Patent Application
20170095830
This Nonprovisional application claims priority under 35 U.S.C. §119 on Patent Application No. 2015-197123 filed in Japan on Oct. 2, 2015, and on Patent Application No. 2016-188320 filed in Japan on Sep. 27, 2016, the entire contents of which are hereby incorporated by reference.
The present invention relates to a coating method, a coating device, and a method of producing a functional film.
There are known methods for coating, with a coating solution, a surface of a film serving as a base material.
Examples of the methods encompass a spin coating method, a spray coating method, a bar coating method, and a gravure coating method. The gravure coating method is carried out by (i) immersing, in a coating solution, a gravure roll having a surface on which unevenness is provided and (ii) causing the gravure roll to come into contact with a base material, so that the base material is coated with the coating solution collected in recesses. The gravure coating method is used for, for example, a step of forming a heat-resistant layer on a porous film base material during the process of producing a heat-resistant separator for a battery.
Patent Literature 1 discloses a method of producing a laminated thermoplastic resin film under certain conditions. With the method, it is possible to prevent a continuous dot-like coating stripe flaw from occurring as a result of fine flaws being connected to each other, which fine flaws are (i) formed on a surface of a film after the surface is coated with a coating solution and (ii) each formed by a resin component spreading in the form of a lower part of a mountain from an aggregate of particles contained in the coating solution.
[Patent Literature 1]
Japanese Patent Application Publication Tokukai No. 2006-297829 (Publication date: Nov. 2, 2006)
However, in some cases, depending on coating conditions, a coating solution is applied so as to be non-uniform in thickness as a result of being applied to have a pattern corresponding to the shape of recesses of a surface of a gravure roll. Furthermore, in some cases, not an entire surface of a film is coated with a coating solution, so that the surface of the film ends up being exposed.
The present invention has been made in view of the problem, and it is an object in accordance with an embodiment of the present invention to provide a coating method, a coating device, and a functional film production method, each of which is intended for uniformly coating an entire surface of a film with a coating solution.
In order to attain the object, a coating method in accordance with an embodiment of the present invention is a reverse gravure coating method of coating a film by use of a gravure roll, in which the following formula is satisfied:
0<a×c/b<113
where (i) a is a diameter (mm) of the gravure roll, (ii) b is a ratio of a circumferential velocity of the gravure roll to a conveyance speed at which the film is conveyed, and (iii) c is a volume (mL/m2) of recesses of the gravure roll, which volume is measured per unit area of a circumferential surface of the gravure roll (hereinafter, “b” may be referred to as “rotation ratio”).
In order to attain the object, a coating device in accordance with an embodiment of the present invention includes: a gravure roll which rotates in a reverse direction which is opposite a direction in which a film is conveyed, the following formula being satisfied:
0<a×c/b<113
where (i) a is a diameter (mm) of the gravure roll, (ii) b is a ratio of a circumferential velocity of the gravure roll to a conveyance speed at which the film is conveyed, and (iii) c is a volume (mL/m2) of recesses of the gravure roll, which volume is measured per unit area of a circumferential surface of the gravure roll.
In order to attain the object, a functional film production method in accordance with an embodiment of the present invention is configured so that the above coating method is used.
With an embodiment of the present invention, it is possible to provide a coating method, a coating device, and a functional film production method, each of which is intended for uniformly coating an entire surface of a film with a coating solution.
The following description will discuss the details of an embodiment of the present invention with reference to
<Configuration of Lithium Ion Secondary Battery>
A nonaqueous electrolyte secondary battery, typically, a lithium-ion secondary battery has a high energy density, and therefore, currently widely used not only as batteries for use in devices such as personal computers, mobile phones, and mobile information terminals, and for use in moving bodies such as automobiles and airplanes, but also as stationary batteries contributing to stable power supply.
