FR2464277A1 - METHOD FOR MAKING NORMALLY HYDROPHOBIC, HYDROPHILIC MICROPOROUS MICROPOROUS FILM AND HYDROPHILIC MICROPOROUS FILM OBTAINED THEREBY - Google Patents

Abstract

THE INVENTION RELATES TO A PROCESS FOR RENDERING A MICROPOREOUS FILM OF NORMALLY HYDROPHOBIC, HYDROPHILIC POLYOLEFIN AS WELL AS A MICROPOREOUS FILM OF HYDROPHILIC POLYOLEFINE OBTAINED BY THIS PROCESS. THIS PROCESS INCLUDES COATING THE SURFACE OF THE MICROPORES WITH AN OPEN-CELL MICROPOROUS FILM OF NORMALLY HYDROPHOBIC POLYOLEFIN HAVING A REDUCED VOLUME DENSITY IN RELATION TO THE VOLUME DENSITY OF A PRECURERING FILM, IN PARTICULARLY FROM WHICH IT IS PRECESSOR. A HYDROPHILIC ORGANIC HYDROCARBON MONOMER HAVING FROM ABOUT 2 TO ABOUT 18 CARBON ATOMS AND INCLUDING AT LEAST A DOUBLE BOND AND AT LEAST ONE POLAR FUNCTIONAL GROUP; AND THE CHEMICAL FIXATION ON THE SURFACE OF THE MICROPORES OF THE MICROPOROUS FILM OF A SUFFICIENT QUANTITY OF THE SAID MONOMER TO PRESERVE THE ABOVE OPEN-CELL NATURE OF THE MICROPORES AND TO OBTAIN AN ADDITION OF ABOUT 0.1 TO ABOUT 10 IN WEIGHT COMPARED TO THE NON-FILM COATED BY IRRADIATION OF THE MICROPOREOUS FILM COATED WITH APPROXIMATELY 1 TO ABOUT 10 MEGARADS OF IONIZING RADIATION. THESE FILMS PRESENT AN IMPROVEMENT IN THE RATE OF WATER FLOW THROUGH THEM AND A REDUCTION IN ELECTRICAL RESISTANCE.

Description

The present invention essentially relates to a method for rendering a

  normally hydrophilic microporous film, hydrophilic and a hydrophilic microporous film obtained by this method. In particular, the present invention relates to a microporous film having a

  permeability & improved water and / or electrical resistance

reduced.

  Recent developments in the field of microporous open-cell polymeric films, exemplified by US Patent Nos. 3,839,516; 3,801,404;

  3,679,538; 3,558,764; and 3,426,754 have in-depth studies

  to discover applications that would exploit the unique properties of these new films. Such films which are indeed a gas permeable water barrier can be used as vents, liquid-gas transfer media, battery separators and plurality.

other jobs.

  A disadvantage of these films, which in the past has limited the number of applications to which they may be employed, has been their hydrophobic nature. This is especially true when polyolefin films, a preferred type of polymeric material often used in the manufacture of microporous films, are employed. Because these films are not "wetted" by water and aqueous solutions they could not be used advantageously in such logic applications as electrochemical separator components by means of

of a filter and the like.

  Several proposals have been mentioned in the past to overcome these problems, as exemplified in US Patents Nos. 3,853,601; 3,231,530; 3,215,486; and Canadian Patent No. 981,991, which utilize a variety of kinds of hydrophilic coating or impregnating agents. Such coating agents or impregnating agents, although effective for a limited period of time, tend to be removed in a relatively short time by solutions which contact the films or

  fibers in which they are present.

  Others have tried to impart a hydrophilic character to a normally hydrophobic microporous film

  by the use of low energy plasma treatments.

  Such plasma treatments are obtained firstly by activating the superficial sites of the microporous film by employing an argon or hydrogen plasma, and then by crosslinking thereon a kind of polymerization by

  appropriate free radical, such as acrylic acid.

  Plasma treatments result in the formation of a

  film having only one surface that is rewettable.

  The surface of the film thus becomes clogged when it is wet, which then inhibits or prevents free flow

  of water & through the inside of the film.

  The inevitable blockage of superficial 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 set

  evidence of substantial shrinkage in drying.

  As previously mentioned, a disadvantage of the plasma treatment and its limited ability to render only the surface of the microporous film wettable. He has

  It has been observed that because of the unusual surface area

  Microporous film of the type described here, a simple surface wettability does not ensure that the film will show certain functional properties such as

  low electrical resistance, and

  water through the film that are comparable to

  known surfactant systems discussed previously.

  The major disadvantage of plasma treatments,

  that is to say, the clogging of the pores and a simple wetting

  Surface area appears to result from a combination of factors, such as the tendency of low energy plasma to be easily deactivated by the high specificity of the microporous film. This reduces the likelihood of activation of an internal site in the microporous film. Similarly there is a competition for the arrival of crosslinkable monomers exerted by the free radical polymer crosslinks which are initially generated on the surface of the film when the crosslinkable monomer contacts any

  first the surface of the plasma-activated microporous film.

  Thus, the crosslinked polymer chains initially present on the surface of the film propagate at a faster and faster rate as the reaction takes place. Therefore, the resulting elongated crosslinked polymer strands that occur on the surface of the

  fila s GP, island or entangler and clog the micro-

  superficial or surface pores in the presence of water.

  Other attempts to provide hydrophilic films by employing plasma treatment are illustrated by the

  Nos. 3,992,495 and 4,046,843.

  Another technique for making polyethylene films that are wettable and suitable for use in electro-battery storage is illustrated by V. A. Agostino and J. Lee, Manufacturing Methods for Grafted-Polyether Battery Batter Segarators,

  "IU.S.I: atlaea-l Technical Information Service", A.D.

  No. 745,571 (1972) Abstract in Chemical Abstract 503148 (1973); V. D'Agostlno and J. Lee, Low Teorie A kaline Battery earators 27 Power Sources Symp. 87-91 (1971), summarized in the Chemical Abstract Review,

  86, 158277f; 19 D'Agostino, J. Lee and G. Orban, Zinc-

  Silver Batteries, (A. Fleisether and J. Lander ed., 1971).

  Such articles discuss or relate to a commercial product known as PERMION ™ developed by RAI Research Corporation. Briefly, the process for preparing this material is a crosslinking of a polyethylene sheet having a thickness of about 2.5410-3 cm using a beta radiation, followed by crosslinking with methacrylic acid in a appropriate solution under gamma radiation Co60. The crosslinked material is washed to remove the homopolymer, then converted to the salt form in hot KOH, further washed to remove the residual base, dried and packaged. The initial crosslinking step creates microcracks at the longitudinal slits in the non-porous polyethylene film that are so small that they are not visible even under the electron microscope. The diameter of these microcracks is estimated to be about 20 angstroms (10-8 cm). The microcracks are then crosslinked with methacrylic acid. The resulting film is therefore not microporous in the sense of the microporous films employed in the present invention which have

  an average size of about 100 to about 5,000 ang-

  Ströms. The extremely small size of the PERMION ™ microcracks generally prohibits mass transfer of species carrying mobile electrons generated by oxidation-reduction reactions across the film at any substantial rate. This is reflected by the relatively high electrical resistances (e.g., 30 to 40 milliohms-about 6.44 cm2) evidenced by such films. In addition, the PERMION! is not dimensionally stable in more than one

  direction as evidenced by a subsequent inflation.

  tantiel. It is well known that non-porous polymer substrates such as polyethylene and polypropylene can be reacted with various monomers such as acrylic acid by employing various types of ionizing radiation as exemplified by US Patent Nos. 2,999,056. ; 3,281,263; 3,372,100; and 3,709,718. Since none of these patents relate to microporous films, however, they do not relate to particular problems.

associated with them.

  Thus, research has continued on obtaining a relatively permanently wettable, hydrophilic microporous film which exhibits low electrical resistance, and increased water flow rates through the microporous film. The present invention has been developed

in response to this research.

  It is therefore an object of the present invention to provide a method for rendering a normally hydrophobic microporous film relatively hydrophilic at all times,

  thus improving its permeability to water.

  Another object of the present invention is to provide a method for reducing electrical resistance

  a microporous film normally hydrophobic.

  It is yet another object of the present invention to provide a hydrophilic microporous film having a

reduced electrical resistance.

  Yet another object of the present invention is to overcome the problems of the prior art discussed above. These and other objects will be apparent to those skilled in the art, as well as the scope, nature and use of the present invention as claimed in the light of the invention.

  of the explanatory description that follows.

