DD153884A5 - MICROPOROESIC FILM WITH IMPROVED WATER THERMALITY AND / OR REDUCED ELECTRICAL RESISTANCE - Google Patents

Abstract

D. Invent. relates to a hydrophilic open-cell, microporous film with improved water permeability and / or reduced electrical resistance. The film of the present invention has: a) an open-celled normally hydrophobic microporous film having a lower bulk density, a mean pore size of about 200 to about 10,000 angstroms, and a surface area of at least 10 meters high, compared to the bulk density of a precursor film / g, and b) has a coating on the surface of the micropores of the microporous film of at least one hydrophilic organic hydrocarbon monomer having at least about 2 to about 18 carbon atoms and the like. at least one double bond having at least one polar functional group, said hydrophilic organic hydrocarbon monomer coating being chemically deposited on the surface of the microporous film micropores by exposing ionizing radiation of from about 1 to about 10 megarads and to an extent sufficient; open-celled nature d. microporous film and an addition of about 0.1 to about 10 wt .-%, based on the weight of the uncoated microporous film to obtain, is fixed. The film may e.g. serve as a battery separator.

Description

Microporous film with improved water permeability and / or reduced electrical resistance

Field of application of the invention

The invention relates to a microporous film having improved water permeability and / or reduced electrical resistance.

Characteristic of the known technical solutions

Recent development work in the field of open-cell microporous polymeric films, such as US Pat. Nos. 3,839,516, 3,801,404, 3,679,538, 3,558,764 and US Pat. 3,426,754 have been concerned with finding applications that can utilize the unique properties of these new films. Such films, which ultimately form a gas-breathing water barrier, can be used as deaerating agents, as gas-liquid transfer agents, as battery separators, and for a variety of other purposes.

A disadvantage of these films _7, which in the past has limited the number of uses of these films, is their hydrophobic nature. This is especially true when 'polyolefin films, a preferred form

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often used in the manufacture of microporous films. Since these films are not "wetted" with water and aqueous solutions, they advantageously can not be used in such logical applications as filaments, electrochemical separator components, and the like. , ,

Various proposals have been made in the past to overcome these problems, such as e.g. in U.S. Patents 3,853,601, 3,231,530 ,. 3-215,486 and Canadian Patent 981,991. Various hydrophilic coating agents or impregnating agents are used. Such coating or impregnating agents, although effective for a limited period of time, tend to be removed in a relatively short time from solutions which come in contact with the films or fibers in which they are present.

Other attempts have been made to impart hydrophilic behavior to a normally hydrophobic microporous film by treatment with a low energy plasma In such plasma treatments, surface areas of the microporous film are first activated by means of an argon or hydrogen plasma and then The plasma treatments result in films that only half a surface that is rewettable, and the surface of the film also clogs when wetted, thereby allowing the free flow of water through the interior the film is hindered or even prevented.

The inevitable clogging of the. Surface pore makes the film unsuitable for a variety of applications,

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increases the electrical resistance of the film and reduces the dimensional stability of the film, which in particular proves by shrinkage during drying.

As mentioned above, a disadvantage of the plasma treatment is its limited ability to wet only the surface of the microporous film. close. It has been found that because of the unusually large surface area of a microporous film of the type described above, a mere surface wettability can not ensure that the film has certain functional properties, such as low electrical resistance, and water transmission rates through the film that are known in the art surface systems described above, has.

It is believed that the major disadvantage of the plasma treatments, namely clogging of the pores and wettability of only the surface, is due to a combination of factors, such as the tendency of the low energy plasma to be deactivated by the large surface area of the microporous film. This reduces the likelihood that an internal region within the microporous film will be activated. In similar. There is a rivalry between graftable monomers exerted by the free radical polymer grafts initially generated at the surface of the film when the graftable monomer first contacts the plasma activated microporous film surface. The graft polymer chains, initially present on the film surface, propagate at a rate which increases as the reaction progresses. As a result, the resulting elongated graft polymer chains formed on the surface of the film clog the micropores on the surface in the presence of water.

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Other attempts have been made to obtain hydrophilic films by treatment with plasma, as illustrated by U.S. Patents 3,992,495 and 4,046,843. ,

Another technique for making polyethylene films wettable and suitable for use in separators for electric batteries is described by the publication of V. D'Agostino and J. Lee, "Manufacturing Methods For High Performance Grafted Polyethylene Battery Separators", U.S. Pat. National Technical Information Service, A.D. Report No. 745,571 (1972), summarized in 78 Chemical Abstract 5031f (1973), by the publication of V. D'Agostino and J. Lee "Low Temperature Alkaline Battery Separators", 27 Power Sources Symp. 87-91 (1976 ) And summarized by the publication by V. D'Agostino, J. Lee and G. Orban of "Zinc-Silver Oxide Batteries" (A. Fleischer and J. Lander ed., 1971).

These articles discuss or refer to exn commercially available product known as PERMION · ¹ · ¹¹ and available from RAI Research Corp. has been developed. Briefly, the process of making these fabrics is by cross-linking a 2.54. 10 ~ - ^ cm polyethylene sheet under beta irradiation and subsequent grafting with methacrylic acid in a suitable solution under Co60 gamma irradiation. The grafted material is then washed to remove the homopolymer, then salified in hot KOH, washed anew to remove the remaining base, dried and finally packed The initial crosslinking step creates microcracks or longitudinal slits in the non-porous Polyethylene film that are so small that they are not visible even under an electron microscope

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Microcracks is set to about 20 angstroms (1O ^- CC) estimated. The microcracks or breaks are then grafted with the methacrylic acid. The resulting film is not microporous in the sense of microporous films of the present invention having an average size of from about 100 to about 5000 angstroms. The extreme

PBRMION ™ small size microbreaks generally prohibit mass transfer of mobile electron-bearing species formed by the oxidation-reduction reactions through the film at a significant rate. This causes the relatively large resistance (e.g., 4.6 to 6.2 milliohm. Per cm ² ) of the films of this type. In addition, the PERMION ™ does not dimensionally in more than one direction as evidenced by appreciable swelling is stable.

It is known that nonporous polymeric substrates such as polyethylene and polypropylene can be reacted with various monomers such as acrylic acid using various types of ionizing radiation as described by U.S. Patents 2,999,056, 3,281,263, 3O72,100 and U.S. Pat 3,709,718 is illustrated. However, since none of these patents relates to microporous films, they do not address the particular problems underlying the invention.

As a result, the search for a relatively permanently wettable hydrophilic microporous film has proceeded with low electrical resistance and better water flow rates through the film. The present invention has been developed in the course of this search.

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Aim of the invention; '·

The aim of the invention is to broaden their field of application and possible uses.

presentation; the essence of the invention

It is therefore the object of the invention to propose a method with which a normally hydrophobic microporous film can be rendered relatively permanently hydrophilic, so that its water permeability becomes better.

Another object of the invention is to propose a method by which the electrical resistance of a normally hydrophobic microporous film can be reduced.

It is another object of the present invention to provide a hydrophilic microporous film having reduced electrical resistance.

The invention further seeks to overcome the problems of the prior art described above.

The invention proposes a method by which a normally hydrophobic microporous polyolefin film can be made hydrophilic and thereby improve the water flow velocity through this film and reduce the electrical resistance of the film. This method is characterized by:

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A) Covering the surface of the micropores of a normally hydrophobic, open-cell, microporous polyolefin film having a bulk density less than that of a precursor film which has been assumed to have an average pore size of from about 200 to about 10,000

Having at least about 10 meters per gram of Angstrom and having at least one hydrophilic organic hydrocarbon monomer having from about 2 to about 18 carbon atoms and at least one double bond and at least one polar functional group? and

b) chemical fixing on the surface of the micropores of the microporous film of an amount of said hydrophilic organic hydrocarbon monomer sufficient to obtain the open-cell nature of said micropores and sufficient to provide from about 0.1 to about 10 wt. %, based on the weight of the uncovered microporous film, by irradiating the covered microporous film with an ionizing radiation of about 1 to about 10 megarads.

