High-strength porous film and process for producing the same

The present invention relates to a high-strength porous film or sheet consisting essentially of a high-molecular-weight polyethylene resin having a viscosity-average molecular weight of not less than 300,000, wherein the said film has a thickness of 5 to 50 .mu.m, an air permeability of 200 to 1,000 sec/100 cc, a porosity of 10 to 50% and a pin puncture strength of not less than 600 gf/25 .mu.m, and a process for producing the said film.

BACKGROUND OF THE INVENTION 
The present invention relates to a high-strength porous film useful for a 
variety of commercial articles or parts thereof such as, typically, 
battery separator, separating membranes such as a precision filter 
membrane, clothing articles such as an air-permeable jumper, sanitary 
articles such as a diaper and sanitary napkin, and the like, and a process 
for producing such a film. More particularly, the present invention 
relates to a high-strength porous film having high surface strength and a 
moderate degree of air permeability, and a process for producing such a 
film. 
The porous films (including sheets) have been widely used for a variety of 
purposes, and many method for producing such porous films have been 
proposed. For instance, the porous film used for battery separator is 
produced generally by once forming a film of a resin composition 
containing an ultra-high-molecular-weight polyethylene and a plasticizer 
by melt extrusion molding, and then dissolving away the plasticizer 
contained in the film with an organic solvent such as isopropanol, 
ethanol, hexane or the like. 
A porous film which is useful for a battery separator with high surface 
strength, especially excellent pin puncture strength has been proposed 
(Japanese Patent Application Laid-Open (KOKAI) No. 7-29563). 
Further enhancement of strength of these porous films is desired. 
Specifically, with speed-up of battery manufacture, it is required still 
more to improve the surface strength of the porous film used for battery 
separator. 
However, by the conventional porous film manufacturing techniques, it was 
difficult to satisfy such required film strength, and especially, it was 
impossible to satisfy both requirements for high film surface strength and 
for a moderate degree of air permeability at the same time. For instance, 
in case of a film having high surface strength such as 600 gf/25 .mu.m or 
more of pin puncture strength, the air permeability (sec/cc) of such film 
increases, such as above 3,000 sec/100 cc and there is a problem that the 
breathability of the film which is a basic property of porous films is 
impaired. Reversely, in case of a porous film with high breathability, the 
surface strength of such film is too poor. 
As a results of the present inventors' strenuous studies for obtaining a 
film with high surface strength and a moderate degree of air permeability, 
it has been found that by melt-extruding a resin composition containing a 
high-molecular weight polyethylene resin having a viscosity-average 
molecular weight of not less than 300,000 and a plasticizer into a film, 
cooling the extruded film, removing the plasticizer contained in the 
obtained film and then stretching the resultant film, when the deformation 
ratio of the machine direction (MD)/transverse direction (TD) being 
controlled within the range of 0.1 to 1 throughout the production process, 
the produced film has specifically a high surface strength and a moderate 
degree of air permeability. The present invention has been attained on the 
basis of this finding. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide a high-strength porous 
film or sheet having high surface strength, for example, a pin puncture 
strength of not less than 600 gf/25 .mu.m and a moderate degree of air 
permeability, for example, an air permeability of 200 to 1000 sec/100 cc, 
and a process for producing such a high-degree porous film or sheet. 
To accomplish the aims, in a first aspect of the present invention, there 
is provided a high-strength porous film or sheet comprising a 
high-molecular-weight polyethylene resin having a viscosity-average 
molecular weight of not less than 300,000, and having a thickness of 5-50 
.mu.m, an air permeability of 200-1,000 sec/100 cc, a porosity of 10-50% 
and a pin puncture strength of not less than 600 gf/25 .mu.m of the film 
thickness. 