  According to a feature of the present invention, there is provided a method for rendering a normally hydrophobic, hydrophilic, polyolefin microporous film by improving the flow velocity of water therethrough, and reducing the electrical resistance thereof. , characterized in that it comprises: (a) coating the surface of the micropores with a normally hydrophobic polyolefin open-cell microporous film characterized by a bulk density or reduced mass relative to the bulk density or by mass of a precursor film from which it is prepared, an average pore size of from about 200 to about 10,000 Angstroms, and a specific surface area of at least about 10 square meters per gram, with at least one hydrophilic organic hydrocarbon monomer having about 2 to about 18 carbon atoms characterized by the presence of at least one double bond and at least one polar functional group; and

  (b) the chemical fixation on the surface of micro-

  pores of the microporous film of an amount of said hydrophilic organic hydrocarbon monomer sufficient to preserve the open cell nature of said micropores and sufficient to obtain an addition of from about 0.1 to about 10%, by weight, based on the weight of the microporous film uncoated by irradiating the microporous film coated with (a) with from about 1 to about 10 megarads of

ionizing radiation.

  According to another characteristic of the present invention there is provided a hydrophilic open-cell microporous film characterized in that it comprises: (a) a normally open-cell, hydrophobic, microporous film characterized by a volume density or a density-reduced density volumetric or bulk of a precursor film from which it is prepared, an average pore size of about 200 to about 10,000 Angstroms and a specific surface area of at least square meters per gram; and (b) coating on the micropore surface of the microporous film of at least one hydrophilic organic hydrocarbon monomer having from about 2 to about 18 carbon atoms characterized by the presence of at least one double bond and at least one a group with a polar function, said hydrophilic organic hydrocarbon monomer of

  coating being chemically bonded to the surface of micro-

  pores of the microporous yarn, by exposure to about 1 to about 10 megarads of ionizing radiation, and in an amount sufficient to preserve the open cell nature of the microporous film and to obtain an addition of about 0.1 to about 10% , by weight, in relation to

  weight of the uncoated microporous film.

  The essence of the present invention lies in the discovery that open cell microporous films can be rendered relatively permanently wettable and / or hydrophilic when the pores thereof are chemically bound with a controlled amount of crosslinkable hydrophilic monomers upon exposure to ionizing radiation while preserving the open cell nature of the microporous film. In addition, such a chemical fixation of the hydrophilic monomers occurs even in these pores. at an internal site in 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 crosslinking that results from exposure to the radiation can be aligned with the pore surface. Therefore, the em-deleting and clogging of pores

is avoided.

  The present invention therefore relates to a micro-film

  BACKGROUND OF THE INVENTION Porous or cellular films can be classified into two general types: a type in which the pores are coated, ie, an iron cell film, and the other type in which the pores are porous. interconnected by tortuous passages which can extend from an external surface or super-liquid surface to another, that is to say, an open cell film. The porous lines of the present invention.

are dedlier type.

  In addition, the pores of porous films of the present invention are microscopic, that is, the details of their configuration or pore arrangement are discernible only by microscopic examination. In fact, the open-cell pores in the films are generally smaller than those that can be measured using a regular light microscope9 because the length

  wave of visible light, which is about 5 000 Ang-

  strOms (an angstrom is equal to 10-10 meters), t longer than the longest plane or surface dimension of the cell or open pore The microporous films of the present invention can be identified, however, by employing electron microscopy that are able to solve structural troubles of

pore below 5,000 Angstroms.

  The microporous films of the present invention are also characterized by a bulk density or reduced mass, sometimes hereinafter simply referred to as "low" density. That is to say, these microporous films have a bulk density or mass in total less than the density or mass of corresponding films composed of identical polymeric material but having no open cells or being devoid of structure comprising voids. The term "volume or bulk density" as used herein means the weight per unit gross or geometric volume of the film where the gross volume is determined by immersion of a known weight of the film in a container partially filled with mercury at room temperature. 250C and at atmospheric pressure. Volumetric increase in

  Mercury level is a direct measure of gross volume.

  This method is known as the mercury volumetric method, and is described in the Encyclopedia of Chemical Technology Journal, Vol. NI 4, page 892 (Interscience

1949).

  Porous films have been produced which have a microporous, open-cell structure and which are

  also characterized by a reduced density.

  Such films having this microporous structure are described, for example, in US Pat. No. 3,426,754, which patent is assigned to the assignee of the present invention and incorporated herein by reference. The preferred method of preparation described herein includes stretching or stretching at ambient temperatures, i.e., cold drawing, of an elastic, crystalline precursor film, to a quantity of about 10 parts. at 300% of its original length, with subsequent stabilization by thermal hardening of the stretched film under tension such that the film is not free of contraction or can contract only of limited value.

  methods of preparing microporous films are exemplary

  US Pat. Nos. 3,558,764; 3,843,762; 3,920,785; British Patent Nos. 1,180,066 and

  1,198,695 which are all incorporated herein by reference.

  While all of the patents mentioned above describe processes for the preparation of normally hydrophobic microporous films that can be rendered hydrophilic

  according to the present invention, microporous films normally

  The preferred hydrophobic compositions are prepared according to the methods described in US Pat. No. 3,801,404 which defines a process for the preparation of microporous films herein referred to as "dry drawing" method and US Pat. No. 3,839,516 which defines a process for the production of microporous films. microporous film preparation referred to herein as a "solvent stretch" method, both of which are incorporated herein by

  reference. Each of these patents reveals alternate routes

  preferred for obtaining a normally hydrophobic microporous film by manipulating a precursor film

  according to specifically defined process steps.

  Preferred precursor films that can be used to prepare microporous films by the "dry stretch" and "solvent stretch" methods are specifically detailed in each of the respective patents above. Thus, the "dry stretching" method uses a non-porous crystalline elastic polymer film having an elastic recovery at zero recovery time (hereinafter defined) when subjected to a standard extension (extension) of 50 % at 250C and 65% relative humidity of at least 40%, preferably at least 50% and

  still preferably at least 80o.

  Elastic recovery as used herein is a measure of the capacity of the formed structure or article such as a film to return to its original dimension after being stretched or elongated, and can be calculated as follows: Elastic recovery (ER)% length when lengthened after elongation - elongation x 100 length added when elongated When a standard elongation of 50% is used to identify the elastic properties of the starting films, such elongation is merely exemplary. In general, such starting films will have elastic recoveries greater than elongations of less than 50%, and somewhat less than elongations substantially greater than 50%, relative to their elastic recovery.

at an elongation of 50%.

  These starting elastic films will also have a percentage of crystallinity of at least 20%, preferably at least 30%, and more preferably at least 50%,

  for example, from about 50 to 90%, or more. The percentage

  crystallinity is determined by the X-ray method described by R.G. Quynn et al in the Journal of

  ApDlied Polymer Science, Vol. 2, 5 pages 166-173 (1959).

  For a detailed discussion of crystallinity and its meaning in polymers, see the book Polymers

  and Resins, Golding (Van Nostrand, 1959).

  Other elastic films considered suitable for the preparation of the precursor films used in the stretching or dry elongation process are described in British Patent No. 1,052,550 published on

December 21, 1966.

  The precursor elastic film used in the preparation of the microporous films by road using the "dry elongation" process should be distinguished from films formed from conventional elastomers such as natural and synthetic rubbers. With such

  classical elastomers the elongation behavior-

  constraint, and particularly the temperature-

  constraint, is governed by the deformation entropy mechanism (rubbery elasticity). The coefficient

  positive temperature of the retraction force, that is

  to say, the decreasing stress with decreasing temperature and the complete loss of elastic properties

  of the glass transition temperature, are conse-

  particular quences of entropic elasticity. The elasticity

  cited precursor elastic films used here, on the other hand, is of a different nature. In qualitative thermodynamic experiments with these elastic precursor films, increasing stress with decreasing temperature (negative temperature coefficient) can be interpreted to mean that the elasticity of these materials is not governed by entropic effects but depends on energy term. More significantly, it has been found that "dry stretch" precursor eelastic films retain their elongation properties at temperatures where the normal entropy eeasticity could not be longer op $ raiveo Thus, it is believed that the mechanism

  elongation of the precursor elastic films

  This is based on enzygelelastic relations.

  and these elastic films can then be referred to as "non-conventional" elastomers. Alternatively, the solvent-bonding method uses a precursor film which may contain at least two components, for example, an amorphous component and a crystalline ooEiposant. the materials, which are of two-component nature, are well suited to the process, the degree of crystallinity of the precursor film may therefore be at least 30%, preferably at least 4% and more preferably at least at least 50% by volume of the precursor film polymers, that is to say the syiithic resinous material from which the precursor films employed in the kills either of the processes according to the present invention include olefinic polymers such as polypropylene, poly (3-methylpentene) 1, poly-4-methylpenitene-1, and copolymers of propylamine, 3-methylbutene-1, 4-methylpentane-1, or ethylene with each other or with minor amounts of other olefins, for example, copolymers of propylene and ethylene, copolymers of a major amount of 3-methylbutene-1 and a minor amount of

  n-straight chain alkene such as n-octene-1, n-hexadecene

  1, n-octadecene-1, or other relatively long chain alkenes, such as copolymers of 3-methylpentene-1 and any of the same n-alkenes mentioned

  previously with respect to 3-methylbutene-1.

