Polymer composition for forming porous film, method for producing porous film, porous film, flexible metal-clad laminate and electronic substrate

Abstract

To provide a polymer composition for forming a porous film which has a low permittivity and a low dielectric dissipation factor and in which the problems of liquid permeability and water absorption have been mitigated and the risk of decrease in mechanical strength has been reduced.SOLUTION: A polymer composition for porous film formation comprises a polymer, a solvent (A), and a solvent (B). The solvent (A) has a boiling point of 100°C or higher, and a 10 mass% solution of the polymer in the solvent (A) has a 25°C viscosity of 150 dPa s or higher. The solvent (B) has a boiling point lower than 100°C, and a 10 mass% solution of the polymer in the solvent (B) has a 25°C viscosity less than 50 dPa s.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a polymer composition for forming a porous film, a method for producing a porous film, a porous film, a flexible metal-clad laminate, and an electronic substrate.

[0002] In recent years, mobile communication devices compatible with the fifth generation mobile communication system (5G) have appeared, and development has begun with an eye on the sixth generation. In order to meet the demand for miniaturization in mobile communication devices and other devices, flexible metal-clad laminates, which are made by bonding metal foil to a flexible polymer film, are often used for printed wiring boards in order to save space.

[0003] On the other hand, in mobile communication devices and the like, the use of high-frequency (GHz order) electrical signals is increasing in order to transmit and process large volumes of information at high speed. High-frequency signals are prone to attenuation, so materials used are required to be designed to suppress transmission loss. In addition, these devices generate a lot of heat, so materials must also be heat-resistant.

[0004] Against this background, polymer films with low dielectric constants and low dielectric loss tangents that can reduce transmission loss have been developed. Polyimide is known as a material with excellent heat resistance, and methods have been proposed for improving the dielectric properties of polyimide films by making the film porous.

[0005] For example, Patent Document 1 proposes a method for producing a low dielectric constant film, for example, a nanoporous polymer film having a low dielectric constant and a thickness of 10 μm or less, which comprises providing a polymer in a solution containing at least two solvents, the difference in boiling point between the lowest boiling point solvent and the highest boiling point solvent being approximately 50° C. or more, to form pores in the film with an average pore size of less than 30 nm, and a nanoporous film produced by this method.

[0006] For example, Patent Document 2 proposes a method for producing a porous polyimide film having dense layers on both sides of the film and a porous layer in the center, which comprises a film-like composition (C) consisting of a homogeneous solution (A) of a polyimide precursor and a poor solvent (B) for the polyimide precursor, where the poor solvent has a boiling point or thermal decomposition point higher than or equal to the imidization initiation temperature of the polyimide precursor, and the film-like composition is imidized by heat treatment; and also proposes a porous polyimide film produced by this method.

[0007] Furthermore, Patent Document 3 proposes a porous low dielectric polymer film in which fine pores are dispersed in a film made of a polymer material, the porosity of the film being 60% or more and the average pore size of the pores being 10 μm or less, and Patent Document 4 proposes a porous low dielectric polymer film in which fine pores are dispersed in a base material layer made of a polymer material, the film being characterized in that a substantially smooth skin layer made of the polymer material of the base material layer is formed on at least one surface of the base material layer. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] Japanese Patent Application Laid-Open No. 2000-154268 [Patent Document 2] Japanese Patent Application Laid-Open No. 2001-151929 [Patent Document 3] Japanese Patent Application Publication No. 2018-21171 [Patent Document 4] Japanese Patent Application Publication No. 2018-21172 Summary of the Invention [Problem to be solved by the invention]

[0009] However, the film of Patent Document 1 is obtained by forming a polymer coating on a substrate, then contacting the substrate with a non-solvent for the polymer to bring about a phase inversion, which requires multi-stage processing for production. In addition, the obtained film has pores even near the surface, which causes problems such as poor liquid permeability and water absorption when subjected to plating processing, and also causes problems such as low mechanical strength. In Patent Document 2, the polyimide precursor is converted into polyimide by heat treatment in the film production process, so it is difficult to control the size of the independent pores. In Patent Documents 3 and 4, the pore size (average pore diameter) of the film actually obtained is 4.4 to 9.8 μm, which means that the problems of permeability to the plating solution and water absorption cannot be avoided, and the film also has problems such as low mechanical strength. [Means for solving the problem]

[0010] The present invention aims to provide a polymer composition for forming a porous film having a low dielectric constant and a low dielectric loss tangent, improved liquid permeability and water absorption, and reduced risk of a decrease in mechanical strength, and also aims to provide such a porous film and a method for producing the porous film.

