JPS62214912A - Manufacture of polyimide film - Google Patents

Abstract

PURPOSE:To restrain the viscosity of polyamide varnish within a given width and permit continuous flow stretching by a method wherein aromatic acid 2-anhydride, dissolved in polar organic solvent, is mixed with the solution of aromatic diamine while the mixture is sent into a reaction vessel kept under a temperature lower than a specified temperature. CONSTITUTION:The solution A of aromatic acid 2-anhydride dissolved in organic solvent such as N,N-dimethylformamide, N,N-diethylforamide or the like is mixed with the solution B of aromatic diamine such as m-phenylenediamine, 4,4-diaminodiphenylpropane or the like in a vessel 3 while the mixture is sent by a liquid sending pump into a reaction vessel 6 kept under a temperature lower than 100 deg.C to effect mixing and polymerizing reaction. Continuously synthesized polyamide acid varnish is made to flow on a base material and is dried, thereafter, is separated from the base material and is dehydrated to make cycloimide.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a method for producing a polyimide film, and more specifically, a method for continuously synthesizing a polyamic acid varnish, and further continuously performing a polyamic acid varnish synthesis step and a casting step. The present invention relates to a method for producing a polyimide film as a process.

[Conventional technology]

Polyimide films are widely used as electrical and electronic insulating materials due to their excellent heat resistance and electrical insulation properties. In recent years, applications such as magnetic vapor deposition tapes have required highly uniform film thickness and surface smoothness.

Since polyimide is insoluble in solvents, has high heat resistance, and has a melting point and a similar decomposition temperature, polyamic acid, which is a precursor of polyimide, is first synthesized in a synthesis kettle. This polyamic acid varnish once synthesized is applied to a T-die, a roll coater,
The film is cast to a certain thickness onto a metal belt or drum using a casting device such as a knife coater, the solvent is dried and removed to form a self-supporting film, and the film is peeled off from the belt or drum.

Next, both ends of the film are fixed with pins or chestnut knobs, and the film is passed through a high-temperature oven to remove residual solvent and undergo dehydration and ring-closing imidization to obtain a polyimide film. In this method, a metal belt or drum is used as the base material, but other materials are used, such as JP-A No. 50-12
No. 7959 discloses a method of casting using a plastic film as a base material, and JP-A-56-103227 discloses a method of casting on a carrier film, and after drying, the carrier film such as a plastic film or metal film is coated with a solvent or A method of removing it by etching has been proposed.

On the other hand, while the synthesized polyamic acid varnish is being stored,
Varnish viscosity may increase or gelatinize over time, and storage stability is problematic. Special Public Service
6 and Japanese Patent Publication No. 48-17465, formic acid, monochloroacetic acid, benzaldehyde, p-aminophenol, p-nitrobenzaldehyde, aromatic mono- and dicarboxylic acids, etc. are added to polyamic acid varnish. A method to stabilize viscosity is proposed.

[Problem that the invention seeks to solve]

Polyamic acid varnish is synthesized by reacting aromatic acid dianhydride and aromatic diamine in an organic polar solvent.
Synthesis is an addition reaction, and the viscosity of the varnish increases significantly, making it extremely difficult to stop the reaction with constant accuracy. For varnishes whose viscosity has become too high, an operation called "chucking" is performed in which the varnish temperature is raised and the varnish is hydrolyzed to adjust the viscosity to a desired level. At this time, when the varnish temperature is increased, the viscosity decreases significantly, and although the desired viscosity is reached in a relatively short time, the viscosity often decreases too much. On the other hand, when the varnish temperature is lowered, the hydrolysis rate is slow, and although it is easier to adjust the viscosity to a desired level, it takes a long time and is not economical. Further, even if these methods are used, it is unavoidable that the varnish viscosity changes depending on the loft. On the other hand, synthesized varnishes have poor storage stability, and if stored near room temperature, their viscosity changes over time, so they must be stored at low temperatures, resulting in high costs.

In addition, the synthesized polyamic acid varnish is shaped into a constant thickness using a T-die or the like and cast onto a base material. At this time, if the varnish viscosity changes, the varnish flow rate changes in the slit direction of the T-die, resulting in a The problem arises that the thickness of the film produced varies.

