US20100040789A1

US20100040789A1 - Porous film production method

Info

Publication number: US20100040789A1
Application number: US12/562,324
Authority: US
Inventors: Koju Ito; Tsukasa Ishihara; Hidekazu Yamazaki
Original assignee: Fujifilm Corp
Current assignee: Fujifilm Corp
Priority date: 2008-08-18
Filing date: 2009-09-18
Publication date: 2010-02-18
Also published as: JP2010070635A; US20110155263A1; US8764866B2; JP5351476B2

Abstract

A coating liquid containing a polymer and a hydrophobic solvent is applied to a support to form a coating layer. In a humidifying process with high humidity, the coating layer is set under an atmosphere having a first dew point higher than a surface temperature of the coating layer. Then, in a humidifying process with low humidity, the coating layer is set under an atmosphere having a second dew point higher than the surface temperature of the coating layer and lower than the first dew point. Thereby, water vapor is condensed from ambient air on the coating layer to generate minuscule water drops. Thereafter, the coating layer is dried to form a porous film having plural pores made by the minuscule water drops as a template. Since the humidifying process with high humidity is performed for a short period of time and then the humidifying process with low humidity is performed, the minuscule water drops are prevented from growing up, to obtain a porous film having minute pores.

Description

FIELD OF THE INVENTION

The present invention relates to a porous film production method.

BACKGROUND OF THE INVENTION

In recent years, increase in integration degree, higher information density, and higher image definition have been desired more and more in the fields of optics and electronics. Therefore, a film used in these fields is strongly required to have a finer structure. The film having a fine structure is also required in a medical field and the like. As films having a fine structure required in the medical field, there are a film as a scaffold for cell culture and a film used as a hemofiltration membrane or an adhesion preventing membrane.
As a film having the fine structure, there is a film in which plural μm-scale minute pores are formed to constitute a honeycomb structure. Such a film is a porous film. As a method for producing such a porous film, there is a condensation method as follows, for example. A solution obtained by dissolving a predetermined polymer into a solvent is applied to a substrate. Then, water vapor is condensed from ambient air on a surface of the solution applied to the substrate, and the solvent is evaporated, to generate minuscule water drops. Thereafter, the minuscule water drops are evaporated. (For example, see Japanese Patent Laid-Open Publications No. 2006-070254 and No. 2001-157574). The film produced by the method is also referred to as a self-organized film because of its formation behavior of a fine structure.
Conventionally, liquid drops as a template for a porous structure, which are mainly water drops, have been supplied utilizing condensation caused by humidified air. This condensation is a natural phenomenon. Therefore, it is difficult to control the condensation actively. Additionally, it is necessary to control temperature finely in order to stably cause condensation. Accordingly, improvement in the methods for forming the porous structure has been required. Furthermore, enormous experiments must be conducted to define conditions for forming pores having a desired diameter and changing an arrangement pitch of the pores. Therefore, it has been difficult to change the size of pores and the arrangement pitch of the pores without any trouble.

SUMMARY OF THE INVENTION

In view of the above, an object of the present invention is to provide a porous film production method capable of changing a size of pores and an arrangement pitch of the pores with facility.
In order to achieve the above and other objects, a porous film production method of the present invention includes the following steps. A coating liquid containing a polymer and a hydrophobic solvent is applied to a support to form a coating layer (herein after referred to as step A) The coating layer is set under an atmosphere having a first dew point Td1 higher than a surface temperature Ts of the coating layer (Td1>Ts) (herein after referred to as step B). The coating layer is set under an atmosphere having a second dew point Td2 higher than the surface temperature Ts of the coating layer and lower than the first dew point Td1 (Td1>Td2>Ts) after the step B (herein after referred to as step C). The coating layer subjected to the step B and the step C is dried to form a porous film having a plurality of pores made by water drops, which are formed by the step B and the step C, as a template for the porous film, (herein after referred to as step D).
A relation between the dew point Td1 and the dew point Td2 preferably satisfies the following condition:
Td1−Td2≧1° C.
Further, when t1 is a remaining time of the coating layer in the step B, and t2 is a remaining time of the coating layer in the step C, the t1 and the t2 preferably satisfy the following condition:
t1<t2
Preferably, the remaining time t1 of the coating layer in the step B is not less than 0.1 seconds and not more than 60 seconds. Further, a thickness of the coating layer before being dried is preferably at most 500 μm.
According to the present invention, it is possible to easily change the size of pores and the arrangement pitch of the pores in the porous film having minute pores.

DESCRIPTION OF THE DRAWINGS

The above and other objects and advantages of the present invention will be more apparent from the following detailed description of the preferred embodiments when read in connection with the accompanied drawings, wherein like reference numerals designate like or corresponding parts throughout the several views, and wherein:

FIG. 1 is an enlarged cross sectional view illustrating a porous film according to an embodiment of the present invention;

FIG. 2 is an enlarged plan view illustrating the porous film;

FIG. 3A is an enlarged cross sectional view illustrating a porous film in which shallow pores are formed according to another embodiment of the present invention, FIG. 3B is an enlarged cross sectional view illustrating a porous film in which deeper pores are formed according to another embodiment of the present invention, and FIG. 3C is an enlarged cross sectional view illustrating a porous film having an inner layer disposed between a porous layer and a support according to another embodiment of the present invention;

FIG. 4 is a schematic side view illustrating a porous film production apparatus;

FIG. 5 is an explanation view illustrating various conditions for a condensation process performed in a second section, in which a first humidified air feeding unit, a second humidified air feeding unit, and a third humidified air feeding unit are utilized to vary a dew point and flow rate of humidified air for each of a first zone, a second zone, and third zone; and