As illustrated in
<Separator>
The separator 12 is provided so as to be sandwiched between the cathode 11 which is a positive electrode of the lithium-ion secondary battery 1 and the anode 13 which is a negative electrode of the lithium-ion secondary battery 1. The separator 12 separates the cathode 11 and the anode 13, and allows lithium ions to move between the cathode 11 and the anode 13. Examples of a material for the separator 12 encompass polyolefin such as polyethylene or polypropylene.
As illustrated in (a) of
However, there are, for example, cases in which the temperature of the lithium-ion secondary battery 1 rises due to excessive charging of the lithium-ion secondary battery 1, a high current caused by short-circuiting of the external device, or the like. In such cases, the separator 12 melts or softens and the pores P are blocked as illustrated in (b) of
However, in a case where a temperature of the lithium-ion secondary battery 1 sharply rises, the separator 12 suddenly shrinks. In this case, as illustrated in (c) of
<Heat-Resistant Separator>
As illustrated in (a) of
According to the configuration illustrated in (a) of
As illustrated in (b) of
The following description will discuss a heat-resistant separator production method in accordance with Embodiment 1.
A method of producing the heat-resistant separator 12a includes: a separator forming step of forming the separator 12; a coating step of coating a surface of the separator 12 with a coating solution to become the heat-resistant layer 4; and a drying step of drying the coating solution so that the coating solution becomes the heat-resistant layer 4. Note that after the heat-resistant layer 4 has been laminated, the heat-resistant separator 12a can be, as necessary, slit into slit heat-resistant separators, each of which has a narrow width such as a product width. In the coating step, the surface of the base material is uniformly coated with the coating solution through wet coating with the use of a gravure coater-based coating device.
Note that Embodiment 1 will discuss the coating step of applying a coating solution to be a heat-resistant layer 4 so that a heat-resistant separator 12a, in which the heat-resistant layer 4 is provided on the surface of the separator 12, is to be produced. However, the coating method in accordance with an embodiment of the present invention is not limited to such a coating step. Alternatively, the separator 12 can be provided with a functional layer other than the heat-resistant layer 4. In such a case, a coating solution corresponding to the functional layer can be applied in a coating step.
The coating solution for use in the coating method in accordance with an embodiment of the present invention includes a filler, a binder, and a solvent.
Examples of the filler encompass a filler made of organic matter and a filler made of inorganic matter. Specific examples of the filler made of organic matter encompass fillers made of (i) a homopolymer of a monomer such as styrene, vinyl ketone, acrylonitrile, methyl methacrylate, ethyl methacrylate, glycidyl methacrylate, glycidyl acrylate, or methyl acrylate, or (ii) a copolymer of two or more of such monomers; fluorine-containing resins such as polytetrafluoroethylene, ethylene tetrafluoride-propylene hexafluoride copolymer, tetrafluoroethylene-ethylene copolymer, and polyvinylidene fluoride; melamine resin; urea resin; polyethylene; polypropylene; and polyacrylic acid and polymethacrylic acid. Specific examples of the filler made of inorganic matter encompass fillers made of calcium carbonate, talc, clay, kaolin, silica, hydrotalcite, diatomaceous earth, magnesium carbonate, barium carbonate, calcium sulfate, magnesium sulfate, barium sulfate, aluminum hydroxide, boehmite, magnesium hydroxide, calcium oxide, magnesium oxide, titanium oxide, titanium nitride, alumina (aluminum oxide), aluminum nitride, mica, zeolite, or glass. The porous layer may contain (i) only one kind of filler or (ii) two or more kinds of fillers in combination.
Among the above fillers, a filler made of inorganic matter is suitable. A filler made of an inorganic oxide such as silica, calcium oxide, magnesium oxide, titanium oxide, alumina, or boehmite is preferable. A filler made of at least one kind selected from the group consisting of silica, magnesium oxide, titanium oxide, and alumina is more preferable. A filler made of alumina or boehmite is particularly preferable. While alumina has many crystal forms such as α-alumina, β-alumina, γ-alumina, and θ-alumina, any of the crystal forms can be used suitably. Among the above crystal forms, α-alumina is the most preferable because it is particularly high in thermal stability and chemical stability.
