JP6507650B2 - Microporous membrane and method for producing the same - Google Patents

Description

  The present invention relates to a method for producing a microporous membrane made of a polypropylene polymer, a microporous membrane obtained by the method, and a storage device using the microporous membrane.

  The synthetic resin microporous membrane is used as a material for various separation membranes and battery separators. Among them, polyolefin resins are useful as raw materials for microporous membranes because they can be easily formed into films, have high chemical resistance, and can be produced at relatively low cost among synthetic resins. In particular, in recent years where development of lithium ion batteries mounted on electronic devices such as PCs and portable terminals and vehicles such as hybrid vehicles and electric vehicles has been actively performed, cost reduction of battery members is also required.

  In recent years when development of lithium ion batteries to be mounted on electronic devices such as PCs and portable terminals and vehicles such as hybrid vehicles and electric vehicles is actively performed, both high quality and low cost of battery members are required. It is done. For this reason, as a microporous film which is a material of a separator for lithium ion batteries in particular, one having an excellent separator function and capable of being manufactured at low cost is desirable. In this sense, polyolefin microporous membranes are of interest.

  The battery separator is a partition immersed in the electrolyte, which mediates ion conduction between the positive and negative electrodes. The form of the pores of the microporous membrane constituting the battery separator affects the charge and discharge efficiency of the battery. From the viewpoint of the charge and discharge efficiency of the battery, it is said that as many pores as possible are present in the microporous membrane for a separator, and furthermore, it is preferable that ions pass through the pores through the membrane in as short time as possible. As an index for evaluating such a state of the microporous membrane, porosity (proportion of volume of pores formed in the membrane) and air permeability (moving time of gas molecules inside pores) are used.

  Patent Literatures 1, 2 and 3 describe methods for producing a microporous film having high porosity and good air permeability by using polyolefin as a weight loss.

  Patent Document 1 describes a method of extruding a raw material containing ultra high molecular weight polyethylene, a plasticizer, and an inorganic fine powder, and stretching the obtained raw film to obtain a microporous membrane.

  Patent Document 2 describes a method of extruding a raw material containing an ultrahigh molecular weight polyolefin, a thermoplastic elastomer, and a solvent, and stretching the obtained raw film to obtain a microporous membrane.

  Patent Document 3 describes a method of extruding a raw material containing a polyolefin and an inorganic filler, and stretching the obtained raw film to obtain a microporous membrane.

  However, the manufacturing methods described in Patent Documents 1 and 2 require a step of removing a plasticizer and a solvent, and there is a problem in manufacturing cost. The manufacturing method described in Patent Document 3 requires a special inorganic filler, and there are problems in terms of cost and weight reduction of the separator.

Unexamined-Japanese-Patent No. 2010-7053 JP 2000-72908 A JP, 2008-94911, A

  Therefore, the inventor of the present invention further examined a method for producing a microporous polyolefin membrane having high porosity and excellent air permeability.

  As a result, it has been found that a production method in which a raw film obtained by extruding a polypropylene polymer is stretched under specific conditions is effective.

  That is, the present invention is as follows.

(Invention 1) A method for producing a microporous membrane, comprising the steps of:
(Step 1) A step of extruding a polypropylene polymer at a draft ratio of 120 or more to form a raw film.
(Step 2) A step of heat treating the raw film obtained in Step 1.
(Step 3) A step of cold-stretching the heat-treated raw film obtained in Step 2 at 1.0 to 1.10 times the length direction at -5 to 45 ° C.
(Step 4) A step of warm-stretching the stretched film after step 3 at a temperature 15 to 65 ° C. lower than the melting point of the polypropylene polymer in the length direction by 1.5 to 4.0 times.
(Step 5) A step of relaxing the warm-stretched film obtained in Step 4 so that the length becomes 0.7 to 1.0 times under heating.

