JPH11106533A - Polyolefin porous membrane - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a polyolefin porous membrane having a high strength and capable of being produced without adding a large amount of solvents to a resin. SOLUTION: A polyolefin composition is prepared, which comprises not less than 1 wt.% ultra-high-molecular-weight polyolefin having a weight average molecular weight of not less than 1×10<6> and not less than 20 wt.%, polyolefin wax having a weight average molecular weight of 700-10,000. 50-90 pts.wt. polyolefin composition is dissolved by heating in 50-10 pts.wt. solvent to prepare a polyolefin solution. The polyolefin solution is extruded and cooled to obtain a sheet-like molding material. The sheet-like molding material is biaxially stretched and then a solvent remaining in the stretched sheet-like molding material is removed by extraction, whereby a polyolefin porous membrane is obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a polyolefin porous membrane containing an ultrahigh molecular weight polyolefin.

[0002]

2. Description of the Related Art Porous plastic membranes such as polyolefins are used for various applications such as separators for electronic components, various filters, moisture-permeable waterproof clothing, reverse osmosis filtration membranes, ultrafiltration membranes, and microfiltration membranes. .

[0003] Conventionally, as a method for producing a polyolefin porous membrane, for example, a mixed extraction method in which a pore-forming agent composed of fine powder of a different polymer or the like is mixed and dispersed in a polyolefin and then the pore-forming agent is extracted, A phase separation method of forming a porous structure by microphase-separating a polyolefin with a solvent, and applying a strain such as stretching to a polyolefin molded body in which a heterogeneous solid is micro-dispersed, thereby breaking the interface between the heterogeneous solids to form pores. Stretching methods for forming and making porous are known. However, conventionally, a polyolefin having a molecular weight of less than about 500,000 is usually used, and thus there is a limit to a reduction in thickness and an increase in strength by stretching.

Therefore, in recent years, ultrahigh molecular weight polyolefins that can be formed into high-strength and high-elasticity films have been developed, and various methods for producing high-strength porous membranes using these have been proposed.

For example, JP-A-58-5228 discloses that an ultrahigh molecular weight polyolefin is dissolved in a non-volatile solvent, a gel such as a fiber or a film is formed from this solution, and the solvent in the gel is converted into a volatile solvent. After extraction and removal with
A method of heating and stretching is disclosed. Also, JP-A-63
No. 2,736,651 discloses that the weight average molecular weight is 5 × 10
A solution of ⁵ or more ultrahigh molecular weight polyolefin is prepared, and the solution is extruded from a die while rapidly cooling the solution to a gelling temperature or less to form a gel-like molded article, and the content of the ultra-high molecular weight polyolefin in the gel-like molded article is reduced. After adjusting to 10 to 90% by weight, the melting point of the ultrahigh molecular weight polyolefin was increased by 10%.
A method is disclosed in which the film is stretched at a temperature of not more than ° C., and thereafter, the residual solvent is removed. In addition, Japanese Unexamined Patent Publication
Japanese Patent No. 64334 discloses a method for producing a polyolefin porous membrane, which contains an ultrahigh molecular weight polyolefin and has a value of (weight average molecular weight / number average molecular weight) in a specific range.

[0006]

However, in the method described in JP-A-58-5228, gels having porous fibers highly swollen with a non-volatile solvent are difficult to stretch with high orientation, Further, since the network structure is enlarged, it is easily broken. Therefore, the resulting film has a drawback that the strength is small and the formed pore diameter is large. On the other hand, in the gel dried after extracting the non-volatile solvent with the volatile solvent, the network structure shrinks and densifies, but the non-uniform evaporation of the volatile solvent easily causes warpage of the raw film, and the gel shrinks. There is a drawback that stretching at a high magnification becomes difficult due to densification.

In the method described in Japanese Patent Application Laid-Open No. 63-273651, it is necessary to prepare a dilute solution of a high molecular weight polyolefin in order to obtain sufficient moldability and stretchability. Is difficult to maintain its shape because it contains an excessive amount of solvent, and a desired porous film cannot be obtained even if it is stretched as it is. Therefore, it is necessary to adjust the amount of the solvent in the sheet by desolvation treatment, and there is a problem in productivity.

