SE450706B - HYDROPHIL MICROPOROS POLYOLEFIN FILM AND PROCEDURE FOR ITS PREPARATION - Google Patents

Description

450 706 Coatings or impregnating agents. Such coatings or impregnating agents, although effective for a limited period of time, tend to be removed in a relatively short time from solutions which come into contact with the films or fibers in which they are present.

Others have attempted to impart a hydrophilic character to a normally hydrophobic microporous film by using low energy plasma treatments. Such plasma treatments are achieved by first activating surface areas of the microporous film using argon or hydrogen plasma and then inoculating a suitable free radical polymerizing moiety, such as acrylic acid. The plasma treatments result in a film that has only one surface that is re-wettable. The surface of the film also becomes clogged when wetted, which then inhibits or prevents the free flow of water through the interior of 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 reduces the dimensional stability of the films as shown by significant shrinkage on drying.

As stated above, a disadvantage of the plasma treatment is its limited ability to make only the surface of the microporous film wettable. It has been found that due to the unusually large surface area of a microporous film of the type described herein, surface wettability alone does not ensure that the film will exhibit certain functional properties such as low electrical resistance and water flow rates through the film comparable to known surfactant systems. as discussed above.

The main disadvantage of the plasma treatments, namely pore clogging and only surface wettability, is also assumed to be a combination of factors, such as the tendency for low energy plasma to be easily deactivated by the large surface area of the microporous film. This reduces the probability that an inner space inside the microporous film will be activated. Similarly, there is competition for incoming graftable monomers exerted by the free radical polymer grafts initially generated at the surface of the film when the graftable polymer first comes into contact with the plasma activated microporous film surface. The graft polymer chains initially present on the film surface thus propagate at an increasing rate as the reaction proceeds. The resulting elongated graft polymer chains present at the surface of the film thus become entangled and clog the surface micropores in the presence of water.

Other attempts to provide hydrophilic films using plasma treatment are illustrated by U.S. Pat. Nos. 3,992,495 and 4,046,843.

Another method of making polyethylene films wettable and suitable for use in electric battery separators is illustrated 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. 745,571 (1972) summarized in Chemical Abstract 78 503lf (1973); V.D'Agostino and J.Lee, Low Temperature Alkaline Battery Separators, 27 Power Sources Symp. 87-9I (1976), summarized in Chemical Abstract 86 1582775; 19 V. D'Agostino, J. Lee and G. Orban, Zinc-Silver Oxido Batteries (A. Fleisch and J. Lander ed, 1971).

Such articles discuss or pertain to a commercial product known as PERMIoNW RAI Research Corp. The method of making this material developed from rial briefly consists of crosslinking a 0.025 mm polyethylene sheet using beta radiation, followed by inoculation with methacrylic acid in a suitable solution under C060 gamma radiation. The grafted material is washed to remove the homopolymer, then converted to salt form in 450 706 hot KOH, washed again to remove the remaining base, dried and packaged. The initial crosslinking step gives rise to microcracks or longitudinal slits in non-porous polyethylene film, which are so small that they are not visible even under an electron microscope.

The diameter of these microcracks is estimated to be approximately ZOÅ (l0 ~ 8cm). The microcracks are then inoculated with methacrylic acid. The resulting film is therefore non-microporous in the sense of the microporous films used in the present invention having an average size of about 200 to about 5000 Å. The extremely small size of the microcracks of Permion (R) generally prevents a mass transfer of mobile electron-bearing parts generated by oxidation-reduction reactions through the film at any significant rate. This is reflected in the relatively high (eg 30 to 40 milliohm-6.45 cmz) electrical resistance values exhibited by films of this type.

(R) In addition, Permion is not dimensionally stable in more than one direction as evidenced by significant swelling.

It is well known that non-porous polymeric substrates, such as polyethylene and polypropylene, can be reacted with various monomers, such as acrylic acid, using various types of ionizing radiation as illustrated by U.S. Pat. Nos. 2,999,056, 3,281,263, 3,372. .100 and 3,709,718.

However, since none of these patents relate to microporous films, they do not address the particular problems associated therewith.

The search for a relatively permanently wettable, hydrophilic, microporous film which exhibits low electrical resistance and improved water flow rates through the microporous film has thus continued. The present invention emerged in response to this application.

Thus, an object of the present invention is to provide a method for making a normally hydro-'l 450 706 fob microporous film relatively permanently hydrophilic thereby improving its walloon pormeabililel.

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

Yet another object of the present invention is to provide a hydrophilic microporous film which has reduced electrical resistance.

A further object is to overcome the problems of the prior art discussed above.

These and other objects, as well as the scope, character and use of the present invention, will be apparent to those skilled in the art from the following detailed description and appended claims.

According to one aspect of the present invention there is provided a process for producing an open cell hydrophilic polyolefinic microporous film having an electrical resistance of less than 30 milliohm per 6.45 cm 2 and a water flow rate thereby greater than 0.01 cm 3 / min. / cm 2 at a pressure difference of 1 atm, from a normally hydrophobic polyolefinic microporous film selected from microporous polyethylene and polypropylene films, which process is characterized in that it comprises the process steps (a) coating the surface of the micropores of a normal hydrophobic open-cell microporous polyethylene or polypropylene film characterized by a reduced bulk density compared to the bulk density of a precursor film from which it is made, an average pore size of from 0,02 to 1,0 μm and a surface area of at least 10 m2 per grams, with at least one hydrophilic organic hydrocarbon monomer having from 2 to 18 carbon atoms characterized by the presence of at least one double bond and at least one n polar functional group, and (b) chemical attachment to the surface of the micropores of the microporous film of an amount of said hydrophilic organic hydrocarbon monomer sufficient to preserve the open cell character of said micropores and sufficient to obtain those of an addition of from 0.1 to 10% by weight, based on the weight of the uncoated microporous film, by irradiating the coated microporous film from (a) with an amount of ionizing radiation of from 1 to 10 megarads for microporous polyethylene films and from 3 to 10 megarads for microporous polypropylene films.

According to another aspect of the present invention, there is provided a hydrophilic open cell microporous film having an electrical resistance of less than 30 milliohm per 6.45 cm 2 and a water flow rate thereby of greater than 0.01 cm fi / min / cm 2 at a pressure difference of 1 atm, which film is characterized in that it comprises: (a) an open cell, normally hydrophobic microporous film made of polyethylene or polypropylene characterized in that it has a reduced bulk density compared to the bulk density of a precursor film from which it is made, a by an average pore size of from 0.02 to 1.0 μm and a surface area of at least 10 m 2 per gram, and (b) a coating on the surface of the micropores of the microporous film of at least one hydrophilic organic hydrocarbon monomer having from 2 to 18 carbon atoms which characterized by the presence of at least one double bond and at least one polar functional group in an amount sufficient to preserve the open cell character of the microporous film and for e an addition of from 0.1 to 10% by weight, based on the weight of the uncoated microporous film, which hydrophilic organic hydrocarbon monomer coating is chemically attached to the surface of the micropores of the microporous film by exposure to an amount of ionizing radiation. of from 1 to 10 megarads for microporous polyethylene films and from 3 to 10 megarads for microporous polypropylene films. The essence of the present invention lies in the fact that it has been found that open-cell microporous films can be made relatively permanently wettable and / or hydrophilic when the pores thereof are chemically bonded with a controlled amount of graftable hydrophilic monomers by exposure to ionizing radiation while preserving the the open-celled character of the microporous film. In addition, such chemical attachment of the hydrophilic monomers also occurs within the pores located in an internal location within the microporous film. It is believed that by controlling the amount of monomer chemically attached to the surface of the micropores, the polymer chain grafting obtained as a result of exposure to radiation can be aligned with the surface of the pores. Therefore, entanglement and clogging of the pores is avoided.

The present invention relates to a hydrophilic microporous film and to a process for producing the same.

