U.S. patent application number 11/126279 was filed with the patent office on 2006-10-12 for microporous polyethylene film having excellent physical properties, productivity, and quality consistency, and method of producing same.
Invention is credited to Byoung Cheon Jo, Chol Ho Lee, Young Keun Lee, Jang Weon Rhee, Jung Moon Sung.
Application Number | 20060228540 11/126279 |
Document ID | / |
Family ID | 37073653 |
Filed Date | 2006-10-12 |
United States Patent
Application |
20060228540 |
Kind Code |
A1 |
Lee; Young Keun ; et
al. |
October 12, 2006 |
Microporous polyethylene film having excellent physical properties,
productivity, and quality consistency, and method of producing
same
Abstract
Disclosed herein is a microporous polyethylene film for a
battery separator and a method of producing the same. The
microporous polyethylene film is made from a resin composition. The
resin composition comprises 100 parts by weight of a polyethylene
composition including 10-50 wt % polyethylene having a weight
average molecular weight of from 2.times.10.sup.5 to less than
5.times.10.sup.5 (component I) and 90-50 wt % diluent (component
II), and 0-150 parts by weight of inorganic powder (component III).
The film has a puncture strength of 0.20 N/.mu.m or more and a gas
permeability (Darcy's permeability constant) of 1.times.10.sup.-5
Darcy or more. The microporous polyethylene film has excellent
physical properties, thus improving the performance and stability
of a battery.
Inventors: |
Lee; Young Keun; (Daejeon,
KR) ; Rhee; Jang Weon; (Daejeon, KR) ; Sung;
Jung Moon; (Seoul, KR) ; Jo; Byoung Cheon;
(Daejeon, KR) ; Lee; Chol Ho; (Daejeon,
KR) |
Correspondence
Address: |
CLARK & BRODY
1090 VERMONT AVENUE, NW
SUITE 250
WASHINGTON
DC
20005
US
|
Family ID: |
37073653 |
Appl. No.: |
11/126279 |
Filed: |
May 11, 2005 |
Current U.S.
Class: |
428/317.9 ;
428/304.4; 428/315.5; 428/319.7 |
Current CPC
Class: |
C08J 5/18 20130101; Y02E
60/10 20130101; H01M 50/411 20210101; Y10T 428/249992 20150401;
Y10T 428/249978 20150401; Y10T 428/249986 20150401; Y10T 428/249953
20150401; C08J 2323/06 20130101 |
Class at
Publication: |
428/317.9 ;
428/304.4; 428/319.7; 428/315.5 |
International
Class: |
B32B 3/26 20060101
B32B003/26; B32B 5/22 20060101 B32B005/22; B32B 27/00 20060101
B32B027/00 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 6, 2005 |
KR |
10-2005-0028531 |
Claims
1. A microporous polyethylene film, made from a resin composition
comprising: 100 parts by weight of a polyethylene composition
including 10-50 wt % polyethylene having a weight average molecular
weight of from 2.times.10.sup.5 to less than 5.times.10.sup.5
(component I) and 90-50 wt % diluent (component II); and 0-150
parts by weight of inorganic powder (component III), wherein, a
puncture strength is 0.20 N/.mu.m or more and a gas permeability
(Darcy's permeability constant) is 1.times.10.sup.-5 Darcy or
more.
2. The microporous polyethylene film as set forth in claim 1,
wherein the weight average molecular weight of the polyethylene is
3.times.10.sup.5-4.times.10.sup.5.
3. The microporous polyethylene film as set forth in claim 1,
wherein the diluent is aliphatic hydrocarbon, cyclic hydrocarbon,
phthalic acid ester, fatty acid having 10-20 carbons, fatty alcohol
having 10-20 carbons, or a mixture thereof.
4. The microporous polyethylene film as set forth in claim 3,
wherein the diluent is nonane, decane, decalin, paraffin oil,
paraffin wax, dibutyl phthalate, dihexyl phthalate, dioctyl
phthalate, palmitic acid, stearic acid, oleic acid, linoleic acid,
linolenic acid, palmitic alcohol, stearic alcohol, oleic alcohol,
or a mixture thereof.
5. The microporous polyethylene film as set forth in claim 4,
wherein the diluent is the paraffin oil having a kinetic viscosity
of 20-200 cSt at 40.degree. C.
6. The microporous polyethylene film as set forth in claim 1,
wherein content of polyethylene (component I) is 20-40 wt %, and
content of the diluent (component II) is 80-60 wt %.
7. The microporous polyethylene film as set forth in claim 1,
wherein the inorganic powder (component III) is selected from the
group consisting of calcium carbonate, silica, barium sulfate,
talc, and a mixture thereof.
8. The microporous polyethylene film as set forth in claim 1,
wherein content of the inorganic powder (component III) is 10-100
parts by weight based on 100 parts by weight of the polyethylene
composition including polyethylene (component I) and the diluent
(component II).
9. A method of producing a microporous polyethylene film,
comprising: (a) melt-extruding a resin composition to form a sheet,
the resin composition comprising: 100 parts by weight of a
polyethylene composition including 10-50 wt % polyethylene having a
weight average molecular weight of from 2.times.10.sup.5 to less
than 5.times.10.sup.5 (component I) and 90-50 wt % diluent
(component II); and 0-150 parts by weight of inorganic powder
(component III); (b) stretching the sheet at a temperature range,
where 30-80 wt % of a crystalline portion of polyethylene is
melted, according to a roll-type stretching manner in a machine
direction, and the machine directionally-stretched sheet at a
temperature range, where 30-80 wt % of the crystalline portion of
polyethylene is melted, according to a tenter-type stretching
manner in a transverse direction, thereby producing a film; and (c)
extracting the diluent from the film, and heat-setting the
resulting film, Wherein, stretching part of a roll stretching
machine used in the roll-type stretching has at least 3 stretching
rolls, speed of each of stretching rolls is 1.02 times.about.a
predetermined value times faster than speed of the immediately
preceding roll, and the predetermined value is gained by
multiplying a total machine direction stretching ratio by 0.9.
10. The method as set forth in claim 9, wherein the weight average
molecular weight of polyethylene is
3.times.10.sup.5-4.times.10.sup.5.
11. The method as set forth in claim 9, wherein the diluent is
aliphatic hydrocarbon, cyclic hydrocarbon, phthalic acid ester,
fatty acid having 10-20 carbons, fatty alcohol having 10-20
carbons, or a mixture thereof.
12. The method as set forth in claim 11, wherein the diluent is
nonane, decane, decalin, paraffin oil, paraffin wax, dibutyl
phthalate, dihexyl phthalate, dioctyl phthalate, palmitic acid,
stearic acid, oleic acid, linoleic acid, linolenic acid, palmitic
alcohol, stearic alcohol, oleic alcohol, or a mixture thereof.
13. The method as set forth in claim 12, wherein the diluent is the
paraffin oil having a kinetic viscosity of 20-200 cSt at 40.degree.
C.
14. The method as set forth in claim 9, wherein content of
polyethylene (component I) is 20-40 wt %, and content of the
diluent (component II) is 80-60 wt %.
15. The method as set forth in claim 9, wherein the inorganic
powder (component III) is selected from the group consisting of
calcium carbonate, silica, barium sulfate, talc, and a mixture
thereof.
16. The method as set forth in claim 9, wherein content of the
inorganic powder (component III) is 10-100 parts by weight based on
100 parts by weight of the polyethylene composition including
polyethylene (component I) and the diluent (component II).
17. The method as set forth in claim 9, wherein an extrusion
temperature is 160-250.degree. C. in the step (a).
18. The method as set forth in claim 9, wherein the sheet is
stretched in a machine direction at a stretching ratio of from 3
times to 10 times, and the sheet is stretched in a transverse
direction at a stretching ratio of from 2 times to 10 times.
19. The method as set forth in claim 18, wherein the sheet is
stretched in a machine direction at a stretching ratio of from 4
times to 8 times, and the sheet is stretched in a transverse
direction at a stretching ratio of from 3 times to 9 times.
20. The method as set forth in claim 9, wherein the number of the
stretching rolls is 4-20.
21. The method as set forth in claim 9, wherein surface roughness
of each of the stretching rolls is 0.2-10 s.
22. The method as set forth in claim 21, wherein the surface
roughness of each of the stretching rolls is 0.3-6 s.
23. The method as set forth in claim 9, wherein a pinch roll is not
used as the stretching rolls when stretching in the machine
direction.
24. The method as set forth in claim 9, wherein stretching
temperatures in the machine and transverse directions are selected
at a temperature range, where 40-70 wt % of the crystalline portion
of polyethylene is melted.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a microporous polyethylene
film and a method of producing the same. More particularly, the
present invention pertains to a microporous polyethylene film and a
method of producing the same, in which a resin composition suitable
for a roll-type stretching in a machine direction is employed, and
a two-step biaxial stretching process, where stretching is
implemented in a transverse direction using a tenter after
stretching is conducted in the machine direction under a condition
that stretching is dispersedly conducted, is conducted, thus
assuring high productivity, quality consistency, and excellent
physical properties. Thereby, the film can improve the performance
and stability of a battery using it.
