Process for producing a porous film

A process for producing a porous film having practically sufficient mechanical strengths, good flexibility, uniform fine pores and high moisture vapor permeability, and further an extremely thin porous film, which process comprises blending 30 to 80 parts by weight of an inorganic fine powder having a specific surface area of 15 m.sup.2 /g or less and an average particle size of 0.4 to 4 .mu.m with 20 to 70 parts by weight of a polyolefin resin, followed by melt-molding the resulting blend into a film and then stretching the film to 2 to 7 times the original length at least in the uniaxial direction.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a process for producing a porous film having 
flexibility and a structure of uniform fine pores and hence superior water 
vapor permeability and water resistance and useful for waterproof 
clothings, waterproof covers, packaging materials, etc. 
2. Description of the Related Art 
There has so far been known a process for producing a porous film by 
blending non-compatible substances such as inorganic fine powders with a 
polyolefin resin in a specified proportion, followed by melt-molding the 
resulting blend into a film or sheet and then uniaxially or biaxially 
stretching the film or sheet. 
However, such a process has had a drawback that since the resin is 
stretched and oriented by stretch processing, the film or sheet increases 
in the hardness so that its flexibility is damaged. 
In order to overcome such a problem, the following processes for producing 
a porous film have been proposed: 
(1) a process of blending a thermoplastic elastomer with a polyolefin resin 
and a filler (Japanese patent application laid-open No. Sho 
59-30833/1984); 
(2) a process of blending a liquid or waxy hydrocarbon polymer with a 
polyolefin resin and a filler (U.S. Pat. No. 4,472,328); and 
(3) a process of blending barium sulfate as an inorganic fine powder with a 
polyolefin resin (G.B. No. 2,151,538). 
However, porous films obtained according to these production processes have 
the following practical drawbacks: 
According to the process (1), the resulting porosity is insufficient and 
the water vapor permeability is inferior. According to the process (2), 
the resulting porous film has a problem that in the high temperature 
atmosphere or after a long time lapse, hydrocarbon polymers bleed out on 
the surface of the resulting film so that the surface is sticky. According 
to the process (3), the resulting porous film has a good flexibility and 
sufficient water vapor permeability, but the stretching stability i.e. the 
high stretchability is inferior. Further, as a problem in common to these 
processes, it is impossible to produce an extremely thin film of about 
20.mu.. Furthermore, another process of blending a third component brings 
about a large increase in cost. 
SUMMARY OF THE INVENTION 
The object of the present invention is to provide a process for producing a 
porous film which comprises a polyolefin resin having a superior solvent 
resistance, and has practically sufficient mechanical strengths and a good 
flexibility and also has uniform fine pores and high moisture vapor 
permeability, and further an extremely thin porous film. 
The present invention resides in a process for producing a porous film 
which comprises blending 30 to 80 parts by weight of an inorganic fine 
powder having a specific surface area of 15 m.sup.2 /g or less and an 
average particle size of 0.4 to 4 .mu.m with 20 to 70 parts by weight of a 
polyolefin resin, these parts by weight being based on 100 parts by weight 
of the blend, followed by melt-molding the resulting blend into a film and 
then stretching the film to 2 to 7 times the original length at least in 
the uniaxial direction. 
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Examples of the polyolefin resin used in the present invention are 
polypropylene, low density polyethylene, linear low density polyethylene, 
high density polyethylene and mixtures thereof. Among these, linear low 
density polyethylene and blends containing linear low density polyethylene 
are particularly preferred. Examples of the comonomer component for linear 
low density polyethylene are generally butene, 4-methylpentene, hexene, 
octene, decene, etc. and among these, 4-methylpentene and octene are 
preferred in the aspect of moldability and physical properties of 
products. 
