CA1327259C

CA1327259C - Breathable microporous film and methods for making same

Info

Publication number: CA1327259C
Application number: CA000606649A
Authority: CA
Inventors: Mitchell K. Antoon, Jr.; David J. Hill
Original assignee: Hercules LLC
Current assignee: Hercules LLC
Priority date: 1988-07-27
Filing date: 1989-07-26
Publication date: 1994-03-01
Anticipated expiration: 2011-03-01
Also published as: CN1041167A; IL91130A0; AU3903489A; MX166789B; NZ230093A; BR8903742A; JPH02127445A; IL91130A; DK370689A; US4923650A; AU608169B2; EP0352802A3; DK370689D0; EP0352802A2

Abstract

BREATHABLE MICROPOROUS FILM AND A METHOD FOR MAKING IT
Abstract of the Disclosure A method for making soft, flexible, microporous films having high tensile strength and high breathability levels for air and water vapor and high hydrostatic resistance to penetration by liquid water, in which a film fabricated by blending a mixture of a polymer or copolymer of an alpha-olefin with 60 to 75% by weight of an inorganic filler or glass beads having particle sizes within the range of 10 to 15 micrometers and calcium stearate as a processing aid is biaxially stretched from about 1.5 to about 7 times in each direction at a temperature of from about 200 to about 160°C, and the moisture level in the blended composition is maintained below 700 parts per million before fabricating the film; the film so made having a Gurley porosity of from 0.1 to 20 seconds; and its use in disposable items such as panty liners, diapers, bed sheets, and hospital gowns, or, if the filler consists of glass beads, its use as a battery separator.

Description

This invention relates to methods for making ~o~t, flexible, microporou~ film~ having high ten~ile ~trength and good permeance or "breathabili~y~' level~ for air and water vapor and high hydrostatic resi~tance to penetration by S liquid water, to ~ilms made by the said methods, and to the u~es o~ such films.
~ t is well known that thermoplastic polymers ca~ be .~illed with inert fillers, cast into sheets, and stretched to form an oriented microporous thermoplastic film that pro-10 vides a desired level of gas or vapor permeance. Method~ -for making such films are described, for example, in U.S.
Patents 3,903,234 and 4,698,372, in UK Patent 5pecification 2,151,S38, and in European published Patent Application 272,026, the latter disclosing films o homopolymers, copolymers, or blendg of alpha-monoolefins having 2 to 10 carbons having an o~ygen and carbon dioxide permeance be- -tween about 77,500 and 155,000,000 cc/m2-day-atmosphere.
U. S. Patent No. 4,698,372 discloses microporous polymeric filmæ for use as substitutes for textiles and having good water vapor transmigsi~n rates and hydrostatic resis~ance to water penetration; the films may consist of certain ethylene copolymers and have a filler loading of about 25-35 volume % of inorganic fillers such as calcium carbonate; they use a processing aid such as stearic acid that i9 referred to as an "antagonizer" and is said to . ~

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reduce the effec~ive sur~ace tenYion of the filler to the approximate level of that of the matri~ polymer. U.S.
Patent No. 3,903,234 discloses gas-per~eab.le biaxially ori-ented film prepared from C2 C10 alpha-monoole~in poly-mers containing 26-50% by weight of inorganic filler part-icles. UK Patent Specification 2,151,538 discloses a method for making water-vapor-permeable oriented ~ilms from poly-olefin~ containing 33-83% by weight of bar:ium sulfate filler, for u~e aæ a leakproof sheet in disposable diapers and sanitary napkins.
In general, the available liquid-barrier films that have an appreeiable level of "breathability", for instance, those sold as diaper liners or covers for panty liner~, do not reach desirable levels of breathability. This can be evaluated quantitatively in term of Gurley porosity numbers, whlch are measured in seconds by ASTM D-726, Method A or Method B. (~heoretically, Gurley numbers measured by Method A are 25 times larger than Gurley numbers measured by Method B, and Method B numbers will be used in the following de-scription and claims). Method B of ASTM D-726 measures the time (in 3econds) for ten milliliters of air to pass khrough one square inch of microporous film under a pressure of 12.2 inches of water. (Method A o~ ASTM D-726 measures the time (in seconds) for 100 milliliters of air to pass through one ~quare inch of microporous film under a pressure of 4.9 inches of water). A low Gurley number signifies that a mi-croporous film offers little resistance to the passage of air (or humid air). Thu~, Gurley numbers, also known as Gurley porosity numbers, are ef~ective measures of "breathability".
The generally available films have Gurley numbers up to 100 seconds and usually above 10 to 20 seconds. The unavailability of soft, flexible, microporous liquid-barrier films having high tensile strength and a higher level of , .

