U.S. patent application number 11/407631 was filed with the patent office on 2007-08-16 for microporous polyolefin film with improved meltdown property and preparing method thereof.
Invention is credited to In-Hwa Jung, Gwi-Gwon Kang, Je-An Lee, Young-Keun Lee, Jang-Weon Rhee.
Application Number | 20070190303 11/407631 |
Document ID | / |
Family ID | 38368907 |
Filed Date | 2007-08-16 |
United States Patent
Application |
20070190303 |
Kind Code |
A1 |
Lee; Young-Keun ; et
al. |
August 16, 2007 |
Microporous polyolefin film with improved meltdown property and
preparing method thereof
Abstract
The present invention is related to microporous polyolefin films
that may be used for battery separators and the methods of
manufacturing the same. These microporous polyolefin films are
characterized by being manufactured in a method comprising the
steps of melt-extruding a composition, comprised of 20-50 weight %
of a resin composition, comprised of 90-98 weight % of polyethylene
(Component I) having a weight average molecular weight of
2.times.10.sup.5-4.times.10.sup.5 and less than 5 weight % of
molecules of which molecular weight is less than 1.times.10.sup.4
and less than 5 weight % of molecules of which molecular weight is
greater than 1.times.10.sup.6, and 2-10 weight % of polypropylene
(Component II) of which weight average molecular weight is
3.0.times.10.sup.4-8.0.times.10.sup.5 and the peak of the melting
point is higher than 145.degree. C., and 80-50 weight % of a
diluent (Component III), to mold in the form of sheets; stretching
the above sheets to mold in the form of films; extracting the
diluent from the above films; and heat-setting the above films.
They are also characterized by having a puncture strength of
greater than 0.14 N/.mu.m, Darcy's permeability constant of greater
than 1.5.times.10.sup.-5 Darcy, closing temperature of microporous
films of lower than 140.degree. C., and melt-down temperature of
higher than 160.degree. C. They can enhance the performance and
stability of batteries using them as well as the productivity of
microporous films owing to their high thermal stability and
superior extrusion compoundability and physical properties.
Inventors: |
Lee; Young-Keun; (Seoul,
KR) ; Rhee; Jang-Weon; (Daejeon-city, KR) ;
Kang; Gwi-Gwon; (Daejeon-city, KR) ; Jung;
In-Hwa; (Cheonansi, KR) ; Lee; Je-An;
(Daejeon-city, KR) |
Correspondence
Address: |
CANTOR COLBURN, LLP
55 GRIFFIN ROAD SOUTH
BLOOMFIELD
CT
06002
US
|
Family ID: |
38368907 |
Appl. No.: |
11/407631 |
Filed: |
April 20, 2006 |
Current U.S.
Class: |
428/304.4 |
Current CPC
Class: |
B29K 2105/04 20130101;
B29K 2023/12 20130101; C08J 2323/06 20130101; H01M 50/411 20210101;
B29C 55/005 20130101; B29K 2023/06 20130101; Y02E 60/10 20130101;
C08J 5/18 20130101; Y10T 428/249953 20150401 |
Class at
Publication: |
428/304.4 |
International
Class: |
B32B 3/26 20060101
B32B003/26 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 14, 2006 |
KR |
10-2006-0013923 |
Claims
1. A method of manufacture of microporous polyolefin films having a
puncture strength of greater than 0.14 N/.mu.m, Darcy's
permeability constant of greater than 1.5.times.10.sup.-5 Darcy,
closing temperature of microporus films of lower than 140.degree.
C., and melt-down temperature of higher than 160.degree. C.
comprising the steps of: melt-extruding a composition, comprised of
20-50 weight % of a resin composition, comprised of 90-98 weight %
of polyethylene (Component I) having a weight average molecular
weight of 2.times.10.sup.5.about.4.times.10.sup.5 and less than 5
weight % of molecules of which molecular weight is less than
1.times.10.sup.4 and less than 5 weight % of molecules of which
molecular weight is greater than 1.times.10.sup.6, and 2-10 weight
% of polypropylene (Component II) of which weight average molecular
weight is 3.0.times.10.sup.4.about.8.0.times.10.sup.5 and the peak
of the melting point is higher than 145.degree. C., and 80-50
weight % of a diluent (Component III), to mold in the form of
sheets; stretching said sheets to mold in the form of films;
extracting said diluent from said films; and heat-setting said
films.
2. Microporous polyolefin films in claim 1, characterized by that
said microporous polyolefin films have a weight average molecular
weight of 2.0.times.10.sup.5.about.4.5.times.10.sup.5 and the peak
of the melting point of higher than 145.degree. C.
3. The method of manufacture of microporous polyolefin films in
claim 1, characterized by that said Component I is
homopolyethylene; or ethylene co-polymer containing less than 20
weight % of alpha-olefin having 3-8 carbon atoms, or the mixture of
said homopolyethylene and said ethylene co-polymer.
4. The method of manufacture of microporus polyolefin films in
claim 1, characterized by that said Component II is
homopolypropylene, which is a polypropylene having the peak of the
melting point of higher than 145.degree. C.; random polypropylene
containing alpha-olefin having 3-8 carbon atoms or their mixtures;
polyprophylene containing alpha-olefin having 2-8 carbon atoms or
co-polymer resin using the combination of said alpha-olefin; or the
mixture of said homopolypropylene, said random polypropylene, and
said polypropylene.
5. The method of manufacture of microporous polyolefin films in
claim 1, characterized by that the ratio of said Component I and
said Component II is 95-97 weight % to 3-5 weight %.
6. The method of manufacture of microporous polyolefin films in
claim 1, characterized by that said Component III is one or more
components selected from aliphatic or cyclic hydrocarbons such as
nonane, decane, decalin, paraffin oil, etc.; phthalic acid esters
such as dibutyl phtalate, dioctyl phthalate, etc.; aromatic others
such as diphenyl ether, etc.: fatty acids having 10 to 20 carbon
atoms such as stearic acid, oleic acid, linoleic acid, linolenic
acid, etc.; fatty acid alcohols having 10 to 20 carbon atoms such
as stearic acid alcohol, oleic acid alcohol, etc.; and one or more
fatty acid esters in which one or more fatty acids selected from
saturated and unsaturated fatty acids having 4 to 26 carbon atoms
in the fatty acid group such as palmitic acid mono-, di-, or
tri-ester, stearic acid mono-, di-, or tri-ester, oleic acid mono-,
di-, or tri-ester, linoleic acid mono-, di-, or tri-ester, etc. are
ester-bonded with alcohols having 1 to 8 hydroxy radicals and 1 to
10 carbon atoms.
7. The method of manufacture of microporous polyolefin films in
claim 1, characterized by that said step of stretching is done
within a temperature range at which 30-80 weight % of the crystal
portion of said Component I is melted in the machine and transverse
directions greater than 3 times each at the total ratio of
stretching of 25-50 times.
8. Microporous polyolefin films manufactured according to said
method of manufacture in claim 1.
9. Microporous polyolefin films manufactured according to said
method of manufacture in claim 2.
10. Microporous polyolefin films manufactured according to said
method of manufacture in claim 3.
11. Microporous polyolefin films manufactured according to said
method of manufacture in claim 4.
12. Microporous polyolefin films manufactured according to said
method of manufacture in claim 5.
13. Microporous polyolefin films manufactured according to said
method of manufacture in claim 6.
14. Microporous polyolefin film manufactured according to said
method of manufacture in claim 7.
Description
TECHNICAL FIELD
[0001] The present invention is related to microporous polyolefin
films having superior melt-down property and methods of
manufacturing the same. More concretely, the present invention is
related to microporous polyolefin films that can enhance the
performance and stability of batteries using these films in that
they have superior extrusion compoundability and physical
properties as well as superior melt-down property and high
productivity. The present invention is also related to the methods
of manufacturing the same.
