JP2003147118A - Method for manufacturing polyimide porous film - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for manufacturing a porous polyimide film having a uniform pore size and a uniform distance between pores. SOLUTION: The method for manufacturing a polyimide porous film having a uniform pore size and a uniform distance between pores comprises casting polyimide precursor dope on a substrate, subsequently forming on the thus- obtained liquid film a protective layer comprising a solvent and a non-solvent, particularly a protective layer of a composition comprising 100 wt.% of the total of 30-70 wt.% of a non-solvent for the polyimide precursor and 70-30 wt.% of a polar solyent, followed by dipping in coagulation liquid to form a film-like precursor, and heating and drying the precursor to give a polyimide.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a polyimide porous membrane having a small difference in pore diameter between front and back.

[0002]

2. Description of the Related Art As a method for producing a polyimide porous film, T. K. Takeichi et. al. High P
erformance Polymers Vol. 11, 1
p- (1999) to synthesize polyurethane-imide,
After that, a method of decomposing the urethane segment by heat treatment at 300 to 400 ° C. to obtain a polyimide porous film is disclosed.

Also, Polymer 36, 1325, 1995, and Polymer
37,5229,1996 discloses a method of obtaining a porous polyimide that decomposes by making a segment copolymer other than polyimide an α-methylstyrene segment that is easily decomposed into a block copolymer or a graft copolymer made of polyimide, and subjecting the segment other than the polyimide to heat treatment. Has been done.

In addition, the inventors of the present invention disclosed in Japanese Patent Laid-Open No. 11-31
Japanese Patent No. 0658 discloses a polyimide porous film having through holes in the film cross section and having no dense layer on the surface, and a method for producing the same. This production method is a method of casting a polyimide precursor solution on a substrate and bringing it into contact with a coagulating solvent via a solvent displacement rate adjusting material to deposit a porous film of the polyimide precursor having the through holes. There is.

In addition, Japanese Patent Laid-Open No. 2001-1 by the present inventors
Japanese Patent Laid-Open No. 45826 discloses a method for producing a polyimide porous membrane, which comprises using a mixed solution of a good solvent and a non-solvent as a polyimide precursor precursor.

As a method for producing a polyimide porous membrane, a simpler method that does not use a solvent substitution rate adjusting material and can be produced at low cost is desired. Further, in the above-mentioned method, when the polyimide precursor dope is cast on the substrate, one of the cast liquid film is solid, and the substrate side face in contact with the substrate and the atmospheric side face on the opposite side are solid, Due to the difference in environmental conditions in which the other directly contacts the coagulation liquid, when the porous film is formed by immersion in the coagulation liquid, the pore diameters and the positions of the pores on both surfaces are often not uniform. When they are used in battery separators or precision filters, they may cause problems in liquid permeability or air permeability. Therefore, there is a demand for a method for producing a polyimide porous film that is homogeneous in quality.

[0007]

SUMMARY OF THE INVENTION An object of the present invention is to provide a method for producing a polyimide porous membrane having a substantially uniform pore diameter and a distance between pores.

[0008]

According to the present invention, a polyimide precursor dope is cast on a substrate, and a protective layer made of a mixed solution of a solvent and a non-solvent is further provided on the liquid film, followed by solidification. The present invention relates to a method for producing a polyimide porous membrane, which comprises immersing in a liquid.

The polyimide precursor dope solution of the present invention is preferably composed of 50 to 90% by weight of the polyimide precursor solution and 50 to 10% by weight of its non-solvent, which is a total of 100% by weight. The polyimide precursor solution is preferably composed of 0.2 to 30% by weight of the polyimide precursor and 99.8 to 70% by weight of the polar organic solvent, which is a total of 100% by weight.

The polyimide precursor of the present invention is a polyamic acid obtained by polymerizing a tetracarboxylic acid component and a diamine component, preferably monomers belonging to an aromatic compound, or a partially imidized polyamic acid thereof. By heat treatment or chemical imidization, the ring can be closed to give a polyimide resin. The polyimide resin is a heat resistant polymer having an imidization ratio of about 50% or more, which will be described later.