The filler has an average particle size of preferably equal to or less than 3 μm, and more preferably 1 μm. Examples of a shape of the filler encompass a spherical shape and a gourd shape. An average particle size of the filler can be calculated by, for example, (i) a method in which any 25 particles are selected by a scanning electron microscope (SEM), respective particle sizes (diameters) of the particles are measured, and an average of the 10 particle sizes is calculated or (ii) a method in which a BET specific surface area is measured, and an average particle size is calculated by spherical approximation based on the BET specific surface area. Note that, in a case where the average particle size is calculated with the use of the SEM and where particles of the filler each have a shape other than a spherical shape, a greatest length of each of the particles is designated as a particle size.
Alternatively, particles to be used can be a combination of two or more kinds which differ from each other in particle diameter and/or specific surface area.
A binder resin to be used for formation of the functional layer has a function of (i) binding together fillers by which the functional layer is constituted and (ii) binding a filler and the base film. The binder resin is preferably a resin which is (i) soluble or dispersible in a solvent to be used for a coating solution and (ii) insoluble in an electrolyte of the battery or (iii) is electrochemically stable when the battery is in normal use. The binder resin is preferably a water-dispersible polymer or a water-soluble polymer because such polymers allow an aqueous solvent to be used as a solvent of a coating solution due to a process and/or an environmental impact. Note that “aqueous solvent” means a solvent which contains water in an amount of equal to or greater than 50% by weight and which contains another solvent such as ethanol and contains an additional component provided that neither dispensability of the water-dispersible polymer nor solubility of the water-soluble polymer is impaired.
Examples of the water-dispersible polymer encompass: polyolefins such as polyethylene and polypropylene; fluorine-containing resins such as polyvinylidene fluoride and polytetrafluoroethylene; fluorine-containing rubbers such as vinylidene fluoride-hexafluoropropylene copolymer and ethylene-tetrafluoroethylene copolymer; rubbers such as styrene-butadiene copolymer and a hydrogenated one thereof, acrylic acid ester copolymer, methacrylic acid ester copolymer, acrylonitrile-acrylic acid ester copolymer, styrene-acrylic acid ester copolymer, ethylene propylene rubber, and polyvinyl acetate; and resins with a melting point or a glass transition temperature of equal to or greater than 180° C., such as polyphenylene ether, polysulfone, polyether sulfone, polyphenylene sulfide, polyetherimide, polyamide imide, polyetheramide, polyamide, and polyester.
Acrylic resins such as acrylic acid ester copolymer, methacrylic acid ester copolymer, acrylonitrile-acrylic acid ester copolymer, and styrene-acrylic acid ester copolymer are preferable because these acrylic resins are each high in property to bond fillers together or bond a filler and a base film together.
Resins with a melting point or a glass transition temperature of equal to or greater than 180° C., such as polyphenylene ether, polysulfone, polyether sulfone, polyphenylene sulfide, polyetherimide, polyamide imide, polyetheramide, and polyester are preferable because these resins have high heat resistance and cause a laminated porous film to increase in property to maintain a shape when heated. Among the heat resistant resins, polyetherimide, polyamide imide, polyetheramide, and polyamide are preferable, and polyamide is more preferable.
Examples of the water-soluble polymer encompass polyvinyl alcohol, polyethylene glycol, cellulose ether, sodium alginate, polyacrylic acid, polyacrylamide, and polymethacrylic acid. Among the water-soluble polymers, cellulose ether is preferable. Specific examples of the cellulose ether encompass carboxymethyl cellulose (CMC), hydroxyethyl cellulose (HEC), carboxy ethyl cellulose, methyl cellulose, ethyl cellulose, cyan ethyl cellulose, and oxyethyl cellulose. Among these, CMC and HEC, which have excellent chemical stability, are particularly preferable. In a case where there are salts, examples of the water-soluble polymer encompass the salts.