[0003] 2. Description of the Related Art
[0004] Having chemical stability and superior physical properties,
a microporous polyolefin film is widely used as various battery
separators, filters, and ultrafiltration membranes.
[0005] The production of the microporous film using polyolefin may
be conducted according to the following three processes. In a first
process, polyolefin is processed into a thin fiber to produce a
nonwoven fabric-shaped microporous film. A second process is a dry
process, in which a thick polyolefin film is prepared and stretched
at low temperatures to create micro cracks between lamellas
corresponding to crystalline portions of the polyolefin to form
micro pores in the polyolefin. A third process is a wet process, in
which polyolefin is compounded with a diluent (a low molecular
weight organic substance having a molecular structure similar to
polyolefin) at high temperatures to form a single phase, phase
separation of polyolefin and diluent is initiated in a cooling
step, and the diluent is extracted to form pores in polyolefin. In
comparison with the first and second processes, the wet process,
corresponding to the third process, produces a thin film having
uniform thickness and excellent physical properties, and thus, the
film produced according to the wet process is widely used for an
isolation membrane of a secondary battery, such as a lithium ion
battery.
[0006] A method of producing a porous film according to a wet
process is disclosed in U.S. Pat. No. 4,247,498. This patent
discloses a method comprising blending polyethylene and a
compatible liquid compound at high temperatures to form a
thermodynamically homogeneous phase solution, and cooling the
solution to initiate solid/liquid or liquid/liquid phase separation
of the polyethylene and the compatible liquid compound, thereby
producing the porous polyolefin film. However, the method does not
use a stretching process.
[0007] In conjunction with the earnest use of a secondary battery,
continuous efforts have been made to improve the productivity and
physical properties of a microporous film employing a wet process.
A representative example uses ultra-high molecular weight
polyolefin (UHMWPO) with a weight average molecular weight of about
1,000,000, or mixes such a UHMWPO with a composition so as to
increase the molecular weight of the composition, or employs a
stretching process so as to improve the strength of the porous
film.
[0008] With respect to this, U.S. Pat. Nos. 5,051,183, 5,830,554,
6,245,272, and 6,566,012 disclose a method of producing a
microporous film, in which a sheet is produced using a composition
mixed with polyolefin having a weight average molecular weight of
500,000 or more and a solvent capable of dissolving polyolefin at
high temperatures, and the sheet is sequentially subjected to a
stretching process and a solvent extraction process. These patents
adopt uniaxial stretching or biaxial stretching in the stretching
process, and a typical tenter, roll, calendar method, or
combination thereof is employed. With respect to the biaxial
stretching, the patents give an extensive description, which
comprises both simultaneous biaxial stretching and two-step biaxial
stretching. However, all of the examples in the patents are limited
to simultaneous biaxial stretching, or give explanations only of
the biaxial stretching, but do not mention stretching temperatures
in machine and transverse directions. In other words, the patents
do not disclose a characteristic of the two-step biaxial stretching
in which the stretching is conducted in the transverse direction
using a tenter after the stretching is conducted in the machine
direction using a roll, a characteristic of simultaneous biaxial
stretching, or a difference between the two characteristics.
[0009] Of commercial microporous polyolefin films, goods which are
considered to have excellent physical properties and are created
through the wet process are classified into goods produced by
conducting stretching after a diluent has been extracted and other
goods produced by conducting stretching before a diluent is
extracted. In the latter, the stretching is easily achieved because
of the softness of the polyolefin, imparted by the diluent.
Furthermore, since the film is made thin by stretching, the diluent
can be easily removed from the film. However, it is extensively
known that, up to now, the production of commercial goods by
stretching before extracting the diluent has been accomplished only
using the simultaneous biaxial stretching method.
[0010] Simultaneous biaxial stretching is a stretching method in
which a sheet made from a composition of polyolefin and diluent is
fixed using chucks (clamping means) for seizing both ends of the
sheet in a form resembling thumbs and index fingers of both hands
coming into contact with lower and upper surfaces of the sheet,
respectively, and the chucks are simultaneously pulled outward in
transverse and machine directions. In this method, since there is
holding power when the chucks seize the sheet, slippage does not
occur during the stretching, and there are no defects on the
surface of a middle portion of the sheet in practice because the
stretching is conducted only while both ends of the sheet are
seized. Accordingly, the film produced using this method can be
used for optical applications.
[0011] A portion of the chuck, which seizes the sheet, has the
shape of a circle or oval having a diameter of 10-30 mm. The
distance in the machine direction between the centers of the chucks
is designed so as to be 20-60 mm. If the sheet is stretched in the
machine direction by a stretching ratio of 6 times, the distance in
the machine direction between the centers of the chucks is 120-360
mm, thus they are spaced apart from each other at an interval from
90 to 350 mm. For example, if circular chucks having a diameter of
10 mm are employed and the distance between the centers of the
chucks is designed to be 15 mm so that the stretching is conducted
at a stretching ratio of 6 times, since the distance between the
centers of the chucks is 90 mm after the stretching, a holding
index is just 10/90 (about 11%) based on the machine direction
length. Therefore, if the thin film is inspected after stretching,
the holding area seized by the chucks is observed to be
significantly reduced. However, since there are no chucks on a
portion which is not held, no holding power is applied thereto, so
it shrinks even though stretching is conducted. Accordingly, there
occurs a major disadvantage in that stretching ratios in the
transverse direction are different from each other between the
portions which are seized and not seized by the chucks. If another
stretching ratio is calculated using the same chuck diameter and
the same distance between the centers of the chucks as the above
example, since the holding index is 10/45 (about 22%) when the
stretching is conducted at the stretching ratio of 3 times, and
10/150 (about 7%) when the stretching is conducted at the
stretching ratio of 10 times, most portions of the sheet are not
held by the chucks after stretching, thus most portions of the
sheet shrink in the transverse direction (see FIG. 1).
[0012] In addition, since the width of the portion that is not
seized by the chuck is narrower than that of the portion that is
seized by the chuck, larger portions, on which marks remain due to
the chucks and which must be removed in all the stretching
processes using the chucks, are cut. This reduces yield during the
stretching process. FIG. 1 illustrates the shape of a sheet after
it is simultaneously biaxially stretched. Since both edges on which
the chuck marks remain must be cut, the effective width of the
practically produced sheet corresponds to W. Since the minimum
width (L) of the portion of the sheet that shrinks because it is
not seized by the chuck is smaller than the distance (H) between
the chucks, the effective width (W) of the sheet is reduced,
resulting in reduced yield.
[0013] Moreover, the chunk used for simultaneous stretching must be
designed to endure two-directional stresses simultaneously applied
in machine and transverse directions during the stretching, thus it
has a complicated structure and a stretching device is costly in
comparison with a chuck used to conduct unidirectional stretching.
Furthermore, it is disadvantageous in that it provides reduced
stretching speed, inconsistent quality, and poor yield due to
structural problems with the stretching device.
[0014] With respect to physical properties of goods, the most
significant disadvantage of the commercial simultaneous biaxial
stretching device is that, since the stretching ratio is fixed
during a designing stage because of the complicated structure and
cost, it is impossible to produce goods requiring the stretching
ratio to be changed. For example, when using the simultaneous
biaxial stretching device in which stretching ratios are set to 6
times in both machine and transverse directions, it is impossible
to conduct stretching so that the stretching ratio is changed to be
5 or 7 times. This means that it is difficult to produce goods
having various physical properties in order to meet various needs
of consumers.
[0015] However, when using a two-step biaxial stretching device,
since it is easy to produce goods requiring stretching ratios to be
changed in machine and transverse directions, it is possible to
produce goods having various physical properties. Additionally, if
a microporous polyolefin film is produced using the two-step
biaxial stretching device, desirably, it is possible to
significantly improve productivity, reduce an installation cost,
and reduce defective fractions when working.
[0016] Hence, in the production of the microporous polyolefin film
using the wet process, if two-step biaxial stretching is employed
as the stretching method before the diluent is extracted, the
disadvantages of the simultaneous biaxial stretching method can be
basically avoided.
[0017] In practice, in the two-step biaxial stretching method, it
is unnecessary to use the chuck when the stretching is conducted in
the machine direction, and the sheet is completely stretched using
only a roll, thus there is no problem with respect to the holding.
Furthermore, as for the chuck used to conduct the stretching in the
transverse direction using a tenter, since the sheet is stretched
in the transverse direction using a rectangular chuck, intervals
between the chucks are not changed in the machine direction after
the stretching is conducted, thus the holding index can be
maintained constant before and after the stretching is
conducted.
