NL8004988A - HYDROFILE MONOMER-TREATED MICROPOROUS FILMS. - Google Patents

Description

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Hydrophilic monomer-treated microporous films.

The invention relates to hydraulic monomer-treated microporous films with improved water permeability and / or reduced electrical resistance.

Recent developments in the field of open-celled microporous polymer films such as e.g. described in U.S. Pat. Nos. 3,839,516, 3,801.1 * 01 *, 3,679 * 538,. 3,558,764βι 3, 26,754, have led to investigations to find applications that could exploit the special properties of these new films. Such films, which are essentially a gas-permeable water block, can be used as drains, gas-liquid transfer media, battery separators and a variety of other applications.

One drawback of these films, which has historically limited the number of applications for which they can be used, is their hydrophobic nature. This is especially true when using polyolefin films, a preferred type of polymeric material often used in the manufacture of microporous films. Since these films are not "wetted" not by water and aqueous solutions, they could successfully be used for such logical applications as filter media, electrochemical separators and the like.

Several proposals have been made in the past to overcome these problems, such as e.g. described in U.S. Pat. Nos. 3,853,601, 3,231,530, 3,215,156, and Canadian Patent 981,991, in which various hydrophilic coatings or impregnants are used.

Such coatings or impregnants, although effective for a limited period of time, tend to be removed in a relatively short time by solutions contacting the films or fibers in which they are present.

Others have attempted to impart a hydrophilic character to a normal hydrophobic microporous film using low energy plasma treatments. Such plasma treatments are accomplished by first activating surface locations of the microporous film using argon or hydrogen plasma and then inoculating a suitable free-radical polymerization group such as acrylic acid thereon.

The plasma treatments result in a film with only a surface that is wettable again. The surface of the film also becomes clogged in a wet state, thereby impeding or preventing the free flow of water through the interior of the film.

The unavoidable clogging of the surface pores renders the film unsuitable for certain filtering applications, increases the electrical resistance of the film and reduces the dimensional stability of the films as evidenced by significant shrinkage on drying.

As mentioned above, a drawback of the plasma treatment is its limited ability to only wettable the surface of the microporous film.

It has been observed that due to the unusually common large surface area of a microporous film of the type described herein, only surface wettability does not guarantee that the film will exhibit certain functional properties such as low electrical resistance and water flow rates through the film. which are comparable to known surfactant systems as discussed above.

The main drawback of plasma treatment, viz. the clogging of the pores and only surface wettability are believed to result from a combination of factors, such as the tendency of the low energy plasma to be easily deactivated by the large surface area of the 8004988 microporous film. This decreases the probability that an internal site in the microporous film is activated. Accordingly, hg competes for the entering enthare monomers with the free-radical polymer grafts originally formed on the surface of the film when the extensible monomer first contacts the plasma-activated microporous film surface. Therefore, the graft polymer chains originally present on the film surface multiply at a continuously higher rate as the reaction proceeds. As a result, the formed elongated graft copolymer chains, which occur on the surface of the film, become entangled and clog the surface micropores in the presence of water.

Other attempts to provide hydrophilic film using a plasma treatment are described in U.S. Pat. Nos. 3,992,995 and U0-6.8U3.

Another technique for wettable polyethylene film and rendering it suitable for use in electrical battery separations is described by 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. 7 ^ 5,571 (1972) summarized in 78 C.A. 5031f (1973); V. D'Agostino and J. Lee, Low Temperature Alka. line Battery Separators, 27 Power Sources Syrap. 87-91 (1976) summarized 25 in 86 C.A. 158277f; 19 V. D'Agostino, J. Lee and G. Orban,

Zinc-Silver Oxide Batteries, (A. Fleischer and J. Lander, ed, 1971)

These articles discuss or relate to a commercial product known as Permion (trademark) developed by RAI Research Corp. Briefly, the method of manufacture of this material consists of cross-linking a 0.025 mm thick polyethylene sheet using β-blasting followed by seeding with methacrylic acid in a suitable solution under C060 γ blasting. The grafted material is washed to remove the homopolymer, then converted to salt form in warm Κ0Η, washed again to remove the residual base, dried 8004988 * and packaged. The original cross-linking step causes micro-cracks or longitudinal slits in non-porous ethylene film that are so small that they are not visible even under an electron microscope. The diameter of these micro cracks 5 is estimated to be approximately 20 µm. (10 “® cm). The micro cracks are then inoculated with the methacrylic acid. The film obtained is therefore not microporous in the sense of the microporous films used according to the invention, and which have an average size of 100-5000 S. The particularly small size of the micro-cracks of Permion (trademark) generally prevent a mass transfer of moving electron-containing species formed by oxidation reduction reactions through the film at a significant rate. This is reflected in the relatively high (e.g., 30-1 * 0 milli-15 ohm-6.2l * cm) electrical resistances exhibited by films of this type. In addition, Permion (trademark) is not dimensionally stable in more than one direction, as evidenced by significant swelling.

It is known that non-porous polymeric substrates, such as polyethylene and polypropylene, can be reacted with different monomers, such as acrylic acid, using different types of ionizing radiation, as described in U.S. Patents 2,999,056, 3,281,263 , 3,372,100 and 3,709,718. However, since none of these patents are directed to microporous films, they are not directed to the particular problems associated therewith.

Therefore, the search has continued for a relatively persistently wettable hydrophilic microporous film, which has a low electrical resistance as well as an improved water flow rate through the microporous film. As a result of this research, the invention was accomplished.

The object of the invention is therefore to provide a method to render a normally hydrophobic microporous film relatively permanently hydrophilic and thereby to improve its water permeability.

Another object of the invention is to provide a method for lowering the electrical resistance of a normally hydrophobic microporous film.

Another object of the invention is to provide a hydrophilic microporous film with reduced electrical resistance.

In addition, the invention aims at overcoming the problems of the prior art discussed above.

The invention now provides a method of rendering a normally hydrophobic polyolefinic microporous film hydrophilic, thereby improving the water flow rate and reducing its electrical resistance by providing a) the surface of the micropores of a normal hydrophobic polyolefinic open cell microporous film having a reduced bulk density compared to the bulk density of a precursor film from which it is made, an average pore size of 200-10,000 S, and a surface area of at least 10 m / g with at least one hydrophilic organic hydrocarbon monomer having 2-18 carbon atoms, containing at least one contains ethylenically unsaturated bond and contains at least one polar functional group; and b) chemically securing to the surface of the micropores of the microporous film an amount of this hydrophilic organic hydrocarbon monomer sufficient to maintain the open cell nature of these micropores and sufficient to obtain an adherence of 0.1- 10% by weight, based on the weight of the uncoated microporous film, is obtained by irradiating the coated microporous film from (a) with 1-10 megarad of ionizing radiation.

The invention also provides a hydrophilic open cell microporous film consisting of: (a) an open cell normal hydrophobic microporous film having a reduced bulk density compared to the bulk density of an 8004988 6 precursor film from which it is made, an average pore size of 200 - 10 000 lb and a surface area of at least 2 10 m / g; and (b) a surface coating on the micro-pores of the microporous film of at least one hydrophilic organic hydrocarbon monomer of 2-18 carbon atoms, containing at least one ethylenically unsaturated bond and at least one polar functional group, the hydrophilic organic hydrocarbon monomer coating is chemically attached to the surface of the micropores of the microporous film by exposure to 1-10 megarad of ionizing radiation, and in an amount sufficient to maintain the open-cell nature of the microporous film and obtain an adhesion of 0.1 -10 wt.%, Based on the weight of the uncoated microporous film.

The essence of the invention lies in finding that open-cell microporous films can be made relatively permanently wettable and / or hydrophilic when their pores are chemically bonded with a set amount of inoculable hydrophilic monomer by exposure to ionizing radiation while the open-cell nature of the microporous film is maintained. In addition, such chemical fixation of the hydrophilic monomers takes place even within these pores internally within the microporous film.