In a case where a nonaqueous solvent is to be used, examples of a nonaqueous solvent that can be used encompass: fluorine-containing resins such as polyvinylidene fluoride; polyvinylidene chloride; and polyacrylonitrile.
These binder resins can be used individually. Alternatively, two or more kinds of these binder resins can be used in a mixed state as necessary.
Although a ratio between a binder resin to a filler in the functional layer is to be decided as appropriate according to the purpose for the use of the functional layer, the weight ratio of the filler to the binder resin is preferably 1 to 100, and more preferably 2 to 99. In particular, in a case where the functional layer is a heat-resistant layer, the weight ratio is preferably 4 to 99.
The coating solution has a viscosity of preferably 10 Cps to 15 Cps, and more preferably 15 Cps to 30 Cps.
As illustrated in (a) of
A gravure coater-based coating method is a coating method in which (i) the gravure roll 20 is immersed in the coating solution 31 so that the coating solution 31 is collected in recesses of the surface of the gravure roll 20, (ii) an excess part of the coating solution 31 on the surface of the gravure roll 20 is scraped off with the use of the doctor blade 32, and then (iii) the separator 12 serving as a base material is pressed against the gravure roll 20 with the use of the guide rolls 16, so that the coating solution collected in the recesses of the gravure roll 20 is transferred to the separator 12. Note that pressure, by which the separator 12 is pressed against the gravure roll 20, can be adjusted as appropriate by tensile force of the separator 12 and by a depth to which the guide rolls 16 presses the separator 12. The depth to which the guide rolls 16 presses the separator 12 can be, for example, 5 mm.
As illustrated in (a) of
As illustrated in (b) of
Note that the shape of each of the recesses 21 of the gravure roll 20 is not limited to these shapes. Alternatively, it is possible to use a gravure roll having recesses 21 of various shapes.
According to the gravure coater-based coating method, a desired amount (weight per unit area) of coating solution can be applied by properly setting coating conditions such as (i) the volume of the recesses 21 of the gravure roll 20, (ii) a rotation speed of the gravure roll 20, and (iii) a diameter of the gravure roll 20.
As illustrated in
In a region in which a rotation ratio is equal to or less than a value at which a corresponding weight per unit area is at a maximum value, the weight per unit area linearly changes with respect to an increase in the rotation ratio, so that it is easy to control the weight per unit area. On the other hand, in a region in which a rotation ratio is greater than a value at which a corresponding weight per unit area is at a maximum value, the weight per unit area non-linearly changes with respect to an increase in the rotation ratio, so that it is difficult to control the weight per unit area. Therefore, a weight per unit area is preferably adjusted by controlling a rotation ratio in a region in which the rotation ratio is equal to or less than a value at which a corresponding weight per unit area is at a maximum value.
Under general coating conditions under which coating has been conventionally carried out, weight per unit area is non-uniform in some cases. This may result in a non-uniform thickness of a heat-resistant layer 4 of a heat-resistant separator to be produced, so that, in some cases, a heat-resistant layer 4 may have a defective appearance.
As illustrated in (b) of
The heat-resistant separator production method in accordance with Embodiment 1 includes a coating step in which (i) a coating solution is uniformly applied, so that a heat-resistant layer 4 after drying has an excellent appearance and (ii) the coating solution is applied under coating conditions for enhancing uniformity in thickness of the heat-resistant layer 4. The heat-resistant separator production method will be described in detail below.