(Invention 2) In step 1, the melting point is in the range of 150 to 170 ° C., and the melt mass flow rate (MFR, measured under the conditions according to JIS K 6758 (230 ° C., 21.18 N)) is 0.1 to 10 g / 10 Characterized by extruding a polypropylene-based polymer which is a propylene-based polymer which is in the range of 1% and optionally contains at least one selected from ethylene and an α-olefin having 4 to 8 carbon atoms Manufacturing method of the invention 1

(Invention 3) The method according to Invention 1 or 2, characterized in that extrusion molding is carried out at a draft ratio of 150 or more in Step 1.

(Invention 4) The method according to any one of Inventions 1 to 3, wherein in the step 3, the film is stretched 1.02 to 1.04 times in the lengthwise direction.

(Invention 5) The method according to any one of Inventions 1 to 4, characterized in that in the step 4, the film is stretched 2.0 to 3.5 times in the lengthwise direction.

(Invention 6) A microporous film obtained by any of the production methods of Inventions 1 to 5.

(Invention 7) The microporous film according to Invention 6, wherein the porosity is 45% or more.

(Invention 8) The microporous membrane of Invention 6 or 7 wherein the air permeability is 350 sec / 100 mL or less.

(Invention 9) The microporous film according to any one of Inventions 6 to 8, which is used as a separator of an electricity storage device.

(Invention 10) The microporous film of Invention 9 wherein the electricity storage device is a lithium ion battery.

(Invention 11) The microporous film of Invention 9 wherein the electricity storage device is a capacitor.

(Invention 12) An electricity storage device comprising the microporous membrane of Invention 9.

(Invention 13) A lithium ion battery comprising the microporous membrane of Invention 10.

(Invention 14) A capacitor comprising the microporous membrane of Invention 11.

  The microporous membrane obtained by the production method of the present invention has a high porosity, while the air permeability is suppressed to a certain value or less. This is presumed to be that a large number of relatively small micropores are formed in the above-mentioned microporous membrane, and the generation of a large pore in which a plurality of pores are connected is suppressed. Such microporous membranes can be expected to exhibit higher selective permeability of the material.

(Raw material of microporous membrane)
The raw material of the microporous membrane of the present invention is a polypropylene polymer, and a homopolymer of propylene or a copolymer obtained by copolymerizing a comonomer corresponds to this. As a polypropylene polymer used by this invention, what has comparatively high crystallinity and whose melting | fusing point is in the range of 150-170 degreeC is preferable, and what has melting | fusing point in the range of 155-168 degreeC is more preferable. The above comonomer is generally at least one selected from ethylene and an α-olefin having 4 to 8 carbon atoms. In addition to these, even if they are copolymerized with branched olefins having 4 to 8 carbon atoms such as 2-methylpropene, 3-methyl-1-butene, 4-methyl-1-pentene, styrenes and dienes. Good.

  The content of the above-mentioned comonomer may be in any range as long as the microporous membrane exhibits the stress damping property described later. Preferably, the amount is 5 parts by weight or less, and particularly preferably 2 parts by weight or less, with respect to 100 parts by weight of the polymer, which is a range to give a highly crystalline polypropylene polymer.

  The melt mass flow rate (MFR, measured under the conditions according to JIS K 6758 (230 ° C., 21.18 N)) of the polypropylene polymer is preferably 0.1 to 10 g / 10 min, more preferably 0. It is in the range of 4 to 5.0 g / 10 min.

  In the raw material of the microporous membrane of the present invention, additives such as a crystal nucleating agent and a filler can be blended. The type and amount of the additive are not limited as long as the porosity is not impaired.

(Method of manufacturing microporous membrane)
The microporous membrane of the present invention is produced by a dry method including the following steps 1 to 5 using the above-mentioned raw materials.

(Step 1: Film Forming Step) In this step, the raw material is extruded to form a raw film. The polypropylene polymer, which is a raw material, is supplied to an extruder, and the polypropylene polymer is melt-kneaded at a temperature above its melting point, and a polypropylene polymer film is extruded from a die attached to the tip of the extruder. The extruder used is not limited. As an extruder, for example, any of a single screw extruder, a twin screw extruder, and a tandem extruder can be used. Any die may be used as long as it is used for film forming. As the dice, for example, various T-shaped dice can be used. The thickness and shape of the raw film are not particularly limited. The ratio (draft ratio) of die slip clearance to raw film thickness is 120 or more, preferably 120 to 300, and more preferably 150 to 250. The thickness of the raw film is not particularly limited, but is generally 10 to 200 μm, preferably 15 to 100 μm.

