Method for producing porous polymer film

Abstract

To provide a method for producing a porous polymer film.SOLUTION: A method for producing a porous polymer film comprises the following steps (a) to (e): (a) preparing a polymer film containing a polyolefin; (b) placing the polymer film, a monomer, and a polymerization initiator in a pressure vessel; (c) introducing carbon dioxide into the pressure vessel; (d) adjusting temperature and pressure in the pressure vessel; and (e) releasing the pressure in the pressure vessel to produce a porous polymer film. In step (d), the pressure is adjusted to a level at or above the critical pressure of carbon dioxide, monomer polymerization is initiated, and phase separation between the polymer resulting from the monomer polymerization and the polyolefin is induced.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present disclosure relates to a method for making a porous polymer film. [Background technology]

[0002] Japanese Patent Application Laid-Open No. 2022-116681 discloses a method for producing an olefin-based resin porous body. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Publication No. 2022-116681 Summary of the Invention [Problem to be solved by the invention]

[0004] A method for forming a porous body using high-pressure carbon dioxide (high-pressure CO2) has been proposed. Specifically, a polymer solution is formed in a pressure vessel. The polymer solution contains a polyolefin and a solvent. High-pressure CO2 is introduced into the pressure vessel. The high-pressure CO2 is a poor solvent for the polymer components. The pressure in the pressure vessel is released, forming a porous body.

[0005] In this method, the thickness of the porous body is determined by the amount of polymer solution added. If the amount of polymer solution is reduced in order to form a thin film, it may not be possible to form a film. In other words, there is room for improvement in terms of thinning.

[0006] An object of the present disclosure is to provide a method for producing a porous polymer film. [Means for solving the problem]

[0007] The technical configuration and effects of the present disclosure will be described below. However, the mechanism of action includes speculation. The mechanism of action does not limit the technical scope of the present disclosure.

[0008] 1. A method for producing a porous polymer film includes the following steps (a) to (e): (a) A polymer film containing polyolefin is prepared. (b) The polymer film, monomer, and polymerization initiator are placed in a pressure vessel. (c) Carbon dioxide is introduced into the pressure vessel. (d) Regulating the temperature and pressure in the pressure vessel. (e) Releasing the pressure in the pressure vessel produces a porous polymer film. In the above step (d), the pressure is adjusted to be equal to or higher than the critical pressure of carbon dioxide, polymerization of the monomer occurs, and phase separation between the polymer produced by the polymerization of the monomer and the polyolefin is induced.

[0009] In the manufacturing method "1" above, the thinning process is separated from the porosity process. That is, in the above (a), a polymer film is prepared in advance. The polymer film can be adjusted to a desired thickness. In the above (d), the polymerization of the monomer can be initiated by cleavage of the polymerization initiator. The polymerization of the monomer generates a second polymer component. Phase separation between the second polymer component and the polyolefin (polymer film) can be induced. The pressure is then released, allowing the porous polymer film to be recovered. In the manufacturing method "1" above, the thickness of the porous polymer film can be adjusted by the thickness of the substrate (polymer film). For example, it is thought that the use of a thin substrate can make the porous polymer film thinner.

[0010] CO2 above its critical pressure can form a high-pressure fluid. The high-pressure fluid (CO2) is thought to be a carrier that transports the monomer into the polymer film. Monomers and polyolefins (polymer films) tend to have very low compatibility. Therefore, it is generally thought that it is difficult for the monomer to penetrate the polyolefin, not only at room temperature and normal pressure, but also at high temperature and pressure. It is thought that the high-pressure CO2 acts as a carrier, allowing the monomer to penetrate the polyolefin. Furthermore, if the reaction system becomes even hotter, the monomer may penetrate the polyolefin even without the carrier (high-pressure CO2). However, in high-temperature environments, the polyolefin may melt. Under conditions where the polyolefin melts, it is thought to be difficult to form a porous polymer film while maintaining the outer shape of the substrate (polymer film).

[0011] 2. The method for producing a porous polymer film according to the above item "1" may include, for example, the following configuration: In the above item (d), the temperature inside the pressure vessel is adjusted to within the range of 60 to 100°C.