[0011] The gist and configuration of the present invention are as follows. [1] A method for producing a polymer, a solvent (A), and a solvent (B), the solvent (A) has a boiling point of 100°C or higher, and a solution in which the polymer is dissolved at a concentration of 10% by mass has a viscosity of 150 dPa s or higher at 25°C; the solvent (B) has a boiling point of less than 100°C, and a solution in which the polymer is dissolved at a concentration of 10% by mass has a viscosity of less than 50 dPa·s at 25°C; A polymer composition for forming a porous film. [2] The polymer composition for forming a porous film according to [1], wherein the solvent (A) is an alkylene glycol monoalkyl ether. [3] The polymer composition for forming a porous film according to [1] or [2], wherein the solvent (A) is at least one selected from the group consisting of propylene glycol 1-monomethyl ether and diethylene glycol monoethyl ether. [4] The polymer composition for forming a porous film according to any one of [1] to [3], wherein the solvent (B) is at least one selected from the group consisting of acetone, ethyl acetate, and tetrahydrofuran. [5] The polymer composition for forming a porous film according to any one of [1] to [4], wherein the difference between the boiling point of the solvent (A) and the boiling point of the solvent (B) is 40° C. or more and 150° C. or less. [6] The polymer composition for forming a porous film according to any one of [1] to [5], wherein the polymer is a polyimide. [7] The polymer composition for forming a porous film according to any one of [1] to [6], wherein the ratio by mass of the solvent (A) to the solvent (B) is 8:2 to 2:8. [8] The polymer composition for forming a porous film according to any one of [1] to [7], wherein the mass of the polymer is 5 parts by mass or more and 30 parts by mass or less relative to 100 parts by mass of the total of the mass of the polymer, the mass of the solvent (A), and the mass of the solvent (B). [9] A step of applying the polymer composition for forming a porous film according to any one of [1] to [8] to a substrate to form a coating film; A step of maintaining the coating film at a temperature of less than 100°C to make it porous; and The process of drying the porous coating by keeping it at a temperature of 100°C or higher. A method for producing a porous membrane, comprising:

[10] The method for producing a porous film according to [9], wherein the step of making the coating film porous is a step of maintaining the coating film at a temperature of 10°C or higher and 80°C or lower.

[11] A method for producing a porous film according to claim 9 or 10, wherein the drying step is a step of drying the porous coating film by maintaining it at a temperature of 150°C or higher and 350°C or lower.

[12] A porous membrane having a porous layer with an average pore size of 0.1 μm or more and 3.0 μm or less, and a skin layer with a thickness of 3.0 μm or more on at least one surface of the porous layer.

[13] The porous membrane of claim 12, wherein the porous layer is made of polyimide.

[14] The porous film of claim 12 or 13, having a dielectric constant of 2.5 or less measured at 10 GHz.

[15] A flexible metal-clad laminate in which a metal foil is laminated on any of the porous films described in

[12] to

[14] .

[16] An electronic substrate having a porous film according to any one of

[12] to

[14] . [Effects of the Invention]

[0012] According to the present invention, a polymer composition for forming a porous film having a low dielectric constant and a low dielectric loss tangent, improved problems of liquid permeability and water absorption, and reduced risk of a decrease in mechanical strength, is provided, along with a method for producing such a porous film. [Brief explanation of the drawings]

[0013] [Figure 1] 1 is a scanning electron microscope (SEM) image (2500x magnification) of a cross section of the porous membrane of Example 3. [Figure 2] FIG. 1 shows a scanning electron microscope (SEM) image of a cross section of the porous membrane of Example 3 and the results of binarization analysis (5000x magnification). DETAILED DESCRIPTION OF THE INVENTION