(Step 2: Heat Treatment Step) This is a step of heat treating the raw film which has finished Step 1. A constant tension in the longitudinal direction is applied to the raw film at a temperature 5 to 65 ° C., preferably 10 to 25 ° C. lower than the melting point of the polypropylene polymer. The tension is preferably such that the length of the raw film is more than 1.0 times and not more than 1.1 times.

(Step 3: Cold Stretching Step) This is a step of stretching the raw film finished step 2 at a relatively low temperature. The stretching temperature is -5 to 45 ° C, preferably 5 to 30 ° C. The stretching ratio is 1.0 to 1.1, preferably 1.00 to 1.08, and more preferably 1.02 or more and less than 1.05 in the longitudinal direction. However, the draw ratio is greater than 1.0. The stretching means is not limited. Known means such as roll stretching and tenter stretching can be used. The number of stages of stretching can be set arbitrarily. It may be one-stage stretching, or two or more stages of stretching may be performed through a plurality of rolls. In the cold drawing step, the molecules of the polypropylene polymer constituting the raw film are oriented. As a result, it is possible to obtain a stretched film having a lamellar part having a dense molecular difference and a region (craz) in which a molecular chain between lamellae is sparse.

(Step 4: Warm Stretching Step) This is a step of stretching the stretched film after step 3 at a relatively high temperature. The stretching temperature is 15 to 65 ° C. lower than the melting point of the polypropylene polymer, preferably 15 to 45 ° C. lower than the melting point of the polypropylene polymer. The stretching ratio is 1.5 to 4.5 times, preferably 2.0 to 4.0 times, more preferably 2.8 to 3.5 times in the longitudinal direction. The stretching means is not limited. Known means such as roll stretching and tenter stretching can be used. The number of stages of stretching can be set arbitrarily. It may be one-stage stretching, or two or more stages of stretching may be performed through a plurality of rolls. In the warm drawing step, the craze formed in step 3 is stretched to generate pores.

(Step 5: Relaxation Step) In this step, the film is relaxed in order to prevent the shrinkage of the film after warm drawing which has finished step 4. The relaxation temperature is a temperature slightly higher than the temperature of warm drawing, and a temperature higher by 0 to 20 ° C. is general. The degree of relaxation is adjusted so that the length of the stretched film after step 4 is finally 0.7 to 1.0.

  In the present invention, the porosity and the air permeability are measured under the following conditions.

(Porosity)
It is the value computed by the following formula about the microporous film slice of width 50 mm x length 120 mm.
Porosity (%) = [1- (section weight) / (section area × resin density × section thickness)] × 100

(Permeability)
Expressed by the time (seconds) that a fixed volume (100 mL) of air passes through the microporous membrane. The microporous membrane was measured by a Gurley tester at a temperature of 23 ° C. ± 2 ° C. and at a humidity of 50% ± 5% by a method based on JIS P8117 and at a temperature of 23 ° C. ± 2 ° C. It is a value.

  By the production method of the present invention, a microporous film having the porosity of 45% or more and the air permeability of 350 sec / mL or less can be obtained. Typically, the porosity is in the range of 45-55% and the air permeability is in the range of 150-350 sec / mL.

  Hereinafter, examples of the production method of the present invention will be shown. The following goods were used as a polypropylene polymer which is a raw material.

  Polypropylene resin 1: Product "Novatec EA9FTD" manufactured by Japan Polypropylene Corporation (melting point 165 ° C, MFR 0.5)

  Polypropylene resin 2: Product "Novatec EA9HD" manufactured by Japan Polypropylene Corporation (melting point 165 ° C, MFR 0.5)

  Polypropylene resin 3: Product "Novatec FY6H" manufactured by Japan Polypropylene Corporation (melting point 165 ° C, MFR 2)

  Polypropylene resin 4: Product "Novatec EA7AD" manufactured by Japan Polypropylene Corporation (melting point: 158 ° C, MFR: 1.5)

  A microporous membrane was produced by the method for producing a microporous membrane of Examples 1 to 8 below. In addition, for the measurement of the air permeability, an air permeability meter (Gare-type densometer) manufactured by Toyo Seiki Seisaku-sho, Ltd. was used.

