JPS61195132A - Production of finely porous membrane of ultra-high-molecular-weight alpha-olefin polymer - Google Patents

Abstract

PURPOSE:To obtain a thin and high-strength membrane having a great number of fine through holes with a distribution, by subjecting a gelatinous molded article of an alpha-olefin polymer to solvent removal treatment to adjust the polymer content and drawing it to remove the remaining solvent. CONSTITUTION:A gelatinous molded article is formed from a solution of an alpha-olefin polymer having >=5X10<6> weight-average molecular weight, preferably 1X10<6>-15X10<6> weight-average molecular weight, at least 10wt% solvent contained in the gelatinous molded article is removed from the molded article, so the gelatinous molded article has 10-90wt%, preferably 20-60wt% alpha-olefin polymer. The molded polymer +10 deg.C, preferably from a crystal dispersion temperature - the melting point, and the remaining solvent contained in the pre- pared drawn molded article is removed. Polypropylene is used as the alpha- olefin polymer. A finely porous membrane has 0.05-50mum thickness.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to a method for producing a microporous ultra-high molecular weight α-olefin polymer membrane.

Conventional technologyPorous membranes are used in a variety of applications, such as battery separators, electrolytic capacitor diaphragms, various filters, and moisture-permeable waterproof clothing.Recently, porous membranes have been used to reduce the size and weight of equipment and improve performance. , thinner and stronger materials are required.

A method for producing a porous film of polypropylene, which is a typical example of an α-olefin polymer, is, for example, by blending polypropylene with an inorganic compound, casting it under high shear force in an area with a temperature gradient, and stretching the cast film. There is a method (Japanese Unexamined Patent Publication No. 74327/1983). However, since the porous membrane obtained by this method uses polypropylene having a molecular weight of less than 500,000, there is a limit to how thin the membrane can be made and how strong it can be increased by stretching.

In addition, ultra-high molecular weight polypropylene, which is expected to have high strength and high elastic modulus, has significantly entangled molecular chains compared to polypropylene having a normal molecular weight, making it difficult to stretch into a thin film using conventional extrusion molding.

On the other hand, a method for producing a molded article of ultra-high molecular weight polypropylene includes, for example, dissolving an ultra-high molecular weight thermoplastic crystalline polymer that is essentially polyethylene or polypropylene in a non-volatile solvent, and molding a gel from this solution. A method for producing a thermoplastic article that is substantially a fiber by heating and stretching a gel containing a non-volatile solvent or a dry gel obtained by extracting and removing the solvent contained in the gel with a volatile solvent (Japanese Patent Laid-Open No. 58-52
No. 28) has been proposed. However, with this method, it is not possible to obtain a microporous film made of an ultra-high molecular weight α-olefin polymer, which has a large number of fine and narrowly distributed through holes, and which is uniform and can be stretched at a high magnification.

Problems to be Solved by the Invention The present invention provides a thin, high-strength, ultra-high-strength material having a large number of fine through-holes with a narrow distribution by stretching a gel of an ultra-high molecular weight a-olefin polymer at a high magnification. The purpose is to obtain a microporous membrane of a molecular weight α-olefin polymer.

Means for Solving the Problems The present inventors have conducted various studies on methods for obtaining ultra-high molecular weight a-olefin polymer microporous membranes, and as a result, the present inventors have developed a method for obtaining ultra-high molecular weight a-olefin polymer microporous membranes. It has been found that the object of the present invention can be achieved by desolventizing a gel-like product and stretching it within a specific range of the amount of α-olefin polymer contained in the gel-like molded product to remove the residual solvent. , completed the invention.

That is, the present invention involves forming a gel-like material from a solution of an α-olefin polymer having a weight average molecular weight of 5×10′ or more,
After removing at least 10 parts by weight of the solvent contained in the gel-like molded product so that the α-olefin polymer contained in the gel-like molded product becomes 10 to 90 parts by weight, the α- This is a method for producing a microporous membrane of an ultra-high molecular weight a-olefin polymer, which is characterized by stretching at a temperature of 10° C. or lower than the melting point of the olefin polymer and removing residual solvent in the stretched product.

The ultra-high molecular weight α-olefin polymer used in the present invention has a weight average molecular weight of 5×10′ or more, preferably 1×
It is in the range of 106 to 15×10. If the weight average molecular weight is less than 5×101, an extremely thin and highly strong microporous membrane cannot be obtained. On the other hand, the upper limit is not particularly limited, but 15X
If the number exceeds 106, it is difficult to form a thin film by stretching. Such ultra-high molecular weight α-olefin polymers include crystalline homopolymers of propylene, 1-butene, 4-methyl-1-pentene, 1-hexene, etc., or polymers with a 10 molar relationship with these α-olefins. The following copolymers with ethylene or other a-olefins may be mentioned.