[0014] <Polymer composition> The porous film-forming polymer composition of the present invention contains a polymer, a solvent (A), and a solvent (B). A porous film can be produced using the porous film-forming polymer composition by applying the composition to a substrate, obtaining a coating, and then removing the solvents (A) and (B). During this process, the solvent (B), which has a low boiling point, evaporates first, forming a skin layer on the surface of the coating film. The concentration of the solvent (A), which has a high boiling point, increases, increasing the viscosity inside the coating film. It is presumed that the containment effect of this skin layer and the increase in viscosity inside the coating film result in small, uniform pores when the solvent (A) evaporates. From this perspective, it is preferable that the solvent (B) is a good solvent for the polymer, and the solvent (A) is a poor solvent for the polymer. Here, the skin layer refers to a layer that is present on the surface side in the cross section of the porous membrane and does not contain pores. The skin layer contributes to improving the mechanical strength of the porous membrane and suppressing the permeability and water absorption of liquids.

[0015] Examples of the polymer include polyimide, polyamide, polyamideimide, polybenzoxazole, and polyphenylene ether, with polyimide being preferred in terms of its good mechanical properties and heat resistance.

[0016] Here, polyimide is a polymer containing imide bonds in the repeating unit. Among polyimides, polyetherimides containing ether bonds in the repeating unit and fluorinated polyimides containing fluorine atoms are preferred.

[0017] From the viewpoint of improving mechanical strength, the polymer preferably has a weight average molecular weight (Mw) of 10,000 or more, more preferably 50,000 or more, and from the viewpoint of improving coating workability, Mw is preferably 1,000,000 or less, more preferably 500,000 or less. Here, Mw is a value determined as a standard polystyrene equivalent value using gel permeation chromatography (hereinafter also referred to as GPC).

[0018] The polyimide can be obtained by reacting a tetracarboxylic acid dihydrate with a diamine and then dehydrating and ring-closing the reaction product. Examples of the tetracarboxylic acid dihydrate include pyromellitic dianhydride, 3,3',4,4'-biphenyltetracarboxylic acid dianhydride, 2,2-bis(2,3-dicarboxyphenyl)-1,1,1,3,3,3-hexafluoropropane dianhydride, 2,2-bis(3,4-dicarboxyphenyl)-1,1,1,3,3,3-hexafluoropropane dianhydride, 3,3',4,4'-benzophenonetetracarboxylic acid dianhydride, bis(3,4-dicarboxyphenyl)ether dianhydride, bis(3,4-dicarboxyphenyl)sulfone dianhydride, etc. The tetracarboxylic acid dianhydrides may be used alone or in combination of two or more. Examples of the diamine include m-phenylenediamine, p-phenylenediamine, 3,4'-diaminodiphenyl ether, 4,4'-diaminodiphenyl ether, 4,4'-diaminodiphenyl sulfone, 3,3'-diaminodiphenyl sulfone, 2,2-bis(4-aminophenoxyphenyl)propane, 2,2-bis(4-aminophenoxyphenyl)hexafluoropropane, 1,3-bis(4-aminophenoxy)benzene, 1,4-bis(4-aminophenoxy)benzene, 2,4-diaminotoluene, 2,6-diaminotoluene, diaminodiphenylmethane, 4,4'-diamino-2,2-dimethylbiphenyl, and 2,2-bis(trifluoromethyl)-4,4'-diaminobiphenyl. The diamines may be used alone or in combination of two or more.

[0019] The solvent (A) and the solvent (B) are solvents in which the polymer is soluble. The solvent (A) has a boiling point of 100°C or higher, and a solution in which the polymer is dissolved in the solvent (A) at a concentration of 10% by mass has a viscosity of 150 dPa·s or higher at 25°C. From the viewpoint of improving the formability of the porous layer, the viscosity is preferably 170 dPa·s or higher. From the viewpoint of improving the workability of coating, the viscosity is preferably 1,000 dPa·s or lower, and more preferably 500 dPa·s or lower. Here, the viscosity is a value measured with an E-type viscometer using a rotor 3°×R14 at a rotation speed of 5 rpm for 30 seconds.