[0002] Microporous polyolefin films have been used widely for
various battery separators, separation filters, microfiltration
membranes, etc. owing to their superior chemical stability and
superior physical properties. Among them, secondary battery
separators require for the highest-level quality along with a high
stability. Recently, it has been required to have thermal stability
for separators in accordance with the trend of the high capacity
and high output of secondary batteries. Particularly, in case of
lithium secondary batteries, there is a danger of explosion due to
melt-down of separators coming from overheating of batteries if the
thermal stability of separators is lowered.
[0003] General methods of manufacture of porous films are
introduced in U.S. Pat. No. 4,247,498. Disclosed in this patent is
the technology of manufacturing microporous polyolefin films by
making a thermodynamically single-phase solution by blending
polyethylene and a compatible liquid compound at a high
temperature, cooling the solution, and performing solid-liquid or
liquid-liquid phase separation of polyethylene and the compatible
solvent during the process of cooling.
[0004] Also disclosed in U.S. Pat. No. 4,539,256 is the basic
method of manufacturing microporous films through extrusion
processing of polyethylene and a compatible liquid compound, and
stretching and extracting them.
[0005] The typical method of improving the strength of microporous
films is to increase the molecular weight of a composition by using
or blending ultrahigh-molecular-weight polyolefins (UHMWPO) having
a weight average molecular weight of about 1,000,000.
[0006] Further, disclosed in U.S. Pat. No. 5,051,183 are
microporous polyolefin films using a composition containing
10.about.50 weight % of polyolefin containing greater than 1% of
ultrahigh-molecular-weight polyolefin having a weight average
molecular weight of greater than 700,000 and 90.about.50 weight %
of a solvent such as a mineral oil, etc., and having a polydisperse
index (weight average molecular weight/number average molecular
weight) of 10.about.300. The method of forming cells is to form
porous films by extruding the above composition to make gel-phase
sheets, stretching the sheets at a temperature between the melting
point of the composition and the melting point +10.degree. C. and
extracting the solvent. However, this method leads to blending of
ultrahigh-molecular-weight polyolefins as well as a wide
distribution of molecular weights and an excessive amount of
polyolefins having large molecular weights. This further leads to
lowering of stretchability since chain entanglement among molecules
may occur seriously. That is, melt-down at a high stretching rate
and high stretching speed or non-stretching phenomenon at a low
stretching rate may occur.
[0007] The methods of solving the above problems include making the
composition soft by increasing the stretching temperature during
stretching or obtaining the same effect as that of increasing the
temperature of the composition by slowing down the stretching
speed. Still, to the contrary, there occurs a problem of lowering
physical properties of the final porous films as the orientation of
the resin becomes minor during stretching and stretching effects
are lowered. Also, films that are made of resins having a wide
distribution of molecular weights generally have many defects due
to molecules having small molecular weights compared to the films
made of resins having a narrow distribution of molecular weights,
thus lowering impact strength and puncture strength. These
phenomena are not exceptional for microporous films, and puncture
strength, which is one of important physical properties of
microporous films, is not sufficiently high if the distribution of
molecular weights becomes wide. That is, the effects of
ultrahigh-molecular-weight polyolefins added to improve physical
properties are not shown sufficiently. Such problems appear in
Japanese Laid-Open Patent No. H06-234876, Japanese Laid-Open Patent
No. H06-212006, and U.S. Pat. No. 5,786,396 that disclose similar
technologies.
[0008] Such problems in processing according to the use of
ultrahigh-molecular-weight polyolefins are general and induce
problems such as increase in extrusion load, lowering of extrusion
compounding with a compatible compound, increase in the load of a
stretching machine during stretching, occurrence of non-stretching,
lowering of productivity according to lowering of the stretching
speed and stretching ratio, etc.
[0009] Further disclosed in U.S. Pat. Nos. 4,588,633 and 4,873,034
are processes of manufacturing microporous films by using
ultrahigh-molecular-weight polyolefins having a weight average
molecular weight of greater than 500,000 and an excessive amount of
a solvent that can dissolve polyolefin at a high temperature, and
going through 2-step solvent extraction process and stretching
process. However, these methods are disadvantageous in that an
excessive amount of a solvent should be used during the process of
extrusion in order to improve compoundability and extrudability
with the solvent, which is a disadvantage of
ultrahigh-molecular-weight polyolefins, and this solvent should be
extracted in the first step and extracted again after
stretching.
[0010] The thermal stability of separators in batteries is
determined according to the closing temperature and melt-down
temperature. The closing temperature is a temperature at which no
more current can flow since minute pores of separators are closed
when the inner temperature of batteries is increased abnormally.
The melt-down temperature is a temperature at which current flows
again owing to melt-down of separators when the temperature of
batteries is increased continuously higher than the closing
temperature. It is preferable that the closing temperature is low
but the melt-down temperature is high for the stability of
batteries. Particularly, the melt-down temperature is a temperature
at which current can be cut off when it is likely to induce the
explosion of batteries, and is very closely related to the
stability of batteries.
[0011] Efforts to improve the thermal stability of separation
membranes have been developed in three directions largely: a method
of cross-linking separation membranes, a method of adding inorganic
compounds, and a method of using heat-resistant resins.
[0012] Among them, the method of cross-linking separation membranes
is shown in U.S. Pat. Nos. 6,127,438 and 6,562,519. This method is
a method of electronic line cross-linking or chemical cross-linking
of films. However, in case of line cross-linking, this method is
disadvantageous in that it is necessary to install line
cross-linking equipment using radiation, the speed of production is
restricted, and there is a deviation in quality coming from uneven
cross-linking. Whereas, in case of chemical cross-linking, this
method is disadvantageous in that the process of extrusion
compounding is complicated, it is likely to have gels generated in
the films due to uneven cross-linking, and it is necessary to
perform long-time high-temperature ageing. That is, the increase in
heat resistance according to the method of cross-linking may bring
about increase in inefficiency during the process of production and
unevenness in quality.
[0013] Still further, disclosed in U.S. Pat. No. 6,949,315 is a
method of improving the thermal stability of separation membranes
by compounding an inorganic material, such as 5-15 weight % of
titanium oxide, to ultrahigh-molecular-weight polyethylene.
However, this method is disadvantageous in that it is likely to
have problems in using ultrahigh-molecular-weight resins as well as
problems of lowering compoundability according to the input of
inorganic materials, and uneven quality and generation of pinholes
according to lowering of compoundability. And physical properties
of films such as impact strength, etc. are lowered due to lack of
compatibility of the interface between the inorganic material and
the polymer resin.
[0014] The typical method of using resins having a superior heat
resistance in order to increase the thermal stability of separation
membranes is to make multi-layered separation membranes through the
lamination of resins having a high melting point.
[0015] Disclosed in U.S. Pat. No. 5,691,077 is a method of
manufacturing 3-layered separation membranes through the lamination
of a polypropylene resin having a superior thermal stability
(having a high melting point) to polyethylene having a superior
closing property (having a low melting point). These separation
membranes produced according to the drying method (a method of
making pores by stretching the resin without a diluent) have not
been used widely due to disadvantages such as uneven stretching,
generation of pinholes, increased deviation in thickness, etc.
during the process of manufacture of the original films along with
the problems of lowered productivity due to the addition of the
lamination process progressed in the separate process as well as
the problem of delamination coming from inferior lamination
although they have superior thermal characteristics. In spite of a
superior heat resistance, the films manufactured according to this
method have lowered strength, permeability, evenness in quality,
and productivity that are essential for separation membranes for
the secondary batteries.
