Note: Descriptions are shown in the official language in which they were submitted.
` ~ ~152942
Background of the Invention
Field of the Invention
This invention relates to microporous film having
improved water permeability andtor reduced electrical
resistance.
Summary of the Prior Art
Recent developments in the area of open celled
microporous polymeric films, exemplified by U.S. Patent Nos.
3,839,516; 3,801,404; 3,679,538; 3,558,764; and 3,426,754,
have instigated studies to discover applications which could
exploit the unique properties of these new films. Such
films which are in effect a gas-breathing water barrier can
be used as vents, gas-liquid transfer mediums, battery
separators and a variety of other uses.
One disadvantage of these films, which in the past
has limited the number of applications to which they may be
put, has been their hydrophobic nature. This is especially
true when polyolefinic films, a preferred type of polymeric
material often employed in the manufacture of microporous
films, are employed. Because these films are not ~wetted"
with water and aqueous solutions they could not be used
advantageously in such logical applications as filter media
electrochemical separator components and the like.
Several proposals have been put forth in the past
to overcome these problems such as exemplified by V-S.
Patent Nos. 3,853,601; 3,231,530 3,215,486; and Canadian
Patent No. 981,991, which utilize a variety of hydrophilic
coating agents or impregn~nts. Such coating agents or
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~ l~SZ94Z ~
impregnants, although effective for a limited period of
time, tend to be removed in a relatively short time by
solutions which contact the films or fibers in which they
are present.
Others have attempted to impart a hydrophilic
character to a normally hydrophobic microporous film by the
use of low energy plasma treatments. Such plasma treatments
are achieved by first activating surface sites of the micro-
porous film using argon or hydrogen plasma, and then grafting
thereto an appropriate free radical polymerizing species,
such as acrylic acid. The plasma treatments result in a
film having only a surface which is re-wettable. The surface
of the film alRo becomes plugged when wet which then inhibits
or prevents the free flow of water through the interior of
the film.
The unavoidable plugging of the surface pores
renders the film unsuitable for certain filter applications,
increases the electrical resistance of the film, and reduces
the dimensional stability of the films as exhibited by
substantial shrinkage on drying.
As stated above, a disadvantage of the plasma
treatment is its limited ability to render only the surface
of the microporous film wettable. It has been observed that
due to the unusually large surface area of a microporous
film of the type described herein, mere surface wettability
does not insure that the film will exhibit certain functional
properties such as low electrical resistance, and water flow
rates through the film which are comparable to known surfac-
tant systems discussed above.
~5294Z
The primary disadvantage of the plasma treatments,
namely, pore plugging and mere surface wettability, are
believed to result from a combination of factors, such as
the tendency of the low energy plasma to be readily deacti-
vated by the high surface area of the microporous film.
This reduces the likelihood that an interior site within the
microporous film will be activated. Similarly there is
competition for incoming graftable monomers exerted by the
free radial polymer grafts which are initially generated at
the surface of the film when the graftable monomer first
contacts the plasma activated microporous film surface.
Thus, the graft polymer chains initially present on the film
surface propagate at an increaaingly faster rate as the
reaction proceeds. Consequently, the resulting lengthened
graft polymer chains which occur at the surface of the film
entangle and plug the surface micropores in the presence of
water.
Other attempts to provide hydrophilic films using
a plasma treatment are illustrated by U.S. Patent Nos.
3,992,495 and 4,046,843.
Another technique for rendering polyethylene films
wettable and suitable for use in electrical battery separators
is illustrated by V. D'Agostino and J. Lee, Manufacturing
Methods For High Performance Grafted-Polyethylene Battery
Separators, U.S. National Technical Information Service,
A.D. Report No. 745,571 (1972) summarized in 78 Chemical
Abstract 5031f (1973); V. D'Agostino and J. Lee, Low
Temperature AlkaIine Battery Separators, 27 Power Sources
Symp. 87-91 (1976), summarized in 86 Chemical Abstract
158277f; 19 V. D'Agostino, J. Lee, and G. Orban, Zinc-Silver
Oxide Batteries, tA. Fleischer and J. ~ander ed, 1971).
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~ 15294Z
Such articles discuss or relate to a commercial
product known as PERMIONTM developed by RAI Research Corp.
Briefly, the method of preparation of this material consists
of crosslinking a one mil polyethylene sheet using beta
radiation, followed by grafting with methacrylic acid in an
appropriate solution under Co60 gamma radiation. The grafted
material is washed to remove the homopolymer, then converted
to the salt form in hot KO~, washed again to remove the
residual base, dried and packaged. The initial crosslinking
step creates microcracks or longitudinal slits in non-porous
polyethylene film which are so small they are not visible
even under an electron microscope. The diameter of these
microcracks is estimated to be about 20 Angstroms ~10 8 cm).
The microcracks are then grafted with the methacrylic acid.
The resulting film is therefore not microporous in the sense
of the microporous films employed in the present invention
which have an average size of about 100 to about 5,000
Angstroms. The extremely small size of the microcracks of
PERMIONTM generally prohibits a mass transfer of mobile electron
carrying species generated by oxidation-reduction reactions
through the film at any substantial rate. This is reflected
in the relatively~high (e.g., 30 to 40 milliohms-in2)
electrical resistances evidenced by films of this type.
Moreover, PERMIONTM is not dimensionally stable in more than
one direction as evidenced by substantial swelling.
It is well known that non-porous polymeric substrates
such as polyethylene and polypropylene may be reacted with
various monomers such as acrylic acid using various types of
ionizing radiation as illustrated by U.S. Patent Nos.
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~S2942 ~
2,999,056; 3,281,263; 3,372,100; and 3,709,718. Since none
of these patents are directed to microporous films, however,
they are not directed to the peculiar problems associated
therewith.
Thus, the search has continued for a relatively
permanently wettable, hydrophilic microporous film which
exhibits low electrical resistance, and improved water flow
rates through the microporous film. The present invention
was developed in response to this search.
It is therefore an object of the present invention
to provide a process for rendering a normally hydrophobic
microporous film relatively permanently hydrophilic thereby
improving its water permeability.
~ t is another object of the present invention to
provide a process for reducing the electrical resistance of
a normally hydrophobic microporous film.
It is still another object of the present invention
to provide a hydrophilic microporous film having reduced
electrical resistance.
It is a still further object to overcome the
problems of the prior art discussed above.
These and other objects, as well as the scope,
nature and utilization of the claimed invention, will be
apparent to those skilled in the art from the following
detailed description and appended claims.
~ 115;294Z ~
Summary of the Invention
In one aspect of the present invention there is
provided a process for rendering a normally hydrophobic
polyolefinic microporous film hydrophilic, improving the
water flow rate therethrough, a~d reducing the electrical
resistance thereof which comprises:
(a) coating the surface of the micropores of a
normally hydrophobic polyolefinic open celled microporous
film characterizea by having a reduced bulk density as
compared to the bulk density of a precursor film from which
it is prepared, an average pore size of from about 200 to
about 10,000 ~ngstroms, and a surface area of at least about
10 square meters per gram, with at least one hydrophilic
organic hydrocarbon monomer having from about 2 to about 18
carbon atoms characterized by the presence of at least one
double bond and at least one polar functional group; and
(b) chemically fixing to the surface of the
micropores of the microporous film an amount of said hydro-
philic organic hydrocarbon monomer sufficient to preserve
the open celled nature of said micropores and sufficient to
obtain an add-on of from about 0.1 to about 10%, by weight,
based on the weight of the uncoated microporous film by
irradiating the coated microporous film of (a) with from
about 1 to about 10 megarads of ionizing radiation.