[0018] The length of the chuck capable of being used to conduct
stretching in the transverse direction is designed to be from 1 to
10 inches. In the chuck designed so as to have a length of 2
inches, the total length of the chuck is 40.8 mm and the interval
between the adjacent chucks is 10 mm, thus the holding index is
about 80.3% (40.8/50.8.times.100). In practice, if the holding
index is 70% or more after the stretching is conducted, there is no
problem with respect to shrinkage of the portion that is not held,
thus it is possible to maintain a predetermined stretching ratio
through the entire sheet and to minimize the chuck mark area of
both edges of the sheet, which is to be removed. Accordingly, it is
possible to increase the yield of the stretching process.
[0019] Moreover, in the roll and the chuck used in the two-step
biaxial stretching, since it is enough to design them so as to
endure only one of the stresses occurring in the machine and
transverse directions, they have a simple structure and increased
holding power in comparison with the chuck used in simultaneous
biaxial stretching, thus they are competitive in terms of
stability, mechanical operation speed, and installation cost.
[0020] With respect to the performance of goods, the most important
advantage of the commercial two-step biaxial stretching device is
that it is possible to produce goods having various physical
properties by variously changing the machine direction stretching
ratio to be a few times as high as the original stretching ratio,
which depends on the number of rolls and constitution of the motor,
and that, in the tentor for stretching in the transverse direction,
since the stretching ratio can be freely changed depending on the
width ranges of an inlet and an outlet, it is possible to produce
goods having various physical properties even though only one
device is employed.
[0021] However, even though the two-step biaxial stretching method
is extensively used to produce a thin film, up to now, it has not
been applied to the stretching process before extraction in a
microporous polyolefin film wet process field. Many reasons may be
given for this. Of them, the primary reason is that a slippery
sheet which is mixed with an excess amount of organic liquid
composition (solvent, plasticizer or the like) must be stretched in
the machine direction using a roll. That is to say, in a sheet
mixed with an excess amount of an organic liquid compound like oil,
since the liquid substance is present on the surface of the sheet,
it is difficult to precisely conduct stretching in the machine
direction using the roll and a typical stretching method because
the sheet slips from the roll. Additionally, since it inevitably
comes into contact with the roll during the stretching, the
physical properties of the microporous film are deteriorated due to
damage to the surface of the sheet caused by frictional force.
Particularly, if the sheet is stretched while being pressed using a
pinching roll so as to prevent the sheet from slipping,
undesirably, the sheet is forcibly made thin and the pore structure
of the sheet is deformed and destroyed.
[0022] U.S. Pat. No. 5,641,565 discloses a technology of producing
a porous polyolefin film, in which an organic liquid compound and
an inorganic filler are added to polyolefin to produce a sheet, the
organic liquid compound and the inorganic filler are removed from
the sheet, and the resulting sheet is stretched. U.S. Pat. No.
5,759,678 discloses that, after a plasticizer is mixed with
polyethylene to produce a sheet, the plasticizer is extracted from
the sheet, and the resulting sheet is subjected to a simultaneous
stretching process or a two-step stretching process to improve
strength. Two patents disclose an example with respect to machine
direction stretching a using a roll. It is noteworthy that, after
the liquid compound or the plasticizer is extracted from the sheet,
the resulting sheet is stretched using the roll without slipping.
However, since the hard sheet from which the compatible organic
liquid compound is removed must be stretched, the stretching ratio
of the sheet is reduced, thus breakage easily occurs, the
stretching ratio is limited, and defective fractions increase due
to small pores. Furthermore, since the sheet which is subjected to
an extraction process before the stretching is conducted is thick,
a disadvantage, such as reduced extraction efficiency, occurs, as
in the production of a microporous polyolefin film through a dry
process.
[0023] Therefore, the present inventors have conducted extensive
studies in order to avoid the problems occurring in the prior art,
resulting in the finding that it is possible to produce a
microporous polyethylene film having excellent physical properties
and productivity, and consistent quality by two-step biaxially
stretching a sheet, which contains an organic liquid compound and
which is difficult to be processed through a conventional two-step
biaxial stretching technology, in such a way that a resin
composition suitable for machine direction stretching using a roll
is employed, and conditions for the machine direction stretching
are controlled.
SUMMARY OF THE INVENTION
[0024] An object of the present invention is to avoid the problems
occurring in a simultaneous biaxial stretching technology, of a
conventional wet process and in a two-step biaxial stretching
technology after extraction of diluent, and to provide a
microporous polyethylene film for batteries, which has excellent
productivity and consistent quality, can produce goods having
various physical properties, has excellent physical properties, and
ensures the stability of a battery.
[0025] Another object of the present invention is to provide a
method of producing the microporous polyethylene film.
[0026] In order to accomplish the above objects, the present
invention provides a microporous polyethylene film made from a
resin composition. The resin composition comprises 100 parts by
weight of a polyethylene composition including 10-50% polyethylene
having a weight average molecular weight of from 2.times.10.sup.5
to less than 5.times.10.sup.5 (component D and 90-50% diluent
(component II), and 0-150 parts by weight of inorganic powder
(component III). The film has a puncture strength of 0.20 N/mm or
more and a gas permeability (Darcy's permeability constant) of
1.times.10.sup.-5 Darcy or more.
[0027] Furthermore, the present invention provides a method of
producing a microporous polyethylene film. The method comprises (a)
melt-extruding a resin composition to form a sheet, the resin
composition comprising 100 parts by weight of a polyethylene
composition that includes 10-50 wt % polyethylene having a weight
average molecular weight of from 2.times.10.sup.5 to less than
5.times.10.sup.5 (component D and 90-50 wt % diluent (component
II), and 0-150 parts by weight of inorganic powder (component III);
(b) stretching the sheet at a temperature range, where 30-80 wt %
of a crystalline portion of polyethylene is melted, according to a
roll-type stretching manner in a machine direction, and the machine
directionally-stretched sheet at a temperature range, where 30-80
wt % of the crystalline portion of polyethylene is melted,
according to a tenter-type stretching manner in a transverse
direction, thereby producing a film; and (c) extracting the diluent
from the film, and heat-setting the resulting film, wherein,
stretching part of a roll stretching machine used in the roll-type
stretching has at least 3 stretching rolls, speed of each of
stretching rolls is 1.02 times.about.a predetermined value times
faster than speed of the immediately preceding roll, and the
predetermined value is gained by multiplying a total machine
direction stretching ratio by 0.9.
BRIEF DESCRIPTION OF THE DRAWINGS
[0028] The above and other objects, features and advantages of the
present invention will be more clearly understood from the
following detailed description taken in conjunction with the
accompanying drawings, in which:
[0029] FIG. 1 illustrates a sheet which is simultaneously biaxially
stretched according to a conventional technology; and
[0030] FIG. 2 illustrates a sheet which is two-step biaxially
stretched according to the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0031] Hereinafter, a detailed description will be given of the
present invention.
[0032] The production of the microporous polyethylene film from
polyethylene is based on the following mechanism.
[0033] A diluent forms a thermodynamically single phase in
conjunction with polyethylene at high temperatures at which
polyethylene is melted. When a solution of polyethylene and the
diluent in the thermodynamically single phase state is cooled to
room temperature, phase separation of polyethylene and the diluent
starts. The single phase is divided into a polyethylene rich phase
consisting mostly of a lamella corresponding to a crystalline
portion of polyethylene, and a diluent rich phase containing the
diluent and, partly, polyethylene which is dissolved in the diluent
at room temperature. Thereafter, a sheet is stretched, the diluent
is extracted with an organic solvent, and the resulting sheet is
set by heating to produce the porous polyethylene film.
[0034] The basic structure of the microporous film depends on the
phase separation. In other words, the pore size and structure of
the final microporous film depend on the size and structure of the
diluent rich phase formed through the phase separation. However,
the final physical properties of the microporous film are
determined in the course of again heating the cooled solid having
the basic structure and subsequently stretching it. That is to say,
the sizes and shapes of the fine pores depend on the stretching
ratio, stretching temperature, and stretching speed. Consequently,
gas and liquid permeabilities as intrinsic physical properties of
the microporous film depend on stretching conditions. In addition
to them, mechanical properties, shrinkage, thickness and the like
depend significantly on the stretching conditions, thus the
stretching process can be considered the most important process in
the production of microporous polyolefin film.
[0035] Accordingly, the present inventors have studied the
disadvantages of the stretching process in a conventional process
of producing a microporous polyolefin film so as to overcome the
disadvantages.
[0036] In the present invention, a resin composition suitable for
machine direction stretching using a roll is employed and
conditions of machine direction stretching are controlled so as to
overcome difficulties in stretching a composition sheet of
polyolefin and the diluent in a conventional two-step biaxial
method before an extraction process is conducted.
[0037] Produced using a resin composition which comprises 100 parts
by weight of a polyethylene composition including 10-50 wt %
polyethylene having a weght average molecular weight of from
2.times.10.sup.5 to less than 5.times.10.sup.5 (component I) and
90-50 wt % diluent (component II), and 0-150 parts by weight of
inorganic powder (component III), the microporous polyethylene film
according to the present invention has a puncture strength of 0.20
N/mm or more and a gas permeability (Darcy's permeability constant)
of 1.times.10.sup.-5 Darcy or more.
