JPH021177B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention provides improved water permeability and/or
MICROPOROUS FILM WITH REDUCED ELECTRICAL RESISTANCE. U.S. Patent No. 3839516, No. 3801404, No.
With recent developments in the field of open-cell microporous polymeric films, as exemplified by Nos. 3679538, 3558764, and 3426754, we find applications that take advantage of the unique properties of these new films. Research has been promoted for this purpose. Films of this type that are in fact gas-permeable water barriers can be used in vents, gas-liquid transfer media, battery separators, and a variety of other applications. One drawback of these films, which has limited the number of applications in which they could be used in the past, was their hydrophobic nature. This means that
This is especially true when using polyolefin films, a preferred type of microporous material commonly used in the production of microporous films. Since these films are not "wetted" with water and aqueous solutions, they cannot be advantageously used in consequential applications such as filter media, electrochemical separator components, etc. Several proposals have been made in the past to overcome these problems, as exemplified by U.S. Pat. A coating or impregnating agent is used. Although such coatings or impregnants are effective for a limited period of time, they tend to be removed relatively quickly by solutions that come into contact with the existing film or fiber. Other attempts have been made to use low energy plasma treatments to impart hydrophilic properties to normally hydrophobic microporous films. Such plasma treatment is carried out by first activating the surface sites of the microporous film using argon or hydrogen plasma and then grafting the film with a suitable free radically polymerizable species, such as acrylic acid. .
Plasma treatment results in a film with only a rewettable surface. The surface of the film also becomes clogged when wetted, thereby inhibiting or inhibiting the free flow of water within the film. The inevitable clogging of the surface pores makes the film unsuitable for certain filter applications, increases the electrical resistance of the film, and also impairs the dimensional stability of the film, as evidenced by substantial shrinkage upon drying. Decrease. As mentioned above, a disadvantage of plasma treatment is its limited ability to wet only the surface of the microporous film. Because microporous films of the type described herein have an unusually large surface area, mere surface wettability does not mean that the films have certain organoleptic properties such as low electrical resistance and comparable to the known surfactant systems mentioned above. It has been recognized that it is not possible to reliably represent the water flow rate in the film that is possible. The major disadvantages of plasma processing, namely pore clogging and mere surface wetting, are believed to result from a combination of factors, such as the tendency of low energy plasmas to be easily quenched by the high surface area of microporous films. This reduces the possibility of activation of internal sites within the microporous film. Similarly, if the graftable monomer is first contacted with a plasma-activated microporous film surface,
There is competition for the introduction of graftable monomers used by the free radical polymer grafting that initially occurs on the film surface. In other words, the graft polymer chains initially present on the film surface grow at a rate that gradually increases as the reaction progresses.
Therefore, the resulting extended graft polymer chains that form on the film surface entangle and clog the surface pores in the presence of water. Other attempts to provide hydrophilic films using plasma processing are illustrated in US Pat. Nos. 3,992,495 and 4,046,843. Other techniques for wetting and making polyethylene films suitable for use in battery separators are described by V. Agostino and J. Lee in ``Manufacturing Methods''. Manufacturing Methods for High Performance Grafted Polyethylene Battery Separators
For High Performance Grafted―
Polyethylene Battery Separators), US National Technical Information Service
Service), AD Report No. 745571 (1972), Abstract 78 Chemical Abstracts (Chemical
Abstract) 5031f (1973); “Low Temperature Alkaline Battery Separators” by V. Dooragostino and J. Lee
Temperature Alkaline Battery Separators),
27 Power Sources Symposium (Power
Sources Sym.) 87-91 (1976), Abstract Chemical.
Abstract 86, 158277f; V. Dooragostino,
J. Lee and G. Orban, “Zinc Silver Oxide Batteries”
Silver Oxide Batteries), edited by A. Fleischer and J. Lander,
1971]. These papers use ``PERMION'' developed by RAI Research Corporation.
This article discusses and relates to commercially available products known as (trade name). In summary, the method of manufacturing this material consists of crosslinking 1 mil polyethylene sheets using beta radiation and then grafting with methacrylic acid in a suitable solution under Co60 gamma radiation. The grafted material is washed to remove homopolymer, converted to the salt form in hot KOH, washed again to remove residual base, dried, and packaged. The initial cross-linking process produces microcracks or longitudinal slits in the non-porous polyethylene film, which are so small that they cannot be seen even under an electron microscope. The diameter of these microcracks is approximately
It is estimated to be 20 angstroms (10 ^-8 cm). The microcracks are then grafted with methacrylic acid. Therefore, the resulting film has an average size of about 100-5000 microporous films used in the present invention.
It is not microporous in the sense of having angstroms. The extremely small size of the "permion" microcracks generally prevents the collective migration of mobile charged species generated by redox reactions through the film at any substantial rate. This is reflected in the relatively high (eg, 30-40 milliohms square inch) electrical resistance exhibited by this type of film. Additionally, Permion lacks dimensional stability in more than one direction, as evidenced by substantial swelling. Non-porous polymeric substrates such as polyethylene and polypropylene are described in U.S. Pat.
It is well known that it is possible to react with various monomers, such as acrylic acid, using various types of ionizing radiation, as specifically shown in US Pat. .
However, since these US patents all relate to microporous films, these patents do not address the unique problems associated therewith. Therefore, research continues into relatively permanent wettable hydrophilic microporous films that exhibit low electrical resistance and improved water flux through the microporous film. The present invention was developed in response to this research. Accordingly, it is an object of the present invention to provide a method for imparting relatively permanent hydrophilic properties to normally hydrophobic microporous films, thereby improving their water permeability. Another object of the present invention is to provide a method for reducing the electrical resistance of microporous films, which are typically hydrophobic. Yet another object of the present invention is to provide a hydrophilic microporous film having reduced electrical resistance. Yet another object is to overcome the problems of the prior art mentioned above. These and other objects and scope, nature and uses of the invention will become apparent to those skilled in the art from the following detailed description. In one aspect of the invention, a method is provided for rendering a normally hydrophobic polyolefin-based microporous film hydrophilic to improve its water flow rate and reduce its electrical resistance, the method comprising: (a) The bulk density is approximately 200-10,000, which is reduced compared to the bulk density of the precursor film used to manufacture the film.