It is believed that by controlling the amount of monomer that is chemically attached to the surface of the micropores, the polymer chain grafting that occurs upon exposure to radiation can be placed along the surface of the pores. This prevents entanglement and clogging of the 30 pores.

The invention is directed to a hydrophilic microporous film and a method for its manufacture.

Porous or cellular films can be classified into two general types: one type in which the 35 pores are not interconnected, ie a closed cell 8004988 λ Λ \ 7 film, and the other type in which the pores are essentially interconnected along complex paths, which can run from one external surface or surface area to another, ie an open-cell film. The porous films of the invention are of the latter type.

Furthermore, the pores of the porous films of the invention are microscopic, i.e. the details of their pore configuration or arrangement is only observable in microscopic examination. In practice, the open-cell pores in the films are generally smaller than those that can be measured using an ordinary light microscope, because the wavelength of visible light, which is about 5000X, is longer than the longest Tlak or surface size of the open cell or pore. However, the microporous films of the invention can be identified by electron microscopic techniques capable of discerning details of the pore structure below 5000 X.

The microporous films of the invention also have a reduced bulk density, which is sometimes referred to simply as "low" density. I.e. that these films have a bulk density or total density less than the bulk density of corresponding films formed of identical polymeric material but have no open-cell or other hollowed structure. The term "bulk density" as used herein refers to the weight per gross or geometric unit volume of the film where the gross volume is determined by immersing a known weight of the film in a vessel partially filled with 30 mercury at 25 ° C and atmospheric pressure. The volumetric increase in the level of the mercury is a direct measure of the gross volume. This method is known as the mercury volume menometer method and is described in the Encyclopedia of Chemical Technology, vol. b3 p. 892 (interscience 19 ^ 9).

Porous films have been prepared which have a micro-8004988 \ 8 open-cell porous structure and which also have a reduced bulk density. For example, films of this microscopic structure are described in U.S. Patent 3, 26, 75 ^.

The preferred method of manufacture described therein includes drawing or stretching at room temperature, ie "cold drawing", a crystalline elastic precursor film in an amount of 10-300% of its original length, with subsequent stabilization by hot melt curing the drawn film under such a tension that the film cannot shrink freely or shrink only to a limited extent. Other methods of making microporous films are e.g. described in U.S. Pat. Nos. 3,558,766, 3,8U3,762, 3,920,785, British Pat. Nos. 1,180,066 and 1,198,695.

Although all of the above patents disclose methods of making normally hydrophobic microporous films that can be rendered hydrophilic in accordance with the invention, the preferred normally hydrophobic microporous films are provided in accordance with the methods described in U.S. Pat. No. 3,801,80U, wherein a Method is described for making microporous films referred to herein as the "dry stretch" method and US Pat. No. 3,839,516, which describes a method for making microporous films referred to herein as "solvent drawing" method.

Each of these patents describes preferred alternative routes for obtaining a normal hydrophobic microporous film by treating a precursor film according to specific processing steps.

The preferred precursor film which can be used in the manufacture of microporous films corresponding to "dry stretch" and "solvent stretch" method are specifically described in detail in the respective patents mentioned above. For example, the "dry stretch" method uses a non-porous, crystalline, elastic polymeric film with an elastic recovery and zero recovery time (defined below) when subjected to a standard stress (elongation) of 50% at 25 ° C and 65 ° C. % relative humidity of at least Ho #, preferably at least 50% and most preferably at least 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 can be calculated as follows: 10 Elastic recovery (ER) # = length length in stretched state - after stretching x -you added length in stretched state

Although a standard tension of 50% is used to identify the elastic properties of the output films, this tension serves only as an example.

Generally, such output films have an elastic recovery ability higher at voltages greater than 50% and somewhat lower at voltages significantly greater than 50%, compared to their elastic recovery ability at 50% stress.

These starting elastic films also contain a percentage crystalline of at least 20 #, preferably at least 30% and most preferably at least 50 #, e.g. 50-90% or more.

^ The percentage of crystalline is determined by the X-ray method as described by R.G. Quynn et al. In J.Appl. Polymer Science, vol. 2, No. 5, pp. 166-173 (1959) · For a detailed discussion of crystallinity and its significance in polymers, see Polymers and Resins, Golding, D. (D. Vanïostrand, 1959).

Other elastic films that are considered suitable for making precursor films used in the dry drawing method are described in British Patent No. 1,052,550.

The precursor elastic film used in the manufacture of the microporous films by the "dry stretch" method is to be distinguished from films formed from classic elastomers, such as the natural and synthetic rubbers. With such classic elastomers 5, the tensile stress-shear stress behavior and in particular the tensile stress-temperature relationship are determined by the entropy mechanism of the deformation (rubber elasticity). The positive temperature coefficient of the retraction force, i.e., decreasing tensile stress at decreasing temperature and complete loss of elastic properties at the glass transition temperature, are special effects of entropy elasticity. The elasticity of the precursor elastic films used herein, on the other hand, is of a different nature. In qualitative thermodynamic tests with these elastic precursor films, increasing tensile stress at decreasing temperature (negative temperature coefficient) can be explained to mean that the elasticity of these materials is not determined by entropy effects but depends on an energy term.

More importantly, it has been found that the "dry stretch" precursor elastic films retain their stretch properties at temperatures where normal entropy elasticity could no longer be effective. Thus, the stretch mechanism of the "dry stretch" precursor elastic films is considered to be based on energy elasticity bond, and these elastic films can then be referred to as "non-classic" elastomers.

Instead, the "solvent stretch" method utilizes a precursor film containing at least two components, i.e., an amorphous component and a crystalline component. Therefore, crystalline materials, which are naturally two components, work well in this process.

The degree of crystallinity of the precursor film should therefore be at least 30 vol., Preferably at least 0 vol. and preferably at least 50 vol.? based on the predecessor film.

The polymers, ie synthetic resin material, from which the precursor film used in either 8004988 11 process are used are the olefin polymers such as polyethylene, polypropylene, poly-3-methylbut-1-one, poly-H-methylpent- 1-ene, as well as copolymers of propylene, 3-methylbut-1-ene, H-methylpent-1-ene or ethylene, with one another or with minor amounts of other olefins, eg copolymers of propylene and ethylene, copolymers of predominant amount of 3-methylbut-1-one and a minor amount of a straight chain n-olefin, such as n-oct-1-one, n-hexadec-1-one, n-octadec- 1-One or another relatively long chain olefin, as well as copolymers of 3-methylpent-1-one and one of the n-olefins mentioned above in connection with 3-methylbut-1-one.

In general, e.g. when propylene homopolymers are intended to be used in the "dry stretch" method, an isostatic polypropylene having a percentage Ί5 crystalline as indicated above, an average molecular weight.

by weight in a range of 100,000-750,000 (e.g.

200,000-500,000) and a melt index (ASTM-D-1238-57T, part 9, p. 38) of 0.1 - 75 (eg 0.5-30), used to provide a film end product with the required physical properties. 20 shelves.

In this specification, olefinic polymer or olefin polymer refers to a polymer which is prepared by polymerizing olefin monomers or olefin monomers via their ethylenic unsaturation.

Preferred polymers for use in the "solvent drawing" method are the polymers used in accordance with the invention as described in U.S. Patent Application Ser. No. W * .805 of June 1, 1979.

For example, a polyethylene homopolymer with a density of

O

0.960-0.965 g / cm, a high melt index of not less than 3 and preferably 3-20, and a broad molecular weight. distribution ratio (Μ / M) of not less than 3.8 and preferably 3.8-13 is preferred when manufacturing a microporous film by the "solvent stretch" method In addition, nucleating agents can be incorporated into the polymer used for manufacturing the precursor film as described in the above-mentioned patent application, in which case the polymers having a melt index of only 0.3 can be used.

The types of equipment suitable for making the precursor films are known in the art.