In a second aspect of the present invention, there is provided a process 
for producing a high-strength porous film or sheet comprising 
melt-extruding and molding a resin composition containing a 
high-molecular-weight polyethylene having a viscosity-average molecular 
weight of not less than 300,000 and a plasticizer, cooling the extruded 
film or sheet, removing the plasticizer contained in the said film or 
sheet, and stretching the said resultant film or sheet, wherein the said 
extrusion molding and stretching are carried out in such a manner that the 
machine direction (MD)/transverse direction (TD) overall deformation ratio 
represented by the following formula is 0.1 to 1: 
EQU MD/TD overall deformation ratio=(DR 
.times..lambda.MD)/(WD.times..lambda.TD) 
wherein 
DR: draft ratio in extrusion molding; 
.lambda.MD: stretch ratio in the machine direction (which ratio is supposed 
to be 1 when the film or sheet is not stretched in the machine direction); 
WD: width-direction deformation ratio in extrusion molding (blow ratio in 
the case of inflation molding); 
.lambda.TD: stretch ratio in the transverse direction (which ratio is 
supposed to be 1 when the film or sheet is not stretched in the transverse 
direction). 
DETAILED DESCRIPTION OF THE INVENTION 
The present invention is described in detail below. The term "film" used in 
the present invention, means both film and sheet. 
The high-molecular-weight polyethylene resin constituting the film 
according to the present invention has a viscosity-average molecular 
weight of not less than 300,000, preferably 500,000 to 4,000,000, more 
preferably 1,500,000 to 3,000,000. As such high-molecular weight 
polyethylene, there is preferably used an ethylene homopolymer having a 
melting temperature of 110.degree.-140.degree. C. because of readiness for 
melt deformation. 
In the present invention, if necessary, it is possible to blend 
polybutene-1, polypropylene or a polyethylene having a viscosity-average 
molecular weight of less than 300,000 in an amount of 100 parts by weight 
or less, preferably 2-80 parts by weight base on 100 parts by weight of 
the high-molecular weight polyethylene having a viscosity average 
molecular weight of not less than 300,000 used in the present invention. 
Examples of the polyethylenes having a viscosity-average molecular weight 
of less than 300,000, which are usable in the present invention, include 
linear or branched low-density polyethylenes (5,000-100,000 in viscosity 
average molecular weight), high-density polyethylenes (from 10,000 to not 
more than 300,000 in viscosity average molecular weight), and polyethylene 
waxes (1,000-5,000 in viscosity average molecular weight). Polybutene-1 
and polypropylene usable in the present invention are those having a 
viscosity average molecular weight not more than 4,000,000. 
When polybutene-1, polypropylene or a polyethylene having a 
viscosity-average molecular weight of less than 300,000 is used in an 
amount exceeding 100 parts by weight based on 100 parts by weight of the 
high-molecular weight polyethylene having a viscosity average molecular 
weight of not less than 300,000 used in the present invention, the object 
of the present invention may not be attained. 
According to the present invention, a mixture of the high-molecular weight 
polyethylene and a plasticizer is melt-extruded and molded into a film, 
and after removing the plasticizer, the obtained film is stretched to 
obtain a porous film. 
The plasticizer used in the present invention has a good compatibility with 
the said high-molecular-weight polyethylene, a melting point lower than 
the melt starting temperature of the said polyethylene and a boiling point 
higher than the melt starting temperature of the said polyethylene, and is 
soluble in the polyethylene-insoluble organic solvents. Examples of such 
plasticizers include higher aliphatic alcohols such as stearyl alcohol and 
ceryl alcohol, n-alkanes such as n-decane and n-dodecane, paraffin waxes, 
liquid paraffins, and kerosine. 
The weight percentage of the high-molecular weight polyethylene and the 
plasticizer used as mixture is variable depending on the desired porosity 
of the molded product. Usually high-molecular weight polyethylene is used 
in 5-60 parts by weight, preferably 10-50 parts by weight based on 100 
parts by weight of the resin composition thereof, and the plasticizer in 
95-40 parts by weight, preferably 90-50 parts by weight, based on 100 
parts by weight of the resin composition thereof. 
