JPH0441702B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to a method for producing a microporous ultra-high molecular weight α-olefin polymer membrane. Conventional technology Porous 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 their 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 an inorganic compound with polypropylene, casting it by applying a high shear force in an area with a temperature gradient, and stretching the cast film. Method (Unexamined Japanese Patent Publication No. 58-74327
Publication No.). However, since the porous membrane obtained by this method uses polypropylene with a molecular weight of less than 500,000, there is a limit to how thin the membrane can be made and the strength can be increased by stretching. In addition, ultra-high molecular weight polypropylene, which is expected to provide high strength and high elastic modulus of membranes, has significantly entangled molecular chains compared to polypropylene having a normal molecular weight, making it difficult to stretch into thin films 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, forming 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. 58-5228) Public bulletin) 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 film having a large number of fine through-holes with a narrow distribution by stretching a gel of an ultra-high molecular weight α-olefin polymer at a high magnification. The object is to obtain a microporous membrane of a molecular weight α-olefin polymer. Means for Solving the Problems The present inventors conducted various studies on methods for obtaining microporous membranes of ultra-high molecular weight α-olefin polymers, and as a result, the present inventors discovered that gels formed from solutions of ultra-high molecular weight α-olefin polymers were obtained. It has been discovered that the object of the present invention can be achieved by removing the residual solvent by desolventizing a gel-like molded product and stretching it within a specific range of the amount of α-olefin polymer contained in the gel-like molded product, and the present invention completed. That is, in the present invention, the weight average molecular weight is 5×
A gel is formed from a solution of a 10 ⁵ or more α-olefin polymer, and at least 10% by weight of the solvent contained in the gel is removed to remove the α-olefin contained in the gel. After the olefin polymer is adjusted to 10 to 90% by weight, the α-olefin polymer is stretched at a temperature lower than the melting point of the α-olefin polymer by 10°C or less, and the residual solvent in the obtained stretched product is removed. This is a method for producing an ultra-high molecular weight α-olefin polymer microporous membrane. The ultra-high molecular weight α-olefin polymer used in the present invention has a weight average molecular weight of 5×10 ⁵ or more, preferably in the range of 1×10 ⁶ to 15×10 ⁶ .
If the weight average molecular weight is less than 5×10 ⁵ , an ultrathin and high-strength microporous membrane cannot be obtained. On the other hand, although the upper limit is not particularly limited, if it exceeds 15×10 ⁶ , it is difficult to make it into a thin film by stretching. Examples of such ultra-high molecular weight α-olefin polymers include propylene, 1
-Crystalline homopolymers of butene, 4-methyl-1-pentene, 1-hexene, etc., or copolymers of these α-olefins with 10 mol% or less of ethylene or other α-olefins. . Among these, ultra-high molecular weight polypropylene mainly composed of propylene is preferred. In addition, various additives such as antioxidants, ultraviolet absorbers, lubricants, anti-blocking agents, pigments, and inorganic fillers may be added to the above-mentioned ultra-high molecular weight α-olefin polymers as necessary. It can be added as long as it does not cause any damage. The solution of the ultra-high molecular weight α-olefin polymer used as a raw material in the present invention has a weight average molecular weight of 5.
It is prepared by heating an α-olefin polymer of ×10 ⁵ or more in a solvent. This solvent is not particularly limited as long as it can sufficiently dissolve the α-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 heating and dissolving is performed while stirring at a temperature at which the α-olefin polymer 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 is 160 to 250.
℃ range. In addition, the concentration of the α-olefin polymer solution varies depending on the molecular weight, but is 1 to 10% by weight.
is preferred. If the concentration is too high, it will be difficult to prepare a uniform solution. Note that α-
It is preferable to add an antioxidant to prevent oxidative deterioration of the olefin polymer. Next, this α-olefin polymer heated solution is extruded into a sheet or tube shape from an appropriately selected die, or cast onto a support, and then heated in a water bath.
below the gelling temperature, preferably in an air bath, solvent, etc.