A conventional film extruder equipped with a shallow channel metering screw and coat hanger die is satisfactory. Generally, the resin is fed into a hopper of the extruder equipped with a screw and jacket with heating elements. The resin is melted and transferred by screw to the mold, from which it is extruded through a slit in the form of a film, from which it is drawn with a take-up or casting roller. More than one take-up roller in different combinations or stages can be used. The die opening or gap width can e.g. are in the range 0.25 ^ -5.08 mm.

Using this type of device, film can be extruded with a draw ratio of 5: 1 to 200: 1, preferably 10: 1 to 50: 1.

The term "peel ratio" or more simply "draw ratio" as used herein is the ratio of the film take-up or take-up speed to the speed of the film exiting the extruder die.

The melting temperature for film extrusion is generally no greater 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. Polypropylene can e.g. are extruded at a melting temperature of 180-270 ° C, preferably 200-200 ° C. Polyethylene can be extruded at a melting temperature of 175-225 ° C.

When the precursor film is to be used in accordance with the "dry stretch" method, the extruding operation is preferably performed under rapid cooling and rapid stripping to obtain maximum elasticity. This can be accomplished 800 4 98 8 * Λ 13 by arranging the take-up roller relatively close to the extruder slit, e.g. within 5 cm and preferably within 2.5 cm. An "air knife" operating at a temperature in the range of e.g. 0-1 + 0 ° C, can be used within 2.5 cm of the slit to fix the film, i.e.

cool quickly and get fixed. The take-up roller can e.g. run at a rate of 3-300 ra / min. preferably 15-150 m / min.

When the precursor film is to be used in accordance with the "solvent drawing" method, the extruding operation is preferably performed under slow cooling to minimize the stretching stress and any associated orientation that may result from a fast fixation. , in order to obtain maximum crystallinity, yet fast enough to prevent the development of large spheres. This can be done by adjusting the distance of the chill roller mount from the extruder gap.

While the above description is directed to slit matrix extruding methods, another method of forming the precursor films contemplated in the invention is the blown film extruding method using a hopper and an extruder, which are much the same as in the slit extruder as above described.

The melt exits the extruder into a matrix, from which it is extruded through a circular opening to form a tubular film of original diameter D ^. Air enters the system through an inlet into the interior of this tubular film and results in inflation of the diameter of the tubular film to a diameter Dg. Aids, such as air rings, can also be arranged to direct the air along the outside of the extruded tubular film to achieve different cooling rates. Tools, such as a crown, can be used to locate the interior of the tube 8004988

1U

shaped film to cool. After some distance, over which the film can cool and cure completely, it is wound on a take-up roller.

When using the blown film method, the draw-off ratio is preferably 5: 1 to 100 µl, the gap 5 0.25-5.08 mm, preferably 1.02-0.25 mm, the Dg / D ^ elevation e.g. 1.0-14.0 and preferably 1.0-2.5 and the recording rate e.g.

9-210 m / min. The melting temperature may be in the above slit die extrusion ranges.

The extruded film can then first be heat-treated or annealed to improve the crystal structure, e.g. by increasing the size of the crystallites and removing imperfections therein. Generally, this annealing is performed at a temperature in the range of 5-100 ° C below the melting point of the polymer for a few seconds to several hours, e.g. 5 sec. up to 2k hours, and preferably 30 sec. up to 2 hours.

The polypropylene is the preferred annealing temperature 100-155 ° C.

An exemplary method of annealing involves placing the extruded film in a tensed or stress-free state in an oven at the desired temperature, in which case the residence time is preferably in the range of 30 seconds . up to 1 hour.

In the preferred embodiments, the partially crystalline precursor film obtained is preferably subjected to one of the two alternative methods as described above to obtain a normally hydrophobic microporous film which can be used in accordance with the invention.

The first preferred method is described in U.S. Patent 3,801ΛθΗ, which is referred to herein as the "dry stretch" method and involves the following steps: (1) cold stretch, i.e. cold drawing the elastic film to form porous areas in the surface that are stretched normally or perpendicular to the stretching direction, 80 0 4 98 8 15 are formed, (2) melt drawing, ie melt drawing the cold stretched film into fibrils and pores or open cells which are stretched parallel to the stretching direction are formed, and then (3) heat or melt cure the resulting porous film under tension, ie, its substantially constant length, to impart stability to the film.

The term "cold stretching" as used herein means stretching or drawing a film beyond its original length at a stretching temperature, i.e.

10 a temperature of the film being stretched is lower than the melting temperature when the film is heated uniformly from a temperature of 25 ° C and at a rate of 20 ° C / min. The term "melt drawing" as used herein refers to stretching above the temperature at which melting begins when the film is heated uniformly from a temperature of 25 ° C and at a rate of 20 ° C / min, but below the normal melting point of the polymer, ie below melting temperature. As known to those skilled in the art, the temperature at which melting begins and the melting temperature can be determined with a standard differential heat analyzer (DTA) or other known device that can measure heat transitions of a polymer.

The temperature at which melting begins varies depending on the nature of the polymer, the molecular weight distribution of the polymer, and the crystalline morphology of the film.

Polypropylene elastic film can e.g. cold stretched at a temperature below 120 ° C, preferably at 10-70 ° C and suitably at room temperature, e.g. 25 ° C. The cold-stretched polypropylene film can then be hot-stretched at a temperature of 120 ° C and below the melting temperature, and preferably at a temperature of 130-150 ° C. The temperature of the film itself being stretched is referred to herein as the stretching temperature. The stretching in these two stages should be consecutive, in the same direction and in that order, ie cold and then warm, but can be done in a continuous, semi-continuous or 8004988 16 discontinuous process, as long as the cold stretched film is not allowed to shrink to any significant degree, ie less than 5% of its cold stretched length before being stretched hot.

The total amount of stretch in the above two stages can be in the range of 10-300%, and preferably it is 50-150%, based on the original length of the elastic film. Furthermore, the ratio of the amount of melt drawing to the total amount of drawing or IQ drawing can be from more than 0.10: 1 to less than 0.99: 1, preferably 0.50: 1 to 0.97: 1, and most preferably 0. 50: 1-0.95: 1. This relationship between the "cold" and "warm" stretch is referred to herein as the "stretch ratio (percent" warm "stretch to percent" total "stretch).

In any stretching operation where heat is to be supplied, the film can be heated with moving rollers, which in turn can be heated by an electrical resistance method, by passing over a heated plate, through a heated liquid, a heated gas or the like.

2o After the two-stage stretching described above, the stretched film is subjected to melt-bonding. This heat treatment can be carried out at a temperature in the range of from 125 ° C to less than the melting temperature and preferably 130-160 ° C for polypropylene; from 75 ° C to less than 25, the melting temperature, preferably 115-130 ° C for polyethylene, and at such a temperature range for other above-mentioned polymers. This heat treatment should be carried out while the film is kept under tension, ie, such that the film is not free to shrink or can only shrink 2Q to an adjustable amount not exceeding 15% of the stretched length, but a tension not so great that the film is stretched more than an additional 15%. Preferably, the tension is such that virtually no shrinkage or stretching takes place, e.g. less than 5% change in the stretched length.

The duration of the heat treatment, which is preferably carried out successively with and after the tensile strengthening, should not exceed 0.1 sec. at the higher tempering temperatures and can generally be in the range of 5 sec. up to 1 hour and preferably 1-30 min.

The curing steps described above can be in the air or in another environment, such as nitrogen, helium or argon.

A second other preferred method of converting the precursor film and microporous film 10 as described in U.S. Patent 3,839,516, and herein referred to as the Mopolvent Stretch "method, consists of the basic steps of (1) contacting the precursor film having at least 2 components (eg an amorphous component and a crystalline component) of one has a small volume than all other components, with a leveling agent for sufficient time to allow adsorption of the swelling agent in the film; (2) stretching of the film in at least one direction while in contact with the swelling agent and (3) maintaining the film in its stretched state during removal of the softening agent If desired, the film can be stabilized by heat-melting or ionizing radiation.

