CH648576A5 - MICROPOROUS PRODUCT WITH OPEN CELLS HYDROPHILIC AND METHOD FOR MAKING A MICROPOROUS FILM NORMALLY HYDROPHOBIC, HYDROPHILIC. - Google Patents

Description

The present invention relates to an open cell microporous product having improved water permeability and / or reduced electrical resistance, and a method for rendering a normally hydrophobic microporous film hydrophilic.

Recent developments in the field of microporous open cell polymer films, exemplified by American patents Nos. 3839516, 3801404, 3679538, 3558764 and 3426754, have studied in depth to discover applications which would exploit the unique properties of these new films. Such films, which are indeed a gas permeable water barrier, can be used as vents, as liquid-gas transfer media, as battery separators and in a variety of other uses.

A disadvantage of these films, which in the past has limited the number of applications for which they can be used, has been their hydrophobic nature. This is especially true when films from

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polyolefin, a preferred type of polymeric material often used in the manufacture of microporous films, are used. Because these films are not wetted by water and aqueous solutions, they could not be advantageously used in such logic applications as electrochemical separator components by means of a filter and the like.

Several proposals have been made in the past to overcome these problems, as exemplified in U.S. patents Nos. 3853601, 3231530, 3215486 and Canadian Patent No. 981991, which use a wide variety of coating types or hydrophilic 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 come into contact with 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 surface sites of the microporous film by using an argon or hydrogen plasma, and then by crosslinking thereon a kind of appropriate free radical polymerization, such as acrylic acid. Plasma treatments result in the formation of a film having only one surface which is rewettable. The surface of the film thus becomes clogged when wet, which then inhibits or prevents the free flow of water through the interior of the film.

The inevitable clogging of 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 evidenced by substantial shrinkage on drying.

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

The major drawback of plasma treatments, ie clogging of the pores and simple surface wettability, seems to result from a combination of factors such as the tendency of low energy plasma to be easily deactivated by 'high specific area of the microporous film. This reduces the likelihood of activation of an internal site in the microporous film. Likewise, there is 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 first comes into contact with the surface of the film. plasma activated microporous film. Thus, the chains of crosslinked polymer initially present on the surface of the film propagate at an increasingly rapid rate as the reaction takes place. Therefore, the resulting elongated crosslinked polymer chains which occur on the surface of the film become entangled or entangled and clog the surface or surface micropores in the presence of water.

Other attempts to provide hydrophilic films using plasma processing are illustrated by U.S. patents Nos. 3992495 and 4046843.

Another technique for 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", US National Technical Information Service, AD Report No. 745571 (1972) summarized in the review "Chemical Abstract", 503 lf, 78 (1973); V. D'Agostino and J. Lee, "Low Temperature Alkaline Battery Separators", 27 Power Sources Symp. 87-91 (1971), summarized in the journal "Chemical Abstract", 86, 158277f; V.

D'Agostino, J. Lee and G. Orban, "Zinc-Silver Batteries" (A. Fleischer and J. Lander ed., 1971).

Such articles discuss or relate to a commercial product known under the name Permion ™ developed by the company RAI Research Corporation. In summary, the process for the preparation of this material consists of crosslinking a polyethylene sheet having a thickness of about 2.5410 ~ 3 cm using ß radiation, followed by crosslinking with methacrylic acid in a suitable solution under radiation and Co60. The crosslinked material is washed to remove the homopolymer, then converted to the salt form in hot KOH, washed again to remove the residual base, dried and packaged. The initial crosslinking step creates microcracks or longitudinal cracks in the non-porous polyethylene film which are so small that they are not visible even under the electron microscope. The diameter of these microcracks is estimated at around 20 Å (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 5000 Å. The extremely small size of Permion ™ microcracks generally prohibits mass transfer of species carrying mobile electrons generated by oxidation-reduction reactions through the film at any substantial speed. This is reflected by the relatively high electrical resistances (for example, 30 to 40 mfl - about 6.44 cm2) evidenced by films of this type. In addition, the Permion ™ is not dimensionally stable in more than one direction as evidenced by substantial inflation.

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 illustrated by American patents Nos. 2999056, 3281263, 3372100 and 3709718. Since none of these patents relates to microporous films, they do not concern the specific problems associated with them.

Thus, research has continued with regard to obtaining a hydrophilic microporous film, relatively permanently wettable, which shows a low electrical resistance and increased water flow rates through the microporous film. The present invention was developed in response to this research.

It is therefore an object of the present invention to provide a method for making a normally hydrophobic microporous film relatively hydrophilic at all times, thereby improving its water permeability.

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

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

According to the present invention, the microporous hydrophilic open cell product is characterized in that it comprises:

a) a normally hydrophobic microporous film with open cells having a reduced density relative to the density of a precursor film from which it is prepared, an average pore size of 200 to 10,000 Å and a specific area of at least 10 m2 / g, and b) a coating on the surface of the micropores of the microporous film with at least one hydrophilic organic hydrocarbon monomer having 2 to 18 carbon atoms containing at least one double bond and at least one group having a polar function , said coating of hydrophilic organic hydrocarbon coating monomer being chemically attached to the surface of the micropores of the microporous film in an amount sufficient to preserve the open cell nature of the microporous film and to represent an addition of 0.1 to 10% by weight , based on the weight of the uncoated microporous film.

According to the present invention, the method for rendering a normally hydrophobic microporous film hydrophilic, by improving the

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speed of flow of water through it, and reducing the electrical resistance thereof is characterized in that it comprises:

a) coating the surface of the micropores with a microporous open cell film of normally hydrophobic polyolefin having a density reduced compared to the density of a precursor film from which it is prepared, an average pore size ranging from 200 to 10,000 Å, and a specific area of at least 10 m2 / g, with at least one hydrophilic organic hydrocarbon monomer having from 2 to 18 carbon atoms and containing at least one double bond and at least one polar functional group, and b) the chemical fixation on the surface of the micropores 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 0.1 to 10% by weight, relative to the weight of the uncoated microporous film by irradiating the coated microporous film from a with from 1 to

10 Mrads of ionizing radiation.

The essence of the present invention lies in the discovery that microporous open cell films can be made relatively permanently wettable and / or hydrophilic when the pores thereof are chemically fixed with a controlled amount of crosslinkable hydrophilic monomers by exposure to ionizing radiation while preserving the open cell nature of the microporous film. In addition, such chemical fixation of hydrophilic monomers occurs even in these pores disposed 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 radiation can be aligned with the surface of the pores. Therefore, tangling and clogging of the pores is avoided.

The present invention therefore relates to a hydrophilic microporous product and its manufacturing process.

Porous or cellular films can be classified into two general types: one type in which the pores are not interconnected, i.e. a closed cell film, and the other type in which the pores are essentially interconnected by tortuous passages which may extend from one external surface or surface region to another, i.e. an open cell film. The porous products of the present invention are of the latter type.

