WO2024236832A1 - Electrostatically assisted coating method using backing roll having internal electrode to which high voltage can be applied - Google Patents

Abstract

The present invention provides a coating method for a wide web, with which it is possible to achieve high-speed and stable coating and excellent uniformity in coating thickness. Disclosed is a method for coating a flexible plastic-based web with a coating liquid, the method comprising: a step in which the web is conveyed to a backing roll for coating and is then passed through a coating point while being supported by the backing roll in such a manner that a second surface of the web is in close contact with a part of the surface of the backing roll, which is rotating and to which a DC voltage is applied, by means of an electrostatic field of the backing roll; and a step in which a first surface of the web is coated with a coating liquid at the coating point by the coating liquid being attracted to the first surface of the web by means of an electrostatic force generated as a result of an arrangement of charges having the same polarity as the applied DC voltage. The backing roll comprises an outermost layer, an internal electrode layer, and an insulating layer, in the stated order, and is configured so that a predetermined voltage can be applied to the internal electrode layer. The outermost layer is a ceramic-based material layer that has a volume resistivity value of 10⁷ Ωcm to 10¹³ Ωcm at 25°C to 100°C.

Description

Electrostatic assisted coating method using a backing roll having an internal electrode to which a high voltage can be applied

The present invention relates to a method for increasing the uniformity of coating thickness and coating speed in a process for applying one or more layers of a coating liquid consisting of a liquid composition, particularly a water-soluble composition, to the surface of a flexible belt-like support (hereinafter referred to as a web) that is supported by a backing roll and runs continuously, by a method such as curtain coating or bead coating, by applying an electrostatic force to the contact line portion (hereinafter referred to as a coating point) where the coating liquid comes into contact with the web, thereby preventing an air layer accompanying the front side (coating surface) of the web from being trapped between the coating liquid and the web and promoting uniform wetting of the coating liquid onto the web.

The present invention also relates to a method for bringing the back side of the web (the side of the backing roll) into close contact with the surface of the backing roll, thereby preventing the air layer accompanying the back side of the web and the roll surface from penetrating between the web and the roll, thereby making it possible to raise the limit of the web speed before running problems such as fluctuations in web tension and speed and oscillation occur.

The present invention is applicable to improving the application method of liquid compositions, mainly water-soluble compositions, in the production of photographic film, photographic paper, magnetic recording tape, adhesive tape, pressure-sensitive and heat-sensitive recording paper, coated paper, etc.

In curtain coating and bead coating, air layers accompany the front and back sides of the web as it moves at high speed.

The thickness of the entrained air layers on the front and back sides increases in proportion to the moving speed of the web, so when a certain limit speed is reached, the uniformity of the coating decreases or the web begins to slip and wobble, making stable coating impossible and making it impossible to increase the coating speed beyond the respective limit speed. In order to increase the speed while maintaining the uniformity of the coating, a method is needed that can simultaneously address the effects of the entrained air layers on the front and back sides of the web.

　On the front side of the web, if the web's moving speed increases and the accompanying air layer increases beyond a certain limit, the air layer will invade under the coating liquid that has been transferred to the web at the coating point, and the coating liquid will not be able to push it out. If the air layer invades under the coating layer, the coating liquid will be prevented from wetting and spreading over the front side of the web, or the air layer will be entrapped in the coating liquid, forming bubbles that will reduce the uniformity of the coating layer, making it impossible to increase the coating speed.

　It has been well known that in conventional curtain coating and bead coating, when an electrostatic field is assisted at the coating point, the electrostatic force of the ions and dipolar molecules in the coating liquid makes it possible to attract and adhere the coating liquid to the web surface, thereby increasing the upper limit of the coating speed. Methods of assisting an electrostatic field at the coating point include: 1) a method in which an electrostatic charge is applied to the web from a corona discharge electrode placed in space, and then the web is passed through the coating point while supported by a grounded coating backing roll; 2) a method in which a DC voltage is applied to a conductive coating backing roll (hereinafter referred to as the backing roll) insulated from the earth, forming an electric field between the surface of the web and the coating liquid at the coating point; and 3) or a combination of these two methods.

For example, Patent Documents 1 to 4 disclose a method 1) in which a dielectric web such as polyester is charged. When a web of a dielectric material that has been given a polar charge bound between its opposing back and front surfaces is supported on the surface of a grounded conductive backing roll and transported to a coating point, the polar charge remaining on the front side of the web attracts the coating liquid and acts to more strongly remove the air layer below the coating liquid, thereby increasing the coating speed. Patent Documents 5 to 7 disclose a method 2) in which a direct current voltage is applied to a conductive backing roll insulated from the earth to charge its surface, the web is supported on that surface and transported to a coating point, and an electric field is applied between the web and the coating liquid, causing the coating liquid to adhere to the web. Patent Document 7 also discloses a method that combines methods 2) and 3).

The effect of applying static charge to the web or the effect of assisting static electricity on the surface of the conductive backing roll is to suppress the phenomenon in which the air layer accompanying the web is caught between the surface of the web and the coating liquid, preventing the coating liquid from becoming wet, and to raise the limit speed at which the transparency and thickness of the coating become non-uniform. In conventional electrostatic auxiliary coatings 1) to 3), the higher the adhesion of the coating liquid to the surface of the web, the more strongly the accompanying air layer is eliminated, and it is known that in all cases, the higher the static charge potential of the surface of the web or the voltage of the backing roll surface is, the higher the coating speed is. However, in electrostatic auxiliary coatings in which a voltage is applied to a conductive backing roll, it is known that if the applied voltage exceeds 1500V, spark discharge from the backing roll to peripheral devices and short-circuit currents penetrating the web are likely to occur. Also, in the method of applying static charge to the web, if the charge potential of the surface of the web becomes high, a short-circuit current penetrating the web may occur between the coating liquid and the surface of the conductive backing roll at a location where the web has pinholes or reduced insulation resistance. For this reason, the surface potential of the web due to the electrostatic charge disclosed in Patent Document 1 is 1200 V or less. In addition, for example, Patent Document 4 discloses an electrostatic auxiliary coating technique in which the polarization effect of a coating liquid containing a surfactant is used in combination, and Patent Document 2 discloses a method of charging the web with a conductive brush, thereby setting the surface potential of the web lower than 700 to 800 V, but the standard recommended surface potential is even lower, at 400 to 500 V. In this way, conventional electrostatic auxiliary coating using a conductive backing roll is unable to apply a sufficient potential to the coating point, and is insufficient in terms of the effect of raising the limit speed before coating problems occur due to an increase in entrained air.

Furthermore, the backside of the web and the surface of the backing roll each have an air layer. Therefore, the combined air from both causes the web to float, and the amount of floating increases as the web speed increases. If the amount of web floating exceeds a certain limit, the web transport becomes unstable, and the increase in the air layer on the backside of the web also causes the coating speed to be unable to be increased.