The microporous surface of a typically hydrophobic polyolefin-based open-cell microporous film characterized by having an average pore size of about 100 ^.ANG . and coated with at least one hydrophilic organic hydrocarbon monomer characterized by the presence of at least one double bond and at least one polar functional group; and (b) a microporous film. The hydrophilic organic hydrocarbon monomer is applied to the surface of the micropores in an amount sufficient to maintain the open cell nature of the micropores and about 0.1% based on the weight of the uncoated microporous film. The coated microporous film of (a) is chemically fixed by irradiation with about 1-10 megarads of ionizing radiation in an amount sufficient to obtain a pick-up of -10% by weight. It consists of things. In another aspect of the invention, a hydrophilic open cell microporous film is provided comprising: a hydrophilic open cell microporous film; (a) has a reduced bulk density compared to the bulk density of the precursor film from which it is manufactured, and has a bulk density of approximately
an open cell usually hydrophobic microporous film characterized by having an average pore size of 200-10000 angstroms and a surface area of at least 10 m ² /g; and (b) at least one double bond and at least one
A coating of the microporous surface of a microporous film with at least one hydrophilic organic hydrocarbon monomer having about 2 to 18 carbon atoms and characterized by the presence of polar functional groups of about 2 to about 18 carbon atoms; The hydrophilic organic hydrocarbon monomer coating retains the open-cell nature of the microporous film upon exposure to about 1-10 megarads of ionizing radiation and maintains the open-cell nature of the microporous film by approximately 0.1 megarads based on the weight of the uncoated microporous film. -Chemically fixed to the microporous surface of the microporous film in an amount sufficient to obtain a 10% by weight impregnation. The essence of the present invention is that the open-cell microporous film
Relatively permanently wettable while retaining the open cell nature of the microporous film when the pores are chemically fixed with controlled amounts of hydrophilic monomers that can be grafted by exposure to ionizing radiation and/or can be hydrophilic. Furthermore, this chemical fixation also occurs within air bubbles located at internal sites within the microporous film. By controlling the amount of monomer that is chemically immobilized on the surface of the pores, the polymer chain grafts generated upon exposure to radiation appear to be aligned on the pore surfaces. Entanglement and clogging of the holes is thus avoided. The present invention relates to a hydrophilic microporous film and a method for producing the same. Porous or cellular films can be classified into two general types. One type is a film in which the pores are not continuous, i.e., a closed cell film, and the other type is a film in which the pores are essentially continuous through tortuous paths extending from one external surface or surface area to another. In other words, it is an open-cell film. The porous film of the present invention is of the latter type. Furthermore, the bubbles in the porous film of the present invention are microporous. That is, the details of the structure or arrangement of these bubbles can only be discerned by microscopy. In fact, open cells in the film are generally smaller than can be measured using an ordinary optical microscope. This is because the wavelength of visible light, which is about 5000 angstroms (one angstrom is one ten-billionth of a meter), is longer than the longest plane or surface dimension of an open cell or cell. However, the microporous film of the present invention can be identified using electron microscopy techniques that can analyze the details of cell structures of 5000 angstroms or less. The microporous films of the present invention are also characterized by reduced bulk density (hereinafter sometimes referred to as "low" density). That is, these microporous films have a bulk or overall density that is lower than that of a comparable film made of the same polymeric material but that is not open celled or has other void structures. . The term “bulk density” is
In this text, it means the total volume of the film or the weight per geometric volume unit when the total volume is measured by immersing the film of known weight in a container partially filled with mercury at 25°C and atmospheric pressure. Use as. The volumetric rise in mercury level is a direct unit of total volume. This method is known as the mercury volumetric method and is described in the Encyclopedia of
Chemical Technology” (Encyclopedia of
Chemical Technology, Volume 4, Page 892 (Interscience, 1949). Porous films have been produced that have a microporous, open cell structure and are also characterized by reduced bulk density. For example, a film having this microporous structure is disclosed in US patent application filed by the present applicant.
No. 3426754, which is incorporated herein for reference. The preferred manufacturing method described in that patent involves stretching or elongating a crystalline, elastic precursor film at ambient temperature by an amount of about 10-300% of its original length, i.e., "cold stretching," and then shrinking the film. This includes stabilizing the stretched film by heat curing it under tension such that it does not shrink or shrinks only to a limited extent.
Other methods of making microporous films are illustrated in US Pat. Although both of the above US and UK patents disclose methods of making normally hydrophobic microporous films that can be made hydrophilic in accordance with the present invention, suitable normally hydrophobic microporous films include US Pat. No. 3,801,404, which defines a method for manufacturing microporous films, referred to in the text as the "dry stretching" method, and which is provided in accordance with the method described in U.S. Pat.
No. 3,839,516, which defines a method for making microporous films referred to as the "solvent stretching" process, both of which are incorporated herein by reference. Each of these patents discloses preferred alternative routes for the production of normally hydrophobic microporous films by manipulating the precursor film according to specified method steps.
Preferred precursor films that can be used to make microporous films according to the "dry stretch" and "solvent stretch" methods are specifically detailed in each of the above-mentioned respective patents. So, "dry stretching"
The method has a zero recovery time (defined below) of at least 40%, preferably at least about 50% when subjected to a standard strain (elongation) of 50% at 25°C and 65% relative humidity.
%, most preferably at least about 80%. Elastic recovery, as used herein, is a measure of the ability of a structure or shaped article, such as a film, to return to its original size after being stretched, and is calculated as follows. Elastic recovery rate (ER)%=
Stretched Length - Stretched Length/Stretched Length x 100 A standard strain of 50% is used to identify the elastic properties of the starting film, but this strain is merely exemplary. Generally, such a starting film is
Compared to the elastic recovery rate at 50% strain, higher elastic recovery rate and substantially 50% at less than 50% strain
Strains above have slightly lower recovery rates. These starting elastic films also have at least 20
%, preferably at least 30%, most preferably at least 50%, such as about 50-90% or more. Crystallinity % is ``Journal・
of Applied Polymer Science”
(Journal of Applied Polymer Science), No. 2
Vol. 5, pp. 166-173 (1959), by the X-ray method described by R.G. Quinn et al. For more information on crystallinity and its significance in polymers, see Golding, D. Van Nostrand, Polymers and Resins.
Nostrand), 1959]. Other elastic films considered suitable for the production of precursor films for use in dry stretching processes are described in British Patent No. 1052550, published December 21, 1966. Precursor elastomeric films used for the production of microporous films by the dry stretching process route are to be distinguished from films formed from classical elastomers such as natural and synthetic rubbers. In such a classical elastic body, the stress-strain relationship, especially the stress relationship, is influenced by the entropy mechanism of strain (rubber elasticity). The positive temperature coefficient of the contractile force, ie the reduced stress with decreasing temperature and the complete loss of elasticity at the glass transition temperature, is a special consequence of entropy-elasticity. On the other hand, the elasticity of the precursor elastic film used herein has different properties. Quantitative thermodynamic experiments with these elastic precursor surf films show that the increasing stress (negative temperature coefficient) with decreasing temperature means that the elasticity of these materials depends on the energy conditions and is not governed by entropic effects. be understood as a thing. More importantly, it has been found that "dry-stretched" precursor elastic films retain their stretching properties at temperatures at which normal entropic elasticity can no longer operate.
That is, the stretching mechanism of "dry-stretched" precursor elastic films can be considered to be based on energy-elasticity relationships, and these elastic films can be referred to as "non-classical" elastic bodies. Alternatively, in the "solvent stretching" method, at least 2
A precursor film is used which must contain seed components, such as an amorphous component and a crystalline component. Therefore, crystalline materials that are binary in nature are the ones that work well in this method. The degree of crystallinity of the precursor film is at least 30
%, preferably at least 40%, most preferably at least 50% (volume % of precursor film). Polymers, i.e. synthetic resinous materials in which precursor films are used in any way according to the invention, include olefin polymers such as polyethylene, polypropylene, poly-3-methylbutene-
1, poly-4-methylpentene-1, and copolymers of propylene, 3-methylbutene-1, 4-methylpentene-1 or ethylene with each other or with small amounts of other olefins, e.g. copolymers of propylene with ethylene. polymer, with a major amount of 3-methylbutene-1 and a minor amount of a straight-chain n-alkene, such as n-octene-1, n-hexadecene-1, n-octadecene-1, or other relatively long-chain alkenes. Olefin polymers such as copolymers of 3-methylpentene-1 and 3-methylbutene-1 with any of the same n-alkenes described above with respect to 3-methylbutene-1 are included. For example, if a propylene homopolymer is generally intended for use in a "dry stretch" process, the % crystallinity as described above, approximately
Weight average molecular weight in the range of 100000-750000 (e.g. about 200000-500000) and about 0.1-75 (e.g. about
Isotactic polypropylene having a melt index of 0.5-30) can be used to obtain a final film product with the desired physical properties. "Olefinic polymer"
and “olefin polymer”
It should be understood that the terms are used interchangeably and are intended to describe polymers made by polymerizing olefin monomers through their unsaturation. . "Solvent stretching"
Polymers suitable for use in the solvent stretch process are disclosed in the applicant's application, John. U.S. Patent Application No. 44,805 (filed 6/1/79), invented by John W. Soehngen, titled "Improved method for producing microporous films from precursor films of controlled crystal structure. A polymer used in accordance with the invention described in "Solvent Stretching Method"), the description of which is hereby incorporated by reference. So, the density of about 0.960-0.965gm/cc,
A polyethylene homopolymer having a high melt index of about 3 or more, preferably about 3-20, and a broad molecular weight distribution ratio (w/n) of about 3.8 or more, preferably about 3.8-13, is made microporous by a "solvent drawing" method. Suitable for producing quality films. Additionally, nucleating agents may be added to the polymers used to make precursor films as described in the above-referenced US patent application, in which case polymers having melt indexes as low as 0.3 may be used. Apparatus of suitable types for forming precursor films are well known to those skilled in the art. For example, shallow groove metering screws and coat
A conventional film extruder fitted with a hanger die is convenient. Generally, the resin is introduced into the hopper of an extruder, which has a jacket and screw with heating elements. The resin is melted and conveyed by a screw to a die, from which the resin is extruded in the form of a film through a slit, and the film is stretched by take-up or casting rolls. More than one take-up roll may be used in various combinations or stages. The die opening or slit width may range, for example, from about 10-200 mils. Using this type of equipment, the film is approximately 5:
It can be extruded at a drawdown ratio of 1-200:1, preferably 10:1-50:1. The term "drawdown ratio" or more simply "draw ratio" as used herein is the ratio of the film take-up or take-off speed to the film release speed at the extrusion die. The melt temperature for film extrusion is generally no more than about 100°C above the melting point of the polymer and no less than about 10°C above the melting point of the polymer. For example, polypropylene is extruded at a melt temperature of about 180-270°C, preferably 200-240°C. Polyethylene is extruded at a melt temperature of about 175-225°C. When the precursor film is to be used according to the "dry stretch" method, the extrusion operation is preferably carried out with rapid cooling and rapid drawdown to obtain maximum elasticity. This is accomplished by having the take-off rail relatively close to the extrusion slot, such as within 2 inches, preferably within 1 inch. For example, use an air knife operating at temperatures between 0°C and 40°C within 1 inch of the slot to rapidly cool the film.