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"breathability", at a reasonable cost, indicates that there is a need 9 particularly in the hygienic product industry, for such films. They are needed, for example, in the ~abrication of disposable products ~uch as ~eminine panty liner~, diapers, bed sheets, and hospital gowns that are cool and comfortable for the wearer. There i8 also a need for uch permeable films, which have an ion-exchange function, for use as battery separators.
According to the invention, a method for making æoft, flexible, microporous films having high tensile ~trength and good permeance or ~breathability~ levels for air and water vapor and high hydrostatic resistance to penetration by liq-uid water, i~cluding the step~ of melt-blending a mixture of a polymer or copolymer of an alpha-ole~in, a particulate filler, and calcium stearate as a processing aid, fabri-cating a film, and biaxially stretching the film, is charac-terized in that the mi~ture contains 60 to 75% by weight of an inorganic filler or glass beads having particle sizes within the range of 10 to 15 micrometers in mean diameter, the moisture level in the blended composition i8 maintained below 700 ppm prior to fabricating the film, and the film is ~tretched in two directions from about l.S to about 7 timeæ
in each direction at a temperature range o~ from about 20 to about 160C.
Unless the moisture level in the composition prior to fabricating the film is maintained below 700 ppm, the film cannot be stretched uniformly, using the amount of filler required according to the invention. Preferably, the moisture level in the blended composition is maintained below 300 ppm.
Also according to the invention, a soft, flexible, microporous film having high tensile trength and good permeance or "breathability" levels for air and water vapor and high hydrostatic resistance to penetration by liquid ': ' ' '` - ~ `

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water, and made by the ~aid met~od according to the inven-tion, i8 further characterized in that it comprises:
20 to 37% by weight of a polymer or copolymer of an alpha-olefin having 1-8 carbon atoms, 60 to 75Z by weight of an inorganic filler or glass beads having particle sizes within the range of of 10 to 15 micrometers in mean diameter, a~d 0.1 to 3% by weight of calcium stearate, and has a Gurley porosity, based on method B, ASTM D-726, of 0.1 second to 20 ~econds.
Preferably the film according to the invention, contains 0 to 2% by weight of a stabilizer.
"Microporous" means that the film contains numerous open pores or channels leading frcm one surface to the opposite surface, such pores being of a size to permit air and water vapor to pass through the film while having good resistance to the penetration of liquid water. Their poros-ity or breathability in Gurley porosity numbers can be ex-pressed in terms of permeance by by dividing the constant 44.64 x 109 seconds by the Gurley number ~Method B) to give the permeance in cc/m2-day-atmosphere. The permeance o~ the microporous film of thi~ invention is greater than 44.64 x 109 seconds- cc/m2-day-atmosphere divided by 20 seconds, namely 2.232 x 109 or 2,232,000,000 cc/m2-day-atmosphere.
The particle size of the filler determines the pore size of the microporous films of this invention. As would be expected, smaller particle sizes produce smaller pores than larger particle sizes. There is no theoretical limi-tation on the size of the filler particles that may be usedin the practice of this invention. However, practical con-siderations impose effective limitations. Preferably, the particle size of the fillers should range from 10 to 25 micrometers in mean diameter, and preferably the filler is , ; .
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calcium carbonate and the particle cize iY abou~ 12.5 micro-m~ters in mean diameter.
Filler-loading determines to a great extent how far the casting must be stretched to attain a given degree of over-all porosity. Below the lower end of the loading range, 60%by weight, where the pores are less numerous, they are les~
interconnected, and the fil~ i insufficiently permeable at the maximu~ draw ratio of about 7 times in each direction according to the invention. Above the higher end o~ the loading range, 75% by weight, either the ~aterial~ will not blend uniformly or the casting mac~e from the composition will no~ stretch the minimum according to the invention, namely, 1.5 times. Although other inorganic fillers may be used, calcium carbonate is pre~erred. However, if the films are to be used as battery separators, glass beads are used as the filler.
The calcium s~eara~e processing aid coats the filler particles, thus assisting in the uniform disper~ion o~ the filler particles, and allowing the composition to be ~tretched to the required degree of orientation.
A varie~y of alpha-olefinic polymeric materials ha~ing 1-8 carbon atoms can be used as matrix polymerg. Any such alpha-olefin that exhibits sufficient tensile yielding and some permanent deformation may be used. The selection of the polymeric material will be ba~ed on the desired proper-ties of the microporous film, as for example, temperature resistance or elastic recovery. Preferred homopolymers are polypropylene (PP), polyethylene (PE) (particularly linear low-density polyethylene (LLDPE)), and polybutylene (PB~.
Copolymers of ethylene with propylene or with an alpha-olefin of 4-8 carbon atoms may of course be used also. For ease in processing, a blend of linear low density polyeth-ylene and polypropylene in a ratio of 95 to 5, or polypro-pylene blended with ethylene- propylene copolymer, is ~ . .. .