In the present invention, various known additives such as antioxidant may 
be added to the starting composition comprising the said polyethylene and 
plasticizer in the said ratios and also containing, if necessary, 
polybutene-1, etc., as mentioned above. These additives are added in an 
amount of not more than 5% by weight, preferably 0.01-5% by weight, based 
on the weight of the resin composition. 
The said starting composition is kneaded and melt extrusion molded by a 
single-screw- or twin-screw extruder. A twin-screw extruder is preferred 
in view of extrusion rate, extrusion stability and kneading power. 
The extrusion molding is usually carried out at 140.degree.-240.degree. C. 
by a known molding method such as T-die molding, inflation molding, etc., 
to form a film having a thickness of usually 5-500 .mu.m, preferably 
10-300 .mu.m. 
This film is cooled and then the plasticizer is removed from the film to 
make the film porous. 
As regards the removal of the plasticizer, there can be used, for instance, 
a known "organic solvent method" in which the plasticizer in the film is 
dissolved with an organic solvent such as isopropanol, ethanol, hexane or 
the like, and extracted away by means of solvent substitution. 
The film which has been made porous by removal of the plasticizer in the 
manner described above is suggested to uniaxial or biaxial stretching for 
the purpose of enhancing its mechanical strength. The stretching can be 
carried out by using any suitable known stretching means such as roll 
stretcher, tenter, etc. When the film is subjected to uniaxial stretching, 
it may be stretched either in the machine direction or in the transverse 
direction. Biaxial stretching may be carried out either by sequential 
biaxial stretching or by simultaneous biaxial stretching. 
In the said extrusion molding and stretching steps in the above-described 
production process, mechanical deformations in both machine direction (MD) 
and transverse direction (TD) are imparted to the said film. The 
mechanical deformations of the film are imparted as the form of melt 
deformation at the time of extrusion molding and as the form of solid 
phase stretch at the time of stretching after plasticizer removal. In the 
process of the present invention, in imparting these mechanical 
deformations, the extrusion molding and stretching conditions are 
controlled by setting the MD/TD overall deformation ratio represented by 
the following formula to the range of 0.1 to 1:1, preferably 0.2 to 0.8:1. 
EQU MD/TD overall deformation ratio=(DR.times..lambda.MD)/(WD.times..lambda.TD) 
wherein 
DR: draft ratio in extrusion molding; 
.lambda.MD: stretch ratio in the machine direction (which ratio is supposed 
to be 1 when the film or sheet is not stretched in the machine direction); 
WD: width-direction deformation ratio in extrusion molding (blow ratio in 
the case of inflation molding); 
.lambda.TD: stretch ratio in the transverse direction (which ratio is 
supposed to be 1 when the film or sheet is not stretched in the transverse 
direction). 
When the MD/TD overall deformation ratio is less than 0.1:1 or more than 
1:1, the air permeability of the produced film deviates from its proper 
range, and this film may not satisfy the standard performance for a porous 
film, particularly for a battery separator. 
In the present invention, the draft ratio in extrusion molding is 
preferably selected from the range of 1-100, the stretch ratio in the 
machine direction is preferably selected from the range of 1-20, the 
width-direction deformation ratio in extrusion molding is preferably 
selected from the range of 1-20, and the stretch ratio in the transverse 
direction is preferably selected from the range of 1-20, and it is 
essential that the MD/TD overall deformation ratio calculated from the 
above formula is within the above-defined range. 