That is, it can be rapidly cooled and solidified. The take-off roll may be rotating at a speed of, for example, 10-1000 ft/min, preferably 50-500 ft/min. When the precursor film is to be used according to the "solvent stretch" method, the extrusion operation can result from rapid cooling sufficient to minimize stress and obtain maximum crystallinity, yet to prevent the development of large spherulites. To minimize any accompanying orientation,
It is preferable to carry out the process with slow cooling. This is accomplished by adjusting the distance of the chill roll take-off from the extrusion slit. Although the above description has been in relation to a slit die extrusion process, an alternative process for the precursor film format contemplated by this invention is an inflation process in which the hopper and extruder are the same as the slit extruder described above. Use one that is substantially the same. From the extruder, the melt enters a die from which it is extruded through an annular slot into a tubular film having an initial diameter _D1 . Air is introduced into the system through the inlet and into the tubular film, and this air increases the diameter of the tubular film.
It has the effect of blowing up to _D2 . Means, such as air rings, may also be provided to direct air around the exterior of the extruded tubular film to provide different cooling ratios. Means such as a cooling mandle may be used to cool the interior of the tubular film. Cool the film completely,
After the curing distance, the film is wound up on a take-up roll. Using the blown film method, the drawdown ratio is preferably 5:1-100:1, the slot opening is 10-200 mils, preferably 40-100 mils, and the D ₂ /D ₁ ratio is, for example, 1.0. -4.0, preferably about
1.0−2.5, and the withdrawal speed is, for example, 30−
700ft/min. The melt temperature may be within the ranges defined above for slot die extrusion. The extruded film can then be heat treated or annealed to first improve the crystal structure, for example by increasing the crystallinity size and removing defects. Generally, this annealing is carried out at a temperature in the range of about 5-100°C below the melting point of the polymer for a few seconds to several hours, such as 5 seconds to 24 hours, preferably about 30 seconds to 2 hours. For polypropylene, the preferred annealing temperature is approximately 100-155
It is ℃. An exemplary method of carrying out the anneal is by placing the extruded film under tension or no tension in an oven at the desired temperature, where the residence time is preferably about 30 seconds to 1 hour. In a preferred embodiment, the partially crystalline precursor film obtained is processed, preferably in one of the two alternative methods described above, to obtain a normally hydrophobic microporous film that can be used in accordance with the present invention. The first preferred method (referred to hereinafter as the "dry stretching" method) as described in U.S. Pat. (2) hot stretching or hot stretching the cold stretched film until fibrils and pores or open cells extending parallel to the stretching direction are formed, and then (3) The process involves heating or thermosetting the resulting porous film under tension, ie, at a substantially constant length, to impart stability to the film. "Cold stretching" used in this text
The term stretching refers to a film in which the stretching temperature (i.e. the temperature of the film being stretched) is below the temperature at which melting begins when the film is heated uniformly at a temperature of 25°C and a rate of 20°C/min. Defined as stretching or pulling at a temperature greater than its original length. As used in this text, the term "hot stretching" refers to the temperature at which melting begins when the film is uniformly heated from 25°C at a rate of 20°C/min.
However, it is defined as stretching below the normal melting point of the polymer, that is, below the temperature at which melting occurs.
As is well known to those skilled in the art, the temperature at which melting begins and the melting temperature can be measured by a standard differential thermal analyzer (DTA) or other known equipment capable of detecting thermal transitions in polymers. The temperature at which melting begins varies with the type of polymer, the molecular weight distribution of the polymer, and the crystalline morphology of the film. For example, polypropylene elastic film is approximately
Cold stretching can be performed at temperatures below 120°C, preferably about 10-70°C, and typically at ambient temperatures, such as 25°C. The cold-stretched polypropylene film can then be hot-stretched at a temperature above about 120°C and below the melting temperature, preferably between about 130°C and 150°C.
Again, the temperature at which the film itself is stretched is referred to herein as the stretching temperature. The stretching in these two steps or stages must be in the same direction and sequential in their order, i.e. cold then hot stretching, but the cold stretched film may be stretched to any significant degree (e.g. before being hot stretched). As long as the stretching does not shrink by 5% or less of the length stretched at room temperature, it can be carried out continuously, semi-continuously or in batches. The total amount of stretching in the above two steps may range from about 10-300%, preferably from about 50-150%, based on the original length of the elastic film. Further, the ratio of the amount of hot stretching to the total amount of stretching or tension is approximately 0.10:
1 or more and 0.99:1 or less, preferably about 0.50:1-
0.97:1, most preferably about 0.50:1-0.95:1
That's fine. This relationship between "normal temperature" and "thermal" stretching is referred to as "extension ratio" in this text.
(% "thermal" elongation vs. % "total" elongation). In any stretching operation in which heat must be supplied, the film can be heated by electrical resistance methods, by passing over heated plates, by means of moving rolls that are heated sequentially via heated liquids, heated gases, etc. After the two-step or two-step stretching described above, the stretched film is thermoset. This heat treatment is approximately 125℃
for polypropylene and at temperatures ranging from below the melting temperature, preferably about 130-160°C, and for polyethylene at about 75°C.
℃ to below the melting temperature, preferably about
115-130°C, and similar temperature ranges for the other polymers mentioned above. This heat treatment holds the film under tension, i.e., under such tension that the film can shrink without shrinkage or only to a controlled extent of no more than about 15% of its stretched length, with the proviso that It must be carried out while keeping the film under no significant tension that would cause it to be further stretched by more than 15%. Preferably, the tension is such that no substantial shrinkage or stretching occurs, such as a change in stretched length of 5% or less. Preferably the period of heat treatment carried out subsequent to and after the tensile operation is at a higher annealing temperature.