[0016] Another method of increasing the thermal stability of
separation membranes is to compound and use resins having a
superior heat resistance, which is disclosed in U.S. Pat. No.
5,641,565. In this method, separation membranes are made by mixing
the resin mixture, in which polyethylene comprised of greater than
10 weight % of molecules having a weight average molecular weight
of greater than 1,000,000 but greater than 5 weight % of molecules
having a weight verage molecular weight of less than 100,000 and
5-45 weight % of polypropylene are mixed, with 30-75 weight % of an
organic liquid compound and 10-50 weight % of an inorganic
material, and extracting the organic liquid. compound and the
inorganic material. This technology needs to have an inorganic
material, polyethylene, and ultrahigh-molecular-weight molecules in
order to prevent lowering of physical properties coming from the
addition of polypropylene, which is a heterogeneous resin. However,
this method may still have problems of an excessive number of
ultrahigh-molecular-weight molecules as well as problems of
lowering of compoundability according to the input of an inorganic
material and uneven quality and generation of pinholes according to
lowering of the compoundability. At the same time, the existence of
low-molecular-weight molecules may lead to widening of the
distribution of molecular weights and lowering of physical
properties. Such example may be found in Japanese Laid-Open Patent
No. H09-259858. It is seen in the preferred embodiment of that
patent that the tensile strength of porous polyethylene films
manufactured according to such method is at a comparatively low
level. Also, this method is disadvantageous in that it has
complicated processes due to the addition of the processes for
extracting and removing the inorganic material used, and a
comparatively large amount of polypropylene is required in order to
obtain sufficient effects.
[0017] The essential characteristics of separation membranes of the
secondary batteries are strength, permeability, even quality, and
productivity, and additionally, thermal stability. However, prior
art described in the above use ultrahigh-molecular-weight resins in
order to increase physical properties or use inorganic materials in
manufacturing separation membranes, and have problems in processing
such as the addition of further processes, etc., and therefore, it
is difficult to meet required characteristics of the separation
membranes of the secondary batteries simultaneously.
SUMMARY OF THE INVENTION
[0018] Accordingly, the inventors of the present invention repeated
extensive studies in order to solve problems with prior art
described in the above, and completed the present invention in
light of the fact that it is possible to manufacture separation
membranes having superior thermal stability (melt-down property) as
well as strength, permeability, even quality, and productivity by
maximizing compounding of a small amount of a propylene resin
having a superior thermal stability without using
ultrahigh-molecular-weight polyethylene or inorganic materials
while maintaining the contents of low-molecular-weight polyethylene
molecules and high-molecular-weight polyethylene molecules
contained in general-molecular-weight polyethylene to be less than
specific contents.
[0019] It is, therefore, an object of the present invention to
provide microporous polyethylene films having a high thermal
stability and superior extrusion compoundability and physical
properties and enabling increase in the performance and stability
of batteries using such microporous membranes and increase in the
productivity of microporous films themselves.
[0020] Microporous polyolefin films according to the present
invention to fulfill the above object are characterized by being
manufactured in a method comprising the steps of melt-extruding a
composition, comprised of 20-50 weight % of a resin composition,
comprised of 90-98 weight % of polyethylene (Component I) having a
weight average molecular weight of
2.times.10.sup.5.about.4.times.10.sup.5 and less than 5 weight % of
molecules of which molecular weight is less than 1.times.10.sup.4
and less than 5 weight % of molecules of which molecular weight is
greater than 1.times.10.sup.6, and 2-10 weight % of polypropylene
(Component II) of which weight average molecular weight is
3.0.times.10.sup.4.about.8.0.times.10.sup.5 and the peak of the
melting point is higher than 145.degree. C. and 80-50 weight % of a
diluent (Component III), to mold in the form of sheets; stretching
the above sheets to mold in the form of films; extracting the
diluent from the above films; and heat-setting the above films.
They are also characterized by having a puncture strength of
greater than 0.14 N/.mu.m, Darcy's permeability constant of greater
than 1.5.times.10.sup.-5 Darcy, closing temperature of microporous
films of lower than 140.degree. C. and melt-down temperature of
higher than 160.degree. C.
[0021] Hereinafter, the method of manufacture of microporous
polyolefin films from polyolefin used in the present invention is
illustrated in more detail below:
[0022] A low-molecular-weight organic material (hereinafter
referred to as a diluent) having a similar molecular structure to
that of polyolefin forms a thermodynamically single phase with
polyolefin at the melting temperature of polyolefin. Phase
separation between polyolefin and the diluent occurs during the
process of cooling if polyolefin and the diluent solution in the
thermodynamically single phase are cooled to a room temperature.
Each phase which is subject to phase separation is comprised of a
polyolefin-rich phase centered on the lamella which is the crystal
portion of polyolefin and a diluent-rich phase comprised of a small
amount of polyolefin melted in the diluent even at a room
temperature and the diluent. Porous polyolefin films are made by
extracting the diluent with an organic solvent after cooling.
[0023] Accordingly, the basic structure of microporous films is
determined during the process of phase separation. That is, the
cell size and structure of the final microporous films are
determined according to the size and structure of the diluent-rich
phase formed during the process of phase separation, and the basic
physical properties of microporous films are affected by the
structure of crystals of polyolefin made during the process of
extraction of the diluent.
[0024] As a result of long-term studies, the inventors of the
present invention found the following fact: a small amount of
polyolefin should exist in the diluent-rich phase as much as
possible in order to make superior microporous films. That is, no
defect should be made in polyolefin, which is the matrix, during
the process of extraction of the diluent, which is affected mostly
by low-molecular-weight polyolefin molecules contained in
polyolefin.
[0025] As a result of making products by using polyolefin having
less low-molecular-weight materials based on the above, it is
possible to make microporous polyolefin films having superior
physical properties and an even cell structure with a resin having
smaller molecular weights than those of the conventional
inventions, thus leading to greatly improved processibility.
[0026] In the present invention, as Component I, 90-98 weight % of
polyethylene, having a weight average molecular weight of
2.times.10.sup.5.about.4.times.10.sup.5 and less than 5 weight % of
molecules having the molecular weight of less than 1.times.10.sup.4
and less than 5 weight % of molecules having the molecular weight
of greater than 1.times.10.sup.6, is used.
[0027] Generally, commercially produced or used polyethylene has
its molecular weights distributed inevitably, and polyethylene
having a weight average molecular weight of over 1.times.10.sup.6
has a part of molecules having the molecular weight of several
thousands. These low-molecular-weight materials have been made
intentionally during the process of production of polyethylene
since they have assumed a role of improving the processibility of
resins having large molecular weights when they have been used for
blown films, blow molding, etc. that have been general uses of
polyethylene commercially. However, during the process of making
microporous polyethylene films, these low-molecular-weight
materials lower completeness of lamella, which is the crystal
portion of polyethylene, in the polyethylene-rich phase, and also
lower the number of tie molecules connecting lamellas, thus
reducing the strength of the entire polyethylene. Further, they
have a high affinity to the diluent and exist mostly in the
diluent-rich phase, and therefore, exist in the interface part of
the cells after extraction, which makes the interface of cells
incomplete, thus lowering porosity. This phenomenon is shown in
molecules having molecular weights of less than 1.times.10.sup.4
and is significantly shown when their content exceeds 5 weight
%.
[0028] Another problem with the existence of molecular weight
distribution is the existence of ultrahigh-molecular-weight
molecules. That is, there exist ultrahigh-molecular-weight
molecules having the molecular weights of over 1.times.10.sup.6 in
polyethylene having a weight average molecular weight of about
1.times.10.sup.5. The existence of such ultrahigh-molecular-weight
molecules assumes the role of increasing the strength of sheets.