The tetracarboxylic acid component and the diamine component are
A polyimide precursor having a logarithmic viscosity (30 ° C., concentration; 0.5 g / 100 mL NMP) of 0.3 or more, particularly 0.5 to 7 is produced by dissolving and polymerizing in approximately the same molar amount in the above organic solvent. It Also, when the polymerization is carried out at a temperature of about 80 ° C. or higher, a partially ring-closed imidized polyimide precursor is produced.

As the aromatic diamine, for example, general formula (1) H ₂ N—Ar (R ₁ ) _m — [A—Ar (R ₁ ) _m ] _n —NH ₂ (1) (wherein the general formula In, Ar is an aromatic ring and R ₁
Alternatively, R ₂ is a substituent such as hydrogen, lower alkyl, lower alkoxy, etc., A is a direct bond, O, S, CO, S
It is a divalent group such as O ₂ , SO, CH ₂ , C (CH ₃ ) ₂ , m is an integer of 0, 1 to 4, and n is an integer of 0, 1 to 3. An aromatic diamine compound represented by

Specific examples of the compound include 4,4'-diaminodiphenyl ether (hereinafter sometimes abbreviated as DADE), 1,4-phenylenediamine, (hereinafter, P
(Abbreviated as PDA), 3,3'-dimethyl
-4,4'-diaminodiphenyl ether, 3,3'-
Diethoxy-4,4′-diaminodiphenyl ether and the like can be mentioned.

The diamine component may be diaminopyridine represented by the general formula (2) H ₂ N- (Py) -NH ₂ (2), specifically, 2,6-diaminopyridine, 3 , 6-diaminopyridine, 2,5-diaminopyridine, 3,4-diaminopyridine and the like.

The tetracarboxylic acid component is 3,
3 ', 4,4'-biphenyltetracarboxylic dianhydride (hereinafter sometimes abbreviated as s-BPDA), 2,
3,3 ′, 4′-biphenyltetracarboxylic dianhydride (hereinafter, sometimes abbreviated as a-BPDA) is preferable, but 2,3,3 ′, 4′− or 3,3 ′, 4
4'-biphenyltetracarboxylic acid or 2,3
It may be a salt of 3 ', 4'- or 3,3', 4,4'-biphenyltetracarboxylic acid or an esterified derivative thereof. The biphenyltetracarboxylic acid component may be a mixture of the above biphenyltetracarboxylic acids.

In addition to the above-mentioned biphenyltetracarboxylic acids, the above-mentioned tetracarboxylic acid component includes pyromellitic acid, 3,3 ', 4,4'-benzophenonetetracarboxylic acid, 2,2-bis (3,3). 4-dicarboxyphenyl) propane, bis (3,4-dicarboxyphenyl)
Sulfone, bis (3,4-dicarboxyphenyl)-
Tetra, bis (3,4-dicarboxyphenyl) thioether, butane tetracarboxylic acid, or tetracarboxylic acids such as their acid anhydrides, salts, or esterified derivatives are 100 mol% based on the total tetracarboxylic acid component.
It may be contained below, in particular, in a proportion of 10 mol% or less.

The solvent for the polyimide precursor solution of the present invention is a polar organic solvent such as N-methylpyrrolidone (NMP), p-chlorophenol (PCP),
Pyridine, N, N-dimethylacetamide (DMA
c), N, N-dimethylformamide (DMF), dimethylsulfoxide (DMSO), tetramethylurea,
Examples thereof include phenol and cresol. Of which
NMP, DMAc and DMSO are particularly preferred.