FIG. 6 is a graph showing a relation between a thickness of the coating layer and a diameter of water drops, in which C1 shows the relation when ΔT1 is made larger, C2 shows the relation when ΔT2 is made smaller, and C3 shows the relation when the diameter of water drops is prevented from increasing by varying ΔT3 in the first zone and the second zone.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, preferred embodiments of the present invention are described in detail. However, the present invention is not limited thereto.
As shown in FIG. 1, a porous film 10 of the present invention includes a porous layer 11 in which a plurality of pores 15 are formed, and a support 12 for supporting the porous layer 11. Although the pores 15 are not communicated with each other in this embodiment, the pores 15 may be communicated with each other. As shown in FIG. 2, the pores 15, each of which has an approximately circular shape, are formed densely to constitute a honeycomb structure as a whole.
As shown in FIG. 1, the pores 15 formed in the porous layer 11 penetrate through the porous layer 11 in its thickness direction. However, the pores 15 may be substituted with shallow pores (dimples) 16 as shown in FIG. 3A, or deep pores (hollows) 17 which do not penetrate through the porous layer 11 in its thickness direction as shown in FIG. 3B. The pores 15 are formed in the porous layer 11, the pores 16 are formed on a porous layer 18, and the pores 17 are formed on a porous layer 19, by controlling a formation process of water drops, as described in detail later. For example, when the water drops are prevented from growing up at an early stage to advance the timing of starting drying of a coating liquid to be described in detail later, the shallow pores 16 are formed. In contrast, when the water drops are caused to grow up sufficiently to retard the timing of starting drying of the coating liquid, the pores become deeper in accordance with the time for growth of the water drops. Accordingly, the pores 17 which are deeper pores or the pores 15 which penetrate through the porous layer 11 in its thickness direction are formed. Additionally, as shown in FIG. 3C, an inner layer 13 may be disposed between the porous layer 11 and the support 12. Note that the components identical to those in FIG. 1 are denoted by the same reference numerals in porous films 20, 21, and 22 in FIGS. 3A to 3C.
The porous layer 11 is formed by application of a first coating liquid 35 (see FIG. 4) containing a first polymer. The support 12 and the inner layer 13 are not essential in the present invention, and are disposed as necessary. Although a two-layer structure including the porous layer 11 and the support 12 as shown in FIGS. 1, 3A, and 3B, and a three-layer structure including the porous layer 11, the support 12, and the inner layer 13 as shown in FIG. 3C are adopted, the present invention is not limited thereto. For example, the support 12 and the inner layer 13 may be omitted during the production of the film or at the time of using the film. Alternatively, the porous layer 11 may be peeled from the support 12 and the inner layer 13, and in this case, only the porous layer 11 or a two-layer structure including the porous layer 11 and the inner layer 13 are used as the porous film.
In a preferred embodiment, the inner layer 13 is used together with the porous layer 11 provided with the support 12. Additionally, the inner layer 13 is effective in a case where the porous layer 11 and the inner layer 13 are peeled from the support 12 and the two-layer structure including the porous layer 11 and the inner layer 13 is used, because the inner layer 13 serves for supporting and protecting the porous layer 11. The inner layer 13 may be formed from either the same material as that of the first polymer or the material different from that of the first polymer. In a case where the inner layer 13 is formed from the same material as that of the porous layer 11, it is possible to increase the thickness of the porous film 22 to give a self-supporting property to the porous film 22. In contrast, in a case where the inner layer 13 is formed from the material different from that of the first polymer, it doesn't matter whether or not the material has solubility to the first coating liquid 35. Note that the number of the inner layer 13 is not limited to one, and may be two or more as necessary. The inner layer 13 is formed by application of a second coating liquid containing a second polymer, for example.
The support 12 is necessary during the production of the film. Additionally, the support 12 may be necessary except the case where the porous layer 11 as a final product has the self-supporting property. The support 12 used during the production of the film is also used in the film as the final product. Alternatively, a support for exclusive use of production of the film may be used. The support for exclusive use of production of the film is a stainless endless belt or drum, a polymer film, or the like, in a case where the films are continuously produced. Additionally, in a case where a support of a cut sheet type is used, the support is a plate made of stainless, glass, polymer, or the like. Such a plate may be also used in the film as the final product in addition to being used during the production of the film.
The porous layer 11 consists of a hydrophobic polymer compound and an amphiphilic compound. Thereby, it is possible to form water drops further uniform in form and size by a production method described later. Although the inner layer 13 is preferably a polymer compound, the inner layer 13 does not always have to be a polymer compound. For example, the inner layer 13 may be monomer, oligomer, or other organic compound, and an inorganic compound such as titanium oxide (TiO₂).
Note that it is effective that the support 12 is a film formed of a polymer compound because of the following reasons. The porous film 10 as a whole has flexibility due to the presence of the support 12. Unlike a porous material obtained by disposing a porous layer on a glass, the film itself has excellent handleability and a degree of freedom of the form at the time of being used. The film having the degree of freedom of the form at the time of being used means the film which can be readily changed into various other forms such as a plane state, a curved state, a state cut into a predetermined form, or the like. Thereby, the porous film 10 can be used as a protection layer for protecting a wound site, a percutaneous absorption agent, adhesion preventing membrane, and the like.
In the amphiphilic compound to be mixed with the first polymer in the porous layer 11, a ratio of the number of hydrophilic group to the number of hydrophobic group is preferably in the range of 0.1/9.9 to 4.5/5.5. Thus, the more minuscule water drops can be formed in a coating layer more densely. Upon increase in the proportion of hydrophilic group, if the ratio of the number of hydrophilic group to the number of hydrophobic group is smaller than the above range, the size of the pores formed in the porous layer becomes various and nonuniform in some cases. Concretely, when a variation coefficient (unit: %) of the pore diameter determined as {(standard deviation of pore diameter)/(average of pore diameter)}×100 is at least 10%, the size of the pores is considered to be nonuniform. Further, when the ratio of the number of hydrophilic group to the number of hydrophobic group is higher than the above range, the arrangement pitch of the pores tends to be irregular. The regularity in arrangement of the pores is related to a variation coefficient (unit: %) of a distance between the centers of the adjacent pores. Concretely, as the variation coefficient of the distance between the centers of the adjacent pores is lower, the pores are arranged more densely. It is preferable that the pores are arranged in the densest state. Note that, the distance between the centers of the adjacent pores means a distance between centers of adjacent pores arbitrarily selected among the pores formed in the porous film.