[0038] The weight average molecular weight of polyethylene is from
2.times.10.sup.5 to less than 5.times.10.sup.5, and preferably,
3.times.10.sub.5-4.times.10.sup.5. When the weight average
molecular weight is less than 2.times.10.sup.5, it is impossible to
produce a microporous film having excellent physical properties.
The load on an extruder increases due to an increase in viscosity
during extrusion and extrusion compoundability is reduced due to a
significant difference in the viscosity of the polyethylene and the
diluent as the weight average molecular weight increases over
5.times.10.sup.5.
[0039] As long as it forms a single phase in conjunction with
polyethylene resin at an extrusion-molding temperature, any organic
liquid compound may be used as the diluent. Examples of the
diluentI include aliphatic or cyclic hydrocarbon, such as nonane,
decane, decalin, paraffin oil, and paraffin wax; phthalic acid
ester, such as dibutyl phthalate, dihexyl phthalate, and dioctyl
phthalate; fatty acid having 10-20 carbons, such as palmitic acid,
stearic acid, oleic acid, linoleic acid, and linolenic acid; fatty
alcohol having 10-20 carbons, such as palmitic alcohol, stearic
alcohol, and oleic alcohol; and a mixture thereof. Of them,
paraffin oil, which is harmless to humans, has a high boiling
point, and contains a small volume of volatile components, is
preferable, and paraffin oil having a kinetic viscosity of 20-200
cSt at 40.degree. C. is more preferable. When the kinetic viscosity
of paraffin oil is more than 200 cSt, there may occur problems,
such as increased load and an inferior surface of the sheet,
because of the high kinetic viscosity in the extruding process, and
since it is difficult to conduct the extraction process, the
productivity is reduced and the gas permeability is reduced due to
the remaining diluent. On the other hand, when the kinetic
viscosity of paraffin oil is less than 20 cSt, it is difficult to
conduct compounding paraffin oil with polyethylene melt in the
extruder during the extrusion-molding process because of a
viscosity difference between paraffin oil and polyethylene
melt.
[0040] The composition of polyethylene and the diluent includes
10-50 wt % polyethylene and 90-50 wt % diluent, and preferably,
20-40 wt % polyethylene and 80-60 wt % diluent. When the
polyethylene content is more than 50 wt %, the porosity and pore
size are reduced, and interconnection between pores is reduced,
thereby greatly reducing the gas permeability. Furthermore, since
the sheet is not soft during the machine direction stretching using
the roll, adhesion strength to the roll is reduced, thus the
stretching is conducted with difficulty, and slippage occurs. When
the content of polyethylene is less than 10 wt %, the
compoundability of the polyethylene and diluent is reduced, and
thus, the composition is extruded in a gel state, bringing about
problems, such as breakage and nonuniform thickness during the
stretching process. As well, since the sheet is very soft, it is
deformed by even weak frictional force, its surface is damaged
during the machine direction stretching using the roll, and it is
made thinner than dictated by a predetermined stretching ratio.
[0041] The inorganic powder acts as a nucleus of a pore, which
functions to initiate pore formation during stretching. The
inorganic powder may be selectively used to increase the porosity
and pore size of the microporous film so as to increase the gas
permeability, and to increase frictional force during the machine
direction stretching using the roll so as to prevent slippage.
Examples of the powder include calcium carbonate, silica, barium
sulfate, talc, and a mixture thereof. For consistent quality, it is
preferable to use calcium carbonate or silica.
[0042] The content of the inorganic powder is 0-150 parts by
weight, preferably 10-100 parts by weight, and more preferably
20-80 parts by weight, based on 100 parts by weight of the
composition of polyethylene and diluent. When the inorganic powder
content is more than 150 parts by weight, elongation of the sheet
is significantly reduced, thus causing breakage during the
stretching, resulting in processing difficulty. Furthermore, since
it is difficult to disperse the inorganic powder in the extruder,
gels are formed, thus creating pores and causing breakage during
stretching.
[0043] Additives, such as an oxidation stabilizer, a UV stabilizer,
and an antistatic agent, may be further added to the resin
composition so as to improve specific functions of the composition,
if necessary.
[0044] The resin composition, which includes polyethylene, diluent,
and, optionally, the inorganic powder, is compounded using a twin
screw compounder, a kneader, or a Banbury mixer and is
melt-extruded to be shaped into a sheet. After polyethylene, the
diluent, and the inorganic powder (optional component) are blended
with each other, they may be fed into the compounder, or may be fed
through feeders separated from each other. An extrusion temperature
is 160-250.degree. C., preferably 180-230.degree. C., and more
preferably 190-220.degree. C. When the extrusion temperature is
lower than 160.degree. C., polyethylene is insufficiently melted
during the extrusion, thus a process load is high and a dispersion
problem occurs. When the extrusion temperature is higher than
250.degree. C., the molecular weight of polyethylene is reduced and
a problem with respect to discoloration occurs due to thermal
oxidation.
[0045] A casting or calendaring process may be applied to produce
the sheet using a melt. The sheet extruded at 160-250.degree. C. is
cooled to room temperature so as to have predetermined thickness
and width.
[0046] The sheet thus produced is stretched in a stretching part of
a stretching machine which includes a preheating part, the
stretching part, and a cooling and heat setting part, and a
plurality of rolls in a machine direction at a stretching ratio of
3 times-10 times, and preferably at a stretching ratio of 4 times-8
times. When a machine direction stretching ratio is less than 3
times, orientation is poor in the machine direction, thus
mechanical properties and gas permeability are reduced. When the
stretching ratio is more than 10 times, there is a high possibility
that breakage will occur during the stretching, and shrinkage of
the final film is undesirably increased. The preheating part, and
the cooling and heat setting part each include one or more rolls,
and one or more pinch rolls (rolls which press upper or lower sides
of the sheet toward the basic rolls at a predetermined pressure and
are paired with the basic rolls so as to hold the upper or lower
sides of the sheet) may be provided to the preheating part and/or
the cooling and heat setting part. The pinch rolls must be pressed
when the sheet is sufficiently cooled to room temperature so as to
prevent the surface of the sheet from being damaged and maintain
the thickness of the sheet.
[0047] The stretching part includes at least 3 rolls, and enables
the stretching to be conducted in such a way that the final machine
direction stretching ratio is divided among the rolls. The
stretching must be uniformly conducted with respect to all of the
rolls of the stretching part so as to minimize slippage of the
sheet during stretching and to prevent damage to the surface of the
sheet and abnormal reduction of the thickness of the sheet. If the
number of rolls of the stretching part in which the stretching is
conducted is less than 3, it is impossible to conduct the
stretching so that the stretching ratio is divided, thus the sheet
slips from the stretching roll during the stretching. Thus, it is
impossible to conduct the stretching at a desired ratio. It is
required to provide the stretching rolls as much as possible within
the stretching part where the stretching is conducted within
allowable ranges of space and cost. It is desirable to use
preferably 4-20 rolls, and more preferably 5-20 rolls.
[0048] Speed of each of stretching rolls in the stretching part is
1.02 times-a predetermined value times faster than speed of the
immediately preceding roll, so as to disperse the stretching. The
predetermined value is obtained by multiplying the total machine
direction stretching ratio by 0.9. For example, if the speed of the
first stretching roll is 1.2 m/min, and the total machine direction
stretching ratio is 6 times, the speed of the second stretching
roll must be 1.224 m/min (1.2 m/min.times.1.02)-6.48 m/min (1.2
m/min.times.5.4), which means 1.02 times-5.4 (6.times.0.9) times
faster than the speed of the first stretching roll. Within the
range, the sheet slips at the corresponding roll do not occur, thus
the stretching is uniformly achieved in the machine direction.
Furthermore, significant reduction of width of the sheet is
prevented during the stretching, thus it is possible to produce
goods having a desired width. The speed of the first stretching
roll of the stretching part is increased based on the speed of the
last roll of the preheating part.
[0049] The surface roughness of the roll of the stretching part is
0.2-10 s, preferably 0.3-6 s, and more preferably 0.4-4 s. If using
a roll having a smooth surface which has roughness less than 0.2 s,
the sheet slips from the roll during stretching, and, if using a
roll having a rough surface which has roughness more than 10 s, the
surface structure of the sheet is damaged during stretching.
[0050] The pinch roll must not be used as the rolls in the
stretching part. If the sheet is stretched in the machine direction
while slippage of the sheet is prevented using the pinch roll, the
pinch roll presses the sheet at a predetermined pressure, thus the
sheet is abnormally made thin, and surface and internal pore
structures are deformed, resulting in deteriorated physical
properties of a microporous film. Accordingly, use of the pinch
roll must be avoided in the stretching part when stretching in the
machine direction using the roll.
[0051] After being uniaxially stretched in the machine direction
according to a roll-type stretching manner, the sheet is stretched
in a transverse direction.