As the non-solvent for the polyimide precursor of the present invention, aliphatic alcohols, ketones, ethers, esters, water and the like can be mentioned, with aliphatic alcohols being particularly preferred. More preferably, it is an aliphatic alcohol having 3 to 8 carbon atoms. These are preferable because they have excellent compatibility with the polar organic solvent of the polyimide precursor. Specifically, 1-propanol, 2-propanol, 1-butanol, 2-butanol, 2-methyl-1-propanol
And tert-butanol are preferred. When a non-solvent such as an alcohol having a carbon number of 2 or less, for example, methanol or ethanol is used, the polyimide precursor solution is immediately polymerized.
Is deposited, and a uniform film may not be formed, which is inconvenient. The non-solvent for the polyimide precursor of the present invention may be polyvalent alcohol, and aliphatic polyvalent alcohol is particularly preferable. Specific examples include alkylene glycol, preferably ethylene glycol, glycerin, and polyalkylene glycol and its derivatives, preferably polyethylene glycol having a molecular weight of 600 or less.

The polyimide precursor dope solution in the present invention is preferably 0.2 to 30% by weight of the polyimide precursor and 99.8 to 70% by weight of a mixed solvent of the solvent and the non-solvent. , And more preferably, the polyimide precursor is 1 to 20% by weight. If the concentration of the polyimide precursor is less than 0.2% by weight, the film strength will be reduced, which is not preferable. When the concentration of the polyimide precursor is more than 30% by weight, it is difficult to form a uniform polymer solution, which is not suitable.

The dope solution in the present invention comprises a polyimide precursor and a mixed solvent consisting of a solvent of the polyimide precursor and a non-solvent, and 50 to 70% by weight of the solvent and 100% by weight of the mixed solvent. The solvent comprises 50 to 30% by weight, particularly preferably the solvent comprises 50 to 65% by weight and the non-solvent comprises 50 to 35% by weight. When the solvent in the mixed solvent is less than 50% by weight,
The polyimide precursor tends to precipitate, and if the solvent content exceeds 70% by weight, it is difficult to form a uniform porous film.

The above dope solution is prepared by adding the non-solvent of the polyimide precursor to the polyimide precursor dissolved in the solvent,
It can be obtained by a composition in which the polyimide precursor is not deposited and is kept in a solution state. The criterion for discrimination is that the solution becomes transparent or translucent to the naked eye.
The composition of the dope is such that the ratio of the solvent and the non-solvent is as described above, and the solution is a storage stable for a long time.

The method for casting the polyimide precursor solution to obtain a liquid film that is a casting film is not particularly limited, but the polyimide precursor solution is used as a base material on a substrate such as a glass plate or a metal plate. Alternatively, a method of casting on a movable belt by a spray method or a doctor blade method, a method of extruding the polyimide precursor solution from a T-type die, and the like can be used. Alternatively, coating or spin casting may be used.

Further, the casting dope solution may contain a surfactant, a flame retardant, a coloring agent, or a reinforcing material such as glass fiber, silicon fiber, carbon fiber or the like. These additives and reinforcing materials may be added to the above polyimide precursor polymerization solution, or may be added to the casting dope solution.

In the present invention, a protective layer made of a mixed liquid of a polar solvent and a non-solvent is further provided on the cast liquid film, and then immersed in a coagulating liquid.

The role of the protective layer is to neither dissolve nor precipitate the polyimide precursor, but to act as a buffer layer for solvent exchange between the liquid film and the coagulating liquid, to slow the phase inversion, and to form a uniform porous layer. It forms a film.

In the protective layer of the present invention, the proportion of the non-solvent is − based on the composition of the mixed solution in which the polyimide precursor is the deposition limit.
The amount is 40 to + 10% by weight. If the proportion of the non-solvent is not higher than the precipitation limit value, the film to be deposited tends to be redissolved, which is not preferable. On the other hand, if the content of the non-solvent is more than 10% by weight based on the precipitation limit composition, the surface pores of the film will be closed and a uniform porous film will not be obtained, which is not preferable. The limiting concentration is determined as follows. A non-solvent is added to a polyimide precursor solution having different concentrations, and the amount of the non-solvent added when the polyimide precursor undergoes phase separation is plotted against the concentration of the polyimide precursor solution. A boundary line is obtained which is a region which is separated into two and which is a one-phase region below the line. As the critical composition, the composition of the solvent and the non-solvent is obtained from the intercept value obtained by extrapolating the boundary line to the polyimide precursor concentration of 0.