The inorganic fine powder used in the present invention is fine particles 
having a specific surface area of 15 m.sup.2 /g or less and an average 
particle size of 0.4 to 4 .mu.m. Examples of the inorganic fine powder 
used in the present invention are calcium carbonate, magnesium carbonate, 
magnesium oxide, barium sulfate, silica, aluminum hydroxide, alumina, etc. 
Among these, precipitated calcium carbonate and precipitated barium 
sulfate are preferred since such materials have a uniform particle 
diameter and specific surface area and when they are blended with the 
resin component and the resulting blend is molded, they have superior 
dispersibility and the blend has superior processability. Further, the 
inorganic fine powder is preferred to have a spherical shape. Precipitated 
barium sulfate is usually obtained for example by the reaction of barium 
sulfide with an aqueous solution of sodium sulfate or the reaction of 
barium sulfide with sulfuric acid. The shape includes amorphous shape, 
spindle shape, plate shape, diamond shape, spherical shape, etc. When the 
concentrations of barium sulfide and SO.sub.4.sup.2-, the mechanical 
conditions of the reaction and the reaction temperature are set under 
specified conditions, precipitated barium sulfate having a desired average 
particle size is obtained. 
Further, the precipitated calcium carbonate is usually formed by 
introducing carbon dioxide into an aqueous suspension of calcium 
hydroxide. It has a cubic shape having an average particle size of 1.mu. 
or less or a spindle shape or an acicular shape having an average particle 
size of 1.mu. or more. When the reaction temperature of the aqueous 
suspension of calcium hydroxide, addition of a particular salt and the 
termination time of the reaction are set under specified conditions, a 
spherical precipitated calcium carbonate having a desired average particle 
size is obtained. 
The specific surface area of the inorganic fine powder is 15 m.sup.2 /g or 
less and a range of 0.5 to 5 m.sup.2 /g is preferred. If the specific 
surface area exceeds 15 m.sup.2 /g, the shape of the inorganic fine powder 
becomes amorphous shape, plate shape or acicular shape so that the 
particle size distribution becomes broad and the stretchability of film 
becomes lowered; hence no good porosity is obtained. Further, the porosity 
of the surface of the fine powder increases and due to the volatile 
components such as moisture attached to the pores, foaming is observed at 
the time of melt-molding the pore size of the resulting film is increased 
and its water-resistant is remarkably reduced. 
The average particle size of the inorganic fine powder is preferably in the 
range of 0.4 to 4 .mu.m, more preferably in the range of 0.6 to 2 .mu.m. 
If the average particle size exceeds 4 .mu.m, the stretchability of the 
film is inferior and stretching breakage occurs prior to uniform 
stretching. Thus, the production stability is inferior to make uniform 
pore-formation impossible. 
On the other hand, if the average particle size is less than 0.4 .mu.m, the 
mineral fine powder cannot be highly filled to be unable to make the 
resulting film porous. 
In the resin composition of the present invention, the quantity of the 
inorganic fine powder is in the range of 30 to 80 parts by weight, 
preferably 30 to 70 parts by weight based on 100 parts by weight of the 
blend. With the quantity of less than 30 parts by weight, no good porosity 
can be obtained in respect of the stretched film. If the quantity exceeds 
80 parts by weight, kneadability, dispersibility and stretchability are 
inferior; thus naturally the water vapor permeability is also inferior and 
the flexibility lowers. 
Next, the process for producing the porous film of the present invention 
will be described. First, an inorganic fine powder is mixed with a 
polyolefin resin in specified proportions. The mixing process has no 
particular limitation. In general, the materials are mixed by means of a 
blender or the like, followed by blending the mixture by means of a 
Banbury mixer or another melt-kneader in advance, thereafter pelletizing 
the blend or not pelletizing it and then sheeting by means of a 
conventional sheet-molding machine. It is possible to optionally add 
various additives such as lubricant e.g. calcium stearate, pigment, 
stabilizers such as those against heat, light and others, plasticizers, 
antistatic agent, etc. 
