solvent to remove
the aliphatic hydrocarbon solvent, to prepare a microporous film;
and heat-treating the microporous film at a temperature not greater
than the melting temperature of the polyethylene.
13. A method of preparing a polyethylene microporous film for a
rechargeable battery separator comprising the steps of: melt-mixing
with 12.8 to about 64.9 wt % of a polyethylene, 0.1 to about 7.2 wt
% of a thermoplastic resin incompatible with the polyethylene, and
35 to about 80 wt % of an aliphatic hydrocarbon solvent to form an
A layer, melt-mixing 20 to about 65 wt % of a polyethylene with 35
to about 80 wt % of an aliphatic hydrocarbon solvent to form B
layers, coextruding the A layer and B layers to laminate B layers
on both surfaces of the A layer, and then cooling the coextruded
layers, to form a gel composition having a laminated structure
comprising B layer/A layer/B layer; biaxially stretching the gel
composition, to prepare a film; extracting the aliphatic
hydrocarbon solvent from the A layer and B layers of the film using
an organic solvent to remove the aliphatic hydrocarbon solvent, to
prepare a microporous film; and heat-treating the microporous film
at a temperature not greater than a melting temperature of the
polyethylene.
14. The method as set forth in claim 12, wherein the biaxial
stretching is conducted at a ratio ranging from 4.times.4 to
8.times.8 at 105 to about 125.degree. C.
15. The polyethylene microporous film as set forth in claim 2,
wherein the polyethylene is selected from the group consisting of
high density polyethylene having an average weight molecular weight
ranging from 100,000 to 500,000, ultrahigh molecular weight
polyethylene having an average weight molecular weight ranging from
1,000,000 to 5,000,000, and a mixture of thereof.
16. The polyethylene microporous film as set forth in claim 15,
wherein the mixture comprises 60 to about 80 wt % of high density
polyethylene having an average weight molecular weight ranging from
100,000 to 500,000, and 20 to about 40 wt % of ultrahigh molecular
weight polyethylene having an average weight molecular weight
ranging from 1,000,000 to 5,000,000.
17. The polyethylene microporous film as set forth in claim 2,
wherein the polyethylene has a melt index of 1 g/10 min or
less.
18. The polyethylene microporous film as set forth in claim 2,
wherein the aliphatic hydrocarbon solvent is selected from the
group consisting of nonane, decane, undecane, dodecane, and liquid
paraffin oil.
19. The polyethylene microporous film as set forth in claim 2,
wherein the polyethylene microporous film is 3 to about 50 .mu.m
thick.
20. The polyethylene microporous film as set forth in claim 2,
wherein the A layer is 1 to about 20 .mu.m thick.
21. The polyethylene microporous film as set forth in claim 2,
wherein the B layer is 1 to about 10 .mu.m thick.
22. The method as set forth in claim 13, wherein the biaxial
stretching is conducted at a ratio ranging from 4.times.4 to
8.times.8 at 105 to about 125.degree. C.
Description
BACKGROUND
[0001] The present invention relates a polyethylene microporous
film for a rechargeable battery separator and a method of preparing
the same, more particularly, the present invention relates the
polyethylene microporous film, which has a laminated structure
comprising B layer/A layer/B layer, formed by melt-mixing with a
polyethylene and an aliphatic hydrocarbon solvent together at
controlled mixing ratios to separately form an A layer and a B
layer having different porosities, and then coextruding the A and B
layers, thus exhibiting excellent mechanical properties, such as
strength and elongation, and high-temperature stability, and the
method of preparing such a polyethylene microporous film.
[0002] As various portable apparatuses, including mobile phones,
notebook PCs, portable videos, PDAs, or portable multimedia
players, have trended toward small sizes and light weights,
rechargeable battery markets have gradually increased. A
rechargeable battery is a chemical cell, which can be
semi-permanently used through continuous repetitive charges and
discharges using an electrochemical reaction, and has been realized
as a lead storage battery, a nickel cadmium battery, a nickel
hydrogen metal battery, a lithium ion battery, a lithium ion
polymer battery, etc. In particular, of these batteries, a lithium
ion battery and a lithium ion polymer battery, each of which has
high voltage and excellent energy density properties, lead the
rechargeable battery markets.
[0003] A lithium rechargeable battery comprises an anode, a cathode
and a separator interposed between the anode and the cathode so as
not to physically contact them. The separator for a lithium ion
battery must have mechanical strength to endure a high-speed
winding process upon manufacture of a battery, be chemically stable
in an electrolyte, and have high-temperature stability to prevent
the generation of short circuits and overcharge. As well, high
capacity, excellent battery properties, stability and high
productivity have been required in recent years.
[0004] Presently, as a separator for a lithium ion battery, a
polyethylene microporous film is typically used. When the
polyethylene microporous film is biaxially stretched, it has
superior mechanical strength and remains chemically stable. In this
way, efforts to improve properties required for the use of the
polyethylene microporous film as a separator for a lithium ion
battery have been thoroughly conducted.
[0005] In this regard, Japanese Patent No. 1848017 discloses a
method of preparing a polyethylene microporous film, which
comprises dissolving polyethylene having average weight molecular
weight (Mw) ranging from 500,000 to 1,500,000 in a solvent using
heat to prepare a solution, forming the solution into a gel sheet,
removing the solvent from the gel sheet so that the solvent is
present in the gel sheet in a proportion of 10-80 wt %,
heat-stretching the gel sheet, and then removing residual solvent.
Japanese Patent No. 2126761 discloses a polyethylene microporous
film, prepared using polyethylene having an Mw of 500,000 or
more.
[0006] In addition, Japanese Patent No. 1759736 or 1918760
discloses an ultrahigh molecular weight .alpha.-olefin polymer
microporous film and a method of preparing the same, the method
comprising preparing a solution of .alpha.-olefin polymer having an
Mw of 500,000 or more into a gel body, removing at least 10 wt % of
solvent from the gel body so that the amount of the .alpha.-olefin
polymer in the gel body is 10.about.90 wt %, stretching the gel
body at a temperature further increased to 10.degree. C. higher
than the melting point of the .alpha.-olefin polymer or lower, and
then removing residual solvent. In addition, Japanese Patent No.
1948121 discloses a polyethylene microporous film, prepared by
forming a solution of polyethylene having an Mw of 500,000 or more
into a gel body, removing the solvent from the gel body so that the
solvent is present in the gel body in a proportion of 80.about.95
wt %, uniaxially stretching the gel body at least twofold thus
causing the magnification area thereof to be increased by at least
ten times at 120.degree. C. or less, and then removing residual
solvent. In addition, Japanese Patent No. 1866164 discloses a
method of preparing a polyolefin microporous film, comprising
loading a solution containing ultrahigh molecular weight polyolefin
having an Mw of 500,000 or more into a die, extruding the solution
from the die while quenching it to 90.degree. C. or less at a
cooling rate of 50.degree. C./min to form a gel body, removing at
least 10 wt % of solvent from the gel body so that the amount of
ultrahigh molecular weight polyolefin in the gel body is
10.about.90 wt %, stretching the gel body at a temperature further
increased to 10.degree. C. higher than the melting point of
ultrahigh molecular weight polyolefin or lower, and then removing
residual solvent.
[0007] Further, Japanese Patent No. 2132327 discloses a polyolefin
microporous film having a porosity of 35.about.95%, an average pore
diameter of 0.001.about.0.2 .mu.m and a fracture strength of 0.2 kg
or more at a width of 15 mm, prepared using a polyolefin
composition including 1 wt % or more of ultrahigh molecular weight
polyolefin having an Mw of 700,000 or more, and a ratio of average
weight molecular weight/number average molecular weight (Mw/Mn) of
10.about.300.
[0008] Further, Japanese Patent No. 2657434 discloses a
polyethylene microporous film and a method of preparing the same,
the method comprising mixing 10.about.50 wt % of a composition,
including 1.about.69 wt % of ultrahigh molecular weight
polyethylene having an Mw of 700,000 or more, 98.about.1 wt % of
high density polyethylene, and 1.about.30 wt % of low density
polyethylene, in which an Mw/Mn ratio of the component containing
ultrahigh molecular weight polyethylene and high density
polyethylene is 10.about.300, with 50.about.90 wt % of a solvent to
prepare a solution, extruding the solution from a die on a cooling
roll to form a gel composition, and then stretching the gel
composition at a temperature further increased to 10.degree. C.
higher than the melting point of the polyethylene composition or
lower. As such, the polyethylene microporous film has a thickness
of 0.1.about.50 .mu.m, a porosity of 35.about.95%, an average pore
diameter of 0.001.about.1 .mu.m, tensile fracture strength of 200
kg/cm.sup.2 or more, and impermeability at temperatures less than
135.degree. C.
[0009] Also, Japanese Patent No. 3351940 discloses a polyolefin
microporous film for a separator of a lithium ion battery, the film
being prepared using a solution comprising 5.about.50 wt % of
polyolefin, having an Mw ranging from 500,000 to 2,500,000 and an
Mw/Mn ratio less than 10, and 95.about.50 wt % of a solvent.
[0010] In addition, Japanese Patent No. 2794179 discloses a
polyethylene microporous film, prepared using high density
polyethylene having an Mw ranging from 400,000 to 2,000,000 and an
Mw/Mn ratio of 25 or less, and Japanese Patent No. 2961387
discloses a polyethylene microporous film, prepared using
polyethylene having a viscosity average molecular weight ranging
from 160,000 to 2,000,000, and Japanese Patent No. 3121047
discloses a microporous film having a three-dimensional network
structure, comprising at least 30 wt % of ultrahigh molecular
weight polyethylene having a viscosity average molecular weight of
2,000,000 or more based on the amount of microporous film.