The web floating amount h (μm) when the air temperature is 25° C. is given by the backing roll radius R (m), the roller speed U _R (m/s), the web speed U _W (m/s), and the web tension T (N/m) in the following formula (1).

(Hashimoto Kyo, "Web Handling: Basic Theory and Applications," p. 73, published by the Processing Technology Research Association)

According to equation (1), the thickness of the entrained air layer entrained in the backside of the web and the roll surface decreases as the web tension T increases, and increases as the roll radius R, web speed _UW, or backing roll speed _UR increases (usually _UW ≒ _UR ).

In electrostatic assisted coating, where a voltage is applied to a conductive backing roll, as the web travel speed increases and the web floats due to the accompanying air layer between the web and the backing roll, the distance from the backing roll surface to the front side of the web increases, and the air layer between the conductive backing roll and the back side of the web reduces the electrostatic capacitance. This reduces the amount of polar charge stored on the web, and the electrostatic force between the web and the coating liquid.

In addition, if the thickness of the entrained air layer on the backside of the web exceeds the sum of the surface roughness of the roll and the surface roughness of the backside of the web, the contact between the web and the surface of the backing roll will decrease, and the force pulling the web will decrease. When the web's pulling force decreases, the tension and speed of the web will fluctuate, causing problems such as the web meandering and scratches on the backside of the web. Therefore, in order to increase the coating speed, it is absolutely necessary to take measures to prevent the web from floating up by the entrained air layer sandwiched between the backside of the web and the backing roll.

The relationship between the web transport speed and the amount of lift calculated from equation (1) is shown in Figures 7 and 8. As shown in Figure 7, it is possible to suppress web lift by increasing the web tension T. However, increasing the web tension increases the load on the web transport system, and increases the amount of deflection of the backing roll and the guide rolls in the web transport path. In addition, when the web is thin, wrinkles are likely to occur, so there are limitations to the method of increasing the web tension.

Also, as shown in Figure 8, the web lift amount decreases when the diameter of the backing roll is reduced. However, when the diameter of the backing roll is reduced, the rigidity of the roll decreases and the amount of deflection increases. Furthermore, as the roll diameter decreases, the number of rotations of the roll increases proportionately, so the faster the web is and the wider the web is, the more the roll vibrates, and vibration patterns appear on the coating film. In roller curtain coating and bead coating for wide webs of 1500 mm or more, the diameter of the backing roll is preferably 100 mm or more, taking into consideration the roll's deflection strength and the area required to install the coating head (applicator). The tension of the web depends on the strength and thickness of the web, but is usually around 75 to 300 N/m, taking into consideration the stretching and deformation of the web. According to Figure 7, when a backing roll with a diameter of 100 mm is used, even if the web tension is set to 150 N/m, the web lift amount is already 6 μm at a moving speed of 100 m/min, and in rolls with smooth surfaces, the traction force decreases, causing fluctuations in tension and speed. Judging from the web lift amounts shown in Figures 7 and 8, if the diameter of the backing roll is set to 100 mm or more, in order to increase the web conveying speed to 100 m/min or more, it is necessary to prevent the web from lifting by using an air layer entrained on the back side of the web.

Patent Document 7 discloses a measure to prevent the web from floating up due to the accompanying air layer on the back side of the web in electrostatically assisted curtain coating, in which a DC voltage is applied to a conductive backing roll with a diameter of 100 mm or more. In the examples of Patent Document 7, circumferential grooves (groove depth 0.15 mm, width 0.43 mm, pitch 1 mm) are formed on the surface of a backing roll with a diameter of 200 mm at a ratio of 10 to 30% of the roll surface area, and the accompanying air layer is discharged into the microgroove, making it possible to suppress the web from floating up and increase the conveying speed. However, the voltage applied to the conductive backing roll is 800 V or less, and the electrostatic auxiliary effect is reduced in the grooves, making it easy for uneven coating to occur. Therefore, the increase in coating speed when microgrooves and electrostatic assistance are used in combination is a maximum of 144 m/min, which is not very large.

Patent No. 2835659 Special Publication No. 01-035702 Japanese Patent Application Publication No. 08-252517 Patent No. 2509316 Special Publication No. 06-009671 Special Publication No. 46-027423 U.S. Pat. No. 6,177,141

"Basic Theory and Applications of Web Handling" by Kyo Hashimoto, published by the Processing Technology Research Association, p. 73

In curtain coating and bead coating, air layers are entrained on both the front side (coating surface) and back side (backing roll side) of the web moving at high speed, and the thickness of these layers increases on both sides as the web moves faster. However, the problems caused by the entrained air layers are different on the front side and back side. On the front side, the coating liquid does not wet the web, but on the back side, the entrained air layer that penetrates between the back side of the web and the backing roll causes the web to float, making the web's running unstable. There is a demand for high-speed coating technology that can simultaneously address these different problems caused by entrained air flows on the front and back sides of the web.

By using a backing roll that can apply a voltage to the roll surface of 1.5 kV or more, preferably up to about 3.5 kV, the dielectric web is polarized more strongly, and the electric field between the web surface and the coating liquid at the contact line between the coating liquid and the web (coating point) can be made much stronger than with conventional methods, so the coating speed can be increased as needed simply by adjusting the applied voltage.

At the same time, the back side of the web, which is strongly polarized by the high voltage of the backing roll, is strongly attracted to the backing roll by the electric field caused by the positive charge on the backing roll surface, pushing out the accompanying air layer and adhering to the surface of the backing roll, making it possible to raise the limit speed at which web running problems occur.

To achieve the above objective, the present invention employs the following method.

The present invention provides an electrostatic auxiliary coating method using a highly safe backing roll that does not generate spark discharge to peripheral equipment, does not generate short circuit current in the web, and does not cause electric shock even if a human body touches the roll surface, even when a high DC voltage of up to 6 kV is applied in a process of applying a liquid composition (especially a water-soluble composition) to a web that is supported by a roll and runs continuously. This backing roll uses highly insulating ceramics for the bottom layer, conductive metal for the internal electrode layer, and a highly resistive ceramic material layer for the outermost layer, and a gap-free three-layer structure consisting of the outermost layer/internal electrode layer/bottom layer is formed on the roll surface. The internal electrode layer is completely electrically shielded from the core metal of the backing roll and peripheral equipment by the outermost layer and the bottom layer. Since the insulation withstand voltage of the outermost layer and the bottom layer is 6 kV or more, spark discharge to peripheral equipment and the generation of short circuit current in the web are completely prevented even when a high DC voltage of up to 6 kV is applied to the internal electrode layer.