The invention also proposes a hydrophilic, open-cell, microporous film which has:

A) an open-celled, normally hydrophobic, microporous film having a lower bulk density, a mean pore size of about 200 to about 10,000 angstroms, and a surface area of about 20 to about 15,000, as compared to the bulk density of a precursor film

has at least 10 m per gram, and "

b) a coating on the surface of the micropores of the microporous film of at least one hydrophilic organic carbon monomer having at least about 2

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5a 109/16

to about 18 carbon atoms and at least one double bond and at least one polar puncture group, said hydrophilic organic hydrocarbon monomer coating being chemically deposited on the surface of the micropores of the microporous film by exposing an ionizing radiation of from about 1 to about 10 megarads, and to an extent sufficient open-celled nature of the microporous film and a more of about 0.1 to about 10% by weight, based on the weight of the uncoated microporous film, is fixed. ,

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The essence of the present invention is based on the discovery that open cell microporous films can be rendered relatively durably wettable and / or hydrophilic if their pores chemically react with a controlled amount of a graftable hydrophilic monomer by exposure to ionizing radiation while maintaining the open cell nature of the microporous film fixed. Such chemical fixation of the hydrophilic monomer takes place. also in the pores, which are located at an internal location in the microporous film. It is believed that by controlling chemically on the surface of the microporous fixed amount of the monomer, the polymer chain grafting effected by exposure of the radiation can be aligned with the area of the pores. As a result, clogging or constriction of the pores does not occur _; on.

Ausfiihrungsbeispiele

As already stated, the invention relates to a hydrophilic microporous film and a method for its production.

Porous or cellular films can be classified into two major classes: a class in which the pores are not interconnected (it is a closed-cell film) and the other class in which the pores are essentially interconnected by tortuous paths Which can extend from one outer surface or surface zone to the other (it is now an open-celled film). The porous films according to the invention belong to the latter class.

Further, the pores of the porous films according to the present invention are microscopic, that is, the details

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their pore structure are recognizable only by a microscopic examination. The open-cell pores in the films are generally smaller than those that can be measured with conventional light microscopes because the wavelength of visible light, which is in the range of 5000 angstroms (one angstrom is one ten-billionth meter), is larger than the longest planes - or 'surface dimension of the open cell or pore. However, the microporous films according to the present invention can be identified by use of the electron microscope technique capable of resolving pore composition details below 5000 angstroms.

The microporous films according to the present invention are characterized by a reduced bulk density, which will hereinafter sometimes be referred to simply as "low" density. This implies that these microporous films have a bulk density lower than the bulk density of corresponding films constructed of identical polymeric material but lacking an open cell or other empty structure. The term "bulk density" as used herein means the weight per unit gross or geometric volume of the film. The gross volume is determined by immersing a known weight of the film in a kettle partially filled with mercury at 25 ° C and atmospheric pressure. The increase in volume of the mercury level is a direct measure of the gross volume. This method is known as the mercury volume measurement method and is described in "Encyclopedia of Chemical Technology", Vol. 4, page 892 (Interscience 1949). ,

The production of porous films with a microporous, open-cell structure and with a reduced

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Bulk density is already known. Films having such a microporous construction are described, for example, in U.S. Patent 3,426,754, which is assigned to the assignee of the present invention and incorporated herein by reference. The preferred manufacturing method described therein contemplates pulling or stretching, ie, "cold drawing", a crystalline, elastic precursor film at ambient temperature by an amount of from about 10 to about 300 % of its original crack length _. and then subsequently to stabilize the drawn film at such a tension by heat fixing that the film can not or only to a limited extent shrink. Other methods of making microporous films are described, for example, in U.S. Patents 3,558,764, 3,843,762 and 3,920,785, and British Patents 1,180,066 and 1,198,695. These patents are hereby incorporated by reference in the present application. ·

While all of the foregoing patents describe methods of making normally hydrophobic microporous films that can be made hydrophilic in accordance with the present invention, the preferred normally hydrophobic microporous films are prepared according to the method of preparing microporous films described in U.S. Patent 3,801,404, US Pat. which is referred to herein as a "dry draw" process, and according to U.S. Patent 3,839,516, which proposes a process for producing microporous films, referred to herein as a "solvent draw" process. Both methods are incorporated here by mentioning these documents. Each of these patents disclose preferred methods of forming a normally hydrophobic microporous film by treating a precursor film in a process step that is specifically defined.

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The preferred precursor films that can be used to make microporous films according to the "dry-stretch" method and the "solvent-stretch" method are described in detail in each of the aforementioned patents. The "dry stretching" ~ "ren procedural uses a non-porous crystalline elastic polymer film having a recovery time to the elastic return to the zero state (defined below) after contact with a geeichnten strain or deformation (elongation) of 50% at 25 ⁰ C and 65 % relative humidity of at least 40 % , preferably at least about 50 %, and most preferably at least about 80%.

The elastic recovery as used herein is a measure of the property of the structure or of the formed article, such as a film, to return to its original size after stretching and can be calculated as follows:

Elastic recovery (ER) %

Length in the stretched __. Length after condition ~ stretch . _nn

Added length in stretched condition

Although a calibrated load or elongation of 50% is used to define the elastic properties of the films used as the starting material, this stress should be considered as an example only. In general, the output movies elastic recovery times that are great at break of less than 50% and at break of more than 50% smaller, always compared to their elastischen'Erholzeit at an elongation of 50% have seen ..

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These elastic films used as the starting material have a percent crystallinity of at least 20 %, preferably at least 30 % and most preferably at least 50 %, for example from 50 to 90 % or more. Percent crystallinity is determined by the X-ray method described by RG Guynn et al in "Journal of Applied Polymer Science", Vol. 2, Wr. 5, pp. 166-173 (1959). is described. For a more detailed discussion of crystallinity and its importance in polymers, reference is made to "Polymers and Resins", Golding (D Van Nostrand, 1959).

Other elastic films which may be considered suitable for the preparation of precursor films and which are used in the "dry draw" process are described in British Patent. 1,052,550 published December 21, 1966.

The elastic precursor film used to make the microporous films by the "dry stretch" method should be distinguished from films formed from classic elastomers such as natural rubbers and synthetic rubbers. In such classical or conventional elastomers, the stress-strain behavior, and in particular the stress-temperature relationship, is determined by the entropy mechanism in the deformation (rubber elasticity). The positive temperature coefficient of the retraction force, i. decreasing load with decreasing temperature and complete loss of elastic properties at <ler. Glass transition temperature are the particular consequences of entropy elasticity. The elasticity of the elastic precursor films used herein is otherwise different. According to qualitative thermodynamic experiments with these elastic precursor films one can choose

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the "decreasing temperature" (negative temperature coefficient) stress that the elasticity of these materials is determined not by entropy effects but by those that depend on an energy term It has been found, in particular, that the "dry stretch" precursor elastic films contribute their stretch properties As a result, the stretching mechanism of the "dry-stretch" precursor elastic films is believed to rely on energy-elasticity relationships, which may therefore be termed "non-classical" or "non-conventional" elastomers The "solvent stretch" process uses a precursor film which must have at least two components, namely an amorphous component and a _< crystalline component Crystalline substances that are inherently two-component are therefore well-suited for this process . The degree 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. , · '

The polymers, e.g. Synthetic resinous materials constituting the precursor films used in each process in the present invention include olefin polymers such as polyethylene, polypropylene, poly-3-methylbutene-1, poly-4-methylpentene-1, and copolymers Propylene, 3-methylbutene-1, 4-methylpentene-1, or it '.

Ethylene, with each other or with small amounts of others

Olefins such as copolymers of propylene and ethylene, copolymers having a major proportion of 3-methylbutene-1 and a minor proportion of straight-chain n-alkenes such as n-1-octenes, n-hexadecene-T, n-octadecene-1, or others relatively long-chain alkenes, as well as copolymers of 3-methylpentene-1 and any of the n-alkenes already mentioned above in connection with 3-methylbutene-1.

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For example, in general, when propylene homopolymers are used for the "dry stretch" or "reverse" process, an isotatic polypropylene having a percent crystallinity as stated above having a weight average molecular weight in the range of about 100,000 to about 750,000 (eg, from 200,000 to 500,000) and a melt index (ASTM-D-1238-57T, Part 9, Page 38) of from about 0.1 to about 75 (eg, from about 0.5 to .30) can be used to obtain a to obtain a finished film product having the required physical properties.

It should be noted that the terms "olefinic polymer" and "olefin polymer" are interchangeable and describe a polymer made by polymerizing olefin monomers by their non-saturation.

Polymers preferred for the "solvent draw" process are those polymers used in the context of the invention described in U.S. Patent Application No. 44,805, issued January 6, 1979 to John-¥. Soehngen has been filed and owned by the assignee of the present invention. This patent application is entitled "Improved Solvent Stretch Process for Preparing Microporous Films from Precursor Films of Controlled Crystalline Structure". The text of this application is herewith included in the scope of the present application. As a result, a polyethylene homopolymer having a density of about 0.960 to about 0.965 g / cm, a hc-melt index of not less than about 3, and preferably from about 3 to about 20, and a broad molecular weight distribution ratio (M _w / M _n ) of not less than about 3.8, and preferably from about 3.8 to about 13 for the manufacture of a microporous film after the ^second stretching process preferably solvents ". Further, nucleating agents may in active

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the polymer used to make the precursor film as described in the Soehngen application incorporated in the present application. In this case, polymers with a melt index of less than 0.3 can be used.