Porous or cellular films can be classified into two general types: a type in which the pores are not interconnected, i.e. a closed cell film, and the second type wherein the pores are substantially interconnected by tortuous paths that may extend from one outer surface or surface area to another, i.e. an open-cell film. The porous films of the present invention are of the latter type.

The pores of the porous films of the present invention are further microscopic, i.e. the details of their pore configuration or arrangement can be discerned only by microscopic examination. In fact, in general, the open-cell pores in the films are smaller than those that can be measured using an ordinary light microscope, since the wavelength of visible light, which is about 5000 Å (one Å is 10-10 m), is longer than the longest planar or surface dimension of the open cell or pore. However, the microporous films of the present invention can be identified using electron microscopy methods capable of dissolving details having a pore structure below 5000 Å. 450 706 The microporous films of the present invention are also characterized by a reduced bulk density, sometimes hereinafter simply referred to as a "low" density.

This means that these microporous films have a bulk density or total density lower than the bulk density of corresponding films composed of identical polymeric material but which have no open cell or other hollow structure. The term "bulk density" as used herein means the weight per unit gross or geometric volume before the film where gross volume is determined by immersing a known amount by weight of the film in a vessel partially filled with mercury at 25 ° C and atmospheric pressure. The volumetric increase in the level of mercury is a direct measure of gross volume. This method is known as the Mercury Volumeometer Method and is described in the Encyclopedia of Chemical Technology, Volume 4, p. 892 (Interscience 1949).

Porous films have been produced which have a microporous, open-cell structure and which are also characterized by a reduced bulk density. Films possessing this microporous structure are described, for example, in U.S. Pat. No. 3,426,754, which is hereby incorporated by reference. The preferred method of production described therein includes drawing or stretching at ambient temperatures, i.e. "Cold drawing" of a crystalline, elastic precursor film to a degree of about 10 to 300% of its original length, with subsequent stabilization by heat curing the drawn film under such a tension that the film cannot shrink or shrink freely to a limited extent. Other methods of making microporous films are exemplified by U.S. Pat. Nos. 3,558,764, 3,843,762, 3,920,785, British Pat. Nos. 180,066 and 1,198,695, all of which are hereby incorporated by reference.

While all of the patents listed above describe processes for producing normally hydrophobic _w __, '_ n_uqr ,, q ,, __ gm ,, m, f, m _ ,, r, _.._, m ,, .. W. ..w., ..._, _. l .->. ,, ,,; ._ ..,,., ___ _.,. From the present invention, microporous films which can be seen with the present invention are provided with the preferred, normally hydrophobic microporous films in accordance with the procedures set forth in U.S. Pat. films, which is referred to herein as the "dry stretching" method4, and the Danish: 3,839,516, which discloses a method for aging microporous films 1, new n U as here E35 Süm jnitgsnedelssträcknings "method, both of which are incorporated herein. a reference. each of these patents! lgna alternative rophobic microporous film in accordance with can be used to conform to the position of microporous NON "dry stretching" and "solvent stretching" methods are specifically described in each of the above and the respective patents. Thus, the "stretching" method utilizes a non-rigid, elastic, elastic polymer which has an elastic return at the recovery time of zero (defined in upon exposure to a standard elongation of 50% will be 65% relative humidity: in "â; 50% and preferably of at least 40% - at least about 80%.

Elastic Here a measure of the ability of the product is used, such as a film to return to its light size after stretching and can be calculated on Elastic recovery length in stretched condition - length added in stretched condition 450 706 10 Although a standard elongation of 50% was used to identify - the elastic properties of the starting films such an elongation is only an example. In general, such starting films will have elastic recovery values that are higher at elongations less than 50% and slightly lower at elongations that are significantly higher than 50% compared to their elastic recovery at a 50% elongation.

These elastic starting films will also have a percent crystallinity of at least 20%, preferably at least 30% and preferably at least 50%, for example about 50 to 90% or more. Percent crystallinity is determined by the X-ray method described by R. G. Quynn et al in the Journal of Applied Polymer Science, Volume 2, No. 5, p. 166-173 (1959).

For a detailed discussion of crystallinity and its significance in the polymer, see Polymers and Resins, Golding (D. Van Nostrand, 1959).

Other elastic films considered suitable for the production of precursor films used in the dry stretching method are described in British Patent Specification 1,052,550 published December 21, 1966. In The elastic precursor film used in the production of the microporous films by the "dry stretching" process path shall be distinguished from films made of classical elastomers, such as natural and synthetic rubber materials. In such classical elastomers, the stress-strain behavior, in particular the stress-temperature relationship, is regulated by the entropy mechanism for deformation (rubber elasticity). The positive temperature coefficient for the retraction force, ie. decreasing stress with decreasing temperature and complete loss of elastic properties at the glass transition temperature, are special consequences of entropy elasticity. On the other hand, the elasticity of the elastic precursor films used here is of a different nature. In qualitative thermodynamic experiments with these elastic precursor 450 706 ll sor films, increasing stress with decreasing temperature (negative temperature coefficient) can be interpreted as meaning that the elasticity of these materials is not regulated by entropy effects but due to an energy term. . Even more importantly, elastic "dry stretching" precursor films have been found to retain their stretching properties at temperatures where normal entropy elasticity can no longer be effective.The stretching mechanism of the elastic "dry stretching" precursor films is thus thought to be based on energy elasticity relationships. and these elastic films can then be referred to as "non-classical" elastomers.

Alternatively, the "solvent stripping" method utilizes a precursor film which must contain at least two components, for example an amorphous component and a crystalline component.

Crystalline materials which by nature are two components thus work well in the process. The degree of crystallinity of the precursor film must therefore be at least 30% by volume, preferably at least 40% by volume and preferably at least 50% by volume of the precursor film.

The polymers, i.e. Synthetic resin-like materials of which the precursor films used in either process in accordance with the process of the present 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 minor amounts of other olefins, for example copolymers of propylene and ethylene, copolymers of a larger 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, as well as copolymers of 3-methylpentene-1 and any of the same n-alkenes previously mentioned in connection with 3-methylbutene-1. For example, in general, when propylene homopolymers are intended to be used in the "dry stretching" method, an isotactic polypropylene having a percent crystallinity as set forth above, a weight average molecular weight ranging from about 100,000 to 750,000 (e.g., about 200 ° 000 to 500,000) and a melt index (ASTM-D-1238-57T, Part 9, Page 38) from about 0.1 to about 75 (for example from about 0.5 to 30) is used to give a final film product which has the required physical properties. It is to be understood that the terms "olefinic polymer" and "olefin polymer" are used interchangeably and are intended to describe a polymer prepared by polymerizing olefin monomers via their unsaturation.

Preferred polymers for use in the "solvent stretching" method are those polymers used in accordance with the invention described in U.S. Patent Application 44,805 filed June 1, 1979 by John W. Soehngen and entitled "Improved Solvent Stretch Process". for Preparing Microporous Films from Precursor Films of Controlled Crystalline Structure "the contents of which are hereby incorporated by reference. A polyethylene homoplomer having a density of from about 0.960 to about 0.965 g / cm 3, a high melt index of not less than about 3 and preferably from about 3 to about 20 and a wide molecular weight distribution ratio (fi w / fi n) of not less than about 3, Thus, 8 and preferably from about 3.8 to about 13 are preferred in the production of a microporous film by the "solvent stretching" method. In addition, nucleating agents may be incorporated into the polymer used to make the precursor film as described in the aforementioned U.S. patent application by Soehngen, in which case the polymers having a melt index as low as 0.3 may be used.

The types of apparatus suitable for making the pre-w 450 7Û6 13 cursor films are well known in the art.