In another aspect of the present invention there
is provided a hydrophilic open celled microporous film which
comprises:
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llS2942
(a) an open celled normally hydrophobic microporous
film characterized by having a reduced bulk density as
compared to the bulk density of a precursor film from which
it is prepared. an average pore size of from about 200 to
about 10,000 Angstroms and a surface area of at least 10
square meters per gram; and
(b) a coating on the surface of the micropores of
the microporous film of at least one hydrophilic organic
hydrocarbon monomer having from about 2 to about 18 carbon
atoms characterized by the presence of at least one double
bond and at least one polar function group, said hydrophilic
organic hydrocarbon monomer coating being chemically fixed
to the surface of the micropores of the microporous film, by
exposure to from about 1 to about 10 megarads of ionizing
radiation, and in an amount sufficient to preserve the open
celled nature of the microporous film and to obtain an add-
on of from about 0.1 to about 10%, by weight, based on the
weight of the uncoated microporous film.
The essence of the present invention lies in the
discovery that open celled microporous films may be rendered
relati~ely permanently wettable and/or hydrophilic when the
pores thereof are chemically fixed with a controlled amount
of graftable hydrophilic monomers by exposure to ionizing
radiation while preserving the open celled nature of the
microporous film. Moreover, such chemical fixation of the
hydrophilic monomers occurs even within those pores located
at an interior site within the microporous film. It is
believed that by controlling the amount of monomer which is
l~S294Z-
chemically fixed to the surface of the micropores, the polymer
chain graft which results upon exposure to radiation can be
aligned with the surface of the pores. Therefore, entangle-
ment and plugging of the pores, is avoided.
~ l~SZ942- ~
_scription of Preferred Embodiments
The present invention is directed to a hydrophilic
microporous film and a process for making the same.
Porous or cellular films can be classified into
two general types: one type in which the pores are not
interconnected, i.e., a closed-cell film, and the other type
in ~hich the pores are essentially interconnected through
tortuous paths which may extend from one exterior surface or
surface region to another, i.e., an open-celled film. The
porous films of the present invention are of the latter
type.
Further, the pores of the porous films of the
present invention are microscopic, i.e., the details of
their pore configuration or arrangement are discernible only
by microscopic examination. In fact, the open cell pores in
the films generally are smaller than those which can be
measured using an ordinary light microscope, because the
wave length of visible light, which is about 5,000 Angstroms
(an Angstrom is one ten-billionth of a meter), is longer
than the longest planar or surface dimension of the open
cell or pore. The microporous films of the present invention
may be identified, however, by using electron microscopy
techniques which are capable of resolving details of pore
structure below 5,000 Angstroms.
The microporous films of the present invention are
also characterized by a reduced bulk density, sometimes
hereinafter referred to simply as a "low" density. That is,
these microporous films have a bulk or overall density lower
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~ ~529~2 ~
than the bulk density of corresponding films composed of
identical polymeric material but having no open-celled or
other voidy structure. The term "bulk density" as used
herein means the weigh~ per unit of gross or geometric
volume of the film where gross volume is determined by
immersing a known weight of the film in a vessel partly
filled with mercury at 25C and atmospheric pressure. The
volumetric rise in the level of mercury is a direct measure
of the gross volume. This method is known as the mercury
volumenometer method, and is described in the EncYclopedia
of Chemical TechnologY, Vo. 4, page 892 IInterscience 1949).
Porous films have been produced which possess a
microporous, open-celled structure, and which are also
characterized by a reduced bulk density. Pilms possessing
this microporous structure are described, for example, in
U.S. Patent No. 3,426,754 which patent is assigned to the
assignee of the present invention and herein incorporated by
reference. The preferred method of preparation described
therein in~olves drawing or stretching at ambient temperatures,
i.e., "cold drawing", a crystalline, elastic precursor film
in an amount of about 10 to 300 percent of its original
length, with subsequent stabilization by heat setting of the
drawn film under a tension such that the film is not free to
shrink or can shrink only to a limited extent. Other methods
of preparing microporous film are exemplified by U.S. Patent
Nos. 3,558,764; 3,843,762; 3,920,785; British Patent Nos.
1,180,066 and 1,198,695 which are all herein incorporated by
reference~
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~152942- ~
While all of the above-listed patents describe
processes for preparing normally hydrophobic microporous
films which may be rendered hydrophilic in accordance with
the present invention, the preferred normally hydrophobic
microporous films are provided in accordance with the
processes described in U.S. Patent No. 3,801,404 which
defines a method for preparing microporous films herein
referred to as the "dry stretch" method and V.S. Patent No.
3,839,516 which defines a method for preparing microporous
films herein referred to as the "solvent stretch" method,
both of which are herein incorporated by reference. Each of
these patents discloses preferred alternative routes for
obtaining a normally hydrophobic microporous film by manipu-
lating a precursor film in accordance with specifically
defined process steps.
The preferred precursor films which may be utilized
to prepare microporous films in accordance with the "dry
stretch" and "solvent stretch" methods are specifically
detailed in each of the above respective patents. Thus, the
~dry stretch" method utilizes a non-porous crystalline,
elastic, polymer film having an elastic recovery at zero
recovery time (hereinafter defined) when subjected to a
standard strain ~extension) of 50 percent at 25C and 65
percent relative humidity of at least 40 percent, preferably
at least about 50 percent and most preferably at least about
80 percent.
Elastic recovery as used herein is a measure of
the ability of structure or shaped article such as a film to
return to its original size after being stretched, and may
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. .
~) 11529424 ~
be calculated as follows:
Elastic Recovery (ER)~
length length
when stretched after stretching
length added when stretched x 100
Although a standard s~rain of 50 percent is used
to identify the elastic properties of the starting films,
such strain is merely exemplary. In general, such starting
films will have elastic recoveries higher at strains less
than 50 percent, and somewhat lower at strains substantially
higher than 50 percent, as compared to their elastic recovery
at a 50 percent strain.
These starting elastic films will alsô ha~e a
percent crystallinity of at least 20 percent, preferably at
least 30 percent, and most preferably at least 50 percent,
e.g., about 50 to 90 percent, or more. Percent crystallinity
is determined by the X-ray method described by R.G. Quynn et
al in the Journal of Applied PolYmer Science, Vol. 2, No. 5,
pp. 166-173 ~1959). For a detailed discussion of crystallinity
and its significance in polymers, see Polymers and Resins,
Golding (D. Van Nostrand, 1959).
Other elastic films considered suitable for preparing
percursor films utilized in the dry stretch method are
described in British Patent No. 1,052,550, published December
21, 1966.
The.precursor elastic film utiiized in the prepara-
tion of the microporous films by the Hdry stretch" process
route should be differentiated from films formed from classical
elastomers such as the natural and synthetic rubbers. With
~SZ94Z
such classical elastomers the stress-strain behavior, and
particularly the stress-temperature relationship, is governed
by entropy-mechanism of deformation (rubber elasticity).
The positive temperature coefficient of the retractive
force, i.e., decreasing stress with decreasing temperature
and complete loss of elastic properties at the glass transition
temperature, are particular consequences of entropy-
elasticity. The elasticity of the precursor elastic films
utilized herein, on the other hand, is of a different nature.
In qualitative thermodyna~ic experiments with these elastic
precursor films, increasing stress with decreasing temperature
(negative temperature coefficient) may be interpreted to
mean that the elasticity of these materials is not governed
by entropy effects but dependent upon an energy term. More
significantly, the "dry stretch" precursor elastic films
have been found to retain their stretch properties at tempera-
tures where normal entropy-elasticity could no longer be
operative. Thus, the stretch mechanism of the "dry stretch"
precursor elastic films is thought to be based on energy-
elasticity relationships, and these elastic films may then
be referred to as "non-classical n elastomers.
Alternatively, the "solvent stretch" method utilizes
a precursor film which mus~ contain at least two components,
e.g., an amorphous component and a crystalline component.
Thus, crystalline materials, which are by natllre two components,
work well with the process. The degree of crystallinity of
the precursor film must therefore be at least 30%, preferably
at least 40% and most preferably at least 50% by volume of
the precursor film.
~lSZ94Z -~
The polymers, i.e, synthetic resinous material
from which the precursor films utilized in either process in
accordance with the present invention include the olefin
polymers, such as polyethylene, polypropylene, poly-3-methyl
butene-l, poly-4-methyl pentene-l, as well as ~opolymers of
propylene, 3-methyl butene-l, 4-methyl pentene-l, or ethylene
with each other or with minor amounts of other olefins,
e.g., copolymers of propylene ana ethylene, copolymers of a
major amount of 3-methyl butene-l and a minor amount of a
straight chain n-alkene such'as n-octene-l, n-hexadecene-l,
n-octadecene-l, or other relatively long chain alkenes, as
well as copolymers of 3-methyl pentene-l and any of the same
n-alkenes mentioned previously in connection with 3-methyl
butene-l.