In addition, the pores of the porous products of the present invention are microporous, that is, the details of their configuration or arrangement can only be discerned by microscopic examination. In fact, the open cell pores in films are generally smaller than those that can be measured using an ordinary light microscope, because the wavelength of visible light, which is about 5000 Å (1 Â equals 10-10 m), is longer than the longest plane or surface dimension of the open cell (or pore). The microporous products of the present invention can be identified, however, using electron microscopy techniques which are capable of solving details of pore structure below 5000 Å.

The microporous products of the present invention also have a reduced density, sometimes hereinafter simply referred to as low density. That is to say that these microporous films have a total density lower than the density of the corresponding films composed of identical polymer material, but having no open cells or being devoid of structure comprising voids. The term density by volume as used herein means the weight per unit of gross or geometric volume of the film, where the gross volume is determined by the immersion of a known weight of the film in a container partially filled with mercury at 25 ° C and at atmospheric pressure. The vo-lumetric increase in the mercury level is a direct measure of the gross volume. This method is known as the volumetric mercury method, and is described in the review "Encyclopedia of Chemical Technology", vol. N ° 4, p. 892 (Interscience 1949).

Porous films have been produced which have a microporous, open cell structure, and which are also characterized by reduced density. These films having this microporous structure are described, for example, in American patent N ° 3426754 assigned to the owner of the present invention and incorporated here by reference. The preferred method of preparation described herein includes stretching or stretching at room temperatures, i.e., cold stretching, an elastic, crystalline precursor film in an amount of about 10 to 300 % of its original length, with subsequent stabilization by thermal hardening of the stretched film under tension such that the film is not free from contraction or can contract only by a limited value. Other methods of preparing microporous films are exemplified by American patents Nos. 3558764, 3843762, 3920785, British patents Nos 1180066 and 1198695 which are all incorporated herein by reference.

While all of the above-mentioned patents describe methods for preparing normally hydrophobic microporous films which can be made hydrophilic according to the present invention, the preferred normally hydrophobic microporous films are made according to the methods described in US Patent No. 3801404 which defines a process for the preparation of microporous films referenced here as a dry stretching process, and the American patent N ° 3839516 which defines a process for the preparation of microporous films referenced here as a solvent stretching process, processes incorporated here by reference. Each of these patents discloses preferred alternative routes for obtaining a normally hydrophobic microporous film by manipulating a precursor film according to specifically defined process steps.

The preferred precursor films which can be used to prepare microporous products according to the dry stretching and solvent stretching processes are specifically detailed in each of the respective patents above. Thus, the dry stretching method uses a non-porous crystalline elastic polymer film having elastic recovery at zero recovery time (hereinafter defined) when subjected to a standard elongation (extension) of 50% to 25 ° C and 65% relative humidity of at least 40%, preferably at least 50% and more preferably at least 80%.

The elastic recovery as used here is a measure of the ability of the structure (or article) formed such as a film to return to its original size after being stretched or stretched, and can be calculated as follows:

Elastic recovery (RE)% =

length when stretched - length after stretching ^ added length when stretched

When a standard 50% stretch is used to identify the elastic properties of the starting films, such stretch is simply exemplary. In general, such starting films will have elastic recoveries greater than elongations less than 50%, and somewhat less than elongations substantially greater than 50%, compared to their elastic recovery at an elongation of 50%.

These elastic starting films will also have a percentage of crystallinity of at least 20%, preferably at least 30%, and more preferably at least 50%, for example, around 50 to 90%, or more. The percentage of crystallinity is determined by the X-ray method described by R.G. Quynn et al. in the "Journal of Applied Polymer Science", vol. 2, No. 5, pp. 166-173 (1959). For a detailed discussion of crystallinity and its meaning in polymers, see the book "Polymers and Resins", Golding (D. Van Nostrand, 1959).

Other elastic films considered suitable for the preparation of precursor films used in the stretching or dry elongation process are described in British Patent No. 1052550 published on December 21, 1966.

The precursor elastic film used in the preparation of microporous products by road using the elongation process

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dry should differ from films formed from conventional elastomers such as natural and synthetic rubbers. With such conventional elastomers, the elongation-stress behavior, and in particular the temperature-stress relationship, is governed by the entropy mechanism of deformation (rubbery elasticity). The positive temperature coefficient of the retraction force, i.e. the decreasing stress with the decreasing temperature and the complete loss of the elastic properties of the glass transition temperature, are particular consequences of the entropic elasticity. The elasticity of the precursor elastic films used here, on the other hand, is of a different nature. In qualitative thermodynamic experiments with these elastic precursor films, an increasing constraint with a 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 an energy term. More significantly, it has been found that the precursor elastic films with dry elongation retain their elongation properties at temperatures where the normal entropy elasticity could not be operative any longer. Thus, it is believed that the elongation mechanism of the precursor elastic films of dry elongation is based on energy-elasticity relationships, and these elastic films can then be referred to as non-conventional elastomers.

Alternatively, the solvent elongation process uses a precursor film which may contain at least two components, for example an amorphous component and a crystalline component. Thus, crystalline materials, which are by nature two components, work well with the process. The degree of crystallinity of the precursor film can therefore be at least 30%, preferably at least 40% and more preferably at least 50% by volume of the precursor film.

The polymers, i.e. the synthetic resinous material from which the precursor films used in the process according to the present invention are prepared, include olefinic polymers such as polyethylene, polypropylene, poly-3-methylpent- ene-1, poly-4-methylpentene-1, as well as 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 major amount of 3-methylbutene-1 and a minor amount of a straight chain n-alkene such as n-octene- 1, n-hexadecene-1, n-octadecene-1, or other relatively long chain alkenes, such as copolymers of 3-methylpentene-1 and any of the same n-alkenes mentioned above with respect to relates to 3-methylbutene-1.

For example, in general when homopolymers of propylene are intended for use in the dry elongation process, an isotactic polypropylene having a percentage of crystallinity as indicated above, an average molecular weight of between approximately 100,000 and 750,000 (for example from about 200,000 to 500,000) and a melt index (ASTM-D-1238-57T, part 9, p. 38) from about 0.1 to about 75 (for example from about 0.5 to 30) , can be used to produce a final film product with the required physical properties.

It should be understood that the terms olefin polymer and olefin polymer are used interchangeably to describe a polymer prepared by the polymerization of olefin monomers by their unsaturation.

Preferred polymers for use in the solvent elongation process are the polymers used according to the invention described in American patent application No. 44805, filed January 6, 1979, by John W. Soehngen and assigned to the holder of the present invention, entitled "Improved solvent elongation process for the preparation of microporous films from precursor films of controlled crystal structure", the description of which is incorporated herein by reference. Thus, a polyethylene homopolymer having a density of from about 0.960 to about 0.965 g / cm3, a high melt index not less than about 3 and preferably about 3

to about 20 and a wide molecular weight distribution ratio (Mw / Mn) of not less than about 3.8 and preferably from about 3.8 to about 13 is preferred in the preparation of a microporous film by the process d elongation with solvent. In addition, nucleating agents can be incorporated into the polymer employed 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 can be employed.