It should not be longer than 0.1 seconds and generally can be within the range of about 5 seconds to 1 hour, preferably about 1 to 30 minutes. The curing step may be carried out in air or other atmospheres such as nitrogen, helium or argon. A second preferred alternative method (referred to herein as the "solvent stretching" method) for converting precursor films into microporous films as described in U.S. Pat. No. 3,839,516 consists of (1) at least two components ( for example,
an amorphous component and a crystalline component), one of which is present in a smaller volume than all the other components, and a swelling agent such that the swelling agent is adsorbed onto the film. The basic steps of contacting for a sufficient period of time; (2) stretching the film in at least one direction while in contact with the swelling agent; and (3) maintaining the film in its stretched state throughout removal of the swelling agent. It includes. The film can be stabilized by heat curing or ionizing radiation, optionally under tension. Generally, a solvent with a Hildebrand solubility parameter that is the same as or near that of the polymer will have a suitable solubility for the stretching methods described herein.
The Hildebrand solubility parameter measures the cohesive energy density. The underlying principle is therefore based on the fact that a solvent with the same cohesive energy density as the polymer has a high affinity for the polymer and is suitable for this purpose. A general set of swelling agents, one of which may be selected as appropriate for a particular polymeric film, includes: lower aliphatic ketones such as acetone, methyl ethyl ketone, cyclohexane; lower fatty acid esters such as ethyl formate;
Butyl acetate, etc.; Halogenated hydrocarbons such as carbon tetrachloride, trichloroethylene, perchloroethylene, chlorobenzene, etc.; Hydrocarbons such as heptane, cyclohexane, benzene, xylene, tetralin, decalin, etc.; Nitrogen-containing organic compounds such as pyridine, formamide, dimethylformamide, etc. ; Examples of ethers include methyl ether, ethyl ether, dioxane, and the like. Mixtures of two or more of these organic solvents may also be used. The swelling agent is carbon, hydrogen, oxygen, nitrogen, halogen,
Preferably, the compound consists of sulfur and contains no more than about 20 carbon atoms, preferably no more than about 10 carbon atoms. The "solvent stretching" step can be carried out at a temperature ranging from above the freezing point of the solvent or swelling agent to below the temperature at which the polymer dissolves (eg, ambient temperature - about 50°C). Precursor films used in "solvent stretching" methods can range in thickness from 0.1 to about 20 mils or even thicker. In a preferred embodiment, the precursor film is described in U.S. Patent Application Serial No. 44801, filed on 6/1/79, entitled "Improved Solvent Stretching Method for Producing Microporous Films," commonly filed by the assignee of the present invention. Stretch in two axial directions according to the following operations. The contents of this U.S. application are incorporated herein by reference. This method defines suitable stretching conditions in the uniaxial direction, thereby improving the air permeability of the uniaxially stretched microporous film. The uniaxially stretched microporous film can then be stretched in the transverse direction to further improve air permeability. Therefore, it is preferred to uniaxially "solvent stretch" the precursor film to about 350% or less, and most preferably 300% or more, of its original length. Typically, no further stretching in the same direction after solvent removal is used. The optional stabilization step may be a heat curing step or a cross-linking step. This heat treatment is carried out at a temperature ranging from about 125°C to below the melting temperature for polypropylene, preferably about 130-150°C; for polyethylene from about 75°C to below the melting temperature, preferably about 115-130°C, and above. The rest of the polymer is carried out in the same temperature range. This heat treatment is carried out while the film is held under tension, i.e., the film does not shrink or may shrink only to a controlled extent of no more than about 15% of its stretched length, provided that the film is further stretched by no more than 15%. It should be carried out while being held under tension that is not too great. Preferably, the tension is such that no substantial shrinkage or stretching occurs, eg, less than a 5% change in stretched length. Preferably, the duration of the heat treatment following and subsequent to the "solvent stretching" operation should be no longer than 0.1 seconds at the higher annealing temperature. Generally it may be within the range of about 5 seconds to 1 hour, preferably about 1 to 30 minutes. The curing step described above is carried out in air or other atmosphere such as nitrogen, helium or argon. If the precursor film is biaxially stretched, the stabilization step must be carried out after, and not before, the transverse stretching. Although the text description and examples relate primarily to the olefin polymers described above, high molecular weight acetal (eg, oxymethylene) polymers are also contemplated by the present invention. Although both acetal homopolymers and copolymers are contemplated, the preferred acetal polymers for purposes of polymer stability are "random" oxymethylene copolymers, which contain Repeating oxymethylene interspersed with —OR— groups (i.e., —CH ₂ —O—)
unit, where R is a divalent group containing at least two carbon atoms directly bonded to each other and located in the chain between the two valences, and any on the R group The substituents are inert, i.e., they do not involve intervening functional groups and do not induce undesired reactions, and the majority of the -OR- units are attached to each end of the oxymethylene group. exists as a single unit. Examples or suitable polymers include copolymers of trioxane and cyclic ethers containing at least two adjacent carbon atoms, such as those disclosed in Walling et al., U.S. Pat. No. 3,027,352. Consolidation is included.
These polymers in film form contain at least 20%;
Preferably it may have a crystallinity of at least 30%, most preferably at least 50%, such as 50-60% or more. Furthermore, these polymers have a melting point of at least 150°C and a number average molecular weight of at least 10,000. For a more detailed description of acetals and oxymethylene polymers, see Formaldehyde, Walter, pp. 175-191 (Reinhold, 1964). Other relatively crystalline polymers to which the invention may be applied are polyalkylene sulfides such as polymethylene sulfide and polyethylene sulfide, polyarylene oxides such as polyphenylene oxide, polyamides such as polyhexamethylene adipamide ( Nylon 66) and polycaprolactam (Nylon 6), all of which are well known to those skilled in the art and need not be further discussed in the text for the sake of brevity. The normally hydrophobic microporous film used in the present invention has a density that, under no tension, is lower than that of a corresponding polymeric material that does not have an open cell structure (e.g., materials made from these). It has a high bulk density. Thus, the film has a bulk density of about 95% or less, preferably 20-40%, of the precursor film. Stated another way, the bulk density is reduced by at least 5% and preferably 60-80%. For polyethylene, the decrease is 30-80
%, preferably 60-80%. Bulk density was about 20-40% of the starting material, and porosity increased by 60-80% due to porosity. When the microporous film is produced by the "dry stretch" or "solvent stretch" method, the final crystallinity of the microporous film is preferably at least 30%, more preferably at least 65% and even more suitably about 70%. -85%
Journal of Applied Polymer Science, Volume 2, No. 5, No. 166.
It is measured by the X-ray method described by RG Quinn et al. on page 173. For a detailed explanation of crystallinity and its significance in polymers, see Polymers and Resins, Golding (S. Van Nostrand,
(1959). The microporous film that can be used in the present invention is about 200-
It may have an average pore size of 10000 A°, typically about 400-5000 A°, more typically about 500-5000 A°. These numbers are
Textile Research Journal
Journal), January 1963, pp. 21-34, or by mercury porosimetry as described in the paper by RG Quinn et al., Geil, "Polymer Single Crystals".
Crystals, p. 69 (Interscience, 1963). When using an electronic micrograph, measurements of bubble length and width are usually made by simply using a flat ruler to directly measure the length and width of the bubbles on the electronic micrograph photographed at 5000-10000x magnification. It can be obtained by Normally, the bubble length values obtained by electron microscopy are approximately equal to the bubble size values obtained by mercury porosimetry. The microporous films used in this invention exhibit surface area within certain predictable limits when produced by either the "solvent stretch" or "dry stretch" methods. Typically such microporous films have a surface area of at least 10 m ² /g, preferably about 15-25 m ² /g.
It can be seen that the surface area is in the range of g. For films formed from polyethylene, the surface area is generally about 10-25 m ² /g, preferably about 20 m ² /g.
in the range of g. Surface area can be measured from nitrogen or krypton gas adsorption isotherms using the method and apparatus described in US Pat. No. 3,262,319. The surface area obtained by this method is usually expressed in m ² /g. To facilitate comparison of various materials, this value can be multiplied by the bulk density (g/cc) of the material to give a surface area expressed in m ² /cc. The typically hydrophobic microporous polymeric film of the present invention to which hydrophilicity is imparted is about 1 mil (0.001 inch).