A conventional film extruder equipped with a feed screw with a shallow groove and a clothes hanger nozzle is satisfactory. The resin is usually introduced into a hopper of the extruder which contains a screw and a jacket provided with heating elements. The resin is melted and transferred by the screw to the nozzle from which it is extruded through a slot in the form of a film from which it is drawn through a take-up or casting roller. More than one pick-up roller in different combinations or steps can be used.

The nozzle opening or slot width may be in the range, for example, about 0.25 to 5 mm.

Using this type of device, film can be extruded with a draw ratio of about 5: 1 to 300: 1, preferably 10: 1 to 50: 1.

The terms "pull-out ratio" or simpler "pull-out ratio" as used herein are the relationship between the film winding or pick-up speed and the speed of the film emerging at the extrusion die.

The melting temperature of film extrusion is generally not higher than about 100 ° C above the melting point of the polymer and not lower than about 10 ° C above the melting point of the polymer.

For example, polypropylene can be extruded at a melting temperature of about 180 to 270 ° C, preferably 200 to 240 ° C.

Polyethylene can be extruded at a melting temperature of about 175 to 225%. When the precursor film is to be used in accordance with the "dry stretching" method, the extrusion process is preferably carried out with rapid cooling and rapid elongation to obtain maximum elasticity. This can be achieved by having the pick-up roller relatively close to the extrusion slot, for example within 5.1 cm and preferably within 2.5 cm. An "air knife" that operates at temperatures of, for example, between 0 and 40 ° C can be used within 2.5 cm of the slot to cool down, ie. quickly cool and fasten the film. The pick-up roller can be rotated, for example, at a speed of 3 to 305 m / minute, preferably 15 to 152 m / minute.

When the precursor film is to be used in accordance with the "solvent stretching" method, the extrusion process is preferably carried out with slow cooling to reduce stress and any associated orientation obtainable from a rapid cooling to a minimum to obtain maximum crystallinity but still sufficient. fast to avoid the development of large spherolites. This can be accomplished by controlling the distance of the cooling roll pickup from the extrusion slot.

While the above description has been directed to slit nozzle extrusion methods, an alternative method of making the precursor films encompassed by the present invention is the film blow extruding method in which a hopper and an extruder are used which are substantially the same as in the slit extruder described above. From the extruder, the melt enters a die from which it is extruded through a circular slot to form a tubular film having an original diameter D1. Air enters the system through an inlet into the interior of the tubular film and causes the effect of inflating the diameter 2. Means, such as air rings, may also be provided for guiding the tern of the tubular film to a diameter D of the air. around the outside of the extruded tubular film to provide different cooling rates.

Means such as a cooling mandrel can be used to cool the interior of the tubular film. After a distance below which the film is allowed to cool completely and harden, it is wound up on a take-up roll. when using the film blowing method, the pull-out ratio is preferably 5: 1 to 100: 1, the slit opening 0.25 to 5 mm, preferably 1 to 2.5 mm, the D2 / D1 ratio is, for example, 1.0 to 4.0, and preferably young är ïüll2 , 5odx recording speed, for example 9 to 213 m / minute. The melting temperature may be within the ranges previously indicated for slit nozzle extrusion.

The extruded film can then be heat treated or run out from the beginning to improve the crystal structure, for example by increasing the size of the crystallites and removing defects therein. This annealing is usually carried out at a temperature in the range of about 5 ° C to 100 ° C below the melting point of the polymer for a period of a few seconds to several hours, for example 5 seconds to 24 hours, and preferably from about 30 seconds to 2 hours. preferred annealing temperature is about 100 to 155 ° C.

An example of a method of performing the annealing is to place the extruded film in a tensioned or stress-free state in an oven at the desired temperature in which case the residence time is preferably in the range of about 30 seconds to 1 hour.

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

The first preferred method described in U.S. Pat. No. 3,801,404, referred to herein as the "dry stretching" method, comprises the steps of cold stretching, i.e. cold drawing, of the elastic film until porous surface regions or areas which are elongated normally or perpendicular to the stretching direction are formed, (2) hot stretching, i.e. hot drawing, of the cold stretched film until fibrils and pores or open cells elongated parallel to the stretching direction are formed and then (3) .heating or heat curing of the resulting porous film under tension, i.e. at substantially constant length, to impart stability to the film.

The term "cold stretching" is defined as used herein as stretching or drawing a film to greater than its original length and at a stretching temperature, i.e. the temperature of the film being stretched, which is less than 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 per minute. The term "hot stretching" is defined as used herein as stretching over the temperature at which melting begins when the film is heated uniformly from a temperature of 25 ° C and at a rate of ZOOC per minute but below the normal melting point of the polymer, i.e. below the temperature at which melting occurs. As is known to those skilled in the art, the temperature at which melting begins and the melting temperature can be determined with a standard differential thermal analyzer (DTA) or by any other known apparatus capable of detecting thermal transitions of a polymer.

The temperature at which melting begins varies with the type of polymer, the molecular weight distribution of the polymer and the crystalline morphology of the film. Elastic polypropylene film can, for example, be cold stretched at a temperature below about 120 DEG C., preferably between about 10 DEG and 70 DEG C. and preferably at ambient temperature, for example 25 ° C. The cold stretched polypropylene film can then be hot stretched at a temperature above about 120 ° C and below the melting temperature, and preferably between about 130 and 150 ° C. Again, the temperature of the film itself, which is stretched here, is referred to as the stretching temperature. The distance in these two steps must be consecutive, in the same direction and in the said order, ie. cold then hot, but may be in a continuous, semi-continuous or batch process, as long as the cold-stretched film is not allowed to shrink to any significant degree, for example less than 5% of its cold-stretched length, before being hot-stretched.

The sum of the total amount of stretch in the above two steps may be in the range of about 10 to 300% and preferably about 50 to 150%, based on the initial length of the elastic film. Furthermore, the ratio of the amount of hot stretching to the sum of the total amount of stretching or drawing may be from above about 0.1: 1 to below 0.99; 1, suitably from about 0.50: l to 0.97: 1 and preferably from about 0.50: l to 0.95: l. This relationship between "cold" and "hot" elongation is referred to herein as the "elongation ratio" (percent "hot" elongation to percent ototal "elongation).

In any stretching processes where heat must be applied, the film can be heated by movable rollers which in turn can be heated by an electrical resistance method, by passage over a heated plate, by a heated liquid, a heated gas or the like. After the two-step stretching described above, the stretched film is thermoset. This heat treatment can be carried out at a temperature in the range of about 125 DEG C. up to below the melting temperature, and preferably about 130 DEG to 160 DEG C. for polypropylene from about 7500 DEG C. to less than the melting temperature and preferably about 115 DEG to 130 DEG C. for polyethylene, and at similar temperature ranges. for other of the above polymers. This heat treatment must be carried out while the film is kept under tension, ie. so that the film cannot shrink freely or can shrink only to a controlled extent which is not greater than about 15% of its elongated length, but not such a great tension that the film is stretched down a further 15%. The tension is preferably such that substantially no shrinkage or elongation occurs, for example less than 5% change in elongated length.

The period of heat treatment which is preferably carried out in succession and after the drawing process should not exceed 0.1 seconds at the higher annealing temperatures and may generally be in the range of about 5 seconds to 1 hour and preferably about 1 to 30 minutes.

The curing steps described above can take place in air, or in other atmospheres, such as nitrogen, helium or argon.

A second preferred alternative method of converting the precursor film to a microporous film disclosed in U.S. Pat. No. 3,839,516 and referred to herein as the "solvent stretching" method comprises the basic steps (1) of contacting the precursor film having at least two components (e.g. an amorphous component and a crystalline component), one of which has a smaller volume than all the other components, with a swelling agent for a sufficient time to allow adsorption of the swelling agent into the film; (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 while removing the swelling agent. Optionally, the film can be stabilized by heat curing under tension or by ionizing radiation. In general, a solvent having a Hildebrand solubility parameter at or near that of the polymer will have a suitable solubility for the drawing process described herein. The Hildebrand solubility parameter measures the cohesive energy density. The underlying principle is thus based on the fact that a solvent with a similar cohesive energy density as a polymer will have a high affinity for the polymer and will be suitable for this process.