[0052] The stretching in the transverse direction is conducted in a
tenter-type stretching manner so that the stretching ratio is 2
times-10 times, preferably 3 times-9 times, and more preferably 4
times-8 times. If the transverse direction stretching ratio is less
than 2 times, orientation is poor in the transverse direction, thus
reducing mechanical properties and gas permeability. If the
stretching ratio is more than 10 times, there is a high possibility
that breakage will occur during stretching, thus undesirably
increasing shrinkage of the final film.
[0053] Stretching temperatures in the machine and traverse
directions depend on shaping conditions in the preceding process,
such as a melting temperature of polyethylene, a concentration and
type of diluent, and cooling conditions of the sheet. The
stretching temperatures in the machine and transverse directions
are selected from a temperature range where 30-80 wt %, and
preferably 40-70 wt %, of the crystalline portion of polyethylene
in the sheet is melted before stretching. When the stretching
temperatures are lower than a temperature where 30 wt % of the
crystalline portion of polyethylene in the sheet is melted,
softness of the sheet is poor, and thus, there is a fair
possibility of breakage during stretching, and unstretching occurs
simultaneously. Furthermore, the sheet is not stretched during the
stretching process in the machine direction using the roll, and
slips from the roll. When the stretching temperatures are higher
than a temperature where 80 wt % of the crystalline portion of
polyethylene is melted, the stretching is easily conducted and the
occurrence of unstretching is reduced, but thickness variation
occurs due to partial over-stretching and the physical properties
of the sheet are significantly reduced because an orientation
effect of the resin is insignificant. Additionally, the sheet is
easily deformed during the stretching process in the machine
direction using the roll, resulting in damage to the surface
thereof. The melting of the crystalline portion according to the
temperature may be evaluated by a differential scanning calorimeter
(DSC) analysis for the film.
[0054] Unlike a simultaneous biaxial stretching process, chucks
positioned at narrow intervals hold both edges of the film right
before releasing the film after stretching, thus there are no
portions of the film that shrink and thus make the width of the
film nonuniform if a two-step biaxial stretching process, where the
machine direction stretching is conducted using the roll under the
above-mentioned conditions and then the transverse direction is
conducted using the tenter, is conducted. Accordingly, the
transverse direction stretching ratio is uniform, and it is
possible to minimize an area that corresponds to both edges of the
film, has chuck marks remaining thereon, and is to be removed,
because the width of the film is uniform after the stretching. As
well, it is impossible to change the stretching ratio in a
commercial simultaneous biaxial stretching machine, but it is
possible to change the stretching ratio in a two-step biaxial
stretching machine, thus it is possible to produce goods having
various physical properties using only one machine.
[0055] The stretched film is extracted with an organic solvent and
dried. Non-limiting, illustrative examples of the available organic
solvent of the present invention include any solvent capable of
extracting the diluent used to extrude polyethylene resin, and
preferably, methyl ethyl ketone, methylene chloride, and hexane,
which have a high extraction efficiency and dry rapidly.
[0056] The extraction may be conducted according to a typical
solvent extraction process, in detail, any one process or a
combination of immersion, solvent spray, and ultrasonic processes.
It is preferable that the amount of remaining diluent be 1 wt % or
less during the extraction.
[0057] Finally, the dried film is subjected to a heat setting
process so as to remove remaining stress to reduce the shrinkage of
the final film. The heat setting process is conducted in such a way
that the film is heated while being clamped to prevent shrinkage of
the film, thereby removing the remaining stress. It is desirable
that a heat-setting temperature is high in order to reduce
shrinkage, but when the heat-setting temperature is very high, a
portion of the film is melted and blocks micropores, thereby
reducing the gas permeability. The heat-setting temperature is
selected from a temperature range where 10-30 wt %, and preferably
15-25 wt %, of the crystalline portion of polyethylene in the film
is melted. It is preferable that a heat-setting time be 1-20
min.
[0058] The microporous polyethylene film produced according to the
present invention has the following physical properties.
[0059] (1) The puncture strength is 0.20 N/.mu.m or more.
[0060] When the microporous film is applied to the battery
separator, if the microporous film has insufficient puncture
strength, defined as the strength of the film to a sharp object,
the film may be torn due to an abnormal surface state of electrodes
or dendrites formed on surfaces of the electrodes during use of the
battery, and thus, a short may occur. When a break point is 320 g
or less, a commercial battery separator is problematic in that
safety is reduced due to occurrence of the short. If the film
having the puncture strength of 0.20 N/.mu.m or more according to
the present invention is used as the thinnest film having is 16
.mu.m among films for the commercial separator, a break point is
higher than 320 g, thus assuring safety in use.
[0061] (2) The gas permeability (Darcy's permeability constant) is
1.times.10.sup.-5 Darcy or more.
[0062] When the gas permeability is less than 1.times.10.sup.-5
Darcy, efficiency of the microporous film is significantly reduced.
Particularly, when the gas permeability is less than
1.times.10.sup.-5 Darcy, in the case in which the microporous film
is applied to the battery separator, charging and discharging
characteristics of the battery are poor and the life of the battery
is reduced. The film having the gas permeability of
1.times.10.sup.-5 Darcy or more according to the present invention
gives the battery excellent charging and discharging
characteristics and low temperature characteristics, and serves to
improve the life of the battery.
[0063] In addition to the above physical properties, the
microporous polyethylene film of the present invention has
excellent extrusion-compoudability, and provides excellent battery
stability.
[0064] Having generally described this invention, further
understanding can be obtained by reference to certain specific
examples and comparative examples which are provided herein for the
purpose of illustration only and are not intended to be limiting
unless otherwise specified.
EXAMPLE
[0065] The weight average molecular weight of the polyethylene was
measured using a high temperature gel permeation chromatograph
(GPC) manufactured by Polymer Lab. Inc.
[0066] The viscosity of the diluent was measured using CAV-4
automatic viscometer manufactured by Cannon Co.
[0067] A composition of polyethylene, a diluent, and inorganic
powder (optional component) was shaped into a sheet using a twin
screw extruder in which .PHI. was 30 mm and L/D was 40:1. The
composition was fed through a hopper after components of the
composition were previously blended to form a slurry, an extrusion
temperature was 210.degree. C., and screws revolved at 200 rpm.
[0068] In order to evaluate the extrusion-compoundability, the
extrudate extruded using a T-shaped die was shaped into a sheet
having a thickness of 100 .mu.m using a casting roll, and the
number of gels in an area of 100 cm.sup.2 was counted. The number
of gels per 100 cm.sup.2 was counted when the extrusion amount per
time, corresponding to the standard capacity of the extruder, was
10 kg/hr, and the results are described in Tables 1 to 5. The
number of gels had to be 50 or less per 100 cm.sup.2 to prevent the
quality of microporous film from being reduced.
[0069] Meanwhile, the resulting composition was extruded using the
above T-shaped die at an extrusion rate of 10 kg/hr into a sheet
having a thickness of about 900 .mu.m, to be stretched.
[0070] The shaped sheet was analyzed using a DSC to evaluate the
melting of a crystalline portion thereof according to temperature.
Analysis conditions included a sample weight of 5 mg and a scanning
rate of 10.degree. C./min.
[0071] The sheet was stretched using a two-step stretching pilot
device and a continuous process, similar to commercially available
devices. The sheet shaped through the extrusion was stretched in a
machine direction using a machine direction stretching machine
which has a plurality of rolls, and was then fed into a tenter-type
transverse direction stretching machine to conduct the transverse
direction stretching. The machine direction stretching and the
transverse direction stretching were implemented so that stretching
conditions, such as a stretching ratio, a stretching temperature,
the number of stretching rolls, a stretching speed, use of a pinch
roll during the stretching, and surface roughness of the stretching
roll, could be changed. The stretching temperature was set within a
temperature range where 30-80 wt % of a crystalline portion of
polyethylene in the sheet was melted based on the analysis results
of the DSC.
[0072] The machine direction stretching machine comprised three
preheating rolls, and three cooling and heat setting rolls before
and after the stretching roll, respectively, and pinch rolls were
used in the first preheating roll and the last heat setting
roll.
[0073] The speed of the machine direction stretching was changed
while the speed of the preheating roll located right before the
first stretching roll was set at 1 m/min.
[0074] Thicknesses of the sheet before/after machine direction
stretching were measured using a typical thickness measuring
device. Before the machine direction stretching was conducted, the
thickness of the sheet was set at 900 .mu.m as described above and;
after the machine direction stretching was conducted, the thickness
of the sheet coincided with a value that was calculated by dividing
the thickness before the stretching by the stretching ratio when
the stretching was desirably achieved.
[0075] Slippage during the machine direction stretching was
evaluated in such a way that, after a straight line was drawn using
a pen on the sheet to be supplied into the stretching roll and on
the surface of the roll coming into contact therewith, it was
observed whether the straight line was broken depending on the
difference between the speeds of the roll and the sheet when the
sheet and the roll were moved and rotated, respectively.