Note that, two or more compounds different from each other can be used as the amphiphilic compound. When two or more compounds are used, it is possible to adjust the size and the position of the water drops at higher precision by the method described later. Moreover, when a plurality of compounds are used as the component of the polymer compound in the porous layer 11, the same effect as the above can be achieved.
As preferable examples of the first and second polymers, there are vinyl-type polymer (for example, polyethylene, polypropylene, polystyrene, polyacrylate, polymethacrylate, polyacrylamide, polymethacrylamide, polyvinyl chloride, polyvinylidene chloride, polyvinylidene fluoride, polyhexafluoropropene, polyvinyl ether, polyvinyl carbazol, polyvinyl acetate, polytetra fluoroethylene, and the like), polyesters (for example, polyethylene terephthalate, polyethylene naphthalate, polyethylene succinate, polybutylene succinate, polylactic acid, and the like), polylactone (for example, polycaprolactone and the like), acetyl cellulose, polyamide or polyimide (for example, nylon, polyamic acid, and the like), polyurethane, polyurea, polybutadiene, polycarbonate, polyaromatics, polysulfone, polyethersulfone, polysiloxane derivative, and the like.
Note that, instead of the second polymer, gelatine, polyvinyl alcohol (PVA), sodium polyacrylate, and the like may be used as the inner layer 13. In this case, even when the porous film 10 is used as the protection layer for protecting a wound site or the percutaneous absorption agent, the porous film 10 is not poisonous, and further the inner layer 13 does not spoil the flexibility of the support 12. Accordingly, the porous film 10 can have excellent handleability and degree of freedom of the form at the time of being used.
As the polymer used as the support, there are the above compounds which are preferable as the first polymer. For the purpose of increasing thickness of the support 12 and giving flexibility to the porous film 10, for example, there are acetyl cellulose, cyclic polyolefin, polyester, poly carbonate, polyurethane, polybutadiene, and the like as the polymer used as the support. By using those, it is possible to prevent increase in the cost even if the thickness of the support 12 is increased. Further, it is possible to obtain the porous film 10 which is not easily broken and has the degree of freedom of the form at the time of being used.
A solvent for the first coating liquid 35 is a first solvent, and is not especially limited as far as it has a hydrophobic character and can dissolve the polymer compound. Examples of the first solvent are aromatic hydrocarbon (for example, benzene, toluene, and the like), halogenated hydrocarbon (for example, dichloromethane, chlorobenzene, carbon tetrachloride, 1-bromopropane, and the like), cyclohexane, ketone (for example, acetone, methyl ethyl ketone, and the like), ester (for example, methyl acetate, ethyl acetate, propyl acetate, and the like), and ether (for example, tetra hydrofuran, methyl cellosolve, and the like). A plurality of compounds among them may be combined to be used together as the solvent. Additionally, a hydrophilic solvent such as alcohol, ketone, and ethers may be added to the compound or the mixture of compounds by a small amount, namely, 20% or less.
Further, in order to reduce adverse influence on the environment to the minimum, a solvent containing no dichloromethane is proposed. In this case, the solvent preferably contains ether with 4 to 12 carbon atoms, ketone with 3 to 12 carbon atoms, ester with 3 to 12 carbon atoms, brominated hydrocarbon such as 1-bromopropane, and the like. The solvent also contains a mixture of them. For example, the mixed organic solvent contains methylacetate, acetone, ethanol, and n-butanol. Note that ether, ketone, ester, and alcohol may have a cyclic structure. A compound having at least two functional groups thereof (that is, —O—, —CO—, —COO—, and —OH) may be used as the solvent.
When two or more kinds of compounds different from each other are used as the solvent, and the proportion of the compounds is adequately adjusted, it is possible to control the formation speed of the water drops, a depth of the water drops entering the coating layer, and the like, as described later.
Preferably, in the first coating liquid 35, the content of the first polymer is in the range between 0.02 pts.wt. or more and 30 pts.wt. or less, if the content of an organic solvent is determined as 100 pts.wt. Thereby, the porous layer 11 having high quality can be formed with high productivity. If the content of the first polymer to the organic solvent of 100 pts.wt. is less than 0.02 pts.wt., it takes too much time to evaporate the organic solvent, because the content rate of the solvent in the solution is too large. Accordingly, the productivity of the porous film 10 is decreased. In contrast, if the content of the first polymer to the organic solvent of 100 pts.wt. exceeds 30 pts.wt., it is impossible to cause the water drops generated by the condensation to change the form of the coating liquid 35 for forming the coating layer, and therefore the surface of the porous layer 11 has unevenness in some cases.
As shown in FIG. 4, a porous film production apparatus 30 of the present invention includes a support feeding unit 31, a coating chamber 32, and a product cutting unit 33. In the support feeding unit 31, the support 12 is drawn from a support roll 34 and fed to the coating chamber 32. In the coating chamber 32, the first coating liquid 35 is applied to the support 12 and dried thereon to form the porous film 10. In the product cutting unit 33, the porous film 10 formed in the coating chamber 32 is cut into a predetermined size to obtain a partly-finished product. The partly-finished product is subjected to various kinds of processing to obtain a finished product.
The support feeding unit 31 and the product cutting unit 33 are used to produce continuously the porous films 10 in large amounts, and in accordance with the production scale, they may be omitted appropriately. Further, instead of using the support roll 34, a cut sheet obtained by cutting the support 12 into sheets may be used in the case of low-volume production.
The coating chamber 32 is partitioned into a first section 41, a second section 42, a third section 43, and a fourth section 44 in this order from an upstream side in a moving direction of the support 12. These sections are partitioned from each other, and each of the sections is in a form of chamber. The first section 41 is provided with a coating die 51. The first coating liquid 35 is applied through the coating die 51 onto the support 12 to form a coating layer 35 a (see FIG. 5). A wet thickness of the coating layer 35 a is at most 500 μm, more preferably in the range of 10 μm to 450 μm, and most preferably in the range of 50 μm to 400 μm. When the wet thickness of the coating layer 35 a is made thinner as described above, the time required for drying the coating layer 35 a is short, and therefore it is impossible to control condensation by drying the support 12. Consequently, as described hereinbelow, the atmospheric dew point is controlled such that the water drops can be formed while the wet thickness of the coating layer 35 a is maintained. The viscosity of the first coating liquid 35 is in the range of 1×10⁻⁴Pa·s to 1×10⁻¹Pa·s, more preferably in the range of 3×10⁻⁴Pa·s to 5×10⁻³Pa·s, and most preferably in the range of 5×10⁻⁴Pa·s to 1×10⁻³Pa·s. As described later, since the condensation time is short, the viscosity of the first coating liquid 35 needs to be low in order to soak minuscule water drops into the coating layer 35 a.