That is, the method of the present invention comprises:
(1) A method for applying a coating liquid comprising a liquid composition to a continuously moving flexible plastic web having first and second opposing surfaces by flowing the coating liquid from an applicator grounded on the first surface of the web, the method comprising the steps of:
a step of conveying the web along a path to a backing roll for coating, and passing the web through a coating point while supporting a second surface of the web by being in close contact with a portion of the surface of the rotating backing roll to which a DC voltage is applied, by an electrostatic field of the backing roll;
and applying the coating liquid to the first surface of the web by electrostatic force generated by the arrangement of charges of the same polarity as the DC voltage applied to the backing roll on the first surface of the web, the coating liquid being attracted to the first surface of the web at the coating point;
The backing roll is provided with an outermost layer to which the web is in close contact, a conductive monopolar internal electrode layer adjacent to the inner side of the outermost layer, and an insulating layer adjacent to the inner side of the internal electrode layer, the backing roll is configured to be able to apply a predetermined voltage to the internal electrode layer, the outermost layer is a ceramic material layer having a volume resistivity of 10 ⁷ to 10 ¹³ Ωcm at 25 to 100°C, the surface of the backing roll is charged with a charge having the same sign as a voltage applied to the internal electrode in a state in which the predetermined voltage is applied to the internal electrode layer, the second surface of the web in contact with the outermost layer by the electrostatic force of the charge is brought into close contact with the outermost layer and rotated and transported, and the coating liquid is attracted to and brought into contact with a first surface of the web during rotation and transport, and deposited on the first surface of the web.

In the coating method,
(2) applying a charge having a polarity opposite to that of the DC voltage applied to the internal electrode layer to a first surface of the web opposite to the second surface before the coating liquid comes into contact with the web during rotational transport;
(3) A DC voltage applied to the internal electrode layers is 0.3 to 6.0 KV;
(4) The diameter of the backing roll is 100 mm or more.
(5) At least one of the insulating layer, the internal electrode layer, and the outermost layer is made of a material formed by a thermal spraying method or a material using either an inorganic or organic binder, and at least one of the insulating layer, the internal electrode layer, and the outermost layer of the backing roll is subjected to a sealing treatment.
(6) The outermost layer is made of a ceramic material, and the ceramic material is an alumina-based, zirconium oxide-based, or magnesium oxide-based ceramic containing a compound selected from titanium oxide, chromium oxide, silicon oxide, manganese oxide, nickel oxide, and iron oxide, or an aluminum oxide-based ceramic containing 5 to 17% by weight of titanium oxide.
(7) The ceramic material includes at least one selected from an aluminum nitride material, a silicon carbide material, and a silicon nitride material, and an organic or inorganic binder.
(8) The thickness of the outermost layer is 50 to 500 μm.
(9) The center line average surface roughness Ra of the outermost layer is 0.01 to 5 μm.
(10) The internal electrode layers are made of a conductive material containing tungsten or molybdenum.
(11) The thickness of the internal electrode layer is 5 to 50 μm.
(12) The insulating layer includes at least one highly insulating material selected from aluminum oxide, aluminum oxide-based materials containing 2 to 4% by weight of titanium oxide, magnesium oxide-based materials, beryllium oxide-based materials, aluminum nitride-based materials, ceramic materials containing silicon nitride, porcelain, and enamel.
(13) The volume resistivity of the insulating layer is 10 ¹³ Ωcm or more.
(14) The thickness of the insulating layer is 50 to 500 μm.
It is characterized by:

The present invention is an electrostatic auxiliary coating method using a highly safe backing roll that does not generate spark discharge to peripheral equipment, does not generate short circuit current in the web, and does not cause electric shock even if a high DC voltage of up to 6 kV is applied in a process of applying a liquid composition (especially a water-soluble composition) to a web that is supported by a roll and runs continuously. This backing roll uses highly insulating ceramics for the bottom layer, conductive metal for the internal electrode layer, and a highly resistive ceramic material layer for the outermost layer, forming a gap-free three-layer structure consisting of the outermost layer/internal electrode layer/insulating layer on the roll surface. The internal electrode layer is completely electrically shielded from the core metal of the backing roll and peripheral equipment by the outermost layer and the bottom layer. Since the insulation withstand voltage of the outermost layer and the bottom layer is 6 kV or more, spark discharge to peripheral equipment is completely prevented even if a high DC voltage of up to 6 kV is applied to the internal electrode layer.

Since the volume resistivity of the high-resistance semiconductor ceramics, which is the outermost layer, is 10 ⁷ to 10 ¹³ Ωcm, when a DC voltage is applied to the internal electrode, charges move from the internal electrode to the outermost surface, and the surface of the outermost layer is charged with the same polarity as the internal electrode. At the same time, the web in contact with the outermost surface of the backing roll is dielectrically polarized by the electric field of the internal electrode, and the internal electric dipole is arranged so that the back side direction is a charge of the opposite polarity to the potential of the internal electrode, and the front side direction is a charge of the same polarity. Then, the coating liquid is attracted to the web surface and adheres to the web by the electrostatic charge of the same polarity as the internal electrode arranged on the web surface side, eliminating the air layer accompanying the web front side, and promoting the wetting of the coating liquid by the web. At the same time, the charge of the opposite polarity to the internal electrode arranged on the back side of the web is attracted to the free charge of the same polarity as the internal electrode that is charged on the surface of the high-resistance semiconductor ceramics, which is the outermost layer of the backing roll, so that the web adheres to the backing roll surface. In this way, in the present invention, the effect of the entrained air layer on both the front and back sides of the web is eliminated, making it possible to greatly increase the coating speed onto the web.

The present invention provides the following effects.
1. By applying a high DC voltage of up to 6 kV to the internal electrode installed on the outer surface of the backing roll, it is possible to coat a liquid composition (especially a water-soluble composition) with electrostatic assistance without generating spark discharge to surrounding equipment or damaging the web due to short circuit current.
2. While applying a high DC voltage to the internal electrode of the backing roll, an external auxiliary electrode is simultaneously placed in the space above the front surface of the web, and a DC voltage of 1.5 to 6 kV of the opposite polarity to the internal electrode is applied to the front surface of the web, thereby enabling electrostatic auxiliary coating, in which a charge of the opposite polarity to the internal electrode is applied to the front surface of the web.
3. Even if the diameter of the backing roll is 100 mm or more, the electrostatic force due to the electrostatic charge on the surface of the high resistance semiconductor ceramic on the outermost surface makes it possible to completely adhere the web to the backing roll, effectively performing electrostatic auxiliary coating on the web surface, and preventing rocking and slipping due to the web floating, making it possible to increase the web conveying speed to 400 m/min or more.
4. The diameter of the backing roll can be increased according to the length of the roll, so that the rigidity of the roll can be increased, making it easier to make the web wider.