[0076] To evaluate width uniformity of the sheet after the machine
direction stretching, whether both edges of the sheet were parallel
to each other was observed after the stretching. If the machine
direction stretching is nonuniformly conducted, the width of the
sheet is nonuniform, thus the edges of the sheet are meandrous.
Although the sheet having nonuniform width is supplied into a
transverse direction stretching tenter, chucks cannot desirably
hold the edges of the sheet to stretch the sheet, thus it is
impossible to conduct stretching in the transverse direction.
[0077] In order to compare stretching ratio uniformities in
two-step biaxial stretching and in simultaneous biaxial stretching
to each other and to compare defective fractions after the
stretching processes, distances between the chucks (H in FIGS. 1
and 2) before and after the stretching processes were measured
using a typical ruler to calculate the stretching ratios, and the
minimum width (L in FIG. 1) of the sheet after the stretching,
which was relevant only to the simultaneous biaxial stretching
process, was measured. After the stretching processes, 30 mm
portions of both edges of the sheet, on which chuck marks remained
and which were unstretched, were cut. After the stretching
processes, the defective fractions were calculated using the
maximum width between the chucks and the effective width after
cutting edge portions of the sheet (a difference between H and W
was expressed as a percentage of H in FIGS. 1 and 2).
[0078] Extraction of the diluent was carried out employing
methylene chloride for 6 min through an immersion method using a
continuous process, and afterwards, the film, from which the
diluent was extracted, was dried with air. The dried film was set
in a frame and then left in a convection oven at 12.degree. C. (the
temperature where 20 wt % of the crystalline portion of
polyethylene was melted) for 90 sec, thereby completing a
heat-setting process.
[0079] Puncture strength, gas permeability, and the like, which
were considered the most important physical properties of the
microporous film, were measured, and the results are described in
the following Tables 1 to 5.
[0080] Measurement of the Physical Properties
[0081] (1) The puncture strength was measured by measuring the
strength of the film when the film was punctured by a pin having a
diameter of 0.5 mm moving at a speed of 120 mm/min.
[0082] (2) Tensile strength was measured according to ASTM
D882.
[0083] (3) Shrinkage was gained by measuring average shrinkage in
both machine and transverse directions after the film was left at
105.degree. C. for 10 min, and expressed as a percentage.
[0084] (4) The gas permeability was measured using a porometer
(CFP-1500-AEL manufactured by PMI Co. Ltd.). Conventionally, the
gas permeability was expressed by a Gurley number, but since the
effect of thickness of the film was not reflected by the Gurley
number, it was difficult to obtain a relative permeability of a
pore structure of the film. To avoid the above disadvantage, in the
present invention, a Darcy's permeability constant was used. The
Darcy's permeability constant was calculated by the following
Equation 1, and nitrogen was used as gas in the present invention.
C=(8 FTV)/(.pi.D.sup.2(P.sup.2-1))
[0085] where, C is the Darcy's permeability constant, F is a flow
rate, T is a sample thickness, V is a viscosity of the gas (0.185
for N.sub.2), D is a sample diameter, and P is pressure.
[0086] An average value of Darcy's permeability constants in a
range of 100-200 psi was used in the present invention.
Example 1
[0087] High density polyethylene having a weight average molecular
weight of 2.2.times.10.sup.5 was used as component I. A paraffin
oil having a kinetic viscosity of 110 at 40.degree. C. was used as
component II. Contents of components I and II were 30 wt % and 70
wt %, respectively.
[0088] Stretching was conducted in machine and transverse
directions at 118 and 119.degree. C. which corresponded to
temperatures at which 50 wt % of a crystalline portion of
polyethylene was melted so that machine and transverse direction
stretching ratios were 6.1 times and 6 times, respectively. The
number of stretching rolls which were used to conduct the
stretching in the machine direction was 5. Stretching speed ratios
(speed ratio between the adjacent rolls in the front and the rear)
of the stretching rolls were set at 1.2-1.5-1.5-1.5-1.5 times. In
this case, the speeds of the rolls were increased to be
approximately 1.2-1.8-2.74.1-6.1 m/min. Surfaces of the stretching
rolls were all coated with chromium so as to have roughness of 0.6
s.
Example 2
[0089] High density polyethylene having a weight average molecular
weight of 4.7.times.10.sup.5 was used as component I. The remaining
conditions were the same as those of example 1.
Example 3
[0090] High density polyethylene having a weight average molecular
weight of 3.5.times.10.sup.5 was used as component I. Contents of
components I and II were 10 wt % and 90 wt %, respectively. The
remaining conditions were the same as those of example 1.
Example 4
[0091] High density polyethylene having a weight average molecular
weight of 3.5.times.10.sup.5 was used as component I. Contents of
components I and II were 50 wt % and 50 wt %, respectively. The
remaining conditions were the same as those of example 1.
Example 5
[0092] High density polyethylene having a weight average molecular
weight of 3.5.times.10.sup.5 was used as component I, and a
paraffin oil having a kinetic viscosity of 20 at 40.degree. C. was
used as component II. 50 parts by weight of calcium carbonate
powder (components III) having an average particle size of 1 .mu.m
was added based on 100 parts by weight of a polyethylene
composition of component I and component II. The remaining
conditions were the same as those of example 1.
Example 6
[0093] High density polyethylene having a weight average molecular
weight of 3.5.times.10.sup.5 was used as component I, and paraffin
oil having a kinetic viscosity of 200 at 40.degree. C. was used as
component II. The remaining conditions were the same as those of
example 1.
Example 7
[0094] High density polyethylene having a weight average molecular
weight of 3.5.times.10.sup.5 was used as component I. Stretching
was conducted in machine and transverse directions at 115 and
117.degree. C. which corresponded to temperatures at which 30 wt %
of a crystalline portion of polyethylene was melted so that machine
and transverse direction stretching ratios were 6.1 times and 6
times, respectively. The remaining conditions were the same as
those of example 1.
Example 8
[0095] High density polyethylene having a weight average molecular
weight of 3.5.times.10.sup.5 was used as component I. Stretching
was conducted in machine and transverse directions at 123.degree.
C. which corresponded to a temperature at which 80 wt % of a
crystalline portion of polyethylene was melted so that machine and
transverse direction stretching ratios were 6.1 times and 6 times,
respectively. The remaining conditions were the same as those of
example 1.
Example 9
[0096] High density polyethylene having a weight average molecular
weight of 3.5.times.10.sup.5 was used as component I. The number of
stretching rolls which were used to conduct the stretching in the
machine direction was set at 3, and stretching speed ratios of the
stretching rolls were set at 1.2-2-2.5 times, thereby dispersing
the total stretching ratio among the stretching rolls to conduct
the stretching in a stepwise manner. The remaining conditions were
the same as those of example 1.
Example 10
[0097] High density polyethylene having a weight average molecular
weight of 3.5.times.10.sup.5 was used as component I. The number of
stretching rolls which were used to conduct the stretching in the
machine direction was set at 10, and stretching speed ratios of the
stretching rolls were set at
1.02-1.02-1.3-1.3-1.3-1.3-1.3-1.3-1.1-1.1 times, thereby dispersing
the stretching. The remaining conditions were the same as those of
example 1.
Example 11
[0098] High density polyethylene having a weight average molecular
weight of 3.5.times.10.sup.5 was used as component I. Stepwise
stretching was conducted in a machine direction using stretching
rolls having a surface treated so as to have roughness of 0.2 s.
The remaining conditions were the same as those of example 1.
Example 12
[0099] High density polyethylene having a weight average molecular
weight of 3.5.times.10.sup.5 was used as component I. Stepwise
stretching was conducted in a machine direction using stretching
rolls having a surface treated so as to have roughness of 10 s. The
remaining conditions were the same as those of example 1.
Example 13
[0100] High density polyethylene having a weight average molecular
weight of 3.5.times.10.sup.5 was used as component I. The remaining
conditions were the same as those of example 1.
Example 14
[0101] High density polyethylene having a weight average molecular
weight of 3.5.times.10.sup.5 was used as component I. Machine and
transverse direction stretching ratios were both set at 7 times.
The remaining conditions were the same as those of example 1.
Example 15
[0102] High density polyethylene having a weight average molecular
weight of 3.5.times.10.sup.5 was used as component I. Machine and
transverse direction stretching ratios were set at 8 and 4 times,
respectively. The remaining conditions were the same as those of
example 1.
Example 16
[0103] High density polyethylene having a weight average molecular
weight of 3.5.times.10.sup.5 was used as component I. Stretching
was conducted in a machine direction at 115.degree. C., which
corresponds to a temperature at which 30 wt % of a crystalline
portion of polyethylene is melted, and stretching was conducted in
a transverse direction at 125.degree. C., which corresponds to a
temperature at which 80 wt % of a crystalline portion of
polyethylene is melted. The remaining conditions were the same as
those of example 1.
Example 17
[0104] High density polyethylene having a weight average molecular
weight of 3.5.times.10.sup.5 was used as component I. Stretching
was conducted in a machine direction at 123.degree. C., which
corresponds to a temperature at which 80 wt % of a crystalline
portion of polyethylene is melted, and stretching was conducted in
a transverse direction at 116.degree. C., which corresponds to a
temperature at which 30 wt % of a crystalline portion of
polyethylene is melted. The remaining conditions were the same as
those of example 1.