(Separator Forming Step)
To a total amount of 100 parts by weight consisting of 70% by weight of ultra-high molecular weight polyethylene powder (340M, manufactured by Mitsui Chemicals, Inc.) and 30% by weight of polyethylene wax (FNP-0115, manufactured by Nippon Seiro Co., Ltd.) having a weight-average molecular weight of 1000, 0.4% by weight of antioxidant (Irg1010, manufactured by Ciba Specialty Chemicals Inc.), 0.1% by weight of antioxidant (P168, manufactured by Ciba Specialty Chemicals Inc.), and 1.3% by weight of sodium stearate were added. Then, a calcium carbonate (manufactured by Maruo Calcium Co., Ltd.) having an average pore diameter of 0.1 μm was added in an amount of 38% by volume relative to a total volume. Then, a resultant powder was mixed with the use of a Henschel mixer. Then, a resultant mixture was molten and kneaded with the use of a biaxial kneader, so that a polyolefin resin composition was obtained.
The polyolefin resin composition was rolled with the use of a pair of rolls each having a surface temperature of 150° C., so that a sheet was obtained. The sheet was immersed in a hydrochloric acid aqueous solution (4 mol/L of hydrochloric acid, 0.5% by weight of nonionic surfactant), so that a calcium carbonate was removed. Then, a resultant sheet was stretched widthwise. This resulted in a separator which had (i) a thickness of 18.2 μm, (ii) a weight per unit area (mass per unit area) of 7.2 g/m2, and an air permeability of 89 seconds/100 ml.
(Coating Step)
(1) Preparation of Coating Solution
A coating solution was produced by the following steps.
First, a CMC solution (CMC concentration: 0.70% by weight relative to CMC solution) as a medium was obtained by dissolving carboxymethyl cellulose (CMC, Cellogen 3H manufactured by Dai-ichi Kogyo Seiyaku Co., Ltd.) in 5% by weight of isopropyl alcohol aqueous solution.
Then, 3500 parts by weight of alumina (AKP3000, manufactured by Sumitomo Chemical Co., Ltd.) relative to 100 parts by weight of the CMC solution as calculated based on CMC was added and mixed. Then, a resultant mixture was processed three times under high-pressure dispersion conditions (60 MPa) with the use of a galling homogenizer, so that a coating solution was prepared. Viscosity of the coating solution measured with the use of a B-type viscometer under conditions of 100 rpm and 23° C. was 20 cP.
(2) Coating Conditions
The following description will discuss, in detail, Examples 1 through 10 and Comparative Examples 1 through 3 as examples of the coating conditions. (a) of
As coating conditions of Example 1, (i) a gravure roll 20 having a diameter of 50 mm and having recesses 21 whose volume per unit area was 100 mL/m2 was used, (ii) a line speed (conveyance speed) of a separator 12 was set to 30 m/min, and (iii) a rotation ratio was set to 60%.
As coating conditions of Example 2, (i) a gravure roll 20 having a diameter of 50 mm and having recesses 21 whose volume per unit area was 100 mL/m2 was used, (ii) a line speed (conveyance speed) of a separator 12 was set to 30 m/min, and (iii) a rotation ratio was set to 80%.
As coating conditions of Example 3, (i) a gravure roll 20 having a diameter of 50 mm and having recesses 21 whose volume per unit area was 100 mL/m2 was used, (ii) a line speed (conveyance speed) of a separator 12 was set to 30 m/min, and (iii) a rotation ratio was set to 150%.
As coating conditions of Example 4, (i) a gravure roll 20 having a diameter of 80 mm and having recesses 21 whose volume per unit area was 60 mL/m2 was used, (ii) a line speed (conveyance speed) of a separator 12 was set to 60 m/min, and (iii) a rotation ratio was set to 80%.
As coating conditions of Example 5, (i) a gravure roll 20 having a diameter of 80 mm and having recesses 21 whose volume per unit area was 60 mL/m2 was used, (ii) a line speed (conveyance speed) of a separator 12 was set to 60 m/min, and (iii) a rotation ratio was set to 100%.
As coating conditions of Example 6, (i) a gravure roll 20 having a diameter of 80 mm and having recesses 21 whose volume per unit area was 60 mL/m2 was used, (ii) a line speed (conveyance speed) of a separator 12 was set to 60 m/min, and (iii) a rotation ratio was set to 150%.