The porous film of the present invention produced in the above described 
manner has a thickness of 5 to 50 .mu.m, preferably 15 to 35 .mu.m. The 
air permeability of the film as measured according to JIS P-8117 is 200 to 
1,000 sec/100 cc, preferably 300 to 1000 sec/100 cc, more preferably 400 
to 900 sec/100 cc. In case of use of the porous film of the present 
invention for a battery separator, if the air permeability of the film is 
less than 200 sec/100 cc, the resistance becomes low and the desired 
performance of the battery may not be attained. On the other hand, if the 
air permeability of the film more than 1,000 sec/100 cc, the resistance 
becomes high and also the desired performance of the battery may not be 
obtained. The porosity of the porous film of the present invention is 10 
to 50%, preferably 30 to 45%. For determining the porosity, the film was 
punched out circularly at 5 parts along the width thereof to obtain the 3 
cm-diameter circular pieces of film, and the thickness of the central 
portions of the punched out pieces of film and the weight thereof were 
measured. The porosity was calculated from the following equation: 
EQU Porosity (%)=(V.rho.-W)/(V.rho.).times.100 
wherein 
V: volume of the film (sum of the 5 pieces of film) 
W: weight (sum of the 5 pieces of film) 
.rho.: material density. 
If the porosity of the porous film of the present invention is less than 
10%, the film may not have a desired level of air permeability, and if the 
porosity is more than 50%, the surface strength of the film may be 
deteriorated. 
The porous film according to the present invention has excellent mechanical 
strength, and is especially high in pin puncture strength. The pin 
puncture strength was measured according to Japanese Agricultural Standard 
(AS) Notification No. 1,019 (pin diameter: 1 mm; tip: 0.5 R; pin 
puncturing speed: 300 mm/min. The pin puncture strength of the film of the 
present invention as determined in the above-described manner is not less 
than 600 gf/25 .mu.m of film thickness, preferably 600 to 1,000 gf/25 
.mu.m of film thickness, more preferably 600 to 800 gf/25 .mu.m of film 
thickness. If the pin puncture strength of the film is less than 600 gf/25 
.mu.m of film thickness, the mechanical strength of film may be low. 
As described above, the high-strength porous film according to the present 
invention has a high surface strength and a moderate degree of air 
permeability that the conventional films of this type could not have and 
that could never be realized with the conventional technologies. Thus, the 
present invention makes it possible to provide a porous film having 
excellent surface strength while maintaining other qualities at high 
levels as well, and when, for instance, the film is practically applied as 
a battery separator, the battery can be assembled at a higher speed in 
comparison with use of the conventional films. 
Such a high-strength porous film can be easily and efficiently produced by 
the process of the present invention in which most characteristically the 
MD/TD overall deformation ratio of the film is kept within a specified 
range throughout the production process.

EXAMPLES 
The present invention is explained in more detail in the following Examples 
and Comparative Examples, but it should be recognized that the scope of 
the present invention is not restricted to these Examples. 
In the following Examples and Comparative Examples, the air permeability of 
the films was determined according to JIS P8117. The porosity was 
calculated from the equation: porosity (%)=(V.rho.-W)/(V.rho.).times.100 
as explained above. The pin puncture strength was measured according to 
Japanese Agricultural Standard (JAS) Notification No. 1,019. 
Example 1 
A mixture comprising 25 parts by weight of an ultra-high-molecular-weight 
polyethylene having a melting point of 135.degree. C. and a 
viscosity-average molecular weight of 2,000,000 and 75 parts by weight of 
stearyl alcohol was extruded by a 40 mm .phi. twin-screw extruder at an 
extrusion temperature of 170.degree. C. and an extrusion rate of 10 kg/hr 
and molded into a film by inflation method. In this molding operation, the 
draft ratio (DR) was 12 and the blow ratio (WD) was 9. 
The obtained film was immersed in 60.degree. C. ethanol for 10 minutes to 
extract away stearyl alcohol. The film was stretched twice the original 
length in the machine direction (MD) at 120.degree. C. by a roll stretcher 
and then further stretched 4 times in the transverse direction (TD) at 
128.degree. C. by a tentering machine to obtain a porous film with a 
thickness of 25 .mu.m. The MD/TD overall deformation ratio in this 
production process is shown in Table 1. 
The pin puncture strength, the air permeability and the porosity of the 
molded product measured by the manners described above are shown in Table 
1. 