[0012] At temperatures above 60°C, the polymerization initiator tends to be easily cleaved. At temperatures below 100°C, the polymer film (polyolefin) tends to be difficult to melt.

[0013] 3. The method for producing a porous polymer film according to the above "1" or "2" may include, for example, the following configuration: In the above (b), in addition to the polymer film, the monomer, and the polymerization initiator, pentane is placed in the pressure vessel.

[0014] The addition of pentane to the reaction system is expected to result in smaller pores in the porous polymer film. Pentane and polyolefins may have a certain degree of compatibility. It is believed that pentane plasticizes the polyolefin, making it easier for the monomer to penetrate the polyolefin and also promoting phase separation. Furthermore, because pentane has a low boiling point, it can volatilize when pressure is released. Therefore, it is believed that pentane is less likely to remain in the porous polymer film.

[0015] 4. The method for producing a porous polymer film according to any one of the above items "1" to "3" may include, for example, the following configuration: In the above (d), the pressure is adjusted to within the range of 10 to 30 MPa.

[0016] At pressures of 10 MPa or higher, CO2 and the monomer are expected to form a homogeneous phase. The formation of a homogeneous phase is expected to promote the penetration of the monomer. Pressures of 30 MPa or lower are expected to broaden the range of pressure vessel options, for example.

[0017] 5. The method for producing a porous polymer film according to any one of the above items "1" to "4" may include, for example, the following configuration: The monomer includes an acrylic monomer. The polymerization initiator includes an azo compound.

[0018] Hereinafter, an embodiment of the present disclosure (hereinafter may be abbreviated as "the present embodiment") and an example of the present disclosure (hereinafter may be abbreviated as "the present example") will be described. However, the present embodiment and the example do not limit the technical scope of the present disclosure. The present embodiment and the example are illustrative in all respects. The present embodiment and the example are non-restrictive. The technical scope of the present disclosure encompasses all modifications within the meaning and scope equivalent to the description of the claims. For example, it is also intended from the beginning that any configuration may be extracted from the present embodiment and arbitrarily combined. [Brief explanation of the drawings]

[0019] [Figure 1] 1 is a schematic flowchart of a method for producing a porous polymer film in the present embodiment. [Figure 2] FIG. 2 is a conceptual diagram illustrating an example of a manufacturing apparatus according to the present embodiment. [Figure 3] 1 is a table showing experimental conditions. [Figure 4] 1 is an SEM image of the sample. DETAILED DESCRIPTION OF THE INVENTION

[0020] -Key terms- "Comprise," "include," "have," and variations thereof are open-ended terms. Open-ended terms may or may not include additional elements in addition to the required elements. "Consisting of" is a closed term. However, even a structure expressed in closed terminology may include additional elements that are normally associated with the technology or that are unrelated to the technology in question. "Consisting essentially of..." is a semi-closed term. Semi-closed terminology allows for the addition of elements that do not substantially affect the basic and novel characteristics of the technology in question.

[0021] Unless otherwise specified, the order of execution of multiple steps, actions, operations, etc. included in various methods is not limited to the order described. For example, multiple steps may proceed simultaneously. For example, multiple steps may occur one after the other.

[0022] All numerical values ​​are modified by the term "about." The term "about" may mean, for example, ±5%, ±3%, ±1%, etc. All numerical values ​​may be approximate values ​​that may vary depending on the application of the subject technology. All numerical values ​​may be expressed with significant figures. Measured values ​​may be average values ​​of multiple measurements unless otherwise specified. The number of measurements may be 3 or more, 5 or more, or 10 or more. In general, the more measurements there are, the more reliable the average value is expected to be. Measured values ​​may be rounded to the nearest significant figure. Measured values ​​may include errors, such as those associated with the detection limits of the measuring device.

[0023] -Method of manufacturing porous polymer film- Figure 1 is a schematic flowchart of a method for producing a porous polymer film according to this embodiment. Hereinafter, the "method for producing a porous polymer film according to this embodiment" may be abbreviated as "this method." This method includes "(a) preparation of a polymer film," "(b) placement of materials," "(c) introduction of CO2," "(d) adjustment of temperature and pressure," and "(e) release of pressure." This method may further include, for example, "(f) washing," etc.