  For example, in general when homopolymers of propylene are contemplated for use in the "dry elongation" process, isotactic polypropylene having a percentage of crystallinity as indicated above, a weight average molecular weight of between about 100,000 and 750,000 (e.g., about 200,000 to 500,000) and a melt index (ASTM-D-1238-57T, part 9 page 38) of about 0.1 to about 75, (e.g., about 0.5 to 30) can be used to give a product

  final film having the required physical properties.

  It is to be understood that the terms "olefin polymer" and "olefin polymer" are used interchangeably for the purpose of describing a polymer prepared by the polymerization of olefin monomers by

their unsaturation.

  Preferred polymers for use in the "solvent elongation" process are the polymers employed according to the invention described in US Patent Application No. 44,805, filed January 6, 1979, by John W. Soehngen and assigned to the assignee of the present invention entitled "Improved solvent elongation process for the preparation of microporous films from precursor films of controlled crystalline structure"

  whose description is incorporated herein by reference.

  Thus, a polyethylene homopolymer having a density of about 0.960 to about 0.965 g / cc, a high melt index of not less than about 3 and preferably about 3 to about 20 and a molecular weight distribution ratio. A large (yw / fn) non-inferior to about 3.8, and preferably from about 3.8 to about 13 is preferred in the preparation of a microporous film by the solvent elongation process. nucleating agents may be incorporated into the polymer used to prepare the precursor film as described in the incorporated Soehngen application, in which case polymers having a melt index as low as 0.3 may

to be employed.

  The types of devices suitable for forming the

  Precursor films are well known in the art.

  For example, a conventional film extruder equipped with a flat channel calibrating nut and a bent die is satisfactory. Generally, the resin is introduced into a hopper of the extruder which contains a nut and a jacket provided with heating elements. The resin is melted and transferred by the nut to the die from which it is extruded through a slot in the form of a film from which it is drawn by a pick-up or molding roll. More than one pick roller in various combinations or stages can be used. The opening of the matrix or the width of the slot may be in the range, for example, ranging

  from about 2.54 × 10 -2 cm to about 0.51 cm.

  By employing this type of apparatus, the film can be extruded at a stretch ratio of about 5: 1 to 200: 1,

preferably 10: 1 to 50: 1.

  The terms "stretching ratio" or simply "draw ratio" as used herein is the ratio of the coiled film or the setting velocity relative to the

  film speed coming out of the extrusion die.

  The melting temperature for the extrusion of the film is, in general, no greater than about 1000 ° C above the melting point of the polymer and not less than about

  C above the melting point of the polymer.

  For example, the polypropylene may be extruded at a melting temperature of about 1800C to 2700C, preferably 2000C to 2400C. Polyethylene can be extruded

  at a melting temperature of about 1750C to 2250C.

  When the precursor film is to be used according to the "dry stretching" method, the extrusion operation is preferably carried out with rapid cooling and rapid stretching so as to obtain a maximum of elasticity. This can be accomplished by having the pick roller relatively close to the extrusion slot, for example, in less than 5.08 cm and preferably less than 2.54 cm. An "air knife" operating at temperatures between, for example, 00C and 400C, can be used in

  space of 2.54 cm from the slit to soak, that is

  to quickly cool and solidify the film. The pickup roller can be rotated, for example, at a speed of about 3 m to 305 m / min, preferably about 15 m / min.

at about 157 m / min.

  When the precursor film is to be used according to the "solvent stretching" method, the extrusion operation is preferably carried out with low cooling, so as to minimize stress and any associated orientation that might result from rapid quenching. to get a maximum of crystallinity but fast enough all

  likewise to avoid an important development of spherulites. This can be accomplished by controlling the distance of the

  quench roll from the extrusion slot.

  While the previous description concerned

  Matrix or slot extrusion processes, an alternative method for forming the precursor films contemplated in the present invention is the blown film extrusion process in which a hopper and an extruder are

  employees who are substantially the same as in

slotted slot described above.

  From the extruder, the melt enters a matrix from which it is extruded into a circular slot to form a tubular film having an initial diameter D1. The air enters the system through an inlet within said tubular film and has an effect of blowing the diameter of the tubular film into a diameter D2. Means such as air cores may be provided to direct air outside the extruded tubular film to provide different cooling rates. Means such as a cooling mandrel may be employed to cool the interior of the tubular film. After a distance during which the film is allowed to cool completely and

  harden, roll up on a pick roller.

  By employing the blown film method, the stretch ratio is preferably from 5: 1 to 100: 1, the aperture of the slit from 2.54 × 10 -2 cm to about 5.1 cm, preferably from about 4.10 to about cm to about 2.5 cm, the ratio D2 / D1, for example, 1.0 to 4.0 and preferably about 1.0 to 2.5,

  and the setting speed, for example from 3 m to 200 m / min.

  Melting temperature can be included in the formulas previously given for the material extrusion.

slit.

  The extruded film can then be initially treated thermomatically or recited to improve the crystalline structure by increasing the crystallite dienation and by removing the imperfections in the surface, this annealing is carried out. at a temperature within the range of from about 50 ° C to about 1000 ° C below the melting point of a polyester for a period of seconds to several hours, for example from 5 seconds to 24 hours, and preferably about 30 to seconds to 2 hrs For the polypropyline, the temperature

  The annealing temperature is about 100 ° C to 155 ° C.

  An e.mmplaire method of performing the annealing consists in placing the extruded film in a constrained or unstressed state in an oven at the desired temperature, in which case the residence time is preferably included.

  in the range of about 30 seconds to 1 hour.

  In preferred embodiments, the resulting partially crystalline precursor film is preferably subjected to one of the two alternative procedures described above to obtain a normally hydrophobic microporous film which can be used according to

present invention.

  The first preferred procedure as disclosed in U.S. Patent No. 3,801,404, hereinafter referred to as a "dry stretching" method, includes cold stretching or stretching steps, i.e., cold stretching the elastic film until porous areas or surface areas that are elongated normal or perpendicular to the elongation direction are formed, (2) stretching or hot stretching, i.e. that is, hot stretching the cold drawn film until the open-ended fibrils and pores or cells elongate parallel to the elongation direction are formed, and finally (3) heat-curing or heat-curing the film porous resulting under tension, that is to say, to a substantially constant length,

  to impart stability to the film.

  The term "cold drawing" as used herein is defined as stretching or stretching of a film to a length greater than its original length and at an elongation temperature, i.e., the temperature of the film being elongated, below the temperature at

  which fusion begins when the film is uniform-

  heated from a temperature of 25 ° C. and at a rate of 200 ° C. per minute. The term "drawing at

  "hot" as used here is defined as stretching beyond

  above the temperature at which fusion begins when the film is uniformly heated from a temperature of 250C and at a rate of 200C per minute, but below the normal melting point of the polymer, i.e. say, below the temperature at which the fusion takes place. As is known to those skilled in the art, the temperature at which the melting begins and the melting temperature can be determined by a standard differential thermal analyzer (DTA), or other known apparatus which can detect thermal transitions.

of a polymer.

  The temperature at which fusion begins varies depending on the type of polymer, the molecular weight distribution of the polymer, and the crystalline morphology of the film. For example, a polypropylene elastic film may be cold extended at a temperature of less than about 1200 ° C, preferably about 100 ° C to 1100 ° C.

246 $ 277

  and suitably at room temperature, for example, 250C. The cold-extended polypropylene film may then be heat-extended at a temperature above about 1250C and below the melting temperature, preferably from about 1300C to 1500C. Again, the temperature of the film itself to lengthen

  is referenced here as the elongation temperature.

  The elongation in these two steps or stages can be consecutive, in the same direction, and in this order, that is to say, cold and then hot, but can be done in a continuous, semi-continuous process. continuously or by water, as long as the cold-drawn film is not allowed to shrink to a significant degree, for example, less than 5% of its cold-drawn length, before being

stretched hot.

  The sum of the total amount of stretching or elongation in the two mentioned steps can be in the range of about 10 to 300% and preferably about 50 to 150%, based on the initial length of the elastic film. Further, the ratio of the amount of elongation to heat to the sum of the total amount of elongation or stretching may be greater by about 0.10: there less than 0.99: 1, preferably about 0, 50: 1

  at 0.97: 1 and more preferably from about 0.50: 1 to 0.95: 1.

  This relationship between "cold" and "hot" elongation

  is referenced here as "the extension report" (for-

  percentage of "hot" extension to percent

of the "total" extension).

  In any stretching operation where heat can be supplied the film can be heated by moving rollers which can in turn be heated by

  a process by electrical resistance, by passing through

  above a heated plate, through a heated liquid,

a heated gas, or the like.