For example, in general when propylene homopolymers
are contemplated for use in the "dry stretchn'method, an
isotatic polypropylene having a percent crystallinity as
indicated above, a weight average molecular weight ranging
from about 100,000 to 750,000 (e.g., about 200,000 to 500,000)
and a melt index (ASTM-D-1238-57T, Part 9, page 38) from
about 0.1 to about 75, (e.g., from about 0.5 to 30), can be
employed so as to give a final film product having the
requisite physical properties.
It is to be understood that the terms "olefinic
polymerN and ~olefin polymer" are used'interchangeably and
are intended to describe a polymer prepared by polymerizing
olefin monomers through their unsaturation.
Preferred polymers for use in the ~solvent stretch"
method are those polymers utilized in accordance with the
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~15294Z'
invention described in U.S. Patent 4,257,997, issued March 24,
1981, by John W. Soehngen and assigned to the Assignee of the
present invention, entitled "Improved Solvent Stretch
Process for Preparing Microporous Films from Precursor
Films of Controlled Crystalline Structure" the disclosure
of which is herein incorporated by reference. Thus, a
polyethylene hompolymer having a density of from about 0.960
to about 0.965 gm/cc, a high melt index of not less than
about 3 and preferably from about 3 to about 20 and a broad
molecular weight distribution ratio (MW/Mn) of not less
than about 3.8 and preferably from about 3.8 to about 13
is preferred in preparing a microporous film by the "solvent
stretch" method. Moreover, nucleating agents may be incor-
porated into the polymer em~loyed to prepare the precursor
film as described in the incorporated Soehngen Application
in which case the polymers having a melt index as low as 0.3
may be employed.
The types of apparatus suitable for forming the
precursor films are well known in the art.
For example, a conventional film extruder equipped
with a shallow channel metering screw and coat hanger die is
satisfactory. Generally, the resin is introduced into a
hopper of the extruder which contains a screw and a jacket
fitted with heating elements. The resin is melted and
transferred by the screw to the die from which it is extruded
through a slit in the form of a film from which it is drawn
by a take-up or casting roll. More than one take-up roll in
various combinations or stages may be used. The die opening
or slit width may be in the range, for example, of about 10
to 200 mils.
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~"
,, .-
~15Z94Z- ~
Using this type of apparatus, film may be extruded
at a drawdown ratio of about 5:1 to 200:1, preferably 10:1
to 50:1.
The terms "drawdown ratio" or more simply, "draw
ratio", as used herein is the ratio of the film wind-up or
take-up speed to the speed of the film issuing at the extrusion
die.
The melt temperature for film extrusion is, in
general, no higher than about 100C above the melting point
of the polymer and no lower than about 10C above the melting
point of the polymer.
For example, polypropylene may be extruded at a
melt temperature of about 180C to 270C, preferably 200C
to 240C. Polyethylene may be extruded at a melt temperature
of about 175 to 225C.
When the precursor film is to be utilized in
accordance with the "dry stretch" method, the extrusion
operation is preferably carried out with rapid cooling and
rapid drawdown in order to obtain maximum elasticity. This
may be accomplis~ed by having the take-up roll relatively
close to the extrusion slot, e.g., within two inches and,
preferably, within one inch. An "air knife" operating at
temperatures ~etween, for example, 0C and 40C, may be
employed within one inch of the slot to quench, i.e., quickly
cool and solidify, the film. The take-up roll may be rotated,
for example, at a speed of 10 to 1000 ft/min, preferably 50
to 500 ft/min.
When the precursor film is to be utilized in
accordance with the ~solvent stretch" method, the extrusion
~?3
~52942
operation is preferably carried out with slow cooling, in
order to minimize stress and any associated orientation
which might result from a fast quench to obtain maximum
crystallinity but yet fast enough to avoid developing large
spherulities. This may be accomplished by controlling the
distance of the chill roll take-up from the extrusion slit.
While the above description has been directed to
slit die extrusion methods, an alternative method of forming
the precursor films contemplated in this invention is the
blown film extrusion method wherein a hopper and an extruder
are employed which are substantially the same as in the slit
extruder described above.
From the extruder, the melt enters a die from
which it i8 extruded through a circular slot to form a
tubular film having an initial diameter D1. Air enters the
system through an inlet into the interior of said tubular
film and has the effect of blowing up the diameter of the
tubu~ar film to a diameter D2. Means such as air rings may
also be provided for directing the air about the exterior of
extruded tubular film so as to provide different cooling
rates_ Means such as a cooling mandrel may be used to cool
the interior of the tubular film. After a distance during
which the film is allowed to completely cool and harden, it
is wound up on a take-up roll.
Using the blown film method, the drawdown ratio is
preferably 5:1 to 100:1, the`slot opening 10 to 200 mils,
preferably 40 to lO0 mils, the D2/D1 ratio, for example, 1.0
to 4.0 and preferably about 1.0 to 2.5, and the take-up
speed, for example, 30 to 700 ft/min. The melt temperature
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~ ~529~Z ~
may be within the ranges given previously for slit die
extrusion.
The extruded film may then be initially heat
treated or annealed in order to improve crystal structure,
e.g., by increasing the size of the crystallites and removing
imperfections therein. General~y, this annealing is carried
out at a temperature in the range of about 5C to lOO~C
below the melting point of the polymer for a period of a few
seconds to several hours, e.g., 5 seconds to 24 hours, and
preferably from about 30 secor.ds to 2 hours. For polypropylene,
the preferred annealing temperature is about 100 to 155C.
An exemplary method of carrying out the annealing
is by placing the extruded film in a tensioned or tensionless
state in an oven at the desired temperature in which case
the residence time is preferably in the range of about 30
seconds to 1 hour.
In the preferred embodiments, the resulting partly-
crystalline precursor film is preferably subjected to one of
the two alternative procedures described above to obtain a
normally hydrophobic microporous film which may be utilized
in accordance with the present invention.
The first preferred procedure as disclosed in U.S.
Patent No. 3,801,404, herein referred to as the "dry stretch"
method includes the steps of cold stretching, i.e., cold
drawing, the elastic film until porous surface regions or
areas which are elongated normal or perpendicular to the
stretch direction are formed, (2) hot stretching, i.e., hot
drawing, the cold stretched film until fibrils and pores or
open cells which are elongated parallel to the stretch
direction are formed, and thereafter ~3) heating or heat-
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~15Z942 -
setting the resulting porous film under tension, i.e., at
substantially constant length, to impart stability to the
film.
The term "cold stretching" as used herein is
defined as stretching or drawing a film to greater than its
original length and at a stretching temperature, i.e., the
temperature of the film being stretched, less than the
temperature at which melting begins when the film is uniformly
heated from a temperature o~ 25C and at a rate of 20C per
minute. The term "hot stretching" as used herein is defined
as stretching above the temperature at which melting begins
when the film is uniformly heated from a temperature of 25C
and at a rate of 20C per minute, but below the normal
melting point of the polymer, i.e., below the temperature at
which fusion occurs. As is known to those skilled in the
art, the temperature at which melting begins and the fusion
temperature may be determined by a standard differential
thermal analyzer (DTA), or by other known apparatus which
can detect thermal transitions of a polymer.
The temperature at which melting begins varies
with the type of polymer, the molecular weight distribution
of the polymer, and the crystalline morophology of the film.
For example, polypropylene elastic film may be cold stretched
at a temperature below about 120C preferably between about
10C and 70C and conveniently at ambient temperature, e.g.,
25C. The cold stretched polypropylene film may then be hot
stretched at a temperature above about 120C and below the
fusion temperature, and preferably between about 130C and
150C. Again, the temperature of the film itself being
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~S294Z
st:retched is referred to herein as the stretch temperature.