[0024] (a) Preparation of polymer films The method includes preparing a polymer film. The polymer film is a substrate for the porous polymer film (target object). The polymer film includes a polyolefin. The polyolefin may be a homopolymer or a copolymer. The polymer film may include a polymer alloy. The polymer film may include, for example, at least one selected from the group consisting of polyethylene (PE) and polypropylene (PP).

[0025] The density of polyolefin is, for example, 900 kg / m 3 More than 925kg / m 3 or more, or 950 kg / m 3 The density of the polyolefin may be, for example, 1000 kg / m or more. 3 Below 950kg / m 3 or less, or 925 kg / m 3The melt mass flow rate (MFR) of the polyolefin may be, for example, 25 g / 10 min or more, 50 g / 10 min or more, 75 g / 10 min or more, 100 g / 10 min or more, or 125 g / 10 min or more. The MFR of the polyolefin may be, for example, 150 g / 10 min or less, 125 g / 10 min or less, 100 g / 10 min or less, 75 g / 10 min or less, or 50 g / 10 min or less. The melting point of the polyolefin may be, for example, 100°C or more, 125°C or more, or 150°C or more. The melting point of the polyolefin may be, for example, 175°C or less, 150°C or less, or 125°C or less.

[0026] The polymer film may be substantially non-porous. The polymer film may be prepared by any method. For example, the polymer film may be prepared by melt extrusion molding or the like. The thickness of the polymer film may be, for example, 1 μm or more, 5 μm or more, or 10 μm or more. The thickness of the polymer film may be, for example, 100 μm or less, 50 μm or less, or 30 μm or less.

[0027] ·Manufacturing equipment 2 is a conceptual diagram showing an example of a manufacturing apparatus in this embodiment. The following operations can be performed, for example, in a manufacturing apparatus 100. The manufacturing apparatus 100 includes a gas cylinder 1, a drying tube 2, a cooling device 3, a filter 4, a pump 5, a first pressure gauge 6, a first safety valve 7, a temperature regulator 8, a check valve 9, a pressure vessel 10, an agitator 11, a heater 12, a second pressure gauge 13, a second safety valve 14, and a wet gas flow meter 15. The manufacturing apparatus 100 further includes a pressure regulator 20, a first valve 21, a second valve 22, and a third valve 23.

[0028] (b) Material placement The process involves placing a polymer film, a monomer, and a polymerization initiator into a pressure vessel 10 .

[0029] ·monomer Polymerization of the monomer may generate a second polymer component in the system. The second polymer component may have lower solvent resistance than polyolefin. The monomer may include, for example, an acrylic monomer. The monomer may include, for example, at least one selected from the group consisting of acrylamide, acrylic acid ester, and methacrylic acid ester. The monomer may include, for example, at least one selected from the group consisting of N-benzyl methacrylamide, N-dodecyl methacrylamide; methyl methacrylate (MMA), ethyl methacrylate, propyl methacrylate, butyl methacrylate, hexyl methacrylate, octyl methacrylate, decyl methacrylate, phenyl methacrylate, benzyl methacrylate; methyl acrylate, ethyl acrylate, propyl acrylate, butyl acrylate, hexyl acrylate, octyl acrylate, decyl acrylate, phenyl acrylate, and benzyl acrylate. The amount of the monomer blended per 1 g of polymer film may be, for example, 15 ml or more, 16 ml or more, 17 ml or more, 18 ml or more, or 19 ml or more. The amount of the monomer blended per 1 g of polymer film may be, for example, 20 ml or less, 19 ml or less, 18 ml or less, 17 ml or less, or 16 ml or less.

