CA1250724A - Process for producing porous films - Google Patents

Abstract

ABSTRACT

A process for producing porous films which comprises blending 100 parts by weight of a polyolefin resin with 50 to 500 parts by weight of barium sulfate preferably having an average particle diameter of 0.1 to 7 µm, melting the resulting resin composition and forming it into a film, and then stretching the film at least uniaxially by a factor of 1.5 to 7. As the polyolefin resin, there may preferably be used a polyethylene resin having a melt index of 0.5 to 7 and a density of 0.915 to 0.935, and more preferably a linear low-density polyethylene resin having a melt index of 0.5 to 8.5 and a density of 0.915 to 0.935. As the linear low-density polyethylene resin, a copolymer of ethylene and hexene and/or octene is particularly preferred.

Description

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I
SPECIFICATION
Title o~ the Invention:
Process for Producing Porous Films Background of the Invention:
a) Field of the Invention:
This invention relates to an improved process for producing porous films. More particularly, it relates to a process for producing porous films which comprises blending a polyolefin resin with barium sulfate as a filler, melting the result-ing resin composition and forming it into a film, and then stretching the film at least uniaxially.

b) Description of the Prior Art:
Conventionally, there are a large number of well-known processes for producing porous films by forming a resin composition comprising a poly-oleFin resin and any of various noncompatible fillers into a fiIm and then stretching this film.
For example, Japanese Patent Laid-Open Nos. 47334/
'82 and 203520/'82 laid open March 18, 1982 and December 13, 1982 (both in the name of Mitsubishi Chemical Industries Ltd.) disclose a process for producing porous films which comprises melting a resin composition obtained by blending a poly-olefin resin with a filler and liquid rubber or a hydroxylated poly-saturated-hydrocarbon, forming the molten resin composition into a sheet or film, ~q ~ ~ S ~ 7 ~ ~

and then stretching this sheet or film. Moreover, Japanese Patent Laid-Open No. 15538/'83 laid open January 28, 1983 in the name of Mitsubishi Chemical Industries Ltd. discloses a process for producing porous films which comprises melting a resin compo-sition obtained by blending a linear low-density polyethylene resin with a filler and a liquid or waxy hydrocarbon polymer, forming the molten resin composition into a sheet or film, and then stretch-ing this sheet or film. However, the fiIms produced by these processes are disadvantageous in that they exhibit surface tackiness due to the aforesaid component contained in addition to the polyolefin resin and the filler and in that they can only be practically used in relatively large thickness because of their low mechanical strength.
Furthermore, it is described in Japanese Patent Laid-Open No. 149303/'83 laid open September 15, 1983 in the name of Kao Corporation that such porous films can be used as a leakproof sheet in disposable diapers. Porous fiIms for use as the leak?roof sheet of a disposable diaper are produced by blending 100 parts by weight of a polyolefin resin with 28 to 200 parts by weight of a filler and 10 to 70 parts by weight of a liquid or waxy hydrocarbon polymer, forming the resulting resin composition into a film, and then stretching this film at least uniaxially by ~Z5~24 a factor of 1.2 or greater. However, this process for producing porous films has the disadvantages that some types of fillers give poor stretchability and hence fail to pro-vide fully uniform pores and that the resulting film tends to produce a disagreeable noise. Moreover, the concurrent use of a hydrocarbon polymer makes this process unsatisfactory because the hydrocarbon polymer tends to bloom to the film surface and causes a sticky sensation.
Such porous films are also useful as a leakproof sheet in sanitary napkins. Conventionally, a sheet of paper which has been rendered liquid-impermeable by means of a synthetic resin such as polyethylene has been used for this purpose. However, the resulting sanitary napkins have the disadvantage of causing a disagreeable sensation during prolonged use because of their lack of permeability to water vapor.