Among these, ultra-high molecular weight polypropylene mainly composed of propylene is preferred. In addition, the above-mentioned ultra-high molecular weight a-
Various additives such as antioxidants, ultraviolet absorbers, lubricants, anti-blocking agents, pigments, and inorganic fillers may be added to the olefin polymer as necessary to the extent that the purpose of the present invention is not impaired. can.

The solution of the ultra-high molecular weight α-olefin polymer which is a raw material in the present invention is prepared by heating and dissolving the above-mentioned α-olefin polymer having a weight average molecular weight of 5×10 6 or more in a solvent. The solvent for the a-olefin polymer is not particularly limited as long as it can sufficiently dissolve the a-olefin polymer. Examples include aliphatic or cyclic hydrocarbons such as nonane, decane, undecane, dodecane, decalin, and paraffin oil, or mineral oil fractions with boiling points corresponding to these, and gel-like molded products that are stable in a solvent-containing state. A non-volatile solvent such as paraffin oil is preferred. The heat dissolution is carried out with stirring at a temperature at which the #a olefin combination is completely dissolved in the solvent. The temperature varies depending on the polymer and solvent used, but for example in the case of polypropylene it ranges from 160 to 250°C. In addition, the concentration of the α-olefin polymer solution varies depending on the molecular weight, but
10 weight is preferred. It is difficult to prepare a homogeneous solution when the concentration is high. In addition, during heating and dissolution, it is preferable to add an antioxidant in order to prevent oxidative deterioration of the α-olefin polymer.

Next, the heated solution of the a-olefin polymer is extruded from an appropriately selected die into a sheet or tube shape, or cast onto a support, and is heated preferably below the gelling temperature in a water bath, air bath, solvent, etc. is gelled by cooling to a temperature of 15-25°C at a rate of at least 50°C/min. The thickness of the gel-like molded product is usually about 11 to 5 cm.

This gel-like molded product is swollen with the solvent used to dissolve the a-olefin polymer, and requires removal of the solvent.

Solvent removal treatment includes methods such as immersing the gel-like molded product in an easily volatile solvent, extracting it, and drying it, compressing it, heating it, or a combination of these methods. Extractive removal using a readily volatile solvent is preferred since the solvent can be removed without significant changes. These easily volatile solvents include Du, pentane, hexane, heptane,
Hydrocarbons such as methylene chloride, chlorinated hydrocarbons such as carbon tetrachloride, fluorinated hydrocarbons such as trifluoroethane, ethers such as diethyl ether and dioxane, and alcohols such as methanol, ethanol, propatool, etc. can be given. These solvents are appropriately selected depending on the solvent used to dissolve the a-olefin polymer, and are used alone or in combination.

In addition, the amount of solvent removed from the gel-like molded product is at least 10% by weight relative to the solvent contained, and the amount of the ultra-high molecular weight α-olefin polymer contained in the gel-like molded product is 10 to 90% by weight. It is necessary to perform a solvent removal treatment to reduce the weight of the product, preferably 20 to 60%. If the amount of solvent removed from the gel-like molded product is less than 10% by weight relative to the solvent contained, and the amount of the a-olefin polymer contained in the gel-like molded product is less than 10% by weight, the net shape of the gel-like molded product is Since the structure is highly swollen with solvent, the gel tends to dissolve during heating and stretching. In addition, uneven stretching tends to occur locally, making it difficult to obtain a stretched product with a uniform thickness, and the pore size distribution of the pores formed in the stretched product is undesirable. Furthermore, it is also unfavorable from the viewpoint of handling, such as oozing of the solvent during stretching. On the other hand, if the a-olefin polymer contained in the gel molded product exceeds 90% by weight, excessive desolvation treatment may cause the network structure of the gel molded product to become too dense, making it difficult to stretch at a high magnification. Moreover, it is difficult to obtain a thin and high-strength stretched product, and both the pore diameter and porosity of micropores formed in the stretched product are undesirably reduced.

The removal amount of the solvent contained in the gel-like molded product can be adjusted by the amount of contact of the easily volatile solvent with the gel-like molded product, the time, the compression pressure of the gel-like molded product, etc.