In the method described in Japanese Patent Application Laid-Open No. 3-64334, it is possible to increase the concentration of the raw material solution to some extent and to improve the stretchability. Since the thickness is low, there is a problem that the thickness easily changes during the manufacturing process. In addition, although the raw material solution has a high concentration, the gel-like molded body still contains 50% by weight or more of a solvent, and it is hard to say that extracting it in a subsequent step is economically preferable.

Accordingly, an object of the present invention is to provide a polyolefin porous membrane which has high strength and can be produced without adding a large amount of solvent to the resin.

[0010]

In order to achieve the above object, the polyolefin porous membrane of the present invention comprises an ultra-high molecular weight polyolefin having a weight average molecular weight of 1 × 10 ⁶ or more.
% By weight and a weight average molecular weight of 700 to 10
It is characterized by being obtained by stretching a molded article mainly comprising a polyolefin composition containing 20% by weight or more of 2,000 polyolefin waxes. By adopting such a configuration, it is possible to impart the high strength and high elasticity characteristic of the high molecular weight polyolefin to the porous membrane, and
Since the polyolefin composition as the raw material resin has sufficient moldability, there is an advantage that the resin can be produced without adding a large amount of solvent to the resin.

In the polyolefin porous membrane, the molded article preferably has a tensile modulus of 100 kg / cm ² or more. According to this preferred example, a change in the film thickness of the molded body during the manufacturing and transporting steps can be reliably reduced, so that a porous film having a uniform film thickness can be obtained.

The polyolefin porous membrane has a thickness of 1 to 50 μm, a porosity of 10 to 90%,
Air permeability expressed by Gurley number is 10 to 10000 seconds / 1
100 cc and a tensile breaking strength of 100 kg / c
It is preferably at least m ² . Such a polyolefin porous membrane is suitable for use as a separator for batteries or electrolytic capacitors, various filters, materials for moisture-permeable waterproof clothing, and the like.

The polyolefin porous membrane of the present invention comprises, for example, 1% by weight or more of an ultrahigh molecular weight polyolefin having a weight average molecular weight of 1 × 10 ⁶ or more and a weight average molecular weight of 700 to
A solution obtained by dissolving 50 to 90 parts by weight of a polyolefin composition containing 10000 polyolefin waxes in an amount of 20% by weight or more in 50 to 10 parts by weight of a solvent is extruded, and the obtained molded body is stretched and then left on the molded body. It can be produced by removing the solvent. here,
“Removing the solvent remaining in the molded article” means that the solvent contained in the molded article is less than 1% by weight.

The polyolefin porous membrane of the present invention comprises 1% by weight or more of an ultrahigh molecular weight polyolefin having a weight average molecular weight of 1 × 10 ⁶ or more and a weight average molecular weight of 700 to 1%.
It can also be produced by stretching a molded product obtained by melt-extrusion of a polyolefin composition containing 40% by weight or more of 0000 polyolefin wax.

If these production methods are adopted, it is not necessary to add a large amount of solvent to the raw material resin, so that solvent extraction in the subsequent step is relatively easy or unnecessary, and it is possible to produce a porous membrane with high production efficiency. it can. In addition, since the molded article contains a small amount of solvent, it is excellent in stretchability, has a high tensile modulus, and hardly causes a change in film thickness, so that a porous film having a uniform film thickness can be obtained.

[0016]

BEST MODE FOR CARRYING OUT THE INVENTION The polyolefin porous membrane of the present invention is obtained by stretching a molded article containing a polyolefin composition having a specific composition. Hereinafter, a method for producing the polyolefin porous membrane of the present invention will be described.

First, a polyolefin composition containing an ultrahigh molecular weight polyolefin having a specific weight average molecular weight and a polyolefin wax at a specific ratio is prepared.

The ultrahigh molecular weight polyolefin has a weight average molecular weight of 1 × 10 ⁶ or more, preferably 1 × 10 ⁶ to 15 × 1.
0 ⁶ is intended. When the weight average molecular weight is less than 1 × 10 ^6, it is difficult to form the entanglement of the molecular chain of the ultra-high molecular weight polyolefin which contributes to the improvement of the stretchability, so that the stretchability is inferior and the film strength cannot be sufficiently improved. On the other hand, the upper limit is not particularly limited, but those exceeding 15 × 10 ⁶ are not preferred because the melt viscosity is extremely high and the moldability is poor.