[0020] Solvent (B) has a boiling point of less than 100°C, and a solution of the polymer dissolved in solvent (B) at a concentration of 10% by mass has a viscosity of less than 50 dPa·s at 25°C. From the viewpoint of good mechanical properties, the viscosity is preferably 5 dPa·s or more. From the viewpoint of improving the formability of the skin layer, the viscosity is preferably 30 dPa·s or less, and more preferably 20 dPa·s or less. Here, the viscosity is a value measured with an E-type viscometer using a rotor 3°×R14 at a rotation speed of 5 rpm for 30 seconds.

[0021] The difference between the boiling point of solvent (A) and the boiling point of solvent (B) is preferably 40° C. or more, more preferably 50° C. or more. The upper limit of the boiling point difference is not particularly limited, but can be 150° C. or less. When at least one of solvent (A) and solvent (B) consists of two or more types, it is preferable that any combination of solvent (A) and solvent (B) satisfies the above-mentioned boiling point difference, and it is more preferable that all combinations of solvent (A) and solvent (B) satisfy the above-mentioned boiling point difference.

[0022] The solvents (A) and (B) can be appropriately selected depending on the polymer. When the polymer is a polyimide such as polyetherimide or polyfluoride imide, the solvent (A) may be an alkylene glycol monoalkyl ether. The alkylene moiety in the alkylene glycol monoalkyl ether may be interrupted by an oxygen atom. Examples include ethylene glycol monomethyl ether (boiling point 124°C), ethylene glycol monoethyl ether (135°C), ethylene glycol mono-n-propyl ether (boiling point 150°C), ethylene glycol monoisopropyl ether (144°C), ethylene glycol mono-n-butyl ether (168°C), diethylene glycol monomethyl ether (194°C), diethylene glycol monoethyl ether (196°C), propylene glycol 1-monomethyl ether (1-methoxy-2-propanol, boiling point 121°C), propylene glycol 2-monomethyl ether (2-methoxy-1-propanol, boiling point 130°C), 1-ethoxy-2-propanol (boiling point 131°C), 2-ethoxy-1-propanol (boiling point 141°C), and 1-propoxy-2-propanol (boiling point 148°C). Among these, 1-methoxy-2-propanol and diethylene glycol monoethyl ether are preferred. The solvent (A) may be used alone or in combination of two or more.

[0023] When the polymer is a polyimide such as polyetherimide or polyfluoride imide, examples of the solvent (B) include acetone (56.5°C), ethyl acetate (77.1°C), and tetrahydrofuran (68°C). Tetrahydrofuran is preferred because of its good solubility in polyimides. The solvent (B) may be used alone or in combination of two or more.

[0024] The ratio of the mass of solvent (A) to the mass of solvent (B) (mass of solvent (A):mass of solvent (B)) is preferably 8:2 to 2:8, more preferably 7:3 to 3:7, from the viewpoint of achieving both a low dielectric constant and good mechanical properties. When two or more types of solvent (A) are used, the mass of the solvent (A) is the total value, and when two or more types of solvent (B) are used, the mass of the solvent (B) is the total value. The same applies hereinafter.

[0025] The mass of the polymer is preferably 5 parts by mass or more, more preferably 10 parts by mass or more, relative to 100 parts by mass of the total of the mass of the polymer, the mass of the solvent (A), and the mass of the solvent (B), from the viewpoint of improving the workability of coating, and is preferably 50 parts by mass or less, more preferably 25 parts by mass or less, from the viewpoint of improving the formability of the porous layer.

[0026] The polymer composition can be produced by mixing a polymer, a solvent (A) and a solvent (B).

[0027] Any additive may be blended into the polymer composition as long as it does not impair the effects of the present invention. Examples of such additives include colorants, thickeners, oxygen scavengers, fluorescent brighteners, surfactants, antioxidants, plasticizers, crosslinking agents, flame retardants, antistatic agents, leveling agents, glass fibers, silicon-based fibers, inorganic powders, and antibacterial agents. The resin composition may be composed of a polymer, a solvent (A) and a solvent (B).