FIG. 2 is a cross-sectional view of an example of the method of the present invention for electrostatically assisted coating using only a backing roll with a high voltage applied to the internal electrode. FIG. 2 is a diagram showing a schematic diagram of charge distribution in part A of FIG. 1 . FIG. 2 is an external view of the method of the present invention seen from an oblique direction. FIG. 11 is a cross-sectional view of an alternative method of the present invention for electrostatically assisted coating using a backing roll with a high voltage applied to the internal electrode in combination with the application of an electrostatic charge to the web. FIG. 5 is a diagram showing a schematic diagram of the charge distribution state in part B of FIG. 4 . FIG. 13 is an external view of a method according to a modified example of the present invention, as viewed from an oblique direction. 1 is a graph showing the change in web lift amount with respect to tension and web speed in a backing roll having a diameter of φ200 mm. 1 is a graph showing the change in web lift with respect to backing roll diameter and web speed at a web tension of 150 N/m.

The method of the present invention is shown in Figures 1 to 3. Figure 2 shows a schematic diagram of the charge distribution in part A shown in Figure 1.

(1) In order to increase the coating capacity, in addition to increasing the web transport speed, it is important to lengthen the backing roll to widen the web. However, when the web tension is constant, the amount of deflection of the roll increases rapidly when the backing roll is lengthened, so in order to prevent the backing roll from deflecting, it is necessary to simultaneously increase the backing roll diameter and increase the rigidity of the roll (the amount of deflection of the roll is proportional to the cube of the roll length and inversely proportional to the fourth power of the second moment of area (almost proportional to the roll diameter)). However, as shown in the above formula (1), increasing the roll diameter also increases the accompanying air layer on the back side of the web proportionally. The increase in the accompanying air layer lowers the upper limit of the web transport speed, so that it becomes impossible to increase the diameter of the backing roll and therefore the roll cannot be lengthened. On the other hand, the present invention can eliminate the influence of the accompanying air layer on the back side of the web, so it is possible to increase the diameter and roll length of the backing roll as necessary to widen the web. Judging from Figure 8, the effect of the entrained air layer becomes significant when the diameter of the web-transport roll is 100 mm or more, so the present invention is more effective when applied to a backing roll with a diameter of 100 mm or more.

The internal electrode layer 3 of the backing roll 1 of the present invention is a conductive layer adjacent to the outermost layer 4 on the inside with no gap and in contact with the outermost layer 4. The internal electrode layer 3 is of a unipolar type and is wired to a high-voltage DC power source 5 so that a DC voltage can be applied, but the internal electrode layer 3 is completely insulated from the earth and the core metal 16 of the backing roll 1 by the insulating layer 2 adjacent to the inside.

Generally, there are two types of internal electrode layer 3, a monopolar type and a bipolar type, depending on the voltage application method. The monopolar type has only a single internal electrode, either positive (+) or negative (-), and a voltage is applied only between this single electrode with respect to the earth. The bipolar type has two or more internal electrodes of different polarity, but when the web, which is the object to be attracted, is connected to the earth, the charge (current) leaks to the earth side and the electrostatic attraction force becomes unstable. In the present invention, the monopolar type is used for the internal electrode layer 3. The internal electrode layer 3 is provided continuously and uniformly over a range wider than the width of the web 7 in the axial direction of the backing roll 1, and further over the entire circumference of the backing roll 1 in the rotation direction of the backing roll 1. If the internal electrode 3 is comb-shaped, band-shaped, or spiral-shaped, the coating layer will have shape unevenness. The internal electrode layer 3 does not have to be formed on both ends of the cylindrical surface of the backing roll 1, which are outside the support range of the web 7.

The outermost layer 4 is a high-resistance ceramic material, and has a volume resistivity of 10 ⁷ to 10 ¹³ Ωcm at 25 to 100° C., more preferably 10 ⁸ to 10 ¹² Ωcm. In the "fields dealing with static electricity," substances with a volume resistivity of 10 ¹³ Ωcm or more are classified as insulators because static electricity does not easily move. Materials with a volume resistivity of less than 10 ⁶ Ωcm are called electrostatic conductors, and static electricity moves easily, becoming equipotential when a voltage is applied. Materials with a volume resistivity of 10 ⁷ to 10 ¹³ are classified in the intermediate region between conductors and insulators, and are classified as "electrostatic semiconductors." It is known that when a voltage is applied, a minute current flows, and free charges move to the surface and become charged in the same way as a conductor. The amount of charge of these free charges depends on the applied voltage. Unlike conductive metal surfaces, on the surface of such highly resistive semiconductor ceramic materials, even when a high voltage of 3.5 to 6 kV is applied, only a very small current of about 0.001 to 10 μA/cm ² flows locally. Therefore, discharge due to free charges on the surface becomes a very small corona and does not develop into an arc discharge, and no spark discharge occurs toward surrounding equipment.

In the present invention, as shown in Figure 2, when a DC voltage is applied to the internal electrode layer 3, free charges 17 of the same polarity as the internal electrode 3 migrate to the surface of the outermost layer 4 and become charged. Then, when the second surface 13 (back side) of the web 7 comes into contact with the surface of the outermost layer 4, the dipoles 18 inside the web 7 are oriented with the opposite polarity to the free charges 17 toward the surface of the outermost layer 4 due to dielectric polarization caused by the electric field of the internal electrode. Then, the web 7 is adhered to the surface of the outermost layer 4 due to the electrostatic force between the charges 17 on the surface of the outermost layer 4 and the opposite charges of the dipoles oriented on the second surface 13 of the web 7. At this time, the first surface 12 (front side) of the web, which is the coating surface, is polarized and charged to the same polarity as the internal electrode by the electrostatic field of the internal electrode 3. This "polar charge" attracts the coating liquid toward the first surface 12 of the web 7 during rotational transport, making it possible to form a uniform coating layer 11 on the first surface 12 of the web 7 without the intrusion of an entrained air layer between the coating liquid 9 and the first surface 12 of the web 7. The higher the voltage applied to the internal electrode 3, the more the amount of polar charge accumulated on the first surface 12 due to dielectric polarization of the web 7 increases, and the stronger the electric field becomes, making it possible to greatly increase the coating speed.

(2) Figures 4 and 5 are side views showing an example of a modified method of the present invention. Figure 5 shows a schematic diagram of the charge distribution state in part B of Figure 4.

The method of the present invention is a modified example in which, in the same device configuration as the method of the present invention, a spatial auxiliary electrode 14 is additionally installed at a position upstream of the coating point, and a high voltage of opposite polarity to the internal electrode 4 is applied to generate a corona discharge, thereby charging the first surface 12 of the web 7 with free charges 19 of opposite polarity to the internal electrode layer 3.