~272~9 pre~erable to polypropylene alone as a matrix material.
The choice of the polymeric material in the matrix, and the cholce of the ~iller-material influences the preferred amounts of the filler and the calcium stearate, the orientation tempera-ture, and the extent of biaxial orientation.
Wi~h a polypropylene/calcium carbonate combination, the mixture preferably contain~ about 33% by weight of polypropylene, and the amount of the calcium carbonate filler preferably is abou~
65% by weight. The amount of calcium ~tearate pref~rably is about 0.5 to about 2.0%, and more preferably 2%. The biaxial orienta-tion of the film preferably is in the range of from about 4 to about 7 times in each direction, more preferably about 5 times, wlth the orlentation temperature pre~erably being from 130 to 150C with about 130C being most preferred.
With a polypropylene/glass beads combination, the mixture preferably contains about 33.5% by weight of poly-propylene, the amount of the glass beads filler preferably being in the range of 55 to 65% by weight/ more preferably 65%. The amount of calcium stearate preferably is about 0.5 to about 2.0%, `~
and most preferably 1.5%. The biaxi.al orientation of the film preferably is in the range of ~rom about 4 to about 7 times in each direction, more preferably about 5 times, with the orienta-tion temperature preferably being from 130 to 150C, with about 135C being mo~t preferred.
~ ith a polybutylene/calcium carbonate combination, the mixture preferably contains about 65 to about 75% by weight of the ~iller, with about 70% of filler and about 28% of polybutylene " ",...~ " .~, `

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being more preferred; in this comblnation the amount of calcium stearate to be used should be in the range of about 0.2 to about 4% by weight, with about 2% being more preferred. This fllm should be biaxially oriented ~rom about 1.5 to about 5 times, more preferably about 4 times, at a temperature range of about 20 to about 105C, about 100C being more preferred.
With a polyethylene/calcium carbonate combination, the amount of filler pre~erably is in the range of abou~ 60 to about 70% by weight, about 70% of filler and about 28% of polyethylene being more preferred. The amount of ~alcium s~earate in this combination preferably is from about 0.1 to about 3.5g, about 0.5 to about 2% being more preferred. Thls fllm preferably is biaxially oriented about 1.5 to about 5 times, with about 4 times being more preferred, in a tempera~ure range of about 20C to about 110C, with about 100C being more preferred.
Preferably the mixture contains from about 0.1 to about 2% by weight of a stabllizer against degradation by exposure to UV
light, oxygen, and heat; it is especially use~ul ln the combinations with the polypropylene and the polyethylene.
After the film composition is prepared, it may be compounded in~o the fllm of this invention by any known method suitable for the melt blending of thermoplastic polymers at temperatures at which the ma~rix polymers are processible. High shear mixing, which can be achieved in a Banbury-type or another high intensity mixer or in continuous mixers such as ex~ruders, is preferred. There is no need to premix ingredients, but this may be done without detriment to the practice of this lnvention and .: .