[0028] <Porous membrane> A porous film can be produced using the polymer composition for forming a porous film of the present invention. For example, the production process includes the following steps: A process of applying the composition to a substrate to form a coating film; A step of maintaining the coating film at a temperature of less than 100°C to make it porous; and The process of drying the porous coating by keeping it at a temperature of 100°C or higher. The polymer composition of the present invention makes it possible to obtain a self-supporting porous membrane.

[0029] The substrate is not particularly limited, and examples thereof include glass, copper-clad laminate, and resin films (polypropylene, polyethylene terephthalate, polyimide). A porous film can be formed on a substrate to form a film with the substrate, or the substrate can be peeled off to form a film. When used as a flexible metal-clad laminate, coating can be performed using a metal foil as a substrate, and a porous film can be formed on the metal foil. Alternatively, coating can be performed using an endless belt (e.g., a metal belt) as a substrate, and a porous film can be formed on the endless belt to obtain a film consisting of the porous film.

[0030] The method for applying the polymer composition to the substrate is not particularly limited, and examples thereof include spin coating, blade coating, slit coating, screen printing, inkjet coating, and application methods using various applicators and dispensers.

[0031] The thickness of the polymer composition applied is not particularly limited and can be selected depending on the intended use of the porous film. For example, it can be 10 μm or more, preferably 25 μm or more from the viewpoint of achieving both low dielectric properties and good mechanical properties. It can also be 100 μm or less, preferably 50 μm or less.

[0032] The method for producing a porous film of the present invention includes a step of making the coated film porous by maintaining the coated film at a temperature of less than 100° C. This step can be carried out under atmospheric pressure or reduced pressure.

[0033] The temperature is preferably equal to or lower than the boiling point of the solvent (B), more preferably equal to or lower than 50° C. The lower limit is preferably equal to or higher than 10° C. The step of making the porous structure is preferably carried out at room temperature (10° C. or higher and 30° C. or lower).

[0034] The time for maintaining the temperature is not limited as long as it is long enough to form pores inside the coating film, but it can be 15 minutes or more, preferably 30 minutes or more. There is no particular upper limit, but from the viewpoint of efficiency, it is preferably 1 hour or less.

[0035] The porosity can be visually confirmed by the opacity of the coating film.

[0036] The production method of the present invention includes a step of drying the porous coating film by maintaining it at a temperature of 100°C or higher. In this step, solvent (A) and solvent (B) present in the coating film are removed. This step can be carried out under atmospheric pressure or reduced pressure.

[0037] The temperature is preferably 150° C. or higher, more preferably 200° C. or higher, from the viewpoint of reducing residual solvent, and is preferably 350° C. or lower, more preferably 300° C. or lower, from the viewpoint of maintaining the shape of the porous layer.

[0038] The time for maintaining the temperature can be 10 minutes or more, preferably 30 minutes or more, from the viewpoint of reducing residual solvent. There is no upper limit, but from the viewpoint of efficiency, it is preferably 1 hour or less.

[0039] Through these steps, a porous membrane is formed on a substrate and can be provided as a film with a substrate. The resulting porous membrane is self-supporting and can be obtained by peeling off the substrate.

[0040] The porous membrane comprises a porous layer having pores with an average pore size of 0.1 μm or more and 3.0 μm or less, and a skin layer on at least one surface of the porous layer. The thickness of the skin layer in the cross section of the porous film can be 3.0 μm or more, preferably 5.0 μm or more, from the viewpoint of achieving both low dielectric properties and good mechanical properties, and the upper limit can be 20 μm or less, preferably 10 μm or less. Here, the thickness of the skin layer can be determined by observing the cross section of the porous membrane with a scanning electron microscope at a magnification of 2500, measuring the thickness of the skin layer at any three points on the image (50 μm × 40 μm), and averaging the results. By controlling the pores and the skin layer in this way, the present invention can suppress the risk of a decrease in mechanical strength and easily avoid problems with liquid permeability and water absorption.