The types of apparatus suitable for forming the precursor films are well known in the art.

For example, a traditional film extruder equipped with a flat channel sizing nut and a curved 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 stretched by a take-up or molding roller. More than one pick roller in various combinations or stages can be used. The opening of the die or the width of the slot may be in the range, for example, ranging from about 2.54-10 ~ 2 to about 0.51 cm.

Using this type of apparatus, the film can be extruded at a stretch ratio of from about 5: 1 to 200: 1, preferably from 10: 1 to 50: 1.

The term stretch ratio or, more simply, stretch ratio as used herein is the ratio of the wound film or the setting speed to the speed of the film exiting the extrusion die.

The melting temperature for extruding the film is generally not more than about 100 ° C above the melting point of the polymer and not less 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. The polyethylene can be extruded at a melting temperature of about 175 to 225 ° C.

When the precursor film is to be used according to the dry stretching process, the extrusion operation is preferably carried out with rapid cooling and rapid stretching so as to obtain maximum 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 in less than 2.54 cm. An air knife, operating at temperatures between for example 0 and 40 ° C, can be used in the space of 2.54 cm from the slot, to soak, that is to say quickly cool and solidify the movie. The pick roller can be rotated, for example, at a speed of about 3 to 305 m / min, preferably about 15 to about 157 m / min.

When the precursor film is to be used according to the solvent stretching process, the extrusion operation is preferably carried out with low cooling, so as to minimize the stress and any associated orientation which could result from rapid quenching to obtain maximum crystallinity, but fast enough all the same to avoid significant development of spherulites. This can be accomplished by controlling the distance of the hardening roller grip from the extrusion slot.

While the foregoing description has related to die or slot extrusion methods, an alternative method for forming the precursor films considered in the present invention is the blown film extrusion method, in which a hopper and an extruder are employed which are substantially the same as in the slotted extruder described above.

From the extruder, the molten mass enters a matrix from which it is extruded into a circular slit to form a tubular film having an initial diameter D ^ Air enters the system through an inlet at l inside said tubular film and has a blowing effect from the diameter of the tubular film to a diameter D2. Means such as air cores can be

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designed to direct air outside 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 during which the film is allowed to cool completely and harden, it is rolled up on a pick roller.

Using the blown film process, the stretch ratio is preferably from 5: 1 to 100: 1, the opening of the slit from 2.54-10 ~ 2 to about 5.1 cm, preferably from 4- 10-2 to about 2.5 cm, the ratio D2 / Dl5 for example, from 1.0 to 4.0 and preferably about 1.0 to 2.5, and the setting speed, for example, of 3 to 200 m / min. The melting temperature can be included in the ranges given above for slit die extrusion.

The extruded film can then be initially heat-treated or annealed so as to improve the crystal structure, for example, by increasing the size of the crystallites and removing imperfections therein. Generally, this annealing is carried out at a temperature in the range of about 5 to 100 ° C below the melting point of the polymer for a period of a few seconds to several hours, for example from 5 s to 24 h, and preferably around 30 s to 2 h. For polypropylene, the preferred annealing temperature is about 100 to 155 ° C.

An exemplary method of carrying out the annealing consists in placing the extruded film in a constrained or unconstrained state in an oven at the desired temperature, in which case the residence time is preferably comprised in the range from approximately 30 s to 1 h .

In the 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 the present invention.

The first preferred procedure as disclosed in U.S. Patent No. 38014Ö4 hereinafter referred to as a dry stretching process comprises the steps of cold stretching or stretching, i.e. cold elastic film until porous regions or areas which are elongated normal to or perpendicular to the direction of elongation are formed, hot stretching or elongation, i.e. hot stretching of the film stretched cold until fibrils and pores or open cells which are elongated parallel to the direction of elongation are formed, and finally heat or thermal hardening of the resulting porous film under tension, i.e. - say at a substantially constant length, to impart stability to the film.

The term cold drawing as used herein is defined as stretching or stretching a film to a length greater than its original length and to an elongation temperature, i.e. the temperature of the stretched film, less than the temperature at which the melting begins when the film is uniformly heated from a temperature equal to 25 ° C and at a speed of 20 ° C / min. The term hot drawing as used herein is defined as stretching above the temperature at which melting begins when the film is uniformly heated from a temperature of 25 ° C and at a rate of 20 ° C / min , but below the normal melting point of the polymer, i.e. below the temperature at which the melting takes place. As is known to those skilled in the art, the temperature at which fusion begins and the melting temperature can be determined by a standard differential thermal analyzer (DTA), or by other known devices which can detect thermal transitions d 'a polymer.

The temperature at which melting begins varies depending on the type of polymer, the molecular weight distribution of the polymer and the crystal morphology of the film. For example, an elastic polypropylene film can be cold stretched at a temperature below about 120 ° C, preferably between about 10 and 110 ° C, and suitably at room temperature, for example 25 ° C. The film of cold-stretched polypropylene can then be hot-stretched at a temperature above about 125 ° C and below the melting temperature, preferably between about 130

and 150 ° C. Again, the temperature of the film itself to be extended is referred to herein as the extension temperature. The elongation, in these two stages or stages, can be consecutive, in the same direction, and in this order, that is to say cold then hot, but s can be done in a continuous, semi- continuous or by water, as long as the cold stretched film is not allowed to contract by a significant degree, for example less than 5% of its cold stretched length, before being stretched hot.

The sum of the total amount of stretch or elongation i ° in the two steps mentioned may be in the range of about 10 to 300%, and preferably about 50 to 150%, relative to the initial length elastic film. In addition, the ratio of the amount of hot stretch to the sum of the total amount of stretch or stretch may be about 0.10: 1 greater than 0.99: 1, less than preferably from about 0.50: 1 to 0.97: 1, and more preferably from about 0.50: 1 to 0.95: 1. This relationship between cold and hot elongations is referred to here as the extension ratio (percentage of hot extension relative to the percentage of total extension).

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

After the two-stage or two-stage stretching described above, the stretched or stretched film is thermally cured. This heat treatment can be carried out at a temperature in the range from about 125 ° C to a temperature below the melting temperature, preferably from about 130 to 160 ° C for polypropylene; from about 75C'C to the temperature below the melting temperature, and preferably from about 115 to 130 ° C for polyethylene; at similar temperature ranges for other polymers mentioned above. This heat treatment should be carried out while the film is kept under tension, that is to say in such a way that the film is not free from contraction or can contract only by a controlled value not greater than about 15% of its stretched length, but not as much tension as it allows to stretch the film more than an additional 15%. Preferably the tension is such that there is substantially no contraction or elongation, for example less than 5% change in the elongated or stretched length.