It has a preferred thickness of about 8 mils. Typically hydrophobic microporous films produced according to the procedure described above are prepared by coating the surface of the micropores of the microporous film with a hydrophilic monomer or mixture thereof, and then coating the coated microporous film with a hydrophilic monomer or mixture thereof. Made wettable and/or hydrophilic by exposure to ionizing radiation. The ionizing radiation chemically fixes the hydrophilic monomer and renders the monomer relatively permanently hydrophilic. The impregnation level or amount of chemically immobilized hydrophilic monomers is adjusted within certain limits in order to avoid clogging of the air bubbles and at the same time allow adjustment of the total porosity of the hydrophilic film. Adjustment of the porosity in turn allows adjustment of the water permeability and electrical resistance of the film. The term “hydrophobic” as used in this text refers to
Approx. water per ^1cm2 of flat film surface under water pressure of
Defined to mean a surface that passes 0.010 ml/min or less. Similarly, the term "hydrophilic" is meant to apply to a surface that will pass more than about 0.01 ml/min/cm ² of water under the same pressure. The hydrophilic monomers that can be used to coat the pore surfaces of the microporous films of the invention have 2-18 carbon atoms and at least one double bond (and therefore a double bond). The bond makes the monomer polymerizable and/or copolymerizable under the influence of ionizing radiation at temperatures that do not adversely affect the microporous film) and at least one polar functional group (e.g. carboxy,
It is an organic hydrocarbon compound characterized by the presence of sulfo, sulfinohydroxy, ammonio, amino and phosphono. The hydrophilic monomers of the invention therefore include hydrocarbon compounds having 2-18 carbon atoms and with one or more polymerizable and/or copolymerizable double bonds, e.g. and unsubstituted carboxylic and dicarboxylic acids and their esters; vinyl and allyl monomers, especially itaconic, malonic, fumaric and crotonic acids 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 acids and styrenes Sulfonic acids; 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 unsubstituted or substituted alkyl, cycloalkyl or aryl vinyl ethers; vinyl-substituted silicones; vinyl-substituted Examples include aromatic or heterocyclic hydrocarbons; diallyl fumarates; diallyl maleates; alkyl-substituted phosphates, phosphites or carbonates; vinyl sulfones, reaction products of ethoxylated nonylphenols and acrylic acid, and the like. Since it is contemplated that the hydrophilic monomer will penetrate into the interior of the microporous film, it is preferable to use a hydrophilic monomer having about 2-14, most preferably about 2-4 carbon atoms. is preferred. Preferred hydrophilic monomers used in the invention are acrylic acid, methacrylic acid and vinyl acetate. The amount of hydrophilic monomer coating the interior surfaces of the cells of the microporous film is adjusted to provide the appropriate degree of impregnation as dictated by the film's intended water flow or electrical resistance end use requirements. As previously mentioned, such adjustment is sufficient to preserve the open-cell nature of the micropores, i.e., to avoid clogging of the pores after the monomer-impregnated microporous film is subjected to the radiation treatment described herein. The surfactant coatings act in a sufficient manner so that the desired properties described above are maintained for longer periods of time than can be obtained using typical surfactant coatings of the prior art. The specific amount of hydrophilic monomer that is chemically immobilized on the surface of the pores is expressed in terms of % pick-up. That is, it is the weight percent of the uncoated microporous film that represents the weight of hydrophilic monomer present in the cured microporous film. Therefore, in order to avoid clogging of the pores of the microporous film described in the main text, the impregnated amount of the hydrophilic monomer chemically fixed on the surface of the micropores should be adjusted to Up to about 10% by weight, generally about 0.1-10%, preferably about 0.5-2.5%, most preferably about 1-2.0% (e.g. 1.5%) (wt%)
Adjust so that The particular pick-up percentage selected will be determined by the end use for which the resulting hydrophilic microporous film will be used and can vary within the wide range of the pick-up percentages mentioned above. For example, if a grafted hydrophilic microporous film is to be used as a battery separator, the % impregnation is a function of the impregnation of hydrophilic monomer, the electrical resistance of the resulting hydrophilic microporous film ( (milliohms per square inch). Electrical resistance, as defined herein, is a measure of a microporous film's ability to conduct electrons. Therefore, as a general rule, the higher the electrical resistance of a microporous film, the less effective the microporous film is as a battery separator. The electrical resistivity of the microporous film is then determined at various % pick-ups of hydrophilic monomer, and the electrical resistivity is plotted as a function of % pick-up. next,
The % impregnation amount is determined based on the desired electrical resistance. As described below, the electrical resistance of the hydrophilic microporous film of the present invention is typically less than about 3 milliohms-in ² , preferably about
10 milliohms—in ² , most preferably about 5 milliohms
-Adjusted to less than in ² . When a grafted hydrophilic microporous film is used as a filter, the percent pick-up of the microporous film is selected based on the amount of water passing through the microporous film as defined herein. Therefore, the amount of water can be plotted as a function of the % pick-up of the hydrophilic monomer, and the appropriate % pick-up
is selected based on the desired amount of water. If the microporous film is intended to be used for a specific purpose, the pore size of the film is selected to act as a barrier for the material to be separated, and the amount of water is adjusted to the selected pore size. Adjust as desired within limits. In other words, the amount of water is approximately 0.01cc/with a pressure difference of approximately 1 atmosphere.
min/cm ² , preferably about 0.05 cc/min/cm ² , most preferably about 0.5 cc/min/cm ² . Thus, the present invention has several advantages over previously produced hydrophobic films. Because it is possible to retain the open-cell nature of the hydrophilic microporous films produced according to the present invention, such films are capable of transporting water through the film by diffusion, which is a slow process. For,
Enables large amounts of water to be transferred through the film. The mass transport effect also contributes to a reduction in electrical resistance. Another advantage of the microporous film of the present invention derives from the chemical immobilization of hydrophilic monomers onto the microporous film,
This extends the duration of the presence of the hydrophilic monomer on the micropore surface for a longer period of time than typical surfactants, especially after repeated washings. Additionally, the dimensional stability of the films of the present invention is also improved. It may be appropriate, if possible, to convert the polar functional groups of the hydrophilic monomers into their most polar form. For example, this converts an acid functional group to a base e.g.