  After the two-stage or two-stage elongation described above, the elongated or stretched film is thermally cured. This heat treatment can be carried out at a temperature in the range of about 1250C to a temperature below the melting temperature, preferably from about 1300C to 1600C for polypropylene; from about 75.degree. C. to below the melting temperature, and preferably from about 1150.degree. C. to 1300.degree. C. for the polyethylene, and to similar temperature ranges for other previously mentioned polymers. This heat treatment should be performed while the film is kept under tension, i.e., such that the film is not free of contraction or can contract only a controlled value of no more than about 15 % of its stretched length, but not as much tension as it

  can stretch the film more than an additional 15%.

  Preferably, the tension is such that substantially no contraction or elongation occurs, for example, less than 5% change in elongated length

or stretched.

  The heat treatment period that is

  preferably carried out sequentially with and after the

  stretching, should not be greater than 0.1 seconds at the higher annealing temperatures and, in general, be in the range of about 5 seconds to 1 hour and preferably

from about 1 to 30 minutes.

  The hardening steps mentioned above can take place in the air, or in other atmospheres

  such as nitrogen, helium or argon.

  A second preferred alternative procedure for converting the precursor film to a microporous film as described in U.S. Patent No. 3,839,516 and referenced herein as a "solvent stretching" method includes the basic steps of (1) contacting. precursor film having at least two components (for example an amorphous component and a crystalline component), one of which is smaller in volume than all other components, with an inflating agent in a time sufficient to allow the adsorption of the swelling agent in 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 or elongated state during removal of the swelling agent. Optionally, the film can be stabilized by a thermal clurcissGment under tension or by a

ionizing radiation.

  Generally, a solvent having a Hildebrand solubility parameter at or near that of the polymer would be a suitable solution for the stretching or elongation process described herein. The Hildebrand solubility parameter measures the energy density of cohesion. Thus, the underlying principle is based on the fact that a solvent having a cohesive energy density similar to a poly-ers crimps a high affinity for this poly era. and

would be adequate for this process.

  Oenal classes of blowing agents from which an appropriate for the particular polyp film may be selected are the lower aliphatic ketones such as acetone, methyl ethyl ketone, cycloheole; lower aliphatic acid esters such as ethyl formate, butyl acetate, etc .; halogenated hydrocarbons such as carbon eztrachloride, trichlorethylene, perchloroethyl, chlorobenzene, etc .; hydrocarbons such as heptane, cyclohexane, benzene, xylene, tetralin, decalin, etc .; organic compounds containing

  such as pyridine, formamide, dimethyl-

  formamide, etc .; ethers such as methyl ether, ethyl ether, dioxane, etc. A mixture of two or more of these organic solvents may also be employed. It is preferred that the blowing agents be a compound composed of carbon, hydrogen, oxygen, nitrogen, halogen, sulfur and contain up to about 20 carbon atoms, preferably up to about 10 carbon atoms. carbon

of carbon.

  The "solvent elongation" step can be carried out at a temperature between about the solidification point of the solvent, or the swelling agent, to the point below the temperature at which the polymer dissolves. (that is, the temperature

ambient at about 50 C).

  The precursor film employed in the "solvent-borne" process can be about

  2.54 × 10 cm to about 5.01.1 ± 2 cm, or even thicker.

  In a preferred embodiment, the precursor film is biaxially extended according to the procedures described in US Patent Application No. 44,801 filed January 6, 1979, entitled "Elongation Process".

  improved solvent for the preparation of micro-films

  porous "and assigned to the assignee of the present invention

  whose description is incorporated herein by reference. This

  The method identifies preferred elongation conditions in a uniaxial direction that leads to improved permeability of the uniaxially stretched microporous film. The microporous film stretched uniaxially can then be stretched

  in a transverse direction to increase the permeability

  even later. Thus, it is preferred that the precursor film be "solvent extended" in a uniaxial direction of no greater than about 350%, and preferably

  greater than 300% of its original length.

  Typically, we do not use the extra elongation

  in the same direction after removing the solvent.

  The optional stabilization step can be either a thermal cure step or a crosslinking step. This heat treatment can be carried out at a temperature of between approximately 125 ° C. and a temperature below the melting temperature and preferably from approximately 130 ° C. to 150 ° C. for propylene, of approximately 75 ° C. less than the melting point, and preferably about 1150 ° C to about C for the polyethylene and similar temperature ranges for other polymers of the aforementioned polymers. This heat treatment should be performed while the film is kept under tension, i.e., so that the film is not free of contraction or can shrink only by a controlled value of not more than about 15% its stretched length, but not at a voltage as great as it can stretch the film more than an additional 15%. Preferably, the tension is such that substantially no contraction or stretch occurs, for example, less than 5% of

  modification of the stretched length.

  The heat treatment period which is preferably carried out sequentially with and after the "solvent stretching" operation should not be greater than 0.1 second at the higher annealing temperatures and, in general, may be in the range from from about seconds to 1 hour and preferably from about 1 to

minutes.

  The hardening steps described above can take place in the air, or in other atmospheres

  such as nitrogen, helium or argon.

  When the precursor film is stretched biaxially the stabilization step should be carried out after

  transverse stretching and not before.

  While this description and examples

  first of all relate to the above-mentioned olefin polymers, the invention also includes high molecular weight acetal polymers, for example,

  oxymethylene polymers. Whereas at the same time

  polymers and copolymers of acetal are encompassed, the preferred acetal polymer with respect to the stability of the polymer is a "random" oxymethylene copolymer,

  which contains oxymethylene recurrence units, that is,

  to say, -CH2 -O -, intermingled with groups -OR-

  in the main polymer chain where R represents a divalent radical containing at least two carbon atoms bonded to one another and positioned in the chain between the two valencies, with all substituents on said radical R being inert, this is that is, those which do not include interfering functional groups and which do not include undesirable reactions, and o a major amount of -OR- units exist as single units attached to the oxymethylene groups on each side. Examples of preferred polymers include copolymers of trioxane and cyclic ethers containing at least 2 adjacent carbon atoms such as the copolymers disclosed in Walling et al. US Patent No. 3,027,352. These film-form polymers may also have a crystallinity of at least 20%, preferably at least 30%, and more preferably at least 50%, for example 50 to 60% or higher. In addition, these polymers have a melting point of at least 150 C and a number average molecular weight of at least 10,000. For a more detailed discussion of acetal and oxymethylene polymers, see the article Formaldehyde, Walter, pages 175-191 (Reinhold

1964).

  Other relatively crystalline polymers to which the invention can be applied are polyalkylene sulphides such as polymethylene sulphide and polyethylene sulphide, polyarylene oxides such as polyphenylene oxide, polyamides such as

  polyhexamethylene adipamide (nylon 66) and the polycapro-

  lactam (nylon 6), all of which are well known in the art

  and do not need to be described here.

  The normally hydrophobic microporous films employed in the present invention, in an unconstrained state, have a density density lowered relative to the density of the corresponding polymeric materials having no open cell structure, for example, those from which they are trained. Thus, the films have a density of no greater than about 95% and preferably 20 to 40% of the precursor film. In other words, the density is reduced by at least 5% and preferably from 60 to 80%. For polyethylene, the reduction is 30 to 80%, preferably 60 to 80%. The volume density is about 20 to 40% of the starting material, the porosity has increased from 60 to 80% due to

pores or orifices.

When the microporem film is prepared by the dry process or the solvent stretching process, the final crystallinity of the microporous film is preferably at least 30%, preferably at least 65%, and still more suitably about 85% as determined by the X-ray method described by RG Iuyu et al. in the Journal of Polymer SQci ciceI volume 2, H 5 ages I 66r173. a detailed discussion of crystallinity and its significance in polyesters, see Pol ers ad Resins, Golding

(S. Vtanostrand, 1959).

  The microporous filaments that can be used in this perverse invention have a mean diameter of between 200 and 10,000 grams, typically from about 100,000 to 5,000,000, even more preferably from about 500 to about 5,000. ALgstrems. These values can be determined by microstructural degradation as described in a paper by RoGe Guyn et al at pages 21-34 of the Textile Research Journal, January 1963 or by the use of electron microscopy as described in the artillery. "Pol Gr Sinae CrstlS", page 69 (Session 1963) .When an electronic micrograph is used, the length and pore width measurements can be obtained by simply using a ruler or meter to directly measure the length and width of the pore. width

  pores on an electrenic micrograph taken habitually

  At an enlargement of 5,000 to 10,000, the pore length values that can be obtained by electron microscopy are approximately equal to the pore size values obtained by

mercury porosimetry.

  The microporous films employed in the present invention will have a specific surface area within certain predictable limits when prepared either by the "solvent stretching" method or by the stretching method.