But the existence of an excessive amount of
ultrahigh-molecular-weight molecules induces the problem of
generation of gels coming from lowering of compoundability due to a
large difference in viscosity from that of the diluent, and makes
the surface of sheets extruded rough. One way of solving this
problem is to increase the extrusion temperature or to increase the
shear rate of the screw configuration of the biaxial element of the
biaxial compounder. However, in this case, the resin is
deteriorated and physical properties are lowered. Since this
problem is shown greatly when the content of
ultrahigh-molecular-weight molecules having the molecular weight of
over 1.times.10.sup.6 exceeds 5 weight %, it is preferable that the
content of ultrahigh-molecular-weight molecules having the
molecular weight of over 1.times.10.sup.6is less than 5 weight
%.
[0029] Accordingly, it is seen in the present invention that it is
possible to make microporous polyethylene films having superior
physical properties and an even cell structure by using
polyethylene having a sufficiently small low-molecular-weight part
as well as a sufficiently small ultrahigh-molecular-weight part,
i.e., polyethylene having less than 5 weight % of molecules having
the molecular weight of less than 1.times.10.sup.4 as well as less
than 5 weight % of molecules having the molecular weight of greater
than 1.times.10.sup.6, and to greatly improve processibility.
[0030] Available polyethylene includes homopolyethylene
(high-density polyethylene) and co-monomers, i.e., polyethylene
characterized by that less than 20 weight % is alpha-olefin having
3-8 carbon atoms such as propylene, butene-1, hexene-1,
4-methylpentene-1, octene-1, etc. or their combination, and their
mixture. If the content of the co-monomer exceeds 20 weight %, the
crystallinity of polyethylene itself is lowered greatly, and the
strength of polyethylene as separation membranes can not be
maintained.
[0031] Polyethylene has superior physical properties and chemical
stability, and is advantageous in that its closing temperature is
lowered if it is used for the matrix of the secondary battery
separators owing to its low melting point. However, this
characteristic brings about a disadvantage of lowering the
melt-down temperature at the same time. In the present invention,
polypropylene is used in order to take advantage of the low closing
temperature of polyethylene and to complement the disadvantage of
the low melt-down temperature simultaneously.
[0032] Polypropylene (Component II) having a weight average
molecular weight of 3.0.times.10.sup.4.about.8.0.times.10.sup.5 and
the peak of the melting temperatures of higher than 145.degree. C.
is used for the present invention, and its content is 2-10 weight
%. 80-50 weight % of the diluent (Component III) is mixed and used
with respect to 20-50 weight % of the resin composition of
polyethylene (Component I) and poypropylene (Component II).
[0033] Polypropylene is a polyolefin-group resin such as
polyethylene. Compared to other heterogeneous resins, polypropylene
has a good interchangeability but is subject to phase separation
thermodynamically. In other words, if polyethylene and
polypropylene are subject to melt-extrusion for the resin only, two
phases are separated, and a phase having a less volume fraction in
the equilibrium state is separated and exists. If 95 weight % of
polyethylene and 5 weight % of polypropylene are subject to
melt-compounding through the extrusion for the resin only, 5 weight
% of polypropylene is completely separated from 95 weight % of
polyethylene forming the matrix and exists in the form of an
island. In order for compounded polypropylene to sufficiently
demonstrate the increasing effect of the melt-down temperature,
polypropylene should have been distributed in a sufficiently minute
size. That is, in order to prevent melt-down of the matrix even at
a temperature higher than the melting temperature of polyethylene
forming the matrix, it is advantageous that compounded
polypropylene is distributed minutely, more advantageous, if it is
connected to each other. In order to maximize such dispersibility
of polypropylene, the weight average molecular weight of
polypropylene is adjusted, and the extrusion method utilizing the
diluent is used in the present invention.
[0034] It is preferable that the weight average molecular weight of
polypropylene is 3.0.times.10.sup.4.about.8.0.times.10.sup.5 for
the minute dispersion of polypropylene. Factors for determining the
dispersion of polypropylene within polyethylene include
thermodynamic factors and physical factors. For thermodynamic
dispersibility, the less the molecular weight of polypropylene is,
the more advantageous it is. But the smaller the molecular weight
of polypropylene becomes, the severer the phenomena of lowering of
the physical properties of separation membranes and of leaching of
low-molecular-weight materials on the surface become. Physical
dispersion is something that is determined by mixing of the screw
in the extruder. What are very critical are the viscosity of the
phase forming the matrix (polyethylene in case of the present
invention) and the viscosity of the phase to be mixed in
(polypropylene in case of the present invention). For superior
dispersion, it is preferable that the viscosity of the phase to be
mixed in is less than the viscosity of the phase forming the
matrix. But if it is too less, friction of the interface becomes
small and dispersion is obstructed. Dispersibility is lowered also
when the viscosity of the phase to be mixed in is large, in which
case the melt-down characteristic is not improved greatly and
extrudability is lowered. More preferably, the viscosity of
polypropylene is 1/1-1/100 of the viscosity of polyethylene forming
the matrix. The weight average molecular weight of polypropylene
satisfying the above condition is grater than 2.0.times.10.sup.5
but less than 4.5.times.10.sup.5. This enables to obtain
significant outcome that physical properties are not lowered even
if polypropylene is added, extrudability as well as dispersibility
are superior, and melt-down characteristic is greatly improved.
[0035] It is also preferable that the content of polypropylene with
respect to that of polyethylene is 2-10 weight %. If the content of
polypropylene is les than 2 weight %, it is difficult to obtain
such effect; and if the content of polypropylene exceeds 10 weight
%, the physical properties of separation membranes may be lowered.
It is because the interface between polyethylene and polypropylene
becomes weak due to thermodynamic non-interchangeability. It is,
therefore, preferable that the content of polypropylene should be
minimized within the range that the required melt-down
characteristic is obtained. The desirable content of polypropylene
is 3-5 weight %.
[0036] Any polypropylene characterized by that the peak of the
melting temperatures is higher than 145.degree. C. may be used for
polypropylene. For example, homopolypropylene, random polypropylene
using alpha-olefin having 3-8 carbon atoms such as ethylene,
butene-1, hexene-1, 4-methylpentene-1, octene-1, etc. as
co-monomers or their combination; and polypropylene containing
alpha-olefin having 3-8 carbon atoms such as ethylene, propylene,
butene-1, hexene-1, 4-methylpentene-1, octene-1, etc. as
co-polymers or co-polymer resin using their combination, and their
mixture may be used. If the peak of melting temperatures is lower
than 145.degree. C. the effect of improving the melt-down
characteristic is not significant.
[0037] Polyethylene (Component I) and polypropylene (Component II)
are melt-extruded along with a diluent (Component III). As
described in the above, the diluent is used for two purposes: to
make cells after it is melt-extruded and stretched, and further,
extracted; and to maximize the dispersion of polypropylene within
polyethylene.
[0038] A low-molecular-weight organic material (diluent) having a
similar molecular structure to that of polyolefin forms a
thermodynamic single pahse with polyolefin at a high temperature at
which polyolefin is melted. That is, the resin mixture of
polyethylene and polypropylene may be compounded in the
thermodynamic single phase if it is used along with a proper
diluent, contrary to melt-extrusion of a resin only. the formation
of a thermodynamic single phase makes the dispersion compounding of
the molecular scale. Rapid cooling of a mixture compounded in the
molecular scale enables maintaining of maximized compounding even
after the mixture is cooled to the solid state, in which case the
effects of improving the melt-down characteristic by compounding of
polypropylene are maximized. Accordingly, the type of a diluent,
the content of the diluent, and speed of cooling after
melt-extrusion are very important factors in the present
invention.