Example 1
(Raw material) Propylene polymer 1 was used. (Step 1) The raw material melt-kneaded by a single-screw extruder was extruded from a T-die at a draft ratio of 159 to produce a raw film having a thickness of 22 μm. (Step 2) Next, the raw film was heat-treated at 150 ° C. (Step 3) The raw film was cold-stretched at 30 ° C. in the lengthwise direction to 1.03 times. (Step 4) The obtained stretched film was warm drawn at a temperature of 145 ° C. in the lengthwise direction by 2.9 times. (Step 5) The stretched film was relaxed at 150 ° C. so that the length of the obtained stretched film was 0.87 times. Thus, a microporous membrane of the present invention having a final thickness of 20 μm was obtained. The porosity and air permeability of the obtained microporous membrane were measured by the above-mentioned method, and the results are shown in Table 1 together with the production conditions.

(Example 2)
(Raw material) Propylene polymer 1 was used. (Step 1) The raw material melt-kneaded by a single-screw extruder was extruded from a T-die at a draft ratio of 159 to produce a raw film having a thickness of 22 μm. (Step 2) Next, the raw film was heat-treated at 150 ° C. (Step 3) The raw film was cold-stretched at 30 ° C. in the lengthwise direction to 1.03 times. (Step 4) The stretched film obtained was warm-stretched 3.0 times in the lengthwise direction at 145 ° C. (Step 5) The stretched film was relaxed at 150 ° C. so that the length of the obtained stretched film was 0.88 times. Thus, a microporous membrane of the present invention having a final thickness of 20 μm was obtained. The porosity and air permeability of the obtained microporous membrane were measured by the above-mentioned method, and the results are shown in Table 1 together with the production conditions.

( Reference example: 3)
(Raw material) Propylene polymer 1 was used. (Step 1) The raw material melt-kneaded by a single-screw extruder was extruded from a T-die at a draft ratio of 159 to produce a raw film having a thickness of 22 μm. (Step 2) Next, the raw film was heat-treated at 150 ° C. (Step 3) The raw film was cold-stretched at 30 ° C. in the lengthwise direction to 1.03 times. (Step 4) The stretched film thus obtained was warm drawn at 145 ° C. in the lengthwise direction by 2.8 times. (Step 5) The stretched film was relaxed at 150 ° C. so that the length of the obtained stretched film was 0.85 times. Thus, a microporous membrane of the present invention having a final thickness of 20 μm was obtained. The porosity and air permeability of the obtained microporous membrane were measured by the above-mentioned method, and the results are shown in Table 1 together with the production conditions.

(Example 4)
(Raw material) Propylene polymer 2 was used. (Step 1) The raw material melt-kneaded by a single-screw extruder was extruded from a T-die at a draft ratio of 159 to produce a raw film having a thickness of 22 μm. (Step 2) Next, the raw film was heat-treated at 150 ° C. (Step 3) The raw film was cold-stretched at 30 ° C. in the lengthwise direction to 1.03 times. (Step 4) The stretched film obtained was warm drawn at 145 ° C. in the lengthwise direction by 3.3 times. (Step 5) The stretched film was relaxed at 150 ° C. so that the length of the obtained stretched film was 0.88 times. Thus, a microporous membrane of the present invention having a final thickness of 20 μm was obtained. The porosity and air permeability of the obtained microporous membrane were measured by the above-mentioned method, and the results are shown in Table 1 together with the production conditions.

(Example 5)
(Raw material) Propylene polymer 3 was used. (Step 1) The raw material melt-kneaded by a single-screw extruder was extruded from a T-die at a draft ratio of 159 to produce a raw film having a thickness of 22 μm. (Step 2) Next, the raw film was heat-treated at 150 ° C. (Step 3) The raw film was cold-stretched at 30 ° C. in the lengthwise direction to 1.03 times. (Step 4) The stretched film obtained was warm-stretched 3.0 times in the lengthwise direction at 145 ° C. (Step 5) The stretched film was relaxed at 150 ° C. so that the length of the obtained stretched film was 0.88 times. Thus, a microporous membrane of the present invention having a final thickness of 20 μm was obtained. The porosity and air permeability of the obtained microporous membrane were measured by the above-mentioned method, and the results are shown in Table 1 together with the production conditions.