In addition, when desolventizing a gel-like molded product using a readily volatile solvent, the gel-like molded product shrinks or bends in three axes as the easily volatile solvent substituted in the gel-like molded product evaporates. In order to prevent this and obtain a smooth original fabric with little shrinkage in the biaxial (longitudinal and transverse) directions, which enables uniform and high stretching, the gel-like molded material is selectively stretched in the thickness direction. Preferably, it shrinks. Its shrinkage rate is 5O1s or more in the thickness direction, preferably 70 inches or more, and 2 (11cm or more) in the biaxial direction.
It is preferable that it is below. Selective shrinkage of the gel-like molded product in the thickness direction can be achieved by, for example, applying an easily volatile solvent to the gel-formed product while it is tightly attached to a smooth support, gripped from two axes, or sandwiched between porous plates. One method is to evaporate it.

Stretching is performed by heating the original fabric of the gel-like molded product that has been subjected to solvent removal treatment, and using the usual tenter method, roll method, inflation method,
Biaxial stretching is performed at a predetermined magnification by a rolling method or a combination of these methods.

Biaxial stretching may be done simultaneously or sequentially.

The stretching temperature is the melting point of the ultra-high molecular weight α-olefin polymer +
The temperature is 10°C or less, preferably in the range from the crystal dispersion temperature to less than the melting point. For example, in the case of polypropylene, 90 to 18
0°C, preferably in the range of 130 to 170°C. If the stretching temperature exceeds the melting point +10° C., the resin will melt excessively and orientation by stretching will not be possible. Furthermore, if the stretching temperature is lower than the crystal dispersion temperature, the resin will not be sufficiently softened and the membrane will easily break during stretching, making it impossible to stretch at a high magnification.

Further, the stretching ratio varies depending on the thickness of the original fabric, but is at least 2 times or more in the uniaxial direction, preferably 5 to 20 times, and 10 times or more in area magnification, preferably 25 to 400 times. If the areal magnification is less than 10 times, it is not preferable because the stretching is insufficient and a thin film with high porosity cannot be obtained. On the other hand, the area magnification is 4
If it exceeds 00 times, it is not preferable because restrictions will arise in terms of stretching equipment, stretching operations, etc.

The microporous membrane after stretching is immersed in the above-mentioned easily volatile solvent to extract and remove the remaining solvent, and then the solvent is evaporated and dried.

In the extraction of the solvent, it is necessary to remove the solvent in the microporous membrane to less than 1 part by weight.

The thickness of the ultra-high molecular weight α-olefin polymer microporous membrane of the present invention can be appropriately selected depending on the application, but usually IIL
It ranges from 0.05 to 50 μm, preferably from 11 to 10 μm.

Further, according to the method of the present invention, ultra-high molecular weight α-
A microporous olefin polymer membrane can be obtained.

Effects of the Invention According to the method of the present invention, it is possible to form a porous, ultra-thin film from an ultra-high molecular weight α-olefin polymer by stretching at a high magnification.

In addition, the ultra-high molecular weight α-olefin polymer microporous membrane obtained is extremely thin and has high strength, which cannot be obtained with conventional normal molecular weight α-olefin polymer microporous membranes, and has a finer average pore diameter. It is sticky and has a narrow pore size distribution.

The ultra-high molecular weight α-olefin polymer microporous membrane produced by the method of the present invention has the above-mentioned excellent properties and is suitable for stain pond separators, diaphragms for electrolytic capacitors, various filters, porous membranes for moisture-permeable and waterproof clothing, etc. So, how about making it smaller and lighter and improving its performance? You can find out.

EXAMPLES Below, examples of the present invention are shown. In addition, the test method in the examples is as follows.

(1) Film thickness: Measure the cross section of the film using a scanning electron microscope.

(2J Breaking strength: A8'rM D882 compliant.

(3) Breaking elongation: Mu8TM D882 compliant.

(4) Average pore size, pore size distribution: The field of view observed with a scanning electron microscope after vacuum-depositing gold on the surface of a microporous membrane was statistically processed using an image analyzer, and the area average pore diameter φ6, number average pore diameter φN1 pore size distribution ( φ8/φN) was determined. Let the value of the number average pore diameter be the average pore diameter.

(5) Porosity: Measured using a mercury porosimeter.

Example 1 Liquid paraffin (64 ca.
t / 40℃) 2゜6-T to 100 parts by weight of the mixed solution
-Butyl-p-cresol 1125 parts by weight and tetrakised methylene-5-(S,5-di-t-butyl-4-hydroxyphenyl)-propionate comethane (125 parts by weight) are mixed with an antioxidant. This mixed solution was filled into an autoclave equipped with a stirrer, heated to 20° C., and stirred for 90 minutes to obtain a homogeneous solution.