[0039] The diluent used in the present invention should form a
single phase with a resin at the melt-extrusion processing
temperature of the resin mixture. Examples of diluents include
aliphatic or cyclic hydrocarbons such as nonane, decane, decalin,
paraffin oil, etc.; phthalic acid esters such as dibutyl phthalate,
dioctyl phthalate, etc.; aromatic ethers such as diphenyl ether,
etc.; fatty acids having 10 to 20 carbon atoms such as stearic
acid, oleic acid, linoleic acid, linolenic acid, etc.; fatty acid
alcohols having 10 to 20 carbon atoms such as stearic acid alcohol,
oleic acid alcohol, etc.; and one or more fatty acid esters in
which one or more fatty acids selected from saturated and
unsaturated fatty acids having 4 to 26 carbon atoms in the fatty
acid group such as palmitic acid mono-, di-, or tri-ester, stearic
acid mono-, di-, or tri-ester, oleic acid mono-, di-, or tri-ester,
linoleic acid mono-, di-, or tri-ester, etc. are ester-bonded with
alcohols having 1 to 8 hydroxy radicals and 1 to 10 carbon atoms.
The kinetic viscosity of a desirable diluent is 0.5 cSt.about.30
cSt at 100.degree. C. If the viscosity of a diluent exceeds 30 cSt,
there may occur problems such as an increased load, inferior
surface of sheets and films, etc. due to a high kinetic viscosity
during the process of extrusion, as well as difficulty in
extraction, lowered productivity, reduced permeability due to the
remaining oil, etc. during the process of extraction. On the other
hand, if the viscosity of a diluent is less than 0.5 cSt, it is
difficult to compound during the process of extrusion due to the
difference in density of the resin melted in the extruder.
[0040] It is preferable that 20.about.50 weight % of the resin
mixture of polyethylene and polypropylene is mixed with 80.about.50
weight % of a diluent in the present invention. If the content of
the resin mixture exceeds 50 weight % (i.e., if the content of the
diluent is less than 50 weight %), it is difficult to form the
single phase of polyethylene and polypropylene since the density of
the molten material becomes greatly high and the speed of
thermodynamic diffusion is lowered. That is, the effect of
improving the melt-down characteristic is lowered. Also, porosity
is reduced and the pore size becomes smaller in the characteristics
of the final films after the extraction of the diluent, and
permeability is lowered greatly since interconnection among cells
becomes minor. On the other hand, if the content of the resin
mixture is less than 20 weight % (i.e., if the content of the
diluent exceeds 80 weight %), the friction between the resin
mixture and the diluent in the extruder is lowered greatly; as a
result of which there may occur problems of melt-down, uneven
thickness, etc. during stretching since compoundability is lowered
and the resin mixture is extruded in the form of a gel without
being compounded thermodynamically with the diluent; and it is
difficult to expect to improve the melt-down characteristic.
[0041] If necessary, general additives for improving specific
functions such as anti-oxidants, UV stabilizers, charging
prevention agents, etc. may be further added to the above
composition.
[0042] For compounding of the above composition, a biaxial
compounder, kneader, Banbury mixer, etc. designed for compounding
of the diluent and polyolefin may be used. The extrusion
temperature shold be higher than the temperature at which the
diluent and the resin may be made in the thermodynamically single
phase but below the temperature at which thermal oxidation is
progressed and physical properties of the resin may be lowered. If
paraffin oil is used for the diluent, the temperature should be
higher than 160.degree. C. but lower than 270.degree. C. The resin
and the diluent may be inputted into the compounder after they are
blended previously, or inputted separately from the feeder
separated.
[0043] Thus compounded molten material is extruded through a die
and molded in the form of sheets while being cooled. All of general
casting, calendering, and water cooling methods may be used for
making molded products in the form of sheets from the molten
material. What is important here is the speed of cooling of the
molten material. The molten material made in the single phase
thermodynamically during the process of melt-down extrusion is
subject to phase separation during the process of cooling. At this
time, phase separation between the resin and the diluent is
progressed simultaneously with phase separation among the resins.
That is, if the speed of cooling of the molten material is too
slow, phase separation between polyethylene and polypropylene is
progressed excessively, and compoundability of polypropylene within
polyethylene is lowered greatly. Then, the effect of improving the
melt-down characteristic according to compounding of polypropylene
is reduced and lowering of physical properties is increased.
Accordingly, the speed of cooling of the molten material should be
faster than 10.degree. C./second, more preferably, faster than
100.degree. C./second.
[0044] Thus molded sheets are stretched in the form of films
through the process of stretching, after which the diluent is
extracted from the films to make microporous films.
[0045] Stretching of the sheets made through compounding,
extrusion, and cooing may be done in the roll-type or tenter-type
differential or simultaneous stretching. Here, it is preferable
that the ratio of stretching is greater than 3 times each in the
machine and transverse directions and the total ratio of stretching
is 25.about.50 times. If the ratio of stretching in one direction
is less than 3 times, the tensile strength, puncture strength, etc.
are lowered since facing in one direction is not sufficient and the
balance in physical properties in the machine and transverse
directions is disturbed. Also, if the total ratio of stretching is
less than 25 times, incomplete stretching occurs; and if it exceeds
50 times, it is likely that puncturing occurs during stretching,
and the ratio of contraction of the final films is increased.
[0046] At this time, the temperature of stretching varies according
to the melting point of polyethylene forming the matrix used and
the concentration and type of the diluent. It is proper that the
optimum temperature of stretching is selected from the temperature
range at which 30.about.80 weight % of the crystal portion of
polyethylene in the above molded products of the films is melted.
If the temperature of stretching is selected from the temperature
range which is lower than the temperature at which 30 weight % of
the crystal portion of polyethylene in the above molded products of
films is melted, stretchability is lowered since softness of the
films is lost, and therefore, it is likely that melt-down occurs
during stretching and non-stretching may occur at the same time. To
the contrary, if the temperature of stretching is selected from the
temperature range which is higher than the temperature at which 80
weight % of the crystal portion is melted, stretching may be done
readily and non-stretching may occur less, but there may occur
deviation in thickness due to partial excessive stretching and
physical properties are lowered greatly since the orientation
effect of the resin is insignificant. the degree of melting of the
crystal portion according to the temperature may be obtained
through differential scanning colorimetry (DSC) of the molded
products of the films.
[0047] Stretched films are then extracted and dried by using an
organic solvent. Organic solvents that may be used in the present
invention are not limited specially, but any solvent that can
extract the diluent used for the extrusion of the resin may be
used. It is preferable to use methyl ethyl ketone, methylene
chloride, hexane, etc. that may be extracted efficiently and dried
promptly. As to the methods of extraction, all general methods of
extraction of solvents such as immersion method, solvent spray
method, ultrasonic method, etc. may be used individually or in
combination with each other. During extraction, the content of the
remaining diluent should be less than 1 weight %. If the content of
the remaining diluent exceeds 1 weight %, physical properties are
lowered and the permeability of films is reduced. The amount (ratio
of extraction) of the remaining diluent depends greatly on the
temperature and time of extraction. It is better that the
temperature of extraction is high to increase the solubility of the
diluent and solvent, but is lower than 40.degree. C. in view of the
safety in boiling of the solvent. However, the temperature of
extraction should be higher than the solidification point of the
diluent at all times since the efficiency of extraction is lowered
greatly if the temperature of extraction is lower than the
solidification point of the diluent. The time of extraction varies
according to the thickness of films to be produced, but 2.about.4
minutes is proper in case of producing 10- to 30.mu.-thick general
microporous films.
[0048] Finally, dried films go through the heat-setting step with
their residual stress removed in order to reduce the ratio of
contraction of the final films. Heat setting is to remove residual
stress by setting the films, adding heat, and holding the films to
be contracted forcefully. It is advantageous for lowering of the
ratio of contraction that the temperature of heat setting is high.