  "dry." Typically it will be found that such micro-films

  porous have a specific surface area of at least 10 m 2 / g. and preferably in the range of 15 to 25 m 2 / g For films formed from polyethylene, the specific surface area is generally in the range of from about 10 to 25 m 2 / g and preferably about 20 m 2 / g. The specific surface area can be determined from nitrogen gas or krypton adsorption isotherms using a method and apparatus described in US Pat. No. 3,262,319. The specific area obtained by this method is usually expressed in square meters

per gram.

  In order to facilitate the comparison of the various materials, this value can be multiplied by the density of the material in grams per cubic centimeter resulting in a specific area expressed in meters

squares per cubic centimeter.

  The normally hydrophobic microporous polymeric films of the present invention which are rendered hydrophilic have a preferred thickness of about

2.54 × 10 cm to about 2.10-2 cm.

  The normally hydrophobic microporous film prepared according to the procedures previously described is rendered wettable and / or hydrophilic by coating the micropore surface of the microporous film with a hydrophilic monomer or a mixture thereof and subsequently exposing the coated microporous film to ionizing radiation. . Ionizing radiation chemically binds the hydrophilic monomer to the surface of the micropores and makes it relatively hydrophilic at all times. The addition or amount of hydrophilic monomer which is chemically fixed is controlled within certain limits to avoid clogging of the pores while at the same time allowing control of the total porosity of the hydrophilic film. Porosity control in turn controls water permeability and resistance

electric film.

  As used herein the term "hydrophobic" is defined as meaning a surface that passes less than about 0.010 millimeters of water per minute per square centimeter of flat film area under a water pressure of about 689.47 cubic centimeters. Similarly, the term "hydrophilic" applies to surfaces that pass more than about 0.01 millimeters of water per minute per square centimeter.

the same pressure.

  Hydrophilic monomers that can be used to coat the pore surface of the microporous film of the

  The present invention is an organic hydrocarbon compound

  with 2 to 18 carbon atoms characterized by the presence of at least one double bond which renders the monomer polymerizable and / or copolymerizable under the influence

  ionizing radiation at a temperature which does not affect

  the microporous film and at least one polar functional group such as a carboxy group,

  sulfo, sulfino hydroxyl, ammonio, amino, and phosphono.

  Thus, the hydrophilic monomers of the present invention include hydrocarbon compounds having from 2 to 18 carbon atoms with one or more polymerizable and / or copolymerizable double bonds, such as substituted and unsubstituted carboxylic or dicarboxylic acids and esters thereof. this; vinyl and allyl monomers, especially itaconic acid, malonic acid, fumaric acid, crotonic acid,

  and their esters or anhydrides, non-acrylic acids

  substituted or substituted alkyls such as acrylic acid and methacrylic acid, acrolein or acrylonitrile;

  unsubstituted acrylates or alkyl-alkyl, cyclo-

  substituted alkyl, aryl, hydroxyalkyl or hydroxyaryl; alkyl-dialkylaminoalkyl acrylates

  substituted, epoxyalkyl acrylates; vinyl acid

  sulfonic acid, and styrene sulfonic acid; vinyl esters such as vinyl acetate and vinyl esters of higher carboxylic acid, alkyl esters substituted with a carboxylic acid alkyl group containing sulfo groups; vinyl ethers, such as unsubstituted ethers or substituted alkyl, cycloalkyl or aryl; substituted vinyl silicones; substituted aromatic or heterocyclic vinyl hydrocarbons; diallyl fumarates; diallylmaleates, substituted alkyl phosphates, phosphites or carbonates; vinylsulfones, the reaction product of

  ethoxylated nonylphenol and acrylic acid and the like.

  Since it is desired that the hydrophilic monomer penetrate the interior of the microporous film, it is preferred to employ hydrophilic monomers having from about 2 to about 14, more preferably from about 2 to about 4 carbon atoms. The preferred hydrophilic monomers employed in the present invention are acrylic acid,

  methacrylic, and vinylacetate.

  The amount of hydrophilic monomer which coats the inner surface of the pores of the microporous film is controlled to obtain the correct degree of addition which is

  determined by the requirements of use of electrical resistance

  final cudgel or water flow sought for the films. As previously described, such control is exercised in a manner sufficient to preserve the open-cell nature of the micropores, i.e., to prevent clogging of the pores, after the microporous monomer-impregnated film is subjected to radiation treatment described herein such that the desired properties discussed above are maintained for a longer period than is otherwise obtainable by employing the surface-active coatings typical of the prior art. The specific amount of the hydrophilic monomer that is chemically attached to the pore surface is expressed in terms of percent of addition, i.e., the weight percent of the uncoated microporous film which represents the weight of the monomer coating.

  hydrophilic which is present in the hardened microporous film.

  Thus, to avoid clogging of the pores of the microporous films described herein, the percentage of addition of the hydrophilic monomer that is chemically attached to the surface of the micropores is controlled to not be greater than about 10%, and generally about 0, 1 to about 10%, preferably about 0.5 to about 2.5%, and more preferably about 1 to about 2.0% (eg 1.5%).

  by weight, beaten on the weight of the non-microporous film

  coated. The particular addition percentage selected will be determined by the end use for which the resulting hydrophilic microporous film is applied and will vary in

  A large percentage of the additions described above.

  For example, when the microporous film hydroDhile rte! Or! must be used as the separator of the electric batto the percentage of addition is chosen on the box of Za. atlantic map of the metroporous film hy rv1a t (on ioms by om2) which is a hydrophilic material: USe ..]. a -; (n: .lS '

  Function of the add-on of the hydrophilic moaomer.

  Electrical resistance as defined here is a measure of the ability of the microporous film to conduct electrons. Therefore, as a general rule, the higher the electrical resistance of the microporous film, the less the iroporous film will be effective as a battery separator. As a result, the electrical resistance of the microporous film is determined at various percentages of addition of the hydrophilic monomer, and an electric resistance curve is made as a function of percent addition. The percentage

  addition is then determined on the basis of

desired electrical energy.

  The electrical resistance of the hydrophilic microporous film of the present invention as described below will generally be controlled to be less than about

  4.65 milliohms / cm2 (milliohms-cm2), preferably below

  than about 1.55 milliohms / óm2, and preferably

  less than about 0.775 milliohms / cm2.

  When the crosslinked hydrophilic microporous film eaTelod snId and eazo; oo aleTqdoJpTq eamouow np eaTelod euuo-.ouoj adnozS ae zlJeAuoo ap aeqTssod ase uleao anbsao ea.9Jd uonb.auuoTuew ap pTidoiddu% sa II Ae9aoTlt9e uemaalesu% s. uoTueaut euaesgad el ep smlT; sap allaeuuoTsueaTp% tlTqu; s el 'eano ua * s9% pdga se2ABeI sep gsqade% uemexTtlnOT.aed sanbTcTd% sjTqoe-oTsuea suae2 sal enb sduel. ap sapoTagd sen2uol sntd ap ans esnaiodoioTm aoBjans BI ans atT4doapXq o aiquouom np aouasgad el ap aginp el puae% Tnb xnaaodoaoTa mIT $ nu aITdoapXq eaqouom np anbTaTo uoTYeXT; and ap elnsgi uoTueauT aeuesgad el ep xnaeodoioTm smIT; sap ae: l.ueAe ea.4n a eanbTaoet9 eouesTo uoTonpgal el enqTlauoo ueamaleSp assem ep% aodsueal np aj; e $, w ue Z ueTq gpgooJd un.sa Tnb uoTsn.jjp aed mItj al saaAeBl. Vnee, p.aodsueai. Do not use the same value as you want to add a% ue% e.azed saTl; tHe apiOuTuE ea.uesgad el uoles gaedgad eltqdoapXg xnaiodoioTu mWT; np sea aAno sealnlleo eajn.eu e aaj.iesgad ep aelqTssod% se 0z TnbsTnid ssed el suep sgaedgid sITl4dOopZq SatT salt ans se2BIueAu sanesntld e uoT4ueAuT euespad el 'TSUTy * eaqdsome I uoiTAueap eGTe.T -uezj ;; Tp UOTSSaed eun zmo / um /, mog'O uoaTaue JneTagdns eaouea,; 9d ep aeooue * ae 'mo / um / .mo o uoaTuae anaTapdns Sl eoualgj9d ep' mzo / um / Co'O0 uoaTuae ineTadinq inod qlgqluoo ael 4ned neep ueemalnoogp xns el TsuTV * seTsToqoT eaod ep suoTsuemTp sep saemTl set suep 9JTsgP emoo tgaluoo * se neea, p luamelnoopp xne. el 4th aeaudgs neTqaeum and ano eaqTaaeq aeun emmoo AT2E inod eTsToqo Ol% is mITj np aeod ep uoTsuemTp el uoTeleTj ep SNQ sap suep xneiodoioTm EITj el aeotldve eaTspp uonbsaol 9aTsgp neap 1uemelnoo9gp xnel np esaq el years TsToqo% is 9Tadoidde UOTTppeBp ae2queoaznod el qe ealTqdojpq eazmouom np uoT4Tppe, p e9e> ueoinod np uoTIouo; eumoo 9oeai. eaj:% ned nee, p 4uemalnoog, p xnem el 'TsUTV TOT TuTggP eammoo xneaodooTm mIT; and saaeeal. nee, p% uewelnooap xne% np eseq el ans TsTOtO% se xneaodoioTm mIT; np uoT.Tppep e2eueoanod el 'e4alT; This can be achieved, for example, by reacting an acidic functional group with a base such as KOH to form the corresponding salt. Thus, the salt form can be obtained for example by soaking or soaking the chemically fixed film in a 2% KOH solution for a period of about 5 to about 30 minutes. This favors

  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 give sufficient control

  the percentage of addition of the hydro-

  phile. The preferred method for efficient and accurate coating of the inner surface of the pores of the microporous film is to contact the microporous film with the vapor of the microporous film.

  monomer that condenses on the microporous surface.