Summary of the Invention:

Accordingly, it is an object of an aspect of the present invention to provide an improved process for pro-ducing porous films.
It is an object of an aspect of the present inven-tion to provide a process for producing porous films which have sufficiently high porosity and thereby exhibit high moisture permeability and gas permeability while retaining excellent water resistance.
It is an object of an aspect of the present inven-_4_ ~S~4 tion to provide a process for producing porous filmswhich are free from surface tackiness, have excellent softness and show little reduction in strength.
It is an object of an aspect of the present invention to provide an improved leakproo~ sheet for use in disposable diapers.
It is an object of an aspect of the present invention to provide an improved leakproof sheet for use in sanitary napkins.
Other objects of the present invention will be apparent from the following description.
An aspect of the invention is as follows:
In a process for producing porous films which comprises melting a resin composition consisting essentially of a polyethylene resin and a filler noncompatible therewith, and with or without a hydrocarbon-free lubricant, forming the molten resin composition into a film and then stretching the film at least uniaxially, the improvement in which the polyethylene resin is either a high-pressure-processed low-density polyethylene resin having a melt index of 0.5 to 7 and a density of 0.915 to 0.935, a linear low-density polyethylene resin having a melt index of 0.5 to ~.5 and a density of 0.915 to Q.935, or a mixture thereof; the filler is barium sulfate having an average particle diameter of O.1 to 7 ~m and the barium sulfate is used in an amount of 50 to 500 parts by weight per 100 parts by weight of the polyethylene resin; and the film is stretched by a factor of 1.5 to 7~
According to another aspect of the present invention, there is provided a process for producing porous films which comprises melting a resin composition consisting essentially of 100 parts by weight of a polyolefin resin and 50 to 500 parts by weight of barium sulfate, fo~ming the molten resin composition into a film, and then stretching the film at least uniaxially by a factor of 1.5 to 7.
The expression "a resin composition consisting essentially of 100 parts by weight o~ a polyolefin resin and 50 to 500 parts by weight of barium sulfate"

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as used herein means that the resin composition may further contain (1) at least one additive selected from common stabilizers, antioxidants, colorants, ultraviolet light absorbents and laubricants and/or

(2), in addition to the barium sulfate, other inorganic fillers (such as calcium carbonate and the like) or common inorganic and organic modifiers in an amount less than that of barium sulfate used (for example, not greater than 20~ based on the amount of barium sulfate used), but the addition of liquid rubber, a hydroxylated poly-saturated-hydrocarbon or a hydrocarbon polymer as described in the aforementioned Japanese Patent Laid-Open Nos.
47334/'82, 203520/'82, 15538/'83 and 149303/'83 should be positively avoided in order to obtain a porous film free from surface tackiness.
According to the present invention, porous films which are free from surface tackiness and have excellent properties and which have been un-obtainable in the prior art can be produced withoutusing any of the above-described additives used in the prior artO This can be accomplished simply by specifying the type of the filler, i~s amount used and preferably its average particle diameter;
preferably using a low-density polyethylene resin having a specific melt index and density and more 5~ 29~

preferably a linear low-density polyethylene resin (in particular, a linear low-density polyethylene resin comprising a copolymer of ethylene and hexene and/or octene) which most preferably has a specific melt index and dansity; and stretching the film by a specific factor.

Detailed Description of the Invention:
The polyolefin resins which can be used in the present invention include homopolymers such as polypropylane, low-density polyethylene, high-density polyethylene, linear low-density polyethylene, polybutylene, etc.; copolymers such as ethylene-propylene copolymer, ethylene-butylene copolymer, ethylene vinyl acetate copolymer, etc.; and blends thereof.
Among these resins, low-density polyethylene resins having a melt index of 0.5 to 7 (0.5 to 8.5 for linear low-density polyethylene resins) and a density of O.91S to 0.935 are preferred. Although it is preferable to use a single resin, any blend of poly-ethylene resins having different densities may be used. More preferably, the melt index is in the range of 1 to 5. If the melt index is less than 0.5 or greater than 7 (8.5 for linear low~density poly-ethylene resins), the resin will have significantly 1~5~72~