The outermost layer 4 of the backing roll 1 has a maximum withstand voltage of 6 kV and a volume resistivity of 10 ⁷ to 10 ¹³ Ωcm, so that even if the web to which the electric charge 19 is applied has pinholes or places with weak insulation resistance, the occurrence of a short-circuit current from the coating liquid at the defective places toward the backing roll 1 is prevented. Therefore, by raising the corona discharge potential of the external auxiliary electrode 14 installed in the space upstream of the coating point as necessary, it is possible to increase the amount of free charge 19 on the first surface 12 of the web 7, and it becomes possible to further increase the coating speed by electrostatic assistance. A voltage of DC (-) 3.5 to 6 kV applied to the external auxiliary electrode is sufficiently effective, and if it is installed at an appropriate distance from peripheral devices, no spark discharge will occur.

Furthermore, the electrostatic force between the charges 17 on the surface of the outermost layer 4 and the charges 19 on the first surface of the web causes the web 7 to adhere even more strongly to the surface of the outermost layer 4, completely preventing the intrusion of the accompanying air flow to the back side of the web 7. Therefore, in line with the increase in coating speed, the web transport speed can also be significantly increased as necessary.

(3) Because the web 7, which is the object to be adhered in the present invention, is flexible, there must be no loose parts on all contact surfaces between the web 7 and the backing roll 1, and a uniform and strong adhesion is required. To obtain the electrostatic force required for electrostatic adhesion of the web 7, a voltage of DC (+) 0.3 to 3.5 kV is usually sufficient for the voltage applied to the internal electrode layer 3. The higher the applied voltage, the stronger the electrostatic auxiliary effect becomes, and if the applied voltage is 0.3 kV or less, the electrostatic force is insufficient. The withstand voltage of the outermost layer 4 is a maximum of 6 kV or more, so no spark discharge occurs even if DC +3.5 kV is applied.

(4) At least one of the insulating layer 4, the internal electrode layer 3, and the outermost layer 2 of the backing roll 1 is made of a material formed by a thermal spraying method or a material using either an inorganic or organic binder, and at least one of them has been subjected to a sealing treatment.

In this invention, thermal spraying is a coating technology in which a coating material is melted or softened by heating, turned into fine particles, accelerated, and collided with the surface of the object to be coated, causing the flattened particles to solidify and deposit, forming a coating. There are various methods of thermal spraying, which are classified according to the material used and the type of heat source, but in the case of this invention, the atmospheric plasma spraying method is particularly suitable.

Furthermore, when a sprayed ceramic material is used for the outermost layer 4 and the outermost layer 2, it is preferable that at least one of the insulating layer 2, the internal electrode layer 3, and the outermost layer 4 is subjected to a sealing treatment. The sprayed ceramic coating is a collection of flat sprayed particles, but there are fine gaps between these sprayed particles, and interconnected pores equivalent to 3 to 10 vol% of the total volume of the sprayed coating are formed. In a normal atmospheric environment, air or liquid containing moisture penetrates these pores, and water molecules are adsorbed on the inner walls of the pores, so that the sprayed coating has extremely low insulation resistance and voltage resistance. By penetrating the sprayed ceramic with an insulating sealant, the interconnected pores disappear, and the volume resistivity becomes stable, and the ceramic material has high voltage resistance and excellent corrosion resistance. Sealing the ceramic material after coating it is possible to carry out even on large backing rolls with a diameter of 200 mm or more used in actual machines, and is a practical method. Materials used for sealing include low-viscosity silicon oligomers (e.g., 8 to 40 mPs, 25°C), low-viscosity epoxy resins (e.g., 80 to 400 mPs, 25°C), polyester resins, lithium silicate aqueous solutions forming inorganic coatings, and metal alkoxides forming inorganic sols, which can be diluted with a solvent to reduce the viscosity of the liquid sealing agents. In the present invention, low-viscosity silicon oligomers and low-viscosity epoxy resins are preferred because they exhibit excellent impregnation properties, insulating performance, and voltage resistance properties. In addition, the thickness range of the sealing layer is preferably such that the entire sprayed layer is sealed. It is important to penetrate the sealing agent into the pores of the sprayed coating to seal the pores, and then heat-set the coating to stabilize the volume resistivity and voltage resistance properties of the sealing agent. Unless the pores inside the sprayed coating are sealed reliably, the insulating layer 2 and the outermost layer 4 cannot be adjusted to their original volume resistivity and voltage resistance.

The volume resistivity of the outermost layer 4 at 25 to 100° C. after sealing is 10 ⁷ to 10 ¹³ Ωcm, preferably 10 ⁸ to 10 ¹² Ωcm, and more preferably 10 ⁹ to 10 ¹² Ωcm. If the volume resistivity is less than 10 ⁷ Ωcm, it becomes impossible to prevent the occurrence of short-circuit current from the coating solution connected to the earth through the web 7 in contact with the surface of the backing roll 1 or spark discharge to peripheral devices. In other words, if the volume resistivity of the outermost layer 4 is 10 ⁷ or less, it becomes impossible to perform electrostatic auxiliary coating in which a high voltage of 1.5 kV or more is applied to the surfaces of the backing roll and the web, which is not preferable.

On the other hand, when the volume resistivity of the outermost layer 4 exceeds 10 ¹³ Ωcm, the outermost layer 4 becomes an insulating dielectric, and the free charge 17 does not substantially flow from the internal electrode layer 3. As a result, the surface of the outermost layer 4 cannot be charged with the free charge 17 of the same polarity as the internal electrode, which is not preferable. When the electrical property of the outermost layer 4 is an insulating dielectric, the dipole 18 is oriented so that the surface direction of the outermost layer 4 becomes a charge of the same polarity as the internal electrode 3 due to the dielectric polarization caused by the voltage of the internal electrode 3. Here, the charge on the surface of the outermost layer 4 is due to the charge fixed to the dipole, and cannot be neutralized and annihilated by the free charge. When the web 7 is separated from the backing roll, a reverse charge remains on the surface of the outermost layer 4 due to peeling electrification. This reverse charge is trapped by the fixed charge at the dipole end of the outermost layer 4 when a voltage is applied to the backing roll, and although the apparent charged potential decreases, it does not disappear and remains as it is. When the surface of the outermost layer 4 becomes like this, the web 7 is repelled from the surface of the outermost layer 4. That is, if the volume resistivity of the outermost layer 4 is higher than ¹⁰ Ωcm, the outermost layer 4 becomes an insulating dielectric, and the peeling charge of the web is not neutralized and remains on the surface of the outermost layer 4, so that the web cannot adhere to the surface of the outermost layer 4, causing problems in the running of the web.