As coating conditions of Example 7, (i) a gravure roll 20 having a diameter of 150 mm and having recesses 21 whose volume per unit area was 60 mL/m2 was used, (ii) a line speed (conveyance speed) of a separator 12 was set to 30 m/min, and (iii) a rotation ratio was set to 100%.
As coating conditions of Example 8, (i) a gravure roll 20 having a diameter of 150 mm and having recesses 21 whose volume per unit area was 60 mL/m2 was used, (ii) a line speed (conveyance speed) of a separator 12 was set to 30 m/min, and (iii) a rotation ratio was set to 120%.
As coating conditions of Example 9, (i) a gravure roll 20 having a diameter of 150 mm and having recesses 21 whose volume per unit area was 30 mL/m2 was used, (ii) a line speed (conveyance speed) of a separator 12 was set to 30 m/min, and (iii) a rotation ratio was set to 200%.
As coating conditions of Example 10, (i) a gravure roll 20 having a diameter of 150 mm and having recesses 21 whose volume per unit area was 60 mL/m2 was used, (ii) a line speed (conveyance speed) of a separator 12 was set to 30 m/min, and (iii) a rotation ratio was set to 250%.
As coating conditions of Comparative Example 1, (i) a gravure roll 20 having a diameter of 50 mm and having recesses 21 whose volume per unit area was 100 mL/m2 was used, (ii) a line speed (conveyance speed) of a separator 12 was set to 30 m/min, and (iii) a rotation ratio was set to 40%.
As coating conditions of Comparative Example 2, (i) a gravure roll 20 having a diameter of 150 mm and having recesses 21 whose volume per unit area was 60 mL/m2 was used, (ii) a line speed (conveyance speed) of a separator 12 was set to 30 m/min, and (iii) a rotation ratio was set to 80%.
As coating conditions of Comparative Example 3, (i) a gravure roll 20 having a diameter of 80 mm and having recesses 21 whose volume per unit area was 100 mL/m2 was used, (ii) a line speed (conveyance speed) of a separator 12 was set to 30 m/min, and (iii) a rotation ratio was set to 70%.
<Results of Evaluation of Coating>
(a) of
Specifically, an index A can be represented by the following Formula (1):
A=a×c/b Formula (1)
where (i) a is a diameter (mm) of a gravure roll, (ii) b is a rotation ratio (%) of the gravure roll, and (iii) c is a volume (mL/m2) of recesses 21 per unit area of the gravure roll.
Appearance scores shown in (a) of
A weight per unit area (Za) of a heat-resistant layer 4 of a heat-resistant separator, which weight per unit area (Za) is shown in the table of (b) of
As coating conditions of Example 4, coating was carried out while adjusting the rotation ratio of the gravure roll so that the weight per unit area was 5.5 g/m2. As coating conditions of Example 5, coating was carried out while adjusting the rotation ratio of the gravure roll so that the weight per unit area was 7.2 g/m2. As coating conditions of Example 6, coating was carried out while adjusting the rotation ratio of the gravure roll so that the weight per unit area was 7.6 g/m2. As coating conditions of Example 9, coating was carried out while adjusting the rotation ratio of the gravure roll so that the weight per unit area was 2.5 g/m2.
As coating conditions of Example 10, coating was carried out while adjusting the rotation ratio of the gravure roll so that the weight per unit area was 5.5 g/m2.
<Preferable Coating Conditions>
(Appearance)
As illustrated in
Note that a rotation ratio b (%) of the gravure roll 20 can be represented by the following Formula (2):
b=0.001×a×n×B/d Formula (2)
where (i) d is a conveyance speed (line speed) (m/min) of the separator 12 and (ii) B is a rotation speed (rpm) of the gravure roll 20.