The second section 42 includes a first humidified air feeding unit 52, a second humidified air feeding unit 53, and a third humidified air feeding unit 54 in this order from the upstream side in the moving direction of the support 12. Humidified air is fed from the first humidified air feeding unit 52 to define a first zone 61 with an atmosphere having high humidity on a surface of the coating layer 35 a formed of the first coating liquid 35, as shown in FIG. 5. Further, humidified air is fed from the second humidified air feeding unit 53 to define a second zone 62 with an atmosphere having low humidity in which the dew point is lower than that of the first zone 61 on the surface of the coating layer 35 a, as shown in FIG. 5. Furthermore, humidified air is fed from the third humidified air feeding unit 54 to define a third zone 63 in which the dew point is equal to or slightly different from that of the second zone 62 on the surface of the coating layer 35 a, as shown in FIG. 5.
The dew point and flow rate of the humidified air fed from the humidified air feeding units 52 to 54 are controlled. Thereby, the minuscule water drops 60 are generated promptly in the first zone 61. The minuscule water drops 60 generated in the first zone 61 can be arranged in the densest state without growing up so much in the second zone 62. Additionally, in a case where the diameter and the arrangement pitch of the water drops 60 are not controlled sufficiently in the second zone 62, the humidified air with a controlled dewpoint is fed to the coating layer 35 a at a controlled flow rate, for the purpose of controlling the diameter and the arrangement pitch of the water drops 60 again in the third zone 63. The number of each of the first to third humidified air feeding units 52 to 54 is not limited to one, and may be two or more. In a case where it is unnecessary to control the diameter and the arrangement pitch of the water drops 60 again precisely, the third zone 63 to be defined by the third humidified air feeding unit 54 may be omitted.
As shown in FIG. 4, the first humidified air feeding unit 52 is provided with a duct 52 c having an outlet 52 a and an inlet 52 b, and a blower 52 d. The blower 52 d adjusts the temperature, the dew point, and the humidity of the humidified air to be fed through the outlet 52 a, and sucks the gas around the coating layer 35 a through the inlet 52 b, for the purpose of circulating gas around the coating layer 35 a. Additionally, the blower 52 d is provided with a filter (not shown) for removing dust from the humidified air. The second and third humidified air feeding units 53 and 54 have the same structure as that of the first humidified air feeding unit 52. Accordingly, the second humidified air feeding unit 53 is provided with a duct 53 c having an inlet and an outlet, and a blower 53 d. The third humidified air feeding unit 54 is provided with a duct 54 c having an inlet and an outlet, and a blower 54 d.
As shown in FIG. 5, the humidified air is fed through the outlet 52 a, and the gas around the coating layer 35 a is sucked through the inlet 52 b by the blower 52 d of the first humidified air feeding unit 52, such that highly humidified air 56 is fed to the surface of the coating layer 35 a on the support 12. Thereby, water vapor is condensed from atmosphere on the surface of the coating layer 35 a to generate a plurality of minuscule water drops 60 having a μm-order diameter on the coating layer 35 a. The first humidified air feeding unit 52 feeds the humidified air to define the first zone 61 with atmosphere having high humidity on the surface of the coating layer 35 a. The time for which the coating layer 35 a passes through the first zone 61 (remaining time) t1 is decided in accordance with the desired diameter of the pores. The remaining time t1 of the coating layer 35 a in the first zone 61 is, for example, in the range of 0.1 sec to 60 sec. preferably in the range of 1 sec to 50 sec, and more preferably in the range of 5 sec to 40 sec.
The surface temperature of the coating layer 35 a is denoted by Ts. When the atmospheric dew point in the first zone 61 is denoted by Td1, the value obtained by subtracting Ts from Td1, namely ΔT1 is set to be more than 0 in the first zone 61. Preferably, ΔT1 is at least 5° C. and at most 30° C. When the ΔT1 is set to be relatively high within the range described above, highly humidified air can be fed to the coating layer 35 a, to condense water vapor from atmosphere on the surface of the coating layer 35 a sufficiently.
When the atmospheric dew point in the second zone 62 is denoted by Td2, the value obtained by subtracting Ts from Td2, namely ΔT2 is set to be more than 0 in the second zone 62. Preferably, ΔT2 is set to be at least 5° C. and at most 30° C., and further, less than ΔT1 by at least 1° C. When ΔT2 is set to be less than ΔT1, the minuscule water drops 60 generated in the first zone 61 can be arranged in the densest state with increased regularity in its arrangement pitch while its minuscule size is maintained and its growth is prevented.
Further, a difference ΔT12 obtained by subtracting the atmospheric dew point Td2 in the second zone 62 from the atmospheric dew point Td1 in the first zone 61 is at least 1° C., and preferably at least 3° C. The time for which the coating layer 35 a passes through the second zone 62 (remaining time) t2 is, for example, in the range of 60 sec to 1000 sec, and preferably in the range of 100 sec to 600 sec. Further, the condition expressed by t1<t2 is satisfied. When t2 is set to be longer than t1 as describe above, the minuscule water drops 60 generated in the first zone 61 can be arranged in the densest state with increased regularity in its arrangement pitch while its minuscule size is maintained and its growth is prevented. Furthermore, when the atmospheric dew point in the third zone 63 is denoted by Td3, the value obtained by subtracting Ts from Td3 is denoted by ΔT3 in the third zone 63. A difference ΔT23 obtained by subtracting the atmospheric dew point Td3 in the third zone 63 from the atmospheric dew point Td2 in the second zone 62 is more than ΔT12, and preferably at least 1° C. and at most 10° C. Accordingly, the condition expressed by ΔT1>ΔT2>ΔT3 is satisfied. Since the conditions for the condensation process performed in the second section 42 include several steps as described above, both of productivity of the film and regularity in the arrangement pitch of the pores can be achieved at the same time.
The first solvent is evaporated in the third section 43. For the evaporation, there are disposed a first air feeding/sucking unit 71, a second air feeding/sucking unit 72, and a third air feeding/sucking unit 73 in this order from the upstream side in the moving direction of the support 12. The first to third air feeding/sucking unit 71 to 73 have respectively the same structure as that of the first humidified air feeding unit 52. The first air feeding/sucking unit 71 is provided with a duct 71 c having an inlet and an outlet, and a blower 71 d. The second air feeding/sucking unit 72 is provided with a duct 72 c having an inlet and an outlet, and a blower 72 d. The third air feeding/sucking unit 73 is provided with a duct 73 c having an inlet and an outlet, and a blower 73 d. Each of the blowers 71 d to 73 d adjusts the temperature and the humidity of the dry air to be fed through the outlet, and sucks the gas around the coating layer 35 a through the inlet, for the purpose of circulating gas around the coating layer 35 a. Accordingly, in the third section 43, the first solvent is evaporated from the coating layer 35 a to form a template for a porous structure by the minuscule water drops 60.