(5) The outermost layer 4 is made of a ceramic material, and the ceramic material is an alumina-based, zirconium oxide-based, or magnesium oxide-based ceramic containing a compound selected from titanium oxide, chromium oxide, silicon oxide, manganese oxide, nickel oxide, and iron oxide, or an aluminum oxide-based ceramic containing 5 to 17 weight percent titanium oxide.

A material that can adjust the volume resistivity to 10 ⁷ to 10 ¹³ Ωcm and has excellent handling properties is preferred as the material for the outermost layer 4 of the present invention. In terms of adhesion and density with the internal electrode layer 2, stability of the volume resistivity, and handling properties, an aluminum oxide-based ceramic material layer containing 5 to 17% by weight, preferably 7 to 15% by weight, of titanium oxide by thermal spraying is the most preferable.

(6) In a similar sense, the outermost layer 4 and insulating layer 2 of the present invention are also preferably non-oxide ceramic material layers using organic or inorganic binders applied by a coating method. As long as the volume resistivity and withstand voltage can be ensured, the outermost layer 4 and insulating layer 2 may be coating layers formed by applying a coating material made of a ceramic material containing an organic or inorganic binder and at least one selected from an organic binder such as polyimide, an inorganic binder such as aluminum phosphate, water glass, or silicon, and an aluminum nitride, silicon carbide, or silicon nitride, which is mixed to have a volume resistivity suitable for the present invention.

(7) The thickness of the outermost layer 4 is preferably 50 μm or more, and more preferably 80 to 500 μm. This lower limit of thickness is the minimum thickness necessary to ensure that the outermost layer 4 does not have pinholes that penetrate to the internal electrode layer 3, and is the thickness that can ensure the withstand voltage against the minimum applied voltage required for stable adhesion of the web 7. This upper limit of thickness is set in consideration of the maximum applied voltage. A higher applied voltage is required to obtain a stronger electrostatic auxiliary effect, and it is necessary to increase the thickness of the outermost layer 4 to increase the withstand voltage. However, since a sufficient effect can be obtained with a voltage of 3.5 kV applied to the internal electrode layer, a maximum withstand voltage of 6 kV is sufficient in consideration of a safety factor. Increasing the thickness of the outermost layer 4 beyond 500 μm at which the withstand voltage is obtained is not preferable because it simply increases the spraying time.

(8) The surface roughness of the outermost layer 4 of the backing roll 1 of the present invention is a center line average surface roughness Ra of 0.01 to 5 μm. A smooth surface of less than 0.01 μm is difficult to obtain using current ceramic layer polishing techniques. The greater the surface roughness of the outermost layer 4, the more stable the web will run at high speeds, but if it exceeds 5 μm, the surface roughness of the outermost layer 4 will be transferred to the back surface of the web, which is not preferable.

(9) The material for the internal electrode layer 3 can be a sintered conductive paste such as tungsten, molybdenum, high-performance activated carbon, copper, or silver, but a sprayed coating of tungsten or molybdenum applied by plasma spraying has high thermal conductivity and is also preferable in terms of handling.

(10) Since the volume resistivity of the outermost layer 4 is as high as 10 ⁷ to 10 ¹³ Ωcm, even when a high voltage of 6 kV is applied to the internal electrode, the total current flowing from the internal electrode to the web surface is very small, at several mA or less. Therefore, a thickness of 5 to 50 μm for the internal electrode layer 3 is sufficient, and it is preferable that the thickness is as thin as possible in order to smooth the boundaries of the internal electrode 3 at both ends of the roll.

(11) As the highly insulating material for the insulating layer 2, a ceramic spray coating made of high purity alumina of 99.6% or more or alumina containing 2-4 wt% titanium oxide, or a coating material made of a ceramic material selected from magnesium oxide, beryllium oxide (BeO), aluminum nitride (AlN), silicon nitride (Si3N4), etc. is preferable. Polymer resins selected from polyimide, polyphenylene oxide, polytetrafluoroethylene (Teflon (registered trademark)), polytrifluoroethylene chloride, polyethylene, polypropylene, polystyrene, etc., porcelain, enamel, and other SiO2-based glass films are also acceptable, but in the case of the present invention, a ceramic spray coating made of high purity alumina of 99.6% or more or alumina containing 2-4 wt% titanium oxide is most preferable in terms of insulation performance, thermal conductivity, handling, price, etc.

(12) The volume resistivity of the insulating layer 2, which is the innermost layer in contact with the core metal 16 of the backing roll 1 of the present invention, is preferably ¹⁰ Ωcm or more. This is to reduce the leakage current from the internal electrode layer 3 to the core metal 16 via the insulating layer 2 to an ineffective level.

(13) The thickness of the insulating layer 2 should be 50 to 500 μm. If it is less than 50 μm, the withstand voltage for applying the minimum voltage of 0.3 kV to the internal electrode layer 3 to obtain the necessary adhesion will be insufficient. To enable the application of a voltage of up to 6 kV to the internal electrode layer, it is desirable for the withstand voltage of the insulating layer 3 to be 6 kV or more, and a thickness of 500 μm is sufficient for this withstand voltage.

(14) The web used in the present invention includes paper, plastic film, resin-coated paper, synthetic paper, etc. Examples of materials used for the plastic film include polyolefins such as polyethylene and polypropylene, vinyl polymers such as polyvinyl acetate, polyvinyl chloride, and polystyrene, polyamides such as 6,6-nylon and 6-nylon, polyesters such as polyethylene terephthalate and polyethylene-2,6-naphthalate, polycarbonates, cellulose acetates such as cellulose triacetate and cellulose diacetate, etc. Resins used for resin-coated paper are typically polyolefins such as polyethylene, but are not necessarily limited to these. The web can have one or several layers that have been pre-coated.

(15) In addition, the term "coating liquid" includes various liquid compositions depending on the application, and can be used to form, for example, a photosensitive emulsion layer, undercoat layer, protective layer, backing layer, antistatic layer or antihalation layer of a photographic material, and an ink absorbing layer in the case of an inkjet receiving medium. A magnetic layer, undercoat layer, lubricating layer, protective layer, backing layer, adhesive layer, colored layer, anti-rust layer, etc. of a magnetic recording medium. These coating liquids can contain a water-soluble binder or an organic binder.

(16) Surfactants can be used to modify the surface tension and coatability of the coating solution. Examples of surfactants that can be used include nonionic surfactants such as polyalkylene oxides and water-soluble adducts of glycidol and alkylphenols, anionic surfactants such as alkylaryl polyether sulfates and sulfonates, amphoteric surfactants such as arylalkyl taurines, N-alkyl and N-acyl β-aminopropionates, saponins, and alkylammonium betaine sulfonates.

(17) A thickener can be used in the coating solution to adjust the viscosity.