The heat treatment period which is preferably carried out sequentially with and after the stretching or elongation operation should not be more than 0.1 s at the higher annealing temperatures and, in general, be included in the range ranging from approximately 5 s to 1 h and preferably from approximately 1 to 30 min.

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

A second preferred alternative procedure for converting the precursor film into a microporous film as described in US Patent No. 3,830,516 and referenced herein as the solvent drawing process includes the following basic steps: 1) bringing the film into contact 55 precursor having at least two components (for example an amorphous component and a crystalline component), one of which is lower in volume than all the other components, with a blowing agent in sufficient time to allow adsorption of the blowing agent in the film; 2) stretching the film in at least one direction while in contact with the blowing agent, and 3) maintaining the film in its stretched or elongated state during removal of the blowing agent. Optionally, the film can be stabilized by thermal hardening under tension or by 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 here. The Hildebrand solubility parameter measures the energy density of cohe

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if we. Thus, the underlying principle is based on the fact that a solvent having a cohesive energy density similar to a polymer would have a high affinity for this polymer and would be suitable for this process.

General classes of blowing agents from which one can choose an agent suitable for the particular polymer film are lower aliphatic ketones such as acetone, methyl ethyl ketone, cyclohexanone; esters of lower aliphatic acid such as ethyl formate, butyl acetate, etc. ; halogenated hydrocarbons such as carbon tetrachloride, tri-chloroethylene, perchlorethylene, chlorobenzene, etc .; hydrocarbons such as heptane, cyclohexane, benzene, xylene, tetralin, decaline, 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 blowing agents are a compound of carbon, hydrogen, oxygen, nitrogen, halogen, sulfur and contain up to about 20 carbon atoms, preferably up to about 10 atoms of carbon.

The solvent elongation step can be carried out at a temperature between about the solidification point of the solvent, or of the swelling agent, up to the point below the temperature at which the polymer dissolves (it i.e. room temperature at around 50 ° C).

The precursor film used in the solvent elongation process can be about 2.54-10 ~ 4 to about 5.01 • 10 ~ 2 cm thick, or even thicker.

In a preferred embodiment, the precursor film is biaxially extended according to the procedures described in American patent application No. 44801 filed January 6, 1979, entitled "Process of elongation with improved solvent for the preparation of microporous films" and transferred to the owner of the present invention, the description of which is incorporated herein by reference. This method identifies preferred elongation conditions in a uniaxial direction which leads to improved permeability of the uniaxially stretched microporous film. The uniaxially stretched microporous film can then be stretched in a transverse direction to increase the permeability even later. Thus, it is preferred that the precursor film is elongated with the solvent in a uniaxial direction not greater than about 350%, and preferably greater than 300% relative to its original length. Typically, the additional elongation is not used in the same direction after removal of the solvent.

The optional stabilization step can be either a thermal hardening step or a crosslinking step. This heat treatment can be carried out at a temperature between about 125 ° C up to a temperature below the melting temperature and preferably about 130 to 150 ° C for propylene, from about 75 ~ C up to a temperature below the melting temperature, and preferably about 115 to 130 ° C for polyethylene and at similar temperature ranges for other polymers among the polymers mentioned above. This heat treatment should be carried out while the film is kept under tension, that is to say so that the film is not free from contraction or can contract only by a controlled value of not more than about 15% of its length stretched, but not at a tension as great as it can stretch the film more than 15% more. Preferably, the tension is such that there is substantially no contraction or stretching, for example less than 5% change in the stretched length.

The heat treatment period which is preferably carried out sequentially with and after the solvent drawing operation should not be greater than 0.1 s at the higher annealing temperatures and, in general, may be in the range from about 5 s to 1 h and preferably about 1 to 30 min.

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

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

While the present description and the examples relate first of all to the olefin polymers mentioned above, the invention also encompasses the high molecular weight acetal polymers, for example the oxymethylene polymers. While both homopolymers and acetal copolymers are encompassed, the preferred acetal polymer with respect to polymer stability is a random oxymethylene copolymer, which contains oxymethylene repeat units, it is ie —CH2 - O—, interspersed with —OR— groups in the main polymer chain where R represents a bivalent radical containing at least 2 carbon atoms linked to each other and positioned in the chain between two valences, with all substituents on said radical R being inert, that is to say those which do not include interfering functional groups and which do not include undesirable reactions, and where a major quantity of units —OR - exist as single units attached to the oxyethylene 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 US Patent No. 3,027,352 to Walling et al. These polymers in film form can also have a crystallinity of at least 20%, preferably at least 30%, and more preferably at least 50%, for example from 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, pp. 175-191 (Reinhold 1964).

Other relatively crystalline polymers to which the invention can be applied are polyalkylene sulfides such as polymethylene sulfide and polyethylene sulfide, polyarylene oxides such as polyphenylene oxide, polyamides such as polyhexamethylene adipamide (Nylon 66) and polycapro-lactam (Nylon 6), all of which are well known in the art and need not be described here.

The normally hydrophobic microporous films used in the present invention, in an unstressed state, have a lowered density density compared to the density of the corresponding polymeric materials having no open cell structure, for example those from which they are formed . Thus, the films have a density not greater than about 95% and preferably from 20 to 40% of the precursor film. In other words, the volume density is reduced by at least 5% and preferably by 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; porosity increased from 60 to 80% due to pores or orifices.

When the microporous film is prepared by the dry stretching process or the solvent stretching process, the final crystallinity of the microporous film is preferably at least 30%, more preferably at least 65%, and still more suitably from about 70 to 85%, as determined by the X-ray method described by RG Quynn et al. in the "Journal of Applied Polymer Science", vol. 2, No. 5, pp. 166-173. For a detailed discussion of crystallinity and its meaning in polymers, see "Polymers and Resins", Golding (S. Van Nostrand, 1959).

The microporous films which can be used in the present invention have an average pore size of 200 to 10,000 Å, typically about 400 to 5,000 Å, even more typically about 500 to about 5,000 Å. These values can be determined by mercury porosimetry as described in an article by RG Quynn et al., On pages 21-34 of the "Textile Research Journal", January 1963, or by the use of electron microscopy as described in the article. de Geil, "Polymer Single Crystals", p. 69

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(Interscience 1963). When using an electronic micrograph, measurements of pore length and width can be obtained by simply using a ruler or meter to directly measure the length and width of the pores on an electronic micrograph usually taken at an enlargement of 5000 to 10000. Generally, the pore length values which can be obtained by electron microscopy are approximately equal to the pore size values obtained by mercury porosimetry.

The microporous films used in the present invention will have a specific area within certain foreseeable limits when they are prepared either by the solvent stretching process or by the dry stretching process. Such microporous films have a specific area of at least 10 m2 / g, and preferably in the range ranging from 15 to 25 m2 / g. For films formed from polyethylene, the specific area is generally in the range from about 10 to 25 m2 / g, and preferably about 20 m2 / g.