Such ultra-high molecular weight polyolefins include homopolymers or copolymers of ethylene, propylene, 1-butene, 4-methyl-1-pentene, 1-hexene and the like. Among them, ultra-high molecular weight polyethylene is preferable, and particularly high-density ultra-high molecular weight polyethylene is preferable because of excellent crystallinity and easy formation of a porous structure.

The content of the ultrahigh molecular weight polyolefin in the polyolefin composition is 1 to 80% by weight, preferably 5 to 70% by weight, based on the whole polyolefin composition. If the content of the ultrahigh molecular weight polyolefin is less than 1% by weight, the stretchability is poor and a high-strength porous film cannot be obtained. On the other hand, if it exceeds 80% by weight, it is difficult to achieve uniformity and high concentration of the polyolefin solution used in the production of the porous membrane, which is not preferable.

The polyolefin wax has a weight average molecular weight of 700 to 10,000, preferably 1,000 to 800.
0. When the weight-average molecular weight is less than 700, the sheet exhibits fluidity at room temperature, and therefore the tensile modulus of the sheet-shaped molded article is low, and the film thickness may be changed in the production process. At the same time, the polyolefin wax may be extracted together with the solvent during the extraction of the solvent after stretching.
On the other hand, when the weight average molecular weight exceeds 10,000, it is difficult to perform uniform stretching in stretching at a high magnification.

As such a polyolefin wax, there may be mentioned a low-polymerization degree polyolefin of the same kind as the above-mentioned ultrahigh molecular weight polyolefin, and a wax mainly composed of polyethylene is particularly preferable.

The content of the polyolefin wax in the polyolefin composition is at least 20% by weight, preferably 20 to 90% by weight, more preferably 40 to 85% by weight. If it is less than 20% by weight, it is not possible to achieve uniformity and high concentration of the polyolefin solution. On the other hand, if the content exceeds 90% by weight, the film is likely to break during stretching, which is not preferable.

The polyolefin composition preferably contains a polyolefin having a weight average molecular weight in the range between the ultrahigh molecular weight polyolefin and the polyolefin wax, and particularly preferably has a weight average molecular weight of 1 × 10 ^5.
It is preferable to contain at least 1 × 10 ⁶ polyolefin. In this case, the content of the polyolefin is usually 5 to 79% by weight based on the entire polyolefin composition,
Preferably, it is 5 to 60% by weight. Examples of such a polyolefin include those of the same type as the above-mentioned ultrahigh molecular weight polyolefin, but a polymer mainly composed of ethylene is particularly preferable, and a high-density polyethylene is particularly preferable because of its excellent crystallinity.

Further, if necessary, various additives such as an antioxidant, an ultraviolet absorber, an antiblocking agent, a pigment, a dye, and an inorganic filler are added to the polyolefin composition within a range not to impair the object of the present invention. May be.

Next, an extrudable flowable material containing the above-mentioned polyolefin composition is prepared. This fluid material can be prepared by a method of heating and dissolving a polyolefin composition as a raw material in a solvent, or a method of heating and melting only the polyolefin composition without adding a solvent.

When the fluid material is prepared using a solvent,
The solvent is not particularly limited as long as it can sufficiently dissolve the polyolefin composition. Examples thereof include aliphatic or cyclic hydrocarbons such as nonane, decane, undecane, dodecane, and paraffin oil, and mineral oil fractions having a boiling point corresponding thereto. It is preferable because a molded article with a stable amount can be obtained.

The mixing ratio of the polyolefin composition and the solvent is 50 to 90 parts by weight of the polyolefin composition, 50 to 10 parts by weight of the solvent, preferably 60 to 90 parts by weight of the polyolefin composition and 10 to 40 parts by weight of the solvent. Parts by weight.
If the polyolefin composition is less than the above ratio, not only is the amount of solvent used not economical, but also the tensile modulus of the molded article becomes low, and the film thickness changes in the manufacturing and transport processes, resulting in a uniform film thickness. It cannot be a porous membrane. On the other hand, if the proportion of the polyolefin composition is higher than the above ratio, a problem may occur in dispersion of the solvent, and it may be difficult to prepare a uniform solution.

The heating dissolution is carried out with stirring at a temperature at which the polyolefin composition is completely dissolved in the solvent. The temperature varies depending on the polyolefin and the solvent to be used. For example, in the case of a composition mainly composed of polyethylene, the range of 140 ° C. to 250 ° C. is appropriate. In addition, upon heating and melting, it is preferable to add an antioxidant to prevent oxidation of the polyolefin.