For example, in the polyimide precursor consisting of BPDA and DPDA, non-solvent: 1-propanol,
The precipitation limit composition of 2-propanol and solvent: NMP is 40% by weight of propanol and 60% by weight of NMP. Similarly, when the solvent is DMAc, propanol 34 is used.
It has a composition of wt% and DMAc 66 wt%. Similarly, when the solvent is DMAc and the non-solvent is ethylene glycol, the precipitation limit composition is 64 wt% for the non-solvent and 36 wt% for the solvent.

The composition of the protective layer is preferably such that the proportion of the non-solvent is -40 to + 10% by weight from the composition of the precipitation limit.
A total of 100% by weight of the non-solvent 30 to 70% by weight and the polar solvent 70 to 30% by weight is preferable. If the proportion of the non-solvent in the mixed solvent of the protective layer is more than 70% by weight, the surface pores of the membrane may be blocked and a uniform porous membrane may not be obtained, which is not preferable. If the proportion of the non-solvent is less than 30% by weight, the surface of the porous membrane may be redissolved and the pores on the surface may be clogged, which is not preferable.

The thickness of the protective layer is not limited, but preferably 5 to 300 μm.

The method of casting the protective layer can be selected from coating, spraying, spin casting and the like. The casting device can be selected from a glass rod, a doctor blade, a T-die extruder and the like.

The liquid film provided with the protective layer is left for 10 seconds to 10 minutes if possible, and then immersed in the coagulating liquid. At that time, the liquid film cast on the protective layer and the coagulating liquid are heated at a temperature of -20 to about 30 ° C.
It is preferable to keep More preferably, -20 to 5
C is preferred. At a temperature of about 30 or less, when the temperature of the protective layer is low, the solvent exchange rate of solution molecules becomes slow and phase conversion occurs, so that the protective layer is excellent as an effect.

The coagulating liquid of the present invention may be any solvent as long as it is a non-solvent for the polyimide precursor, such as water or alcohol.
However, the coagulation liquid of the present invention is preferably a mixed solution of 50 to 90% by weight of the non-solvent of the polyimide precursor and 50 to 10% by weight of the solvent, and a total of 100% by weight. More preferably, the non-solvent is 55 to 90% by weight, and the solvent is 45 to 10% by weight. When the amount of the non-solvent is less than 50% by weight, it takes a long time to form a solid film, and only a film having low dimensional stability can be obtained. If the proportion of the non-solvent is more than 70% by weight, the film formation is too rapid due to the solvent exchange with the solution of the protective layer or the solution of the liquid film, and a uniform porous film cannot be formed. However, it is preferable that the composition of the non-solvent of the coagulating liquid is larger than the composition of the non-solvent of the protective layer. If the composition is smaller than the composition of the non-solvent of the protective layer, the polyimide precursor is redissolved, which is not preferable. Since the coagulation liquid having the above composition has a composition close to that of the polyimide precursor dope or the protective layer solution, the coagulation reaction is slow, and the phase separation is performed at a very slow rate. Instead, a uniform porous film is formed from the surface to the deep part and finally to the surface of the substrate.

The obtained polyimide precursor porous film is preferably washed with a non-solvent for the purpose of fixing the structure. The portion adhered or swollen on the film is washed with a nonsolvent of the polyimide precursor to fix the formed structure. The non-solvent used is an aliphatic alcohol of the same type as the coagulation liquid component,
Examples thereof include aliphatic alcohols having 3 or less carbon atoms, aliphatic ketones and water.

The thickness of the polyimide precursor porous film of the present invention is preferably 5 to 300 μm, more preferably 5 to 200 μm. If the thickness is less than 5 μm, the mechanical strength will be poor and handling will be difficult in the subsequent steps.
If the thickness is thicker than 300 μm, a solid film is formed before the solvent and the non-solvent permeate sufficiently, so that the pores may become small or the pores may not be opened, and the pore diameter may not be uniform.