The film is generally made by calendering, casting or extrusion, but among 
these, extrusion using a circular die or a T-die is preferred. The 
extruded sheet is then at least in the uniaxial direction stretched in a 
stretching ratio of 2 to 7 times at the softening temperature or lower of 
the polyolefin resin according to a known process. Among the above range 
of the stretching ratio, 4 to 6 times are preferred. If the stretching 
ratio is less than 2 times, it is difficult to obtain a good water vapour 
permeability, while if it exceeds 7 times, stretching breakage occurs to 
make stabilized production impossible. 
The present invention will be described in more detail by way of Examples. 
Physical properties of the film were evaluated according to the following 
methods: 
Specific surface area (m.sup.2 /g): measured according to BET adsorption 
method. 
Average particle size (.mu.m): measured by means of an instrument for 
measuring the powder surface area (manufactured by Shimazu Seisakusyo), by 
filling its sample (3 g) in a sample cylinder of 2 cm.sup.2 .times.1 cm 
and measuring the time of air permeation (5 cc) under 50 mm water 
pressure. 
Tensile strength at break (kg): according to JIS P-8113 using samples of 25 
mm wide.times.100 mm long; grip separation rate 200 mm/min., the tensile 
strength at break was measured MD (machine direction) and in TD (traverse 
direction to MD), respectively. 
Water vapor permeability: measured according to ASTM-E-96-66. 
Softness: Evaluation was made by hand touch as follows: 
A: very soft 
B: somewhat soft 
C: considerably hard

EXAMPLES 1.about.9 
Inorganic fine powders [precipitated barium sulfate (Examples 1.about.4 and 
8.about.9), precipitated calcium carbonate (Examples 5 and 6) or magnesium 
oxide (Example 7)] having a specific surface area and an average particle 
size indicated in Table 1 were added to a linear low density polyethylene 
of MI=2 (L-LDPE (Examples 1.about.7)), a low density polyethylene of MI=5 
(LDPE (Example 8)) or a polypropylene of MI=1.5 (PP (Example 9)) in 
quantities indicated in Table 1, followed by blending the mixture by means 
of Henschel mixer (tradename), pelletizing the blend, making the pellets 
into a film by extrusion, and then uniaxially roll-stretching the film to 
2 to 7 times the original length at 50.degree. C. to obtain a porous film 
of 20 .mu.m thick. The physical properties of the film were then measured. 
The results are shown in Table 1. 
EXAMPLE 10 
A film made from the same composition as in Example 2 was stretched 
(3.times.3) times in the longitudinal direction simultaneously with the 
traverse direction at 70.degree. C. by means of a biaxially stretching 
machine to obtain a porous film of 20 .mu.m thick. The evaluation results 
of its physical properties are shown in Table 1. 
EXAMPLE 11 
Twenty % by weight of a linear low polyethylene (L-LDPE) of MI=2, 20% by 
weight of a low density polyethylene (LDPE) of MI=5 and 60% by weight of 
precipitated barium sulfate having a specific surface area of 4.1 m.sup.2 
/g and an average particle size of 0.8 .mu.m were blended to obtain a 
porous film in the same manner as in Example 1. The evaluation results of 
its physical properties are shown in Table 1. 
COMATIVE EXAMPLES 1.about.6 
Porous films were prepared in the same manner as in Example 1 except that 
an inorganic fine powders [precipitated barium sulfate (Comparative 
Examples 1.about.3 and 5.about.6) or calcium carbonate (Comparative 
Example 4)] under varied kinds, filled quantities and stretching 
conditions as indicated in Table 1 was blended with a L-LDPE in varied 
quantities. The evaluation results of their physical properties are shown 
in Table 1. In Comparative Example 1, since the quantity of fine powders 
added is less than 30%, the porosity lowers and the water vapor 
permeability is small. In Comparative Example 2, since the quantity of 
fine powders added exceeds 80%, the stretchability lowers and stretching 
breakage occurs in a stretching ratio of 1.5 times. In Comparative 
Examples 3 and 4, since the specific surface area of the fine powder 
exceeds 15 m.sup.2 /g, the stretchability lowered and foaming was observed 
at the time of extrusion by means of a sheet molding machine, and split. 