Additionally, various techniques are reported to control the
properties of a polyethylene microporous film by adjusting the
molecular weight and the amount of ultrahigh molecular weight
polyethylene having an Mw of 1,000,000 or more [Japanese Patent
Nos. 3258737, 3333287, 3497569, and 3486785], in which the
polyethylene microporous film has a monolayer structure.
[0011] On the other hand, Japanese Patent No. 3195120 discloses a
microporous film having a vein-shaped open pore structure
comprising microfibrils, having a porosity of 30.about.70%, tensile
strength of 1,000 kg/cm.sup.2 or more and an average micropore
diameter of 0.1.about.3 .mu.m, prepared using high molecular weight
polyethylene having a limiting viscosity [.eta.] of 5 dl/g or
more.
[0012] Korean Patent No. 371390 discloses a separator having a
multi-layered structure, formed by laminating a plurality of
polymer layers including a) a polypropylene layer, b) a
polyethylene layer, and c) a tie layer, including polypropylene
having an electrophilic functional group and polyethylene having a
nucleophilic functional group, which is chemically bonded
therewith.
[0013] Korean Patent No. 409019 discloses a multi-layered
microporous film, comprising a) a support layer of polymer film,
having a pore size of 0.001.about.100 .mu.m and a thickness of
1.about.50 .mu.m, and b) a shutdown layer formed on either one or
both surfaces of the support layer, including a polymer having a
melting point 40.about.75.degree. C. lower than that of the polymer
of the support layer, and having a pore size of 0.001.about.100
.mu.m and a thickness of 0.01.about.20 .mu.m, and a method of
preparing such a microporous film.
[0014] In addition, Korean Patent No. 263919 discloses a
microporous laminated film, which comprises a first polymer layer,
including a polyethylene copolymer, and a second polymer layer,
including a polypropylene copolymer and formed on at least one
surface of the first polymer layer, and a method of preparing such
a film. As such, the polyethylene copolymer results from
copolymerization of polyethylene, including 10 wt % or less of
polyethylene having a molecular weight of 1,000,000 or more and 60
wt % or less of polyethylene having a molecular weight of 10,000 or
less, and any one or more selected from the group consisting of
methylpentene, propylene, butene, pentene, hexene, and octene. The
polypropylene copolymer is obtained by copolymerzing polypropylene,
including 30 wt % or less of polypropylene having a molecular
weight of 1,000,000 or more and 40 wt % or less of polypropylene
having a molecular weight of 10,000 or less, and any one or more
selected from the group consisting of methylpentene, propylene,
butene, pentene, hexene, and octane.
[0015] In addition, Korean Patent Laid-open Publication No.
2000-51312 discloses a polypropylene microporous film for a battery
separator, comprising 25.about.55 wt % of a polypropylene
homopolymer having a melt flow index of 0.1.about.2.0 g/10 min,
3.about.30 wt % of polypropylene terpolymer having a melt flow
index of 3.about.10 g/10 min, 30.about.70 wt % of paraffin oil, and
0.05.about.0.2 wt % of an antioxidant. Also, Korean Patent
Laid-open Publication No. 2000-51313 discloses a polyethylene
microporous film for a battery separator, comprising 20.about.40 wt
% of high density polyethylene having a melt flow index of
0.2.about.0.5 g/10 min and a density of 0.960.about.0.969
g/cm.sup.3, 4.about.20 wt % of high density polyethylene having a
melt flow index of 0.02.about.0.1 g/10 min and/a density of
0.950.about.0.958 g/cm.sup.3, 40.about.70 wt % of paraffin oil,
5.about.15 wt % of DOP plasticizer, 0.1.about.0.5 wt % of a
nucleation agent, and 0.1.about.0.5 wt % of an antioxidant.
However, a plurality of separators for lithium ion rechargeable
batteries, which is conventionally used, has not yet overcome
difficulties in being manufactured into a thin film due to
decreasing mechanical strength when permeability is increased, or
due to decreasing permeability when mechanical strength is
increased, from the point of view of the physical properties of the
microporous film.
[0016] As mentioned above, almost all of the microporous films are
mainly composed of a polyethylene. Hence, when current is
drastically increased due to external or internal short circuits,
the internal temperature of the battery drastically increases.
Eventually, the microporous film of the battery may become
deformed, and thus the stability of the battery is difficult to
maintain. To overcome such problems, methods of preparing a
microporous film comprising a polyethylene layer and a
polypropylene layer laminated thereon or of variously coating the
surface of the microporous film have been reported. However, these
methods are disadvantageous because additional processes are
required, causing other problems. That is, when the polypropylene
layer is laminated by a dry process, film properties may be
degraded or become non-uniform. Likewise, the coating process may
cause problems with the stability of a battery, on account of the
possibility of contamination. Therefore, a polyethylene film for a
separator having higher stability is urgently required.
SUMMARY
[0017] Leading to the present invention, intensive and thorough
research into a polyethylene film for a rechargeable battery
separator having higher stability, carried out by the present
inventors aiming to avoid the problems encountered in the related
art, resulted in the finding that a polyethylene and an aliphatic
hydrocarbon solvent may be melt-mixed together at controlled mixing
ratios, to separately form an A layer and a B layer having
different porosities, after which the A and B layers are
coextruded, thereby preparing a polyethylene microporous film
having a laminated structure comprising B layer/A layer/B layer,
and in particular, when the A layer may further contain a
thermoplastic resin incompatible with the polyethylene, pores may
be additionally formed during a stretching process, thereby
preparing a polyethylene microporous film having a laminated
structure comprising B layer/A layer/B layer, which is advantageous
because the state of the pores is desirably designed, porosity is
increased, and permeability and mechanical strength are also
improved.
[0018] Therefore, an object of the present invention is to provide
a polyethylene microporous film for a rechargeable battery
separator, which is imparted with a laminated structure comprising
B layer/A layer/B layer by melt-mixing a polyethylene with an
aliphatic hydrocarbon solvent at controlled mixing ratios, to
separately form an A layer and a B layer having different
porosities, and then coextruding the A and B layers.
[0019] Another object of the present invention is to provide a
method of preparing such a polyethylene microporous film.
[0020] In order to accomplish the above objects, the present
invention provides a polyethylene microporous film for a
rechargeable battery separator, comprising an A layer formed by
melt-mixing 20.about.40 wt % of a polyethylene with 60.about.80 wt
% of an aliphatic hydrocarbon solvent; and B layers formed by
melt-mixing 45.about.65 wt % of a polyethylene with 35.about.55 wt
% of an aliphatic hydrocarbon solvent, which are laminated on both
surfaces of the A layer, in which the A layer and B layers are
coextruded to form a laminated structure comprising B layer/A
layer/B layer.
[0021] In addition, the present invention provides a polyethylene
microporous film for a rechargeable battery separator, comprising
an A layer formed by melt-mixing 12.8.about.64.9 wt % of a
polyethylene, 0.1.about.7.2 wt % of a thermoplastic resin
incompatible with the polyethylene, and 35.about.80 wt % of an
aliphatic hydrocarbon solvent; and B layers formed by melt-mixing
20.about.65 wt % of a polyethylene with 35.about.80 wt % of an
aliphatic hydrocarbon solvent, which are laminated on both surfaces
of the A layer, in which the A layer and B layers are coextruded to
form a laminated structure comprising B layer/A layer/B layer. As
such, the polyethylene microporous film has a first melting
temperature of 125.about.145.degree. C., and a second melting
temperature of 175.about.235.degree. C.
[0022] In the polyethylene microporous film, the polyethylene may
be selected from the group consisting of high density polyethylene
having an Mw ranging from 100,000 to 500,000, ultrahigh molecular
weight polyethylene having an Mw ranging from 1,000,000 to
5,000,000, and a mixture thereof.
[0023] In this case, the mixture may comprise 60--80 wt % of high
density polyethylene having an Mw of 100,000-500,000, and
20.about.40 wt % of ultrahigh molecular weight polyethylene having
an Mw of 1,000,000.about.5,000,000.
[0024] The polyethylene may have a melt index of 1 g/10 min or
less.
[0025] The thermoplastic resin incompatible with the polyethylene
may be, for example, copolyester or nylon 6.
[0026] The aliphatic hydrocarbon solvent may be any one selected
from the group consisting of nonane, decane, undecane, dodecane,
and liquid paraffin oil.
[0027] The polyethylene microporous film for a rechargeable battery
separator may have a thickness of 3.about.50 .mu.m, in which the A
layer may preferably be 1.about.20 .mu.m thick and the B layer may
preferably be 1.about.10 .mu.m thick.
[0028] Further, the present invention provides a method of
preparing a polyethylene microporous film for a rechargeable
battery separator, the method comprising melt-mixing 20.about.40 wt
% of a polyethylene with 60.about.80 wt % of an aliphatic
hydrocarbon solvent to form an A layer, melt-mixing 45.about.65 wt
% of a polyethylene with 35.about.55 wt % of an aliphatic
hydrocarbon solvent to form B layers, coextruding the A layer and B
layers to laminate B layers on both surfaces of the A layer, and
then cooling the coextruded layers, to form a gel composition
having a laminated structure comprising B layer/A layer/B layer;
biaxially stretching the gel composition, to prepare a film;
extracting the aliphatic hydrocarbon solvent from the A layer and B
layers of the film using an organic solvent to remove the aliphatic
hydrocarbon solvent, to prepare a microporous film; and
heat-treating the microporous film at a temperature not higher than
the melting temperature of the polyethylene.