The index A can be represented by the following Formula (3) based on the Formula (1) and the Formula (2):
A=(c×d)/(0.001×B×π) Formula (3)
According to the Formula (3), a higher rotation speed B of the gravure roll 20, a slower line speed d, and a smaller volume c of the gravure roll result in a smaller index A. In addition, the index A is directly proportional to a value (c/B) obtained by dividing the line speed d (m/min) by the rotation speed B (rpm) of the gravure roll 20. This is considered to be due to the following factor: In a case where the line speed (m/min) is high with respect to the rotation speed (rpm) of the gravure roll 20 and where the amount of coating solution 31 per unit area of the gravure roll 20 is large, the coating solution 31, which has been released from the recesses 21 by surface tension and has been stuck on the separator 12, is separated from the gravure roll 20 while, without being made uniform by the protrusions, maintaining its shape corresponding to the shape of the recesses 21. Therefore, in order to prevent the shape of the grooves of the gravure roll from being transferred, it is preferable to cause the coating solution 31, which is stuck on the separator 12, to be made uniform so as to be more flat. In order to make the coating solution 31 uniform in such a way, it is preferable to set coating conditions under which an index A becomes small.
As illustrated in
Therefore, the coating step is preferably carried out under coating conditions under which the index A is greater than 0 and less than 113 (0<A<113). This allows a heat-resistant layer 4 to be formed while allowing the entire surface of the separator 12 to be uniformly coated with a coating solution without exposing the separator 12.
The appearance score was equal to or greater than 1 in a case where the index A was equal to or less than 90. Therefore, the coating step is preferably carried out under coating conditions under which the index A is equal to or less than 90 (A≦90). This allows a heat-resistant layer 4 to be formed while allowing the entire surface of the separator 12 to be more uniformly coated with a coating solution.
The appearance score was 2 in a case where the index A was equal to or greater than 32 and equal to or less than 63. Therefore, the coating step is preferably carried out under coating conditions under which the index A is equal to or greater than 32 and equal to or less than 63 (32≦A≦60). This allows a heat-resistant layer 4 to be formed while allowing a coating solution to be uniformly applied without causing a stripe pattern, which corresponds to the shape of the recesses 21 of the gravure roll 20, to be generated.
As illustrated in (b) of
(Relationship Between Conveyance Speed and Index A)
In a case where conveyance tension is excessively small in the coating step in which the separator 12 is coated while being conveyed, the separator 12 becomes wrinkled. In a case where conveyance tension is excessively large in the coating step, there is a risk of tearing the separator 12.
Therefore, in order to coat a film while conveying the film at a proper conveyance tension, a conveyance speed (line speed) in the coating step is preferably set to a speed falling within a range of approximately 20 m/min to 60 m/min. A rotation speed, a diameter, and a volume of a gravure roll are set according to the line speed, and will be described in detail below.
(Rotation Speed)
As described above with reference to
In order to adjust a weight per unit area in a region in which the weight per unit area particularly linearly changes with respect to an increase in a rotation ratio, the rotation ratio is preferably equal to or less than 150%, and particularly preferably equal to or less than 120%. If the rotation ratio is set to an excessively small value, then not an entire surface of a separator being conveyed can be uniformly coated. Therefore, in order to uniformly apply a coating solution, the rotation ratio is preferably equal to or greater than 40%, and particularly preferably equal to or greater than 60%.
The rotation ratio is preferably set to fall within the above ranges by adjusting the rotation speed of the gravure roll 20 according to a line speed.
(Roll Diameter)
The diameter of the gravure roll 20 can be set as appropriate. Note, however, that in order to carrying out coating at a desired rotation ratio, the gravure roll 20 needs to be (i) rotated faster if the diameter of the gravure roll 20 is smaller and (ii) rotated slower if the diameter of the gravure roll 20 is larger.