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After the ingredients of the composition of this invention have been melt blended, the moisture level of this blend is then maintatned below the critical level of 700 parts per million (ppm) (more preferably below 300 ppm), preferably by simultaneously cooling and maintaining the moisture level of the blended and extruded composition by . .

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~lowing air over it on a moving conveyor b01t. This air-cooling method yields strands and pellets that have residual moisture levels far below the levels achieved by the proces~ o~ cooling by immersion in a water bath that i9 conventionally used.
The strands were then pelletized using conventional techni~ues. To accurately achieve this moisture level, ~ensitive moiYture measureme~t i9 required. For example, a Coulometric Karl Fischer titration method (u~ing the Brink-man Model 652 R~ Coulometer) can be used for evaluating themoi~ture level in the formulation~.
After blending and establishing the moisture level, the compoqition i3 converted into any convenient form for processing into film, including pellets or sheets. The ~ilm fabrication can be accomplished by any conventional tech-nique including extrusio~ casting, compression moldin~, flat film extrusion, or blown film extrusion.
A~ter the film is fabricated into its desired form, it is then biaxially oriented by stretching by any of the well known techniques in the art including, by hydraulics, by pinch rolls moving at diffexent rates, or by tentering.
Biaxial stretching can be performed sequentially or simul-taneously. Sequential biaxial stretching is preferred when using the tentering operation.
Another process for maintaining the desired moisture - -level is to employ vacuum-drying in order to reduce the moisture level in too-wet pellets to acceptable levels according to the invention, (below 700 ppm, and more prefer-ably below 300 ppm). In this case, pellets composed of polymer plus filler are made using the conventional water-bath-cooling process, which produces is excessive residual moisture levels. These too-wet pellets are subjected to a paxtial vacuum, preferably with some heating to speed the process, for a period of time until the moisture content is . .
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within acceptable limits as defined above. This process is not preferred since the extra step of vacuum-drying is required.
Yet another procesæ of maintaining the desired moisture level i~ by charging the hot melt directly to the e~truder that extrudes the casting from a die. In t:hiX case, the molten composi~ion i8 never exposed to water and has a low re~idual moisture level as defined above. Therefore, a smooth and highly-orientable ca~ting will be ~ormed.
The stretch ratio of at least two times the original forming dimensions is ~igni~icant to producing a film having at least 30% of pores resulting in relatively high density films. ~owever, to produce relatively low density films, it i~ pre~erred that the film be stretched to at lea~t 3 to 8 times its original forming dimensio~s in mutually perpen-dicular directions, resulting in a film having about 40 to 70% pores. .
Stretching is of course effected above the glass tran-sition temperature of the matrix polymer (preferably at least 20C above) and below the melting temperature of the matrix polymer, eæpecially within 10C of that temperature, depending to some degree on the rate of stretching. Differ-ent polymers and compositions ther.eof exhibit di~ferent elastic and viscoelastic behavior. Thus, different amounts of ætretching must be imposed on dif~erent samples in order to obtain the same permeability properties. Obviously, the film mu~t be stretched beyond its yield point in order to attain the permanent deformation necessary for the formation o~ porosity.
For a given composition, a greater degree of stretch results in greater overall porosity. Higher overall poros-ity can be attained by adding more filler and stretching the same amount or possibly less.

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In a preferred embodiment~ the invention provides a method of preparing a brea~hable microporous polymeric film having the composition of 20 to 37% by weight of a polymer or copolymer of an alpha-olefin having 1 to 8 carbons or mixtures thereof, 60 to 75% by weight of calcium carbonate or ylass beads, 0.1 to 3.0 by weight of calcium stearate and, optionally, 0 to 2~ by weight of a UV light, oxygen, and heat stabilizer, which method comprises preparing a melt blend of the ingredients, maintaining the moisture level in the melt blend below 70Q ppm prior to extrudlng a casting, and based on the polymer or copolymer used, stretching the casting in two directions of from 1.5 to 7 times in each directlon in a temperature range of from about 20 to about 160C
thereby producing the breathable microporous film having a Gurley Porosity of from 0.1 to 85 seconds so that the fil~ has good air and water vapor transmission rates but is substantially impenetra-ble by liquid water.