Generally, a solvent with a Hildebrand solubility parameter at or near that of the polymer will have a solubility suitable for the drawing process described herein. The Hildebrand solubility parameter measures the cohesion energy density. Thus, the underlying principle is based on the fact that a solvent with a corresponding cohesion energy density as a polymer would have a high affinity for that polymer and would be suitable for this process.

General classes of swelling agents from which one can choose a suitable one for the particular polymer film are short chain aliphatic ketones such as acetone, methyl ethyl ketone, eyolohexanone; short chain aliphatic acid esters, such as ethyl 800 4 98 8 18 formate, butyl acetate; halogenated hydrocarbons, such as carbon tetrachloride, trichlorethylene, perchlorethylene and chlorobenzene; hydrocarbons, such as heptane, cyclohexane, henzene, xylene, tetralin and decalien; nitrogen containing organic compounds such as pyridine, formamide and dimethylformamide; ethers, such as methyl ether, ethyl ether and dioxane.

A mixture of two or more of these organic solvents can also be used.

It is preferred that the swelling agents be a compound consisting of carbon, hydrogen, oxygen, nitrogen, halogen, sulfur and contain up to 20 carbon atoms, preferably up to 10 carbon atoms.

The "solvent stretch" step can be performed at a temperature in the range from above the freezing point of the solvent or swelling agent, to the point below the temperature at which the polymer dissolves (i.e., room temperature to 50 ° C).

The precursor film used in the "solvent drawing" process may have a thickness in the range of 0.0025-0.5 mm or more.

In a preferred embodiment, the precursor film is biaxially stretched according to the method described in US patent application Ser. No. 801, filed June 1, 1979. In that process, preferred stretching conditions are reported in a uniaxial direction, which lead to improved permeability of the uniaxial stretched microporous film.

The biaxially stretched microporous film can then be stretched in a transverse direction to further improve permeability. It is therefore preferable that the precursor film be "solvent stretched" in a uniaxial direction 30 by no more than 350% and most preferably 300% more than its original length. Typically, additional stretching in the same direction after solvent removal is not used.

The desired stabilization step can be either a melt curing step or a cross-linking step. This heat treatment can be carried out at a temperature in the range from 125 ° C to less than the melting temperature and preferably 130-150 ° C for polypropylene; from 75 ° C to less than the melting temperature and preferably 115-130 ° C for polyethylene and at such a temperature range for other above-mentioned polymers. This heat treatment should be carried out while the film is kept under tension, ie such that the film is not free to shrink or can shrink only by a limited amount of no more than 15% of its stretched length, but not so great tension that the film is stretched more than an additional 15%. Preferably the tension is such that practically no shrinking or stretching takes place, eg less than 5% change in the stretched length.

The time of the heat treatment, which is preferably performed subsequent to and after the "solvent stretch" operation, should not exceed 0.1 sec.

at the higher annealing temperatures and is generally in the range of 5 sec. up to 1 hour and preferably 1-30 min.

The above-described curing steps can take place in the air or in another environment, such as nitrogen, helium or argon.

When the precursor film is biaxially stretched, the stabilization step should be performed after cross-stretching and not before.

While the description and examples are primarily directed to the above olefin polymers, the invention also relates to high molecular weight. acetal, e.g. oxymethylene, polymers. Although both acetal homopolymers and copolymers are contemplated, the preferred acetal polymer for polymer stability purposes is a "random" oxymetbylene copolymer containing repeating oxymethylene groups, ie -CH 2 -O-, interrupted by - 0R - groups in the main polymer chain wherein 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 35, any substituents on this R group being 80 0 4 98 8 20 inert, ie substituents which are not contain interfering functional groups which do not cause undesired reactions and wherein a majority of the OR units exist as single units bound on both sides to oxymethylene groups. Examples of preferred polymers are copolymers of trioxane and cyclic ethers containing at least two adjacent carbon atoms, such as the copolymers described in U.S. Patent 3,027,352. These film-form polymers may also have a crystallinity of at least 20%, preferably at least 30%, and most preferably at least 50%, e.g. Own 50-60% or higher. In addition, these polymers have a melting point of at least 150 ° C and a numerical average molecular weight. of at least 10,000. For a more detailed discussion of acetal and oxymethylene polymers see formaldehyde, Walter, 15 pp. 175-191 (Reinhold 196U).

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), all of which are known from the prior art of the technique and, for brevity's sake, need not be further described here.

The normally hydrophobic microporous films used in the invention have a reduced bulk density in an unstressed state compared to the density of corresponding polymeric materials having no open-cell structure, e.g. the materials from which they are formed. Thus, the films have a bulk density of not more than 95% and preferably 20-1 * 0! of the precursor film.

In other words, the bulk density is reduced by at least 5% and preferably 60-80%. For polyethylene, the reduction is 30-80%, preferably 60-80%. When the bulk density is 20-1 * 0% of the starting material, the porosity is increased by 60-80% through the pores or holes.

When the microporous film is prepared 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 most suitably 70-85% as determined by the X-ray method described by RG Quynn et al. In J. Appl. Polymer Science, vol. 2, No. 5, pp. 166-173. For a more detailed discussion of crystallinity and its significance in polymers, see Polymers and Resins, Golding (S. Van Nostrand, 1959).

The microporous films that can be used in the invention may also have an average pore size of 20-10,000, typically 1 * 00-5000 S, and most typically 500-5000 S. These values can be determined by mercury porosimetry, such as described in an article by RG Quynn et al., Pp. 21-3¾ in Textile Research Journal, January 15, 1963, or using electron microscopy, as described in Geil's Polymer Single Crystals, p. 69 (Interscience 1963). When using an electron micrograph, pore length and width measurements can be obtained by simply using a ruler to directly measure the length and width of the pores on an electron micrograph, usually using an enlargement of 5000-10000. In general, the pore length values are which can be obtained by electron microscopy approximately equal to the pore size values obtained by mercury porosimetry.

The microporous films used in the invention have a surface within certain predictable limits when prepared by either the "solvent stretch" method or the "dry stretch" method. Typically, such microporous films have been found to have a surface of at least 10 m / g and preferably in the range of 15-25 m / g. For films formed from polyethylene, the surface is generally in the range of 10-25 m2 / g and preferably 20 m2 / g.

The area can be determined from nitrogen 8004988 22 or cryptone gas adsorption isotherms using a method and equipment as described in U.S. Patent 3,262,319. The surface area obtained by this method is usually expressed as in m / g.

5 To facilitate comparison of different materials, this value can be multiplied

O

filled with the bulk density of the material in g / cm, which leads to a surface area, expressed in m / cm.

The normally hydrophobic microporous polymer films 10 of the invention, which are rendered hydrophilic, preferably have a thickness of 0.025-0.203 mm.

The normally hydrophobic microporous film, prepared according to the methods described above, becomes wettable and / or hydrophilic by coating the surface of the micro-pores of the microporous film with a hydrophilic monomer or a mixture thereof and then exposing the coated microporous film ionizing radiation.

The ionizing radiation chemically bonds the hydrophilic monomer to the micropore surface and makes it relatively permanently hydrophilic. The attached amount of hydrophilic monomer which is chemically fixed is set within certain limits to prevent pore clogging while simultaneously controlling the total porosity of the hydrophilic film. The control of the porosity in turn makes it possible to control the water permeability and electrical resistance of the film.

As used herein, the term "hydrophobic" is used in the sense of a surface that allows less than 0.010 ml of water / min / cm of flat film surface to pass under a water pressure of 7 kg / cm. Accordingly, the term "hydrophilic" refers to surfaces , which pass more than 0.01 ml of water / min / cm at this pressure.

The hydrophilic monomers that can be used to coat the pore surface of the micro-porous film of the invention are organic hydrocarbon compounds of 2-18 carbon atoms, containing at least one ethylenically unsaturated bond, the monomers poly- merisible and / or copolymerizable under the influence of ionizing radiation at a temperature which does not adversely affect the microporous film, and at least one polar functional group, such as a carboxy, sulfo, sulfino, hydroxy, ammonia or phosphono group.