The devices suitable for making the precursor films are well known in the art.

For example, a conventional film extruder with a flat channel measuring screw and a garment extruder mold is satisfactory. In general, the resin is introduced into a hopper of the extruder, which contains a screw and a jacket with heating elements. The resin is melted and conveyed by the screw to the mold, from which it is injection-molded through a slot in the form of a film. Thereafter, it is withdrawn by a take-off or form roller. More than one pickup roll in different combinations or levels can be used. The die or extrusion die opening or slot width may be in the range of 0.254-mm to 5.8 inches. -

Using this type of device, the film may be extruded at a draw ratio of about 5: 1 to 200: 1, preferably 10: 1 to 50: 1.

As used herein, the term "draw ratio" or simple "draw ratio" is the ratio of the film take-up or take-off rate to the speed of the film at the exit of the extrusion die or die.

The melting temperature for the Filrnstrangpressen is generally not higher than about -100 ⁰ C above the melting point of the .Polymers and not less than about 10 ⁰ C above the melting point of the polymer.

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For example, polypropylene may be at a melt temperature of about 180 ⁰ C to 270 ⁰ C, preferably from 200 ⁰ C to 240 ⁰ C, are extruded. · Polyethylene can be extruded at a melt temperature of about 175 ° C to 225 ⁰ C ·. ·

When the precursor film is used in the context of the "dry drawing" process, the extrusion step is preferably carried out with rapid cooling and a rapid draw to obtain maximum elasticity. This can be accomplished by placing the take-off roll relatively close to the extrusion die, eg at a distance within 5.08 cm, and preferably at a distance of 2.54. install. An "air knife" operating at temperatures between, for example, ⁰ ° C and 40 ° C can be used within a 2.54 cm distance from the nip to quench the film, ie, cool and solidify quickly. The pickup roller can, for example rotate at a speed of 3 ₅ 04-304 m / min, preferably from 15} 24 to 152.4 m / min.

When the precursor film is to be used in conjunction with the "solvent stretch" process, the extrusion step is preferably carried out with slow cooling to minimize stresses and associated alignment disturbances from rapid quenching to maximum retention Crystallinity could result. However, the cooling must be fast enough to prevent the formation of large spherical bodies.

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prevent it from happening. This can be achieved by monitoring the distance of Kühlröllenabhahme from the extrusion slot.

While the foregoing description has been directed to extrusion molding processes using extrusion dies, it is also within the scope of the present invention

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it is possible to use, as an "alternative method of making the precursor films, the blown film extrusion process using a hopper and an extruder substantially the same as those used in the prior art extruder described above.

From the extruder, the melt enters a. Extruder die from which it passes through a circular slot to form a tubular film having an initial inside diameter D ,. is injected. Air entering the system through an inlet leading into the interior of this tubular film causes the inflating of the diameter of the tubular film to a diameter D0. Means such as air rings may also be used to direct the air to the exterior of the extruded tubular film Means, such as a cooling mandrel, may be used to cool the interior of the tubular film, and following a distance in which the film is allowed to fully cool and harden, it is wound on a take-off roll.

In the extrusion blow molding process for the production of films, the draw ratio is preferably 5: 1 to 100: "1. The Schiit ζ opening is 0.254 to 5.08 mm, preferably 1.016 to 2.54 mm. The Dp / D ^ ratio is for example, in the range of 1.0 to 4.0, and preferably from about 1.0 to 2.5, while the rate of decrease is, for example, in the range of 9.144 to 213 ₅ 36 m / min: The melt spinning temperature or melting temperature may be in the ranges lie, which have been stated in the foregoing on the occasion of slit extrusion.

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The extruded film may then be first heat treated or annealed to improve the crystal structure, eg, by increasing the size of the crystal bodies and removing impurities therein. In general, the annealing at a temperature in the range of about 5 ° C 'to 100 ⁰ C below the melting point of the polymer for a period of several seconds to several hours, for example 5 seconds · to 24 hours and preferably from about 30 seconds to 2 Hours, performed. For polypropylene, the preferred annealing temperature is in the range of about 100 to 155 ° C.

An exemplary method of performing the annealing is then obtained when the extruded film is subjected in a tensioned or de-energized state to an oven of the particular temperature. In this case, the retention time is preferably in the range of 30 seconds to 1 hour. ,

In the preferred embodiments, the formed, partially crystalline precursor film is preferably subjected to one or two alternative procedures described above to obtain a normally hydrophobic microporous film which may be used in the context of the present invention.

The first preferred procedure, as described in U.S. Patent 3,801,404, referred to herein as a "dry-stretch" method, involves the precursor steps of cold stretching, i. cold drawing of the elastic film. until porous surface areas or zones elongated in the normal direction or transverse to the stretching direction, of hot stretching, i. Thermoforming of the cold stretched film until the formation of fibers and pores or open cells which are parallel to the direction of stretching, and thereafter heating or

tit -

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Heat-curing the formed porous films under tension, i. at a substantially constant length to make the film stable. ·

The term "cold heading" as used herein is defined as stretching or drawing a film beyond its initial length and at a stretching temperature (the temperature of the film being stretched) below the temperature at which melting begins, when the film is uniformly heated from a temperature of 25 ° C and heated at a rate of 20 ⁰ C per minute. The term "Warrastrecken" ,, as used herein is defined as stretching or elongation above the temperature at which melting begins when the film uniformly νδη a temperature of 25 ° C starting and at a Geschwindigtkeit of 20 ⁰ C, is heated per minute, but below the normal melting point of the polymer, ie below the temperature at which the fusion takes place. It is known to those skilled in the art that the temperature at which melting begins and the fusion temperature can be determined using a standard differential heat analyzer (DTA) or any other suitable device that can detect heat transfer of a polymer.

The "temperature at which melting begins will vary with the nature of the polymer, the molecular weight distribution of the polymer, and the crystalline structure of the film." For example, a polypropylene elastic film may be at a temperature of less than about 120 ^° C, preferably between about 10 ⁰ C and 70 ⁰ C and are best, cold-stretched at ambient temperature, eg at 25 ° C. the cold stretched polypropylene film can then at a temperature above about 120 ⁰ C and below the fusion or fusion temperature, and preferably between about 130 ⁰ C and 150 ⁰ C are hot stretched. The temperature-

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The temperature of the film itself, in which this is stretched, is referred to here as "stretching temperature". The stretching in these two steps or stages must be carried out successively, ie in the same direction and in this order, ie first cold and then warm. However, stretching may also be accomplished in a continuous or semi-continuous or batch process, as long as the cold-stretched film is prevented from appreciably shrinking, eg, more than 5 % of its cold-stretched. Length before being warmed up.

The total stretch amount achieved in the two steps may be in the range of about 10 to 300 % and preferably 50 to 100 % based on the original length of the elastic film. Further, the ratio of the degree of hot stretching to the total degree of stretching may range from greater than about 0.10: 1 to less than 0.99: 1> preferably in the range of from 0.50: 1 to 0.97: 1 and by Preferably, from about 0.50: 1 to 0.95: 1 »lie. The relationship between the "KaIt" and "Warm" ranges is referred to herein as "expansion ratio" x '(percent "warm" extension to percent "total" extension).

In each stretching step in which heat must be supplied, the film may be heated by moving rollers, which in turn are heated by electrical resistance, by running over a heated plate, by a heated liquid, by heated gas or the like. '

After the above two stages or stretching steps, the stretched film is heat-cured. This heat treatment is carried out at a temperature in the range of approximately 125 ° C up to a temperature below the fusion temperature.

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temperature "and preferably polypropylene 130 ⁰ C to 16O ° C, said from about 75 ° C up to a temperature below the fusion temperature of polyethylene ;, but preferably in the range of 115 ° C to 13Q ⁰ C, and in other in the foregoing performed polymers at similar temperature ranges. This heat treatment should be carried out while the film is held under tension, DH SQ ₅ that the film can not shrink or only to a controlled, not more than 15% of its stretched · length can shrink amount forming extent. However, the tension should not be so great that the film is stretched by more than an additional 15 % Preferably, the tension should be chosen so that there is essentially no shrinkage or

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Stretching occurs, that is, "the change in the stretched length is less than 5%.

The period of heat treatment, which is preferably performed sequentially with and after the drawing operation, should not last longer than 0.1 second at the high annealing temperatures, and generally within the range of about 5 seconds to 1 hour, and preferably from 1 to 30 Minutes are.