The fourth section 44 includes a first air feeding/sucking unit 83, a second air feeding/sucking unit 84, a third air feeding/sucking unit 85, and a fourth air feeding/sucking unit 86, for the purpose of evaporating the minuscule water drops 60. Due to the evaporation, a plurality of minute pores are formed in the coating layer 35 a to obtain the porous film 10. The first to fourth air feeding/sucking unit 83 to 86 have respectively the same structure as that of the first air feeding/sucking unit 71, and therefore the detailed description thereof will be omitted.
As described above, the support 12 passes through the first to fourth sections 41 to 44 to form a porous layer 11 on the support 12. At first, the coating liquid 35 is applied to the support 12 to form the coating layer 35 a in the first section 41. Next, as shown in FIG. 5, the water vapor is condensed from atmosphere on the surface of the coating layer 35 a in the first zone 61 of the second section 42. After the condensation, the minuscule water drops 60 are generated while being prevented from growing up in the second and third zones 62 and 63 of the fourth section 42. The minuscule water drops 60 are arranged densely to achieve the densest state. In the third section 43, the solvent is evaporated from the coating layer 35 a in which the minuscule water drops 60 are arranged in the densest state such that the template for a porous structure is formed by the minuscule water drops 60. In the fourth section 44, the minuscule water drops 60 are evaporated together with the solvent to be dried, and thereby a plurality of the pores 15 are formed by the template for a porous structure formed by the minuscule water drops 60. Note that, in FIG. 5, the minuscule water drops 60, the coating layer 35 a, and the like are shown emphatically, for the purpose of showing the minuscule water drops 60.
The flow rate of air fed from each of the units 52 to 54, 71 to 73, and 83 to 86 is preferably set in accordance with the moving speed of the support 12. The relative speed of the flow rate of air fed from the above units with respect to the moving speed of the support 12 is preferably within the range between 0.02 m/s or more and 2 m/s or less, more preferably within the range between 0.05 m/s or more and 1.5 m/s or less, and most preferably within the range between 0.1 m/s or more and 0.5 m/s or less. In a case where the relative speed is less than 0.02 m/sec, the coating layer 35 a is unfavorably introduced to the fourth section 44 before the water drops are arranged in the densest state, in some cases. In contrast, in a case where the relative speed exceeds 2 m/sec, an exposed surface of the coating layer 35 a loses smoothness, or condensation does not sufficiently proceed, in some cases. The moving speed of the support 12 is within the range of 10 mm/min to 10,000 mm/min, for example, and preferably within the range of 20 mm/min to 2,000 mm/min.
A plurality of rollers 90 are disposed with an appropriate pitch in each of the sections 41 to 44. Only main rollers 90 are shown, and other rollers 90 are omitted in the drawing. The rollers 90 include driving rollers and freely rotating rollers. The driving rollers are disposed with an appropriate pitch in each of the sections 41 to 44, and thereby the support 12 is conveyed at a constant moving speed in each of the sections 41 to 44. Further, the temperature of each of the rollers 90 is independently controlled by a not-shown temperature controller in each of the sections 41 to 44. Each of the processes for drying the coating layer, condensation, growing the water drops, and forming the pores, is conducted under optimal conditions. Additionally, a not-shown temperature controlling plate is disposed between the adjacent rollers 90 so as to be in proximate to the support 12 and reverse to the coating layer 35 a. The temperature of the temperature controlling plate is controlled such that the temperature of the support becomes an appropriate value.
A not-shown solvent recovery device is disposed in each of the sections 41 to 44 of the coating chamber 32 so as to recover the solvent in each of the sections 41 to 44. The recovered solvent is refined in a not-shown refining device to be reused.
Next, the operation of the preferred embodiment is described hereinbelow. As shown in FIG. 4, the first coating liquid 35 is discharged to the support 12 through the coating die 51 in the first section 41, and as shown in FIG. 5, the first coating liquid 35 becomes the coating layer 35 a on the support 12. The thickness of the coating layer 35 a before being dried, (wet thickness) is a predetermined value within the range between 0.01 mm or more and 1 mm or less. Note that, even when the thickness of the coating layer 35 a is within the range between 0.01 mm or more and 1 mm or less, variation in thickness of the coating layer 35 a makes it impossible to form the water drops having a uniform diameter in some cases. Further, when the thickness of the coating layer 35 a is less than 0.01 ml, it is impossible to form the coating layer 35 a itself evenly on the support 12, and the first coating liquid 35 is partially repelled on the support 12 in some cases. Thereby, the coating layer 35 a may not be able to cover the support 12 entirely. In contrast, when the thickness of the coating layer 35 a exceeds 1 mm, it takes too much time to dry the coating layer 35 a, and therefore the production efficiency may be decreased in some cases.
During the condensation process performed in the second section 42, the highly humidified air 56 is fed to the coating layer 35 a through the outlet 52 a of the first humidified air feeding unit 52 in the first zone 61. Further, the gas around the coating layer 35 a is sucked through the inlet 52 b to be discharged. Here, the dew point of the humidified air fed through the outlet 52 a is denoted by Td1, and a value obtained by subtracting Ts from Td1 is denoted by ΔT1. In this case, at least one of the values Td1 and Ts is controlled such that ΔT1 is within the range between 3° C. or more and 30° C. or less. The surface temperature Ts of the coating layer 35 a can be measured by, for example, a non-contact thermometer (such as an infrared thermometer commercially available) that is disposed near a conveying path of the coating layer 35 a. When the value ΔT is less than 3° C., the water drops 60 are hardly generated. In contrast, when the value ΔT exceeds 30° C., the water drops 60 are generated suddenly. In this case, the water drops 60 become nonuniform in size, and otherwise, the water drops, which should be arranged in two dimensional arrangement (in a matrix manner), are arranged in three dimensional arrangement in which one of the water drops 60 overlaps on the other one.
The coating layer 35 a is conveyed through the first zone 61 for the predetermined short time t1, and thereby each of the minuscule water drops 60 generated due to the condensation grow up to have a diameter of as small as 50 nm to 5 μm, for example, at the end of the first zone 61. Then, the coating layer 35 a is dried utilizing the minuscule water drops as the template for a porous structure during a solvent drying process performed in the third section 34 and a water drop drying process performed in the fourth section 44. Thereby, the porous film 10 having minute pores whose diameter is within the range of 50 nm to 5 μm can be obtained.