General classes of swelling agents from which a suitable one for the particular polymer film can be selected are lower aliphatic ketones, such as acetone, methyl ethyl ketone, cyclohexanone; lower aliphatic acid esters such as ethyl formate, butyl acetate, etc .; halogenated hydrocarbons such as carbon tetrachloride, trichlorethylene, perchlorethylene, chlorobenzene, etc .; hydrocarbons such as heptane, cyclohexane, benzene, xylene, tetralin, decalin, etc .; nitrogen-containing organic compounds such as pyridine, formamide, dimethylformamide, etc.; ethers such as methyl ether, ethyl ether, dioxane, etc .; A mixture of two or more of these organic solvents can also be used.

It is preferred that the swelling agent is a compound composed of carbon, hydrogen, oxygen, nitrogen, halogen, sulfur and contains up to about 20 carbon atoms, preferably up to about 10 carbon atoms. The "Solvent Stretching" step can be performed at a temperature in the range from above the freezing point of the solvent, or swelling agent, to a point below the temperature at which the polymer dissolves (ie, ambient temperature to about 50 ° C). .

The precursor film used in this "solvent stretching" process can range from 0.0025 to about 0.5 mm or even thicker. In a preferred embodiment, the precursor film is biaxially stretched in accordance with the methods described in U.S. Patent Application 44,801, filed June 1, 1979, entitled "Improved Solvent Stretching Process for Preparing Microporous Films," which is hereby incorporated by reference. This method identifies preferred stretching conditions in a uniaxial direction which lead to improved permeability of the uniaxial stretch. The uniaxially stretched microporous film can then be stretched in a transverse direction to further improve the permeability, Thus, it is preferred that the precursor film be "solvent stretched" in a uniaxial direction not greater than about 350% and preferably 300% greater than its original length. Further stretching in the same direction after solvent removal was typically not used.

The optional stabilization step can be either a heat curing step or a crosslinking step. This heat treatment can be carried out at a temperature in the range from about 125 ° C up to lower than the melting temperature and preferably about 130 to 150 ° C for polypropylene; from about 75 ° C up to lower than the melting temperature, and preferably about 115 to 130 ° C for polyethylene and at similar temperature ranges for other of the above polymers. This heat treatment must be carried out while the film is kept under tension, ie. so that the film cannot shrink freely or can only shrink to a controlled extent not greater than about 15% of its elongated length, but not so great a tension that the film is stretched more than a further 15%.

The tension is preferably such that essentially no shrinkage or elongation occurs, for example less than 5% change in elongated length.

The period of heat treatment which is preferably carried out in conjunction with and after the "solvent stretching" process should not exceed 0.1 seconds at the higher effluent temperatures and may generally be in the range of about 5 seconds to 1 hour and preferably about l to 30 minutes.

The curing steps described above may take place in air or in another atmosphere, such as nitrogen, helium or argon.

When the precursor film is stretched biaxially, the stabilization step must be performed after transverse stretching and not before.

While the present description and examples are directed primarily to the aforementioned olefin polymers, the invention also encompasses acetally, for example, the high molecular weight oxymethylene polymers. While including both acetal homopolymers and copolymers, the preferred acetal polymer for the purposes of polymeric stability is a "random" oxymethylene copolymer containing recurring oxymethylene, i.e. -CH 2 -O moieties mixed with -OR 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, all substituents on said R group being inert, i.e. those which do not include hindering functional groups and which do not more to induce undesirable reactions, and where a major amount of the -OR moieties exist as single moieties attached to oxymethylene groups on each side.Examples Preferred polymers include copolymers of trioxane and cyclic ethers containing at least two adjacent carbon atoms, such as the copolymers which described in U.S. Pat. No. 3,027,352. These polymers in film form may also have a crystallinity of at least 20%, for preferably at least 30% and most preferably at least 50%, for example 50 to 60% or higher. These polymers further have a melting point of at least 150 ° C and a number average molecular weight of at least 10,000. For a more detailed discussion of acetal and oxymethylene polymers, see Formaldehyde, Walter, p. 175-191 (Reinhold 1964). Other relatively crystalline polymers to which the present 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 (all of which are well known in the art and nylon 6). here need not be described further for the sake of simplicity.

The normally hydrophobic microporous film used in the present invention, in a stress-free state, has reduced bulk density compared to the density of corresponding polymeric materials having no open-cell structure, such as those from which they are made. The films thus have a bulk density not greater than 95% and preferably 20-40% of the precursor film. In other words, the bulk density is reduced by at least 5% and preferably 60 to 80%. For polyethylene, the reduction is 30-80%, preferably 60-80%. The bulk density is about 20-40% of the starting material, the porosity has increased by 60 to 80% depending on the pores or holes.

When the microporous film is prepared by the "dry stretching" or "solvent stretching" methods, the final crystallinity of the microporous film is preferably at least 30%, more preferably at least 65% and most preferably about 70 to 85% determined by the X-ray method used. described by RG Quynn et al in The Journal of Applied Polymer Science, Volume 2, No. 5, pp. 166 - 173. For a 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 present invention can also have an average pore size of from about 200 to 10,000 Å, in typical Full from about 400 to 5,000 Å and even more typically about 500 to about 50,000 Å. determined by mercury porosimetry as described in an article by R @ G. Quynn et al at pages 21-34 of the Textile Research Journal, January 1963 or using electron microscopy as described in Geil's Polymer Single Crystals, page 69 (Interseience 1963).

When an electron micrograph is used, 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 taken, usually at 5,000 to 10,000 times magnification. In general, the pore length values obtainable by electron microscopy are approximately equal to the pore size values obtained by mercury porosimetry.

The microporous films used in the present invention will exhibit a surface area within certain predictable limits when produced by either the "solvent stretching" method or the "dry stretching" method. Such microporous films will typically be found to have a surface area of at least 10 m 2 / g and preferably in the range of about 15 to about 25 m 2 / g. For films made of polyethylene, the surface area generally ranges from about 10 to 25 m 2 / g and preferably about 20 m 2 / g.

The surface area can be determined from nitrogen or krypton gas adsorption isotherms using a method and apparatus described in U.S. Pat. No. 3,262,310. The surface area obtained by this method is usually expressed as square meters per gram.

To facilitate a comparison of different materials, this value can be multiplied by the bulk density of the material in grams per cm, resulting in a surface area expressed as square meters per cubic centimeter.

The normally hydrophobic microporous polymer films of the present invention which are rendered hydrophilic have a preferred, ..............'... »-_....._...... ..... ..........-.._...._-...._._..., 450 vos '24 thickness of from about 0.025 mm to about 0, 2 mm.

The normally hydrophobic microporous film prepared in accordance with the methods described above is made wettable and / or hydrophilic by coating the surface of the micropores of the microporous film with a hydrophilic monomer or mixture thereof and then exposing the coated microporous film to ionizing. radiation. The ionizing radiation chemically attaches the hydrophilic monomer to the micropore surface and makes it relatively permanently hydrophilic. The addition or amount of hydrophilic monomer that is chemically attached is controlled within certain limits to avoid clogging the pores while simultaneously allowing control of the overall porosity of the hydrophilic film. Regulation of porosity, in turn, allows regulation of the water permeability and electrical resistance of the film.

The term "hydrophobic" is defined as used herein as a surface that permits less than about 0.010 ml of water per minute per cm 2 of flat film surface under a water pressure of 7.0 kp / cm 2. Similarly, the term "hydrophilic" refers to those surfaces which allow more than about 0.01 ml of water per minute per cm 2 to pass at the same pressure.