decreased formability into a film and may fail to yield a film of small and uniform thickness.
On the other hand, if the density is lower than 0.915 or higher than 0.935, the resin will have decreased stretchability and increased rigidity and may fail to yield a soft film.
Particularly preferred polyolefin resins are linear low-density polyethylene resins. Lineax low-density polyethylene resins are copolymers of ethylene and one or more a-olefins, and differ from low-density polyethylene resins prepared by the conventional high-pressure process. Lineax low-density polyethylene resins are prepared by the low-pressure process, and useful a-olefins include butene, hexene, octene, dPcene and the like. The difference between high-pressure-processed low-density polyethylene resins and low-pressure-processed low-density poly-ethylene resins lies in the fact that, when seen from the viewpoint of chemical structure, the former ones are highly branched polymers while the latter ones are straight-chain polymers Among such liear low~density polyethylene resins, copolymers of ethylene and hexene and/or octene are particularly preferred.
In the practice of the present invention, it is preferable to use barium sulfate having an ~S~24 average particle diameter of 0.1 to 7 ~m and more preferably 0.5 to 5 ~m. If the average particle diameter is smaller than 0.1 ~m, well-defined pores cannot be obtained. On the contrary, if it is larger than 7 ~m, the resulting film will have poor stretchability and, therefore, well-defined pores can hardly be obtained just as in cases where the average particle diameter is too small.
The barium sulfate is used in an amount of 50 to 500 parts by weight, and preferably 100 to 400 parts by weight, per 100 parts by weight of the polyolefin resin. If the amount of barium sulfate used is less than 50 parts by weight, sufficiently high porosity cannot be obtained, while if it is greater than 500 parts by weight, the resulting film cannot be fully stretched because of its increased rigidity and, therefore, will show a reduction in porosity.
It is preferable to subject the barium sulfate to surface treatment with a fatty acid or a metallic salt thereof, silicone, silane, a resin acid or the like, because this treatment is effective in improving its dispersibility in the resin and producing well-defined pores.
So far as the effects of the present inven-tion are not impaired, other inorganic fillers such as `` ~IL25~17z~
_ 9 _ calcium carbonate and the like or common inorganic and organic modifiers may be used in addition to the barium sulfate. However, these additives should be us~d in an amount of not greater than 20% based on the amount of barium sulfate used.
Now, the pres~nt process for producing porous films will be specifically described hereinbelow.
According to the need, at least one additive selected from stabilizers, antioxidants, colorants, ultraviolet light absorbents and lubricants is added to a polyolefin resin and barium sulfate. These ingredients are mixed with a Henschel mixer, super-mixer or tumbling mixer. Thereafter, using an ordinary single-screw or twin-screw extruder, the resulting mixture is blended and pelletized. Then, using an inflation extruder or T-die extruder, these pelle~s (alone or in admixture with polyolefin resin pellets) are melted at a temperature higher than the melting point of the polyolefin resin (prefer-ably, by 20C or more) and lower than the decomposition temperature thereof, and formed into a film. In some cases, the aforesaid mix~ure may be directly formed into a film with an extruder, instead of being pelletized. Subsequently, the film is a~ least unaxially stretched by a factor of 1.5 to 7 according ~S()72~

to a conventional technique such as roll stretching, tentering or the like. This stretching may be performed in steps and~or in two or more directions.
In the case of biaxial stretching, however, it is preferabl~ to stretch the film simultaneously in the two directions. In order to enhance the morphological stability o~ pores, the stretched film may be annealed by heating.
~he porosity is determined by the amount of barium sulfate used, the stretching factor and the like. If the stretching factor is less than 1.5, sufficiently high porosity cannot be obtained, while if it is greater than 7, a porous film cannot be steadily produced because of its frequent breakage during the stretching process.-Porous films produced by the process of thepresent invention are characterized by high porosity, excellent softness and little reduction in strength.
Moreover, since the good affinity between the polyolefin resin and barium sulfate provides good stretchability, not only good workability but also an even distribution of pores can be achieved and, therefore, a porous film can be produced steadily.
Especially when a linear low-density polyethylene resin is used as the base resin, the resulting film will show very little reduction in strength. Thus, it ~ZS~4 is possible to produce porous films which are thinner (for example, approximately 10 ~m in thickness) than those produced by the prior-art processes.
Furthermore, since the resin composition does not contain any of the liquid rubber, hydroxylated poly-saturated-hydrocarbons and hydrocarbon polymers used in the prior~art processes, the resulting porous film is free from surface tackiness.
Thus, the porous films of the present invention have sufficiently high porosity and hence exhibit good moisture permeability and gas permeability while retaining excellent water resis-tance, so that they can be used in clothing and sanitary applications. In addition, they can also be used as a filtering medium owing to their even distribution of pores.
The present invention is further illustrated by the following examples. However, these examples a~e given for purposes of illustration only and are not to be construed to limit the scope of the inven-tion.
In the examples, melt index (MI) was determined according to ASTM D-1238 and density was determined according to ASTM D-1505.
Also in the examples t film properties were evaluated according to the following procedures:

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~1) Strength and elongation Using a Tensilon tensile testing machine, a piece of film measuring 25 mm (wide) x 100 mm tlong) is tested at a straining rate of 200 mm/min. Its strength and elongation at breakage are determined with respect to the machine direction (MD) and the transverse direction (TD).

(2) Moisture permeability Moisture permeability is tested according to ASTM E36(Method D).

(3) Softness Softness is evaluated ~y the feel and rated according to the following cristeria:

A = Very soft and smooth B = Rather soft and smooth C = Hard and rough Examples 1-19 and Comparative Examples 1-9 Each of the fillers given in Table 1 was added to the corresponding base resin in the amount given in Table 1, and mixed therewith by means of a Henschel mixer. Thereafter, using a twin-screw mixer, the resulting mixture was intimately blended ~LZ5~7~4~

and formed into pellets. Then, using a T-die extruder, these pellets were melted at a temperature 80C higher than the melting point of the base resin and formed into a film. This film was uniaxially or biaxially (Example 3~ stretched by the factor given in Table 1 to obtain a porous film having the thickness given in Table 1. However, the film was not stretched in Comparative Example 1 and could not be stretched into a porous film in Comparative Examples 2 and 9. In Comparative Examples 5 and 8, the film could only be stretched by a factor of up to 2. In Comparative Examples 3 and 7, no sampling was possible because of frequent breakage during the stretching process.
The strength, elongation, moisture permea-bility and softness of the porous films thus obtained were evaluated according to the above-described procedures and the results are shown in Table 1.

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Table 1 . .
_ . __ _ _ Base resin _ _ T 1) Trade name Melt Density YP (manufacturer) ( /10 min) (g/cm3) ~ g. _ REXLON F-41*(Nippon Example 1LDPE Petxochemicals Co., 5.0 0.923 . ._ _ ~ _ " 2 ll .. .. ..
. . _ .. __ . _ .. 3 ., .. .. ..
. ... _ MIRASON 45*~Mitsui .. ~ .. Polychemicals Co., 1.5 0.920 Ltd.~
_ . _ ., _ . _ .. 5 .. .. .. ., . . __ . . _ UBE POLYETHYLENE
" 6 ll F0191*(Ube Kosan 0.9 0.912 K.K.) ... _ . _ Neo-zex 4330*(Mitsui Petrochemical Indus-7 .. tries, Inc.)/UBE 3.0 0.940 . (Ube Kosan K.K.)=2/1 .. . .~ _ _ " 8 L-LDPE Unicar Co., Ltd.) 1.0 0.920 _ _ _ .................................. _ " .9~ .. ._ _"_ .. ". ._ _ Ultzex 202OL*(Mitsui " 10 ~ Petrochemical Indus- 2.1 ..
tries, Inc.) _ " 11 .. .. " "
_ " 12 .. .. ,. "
. . _ " 13 .. .. ~ ,. " _ * trade marks or trade designations ~L~S~7~

Ta~le 1 (Cont'd) .

Fiiïer _ _ Average 2) Stretchingl Film Type particle dia- Amount factor I thickness meter (~m) (phr) __ (~m) _ BaSO4 0.8 150 4 ¦ 40 . 5 _ ~
_ .. .. 2 X 2 ~ _ ,. 1.2 130 . 5 50 _ ,. 5.0 120 . ll ,. _ .~ 1.2 ,- 33) I ..
. . .__ _ .. .. 100 4 I ..