Example 2 
A mixture comprising 20 parts by weight of an ultra-high-molecular-weight 
polyethylene having a melting point of 135.degree. C. and a 
viscosity-average molecular weight of 2,000,000 and 80 parts by weight of 
a paraffin wax (average molecular weight=389) was extruded at 170.degree. 
C. and at a rate of 10 kg/hr by a 40 mm .phi. twin-screw extruder and 
inflation molded into a film by the inflation method. In this molding 
operation, DR=12, WD=6. 
The obtained film was immersed in 60.degree. C. hexane for 10 minutes to 
extract away the paraffin wax. Then the film was stretched 7 times in the 
transverse direction by a tentering machine to obtain a porous film with a 
thickness of 26 .mu.m. The MD/TD overall deformation ratio and the 
properties of the obtained porous film are shown in Table 1. 
Example 3 
A mixture comprising 25 parts by weight of an ultra-high-molecular-weight 
polyethylene having a melting point of 135.degree. C. and a 
viscosity-average molecular weight of 2,000,000 and 75 parts by weight of 
stearyl alcohol was extruded at 170.degree. C. and at a rate of 10 kg/hr 
by a 40 mm .PHI. twin-screw extruder and molded into a film by T-die 
method. In this molding operation, DR=5 and the deformation rate in the 
width direction (WD) was 1. 
The obtained film was immersed in 60.degree. C. ethanol for 10 minutes to 
extract away steary alcohol. Then the film was stretched 6 times in the 
transverse direction at 128.degree. C. by a tentering machine to obtain a 
porous film with a thickness of 25 .mu.m. The MD/TD overall deformation 
ratio and the properties of the obtained porous film are shown in Table 1. 
Comparative Example 1 
The procedure of Example 1 was carried out except that the film was 
stretched 3 times in the machine direction by a roll stretcher and further 
stretched 3 times in the transverse direction by a tentering machine to 
obtain a porous film. The MD/TD overall deformation ratio and the 
properties of the produced film are shown in Table 1. 
Comparative Example 2 
The procedure of Example 1 was carried out except that a mixture comprising 
20 parts by weight of an ultra-high-molecular-weight polyethylene and 80 
parts by weight of stearyl alcohol was extruded and molded by inflation 
method at a draft ratio of 20 and a blow ratio of 4. After removal of 
stearyl alcohol by extraction, the molded film was stretched 5 times in 
the machine direction at 125.degree. C. by a roll stretcher but not 
stretched in the transverse direction to obtain a porous film with a 
thickness of 27 .mu.m. The MD/TD overall deformation ratio and the 
properties of the obtained porous film are as shown in Table 1. 
TABLE 1 
______________________________________ 
Exam- Exam- Exam- Comp. Comp. 
ple ple ple Example 
Example 
1 2 3 1 2 
______________________________________ 
Draft ratio in 
12 12 5 12 20 
extrusion molding 
Width-direction 
9 6 1 9 4 
deformation ratio in 
extrusion molding 
(Note 1) 
Stretch ratio in 
2 1 1 3 5 
machine direction 
(Note 2) 
Stretch ratio in 
4 7 6 3 1 
transverse direction 
(Note 2) 
MD/TD overall 
0.7 0.3 0.8 1.3 25 
deformation ratio 
Film thickness (.mu.m) 
25 26 25 25 27 
Pin puncture 
700 650 620 400 320 
strength (g/25 .mu.m) 
Air permeability 
450 780 500 120 3500 
(s/100 cc) 
Porosity (%) 
45 42 38 65 35 
______________________________________ 
Note 1: In the case of inflation molding, the widthdirection deformation 
ratio corresponds to blow ratio. 
Note 2: The MD/TD deformation ratio is calculated by supposing that the 
stretch ratio in the direction where actually no stretching was made is 1 
 
As seen from Table 1, it is possible in accordance with the present 
invention to produce a porous film having a high surface strength as well 
as a moderate degree of air permeability.