The hydrophilic monomers that can be used to coat the pore surface of the microporous film of the present invention are organic hydrocarbon compounds having 2 to 18 carbon atoms which are characterized by the presence of at least one double bond, which makes the monomers polymerizable and / or copolymerizable under the action. of ionizing radiation at a temperature that will not adversely affect the microporous film and at least one polar functional group, such as carboxy, sulfo, sulphinohydroxyl, ammonio, amino and phosphono.

Accordingly, the hydrophilic monomers of the present invention include hydrocarbon compounds having from 2 to 18 carbon atoms having one or more polymerizable and / or copolymerizable double bonds, such as substituted and unsubstituted carboxylic or dicarboxylic acids and esters thereof, vinyl- and allyl monomers, in particular itaconic acid, malonic acid, fumaric acid and crotonic acid and their esters or anhydrides, unsubstituted or alkyl-substituted acrylic acids such as acrylic acid and methacrylic acid, acrolein or acrylonitrile; unsubstituted or alkyl-substituted alkyl or cycloalkyl or aryl or hydroxyalkyl or hydroxyaryl acrylates, alkyl-substituted dialkylaminoalkyl acrylates, epoxyalkyl acrylates; vinylsulfonic 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; the silicones, 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 intended to penetrate the interior of the microporous film, it is preferred to use hydrophilic monomers having from about 2 to about 14, preferably from about 2 to about 4 carbon atoms.

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

The amount of hydrophilic monomer coating the inner surface of the pores of the microporous film is controlled to achieve the appropriate degree of addition, which is determined by the intended end use requirements of the films for water flow or electrical resistance. As described above, such regulation is exercised in a manner sufficient to preserve the open cell character of the micropores, i.e.. avoiding clogging of the pores, after the monomer-impregnated microporous film is subjected to the radiation treatment described herein so that the desired properties discussed above are maintained for a longer duration than can otherwise be obtained using typical prior art coatings.

The particular amount of hydrophilic monomer that is chemically attached to the surface of the pores is expressed in terms of percent addition, i.e. the weight percent of the uncoated microporous film that represents the weight of the hydrophilic monomer coating present in the cured microporous film.

Accordingly, to avoid porous clogging of the microporous films described herein, the percentage of addition of the hydrophilic monomer chemically adhered to the surface of the micropores is controlled so that it is not greater than about 10%, and generally from about 0.1 to about 10%, preferably from about 0.5 to about 2.5%, and most preferably from about 1 to about 2.0 weight percent (e.g., 1.5 weight percent) by weight of the uncoated microporous film.

The particular percentage of additive selected will be determined by the end use for which the resulting hydrophilic microporous film is used and will vary within the wide range of additive percentages described above.

For example, when the grafted hydrophilic microporous film is intended to be used as an electric battery separator, the percentage of addition is selected on the basis of the electrical resistance of the resulting hydrophilic microporous film (in milliohm per 6.45 cm 2) which constitutes a function of percent addition of the hydrophilic monomers.

Electrical resistance is defined here as a measure of the ability of the microporous film to conduct electrons. Consequently, as a general rule, the higher the electrical resistance of the microporous film, the less efficient the microporous film will be as a battery separator.

The electrical resistance of the microporous film is thus determined at different addition percentage values of the hydrophilic monomer and a deposition of electrical resistance as a function of percentage addition is made. The percentage of additive is then determined on the basis of the desired electrical resistance.

The electrical resistance of the hydrophilic microporous film of the present invention described hereinafter will generally be controlled to be less than about 30 milliohm per 6.45 cm 2, preferably less than about 10 milliohm per 6.45 cm 2. cm2 and most preferably less than about 5 milliohm per 6.45 cmz.

Ef When the grafted hydrophilic microporous film is to be used as a filter, the percentage of addition of the microporous film is selected on the basis of the water flow rate through the microporous film defined here.

Accordingly, the water flow rate can be plotted as a function of percent addition of the hydrophilic monomer and the appropriate percentage addition 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 so that it acts as a barrier layer for the material to be separated and the water flow rate is regulated as desired within the limits of the pore size selected. Thus, the water flow rate can be controlled to be greater than about 0.01 cm 3 / minute / cm 2, preferably greater than about 0.05 cm 3 / minute / cm 2, and most preferably greater than about 0.5 cm 3 / minute / cm 2 at a pressure differential of about 1 atmosphere. Thus, the present invention has several advantages over hydrophilic films previously produced. Since it is possible to preserve the open cell character of the hydrophilic microporous film produced in accordance with the present invention, such films allow a mass transport of water through the film as opposed to transport of water through the film by diffusion which is a very long time. same process. The mass transport effect also contributes to the reduction in electrical resistance. Another advantage of the microporous films of the present invention is obtained as a result of the chemical attachment of the hydrophilic monomer to the microporous film, which prolongs the duration of the presence of the hydrophilic monomer on the microporate over longer periods of time than typical surfactants. , especially after repeated washes. In addition, the dimensional stability of the films of the present invention is also improved.

It is worth mentioning that it is preferred to convert the polar functional group of the hydrophilic monomer to its most polar form whenever possible. This can be achieved, for example, by reacting an acid functional group with a base such as KOH to form the corresponding salt. The salt form can thus be obtained in this case by soaking the chemically attached film in a 2% solution of KOH for a time of from about 5 to about 30 minutes. This improves wettability and increases the flexibility of the film.

Any of the well-known coating methods can be used to coat the microporous film provided that such methods provide a sufficiently accurate control of the percentage addition of the hydrophilic monomer.

The preferred method of 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 microporous surface.

The percent addition can be controlled by controlling the equilibrium vapor pressure (ie, the vapor pressure at which the condensation rate of the monomer vapor is equal to its evaporation rate at a given temperature) for the hydrophilic monomer and the time during which the hydrophilic monomer is in contact with the microporous film.

The equilibrium vapor pressure required to achieve the desired amount of hydrofilm monomer coating at a constant temperature is determined from curves_for percent addition as a function of time at different equilibrium vapor pressures while keeping the temperature constant. The percentage of additive selected as described above depends on the particular property which is sought to impart to the microporous film.

The equilibrium vapor pressure required to achieve the appropriate percentage of addition is readily determined from the curves described above. The contact time of the monomer rim with the microporous film is also determined in a similar manner.

Thus, in a continuous process, the microporous film is passed through a chamber containing the hydrophilic monomer vapor at the appropriate equilibrium vapor pressure and temperature.

The duration of contact of the microporous film with the hydrophilic monomer vapor is controlled by setting the web length and line velocity of the microporous film through the vapor. The vapor coating procedure can obviously only be used when the critical temperature of the hydrophilic monomer (ie the temperature at which a liquid monomer cannot exist independently of pressure) is above the temperature at which the properties of microporous film would be adversely affected at the special contact time 450 706 30 used.

It is preferred that the steam coating process be carried out at atmospheric pressure at a temperature which gives the appropriate vapor pressure.

Typically, the temperatures at which the vapor coating process is performed when atmospheric pressure is used will range from about 50 to about 170 ° C when the hydrophilic monomer used is acrylic acid, methacrylic acid or vinyl acetate.

Negative pressure and overpressure can also be used with appropriate adjustments in temperature and contact time.

It is to be understood that since the percentage of additive is determined after the curing treatment, an amount of hydrophilic monomer in excess of the percentage of additive is applied from the beginning to the microporous film to compensate for loss of the monomer that may occur during the curing treatment.

Other suitable methods by which the microporous film can be coated with the hydrophilic monomer include dissolving the hydrophilic monomer in an evaporable solvent, such as methylene chloride, to prepare a pad bath.

The immersion bath can then be used in a reverse roll coating process. In this method, a metering roller is placed partially in the immersion bath of the monomer coating solution.