The stretching in these two steps or stages must be consecutive,
in the same direction, and in that order, i.e., cold then
hct, but may be done in a continuous, semi-continuous, or
batch process, as long as the cold stretched film is not
allowed to shrink to any significant degree, e.g., less than
5 percent of its cold stretched length, before being hot
stretched.
The sum total amount of stretching in the above
two steps may be in the range of about 10 to 300 percent and
preferably about 50 to 150 percent, based on the initial
length of the elastic film. Yurther, the ratio of the
amount of hot strétching to the sum total amount of stretching
or drawing may be from above about 0.10:1 to below 0.99:1,
preferably from about 0.50:1 to 0.97:1, and most preferably
from about ~50:1 to 0.95:1. This relationship between the
"cold" and "hot" stretching is referred to herein as the
"extension ratio" (percent "hot" extension to the percent
n total" extension).
In any stretchinq operations where heat must be
supplled the film may be heated by moving rolls which may in
turn be heated by an electrical resistance method, by passage
over a heated plate, through a heated liquid, a heated gas,
or the like.
After the above-described two stage or two step
stretching, the stretched film is heat set. This heat
treatment may be carried out at a temperature in the range
from about 125C up to less than the fusion temperature, and
preferably about 130 to 160C for polypropylene; from about
-21-
llszs~z
75C up to less than fusion temperature, and preferably
about 115C to 130C, for polyethylene, and at similar
temperature ranges for other of the above mentioned polymers.
This heat treatment should be carried out while the film is
being held under tension, i.e., such that the film is not
free to shrink or can shrink to only a controlled extent not
greater than about 15 percent of its stretched length, but
not so great a tension as to stretch the film more than an
additional 15 percent. Preferably, the tension is such that
substantially no shrinkage or stretching occurs, e.g., less
than 5 percent change in stretched length.
The period of heat treatment which is preferably
carried out sequentially with and after the drawing operation,
should not be longer than 0.1 second at the higher annealing
temperatures and, in general, may be within the range of
about 5 seconds to 1 hour and preferably about 1 to 30
minutes.
The above described setting steps may take place
in air, or in other atmospheres such as nitrogen, helium or
argon.
A second preferred alternative procedure for
converting the precursor film to a microporous film as
described in U.S. Patent No. 3,839,516 and herein referred
to as the "solvent stretch" method includes the basic steps
of (1) contacting the precursor film having at least two
components (e.g. an amorphous component and a crystalline
component), one of which is lesser in volume than all the
other components, with a swelling agent for sufficient time
to permit adsorption of the swelling agent into the film:
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~52942 ~
(2) stretching the film in at least one direction while in
contact with swelling agent, and (3) maintaining the film in
its stretched state during ~emoval of the swelling agent.
Optionally, the film may be stabilized by heat-setting under
tension or by ionizing radiation.
Generally, a solvent~having a Hildebrand solubility
parameter at or near that of the polymer would have a solu-
bility suitable for the drawing process described herein.
The Hildebrand solubility parameter measures the cohesive
energy density. Thus, the underlying principle relies on
the fact that a solvent with a similar cohesive energy
density as a polymer would have a high affinity for that
polymer and would be adequate for thi~ process.
General classes of swelling agents from which one
appropriate for the particular polymeric film may be chosen
are lower aliphatic ketones such as acetone, methyl ethyl-
ketone, cyclohexanone; lower aliphatic acid esters such as
ethyl formate, butyl acetate, etc.; halogenated hydrocarbons
such as carbon tetrachloride, trichloroethylene, perchloro-
ethylene, chlorobenzene, etc.; hydrocarbons such as heptane,
cyclohexane, benzene, xylene, tetraline, decaline, etc.;
nitrogen-containing organic compounds such as pyridine,
formamide, dimethylformamide, etc.; ethers such as methyl
ether, ethyl ether, dioxane, etc. A mixture of two or more
of these organic solvents may also be used.
It is preferred that the swelling agents be a
compound composed of carbon, hydrogen, oxygen, nitrogen,
halogen, sulfur and contain up to about 20 carbon atoms,
preferably up to about 10 carbon atoms.
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~15294Z
The "solvent stretching" step may be conducted at
a temperature in range from above the freezing point of the
solvent, or swelling agent, to a point below the temperature
at which the polymer dissolves (i.e., ambient temperature to
about 50C).
The precursor film employed in the "solvent stretch"
process may range from 0.1 to about 20 mils, or even thicker.
In a preferred embodiment the precursor film is
biaxially stretched in accordance with the procedures disclosed
in U. S. Patent 4,257,997, issued March 24, 1981, entitled
"Improved Solvent Stretching Process for Preparing Microporous
Films" and assigned to the assignee of the present invention
the disclosure of which is herein incorporated by reference.
This process identifies preerred stretching conditions in a
uniaxial direction which lead to improved permeability of
the uniaxially stretched microporous film. The uniaxially
stretched microporous film can then be stretched in a trans-
verse direction to improve the permeability even further. Thus,
it is preferred that the precursor film be "solvent stretched"
in a uniaxial direction not greater than about 350%, and most
preferably 300% greater than its original length. Typically,
additional stretching in the same direction after the solvent
removal is not employed.
The optional stabilizing step may be either a
heat-setting step or a cross-linking step. This heat treatment
may be carried out at a temperature in the range from about
125C up to less than the fusion temperature and prefer-
ably about 130 to 150C for polypropylene; from about
~ ~ 5Z9 42
75C up to less than fusion temperature, and preferably
about 115 to 130C for polyethylene and at similar temperature
ranges for other of the above mentioned polymers. This heat
treatment should be carried out while the film is being held
under tension, i.e., such that the film is not free to
shrink or can shrink to only a controlled extent not greater
than about 15 percent of its stretched length, but not so
great a tension as to stretch the film more than an additional
15 percent. Preferably, the tension is such that substantially
no shrinkage or stretching occurs, e.g., less than 5 percent
change in stretched length.
The period of heat treatment which is preferably
carried out sequentially with and after the "solvent stretching"
operation, shouldn't be longer than 0.1 second at the higher
annealing temperatures and, in general, may be within the
range of about 5 seconds to 1 hour and preferably about 1 to
30 minutes.
The above described setting steps may take place
in air, or in other atmospheres such as nitrogen, helium or
argon.
When the precursor film is biaxially stretched the
stabilizing step should be conducted after transverse stretching
and not before.
While the present disclosure and examples are
directed primarily to the aforesaid olefin polymers, the
invention also contemplates the high molecular weight acetal,
e.g., oxymethylene, polymers. While both acetal homopolymers
and copolymers are contemplated, the preferred acetal polymer
for purposes of polymer stability is a "random" oxymethylene
-25-
~15Z942^
copolymer, which contains recurring oxymethylene, i.e.,
- -CH2 - O - , units interspersed with ---OR groups in
the main polymer chain where R is a divalent radical containing
at least two carbon atoms directly linked to each other and
positioned in the chain between the two valences, with any
substituents on said R radical being inert, that is, those
which do not include interfering functional groups and which
will not induce undesirable reactions, and where a major
amount of the ---OR units exist as sin~le units attached
to oxymethylene groups on each side. Examples or preferred
polymers include copolymers o~ trioxane and cyclic ethers,
containing at least two adjacent carbon atoms such as the
copolymers disclosed in U.S. Patent No. 3,027,352 of Walling
et al. These polymers in film form may also have a crystal-
linity of at least 20 percent, preferably at least 30 percent,
and most preferably at least 50 percent, e.g., 50 to 60
percent or higher. Further, these polymers have a melting
point of at least 150C. and a number average molecular
weight of at least 10,000. For a more detailed discussion
of acetal and oxymethylene polymers, see FormaldehYde,
Walter, pp. 175-191, (Reinhold 1964).
Other relatively crystalline polymers to which the
invention may be applied are the polyalkylene sulfides such
as polymethylene sulfide and polyethylene sulfide, the
polyarylene oxides such as polyphenylene oxide, the polyamides
such as polyhexamethylene adipamide (nylon 66) and poly-
caprolactam (nylon 6), all of which are well known in the
art and need not be described further herein for the sake of
brevity.