The steps of heat-curing or fixing described above may be carried out in air, in other atmospheres such as nitrogen, helium or argon.

A second preferred alternative approach to converting the precursor film into a microporous film, described in US Pat. No. 3,839,516, referred to herein as the "solvent stretch" process, provides as a basic operation (firstly) the precursor film containing at least has two components (an amorphous component and a crystalline component) of

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one being smaller in volume than the other in contact with a swelling agent for a period of time which allows the swelling agent to be incorporated into the film, and (second) to stretch or stretch the film in at least one direction while with the swelling agent in contact, and (third) to hold the film in this stretched state during the removal of the swelling agent. If desired, the film can be stabilized under tension by heat curing or by ionizing radiation.

In general, a solvent having a Hildebrand solubility parameter equal to or close to that of the polymer will have suitable solubility for the draw process described herein. The Hildebrarid solubility parameter measures the cohesive energy density. The underlying principle is based on the fact that a solvent with a similar cohesive energy density as a polymer has a high affinity for this polymer and is suitable for this process.

General classes of swelling agents, one of which may be chosen for the particular polymeric film, are lower aliphatic ketones such as acetone, methyl ethyl ketone, cyclohexanone; lower aliphatic ester acids

such as "ethyl formate, butyl acetate, etc., halogenated hydrocarbons such as carbon tetrachloride, trichlorethylene, perchlorethylene, chlorobenzene, etc., hydrocarbons such as heptane, cyclohexane, benzene, xylene, tetralin, decalin, etc., nitrogen-containing organic

"Compounds such as pyridine, formamide, dimethylformamide, ti rt

etc. ; Athere v / ie methyl ether, ethyl ether, dioxane, u.s.v /. A mixture of two or more of these organic solvents may also be used.

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The swelling agents may preferably be compounds consisting of carbon, hydrogen, oxygen, nitrogen, halogen, sulfur and containing up to about 20, preferably up to about 10, carbon atoms.

The "solvent stretching" step may be conducted at a temperature ranging from above the freezing point of the solvent or swelling agent to a point below the temperature at which the polymer dissolves (ie, at ambient temperature to about 50 ^° C).

The precursor film used in the "solvent draw" process may have a thickness in the range of 0.0025 mm to about 0.5 mm or more. ,

In a preferred embodiment, the precursor film is biaxially stretched in the manner described in U.S. Patent Application No. 44,801 filed January 6, 1979, entitled "Improved Solvent Stretching Process for Preparing Microporous Films." This application has been assigned to the assignee of the present invention and is incorporated by reference into the present application This method demonstrates the preferred stretch conditions for single-direction stretching which results in improved permeability of the uniaxially stretched microporous film uniaxially stretched microporous film may then be stretched transversely to further improve permeability, Preferably, the precursor film will be in a uniaxial direction by not more than 350 % _m , most preferably not more than 300 ° / ί , based on its original length , in solvent g stretched.

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Typically, additional stretching in the same direction after removal of the solvent is not undertaken.

The optimum stabilization step may be either a heat-hardening step or a cross-linking step. This heat treatment may be performed up to 150 ⁰ C for polypropylene, at a temperature in the range of about 125 ^D C up to a temperature below the fusion temperature and .vorzugsweise in a range of about 13O ° C. In this polyethylene range of about 75 ⁰ C can range up to a temperature below the fusion temperature and preferably from about 115 ° C to 130 ⁰ C. Other above-mentioned polymers have similar temperature ranges. This heat treatment may be carried out while maintaining the film under tension, ie, such that the film is not free to shrink, or shrink only to a certain and controlled extent, which does not exceed more than 15% of its stretched length. However, the tension should not be so great that the film is stretched by more than an additional 15 % . Preferably, the tension is chosen so that substantially no shrinkage

ti

or stretches, ie that only changes in the stretched length less than 5 % are allowed.

The duration of the heat treatment, which is preferably carried out successively with or after the "solvent stretch" step, should not be longer than. For at least 0.1 second at the higher annealing temperatures and generally within the range of about 5 seconds to 1 hour and preferably from about 1 to 30 minutes.

The heat-hardening steps described above can be used in air, in other atmospheres such as nitrogen _? Helium or argon can be performed.

223 7 27

If the precursor film has been stretched in two axes, the stabilizing step may be performed after stretching in the transverse direction and not beforehand.

However, while the present specification and examples are directed primarily to the aforementioned olefin polymers, the invention also relates to high molecular weight acetals, e.g. Oxymethylene, polymers. Although both acetal homopolymers and copolymers are concerned, the preferred acetal polymer in polymer stability is a "random" oxymethylene copolymer containing recurring oxymethylene moieties such as -CH 2 -O-, between which are -OR groups in the main polymer chain, where R is a Diradical with at least two carbon atoms linked together and in-chain between two valencies, with any substitution for the R radical that is in R, ie which has no intervening functional groups and does not undergo unwanted reactions, and wherein a greater proportion of the -OR groups are present as Einzaleinheiten, which are connected to the oxymethylene groups on each side. Examples or preferred polymers include copolymers of tri-

oxane and cyclic ether which contain at least two adjacent carbon atoms, such as the copolymers described in U.S. Patent 3,227,352 to Walling et al. These polymers in film form may also have a crystallinity of at least 20 % , preferably at least 30 %, and most preferably at least 50 % , eg 50 to 60 % or more. Further, these polymers have a melting point of at least 150 ⁰ C and a number average molecular weight of at least 10,000. For a more detailed discussion of acetal and oxymethylene polymers, reference is made to "Formaldehyde", Walter, pages 175-191 (Reinhold 1964).

    - S6 -

223 727

Other relatively crystalline polymers to which the invention can 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 polycaprolactam (nylon 6), which are all well known and therefore no longer be described here '.

The normally micro-hydrophobic microporous films used in the present invention have, in a de-energized state, a bulk density which is less than the density of corresponding polymeric materials having no open-cell structure, eg, the materials from which the films have been formed. As a result, the films have a bulk density no greater than about 95 % and preferably 20 to 40 % relative to the precursor films. In other words, the bulk density is smaller by at least 5 % and preferably 60 to 80 % . For polyethylene, the reduction is 30 to 80 % , preferably 60 to 80 %. The bulk density is about 20 to 40 % of the starting material and the porosity has increased by 60 to 80 % because of the holes or pores.

When the microporous films are prepared by the "dye draw" method or the "solvent draw" method, the final crystallinity of the microporous film is preferably at least 30 %, preferably at least 65 % and most preferably at 70-85 %. These values can be determined by the X-ray method described by RG Quynn et al in "Journal of Applied Polymer Science", Vol. 2, No. 5, pp. 166-173. For a more detailed discussion of crystallinity and its importance in polymers, reference is made to "Polymers and Resins", Golding (S Van Nostrand, 1959).

223 727

The microporous films that can be used in the present invention may have an average pore size of about 200 to 10,000 angstroms, typically about 400 to 5,000 angstroms, and more preferably about 500 to about 5,000 angstroms. These values can be determined by the mercury porosimetry method described in an article by R.G, Guynn et al, pages 21-34 of "Textile Research Journal", January 1963? or by using electron microscopy as described in "Polymer Single Crystals" by Geil, page 69 (Interscience 1963). When an electron micrograph is used, pore length and width measurements can be made by simply using a rule The length and width of the pores can be measured directly on an electron micrograph, which has usually been recorded at 5,000 to 10,000 times magnification. In general, the pore length values obtainable by electron microscopy are approximately equal to the pore values can be obtained with mercury porosimetry.

The microporous films used in the present invention have a surface area within certain predictable limits when prepared by the "solvent draw" process or the "dry draw" process.

It has been found that typically such micro-

p porous films have a surface area of at least 10 meters per meter

ρ grams, preferably in the range of about 15 to 25 meters per gram. For films formed from polyethylene, the surface has a value that

generally in the range of about 10 to 25 meters per

And preferably 20 microns per gram.

The surface can be prepared using nitrogen or krypton gas absorption isotherms using a

223 7 27

and described by U.S. Patent 3,262,319 ", a surface which can be determined by this method is usually expressed in m / g.

To facilitate the comparison of different substances, this value is multiplied by the bulk density of the substance in g / cm leading to a surface expressed in m / cnr. ,

The normally hydrophobic microporous polymeric films of the present invention which have been rendered hydrophilic have a preferred thickness of. about 0.025 mm to about 0.2 mm.