(Example 6)
(Raw material) Propylene polymer 3 was used. (Step 1) The raw material melt-kneaded by a single-screw extruder was extruded from a T-die at a draft ratio of 159 to produce a raw film having a thickness of 22 μm. (Step 2) Next, the raw film was heat-treated at 150 ° C. (Step 3) The raw film was cold stretched by 1.04 times in the lengthwise direction at 30 ° C. (Step 4) The stretched film obtained was warm-stretched 3.0 times in the lengthwise direction at 145 ° C. (Step 5) The stretched film was relaxed at 150 ° C. so that the length of the obtained stretched film was 0.88 times. Thus, a microporous membrane of the present invention having a final thickness of 20 μm was obtained. The porosity and air permeability of the obtained microporous membrane were measured by the above-mentioned method, and the results are shown in Table 1 together with the production conditions.

(Example 7)
(Raw material) Propylene polymer 4 was used. (Step 1) The raw material melt-kneaded by a single-screw extruder was extruded from a T-die at a draft ratio 205 to produce a raw film having a thickness of 22 μm. (Step 2) Next, the raw film was heat-treated at 150 ° C. (Step 3) The raw film was cold-stretched at 30 ° C. in the lengthwise direction to 1.07 times. (Step 4) The obtained stretched film was warm drawn at 128 ° C. in the lengthwise direction by 3.2 times. (Step 5) The stretched film was relaxed at 150 ° C. so that the length of the obtained stretched film was 0.88 times. Thus, a microporous membrane of the present invention having a final thickness of 20 μm was obtained. The heat shrinkage and porosity of the obtained microporous membrane were measured by the above-described method, and the results are shown in Table 1 together with the production conditions.

(Example 8)
(Raw material) Propylene polymer 4 was used. (Step 1) The raw material melt-kneaded by a single-screw extruder was extruded from a T-die at a draft ratio 205 to produce a raw film having a thickness of 22 μm. (Step 2) Next, the raw film was heat-treated at 150 ° C. (Step 3) The raw film was cold-stretched at 30 ° C. in the lengthwise direction to 1.07 times. (Step 4) The obtained stretched film was warm drawn at 128 ° C. in the lengthwise direction by 3.2 times. (Step 5) The stretched film was relaxed at 150 ° C. so as to have a length of 0.92 times. Thus, a microporous membrane of the present invention having a final thickness of 20 μm was obtained. The heat shrinkage and porosity of the obtained microporous membrane were measured by the above-described method, and the results are shown in Table 1 together with the production conditions.

  The microporous film was manufactured by the manufacturing method of the microporous film of the following Comparative Examples 1-3 for comparison. In addition, for the measurement of the air permeability, an air permeability meter (Gare-type densometer) manufactured by Toyo Seiki Seisaku-sho, Ltd. was used.

(Comparative example 1)
(Raw material) Propylene polymer 4 was used. (Step 1) The raw material melt-kneaded by a single-screw extruder was extruded from a T-die at a draft ratio of 159 to produce a raw film having a thickness of 23 μm. (Step 2) Next, the raw film was heat-treated at 150 ° C. (Step 3) The original film was cold-drawn at 1.07 times in the longitudinal direction at 30 ° C (step 4) and the obtained stretched film was hot-drawn 3.2 times in the lengthwise direction at 145 ° C. (Step 5) The stretched film was relaxed at 150 ° C. so that the length of the obtained stretched film was 0.88 times. Thus, a comparative microporous membrane with a final thickness of 20 μm was obtained. The evaluation results are shown in Table 1 together with the production conditions.