[0029] In addition, the present invention provides a method of
preparing a polyethylene microporous film for a rechargeable
battery separator, the method comprising melt-mixing with
12.8.about.64.9 wt % of a polyethylene, 0.1.about.7.2 wt % of a
thermoplastic incompatible with the polyethylene, and 35.about.80
wt % of an aliphatic hydrocarbon solvent to form an A layer,
melt-mixing 20.about.65 wt % of a polyethylene with 35.about.80 wt
% of an aliphatic hydrocarbon solvent to form B layers, coextruding
the A layer and B layers to laminate B layers on both surfaces of
the A layer, and then cooling the coextruded layers, to form a gel
composition having a laminated structure comprising B layer/A
layer/B layer; biaxially stretching the gel composition; to prepare
a film; extracting the aliphatic hydrocarbon solvent from the A
layer and B layers of the film using an organic solvent to remove
the aliphatic hydrocarbon solvent, to prepare a microporous film;
and heat-treating the microporous film at a temperature not higher
than the melting temperature of the polyethylene.
[0030] In the method of preparing the polyethylene microporous
film, the biaxial stretching process is conducted at a ratio
ranging from 4.times.4 to 8.times.8 at 105.about.125.degree. C.
[0031] Additional features and advantages are described herein, and
will be apparent from, the following Detailed Description and the
figures.
BRIEF DESCRIPTION OF THE FIGURES
[0032] FIG. 1 is a sectional view showing the structure of a
polyethylene microporous film for a rechargeable battery separator
of the present invention.
DETAILED DESCRIPTION
[0033] Hereinafter, a detailed description will be given of the
present invention.
[0034] The present invention provides a polyethylene microporous
film for a rechargeable battery separator, which has a laminated
structure comprising B layer/A layer/B layer, obtained by
melt-mixing with a polyethylene and an aliphatic hydrocarbon
solvent at controlled mixing ratios, to separately form an A layer
and a B layer having different porosities, after which the A and B
layers are coextruded.
[0035] According to a first embodiment of the present invention, a
polyethylene microporous film for a rechargeable battery separator
is provided, which comprises an A layer formed by melt-mixing
20.about.40 wt % of a polyethylene with 60.about.80 wt % of an
aliphatic hydrocarbon solvent, and B layers formed by melt-mixing
45.about.65 wt % of a polyethylene with 35.about.55 wt % of an
aliphatic hydrocarbon solvent, which are laminated on both surfaces
of the A layer, in which the A layer and B layers are coextruded to
form a laminated structure comprising B layer/A layer/B layer.
Also, a method of preparing such a polyethylene microporous film is
provided.
[0036] In addition, according to a second embodiment of the present
invention, a polyethylene microporous film for a rechargeable
battery separator is provided, which comprises an A layer formed by
melt-mixing 12.8.about.64.9 wt % of a polyethylene, 0.1.about.7.2
wt % of a thermoplastic resin incompatible with the polyethylene,
and 35.about.80 wt % of an aliphatic hydrocarbon solvent, and B
layers formed by melt-mixing 20.about.65 wt % of a polyethylene
with 35.about.80 wt % of an aliphatic hydrocarbon solvent, which
are laminated on both surfaces of the A layer, in which the A layer
and B layers are coextruded to form a laminated structure
comprising B layer/A layer/B layer. Also, a method of preparing
such a polyethylene microporous film is provided.
[0037] The polyethylene used in each of the polyethylene
microporous films, according to the first and second embodiments of
the present invention, is not particularly limited as long as it is
high density polyethylene suitable for use in a rechargeable
battery separator. Preferably, high density polyethylene, having an
Mw ranging from 100,000 to 500,000, or an ultrahigh molecular
weight polyethylene, having an Mw of 1,000,000 or more, and
preferably, of 1,000,000.about.5,000,000, is used alone or in
mixtures thereof. More preferably, high density polyethylene having
an Mw of 200,000.about.400,000 is used. In the present invention,
for comparison under the same conditions, high density polyethylene
having an Mw of 400,000 is used, but ultrahigh molecular weight
polyethylene having an Mw more than 400,000 or high density
polyethylene having an Mw less than 400,000 may be used. If the Mw
is less than 100,000, mechanical properties are decreased. On the
other hand, if the Mw exceeds 500,000, process efficiency may be
decreased. In the case where the mixture of high density
polyethylene and ultrahigh molecular weight polyethylene is used,
the mixture comprises 60.about.80 wt % of high density polyethylene
having an Mw ranging from 100,000 to 500,000 and 20.about.40 wt %
of ultrahigh molecular weight polyethylene having an Mw of
1,000,000.about.5,000,000.
[0038] In addition, an Mw/Mn ratio of the polyethylene of the
present invention, which is regarded as a parameter showing the
uniformity of a molecular weight distribution, ranges from 4 to 10.
If this ratio is less than 4, the polyethylene has low melt
strength. Meanwhile, if the ratio exceeds 10, a low molecular
weight component is cut during a stretching process, and thus, the
resultant film may have its overall strength decreased. Further,
the polyethylene has a melt index of 1 g/10 min or less.
[0039] In the second embodiment of the present invention, as the
thermoplastic resin incompatible with the polyethylene, any resin
may be used as long as it is a polymer resin having a melting
temperature of 175.about.235.degree. C. Preferably, copolyester or
nylon 6 may be selectively used. Hence, an A layer (an internal
layer) is formed by further including the thermoplastic resin
having a melting temperature higher than that of the polyethylene.
As such, since the thermoplastic resin functions to form pores
during a stretching process and is not melted at high temperatures,
the resultant polyethylene microporous film can have
high-temperature stability. Thus, when overheated, the film is not
melted, and thus the generation of short circuits may be
retarded.
[0040] The aliphatic hydrocarbon solvent is used to dissolve the
polyethylene in the presence of heat to form a gel composition, and
preferably is any one selected from the group consisting of nonane,
decane, undecane, dodecane, and liquid paraffin oil. More
preferably, non-volatile liquid paraffin oil is used, to obtain a
gel composition having a uniform solvent content.
[0041] In the solution comprising a polyethylene and a solvent,
according to the first embodiment of the present invention, or the
solution comprising a polyethylene, a thermoplastic resin
incompatible with the polyethylene, and a solvent, according to the
second embodiment, a solid content is preferably 20.about.65 wt %.
As such, if the solid content is smaller than 20 wt %, a great
amount of solvent should be used, and also, swelling and neck-in
phenomena occur in a die lip when forming a sheet, and thus a large
film is difficult to manufacture. On the other hand, when the solid
content exceeds 65 wt %, porosity is decreased. In addition, to
prevent the polyethylene from being decomposed by oxidation, the
solution may include an antioxidant, if necessary. Although the
film further including an antioxidant is exemplified in the present
invention, the present invention is not limited thereto.
[0042] FIG. 1 is a sectional view showing the structure of a
polyethylene microporous film for a rechargeable battery separator,
prepared using the method of the present invention, in which the
polyethylene microporous film has a laminated structure comprising
B layer/A layer/B layer.
[0043] The polyethylene microporous film for a rechargeable battery
separator, according to the first and second embodiments of the
present invention, has a thickness of 3.about.50 .mu.m, and
preferably of 3.about.30 .mu.m. When the microporous film is
thinner than 3 .mu.m, the mechanical strength of the film becomes
insufficient. On the other hand, when the film is thicker than 50
.mu.m, limitations are imposed on reducing the size of the battery
and decreasing the weight thereof.
[0044] More specifically, the A layer is preferably 1.about.20
.mu.m, and the B layer is preferably 1.about.10 .mu.m.
[0045] According to the first embodiment of the present invention,
the polyethylene microporous film having a laminated structure
comprising three layers of B layer/A layer/B layer has a porosity
of 20.about.60%, and preferably of 30.about.50%. If porosity is
less than 20%, permeability becomes insufficient. Meanwhile, a
porosity exceeding 60% leads to insufficient mechanical
strength.
[0046] In addition, in the polyethylene microporous film having a
laminated structure comprising three layers of B layer/A layer/B
layer, according to the second embodiment of the present invention,
the A layer (an internal layer) further includes a thermoplastic
resin, which has a melting temperature higher than that of the
polyethylene and functions to form pores during stretching process
of the film, thus assuring high-temperature stability. Therefore,
the A layer as an internal layer functions to increase a meltdown
temperature, while the B layer as an external layer functions to
decrease a shutdown temperature. That is, small pores in the B
layer as an external layer act to shorten a shutdown time, and the
incompatible resin in the A layer as an internal layer acts to
delay a meltdown time. The microporous film resists melting for a
long time upon overheating, resulting in the retardation of the
generation of short circuits.
[0047] Accordingly, the polyethylene microporous film according to
the second embodiment of the present invention has a porosity of
25.about.95%, and preferably of 30.about.90%. Porosity less than
25% leads to insufficient permeability, whereas porosity exceeding
95% leads to insufficient mechanical strength. Especially, the
pores can be formed during a stretching process due to the
thermoplastic resin used along with the polyethylene, as well as
during a typical formation process of micropores due to a solvent,
thereby increasing the porosity of the polyethylene microporous
film of the present invention.
[0048] Further, the polyethylene microporous film according to the
second embodiment has a first melting temperature of
125.about.145.degree. C. and a second melting temperature of
175.about.235.degree. C., thus exhibiting high-temperature
stability.