The normally hydrophobic microporous film prepared according to the methods described above is rendered wettable and / or hydrophilic by coating or covering the surface of the micropores of the microporous film with a hydrophilic monomer or mixtures thereof and then exposing the coated microporous films to ionizing radiation. The ionizing radiation chemically fixes the hydrophilic monomer on the microporous surface and makes it relatively hydrophilic for a relatively long time. The addition of hydrophilic monomer which has been chemically fixed is monitored within certain limits to avoid clogging of the pores, while at the same time permitting control of the overall porosity of the hydrophilic film. The control

I!

or monitoring the porosity in turn allows control of the water permeability and electrical resistance of the film.

X The term "hydrophobic" as used herein refers to a surface that transmits less than 0.010 milliliters of water per minute and per square centimeter of flat film surface at a water pressure of 7 kg / cm. The term

223 727

"hydrophilic" refers to those surfaces through which more than 0.1 milliliter of water per minute and per square meter can pass at the same pressure.

Il

The hydrophobic monomers that can be used as a coating for the pore surface of the microporous film according to the present invention are organic water off compounds having 2 to 18 'hydrocarbon atoms and having at least one double bond which polymerizes the monomers and / or copolymerizable under the Influence of ionizing radiation at a temperature that does not adversely affect the microporous film, and having at least one polar functional group such as carboxyl, sulfo, sulfino hydroxyl, ammonium, amino and phosphonium.

As a result, the hydrophilic monomers of the present invention include hydrocarbon compounds having from 2 to 18 carbon atoms, having one or more polymerizable and / or copolymerizable double bonds. Preferred substituted or unsubstituted carboxylic or dicarboxylic acids and esters thereof; Vinyl and allyl monomers, in particular 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, vinylsulfonic acid. and styrenesulfonic acid; Vinyl esters like vinyl. acetates and higher carboxylic acid vinyl esters, alkyl-substituted vinyl esters of carboxylic acids with sulfone groups; Vinyl ethers such as unsubstituted or substituted alkyl, cycloalkyl or aryl vinyl ether; vinyl-substituted silicon; vinyl-substituted aromatic or heterocyclic Kdilaiwasserstoffatome; Diallylfuraarates, diallyl maleates; alkyl-substituted phosphates, phosphites or carbonates;

223 7 27

Vinylsulfones, the reaction products of ethoxylated monylphenol and acrylic acid, and the like.

Since it is intended that the hydrophilic monomers penetrate into the interior of the microporous film, it is preferable to use hydrophilic monomers having about 2 to 14, preferably about 2 to 4, hydrocarbon atoms.

The preferred hydrophilic monomers used according to the invention are acrylic acid, methacrylic acid and vinyl acetate.

The level of hydrophilic monomer that covers the inner surface of the pores of the microporous film is monitored to obtain the proper level of additive as determined by the desired water flow or electrical resistance of the films according to application requirements. As described above

Il

the monitoring or control is carried out with sufficient care to preserve the open-cell nature of the micropores, i. to prevent clogging of the pores after the monomer-impregnated microporous film has been subjected to the radiation treatment described above, so that the desired properties listed above are maintained for a period longer than that otherwise obtained using typical surface coatings of the prior art. The specific amount of hydrophilic monomer that can be chemically fixed on the surface of the pores is expressed in terms of percent additive, i. by the weight percentage of the uncoated microporous film that represents the weight of the hydrophilic monomer coating present in the treated microporous film.

"A.

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In order to avoid clogging of the pores of the microporous films, the percent addition of hydrophilic monomer chemically fixed on the surface of the micropores is controlled and monitored to be no greater than 10 % and generally in the range of about zero , 1 to about 10 %, preferably between 0.5 to about 2.5 %, and most preferably from about 1 to about 2 % (eg, 1.5 %) by weight based on the weight of the uncovered microporous film. -

The selected particular percent additive may be determined by the end use intended for the finally formed hydrophilic microporous film and may vary within a wide range of additional percentages described above. '·

If e.g. the grafted hydrophilic microporous film is intended for use as a separator in an electric battery, the percentage additive is chosen according to the electrical resistance of the hydrophilic microporous film formed (in milliohms per cm), since this resistance is a function of the percent additive of hydrophilic Monomer is.

The electrical resistance is to conduct here as a measure of the property of the microporous film, electron defined, _A a result, can be regarded as a general rule, the electrical resistance of the microporous film is so large, is, the less effective the microporous film as a battery separator.

The electrical resistance of the microporous film is determined at various additive percentages of the hydrophilic monomer and the electrical resistance is plotted as a function of the percent additive. The percentage addition will then be based on the desired

Vb

determined electrical resistance.

The electrical resistance of the hydrophilic microporous film according to the present invention, as will be described below, is generally controlled and monitored to be smaller than

2 2

4.6 milliohms per cm, preferably 1.5 milliohms per cm

and most preferably 0.75 milliohms per cm.

When the grafted hydrophilic microporous film is used as a filter, the percentage addition of the microporous film becomes the water flow rate. through the microporous film.

The water flow rate may be plotted as a function of the percent additive of hydrophilic monomer and the appropriate percent additive selected according to the desired water flow rate.

When the microporous film is to be used for filtration, the pore size of the film is selected to serve as a barrier to the materials to be separated, and the water flow rate is controlled to be within the limits of the selected pore size. The water flow rate can be controlled to be greater than approximately

3 2

0.01 cm per minute per cm, preferably greater than about 0.05 cnr / min / cm, and most preferably greater than about 0.5 cnr / min / cm at a differential pressure of about one atmosphere.

As a result, the present invention offers a number of advantages over hydrophilic films that have heretofore been made. Since it is possible to study the open-cell nature of the hydrophilic microporous film, the

223 7 27

According to the present invention, such films permit mass transport of water through the film, which differs from the transport of water through the film by diffusion, which is a significantly slower process Another advantage of the microporous film according to the present invention results from the chemical fixation of the hydrophilic monomer on the microporous film, which allows the time of the presence of hydrophilic monomer on the microporous surface for a long time, This was the case with typical surface active agents, in particular after repeated washing.

In addition 'comes that the resistance of the film according to the invention in terms of its dimensions yet. is improved.

It should be added that it is advantageous to bring the polar functional group of the hydrophilic monomer to its most polar form. This can be achieved, for example, by reacting an acid functional group with a base such as KOH to obtain the corresponding salt. The salt is maintained by moistening or soaking the chemically fixed film in a 2 % ± KOH solution over a period of about 5 to about 30 minutes. This promotes wettability and increases the

Suppleness of the film.

/

ti

Any of the known methods of coating may be used to coat the microporous film, provided that these methods allow sufficiently precise monitoring or control of the percent additive of hydrophilic monomer.

The preferred method for accurately and effectively covering the inside surface of the pores of the microporous film

223 7 27

consists in contacting the microporous film with a vapor of a monomer condensing on the microporous surface.

The percent additive may be controlled or monitored by monitoring the equilibrium vapor pressure (ie, the vapor pressure at which the condensation rate of the monomer vapor is equal to its rate of vaporization at a given temperature) of the hydrophilic monomer and the time the hydrophilic monomer with the microporous film is in contact. , ·

The equilibrium vapor pressure necessary to obtain the desired amount of hydrophilic monomer coating at a constant temperature is determined by plots of the percent additive versus. time at various equilibrium vapor pressures at constant temperature. The as in the previous. The selected percent additive described above depends on the particular property that one wishes to give to the microporous film.

The equilibrium vapor pressure to obtain the appropriate percent additive can be easily determined using the curves and graphs described above. The time during which the monomer vapor is in contact with the microporous film is similarly determined.

In a continuous process, the micropositive film is continuously passed through a chamber containing Jr hydrophil 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 or controlled by controlling the pathlength and line speed of the microporous film through the vapor.

- 30 -

223 7 2?

Obviously, the vapor deposition technique can only be used if the critical temperature of the hydrophilic Mo-monomer (ie the temperature at which a liquid monomer can not exist regardless of the pressure) is above the temperature at which the property of the microporous film are adversely affected at the particular contact time used.

Preferably, the vapor deposition technique is performed at atmospheric pressure and at a temperature that provides the appropriate vapor pressure. :

Typically, the temperature at which the vapor-coating technique is performed varies as the atmospheric pressure verv / ends, is from about 50 to about 170 ⁰ C, when the hydrophilic monomer is used, acrylic acid, methacrylic or vinyl acetate is.

Subatmospheric or superatmospheric pressure may also be used, along with appropriate temperature and contact time settings.

Since the percent additive is determined after the cure, an amount of hydrophilic monomer that is greater than the percent additive is initially applied to the microporous film to compensate for monomer losses that may occur during the cure.