As the coating layer 35 a is moved from the first zone 61 to the second zone 62 in the second section 42, the ΔT1 changes from a high value to a low value in the moving direction of the support 12. A graph in FIG. 6 shows the thickness of the coating layer 35 a and the diameter of the minuscule water drop 60 when ΔT obtained by subtracting Ts from Td is changed. The graph in FIG. 6 has a horizontal axis representing the thickness of the coating layer 35 a, and a vertical axis representing the diameter of the minuscule water drop 60. In FIG. 6, S represents a point at which the humidifying process is started. A curve line C1 shown by the solid line represents a relation between the thickness of the coating layer 35 a and the diameter of the minuscule water drop in a case where the minuscule water drops 60 are formed consistently in a highly humidifying process performed in the first zone 61. A curve line C2 shown by the dashed line represents the relation between the thickness of the coating layer 35 a and the diameter of the minuscule water drop 60 in a case where the minuscule water drops 60 are formed consistently in a low humidifying process performed in the second zone 62. A curve line C3 shown by the chain double-dashed line represents the relation between the thickness of the coating layer 35 a and the diameter of the minuscule water drop 60 in a case where the minuscule water drops 60 are generated in a short period of time in a highly humidifying process performed in the first zone 61, and then the minuscule water drops 60 are prevented from growing up in the low humidifying process performed in the second zone 62, according to the present invention. In FIG. 6, a point P corresponds to a point at which the coating layer 35 a enters the second zone 62 for performing the low humidifying process from the first zone 61 for performing the highly humidifying process.
In this embodiment, the relation between the thickness of the coating layer 35 a and the diameter of the minuscule water drop 60 is shown by a lower portion of the curve line C1 which extends from the origin to the point P, and C3 which intersects with the curve line C1 at the point P. Accordingly, the minuscule water drops 60 are generated at an earlier stage, and the growth of the minuscule water drops 60 can be stopped. Therefore, according to this embodiment, it is possible to form the minuscule water drops 60 more easily in comparison with the conventional methods. Further, the timing for switching the first zone 61 and the second zone 62 is appropriately changed, and therefore it is possible to set the diameter of the minuscule water drops 60 appropriately. Furthermore, the time for feeding the low humidified air and the atmospheric dew point thereof are controlled in the second zone 62 and the third zone 63, and therefore it is possible to change the density of the formed minuscule water drops 60. Thereby, it is possible to form the minuscule water drops 60 with more uniform arrangement pitch and with a desired degree of density in comparison with the conventional methods.
In the second section 42, the surface temperature Ts of the coating layer 35 a is controlled by adjusting the temperature of the rollers 90. In stead of/in addition to this, a temperature controlling plate (not shown) may be disposed in proximity to a surface reverse to the coated surface of the support 12, for the purpose of controlling the surface temperature Ts of the coating layer 35 a. The dew point Td is controlled by varying conditions for humidified air fed from the first to third humidified air feeding units 52 to 54.
In the third section 43, the first solvent contained in the coating layer 35 a is evaporated while the form and size of the water drops generated due to the condensation in the second section 42 are maintained by using the first to third air feeding/sucking units 71 to 73. Note that the number of the air feeding/sucking units 71 to 73 is not limited to three, and may be at least one. The structure of each of the first to third air feeding/sucking units 71 to 73 is the same as that of the humidified air feeding unit 52, however is not limited to this. The temperature of air fed from the first to third air feeding/sucking units 71 to 73 is set to 25° C., and the flow rate thereof is set to 0.3 m/s, for example. In the above conditions, the solvent contained in the coating layer 35 a is evaporated while the form and size of the water drops contained in the coating layer 35 a are maintained, to accelerate drying of the coating layer 35 a.
In the fourth section 44, at least one of the surface temperature Ts and the atmospheric dew point Td is controlled such that the surface temperature Ts is higher than the atmospheric dew point Td. The surface temperature Ts is controlled by the temperature controlling plate (not shown). The atmospheric dew point Td is controlled by adjusting the conditions of dry air fed through the outlet of each of the air feeding/sucking units 83 to 86. The surface temperature Ts is measured by a thermometer similar to the above disposed in proximate to the coating layer 35 a. When Ts is set to be higher than Td, the growth of the water drops is stopped to evaporate the water drops, and therefore it is possible to produce a porous film having uniform pores. Note that when Ts is equal to or less than Td1, water vapor is further condensed from ambient air on the water drops, and the formed porous structure is destroyed in some cases, thus causing an unfavorable result.
The main object to be achieved in the fourth section 44 is to evaporate the water drops. However, the solvents which have not been evaporated until reaching the fourth section 44 are also evaporated in the fourth section 44.
The evaporation process of the water drops in the fourth section 44 may be performed by using a reduced-pressure drying device or a so-called 2D nozzle instead of using the first to fourth air feeding/sucking units 83 to 86. The reduced-pressure drying makes it possible to facilitate adjustment of the evaporation speeds of the organic solvent and the water drops, respectively. Thereby, it is possible to improve the evaporation of the organic solvent and the water drops such that the water drops are favorably formed in the coating layer 35 a. Accordingly, it is possible to form pores whose size and form are adjusted in a position corresponding to the water drops. The 2D nozzle consists of air feeding nozzle portions and outlets for sucking the gas around the coating layer 35 a to be discharged. The air feeding nozzle portions and outlets are alternately disposed in the moving direction of the support 12.
The methods for applying the first coating liquid 35 to the support 12 are as follows. In a first method, the coating liquid is discharged onto a support disposed stationary and spread over the support. In a second method, the coating liquid is applied to the support 12 by an inkjet coating unit. In a third method, the coating liquid is discharged through a coating die onto a moving support. A fourth method is a dip-coating method, for example. All of the methods may be applicable to the present invention. The first method and the second method are generally suitable for high-mix low-volume production. The third method and the fourth method are generally suitable for mass production. In any method, if the coating liquid is continuously discharged, a long porous film can be obtained, and in contrast, if the coating liquid is intermittently discharged, porous films having a predetermined length can be obtained.
In a case where a support of a cut sheet type is used instead of using the belt-like continuous support 12 as shown in FIG. 4, the support of a cut sheet type is caused to pass through the first to fourth sections 41 to 44 in order of precedence as in the case of the continuous support. Thereby, the porous film can be obtained. Additionally, in a case where the support of a cut sheet type is used, the number of coating chambers may be one, such that application of the first coating liquid for forming the porous layer, a condensation process which necessitates at least the first zone and the second zone, and drying process are performed in the coating chamber.