When the flowable material is prepared by melting,
The content of the polyolefin wax in the polyolefin composition is 40% by weight or more, preferably 40 to 70% by weight.
And If the polyolefin wax is less than 40% by weight, the melt viscosity of the polyolefin composition becomes high,
This is because molding and uniform stretching become difficult.

The heating and melting are carried out with stirring at a temperature at which the polyolefin composition uniformly melts with the polyolefin wax. The temperature varies depending on the polyolefin used. For example, in the case of a composition mainly composed of polyethylene, the temperature is in the range of 140 ° C to 250 ° C. In addition, it is preferable to add an antioxidant also in heating and melting to prevent oxidation of the polyolefin.

Next, the flowable material of the polyolefin composition is extruded and molded. Usually, it is formed into a sheet using a rectangular die-shaped sheet die, but it may be formed into a tube using a double cylindrical hollow die, an inflation die or the like. Die temperature is usually 14
0 to 250 ° C., and the die gap is usually 0.1 to 5
mm. The extrusion speed is usually 0.2 to 5 m / min.

The flowable material extruded from the die is cooled to obtain a molded body. Cooling is performed until the temperature becomes lower than the softening temperature of the polyolefin composition. As a cooling method, a method of directly contacting cold air, cooling water, or other cooling medium,
For example, a method of contacting a roll cooled with a refrigerant can be used. The fluid material extruded from the die is
Before or during cooling, it may be taken to a roll,
When the take-up ratio is 10 or more, the neck-in becomes large and breakage tends to occur during stretching, so the take-up ratio is set to 1 to 10, preferably 1 to 5.

The molded article obtained as described above has a relatively high tensile modulus, preferably 100 kg / cm ² or more. Such a molded product is less likely to change in film thickness in a later manufacturing process or the like, and can have a uniform porous film thickness.

Next, this molded body is stretched. Stretching can be performed by the usual tenter method, roll method, inflation method, rolling method, or a combination thereof. Stretching is preferably performed in the biaxial direction, since a fine fiber structure is obtained and the piercing strength of the membrane is improved.
In this case, either vertical or horizontal simultaneous stretching or sequential stretching may be employed, but simultaneous biaxial stretching is particularly preferred.

The stretching temperature is not more than a temperature (Tm + 10 ° C.) higher by 10 ° C. than the melting point (Tm ° C.) of the ultrahigh molecular weight polyolefin, preferably in a range from the softening temperature to the melting point. For example, in the case of polyethylene, it is in the range of 90 to 140C, preferably 100 to 130C. Stretching temperature is Tm + 10
When the temperature exceeds ℃, it becomes difficult to realize the orientation of molecular chains by stretching due to melting of the resin. If the stretching temperature is lower than the softening temperature, the softening of the resin is insufficient, so that the film is easily broken in the stretching and it is difficult to stretch at a high magnification.

The stretching ratio varies depending on the film thickness of the molded product, but is usually at least 2 times or more, preferably 3 to 20 times in the uniaxial direction, 10 times or more, and preferably 20 to 400 times in area ratio. I do. If the area ratio is less than 10 times, stretching is insufficient, and it is difficult to obtain a porous membrane having high tensile elasticity and high strength. On the other hand, when the area magnification exceeds 400 times, there is a drawback that restrictions are imposed on the stretching apparatus, the stretching operation, and the like.

When a fluid material containing a solvent is used, the solvent remaining in the stretched molded article after stretching is removed by washing with a solvent. As the washing solvent, a solvent that dissolves the solvent without dissolving the polyolefin composition, and preferably has high volatility, is used. For example, hydrocarbons such as heptane and hexane, alcohols such as 1-butanol and 1-propanol, ketones such as methyl ethyl ketone and methyl isobutyl ketone, chlorinated hydrocarbons such as dichloroethane and carbon tetrachloride, and esters such as ethyl acetate and butyl acetate. And the like. These solvents are selected according to the type of the solvent used for dissolving the fluid material, and used alone or as a mixture. The washing method can be performed by immersing the membrane in a solvent to extract the solvent, showering the solvent, or a combination thereof.

The washing is performed until the residual solvent in the stretched molded product becomes less than 1% by weight. Thereafter, the washing solvent is dried to obtain a polyolefin porous membrane. The washing solvent can be dried by a method such as heat drying, air drying, or vacuum drying.