_ . 30 _ .. 150 . . _ -lS------- _ . . _ _ ._ - .. 4.2 50 6.5 40 .. 0.8 _ 150~ 5 --- _ _ 0.5 100 . 7 ._10 _ 300 . 3 40 _ ~25~

Table 1 (Cont'a) . _ __ ._ _ (kg/25 mm) Elongation (%) Moisture permeability Soft-= _ ~D MD rD (g/m /24 hr) ness 6.0 1.5 80 350 3,500 A

6.5 1.3 60 300 4,200 A

4.9 4.5 200 200 3,900 A

5.5 1.5 120 450 3,000 A
.___ .. _ __ . ._ 5.0 1.3 100 390 n A

3.9 1.0 55 390 1,100 ¦ B
. ... _ .__ ~

4.5 .. 50 200 2,000 C

3.5 1.2 100 420 3,900 C
. _ ___ . .... _ 2.5 0.880 400 5,000 A
... _ ._ _ _ .____ 7.8 1 4 65 170 2,800 a

6.5 1 1.8 130 510 7,530 A
2.3 1 0.7 !40 120 8,500 A
5.3 1.5 78 360 5,500 A

~2S~37Z4 Table 1 (Cont'd) _ . ~ _ Base resin . _ _ ,_ _.. __ _ 1)Trade name Melt Density Type (manufacturer) lndex 3 (g/l0 mln) ~g/cm ) . ~ __ .. ___ _ Ultzex 3010F*(Mitsui Example 14 L-LDPE Petrochemical Indus- 1.3 0.930 tries, Inc.) _ _ . ._ _ . _ Ultzex 2020L*(Mitsui - 15 ll Petrochemical Indus- 2.l 0.920 tries, Inc.) , _ _ _ Ultzex 20100J*
" 16 ll (Mitsui Petrochemical 8.0 0.920 Industries, Inc.) _ ._ . . _ ..... ._ _ __ _ Hi-zex ~Z5000S*
" 17 HDPE (Mitsui Petrochemical 0.9 0.954 Industries, Inc.) .. __ _ _ ._ ............................. _ MITSUI NOBLEN JS-G*
" 18 PP (Mitsui Toatsu 1.5 0.890 Chemicals, Inc.) . , _ MITSUI NOBLEN MJS-G
" 19 EPC (Mitsui To~tsu ll ,.
Chemicals, Inc.) _ _ .

* trade marks or trade designations lZ5~

Table 1 (Cont'd) -. _ ._ . _ _ Filler l r-_ Average _ Stretching ¦ Film Type particle dia- Amount factor I thickness meter (~m) (Phr) l (~m) __ _ ... __ _ BaSO4 5.0 150 5 50 _ ~. 0.5 500 2 40 _ _ .. _ _ _ b ~ ~ -_ . _ ._ . _ _ ~` = ~-~LZ5~

Table 1 (Cont'd) . .

.
Strength Elongation (~) Moisture _ . permeability Soft-MD TD MD TD (g/m /24 hr) ness 5.1 1.4 110 400 6,700 A
.__ ._ .
; D 1.0 35 ~ 5,000 A

5.5 1.4 105 420 4,700 A

~ ~ - 2.2 ' ~,~0~ C -10.3 1 2.5 90 1 350 4,900 C
.. .. _ .__ 9.1 2.3 88 1 520 14,000 C

~2~ Z4 Table 1 (Cont'd) . . . . . . _ _ .
Base resin _ _ _ _ , __ _ Type ) Trade name index Density (manufacturer) (g/10 min) (g/cm3) ._ . ._ _ _ t- .~

6 ..NUCG-5511*(Nippon 1 0 Unicar Co., Ltd.) " 7 __ _ " 8 .._ '' ~ - _ ~ = _ PETROSEN 207*(Toyo .. g LDPESoda Manufacturing 8 0.924 __ Co., Ltd. _ _ Notes: 1) LDPE = low-density polyethylene.
L-LDPE= linear low-density polyethylene.
PP = Polypropylene EPC = ethylene-propylene copolymer.
2) Parts by weight of the filler per 100 parts by weight of the base rein.
3), 5) & 7) The maximum value at which stretching can be performed steadily.
4) ~ 6) No sampling was possible because of frequent breakage during the stretching process.