[0049] Since the porosity, which represents the degree of pore
formation, is affected by the conditions of an extraction process,
it was assayed as a numerical value obtained by removing the
plasticizer through extraction using methylene chloride at room
temperature in a suppressed state.
[0050] According to the polyethylene microporous film, according to
the first and second embodiments of the present invention, has gas
permeability of 104,000 sec, and preferably, of 50.about.1,000 sec.
As such, if gas permeability is less than 10 sec, pore diameter is
too large. If gas permeability exceeds 4,000 sec, permeability is
insufficient.
[0051] In addition, the polyethylene microporous film, according to
the first and second embodiments of the present invention, has a
pore diameter of 0.003.about.0.3 .mu.m, and preferably of
0.01.about.0.1 .mu.m. When a pore diameter is smaller than 0.003
.mu.m, permeability becomes insufficient. Meanwhile, when a pore
diameter exceeds 0.3 .mu.m, the interruption of current flow due to
melt effects occurs late, and short circuits may be generated by
precipitated resin crystals and decomposed active materials. Hence,
the resultant microporous film becomes unsuitable for use in a
battery.
[0052] In addition, the polyethylene microporous film for a
rechargeable battery separator of the first and second embodiments
of the present invention has a laminated structure, and thus
exhibits mechanical properties superior to a polyethylene
microporous film having a monolayer structure. As such, the
polyethylene microporous film of the present invention has a
strength of 10.about.20 kg/mm.sup.2, and preferably, of
13.0.about.17.0 kg/mm.sup.2, therefore exhibiting excellent
mechanical properties.
[0053] Further, the present invention provides a method of
preparing a polyethylene microporous film according to the first
embodiment. Preferably, the method of the present invention
comprises melt-mixing 20.about.40 wt % of a polyethylene with
60.about.80 wt % of an aliphatic hydrocarbon solvent to form an A
layer, melt-mixing 45.about.65 wt % of a polyethylene with
35.about.55 wt % of an aliphatic hydrocarbon solvent to form B
layers, coextruding the A layer and B layers to laminate B layers
on both surfaces of the A layer, and then cooling the coextruded
layers, to form a gel composition having a laminated structure
comprising B layer/A layer/B layer; biaxially stretching the gel
composition, to prepare a film; extracting the aliphatic
hydrocarbon solvent from the A layer and B layers of the film using
an organic solvent to remove it, to form a microporous film; and
heat-treating the microporous film at a temperature not higher than
the melting temperature of the polyethylene.
[0054] In addition, the present invention provides a method of
preparing a polyethylene microporous film according to the second
embodiment. Preferably, the method of the present invention
comprises melt-mixing with 12.8.about.64.9 wt % of a polyethylene,
0.1.about.7.2 wt % of a thermoplastic resin incompatible with the
polyethylene, and 35.about.80 wt % of an aliphatic hydrocarbon
solvent together to form an A layer, melt-mixing with 20.about.65
wt % of a polyethylene and 35.about.80 wt % of an aliphatic
hydrocarbon solvent to form B layers, coextruding the A layer and B
layers to laminate B layers on both surfaces of the A layer, and
then cooling the coextruded layers, to form a gel composition
having a laminated structure comprising B layer/A layer/B layer;
biaxially stretching the gel composition, to prepare a film;
extracting the aliphatic hydrocarbon solvent from the A layer and B
layers of the film using an organic solvent to remove it, to form a
microporous film; and heat-treating the microporous film at a
temperature not higher than the melting temperature of the
polyethylene.
[0055] Specifically, in the formation process of the gel
composition, according to the first embodiment, 20.about.40 wt % of
the polyethylene and 60.about.80 wt % of the aliphatic hydrocarbon
solvent are melt-mixed together at 160.about.220.degree. C. and
150.about.250 rpm using a first intermeshing corotating twin-screw
extruder to form an A layer. Separately, 45.about.65 wt % of the
polyethylene and 35.about.55 wt % of the aliphatic hydrocarbon
solvent are melt-mixed together at 160.about.220.degree. C. and
150.about.250 rpm using a second intermeshing corotating twin-screw
extruder, to form a B layer. As such, the extrusion temperature of
the first and second twin-screw extruders is controlled, therefore
resulting in uniform flow between layers.
[0056] According to the second embodiment, 12.8.about.64.9 wt % of
the polyethylene, 0.1.about.7.2 wt % of the thermoplastic resin
incompatible with the polyethylene, and 35.about.80 wt % of the
aliphatic hydrocarbon solvent are melt-mixed together at
180.about.280.degree. C. and 100.about.300 rpm using a first
intermeshing corotating twin-screw extruder, to form an A layer.
Separately, 20.about.65 wt % of the polyethylene and 35.about.80 wt
% of the aliphatic hydrocarbon solvent are melt-mixed together at
180.about.280.degree. C. and 100.about.300 rpm using a second
intermeshing corotating twin-screw extruder, to form a B layer. As
such, the control of the extrusion temperature of the first and
second twin-screw extruders is important to obtain uniform flow
between layers.
[0057] Then, the A and B layers are coextruded from a T die through
feed blocks so that B layers are laminated on both surfaces of the
A layer, to form a laminated structure comprising B layer/A layer/B
layer, which is then cooled on a cooling roll, thus preparing a gel
composition having a structure comprising B layer/A layer/B
layer.
[0058] In the subsequent stretching process, the gel composition
thus formed is stretched using sequential biaxial stretching or
simultaneous biaxial stretching techniques. The stretching process
is preferably conducted at a temperature not higher than the
melting temperature of polyethylene, and more preferably, biaxial
stretching is conducted at 105.about.125.degree. C. and a ratio
ranging from 4.times.4 to 8.times.8. In particular, the
polyethylene microporous film for a rechargeable battery separator
according to the second embodiment is advantageous because pores
are formed during a stretching process, due to the use of the
thermoplastic resin incompatible with the polyethylene. This
process is carried out along with typical pore formation due to the
solvent, resulting in increased porosity. In addition, since the
thermoplastic resin has a high melting temperature, it does not
melt even at high temperatures.
[0059] In the subsequent pore formation process, the aliphatic
hydrocarbon solvent is removed from the biaxially stretched film
using an organic solvent, therefore obtaining a microporous film.
As such, when paraffin oil or dioctylphthalate is used as the
aliphatic hydrocarbon solvent, an organic solvent such as methylene
chloride or methylethylketone may be used. Further, in the case
where the aliphatic hydrocarbon solvent, such as decaline having a
low boiling point, is used, it may be sufficiently removed even if
the film is heated and dried at a temperature not higher than the
melting temperature of polyethylene, therefore obtaining a
polyethylene microporous film.
[0060] In the subsequent heat treatment process, to improve
permeability and dimensional stability, the polyethylene
microporous film is treated at a temperature not higher than the
melting temperature of polyethylene.
[0061] A better understanding of the present invention may be
obtained in light of the following examples, which are set forth to
illustrate, but are not to be construed to limit the present
invention.
[0062] Preparation of Polyethylene Microporous Film According to
the 1.sup.st Embodiment
EXAMPLE 1
[0063] A high density polyethylene having an Mw of 400,000 was dry
mixed with 1 wt % Irganox (1010, available from Ciba Geigy Co.
Ltd.), serving as an antioxidant, based on the amount of the high
density polyethylene, to prepare a resin composition. Then, the
resin composition was loaded into a first twin-screw extruder, and
paraffin oil was injected into a side feed of the twin-screw
extruder using a pump. The amounts of resin composition and
paraffin oil added are shown in Table 1 below. Subsequently, an
extrusion process was conducted at 180.degree. C. and a screw
rotation rate of 200 rpm, to form an A layer having a thickness of
2 .mu.m.
[0064] In addition, a high density polyethylene having an Mw of
400,000 was dry mixed with 1 wt % Irganox antioxidant based on the
amount of the high density polyethylene, to prepare a resin
composition. Then, the resin composition was loaded into a second
twin-screw extruder, and paraffin oil was injected into a side feed
of the twin-screw extruder using a pump. The amounts of resin
composition and paraffin oil added are shown in Table 1 below.
Thereafter, an extrusion process was conducted at 200.degree. C.
and a screw rotation rate of 200 rpm, to form a B layer having a
thickness of 1 .mu.m.
[0065] Subsequently, the A and B layers were coextruded from a
T-die through feed blocks, to form a laminated structure comprising
three layers, that is, B layer/A layer/B layer, which was then
cooled on a cooling roll, thereby forming a gel composition
comprising B layer/A layer/B layer. The gel composition was loaded
into a simultaneous biaxial stretching apparatus, stretched to a
5.times.5 ratio at 120.degree. C., treated so that the paraffin oil
was removed from the stretched gel composition through extraction
using methylene chloride as an organic solvent, and then heat
treated at 120.degree. C., thus preparing a 4 .mu.m thick
microporous film having a laminated structure comprising B layer/A
layer/B layer.
EXAMPLES 2.about.9
[0066] A 4 .mu.m thick microporous film having a laminated
structure comprising B layer/A layer/B layer was prepared in the
same manner as in Example 1, with the exception that the amounts of
resin composition and paraffin oil added were changed as shown in
Table 1 below.
COMPARATIVE EXAMPLE 1
[0067] A high density polyethylene having an Mw of 400,000 was dry
mixed with 1 wt % Irganox antioxidant based on the amount of the
high density polyethylene, to prepare a resin composition, which
was then loaded into a twin-screw extruder, after which paraffin
oil was injected into a side feed of the twin-screw extruder using
a pump. The amounts of resin composition and paraffin oil added are
shown in Table 1 below. Subsequently, an extrusion process was
conducted at 190.degree. C. and a screw rotation rate of 200 rpm,
to obtain a predetermined layer.