A second driven roller guides an uncoated hydrophobic microporous film web through the gap formed by it and the metering roller. The two rollers, which are preferably driven separately, rotate in the same direction so that the coated film web is guided in the direction from which the uncoated film originates. The amount of monomer coating deposited on the film is a function of the difference in speed of the metering roller and the second film driving roller and also of the size of u; 450 706 31 the gap formed by the two rollers.

Alternatively, a pinch selection method can be used. In this method, the film is passed into a dip bath of the monomer coating solution and clamped between two clamping rollers located downstream of the bath. The amount of coating thus constitutes a function of the gap size between the two clamping rollers and the pressure exerted between them.

Another alternative method of coating the hydrophobic microporous substrate film is the wire winding feed rod method. This method is the same as the nip roll method except that after being coated by conduction through a bath of the monomer coating solution, the microporous film is clamped between a pair of wire winding feed rods which control the amount of coating applied thereto by the configuration of the wires which are wrapped around the feed rods.

In the reverse roll, pinch roll and wire winding coating processes, the amount of hydrophilic monomer initially applied to the microporous film is a function of one or two variables discussed in the description of the methods. In addition, in all three methods, the amount of monomer coating is a function of the concentration of the monomer in the dip bath. The hydrophilic monomer dip bath is provided by dissolving the monomer in a common organic solvent having a boiling point lower than the boiling point of the hydrophilic monomer used as in addition to methylene chloride, acetone, methanol, ethanol and isopropanol.

The concentration of the hydrophilic monomer in the immersion bath is controlled to achieve the application of the appropriate amount of monomer upon evaporation of the solvent. In general, the monomer concentration in the dip bath can range from about 1% to about 30% by weight, preferably from about 450% to about 15% by weight, based on the weight of the bath.

When the hydrophilic monomers are coated on the microporous film using a dip bath, the solvent present therein is removed by passing the coated film through a dryer. The temperature of the dryer must be high enough to evaporate only the solvent, leaving the monomer deposited on the micropore surface of the film. After the pores of the microporous film have been coated with the hydrophilic monomer and solvent if used is removed therefrom, the coated microporous film is subjected to ionizing radiation for chemical attachment of the hydrophilic monomer to the normally hydrophobic microporous surface and to make it hydrophilic.

Upon exposure to ionizing radiation, a number of possible mechanisms or combinations thereof may act to achieve the desired effect. The hydrophilic monomers may thus be chemically bonded to the microporate surface and / or may, by polymerization and / or copolymerization, form a polymer. layer or sheath chemically intimately bonded to the micropore surface, such as by random chemical bonding of the sheath surface to the micropore surface, and / or physically, as a result of the limiting effect on the polymer sheath of the contour of the micropore surface. The term "chemical attachment" is intended to encompass all of the above mechanisms or combinations thereof.

Without being limited to the particular mechanisms by which the desired improvement can be achieved, the properties of the normally hydrophobic microporous film are modified in one or more ways depending on the percentage addition of the hydrophilic monomer. The resulting radiation-treated microporous film is thus rendered hydrophilic over a longer period of use than is otherwise obtained by typical coatings of the octane part, and yet the open cell character of the film can be preserved for use in the microporous film in such applications where mass transport and low electrical resistance is required.

As stated above, chemical attachment of the hydrophilic monomers to the normally hydrophobic microporous film is achieved by exposing the hydrophilic monomer impregnated microporous film to ionizing radiation.

Ionizing radiation is defined herein as consisting essentially of the type that provides emitted particles or photons that have an internal energy sufficient to produce ions and break chemical bonds, thereby inducing free radical reactions between the hydrophilic monomers used and between the monomers and the micropore surface. as described herein. Ionizing radiation is conveniently available in the form of ionizing particle radiation, ionizing electromagnetic radiation and actinic light.

The term "ionizing particle radiation" has been used to denote the emission of electrons or highly accelerated nuclear particles such as protons, neutrons, alpha particles, deuterons, beta particles or their analogues, directed in such a way that the particle is pushed into the mass which shall be irradiated. Charged particles can be accelerated by means of voltage gradients by means of such devices as an extended low energy (ie 200 KeV) electron beam generator, such as ELECTROCURTAINÜK) Corporation, accelerators with resonant chambers, Van Der made of Energy Sciences Graaff generators, betatrons, synchrotroners, etc. Neutron radiation can be produced by bombarding a selected light metal, such as beryllium, with high energy positive particles. Particle radiation can also be obtained by using a nuclear reactor, radioactive isotopes or other natural or synthetic radioactive materials.

"Ionizing electromagnetic radiation" is produced when a metal target, such as tungsten, is bombarded with electrons with. appropriate energy. This energy is imparted to the electrons by potential accelerators of over 0.1 million electron volts (mev.).

In addition to radiation of this type, commonly referred to as X-rays, an ionizing electromagnetic radiation suitable for practicing the present invention can be obtained by means of a nuclear reactor or by using natural or synthetic radioactive material, for example cobalt 60.

The hydrophilic monomers described herein will also undergo chemical attachment by exposure to actinic light.

The use of wavelengths at which sensitivity to actinic light occurs is generally about 1800 to 4000 Angstroms. Various suitable sources of actinic light are available in the art, including, for example, quartz mercury lamps, ultraviolet arc lamp carbon and high flask initiators. used when actinic light lantern was used.

The preferred source of ionizing radiation is the extended electron beam generator as described in U.S. Pat. Nos. 3,702,412, 3,745,396 and 3,769,600, which are hereby incorporated by reference. The procedures for accurate process control are sufficiently developed to allow the conversion of results from one type of radiation to another and the appropriate adjustment of the procedures described herein can be used interchangeably in the manufacture of any desired product.

A radiation dose of about 1.0 to about 10 megarads (mrad.) And preferably from about 2 to about 5 mrad. (e.g. 3 mradu) was used to achieve chemical attachment. A mega line is a million lines. One row is the amount of high-energy ionizing radiation which gives an absorption of I 100 erg of energy per gram of absorbent material. This unit is widely accepted as a suitable means of measuring the radiation absorption of materials; L The lowest ionizing radiation is used preferably to achieve chemical attachment due to economic considerations, excessive doses (ie greater than about 20 mrad. at a single exposure) should be avoided to avoid excessive heating and shrinkage of the material and degradation of the hydrophilic monomers and the microporous film. However, if the dose is all small, the chemical attachment will not be the hydrophilic monomer. The minimum radiation dose that will achieve chemical attachment of the hydrophilic monomer will vary depending on the type of polymer used to make the hydrophilic monomer. microporous filmi eilenda vymeren is polyethylene, the radiation doe shall not be less than for polypropylene radiation doses should not be less than about 2 mrad and preferably not less than about 3 mrad.

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

Since oxygen tends to inhibit the formation of the hydrophilic monomer to the microporous film, it is preferable to carry out the radiation treatment of the microporous film under an inert atmosphere, such as nitrogen or other inert gas. .--- ..>,: = ..-., -., ...- r>: -..-. ... ~ .- ~ ..-..... ~ -. ,.-..., -. _... ...,; ,. . , _. _. . i, - I ~ / lïtïóí QUAIJTY 450 706 se As previously discussed, the percentage of additives can be selected on the basis of the electrical resistance of the microporous film. Electrical resistance (DC method) of a microporous film is determined by soaking a sample of the microporous film having a known surface area (for example 0.2 x 6.45 cm 2) in a 40% (w / w) solution of KOH in water under 24 hours.

The resulting sample is then placed between cadmium test electrodes (ie, anode and a cathode) immersed in an electrolyte of a 40% (w / w) solution of KOH in water and a direct current of known current (eg 40 milliamperes) is passed through the cell between the electrodes. The potential drop across the film (E) is measured with an electrometer. The potential drop across the cell without the microporous film placed therein (E ') is also determined using the same current.

The electrical resistance of the microporous film was then determined using the equation: (E '- E) A E.R. = --- «- I where A is the surface area of the exposed film in square inches (6.45 cmz), I is the current across the cell in milliamps.