[0030] Polymerization initiator The polymerization initiator initiates the polymerization of the monomer. In this production method, for example, a thermal polymerization initiator or the like can be used. The polymerization initiator may include, for example, an azo compound or the like. An azo compound refers to an organic compound in which two organic groups are linked by an azo group (RN=N-R'). The azo compound may include, for example, at least one selected from the group consisting of azonitrile, azoester, azoamide, azoamidine, and azoimidazoline. The polymerization initiator may include, for example, azobisisobutyronitrile (AIBN) or the like. The polymerization initiator may be, for example, oil-soluble. The amount of the polymerization initiator mixed per 1 ml of monomer may be, for example, 0.010 g or more, 0.011 g or more, 0.012 g or more, 0.013 g or more, 0.014 g or more, or 0.015 g or more. The amount of the polymerization initiator mixed per 1 ml of monomer may be, for example, 0.020 g or less, 0.015 g or less, 0.014 g or less, 0.013 g or less, or 0.012 g or less.

[0031] ·solvent A solvent may be added to the system. The solvent may be compatible with the polymer film (polyolefin). The solvent may plasticize the polyolefin. The solvent may include, for example, an alkane having 5 to 10 carbon atoms. The solvent may include, for example, a linear alkane, a branched alkane, a cyclic alkane, etc. The solvent may include, for example, at least one selected from the group consisting of pentane (n-pentane, isopentane, neopentane), hexane, heptane, octane, nonane, and decane. The amount of solvent added per 1 g of polymer film may be, for example, 25 ml or more, 50 ml or more, 75 ml or more, or 100 ml or more. The amount of solvent added per 1 g of polymer film may be, for example, 150 ml or less, 125 ml or less, 100 ml or less, or 75 ml or less.

[0032] (c) Introduction of CO2 The method includes introducing CO2 into a pressure vessel 10. For example, a gas cylinder 1 provides the CO2. The CO2 is supplied via a drying tube 2 and a cooling device 3 to a pump 5. The pump 5 may compress the CO2. After compression, the CO2 may form a high-pressure fluid.

[0033] The temperature of the compressed CO2 may be adjusted. For example, a temperature adjustment device 8 may adjust the temperature of the CO2. For example, the CO2 may be heated while being stirred in the temperature adjustment device 8. After the pressure and temperature are adjusted, the CO2 may be introduced into the pressure vessel 10. After the CO2 is introduced, for example, a stirrer 11 may stir the system. The stirring time may be, for example, 1 minute or more, 5 minutes or more, or 10 minutes or more. The stirring time may be, for example, 20 minutes or less.

[0034] (d) Temperature and pressure regulation The method includes adjusting the temperature and pressure within the pressure vessel 10. Adjusting the temperature and pressure can cause polymerization of the monomer. For example, when the monomer is MMA, polymethyl methacrylate (PMMA) can be produced as the second polymer component. Furthermore, phase separation between the second polymer component and the polyolefin can be induced.

[0035] For example, a heater 12 may heat the pressure vessel 10. The temperature inside the pressure vessel 10 (reaction temperature) may be adjusted to, for example, the critical temperature of CO (31.1°C) or higher. The reaction temperature may be adjusted to, for example, the decomposition temperature of the polymerization initiator or higher. Polymerization of the monomer may occur due to cleavage of the polymerization initiator. The reaction temperature may be, for example, 40°C or higher, 60°C or higher, 70°C or higher, or 80°C or higher. The reaction temperature may be adjusted to, for example, lower than the melting point of the polyolefin. The reaction temperature may be, for example, 120°C or lower, 110°C or lower, or 100°C or lower. The reaction temperature may be, for example, 60 to 100°C.

[0036] The pressure inside the pressure vessel 10 (reaction pressure) is adjusted to be equal to or higher than the critical pressure of CO2 (7.38 MPa). Within the pressure vessel 10, CO2 may form a high-pressure fluid. Within the pressure vessel 10, CO2 may form a supercritical fluid. CO2 can form a supercritical fluid under conditions of equal to or higher than its critical temperature and pressure. The formation of a supercritical fluid by CO2 may also promote the transport of the monomer. The reaction pressure may be, for example, 10 MPa or higher, 15 MPa or higher, 20 MPa or higher, 25 MPa or higher, or 30 MPa or higher. The reaction pressure may be, for example, 35 MPa or lower, 30 MPa or lower, 25 MPa or lower, or 20 MPa or lower.