[Means for solving problems]

The inventors of the present invention have arrived at the present invention as a result of extensive research into economically producing a film with uniform thickness and excellent surface smoothness using a polyamic acid varnish with little change in viscosity.

That is, the method for producing a polyimide film of the present invention involves casting a polyamic acid varnish synthesized from an aromatic acid dianhydride and an aromatic diamine onto a base material, drying it, peeling it off from the base material, and performing dehydration and ring-closing imidization. In the method for producing a polyimide film, a solution of an aromatic acid dianhydride dissolved in an organic polar solvent and a solution of an aromatic diamine are mixed, and the mixture is sent to a reaction vessel kept at 100° C. or lower using a liquid pump, The present invention is characterized in that a polyamic acid varnish is continuously synthesized by carrying out mixing and polymerization reactions, and that the process of synthesizing the polyamic acid varnish and the process of casting it onto a substrate are continuous processes.

Examples of the organic polar solvent used in the present invention include N2N-dimethylformamide, N,N-diethylformamide,
N, N-dimethylacetamide, N.

N-diethylacetamide, N-methyl-2-pyrrolidone, N-methylcaprolactam, dimethylsulfoxide, pyridine, N,N-dimethylmethoxyacetamide,
Usually known compounds such as hexamethylphosphoramide, dimethylsulfone, tetramethylsulfone, etc. are used singly or in combination.
Used as a mixture of more than one species. Furthermore, these organic polar solvents can be used in appropriate combination with nonsolvents such as benzene, toluene, xylene, benzonitrile, dioxane, cyclohexane, and the like.

As the aromatic acid dianhydride used in the present invention, aromatic tetracarboxylic dianhydride is used, such as pyromellitic dianhydride, 2,3,6.7-naphthalenetetracarboxylic dianhydride, 3 .3'.

4.4'-diphenyltetracarboxylic dianhydride, 1.
2,5.6-naphthalenetetracarboxylic dianhydride, 2
．． 2',3.3'-diphenyltetracarboxylic dianhydride, 2.2-bis(3,4-dicarboxyphenyl)brobannihydride, bis(3,4-dicarboxyphenyl)sulfonic dianhydride, 3. 4,9.10-perylenetetracarboxylic dianhydride, bis(3,4-dicarboxyphenyl)ethane dianhydride, naphthalene-1,2°4.5
-Tetracarboxylic dianhydride, naphthalene-1,4,5
, 8-tetracarboxylic dianhydride, 2.2-bis-(2
, 3-dicarboxyphenyl)propanihydride, 1.
1-bis(2,3-dicarboxyphenyl)ethane dianhydride, 1,1-bis(3,4-dicarboxyphenyl)
Ethane dianhydride, bis(2,3-dicarboxyphenyl)ethane dianhydride, bis(3,4-dicarboxyphenyl)sulfone dianhydride, benzene-1°2, 3. 4
-tetracarboxylic dianhydride, 3,4°3',4'-benzophenone tetracarboxylic dianhydride, etc.
Pyromellitic dianhydride, 3.3',4.4'-diphenyltetracarboxylic dianhydride, 3.4.3',4'-
Benzophenone tetracarboxylic dianhydride or a mixture of two or more thereof is preferred, and pyromellitic dianhydride is most preferred.

The aromatic diamine used in the present invention is Hx N-R-
It is shown by the structural formula of NH2, R is a divalent aromatic group,
It is selected from a phenylene group, a natalene group, a biphenylene group, and a group represented by the following structural formula.

[In the formula, R' is an alkylene group having 1 to 4 carbon atoms, -0-1
-S-1-3O□-5R.

R10 R.

-o-po-o-, R+ and Rz are aliphatic groups or aromatic groups.

] Specific examples include m-phenylenediamine, p-phenylenediamine, 4,4'-diaminodiphenylpropane, 4,4'-diaminodiphenylmethane, benzidine, 4,4'-diaminodiphenyl sulfide, 4.4'
-Diamino diphenyl sulfone, 3.3'-diaminodiphenylsulfone, 4.4'-diaminodiphenyl ether, 2゜6-diamitubiridine, bis(4-aminophenyl)diethylsilane, bis(4-aminophenyl)
Diphenylsilane, 3,3'-dichlorobenzidine, bis(4-aminophenyl)ethylphosphine oxide, bis(4-aminophenyl)phenylphosphine oxide, bis(4-aminophenyl)-N-phenylamine, bis (4-aminophenyl)-N-methylamine, 1,5-diaminonaphthalene, 3,3'-dimethyl-
Examples thereof include 4°4'-diaminodiphenyl, 3,3'-dimethoxybenzidine, and mixtures of two or more thereof. Preferred among these aromatic diamines are 4.4'-diaminodiphenyl ether, 4.4'
-diaminodiphenylpropane, 4,4'-diaminodiphenylmethane, p-phenylenediamine, and mixtures of two or more thereof, with 4,4'-diaminodiphenyl ether and p-phenylenediamine being most preferred.