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~5294Z
The normally hydrophobic microporous films employed
i.n the present invention, in a tensionless state, have a
lowered bulk density compared with the density of correspond-
ing polymeric materials having no open-celled structure,
e.g., those from which it is formed. Thus, the films have a
bulk density no greater than about 95 percent and preferably
20 to 40 percent of the precursor film. Stated another way,
the bulk density is reduced by at least 5 percent and prefer-
ably 60 to 80 percent. For polyethylene, the reduction is
30 to 80 percent, preferably 60 to 80 percent. The bulk
density is about 20 to 40 percent of the starting material,
the porosity has been increased by 60 to 80 percent because
of the pores or holes.
When the microporous film is prepared by the "dry-
stretch" or solvent "stretch methods~ the final crystallinity
of the microporous film is preferably at least 30 percent,
more preferably at least 65 percent, and more suitably about
70 to 85 percent, as determined by the X-ray method described
by R.G. Quynn et al in the Journal of Applied PolYmer Science,
Vol 2, No. 5, pp. 166-173. For a detailed discussion of
crystallinity and its significance in polymers, see Polvmers
and Resins, Golding (S. Van Nostrand, 1959).
The microporous films which can be employed in the
present invention may also have an average pore size of from
about 200 to 10,000 A, typically from about 400 to 5000 A,
and more typically about 500 to about 5000 A. These values
can be determined by mercury porosimetry as described in an
article by R. G. Quynn et al, on pages 21-34 of Textile
Research Journal, January, 1963 or by the use of electron
microscopy as described in Geil's Polvmer Sinqle CrYsta
~15294Z' '`'`f~)
p. 69 (Interscience 1963). When an electron micrograph is
employed pore length and width measurements can be obtained
by simply utilizing a ruler to directly measure the length
and width of the pores on an electron micrograph taken
usually at 5,000 to 10,000 magnification. Generally, the
pore length values obtainable by electron microscopy are
approximately equal to the pore size values obtained by
mercury porosimetry.
The microporous films employed in the present
invention will exhibit a surface area within certain predict~
able limits when prepared by either the n solvent stretch"
method or the "dry stretch" method. Typically such micro-
porous films will be found to have a surface area of at
least 10 sq.m/gm and preferably in the range of about 15 to
25 sq.m/gm. For films formed from polyethylene, the surface
area generally ranges from about lO to 25 sq.m/gm. and
preferably about 20 sq.m/gm.
Surface area may be determined from nitrogen or
krypton gas adsorption isotherms using a method and apparatus
described in U.S. Pat. No. 3,262,319. The surface area
obtained by this method is usually expressed as square
meters per gram.
In order to facilitate comparison of various
materials, this value can be multiplied by the bulk density
of the material in grams per cc. resulting in a surface area
expressed as square meters per cc.
The normally hydrophobic microporous polymeric
films of the instant invention which is rendered hydrophilic
has a preferred thickness of from about 1 mil (.001 inch) to
about 8 mills.
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~15294Z-
The normally hydrophobic microporous film prepared
in accordance with the procedures described above is rendered
wettable, and/or hydrophilic by coating the surface of the
micropores of the microporous film with a hydrophilic monomer
or mixture thereof and subsequently exposing the coated
microporous film to ionizing radiation. The ionizing radiation
chemically fixes the hydrophilic monomer to the micropore
surface and renders it relatively permanently hydrophilic.
The add-on or amount of hydrophilic monomer which is chemically
fixed is controlled within certain limits to avoid plugging
of the pores while at the same time permitting control of
thç total porosity of hydrophilic film. Control of porosity
in turn permits control of the water permeability and elec-
trical resistance of the film.
As used herein the term "hydrophobic" is defined
as meaning a surface which passes less than about 0.010
milliliters of water per minute per cm2 of flat film surface
under a water pressure of 100 psi. Likewise the term "hydro-
phylic" is meant to apply to those surfaces which pass
greater than about 0.01 milliliters of water per minute per
cm2 at the same pressure.
The hydrophilic monomers which may be employed to
coat the pore surface of the microporous film of the present
invention are organic hydrocarbon compounds having from 2 to
18 carbon atoms characterized by the presence of at least
one double bond which renders the monomers polymerizable,
and/or co-polymerizable under the influence of ionizing
radiation at a temperature which would not adversely affect
the microporous film and at least one polar functional group
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~L152942
such as carboxy, sulfo, sulfino hydroxyl, ammonio, amino, and
phosphono.
Accordingly, the hydrophilic monomers of the
present invention include hydrocarbon compounds having from
2 to 18 carbon atoms with one or more polymerizable and/or
copolymerizable double bonds s~ch as substituted and unsub-
stituted carboxylic or dicarboxylic acids and esters thereof;
vinyl and allyl monomers, particularly itaconic acid, malonic
acid, fumaric acid, and crotonic acid, and.their esters or
anhydrides, unsubstituted or alkyl-substituted acrylic acids
such as acrylic acid and methacrylic acid, acrolein, or
acrylonitrile; unsubstituted or alkyl-substituted alkyl,
cycloalkyl, aryl, hydroxyalkyl, or hydroxyaryl acrylates,
alkyl-substituted dialkylaminoalkyl acrylates, epoxyalkyl
acrylates; vinyl sulfonic acid, and styrene sulfonic acid;
vinyl esters such as vinyl acetate and higher carboxylic
acid vinyl esters, alkyl substituted vinyl esters of carboxylic
acids containing sulfo groups; vinyl ethers, such as unsub-
stituted or substituted alkyl, cycloalkyl or aryl vinyl
ethers; vinyl-substituted silicones; vinyl-substituted
aromatic or heterocyclic hydrocarbons; diallyl fumarates;
diallyl maleates; alkyl-substituted phosphates, phosphites
or carbonates; vinyl sulfones, the reaction product of ..
ethoxylated monylphenol and acrylic acid and the like.
Since it is intended that the hydrophilic monomer
penetrate to the interi~r of the microporous film it is
preferred to employ hydrophilic monomers having from about
2 to about 14, most preferably from about 2 to about 4
carbon atoms.
-30-
~S2942`
The preferred hydrophilic monomers employed in the
present invention are acrylic acid, methacrylic acid, and
vLnyl acetate.
The amount of hydrophilic monomer which coats the
interior surface of the pores of the microporous film is
controlled to achieve the proper degree of add-on which is
determined by the films intended water flow or electrical
resistance end use requirements. As described above, such
control is exerted in a manner sufficient to preserve the
open celled nature of the micropores, i.e., avoid plugging
of the pores, after the monomer impregnated microporous film
is subjected to the radiation treatment described herein
such that the desired properties discussed above are maintained
for a longer duratlon than i9 otherwise obtainable using the
typical surfactant coatings of the prior art. The specific
amount of hydrophilic monomer which is chemically fixed onto
the surface of the pores is expressed in terms of percent
add-on, i.e., that percent by weight of the uncoated micro-
porous film which represents the weight of the hydrophilic
monomer coating which is present in the cured microporous
film.
Accordingly, to avoid pore plugging of the micro-
porous films described herein, the percent add-on of the
hydrophilic monomer which is chemically fixed to the surface
of the micropores is controlled to be not greater than about
10~, and generally from about 0.1 to about 10%, preferably
from about O.S to about 2.5%, and most preferably from about
1 to about 2.0% (e.g., 1.5%) by weight, based on the weight
of the uncoated microporous film.
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~ l~5Z99~2` o~3
~ he particular percent add-on selected will be
determined by the end use for which the resulting hydrophilic
microporous film is employed and will vary within the broad
range of add-on percentages described above.
For example, when the grafted hydrophilic microporous
film is intended to be used as~an electrical battery separator
the percent add-on is selected on the basis of the electrical
resistance of the resulting hydrophilic microporous film (in
milliohms per square inch) which is a function of percent
add-on of the hydrophilic monomer.