Cool to a temperature of 15-25°C at a rate of at least 50°C/min to form a gel. The thickness of the gel-like molding is usually
It is molded to about 0.1 to 5 mm. This gel-like molded product is swollen with the solvent used when the α-olefin polymer is dissolved, 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. Examples of easily volatile solvents include hydrocarbons such as pentane, hexane, and heptane, chlorinated hydrocarbons such as methylene chloride and carbon tetrachloride, fluorinated hydrocarbons such as trifluoroethane, and ethers such as diethyl ether and dioxane. and other alcohols such as methanol, ethanol, and propanol. These solvents are appropriately selected depending on the solvent used to dissolve the α-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 based on the dissolving power 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. %, preferably 20 to 60% by weight. If the amount of solvent removed from the gel-like molded product is less than 10% by weight based on the solvent contained, and the α-olefin polymer contained in the gel-like molded product is less than 10% by weight,
Since the network structure of the gel-like molded product is highly swollen by the solvent, the gel is likely 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 pores formed in the stretched product becomes undesirable. Furthermore, it is also unfavorable from the viewpoint of handling, such as oozing of the solvent during stretching. On the other hand, excessive desolvation treatment in which the α-olefin polymer contained in the gel molded product exceeds 90% by weight causes the network structure of the gel molded product to become too dense, making it difficult to stretch at a high magnification. It is difficult to obtain thin, high-strength stretch molded products.
This is undesirable because both the pore diameter and porosity of the micropores formed in the stretched product decrease. The amount of solvent contained in the gel-like molded product removed is
It can be adjusted by the amount of contact of the easily volatile solvent with the gel-like molded product, the time, or the compression pressure of the gel-like molded product. 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. To prevent this,
In order to obtain a smooth raw fabric that can be stretched uniformly and at high magnification, with minimal shrinkage in the biaxial (longitudinal and horizontal) directions,
It is preferable to selectively shrink the gel-like molded product in the thickness direction. Its shrinkage rate is 50% or more in the thickness direction, preferably 70% or more, and 20% or more in the biaxial direction.
% or more. 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-like molded 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 carried out by heating the original fabric of the gel-like molded product that has been subjected to solvent removal treatment, and biaxially stretching it at a predetermined magnification using the usual tenter method, roll method, inflation method, rolling method, or a combination of these methods. do. Biaxial stretching may be done simultaneously or sequentially. The stretching temperature is below the melting point of the ultra-high molecular weight α-olefin polymer +10° C., preferably in the range from the crystal dispersion temperature to below the melting point. For example, in the case of polypropylene, the temperature is 90-180℃, more preferably 130-170℃.
℃ range. If the stretching temperature exceeds the melting point +10°C, the resin will melt excessively, making orientation by stretching impossible. 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. In addition, 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 times or more.
~20x, surface magnification 10x or more, preferably 25-400
It's double. 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, if the area magnification exceeds 400 times, the stretching device
This is not preferable because it imposes restrictions on stretching operations and the like. 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. The solvent extraction requires removing the solvent in the microporous membrane to less than 1% by weight. The thickness of the ultrahigh molecular weight α-olefin polymer microporous membrane of the present invention can be appropriately selected depending on the application, but is usually 0.05 to 50 μm, preferably 0.1 to 50 μm.
It is in the range of 10 μm. In addition, according to the method of the present invention, ultra-high molecular weight α-olefin polymer particles having fine through holes having an average pore diameter of 0.01 to 1 μm, a porosity of 30 to 90%, and a breaking strength of 100 Kg/cm ² or more are produced. A porous membrane can be obtained. Effects of the Invention According to the method of the present invention, it is possible to transform an ultra-high molecular weight α-olefin polymer into a porous film by high-magnification stretching. 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 also has a finer average pore diameter. , 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 battery separators, diaphragms for electrolytic capacitors, various filters, porous membranes for moisture-permeable and waterproof clothing, etc. It is possible to reduce the size and weight and improve the performance. Examples Examples of the present invention are shown below. In addition, the test method in Examples is as follows. (1) Film thickness: Measuring the cross section of the film using a scanning electron microscope. (2) Breaking strength: Based on ASTM D882 (3) Breaking elongation: Based on ASTM D882 (4) Average pore size, pore size distribution: Regarding the field of view observed using a scanning electron microscope after vacuum-depositing gold on the surface of a microporous membrane, Statistical processing was performed using an image analyzer to determine the area average pore diameter φ _s , number average pore diameter φ _N , and pore size distribution (φ _s /φ _N ). 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 cst/40
°C) 100 parts by weight of the mixed solution, 0.125 parts by weight of 2,6-di-t-butyl-p-cresol and tetrakis[methylene-3-(3.5-di-t-butyl-4-hydroxyphenyl)-propionate]methane. Add 0.25 parts by weight of antioxidant and mix. Fill this mixed solution into an autoclave equipped with a stirrer and heat it to 200°C.