This solution was filled into a heated mold and rapidly cooled to 15° C. to form a gel-like sheet with a thickness of 2 days.

After immersing this gel-like 7-t in methylene chloride for 60 minutes, the methylene chloride was evaporated and dried while it was attached to a smooth plate, and the polypropylene material was 19.4% by weight and the shrinkage rate in the thickness direction was 79. .4 original fabric sheets were obtained.

The obtained raw sheet was set in a biaxial stretching machine, and the temperature was 1.
Simultaneous biaxial stretching was performed at 50° C., at a speed of 30 ct11/min, and at a magnification of 8×8. After washing the obtained stretched membrane with methylene chloride to extract and remove the remaining liquid paraffin,
After drying, a microporous polypropylene membrane was obtained. Its characteristics are shown in Table-1.

Examples 2 to 6 Microporous polypropylene membranes were obtained in the same manner as in Example 1, except that the gel-like sheet molded in Example 1 was formed under the conditions shown in Table 1. These characteristics are also listed in Table-1.

Example 7 A microporous polypropylene membrane was obtained in the same manner as in Example 1, except that the gel-like sheet molded in Example 1 was stretched one after another under the conditions shown in Table 1.

These characteristics are also listed in Table-1.

Comparative Example 1 Microporous polypropylene was produced in the same manner as in Example 1, except that the gel sheet formed in Example 1 was set in a biaxial stretching machine without removing the solvent, and the film was formed under the conditions shown in Table 1. A membrane was obtained. Its characteristics are also listed in Table-1. The resulting microporous membrane had a wide average pore size distribution and was non-uniformly stretched, as shown in Table 1. In addition, the surface of the film immediately after stretching was covered with excess solvent that oozed out, causing pooling and dripping in some places, and a large amount of solvent was wasted in cleaning the film.

Example 8 A microporous polypropylene membrane was obtained in the same manner as in Example 1, except that a liquid paraffin solution containing 2-0 weight percent of polypropylene was prepared and the membrane was formed under the conditions shown in Table 1. Ta. These characteristics are also listed in Table-1.

Example 9 A liquid paraffin solution of &0 weight ratio was prepared using polypropylene of Mw = 2.5 x 10' instead of the polypropylene of Mw = 4.7 x 10' used in Example 1, and each condition shown in Table 1. A microporous polypropylene membrane was obtained in the same manner as in Example 1, except that the membrane was formed in the following manner. These characteristics are also listed in Table-1.

Comparative Example 2 Same as Example 1 except that 9.0% by weight of liquid paraffin in the gel sheet formed from the polypropylene solution prepared in Example 9 was removed and the film was formed under the conditions shown in Table 1. A microporous polypropylene membrane was obtained. These characteristics are also listed in Table-1. The obtained microporous membrane is
The average pore size distribution was wide and the stretching was non-uniform.

In addition, the surface of the film immediately after stretching was covered with excess solvent that oozed out, causing puddles and sag in some places.

Comparative Example 3 Polypropylene was produced in the same manner as in Example 1, except that 50% by weight of liquid paraffin in the gel-like sheet formed from the polypropylene solution prepared in Example 8 was removed and the film was formed under the conditions shown in Table 1. A microporous membrane was obtained. The obtained microporous membrane had a wide average pore size distribution and was non-uniformly stretched. In addition, the surface of the film immediately after stretching was covered with excess solvent that oozed out, causing puddles and sag in some places.

Comparative Example 4 After immersing the gel-like sheet formed in Example 1 in a large amount of methylene chloride for 60 minutes, the methylene chloride was evaporated and dried while the smooth plate was still attached to the liquid paraffin. Set the gel-like sheet containing no gel in a biaxial stretching machine, stretch at a temperature of 110 to 170°C, and at a speed of 30c.
Attempts were made to stretch each film at a rate of In/min, but due to uneven stretching and breakage, it was not possible to stretch at a magnification of 3x3 or more.

Claims (2)

[Claims] (1) Form a gel from a solution of an α-olefin polymer having a weight average molecular weight of 5 x 10^5 or more, remove at least 10% by weight of the solvent contained in the formed gel, and After the α-olefin polymer contained in the gel-like molded product is adjusted to 10 to 90% by weight, it is stretched at a temperature below the melting point of the α-olefin polymer +10°C, and the resulting stretched molded product is 1. A method for producing a microporous ultra-high molecular weight α-olefin polymer membrane, which comprises removing residual solvent contained therein. (2) The method according to claim 1, wherein the α-olefin polymer is polypropylene.