If the organic content of aromatic acid dianhydride and aromatic diamine exceeds %, the viscosity becomes too high and it becomes difficult to handle. The most preferred range is 10-20% by weight.

The moisture content of the solution of aromatic acid dianhydride and the solution of aromatic diamine pre-dissolved in the polar solvent of the batter is 5000p.
It is preferable to set it to below micrometer.

This is because when the water content exceeds 5000 pI)m, the acid anhydride opens its ring and does not react with the amine, and the generated polyamic acid undergoes hydrolysis and becomes lower in molecular weight, so the varnish viscosity does not increase. This is because only polyamic acid varnishes with poor film-forming ability can be obtained. More preferably, 110
00 pp or less is preferable.

The aromatic acid dianhydride and aromatic diamine dissolved in the organic polar solvent are filtered if necessary, and approximately equimolar amounts are supplied to a mixing container using a metering pump, etc., and then mixed using a stirring rod with a propeller, etc. Mix. This mixed solution is sent to a reaction vessel for polymerization reaction using a liquid sending pump.

The reaction vessel is a continuous tubular vessel, such as a metal pipe, and stainless steel, iron, or copper pipes coated with Teflon are particularly preferable, and resin pipes reinforced with metal wire can also be used. The diameter of the pipe is determined by taking into consideration the pressure loss of the varnish inside the pipe and the amount of liquid to be sent. The length of the pipe is determined by the amount of liquid to be fed, the solid concentration of the varnish in the pipe, the reaction temperature, the monomer composition, and the desired viscosity of the polyamic acid varnish. The longer the length of the pipe, the higher the viscosity of the polyamic acid varnish. These tubular containers are designed to have low pressure loss, are piped to minimize stagnation, and are connected using flanges, screws, welding, etc. to make them compact, and can be used in liquids whose temperature is controlled to below 100°C. Soak in. The viscosity change of polyamic acid varnish over time initially increases gradually and then gradually increases rapidly. Therefore, in order to suppress this sudden viscosity change, the continuous tubular reaction vessel is divided into sections, and the latter stage of the varnish flow path is divided into the former stage. Lower temperatures may be maintained to keep the polymerization reaction rate low. Alternatively, the temperature in the latter stage may be made extremely higher than that in the former stage, so that the rate of hydrolysis reaction due to water in the varnish in the tube is faster than the rate of polymerization reaction, and the viscosity may be kept low.
The tubular reaction vessel may be divided into several blocks, each of which may be controlled at a different temperature.

The reaction temperature of the polymerization reaction is preferably 0 to 100°C, particularly 0 to 100°C.
~60°C is preferred. If the temperature is lower than 0°C, the reaction will take a long time, which is undesirable. If the temperature exceeds 100°C, the polyamic acid will be hydrolyzed by the moisture in the varnish, and the viscosity of the resulting polyamic acid varnish will not increase, resulting in poor film-forming ability. This is not desirable.

The pressure loss of the liquid pump is expressed by the following simple formula. The higher the viscosity of the polymerized polyamic acid varnish, the larger the amount of liquid to be fed, and the longer the piping, the higher the liquid feeding pressure will be.

πR4 ΔP: Pressure loss (g/cm) Q: Liquid delivery amount (cm3/5ec) L: Piping length (cm) R: Piping diameter (cm) η: Varnish viscosity Therefore, high discharge pressure for a liquid pump It is preferable to use a gear pump that produces .

A mixing device is installed to thoroughly mix the monomer solution sent by the liquid pump. For this purpose, glass bulbs, metal rods, or cylindrical or prismatic objects may be used for mixing, but in order to achieve more uniform mixing with less pressure loss, a stationary mixing device is recommended. It is more preferable to install an in-tube mixer. Static in-tube mixers include Toshi's Himixer and Sluzer Brothers' static mixing unit.