This is achieved by reaction with KOH to form the corresponding salt. So, the salt form, in this case, has a chemically fixed film of about 5-30
This can be achieved by soaking in a 2% solution of KOH for minutes. This enhances the film's wettability and increases its flexibility. Any of the well known coating methods can be used to coat the microporous film. However, such a method requires that the percentage impregnation of the hydrophilic monomer can be controlled with sufficient accuracy. A preferred method of accurately and thoroughly coating the internal surfaces of the cells of a microporous film is to contact the microporous film with a monomer vapor that condenses on the microporous surfaces. The % pick-up is determined by the equilibrium vapor pressure of the hydrophilic monomer (i.e., the rate of condensation of the monomer vapor is equal to its rate of evaporation at a given temperature) and the rate at which the hydrophilic monomer contacts the microporous film. It can be controlled by adjusting the time that the The equilibrium vapor pressure required to obtain the desired amount of hydrophilic monomer coating at a constant temperature can be determined by plotting the percent pick-up versus time at various equilibrium vapor pressures while keeping the temperature constant. Desired. The percentage pick-up selected as described above will depend on the particular properties sought to be imparted to the microporous film. The equilibrium vapor pressure required to obtain the appropriate percent pick-up is easily determined from the above plot. The contact time between the monomer vapor and the microporous film is also determined in a similar manner. Thus, in a continuous process, the microporous film is passed continuously through a chamber containing hydrophilic monomer vapor at the appropriate equilibrium vapor pressure and temperature. The contact time of the microporous film with the hydrophilic monomer vapor is controlled by adjusting the path length of the microporous film in the vapor and the line speed. Clearly, the vapor coating technique is important because the critical temperature of the hydrophilic monomer (i.e., the temperature at which no liquid monomer can exist, independent of pressure) is due to the properties of the microporous film at the particular contact time used. It may only be used below the temperature at which it is adversely affected. Preferably, the steam-coating technique is carried out at atmospheric pressure and at a temperature that produces a suitable steam pressure. Typically, the temperature at which the steam coating technique is carried out using atmospheric pressure is such that when the hydrophilic monomer is acrylic acid, methacrylic acid or vinyl acetate,
The temperature changes from about 50℃ to 170℃. Subatmospheric and superatmospheric pressures can also be used with appropriate adjustment of temperature and contact time. Since the % pick-up is determined after the curing process, an amount in excess of the % pick-up of hydrophilic monomer is initially applied to the microporous film to compensate for any loss of monomer that may occur during the curing process. Another suitable method by which a microporous film may be coated with a hydrophilic monomer is by dissolving the hydrophilic monomer in a volatile solvent to form a padding bath. Examples of the solvent include methylene chloride. The padding bath is then used in a reverse roll coating technique. In this method, a doctor roll is partially placed in a padding bath of monomer coating solution. A second driven roll directs the uncoated hydrophobic microporous film web through a nip formed by itself and the doctor roll. The two rolls, preferably driven separately, rotate in the same direction so that the coated film web is guided in the direction in which the uncoated film emerges. The monomer coverage deposited on the film is a function of the speed of the doctor roll and second film drive roll and the difference in size of the nip formed by the two rolls. Alternatively, a squeeze roll method may be used. In this method, the film is introduced into a padding bath of monomer coating solution and squeezed between two downwardly positioned squeeze rolls. Coverage is a function of the gap size between the two squeeze rolls and the pressure exerted between them. Another alternative method for coating a substrate hydrophobic microporous film is the wire wound metered rod method. This method is the same as the squeeze roll method. However, the microporous film after being coated by being guided through a monomer coating solution bath is squeezed between a pair of wire-wound metering rods, and these rods are arranged by an arrangement of wire windings around the metering rods. The exception is that the amount of coverage is adjusted. In 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 of the variables discussed in the description of these methods. Furthermore,
In all three methods, monomer coverage is also a function of the concentration of monomer in the padding bath. The hydrophilic monomer padding bath uses monomers containing common organic solvents with a boiling point lower than that of the hydrophilic monomer, such as methylene chloride, as well as acetone,
Provided by dissolving in methanol, ethanol and isopropanol. The concentration of hydrophilic monomer in the padding bath is adjusted to obtain the appropriate amount of monomer applied during solvent evaporation. Generally, the monomer concentration of the padding bath is about 1-30%, preferably about 5-15%, based on the weight of the bath.
(weight). When a microporous film is coated with a hydrophilic monomer using a padding bath, the solvent present therein is removed by passing the coated film through a dryer. The temperature of the dryer evaporates only the solvent,
The temperature must be high enough to leave the deposited monomer on the microporous surface of the film. If the bubbles of the microporous film are coated with the hydrophilic monomer and, if present, the coated microporous film is exposed to ionizing radiation to remove the hydrophilic monomer after removing the solvent. It is chemically fixed to the usually hydrophobic micropore surface and makes the surface hydrophilic. When exposed to ionizing radiation, many possible mechanisms or combinations thereof may be activated to achieve the desired effect. The hydrophilic monomer then becomes chemically bound to the micropore surface and/or
Monomeric layers or sleeves may be formed by polymerization and/or copolymerization. This layer or sleeve is formed by chemically and/or physically as a result of a confinement effect on the polymeric sleeve by the contour of the microporous surface, e.g. by random chemical bonding of the sleeve surface to the microporous surface. tightly bonded to the surface. "chemical fixation"
The term fixation is intended to encompass all of the above mechanisms or combinations thereof. Without being limited to the particular mechanism by which the desired improvement may be achieved, the properties of the typically hydrophobic microporous film are modified in one or more ways based on the percent pick-up of hydrophilic monomer. The resulting radiation-treated microporous film is thus rendered hydrophilic for a longer period of use than would otherwise be obtained with a typical surfactant coating. Furthermore, the open cell property of the film is
Microporous films are reserved for use in applications where high volume transport and low electrical resistance are required. As mentioned above, chemical fixation of hydrophilic monomers to normally hydrophobic microporous films is accomplished by exposing the hydrophilic monomer-impregnated microporous film to ionizing radiation. Ionizing radiation is defined in the text as generating ions and breaking chemical bonds, thereby inducing free radical reactions between the hydrophilic monomers used and between the monomers and the microporous surfaces as described in the text. is defined as consisting essentially of a type that provides emitting elementary particles or photons with sufficient internal energy to do so. Ionizing radiation may be conveniently utilized in the form of ionizing particle radiation, ionizing electromagnetic radiation, and actinic radiation. The term "ionizing particle radiation" refers to electrons or highly accelerated nucleons that are oriented in such a way that the particles are projected onto the irradiated mass, such as protons, neutrons, alpha particles, dieuterones, beta particles, etc. Used to refer to the release of particles or their analogs. The charged particles can be produced using, for example, a low-energy (i.e. ^, 200 KeV) elongated electron beam oscillator (e.g., ElectroCurtain Tm manufactured by Energy Sciences Corporation).
(ELECTROCURTAIN ^Tm ), accelerator with resonance chamber,
Acceleration can be achieved with the aid of potential gradients by devices such as van degraff generators, betatrons, synchrotrons, cyclotrons, etc.
Neutron radiation can be generated by bombarding selected light metals, such as beryllium, with high energy positive subatomic particles. Particle radiation can also be obtained using atomic piles, radioactive isotopes or other natural or artificial radioactive materials. "Ionizing electromagnetic radiation" is generated by bombarding a metal target, such as tungsten, with electrons of appropriate energy. This energy is
A potential of 0.1 million electron volts (mev.) or more is given to electrons by an accelerator. In addition to this type of radiation, the ionizing electromagnetic radiation suitable for the implementation of the invention, commonly called X-rays, can be obtained using nuclear reactors (piles) or by the use of natural or artificial radioactive materials, such as cobalt-60. The hydrophilic monomers described herein can also undergo chemical fixation by exposure to actinic radiation. Generally, the wavelength used at which sensitivity to actinic radiation occurs is approximately 1800-4000 angstrom units. A variety of suitable sources of actinic radiation are available in the art, including quartz mercury lamps, ultraviolet core carbon arcs, and high flash lamps. When using actinic radiation, an initiator may be used. Suitable sources of ionizing radiation are e.g.
3702412, 3745396 and 3769600, which patents are incorporated herein by reference. Technology for precise process control
It has been sufficiently developed to allow the conversion of the results of one type of radiation to another, and appropriate adjustment of the techniques described herein can be used interchangeably for the production of any desired product. . about 1.0-10 megarads (mrad), preferably about 2
Achieve chemical fixation using a radiation dose of -5 mrad (e.g. 3 mrad). A megarad is a million rad. One rad is the amount of ionizing high-energy radiation that results in the absorption of 100 ergs of energy per gram of absorbent material. This unit is widely used as a customary means of measuring radiation absorption by materials. Preferably, chemical fixation is achieved using minimal ionizing radiation due to economic considerations. Excessive amounts (ie, greater than about 20 mrads in a single exposure) should be avoided to avoid overheating and shrinkage of the film and degradation of the hydrophilic monomer and microporous film. If the amount is too low, the hydrophilic monomers through which chemical fixation is achieved will be rapidly lost. The minimum radiation dose for chemical immobilization of hydrophilic monomers varies depending on the type of polymer used to make the microporous film. So, for example, if the polymer is polyethylene, the radiation dose should not be less than about 1 mrad, while for polypropylene the radiation dose should not be less than about 2 rads, preferably about 3 rads. Although room temperature may be conveniently used for irradiation, elevated temperatures may also be used. Since oxygen tends to inhibit grafting of hydrophilic monomers onto the microporous film, radiation treatment of the microporous film is preferably carried out under an inert atmosphere, such as nitrogen or other inert gas. As mentioned above, the percent pick-up can be selected based on the electrical resistance of the microporous film. Electrical resistance of microporous films (DC method) is determined by injecting a sample of microporous film with a known surface area (e.g., 0.2 square inches) into 40% by weight KOH for 24 hours.