The specific area can be determined from the adsorption isotherms of nitrogen or krypton gas using a method and an apparatus described in US Patent No. 3262319. The specific area obtained by this process 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 volume density of the material in grams per cubic centimeter resulting in a specific area expressed in square meters per cubic centimeter.

Normally hydrophobic microporous polymer films which are rendered hydrophilic have a preferred thickness of about 2.54-10 I-3 to about 2-10 ~ 2 cm.

The normally hydrophobic microporous film prepared according to the procedures described above is made wettable, and / or hydrophilic by coating the surface of the micropores with the microporous film with a hydrophilic monomer or a mixture thereof and, subsequently, by exposing the microporous film coated with ionizing radiation. Ionizing radiation chemically fixes the hydrophilic monomer on the surface of the micropores and makes it relatively hydrophilic at all times. The addition or the 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 the total porosity of the hydrophilic film to be controlled. The porosity control, in turn, makes it possible to control the water permeability and the electrical resistance of the film.

As used herein, the term hydrophobic is defined to mean a surface which passes less than about 0.010 mm of water per minute per square centimeter of flat film surface under a water pressure of about 689.47 pz. Similarly, the term hydrophilic applies to surfaces that pass more than about 0.01 mm of water per minute per square centimeter at the same pressure.

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

Thus, the hydrophilic monomers which can be used 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; vinyl and allyl monomers, particularly itaconic acid, malonic acid, fumaric acid, crotonic acid and their esters or anhydrides; unsubstituted acrylic acids or substituted alkyls such as acrylic acid and methacrylic acid, acrolein or acrylonitrile; unsubstituted or alkylalkyl, cycloalkyl, aryl, hydroxyalkyl or hydroxyaryl acrylates; substituted alkyl diaminoalkyl acrylates, epoxyalkyl acrylates; vinylsulfonic acid, and styrenesulfonic acid; vinyl esters such as vinyl acetate and vinyl esters of higher carboxylic acid, vinyl esters substituted with an alkyl group of carboxylic acid containing sulfo groups; vinyl ethers, such as unsubstituted or alkyl-, cycloalkyl- or aryl-substituted ethers; vinyl substituted silicones; vinyl substituted aromatic or heterocyclic hydrocarbons; di-allyl fumarates; diallylmaleates, alkyl substituted phosphates, phosphites or carbonates; vinyl sulfones, the reaction product of ethoxylated nonylphenol and acrylic acid and the like.

Since it is desired that the hydrophilic monomer penetrate inside the microporous film, it is preferred to use hydrophilic monomers having from 2 to 14, preferably from 2 to 4 carbon atoms. The preferred hydrophilic monomers are acrylic acid,

methacrylic acid and vinyl acetate.

The quantity of hydrophilic monomer which coats the interior surface of the pores of the microporous film is controlled in order to obtain the correct degree of addition which is determined by the employment requirements of final electrical resistance or water flow desired for movies. As previously described, such control is exercised in a manner sufficient to preserve the open cell nature of the micropores, i.e., to avoid clogging of the pores, after the microporous film impregnated with monomer has been subjected to the radiation treatment described herein, so that the desired properties discussed above are maintained for a longer period of time than can otherwise be obtained by employing the surfactant coatings typical of the prior art. The specific amount of the hydrophilic monomer which is chemically attached to the surface of the pores is expressed in terms of percent addition, i.e. the percentage by weight of the hydrophilic monomer coating which is present in the hardened microporous film.

Thus, to avoid clogging of the pores of the microporous films described here, the percentage of addition of the hydrophilic monomer which is chemically fixed to the surface of the micropores is controlled not to be greater than 10%, from 0.1 to 10% , preferably from about 0.5 to about 2.5%, and more preferably from about 1 to about 2.0% (e.g. 1.5%) by weight, based on the weight of the uncoated microporous film .

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

For example, when the crosslinked hydrophilic microporous film is to be used as an electric battery separator, the percentage of addition is chosen based on the electrical resistance of the resulting hydrophilic microporous film (in milliohms per square centimeter) which is a function of the percentage of addition of the hydrophilic monomer.

The electrical resistance as defined here is a measure of the capacity of the microporous film to conduct the electrons. Therefore, as a general rule, the higher the electrical resistance of the microporous film, the less effective the microporous film will be as a battery separator.

Thus, the electrical resistance of the microporous film is determined at various percentages of addition of the hydrophilic monomer and an electrical resistance curve is produced as a function of the percentage of addition. The percentage of addition is then determined based on the desired electrical resistance.

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 mfl / cm2, preferably less than about 1.55 mfi / cm2, and more preferably less than about 0.775 m £ 2 / cm2.

When the crosslinked hydrophilic microporous film is used as a filter, the percentage of addition of the microporous film is chosen based on the rate of flow of water through the microporous film as defined here.

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Thus, the water flow rate can be plotted as a function of the percentage of addition of the hydrophilic monomer, and the appropriate percentage of addition is chosen based on the desired water flow rate.

When it is desired to use the microporous film for filtration purposes, the pore size of the film is chosen to act as a barrier for the material to be separated and the water flow rate is controlled as desired within the limits of the dimensions. of selected pores. Thus, the rate of water flow can be controlled to be greater than approximately 0.01 cm3 / min / cm2, preferably greater than approximately 0.5 cm3 / min / cm2, and more preferably greater than approximately 0, 5 cm3 / min / cm2 at a differential pressure of approximately 1 atm.

Thus, the present invention has several advantages over the hydrophilic films prepared in the past. Since it is possible to preserve the open cell nature of the hydrophilic microporous product prepared according to the present invention, such films allow a mass transport of water through the film, in contrast to the transport of water through the film by diffusion which is a much slower process. The effect of mass transport also contributes to the reduction in electrical resistance. Another advantage of the microporous films of the present invention results from the chemical attachment of the hydrophilic monomer to the microporous film which extends the duration of the presence of the hydrophilic monomer on the microporous surface over longer periods of time than the typical surfactants, especially after repeated washing.

In addition, the dimensional stability of the products of the present invention is also improved.

It is appropriate to mention that it is preferred, where possible, to convert the polar functional group of the hydrophilic monomer to its most polar form. This can be done, 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 approximately 5 to approximately 30 min. This promotes 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 sufficiently precise control over the percentage of addition of the hydrophilic monomer.

The preferred method for efficient and precise coating of the internal surface of the pores of the microporous film is to bring the microporous film into contact with the vapor of the monomer which condenses on the microporous surface.

The percentage of addition can be controlled by controlling the vapor pressure at equilibrium (i.e. the vapor pressure in which the condensation rate of the monomer vapor 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 hydrophilic monomer coating at a constant temperature can be determined from percentage addition curves over time at various equilibrium vapor pressures, whatever keeping the temperature constant. The percentage of addition chosen as described above depends on the particular property sought to impart to the microporous film.