Comparative Example 1
[0105] A composition which was required to form a sheet was the
same as that in example 13, and stretching was conducted in a
simultaneous biaxial stretching pilot plant. In consideration of
commercial availability, a continuous process was employed.
Stretching was conducted at 118.degree. C., which corresponds to a
temperature at which 50 wt % of a crystalline portion of
polyethylene is melted, so that a stretching ratio was 36 times
(machine direction.times.transverse direction=6.times.6). After the
stretching, the remaining processes were conducted through a
procedure that was the same as the examples.
Comparative Example 2
[0106] High density polyethylene having a weight average molecular
weight of 1.9.times.10.sup.5 was used as component I. The remaining
conditions were the same as those of example 1.
Comparative Example 3
[0107] High density polyethylene having a weight average molecular
weight of 5.1.times.10.sup.5 was used as component I. The remaining
conditions were the same as those of example 1.
Comparative Example 4
[0108] High density polyethylene having a weight average molecular
weight of 3.5.times.10.sup.5 was used as component I. Contents of
the components I and II were 7 wt % and 93 wt %, respectively. The
remaining conditions were the same as those of example 1.
Comparative Example 5
[0109] High density polyethylene having a weight average molecular
weight of 3.5.times.10.sup.5 was used as component I. Contents of
the components I and II were 55 wt % and 45 wt %, respectively. 50
parts by weight of calcium carbonate powder having an average
particle size of 1 .mu.m was used as component III based on 100
parts by weight of a polyethylene composition of component I and
the component II. The remaining conditions were the same as those
of example 1.
Comparative Example 6
[0110] High density polyethylene having a weight average molecular
weight of 3.5.times.10.sup.5 was used as component I. A paraffin
oil having a kinetic viscosity of 10 at 40.degree. C. was used as
component II. The remaining conditions were the same as those of
example 1.
Comparative Example 7
[0111] High density polyethylene having a weight average molecular
weight of 3.5.times.10.sup.5 was used as component I, and a
paraffin oil having a kinetic viscosity of 250 at 40.degree. C. was
used as component II. The remaining conditions were the same as
those of example 1.
Comparative Example 8
[0112] High density polyethylene having a weight average molecular
weight of 3.5.times.10.sup.5 was used as component I. Stretching
was conducted in a machine direction at 112.degree. C., which
corresponds to a temperature at which 10 wt % of a crystalline
portion of polyethylene is melted so that a machine direction
stretching ratio was 6.1 times. The remaining conditions were the
same as those of example 1.
Comparative Example 9
[0113] High density polyethylene having a weight average molecular
weight of 3.5.times.10.sup.5 was used as component I. Stretching
was conducted in machine and transverse directions at 126 and
127.degree. C., which corresponds to temperatures at which 90 wt %
of a crystalline portion of polyethylene is melted. The remaining
conditions were the same as those of example 1.
Comparative Example 10
[0114] High density polyethylene having a weight average molecular
weight of 3.5.times.10.sup.5 was used as component I. The number of
stretching rolls which were used to conduct stretching in the
machine direction was set at 2, and stretching speed ratios of the
stretching rolls were set at 1.54 times, thereby dispersing the
total stretching ratio among the stretching rolls to conduct the
stretching in a stepwise manner. The remaining conditions were the
same as those of example 1.
Comparative Example 11
[0115] High density polyethylene having a weight average molecular
weight of 3.5.times.10.sup.5 was used as component I. The number of
stretching rolls which were used to conduct the stretching in the
machine direction was set at 5, and stretching speed ratios of the
stretching rolls were set at 1.01-5.46-1.03-1.03-1.03 times,
thereby dispersing the total stretching ratio among the stretching
rolls to conduct the stretching in a stepwise manner. The machine
direction stretching ratio was set at 6 times. The remaining
conditions were the same as those of example 1.
Comparative Example 12
[0116] High density polyethylene having a weight average molecular
weight of 3.5.times.10.sup.5 was used as component I. The number of
stretching rolls which were used to conduct the stretching in the
machine direction was set at 5, and stretching speed ratios of the
stretching rolls were set at 1.2-1.5-1.5-1.5-1.5 times, thereby
dispersing the total stretching ratio among the stretching rolls to
conduct the stretching in a stepwise manner. A pinch roll was used
in the first stretching roll operated at the stretching ratio of
1.2 times so as to press a sheet to prevent the sheet from slipping
therefrom. The remaining conditions were the same as those of
example 1.
Comparative Example 13
[0117] High density polyethylene having a weight average molecular
weight of 3.5.times.10.sup.5 was used as component I. Stretching
was conducted stepwise in a machine direction using stretching
rolls having a surface treated so as to have roughness of 0.1 s.
The remaining conditions were the same as those of example 1.
Comparative Example 14
[0118] High density polyethylene having a weight average molecular
weight of 3.5.times.10.sup.5 was used as component I. Stretching
was conducted stepwise in a machine direction using stretching
rolls having a surface treated so as to have roughness of 12 s. The
remaining conditions were the same as those of example 1.
TABLE-US-00001 TABLE 1 Item Unit Ex. 1 Ex. 2 Ex. 3 Ex. 4 Ex. 5 Ex.
6 High density Mw g/mol 2.2 .times. 10.sup.5 4.7 .times. 10.sup.5
3.5 .times. 10.sup.5 3.5 .times. 10.sup.5 3.5 .times. 10.sup.5 3.5
.times. 10.sup.5 polyethylene Content wt % 30 30 10 50 30 30
(component I) Paraffin oil 40.degree. C. cSt 110 110 110 110 20 200
(component II) viscosity Content wt % 70 70 90 50 70 70 Calcium
Content pbw* -- -- -- -- 50 -- carbonate (component III) Number of
gels(10 kg/hr) #/100 9 14 10 12 14 9 cm.sup.2 Surface of sheet(10
kg/hr) -- Good Good Good Good Good Good Machine Temperature
.degree. C. 118 118 115 121 117 118 direction Melting of % 50 50 50
50 50 50 stretching crystalline conditions portion No. of No. 5 5 5
5 5 5 stretching rolls Pinch roll No. -- -- -- -- -- -- used
Roughness of S 0.6 0.6 0.6 0.6 0.6 0.6 roll Stretching Times 6.1
6.1 6.1 6.1 6.1 6.1 ratio Stretching Times 1.2-1.5- 1.2-1.5-
1.2-1.5- 1.2-1.5- 1.2-1.5- 1.2-1.5- speed ratio of 1.5-1.5-
1.5-1.5- 1.5-1.5- 1.5-1.5- 1.5-1.5- 1.5-1.5- each 1.5 1.5 1.5 1.5
1.5 1.5 stretching roll Thickness before/after .mu.m 900/150
900/147 900/152 900/147 900/149 900/147 machine direction
stretching Slippage during machine -- None None None None None None
direction stretching Width uniformity after -- Good Good Good Good
Good Good machine direction stretching Transverse Temperature
.degree. C. 119 119 116 122 118 119 direction Melting of % 50 50 50
50 50 50 stretching crystalline conditions portion Stretching Times
6 6 6 6 6 6 ratio Puncture strength N/.mu.m 0.21 0.28 0.21 0.28
0.22 0.25 Gas permeability 10.sup.-5 1.8 1.7 2.1 1.6 3 1.7 Darcy
*parts by weight based on 100 parts by weight of polyethylene
composition of the components I and II
[0119] TABLE-US-00002 TABLE 2 Item Unit Ex. 7 Ex. 8 Ex. 9 Ex. 10
Ex. 11 Ex. 12 High density Mw g/mol 3.5 .times. 10.sup.5 3.5
.times. 10.sup.5 3.5 .times. 10.sup.5 3.5 .times. 10.sup.5 3.5
.times. 10.sup.5 3.5 .times. 10.sup.5 polyethylene Content wt % 30