Electrical resistance as defined herein is a
measure of the ability of the microporous film to conduct
electrons. Consequently, as a general rule the higher the
electrical resistance of the microporous film the less
effective the microporous film will be as a battery separator.
Thus, the electrical resistance of the microporous
film is determined at various add-on percentages of the
hydrophilic monomer and a plot of electrical resistance as a
function of percent add-on is made. The percent add-on is
then determined on the basis of the desired electrical
resistance.
The electrical resistance of the hydrophilic
microporous film of the present invention as hereinafter
described will generally be controlled to be less than about
30 milliohms per square inch (milliohms-in. ), preferably
less than about 10 milliohms-in.2, and most preferably less
than about 5 milliohms-in2 inch.
When the grafted hydrophilic microporous film is
to be used as a filter, the percent add-on of the microporous
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llSZ942`
film is selected on the basis of the water flow rate through
the microporous film as herein defined.
Accordingly, the water flow rate may be plotted as
~ function of percent add-on of the hydrophilic monomer and
the appropriate percent add-on is selected on the basis of
the desired water flow rate.
When the microporous film is intended to be used
for filtration purposes the pore size of the film is selected
to act as a barrier for the material to be separated and the
water flow rate is controlled as desired within the limits
of the pore size selected. Thus, the water flow rate can be
controlled to be greater than about .01 cc/min/cm2, preferably
greater than about .05 cc/min/cm2, and most preferably
greater than about .5 cc/min~cm2 at a pressure differential
of about one atmosphere.
Thus, the present invention has several advantages
over hydrophilic films prepared in the past. Since it is
possible to preserve the open celled nature of the hydrophilic
microporous film prepared in accordance with the present
invention such films permit a mass transport of water through
the film as opposed to transport of water through the film
by diffusion which is a much slower process. The mass
transport effect also contributes to the reduction in elec-
trical resistance. Another advantage of the microporous
films of the present invention results from the chemical
fixation of the hydrophilic monomer to the microporous film
which extends the duration of the presence of the hydrophilic
monomer on the micropore surface over longer periods of time
than typical surfactants, particularly after repeated washings.
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1152942
~oreover, the dimensional stability of the films of the
p;resent invention is also improved.
It is appropriate to mention that it is preferred
when possible to convert the polar functional group of the
hydrophllic monomer to its most polar form. This can be
achieved, for example, by reac~ing an acid functional group
with a base such as KOH to form the correspondin~ salt.
Thus, the salt form can be achieved in this instance by
soaking the chemically fixed film in a 2% solution of KOH
for a period of from about 5 to about 30 minutes. This
enhances the wettability and increases the flexibility of
the film.
Any of the well known coating methods may be
employed to coat the microporous f~lm, provided such methods
supply a sufficiently accurate control of the percent add-on
of the hydrophilic monomer.
The preferred method for accurately and efficiently
coating the interior surface of the pores of the microporous
film is to contact the microporous film with a vapor of the
monomer which condenses on the micropore surface.
The percent add-on can be controlled by controlling
the equilibrium vapor pressure (i.e. the vapor pressure
wherein the rate of condensation of the monomer vapor equals
its rate of vaporization at a given temperature) of the
hydrophilic monomer and the time during which the hydrophilic
monomer is in contact with the microporous film.
The equilibrium vapor pressure requixed to achieve
the desired amount of the hydrophilic monomer coating at a
constant temperature is determined from plots of percent
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.
1152942
add-on versus time at various equilibrium vapor pressures
while keeping the temperature constant. The percent add-on
selected as described above depends upon the particular
property sought to be imparted to the micrporous film.
The equilibrium vapor pressure required to achieve
the appropriate percent add-on is easily determined from the
above described plots. The contact time of the monomer
vapor with the microporous film is also determined in a
similar manner.
Thus, in a continuous process the microporous film
is continuously passed through a chamber containing the
hydrophilic monomer vapor at the appropriate equilibrium
vapor pressure and temperature. The duration of contact of
the microporous film with the hydrophilic monomer vapor is
controlled by adjusting the path lenght and line speed of
the microporous film through the vapor.
Obviously the vapor coating technique can only be
employed when the critical temperature of the hydrophilic
monomer (i.e., the temperature at which a liquid monomer
cannot exist regardless of pressure) is above the temperature
at which the properties of microporous film would be adversely
affected at the particular contact time employed.
- It is preferred that the vapor-coating technique
be conducted at atmospheric pressure at a temperature which
yields the appropriate vapor pressure.
Typically the temperatures at which the vapor
coating technique is conducted when atmospheric pressure is
employed will vary from about 50 to about 170C. when the
hydrophilic monomer employed is acrylic acid, methacrylic
acid, or vinyl acetate.
-35-
~52942` ~
Sub-atmospheric and supra atmospheric pressure may
also be employed with appropriate adjustments in temperature
and contact time.
It should be understood that since the percent
add-on is determined after the cure treatment, an amount of
hydrophylic monomer in excess of the percent add-on is
initially applied to the microporous film to compensate for
loss of the monoMer which may occur during the cure treatment.
Other suitable methods by which the microporous
film may be coated with the hydrophilic monomer include
dissolving the hydrophilic monomer in a vaporizable solvent
such as methylene chloride to-form a pad bath.
The pad bath may then be employed in a reverse
roll coating technique. In thi~ method a doctor roll is
disposed partially in the pad bath of the monomer coating
solution. A second driven roll guides an uncoated hydrophobic
microporous film web through the nip formed by itself and
the doctor roll. The two rolls which are preferably separately
driven, rotate in the same direction so that the coated film
web is guided in the direction from whence the uncoated film
originates. The amount of monomer coating disposed on the
.
film is a function of the difference in speed of the doctor
roll and the second film driving roll and also the size of
the nip formed by the two rolls.
Alternatively, a squeeze roll method may be employed.
The film in this method is guided into a pad bath of the
monomer coating solution and squeezed between two squeeze
rolls disposed downstream thereo~. The amount of coating is
thus a function of the gap size between the two squeeze
rolls and the pressure exerted therebetween.
-36-
~15iZ942`
Another alternative method for coating the substrate
hydrophobic microporous film is the wire wound metering rod
method. This method is the same as the squeeze roll method
except that the microporous film after being coated by being
guided through a bath of the monomer coating solution is
squeezed between a pair of wire wound metering rods which
control the amount of coating disposed thereon by the con-
figuration of the wires wound around the metering rods.
In the reverse roll, squeeze roll, and wire wound
coating techniques the amount of hydrophilic monomer initially
applied to the microporous film is a function of one or two
variables discussed in the description of the methods. In
addition, in all three methods the amount of the monomer
coating is al 80 a function of the concentration of the
monomer in the pad bath. The hydrophilic monomer pad bath
is provided by dissolving the monomer in a common organic
solvent which has a boiling point lower than the bo ling
point of the hydrophilic monomer employed, such as in addition
to methylene chloride, acetone, methanol, ethanol, and
isopropanol.
The concentration of the hydrophilic monomer in
the pad bath is controlled to achieve application of the
appropriate amount of monomer upon evaporation of the solvent.
Generally the monomer concentration in the pad bath can vary
from about 1 to about 30%, and preferably from about 5 to
about 15% by weight, based on the weight of the bath.
When the hydrophilic monomers are coated on the
microporous film using a pad bath the solvent present therein
is removed by passing the coated film through a drier. The
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115294Z
te~mperature of the drier should be high enough to evaporate
the solvent only thereby leaving the monomer deposited on
t}!e micropore surface of the film.
After the pores of the microporous film have been
coated with the hydrophilic monomer and solvent if any,
removed therefrom, the coated microporous film is subjected
to ionizing radiation to chemically fix the hydrophilic
monomer to the normally hydrophobic micropore surface and
render it hydrophilic.
When subjected tO ionizing radiation a number of
possible mechanisms or combinations thereof may operate to
achieve the desired effect. Thus, the hydrophilic monomers
may become chemically attached to the micropore surface
and/or may form by polymerization andtor copolymerization a
polymeric layer or sleeve which is intimately bonded to the
micropore surface chemically, such as by random chemical
attachment of the sleeve surface to the micropore surface,
and/or physically, as a result of the confining effect on
the polymeric sleeve of the contour of the micropore surface.