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

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

Obviously, the vapor coating technique can also be employed when the critical temperature of the hydrophilic monomer (i.e. the temperature at which a liquid monomer cannot exist independently of the pressure) is higher than the temperature at which the properties of the microporous film would be adversely affected at the particular contact time employed.

It is preferred that the vapor coating technique is conducted at atmospheric pressure at a temperature which gives the appropriate vapor pressure.

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

Pressures below atmospheric pressure and above atmospheric pressure may also be employed with appropriate adjustments with respect to temperature and 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 percentage of addition is initially applied to the microporous film to compensate for the loss of the monomer which may occur during the hardening or crosslinking or vulcanization treatment.

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 chloride to form a buffer bath.

The buffer bath can then be used in an inverted roller coating technique. In this process, a repair roller is partially disposed in the buffer bath of the monomer coating solution. A second driven roller guides an uncoated hydrophobic microporous film strip through the nip or throttle formed by itself and the repair roller. The two rollers, which are preferably driven separately, rotate in the same direction, so that the ribbon of coated film is guided in the direction from which the uncoated film originates. The amount of coating monomer placed on the film is a function of the difference in speed of the repair roller and of the second roller driving the film, and also the size of the constriction formed by the two rollers.

Alternatively, a compression roller method can be used. In this process, the film is guided in a buffer bath of the monomer coating solution and compressed between two compressing rollers arranged downstream. The amount of coating is thus a function of the size of the space between the two compression rollers and the pressure exerted between them.

Another alternative method for coating the hydrophobic microporous film forming the substrate is the method using a wire winding rod or measuring rod. This process is the same as the process using road rollers, except that the microporous film, after having been coated while being guided through a bath of the monomer coating solution, is compressed between a pair of tie rods, or rods, for measuring coils of wire which control the amount of coating placed on them by the configuration of the wire coils around the tie rods, or rods.

In the techniques of coating by reverse rollers, by compression rollers and by winding of wire, the quantity 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 addition, in all three methods, the amount of monomer coating is also a function of the concentration of the monomer in the buffer bath. The hydrophilic monomer buffer bath is produced by dissolving the monomer in a usual organic solvent which has a boiling point below the point

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boiling of the hydrophilic monomer used such as, in addition to methylene chloride, acetone, methanol, ethanol and isopropanol.

The concentration of the hydrophilic monomer in the buffer bath is controlled in order to carry out the application of the appropriate quantity of the monomer by evaporation of the solvent. Generally, the concentration of monomer in the buffer bath can vary between about 1 and about 30%, and preferably between about 5 and about 15% by weight, relative to the weight of the bath.

When the hydrophilic monomers are coated on the microporous film using a buffer bath, the solvent present therein is removed by passing the coated film through a dryer. The temperature of the dryer should be high enough to evaporate only the solvent, thus leaving the monomer deposited 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, has been removed therefrom, the coated microporous film is subjected to ionizing radiation to chemically fix the hydrophilic monomer to the normally hydrophobic microporous surface and to make it hydrophilic.

When subjected to ionizing radiation, a large number of possible mechanisms or combinations can operate to achieve the desired effect. Thus, the hydrophilic monomers can become chemically bound to the microporous surface and / or can form, by polymerization and / or copolymerization, a polymeric layer (or sleeve) which is intimately bound chemically to the microporous surface, for example by a statistical chemical attachment. from the surface of the sleeve to the microporous surface,

and / or physically, as a result of the entrapment effect on the polymeric sleeve of the contour of the microporous surface. The term chemical fixation encompasses all of the above 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 depending on the percentage of addition of the hydrophilic monomer.

Thus, the resulting radiation treated microporous film is made hydrophilic over a longer period of use than when otherwise obtained by coatings with typical surfactants, 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 low electrical resistance are required.

As indicated above, the chemical attachment of 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 which provides emitted particles or photons having sufficient intrinsic energy to produce ions and break chemical bonds and thus induce reactions by free radicals between the hydrophilic monomers employed and between monomers and the microporous surface, as described here. Ionizing radiation is suitably available in the form of radiation by ionizing particles, electromagnetic ionizing radiation and actinic light.

The term ionizing particle radiation has been used to refer to the emission of electrons or highly accelerated nuclear particles such as protons, neutrons, α particles, deuterons, ß 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 using voltage gradients by devices such as a low energy elongated electron beam generator (ie 200 keV) such as the manufactured Electrocurtain ™ device by Energy Sciences Corporation, accelerators with resonance chambers, Van der Graaff generators, beta-trons, synchrotrons, cyclotrons, etc. Neutron radiation can be produced by bombarding a selected light metal such as beryllium with positive particles of high energy. Particle radiation can also be obtained by the use of an atomic cell, radioactive isotopes 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 given to the electrons by accelerators with a potential greater than 0.1 MeV. In addition to this type of irradiation, commonly called X-rays, ionizing electromagnetic irradiation suitable for the practice of the present invention can be obtained by means of a nuclear reactor (battery) or by the use of a natural or synthetic radioactive material, for example cobalt 60.

The hydrophilic monomers described here 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 1800 to 4000 Å units. Various suitable sources of actinic light are available in the art, including, for example, quartz mercury lamps, ultraviolet carbon core arcs and high flash lamps. Initiators can be used when actinic light is used.

The preferred source of ionizing radiation is the elongated electron beam generator as described in US Patents Nos. 3702412, 3745396 and 3769600, which are thus incorporated by reference.

Techniques for precise process control are sufficiently developed to allow conversion of results from one type of radiation to another, and adequate adjustment of the techniques described herein can be used interchangeably in the production of any desired product.

A radiation dosage of about 1.0 to about 10 Mrads, and preferably about 2 to about 5 Mrads, for example 3 Mrads, is used to achieve chemical fixation. A mega-rad is a million rads. A rad is the amount of high energy ionizing radiation that produces an absorption of 100 ergs of energy per gram of absorbent material. This unit is widely accepted as a suitable means of measuring radiation absorption by a material.

Preferably, minimum ionizing radiation is used to achieve chemical fixation due to economic considerations. Excessive dosages (i.e. greater than about 20 Mrads at a single exposure) should be avoided so that there is no excess heating and contraction of the film, no degradation of the hydrophilic monomers and microporous film. If the dosage is too low, however, chemical fixation will not occur and the hydrophilic monomer will be quickly lost.

The minimum dosage of radiation that will result in chemical attachment of the hydrophilic monomer will vary depending on the type of polymer used to prepare the microporous film. Thus, for example, so when the polymer is polyethylene, the radiation dosage should not be less than about 1 Mrad while for polypropylene, the radiation dosage should not be less than about 2 Mrads and, preferably , not less than about 3 Mrads.

Ambient temperatures can be employed satisfactorily for irradiation, although high temperatures can also be employed.

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

As previously discussed, the percentage of addition can be chosen based on the electrical resistance of the microporous film.