* trade marks or trade designations ~5~24 Table ' (Cont'd) . . _ . .
- ._ , ~ _ Filler . I
Average21 I Stretching I Film Type particle dia- Amount l factor Ithickness meter (~ml(Phr) l l (~m) .__ ._ ~ _ BaSO4 0.8 150 ¦ Unstretch- 50 .. _ _ .. .. 600 ¦ Unstretch-.. _ _ _ 150 84~- _~ -CaCO3 1.0 .. ¦ 4 70 . ._ _ _ beads 1.2 120 _ _ _ BaSO4 .. 30 ¦ 5 40 _ 100 86) _ _ beads . _ 27) 60 _ BaSO4 l30 ACI _ _ 12S~7~

Table 1 (Cont'd) Strength Elongation (~) Moisture (kg/25 mm) . ~ permeability Soft-MD TD MD TD (g/m /24 hr) ness . ._ _ ..
2.5 2.3 570 480 15 A
.
_ _ _ _ _ _ __ _ _ ~ _ 4.3 0.8 20 100 3,000 C
1.5 0.7 140 290 30 C
. _ _ _ _ 4.5 1.0 105 430 15 C
- _ _ __ . . . . _ 1.0 0.5 40 100 40 C
. _ _ .. ... _ __ _ _ _ _ .__ . _ . _ 12~iU~

Porous films produced by the proces~ of the present invention have high porosity and excellent softness and, moreover, show little reduction in strength, so that they are very suitable for use as a leakproof sheet in disposable diapers. Although such a leakproof sheet is generally used as the outermost layer of a disposable diaper, a material (such as a common punched film or sterically embossed sheet) not impairing its moisture permeability may be super-posed on the outer side of the leakproof sheet toimpart a cloth-like appearance thereto.
In such disposable diapers, there may be used any of common liquid absorbers including fluff composed of pulp fibers, such fluff wrapped in absorbent paper or the like, polymeric absorbers having high water absorbency, and the like.
As the liquid-permeable sheet which will be in direct contact with the skin, there may preferably be used a non-woven fabric composed of polyester fibers, nylon fibers, polyolefin fibers or the like.
In addition, there may be used pressure-sensitive tapes for fastening the diaper, and elastic members (such as rubber members) provided along the side edges to prevent leakage.
Disposable diapers are made by placing a 3LZS~37~

liquid absorber on the aforesaid leakproof sheet, and stacking a liquid-permeable sheet thereon.
In disposable diapers using the porous film of the present invention as the leakproof sheet, the outermost leakproof sheet has a large number of pores. Since these pores allow water vapor to pass therethrough while retaining water droplets, the skin of the infant is not dampened but kept in a dry state, resulting in little tendency to develop diaper rash. Moreover, they also have the advantages of being hardly torn because of their high strength and being soft enough to produce no disagreeable noise.
The following examples illustrate the use of the porous film of the present invention as a leakproof sheet in disposable diapers.

Exam les 20-22 and Comparative Examples 10-14 p Barium sulfate having an average particle diameter of l.S ~m or each of the other fillers given in Table 2 was added to 100 parts by weight of low-density polyethylene (LDPE) having a melt index (~$I) of 3 (Examples 20 and 21) or linear low-density polyethylene (L-LDPE) having a melt index (MI) of 5 (Example 22 and Comparative Examples 10-14) in -the amount given in Table 2, and mixed therewith by means of a ~enschel mixer. Thereafter, using a ~LZ~72~