However, in a case where the rotation speed of the gravure roll 20 is set to an excessively high value or to an excessively small value, a weight per unit area becomes less stable. Therefore, the diameter of the gravure roll 20 is preferably equal to or greater than 20 mm and equal to or less than 180 mm, and particularly preferably equal to or greater than 30 mm and equal to or less than 150 mm.
(Volume)
The volume of the recesses 21 of the gravure roll 20 can be set as appropriate. Note, however, that in a case where the volume is set to an excessively small value, the gravure roll 20 needs to be rotated fast in order for coating to be carried out with a desired weight per unit area. In a case where the volume is set to a large value, there is a risk of impairing the uniformity of the weight per unit area.
Therefore, the volume of a gravure roll is equal to or greater than 10 mL/m2, preferably equal to or less than 120 mL/m2, more preferably equal to or greater than 20 mL/m2 and equal to or less than 100 mL/m2, and particularly preferably equal to or greater than 60 mL/m2.
It is possible to form a heat-resistant layer 4 by uniformly coating the entire surface of the separator 12 with a coating solution through selecting proper coating conditions under which the index A falls within the above-described numerical range based on preferable numerical ranges of the rotation ratio, the diameter, and the volume of the gravure roll 20.
In order to carry out coating while conveying the film at a proper conveyance tension, as described above, (i) the rotation ratio is particularly preferably equal to or less than 120%, (ii) the diameter of the gravure roll 20 is particularly preferably equal to or greater than 30 mm, and (iii) the volume of the recesses 21 of the gravure roll 20 is particularly preferably equal to or greater than 60 mL/m2. Therefore, the index A is particularly preferably equal to or greater than 15.
A coating method in accordance with an embodiment of the present invention is a reverse gravure coating method of coating a film by use of a gravure roll, in which the following formula is satisfied:
0<a×c/b<113
where (i) a is a diameter (mm) of the gravure roll, (ii) b is a ratio of a circumferential velocity of the gravure roll to a conveyance speed at which the film is conveyed, and (iii) c is a volume (mL/m2) of recesses of the gravure roll, which volume is measured per unit area of a circumferential surface of the gravure roll (hereinafter, “b” may be referred to as “rotation ratio”).
With the method, an entire surface of the film can be uniformly coated with a coating solution without exposing the film.
The coating method is preferably configured so that the following formula is satisfied:
a×c/b≦90.
This more reliably allows the entire surface of the film to be uniformly coated with a coating solution without exposing the film.
The coating method is preferably configured so that the following formula is satisfied:
15≦a×c/b.
This allows the entire surface of the film to be uniformly coated with a coating solution while conveying the film at a proper conveyance tension without exposing the film.
The coating method is preferably configured so that the following formula is satisfied:
32≦a×c/b≦63.
This allows a coating solution to be applied without causing a pattern, which corresponds to the shape of the recesses of the gravure roll, to be generated.
The coating method can be configured so that a surface of the gravure roll is provided with a plurality of grooves constituting the recesses.
The coating method is preferably configured so that the following formula is satisfied:
20≦a≦180.
A coating device in accordance with an embodiment of the present invention includes: a gravure roll which rotates in a reverse direction which is opposite a direction in which a film is conveyed, the following formula being satisfied:
0<a×c/b<113
where (i) a is a diameter (mm) of the gravure roll, (ii) b is a ratio of a circumferential velocity of the gravure roll to a conveyance speed at which the film is conveyed, and (iii) c is a volume (mL/m2) of recesses of the gravure roll, which volume is measured per unit area of a circumferential surface of the gravure roll.
In order to attain the object, a functional film production method in accordance with an embodiment of the present invention is configured so that the above coating method is used.
[Additional Remarks]
The present invention is not limited to the description of the embodiments, but can be altered in many ways by a person skilled in the art within the scope of the claims. An embodiment derived from a proper combination of technical means disclosed in different embodiments is also encompassed in the technical scope of the present invention.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2015-197123 | Oct 2015 | JP | national |
| 2016-188320 | Sep 2016 | JP | national |