A plurality of pulps may be installed at intervals in a continuous tubular reaction vessel, and a polyamic acid varnish outlet may be provided to take out the polyamic acid varnish that has reached a desired viscosity. This is based on the monomer composition of the varnish being delivered,
When the amount of liquid delivered, monomer purity and water content are different,
This is to adjust the reaction time and obtain a polyamic acid varnish with a desired viscosity. The synthesized polyamic acid varnish may be mixed and further homogenized by installing the above-mentioned static type in-tube mixer at the outlet from the continuous tubular reaction vessel. This is to make the reaction uniform since the flow velocity is different between the center and the periphery of the cross section of the tubular reaction vessel and the reaction occurs non-uniformly. Furthermore, a filtration device may be installed at the outlet to filter out foreign substances and gel-like substances. If necessary, an in-line viscometer may be installed in the middle of the continuous tubular reaction vessel or near the outlet.

The polyamic acid varnish that is continuously synthesized and taken out from the outlet is continuously led to the liquid receiver of the casting device, and then cast onto the substrate to a uniform thickness. A metal belt or drum is used as the base material. Moreover, a plastic film can also be used as a base material. The plastic film base material is preferably a biaxially stretched polypropylene film or a polyester film with an average surface roughness of 0.05 μm or less. This is because the polyimide film can be easily peeled off from the base material, and the surface of the obtained polyimide film becomes smooth. Among commercially available films, those for vapor deposition and capacitor use are preferred, and those without corona discharge treatment are preferred. This is because the base film that has been subjected to corona discharge treatment has good adhesion, making it extremely difficult to separate the base film and the polyamic acid film. In addition, uniaxially stretched or unstretched films shrink more than biaxially stretched films at the same temperature, and must be dried at lower temperatures, which is not economical. In addition, polyetherketone, polyimide film, polycarbonate, polyarylate, polysulfone, polyetherimide, polyethersulfone film, etc. can also be used.

The average surface roughness of the polyamic acid film peeled from the base material correlates with the surface roughness of the base material surface, and the opposite surface of the base material becomes a film with a smooth surface. When a polyamic acid film with a transferred base material surface is heat-treated, the surface roughness of the resulting polyimide film becomes smaller than that of the base material surface, but if the average surface roughness of the base material exceeds 0.05 μm, the polyamic acid film The average surface roughness of the film becomes too large, making it difficult to obtain a polyimide film with a smooth surface. It is more preferable to use a base material with an average surface roughness of 0.03 μm or less.

A T-die is generally used as the casting device, but a coating device that shapes the varnish to a constant thickness depending on the gap between the casting device and the substrate is more preferable. This is because there is less varnish retention and the thickness can be easily adjusted. Examples of such coating machines include air knife coaters, wire bar coaters, and roll coaters.

The weight loss on heating of a polyamic acid film containing a solvent that can be peeled off from a base material is carried out by heating at 300° C. for 30 minutes, and it is preferable that the weight change before and after heating is in the range of 10 to 45% by weight. If the loss on heating exceeds 45% by weight, the self-supporting properties of the film will be weak and the film will stretch during peeling, which is not preferable. Moreover, if it is less than 10% by weight, the releasability from the base material becomes very poor.

The polyamic acid film peeled from the base material is held at both ends in a bottle or clip and heat-treated at a high temperature of 200° C. or higher to dehydrate and form a ring-closing imidation, after which it is removed from the pin or clip, the edges are capped, and the film is rolled up.

[Effect]

The feature of the present invention is that a continuously synthesized polyamic acid varnish is used, and the polyamic acid synthesis step and the film forming step are made continuous, thereby eliminating the following drawbacks of the conventional method.

Polyamic acid varnishes obtained by conventional batch methods have a problem in that the viscosity of the varnish increases or decreases over time during storage. When producing polyimide films using these varnishes, the viscosity of the varnish varies from batch to batch, so a film thickness of 1 lfl must be produced for each batch. Adjusting the thickness requires a large amount of varnish, making it uneconomical. on the other hand,
In order to maintain the synthesized varnish at a constant viscosity, the varnish must be stored at a low temperature, which increases costs due to factors such as equipment and energy. In addition, additives such as the aforementioned carboxylic acids are added to improve the storage stability of varnishes, but in this case, the additives may cause the product to become colored, resulting in a decline in commercial value, or the additives may cause a decline in physical properties. I may invite you.