YOUR. is the electrical resistance of the microporous film in milliohms per square inch and E 'and E are as described.

The water permeability or water flow rate of the hydrophilic microporous film of the present invention is determined by measuring the rate of flow of water through a specific surface area of film while the water is under a differential pressure of an atomic sphere. The water flow rate is thus expressed in volume units of water in cm 3 per minute per cm 2 of film surface, i.e. cm 3 / minute / cm 2. The air permeability of the microporous films of the present invention is determined by the Gurley test, i.e. according to ASTM D 726 by mounting a film having a surface area of one square inch in a standard Gurley densometer. The film is then subjected to a standard differential pressure (pressure drop across the film) of 30.99 cm 3 of water. The time in seconds required to pass 10 cm3 of air through the film is a measure of permeability. A Gurley value greater than about 1.5 minutes is an indication that the pores are clogged.

The hydrophilic microporous films of the present invention find many varying uses. In particular, they find use in areas where controlled passage of moisture through a film or surface is desired. In addition, films made in accordance with the present invention can be used as filter membrane supports or filters suitable for separating ultrafine materials from various liquids and as battery separators. The invention is further illustrated by the following examples in which all parts and percentages are by weight unless otherwise indicated.

ExEMPr__1_ Part A The following discussion illustrates the preparation of a normally hydrophobic polyolefinic microporous film by the "dry stretching" method illustrated by U.S. Pat. No. 3,801,404.

Crystalline polypropylene with a melt index of 0.7 and a density of 0.92 is melt extruded at 230 DEG C. through a 20.3 cm wear hanger type slitting nozzle using a 2.54 cm extruder with a shallow feed screw. The length to diameter ratio of the extruder barrel is 24/1. The extrudate is extracted very rapidly to a melt extraction needle of 150 and brought into contact with a rotating casting roller held at 5090 and 1.9 cm from the edge of the nozzle. The 'surface area of young.

Qi contact with the river microp __: __ j aflde stretch A sample of this film ug fl is expelled with air. . n -o- q,; “._. _ nlng at len u_nnaer unre "ar åu mlnuz fi r, nen and allowed to cool fl The sample of ut an outlet elas' cold stretch and warm" those of 0,5Û: l and then heat treated under voltage, ie, _. _ ",. _ o _ _. _. constant length at 145 C 1 lb mlnte "1 J 0 the original length of the smooth elastic film, The resultant size of about 59.6% of the film has an average of 3000 Angstroms, a crystallinity of nngeier and a 4 microporous film is 0.025 mm gel B A continuous roll of it with a length of 30,5 m and a width of 15, 2 cm, prepared in corresponding ~ isacrylic acid (ie, 100%) an additive The impregnated elongated elec fi ~ -. *; ^ _fv: l length) with a line speed ktron.tråle enera ~ 39 iorn is set so provided at the used line speedü '_-. unae: lönsäïeä of the device and the acid content of the atmosphere is kept below 500 ppm by enclosing a chamber flushed with nitrogen. _ __;., __., .__..., _... _, ...._ .. fl ..... ~ .. l .., .... , .... »V-_ - - - ~ 450 706 39 A sample of cured dry microporous film is then tested for wettability by means of the drip test. The drip test is carried out by a 0.6 ml drop of 2% KOH in water is placed on the surface a in the film. The film is then observed visually.

If the part of the film on which the drop is placed becomes translucent and the opposite side of the film on which the drop is placed appears wet, the film is determined to be wet (in Table 1, denoted by the comment "yes"). The drip test is performed on a stand-alone sample of film immediately after irradiation and on a sample that has been stored for one week at room temperature.

Several other samples are taken from the cured microporous film roll and tested for electrical resistance in the manner described herein after soaking in a 40% (w / w aqueous solution of KOH maintained at 60 ° C for 1 hour) for 24 hours, 4 days resp. 8 days and the results are averaged.

The soaking of the film sample in hot KOH during increasing periods of time mimics aging over long periods of time.

The surface area of each sample tested for electrical resistance is 0.2 square inches and the current used is 40 milliamperes of direct current.

The results are summarized in Table I.

Samples of the cured microporous film roll are also tested for water flow after soaking in a 31% aqueous solution of KOH kept at 60 DEG C. for 24 hours by placing each sample having a surface area of 11.3 cm 2 in a polypropylene filter housing. The millipore filter is filled with water and pressurized to an atmospheric pressure differential between both surfaces and the film sample. Water is collected as it passes through the microporous film over a 5 minute period.

The results are converted to cm3 of collected water per minute per cmz of film surface.

The results are summarized in Table I.

Jäèi fi- 'EXERCISE EXAMPLE in Example 1 is repeated except that the radiation dose used for curing is reduced to 1 megarad. The results are summarized in Table I. An uncoated microporous film prepared in accordance with Example 1 is also exposed to ionizing radiation and tested in the same manner as described in Example 1 and serves as a control sample. The results are summarized in Table 1.

As can be seen from the results in Table I, a 1 megarad dose is assumed to be too small to achieve effective curing of a microporous polypropylene film as evidenced by the infinite electrical resistance of the microporous film in the comparative example. The positive wettability test for the comparative example on the stand-alone sample is assumed to be due to residual acrylic acid which has not been chemically attached by the irradiation. The negative wettability of the film in this comparative example after one week is assumed to be due to loss of the acrylic acid during the storage period of one week. This is confirmed by the infinite electrical resistance of the film, after one week of storage.

It can also be seen that the electrical resistance of the film of Example 1 increases after 4 days of exposure to hot KOH and then decreases slightly after 8 days of exposure. It is assumed that the decline after 8 days of aging may be due to the complete conversion of the acrylic oval to its salt form, while after 4 days of aging the salt conversion is only partially completed. Complete conversion of the acrylic ointment to its salt form thus proves to lower the electrical resistance. 450 706 41 TAße¿g_; Example l Comparison- Control- see example sample Dose (megarad) 3 l 3 Line speed (m / min) 9.1 6.1 9.1 Wettability (drip test) 7 independent sample yes yes ND after 1 week I yes' no ND Electrical resistance (milliohm / square inch (6.4: cmz) 41 hours 5.00 24 hours 6.87 4 days 10.6 8 days 7.1 Gurley (air) '(Gurley ~ seconds) 9.1 ND 10 water monofilament ( cnP / Inin / cmz) o, 35 o ND ND = Not determined §§ EXAMPLE 2 Eellè The following discussion illustrates the production of a microporous polyethylene film by the "solvent stretching" method.

Crystalline polyethylene having a melt index of 5.0, a weight average molecular weight of about 80,000, a density of 0.960 g / cm cool by rapid cooling in air at 25oC. A sample of the resulting precursor film is then immersed for 1 minute in trichlorethylene at 70 ° C and then ironed while immersed in trichlorethylene maintained at a temperature of q-- ..........._..... ._....... ....-...._.....-..._.._..._.........- W. _, 450 706 42 70OC with an elongation rate of 150% / min to 4 times its original length (ie 300% total elongation). The trichlorethylene is then removed by evaporation and the sample is stretched in 2 cross-sections to the machine direction at a degree of elongation of about 50% and allowed to air dry in the stretched state. Drying is performed at ZSOC.

The resulting microporous film exhibits a crystallinity of about 60%, an average pore length of about 5000 Angstroms and a surface area between about 10 and about 2 "far-25 m / g.