[0041] The average pore size of the pores in the porous layer is preferably 0.1 μm or more and 3.0 μm or less from the viewpoint of good mechanical properties and suppression of liquid penetration. Here, the average pore size can be determined by observing the cross section of a porous membrane with a scanning electron microscope, binarizing the scanning electron microscope image of the pores contained in one field of view (25 μm × 20 μm) in an image at a magnification of 5000, identifying the pores, and then analyzing the image to calculate the pore size as a circle equivalent diameter, and then determining the average value. ImageJ can be used as the analysis software, and pore sizes less than 0.1 μm are not included in the calculation of the average pore size.

[0042] In order to reduce the dielectric constant of the porous film, the porosity of the porous film is preferably 10% or more, more preferably 20% or more. The upper limit can be set to 70% or less, preferably 60% or less. The specific gravity of the non-porous film substantially corresponds to the specific gravity of the polymer constituting the porous film. Here, the porosity was calculated using the following formula. Porosity (%) = (1 - specific gravity of porous membrane / specific gravity of non-porous membrane) x 100

[0043] The dielectric constant of the porous film measured at 10 GHz can be 2.5 or less, and preferably 2.0 or less, and the dielectric loss tangent of the porous film measured at 10 GHz is preferably 0.01 or less.

[0044] According to the present invention, when the polymer used is colorless and transparent, a porous film with excellent whiteness can be obtained. The OD value of the porous film can be 0.2 or more and 1 or less.

[0045] The porous membrane of the present invention can be a substrate-attached film or a laminate containing the porous membrane.The laminate can be a laminate obtained by laminating the porous membrane of the present invention with another porous membrane or the porous membrane of the present invention with another film by thermal lamination or adhesive.For example, wiring can be formed on the porous membrane of the present invention or on another film, and the porous membrane of the present invention can be laminated on the wiring.

[0046] The porous film of the present invention is useful as a low-dielectric constant film by taking advantage of its excellent dielectric properties, and can be made into a flexible metal-clad laminate in which a metal foil is laminated on the porous film, or an electronic substrate equipped with the porous film. The porous film is suitable for high-frequency antenna substrates and flexible substrates for high-speed transmission. Furthermore, when the polymer used is colorless and transparent, a porous film with excellent whiteness can be obtained, and therefore it can also be applied to a reflective film for LEDs, etc. [Example]

[0047] The present invention will be described in more detail using examples, but the present invention is not limited to these examples. In the following, "parts" and "%" are all by mass unless otherwise specified.

[0048] Measurements and evaluations in the examples were carried out as follows.

[0049] (Weight average molecular weight) The weight average molecular weight (Mw) was measured by GPC (gel permeation chromatography) and calculated in terms of polystyrene.

[0050] (viscosity) The polymer was dissolved in a solvent to prepare a solution with a concentration of 10% by mass, and the viscosity was measured at 25°C using an E-type viscometer (manufactured by Toyo Seiki Co., Ltd.) with a rotor of 3°×R14 at a rotation speed of 5 rpm for 30 seconds to determine the viscosity.

[0051] (Thickness) After gold (Au) was deposited on both sides of the porous film, it was cut with a razor to expose the cross section, and carbon was deposited on it. Observation was performed using a scanning electron microscope (manufactured by JEOL Ltd.) at a magnification of 2500, and the thickness of the target was measured at any three points on the image and averaged to determine the average thickness.

[0052] (Average pore size) The porous membrane was cut with a razor to expose the cross section, and carbon was vapor-deposited. Observation was performed using a scanning electron microscope (manufactured by JEOL Ltd.) at a magnification of 5000. 20 holes were randomly selected from the holes in one field of view, and their equivalent circle diameters were measured and averaged. The selected holes did not include any holes with a diameter of less than 0.1 μm.

[0053] (porosity) The specific gravity was calculated using the following formula by measuring the weight of a porous membrane cut into a 3 cm square using an electronic balance and measuring the membrane thickness using a micrometer. Specific gravity (g / cm 3 ) = weight / (film thickness x area) The porosity was calculated using the following formula. Porosity (%) = (1 - specific gravity of porous membrane / specific gravity of non-porous membrane) x 100

[0054] (Dielectric properties) The dielectric constant and dielectric loss tangent at 10 GHz were measured using an ENA network analyzer (Agilent Technologies) and an SPDR resonator.

