Electrostatically assisted coating method using a backing roll having an internal electrode capable of applying high voltage

Abstract

A coating method for a wide web is provided that enables high-speed and stable coating and has excellent uniformity of coating thickness. A method for coating a coating liquid on a flexible plastic-based web comprises the following steps: conveying the web to a backing roll for coating; passing the web through a coating point while supporting the second surface of the web by the electrostatic field of the backing roll to a part of the surface of a rotating backing roll to which a DC voltage is applied; and attracting the coating liquid at the coating point and applying it to the first surface of the web by the electrostatic force generated by the coordination of charges of the same polarity as the DC voltage applied to the first surface of the web. The backing roll is configured to sequentially have an outermost layer, an inner electrode layer, and an insulating layer, and to apply a predetermined voltage to the inner electrode layer. The outermost layer is a ceramic-based material layer having a volume resistivity of 107 to 1013 Ωcm at 25 to 100°C.

Description

Technology Field

[0001] The present invention relates to a method for applying one or more layers of a coating solution, composed of a liquid composition, particularly a water-soluble composition, to the surface of a flexible strip-shaped support (hereinafter referred to as a web) that is supported by a backing roll and runs continuously, by means of a curtain coating or bead coating method, wherein an electrostatic force is applied to a contact line portion (hereinafter referred to as a coating point) where the coating solution contacts the web, thereby preventing an air layer accompanying the front side (coating surface) of the web from entering between the coating solution and the web, and promoting the uniform wetting of the web by the coating solution, thereby increasing the uniformity of the coating thickness and the coating speed.

[0002] Furthermore, the present invention relates to a method for preventing the air layer associated with the web's rear side (backing roll side) from penetrating between the web and the roll by pressing the rear side of the web (backing roll side) against the backing roll surface, thereby enabling the limit of web speed to be raised until driving troubles such as fluctuations in web tension and speed, and oscillations occur.

[0003] The present invention is applicable to the improvement of a method for applying a liquid composition, mainly a water-soluble composition, in the manufacture of photographic films, photographic printing paper, magnetic recording tapes, adhesive tapes, pressure-sensitive and thermal recording papers, coated papers, etc. Background Technology

[0004] In curtain coatings or bead coatings, the front and rear sides of the high-speed moving web are each accompanied by an air layer.

[0005] Since the thickness of the accompanying air layer on the front and rear sides increases in correspondence with the web's movement speed, reaching a certain limit speed in each case results in a decrease in coating uniformity or causes web slip and oscillation, making it impossible to achieve stable coating and preventing the coating speed from being increased beyond that respective limit speed. To increase speed while maintaining coating uniformity, a method is required to simultaneously address the influence of the accompanying air layer on both the front and rear sides of the web.

[0006] Regarding the front side of the web, if the web's movement speed increases and the accompanying air layer grows beyond a certain limit, the air layer penetrates beneath the coating liquid transferred to the web at the coating point and cannot be pushed out by the liquid. When the air layer penetrates beneath the coating layer, it hinders the coating liquid from spreading while wetting the front side of the web, or the air layer enters the liquid and forms bubbles, thereby reducing the uniformity of the coating layer and making it impossible to increase the coating speed.

[0007] Conventionally, in curtain coating or bead coating, it is well known that if an electrostatic field is applied to the coating point, the coating solution can be attracted and adhered to the web surface by electrostatic force on ions or dipolar molecules in the coating solution, thereby increasing the upper limit of the coating speed. As a method of applying an electrostatic field to the coating point, 1) a method of applying an electrostatic charge to the web from a corona discharge electrode placed in space and then passing it through the coating point while supported by a grounded coating backing roll, 2) a method of applying a DC voltage to a conductive coating backing roll (hereinafter backing roll) insulated from earth to form an electric field between the surface of the web at the coating point and the coating solution, 3) or a method of combining these two methods.

[0008] For example, Patent Documents 1 to 4 disclose a method of 1) for imparting a charge to a dielectric web, such as polyester. When a dielectric web with a polar charge imparted between the opposing back and front surfaces is supported on a grounded conductive backing roll surface and transported to a coating point, the polar charge remaining on the front side of the web attracts the coating liquid, acting to more strongly eliminate the air layer beneath the coating liquid, and consequently increases the coating speed. Furthermore, Patent Documents 5 to 7 disclose a method of 2) in which a DC voltage is applied to a conductive backing roll insulated from 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 to adhere the coating liquid to the web. Additionally, Patent Document 7 discloses a method combining 2) and 3).