  The percentage of addition can be controlled in

  controlling the vapor pressure at equilibrium (ie

  the vapor pressure in which the condensation rate of the vapor of the monomer is equal to its vaporization rate at a given temperature) of the hydrophilic monomer and the time during which the hydrophilic monomer

  is in contact with the microporous film.

  The equilibrium vapor pressure required to obtain the desired amount of the hydrophilic monomer coating at a constant temperature is determined from time addition curves with respect to time at various equilibrium vapor pressures while keeping the constant temperature. The addition percentage chosen as described above depends on the property

  particular sought to impart to the microporous film.

  The equilibrium vapor pressure required to obtain the appropriate percentage of addition is easily

  determined from the previously mentioned curves.

  The contact time of the monomer vapor with the film

  microporous is also determined in a similar manner.

  Thus, in a continuous process the microporous film is continuously passed through a chamber containing the hydrophilic monomer vapor at the vapor pressure

  at the proper balance and the appropriate temperature.

  The contact time of the microporous film with the hydrophilic monomer vapor is controlled by adjusting the length of passage and the linear velocity of the microporous film to

through the steam.

  Obviously the steam coating technique can also be used when the temperature

  critical of the hydrophilic monomer (ie, the temperature

  to which a liquid monomer can not exist

  regardless of the pressure) is greater than the temperature

  to which the properties of the microporous film would be adversely affected to the particular contact time

employee.

  It is preferred that the steam coating technique be conducted at atmospheric pressure to a

  temperature which gives the appropriate vapor pressure.

  Typically the temperatures at which the steam coating technique is conducted when the atmospheric pressure is employed will vary from about 50 to about 1700C when the hydrophilic monomer employed is the acid.

  acrylic, methacrylic acid or vinylacetate.

  Pressures below the atmospheric pressure

  and above atmospheric pressure can also be used with appropriate adjustments

  regarding the temperature and the contact time.

  It will be understood that since the percentage of addition is determined from the curing treatment, an amount of hydrophilic monomer in excess of the percent addition is initially applied to the microporous film to compensate for the monomer loss that may occur during treatment. curing or crosslinking or vulcanization. Other suitable methods by which the microporous film can be coated with the hydrophilic monomer include dissolving the hydrophilic monomer in a vaporizable solvent such as

  methylene to form a buffer bath.

  The buffer bath can then be used in a roller coating technique. In this process a repair roller is arranged partially.

  in the buffer bath of the monomer coating solution.

  A second driven roller guides a microporous film ribbon through the nip or constriction formed by it-mee and the repair roller. The two-blades, which preferably serve separately, rotate in the same direction so that the coated film ribbon is guided in the direction from which the non-coated film originates. The amount of monomer

  revteclive on the film and the fenfoo of the diff

  the vitecse of the repair roller and the second roller driving the film and also the dimension of

  the 4tra ^ gl @ ment formed by the two rollers.

  Alternately, a process by rolls, compression can be used. In this process the film is guided in a buffer bath of the monomer coating solution and co-impregnated between two compression rollers arranged downstream. The amount of coating is thus a function of the size of the space between the two rolls of

  compression and exercised pession between them.

  An alternative method for coating the hydrophobic microporous substrate film and the wire rod pulling method or rod. This process is the same as the compressor roll process except that the microporous film after having been coated by being guided through a bath of the monomer coating solution is compressed between a pair of coiled wire rods or test rods which control amount of coating disposed on them by the configuration of the

  windings of wire around the tie rods or measuring rods.

  In reverse roll coating, roll-roll, and yarn winding, the amount of hydrophilic monomer initially applied to the microporous film is a function of one or more

  variables discussed in the description of the processes. In

  furthermore, in all three methods the amount of monomer coating is also a function of the monomer concentration in the buffer bath. The hydrophilic monomer buffer bath is made by dissolving the monomer in a usual organic solvent which has a boiling point below the boiling point of the hydrophilic monomer employed, such as, in addition to methylene chloride, acetone,

  methanol, ethanol and isopropanol.

  The concentration of the hydrophilic monomer in the buffer bath is controlled to achieve the application of the

  appropriate amount of the monomer by evaporation of the solvent.

  Generally the monomer concentration in the buffer bath may vary from about 1 to about 30%, and preferably from about 5 to about 15% by weight,

relative to the weight of the bath.

  When the hydrophilic monomers are coated on the microporous film by employing a buffer bath, the solvent present 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 deposited monomer on the microporous surface of the film. After the pores of the microporous film have been coated with the hydrophilic monomer and the solvent, if any, removed therefrom, the coated microporous film is subjected to ionizing radiation to chemically bind the hydrophilic monomer to the microporous surface. normally

  hydrophobic and to make it hydrophilic.

  When subjected to ionizing radiation a large number of possible mechanisms or combinations can be made to achieve the desired effect. Thus, the hydrophilic monomers can become chemically bound to

  the microporous surface and / or can form by polymer

  and / or copolymerization a polymer layer or sleeve which is intimately chemically bonded to the microporous surface, such as by a chemical attachment

  statistic of the surface of the sleeve at the micro-surface

  porous, and / or physically, as a result of the effect of imprisonment on the polymeric sleeve of the contour of the microporous surface. The term "chemical fixation" is meant to encompass all the aforementioned mechanisms or combinations. 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 according to the

  percentage of addition of the hydrophilic monomer.

  Thus, the resultant radiation-treated microporous film is rendered hydrophilic over a longer period of use than is otherwise obtained by typical surfactant coatings and already the open-cell nature of the film can be preserved. for the use of microporous film in these applications where mass transport and resistance are required

weak electric.

  As indicated above, the chemical fixing of the hydrophilic monomers to the normally hydrophobic microporous film is carried out by exposing the microporous film

  impregnated with hydrophilic monomer to ionizing radiation.

  Ionizing radiation is defined here to consist essentially of the type that provides particles

  or emitted photons with intrinsic energy sufficient

  to produce ions and break the chemical bonds and thus induce free radical reactions between the hydrophilic monomers employed and between

  the monomers and the microporous surface as described herein.

  Ionizing radiation is suitably available in the form of ionizing particle radiation, ionizing electromagnetic radiation, and light

actinic.

  The term "ionizing particle radiation N" has been used to designate 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 projected in the mass to be irradiated, charged particles can be accelerated by means of

  voltage gradients by devices such as a genera-

  elongated low energy electron beams (i.e. 200 KeV) such as ELECTROCURTAINTM manufactured by Energy Sciences Corporation, accelerators with resonance chambers, Van der Graaff generators, betatrons, synchrotrons, cyclotrons, etc. Neutron radiation can be produced by bombarding a selected light metal such as

  beryllium with positive particles of high energy.

  Radiation per particle can also be obtained

  by the use of an atomic pile, radioisotopes

  active or other natural or synthetic radioactive materials. * Electromagnetic ionization radiation * is produced when a metal target, such as tungsten, is bombarded with electrons of appropriate energy. This energy is imparted to electrons by accelerators with potential greater than 0.1 million electron volts (mev). In addition to irradiation of this type,

  commonly referred to as X-ray, an electron irradiation

  An ionizing magnetic unit suitable for the practice of the present invention may be obtained by means of a nuclear reactor (battery) or by the use of a radioactive material.

  natural or synthetic, for example, cobalt 60.

  The hydrophilic monomers described herein will also support chemical fixation by exposure to actinic light. In general, the use of wavelengths in which sensitivity to actinic light occurs

  is approximately 1,800 to 4,000 angstrom units.

  Various suitable sources of actinic light are available in the art including, by way of example, quartz mercury lamps, ultraviolet carbon core arcs, and high flash lamps. Initiators may be employed when actinic light is employed. The preferred source of ionizing radiation is the elongated electron beam generator as described in U.S. Patent Nos. 3,702,412; 3,745,396;

  and 3,769,600 which are hereby incorporated by reference.