Example 1

Next, examples of the present invention are described hereinbelow. In Example 1, in the porous film production apparatus 30 as shown in FIG. 4, the condensation process was performed by using the first to third humidified air feeding units 52 to 54, and thereby the minuscule water drops 60 were formed in the first to third zones 61 to 63. The atmospheric dew point and the flow rate of the humidified air were different for each of the first to third zones 61 to 63. The support was a film formed of polyethylene terephthalate (PET). The composition of the first coating liquid 35 was as follows. The thickness of the coating layer was 300 μm. Polystyrene having the number average molecular weight (Mn) of 200,000 and amphiphilic polyacrylamide shown in Chemical Formula 1 were mixed in a ratio by weight of 10:1, to obtain a solute for the first coating liquid 35. The solvent was dichloromethane. The first coating liquid 35 was prepared such that the concentration of the polymer was 0.2 wt %. The first coating liquid 35 was preserved in a tank such that the temperature thereof was kept at approximately 15° C. The surface temperature of the support 12 was set to 10° C. by a peltier unit.
At first, various parameters were changed, namely various conditions of the humidified air were adjusted, such that the atmospheric dew point Td1 became 22° C. and the flow rate of the highly humidified air became 0.1 m/s by the first humidified air feeding unit 52 in the first zone 61. The time for which the coating layer 35 a passes through the first zone 61 (remaining time) t1 was 5 seconds. Next, various parameters were changed such that the atmospheric dew point Td2 became 14° C. and the flow rate of the humidified air became 0.1 m/s by the second humidified air feeding unit 53 in the second zone 62. The time for which the coating layer 35 a passes through the second zone 62 (remaining time) t2 was 40 seconds. At last, various parameters were changed such that the atmospheric dew point Td3 became 12° C. and the flow rate of the humidified air became 0.3 m/s by the third humidified air feeding unit 54 in the third zone 63. The time for which the coating layer 35 a passes through the third zone 63 (remaining time) t3 was 95 seconds. The temperature difference ΔT12 in atmospheric dew point between the first zone 61 and the second zone 62 was 8° C.
The condition of the dew point and the remaining time in each zone are shown in Table 1. As described above, at first, water vapor was condensed from ambient air on the surface of the coating layer 35 a in a short period of time in the first zone 61, and then the minuscule water drops 60 generated due to the condensation were prevented from growing up and arranged in the densest state while the diameter of each of the water drops 60 was maintained in the second zone 62, and lastly final adjustment was performed in the third zone 63. Accordingly, it was possible to arrange the minuscule water drops 60 having a desired diameter at a desired arrangement pitch. Finally, the porous film having the pores whose diameter was 0.8 μm on average was obtained.
Evaluation was made as to the porous film obtained in each of examples and comparative examples in accordance with the following criteria.
(1) Uniformity in Pore Size
“Uniform”; Variation coefficient of the pore diameter is less than 5%.
“Slightly nonuniform”; Variation coefficient of the pore diameter is in the range between 5% or more and less than 10%.
“Nonuniform”; Variation coefficient of the pore diameter is more than 10%.
(2) Degree of Unevenness in Arrangement Pitch of Pores
The degree of unevenness in the arrangement pitch of the pores is evaluated based on the variation coefficient of the distance between the centers of the adjacent pores according to the following criteria. As the variation coefficient of the distance between the centers of the adjacent pores is lower, the pores are arranged more densely as described above. Further, as the pores are arranged more densely, the arrangement pitch of the pores is more uniform. Accordingly, as the variation coefficient of the distance between the centers of the adjacent pores is lower, the arrangement pitch of the pores is more uniform. Note that, when the pore sizes are nonuniform, it is natural that there is unevenness in the arrangement pitch of the pores. In contrast, when the pore sizes are uniform, there are a case in which there is unevenness in the arrangement pitch of the pores, and a case in which there is no unevenness in the arrangement pitch of the pores.
“No unevenness”; Variation coefficient of the distance between the centers of the adjacent pores is less than 5%.
“Slight unevenness”; Variation coefficient of the distance between the centers of the adjacent pores is in the range between 5% or more and less than 10%.
(3) Average Pore Diameter
“Extremely small”; Average pore diameter is less than 1 μm.
“Slightly large”; Average pore diameter is in the range between 1 μm or more and less than 3 μm.
“Large”; Average pore diameter is more than 3 μm.
In accordance with the evaluation results in (1) to (3) described above, comprehensive evaluation was made as follows.
A (Excellent): The pore sizes were uniform, no unevenness was observed in the arrangement pitch of the pores, and the average pore diameter was extremely small.
B (Good): Although the average pore diameter was slightly large, the pore sizes were uniform, and no unevenness was observed in the arrangement pitch of the pores.
C (Passed): The average pore diameter was slightly large, and the pore sizes were slightly nonuniform but were allowable, and slight unevenness was observed in the arrangement pitch of the pores but was allowable.
D (Failure): The pore sizes were nonuniform, and streaks and defects were observed.
Note that, the streaks indicate a state in which the pores are formed so as to be stacked on one another in the thickness direction of the porous film. Such a state is caused, because too much water drops are generated and stacked on one another in the thickness direction of the porous film in the production process of the film. Additionally, the defects indicate a state in which there are spaces containing no pores, namely, a state in which the distance between the adjacent pores is too large, or a state in which a pore larger than the other peripheral pores is formed. Such a state, in which a pore larger than the other peripheral pores is formed, is caused, because a plurality of water drops are integrated into one water drop in the production process of the film.
According to Example 1, it was possible to form minute pores whose diameter was 0.8 μm on average, and uniformity in diameter of the pores was excellent, and further no unevenness was observed in the arrangement pitch of the pores. Accordingly, the evaluation rank was A.

Example 2

In Example 2, the atmospheric dew point Td1 was 22° C., and remaining time t1 was 10 seconds in the first zone 61. The atmospheric dew point Td2 was 12° C., and the remaining time t2 was 130 seconds in the second zone 62. Other conditions were the same as those in Example 1. The third zone 63 was omitted in Example 2. In Example 2, although the dew point was controlled at two stages, the porous film having the pores whose diameter was 1.2 μm on average was obtained. The uniformity in diameter of the pores in the obtained porous film was excellent, and no unevenness was observed in the arrangement pitch of the pores. However, the average diameter of the pores was larger than 0.8 μm that was the average diameter of the pores in Example 1. Accordingly, the evaluation rank was B.