But if it is too high, the films are melted partially and
permeability is lowered as minute pores thus formed are clogged. It
is preferable that the temperature of heat setting is selected from
the temperature range at which 10.about.30 weight % of the crystal
portion of the films is melted. If the temperature of heat setting
is selected from the temperature range which is lower than the
temperature at which 10 weight % of the crystal portion of the
above films is melted, it is not effective to remove residual
stress of the films as the reorientation of polyethylene molecules
within the films is minor; and if it is selected from the
temperature range which is higher than the temperature at which 30
weight % of the crystal portion of the films is melted, minute
pores are clogged and permeability is lowered due to partial
melting.
[0049] Here, the time of heat setting should be short relatively if
the temperature of heat setting is high, and if the temperature of
heat setting is low, it may be made long relatively. It is
preferable that the time of heat setting is for about 20 seconds to
2 minutes if the tenter-type continuous heat-setting equipment is
used. Most preferably, the time of heat setting is for about 1 to 2
minutes in the temperature range at which 10.about.15 weight % of
the crystal portion of the films is melted, or for about 20 seconds
to 1 minute in the temperature range at which 15.about.30 weight %
of the crystal portion of the films is melted.
[0050] High-density microporous polyethylene films of the present
invention manufactured as described in the above have the following
physical properties: [0051] (1) Puncture strength is greater than
0.14 N/.mu.m.
[0052] Puncture strength is a value showing the strength of films
with respect to that of sharp articles. When the films are used for
battery separators, if the puncture strength is not sufficient, the
films are broken due to abnormality on the surface of electrodes or
dendrite occurring on the surface of electrodes when using
electrodes, and short may occur. The films having the puncture
strength of greater than 0.14 N/.mu.m according to the present
invention may be used safety for the secondary battery separators.
[0053] (2) Darcy's permeability constant is greater than
1.5.times.10.sup.-5 Darcy.
[0054] It is better to have a high gas permeability. If Darcy's
permeability constant is less than 1.5.times.10.sup.-5 Darcy, the
efficiency as porous films is lowered, and the ion permeability and
charging/discharging characteristics in batteries are lowered. That
is, the films having the Darcy's permeability constant of greater
than 1.5.times.10.sup.-5 Darcy according to the present invention
have superior charging/discharging characteristics, superior
low-temperature characteristics, and long lifetime of batteries.
[0055] (3) Closing temperature of microporous films is lower than
140.degree. C. and melt-down temperature is higher than 160.degree.
C.
[0056] Closing temperature is a temperature at which no more
current flow as minute pores of batteries are clogged when the
internal temperature of batteries is increased abnormally.
Melt-down temperature is a temperature at which current flows again
as separators are melted down when the temperature of batteries is
increased continuously higher than the closing temperature. It is
better that the closing temperature is low and the melt-down
temperature is high for the stability of batteries. Particularly,
melt-down temperature is a temperature at which current may be cut
off under the circumstance that the explosion of batteries may
occur, and is most closely related to the stability of batteries.
The microporous films according to the present invention have the
closing temperature of below 140.degree. C. and maintain the low
closing temperature of microporous polyethylene films. At the same
time, they have the melt-down temperature of higher than
160.degree. C. which is higher than 145.degree. C. i.e., the
melt-down temperature of the case that polyethylene is used singly,
by greater than 15.degree. C. thus improving the thermal stability
of batteries remarkably.
[0057] The microporous films according to the present invention
having the above characteristics have a high thermal stability as
well as superior extrusion compoundability and physical properties,
and therefore, can improve the performance and stability of the
batteries using them as well as the productivity of microporous
films.
DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION
[0058] The present invention is illustrated in more detail in terms
of the following preferred embodiments:
[0059] Molecular weights of polyethylene and polypropylene and the
distribution of molecular weights were measured with
high-temperature GPC (Gel Permeation Chromatography, Model GPC-210)
of Polymer Laboratory Company. The calibration of molecular weights
was done by measuring the standard samples of polyethylene and
polypropylene of Polymer Laboratory Company and correcting Q-factor
after obtaining calibration curves by using standard polystyrene
samples (EasiCal PSI-A, B) of Polymer Laboratory Company.
[0060] The viscosity of the diluent was measured with CAV-4
Automatic Viscometer of Cannon Company.
[0061] Polyethylene and the diluent were compounded in a .phi.=30
mm biaxial compounder. the temperature of extrusion was
180.about.240.degree. C. and the residence time was for 3 minutes.
The extruded molten material was extruded from T-shaped dies and
molded in the form of sheets by using a casting roll. At this time,
the thickness of sheets was adjusted to 600.about.1,200 .mu.m in
order to adjust the thickness of the films to 16.+-.1 mm after the
final stretching, extraction, and heat setting. In order to see
whether there were gels due to inferior melting and compounding,
200- .mu.m-thick films were manufactured separately, and the number
of gels in the area of 2,000 cm.sup.2 was counted. For the
manufacture of high-quality microporous films, the number of gels
per 2,000 cm.sup.2 should be less than 20, which was defined to be
the case of superior extrusion compoundability. The sheets were
cooled by cooling the casting roll with water at a speed of
200.degree. C./second.
[0062] The sheets molded were analyzed with a DSC (Differencial
Scanning Calorimeter) in order to analyze the phenomenon of melting
of the crystal portion according to the temperature of stretching
during stretching. The conditions for analysis were 5 mg for the
weight of samples and 10.degree. C./minute for the rate of
scanning.
[0063] Stretching of the sheets was progressed in terms of
simultaneous stretching in a tenter-type stretching machine with
the ratio of stretching of 6.times.6 at the speed of stretching of
2.0 m/minute while changing the temperature of stretching. The
temperature of stretching according to the present invention was
determined within the temperature range at which 30.about.80 weight
% of the crystal portion of polyethylene in the molded products of
the films was melted based on the results of DSC.
[0064] The diluent was extracted in the immersion method by using
methylene chloride for 4 minutes.
[0065] Heat setting was performed in a convention oven at
122.degree. C. for 1 minute and 30 seconds after drying the films,
from which the diluent was extracted, in the air, and fixing the
films to a frame.
[0066] Tensile strength, puncture strength, gas permeability,
closing temperature of separation membranes, and melt-down
temperature of the films thus manufactured, that were the most
important physical properties of microporous films, were measured,
and the results were shown in the following tables: [0067] (1)
Tensile strength was measured with ASTM D882. [0068] (2) Puncture
strength was measured in terms of the strength of melt-down of the
films by a 0.5-mm-diameter pin at a speed of 120 mm/minute. [0069]
(3) Gas permeability was measured with a porometer (CFP-1500-AEL of
PMI Company). Although gas permeability is indicated in terms of
Gurley number generally, it is difficult to compute the relative
permeability according to the cell structure of the films
themselves since the affect of the thickness of the films is not
corrected in employing the Gurley number. In order to solve this
problem, Darcy's permeability constant was used in the present
invention. Darcy's permeability constant may be obtained according
to the following Equation 1, where nitrogen is used in the present
invention: C=(8F T V)/(D.sup.2 (p.sup.2-1)) [Equation 1] where
C=Darcy's permeability constant [0070] F=Flow rate [0071]
T=Thickness of a sample [0072] V=Viscosity of a gas (0.185 for
N.sub.2) [0073] D=Diameter of a sample [0074] P=Pressure
[0075] In the present invention, an average value of Darcy's
permeability constants in the region of 100.about.200 psi was used.