twin-screw mixer, the resulting mixture was intima-tely blended and formed into pellets. Then, using a T-die extruder, these pellets were melted at 130C and formed into a film. This film was uniaxially stretched between a preheating roll heated to 50C
and a ^qtretching roll by the factor given in Table 2 to obtain a porous film having a thickness of 50 ~m.
Properties of the porous films thus obtained were evaluated and the results are shown in Table 2.
Disposable diapers were made by placing a filling of fluffy pulp and a non-woven polyester fabric on each of the porous films obtained in Examples 20-22 and Compara~ive Examples 10, 13 and 14, and then providing it with pressure-sensitive tapes and rubber members.
The disposable diapers of Examples 20-22 were superior in strength, moisture permeability and softness to those of Comparative Examples 10, 13 and 14, 50 that they produced only a slight rustling noise during use and felt comfortable to the touch.
When these disposable diapers were tested by using them practically in infants, those of Examples 20-22 caused no rash on the skin of the wearer. In contrast, the disposable diapers of Comparative Examples 10, 13 and 14 caused an extensive rash or a slight rash (Comparative Example 13).

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Example 23 Using a biaxial stretching machine heated to 70C, the unstretched film formed in Example 20 was simultaneously stretched in both the machine and the transverse direction (by factors of 2 x 2) to obtain a porous film having a thickness of 50 ~m.
Properties of this porous film were evaluated and the results are shown in Table 2. Disposable diapers using this porous film as the leakproof sheet exhibited as good performance as those of Example 20.

Example 24 The same resin composition as used in Example 22 was formed into a film. Using a roll heated to 50C, this film was uniaxially stretched by a factor of 4 to obtain a porous film having a thickness of 15 ~m. Properties of this porous film were evaluated and the results are shown in Table 2.
On the outer side of this porous film was superposed a LDPE film of 70 ~m thickness having apertures of 1 mm diameter (20/cm2) all over its surface. Disposable diapers using this composite material as the leakproof sheet also exhibited as good performance as those of Examples ~0-23.

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Table 2 _ .
Base resin Filler _ Amount Amount Type (parts by Type (parts by weight) weight) _ Example 20 LDPE 100 BaSO4 150 _ _ . . ___ _ ' 22 L-LDPE ll .. ~00 . ._ . _ . . _ " 23 LDPE .. .. 150 " 24 L~LDPE 200 _ Comparative .. -:~ _ " 11 .. .. .. 6Q0 . __ _ _ " 12 .. .. " __ 150 _ " 13 . CaCO3 _ " 14 beads 120 _ *) An attempt was made to stretch the film by a factor of 8, but no sampling was possible because of frequent breakage during the stretching processO
**) The film could not be stretched by a factor of more than 2.

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Table 2 (Cont'd) _ _ Strength Moisture Stretching ~MD/TD] permeability ~Softness (kg/25 mm) (g/m /24 hr) .. _ .. _ ._ 4 6.0/1.5 . 3,500 , . .. _ 6.5/1.3 4,200 A
35.5/1.3 4,600 _ 2 X 2 4.9/4.5 3,900 A
6.7/0.9 4,700 _ __ Unstretch- 2.3/2~2 15 A
, Unstretch- _ 8*) _ _ 44.3/0.8 3,000 C
**) .__ _ . 1.5/0.7 _ _ ~L~S~ 4 Comparative Exam~e 15 .
120 parts by w~ight of calcium carbonate having an average particle diameter of l.2 ~m and 20 parts by weight of a hydroxylated poly-saturated-hydrocarbon (Liquid Polybutadiene GI-2000; Nippon Soda Co., Ltd.) were added to lO0 parts by weight of linear low-density polyethylene IL-LDPE) having a melt index (MI) of 5, and mixed therewith by means of a Henschel mixer. Thereafter, using a twin-screw mixer, the resulting mixture Wa5 intimatelyblended and formed into ~ellets. Then, using a 40 mm~
inflation extruder, these pellets were formed into a film. This film was roll stretched at 80C by a factor of 3.0 to obtain a porous film having a thick-ness of 50 ~m. This porous film varied in moisturepermeability according to th~ location and exhibited a slight degree of surface tackine~s. Disposable diapers using this porous film as the leakproof sheet caused a slight rash on the skin of infants.

Comparative Example 16 The procedure of Com~arative Example 15 was repeated except that liquid Polybutadiene (Nisso PBG;
Nippon Soda Co., Ltd.) or rubbery EPR (Toughmer (19 P0480; ~itsui Petrochemical Industries, Inc.) was used as the hydroxyla~ed poly-saturated-hydrocarbon.