(17) Coating applicators to which the present invention can be applied include bead coating applicators, curtain coating applicators, extrusion coating applicators, and slide extrusion coating applicators.

[Measurement method]
The methods for measuring physical properties used in the present invention are as follows.

1. Center line average roughness Ra
When the surface roughness of the backing roll can be measured while it is in the rolled state, the average roughness Ra of the surface is measured with a portable surface roughness meter. The measuring device is in accordance with JIS B0651, and a stylus with a cone shape with an apex angle of 60 degrees and a radius of curvature of the spherical tip of 2 μm is used. The measuring method is in accordance with JIS B0601-2013, and the measurement is performed under the condition of a cutoff value of 0.8 mm to obtain the center line average roughness Ra. The measuring device used is a Surfcorder SE1700α from Kosaka Laboratory Co., Ltd.

2. Volume resistivity The volume resistivity of the outermost layer 4 on the surface of the backing roll 1 is measured in accordance with JIS C2141-1992, a test method for ceramic materials for electrical insulation. A main electrode with an outer diameter of φ26 mm and a guard electrode with an outer diameter of φ48 mm and an inner diameter of φ38 mm are cut out from a conductive adhesive sheet, and are attached to the curved surface of the outermost layer 4 so that the guard electrode and the main electrode are concentric to conform to the three-terminal method of JIS C2141. A conductor with a terminal of the conductor attached with conductive adhesive tape is used as the contact point on the surface of the main electrode and the guard electrode. The internal electrode layer 3 of the backing roll is used as the counter electrode. The value measured 1 minute after the start of the measurement is adopted. The measuring instrument used is a super insulation meter R-503 manufactured by Kawaguchi Electric Works. A DC voltage of 500 V is applied between the main electrode and the internal electrode layer 3 and between the guard electrode, and the volume resistance R _V (= V/I) is calculated from the current value I (A) flowing through the main electrode and the counter electrode (internal electrode layer 3) at that time, and the volume resistivity ρ _V is calculated by the following formula.

ρ _V (Ωcm)=R _V ×A/d
A = π × D ² /4
Where:
V: Applied voltage (V)
R _V : Volume resistance (Ω)
ρ _V : Volume resistivity (Ωcm)
A: area of main electrode (cm ² )
D: Main electrode outer diameter (cm)
d: outermost layer thickness (cm)
The thickness d is measured by a magnetic or eddy current thickness gauge, such as Fisher Instruments' magnetic/eddy current thickness gauge FMP20.

Example 1
In the method of FIG. 1, electrostatically assisted curtain coating according to the method of the present invention was carried out by continuously conveying a web 7 to a backing roll 1 and applying a DC voltage only to the internal electrode 3 of the backing roll.

The backing roll 1 used here was manufactured as follows.

The entire surface of the center of the body of a cylindrical core 16 made of steel and having a diameter of 200 mm was sandblasted, and then a 50 μm thick 80 wt % Ni/20 wt % Cr alloy was laminated by plasma spraying as a bonding layer to improve the adhesion of the ceramic layer. Subsequently, 250 μm of aluminum oxide (alumina) (99.6 wt % Al ₂ O ₃ ) was laminated by plasma spraying as a high insulating material on the surface of the bonding layer, and then this sprayed alumina layer was sealed with a low viscosity epoxy resin to form an insulating layer 2 having a volume resistivity of 10 ¹⁴ Ωcm or more. A 30 μm thick tungsten (W) was laminated by plasma spraying on the insulating layer 2 to form an internal electrode layer 3. At this time, a 20 mm wide mask was applied from both ends of the backing roll 1 in the width direction so that the internal electrode layer 3 was not formed in this range. After removing the masks from both ends, an alumina-based ceramic material containing 10 wt % titanium oxide (TiO ₂ ) and the remaining 90 wt % alumina (Al ₂ O ₃ ) was laminated by plasma spraying on both ends and on the upper surface of the internal electrode layer 3, i.e., the entire cylindrical surface of the backing roll 1, to a thickness of 400 μm as the outermost layer 4. Similarly, the alumina-based ceramic material layer was sealed together with the tungsten layer of the internal electrode layer 3 with a low-viscosity epoxy resin to obtain the outermost layer 4 with a volume resistivity of 5.6×10 ¹⁰ Ωcm.

The outermost layer 4 after the sealing treatment was polished with a diamond grindstone to a residual thickness of 300 μm and a surface roughness Ra of 0.05 μm after polishing. In order to enable electrical connection to the internal electrode layer 3, a rectangular mask of 20 mm x 30 mm was applied to one end of the roll when the outermost layer 4 was sprayed, and the internal electrode layer 3 was exposed at the end of the roll with a width of 20 mm x 10 mm after the outermost layer 4 was sprayed. A slip ring 6 was connected to the exposed internal electrode layer 3. A DC high voltage power supply 5 was connected to the slip ring 6, and a DC voltage of (+) 3.5 kV was applied to the internal electrode 3. Here, when a voltage was applied to the internal electrode, no current that could be felt by the human body flowed even when the surface of the outermost layer 4 was touched with a finger.

Web 7 is a 100 μm thick polyethylene terephthalate film with a 0.3 μm thick gelatin undercoat layer. Web 7 was placed in a heating furnace at 90-100°C and alternately brought into contact with earthed rolls on both sides to remove charge until the surface potential was below ±50 V, after which it was cooled to 25°C.

The gelatin composition was applied so that the thickness of the wet coating layer 11 was 60 μm, with a curtain height of the coating liquid 9 of 10 cm, a coating angle of 30° forward from the top of the roll, a tension of the web 7 of 150 N/m, and a conveying speed of the web 7 of 400 m/min. The coating liquid used was a 15% aqueous gelatin solution containing 0.1% sodium dodecyl benzene sulfonate, and the low shear viscosity was adjusted to 100 mPa·s using a thickener, and the flow rate of the coating liquid was set to 4 cc/sec per 1 cm of coating width.

The back surface 12 of the web 7 was in close contact with the surface of the backing roll 1, and the web traveled stably. Coating could be continued without any coating problems caused by the air layer accompanying the surface of the web 7 being drawn under the coating liquid, and the wet coating layer 11 had good thickness uniformity and a smooth surface. No spark discharge occurred from the back roll to the surroundings, and no short-circuit current was generated that penetrated the web 7 from the coating liquid 9 toward the roll surface.

(Comparative Example 1)
When coating was performed with the same configuration and conditions as in Example 1 with the voltage to the internal electrode 3 set to 0 (zero), the curtain of coating liquid 9 alone was unable to eliminate the entrained air layer, and the coating liquid did not wet the surface 12 of the web 7, making uniform coating impossible. In addition, the web 7 floated up from the backing roll, and the traction force of the web 7 by the backing roll 1 decreased, causing the tension and speed of the web 7 to fluctuate, making the running unstable, and therefore coating could not be sustained.