The electrical resistance (direct current process) of a microporous film 65 is determined by quenching or immersing a sample of the microporous film having a known area or surface (for example 1.29 cm 2) in a KOH solution at 40% by weight in water for 24 h. The resulting sample is then placed

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between working cadmium electrodes (i.e. an anode and a cathode) immersed in an electrolyte of a 40% by weight KOH solution in water, and a direct current of amperes is passed - known current (for example 40 mA) through the cell between the electrodes. The drop in potential across the film (E) is measured with an electrometer. The drop in potential across the cell without the microporous film disposed therein (E ') is also determined using the same current.

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

R.E. = (E ~ E> A I

where A represents the area or surface of the exposed film in square centimeters, I represents the current through the cell in milliamers, RE represents the electrical resistance of the microporous film in milliohms per square centimeter, and E 'and E are as described.

The water permeability, or water flow rate, of the hydrophilic microporous film of the present invention is generally determined by measuring the water flow rate through a specific area (or surface) of the film , while the water is under a differential pressure of 1 atm. Thus, the water flow rate is expressed in units of volume of water in cubic centimeters per minute per square centimeter, of the surface or the area of the film (cm3 / min / cm2).

The air permeability of the microporous product of the present invention can be determined by the Gurley test, that is to say according to standard ASTM D 726 by mounting a film having an area (or surface) of approximately 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 the permeability. A Gurley value greater than about 1.5 min is an indication that the pores are clogged.

The hydrophilic microporous products of the present invention find many diverse uses. In particular, they are used in fields where the controlled passage of moisture through a film or a surface is desired. Furthermore, products according to the present invention can be used as a filter membrane support or filters useful in the separation of ultrafine materials from various liquids and as battery separators.

Other objects, characteristics and advantages of the present invention will become apparent in the light of the explanatory description which will follow, given with reference to the following examples given simply by way of illustration and which in no way limit the scope of the present invention. All parts and percentages in the examples as well as in the description and claims are by weight unless otherwise indicated.

Example 1:

Part A

This discussion illustrates the preparation of a microporous normally hydrophobic polyolefin film by the dry stretching method as illustrated by US Patent No. 3801404.

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 flat channel measuring nut. The ratio of the length to the diameter of the extruder bar is 24/1. The extrudate is stretched very quickly to a melt stretch ratio of 150, and contacted with a rotating molding roller maintained at 50 ° C, and at a distance of 1.905 cm from the lip of the matrix. The film produced in this way is found to have 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 constraint at 140 ° C. for approximately 30 min, removed from the oven and allowed to cool.

The annealed elastic film sample is then subjected to cold drawing and hot drawing at an extension rate of

0.50: 1, and then thermally cured under tension or stress, i.e. at a constant length, at 145 ° C for 10 min in air. The cold drawing part is carried out at 25 ° C, the hot drawing part is carried out at 145 ° C, and the total draw is 100%, based on the original length of the elastic film. The resulting film has an average pore size length of about 3000 Å, a crystallinity of about 50.6% and a specific area of about 8.54 m2 / 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 according to part A, is then passed through a bath of glacial acrylic acid (i.e. 100%) and then through a compression roller to obtain an addition of 1.5% after crosslinking, hardening or vulcanization, by weight, relative to the weight of the film before impregnation as determined by infrared analysis. The impregnated film is then passed below the window of an elongated electron beam generator (i.e., 60.96 cm in length) at a linear speed of 6.096 m / min. The electron beam generator is adjusted to deliver a dose of 3 Mrads at the linear speed used.

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

A crosslinked dry microporous film sample is then tested for wettability by the drop test. The drop test is carried out by placing a drop of 0.6 ml of 2% KOH in water on the surface of the film. The film is then observed visually. If the portion of the film on which the drop is placed becomes translucent and if the opposite side of the film on which the drop is placed appears wet, the film is determined to be wet (as indicated in Table I by the comment: yes). The drop test is carried out on a closed line sample of the film immediately after irradiation, and on a sample which has been stored for one week at room temperature.

Several other samples are taken from the roll of crosslinked microporous film and tested for electrical resistance as described here, after quenching in an aqueous solution at 40% by weight of KOH maintained at 60 ° C. for 1 h, 24 h, 4 d and 8 d respectively, and we give the average of the results. Soaking the film sample in hot KOH for progressively longer periods of time simulates aging over long periods of time.

The area or surface of each sample tested for electrical resistance is 1.19 cm2, and the current used is 40 mA in direct current.

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

The results are summarized in the form of a diagram in Table I.

Samples of the crosslinked microporous film roll are also tested for water flow after immersion or quenching in a 31% aqueous KOH solution maintained at 100 ° C for 24 h, placing each sample having a 11.3 cm2 area (or area) 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. Water is collected as it passes through the microporous film for a period of 5 min. The results are converted to cubic centimeters of water collected per minute per square centimeter of film surface.

The results are summarized in Table I.

Comparative example:

Example 1 is repeated, except that the dose of radiation used for crosslinking is reduced to 1 Mrad. We summarize the results5

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States 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 in Table I, it is believed that a dose of 1 Mrad is too low to effect effective crosslinking in a microporous polypropylene film, as indicated by the infinite electrical resistance of the microporous film of the comparative example. The positive wettability test of the comparative example on the closed line sample is believed to be due to residual acrylic acid which has not been chemically fixed by 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 d of exposure to hot KOH and then drops slightly after 8 d of exposure. It is believed that the fall after 8 d of aging may be due to the complete conversion of the acrylic crosslinking to its salt form while, after 4 d of aging, the conversion of the salt is only partially complete. Thus, the complete conversion of the acrylic crosslinking into its salt form appears to decrease the electrical resistance.

Table I

Example 1

Comparative example

Control

Dose (Mrads)

3

1

3

Linear speed (m / min)

9.14

6.09

9.14

Wettability

(drop test)

closed line yes yes

ND

after 1 week yes no

ND

Electrical resistance

(mfl / cm2)

1 hr

0.775

00

00

24h

1.065

00

4 d

1,643

00

8 d

1,100

00

Gurley (air) (Gurley-s)

9.1

ND

10

Water flow

(cm3 / min / cm2)

0.35

0

ND

ND: not determined.

Example 2:

Part A

This discussion illustrates the preparation of a microporous polyethylene film by the solvent stretching process.

Crystalline polyethylene having a melt index of 5.0, an average molecular weight of about 80,000, a density of 0.960 g / cm3, and a molecular weight distribution rate of about 9.0 is prepared by method d extruding the blown film to form a precursor film (7.62 cm thick) and allowed to cool by quenching in air at 25 ° C. A sample of the resulting precursor film is then immersed for a period of 1 min in tri-chloroethylene at 70 ° C, then stretched, while being immersed in tri-chloroethylene maintained at the temperature of 70 ° C, at a stress rate of 150% / min up to 4 times its initial length (c i.e. total stretch of 300%). The trichlorethylene is then removed by evaporation and the sample is drawn in the direction of the transverse machine to a degree of stretching of about 50% and allowed to dry in the air in the drawn state. Drying is carried out at 25 ° C.