The obtained polyolefin porous membrane is preferably subjected to a heat treatment in a temperature range from a softening temperature to a melting point in order to improve dimensional stability and the like.

The obtained polyolefin porous membrane can be further subjected to hydrophilic treatment by plasma irradiation, surfactant impregnation, surface grafting, etc., if necessary.

The properties of the polyolefin porous membrane of the present invention obtained by the method described above can be adjusted according to the use depending on the production conditions and the like. Porosity 10 to 90%, air permeability (Gurley number) 10 to 10000 seconds / 100 cc,
The tensile breaking strength is preferably set to 100 kg / cm ² or more.

[0043]

EXAMPLES Ultra high molecular weight polyethylene having a weight average molecular weight of 2.0 × 10 ⁶ , polyethylene having a weight average molecular weight of 4.9 × 10 ⁵ , and a weight average molecular weight of 1,000, 4000 or 7
000, and liquid paraffin (weight average molecular weight 450) as a solvent, and these were mixed at the weight ratio shown in Table 1 (Sample Nos. 1 to 6). The above weight average molecular weight was determined using a GPG apparatus manufactured by Waters Corporation, GMH-6 manufactured by Tosoh Corporation, 1,4-dichlorobenzene as a solvent, a temperature of 135 ° C., and a flow rate of 1.0 ml / g. It is a value measured by the gel permeation chromatography (GPC) method in minutes.

Then, tetrakis [methylene-3- (3,5-di-tert-butyl-) was added to 100 parts by weight of this mixture.
4-hydroxyphenyl) -propionate] methane (“Irganox 1010” manufactured by Ciba-Geigy)
3 parts by weight were added as an antioxidant and mixed.

This mixture was heated for 200 minutes with a stirring and melting apparatus.
C. to obtain a uniform fluid material. This fluid material is extruded from a T-die by an extruder having a diameter of 45 mm,
The sheet was taken out by a cooling roll to obtain a sheet-like molded product. The sheet-like molded product was set in a biaxial stretching machine, and was simultaneously biaxially stretched at a temperature of 115 ° C. and a stretching speed of 5 mm / sec at a stretching ratio shown in Table 1. After stretching, the sample No. Samples other than 3 were washed with ethyl acetate to extract and remove residual liquid paraffin, and dried.

The resulting polyethylene porous membrane (sample N
o. Table 1 shows the characteristics of 1) to 6). In addition, the measuring method of each characteristic is as follows. Film thickness: measured with a 1/10000 linear gauge. Tensile modulus: Measured according to ASTM D882. Tensile breaking strength: Using a 10 mm wide strip-shaped test piece, AS
It measured according to TMD882. Air permeability: Measured by a Gurley-type air permeability meter. Porosity: Measured by a mercury porosimeter.

[0047]

[Table 1]

As shown in Table 1, in the sample to which the polyolefin wax was not added (No. 6), the tensile breaking strength was 250 kg / cm ² , and the air permeability was 300 seconds /
Although it became 100 cc, in each sample (No. 1 to 5) to which polyethylene wax was added, 200 kg / c
while maintaining the m ² or more of tensile strength, air permeability 500
It was possible to obtain a polyolefin porous membrane having a speed of 100 cc / sec or more.

[0049]

As described above, the polyolefin porous membrane of the present invention comprises, as a main component, a polyolefin composition containing a specific ratio of an ultrahigh molecular weight polyolefin having a specific weight average molecular weight and a polyolefin wax. Since it is obtained by stretching a molded article, it has high strength, and the resin as a raw material has sufficient moldability, so that it can be produced without adding a large amount of solvent. Therefore, it is possible to adopt a production method in which a molded article is prepared from a high-concentration solution of a polyolefin composition of 50 to 90% by weight or a melt containing no solvent at all and stretched.

Claims (2)

[Claims] 1. A polyolefin composition containing 1% by weight or more of an ultrahigh molecular weight polyolefin having a weight average molecular weight of 1 × 10 ⁶ or more and 20% by weight or more of a polyolefin wax having a weight average molecular weight of 700 to 10,000. A porous membrane made of polyolefin, obtained by stretching a molded article. 2. The molded article has a tensile modulus of elasticity of 100 kg.
² / cm ² or more.