[0037] The reaction temperature and reaction pressure may be maintained for, for example, 1 minute or more, 5 minutes or more, or 10 minutes or more, and may be maintained for, for example, 60 minutes or less, 30 minutes or less, or 10 minutes or less.

[0038] In this manufacturing method, high-pressure CO2 acts as a carrier, allowing the monomer to penetrate the polymer film (polyolefin). Once the monomer has penetrated the polymer film, a second polymer component is produced. Furthermore, phase separation between the second polymer component and the polyolefin is induced. This phase separation is thought to form the basis for the pore structure.

[0039] (e) Pressure release The method includes producing a porous polymer film by releasing the pressure in the pressure vessel 10. For example, the pressure vessel 10 may first be cooled using a water-cooled chiller (not shown) or the like. The temperature inside the pressure vessel 10 may be cooled to, for example, 15 to 25°C. After cooling, the temperature of 15 to 25°C may be maintained for, for example, 20 to 40 minutes.

[0040] The temperature inside the pressure vessel 10 may then be adjusted to about 40° C. to prevent the formation of dry ice, which may damage the pore structure.

[0041] For example, the pressure inside the pressure vessel 10 is gradually reduced in pressure while the temperature inside the pressure vessel 10 is maintained at about 40°C. The pressure reduction time may be, for example, about 10 to 30 minutes. During the pressure reduction process, the solvent and CO2 volatilize and are released outside the system. After the pressure inside the pressure vessel 10 reaches normal temperature and pressure, the polymer film can be recovered.

[0042] (f) Cleaning This manufacturing method may include washing the polymer film. For example, residual monomers and the second polymer component can be removed by washing. A solvent that does not easily dissolve polyolefin but easily dissolves the second polymer component can be selected as the washing solvent. For example, acetone or the like can be used. Acetone can dissolve, for example, PMMA. For example, the polymer film can be shaken in the solvent. The shaking time can be, for example, about 24 hours. In this manner, a porous polymer film can be produced.

[0043] -Porous polymer film- The porous polymer film can be used, for example, as a battery separator. That is, the present disclosure also provides a method for producing a battery separator. The thickness of the porous polymer film can be, for example, 1 μm or more, 5 μm or more, 10 μm or more, or 15 μm or more. The thickness of the porous polymer film can be, for example, 100 μm or less, 50 μm or less, or 30 μm or less.

[0044] The porous polymer film has a polymer skeleton. The polymer skeleton may be connected in the form of a three-dimensional network. First pores may be formed in the gaps of the polymer skeleton, interconnected in the form of a three-dimensional network. The first pores may be formed by interconnecting a plurality of bubble-like pores. The polymer skeleton itself may also be porous. That is, second pores (micropores) may be formed in the polymer skeleton. The maximum Feret diameter of the first pores may be, for example, more than 1 μm, 5 μm or more, 10 μm or more, or 20 μm or more. The maximum Feret diameter of the first pores may be, for example, 50 μm or less, 25 μm or less, 15 μm or less, or 10 μm or less. The maximum Feret diameter of the second pores may be, for example, 1 μm or less, 0.5 μm or less, 0.1 μm or less, or 0.05 μm or less. The maximum Feret diameter of the second pores may be, for example, 0.01 μm or more, 0.05 μm or more, or 0.1 μm or more. The maximum Feret diameter of the pores can be measured in a scanning electron microscope (SEM) image of the porous polymer film. [Example]

[0045] -experiment- Sample preparation The following materials were prepared: Polyolefin: Tosoh PE "Petrothene 353" Polyolefin: Prime Polymer homo PP "J137G" Monomer: MMA Polymerization initiator: AIBN Solvent: n-pentane