(Comparative example 2)
(Raw material) Propylene polymer 4 was used. (Step 1) The raw material melt-kneaded by a single-screw extruder was extruded from a T-die at a draft ratio 114 to produce a raw film having a thickness of 23 μm. (Step 2) Next, the raw film was heat-treated at 150 ° C. (Step 3) The original film was cold-drawn at 1.07 times in the length direction at 30 ° C (step 4) and the obtained stretched film was hot-drawn at 2.6 times in the length direction at 145 ° C. (Step 5) The stretched film was relaxed at 150 ° C. so that the length of the obtained stretched film was 0.88 times. Thus, a comparative microporous membrane with a final thickness of 20 μm was obtained. The evaluation results are shown in Table 1 together with the production conditions.

(Comparative example 3)
(Raw material) Propylene polymer 4 was used. (Step 1) The raw material melt-kneaded by a single-screw extruder was extruded from a T-die at a draft ratio 114 to produce a raw film of 22 μm in thickness. (Step 2) Next, the raw film was heat-treated at 150 ° C. (Step 3) The original film was cold-drawn at 1.07 times in the longitudinal direction at 30 ° C (step 4) and the obtained stretched film was hot-stretched at 128 ° C in the longitudinal direction 3.2 times. (Step 5) The stretched film was relaxed at 150 ° C. so that the length of the obtained stretched film was 0.88 times. Thus, a comparative microporous membrane with a final thickness of 20 μm was obtained. The evaluation results are shown in Table 1 together with the production conditions.

  In Comparative Example 1, the warm drawing in step 4 was performed at a temperature higher than a predetermined temperature. The air permeability of the resulting microporous membrane was low, and the desired microporous membrane was not obtained. In Comparative Examples 2 and 3, the draft ratio in Step 1 is too small. The resulting microporous membrane has low porosity and poor air permeability. In the microporous membrane of the present invention, it is predicted that pores having a relatively small pore size are present at high density. Such micropore morphology is also reflected in the permeability per thickness of the microporous membrane.

  The microporous membrane exhibiting high selective permeability to the substance obtained by the production method of the present invention can be used as a substance-permeable material with higher added value. In addition, since it is considered that the generation of defects such as large pores is suppressed in the method for producing a microporous membrane of the present invention, a material which can stably maintain material permeability and has a stable mechanical strength is produced. It is thought that The obtained microporous film is useful as a separator of electricity storage devices such as batteries and capacitors which are required to have high performance and reliability such as high ion conductivity and strength. The obtained microporous membrane is particularly suitable for a lithium ion battery separator which is required to have low cost, high performance and high durability.

Claims (3)

A fine structure having a porosity of 49% to 55%, an air permeability of 150 to 350 sec / 100 mL, and an air permeability per thickness of 10.0 to 16.0 sec / 100 mL / μm, including the following steps: Method for producing a porous membrane
(Step 1) A step of extruding a polypropylene polymer at a draft ratio of 120 or more to form a raw film.
(Step 2) The raw film obtained in Step 1 is subjected to a tension of a certain size such that the length of the raw film in the length direction of the raw film is more than 1.0 times and not more than 1.1 times. And heat treating at a temperature 5 to 65 ° C. lower than the melting point of the polypropylene polymer .
(Step 3) A step of cold-stretching the heat-treated raw film obtained in Step 2 at 1.0 to 1.04 times in the length direction at -5 to 45 ° C.
(Step 4) A step of warm-stretching the stretched film after step 3 at a temperature 15 to 65 ° C. lower than the melting point of the polypropylene polymer in the length direction by 2.9 to 3.3 times.
(Step 5) A step of relaxing the warm-stretched film obtained in Step 4 so that the length becomes 0.7 to 1.0 times under heating.   In step 1, the melting point is in the range of 150 to 170 ° C., and the melt mass flow rate (MFR, measured under the conditions according to JIS K 6758 (230 ° C., 21.18 N)) is in the range of 0.1 to 10 g / 10 min And extruding a polypropylene-based polymer, which is a propylene-based polymer, optionally containing at least one selected from ethylene and an α-olefin having 4 to 8 carbon atoms. The manufacturing method according to Item 1.   The method according to claim 1 or 2, wherein in step 1, extrusion molding is performed at a draft ratio of 150 or more.