Example 2
In the same coating apparatus as in Example 1, the web 7 was transported to the backing roll 1 having a diameter of 200 mm, the voltage applied to the internal electrode 3 was set to +3.5 kV, and a tungsten wire electrode 14 having a diameter of 0.5 mm was additionally installed at a distance of 10 mm from the web as shown in FIG. 2, a voltage of -3 kV was applied, and a negative charge (-) was deposited from the electrode 14 onto the first surface of the web 7, thereby performing electrostatically assisted curtain coating according to the method of the modified example of the present invention.

Web 7 is a 100 μm thick polyethylene coated paper with a 0.6 μm thick gelatin undercoat layer. Web 7 is placed in a heating furnace at 90-100°C, where it is alternately brought into contact with earthed rolls on both sides to remove charge until the surface potential is below ±50V, and then cooled to 25°C.

The gelatin composition was applied to a wet thickness of 35 μm at a curtain height of 10 cm, a coating angle of 30° forward from the roll apex, a tension of 150 N/m on the web 7, and a web conveying speed of 400 m/min. The coating solution 9 was a 12% gelatin aqueous solution containing 0.1% sodium dodecyl benzene sulfonate, and the low shear viscosity was adjusted to 21 mPa·s with a thickener. The flow rate of the coating solution was set to 2.3 cc/sec per cm of coating width. The second surface 13 of the web 7 was in close contact with the surface of the backing roll 1, and the running was stable. The first surface 12 of the web 7 could continue coating stably without causing coating problems due to the air layer accompanying the first surface 12 of the web 7 being drawn into the coating solution, and the thickness uniformity and surface smoothness of the wet coating layer 11 were both good. No spark discharge or glow discharge occurred from the back roll to the surroundings, and no short-circuit current was generated that penetrated the web 7 from the coating liquid 9 to the roll surface.

(Comparative Example 2)
Under the same configuration conditions as in Example 2, when coating was performed with the voltage to the internal electrode 3 set to 0 (zero), the coating liquid 9 was able to form a wet coating layer 11 that was in close contact with the entire first surface 12 of the web. However, an entrained air layer entered between the back surface 13 of the web and the backing roll, causing the web 7 to float, which resulted in the tension and speed of the web 7 becoming unstable and causing it to oscillate and run unstably, making it impossible to continuously form a wet coating layer 11 with a smooth surface.

1 Backing roll 2 Insulating layer 3 Internal electrode (+)
4 Outermost layer 5 DC high voltage power supply (for internal electrodes)
6 Slip ring 7 Band-shaped support (web)
8. Applicator (application head)
9: Curtain of coating liquid 10: Coating liquid contact line (coating point)
11 Wet coating layer 12 Web first surface (surface)
13 Web second surface (reverse surface)
14 Spatial auxiliary electrode (-)
15 DC high voltage power supply (for space auxiliary electrode)
16 Roll core metal 17 Free charge (+)
18 Dipole inside the web 19 Surface charge (-)

Claims (14)

1. A method for applying a coating liquid comprising a liquid composition to a continuously moving flexible plastic-based web having opposed first and second surfaces by flowing a coating liquid from an applicator grounded on a first surface of the web, the method comprising the steps of:
a step of conveying the web along a path to a backing roll for coating, and passing the web through a coating point while supporting a second surface of the web by being in close contact with a portion of the surface of the rotating backing roll to which a DC voltage is applied, by an electrostatic field of the backing roll;
and applying the coating liquid to the first surface of the web by electrostatic force generated by the arrangement of charges of the same polarity as the DC voltage applied to the backing roll on the first surface of the web, by attracting the coating liquid at the coating point,
The backing roll is provided with an outermost layer to which the web is in close contact, a conductive monopolar internal electrode layer adjacent to the inner side of the outermost layer, and an insulating layer adjacent to the inner side of the internal electrode layer, the backing roll is configured to be able to apply a predetermined voltage to the internal electrode layer, the outermost layer is a ceramic material layer having a volume resistivity of 10 ⁷ to 10 ¹³ Ωcm at 25 to 100°C, the surface of the backing roll is charged with a charge having the same sign as a voltage applied to the internal electrode layer in a state in which the predetermined voltage is applied to the internal electrode layer, the second surface of the web in contact with the outermost layer by the electrostatic force of the charge is brought into close contact with the outermost layer and rotated and transported, and the coating liquid is attracted to and brought into contact with a first surface of the web during rotation and transport, and deposited on the first surface of the web. The coating method according to claim 1, in which a charge of a polarity opposite to the DC voltage applied to the internal electrode layer is applied to a first surface of the web opposite to the second surface before the coating liquid comes into contact with the web during rotational transport. The coating method according to claim 1, wherein the DC voltage applied to the internal electrode layer is 0.3 to 6.0 KV. The coating method according to claim 1, wherein the diameter of the backing roll is 100 mm or more. The coating method according to claim 1, wherein at least one of the insulating layer, the internal electrode layer, and the outermost layer is a material formed by a thermal spraying method or a material using either an inorganic or organic binder, and at least one of the insulating layer, the internal electrode layer, and the outermost layer of the backing roll is subjected to a sealing treatment. The coating method according to claim 1, wherein the outermost layer is made of a ceramic material, and the ceramic material is an alumina-based, zirconium oxide-based, or magnesium oxide-based ceramic containing a compound selected from titanium oxide, chromium oxide, silicon oxide, manganese oxide, nickel oxide, and iron oxide, or an aluminum oxide-based ceramic containing 5 to 17% by weight of titanium oxide. The coating method according to claim 1, wherein the outermost layer is made of a ceramic material, and the ceramic material includes at least one selected from aluminum nitride, silicon carbide, and silicon nitride, and an organic or inorganic binder. The coating method according to claim 1, wherein the thickness of the outermost layer is 50 to 500 μm. The coating method according to claim 1, wherein the centerline average surface roughness Ra of the outermost layer of the backing roll is 0.01 μm to 5 μm. The coating method according to claim 1, wherein the internal electrode layer is made of a conductive material containing tungsten or molybdenum. The coating method according to claim 1, wherein the thickness of the internal electrode layer is 5 to 50 μm. The coating method according to claim 1, wherein the insulating layer includes at least one highly insulating material selected from aluminum oxide, aluminum oxide containing 2 to 4 weight percent titanium oxide, magnesium oxide, beryllium oxide, aluminum nitride, or ceramic material containing silicon nitride, porcelain, and enamel. The coating method according to claim 1 , wherein the insulating layer has a volume resistivity of 10 ¹³ Ωcm or more. The coating method according to claim 1, wherein the thickness of the insulating layer is 50 to 500 μm.

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