  Precise process control techniques are sufficiently developed to allow the conversion of results from one type of radiation to another and an adequate adjustment of the techniques described herein can be used interchangeably in the production of any desired product. A radiation dosage of from about 1.0 to about 10 m6garads (mrad) and preferably from about 2 to about mgarads (e.g., 3 mrad.) Is used to obtain

  chemical fixation. A megadar is a million rads.

  A rad is the high energy ionizing radiation which produces an absorption of 100 ergs of energy per part of absorbing material. This unit is widely accepted as a suitable means of measuring absorption

of radiation by a material.

  Preferably, minimal ionizing radiation is used to obtain the chemical fixation due to

  economic considerations. Excessive dosages (ie

  ie greater than about 20 megarads at single exposure) should be avoided to avoid excess heating and film contraction and degradation of hydrophilic monomers and microporous film. If the dosage is too low, however, chemical fixation will not occur and the hydrophilic monomer will be rapidly lost. The minimum radiation dosage that will result in chemical uptake of the hydrophilic monomer will vary depending on the type of polymer used to prepare the microporous film. So, for example, when the polymer is polyethylene the radiation dosage should not be

  less than about 1 megarad while for the poly-

  propylene the radiation dosage should not be

  less than about 2 megarads and preferably not

less than about 3 megarads.

  Ambient temperatures can be used satisfactorily for irradiation although

  High temperatures can also be used.

  Since oxygen tends to inhibit the crosslinking of the hydrophilic monomer on the microporous film, the radiation treatment of the microporous film under an inert atmosphere such as nitrogen or other inert gas is preferred.

  As discussed previously, the percentage of addi-

  can be chosen on the basis of resistance

electric microporous film.

  The electrical resistance (DC method) of a microporous film is determined by quenching or immersing a microporous film sample having a known area or area (e.g., 1.29 cm) in a microporous film solution. KOH at 40% by weight in water for 24 hours. The resulting sample is then disposed between working cadmium electrodes (i.e., anode and cathode) immersed in an electrolyte of a 40 wt% KOH solution in water and past

  a continuous current of known amperage (for example 40 milli-

  amperes) through the cell between the electrodes. The potential drop across the film (E) is measured with an electrometer. The potential drop across the cell without the microporous film disposed therein (E ') is

  also determined by using the same current.

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

R.E. = (E '- E) A

  Where A represents the area or surface of the exposed film in cm 2, I represents the current through the cell in milliamps, R.E. represents the electrical resistance of the microporous film in milliohms per cm 2

and Et 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 water flow through a specific area or surface of the film while the water is under a differential pressure of an atmosphere. Thus, the water flow rate is expressed in units of water volume in cubic centimeters per minute per square centimeter of the area or area of the film, i.e., in cm 3 / min / cm 2 The air permeability of the microporous film of the present invention is determined by the Gurley test, i.e., according to ASTM D 726, by mounting a film having an area or area of about 7, 46 cm2 in a standard Gurley densometer. The film is subjected to a standard differential pressure (the pressure drop across the film) of about 30.9 cm of water. The time in seconds required to pass 10 cm3 through the film is an indication of permeability. A Gurley value greater than about 1.5 minutes is an indication that

pores are clogged.

  The hydrophilic microporous films of the present invention find many variable uses. In particular, utility is found in areas where the controlled passage of moisture through a film or surface is desired. In addition, films made according to the present invention can be used as a filter membrane support or filters useful in the separation of ultra-fine materials from various

  liquids and as battery separators.

  Other objects, features and advantages of the present invention will become apparent in light of the

  explanatory description which will follow made reference

  the following examples given merely as illustrations

  and in no way limit the scope of the present invention. All parties and

  percentages in the examples as well as in the description

  and the claims are by weight unless indicated

otherwise.

EXAMPLE 1

PART A

  This discussion illustrates the preparation of a microporous polyolefin film normally hydrophobic by the method of "dry stretching" as illustrated by the patent

American No. 3,801,404.

  Crystalline propylene having a melt index of 0.7 and a density of 0.92 is melt extruded at 230 ° C through a 20.30 cm slit matrix of the hanger type using an extruder of 2, 54 cm with a measuring nut with flat channel The length ratio

  relative to the diameter of the extruder bar is 24/1.

  The extrudate is stretched very rapidly to a melt stretch ratio of 150, and contacted with a rotating molding roll held at 50 ° C, and

  distance of 1.905 cm from the lip of the matrix.

  It is found that the film produced in this way has the following properties: thickness: 0.00508 cm; recovery of an elongation of 50% at 25 ° C, 50.3%, crystallinity: 59.6% A sample of this film is annealed in the oven with air under a slight stress at 1400C for about

  minutes, removed from the oven and allowed to cool.

  The sample of the annealed elastic film is then subjected to cold drawing and hot stretching at an extension ratio of 0.50: 1, and then heat-cured under tension or stress, i.e., at a length

  constant, at 1450C for 10 minutes in the air. The-

  cold draw portion is conducted at 250C, the hot stretch portion is conducted at 145C, and the total draw

  is 100%, based on the original length of the elastic film.

  The resulting film has an average pore size of about 3,000 Angstroms, a crystallinity

  about 59.6%, and a specific surface area of about 8.54m2 / g.

  The thickness of the microporous film is 2.54 × 10 -3 cm.

PART B

  A continuous roll of microporous film 3.048 m long and 15.24 cm wide, prepared in Part A, was then passed through a bath of glacial acrylic acid (i.e. 100%) and then through a compression roll to obtain an addition of 1.5% after crosslinking, curing or vulcanization, by weight, based on the weight of the film before impregnation as determined by infrared analysis. The impregnated film was then passed under the window of an elongated electronic-format generator (ie, 60.96 cm

  of length) at a linear speed of 6.096 m per minute.

  The electron beam generator is set to provide a dose of 3 megarads at the linear speed employed. The oxygen content of the atmosphere below the door window and in contact with the microporous film is kept below 500 ppm by

  including the fen-ie in a chamber purged with nitrogen.

  A cross-linked dry microporous film sample is then tested for wettability by the drop test. The drop test is conducted by placing 0.6 ml of 2% KOH in water on the surface of the film. The film is then visually observed. If the portion of the film on which the drop is placed becomes translucent and the opposite side of the film on which the drop is placed appears wet, the film is determined as

  being wet (as shown in Table 1 by the commentary

  Say yes. ") The drop test is conducted on a closed-line sample of the film immediately after irradiation, and on a sample that has been stored for a period of time.

  one week at room temperature.

  Several other samples are taken from the cross-linked microporous film roll and tested for electrical resistance as described herein after quenching in 40% by weight aqueous KOH solution maintained at 600 ° C. for 1 hour, 24 hours, 4 days and

  8 days respectively and we give the average of the results.

  Quenching the film sample in hot KOH for progressively longer periods of time simulates aging over long periods of time. The area or area of each sample tested for electrical resistance is 1.19 cm2, and the

  Current used is 40 milliamperes DC.

  Three samples of the cross-linked microporous film roll are tested for flow permeability

  of air according to Gurley ASTM-D-726B.

  The results are summarized in chart form in Table I. Samples of the crosslinked microporous film roll are also tested for water flow after immersion or quenching in 31% aqueous KOH solution. maintained at 1000C for 24 hours by placing each sample having an area or area of 11.3 cm 2 in a millipore filter frame. The millipore filter is filled with water and pressurized to a differential atmosphere between the two surfaces of the film sample. The water is collected as it passes through the microporous film for a period of 5 minutes. The results are converted into cubic centimeters of water collected per minute per cm 2 of film area. The results are summarized in Table I.

COMPARATIVE EXAMPLE

  Example 1 is repeated except that the radiation dose employed for the crosslinking is reduced to 1 megarad. The results are summarized in Table I. An uncoated microporous film prepared according to Example 1 is also exposed to ionizing radiation and tested in the same manner as described in Example 1 and used as a control. The results are summarized in Table I. As can be seen from the results of Table I, it is believed that a 1 megarad dose is too low to effect effective crosslinking in microporous polypropylene film as indicated by electrical resistance. infinite microporous film of the comparative example. The positive wettability test of the Comparative Example on the closed line sample is believed to be residual acrylic acid which has not been chemically fixed by the irradiation. It is believed that the negative wettability of the film of this comparative example after one week is due to the loss of acrylic acid during the one week storage period. This is confirmed by the infinite electrical resistance of the film

after a 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 drops slightly after 8 days of exposure. It is believed that the drop after 8 days of aging may be due to the complete conversion of the acrylic crosslinking into its salt form while after 4 days of aging the conversion of the salt is only partially complete. Thus, the complete conversion of acrylic crosslinking in its salt form appears to decrease the electrical resistance.

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made at 25 C.