Several microporous film samples with a thickness of 0.025 mm prepared in accordance with Part A are dipped in isacrylic acid (100%) and the film becomes translucent. The samples are allowed to dry until the film assumes a white opaque appearance, which appearance indicates the appropriate monomer coating capable of achieving an addition of about 1.5%. The resulting samples were then passed under an extended electron beam curtain (ie, 61 cm long) and irradiated with varying radiation doses while maintaining the atmosphere under the window and in contact with the microporous film at less than 500 ppm oxygen in the manner used in Example The resulting cured films are tested for Gurly air flow, electrical resistance (after immersion for 24 hours in a 40% KOH solution at 6000) and wettability by the drop test in the manner described in Example 1. The results are summarized in Table II as experiments l-8. The percentage addition of resulting cured film samples is also determined by IR analysis and the results are shown in Table II.

As can be seen from the results in Table 1 experiments 4 and 7, the electrical resistance is infinite and the samples are not wetted. The results are assumed to be attributable to the use of a radiation device that is too low to achieve a chemical attachment. It is assumed that the hydrophilic monomer evaporates as shown by the absence of any measurable additive with a resulting loss of properties.

The film samples in experiments 1, 2, 5, 6 and 8 show low electrical resistance and are wettable. The higher electrical resistance of the film sample in Experiment 3 is assumed to be due to a visually observed inhomogeneity in the film. In addition, these film samples in which chemical attachment occurs, as evidenced by a 1.5% addition, show only a slight decrease in air permeability.

This indicates that the pores remain unsealed after chemical attachment. _ _ ___..-., ...., -, ..............__, ..., _..-.._.._. ._ _ riff. _! _ “/ Ur- 4 Ü cmmå flfi m fl nmu fl nwmošo fiflfl mm mm onwn mwmmwøm wnmuwuoš uxm fl nuxm fi w» mms m fi wß.o mm.ø o_mH H m. Fl m fi mq mw_ @ owåo N \ ~ Ü ß m H m flfl WM mn w @ ~ @ ow_o oko fi M m & H m mmm wmmø mmåo ~ \ HQ w w fi @ w ~ ø wmäo mqwm M mä fi m m fi ww. @ Wåo 0. » H m_H N m fi wäo w_o m ~ æ m mà fi H Awšu mwßu W 1 \ Ed0 fifi H fi2 v ^ JmummmEv m flfl n fi mnvm Hæuww www »Lwywü mnßm H L W wâmumm fl .mc mmonw ^ wmw |

Claims (15)

4 5 Û 7 0 6 then PATßNTKRAv_ 1. A process for producing an open-cell hydrophilic polydine-finic microporous film having an electrical resistance of less than 30 milliohm per 6.45 cm2 and a water flow rate thereby greater than 0,01 cm min / min / cm2 at a pressure - difference from l atm, from a normally hydrophobic polyolefinic microporous film selected from microporous polyethylene and polypropylene films, characterized in that it comprises the process steps. (a) coating the surface of the micropores of a normally hydrophobic open-cell microporous polyethylene or polypropylene film which is characterized by having a reduced bulk density compared to the bulk density of a precursor film from which it is made, an average pore size of from 0.02 to 1.0 μm and a surface area of at least 10 m2 per gram, with at least one hydrophilic organic hydrocarbon monomer having from 2 to 18 carbon atoms characterized by the presence of at least one double bond and at least one polar functional group, and (b) chemical attachment to the surface of the micropores of the microporous film of an amount of said hydrophilic organic hydrocarbon monomer which is sufficient to preserve the open cell character of said micropores and sufficient to obtain an addition of from 0.1 to 10% by weight, based on the weight of the uncoated microporous film, by irradiating the coated microporous film from (a) with an amount of ionizing radiation of from 1 to 10 megarads for microporous polyethylene films and from 3 to 10 megarads for microporous polypropylene films. 2. A process according to claim 1, characterized in that the hydrophilic organic hydrocarbon monomer has from 2 to 14 carbon atoms and at least one polar functional group selected from carboxyl, sulfo, sulphino, hydroxyl, ammonio, amino and phosphono. 450 7060 46 3. A process according to claim 1, characterized in that the hydrophilic organic hydrocarbon monomer is selected from unsubstituted and alkyl-substituted acrylic acids, vinyl esters, vinyl ethers and mixtures thereof. 4. A process according to claim 1, characterized in that the hydrophilic organic hydrocarbon monomer is selected from acrylic acid, methacrylic acid, vinyl acetate and mixtures thereof. 5. A method according to claim 1, characterized in that the normally hydrophobic microporous film is provided by polymers selected from polyethylene and polypropylene and "prepared by the" solvent stretching "or" dry stretching "method. 6. by chemically attaching the surface of the micropores of the normally hydrophobic microporous film with an addition of hydrophilic organic hydrocarbon monomer of from 0.5 to 2.5% by weight based on the weight of the uncoated film. A method according to claim 1, known as a 7. A process according to claim 1, characterized in that the normally hydrophobic microporous film has a crystallinity greater than 30%, an average pore size of from 0.04 to 0.5 μm, the hydrophilic organic hydrocarbon monomer is selected from acrylic acid, methacrylic acid and vinyl acetate, and the addition of the hydrophilic monomer to the surface of the micropores of the microporous film is in an amount of from 0.5 to 2.5% by weight, based on the weight of the uncoated microporous film. Hydrophilic open-cell microporous film having an electrical resistance of less than 30 milliohm per 6.45 cm2 and a water flow rate thereby greater than 0,01 cm min / min / cm2 at a pressure difference of 1 atm, characterized in that it comprises: (a) an open-cell, normally hydrophobic microporous film made of polyethylene or polypropylene characterized by having a reduced bulk density compared to the bulk density of a precursor film from which it is made, an average pore size of from 0.02 to 1.0 μm and a surface area of at least 10 m 2 per gram, and (b) a coating on the surface of the micropores of the microporous film of at least one hydrophilic organic hydrocarbon monomer having from 2 to 18 carbon atoms characterized by the presence of at least one double bond and at least a polar functional group in an amount sufficient to preserve the open cell character of the microporous film and to obtain an addition of from 0.1 to 10% by weight, based on the weight of the uncoated microporous film, which hydrophilic organic hydrocarbon monomer coating is chemically attached to the surface of the micropores of the microporous film by exposure to an amount of ionizing radiation of from 1 to 10 megarads for microporous polyethylene films and from 3 to 10 megarads for microporous polypropylene films. Hydrophilic microporous film according to Claim 8, characterized in that the hydrophilic organic hydrocarbon monomer contains from 2 to 14 carbon atoms and at least one polar functional group selected from carboxyl, sulfo, sulfino, hydroxyl, ammonio, amino and phosphono . Hydrophilic microporous film according to claim 8, characterized in that the hydrophilic organic hydrocarbon monomer is selected from unsubstituted and alkyl-substituted acrylic acids, vinyl esters and mixtures thereof. 11. ll. Hydrophilic microporous film according to claim 8, characterized in that the hydrophilic organic hydrocarbon monomer is selected from acrylic acid, methacrylic acid, vinyl acetate and mixtures thereof. Hydrophilic microporous film according to claim 8, characterized in that the normally hydrophobic polymeric microporous film is selected from polypropylene and polyethylene and is produced by the "solvent stretching" or "dry stretching" method. . 13. l3. Hydrophilic microporous film according to claim 8, characterized in that the hydrophilic monomer is chemically attached to the surface of the micropores with an addition of from 0.5 to 2.5% by weight, based on the weight of the uncoated microporous film. Hydrophilic microporous film according to Claim 8, characterized in that the normally hydrophobic microporous film is produced by the "solvent" or "dry stretching" method and has a crystallinity of at least 30% and an average pore size of from 0.04 to 0.5 μm and wherein the hydrophilic organic hydrocarbon monomer coating is selected from unsubstituted and alkyl-substituted acrylic acids, vinyl esters and mixtures thereof and is added to the microporous surface of the microporous film in an amount of from 0.5 to 2 .5% by weight, based on the weight of the uncoated microporous film. A hydrophilic microporous film according to claim 14, characterized in that the hydrophilic monomer is selected from acrylic acid, methacrylic acid, vinyl acetate and mixtures thereof.