Accordingly, the hydrophilic monomers of the invention include hydrocarbon compounds having from 2 to 18 carbon atoms with one or more polymerizable and / or copolymerizable ethylenically unsaturated bonds, such as substituted and unsubstituted carbon or dicarboxylic acids and esters thereof; vinyl and allyl monomers, especially itaconic acid, malonic acid, fumaric acid and crotonic acid, as well as esters and anhydrides thereof, unsubstituted or alkyl substituted acrylic acids, such as acrylic acid and methacrylic acid, acrolein or acrylonitrile; unsubstituted or alkyl substituted alkyl, cycloalkyl, aryl, hydroxyalkyl or hydroxyaryl acrylates, alkyl substituted dialkylaminoalkyl acrylates, epoxyalkyl acrylates; vinyl sulfonic acid and styrene sulfonic acid; vinyl esters such as vinyl acetate and higher carboxylic acid vinyl esters, alkyl substituted vinyl esters of carboxylic acids containing sulfo groups; vinyl ethers, such as unsubstituted or substituted alkyl, cycloalkyl or aryl vinyl ethers; vinyl substituted silicone; vinyl substituted aromatic or heterocyclic hydrocarbons; diallyl fumarates; diallyl maleates; alkyl substituted phosphates, phosphites or carbonates; vinyl sulfones ,; the reaction product of ethoxylated nonylphenol and acrylic acid and the like.

Since the hydrophilic monomer is contemplated to penetrate the interior of the microporous film, it is preferred to use hydrophilic monomers having 2-1U, most preferably 2-k carbon atoms.

The hydrophilic monomers preferably used in the invention are acrylic acid, methacrylic acid and vinyl acetate.

8004988 2k

The amount of hydrophilic monomer that coats the inner surface of the pores of the microporous film "is adjusted to obtain the appropriate degree of adhesion" determined by the intended water flow or electrical resistance required for the end use of the films.

As described above, such control is accomplished in a manner sufficient to maintain the open-cell nature of micropores, ie, preventing clogging of the pores, after the monomer-impregnated microporous film has been subjected to the radiation treatment as described herein such that the desired properties as discussed above are maintained for a longer period of time than can otherwise be obtained using the prior art characteristic surfactant coatings. The specific amount of hydrophilic monomer that is chemically bonded to the surface of the pores is expressed as percent adhesion, ie the weight of the uncoated microporous film, which represents the weight of the hydrophilic monomer coating present in the cured 2Q microporous film .

Therefore, in order to avoid clogging pores of the microporous film described herein, the percent adhesion of the hydrophilic monomer, which is chemically bonded to the surface of the micropores, is controlled so that it does not exceed 10 wt% and in the generally 0.1-10 wt%, preferably 0.5-2.5 wt%, and most preferably 1-2.0 wt% (e.g.

1.5% by weight, based on the weight of the uncoated microporous film.

The determined percentage of adhesion selected is determined by the end use for which the obtained hydrophilic microporous film is used and varies within the wide range of adhesion percentages described above.

When e.g. the grafted hydrophilic microporous film is intended to be used as an electric 800 4 98 8 25 battery separator, the percent adhesion is chosen based on the electrical resistance of the obtained hydrophilic microporous film (in milliohm per 6.1 + 5 ca) which is a function of the percent adhesion of the hydrophilic monomer.

5. Electrical resistance is defined herein as a measure of the microporous film's ability to conduct electrons. Therefore, the microporous film as a battery separator will be less effective the generally higher the electrical resistance of the microporous film.

For example, the electrical resistance of the microporous film is determined at different adhesion percentages of the hydrophilic monomer and the electrical resistance is plotted as a function of the percent adhesion. The percentage of adhesion is then determined based on the desired electrical resistance.

The electrical resistance of the hydrophilic microporous film according to the invention as described below is generally adjusted to be less than 20 milliohms per 6.1 + 5 cm, preferably less than 10 milli-2 ohms per 6.1 + 5 cm and preferably less than 5 milliohm per 6.1 + 5 cm.

When the grafted hydrophilic microporous film is to be used as a filter, the percent adhesion of the microporous film is selected based on the water flow rate through the microporous film, as defined below.

Therefore, the water flow rate can be plotted as a function of the percentage adhesion of the hydrophilic monomer and the appropriate percentage adhesion is selected based on the desired water flow rate.

When the microporous film is intended to be used for filtration purposes, the pore size of the film is selected to act as a barrier for the material to be separated and the water flow rate is set as desired within the 80 ° limits. 4 98 8 26 selected pore size. For example, the water flow rate can be set as more than 0.1 ca / min./cm, preferably more 3 2 3.2 than 0.05 cm / min./cm, and most preferably more than 0.5 cm / min. cm at a pressure difference of about 1 atm.

Therefore, the invention offers several advantages over prior art hydrophilic films. Since it is possible to maintain an open-cell nature of the hydrophilic microporous films prepared according to the invention, such films allow a mass transport of water through the film, as opposed to transport of water through the film by diffusion, which slower process. The mass transport action also contributes to the reduction of the electrical resistance. Another advantage of the microporous film according to the invention is due to the chemical adhesion of the hydrophilic monomer to the microporous film, extending the duration of the presence of the hydrophilic monomer on the micropore surface over a longer period of time than that for characteristic surfactants, especially after repeated washing. Moreover, the dimensional stability of the films according to the invention is also improved.

It is recommended to mention here that it is preferable to convert the polar functional group of the hydrophilic monomer to its strongest polar form, if possible. This can e.g. are accomplished by reacting an acidic functional group with a base, such as KOH, to form the corresponding salt. Thus, in this case, the salt form can be obtained by drinking the chemically bonded film in a 2% KOH solution for a period of 5-30 minutes. This increases the wettability and improves the flexibility of the film.

All known coating methods can be used to coat the microporous film, provided such methods provide sufficiently precise control of 8004988 27 percent adhesion of the hydrophilic monomer,

The preferred method for accurately and efficiently coating the inner surface of the pores of the microporous film is to contact the microporous film with a vapor of the monomer which condenses on the micropore surface.

The percent adhesion can be controlled by controlling the equilibrium vapor pressure (ie vapor pressure at which the condensation rate of the monomer vapor equals its evaporation rate at a given temperature) of the hydrophilic monomer and the length of time the hydrophilic monomer is in contact with the microporous film .

The equilibrium vapor pressure required to obtain the desired amount of hydrophilic monomer coating at a constant temperature is determined by plotting the percent adhesion versus time at various equilibrium vapor pressures while keeping the temperature constant. The percentage of adhesion selected as described above depends on the particular property desired to be imparted to the microporous film.

The equilibrium vapor pressure required to obtain the appropriate percentage of adhesion is easily determined from the graphical expansions described above.

The contact time of the monomer vapor with the microporous film 25 is also determined in such a manner.

Thus, in a continuous process, the microporous film is continuously passed through a chamber containing the hydrophilic monomer vapor at the appropriate equilibrium vapor pressure and temperature. The contact time of the microporous film with the hydrophilic monomer vapor is controlled by adjusting the web length and the line speed of the microporous film through the vapor.

Apparently, the vapor coating technique can only be used when the critical temperature of the hydrophilic monomer (ie, the temperature at which a liquid monomer cannot exist regardless of the pressure) is above 8004988 28 temperature at which the properties of the macroporous film would be adversely affected the determined contact duration used.

It is preferred that the vapor coating technique be performed at atmospheric pressure and at a temperature that provides the desired vapor pressure.

Typically, the temperature at which the vapor coating technique is performed using atmospheric pressure ranges from 50-170 ° C when the hydrophilic monomer used is acrylic acid, methacrylic acid or vinyl acetate.

Reduced or elevated pressure can also be used with associated temperature and contact time settings.