Other suitable procedures by which the hydrophilic monomer may be coated on the microporous film include the solution of the hydrophilic monomer in a volatilizable solvent such as methylene chloride to form a base bath. ''

The basic bath or upholstery bath can also be used with an opposite rolling technique. In this

223 727

A metering roller is partially disposed in the base or pad bath of the monomer coating solution. A second driven roller passes an uncoated band of the hydrophobic microporous film through the nip formed by this second roller and the metering roller. The two rollers, which are preferably driven separately, rotate in the same direction.

Il

so that the film tape provided with the coating is guided in the direction from which the uncovered or uncoated film originates. The amount of monomer coating applied to the film is a function of the difference in speed of the metering roller and the second roller driving the film and also the size of the nip formed between these two rollers. Alternatively, a squeeze roller method may be used as the alternative option. In this process, the film is passed into a pad bath of the monomer coating solution and squeezed between two downstream squeeze rolls. The amount or amount of coating herein is a function of the gap size between the two nip rolls and the pressure applied therein.

Another alternative procedure for coating or coating the hydrophobic microporous film is the wire-wound dosing rod method. This method is similar to the nip roll method except that the microporous film after coating is squeezed by passing it through a bath of a monomer coating solution or monomer coating solution between a pair of wire-twisted dosing bars that measure the amount of coating or coating through the configuration Check the wire wrapping around the dosing rods.

In the reverse roll method, the Quets.ch rolling method, and the wire-wound coating method, the amount or amount of the hydrophilic monomer initially applied to the microporous film is a function of one

223 7 27

or of two variables that have been discussed in describing these procedures. In addition, it should be noted that for all three process iron the amount of monomer coating or monomer coating is also a function of the concentration of the monomer in the cushion bath. The cushion bath of the hydrophilic monomer is prepared by dissolving the monomer in a common organic solvent having a boiling point lower than the boiling point of the hydrophilic monomer used, e.g. as admixture to methylene chloride, acetone,

Il

Methanol, ethanol and isopropanol.

The concentration of the hydrophilic monomer in the cushion bath or base bath is monitored so that the appropriate amount of monomer is applied upon evaporation or evaporation of the solvent. In general, the monomer concentration in the bath is between about Λ to about 30 %, and preferably between about 5 and about 15% by weight, based on the weight of the bath.

When the hydrophilic monomer has been coated or coated with the microporous film using a cushioning bath, the solvent contained therein is removed by passing the coated film through a dryer. The temperature of the dryer should be high enough for the solvent to evaporate without the monomer being deposited on the microporous surface of the film disappearing.

After the pores of the microporous film have been coated or coated with the hydrophilic monomer and the solvent, if any, has been removed therefrom, the coated or overcoated microporous film is subjected to ionizing radiation to form the hydrophilic monomer normally To fix hydrophobic micropores and make them hydrophilic.

  , 2237 2?

For ionizing radiation, a number of possible devices or combinations can be used to produce the desired effect. Thus, the hydrophilic monomer may be chemically bound to the microporous surface and / or formed by polymerization and / or copolymerization a polymeric layer which is intimately bonded to the microporous surface, in a chemical manner e.g. by chemically bonding the layer surface to the microporous surface, and / or in a physical manner as a result of the inclusion effect on the polymeric layer of the periphery of the microporous surface. The term "chemical fixation" includes all of the aforementioned mechanisms or combinations thereof;

Without being limited to the particular mechanism by which the desired improvement can be achieved, the properties of the normally hydrophobic microporous film can be altered in one or more ways, depending on the percent addition of the hydrophilic monomer.

The irradiated microporous film is rendered hydrophilic over a period of time longer than that achievable with conventional surfactant coatings. Nevertheless, the open-cell nature of the film is preserved for use in such applications requiring bulk transport and low electrical resistance.

As noted above, the initial fixation of the hydrophilic monomer on the normally hydrophobic microporous film is accomplished by exposing the hydrophilic monomer impregnated microporous ionizing radiation film.

Ίο

223 7 27

Essentially, this ionizing radiation is of the type that provides emitted particles or photons having internal energy sufficient to form ions and break chemical bonds, thereby freeing up free radical reactions between the hydrophilic monomer and monomer and solvent used the micropore surface as described above. Ionizing radiation is generally present as ionizing particle radiation, ionizing electromagnetic radiation and actinic light. ,

The term "ionic particle radiation" is used herein to refer to the emission of electrons or highly-accelerated core particles such as protons, neutrons, alpha particles, deuterons, beta particles, or their counterparts, which are directed to direct the particle toward it Charged particles can be transmitted by means of voltage gradients with devices such as a low energy (ie, 200 KeW) long electron beam generator, such as the ELECTRO-CURTAIN manufactured by Energy Sciences Corporation, Resonant Chamber Accelerators, Van Der Graaff Generators, Betatron , Synchrotrons, cyclotrons, etc. Neutron radiation can be generated by bombarding a selected light metal, such as beryllium with positive high energy particles. Particle radiation is also obtained by using atomic reactors, radioactive isotopes or other natural or synthetic ones radioactive n fabrics.

"Ionizing electromagnetic radiation" is produced when a metallic target, such as tungsten, is bombarded with electrons of suitable energy. This energy is transmitted to the electrons by potential accelerators of over 0.1 million electron volts (mev.). In addition to radiation of this type, commonly referred to as X radiation, ionizing electromagnetic radiation,

223 727

which is suitable in the context of the present invention, can be produced by nuclear reactors or by using natural or artificial radioactive material, e.g. Cobalt 60.. '. '.

The hydrophilic monomers described herein are chemically fixed by exposure to actinic light. In general, actinic light sensitivity wavelengths are used in the range of approximately 1,800 to 4,000 angstrom units Various other sources of actinic light are available in the art, such as quartz mercury lamps, ultraviolet pierced carbon sheets, and High-intensity lamps · Initiators can be used when actinic light is used.

The preferred source of ionizing radiation is an electron beam generator as described in U.S. Patents 3,702,412, 3,745,396 and 3,769,600. These patents are incorporated by reference into the present application

t.

The techniques for precise control of the process are sufficiently developed to permit conversion of the results of one type of radiation into the other and adequate adaptation of the techniques described herein can be used interchangeably in the preparation of any desired product. ,

A beam termination of about 1 to 10 megarads. '(Mrad) and preferably from about 2 to about 5 Mrad (e.g., 3 Mrad) can be made to obtain a chemical fixation. A megarad equals one millionth wheel. One wheel is the amount of high-energy ionizing radiation that causes an absorption of 100 ergs of energy per gram of absorbent material. These

223 7 27

Unit is known as a suitable means of measuring the radiation absorption of a substance.

Preferably, the minimal ionizing radiation for chemical fixation is used from co-terminations. Excessive dosages (i.e., dosages greater than 20 Mrads for a single exposure) should be avoided to avoid excessive heating and shrinking of the film and deterioration of the hydrophilic monomer of the microporous film. However, if the dosage is too small, the chemical fixation is not complete and the hydrophilic monomer is lost quickly.

The minimum radiation dose necessary for chemical fixation of the hydrophilic monomer will vary depending on the type of polymer used to make the microporous film. If e.g. the polymer is made of polyethylene, the radiation dosage should not be less than about 1 Mrad. For polypropylene, the radiation dose should not be less than about 2 Mrad, and preferably not less than 3 Mrad.

Room temperatures can be satisfactorily used in the irradiation, although high temperatures are also usable.

Since oxygen tends to hinder the grafting or application of the hydrophilic monomer to the microporous film, it is preferable to perform the irradiation treatment of the microporous film in an inert atmosphere such as nitrogen or other inert gas.

As previously described, the percent additive may be selected according to the electrical resistivity of the microporous film.

ft

The electrical resistance (DC method) of the microporous film is determined by introducing a sample of the microporous film having a known surface area (1.3

p .

cm) into a 40 weight percent solution of KOH in water for 24 hours. , The sample obtained thereafter is then inserted between cadmium electrodes (i.e., an anode and a cathode). immersed in an electrolyte of 40% solution of KOH in water, and a direct current of known strength (e.g., 40 milliamps) is then passed through the cell between the electrodes. The voltage drop across the film (E) is measured with a meter. The voltage drop across the cell without the microporous film present therein (E ') is also determined at the same current.

  ·

The electrical resistance of the microporous film is then determined according to the following equation:

(E '- E) A

HE. =

I.

2 _

w.orin A is the surface of the exposed film in cm, * i is the current through the cell in Milliamper, E.R. the electrical

Resistance of the microporous film in milliohms per cm and E 'and E mean the voltage drops described above.