  The resultant microporous film has a crystallinity of about 60%, an average pore length of about 5,000 Angstroms, and a specific surface area of about 10 to about 25 m2 / g.

PART B

  Several microporous film samples having a thickness of 2.54 × 10 3 cm prepared according to Part A are immersed in glacial acrylic acid (100%) and the film becomes translucent. The samples are allowed to dry until they have a white opaque appearance which appearance is indicative of the appropriate amount of monomer coating capable of effecting an addition of about 1.5%. The resulting samples are then passed beneath an elongated electron beam curtain (i.e. 60.96 cm in length) and irradiated with varying doses of radiation while maintaining the atmosphere under the window and in contact with the microporous film with less than 500 ppm oxygen as used in Example 1. The resulting crosslinked films are tested by Gurley airflow, the electrical resistance (after immersion for 24 hours in a 40% KOH solution at 60 ° C) and wettability

  by the drop test as described in Example 1.

  The results in Table II are summarized as Tests 1 to 8. The percentage of addition of the resulting crosslinked film samples is also determined by

  infrared analysis and the results are shown in Table II.

  As can be seen from the results in Table II for Tests 4 and 7, the electrical resistance is infinite and the samples do not get wet. These results are thought to be attributable to the use of too low a dosage of radiation to achieve chemical fixation. Therefore, it is believed that the hydrophilic monomer evaporates as evidenced by the lack of any measurable addition with resulting loss.

properties.

  The film samples from runs 1, 2, 5, 6 and 8 show low electrical resistance and are wettable. It is believed that the higher electrical resistance of the test 3 film sample is due to a visually observed heterogeneity of the film. In addition, those film samples in which the chemical fixation occurs, as indicated by a 1.5% addition, show only a slight drop in air permeability. This indicates that the pores remain unobstructed

after a chemical fixation.

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  in the previous description. Of course, the invention

  is not limited to the described embodiments which have been given by way of example. In particular, the invention comprises all the means constituting technical equivalents of the means described and their combinations if they are executed according to its spirit and implemented in the context of protection

as claimed.

R E V E N D IC A T I ONS

  A process for rendering a microporous, normally hydrophobic, hydrophilic polyolefin film by improving the rate of water flow therethrough and reducing the electrical resistance thereof, characterized in that it comprises: a) the surface coating of the micropores of a normally hydrophobic polyolefinic open-cell microporous film characterized by a reduced volume or mass density in comparison with the bulk or bulk density of a precursor film from which it is prepared, an average pore size of about 200 to about 10,000 Angstroms, and a specific surface area of at least about 10 square meters per gram, with at least one hydrophilic organic hydrocarbon monomer

  having from about 2 to about 18 carbon atoms

  risoed by the presence of at least one double bond and at least one polar functional group; and (b) chemically binding to the micropore surface of the microporous film an amount of said hydrophilic organic hydrocarbon monomer sufficient to preserve the open-cell nature of said micropores and sufficient to obtain an addition of from about 0.1 to about 10% , by weight, based on the weight of the uncoated microporous film, by irradiating the microporous film coated with (a) with from about 1 to about

megarads of ionizing radiation.

The process according to claim 1, characterized in that the hydrophilic organic hydrocarbon monomer has from about 2 to about 14 carbon atoms and at least one polar functional group selected from the group consisting of carboxyl, sulfo, sulfino, hydroxyl

ammonio, amino and phosphono.

  3. A process according to claim 1, characterized in that the aforementioned hydrophilic organic hydrocarbon monomer is selected from the group consisting of unsubstituted acrylic acids and substituted alkyl, esters

  vinyl, vinyl ethers and mixtures thereof.

  4. A process according to claim 1, characterized in that the hydrophilic organic hydrocarbon monomer is selected from the group consisting of acrylic acid,

  methacrylic acid, vinyl acetate and mixtures thereof.

  5. A process according to claim 1, characterized in that the above-mentioned hydrophobic microporous film is prepared from polymers selected from the group consisting of polyethylene and polypropylene and is prepared by the "solvent stretching" process.

  or the method of dry stretching.

  6. A process according to claim 1, characterized in that the surface of the micropores of the normally hydrophobic microporous film is chemically fixed with an addition of hydrophilic organic hydrocarbon monomer of from about 0.5 to about 2.5% by weight, by report to

weight of the uncoated film.

  7. A process for rendering a microporous film normally hydrophobic with open cells, hydrophilic by increasing the rate of flow of water therethrough and reducing the electrical resistance thereof, characterized in that it comprises (a) coating the surface of the micropores with a normally open cell hydrophobic microporous film prepared from polymers selected from the group consisting of polyethylene and polypropylene, said film being characterized by a reduced volume density in comparison with the density of a film

  precursor from which it is prepared, a crystallization

  greater than about 30%, an average pore size of about 400 to about 5,000 Angstroms, and a surface area of at least about 10 square meters per gram, with at least one hydrophilic organic hydrocarbon monomer selected from the group consisting of acrylic acid, methacrylic acid, vinyl acetate; and (b) chemically attaching to the surface of the micropores of the microporous film an amount of said hydrophilic monomer sufficient to preserve the open cell nature of said micropores and sufficient to obtain an addition of from about 0.1 to about 10%,

  weight, relative to the weight of the non-microporous film

  coated by irradiating the microporous film coated with (a) with from about 1 to about 10 megarads of ionizing radiation. 8. Microporous film with open cell hydrophilic characterized in that it comprises: (a) a microporous film normally hydrophobic open cell characterized by a reduced density in comparison with the density of a precursor film from which it is prepared, an average pore size of about 200 to about 10,000 Angstroms and a specific area of about 10 square meters per gram; and (b) coating on the micropore surface of the microporous film of at least one hydrophilic organic hydrocarbon monomer having from about 2 to about 18 carbon atoms characterized by the presence of at least one double bond and at least one a polar functional group, said hydrophilic organic hydrocarbon monomer coating being chemically attached to the micropore film surface of the microporous film by exposure to about 1 to about 10 megarads of ionizing radiation, and in an amount sufficient to preserve the open-cell nature of the microporous film; microporous film and to obtain an addition of from about 0.1 to about 10%, by weight, relative to

  weight of the uncoated microporous film.

  9. Microporous film hydrophilic according to the

  cation 8, characterized in that said hydrophilic organic hydrocarbon monomer has from about 2 to about 14 carbon atoms and at least one polar functional group selected from the group consisting of carboxyl, sulfo, sulfino, hydroxyl, ammonio, amino, and phosphono. 10. The hydrophilic microporous film according to claim 8, characterized in that the abovementioned hydrophilic organic hydrocarbon monomer is chosen from the group consisting of unsubstituted and alkyl substituted acrylic acids, vinyl esters,

  vinyl ethers and their mixtures.

  11. The hydrophilic microporous film according to claim 8, characterized in that the above-mentioned hydrophilic organic hydrocarbon monomer is chosen from the group consisting of acrylic acid,

  methacrylic acid, vinyl acetate and mixtures thereof.

  12. The hydrophilic microporous film according to claim 8, characterized in. the normally hydrophobic polymeric microporous film is selected from the group consisting of polypropylene and polyethylene and is prepared by the "solvent stretch" method or

the "dry stretching" method.

  13. The hydrophilic microporous film according to claim 8, characterized in that the aforementioned hydrophilic monomer is chemically fixed on the surface of the micropores at an addition of approximately 0.5 to approximately 2.5% by weight, relative to the weight of the uncoated microporous film.

  14.- Microporous film with open cells

  phile, characterized in that it comprises: (a) a normally hydrophobic open cell microporous film prepared by the "solvent stretching" method or the "dry stretching" method from

  polymers selected from the group consisting of

  ethylene, and polypropylene, said film being characterized by a reduced volume density with respect to the volume density of a precursor film from which it is prepared, a crystallinity of at least 30%, an average pore size of about 400 to about 5,000 Angstroms, and a specific area of at least about 10 square meters per gram; and (b) coating on the micropore surface of the microporous film of at least one hydrophilic organic hydrocarbon monomer having from about 2 to about 18 carbon atoms selected from the group consisting of unsubstituted and alkyl-substituted acrylic acids vinyl esters, vinyl ethers, and mixtures thereof, said hydrophilic organic hydrocarbon monomer coating being chemically attached to the micropore surface by exposure to ionizing radiation of from about 2 to about 5 mgaradse and in an amount sufficient to preserve the open-cell nature of the microporous film icoropores and to obtain an addition of from about 0.1 to about 10% by weight, by

  relative to the weight of the uncoated microporous film.

  15. The hydrophilic microporous film according to claim 14, characterized in that the preferred trophilophilic monomer is chosen from the group consisting of acrylic acid, methacrylic acid, vinyl acetate and mixtures thereof, which Hydrophilic monomer is present on the surface of the microporesE in an amount of about 0.5% to about 2% by weight based on the weight

uncoated microporous film.