The cleaned polyimide precursor film is attached to a pin tenter or the like. The polyimide precursor film is dried and imidized immediately after adjusting the degree of attachment so that the polyimide precursor film does not break due to heat shrinkage when the temperature rises. Temperature 50-100 ℃
With a hot air dryer, hot air oven, etc. to dry continuously or discontinuously,
After that, the temperature can be further raised to perform imidization.
The imidization can be thermal imidization or chemical imidization. The chemical imidization is performed by using a fatty acid anhydride or an aromatic acid anhydride as a dehydrating agent and imidizing with a tertiary amine such as triethylamine as a catalyst. Japanese Patent Laid-Open No. 4-33
As disclosed in Japanese Patent No. 9835, imidazole, benzimidazole or a substituted derivative thereof may be used.

In the case of thermal imidization, the dried polyimide precursor is preferably heated to 280 to 500 ° C. to perform thermal imidization. The temperature may be raised stepwise or may be raised to a predetermined temperature in one step. 5 at 280 to 500 ° C. in air, preferably in an inert atmosphere
Hold for 240 minutes. Then, the temperature is lowered to room temperature to obtain a polyimide porous film.

According to the present invention, a substantially uniform porous film having a small difference in pore diameter on both sides can be obtained, and 1) the difference between the average pore diameters of the atmosphere side surface and the substrate side surface is the average pore diameter. It is smaller than 200% based on the small average value. The average pore diameter refers to the number average value of the diameters of the pores when viewed from the surfaces of the atmosphere side surface and the substrate side surface, respectively. The difference in hole diameter between the atmosphere side surface and the substrate side surface is 200% based on the value of the surface with the smaller hole diameter.
%, And more preferably less than 150%.
If it is more than 200%, the pore sizes on both sides are different,
When it is used for a battery separator or a filter, it is inconvenient because the difference in pore diameter between both sides is remarkable and the liquid permeation performance or air permeation performance is different.

Furthermore, according to the present invention, it is possible to obtain a uniform porous membrane having pores of substantially uniform diameter on each surface.
2) The variation coefficient of the hole diameter on each of the atmosphere side surface and the substrate side surface is smaller than 70%. The coefficient of variation (CV) of the distribution of pore diameters is expressed by the ratio of the standard deviation (σ) from the individual pore values (Xi) measured in the same manner as in the previous section to the number average value (Xn). , Is calculated by the following formula. CV (%) = (σ / Xn) × 100 The variation coefficient of the distribution of pore diameter can be used as a measure of the uniformity of pore diameter, and the variation coefficient of pore diameter is preferably 60% or less. If the coefficient of variation of the pore diameter is larger than 70%, the variation of the pore diameter becomes large, so that when it is used for a filter or the like, the uniformity of the filtration performance is deteriorated, which is not preferable.

Furthermore, according to the present invention, it is possible to obtain a substantially uniform porous film in which the distance between the pores is uniform on each surface, and 3) between the center of gravity of the pores on the atmosphere side surface and the substrate side surface, respectively. The coefficient of variation of distance is less than 50%. The distance between center of gravity (li) is the distance from the center of gravity of one hole to the center of gravity of the adjacent hole, which is a measure of uniform hole position. Can be If the coefficient of variation of the distance between the center of gravity of the holes is greater than 50%, the hole positions will vary, so when used in a filter or separator, non-uniform pressure will occur, and in some cases,
The porous film may be damaged, which is not preferable. More preferably, the coefficient of variation of the distance between the center of gravity of the holes is less than 45%. This value can be similarly obtained by replacing the hole diameter value with the center-of-gravity distance value in the above equation.

Furthermore, according to the present invention, a fine range of pore diameter can be obtained on each surface. That is, the average pore diameter is 0.05 to 5 μm on the atmosphere side surface (A surface) and the substrate side surface (B surface). More preferably, 0.08
Is about 3 μm. If the average pore size is too small, for example, the filtration rate will be poor when used in a filter. 5 μm
If it is larger, the collection performance is inferior, so the above range is preferable.