The water permeability or water flow rate of the hydrophilic microporous film according to the present invention is determined by measuring the rate of water flow through a specific surface of the film while keeping the water under a differential pressure of one atmosphere. The water flow rate is then expressed in units per volume of water in cm ³ / min / cm ^{2 of} film surface, ie in cm ³ / min / cm.

223 7 27

The air permeability of the microporous films according to the present invention is determined by the Gurley test, ie, according to ASTM D 726, by placing a film having a surface of 6.46 cm in a Gurley densified gauge. The film is then subjected to a calibrated differential pressure (the pressure drop across the film) of a 31 cm water column (Fig. 2f). The time in seconds necessary for 10 cm of air to pass through the film is used as a guide Permeability _: A Gurley value of no more than 1.5 minutes is an indication that the pores are clogged.

The hydrophilic microporous films according to the invention find many different applications. In particular, their utility has been demonstrated in areas where the controlled passage of moisture through a film or surface is desired. Furthermore, films prepared according to the invention can be used as filter membrane supports or filters which are to be usable for the separation of ultrafine substances from different liquids, and as battery separators.

The present invention will be further illustrated by the following examples. All parts and percentages in the examples and also in the specification and claims are by weight unless otherwise specified .

Example 1 '

Part A / .· .

This discussion illustrates the preparation of a normally hydrophobic polyolefinic microporous film by the "dry draw" method as described in U.S. Patent 3,801,404.

223 7 27

Crystalline. Polypropylene with a Schelzindex of 0.7 and a density of 0.92 are melt-extruded at a temperature of 230 ⁰ C through a nozzle with a slit of 20.32 cm and hanger-type, the one. 2.54 cm extruder with a flat dosing screw used. The ratio of the length to the diameter of the extrusion press drum is 24/1. The extruded fabric is peeled off very rapidly at a melt removal ratio of 150 and brought into contact with a rotating forming roll which is maintained at a temperature of 50 ° and located at a distance of 1.9 cm from the lips of the die or injection mold , The film thus prepared has the following properties: Thickness: 0.00508 cm; Recovery -lgen from a 50% elongation at 25 ⁰ C: 50.3%; Crystallinity: 59.6 %.

A sample of this film is annealed in an oven with air at a gentle tension at 140 ^° C. for a period of 30 minutes, then taken out of the oven and cooled. , ·.

The sample of the annealed elastic film is then cold stretched and hot stretched at a stretch ratio of 0.50: 1, and then heat set under tension, ie. at constant 'length and 145 ⁰ C' during 10 minutes in 'air. The Kaltziehschritt is carried out at 25 ⁰ C, the hot drawing step at 145 ⁰ C and the total tensile is 100 % based on the original length of the elastic film. The resulting film has a mean pore size of about 3,000 angstroms, a crystallinity of about 59.6 %, and a surface area of about 8j54 m / g. The thickness of the microporous film is 0.00254 cm.

part B

A continuous role of the microporous film of a

Length of 30.43 m and a width of 15.24 cm, which has been prepared according to Part A, is then passed through a bath of glacial acetic acid (ie 100%) and then over a squeezing roller to give an addition of 1, 5 weight s-? 6 after curing, based on the weight of the film before impregnation, as determined by IR analysis.

The impregnated film is then passed under the window of an electron-beam generator (60.96 cm long) at a line speed of 6.096 m / min. The electron beam generator is set to deliver an amount of 3 Mrad at the line speed used.

The oxygen level of the atmosphere under the curtain window and in contact with the microporous film will be below 500 ppm by inclusion of the window in one. held with nitrogen purged chamber.

A sample of the cured dry microporous film is then tested for wettability with the drop test. The drop test is performed by placing a 0.6 mm drop of 2% KOH solution in water on the surface of the film. The movie is then watched. If the portion of the film on which the drop has been applied is translucent, then _y and the side of the film bearing it on the opposite side of the film will appear to be wet, the film will be considered wet or wetted in Table 1 with "yes.") The drop test is carried out with an out of line sample of the film immediately after irradiation, and with a sample stored for one week at room temperature. '

Various other samples are taken from the role of the

and tested for electrical resistance in the manner described herein by placing it in a 40 weight percent aqueous solution of KOH maintained at a temperature of 60 ° C for a period of one hour, 24 hours, 4 hours Days and 8 days and averages the received values. Immersion of the film sample in hot KOH at increasingly longer times simulates exposure over extended periods of time.

The surface of each on their electrical resistance

The sample tested is 1.29 cm and the DC used has a value of 40 milliamps.

Three samples of the roll of cured microporous film are tested for air permeability according to the Gurley test ASTM-D-726B.

The results are shown in Table 1.

Samples of the roll of hoarded microporous film are also tested for water flow after being immersed in a 31 % aqueous solution of KOH maintained at a temperature of 60 ° C for 24 hours when each is intakes Sample, which has a surface of

2 11.3 cm, inserted into a Millipore filter housing. The Millipore filter is filled with water and pressurized to an atmosphere between both surfaces of the film sample. The water passed through the microporous film is collected for a period of 5 minutes. The results are converted into cnr / min / cm of film surface collected water.

The results are given in Table 1.

223 7 27

Comparative example

Example 1 is repeated, except that the radiation dose used to cure is reduced to one megarad. The results are listed in Table 1. An uncoated microporous film made according to Example 1 is also exposed to ionizing radiation and tested or tested in the same manner as in Example 1 and serves as a control. The results are given in Table 1.

As can be seen from Table 1, a 1-megad dose seems to be too small to achieve effective cure of the microporous polypropylene film, as indicated by the infinitesimal electrical resistance of the microporous film of the Comparative Example. The positive wetting test of the comparative example for the off-line sample appears to be due to the residual acrylic acid which has not been chemically fixed by the irradiation. The negative wettability of the film of this comparative example after one week appears to have been caused by the loss of acrylic acid during the one week storage period. This is confirmed by the infinitesimal electrical resistance of the film after one week of storage »

It can also be seen that the electrical resistance of the film of Example 1 increases after 4 days exposure to hot KOH and slightly decreases after 8 days of exposure. It is believed that after 8 days exposure, the waste is due to the complete conversion of the acrylic grafting to its salt form, while after 4 days of soaking, the salt conversion has only partially occurred. As a result, complete conversion of the acrylic graft to its salt form appears to reduce electrical resistance.

223 727

Example 2.

Part A

This discussion illustrates the preparation of a microporous polyethylene film by the "solvent draw" method. · ·

Crystalline polyethylene with a melt index of 5? A weight average molecular weight of about 80,000, a density of 0.960 g / cnr and a molecular weight distribution ratio of about 9 "0" is prepared by the combined extrusion blow molding method to form a precursor film (0.0762 cm thick) and quenching in air at 25 ° C cooled. A sample of the precursor film thus formed is then immersed in trichlorethylene at 70 ^° C. for one minute and then immersed in trichlorethylene, at a temperature of 70 ^° C. and a strain rate of 150 % per minute to four times its original length (ie 300 % total elongation or Strec? Nir_g), - The trichlororrhetyl ¹ ! is then removed by evaporation and the sample is stretched in the cross-machine direction with a draft of about 50 % and dried in air in the stretched state. The drying is carried out at 25 ° C.

The microporous film thus formed has a crystallinity of about 60 %, an average pore length of about 5,000 angstroms, and a surface area of between about 10 and about 25 m / g.

part B

Several samples of the microporous film of 0.00254 cm thickness, prepared according to Part A, are immersed in icyacylic acid (100 %) and the film becomes translucent. The samples may dry until the film turns white

opaque appearance, suggesting that the appropriate amount of monomer coating is present to give an addition of about 1.5 % . The formed samples are then moved underneath an electron beam curtain (60.96 cm long) and irradiated with varying doses of radiation "the atmosphere below the window and in contact with the microporous film on a sweetener worth less than already described in Example 1 "manner is held. The obtained cured films are (C-for 24 hours after immersion in a 40% KOH solution at 60 ⁰⁾ and its wettability by dem'Tropfentest described on the Gurley air flow, the · electric resistance on the occasion of Example 1 in the Way tested. The results are listed in Table 2 as Experiments 1 to 8. The percent addition of the cured film samples obtained is determined by infrared analysis. The results are shown in Table 2.

As can be seen from the results of experiments 4 and 7 of Table 1, the electrical resistance is unlimited and the samples do not moisten. It is believed that these results are attributable to the use of a radiation dose that is too low to cause chemical fixation. As a result, it is believed that the "hydrophilic monomer evaporates as can be concluded from the lack of measurable additive with a consequent loss of properties.