In Comparative Example 5, since the average particle size of fine powders 
exceeds 4 .mu.m, the stretchability lowered so that it is impossible to 
effect a stabilized production in a stretching ratio of 2.0. In 
Comparative Example 6, since the stretching ratio is less than twice, no 
sufficient water vapor permeability can be obtained. 
Since the film of the present invention is sufficiently porous, the water 
vapor permeability and air permeability are good and also the water 
resistance is superior. Particularly since its flexibility is good to 
afford a soft hand, it is suitable for clothings, particularly for 
sanitary use application. As compared with prior art, it is possible to 
produce even an extremely thin film of 20 .mu.m thickness or less; thus 
the resulting porous film scarcely has use application where it is used as 
a single product, but its main use application is directed to its 
lamination onto non-woven fabric, pulp, nylon taffeta, etc. The thinner 
the film, the less the cost, and further, a specific feature is exhibited 
that at the time of clothing, the fitting feeling due to the thickness of 
the laminate is not uncomfortable. 
TABLE 1 
__________________________________________________________________________ 
Inorganic fine powders 
Basic resin Specific Average Tensile strength 
Water 
Amount surface 
particle 
Amount 
Stretch 
at break 
vapor 
added area size added 
ratio 
(kg) permeability 
Kind (wt %) 
Kind (m.sup.2 /g) 
(.mu.m) 
(wt %) 
(times) 
MD TD (g/m.sup.2 24 
Flexibility 
__________________________________________________________________________ 
Example 
1 L-LDPE 
30 Precipitated 
4.1 0.8 70 2 3.0 1.0 2000 A 
BaSO.sub.4 
2 " 40 Precipitated 
" " 60 5 4.0 0.5 3500 A 
BaSO.sub.4 
3 " 65 Precipitated 
" " 35 7 5.5 0.3 2500 A 
BaSO.sub.4 
4 " 50 Precipitated 
8.0 0.5 50 6 5.0 0.4 3200 A 
BaSO.sub.4 
5 " 40 Precipitated 
14.0 0.5 60 4 5.8 0.4 2200 B 
CaCO.sub.3 
6 " 60 Precipitated 
5.5 3.0 40 5 4.2 0.5 2300 B 
CaCO.sub.3 
7 " 50 MgO 7 1.1 50 5 4.3 0.4 2700 B 
8 LDPE 40 Precipitated 
4.1 0.8 60 4 3.0 0.3 1800 B 
BaSO.sub.4 
9 PP 30 Precipitated 
4.1 0.8 70 6 6.0 0.3 3200 B 
BaSO.sub.4 
10 L-LDPE 
40 Precipitated 
4.1 0.8 60 3 .times. 3 
4.5 3.8 7000 A 
BaSO.sub.4 
11 L-LDPE 
40 Precipitated 
4.1 0.8 60 6 4.5 0.3 3300 A 
LDPE BaSO.sub.4 
Compara- 
tive ex. 1 
L-LDPE 
80 Precipitated 
4.1 0.8 20 8 6.5 0.1 500 C 
BaSO.sub.4 
2 " 10 Precipitated 
4.1 0.8 90 
BaSO.sub.4 
3 " 50 Precipitated 
18.3 0.3 50 
BaSO.sub.4 
4 " 70 CaCO.sub.3 
16.5 1.0 30 
5 " 60 Precipitated 
0.8 4.5 40 2 2.8 0.8 1500 B 
BaSO.sub.4 
6 " 40 Precipitated 
4.1 0.8 60 1.5 2.5 1.5 100 C 
BaSO.sub.4 
__________________________________________________________________________