[0009] The effect of imparting electrostatic charge to the web, or the effect of assisting electrostatic charge on the surface of a conductive backing roll, refers to suppressing the phenomenon where an air layer accompanying the web gets caught between the front side of the web and the coating liquid, preventing the coating liquid from wetting, and increasing the limiting speed until the transparency or thickness of the coating becomes non-uniform. In the conventional electrostatic assisting coatings of 1) to 3), it is known that the coating speed increases as the electrostatic charging potential of the front side of the web or the voltage of the backing roll surface increases, because the more the adhesion force of the coating liquid to the front side of the web increases, the more strongly the accompanying air layer is excluded. However, in electrostatic assisting coatings that apply voltage to a conductive backing roll, it is known that if the applied voltage exceeds 1500V, spark discharge from the backing roll to peripheral equipment and short-circuit current penetrating the web are likely to occur. Furthermore, regarding the method of imparting electrostatic charge to the web, if the charge potential on the front side of the web increases, a short-circuit current may occur penetrating the web between the coating liquid and the surface of the conductive backing roll in places where there are pinholes in the web or where the insulation resistance is reduced. For this reason, the surface potential of the web due to the charge disclosed in Patent Document 1 is 1200V or less. In addition, for example, Patent Document 4 discloses an electrostatic assist coating technology in which the surface potential of the web is set lower than 700-800V by combining the polarization effect of a coating liquid containing a surfactant, and Patent Document 2 discloses an electrostatic assist coating technology in which the surface potential of the web is set lower than 700-800V by combining the method of charging the web with a conductive brush; however, the surface potential that is standardly recommended is lower, at 400-500V. As such, conventional electrostatic assist coating using a conductive backing roll is not capable of applying a sufficient potential to the coating point, and thus the effect of raising the limit speed before coating failure occurs due to an increase in entrained air is insufficient.

[0010] Furthermore, the back side of the web and the surface of the backing roll each entail an air layer. Consequently, the web is lifted by the combined entailed air from both sides, and the amount of lifting increases as the web speed increases. If the amount of web lifting exceeds a certain limit, there is a problem where the web's transport becomes unstable, and the increase in the entailed air layer on the back side of the web also becomes a cause preventing the coating speed from being increased.

[0011] The web buoyancy h (㎛) is given by backing roll radius R (m) and roller speed U when the air temperature is 25℃. R (m / s), Web Speed ​​U W (m / s), from web tension T (N / m), is given by the following equation (1).

[0012]

[0013] (Hiroshi Hashimoto, *Basic Theory and Application of Web Handling*, p. 73, published by the Processing Technology Research Group)

[0014] (1) According to the equation, the thickness of the accompanying air layer that is rolled up on the back side of the web and the roll surface decreases as the web tension (T) increases, and the roll radius R and web speed U W or back roll speed U R It increases as this gets bigger (usually U W ≒U R ).

[0015] In an electrostatically assisted coating in which voltage is applied to a conductive backing roll, as the web's transport speed increases and the web rises 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 capacitance decreases due to the air layer between the conductive backing roll and the back side of the web. Because of this, the amount of polar charge accumulated on the web decreases, and the electrostatic force between the web and the coating solution decreases.

[0016] Furthermore, if the thickness of the accompanying air layer on the back side of the web exceeds the sum of the surface roughness of the roll and the surface roughness of the web's back side, contact between the web and the backing roll surface decreases, thereby reducing the traction force on the web. A decrease in web traction force causes fluctuations in web tension and speed, leading to web meandering and problems such as scratches on the back side of the web. Therefore, to increase coating speed, it is essential to implement measures to prevent the web from lifting due to the accompanying air layer sandwiched between the back side of the web and the backing roll.

[0017] (1) The relationship between the web return speed and the amount of buoyancy calculated from the equation is shown in FIGS. 7 and FIGS. 8. As shown in FIGS. 7, it is possible to suppress the buoyancy of the web by increasing the web tension (T). However, increasing the web tension increases the load on the web return system and increases the deflection of the backing roll and the guide roll in the web return path. In addition, there is a limit to the method of increasing the web tension because wrinkles are likely to occur when the web is thin.

[0018] In addition, as shown in Fig. 8, reducing the diameter of the backing roll reduces the amount of web lift. However, reducing the diameter of the backing roll reduces the rigidity of the roll, resulting in an increased amount of deflection. Furthermore, as the roll diameter decreases, the rotational speed of the roll increases proportionally; therefore, as the web becomes faster and wider, the roll vibrates, causing vibration patterns to occur in the coating film. For roller curtain coating or bead coating on wide webs of 1500 mm or more, considering the area required to secure the deflection strength of the roll and install the coating head (applicator), a backing roll diameter of 100 mm or more is preferable. Web tension depends on the strength and thickness of the web, but considering the elongation and deformation of the web, it is typically around 75 to 300 N / m. According to FIG. 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 amount of web lifted is already 6 μm at a travel speed of 100 m / min, and in the case of a roll with a smooth surface, the traction force decreases, causing fluctuations in tension and speed. Judging from the amount of web lifted shown in FIG. 7 and 8, when the diameter of the backing roll is 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 due to the air layer accompanying the back side of the web.