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In the ~ollowing examples, all parts, proportions, and percentages are by weight unles~ otherwi~e indicated.
In Examples 1-8, the ingredients (listed in Table lA) were blended at room temperature and compounded in a twin-screw extruder; strands were extruded at in a temperaturerange of 243 to 265OC. The strands were then air cooled (except that in Examples 6-8 they were water cooled) and pelletiz~d. The pellets were vacuum dried for 24 hours at 80OC. (except that Examples 7 and 8 they were vacuum dried for 8 hours at 70~C.). U~ing a melt temperature of 278 to 282OC (478 to 5400F), the pellets were extruded by a single screw extruder through a six inch wide slit die onto a cast-ing roll maintained at about 65C (except Examples 6-8 were maintained at about 18-24C) so as to form a 15 mil thick casting. Using a T.M. Long stretcher, square pieces having the dimensions 2 x 2 inches from the casting were biaxially oriented by stretching 4 times in the machine direction and 4 times in the transverse direction (except that in E~ample 8 it was stretched 2x by 2x) a~ 100C, producing the product as set forth in the following Table 1.
The ingredients for Examples 9-14 are listed in Table 1 B. In Examples 9 and 10, the ingredien~s were blended together on a 2-roll mill at 200C.; this blend was com-pression molded at 215C. to yield 30 mil thick plaques.
Two inch by two inch portions of t:he plaques were biaxially oriented by stretching 5 times in the machine direction and 5 time~ in the transverse direction on a T. M. Long stretch-er at 140C. to make the film as described in Table lB.
In Examples 11 and 14, the ingredients were compounded in a twin screw extruder at 225-250C.; the extrudate was pelletized and cast on a casting extruder at 180-230C.
For Example 11 since much strand breakage and non-uniformity was observed during the pelletizing step, the casting could not ~e stretched at 140C. on the T. M. Long stretcher; the ,~
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casting was too brittle. For ~xample 14, 2 ~ 2 inc~ por-tions of the casting were æ~retched 4.5X by 4.5X at 140C.
on the T. M. Long stretcher.
In Example 12 and 13, the ingredients were blended by a twin-~crew e~truder and were extruded by a single screw e~truder and slit die to form a 30 mil casting; the casting wa~ stretched 5X by 5X on a T. ~. Long stretcher to form the ~ilm.

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All of the resulting product~ of the E~amples were opaque white films.
Examples la, b, and c ~how that for the system LLDP~/
CaC03 films, 70% by weight o~ the filler gives much lower Gurley number (i.e., high breathability) than 65% by weight of the filler. ~amples la, b a~d c show that breathability is the best in the more highly oriented fi:Lms. Similarly, 65% by weight of the filler (Example 2) gives a much lower Gurley number than 60% filler, (Example 3). Example 4 compared to Example 1 show~ that ~he processibili~y o~ the formulation i~ improved by adding 1.5% o.~ calcium stearate ~Example 1) instead of 0.6% calciu~ æteara~e (Example 4);
further, in comparative Example 4, die depo~its and melt fractures were excessive and caused constant breakage o~ the extruded molten strands. Thus, the material could not be pelletized and extruded into castings suitable ~or orien-tation. Example 5 demonstrates the advantage of using a small amount of polypropylene additive in the LLDPE to reduce die lines. Regions of melt fracture (die lines) thinner than the rest of the casting were greatlv reduced compared to the melt fracture regions commonly observed in compositions such as those in Examples la, b and c.
Example 6 shows that, for po:l.ybutene/calcium carbonate film, 70% by weight of the filler gives much lower Gurley number (i.e., high breathability) than 50% by weight of the filler in Example 7. Example 8, compared to Example 6, shows that 1.5% of calcium stearate allows much easier processing to a porous film than if no calcium stearate is u~ed (Example 8). The film prepared in comparative Example 8 had many visible pinholes and was extremely rough. Gurley mea-surements were not possibler Castings made from this compo-sition could not be oriented 4 times by 4 times at the temp-erature of 100C. ~xample 9 shows that, for polypropy~ene/
calcium caxbonate films, 60% by weight of the filler gives a , .