[0068] The layer thus obtained was extruded from a T-die, and then
cooled on a cooling roll, thereby forming a gel composition. The
gel composition was loaded into a simultaneous biaxial stretching
apparatus, stretched to a 5.times.5 ratio at 120.degree. C.,
treated so that the paraffin oil was removed from the stretched gel
composition through extraction using methylene chloride as an
organic solvent, and then heat treated at 120.degree. C., thus
preparing a 4 .mu.m thick microporous film having a monolayer
structure.
COMPARATIVE EXAMPLES 2.about.3
[0069] A 4 .mu.m thick microporous film having a monolayer
structure was prepared in the same manner as in Comparative Example
1, with the exception that the amounts of resin composition and
paraffin oil added were changed as shown in Table 1 below.
[0070] The processes and properties of the microporous films
prepared in Examples 1.about.9 and Comparative Examples 1.about.3
are summarized in Table 1 below.
EXAMPLE 10
[0071] A high density polyethylene having an Mw of 400,000 was dry
mixed with 1 wt % Irganox antioxidant based on the amount of the
high density polyethylene, to prepare a resin composition. Then,
the resin composition was loaded into a first twin-screw extruder,
and paraffin oil was injected into a side feed of the twin-screw
extruder using a pump. The amounts of resin composition and
paraffin oil added are shown in Table 2 below. Subsequently, an
extrusion process was conducted at 180.degree. C. and a screw
rotation rate of 200 rpm, to form an A layer 4 .mu.m thick.
[0072] In addition, a high density polyethylene having an Mw of
400,000 was dry mixed with 1 wt % Irganox antioxidant based on the
amount of the high density polyethylene, to prepare a resin
composition. Then, the resin composition was loaded into a second
twin-screw extruder, and paraffin oil was injected into a side feed
of the twin-screw extruder using a pump. The amounts of resin
composition and paraffin oil added are shown in Table 2 below.
Subsequently, an extrusion process was conducted at 200.degree. C.
and a screw rotation rate of 200 rpm, to form a B layer 2 .mu.m
thick.
[0073] Thereafter, the A and B layers were coextruded from a T-die
through feed blocks, to form a laminated structure comprising three
layers, that is, B layer/A layer/B layer, which was then cooled on
a cooling roll, thereby forming a gel composition comprising B
layer/A layer/B layer. The gel composition was loaded into a
simultaneous biaxial stretching apparatus, stretched to a 6.times.6
ratio at 120.degree. C., treated so that the paraffin oil was
removed from the stretched gel composition through extraction using
methylene chloride as an organic solvent, and then heat treated at
120.degree. C., thus preparing an 8 .mu.m thick microporous film
having a laminated structure comprising B layer/A layer/B
layer.
EXAMPLES 11.about.18
[0074] An 8 .mu.m thick microporous film having a laminated
structure comprising B layer/A layer/B layer was prepared in the
same manner as in Example 10, with the exception that the amounts
of resin composition and paraffin oil added were changed as shown
in Table 2 below.
COMPARATIVE EXAMPLE 4
[0075] A high density polyethylene having an Mw of 400,000 was dry
mixed with 1 wt % Irganox antioxidant based on the amount of the
high density polyethylene, to prepare a resin composition, which
was then loaded into a twin-screw extruder, after which paraffin
oil was injected into a side feed of the twin-screw extruder using
a pump. The amounts of resin composition and paraffin oil added are
shown in Table 2 below. Subsequently, an extrusion process was
conducted at 190.degree. C. and a screw rotation rate of 200 rpm,
to obtain a predetermined layer.
[0076] The layer thus obtained was extruded from a T-die, and then
cooled on a cooling roll, thereby forming a gel composition. The
gel composition was loaded into a simultaneous biaxial stretching
apparatus, stretched to a 6.times.6 ratio at 120.degree. C.,
treated so that the paraffin oil was removed from the stretched gel
composition through extraction using methylene chloride as an
organic solvent, and then heat treated at 120.degree. C., thus
preparing an 8 .mu.m thick microporous film having a monolayer
structure.
COMPARATIVE EXAMPLES 5.about.6
[0077] An 8 .mu.m thick microporous film having a monolayer
structure was prepared in the same manner as in Comparative Example
4, with the exception that the amounts of resin composition and
paraffin oil added were changed as shown in Table 2 below.
[0078] The processes and properties of the microporous films
prepared in Examples 10.about.18 and Comparative Examples 4.about.6
are 5 summarized in Table 2 below.
EXAMPLE 19
[0079] A high density polyethylene having an Mw of 400,000 was dry
mixed with 1 wt % Irganox antioxidant based on the amount of the
high density polyethylene to prepare a resin composition. Then, the
resin composition was loaded into a first twin-screw extruder, and
paraffin oil was injected into a side feed of the twin-screw
extruder using a pump. The amounts of resin composition and
paraffin oil added are shown in Table 3 below. Subsequently, an
extrusion process was conducted at 180.degree. C. and a screw
rotation rate of 200 rpm, to form an A layer 6 .mu.m thick.
[0080] In addition, a high density polyethylene having an Mw of
400,000 was dry mixed with 1 wt % Irganox antioxidant based on the
amount of the high density polyethylene, to prepare a resin
composition. Then, the resin composition was loaded into a second
twin-screw extruder, and paraffin oil was injected into a side feed
of the twin-screw extruder using a pump. The amounts of resin
composition and paraffin oil added are shown in Table 3 below.
Subsequently, an extrusion process was conducted at 200.degree. C.
and a screw rotation rate of 200 rpm, to form a B layer 3 .mu.m
thick.
[0081] Thereafter, the A and B layers were coextruded from a T-die
through feed blocks, to form a laminated structure comprising three
layers, that is, B layer/A layer/B layer, which was then cooled on
a cooling roll, thereby forming a gel composition comprising B
layer/A layer/B layer. The gel composition was loaded into a
simultaneous biaxial stretching apparatus, stretched to a 7.times.7
ratio at 120.degree. C., treated so that the paraffin oil was
removed from the stretched gel composition through extraction using
methylene chloride as an organic solvent, and then heat treated at
120.degree. C., thus preparing a 12 .mu.m thick microporous film
having a laminated structure comprising B layer/A layer/B
layer.
EXAMPLES 20.about.-27
[0082] A 12 .mu.m thick microporous film having a laminated
structure comprising B layer/A layer/B layer was prepared in the
same manner as in Example 19, with the exception that the amounts
of resin composition and paraffin oil added were changed as shown
in Table 3 below.
COMPARATIVE EXAMPLE 7
[0083] A high density polyethylene having an Mw of 400,000 was dry
mixed with 1 wt % Irganox antioxidant based on the amount of the
high density polyethylene to prepare a resin composition, which was
then loaded into a twin-screw extruder, after which paraffin oil
was injected into a side feed of the twin-screw extruder using a
pump. The amounts of resin composition and paraffin oil added are
shown in Table 3 below. Subsequently, an extrusion process was
conducted at 190.degree. C. and a screw rotation rate of 200 rpm,
to obtain a predetermined layer.
[0084] The layer thus obtained was extruded from a T-die and cooled
on a cooling roll, to form a gel composition. The gel composition
was loaded into a simultaneous biaxial stretching apparatus,
stretched to a 7.times.7 ratio at 120.degree. C., treated so that
the paraffin oil was removed from the stretched gel composition
through extraction using methylene chloride as an organic solvent,
and then heat treated at 120.degree. C., thus preparing a 12 .mu.m
thick microporous film having a monolayer structure.
COMPARATIVE EXAMPLES 8.about.9
[0085] A 12 .mu.m thick microporous film having a monolayer
structure was prepared in the same manner as in Comparative Example
7, with the exception that the amounts of resin composition and
paraffin oil added were changed as shown in Table 3 below.
[0086] The processes and properties of the microporous films
prepared in Examples 19.about.27 and Comparative Examples 7.about.9
are summarized in Table 3 below.
EXPERIMENTAL EXAMPLE 1
[0087] Measurement of Properties
[0088] Thickness of Microporous Film
[0089] The thickness of each of the polyethylene microporous films
prepared in Examples 1.about.27 and Comparative Examples 1.about.9,
according to the first embodiment of the present invention, was
measured using a contact thickness gauge (MITUTOYO). The results
are given in Tables 1 to 3.
[0090] The thickness of each of the polyethylene microporous films
of the present invention was confirmed to be 4 .mu.m, 8 .mu.m, and
12 .mu.m. While measuring the thickness of the microporous film,
the following porosity and mechanical properties were also
measured.
[0091] Measurement of Porosity
[0092] A 5 cm.times.5 cm sample of each of the polyethylene
microporous films prepared in Examples 1.about.27 and Comparative
Examples 1.about.9, according to the first embodiment, was cut, and
the volume and weight thereof were measured. The measured values
were substituted into Equation 1 below, to calculate porosity (%).
The results are given in Tables 1 to 3 below.
[0093] In Tables 1 to 3, total porosity is represented by a
measured value, while partial porosity is difficult to measure in
practice and is thus represented by a calculated value resulting
from the total porosity varying with the content of oil. Porosity
.times. .times. ( % ) = [ void .times. .times. volume , cm 3 total
.times. .times. film .times. .times. volume , cm 3 ] .times. 100
Equation .times. .times. 1 ##EQU1##
[0094] Wherein a void volume is a total film volume
(cm.sup.3)--film weight (g)/resin density (g/cm.sup.3), in which a
resin density is 0.95 g/cm.sup.3.