[0055] (OD value) The intensity of incident light and transmitted light of the porous film was measured using an optical densitometer (361T Visual; manufactured by X-Rite), and the light-blocking OD value was calculated using the following formula. OD value = log 10 (I0 / I) I0: Incident light intensity I: transmitted light intensity

[0056] (growth rate) The porous membrane was cut into strips (50 mm x 5 mm) and stretched using a tensile tester (Shimadzu Corporation) with a chuck width of 30 mm and a stretching speed of 30 mm / min. The top three values ​​were averaged to obtain the elongation percentage.

[0057] (Liquid permeability) The porous membrane was cut to expose the cross section. After immersion in a red penetrant (Taiyo Bussan, Japan Red Check NRC-AL2450) for 5 minutes, the penetrant adhering to the surface was wiped off. The porous membrane was further cut perpendicular to the exposed cross section, and the immersion length was evaluated using an optical microscope.

[0058] The polyimides used in the examples are as follows: Kawamura Industries KPI-MX300F (Mw: 300,000, specific gravity 1.53 g / cm 3 ) [ka]

[0059] The viscosity of a solution in which polyimide was dissolved at a concentration of 10% by mass, measured using the solvent used in the examples and the viscosity measurement method described above, is as follows. [Table 1]

[0060] Example 1 15 g of polyimide was added to a mixed solvent containing 68 g of tetrahydrofuran (THF) and 17 g of propylene glycol 1-monomethyl ether (PGME), and the mixture was mixed until homogeneous to obtain a polymer solution. The obtained solution was applied to a soda glass using an applicator in a thickness such that the final film thickness after drying would be 25 μm, forming a coating film on the soda glass. The soda glass on which the coating film was formed was left to stand in a thermostatic chamber set at 25° C. for 30 minutes. After the holding period, the presence or absence of a porous structure in the coating film was confirmed by visually checking for opacity. The coating was then dried in a hot air oven at 200° C. for 30 minutes. The resulting coating was then subjected to various evaluations. The results are shown in Table 2.

[0061] <Examples 2 to 5, Comparative Examples 1 and 2> A membrane was obtained in the same manner as in Example 1, except that the mass ratio of THF to PGME was changed as shown in Table 2, and various evaluations were performed. Figures 1 and 2 show a scanning electron microscope (SEM) image of the cross section of the porous membrane of Example 3 and the results of binarization using ImageJ. The average pore size was calculated based on Figure 2(c). The white area above the skin layer in Figure 1 is a gold (Au) vapor-deposited film that was applied to the membrane surface for cross-sectional observation.

[0062] [Table 2]

[0063] From Table 2, it can be seen that the porous films of the examples have low dielectric constants and low dielectric tangents, and in particular, in Examples 2 and 3 where the ratio of the mass of solvent (A) to the mass of solvent (B) is 3:7 to 7:3, the dielectric properties are good and the porosity is controlled, so that the problem of liquid permeability is avoided and the risk of a decrease in mechanical strength is reduced.

[0064] Example 6 10 g of polyimide was added to a mixed solvent containing 45 g of acetone (THF) and 45 g of propylene glycol 1-monomethyl ether (PGME), and the mixture was mixed until homogeneous to obtain a polymer solution. The obtained solution was applied to a soda glass using an applicator in a thickness such that the final film thickness after drying would be 25 μm, forming a coating film on the soda glass. The soda glass on which the coating film was formed was left to stand in a thermostatic chamber set at 25° C. for 30 minutes. After the holding period, the presence or absence of a porous structure in the coating film was confirmed by visually checking for opacity. The coating was then dried in a hot air oven at 200°C for 30 minutes. The cross section of the resulting film was observed under a scanning electron microscope to confirm whether the resulting film had a porous layer with an average pore size of 0.1 μm to 3.0 μm and a skin layer with a thickness of 3.0 μm or more. The elongation of the film was also measured. ◎: The specified porous membrane was obtained. The membrane elongation was 5% or more. Good: A porous membrane was obtained as expected. The membrane elongation was less than 5%. ×: A non-porous membrane was obtained, or a free-standing porous membrane could not be prepared. The results are shown in Table 3.