much lower Gurley nu~ber than ~hen only 50% by weight of the filler i8 uaed (Exa~ple lO). Example 11 compared to Example 9, 6how~ that using a ble~d of polypropylene and ethylene-propylene copolymer gives better proce sing than if pure polypropylene is ubstitut~d for the blend (~xample 11) because the caating of E~ample 11 would not ~tretch to form film at 140C. Example 12 shows that high calcium stearate levels greatly improves processibility compared to a low calcium stearate level in Example 13 because the resulting film had large visible pin holes and wa~ e~tremely rough.
Example 14 demonstrates a breathable composition co~posed of polypropylene and glass bead filler.

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Claims

1. A method for making soft, flexible, microporous films having high tensile strength and good permeance or "breathability"
levels for air and water vapor and high hydrostatic resistance to penetration by liquid water, including the steps of melt-blending a mixture of a polymer or copolymer of an alpha-olefin, a particulate filler, and calcium stearate as a processing aid, fabricating a film, and biaxially stretching the film, charac-terized in that the mixture contains 60 to 75% by weight of an inorganic filler or glass beads having particle sizes within the range of 10 to 15 micrometers in mean diameter, the moisture level in the blended composition is maintained below 700 parts per million prior to fabricating the film, and the film is stretched in two directions from about 1.5 to about 7 times in each direction at a temperature range of from about 20° to about 160°C.

2. A method for making microporous films as claimed in claim 1, further characterized in that the particle size of the filler is from 10 to 25 micrometers in mean diameter.

3. A method for making microporous films as claimed in claim 1, further characterized in that the filler is calcium carbonate and the particle size is about 12.5 micrometers in mean diameter.

4. A method for making microporous films as claimed in claim 1, further characterized in that the moisture level in the blended composition is maintained below 300 parts per million prior to fabricating the film.

5. A Method for making microporous films as claimed in claim 1, further characterized in that the blended and extruded composition is cooled and its moisture level is simultaneously maintained by flowing air over it on a moving conveyor belt.

6. A method for making microporous films as claimed in any one of claims 1 to 5, further characterized in that the polymer or copolymer of an alpha-olefin is an alpha-olefin having 1 to 8 carbon atoms or a copolymer of ethylene with propylene or with an alpha-olefin of 4 to 8 carbon atoms.

7. A method for making microporous films as claimed in claim 6, further characterized in that the polymer or copolymer of an alpha-olefin is polypropylene polyethylene, polybutylene, a blend of linear low-density polyethylene and polypropylene, or a blend o polypropylene with ethylene-propylene copolymer.

8. A method for making microporous films as claimed in claim 7, further characterized in that the polymer or copolymer of an alpha-olefin is linear low-density polyethylene.

9. A method for making microporous films as claimed in any one of claims 1 to 5, 7 and 8, further characterized in that the mixture contains about 20 to 37% by weight of polypropylene, and the filler is calcium carbonate.

10. A method for making microporous films as claimed in claim 9, further characterized in that the mixture contains about 33% by weight of polypropylene, about 65% by weight of calcium carbonate, about 0.5 to 2% by weight of calcium stearate, and the casting is stretched in two directions 5 to 7 times at a tempera-ture of about 150 to 130°C.

11. A method for making microporous films as claimed in claim 9, further characterized in that the mixture contains about 2% by weight of calcium stearate, the moisture level is maintained below 300 ppm, and the casting is stretched in two directions 5 times in each direction at a temperature of about 130°C.

12. A method for making microporous films as claimed in any one of claims 1 to 5, 7 and 8. further characterized in that the mixture contains about 20 to 37% by weight of polypropylene, and the filler is glass beads.

13. A method for making microporous films as claimed in claim 12, further characterized in that the mixture contains about 33.5% by weight of polypropylene, about 65% by weight of glass beads, about 0.5 to 2% by weight of calcium stearate, and the casting is stretched in two directions 5 to 7 times at a temperature of about 130 to 150°C.