Therefore, according to the present invention, the porosity is 20 to 80% and the average pore diameter is 0.0, although it varies depending on the manufacturing conditions.
5 to 5 μm, and further, based on the smaller difference in average pore diameter between the atmosphere side surface (A surface) and the substrate side surface (B surface), 2
It is possible to obtain a uniform polyimide porous membrane in which the coefficient of variation of the hole diameter is less than 70% and the coefficient of variation of the distance between the center of gravity of the holes is less than 50% in each surface.

The polyimide porous film has one layer or two layers.
You may use it combining two or more layers. By combining two or more layers, it can be used as a reinforcing material or a thick material depending on the application. Further, it may be used in combination with other materials, other polymer films, fibers and inorganic substances.

[0043]

EXAMPLES The present invention will now be described in detail with reference to examples, but the present invention is not limited to these examples. The tests and evaluation methods in Examples and Comparative Examples are as follows.

Porosity The thickness and weight of a porous film cut into a predetermined size were measured, and the porosity was calculated from the weight per unit area by the following formula. In the following equation, S means the area of the porous film, d means the film thickness, W means the measured weight, D means the density of the polyimide, and the density of the polyimide is 1.34 g / cm ³ . Porosity (%) = 100-100 × (W / D) / (S ×
d)

Air permeability Measured according to JIS P8117. The air permeability was expressed as a Galley value. The average pore diameter and the coefficient of variation of pores on the surface of the membrane were observed with a scanning electron microscope. The major axis and minor axis of the hole were measured, the area thereof was calculated, and the equivalent diameter of the circle was calculated. The number average pore size was determined for 100 or more holes, and the coefficient of variation was determined. Atmosphere side (A side) and substrate side (B side)
Was measured on each side.

Average Distance Between Centers of Gravity and Coefficient of Variation The distance between the centers of gravity of the pores on the surface of the film and the nearest pore was read from a scanning electron micrograph at an arbitrary 100 points. The average distance between center of gravity and its coefficient of variation were calculated. The measurement was performed on each of the A surface and the B surface.

The puncture strength sample was fixed to a circular hole holder having a diameter of 11.28 mm and an area of 1 cm ² , and the tip shape was 0.5 R and the diameter was 1 mmφ.
The dollar was lowered at a speed of 2 mm / sec and pierced, and the penetration load was measured.

Smoothness The smoothness of the porous film was qualitatively evaluated visually. ◯: The surface was smooth. Δ: The surface was slightly uneven. X: Roughness was observed.

Example 1 Using s-BPDA as a tetracarboxylic acid component and PPDA as a diamine component, PP for s-BPDA was used.
It was dissolved in NMP so that the molar ratio of DA was 0.994 and the total weight of the monomer components was 15% by weight.
Polymerization was performed at 0 ° C. for 6 hours to obtain a polyimide precursor solution. 1-Propanol was added to a part of the polyimide precursor solution, the polyimide precursor was 9.7% by weight, the mixed solvent was 90.3% by weight, NMP in the mixed solution was 59% by weight, 1-propanol. Of the dope solution was prepared, and the dope solution was cast on a glass plate to a thickness of about 100 μm. Subsequently, the temperature was kept at -10 ° C.
40% by weight of 1-propanol kept at the same temperature, NMP6
A protective layer of a 0% by weight mixed solution was provided on the polyimide precursor liquid film with a glass rod to form a protective layer having a thickness of about 20 μm. Then, 1-propanol 6 kept at a temperature of -10 ° C
A polyimide precursor film was obtained by immersing the film in a coagulation liquid consisting of 6.7% by weight and 33.3% by weight NMP for 15 minutes. Subsequently, the polyimide precursor film was washed with water at room temperature, the glass plate was peeled off, and then dried at a temperature of 80 ° C. to obtain a polyimide precursor film. Immediately, the polyimide precursor film was put on a pin tenter, placed in a hot air dryer at a temperature of 400 ° C., and subjected to thermal imidization for 40 minutes. Thus, a polyimide film was obtained.