Compared to polyamic acid varnishes synthesized by conventional batch methods, the present invention uses continuous synthesis to obtain polyamic acid varnishes with a constant composition and constant precision. Because of this, there is no change in viscosity, and the film thickness only needs to be adjusted once, resulting in a film with good thickness accuracy.

Furthermore, since the synthesized polyamic acid varnish is used immediately in the film forming process, there is no need to store it at low temperatures, and there is no need to add additives to improve storage stability.

〔Example〕

Hereinafter, the present invention will be explained in more detail with reference to Examples.
The invention is not limited to this example.

EXAMPLE A polyimide film was manufactured according to the polyimide film manufacturing process shown in FIG. 1 of the accompanying drawings.

A solution of 1,565 g of pyromellitic dianhydride dissolved in 8,500 g of N,N-dimethylformamide and a solution of 1,435 g of diaminodiphenyl ether dissolved in 8,500 g of N,N-dimethylformamide were prepared. If it was difficult to dissolve, it was heated, and after dissolution, the temperature was lowered to room temperature. Solutions A and B are each supplied at a constant weight below the solution surface in the mixing tank 3 from the metering pump 1.2 in FIG.
The mixture was stirred and mixed using a stirring motor with a propeller to prevent air from entering. This mixed solution was pumped into a continuous tubular reaction vessel 6 through a static pipe mixer 5 using a gear pump 4 connected to the lower surface of the mixing tank 3 through a stainless steel pipe.

The amount of liquid sent by gear pump 4 (KHP type (maximum discharge pressure 400 kg/c+J) manufactured by Kawasaki Heavy Industries ■) was determined by using a static mixing unit with 5 elements (manufactured by Sluzer Brothers) connected to the gear pump. The continuous tubular reaction vessel 6 was a copper tube with an outer diameter of 25 mm, a wall thickness of 2.3 mm, and a Teflon coated inner surface, and a total length of 20 m. The tubular reaction vessel 6 was immersed in a medium 7 whose temperature was adjusted to 30°C, and the same static type in-tube mixer 8 as in 5 was installed at the outlet of the steel pipe, leading to a roll coater liquid receiver 9.

The amount of liquid fed was 150 cm'/min, and the viscosity of the obtained polyamic acid varnish in the liquid receiver was 500 poise (2
5°C), and the reduced specific viscosity is 1°3 dl/g (0.3 dl/g in DMF).
1 g/di, 25°C).

Using a biaxially stretched polypropylene film (trade name: TORAYFAN BO) (thickness: 30 μm, average surface roughness: 0.02 μm) on the east side as the base material 11, polyamic acid varnish was applied to the base material 11 at a constant thickness using a roll coater 10. It spread to The cast polyamic acid film on the base material 11 was dried in a drying oven 12. The heating loss of the polyamic acid film after drying (300° C., 30 minutes) was 29% by weight.

The polyamic acid film released from the drying oven 12 and peeled from the base material 11 is passed through a heat treatment oven 13 with a temperature distribution that rises stepwise from 200 to 400°C to undergo dehydration and ring-closing imidization, resulting in a film with a thickness of 25 μm. A polyimide film 14 was obtained. Table 1 shows the properties of the obtained polyimide film.

Table 2 shows the viscosity of the polyamic acid varnish synthesized by the continuous synthesis method obtained by continuous operation time, and the film thickness and thickness accuracy of the polyimide film obtained by continuous casting after varnish synthesis. Ta.

Table 1 below: +3Qσn/XX100 σn: Standard deviation of thickness (Measured at 25° C.)”: +3yn/XxlO0 The viscosity of the polyamic acid varnish hardly changed depending on the continuous operation time, and a film with a constant thickness was obtained with high accuracy.

Comparative Example Solution A shown in Example was placed in a reaction vessel, and solution B was added dropwise thereto while stirring to synthesize a polyamic acid varnish (viscosity 500 poise (25°C)) at 30°C by a batch method. This was casted, dried, and
The physical properties of the polyimide film obtained by heat treatment were comparable to those of the polyimide film obtained by the continuous synthesis method shown in Table 1.