Measured by immersion in an aqueous solution. The resulting sample was then placed into a working cadmium electrode (i.e.
anode and cathode), and a direct current of known amperage (e.g., 40 milliamps) is passed through the cell between the electrodes. Potential drop across the film (E)
is measured with an electrometer. The potential drop (E') across the cell without the microporous film placed therein is measured using the same current. Next, the electrical resistance of the microporous film is determined using the following equation. ER = (E' - E) A/I where A is the surface area of the exposed film (in square inches), I is the current through the cell (milliamperes), and ER is the electrical resistance of the microporous film ( milliohms per square inch), and E' and E are as described above. The water permeability or water flow rate of the hydrophilic microporous film of the present invention is determined when water is placed under a pressure difference of 1 atmosphere,
It is determined by measuring the flow rate of water through a specific surface area of the film. The water flow rate is then expressed in units of the amount of water in cm ³ per minute per cm ³ of the film surface, ie cc/min/cm ² . The air permeability of the microporous film of the present invention was determined by the Gurley test, i.e., ASTM D726
It is measured by attaching a film having an area of 1 square inch to a standard Gurley densometer. water on film
Apply a standard differential pressure (pressure drop across the film) of 12.2 inches. The time (in seconds) it takes for 10 cm ³ of air to pass through the film is a measure of air permeability. about
A Gurley value of 1.5 minutes or more indicates that the pores are clogged. The hydrophilic microporous films of the present invention have many different uses. In particular, they have found utility in areas where controlled passage of moisture through a film or surface is desired. Additionally, films made in accordance with the present invention may be used as filter membrane supports or filters useful in separating ultrafine materials from various liquids, and as battery separators. The present invention will be explained in more detail with reference to the following examples. All parts and percentages in the examples and text are by weight unless otherwise specified. EXAMPLE 1 Part A This example illustrates the production of a normally hydrophobic polyolefin-based microporous film by the "dry stretch" process exemplified in US Pat. No. 3,801,404. Crystalline polypropylene with a melt index of 0.7 and a density of 0.92 was coated using a 1-inch extruder with a shallow metering screw.
Melt extrude at 230°C through a hanger type 8 inch slit die. The length to diameter ratio of the extruder barrel is 24/1. The extrudate is very rapidly drawn down to a melt draw down ratio of 150 and contacted with a rotating casting roll maintained at 0.75 inches from the lip of the die at 50°C. It can be seen that films produced in this manner have the following properties: thickness
Recovery rate from 50% elongation at 0.002 inches and 5℃
50.3%, crystallinity 59.6%. Samples of this film are oven annealed in air with slight tension at 140° C. for approximately 30 minutes, removed from the oven, and allowed to cool. The annealed elastic film samples are then subjected to cold and hot stretching at a stretch ratio of 0.50:1 and then heat cured under tension or constant length at 145° C. for 10 minutes in air. The room temperature stretching part was carried out at 25℃,
And the hot stretching part was carried out at 145℃, and the total stretching was 100% based on the original length of the elastic film.
It is. The resulting film has an average cell size length of about 3000 angstroms, a crystallinity of about 59.6%, and a surface area of about 8.54 m ² /g. The thickness of the microporous film is 1 mil. Part B A continuous roll of microporous film 100 feet long and 6 inches wide prepared according to Part A was passed through a bath of glacial acrylic acid (i.e., 100%) and then passed through a squeeze roll and measured by IR analysis. to obtain an impregnated amount of 1.5% by weight after curing based on the weight of the film before impregnation. The impregnated film is then passed under the window of an elongated electron beam generator (ie, 24 inches long) at a line speed of 20 feet/minute. A line speed using an electron beam generator is set to give a radiation dose of 3 megarads. Under the curtain window, the oxygen content of the atmosphere in contact with the microporous film is maintained below 500 ppm by sealing the window in a nitrogen-swept chamber. A sample of the cured dry microporous film is then tested for wettability in a drop test. The drop test is performed by placing a 0.6 m drop of a 2% KOH aqueous solution on the surface of the film. Next, observe the film with the naked eye. The film is determined to be swollen when the part of the film on which the water droplets are placed becomes translucent and the opposite side of the film on which the water droplets are placed appears wet (represented by the annotation "yes" in Table 1). as). Drop tests are performed on off-line samples of the film immediately after irradiation and on samples stored for one week at room temperature. Several other samples were taken from the cured microporous film roll and tested for 1 hour, 24 hours, 4 days and 8 hours.
After being immersed in a 40% by weight aqueous solution of KOH maintained at 60° C. for days, it is tested for electrical resistance as described in the text. Average the results. Immersion of the film sample in hot KOH for progressively longer times simulates long-term aging. The surface area of each sample tested for electrical resistance is
It is 0.2 square inches, and the current used is 40 mA DC. Three samples of cured microporous film rolls are tested for airflow per Gurley test ASTM-D-726B. The results obtained are summarized in chart format in the table below. Samples of cured microporous film rolls, each sample having a surface area of 11.3 cm ² , were placed in a millipore filter housing for 24 hours.
Test for water flow after immersion in a 31% aqueous solution of KOH maintained at 60°C. The Millipore filter is filled with water and pressurized to an atmospheric pressure differential across the film sample. Water is collected as it passes through the microporous film over a 5 minute period. The results are converted to cc of water sampled per cm ² of film surface. The results are summarized in table. Comparative Example Example 1 is repeated except that the radiation dose used for curing is reduced to 1 megarad. An uncoated microporous film prepared according to Example 1 was also exposed to ionizing radiation in the same manner as described in Example 1, tested, and used as a control. The results are summarized in table. As is clear from the data in the table, the amount of 1 megarad is too low to achieve effective curing of the polypropylene microporous film as indicated by the variable electrical resistance of the microporous film of the comparative example. It is considered too much. The positive swelling test of the comparative example for the off-line samples is believed to be due to residual acrylic acid that was not chemically fixed by irradiation. The negative wettability of the film of this comparative example after one week is believed to be due to the loss of acrylic acid over the one week storage period. This is confirmed by the variable electrical resistance of the film after one week of storage. It can be seen that the electrical resistance of the film of Example 1 increases after 4 days of exposure to hot KOH and then decreases slightly after 8 days of exposure. It is believed that the decrease after 4 days of aging is due to complete conversion of the acrylic graft to its salt form, whereas after 4 days of aging, salt conversion is only partially complete. Complete conversion of the acrylic graft to its salt form is therefore expected to reduce electrical resistance.

Table Example 2 Part A This description illustrates the production of polyethylene microporous films by the "solvent stretching" method. Melt index 5.0; weight average molecular weight approx.
80000, density 0.960g/cc and molecular weight distribution ratio approx.
9.0 is produced by blown film extrusion to form a precursor film (3 mm thick) and then allowed to cool by quenching with air at 25°C. A sample of the resulting precursor film was then immersed in trichlorethylene at 70°C for 1 minute and then strained at a strain rate of 150%/min while still immersed in trichlorethylene maintained at a temperature of 70°C. (i.e., 300% full stretch). The trichlorethylene is evaporated off, the sample is stretched in the cross machine direction to approximately 50% stretch, and air-dried in the stretched state. Drying is carried out at 25°C. The resulting microporous film has a crystallinity of about 60%,
It exhibits an average cell length of about 5000 angstroms and a surface area of about 10-25 m ² /g. Part B Soak several 1 mil thick microporous film samples prepared according to Part A in glacial acrylic acid (100%);
The film becomes translucent. Allow the sample to dry until the film has a white, opaque appearance. This appearance is approx.