[0095] Measurement of Mechanical Properties
[0096] Each of the polyethylene microporous films prepared in
Examples 1.about.27 and Comparative Examples 1.about.9, according
to the first embodiment, was measured for strength (kg/mm.sup.2)
and elongation (%) using an ASTM D882 method. The results are given
in Tables 1 to 3 below. TABLE-US-00001 TABLE 1 Amount Porosity
Strength Elongation (wt %) (%) (kg/mm.sup.2) (%) Ex. No. Layer
Thick. (.mu.m) PE Oil Part. Total MD/TD MD/TD Structure 1 B 1 62 38
23 28 16.3/16.5 143/141 Multilayer A 2 37 63 33 B 1 62 38 23 2 B 1
55 45 28 30 16.7/16.1 152/155 Multilayer A 2 37 63 33 B 1 55 45 28
3 B 1 48 52 32 32 15.3/15.1 161/168 Multilayer A 2 37 63 33 B 1 48
52 32 4 B 1 62 38 23 31 15.5/15.9 175/170 Multilayer A 2 30 70 38 B
1 62 38 23 5 B 1 55 45 28 33 14.7/14.5 183/187 Multilayer A 2 30 70
38 B 1 55 45 28 6 B 1 48 52 33 36 13.2/13.9 185/181 Multilayer A 2
30 70 38 B 1 48 52 33 7 B 1 62 38 23 33 13.7/13.2 192/197
Multilayer A 2 23 77 43 B 1 62 38 23 8 B 1 55 45 28 35 12.1/12.8
215/212 Multilayer A 2 23 77 43 B 1 55 45 28 9 B 1 48 52 32 38
12.6/12.1 223/225 Multilayer A 2 23 77 43 B 1 48 52 32 C. 1 -- 4 55
45 -- 28 7.3/7.7 117/111 Monolayer C. 2 -- 4 45 55 -- 32 6.5/6.2
131/136 Monolayer C. 3 -- 4 35 65 -- 35 5.2/4.8 157/164
Monolayer
[0097] TABLE-US-00002 TABLE 2 Amount Porosity Strength Elongation
(wt %) (%) (kg/mm.sup.2) (%) Ex. No. Layer Thick. (.mu.m) PE Oil
Part. Total MD/TD MD/TD Structure 10 B 2 62 38 27 33 18.5/18.3
131/133 Multilayer A 4 37 63 39 B 2 62 38 27 11 B 2 55 45 32 36
18.1/18.7 145/142 Multilayer A 4 37 63 39 B 2 55 45 32 12 B 2 48 52
37 38 17.4/17.2 152/159 Multilayer A 4 37 63 39 B 2 48 52 37 13 B 2
62 38 27 35 17.6/17.9 166/161 Multilayer A 4 30 70 44 B 2 62 38 27
14 B 2 55 45 32 38 16.5/16.7 177/173 Multilayer A 4 30 70 44 B 2 55
45 32 15 B 2 48 52 37 41 15.9/15.2 171/175 Multilayer A 4 30 70 44
B 2 48 52 37 16 B 2 62 38 27 38 15.8/15.3 183/188 Multilayer A 4 23
77 49 B 2 62 38 27 17 B 2 55 45 32 40 14.2/14.9 206/203 Multilayer
A 4 23 77 49 B 2 55 45 32 18 B 2 48 52 37 43 14.6/14.1 213/215
Multilayer A 4 23 77 49 B 2 48 52 37 C. 4 -- 8 55 45 -- 32
11.7/11.3 121/127 Monolayer C. 5 -- 8 45 55 -- 37 10.6/10.3 142/147
Monolayer C. 6 -- 8 35 65 -- 41 9.8/9.2 164/177 Monolayer
[0098] TABLE-US-00003 TABLE 3 Amount Porosity Strength Elongation
(wt %) (%) (kg/mm.sup.2) (%) Ex. No. Layer Thick. (.mu.m) PE Oil
Part. Total MD/TD MD/TD Structure 19 B 3 62 38 31 38 16.3/16.5
145/149 Multilayer A 6 37 63 45 B 3 62 38 31 20 B 3 55 45 36 41
16.7/16.1 153/151 Multilayer A 6 37 63 45 B 3 55 45 36 21 B 3 48 52
41 43 15.3/15.1 163/168 Multilayer A 6 37 63 45 B 3 48 52 41 22 B 3
62 38 31 40 15.5/15.9 170/179 Multilayer A 6 30 70 50 B 3 62 38 31
23 B 3 55 45 36 43 14.7/14.5 184/188 Multilayer A 6 30 70 50 B 3 55
45 36 24 B 3 48 52 41 45 13.2/13.9 189/180 Multilayer A 6 30 70 50
B 3 48 52 41 25 B 3 62 38 31 43 13.7/13.2 193/197 Multilayer A 6 23
77 55 B 3 62 38 31 26 B 3 55 45 36 46 12.1/12.8 211/219 Multilayer
A 6 23 77 55 B 3 55 45 36 27 B 3 48 52 41 48 12.6/12.1 233/238
Multilayer A 6 23 77 55 B 3 48 52 41 C. 7 -- 12 55 45 -- 36
13.7/13.3 141/151 Monolayer C. 8 -- 12 45 55 -- 41 12.6/12.3
162/167 Monolayer C. 9 -- 12 35 65 -- 47 11.8/11.2 184/186
Monolayer
[0099] As is apparent from Tables 1 to 3, the microporous films
having a structure comprising B layer/A layer/B layer and
thicknesses ranging from 4 to 12 .mu.m prepared in Examples of the
present invention, had mechanical properties of strength and
elongation superior to microporous films of Comparative Examples
having a monolayer structure and same thicknesses.
[0100] Preparation of Polyethylene Microporous Film According to
the 2.sup.nd Embodiment
EXAMPLE 28
[0101] A high density polyethylene (PE-1) having an Mw of 400,000
and a copolyester resin were dry mixed with 0.5 wt % Irganox (1010,
available from Ciba Geigy Co. Ltd.), serving as an antioxidant,
based on the amount of the high density polyethylene, to prepare a
resin composition. Then, the resin composition was loaded into a
first twin-screw extruder, and paraffin oil was injected into a
side feed of the twin-screw extruder using a pump. The amounts of
resin composition and paraffin oil added are shown in Table 4
below. Subsequently, an extrusion process was conducted at
240.degree. C. and a screw rotation rate of 200 rpm, to form an A
layer.
[0102] In addition, a high density polyethylene having an Mw of
400,000 was dry mixed with 1 wt % Irganox antioxidant based on the
amount of the high density polyethylene, to prepare a resin
composition. The resin composition was loaded into a second
twin-screw extruder, and paraffin oil was injected into a side feed
of the twin-screw extruder using a pump. The amounts of resin
composition and paraffin oil added are shown in Table 4 below.
Subsequently, an extrusion process was conducted at 240.degree. C.
and a screw rotation rate of 200 rpm, to form a B layer.
[0103] Thereafter, the A and B layers were coextruded from a T-die
through feed blocks, to form a laminated structure comprising three
layers, that is, B layer/A layer/B layer, which was then cooled on
a cooling roll, thereby forming a gel composition comprising B
layer/A layer/B layer. The gel composition was loaded into a
simultaneous biaxial stretching apparatus, stretched to a 6.times.6
ratio at 120.degree. C., treated so that the paraffin oil was
removed from the stretched gel composition through extraction using
methylene chloride as an organic solvent, and then heat treated at
120.degree. C., thus preparing a 16 .mu.m thick microporous film
having a laminated structure comprising B layer/A layer/B
layer.
EXAMPLES 29.about.30
[0104] A 16 .mu.m thick microporous film having a laminated
structure comprising B layer/A layer/B layer was prepared in the
same manner as in Example 28, with the exception that the amounts
of resin composition and paraffin oil added were changed as shown
in Table 4 below.
EXAMPLE 31
[0105] A high density polyethylene (PE-2) having an Mw of 300,000
and a copolyester resin were dry mixed with 0.5 wt % Irganox
antioxidant based on the amount of the high density polyethylene,
to prepare a resin composition, which was then loaded into a first
twin-screw extruder, after which paraffin oil was injected into a
side feed of the twin-screw extruder using a pump. Subsequently, an
extrusion process was conducted at 240.degree. C. and a screw
rotation rate of 200 rpm, to form an A layer.
[0106] In addition, a high density polyethylene (PE-2) having an Mw
of 300,000 was dry mixed with 0.5 wt % Irganox antioxidant based on
the amount of the high density polyethylene, to prepare a resin
composition, which was then loaded into a second twin-screw
extruder, after which paraffin oil was injected into a side feed of
the twin-screw extruder using a pump. Subsequently, an extrusion
process was conducted at the same temperature and screw rotation
rate mentioned above, to form a B layer.
[0107] Thereafter, the A and B layers were coextruded from a T-die
through feed blocks, to form a laminated structure comprising three
layers, that is, B layer/A layer/B layer, which was then cooled on
a cooling roll, thereby forming a gel composition comprising B
layer/A layer/B layer. The gel composition was loaded into a
simultaneous biaxial stretching apparatus, stretched to a 7.times.7
ratio at 120.degree. C., treated so that the paraffin oil was
removed from the stretched gel composition through extraction using
methylene chloride as an organic solvent, and then heat treated at
120.degree. C., thus preparing a 16 .mu.m thick microporous film
having a laminated structure comprising B layer/A layer/B
layer.