The term "chemical fixation" is intended to embody all of
the above mechanisms or combinations thereof.
Without being restricted to the particular mechanisms
by which the desired improvement may be achieved the properties
of the normally hydrophobic microporous film are modified in
one or more ways depending upon the percent add-on of the
hydrophilic monomer.
Thus, the resulting radiation treated microporous
film is rendered hydrophilic over a longer period of use
than is otherwise obtained by typical surfactant coatings
-38-
~ ~1529~2 ~
and yet the open celled nature of the film can be preserved
for use of the microporous film in those applications where
mass transport and low electrical resistance is required.
As indicated above chemical fixation of the hydro-
philic monomers to the normally hydrophobic microporous film
is achieved by exposing the hydrophilic monomer impregnated
microporous film to ionizing radiation.
Ionizing radiation is herein defined to consist
essentially of the type which provides emitted particles or
photons having an intrinsic energy sufficient to produce
ions and break chemical bonds and thereby induce free radical
reactions between the hydrophilic monomers employed and
between the monomers and the micropore surface as described
herein. Ionizing radiation is conveniently available in the
form of ionizing particle radiation, ionizing~electromagnetic
radiation, and actinic light.
The term "ionizing particle radiation~ has been
used to designate the emission of electrons or highly accel-
erated nuclear particles such as protons, neutrons, alpha-
particles, deuterons, beta-particles, or their analogs,
directed in such a way that the particle is projected into
the mass to be irradiated. Charged particles can be accel-
erated by the aid of voltage gradients by such devices as
a low energy (i.e., 200 KeV) elongated electron beam
generator such as the ELECTROCURTAINTm manufactured by
Energy Sciences Corporatio~, accelerators with resonance
chambers, Van Der Graaff generators, betatrons, synchrotrons,
cyclotrons, etc. Neutron radiation can be produced by
bombarding a selected light metal such as beryllium with
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~52942
positive particles of high energy. Particle radiation can
also be obtained by the use of an atomic pile, radioactive
isotopes or other natural or synthetic radioactive materials.
"Ionizing electromagnetic irradiation" is produced
when a metallic target, such as tungsten, is bombarded with
electrons of suitable energy. This energy is conferred to
the electrons by potential accelerators of over 0.1 million
electron volts (mev.) In addition to irradiation of this
type, commonly called X-ray, an ionizing electromagnetic
irradiation suitable for the practice of this invention can
be obtained by means of a nuclear reactor (pile) or by the
use of natural or synthetic radioactive material, for example,
cobalt 60.
The hydrophilic monomers described herein will
also undergo chemical fixation by exposure to actinic light.
In general, the use of wave lengths in which sensitivity to
actinic light occurs is approximately 1,800 to 4,000 angstrom
units. Various suitable sources of the actinic light are
available in the art including by way of example, quartz
mercury lamps, ultraviolet cored carbon acrs, and high flash
lamps. Initiators may be employed-when actinic light is
used.
The preferred source of ionizing radiation is the
elongated electron beam generator such as described in U.S.
Patent Nos. 3,702,412; 3,745,396; and 3,769,600.
The techniques for accurate process control are
sufficiently developed to permit conversion of results from
one type of radiation to the other and adequate adjustment
-40-
~? ~152942 ~
of the techniques described herein may be employed inter-
changeably in the production of any desired product.
A radiation dosage of about 1.0 to about 10 megarads
(mrad.), and preferably from about 2 to about 5 mrad. (e.g.,
3 mrad.) i5 employed to achieve chemical fixation. A megarad
is one million rads. A rad is`the amount of ionizing high
energy radiation which produces an absorption of 100 ergs of
energy per gram of absorbing material. This unit is widely
accepted as a convenient means of measuring radiation absorption
by material.
Preferably, the minimum ionizing radiation is
employed to achieve chemical fixation due to economic con-
siderations. Exces~ive dosages (i.e., greater than about 20
mrads. at a single exposure) should be avoided to avoid
excess heating and shrinkage of the film and degradation of
the hydrophilic monomers and the microporous film. If the
dosage is too small, however, the chemical fixation will not
be achieved and the hydrophilic monomer will quickly be
lost.
The minimum radiation dosage which will achieve
chemical fixation of the hydrophilic monomer will vary
depending on the type of polymer employed to prepare the
microporous film. Thus, for example, when the polymer is
polyethylene the radiation dosage should not be less than
about l mrad. while for polypropylene the radiation dosage
should not be less than about 2 mrad. and preferably not
less than about 3 mrad.
Room temperatures may be employed satisfactorily
for irradiation although elevated temperatures may also be
employed.
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~ ~15294Z ~
Since oxygen tends to inhibit grafting of the
h~drophilic monomer to the microporous film it is preferred
to conduct the radiation treatment of the microporous film
under an inert atmosphere such as nitrogen or other inert
gas.
As discussed earlier~ the percent add-on may be
selected on the basis of the electrical resistance of the
microporous film.
Electrical resistance (direct current method) of a
microporous film is determined by soaking a sample of the
microporous film having a known surface area (e.g., 0.2 sq.
inches) in a 40% by weight, solution of KOH in water for 24
hours. The resulting sample is then disposed between
wor~ing cadium electrodes ~i.e., anode and a cathode)
immersed in an electrolyte of a 40%, by weight, solution of
XOH in water and a direct current of known amperage (e.g. 40
milliamperes) is passed through the cell between the electrodes.
The potential drop across the film (E) is measured with an
electrometer. The potential drop across the cell without
the microporous film disposed therein (E') is also determined
using the same current.
The electrical resistance of the microporous film
is then determined using the equation:
E.R. = (E' - E)A
where A is the surface area of the exposed film in square
inches, I is the current across the cell in milliamperes, E.R.
is the electrical resistance of the microporous film in
milliohms per square inch, and E' and E are as described.
-42-
1152942
The water permeability or the water flow rate of
the hydrophilic microporous film of the present invention is
determined by measuring the rate of flow of water through a
specific surface area of film while the water is under a
differential pressure of one atmosphere. Thus, the water
flow rate is expressed in units of volume of water in cubic
centimeters per minute per square centimeter of film surface
i.e., cc/minute/cm2.
The air permeability of the microporous films of
the present invention is determined by the Gurley test,
i.e., according to ASTM D 726 by mounting a film having an
area of one square inch in a standard Gurley densometer.
The film is subject to a standard differential pressure ~the
pressure drop across the film) of 12.2 inches of water. The
time in seconds required to pass lOcm3 of air through the
film i8 an indication of permeability. A Gurley value of
greater than about 1.5 minutes is an indication that the
pores are plugged.
The hydrophilic microporous films of this invention
find many varying uses. In particular, usefulness is found
in areas where the controlled passage of moisture through a
film or surface is desired. Furthermore, films made according
to this invention can be used as filter membrane supports or
filters useful in separating ultrafine materials from various
liquids and as battery separators.
The present invention is further illustrated by
the following examples. All parts and percentages in
the examples as well as in the specification and claims are
by weight unless otherwise specified.
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~152942 ~
EXAMPLE 1
PART A
This discussion illustrates the preparation of a
normally hydrophobic polyolefinic microporous film by the
"dry stretch" method as illustrated by U.s. Patent No.
3,801,404.
Crystalline polypropylene having a melt index of
0.7 and a density of 0.92 is melt extruded at 230C through
an 8 inch slit die of the coat hanger type using a 1 inch
extruder with a shallow metering screw. The length to
diameter ratio of the extruder barrel is 24/1. The extrudate
is drawn down very rapidly to a melt drawdown ratio of 150,
and contacted with a rotating casting roll maintained at
50C, and 0,75 inche6 from the lip of the die. The film
produced in this fashion is found to have the following
properties: thickness, 0.002 inches; recovery from 50
percent elongation at 25C, 50.3 percent; crystallinity,
59.6 percent.
A sample of this film is oven annealed with air
with a slight tension at 140C for about 30 minutes, removed
from the oven and allowed to cool.