The thickness of the obtained polyimide film is 10 μm,
The porosity was 65% and the puncture strength was 65 g. When observed with a scanning microscope, the average pore size is 0.32μ on the atmosphere surface side.
m, the coefficient of variation is 43.3%, the side surface of the substrate has an average pore diameter of 0.2.
The coefficient of variation was 2 μm and the coefficient of variation was 33.0%. Average distance between centers of gravity of holes on the atmosphere side 0.88 μm, coefficient of variation 31.0
%, The average distance between the centers of gravity of the holes on the side surface of the substrate was 0.96 μm, and the coefficient of variation was 43.8%. The ratio of the difference between the average hole diameters of the atmosphere side surface and the substrate side surface was 45% based on the value of the substrate side surface. Both sides had a smoothness of ◯ and exhibited a uniform porous film. Scanning electron micrographs of the air side of the film and the side of the substrate are shown in FIGS.

Comparative Example 1 A polyimide film was obtained in the same manner as in Example 1 except that the coagulating liquid was 100% by weight of 1-propanol and no protective layer was provided. The film thickness was 11 μm, the porosity was 60%, the average pore diameter on the atmosphere side was 0.07 μm, and the average pore diameter on the substrate side was 0.55 μm. The hole diameters on the atmosphere side and the substrate side are
The difference between the two surfaces was large at 657% based on the hole diameter on the side surface of the substrate.

Example 2 The polymerization solvent was changed to DMAc, and the dope solution, protective layer solution,
A polyimide film was obtained in the same manner as in Example 1 except that the solvent of the coagulation liquid was changed to DMAc and the non-solvent was changed to 2-propanol, and the protective layer was changed to 34 wt% of the non-solvent and 66 wt% of the solvent.

The obtained polyimide film has a film thickness of 11 μm,
Porosity is 73% and air permeability is 1000 seconds / 100c
m ³ , and the puncture strength was 71 g. When observed with a scanning microscope, the average pore diameter was 0.54 μm and the coefficient of variation was 33.4% on the atmosphere side, and the average pore diameter was 0.23 μm and the coefficient of variation was 34.2% on the substrate side surface. The average distance between the centers of gravity of the holes on the side of the atmosphere is 0.80 μm, the coefficient of variation is 25.0%, the average distance between the centers of gravity of the holes on the side of the substrate is 0.90 μm, and the coefficient of variation is 45.
It was 3%. The ratio of the difference between the average hole diameters of the atmosphere side surface and the substrate side surface was 135% based on the value of the atmosphere side surface. Both sides had a smoothness of ◯ and exhibited a uniform porous film.

Comparative Example 2 A polyimide film was obtained in the same manner as in Example 1 except that the protective layer was not provided. The film thickness was 11 μm, the porosity was 69%, the atmosphere side average pore diameter was 0.06 μm, and the substrate side average pore diameter was 0.58 μm. The hole diameters on the atmosphere side and the substrate side are
There was a large difference between both surfaces, which was 867% based on the air-side hole diameter.

Example 3 33% by weight of ethylene glycol in the protective layer, DMAc67
A polyimide film was obtained in the same manner as in Example 2 except that the weight% was used, the coagulating liquid was water, the temperature was 0 ° C., and the thickness of the liquid film was 500 μm. The obtained polyimide film had a film thickness of 49 μm, a porosity of 69%, and an air permeability of 280 seconds / 10.
The puncture strength was 0 cm ³ and 162 g. When observed with a scanning microscope, the average pore diameter is 0.27 μm on the atmosphere side, the coefficient of variation is 34.2%, and the average pore diameter is 0.24 μ on the substrate side surface.
m, the coefficient of variation was 35.7%. The average distance between centers of gravity of holes on the atmosphere side was 0.49 μm, the coefficient of variation was 24.9%, the average distance between centers of gravity of holes on the side of the substrate was 0.51 μm, and the coefficient of variation was 20.9%. The ratio of the difference between the average hole diameters of the atmosphere side surface and the substrate side surface was 13% based on the value of the substrate side surface. Both sides had a smoothness of ◯ and exhibited a uniform porous film.