EXAMPLES 32.about.33
[0108] A 16 .mu.m thick microporous film having a laminated
structure comprising B layer/A layer/B layer was prepared in the
same manner as in Example 31, with the exception that the amounts
of resin composition and paraffin oil added were changed as shown
in Table 4 below.
EXAMPLE 34
[0109] A high density polyethylene (PE-3) having an Mw of 200,000,
ultrahigh molecular weight polyethylene (PE-4) having an Mw of
2,000,000, and a copolyester resin were dry mixed with 0.5 wt %
Irganox antioxidant based on the amount of the high density
polyethylene, to prepare a resin composition, which was then loaded
into a first twin-screw extruder, after which paraffin oil was
injected into a side feed of the twin-screw extruder using a pump.
Subsequently, an extrusion process was conducted at 240.degree. C.
and a screw rotation rate of 200 rpm, to form an A layer.
[0110] In addition, high density polyethylene (PE-3) having an Mw
of 200,000 and ultrahigh molecular weight polyethylene (PE-4)
having an Mw of 2,000,000 were dry mixed with 0.5 wt % Irganox
antioxidant based on the amount of the high density polyethylene,
to prepare a resin composition, which was then loaded into a second
twin-screw extruder, after which paraffin oil was injected into a
side feed of the twin-screw extruder using a pump. Subsequently, an
extrusion process was conducted at the same extrusion temperature
and screw rotation rate mentioned above, to form a B layer.
[0111] Thereafter, the A and B layers were coextruded from a T-die
through feed blocks, to form a laminated structure comprising three
layers, that is, B layer/A layer/B layer, which was then cooled on
a cooling roll, thereby forming a gel composition comprising B
layer/A layer/B layer. The gel composition was loaded into a
simultaneous biaxial stretching apparatus, stretched to a 5.times.5
ratio at 120.degree. C., treated so that the paraffin oil was
removed from the stretched gel composition through extraction using
methylene chloride as an organic solvent, and then heat treated at
120.degree. C., thus preparing a 16 .mu.m thick microporous film
having a laminated structure comprising B layer/A layer/B
layer.
EXAMPLES 35.about.36
[0112] A 16 .mu.m thick microporous film having a laminated
structure comprising B layer/A layer/B layer was prepared in the
same manner as in Example 34, with the exception that the amounts
of resin composition and paraffin oil added were changed as shown
in Table 4 below. TABLE-US-00004 TABLE 4 Preparation of
Polyethylene Microporous Film Amount Ex. Thick. (wt %) Copoly- No.
(.mu.m) PE-1 PE-2 PE-3 PE-4 ester Oil Structure 28 16 40 -- -- --
-- 60 Multilayer 39.6 -- -- -- 0.4 60 40 -- -- -- -- 60 29 16 40 --
-- -- -- 60 Multilayer 39 -- -- -- 1 60 40 -- -- -- -- 60 30 16 40
-- -- -- -- 60 Multilayer 38 -- -- -- 2 60 40 -- -- -- -- 60 31 16
-- 55 -- -- -- 45 Multilayer -- 54.5 -- -- 0.5 45 -- 55 -- -- -- 45
32 16 -- 55 -- -- -- 45 Multilayer -- 53.6 -- -- 1.4 45 -- 55 -- --
-- 45 33 16 -- 55 -- -- -- 45 Multilayer -- 52.3 -- -- 2.7 45 -- 55
-- -- -- 45 34 16 -- -- 21 9 -- 70 Multilayer -- -- 20.8 8.9 0.3 70
-- -- 21 9 -- 70 35 16 -- -- 21 9 -- 70 Multilayer -- -- 20.5 8.8
0.7 70 -- -- 21 9 -- 70 36 16 -- -- 21 9 -- 70 Multilayer -- --
20.0 8.5 1.5 70 -- -- 21 9 -- 70
EXAMPLE 37
[0113] A high density polyethylene (PE-1) having an Mw of 400,000
and nylon 6 were dry mixed with 0.5 wt % Irganox (1010, available
from Ciba Geigy Co. Ltd.), serving as an antioxidant, based on the
amount of the high density polyethylene, to prepare a resin
composition. Then, the resin composition was loaded into a first
twin-screw extruder, and paraffin oil was injected into a side feed
of the twin-screw extruder using a pump. The amounts of resin
composition and paraffin oil added are shown in Table 5 below.
Subsequently, an extrusion process was conducted at 240.degree. and
a screw rotation rate of 200 rpm, to form an A layer.
[0114] In addition, a high density polyethylene having an Mw of
400,000 was dry mixed with 1 wt % Irganox antioxidant based on the
amount of the high density polyethylene, to prepare a resin
composition, which was then loaded into a second twin-screw
extruder, after which paraffin oil was injected into a side feed of
the twin-screw extruder using a pump. The amounts of resin
composition and paraffin oil added are shown in Table 5 below.
Subsequently, an extrusion process was conducted at 240.degree. C.
and a screw rotation rate of 200 rpm, to form a B layer.
[0115] Thereafter, the A and B layers were coextruded from a T-die
through feed blocks, to form a laminated structure comprising three
layers, that is, B layer/A layer/B layer, which was then cooled on
a cooling roll, thereby forming a gel composition comprising B
layer/A layer/B layer. The gel composition was loaded into a
simultaneous biaxial stretching apparatus, stretched to a 6.times.6
ratio at 120.degree. C., treated so that the paraffin oil was
removed from the stretched gel composition through extraction using
methylene chloride as an organic solvent, and then heat treated at
120.degree. C., thus preparing a 16 .mu.m thick microporous film
having a laminated structure comprising B layer/A layer/B
layer.
EXAMPLES 38.about.39
[0116] A 16 .mu.m thick microporous film having a laminated
structure comprising B layer/A layer/B layer was prepared in the
same manner as in Example 37, with the exception that the amounts
of resin composition and paraffin oil added were changed as shown
in Table 5 below.
EXAMPLE 40
[0117] High density polyethylene (PE-2) having an Mw of 300,000,
and nylon 6 were dry mixed with 0.5 wt % Irganox antioxidant based
on the amount of the high density polyethylene, to prepare a resin
composition. Then, the resin composition was loaded into a first
twin-screw extruder, and paraffin oil was injected into a side feed
of the twin-screw extruder using a pump. Subsequently, an extrusion
process was conducted at 240.degree. C. and a screw rotation rate
of 200 rpm, to form an A layer.
[0118] In addition, high density polyethylene (PE-2) having an Mw
of 300,000 was dry mixed with 0.5 wt % Irganox antioxidant based on
the amount of the high density polyethylene, to prepare a resin
composition. Then, the resin composition was loaded into a second
twin-screw extruder, and paraffin oil was injected into a side feed
of the twin-screw extruder using a pump. Subsequently, an extrusion
process was conducted at the same extrusion temperature and screw
rotation rate mentioned above, to form a B layer. The amounts of
addition of the resin composition and the paraffin oil are shown in
Table 5 below.
[0119] Thereafter, the A and B layers were coextruded from a T-die
through feed blocks, to form a laminated structure comprising three
layers, that is, B layer/A layer/B layer, which was then cooled on
a cooling roll, thereby forming a gel composition comprising B
layer/A layer/B layer. The gel composition was loaded into a
simultaneous biaxial stretching apparatus, stretched to a 7.times.7
ratio at 120.degree. C., treated so that the paraffin oil was
removed from the stretched gel composition through extraction using
methylene chloride as an organic solvent, and then heat treated at
120.degree. C., thus preparing a 16 .mu.m thick microporous film
having a laminated structure comprising B layer/A layer/B
layer.
EXAMPLES 41.about.-42
[0120] A 16 .mu.m thick microporous film having a laminated
structure comprising B layer/A layer/B layer was prepared in the
same manner as in Example 40, with the exception that the amounts
of resin composition and paraffin oil added were changed as shown
in Table 5 below.
EXAMPLE 43
[0121] High density polyethylene (PE-3) having an Mw of 200,000,
ultrahigh molecular weight polyethylene (PE-4) having an Mw of
2,000,000, and a copolyester resin were dry mixed with 0.5 wt %
Irganox antioxidant based on the amount of the high density
polyethylene to obtain a resin composition. Then, the resin
composition was loaded into a first twin-screw extruder, and
paraffin oil was injected into a side feed of the twin-screw
extruder using a pump. Subsequently, an extrusion process was
conducted at 240.degree. C. and a screw rotation rate of 200 rpm,
to form an A layer.
[0122] In addition, high density polyethylene (PE-3) having an Mw
of 200,000 and ultrahigh molecular weight polyethylene (PE-4)
having an Mw of 2,000,000 were dry mixed with 0.5 wt % Irganox
antioxidant based on the amount of the high density polyethylene,
to obtain a resin composition. Then, the resin composition was
loaded into a second twin-screw extruder, and paraffin oil was
injected into a side feed of the twin-screw extruder using a pump.
Subsequently, an extrusion process was conducted at the same
extrusion temperature and screw rotation rate mentioned above, to
form a B layer. The amounts of resin composition and paraffin oil
added are shown in Table 5 below.
[0123] Thereafter, the A and B layers were coextruded from a T-die
through feed blocks, to form a laminated structure comprising three
layers, that is, B layer/A layer/B layer, which was then cooled on
a cooling roll, thereby forming a gel composition comprising B
layer/A layer/B layer. The gel composition was loaded into a
simultaneous biaxial stretching apparatus, stretched to a 5.times.5
ratio at 120.degree. C., treated so that the paraffin oil was
removed from the stretched gel composition through extraction using
methylene chloride as an organic solvent, and then heat treated at
120.degree. C., thus preparing a 16 .mu.m thick microporous film
having a laminated structure comprising B layer/A layer/B
layer.