The film examples of experiments 1, 2, 5? Figures 6 and 8 show too little electrical resistance and are wettable or wettable. The higher electrical resistance of the Experiment 3 film sample appears to be due to an observed inhomogeneity in the. Attributed to film. Further, the film samples which have been chemically fixed as indicated by a 1.5% additive show only a slight drop in their air permeability. The

" ⁵¹ " 223 7 27

means that the pores after chemical fixation

not clogged. "

The principles, preferred embodiments and modes of the present invention have been described in the foregoing. However, the invention is not limited to the particular embodiments described. Modifications and changes may be made by those skilled in the art without departing from the scope of the invention.

sz.

«FS -

BLACKBOARD

example 1

Comparative example

control

Dose (megarad)

Line speed (m / min)

9.14 6.096

9.14

wettability Yes (Drop Test) There out of line after 1 week 0.77 1.06 1.64 1.10 Electrical resistance ₉ (milliohms / cnr) 9.1 1 hour 24 hours 4 days 8 days 0.35 Gurley (Air) (Gurley Seconds) Water flow (cm / min / cm)

No

OO

FD ND

ND

ND = Not determined

TABLE 2

Addition of acrylic acid after which irradiation be-no. Right

Radiation dose (megarad)

Electrical resistance

Gurley (ASTM-D-726).

(Milliohm / cm) Before After

Be-ii.otzt drop test

.1 · 1.5 3 1.32 0,4 ' 0.6 Yes 2 , 1.5 1 1.08 0.4. , 0.46 · there 3 1.5 3 56.3 *> 0.38 0.80 Yes 4 0 1.2 0.35 0.34. No 5 1.5 3 1.55 0.40 0.68 there 6 1.5 1 0.697 0.40 0.76 there 7 0 1.2 0.40 0.48. No_ 8th 1.5. 1 '2.01 0.35 0.76 there

It is believed that the high electrical resistance results from inhomogeneity in the film.

Claims (11)

invention claim A method of hydrophilizing a normally hydrophobic microporous polyolefin film to improve the water flow rate through the film and to reduce its electrical resistance, characterized by: a) Covering the surface of the micropores of a normally hydrophobic? open-celled microporous polyolefin film having a bulk density less than that of a precursor film of which it is based has a mean pore size of about 200 to about 10,000 angstroms and a surface area of at least about 10 m / g, with at least one hydrophilic, organic hydrocarbon monomer having from about 2 to about 18 carbon atoms and at least one double bond and at least one polar functional group, and b) Chemical fixation on the surface of the micropores of the microporous film of a narrow one of said ) i. hydrophilic organic hydrocarbon monomer sufficient to obtain the open cell nature of these micropores and sufficient to provide an addition of from about 0.1 to about 10% by weight, based on the weight of the uncoated or coated microporous film, of the coated one microporous film of a) with an ionizing radiation of about 1 to about 10 megarads. 2. The method according to item 1, characterized in that the hydrophilic organic hydrocarbon monomer un ~ -Jl 223 7 27 58 109/16 danger of having 2 to about 14 hydrocarbon atoms and at least one polar functional group consisting of
is selected from the group consisting of carboxyl, sulfo, sulfo in hydroxyl, ammonium, amino and phosphonium. 3. Method according to item 1, characterized in that "
the hydrophilic organic hydrocarbon monomer is selected from the group consisting of unsubstituted and alkyl-substituted acrylic acid, vinyl esters, vinyl ethers • and mixtures thereof. 4. The method according to item 1, characterized in that -that
the hydrophilic organic hydrocarbon monomer is selected from the group consisting of acrylic acid, methacrylic acid, vinyl acetate and mixtures thereof. 5. The method according to item 1, characterized in that
the normally hydrophobic microporous film is prepared OUT_ polymers that are selected from the group consisting of polyethylene, polypropylene, and after the "solvent middle lstreck ^r? -Ve rf Ahren or" dry stretch "method is prepared. 6. The method according to item 1, characterized in that
the surfaces of the micropores of the normally hydrophobic microporous film are chemically fixed with an addition of hydrophilic organic hydrocarbon monomer of about 0.5 to about 2.5% by weight, based on the weight of the uncoated film. 7. Method of rendering an open-celled, usually hydrophobic, microporous film hydrophilic, increasing the water flow rate through this film • "-58 109/16 to improve and reduce the electrical resistance of the film, characterized by: a) Bedec'ken or coating the surface of the micropores of a normally hydrophobic, open-cell, microporous film, which has been made of polymers which have been selected from the group with polyethylene and polypropylene, wherein the film has a bulk density, the smaller is, as the bulk density of a precursor film, assumed to have a crystallinity of not less than about 30 %, a mean pore size of about 400 to about 5,000 angstroms and a surface area of at least about 10 m / g, with at least one hydrophilic organic hydrocarbon monomer selected from the group consisting of acrylic acid, methacrylic acid, vinyl acetate, and. b) cfiemic fixing on the surface of the micropores of the microporous film an amount of said hydrophilic monomer sufficient to preserve the open-celled nature of said micropores and sufficient to have an addition thereto of from about 0.1 to about 10% by weight; based on the weight of the uncoated microporous film _? by irradiating the coated or coated microporous film of a) with an ionizing radiation of about 1 to 10 megarad. 8. Hydrophilic, open-cell, microporous film, marked -.... distinguished by having: , a) an open - cell, normally hydrophobic, microporous film that exhibits a bulk density of , 223 7 27 109/16 Precursor film, lower bulk density, a mean pore size of about 200-200 angstroms and a surface ρ. of at least 10 m / g, and b) a coating on the surface of the micropores of the microporous film of at least one hydrophilic organic hydrocarbon monomer having at least about 2 to about 18 carbon atoms and at least one double bond having at least one polar functional group, said hydrophilic organic hydrocarbon monomer coating chemically on the surface of the Microporous micropores of the microporous film by exposing an ionizing radiation of about 1 to about 10 megarads and to an extent sufficient, the open-celled nature of the microporous film and an addition 'of about 0.1 to about 10% by weight, based on the weight of uncoated ^ microporous film is obtained. A hydrophilic microporous film according to item 8, characterized in that said hydrophilic organic hydrocarbon monomer has about 2 to about 14 carbon atoms and at least one polar functional group selected from the group consisting of carboxyl, sulfo, sulfino, hydroxyl, ammonium, amino and phosphonium, The hydrophilic microporous film of item 8, characterized in that the hydrophilic organic hydrocarbon monomer is selected from the group consisting of unsubstituted and alkyl substituted acrylic acid esters, vinyl esters and mixtures thereof. 223 7 27 58 109/16 11 . A hydrophilic microporous film according to item 8, characterized in that the hydrophilic organic hydrocarbon monomer is selected from the group consisting of acrylic acid, methacrylic acid, vinyl acetate and mixtures thereof. , , The hydrophilic microporous film of item 8, characterized in that the normally hydrophobic polymeric microporous film is selected from the group consisting of polypropylene and polyethylene prepared by the "solvent draw" method or the "dry draw" method. o ^v A hydrophilic microporous film according to item 8, characterized in that the hydrophilic monomer has been chemically fixed on the surface of the micropores in an amount of about 0.5 to about 2.5% by weight based on the weight of the uncovered microporous film is. 14 * Hydrophilic, open-cell, microporous film, characterized dadu.rch that it possesses: a) a normally hydrophobic, open-cell, microporous film made by the "loose-laminate" process or the "dry-drawing" process from polymers selected from the group consisting of polyethylene and polypropylene, said film having a face the bulk density of a precursor film which has been assumed to have a lower bulk density, a crystallinity of at least 30 %, an average pore size of about 400 to 5,000 angstroms, and a surface area of at least about 10 m / g, and .58 109/16 b) a coating on the microporous surface of the microporous film of at least one hydrophilic organic hydrocarbon monomer having from about 2 to about 18 carbon atoms and selected from the group consisting of unsubstituted and alkyl substituted acrylic acids, vinyl esters, and mixtures thereof, wherein said hydrophilic organic Hydrocarbon monomer overcoat chemically on the surface of the micropores by exposing an ionizing radiation of from about 2 to about 5 megarads and to a level sufficient to increase the open cell nature of the microporous film and an additive thereof from about 0.1 to about 10% by weight is fixed to the weight of the uncoated microporous film. A hydrophilic microporous film according to item i4, characterized in that the hydrophilic monomer is selected from the group consisting of acrylic acid, methacrylic acid, vinyl acetate and mixtures thereof, the hydrophilic monomer being present on the surface of the micropores in an amount of about 0.5 to about 2 % By weight, based on the weight of the uncovered microporous film.