This shows the appropriate amount of monomer that can achieve an impregnation amount of 1.5%. The resulting sample was then passed directly under an extended electron beam curtain (i.e., 24 inches long) and the atmosphere in contact with the microporous film under the window was changed to oxygen in the manner used in Example 1.
Irradiate with various radiation doses while maintaining the radiation below 500 ppm. The resulting cured film is tested for Gurley air flow, electrical resistance (after immersion in 40% KOH solution for 24 hours at 60°C) and wettability by drop test in the manner of Example 1. Operation 1 with results in table
This is summarized as 8. The percent pick-up of the resulting cured film samples was also determined by infrared analysis, and the results are shown in Table 1. As can be seen from the results of runs 4 and 7 in Table 1, the electrical resistance is variable and the sample is not wet. These results may be attributed to the use of radiation doses that were too low to achieve chemical fixation. It is therefore believed that the hydrophilic monomer evaporates, as evidenced by the lack of any measurable pick-up with consequent loss of properties. Film samples from runs 1, 2, 5, 6 and 8 exhibit low electrical resistance and are wettable. The higher electrical resistance of the Run 3 film samples is likely due to the macroscopically observed film inhomogeneity.
Additionally, these film samples undergoing chemical fixation show only a slight decrease in air permeability, as indicated by 1.5% pick-up. From this,
It becomes clear that the bubbles remain unclogged even after chemical fixation.

[Table] *The high electrical resistance is thought to be due to the non-uniformity of the film.
The principles, preferred embodiments, and mode of operation of the invention are detailed in the preamble. However, the invention is not intended to be limited to the particular forms disclosed. Modifications and changes may be made by those skilled in the art without departing from the spirit of the invention.

Claims (1)

[Scope of Claims] 1. In a method for making a normally hydrophobic polyolefin microporous film hydrophilic, improving its water flow rate and reducing its electrical resistance: (a) A cover of a precursor film for producing the same; Umbrella density decreased compared to density, approximately 200 to 10,000
The microporous surface of the typically hydrophobic polyolefin-based open-cell microporous film is characterized by having an average pore size of about 2 to 18 carbon atoms and a surface area of at least about 10 m ² /g. At least one hydrophilic organic hydrocarbon monomer characterized by the presence of at least one double bond and at least one polar functional group
and (b) applying the hydrophilic organic hydrocarbon monomer to the surface of the micropores of the microporous film in an amount sufficient to maintain the open-cell nature of the micropores, and (b) applying the hydrophilic organic hydrocarbon monomer to the surface of the micropores of the microporous film in an amount sufficient to maintain the open-cell nature of the micropores; Approximately 0.1-10% by weight based on the weight of the microporous film
A process characterized in that the coated microporous film of (a) is chemically fixed by irradiating it with about 1 to 10 megarads of ionizing radiation in an amount sufficient to obtain an impregnation level of . 2 The hydrophilic hydrocarbon monomer has about 2 to 14 carbon atoms and at least one polar functional group selected from the group consisting of carboxyl, sulfo, sulfino, hydroxy, ammonio, amino and phosphono.
The method according to claim 1, comprising: 3. The method of claim 1, wherein the hydrophilic organic hydrocarbon monomer is selected from the group consisting of unsubstituted and alkyl substituted acrylic acids, vinyl ethers, vinyl esters, and mixtures thereof. 4. The method of claim 1, wherein the hydrophilic organic hydrocarbon monomer is selected from the group consisting of acrylic acid, methacrylic acid, vinyl acetate, and mixtures thereof. 5. The normally hydrophobic microporous film is provided from a polymer selected from the group consisting of polyethylene and polypropylene and is produced by a "solvent stretching" or "dry stretching" process.
The method described in section. 6 The microporous surface of the normally hydrophobic microporous film is chemically fixed with a hydrophilic organic hydrocarbon monomer impregnation amount of about 0.5 to 2.5% by weight based on the weight of the uncoated film. The method described in Scope 1. 7. The coating step (a) involves the microporous surface of an open-cell, normally hydrophobic, microporous film made from a polymer selected from the group consisting of polyethylene and polypropylene (so that said film is similar to the precursor film from which it is made). an acrylic material (characterized by a reduced bulk density compared to that of an acrylic material), a crystallinity of about 30% or more, an average pore size of about 400 to 5000 angstroms, and a surface area of at least about 10 m ² /g. The method according to claim 1, which comprises coating with at least one hydrophilic organic hydrocarbon monomer selected from the group consisting of acid, methacrylic acid, and vinyl acetate. 8 (a) Bulk density reduced compared to the bulk density of the precursor film used to manufacture it, approximately 200 ~
an open cell usually hydrophobic microporous film characterized by having an average pore size of 10000 angstroms and a surface area of at least 10 m ² /g; and (b) having about 2 to 18 carbon atoms and at least one A coating on the microporous surface of a microporous film of at least one hydrophilic organic hydrocarbon monomer characterized by the presence of a double bond and at least one polar functional group (and thus the hydrophilic organic carbon monomer) The hydrogen monomer coating is sufficient to retain the open cell nature of the microporous film upon exposure to about 1 to 10 megarads of ionizing radiation, and is chemically fixed to the microporous surface of the microporous film in an amount sufficient to obtain an impregnation level of about 0.1 to 10% by weight. 9. Claims in which the hydrophilic organic hydrocarbon monomer has about 2 to 14 carbon atoms and at least one polar functional group selected from the group consisting of carboxyl, sulfo, sulfino, hydroxy, ammonio, amino and phosphono. Hydrophilic microporous film according to item 8. 10. The hydrophilic microporous film of claim 8, wherein the hydrophilic organic hydrocarbon monomer is selected from the group consisting of unsubstituted and alkyl-substituted acrylic acids, vinyl esters, and mixtures thereof. 11 The hydrophilic organic hydrocarbon monomer is acrylic acid,
9. The hydrophilic microporous film according to claim 8, which is selected from the group consisting of methacrylic acid, vinyl acetate, and mixtures thereof. 12 The normally hydrophobic polymeric microporous film is selected from the group consisting of polyethylene and polypropylene and is "solvent stretched" or "dry stretched".
9. A hydrophilic microporous film according to claim 8, which is produced by a method. 13. The hydrophilic monomer is chemically immobilized on the microporous surface at an impregnation level of about 0.5 to 2.5% by weight based on the weight of the uncoated microporous film.
The hydrophilic microporous film described in . 14. An open-cell, usually hydrophobic, microporous film produced by a "solvent stretching" or "dry stretching" process from a polymer selected from the group consisting of polyethylene and polypropylene. Reduced bulk density compared to the bulk density of the precursor film, at least
30% crystallinity, an average pore size of about 400-5000 angstroms, and a surface area of at least about 10 m ² /g); and the coating on the microporous surface of the microporous film is non-substituted. and alkyl-substituted acrylic acids, vinyl esters, and mixtures thereof, about 2
At least one hydrophilic organic hydrocarbon monomer having ~18 carbon atoms, such that the hydrophilic organic hydrocarbon monomer coating becomes microporous upon exposure to about 2 to 5 megarads of ionizing radiation. chemically fixed to the surface of the microporous film in an amount sufficient to maintain the open-cell nature of the microporous film and to obtain an impregnation amount of about 0.1 to 10% by weight based on the weight of the uncoated microporous film. 9. The hydrophilic microporous film according to claim 8, characterized in that the hydrophilic microporous film comprises: 15 The hydrophilic monomer is selected from the group consisting of acrylic acid, methacrylic acid, vinyl acetate, and mixtures thereof, such that the hydrophilic monomer has an amount of about 0.5% based on the weight of the uncoated microporous film. 15. The hydrophilic microporous film of claim 14, wherein the hydrophilic microporous film is present on the surface of the micropores in an amount of ~2% by weight.