EXAMPLES 44.about.45
[0124] A 16 .mu.m thick microporous film having a laminated
structure comprising B layer/A layer/B layer was prepared in the
same manner as in Example 43, with the exception that the amounts
of resin composition and paraffin oil added changed as shown in
Table 5 below. TABLE-US-00005 TABLE 5 Preparation of Polyethylene
Microporous Film Amount Ex. Thick. (wt %) No. (.mu.m) PE-1 PE-2
PE-3 PE-4 Nylon 6 Oil Structure 37 16 40 -- -- -- -- 60 Multilayer
39.6 -- -- -- 0.4 60 40 -- -- -- -- 60 38 16 40 -- -- -- -- 60
Multilayer 39 -- -- -- 1 60 40 -- -- -- -- 60 39 16 40 -- -- -- --
60 Multilayer 38 -- -- -- 2 60 40 -- -- -- -- 60 40 16 -- 55 -- --
-- 45 Multilayer -- 54.5 -- -- 0.5 45 -- 55 -- -- -- 45 41 16 -- 55
-- -- -- 45 Multilayer -- 53.6 -- -- 1.4 45 -- 55 -- -- -- 45 42 16
-- 55 -- -- -- 45 Multilayer -- 52.3 -- -- 2.7 45 -- 55 -- -- -- 45
43 16 -- -- 21 9 -- 70 Multilayer -- -- 20.8 8.9 0.3 70 -- -- 21 9
-- 70 44 16 -- -- 21 9 -- 70 Multilayer -- -- 20.5 8.8 0.7 70 -- --
21 9 -- 70 45 16 -- -- 21 9 -- 70 Multilayer -- -- 20.0 8.5 1.5 70
-- -- 21 9 -- 70
COMPARATIVE EXAMPLE 10
[0125] A high density polyethylene (PE-1) having an Mw of 400,000
was dry mixed with 1 wt % Irganox antioxidant based on the amount
of the high density polyethylene to obtain a resin composition.
Then the resin composition was loaded into a twin-screw extruder,
and paraffin oil was injected into a side feed of the twin-screw
extruder using a pump. The amounts of resin composition and
paraffin oil added are shown in Table 6 below. Subsequently, an
extrusion process was conducted at 190.degree. C. and a screw
rotation rate of 200 rpm, to obtain a predetermined layer. The
layer thus obtained was extruded from a T-die, and then cooled on a
cooling roll, thereby forming a gel sheet. The sheet was stretched
to a 6.times.6 ratio at 120.degree. C. using a simultaneous biaxial
stretching apparatus, treated so that the paraffin oil was removed
from the stretched gel composition through extraction using
methylene chloride as an organic solvent, and then heat treated at
120.degree. C., thus preparing a 16 .mu.m thick microporous film
having a monolayer structure.
COMPARATIVE EXAMPLE 11
[0126] A 16 .mu.m thick microporous film having a monolayer
structure was prepared in the same manner as in Comparative Example
10, with the exception that the amounts of resin composition and
paraffin oil added were changed as shown in Table 6 below, and the
stretching ratio was changed to 7.times.7.
COMPARATIVE EXAMPLE 12
[0127] A 16 .mu.m thick microporous film having a monolayer
structure was prepared in the same manner as in Comparative Example
10, with the exception that the amounts of resin composition and
paraffin oil added were changed as shown in Table 6 below, and the
stretching ratio was changed to 5.times.5. TABLE-US-00006 TABLE 6
Preparation of Polyethylene Microporous Film Amount C. Ex. Thick.
(wt %) No. (.mu.m) PE-1 PE-2 PE-3 PE-4 Oil Structure 10 16 40 -- --
-- 60 Monolayer 11 16 -- 55 -- -- 45 Monolayer 12 16 -- -- 21 9 70
Monolayer
[0128] EXPERIMENTAL EXAMPLE 2
[0129] Measurement of Properties
[0130] Thickness of Microporous Film
[0131] The thickness of each of the polyethylene microporous films
prepared in Examples 28.about.45 and Comparative Examples
10.about.12 according to the second embodiment of the present
invention was measured using a contact thickness gauge(MITUTOYO).
The results are given in Tables 7 to 9. The thickness of each of
the polyethylene microporous films was confirmed to be 16 .mu.m.
While measuring the thickness of the microporous film, the
following porosity and mechanical properties were also
measured.
[0132] Measurement of Porosity
[0133] A 10 cm.times.10 cm sample of each of the polyethylene
microporous films prepared in Examples 28.about.45 and Comparative
Examples 10.about.12 according to the second embodiment was cut,
and the volume and weight thereof were measured. The measured
values were substituted into Equation 1, to calculate porosity (%).
The results are given in Tables 7 to 9 below.
[0134] Measurement of Strength and Elongation
[0135] Each of the polyethylene microporous films prepared in
Examples 28.about.45 and Comparative Examples 10.about.12 according
to the second embodiment was measured for strength (kg/mm.sup.2)
and elongation (%) using an ASTM D882 method. The results are given
in Tables 7 to 9 below. TABLE-US-00007 TABLE 7 Melting Ex. Porosity
Strength (kg/mm.sup.2) Elongation (%) Temp. (.degree. C.) No. (%)
MD PE-2 PE-3 PE-4 1.sup.st 2.sup.nd 28 43 14.2 13.8 163 172 138 221
29 46 13.2 14.2 175 184 138 221 30 51 13.5 13.1 181 192 139 223 31
40 13.7 14.5 79 80 138 222 32 44 15.1 14.1 85 90 138 222 33 48 13.1
13.3 89 102 139 223 34 41 16.3 13.1 142 240 136 222 35 46 15.6 13.3
158 231 135 221 36 51 15.8 13.5 168 257 138 223
[0136] TABLE-US-00008 TABLE 8 Melting Ex. Porosity Strength
(kg/mm.sup.2) Elongation (%) Temp. (.degree. C.) No. (%) MD PE-2
PE-3 PE-4 1.sup.st 2.sup.nd 37 42 14.6 13.4 161 179 138 223 38 26
14.3 14.8 171 187 138 223 39 52 13.4 13.0 188 182 139 222 40 39
13.2 14.7 88 73 139 223 41 43 15.3 14.3 81 96 138 223 42 48 13.1
13.5 82 107 138 222 43 41 16.3 13.2 143 237 135 224 44 45 15.5 13.7
158 244 137 222 45 50 15.1 13.2 138 257 137 224
[0137] TABLE-US-00009 TABLE 9 Melting Ex. Porosity Strength
(kg/mm.sup.2) Elongation (%) Temp. (.degree. C.) No. (%) MD PE-2
PE-3 PE-4 1.sup.st 2.sup.nd 10 39 11.2 10.8 153 162 139 -- 11 35
10.1 11.1 69 70 138 -- 12 38 13.3 10.5 138 230 136 --
[0138] As is apparent from Tables 7-9, the microporous films having
a laminated structure comprising B layer/A layer/B layer, prepared
in Examples 28.about.45, had mechanical properties of strength and
elongation superior to microporous films of Comparative Examples
10.about.12 having a monolayer structure and same thicknesses.
[0139] In addition, the polyethylene microporous films having a
laminated structure comprising B layer/A layer/B layer, prepared in
Examples 28.about.45, had higher porosity than the polyethylene
microporous films prepared in Comparative Examples 10.about.12.
[0140] This is because polyester or nylon 6, incompatible with
polyethylene, is selectively used in Examples 28.about.45. Thereby,
pores can be formed not only through a typical pore formation
process but also through additional pore formation during a
stretching process due to the incompatible resin, therefore
resulting in increased porosity.
[0141] Further, the polyethylene microporous film having a
laminated structure comprising B layer/A layer/B layer, prepared
using the thermoplastic resin having a melting temperature higher
than that of the polyethylene in Examples 28.about.45, has a first
melting temperature of 125.about.145.degree. C. and a second
melting temperature of 175.about.235.degree. C., and thus can be
confirmed to have high-temperature stability. Hence, the
polyethylene microporous film of the present invention does not
melt upon overheating, and can delay the occurrence of short
circuits.
[0142] As described above, the present invention provides a
polyethylene microporous film for a rechargeable battery separator
and a method of preparing the same. According to a first embodiment
and a second embodiment of the present invention, a polyethylene
and an aliphatic hydrocarbon solvent are melt-mixed to separately
form an A layer and a B layer having different porosities, after
which the A and B layers are coextruded, thus preparing a
polyethylene microporous film for a rechargeable battery separator
having a laminated structure comprising B layer/A layer/B
layer.
[0143] Since the polyethylene microporous film thus prepared has a
laminated structure, it has mechanical properties, such as strength
and elongation, superior to a film having a monolayer structure. In
particular, the polyethylene microporous film, according to the
second embodiment, comprises an A layer further including a
thermoplastic resin having a melting temperature higher than that
of the polyethylene, in which the thermoplastic resin functions to
form pores during a stretching process, thus increasing the
porosity thereof and exhibiting high-temperature stability.
[0144] In addition, the method of preparing the polyethylene
microporous film of the present invention is advantageous because
process stability is exhibited upon preparation of a microporous
film, and thin film articles can be produced.
[0145] It should be understood that various changes and
modifications to the presently preferred embodiments described
herein will be apparent to those skilled in the art. Such changes
and modifications can be made without departing from the spirit and
scope of the present subject matter and without diminishing its
intended advantages. It is therefore intended that such changes and
modifications be covered by the appended claims.
* * * * *