Example 3

In Example 3, the atmospheric dew point Td1 was 19° C., and the remaining time t1 was 65 seconds in the first zone 61. The atmospheric dew point Td2 was 17° C., and the remaining time t2 was 35 seconds in the second zone 62. The atmospheric dew point Td3 was 16° C., and the remaining time t3 was 40 seconds in the third zone 63. Ts was 15° C., ΔT12 was 2° C., and ΔT2 was 2° C. Other conditions were the same as those in Example 1. In Example 3, the porous film having the pores whose diameter was 2.5 μm on average was obtained. In Example 3, the remaining time t1 in the first zone 61 with high humidity exceeded 60 seconds, and concretely, the remaining time t1 was 65 seconds. However, the remaining time t2 in the second zone 62 with low humidity and the remaining time t3 in the third zone 63 with low humidity were 75 seconds in total, namely, the remaining times t2 and t3 in the second and third zones 62 and 63 with low humidity in total were longer than t1 in the first zone 61 with high humidity. Accordingly, the porous film having the pores whose diameter was as small as approximately 2.5 μm on average was obtained. Note that, in Example 3, the uniformity in diameter of the pores in the obtained porous film was not so excellent, and slight unevenness was observed. Although there is no problem in the practical use of the porous film obtained in Example 3, the pore diameters were nonuniform, and slight unevenness was observed in the arrangement pitch of the pores. Accordingly, the evaluation rank was C.

Example 4

In Example 4, the atmospheric dew point Td1 was 18° C., and remaining time t1 was 15 seconds in the first zone 61. The atmospheric dew point Td2 was 17° C., and the remaining time t2 was 60 seconds in the second zone 62. The third zone 63 was omitted in Example 4. The thickness of the coating layer 35 a was 200 μm. Ts was 15° C., ΔT12 was 1° C., and ΔT2 was 2° C. Other conditions were the same as those in Example 1. In Example 4, the porous film having the pores whose diameter was 1.0 μm on average was obtained. In Example 4, although the temperature difference ΔT12 between the atmospheric dew point Td1 in the first zone 61 with high humidity and the atmospheric dew point Td2 in the second zone 62 with low humidity was 1 C that was a lower limit, the porous film having pores whose diameter was 1.0 μm on average was obtained. The uniformity in diameter of the pores in the obtained porous film was excellent, and no unevenness was observed. However, the average diameter of the pores was larger than 0.8 μm that was the average diameter of the pores in Example 1. Accordingly, the evaluation rank was B.

Comparative Example 1

The atmospheric dew point Td1 was 22° C., and the remaining time t1 was 140 seconds in the first zone. The second zone and the third zone with low humidity were omitted in Comparative Example 1. Other conditions were the same as those in Example 1. The average diameter of the pores in the obtained porous film was 3.5 μm. Namely, it was impossible to obtain the porous film having desired pores whose diameter was at most 3 μm in Comparative Example 1. Additionally, the pore diameters were nonuniform, and streaks and defects were observed. Accordingly, the evaluation rank was D. As a result, it was found that, it is not sufficient to use only the first zone with high humidity to make the pores minute.

Comparative Example 2

The atmospheric dew point Td1 was 18° C., and remaining time t1 was 5 seconds in the first zone. The atmospheric dew point Td2 was 23° C., and the remaining time t2 was 135 seconds in the second zone. The temperature difference ΔT12 in atmospheric dew point between the first zone and the second zone was −5° C. The third zone was omitted in Comparative Example 2. Other conditions were the same as those in Example 1. The average diameter of the pores in the obtained porous film was 12 μm. Namely, it was impossible to obtain the porous film having desired pores whose diameter was at most 3 μm in Comparative Example 2. Additionally, the pore diameters were nonuniform, and streaks and defects were observed. Accordingly, the evaluation rank was D. As a result, it was found that, if the condensation process is performed in the first zone with low humidity and in the second zone with high humidity, it is difficult to make the pores minute.
The temperature conditions, the remaining times, the average diameter of the pores, and evaluation rank in Examples 1 to 4 and Comparative Examples 1 and 2 are shown in Table 1 below.

TABLE 1

Ex 1	Ex 2	EX 3	Ex 4	Com 1	Com 2

Td1 (° C.)	22	22	19	18	22	18
t1 (s)	5	10	65	15	140	5
Td2 (° C.)	14	12	17	17	—	23
t2 (s)	40	130	35	60	—	135
Td3 (° C.)	12	—	16	—	—	—
t3 (s)	95	—	40	—	—	—
Ts (° C.)	10	10	15	15	10	10
Film thickness (μm)	300	300	300	200	300	300
ΔT12 (° C.)	8	10	2	1	—	−5
ΔT2 (° C.)	4	2	2	2	—	13
Average diameter of	0.8	1.2	2.5	1.0	3.5	12.0
pores (μm)
Evaluation rank	A	B	C	B	D	D

Various changes and modifications are possible in the present invention and may be understood to be within the present invention.

Claims

1. A porous film production method comprising the steps of:

(A) applying a coating liquid containing a polymer and a hydrophobic solvent to a support to form a coating layer;

(B) setting said coating layer under an atmosphere having a first dew point Td1 higher than a surface temperature Ts of said coating layer (Td1>Ts);

(C) setting said coating layer under an atmosphere having a second dew point Td2 higher than said surface temperature Ts of said coating layer and lower than said first dew point Td1 (Td1>Td2>Ts) after said step B; and

(D) drying said coating layer subjected to said step B and said step C to form a porous film having a plurality of pores made by water drops as a template for said porous film, and said water drops being formed by said step B and said step C.

2. A porous film production method as defined in claim 1, wherein a relation between said dew point Td1 and said dew point Td2 satisfies the following condition:

Td1−Td2≧1° C.

3. A porous film production method as defined in claim 2, wherein t1 is a remaining time of said coating layer in said step B, t2 is a remaining time of said coating layer in said step C, and said t1 and said t2 satisfy the following condition:

t1<t2

4. A porous film production method as defined in claim 3, wherein said remaining time t1 of said coating layer in said step B is not less than 0.1 seconds and not more than 60 seconds.

5. A porous film production method as defined in claim 4, wherein a thickness of said coating layer before being dried is at most 500 μm.