[0076] (4) The closing temperature of separation membranes and
melt-down temperature were measured in simple cells, in which the
impedance of separation membranes may be measured. Simple cells wre
assembled by positioning separation membranes between two graphite
electrodes having channels and closely attaching copper plates,
assuming the role of collectors , to both sides of graphite
electrodes. Electric resistance value was measured by an impedance
analyzer by connecting wires to the copper plates after inputting
an electrolyte solution to the assembled cells through graphite
electrode channels so that the separation membranes were soaked
sufficiently. Alternating current (5 mV) of 1 kHz was used for the
measurement of the electric resistance value.
[0077] A liquid electrolyte in which hexafluorophosphate
(LiPF.sub.6) is diluted in an electrolyte solution, in which
ethylene carbonate and propylene carbonate were mixed at a volume
ratio of 1:1, to have a concentration of 1 mole was used for the
electrolyte solution. The electric resistance value was measured
after inputting the electrolyte solution while increasing the
temperature of cells from 25.degree. C. to 200.degree. C. at a
speed of 5.degree. C./(minute. The electric resistance value was
maintained to be about 0.5-5 .OMEGA. continuously, and then, was
increased to about 500.about.several thousands .OMEGA. abruptly at
a specific temperature, which was defined to be the closing
temperature. If the temperature of the cells was increased
continuously, the electric resistance value was reduced again. The
temperature at which the electric resistance value of the cells was
lowered below 100 .OMEGA. was defined to be the melt-down
temperature. It was not possible to measure the electric resistance
at a temperature higher than 200.degree. C. since the vaporization
of the electrolyte solution occurred at that temperature, and the
melt-down temperature was marked to be over 200.degree. C. if the
electric resistance was not lowered below 100 .OMEGA. even at
200.degree. C.
Preferred Embodiment 1
[0078] High-density polyethylene not containing a co-monomer but
having a weight average molecular weight of 3.0.times.10.sup.5 and
containing 3.5 weight % of molecules having the molecular weight of
lower than 10.sup.4 and 4.9 weight % of molecules having the
molecular weight of greater than 10.sup.6 was used for Component I.
And homopolypropylene having a weight average molecular weight of
5.7.times.10.sup.5 and melting point of 162.degree. C. was used for
Component II. The contents of Component I and Component II were 95
weight % and 5 weight %, respectively. Paraffin oil of which
100.degree. C. kinetic viscosity was 11 cSt (Component A in the
following table) was used for Component III, and the content of
Component III with respect to the entire content of Components I,
II, and III was 70 weight %. The temperatures of both of the
extruder and die were 180.degree. C.
[0079] Stretching was done at 115.degree. C. in order to adjust the
ratio of melting of the crystal portion to 30 weight % during
stretching. Other conditions for stretching and processing were the
same as described in the above.
Preferred Embodiment 2
[0080] High-density polyethylene not containing a co-monomer but
having a weight average molecular weight of 2.5.times.10.sup.5 and
containing 4.8 weight % of molecules having the molecular weight of
lower than 10.sup.4 and 4.5 weight % of molecules having the
molecular weight of greater than 10.sup.6 was used for Component I.
And homopolypropylene having a weight average molecular weight of
2.9.times.10.sup.5 and melting point of 162.degree. C. was used for
Component II. The contents of Component I and Component II were 97
weight % and 3 weight %, respectively. A diluent in which dibutyl
phthalate and paraffin oil of which 100.degree. C. kinetic
viscosity was 11 cSt were mixed at a ratio of 1:2 (Component B in
the following table) was used for Component III, and the content of
Component III with respect to the entire content of Components I,
II, and III was 60 weight %. The temperatures of both of the
extruder and die were 240.degree. C.
[0081] Stretching was done at 120.degree. C. in order to adjust the
ratio of melting of the crystal portion to 50 weight % during
stretching.
Preferred Embodiment 3
[0082] High-density polyethylene having a weight average molecular
weight of 2.9.times.10.sup.5, containing 4.9 weight % of molecules
having the molecular weight of lower than 10.sup.4 and 3.0 weight %
of molecules having the molecular weight of greater than 10.sup.6,
and using 0.1% of butene-1 as a co-monomer was used for Component
I. And homopolypropylene having a weight average molecular weight
of 2.5.times.10.sup.5 and melting point of 164.degree. C. was used
for Component II. The contents of Component I and Component II were
90 weight % and 10 weight %, respectively. A diluent in which
dibutyl phthalate and paraffin oil of which 100.degree. C. kinetic
viscosity was 11 cSt were mixed at a ratio of 1:2 (Component B in
the following table) was used for Component III, and the content of
Component III with respect to the entire content of Components I,
II, and III was 65 weight %. The temperatures of both of the
extruder and die were 240.degree. C.
[0083] Stretching was done at 118.degree. C. in order to adjust the
ratio of melting of the crystal portion to 40 weight % during
stretching.
Preferred Embodiment 4
[0084] High-density polyethylene not containing a co-monomer but
having a weight average molecular weight of 3.0.times.10.sup.5 and
containing 3.5 weight % of molecules having the molecular weight of
lower than 1 and 4.9 weight % of molecules having the molecular
weight of greater than 10.sup.6 was used for Component I. And
random polypropylene having a weight average molecular weight of
3.5.times.10.sup.5 and melting point of 157.degree. C., as 0.3
weight % of ethylene was used for a co-monomer, was used for
Component II. The contents of Component I and Component II were 95
weight % and 5 weight %, respectively. Paraffin oil of which
100.degree. C. kinetic viscosity was 11 cSt (Component A in the
following table) was used for Component III, and the content of
Component III with respect to the entire content of Components I,
II, and III was 70 weight %. The temperatures of both of the
extruder and die were 180.degree. C.
[0085] Stretching was done at 119.degree. C. in order to adjust the
ratio of melting of the crystal portion to 60 weight % during
stretching.
Preferred Embodiment 5
[0086] High-density polyethylene not containing a co-monomer but
having a weight average molecular weight of 2.5.times.10.sup.5 and
containing 4.8 weight % of molecules having the molecular weight of
lower than 10.sup.4 and 4.5 weight % of molecules having the
molecular weight of greater than 10.sup.6 was used for Component I.
And impact polypropylene having a weight average molecular weight
of 4.2.times.10.sup.5, containing 15% of ethylene-propylene
co-polymer in homopolypropylene, and having a melting point of
162.degree. C. was used for Component II. The contents of Component
I and Component II were 95 weight % and 5 weight %, respectively.
Paraffin oil of which 100.degree. C. kinetic viscosity was 11 cSt
(Component A in the following table) was used for Component III,
and the content of Component III with respect to the entire content
of Components I, II, and III was 75 weight %. The temperatures of
both of the extruder and die were 240.degree. C.
[0087] Stretching was done at 117.degree. C. in order to adjust the
ratio of melting of the crystal portion to 50 weight % during
stretching.
Comparative Example 1
[0088] Polyethylene having a weight average molecular weight of
1.8.times.10.sup.5, containing 22.0 weight % of molecules having
the molecular weight of lower than 10.sup.4 and 1.5 weight % of
molecules having the molecular weight of greater than 10.sup.6, and
using 0.8% of butene-1 as a co-monomer was used for Component I.
And homopolypropylene having a weight average molecular weight of
2.5.times.10.sup.5 and melting point of 162.degree. C. was used for
Component II. The contents of Component I and Component II were 99
weight % and 1 weight %, respectively. Paraffin oil of which
100.degree. C. kinetic viscosity was 11 cSt (Component A in the
following table) was used for Component III, and the content of
Component III with respect to the entire content of Components I,
II, and III was 70 weight %. The temperatures of both of the
extruder and die were 180.degree. C.
[0089] Stretching was done at 114.degree. C. in order to adjust the
ratio of melting of the crystal portion to 25 weight % during
stretching.