Furthermore, polyamic acid varnish was synthesized five times by the batch method in the same manner as above, and the viscosity of the polyamic acid varnish at the time of synthesis, the viscosity of the polyamic acid varnish at the time of use (during production of polyamic acid film), and the viscosity of the polyamic acid varnish obtained The thickness and thickness accuracy were measured. The measurement results are shown in Table 3.

In Table 3, Nos. 1 to 5 were cast on the same day, but when trying to cast the synthesized polyamic acid varnish continuously,
When varnishes with different viscosity are added, the varnish before replenishment and the varnish added do not mix and create a boundary, and the boundary can be identified even after casting (the film thickness differs at the boundary), so the varnish is continuous. It could not be spread.

Therefore, each varnish was cast individually, and after completion of casting, the next varnish was cast after thorough cleaning. During cleaning, the gap used to shape the varnish to a constant thickness was slightly distorted, so the film thickness was adjusted each time.

As shown in Table 3, for polyamic acid varnish synthesized by the batch method, it is extremely difficult to maintain a constant varnish viscosity during synthesis.Furthermore, the varnish viscosity changes over time during storage, and The viscosity of the varnish differs from the viscosity at the time of synthesis. Furthermore, when the coating machine is additionally refilled with varnishes of different viscosities, the varnish before refilling and the refilled varnish do not mix, creating a boundary, which can be identified even after casting. This continues until most of the varnish on one side is consumed, and the film thickness differs at the boundary, resulting in poor thickness accuracy. For this reason, replenishing varnishes with different viscosities will worsen the thickness accuracy. In addition, to prevent this, the method is to use varnish for each batch, clean it, replenish it with varnish of different viscosity, adjust the thickness, and then cast it, but adjusting the thickness requires a large amount of varnish and time. It is expensive and not economical.

〔Effect of the invention〕

By using the production method of the present invention, the viscosity of the polyamic acid varnish can be suppressed within a certain range, there is no need to store it at low temperatures, it can be cast continuously, and films with good thickness accuracy can be continuously produced economically. It will be done. Also, compared to the batch method,
The process of putting the synthesized varnish into a container, the process of moving it, and the process of taking it out from the container can be omitted, and there is no loss due to varnish adhesion to the container wall, etc., and the varnish yield is improved.

[Brief explanation of drawings]

FIG. 1 is a schematic diagram showing the process of manufacturing a polyimide film in an example of the present invention. Explanation of symbols A Aromatic acid dianhydride solution B Aromatic diamine solution ■ Metering pump 2 Metering pump 3 Mixing tank
4 Gear pump 5 Static in-tube mixer 6 Tubular reaction vessel 7 Temperature-controlled medium 8 Static in-tube mixer 9 Liquid receiver IO roll coater 11 base film 12 Drying oven 13 Heat treatment oven 14 Polyimide film

Claims (1)

[Claims] 1. A polyamic acid varnish synthesized from an aromatic acid dianhydride and an aromatic diamine is cast onto a base material, dried, and then peeled off from the base material and subjected to dehydration and ring-closing imidization to form a polyimide film. In the manufacturing method, a solution of an aromatic acid dianhydride dissolved in an organic polar solvent and a solution of an aromatic diamine are mixed, and the mixture is sent to a reaction vessel kept at 100 ° C. or lower using a liquid pump.
A method for producing a polyimide film, characterized in that a polyamic acid varnish is continuously synthesized by performing a mixing and polymerization reaction, and the step of synthesizing the polyamic acid varnish and the step of casting it onto a substrate are continuous steps. . 2. The method for producing a polyimide film according to claim 1, wherein the mixing device for mixing the contents of the reaction container is a static in-tube mixer, and the liquid feeding pump is a gear pump. 3. The base material is a biaxially stretched polypropylene film or polyester film, and its average surface roughness is 0.05.
The method for producing a polyimide film according to claim 1, wherein the polyimide film has a particle size of .mu.m or less. 4. The water content of the aromatic acid dianhydride solution and the aromatic diamine solution dissolved in the organic polar solvent is 5000p.
The method for producing a polyimide film according to claim 1, wherein the polyimide film has a particle size of .mu.m or less. 5. The method for producing a polyimide film according to claim 1, which uses a coating machine that shapes the varnish to a constant thickness by a gap between the casting device and the base material as the casting device in the casting step.