The sample of the annealed elastic film is then
subjected to cold stretching and hot stretching at an extension
ratio of 0.50:1, and thereafter heat set under tension,
i.e., at constant length, at 145C for 10 minutes in air.
Th- cold drawing portion is conducted at 25C, the hot
drawing portion is conducted at 145C, and total draw is 100
percent, based on the original length of the elastic film.
The resulting film has an average pore size length of about
l~S29~2
3,000 ~ngstroms, a crystallinity of about 59.6%, and a
surface area of about 8.54m2/gm. The thickness of the
microporous film is 1 mil.
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~15299~2` ~
P~RT B
A continuous roll of the microporous film 100 feet in
length and 6 inches width, prepared in accordance with Part
A is then passed through a bath of glacial acrylic acid
~i.e. 100%) and then through a squeeze roll to obtain an
add-on of 1.5% after cure, by weight, based on the weight of
the film prior to impregnation as determined by I.R. analysis.
The impregnated film is then passed beneath the window of an
elongated electron beam generator (i.e., 24 inches in
length) at a line speed of 20 feet per minute. The election
beam generator is set to provide a dose of 3 megarads at the
line speed employed.
The oxygen content of the atmosphere below the
window of the curtain and in contact with the microporous
film is maintained below 500 ppm by enclosing the window in
a chamber purged with Nitrogen.
A sample of cured dry microporous film is then
tested for wettability by the drop test. The drop test is
conducted by placing a .6 ml. drop of 2% KOH in water on the
surface of the film. The film is then visually observed.
If thç portion of the film on which the drop is placed
becomes translucent and the opposite side of the film on
which the drop is placed appears wet the film is determined
to be wetted tas signified at Table 1 by the comment "yes").
The drop test is conducted on an off-line sample of film
immediately after irradiation, and on a sample which has
been stored for one week at room temperature.
Several other samples are taken from the cured
microporous film roll and tested for electrical resistance
in the manner described herein after soaking in a 40%, by
-46-
1152~4Z
weight, aqueous solution of KOH maintained at 60C for one
hour, 24 hours, 4 days and 8 days respectively and the results
averaged. The soaking of the film sample in hot KOH for
progressively longer times simulates aging over long periods
of time.
The surface area of èach sample tested for electrical
resistance is 0.2 in2, and the current employed is 40
milliamperes direct current.
Three samples of the cured microporous film roll
are tested for air flow permeability in accordance with
Gurly test ASTM-D-726B.
The results are summarized in chart form in Table I.
Samples of the cured microporous film roll are
also tested for water flow after soaking in a 31% aqueous
solution of KOH maintained at 60C for 24 hours by placing
each sample having a surface area of 11.3 cm2 in a millipore
filter housing. The millipore filter is filled with water
and pressurized to an atmosphere differential between both
surfaces of the film sample. Water is collected as it
passes through the microporous film over a 5 minute period.
The results are converted to cc's of water collected per
minute per cm2 of film surface.
The results are summarized in Table I.
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~i;294Z ~
COMPA2~TIVE EXAMPLE
Example 1 is repeated with the exception that the
radiation dose employed for curing is reduced to 1 megarad.
The results are summarized at Table 1. An uncoated microporous
film prepared in accordance with Example 1 is also exposed
to ionizing radiation and tested in the same manner as
described in Example 1 and serves as a Control. The results
are summarized at Table 1.
~ s may be seen from the data of Table I, a 1
megarad dose is believed to be too small to achieve effective
curing of a polypropylene microporous film as indicated by
the infinite electrical resistance of the microporous film
of the comparative Example. The positive wettability test
of the Comparative Example on the off l~ne sample is believed
to be due to residual acrylic acid which has not been chemi-
cally fixed by the irradiation. The negative wettability of
the film of this Comparative Example after one week is
believed to be due to loss of the acrylic acid over the
storage period of one week. This is confirmed by the infinite
electrical resistance of the film after 1 week storage.
It can also be seen that the electrical resistance
of the film of Example 1 increases after 4 days of exposure
to hot KOH and then drops slightly after 8 days of exposure.
It is believed that the drop after 8 days of aging may be
due to the complete conversion of the acrylic graft to its
salt form while after 4 days of aging the salt conversion is
only partially completed. Thus, complete conversion of the
acrylic graft to its salt form appears to lower electrical
resistance.
-48-
llS29~2 `~
TABLE I
. Comparative
Example 1 Example Control
Dose (megarads) 3 1 3
Line speed
(ft/min) 30 20 30
Wettability
(drop test)
Off lineyes yes ND
after 1 week yes no ND
Electrical
resistance
~milliohms/in2)
1 hour5.00 cx~ C~
24 hours6.87 ~Ya
4 days10.6
8 days 7.1
Gurley (air)
~Gurley-seconds) 9.1 ND 10
Water flow 2
(cc/min/cm ) 0.35 0 ND
ND = Not Determined
-
-49-
1152942 `
EXAMPLE 2
PART A
.
This discussion illustrates the preparation of a
polyethylene microporous film by the Usolvent stretch~
method.
Crystalline polyethylene having a melt index of
5.0; a weight average molecular weight of about 80,000, a
density of 0.960 gm/cc, and a molecular weight distribution
ratio of about 9.0 is prepared by the blown film extrusion
method to form a precursor film (3 mils thick) and allowed
to cool by quenching in air at 25C. A sample of the result-
ing precursor film i8 then immersed for a period of 1 minute
in trich~oroethylene at 70C and subsequently stretched,
while immersed in trichloroethylene maintained at a tempera-
ture of 70C, at a strain rate of 150%/min. to 4 times its
initial length li.e., 300% total stretch). The trichloro-
ethylene is then removed by evaporation and the sample is
stretched in the cross machine direction to a degree of
stretch of about 50% and allowed to dry in air in the stretched
state. Drying is carried out at 25C.
The resulting microporous film exhibits a crystal-
linity of about 60%, an average pore length of about 5000
Angstroms, and a surface area between about 10 and about 25
sq. m/gm.
-50-
~ ~152942 ~3
P~RT B
Several microporous film samples 1 mil in thickness
prepared in accordance with Part A are dipped in glacial
acrylic acid (100%) and the film becomes translucent. The
samples are permitted to dry until the film takes on a white
opaque appearance which appearance is indicative of the
appropriate amount of monomer coating capable of achieving
an add-on of about 1.5%. The resulting samples are then
passed beneath an elongated electron beam curtain (i.e. 24
inches length) and irradiated with varying doses of radiation
while maintaining the atmosphere under the window and in
contact with the microporous film at less than 500 ppm of
oxygen in the manner employed in Example 1. The resulting
cured films are tested for Gurly air flow, electrical resis-
tance (after submersion for 24 hours in a 40% ROH solution
at 60C) and wettability by the drop test in the manner
described in Example 1. The results are summarized at Table
II as runs 1 to 8. The percent add-on of resulting cured
film samples is also determined by infrared analysis and the
results shown at Table II.
As may be seen from the results of Table I runs 4
and 7, the electrical resistance is infinite and the
samples fail to wet. These results are believed to be
attributable to the use of too low a radiation dosage to
achieve chemical fixation. Consequently, it is believed
that the hydrophilic monomer evaporates as evidenced by the
lack of any measurable add-on with a resultant loss of
properties.
--51--
~5Z94:~
The film samples of runs 1, 2, 5, 6, and 8 exhibit
low electrical resistance and are wettable. The higher
electrical resistance of the film sample of run 3 is be-
lieved to be due to a visually observed inhomogeneity in the
film. Moreover, these film samples wherein chemical fixa-
tion occurs, as indicated by a~1.5~ add-on, exhibit only a
slight drop in air permeability. This indicates that the
pores remain unplugged after chemical fixation.
~ 1152942 ~
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152942 -~
The principles, preferred embodiments and modes of
operation of the present invention have been described in
the foregoing specification. The invention which is intended
to be protected herein, however, is not to be construed as
limited to the particular forms disclosed. Variations and
changes may be made by those skilled in the art without
departing from the spirit of ~he present invention.
~ -54-