[0065] Membranes were obtained and various evaluations were carried out in the same manner as in Example 6, except that the solvents shown in Table 3 were used. The results are shown in Table 3. In each example, a predetermined porous membrane was obtained, similarly to Examples 2 to 5.

[0066] [Table 3] TIFF0007751974000005.tif245105TIFF0007751974000006.tif245104TIFF0007751974000007.tif245116 [Industrial Applicability]

[0067] The polymer composition of the present invention can form a porous film having a low dielectric constant and a low dielectric loss tangent, which is improved in terms of liquid permeability and water absorption, and which is less susceptible to a decrease in mechanical strength, and is therefore highly useful in industry.

Claims

1. The composition comprises one or more polymers selected from the group consisting of polyimide, polyamide, polyamideimide, and polybenzoxazole, a solvent (A), and a solvent (B), the solvent (A) has a boiling point of 100°C or higher, and a solution in which the polymer is dissolved at a concentration of 10% by mass has a viscosity of 150 dPa s or higher at 25°C; The solvent (B) has a boiling point of less than 100°C, and a solution in which the polymer is dissolved at a concentration of 10% by mass has a viscosity of less than 50 dPa s at 25°C. A polymer composition for forming a porous film.

2. 2. The polymer composition for forming a porous film according to claim 1, wherein the solvent (A) is an alkylene glycol monoalkyl ether.

3. 3. The polymer composition for forming a porous film according to claim 1, wherein the solvent (A) is at least one selected from the group consisting of propylene glycol 1-monomethyl ether and diethylene glycol monoethyl ether.

4. 4. The polymer composition for forming a porous film according to claim 1, wherein the solvent (B) is at least one selected from the group consisting of acetone, ethyl acetate, and tetrahydrofuran.

5. 5. The polymer composition for forming a porous film according to claim 1, wherein the difference between the boiling point of the solvent (A) and the boiling point of the solvent (B) is 40°C or more and 150°C or less.

6. The polymer composition for forming a porous film according to any one of claims 1 to 5, wherein the polymer is a polyimide.

7. 7. The polymer composition for forming a porous film according to claim 1, wherein the ratio of the mass of the solvent (A) to the mass of the solvent (B) is 8:2 to 2:

8.

8. The polymer composition for forming a porous film according to any one of claims 1 to 7, wherein the mass of the polymer is 5 parts by mass or more and 30 parts by mass or less, relative to 100 parts by mass of the total of the mass of the polymer, the mass of the solvent (A), and the mass of the solvent (B).

9. A step of applying the polymer composition for forming a porous film according to any one of claims 1 to 8 to a substrate to form a coating film; A step of maintaining the coating film at a temperature of less than 100°C to make it porous; and A process of drying the porous coating film by keeping it at a temperature of 100°C or higher. A method for producing a porous membrane, comprising:

10. The method for producing a porous film according to claim 9, wherein the step of making the coating film porous is a step of maintaining the coating film at a temperature of 10°C or higher and 80°C or lower.

11. The method for producing a porous film according to claim 9 or 10, wherein the drying step is a step of drying the porous coating film by maintaining it at a temperature of 150°C or higher and 350°C or lower.

12. A porous film formed by forming the polymer composition for forming a porous film according to claim 1, A step of applying the polymer composition for forming a porous film according to claim 1 to a substrate to form a coating film; the coating film is obtained by a production method including a step of maintaining the coating film at a temperature of less than 100°C to make it porous, and a step of maintaining the porous coating film at a temperature of 100°C or higher to dry it; A porous membrane comprising a porous layer having an average pore size of 0.1 μm or more and 3.0 μm or less, and a skin layer having a thickness of 3.0 μm or more on at least one surface of the porous layer.

13. The porous membrane of claim 12 , wherein the porous layer comprises a polyimide.

14. 14. The porous film according to claim 12 or 13, having a dielectric constant of 2.5 or less measured at 10 GHz.

15. A flexible metal-clad laminate comprising the porous film according to any one of claims 12 to 14 and a metal foil laminated thereon.

16. An electronic substrate comprising the porous film according to any one of claims 12 to 14.

Images