30 30 30 30 30 (component I) Paraffin oil 40.degree. C. cSt 110 110
110 110 110 110 (component II) viscosity Content wt % 70 70 70 70
70 70 Number of gels(10 kg/hr) #/100 11 10 12 11 10 10 cm.sup.2
Surface of sheet(10 kg/hr) -- Good Good Good Good Good Good Machine
Temperature .degree. C. 115 123 118 118 118 118 direction Melting
of crystalline % 30 80 50 50 50 50 stretching portion conditions
No. of stretching rolls No. 5 5 3 10 5 5 Pinch roll used No. -- --
-- -- -- -- Roughness of roll S 0.6 0.6 0.6 0.6 0.2 10 Stretching
ratio Times 6.1 6.1 6 6.1 6.1 6.1 Stretching speed ratio Times
1.2-1.5- 1.2-1.5- 1.2-2- 1.02 * 2- 1.2-1.5- 1.2-1.5- of each
stretching roll 1.5-1.5- 1.5-1.5- 2.5 1.3 * 6- 1.5-1.5- 1.5-1.5-
1.5 1.5 1.1 * 2 1.5 1.5 Thickness before/after machine .mu.m
900/150 900/150 900/150 900/150 900/147 900/152 direction
stretching Slippage during machine direction -- None None None None
None None stretching Width uniformity after machine -- Good Good
Good Good Good Good direction stretching Transverse Temperature
.degree. C. 117 123 119 119 119 119 direction Melting of
crystalline % 30 80 50 50 50 50 stretching portion conditions
Stretching ratio Times 6 6 6 6 6 6 Puncture strength N/.mu.m 0.26
0.21 0.24 0.25 0.25 0.23 Gas permeability 10.sup.-5 1.4 2.1 1.7 1.8
1.6 1.9 Darcy
[0120] TABLE-US-00003 TABLE 3 Item Unit Ex. 13 Ex. 14 Ex. 15 Ex. 16
Ex. 17 Co. Ex. 1 High density Mw g/mol 3.5 .times. 10.sup.5 3.5
.times. 10.sup.5 3.5 .times. 10.sup.5 3.5 .times. 10.sup.5 3.5
.times. 10.sup.5 3.5 .times. 10.sup.5 polyethylene Content wt % 30
30 30 30 30 30 (component I) Paraffin oil 40.degree. C. cSt 110 110
110 110 110 110 (component II) viscosity Content wt % 70 70 70 70
70 70 *Total Temperature .degree. C. 118/119 118/119 118/119
115/125 123/116 118 stretching Melting of % 50/50 50/50 50/50 30/80
80/30 50 conditions crystalline portion Stretching ratio Times
6.1/6 7/7 8/4 6.1/6 6.1/6 6 Machine direction length of mm 40 40 40
40 40 25 chuck Machine direction length mm 50 50 50 50 50 150
between centers of chucks after stretching Machine direction
holding % 80 80 80 80 80 17 index of chuck Sheet width between
chucks mm 220 220 220 220 220 220 before stretching Maximum width
between mm 1320 1540 880 1320 1320 1320 chucks after stretching
Actual maximum stretching Times 6 7 4 6 6 6 ratio in transverse
direction Minimum width after mm 1320 1540 880 1320 1320 1180
stretching Actual minimum stretching Times 6 7 4 6 6 5.1 ratio in
transverse direction Remaining width gained by mm 1260 1480 820
1260 1260 1120 cutting both edges after stretching Defective
fractions after % 4.5 3.9 6.8 4.5 4.5 15.2 stretching Puncture
strength N/.mu.m 0.25 0.31 0.24 0.25 0.24 0.23 Tensile MD
Kg/cm.sup.2 1400 1630 1810 1500 1320 1380 strength TD Kg/cm.sup.2
1350 1530 850 1150 1390 1150 Shrinkage % 3.5 4.5 2.5 3.1 3.7 3 Gas
permeability 10.sup.-5 1.7 1.8 1.9 2.2 2.5 1.8 Darcy *Total
stretching conditions: conditions with respect to machine
direction/transverse direction are described together in Table
3
[0121] TABLE-US-00004 TABLE 4 Item Unit Co. Ex. 2 Co. Ex. 3 Co. Ex.
4 Co. Ex. 5 Co. Ex. 6 Co. Ex. 7 High density Mw g/mol 1.9 .times.
10.sup.5 5.1 .times. 10.sup.5 3.5 .times. 10.sup.5 3.5 .times.
10.sup.5 3.5 .times. 10.sup.5 3.5 .times. 10.sup.5 polyethylene
Content wt % 30 30 7 55 30 30 (component I) Paraffin oil 40.degree.
C. cSt 110 110 110 110 10 250 (component II) viscosity Content wt %
70 70 93 45 70 70 Calcium Content Parts by -- -- -- 50 -- --
carbonate weight (component III) Number of gels #/100 4 55 3 60 80
90 (10 kg/hr) cm.sup.2 Surface of sheet -- Good Poor Good Poor Poor
Poor (10 kg/hr) Machine Temperature .degree. C. 118 118 114 121 117
118 direction Melting of % 50 50 50 50 50 50 stretching crystalline
conditions portion No. of No. 5 5 5 5 5 5 stretching rolls Pinch
roll No. -- -- -- -- -- -- used Roughness of S 0.6 0.6 0.6 0.6 0.6
0.6 roll Stretching Times 6.1 6.1 6.1 6.1 6.1 6.1 ratio Stretching
Times 1.2-1.5- 1.2-1.5- 1.2-1.5- 1.2-1.5- 1.2-1.5- 1.2-1.5- speed
ratio 1.5-1.5- 1.5-1.5- 1.5-1.5- 1.5-1.5- 1.5-1.5- 1.5-1.5- for
each 1.5 1.5 1.5 1.5 1.5 1.5 stretching roll Thickness before/after
.mu.m 900/140 900/155 900/135 Non- 900/149 900/147 machine
direction uniform stretching Slippage during machine -- None None
None Slip None None direction stretching Width uniformity after --
Good Good Good Poor Poor Poor machine direction stretching
Transverse Temperature .degree. C. 119 119 115 Transverse 118 119
direction direction stretching stretching conditions Melting of %
50 50 50 Impossible 50 50 crystalline portion Stretching Times 6 6
6 -- 6 6 ratio Puncture strength N/.mu.m 0.18 0.29 0.19 -- 0.22
0.25 Gas permeability 10.sup.-5 1.8 1.7 0.8 -- 1.9 1.7 Darcy
[0122] TABLE-US-00005 TABLE 5 Item Unit Co. Ex. 8 Co. Ex. 9 Co. Ex.
10 Co. Ex. 11 Co. Ex. 12 Co. Ex. 13 Co. Ex. 14 High density Mw
g/mol 3.5 .times. 10.sup.5 3.5 .times. 10.sup.5 3.5 .times.
10.sup.5 3.5 .times. 10.sup.5 3.5 .times. 10.sup.5 3.5 .times.
10.sup.5 3.5 .times. 10.sup.5 polyethylene Content wt % 30 30 30 30
30 30 30 (component I) Paraffin oil 40.degree. C. cSt 110 110 110
110 110 110 110 (component II) viscosity Content wt % 70 70 70 70
70 70 70 Number of gels #/100 10 11 12 12 14 10 11 (10 kg/hr)
cm.sup.2 Surface of sheet -- Good Good Good Good Good Good Good (10
kg/hr) Machine Temperature .degree. C. 112 126 118 118 118 118 118
direction Melting of % 10 90 50 50 50 50 50 stretching crystalline
conditions portion No. of No. 5 5 2 5 5 5 5 stretching rolls Pinch
roll No. -- -- -- -- 1 -- -- used Roughness S 0.6 0.6 0.6 0.6 0.6
0.1 12 of roll Stretching Times 6.1 6.1 6 6 6.1 6.1 6.1 ratio
Stretching Times 1.2-1.5- 1.2-1.5- 1.5-4 1.01-5.46- 1.2-1.5-
1.2-1.5- 1.2-1.5- speed ratio 1.5-1.5- 1.5-1.5- 1.03-1.03- 1.5-1.5-
1.5-1.5- 1.5-1.5- for each 1.5 1.5 1.03 1.5 1.5 1.5 stretching roll
Thickness before/ .mu.m Non- 900/135 Non- 900/190 900/110 Non-
900/152 after machine uniform uniform uniform direction stretching
Slippage during -- Slip None Slip Slip None Slip None machine
direction stretching Width uniformity -- Non- Good Non- Non- Good
Non- Good after machine uniform uniform uniform uniform direction
stretching Transverse Temperature .degree. C. Transverse 126
Transverse Transverse 119 Transverse 119 direction direction
direction direction direction stretching stretching stretching
stretching stretching conditions Melting of % Impossible 90
Impossible Impossible 50 Impossible 50 crystalline portion
Stretching Times -- 6 -- -- 6 -- 6 ratio Puncture strength N/.mu.m
-- 0.17 -- -- 0.29 -- 0.18 Gas permeability 10.sup.-5 -- 0.7 -- --
0.5 -- 0.9 Darcy
[0123] As described above, in the present invention, since a
two-step biaxial stretching process in which stretching is
conducted in a machine direction so that the stretching is
dispersed among rolls is employed, it is possible to produce a
microporous polyethylene film which has high productivity,
consistent quality, high puncture strength, high gas permeability,
and low shrinkage, thus assuring excellent electric stability.
Therefore, the microporous polyethylene film according to the
present invention can be usefully applied to battery separators and
various filters. Furthermore, in the present invention, since the
stretching can be conducted at variable stretching ratios, it is
possible to produce the microporous polyethylene film having
various physical properties.
[0124] Although the preferred embodiments of the present invention
have been disclosed for illustrative purposes, those skilled in the
art will appreciate that various modifications, additions and
substitutions are possible, without departing from the scope and
spirit of the invention as disclosed in the accompanying
claims.
* * * * *