14. A method for making microporous films as claimed in claim 13, further characterized in that the mixture contains about 2% by weight of calcium stearate, the moisture level is maintained below 300 ppm, and the casting is stretched in two directions 5 times in each direction at a temperature of about 130°C.

15. A method for making microporous films as claimed in any one of claims 1 to 5, 7 and 8, further characterized in that the mixture contains about 65 to 75% by weight of polybutylene, and the filler is calcium carbonate.

16. A method for making microporous films as claimed in claim 15, further characterized in that the mixture contains about 28% by weight of polybutylene, about 70% by weight of calcium carbonate, about 0.2 to 4% by weight of calcium stearate, and the casting is stretched in two directions 1.5 to 5 times at a temperature of about 20 to 105°C.

17. A method for making microporous films as claimed in claim 16, further characterized in that the mixture contains about 2% by weight of calcium stearate, the moisture level is maintained below 300 ppm, and the casting is stretched in two directions 4 times in each direction at a temperature of about 100°C.

18. A method for making microporous films as claimed in any

19 one of claims 1 to 5, 7 and 8, further characterized in that the mixture contains about 60 to 70% by weight of polyethylene, and the filler is calcium carbonate.

19. A method for making microporous films as claimed in claim 18, further characterized in that the mixture contains about 28% by weight of polyethylene, about 70% by weight of calcium carbonate, about 0.1 to 3.5% by weight of calcium stearate, and the casting is stretched in two directions 1.5 to 5 times at a temperature of about 20 to 105°C.

20. A method for making microporous films as claimed in claim 19, further characterized in that the mixture contains about 0.5 to 2% by weight of calcium stearate, the moisture level is maintained below 300 ppm, and the casting is stretched in two directions 4 times in each direction at a temperature of about 100°C.

21. A method for making microporous films as claimed in any one of claims 1 to 5, 7, 8, 10, 11, 13, 14, 16, 17, 19 and 20, further characterized in that the mixture contains from about 0.1 to about 2% by weight of a stabilizer against degradation by exposure to UV light, oxygen and heat.

22. A biaxially oriented microporous FILM comprising a polymer or copolymer of an alpha-olefin, a particulate filler, and calcium stearate, characterized in that it is made by the method as claimed in any one of claims 1 to 5, 7, 8, 10, 11, 13, 14, 16, 17, 19 and 20 and further characterized in that it has a Gurley porosity, based on method B, ASTM D-726, of 0.1 second to 20 seconds so that the film has good air and water vapor transmission rates but is substantially impenetrable by liquid water.

23. Use of the microporous film as claimed in claim 22, as a cover sheet for panty liners or a diaper.

24. Use of microporous film as claimed in claim 22, as a bed sheet or a hospital gown.

25. Use of the microporous film as claimed in claim 22, as an ion-permeable separator in a liquid-containing battery.

26. A method of preparing a breathable microporous polymeric film having the composition of 20 to 37% by weight of a polymer or copolymer of an alpha-olefin having 1 to 8 carbons or mixtures thereof, 60 to 75% by weight of calcium carbonate or glass beads, 0.1 to 3.0% by weight of calcium stearate and, optionally, 0 to 2%
by weight of a UV light, oxygen, and heat stabilizer, which method comprises preparing a melt blend of the ingredients, maintaining the moisture level in the melt blend below 700 ppm prior to extruding a casting, and based on the polymer or copolymer used, stretching the casting in two directions of from 1.5 to 7 times in each direction in a temperature range of from about 20° to about 160°C thereby producing the breathable microporous film having a Gurley porosity of from 0.1 to 85 seconds so that the film has good air and water vapor transmission rates but is substantially impenetrable by liquid water.

27. The method of claim 26 wherein the moisture level is maintained below 300 ppm.

28. The method of claim 27 wherein the composition has 65 to 75% of calcium carbonate.

29. The method of claim 28 wherein the polymer is polyethylene or polybutylene and the casting is stretched in two direction from 1.5 to 5 times in each direction in a temperature range of from about 20° to about 105°C.

30. The method of claim 28 wherein the polymer is poly-propylene and the casting is stretched in two directions from 4 to 7 times in each direction in a temperature range of from about 130° to about 160°C.