The resulting microporous film has a crystallinity of about 60%, an average pore length of about 5000 Å, and a specific area between about 10 and about 25 m2 / g.

Part B

Several samples of microporous film 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 were allowed to dry until the film assumed an opaque white appearance indicative of the appropriate amount of monomer coating capable of adding about 1.5%. The resulting samples are then passed under 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 of oxygen in the manner used in Example 1. The resulting crosslinked films are tested by the Gurley air flow, the electrical resistance (after immersion for 24 h in a 40% KOH solution at 60 ° C) and wettability by the drop test as described in Example 1.

The results are summarized in Table II as runs 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 believed to be due 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 a resulting loss of properties.

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

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Table II

Test

Additive acrylic acid

Radiation dose

Electrical resistance

Gurley (ASTM-D-726)

Wettability

No.

after irradiation (%)

(Mrads)

(mfi / cm2)

Before

After

(drop test)

1

1.5

3

1.317

0.4

0.6

Yes

2

1.5

1

1.085

0.4

0.46

Yes

3

1.5

3

56,420 *

0.38

0.80

Yes

4

0

'/ 2

0.35

0.34

no

5

1.5

3

1,550

0.40

0.68

Yes

6

1.5

1

0.697

0.40

0.76

Yes

7

0

'/ 2

0.40

0.48

no

8

1.5

1

2015

0.35

0.76

Yes

* It is believed that the high electrical resistance is due to heterogeneity in the film.

Claims (15)

648,576 2 1. Hydrophilic open cell microporous product, characterized in that it comprises: a) a normally hydrophobic microporous film with open cells having a reduced density compared to the density of a precursor film from which it is prepared, an average pore size of 200 to 10,000 Å and a specific area of at least at least 10 m2 / g, and b) a coating on the surface of the micropores of the microporous film with at least one hydrophilic organic hydrocarbon monomer having 2 to 18 carbon atoms containing at least one double bond and at least one polar functional group, said coating of hydrophilic organic hydrocarbon monomer being chemically attached to the surface of the micropores of the microporous film in an amount sufficient to preserve the open cell nature of the microporous film and to represent an addition of 0.1 to 10%, by weight, relative to the weight of the uncoated microporous film. 2. Hydrophilic microporous product according to claim 1, characterized in that the above-mentioned hydrophilic organic hydrocarbon monomer has from 2 to 14 carbon atoms and at least one polar functional group chosen from the group consisting of a carboxyl, sulfo, sulfino group , hydroxyl, ammonio, amino and phosphono. 3. Hydrophilic microporous product according to claim 1, characterized in that the aforementioned hydrophilic organic hydrocarbon monomer is chosen from the group consisting of unsubstituted and alkyl substituted acrylic acids, vinyl esters, vinyl ethers and their mixtures. 4. Hydrophilic microporous product according to claim 1, characterized in that the aforementioned hydrophilic organic hydrocarbon monomer is chosen from the group consisting of acrylic acid, methacrylic acid, vinyl acetate and their mixtures. 5. Hydrophilic microporous product according to claim 1, characterized in that the normally hydrophobic polymer microporous film is chosen from the group consisting of polypropylene and polyethylene. 6. Hydrophilic microporous product according to claim 1, characterized in that the aforementioned hydrophilic monomer is chemically fixed on the surface of the micropores in an amount of 0.5 to 2.5% by weight, relative to the weight of the uncoated microporous film. 7. Hydrophilic microporous product according to claim 1, characterized in that it comprises: a) a normally hydrophobic open cell microporous film of polyethylene or polypropylene, said film having a crystallinity of at least 30%, an average pore size of 400 to 5000 Å and a specific area of at least 10 m2 / g , and b) a coating on the surface of the micropores of the microporous film with at least one hydrophilic organic hydrocarbon monomer having from 2 to 18 carbon atoms chosen from the group consisting of unsubstituted and alkylated acrylic acids, vinyl esters, vinyl ethers and their mixtures. 8. Hydrophilic microporous product according to claim 7, characterized in that the aforementioned hydrophilic monomer is chosen from the group consisting of acrylic acid, methacrylic acid, vinyl acetate and their mixtures, hydrophilic monomer present on the surface of the micropores in an amount of 0.5 to 2% by weight relative to the weight of the uncoated microporous film. 9. Method for rendering a normally hydrophobic microporous film hydrophilic by improving the rate of flow of water therethrough and by reducing the electrical resistance thereof, characterized in that it comprises: a) coating the surface of the micropores with a normally hydrophobic open cell microporous film having a density or reduced mass compared to the density or mass of a precursor film from which it is prepared, a dimension with an average pore of 200 to 10,000 Å, and a specific area of at least 10 m2 / g, with at least one hydrophilic organic hydrocarbon monomer having from 2 to 18 carbon atoms and containing at least one double bond and at least one group polar functional, and b) the chemical fixation on the surface of the micropores 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 0.1 to 10%, by weight, relative to the weight of the uncoated microporous film, by irradiating the microporous film coated with a with from 1 to 10 Mrads of ionizing radiation. 10. The method of claim 9, characterized in that the hydrophilic organic hydrocarbon monomer has from 2 to 14 carbon atoms and at least one polar functional group chosen from the group consisting of a carboxyl, sulfo, sulfino, hydroxyl, ammonio group, amino and phosphono. 11. The method of claim 9, characterized in that the aforementioned hydrophilic organic hydrocarbon monomer is chosen from the group consisting of unsubstituted and alkyl substituted acrylic acids, vinyl esters, vinyl ethers and their mixtures. 12. Method according to claim 9, characterized in that the hydrophilic organic hydrocarbon monomer is chosen from the group consisting of acrylic acid, methacrylic acid, vinyl acetate and their mixtures. 13. The method of claim 9, characterized in that the aforementioned normally hydrophobic microporous film consists of polymers chosen from the group consisting of polyethylene and polypropylene. 14. Method according to claim 9, 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 0.5 to 2.5% by weight, relative to the weight of the film not coated. 15. Method according to claim 9, characterized in that it comprises: a) coating the surface of the micropores with a normally hydrophobic microporous film with open cells prepared from polymers chosen from the group consisting of polyethylene and polypropylene, said film having a crystallinity greater than 30%, an average pore size of 400 to 5000 Å, and a specific area of at least 10 m2 / g, with at least one hydrophilic organic hydrocarbon monomer chosen from the group consisting of acrylic acid, methacrylic acid, vinyl acetate, and b) chemical fixation on the surface of micropores of the microporous film of an amount of said hydrophilic monomer sufficient to preserve the open cell nature of said micropores and sufficient to obtain an addition of 0.1 to 10%, by weight, relative to the weight of the uncoated microporous film, by irradiation microporous film coated with a with from 1 to 10 Mrads of ionizing radiation.