A homogeneous solution was obtained by stirring for 90 minutes. 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 mm. After immersing this gel-like sheet in methylene chloride for 60 minutes, the methylene chloride was evaporated and dried while it was attached to a smooth plate, resulting in an original sheet with a polypropylene content of 19.4% by weight and a shrinkage rate of 79.4% in the thickness direction. Got a sheet. The obtained raw sheet was set in a biaxial stretching machine and subjected to simultaneous biaxial stretching at a temperature of 150° C., a speed of 30 cm/min, and a magnification of 8×8. The obtained stretched membrane was washed with methylene chloride to extract and remove residual liquid paraffin, and then dried to obtain a microporous polypropylene membrane. Its characteristics are shown in Table-1. Examples 2 to 6 The gel-like sheets molded in Example 1 are shown in the table below.
A microporous polypropylene membrane was obtained in the same manner as in Example 1, except that the membrane was formed under the conditions shown in Example 1. These characteristics are also listed in Table-1. Example 7 The gel-like sheet molded in Example 1 is shown in the table below.
A microporous polypropylene membrane was obtained in the same manner as in Example 1, except that the stretching was carried out successively under the conditions shown in Example 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-like sheet formed in Example 1 was set in a biaxial stretching machine without removing the solvent, and a film was formed under the conditions shown in Table 1. A membrane was obtained. Its characteristics are also listed in Table-1. The obtained microporous membrane is shown in Table-
As shown in Figure 1, 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 accumulations and drips in some places, and a large amount of solvent was required to clean them. Example 8 In Example 1, 2.0% by weight of polypropylene
A microporous polypropylene film was obtained in the same manner as in Example 1, except that a liquid paraffin solution containing the following was prepared and the film was formed under the conditions shown in Table 1. These characteristics are also listed in Table-1. Example 9 A 6.0% by weight liquid paraffin solution was prepared using polypropylene with w = 2.5 x 10 ⁶ instead of polypropylene with w = 4.7 x 10 ⁶ 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 The procedure was the same as in Example 1 except that 9.0% by weight of the liquid paraffin in the gel-like 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. Table 1 shows this characteristic.
Also listed. The resulting 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 pooling and sag in some places. Comparative Example 3 Determination of liquid paraffin in a gel-like sheet formed from the polypropylene solution prepared in Example 8.
A microporous polypropylene membrane was obtained in the same manner as in Example 1, except that 50% by weight was removed and the membrane was formed under the conditions shown in Table 1. The resulting 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 pooling and sag in some places. Comparative Example 4 A gel-like sheet formed in Example 1 was immersed in a large amount of methylene chloride for 60 minutes, and then attached to a smooth plate and the methylene chloride was evaporated to dry. Set the gel-like sheet without any heat in the biaxial stretching machine and set the stretching temperature to 110.
Stretching was attempted at a temperature of ~170° C. and a speed of 30 cm/min, but due to uneven stretching and breakage, it was not possible to stretch at a magnification of 3×3 or more. 【table】

Claims (1)

[Claims] 1. Molding a gel from a solution of an α-olefin polymer having a weight average molecular weight of 5×10 ⁵ or more, and removing at least 10% by weight of the solvent contained in the gel-like molded product. The α-olefin polymer contained in the gel-like molded product is adjusted to 10 to 90% by weight, and then stretched at a temperature equal to or lower than the melting point of the α-olefin polymer by 10°C. Ultra-high molecular weight α characterized by removing residual solvent contained in molded products
- A method for producing an olefin polymer microporous membrane. 2. The method according to claim 1, wherein the α-olefin polymer is polypropylene.