Comparative Example 2
[0090] High-density polyethylene not containing a co-monomer but
having a weight average molecular weight of 3.0.times.10.sup.5 and
containing 3.5 weight % of molecules having the molecular weight of
lower than 10.sup.4 and 4.9 weight % of molecules having the
molecular weight of greater than 10.sup.6 was used for Component I.
And homopolypropylene having a weight average molecular weight of
2.5.times.10.sup.5 and melting point of 162.degree. C. was used for
Component II. The contents of Component I and Component II were 85
weight % and 15 weight %, respectively. Paraffin oil of which
100.degree. C. kinetic viscosity was 11 cSt (Component A in the
following table) was used for Component III, and the content of
Component III with respect to the entire content of Components I,
II, and III was 70 weight %. The temperatures of both of the
extruder and die were 180.degree. C.
[0091] Stretching was done at 121.degree. C. in order to adjust the
ratio of melting of the crystal portion to 85 weight % during
stretching.
Comparative Example 3
[0092] Polyethylene having a weight average molecular weight of
2.3.times.10.sup.5, and containing 11.6 weight % of molecules
having the molecular weight of lower than 10.sup.4 and 6.4% weight
% of molecules having the molecular weight of greater than
10.sup.6, and further containing 0.6 weight % of butene-1 as a
co-monomer was used for Component I. Component II was not used.
Paraffin oil of which 100.degree. C. kinetic viscosity was 11 cSt
(Component A in the following table) was used for Component III,
and the content of Component III with respect to the entire content
of Components I and III was 85 weight %. The temperatures of both
of the extruder and die were 180.degree. C.
[0093] Stretching was done at 116.degree. C. in order to adjust the
ratio of melting of the crystal portion to 60 weight % during
stretching.
Comparative Example 4
[0094] The mixture of 25 weight % of ultrahigh-molecular-weight
polyethylene having a weight average molecular weight of
8.8.times.10.sup.5 and 75 weight % high-density polyethylene having
a weight average molecular weight of 1.8.times.10.sup.5 was used
for Component I. The mixture contained 9.4 weight % of molecules
having the molecular weight of lower than 10.sup.4, 10.5 weight %
of molecules having the molecular weight of greater than 10.sup.6,
and 0.6% butene-1 as a co-monomer. Ter-polypropylene having a
weight average molecular weight of 3.1.times.10.sup.5 and a melting
point of 131.degree. C. as 3.2 weight % of ethylene and 5.0 weight
% of butene-1 were used as co-monomers was used for Component II.
The contents of Component I and Component II were 95 weight % and 5
weight %, respectively. Paraffin oil of which 100.degree. C.
kinetic viscosity was 11 cSt (Component A in the following table)
was used for Component III, and the content of Component III with
respect to the entire content of Components I, II, and III was 70
weight %. The temperatures of both of the extruder and die were
180.degree. C.
[0095] Stretching was done at 119.degree. C. in order to adjust the
ratio of melting of the crystal portion to 60 weight % during
stretching.
Comparative Example 5
[0096] Polyethylene using 0.8% of butene-1 as a co-monomer and
having a weight average molecular weight of 1.8.times.10.sup.5 and
containing 22.0 weight % of molecules having the molecular weight
of lower than 10.sup.4 and 1.5 weight % of molecules having the
molecular weight of greater than 10.sup.6 was used for Component I.
And talcum powder, which was an inorganic material, having the size
of 0.2-10 mm was used for Component II. The contents of Component I
and Component II were 95 weight % and 5 weight %, respectively.
Paraffin oil of which 100.degree. C. kinetic viscosity was 11 cSt
(Component A in the following table) was used for Component III,
and the content of Component III with respect to the entire content
of Components I, II, and III was 40 weight %. The temperatures of
both of the extruder and die were 180.degree. C.
[0097] Stretching was done at 120.degree. C. in order to adjust the
ratio of melting of the crystal portion to 30 weight % during
stretching. TABLE-US-00001 TABLE 1 Preferred Embodiments
Manufacturing Conditions Units 1 2 3 4 5 Polyethylene Mw g/mol 3.0
.times. 10.sup.5 2.5 .times. 10.sup.5 2.9 .times. 10.sup.5 3.0
.times. 10.sup.5 2.5 .times. 10.sup.5 (Component I) Mw <
10.sup.4 wt % 3.5 4.8 4.9 3.5 4.8 Mw > 10.sup.6 wt % 4.9 4.5 3.0
4.9 4.5 Co-Monomer wt % 0.0 0.0 0.1 0.0 0.0 Content in wt % 95 97
90 95 95 Components I + II Polypropylene Mw g/mol 5.7 .times.
10.sup.5 2.9 .times. 10.sup.5 2.5 .times. 10.sup.5 3.5 .times.
10.sup.5 4.2 .times. 10.sup.5 (Component II) Melting Point .degree.
C. 162 162 164 157 162 Content in wt % 5 3 10 5 5 Components I + II
Diluent Type -- A B B A B (Component III) Content in wt % 70 60 65
70 75 Components I + II + III Stretching Temperature .degree. C.
115 120 118 119 119 Melting of % 30 50 40 60 70 Crystals Extrusion
Processibility -- Sup. Sup. Sup. Sup. Sup. Puncture Strength N/mm
0.23 0.20 0.22 0.17 0.15 Air Permeability Darcy 1.9 2.5 2.1 3.0 3.7
(.times.10.sup.-5) Thermal Closing .degree. C. 136 136 136 135 134
Characteristic Temperature Melt-Down 193 183 >200 178 187 (sup.:
superior)
[0098] TABLE-US-00002 TABLE 2 Comparative Examples Manufacturing
Conditions Units 1 2 3 4 5 Polyethylene Mw g/mol 1.8 .times.
10.sup.5 3.0 .times. 10.sup.5 2.3 .times. 10.sup.5 8.8 .times.
10.sup.5 1.8 .times. 10.sup.5 (Component I) (25 wt %) 1.8 .times.
10.sup.5 (75 wt %) Mw < 10.sup.4 wt % 22.0 3.5 11.6 9.4 22.0 Mw
> 10.sup.6 wt % 1.5 4.9 6.4 10.5 1.5 Co-Monomer wt % 0.8 0.0 0.6
0.6 0.8 Content in wt % 99 85 100 95 95 Components I + II
Polypropylene Mw g/mol 2.5 .times. 10.sup.4 2.5 .times. 10.sup.5 --
3.1 .times. 10.sup.5 Talc (Component II) Melting Point .degree. C.
162 164 -- 131 Content in wt % 1 15 -- 5 5 Components I + II
Diluent (Component Type -- A A A A A III) Content in wt % 70 70 85
70 40 Components I + II + III Stretching Temp. .degree. C. 114 121
116 119 120 Melting of % 25 85 60 60 30 Crystals Extrusion
Processibility -- Sup. Sup. Inf. Inf. Inf. Puncture Strength N/mm
0.25 0.11 0.13 0.22 0.25 Air Permeability Darcy 1.3 4.0 4.2 2.0 0.8
(.times.10.sup.-5) Thermal Closing .degree. C. 135 136 137 135 135
Characteristic Temp.re Melt- 149 >200 145 140 154 Down (sup.:
superior, inf.: inferior)
[0099] As shown in the above Tables 1 and 2, high-density
microporous polyolefin films manufactured according to the present
invention have a high thermal stability and superior extrusion
compoundability and physical properties. And therefore, they are
applicable usefully to not only separation membranes for batteries
but also various separation membranes since they can enhance the
performance and stability of the batteries using them as well as
the productivity of microporous films. While certain present
preferred embodiments and comparative examples of the invention
have been shown and described, it is to be distinctly understood
that the invention is not limited thereto but may be otherwise
variously embodied and practiced within the scope of the following
claims.
* * * * *