It will be appreciated that because the percentage of adhesion is determined after the curing treatment, an amount of hydrophilic monomer is originally applied to the microporous film greater than the percentage of adhesion, to account for monomer losses that may occur during the curing treatment.

Other suitable methods by which the microporous film can be coated with the hydrophilic monomer are to dissolve the hydrophilic monomer in an evaporated solvent, such as methylene chloride, to form a dab bath.

The dep bath can then be used in a reverse roll coating technique. By this method, a doctor roller is partially placed in the dab bath of the monomer coating solution. A second driven roll passes an uncoated hydrophobic microporous film web through the nip formed between this roll and the doctor roll. The two rollers, which are preferably driven separately, rotate in the same direction, so that the coated film web is fed in the direction from which the uncoated film comes. The amount of monomer coating 35 applied to the film is a function of the speed difference of the doctor roll and the second film driving roll and also the size of the nip formed between the two rollers.

A pressure rolling method 5 can be used instead. In this method, the film is passed into a dab bath of the monomer coating solution and expressed between two pressure rollers disposed downstream thereof.

The amount of coating is therefore a function of the size of the gap between the two pressure rollers and the pressure exerted between them.

Another method of coating the substrate hydrophobic microporous film is the wire wrapped metering rod method. This method is the same as the pressure roll method, except that the microporous film coating is coated by passing through a bath of the monomer coating solution is expressed between a pair of wire wrapped metering rods which control the amount of coating applied thereto, by the configuration of the wires wrapped around the dosing rods.

In the retaining roll, pressure roll and wire-wrapped metering rod coating techniques, the amount of hydrophilic monomer originally applied to the microporous film is a function of one or two variables, as discussed in the description of the methods. In addition, in all three methods, the amount of the monomer coating is also a function of the concentration of the monomer in the dep bath. The hydrophilic monomer dep bath is obtained by dissolving the monomer in a conventional organic solvent, which has a boiling point lower than the boiling point of the hydrophilic 20 monomer used, such as in addition to methylene chloride, acetone, methanol, ethanol and isopropanol.

The concentration of the hydrophilic monomer in the dep bath is controlled to obtain application of the appropriate amount of monomer upon evaporation of the solvent.

Generally, the monomer concentration in the dep bath can range from 800 to 988 wt. 1-30 wt.%, Preferably 5-15 wt.%, Based on the weight of the bath.

When the hydrophilic monomers are coated onto the microporous film using an immersion bath, the solvent contained therein is removed by passing the coated film through a dryer. The dryer temperature should be high enough to evaporate only the solvent, leaving the monomer deposited on the microporous surface of the film.

After the pores of the microporous film are coated with the hydrophilic monomer and any solvent is removed therefrom, the coated microporous film is subjected to ionizing radiation to chemically bond the hydrophilic monomer to the normally hydrophobic microporous surface and to hydrophilize. to make.

When exposed to ionizing radiation, a number of similar mechanisms or combinations thereof can act to achieve the intended effect. For example, the hydrophilic monomers may become chemically bonded to the microporous surface and / or may form a polymer layer or sleeve chemically bonded to the microporous surface by polymerization and / or eopolymerization, such as by random chemical bonding of the sleeve. surface at the microporous surface, and / or physically due to the limiting effect on the polymer sleeve of the periphery of the microporous surface. The term "chemical adhesion" is intended to include all of the above mechanisms or combinations thereof.

Without wishing to limit ourselves to the particular mechanisms by which the desired improvement can be obtained, the properties of the normally hydrophobic microporous film are modified in one or more ways depending on the percent adhesion of the hydrophilic monomer.

Thus, the obtained radiation-treated microporous film is rendered hydrophilic over a longer period of use than is otherwise obtained with characteristic surfactant coatings and yet the open-cell nature of the film can be maintained to use the microporous film for those applications where mass transport 5 and low electrical resistance are required.

As indicated above, chemical adhesion of the hydrophilic monomers to the normally hydrophobic microporous film is accomplished by exposing the hydrophilic monomer impregnated microporous film to 1Q ionizing radiation.

Ionizing radiation is defined herein as essentially consisting of the type that provides emitted particles or photons with an intrinsic energy sufficient to form ions and break chemical bonds thereby initiating free radical reactions between the hydrophilic used monomers and between the monomers and the microporous surface as described herein. Ionizing radiation is readily available in the form of ionizing particle radiation, ionizing electromagnetic radiation and 20 actinic light.

The term "ionizing particle radiation" has been used to indicate the emission of electrons or highly accelerated nuclear particles such as protons, neutrons, alpha particles, deuterons, beta particles or the like, which are directed in such a way that the particle in the mass to be irradiated is flung. Charged particles can be accelerated using voltage gradients through devices such as a low energy (i.e., 200 KeV) extended electron beam generator, such as the electrocurtain (trademark) (manufactured by Energy 2o Sciences Corporation, accelerators with resonance chambers, van der

Dig generators, beta patterns, synchrotrons, cyclotrons. Neutron radiation can be formed by bombarding a certain light metal, such as beryllium, with high energy positive particles. Particle radiation can also be obtained using a kerareactor, radioactive isotopes, or other natural or synthetic radioactive materials.

"Ionizing electromagnetic radiation" is formed from a metal target, such as tungsten, bombarded with electrons of suitable energy. This energy is imparted to the electrons by potential accelerators of more than 0.1 million electrovolts (mev). Nearly the radiation of this type, commonly referred to as X-rays, an ionizing electromagnetic radiation suitable for the practice of the invention can be obtained by means of a nuclear reactor or by using natural or synthetic radioactive material, e.g. cobalt 6θ.

The hydrophilic monomers described herein also undergo chemical bonding by exposure to actinic light. In general, wavelengths in which sensitivity to actinic light occurs are used, in the range of 1800 - HOOO 2. Various suitable sources of actinic light are available in this field, including, for example, quartz kvik lamps, ultraviolet core carbon arcs, and strong flash lamps. When actinic light is used, starters can be used.

The preferred source of ionizing radiation is the elongated electron beam generator as described in U.S. Pat. Nos. 3,702Λ12, 3.7 ^ 5,396 and 3,769,600.

The precise process control techniques have been sufficiently developed to allow conversion of results from one type of radiation to another, and appropriate adjustment of the techniques described herein can be used interchangeably in the preparation of any desired product.

A radiation dose of 1.0-10 megarad (mrad.) And preferably 2-5 mrad (e.g. 3 mrad) is used to effect chemical bonding. A megarad is 1 million rads. A wheel is the amount of ionizing high energy radiation that causes an absorption of 100 very energy per g absorbent material. This unit is widely accepted as a suitable means of measuring radiation absorption by 8004988 33 material.

Preferably, for economic reasons, the minimum ionizing radiation is used to effect chemical bonding. Excessive dosages (i.e., more than 5 mrads in a single exposure) should be avoided to prevent excessive heating and shrinkage of film and degradation of the hydrophilic monomers and the microporous film.

However, when the dosage is too small, the chemical adhesion is not established and the hydrophilic monomer 10 is quickly lost.

The minimum radiation dose, which establishes chemical adhesion of the hydrophilic monomer, varies depending on the nature of the polymer used to make the microporous film. For example, when the polymer is polyethylene, the radiation dose should be not less than 1 mrad, while for polypropylene, the radiation dose is not less than 2 mrad. and preferably not less than 3 mrad. must be.

Room temperature can be used satisfactorily for irradiation, although elevated temperature can also be used.

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

As discussed previously, the percent adhesion can be selected based on the electrical resistance of the microporous film.

Electrical resistance (DC method) of a microporous film is determined by soaking a sample of a microporous film with a known surface (eg 1.29 cm) in a Uo weight solution of KOH in water for 2 hours. . The resulting sample is then placed between working cadmium electrodes (i.e., anode and cathode), 8Ό0 4 98 8

3U

immersed in an electrolyte of a 1 * 0 wt / S solution of KOH in water and a DC current of known strength (e.g., mA) is passed through the cell between the electrodes. The potential decay across the film (E) is measured with an electrometer. The potential decay across the cell without the microporous film deposited therein (E ') is also determined with the same current.