Comparative Example 3 A polyimide film was obtained in the same manner as in Example 3 except that the protective layer was not provided. The thickness of the obtained polyimide film is 51 μm.
m, the porosity is 65%, the average pore size on the atmosphere side is 3.0μ
The average pore size on the side surface of the substrate was 0.8 μm. The hole diameters on the atmosphere side and substrate side are 27
There was a large difference between the two surfaces, which was 5%.

Example 4 The solvent of the dope solution was 65% by weight of DMAc and 3% of glycerin.
A polyimide film was obtained in the same manner as in Example 3, except that the protective layer was 5 wt%, ethylene glycol was 55 wt%, and DMAc was 45 wt%. The obtained polyimide film had a film thickness of 53 μm, a porosity of 67%, and a puncture strength of 160 g. When observed with a scanning microscope, the average pore size was 1.4 μm on the atmosphere side, the coefficient of variation was 38.1%, the average pore size was 1.03 μm on the substrate side, and the coefficient of variation was 23.4%.
Average distance between the centers of gravity of the holes on the atmosphere side is 1.75 μm, coefficient of variation is 3.0%, average distance between the centers of gravity of the holes on the substrate side is 1.39
μm, the coefficient of variation was 17.5%. The ratio of the difference between the average hole diameters of the atmosphere side surface and the substrate side surface was 36% based on the value of the substrate side surface. Both sides had a smoothness of ◯ and exhibited a uniform porous film.

Comparative Example 4 A polyimide film was obtained in the same manner as in Example 4 except that the protective layer was not provided. The obtained polyimide film was almost dense with very few holes on the atmosphere side.

[0059]

INDUSTRIAL APPLICABILITY The present invention can provide a method for producing a polyimide porous film having a small difference in pore diameter between the atmosphere surface and the substrate surface, and uniform pore diameter and distance between pore centroids. The polyimide porous membrane obtained by the present invention has uniform liquid permeability on both sides, and can be provided as a separator for batteries, a precision filter, and a raw material for carbonized membranes for fuel cells.

[Brief description of drawings]

FIG. 1 is an electron micrograph of an atmosphere side surface of a polyimide porous film obtained in Example 1, which is an example of the present invention.

FIG. 2 is an electron micrograph of a substrate side surface of the polyimide porous film obtained in Example 1.

FIG. 3 is an electron micrograph of the atmosphere side surface of the polyimide porous film obtained in Example 4, which is an example of the present invention.

FIG. 4 is an electron micrograph of a side surface of a substrate of a polyimide porous film obtained in Example 4.

Continued front page    F-term (reference) 4F074 AA74 BA72 BA73 CB33 CB37                       CB45 CC28Y DA47 DA54

Claims (4)

[Claims] 1. A polyimide precursor dope is cast on a substrate, a protective layer made of a mixed liquid of a solvent and a non-solvent is further provided on the liquid film, and then immersed in a coagulating liquid. Method for producing a polyimide porous membrane. 2. The method for producing a polyimide porous film according to claim 1, wherein the composition of the protective layer is 100% by weight of 30 to 70% by weight of the non-solvent of the polyimide precursor and 70 to 30% by weight of the polar solvent. . 3. The method for producing a polyimide porous membrane according to claim 1, wherein the non-solvent as a component of the protective layer is an aliphatic monovalent alcohol having 3 to 5 carbon atoms. 4. The polyimide according to claim 1 or 2, wherein the non-solvent of the component of the protective layer is a polyvalent alcohol selected from alkylene glycol, glycerin, and polyalkylene glycol and its derivatives. A method for manufacturing a porous membrane.