12~ 4 Thus; there were obtained films having a thickness of 50 ~m. These porous films exhibited surface tackiness and varied in moisture permeability according to the location. Disposable diapers using each of these porous films as the leakproof sheet caused a slight rash on the skin of infants.
Porous films obtained by the process of the present invention have high porosity and excellent softness and, moreover, show little reduction in strength in spite of their small thickness, so that they are very suitable for use as a leakproof sheet in sanitary napkins. Conventional sanitary napkins are so constructed that a liquid absorber such as fluffy pulp, cotton, absorbent resin or the like is partially covered with a film of paper having been rendered liquid-impermeable by treatment with a synthetic resin such as polyethylene or the like and the resulting structure is then wrapped in a non-woven fabric. In sanitary napkins using the porous film of the present invention as the leakproof sheet, this leakproof sheet has a large number of pores which allow water vapor to pass therethough. Accord-ingly, they can keep the skin of the user in a dry state and cause no disagreeable sensation even during prolonged use.
The following examples illustrate the use of ~zs~

the porous film of the present invention as a leakproof sheet in sanitary napkins.

Examples 25-27 -Barium sulfate having an average particle diameter of 0.8 ~m was added to 100 parts by weight of linear low-density polyethylene (L-LDPE) having a melt index (MI) of 2.1 in the amount given in Table 3, and mixed therewith by means of a Henschel mixer.
Thereafter, using a twin-screw mixer, the resulting mixture was intimately blended and formed into pellets. Then, using a T-die extruder, these pellets were melted at 230C and formed into a film. This film was uniaxially stretched between a preheating roll heated ~o 80C and a stretching roll by the factor given in Table 3 to obtain a porous film having a thickness of 20 ~m. Properties of this porous film were evaluated and the results are shown in Table 3. Sànitary napkins were made by covering a filling of fluffy pulp partially with each cf the porous films obtained in Examples 25-27, wrapping the resulting structure in a non-woven fabric and then heat sealing its overlapping portions.
When these sanitary napkins and commercially available ones having a liquid-impermeable film of polyethylene-coated paper were comparatively tested by using them lZS~)~7~2~

practically for prolonged periods of time, the sanitary napkins in accordance with the present invention did not cause a disagreeable, stuffy sensation.

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Claims (8)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. In a process for producing porous films which comprises melting a resin composition consisting essentially of a polyethylene resin and a filler noncompatible therewith, and with or without a hydrocarbon-free lubricant, forming the molten resin composition into a film and then stretching the film at least uniaxially, the improvement in which the polyethylene resin is either a high-pressure-processed low-density polyethylene resin having a melt index of 0.5 to 7 and a density of 0.915 to 0.935, a linear low-density polyethylene resin having a melt index of 0.5 to 8.5 and a density of 0.915 to 0.935, or a mixture thereof; the filler is barium sulfate having an average particle diameter of 0.1 to 7 µm and the barium sulfate is used in an amount of 50 to 500 parts by weight per 100 parts by weight of the polyethylene resin; and the film is stretched by a factor of 1.5 to 7.

2. A process as claimed in claim 1 wherein the high-pressure-processed low-density polyethylene resin is a single resin.

3. A process as claimed in claim 1 wherein the high-pressure-processed low-density polyethylene resins have different densities.

4. A process as claimed in claim 1 wherein the melt index of the high-pressure-processed polyethylene resin is in the range of 1 to 5.

5. A process as claimed in claim 1 wherein the melt index of the linear low-density polyethylene resin is in the range of 0.5 to 7.

6. A process as claimed in claim 1 wherein the average particle diameter of the barium sulfate is in the range of 0.5 to 5 µm.

7. A process as claimed in claim 1 wherein the barium sulfate is used in an amount of 100 to 400 parts by weight per 100 parts by weight of the polyethylene resin.

8. A process as claimed in claim 1 wherein the barium sulfate has been subjected to surface treatment with a fatty acid or a metallic salt thereof, silicone, silane or a resin acid.