The electrical resistance of the microporous film is then determined by the equation:

10 E.R. = "E) A

2 where A is the area of the irradiated film in 6.1 * 5 cm,

1 the current through the cell is in mA, E.R. the electrical resistance of the microporous film is in milliohm per 6.1 * 5 cm and E and E

have the above meaning.

15

The water permeability or the water flow rate of the hydrophilic microporous film according to the invention is determined by measuring the flow rate of the water through a specific surface of the film, while the water is under a pressure difference of 1 atmosphere. For example, the water flow rate is expressed in units of volume of water in cm per min per cm film area, i.e. cm / min./cm.

The air permeability of the microporous film according to the invention is determined by the Gurley test, i.e. according to ASTM D 726, by tensioning a film with a surface area of 6.1 * 5 cm in a standard Gurley densometer. The film is subjected to a standard pressure difference (the pressure drop across the film) of 31 cm of water. The time in sec. required to pass 10 cm of air through the film is an indication of permeability. A Gurley value of more than 1.5 min. Is an indication that the pores are clogged.

The hydrophilic microporous films of the invention can be used in many different ways. In particular, they can be applied to areas where the adjusted passage of moisture through a film or surface is desired 800 4 98 8 35. In addition, films made according to the invention can be used as filter membrane support or filters useful for separating ultrafine materials from different liquids and as battery partitions.

The examples below serve to further illustrate the invention. All parts and percentages in the examples as well as in the description and in the claims are on a basis basis unless otherwise stated.

Example I

Part a

This illustrates the manufacture of a normal hydrophobic polyolefinic macroporous film by the "dry stretch" method as illustrated in U.S. Patent 3,801,404.

Crystalline polypropylene with a melt index of 0.7 and a density of 0.92 is melt extruded at 230 ° C through a 20 cm slit of the clothes hanger type using a 2.5 cm extruder with a shallow dosing screw. The length to diameter ratio of the extruder tube is 24/1. The extrudate is subtracted very quickly to a melt pull-off ratio of 150 and contacted with a rotary casting roll held at 50 ° C and 1.9 cm from the edge of the die. The film formed in this way appears to have the following properties: thickness 0.05 cm; recovery of 50% stretching at 25 ° C, 50.3% crystallinity, 59.6%.

A sample of this film was annealed in an oven with low voltage air for 30 min.

30 at 140 ° C, taken out of the oven and the film allowed to cool.

The sample of the annealed elastic film is then subjected to cold drawing and hot drawing at a draw ratio of 0.50: 1 and then heat cured under tension, i.e., at constant length for 10 min at 145 ° C in air. The cold drawing section is carried out at 25 ° C, the 800 4 98 8 36 hot drawing section is carried out at 1.5 ° C and the total drawing is 100% y based on the original length of the elastic film. The obtained film has an average pore size length of 3000 S, a crystallinity of 59 * 6%,

O

5 and a surface area of 8.5 µm / g. The thickness of the microporous film is 0.025 mm.

Part b

A continuous roll of the microporous film 30 m long and 15 cm wide, manufactured in accordance with part a, is then passed through an anhydrous acrylic acid bath (ie 100%) and then through a pressure roller to obtain an adhesion after hardening 1.5 wt.%, based on the weight of the film prior to impregnation, as determined by infrared analysis. The impregnated film 15 is then passed under the window of an elongated electron beam generator (i.e., 60 cm in length) at a line speed of 6 m / min. The electron beam generator is adjusted to provide a dose of 3 megarads at the line speed used.

The oxygen content of the atmosphere below the window of the curtain and in contact with the microporous film is kept below 500 ppm by enclosing the window in a chamber purged with nitrogen.

A sample of cured dry microporous film is then tested for wettability by the drop test. The drop test is performed by applying a 0.6 ml drop of 2% KOHLin water to the surface of the film. The film is then observed with the naked eye. When the portion of the film on which the drop is applied becomes translucent, and the other side of the film on which the drop is applied looks wet, the film is considered wetted (as indicated in Table A with the comment " yes ”) The drop test is performed on an off-line sample of the film immediately after irradiation and on a sample stored at room temperature for one week.

800 4 98 8 37

Several other samples are taken from the cured microporous film roll and tested for electrical resistance in the manner described herein after soaking in a 100 wt% aqueous solution of KOH held at 60 ° C for 1 hour, 2 hours. , ^ days and 8 days, and results are averaged. Soaking the film sample in hot KOH for an increasingly long period of time simulates long-term aging.

The area of each sample tested for electrical resistance is 1.29 cm and the current used is 0 mA mA.

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

15 The results are summarized in tabular form in Table A.

Samples of the cured microporous film roll are also examined for water flow after soaking in a 31 # solution of KOH held at 60 ° C for 2 hours by each 11.3 cm area sample in a milliporous filter housing to place. The milliporous filter is filled with water and pressurized to an atmospheric difference between both surfaces of the film sample. Water is collected that passes through the microporous film over a period of 5 min.

. 3 25 The results are converted m cm water collected 2 per min per cm film area. The results are summarized in Table A.

Comparative example

Example I is repeated except that the radiation dose used for curing is reduced to 1 megarad. The results are summarized in Table A. An uncoated microporous film prepared according to Example I is also exposed to ionizing radiation and tested in the same manner as described in Example I and serves as a control. The results are summarized in Table A.

800 4 98 8 38

As can be seen from the data in Table A, a dose of 1 megarad is considered too small to achieve effective curing of a polypropylene microporous film as evidenced by the infinite electrical resistance of the microporous film of the comparative example. The positive wettability test of the comparative example on the withdrawn sample is believed to result from residual acrylic acid, which is not chemically attached by the irradiation. The negative wettability of the film according to this comparative example after 1 week is considered to be due to the loss of the acrylic acid during the 1 week storage period. This is confirmed by the infinite electrical resistance of the film after 1 week of storage.

It can also be seen that the electrical resistance of the film of Example I increases after k days of exposure to warm KOH and then decreases somewhat after 8 days of exposure. It was believed that the drop after 8 days of aging may be due to the complete conversion of the acrylic seed to its salt form, while after H days of aging the salt deposition is only partially completed. Thus, complete conversion of the acrylic inoculation into its salt form appears to decrease the electrical resistance.

8004988 39

Table A

Example Comparative Example I Example Control

Dose (megarad) 3 1 3 line speed (m / min) 10 6.7 10

Line wettability (drip test) of the line yes yes not determined after 1 week yes no not determined

Electric spring setting (milliohm / 6.1 * 5 ca2) 1 hour 5.00 <=? = Ȣ 0 2k hours 6.87 ^ H days 10.6 e * 3 8 days 7.1

Gurley (air) (Gurley sec.) 9.1 not determined 10

Water flow 2 2 (cm / min / cm) 0.35 O not determined

Example II

part a

Described herein is the manufacture of a polyethylene-moroporous film by the "solvent stretch" method.

Crystalline polyethylene with a melt index of 5.0; an average molecular weight. on a weight basis of 80 000 and

O

a density of 0.960 g / cm and a molecular weight. 9.0 distribution ratio is prepared by the blown film extruding method to form a precursor film (0.076 mm thickness) which is allowed to cool by quenching in air at 25 ° C. A sample of the resulting precursor film is then immersed in trichlorethylene at 70 ° C for 1 min and then stretched while immersed in trichlorethylene held at a temperature of 70 ° C at a drawing rate of 150 µm / min. up to k times its original length (i.e., 300% total stretch). The trichlorethylene is then removed by evaporation and the sample is stretched in the direction of the machine through the direction of 8004988 high stretch of about 50% and allowed to air dry in the stretched state. Drying is performed at 25 ° C.

The obtained microporous film exhibits a crystallinity of about 60%, an average pore length of about 5000 & a surface of 10-25 m / g.