[0046] The experimental conditions are shown in Figure 3. Samples No. 1 to No. 10 were manufactured according to the following procedures (1) to (7). (1) A polymer film made of PE is prepared. Note that the above-mentioned homo-PP may be used instead of the above-mentioned PE. (2) The manufacturing apparatus 100 shown in Figure 2 is prepared. The materials shown in Figure 3 are sealed in a pressure vessel 10. CO2 is introduced into the pressure vessel 10. (3) The temperature and pressure inside the pressure vessel 10 are adjusted to the temperature and pressure shown in the "Reaction Conditions" item of FIG. 3. After the start of adjustment of the reaction conditions, the inside of the pressure vessel 10 is stirred for about the first 5 minutes. (4) The pressure vessel 10 is cooled to about 20 °C by water cooling. After cooling, the state is maintained for about 30 minutes. (5) While the pressure vessel 10 is heated to 40 °C, the inside of the pressure vessel 10 is slowly depressurized. The depressurization time is about 20 minutes. (6) After the inside of the pressure vessel 10 reaches normal temperature and pressure, the pressure vessel 10 is opened to recover the sample. (7) The sample is washed with acetone. For example, the sample is subjected to a shaking treatment in acetone for 24 hours. After washing, the sample is dried.

[0047] · Evaluation The degree of porosification was evaluated by visual observation. The polymer film (substrate) before porosification is transparent. When the polymer film becomes porous, it whitens. This is thought to be due to diffuse reflection of light. The degree of porosification can be evaluated by the degree of whitening. In the "Porosification" item in FIG. 3, the degree of porosification is in the order of (small) C < B < A (large).

[0048] · Results In No. 1 and No. 2, the formation of pores was insufficient. In No. 1 and No. 2, no monomer and polymerization initiator were used.

[0049] In No. 3, No. 4, and No. 5, the formation of pores was confirmed. In No. 3, No. 4, and No. 5, a monomer and a polymerization initiator were used.

[0050] In No. 6, the formation of pores was insufficient. In No. 6, no monomer and polymerization initiator were used.

[0051] Pore ​​formation was confirmed in samples No. 7, No. 8, and No. 9. Figure 4 shows SEM images of the samples. For example, the pores in No. 9 are smaller than those in No. 4. A solvent (n-pentane) was used in No. 9.

[0052] In Nos. 3, 4, 5, 7, 8, and 9, there is a tendency for porosity to be promoted when the reaction temperature is in the range of 60 to 100°C. This is thought to be because polymerization of the monomer is more likely to begin at reaction temperatures of 60°C or higher.

[0053] In No. 10, the polymer film melted, presumably due to excess solvent. [Explanation of symbols]

[0054] 1 gas cylinder, 2 drying tube, 3 cooling device, 4 filter, 5 pump, 6 first pressure gauge, 7 first safety valve, 8 temperature adjustment device, 9 check valve, 10 pressure vessel, 11 stirring device, 12 heater, 13 second pressure gauge, 14 second safety valve, 15 wet gas flow meter, 20 pressure regulator, 21 first valve, 22 second valve, 23 third valve, 100 manufacturing equipment.

Claims

1. (a) providing a polymer film comprising a polyolefin; (b) placing the polymer film, monomer, and polymerization initiator in a pressure vessel; (c) introducing carbon dioxide into the pressure vessel; (d) adjusting the temperature and pressure within the pressure vessel; and (e) releasing the pressure in the pressure vessel to produce a porous polymer film; Including, In the above (d), The pressure is adjusted to be equal to or greater than the critical pressure of carbon dioxide; Polymerization of the monomers occurs; and Phase separation is induced between the polymer produced by polymerization of the monomer and the polyolefin. A method for producing a porous polymer film.

2. In the step (d), the temperature in the pressure vessel is adjusted to a range of 60 to 100°C. The method for producing the porous polymer film according to claim 1 .

3. In the step (b), pentane is placed in the pressure vessel in addition to the polymer film, the monomer, and the polymerization initiator. A method for producing the porous polymer film according to claim 1 or 2.

4. In (d), the pressure is adjusted to a range of 10 to 30 MPa. A method for producing the porous polymer film according to claim 1 or 2.

5. The monomer includes an acrylic monomer, and The polymerization initiator includes an azo compound. A method for producing the porous polymer film according to claim 1 or 2.

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