Part h

Several 0.025 mm thick microporous film samples, manufactured in accordance with part a, are dipped in anhydrous acrylic acid (100%) and the film becomes translucent * The samples are allowed to dry until the film acquires a white opaque appearance, and this appearance is a designation of the appropriate amount of monomer coating capable of achieving approximately 1.5 L adhesion. The resulting samples are then passed under an extended electron beam curtain (ie, 30 cm long) and irradiated with different doses of radiation, while the atmosphere under the window and in contact with the microporous film is kept at less than 500 ppm oxygen on the method as used in Example X. The cured films obtained are tested for Gurley air flow, electrical resistance (after immersion in a 1 * 0 *-Κ0Η solution at 60 ° C for 2 h and wettability according to the drop test in the manner described in example I. The results are summarized in table B as loop no * s 1-8.

The percent adhesion of the obtained cured film samples is also determined by infrared analysis and the results are shown in Table B.

As can be seen from the results in Table B, loops k and 7, the electrical resistance is infinite and the 30 samples do not get wet. These results are believed to be attributed to the use of too low a dose of radiation to effect chemical adhesion. It is therefore believed that the hydrophilic monomer evaporates, as evidenced by the lack of any measurable adhesion with a consequent loss of properties.

800 4 98 8 1 * 1

The film samples of loops 1, 2, 5, 6 and 8 show little electrical resistance and are wettable.

The higher electrical resistance of the film sample from loopno. 3 is attributed to a visually perceived inhomogeneity 5 in the film. Moreover, these film samples in which chemical adhesion takes place, as can be seen from a 1.5% adhesion, show only a slight drop in air permeability. This indicates that the pores remain unblocked after chemical bonding.

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Claims (11)

1. A method of hydrophilizing a normal hydrophobic polyolefinic nicroporous film, improving its water flow rate and decreasing its electrical resistance, characterized in that (a) the surface of the micropores of a normal hydrophobic polyolefinic open cell microporous film, which has a reduced bulk density compared to the bulk density of a precursor film from which it is made, an average pore size of 200-10,000 & coated on a surface of at least 10 m / g with at least one hydrophilic organic hydrocarbon monomer having 2-18 carbon atoms, which contains at least one ethylenically unsaturated bond and at least one polar functional group, and 15 (b) chemically fixes on the surface of the micropores of the microporous film an amount of this hydrophilic organic hydrocarbon monomer sufficient to be of open-cell nature of the micropores and is sufficient for obtaining an adhesion of 0.1-10 wt.%, based on the weight of the uncoated porous film, by irradiating the coated microporous film from (a) with 1-10 megarad of ionizing radiation. 2. Process according to claim 1, characterized in that a hydrophilic organic hydrocarbon monomer containing 2-1k carbon atoms is used and as a polar functional group at least one carboxy, sulfo, sulfino, hydroxy, ammonio, amino or phosphono group. 3. Process according to claim 1 or 2, characterized in that an unsubstituted or alkyl-substituted acrylic acid, a vinyl ester, vinyl ether or mixture thereof is used as the hydrophilic organic hydrocarbon monomer. The process according to one or more of the preceding claims, characterized in that the hydrophilic organic hydrocarbon monomer used is acrylic acid, methacrylic acid, vinyl acetate or a mixture thereof. 80 0 4 98 8 1Λ Process according to one or more of the preceding claims, characterized in that the normally hydrophobic microporous film is manufactured from polyethylene or polypropylene according to the "solvent stretch" or "dry stretch" method. Method according to one or more of the preceding claims, characterized in that the surface of the micropores of the normally hydrophobic microporous film is chemically fixed with an adherence of hydrophilic organic hydrocarbon monomer in an amount of 0.5- 2.5 wt%, based on the 1Q weight of the uncoated film. 7. A method of hydrophilizing an open-cell, normally hydrophobic, microporous film, improving its water flow rate and decreasing its electrical resistance, characterized in that 12 (a) the surface of the micropores is of an open-cell normal hydrophobic microporous film made from polyethylene or polypropylene, which has a reduced bulk density compared to the bulk density of a precursor film from which it is made, a crystallinity of more than 30%, an average pore size of 0000-5000 en and a surface of at least 10 m / g, coated with at least one of the hydrophilic organic hydrocarbon monomers acrylic acid, methacrylic acid and vinyl acetate, and (b) an amount of the hydrophilic monomer 22 that is sufficient to maintain the open-cell nature of the micropores and sufficient is to chemically tighten an adherence of 0.1-10 wt / lb based on the weight of the uncoated microporous film p the surface of the micropores of the microporous film, by irradiating the coated microporous 2Q film from step (a) with 1-10 megarad ionizing radiation. 8. Hydrophilic open-cell microporous film consisting of, (a) an open-cell normal hydrophobic microporous film with a reduced bulk density compared to 22 with the bulk density of a precursor film from which it is made 800 4 98 8, an average pore size of 200 - 10,000 S and a surface of at least 10 m / g and (b) a surface coating on the micropores of the microporous film of at least one hydrophilic organic hydrocarbon monomer of 2-18 carbon atoms containing at least one ethylenically unsaturated bond and at least one polar functional group wherein the hydrophilic organic hydrocarbon monomer coating is chemically attached to the surface of the micropores of the microporous film by exposure to 1-10 megarad of ionizing radiation and in an amount sufficient to maintain the open-cell nature of the microporous film and obtaining an adherence of 0.1-10% by weight based on the weight of the uncoated microporous film. 9. A hydrophilic microporous film according to claim 8, characterized in that the hydrophilic organic hydrocarbon monomer contains 2-1U carbon atoms and at least one polar functional group consisting of a carboxy, sulfo, sulfino, hydroxy, ammnio, amino or phosphono group. 10. A hydrophilic microporous film according to claim 8, characterized in that the hydrophilic organic hydrocarbon monomer consists of an unsubstituted or alkyl-substituted acrylic acid, an unsubstituted or alkyl-substituted vinyl ester or a mixture thereof. 11. A hydrophilic microporous film according to claim 8, characterized in that the hydrophilic organic hydrocarbon monomer consists of acrylic acid, methacrylic acid, vinyl acetate or a mixture thereof. 12. A hydrophilic microporous film according to claim 8, characterized in that the normally hydrophobic polymeric microporous film consists of polypropylene or polyethylene and is manufactured by the "solvent stretch" or "dry stretch" method. 13. Hydrophilic microporous film according to claim 8, characterized in that the hydrophilic monomer is chemically fixed on the surface of the micropores with an adhesion 800 4 98 8 1 * 6 of 0.5-2.5 wt%, " based on the weight of the uncoated microporous film 1U Hydrophilic open cell microporous film consisting of 5 (a) a normal hydrophobic open cell microporous film, prepared by the "solvent stretch" or "dry stretch" method of polyethylene or polypropylene, the film has a reduced bulk density compared to the bulk density of a precursor film from which it is made, has a crystallinity of at least 30%, has an average pore size of UOO-5000 lb and a surface o of at least 10 m / g, and ( b) a coating on the micropore surface of the microporous film of at least one hydrophilic organic hydrocarbon monomer of 2-18 carbon atoms, consisting of an unsubstituted-or-alkyl substituted acrylic acid, an unsubstituted or alkyl substituted vinyl ester or a mixture thereof, wherein the hydrophilic organic hydrocarbon monomer coating is chemically attached to the surface of the micropores by exposure to 2-5 megarads of ionizing radiation and in an amount sufficient to maintain the open-cell nature of the micropores of the microporous film and obtain an adhesion of 0.1-10% by weight, based on the weight of the uncoated microporous film. Hydrophilic microporous film according to claim 1, characterized in that the hydrophilic monomer consists of acrylic acid, methacrylic acid, vinyl acetate or a mixture thereof, the hydrophilic monomer being present on the surface of the micropores in an amount of 0.5-2 wt. $ 30 based on the weight of the uncoated microporous film. 16. Hydrophilic microporous film as described herein. 80 0 4 98 8