[0019] In an electrostatic assist curtain coating in which a DC voltage is applied to a conductive backing roll with a diameter of 100 mm or more, a countermeasure against the web rising due to an accompanying air layer on the back side of the web is disclosed in Patent Document 7. In an embodiment 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 to suppress the rising of the web, thereby enabling an increase in the conveying speed. However, the voltage applied to the conductive backing roll is 800 V or less, and since the electrostatic assist action decreases in the groove area, coating non-uniformity is prone to occur. Therefore, when the microgroove and electrostatic assist are used in combination, the increase in coating speed is not very large, at most 144 m / min. Prior art literature

[0020] Japanese Patent No. 2835659, Japanese Patent Publication No. Hei 01-035702, Japanese Patent Publication No. Hei 08-252517, Japanese Patent No. 2509316, Japanese Patent Publication No. Hei 06-009671, Japanese Patent Publication No. Sho 46-027423, and U.S. Patent No. 6177141 specification

[0021] Hiroshi Hashimoto, *Basic Theory and Applications of Web Handling*, published by the Processing Technology Research Group, p. 73 The problem to be solved

[0022] In curtain coating or bead coating, the front side (coating side) and rear side (backing roll side) of a high-speed moving web are accompanied by air layers on both sides, and the thickness of these layers increases on both sides as the web's movement speed increases. However, the problems caused by these accompanying air layers differ between the front and rear sides. On the front side, the phenomenon is that the coating liquid fails to wet the web, whereas on the rear side, the web lifts due to the accompanying air layer penetrating between the rear side of the web and the backing roll, causing the web's movement to become unstable. High-speed coating technology capable of simultaneously addressing these different problems caused by the accompanying air flow on the front and rear sides of the web is required.

[0023] By using a backing roll capable of increasing the voltage applied to the roll surface to 1.5 kV or higher, preferably up to about 3.5 kV, the dielectric web is more strongly polarized, and the electric field between the web surface and the coating liquid at the contact line (coating point) between the coating liquid and the web can be made much stronger than in conventional methods, so the coating speed can be increased as needed simply by adjusting the applied voltage.

[0024] At the same time, the rear side of the web, strongly polarized by the high voltage of the backing roll, is strongly attracted to the backing roll by the electric field caused by the electrostatic charge on the backing roll surface, pushing out the accompanying air layer and adhering to the surface of the backing roll, thereby making it possible to increase the limit speed until the web's driving trouble occurs. means of solving the problem

[0025] To achieve the above objective, the present invention employs the following method.

[0026] The present invention provides an electrostatic assisted coating method using a highly stable backing roll in a process of applying a liquid composition (particularly a water-soluble composition) to a web that is continuously driven while supported by a roll, wherein spark discharge to surrounding equipment and short-circuit current in the web are not generated even when a high DC voltage of up to 6 kV is applied, and electric shock does not occur even if a human body comes into contact with the roll surface. This backing roll forms a gapless three-layer structure on the roll surface consisting of an outermost layer, an inner electrode layer, and a bottom layer by using a high-insulating ceramic in the bottom layer, a conductive metal in the inner electrode layer, and a high-resistance ceramic-based material layer in the outermost layer. The inner electrode layer is electrically completely shielded from the core of the backing roll and surrounding equipment by the outermost layer and the bottom layer. Since the dielectric breakdown voltage of the outermost layer and the bottom layer is 6 kV or higher, spark discharge to surrounding 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 inner electrode layer.

[0027] That is, the method of the present invention is,

[0028] (1) A method of applying a coating solution made of a liquid composition by discharging it from an applicator that is grounded to the first surface of a continuously moving flexible plastic web having opposing first and second surfaces,

[0029] A process of conveying the above web along a path to a backing roll for coating, and passing the web through a coating point while supporting and adhering a second surface of the web to a part of the surface of the rotating backing roll to which a DC voltage is applied using the electrostatic field of the backing roll; and

[0030] The process includes attracting the coating liquid at the coating point and applying it to the first surface of the web using electrostatic force generated by coordinating charges of the same polarity as the DC voltage applied to the backing roll on the first surface of the web.

[0031] The backing roll comprises an outermost layer to which the web adheres, a conductive unipolar inner electrode layer adjacent to the inner side with respect to the outermost layer, and an insulating layer adjacent to the inner side with respect to the inner electrode layer, and is configured to apply a predetermined voltage to the inner electrode layer, and the outermost layer has a volume resistivity of 10 at 25 to 100°C 7 ~10 13 The coating method is characterized by a ceramic material layer having Ωcm, wherein the surface of the backing roll is in a state where the predetermined voltage is applied to the inner electrode layer, the outermost layer is charged with a charge having the same sign as the voltage applied to the inner electrode, the second surface of the web in contact with the outermost layer is pressed against the outermost layer by the electrostatic force of the charge and rotated and transported, and the coating liquid is pulled and brought into contact with the first surface of the web while it is rotating and transported, and immersed in the first surface of the web.

[0032] And, in the above application method,

[0033] (2) A charge of opposite polarity to the DC voltage applied to the internal electrode layer is applied to the first surface opposite to the second surface of the web before the coating liquid comes into contact with the web being rotated and transported, and

[0034] (3) The DC voltage applied to the internal electrode layer is 0.3 to 6.0 KV, and

[0035] (4) The diameter of the backing roll is 100 mm or more, and

[0036] (5) At least one of the insulating layer, the inner 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 inner electrode layer, and the outermost layer of the backing roll is treated with a hole sealing process.

[0037] (6) The outermost layer is made of a ceramic material, and the ceramic material is an alumina-based, zirconium-based, or magnesium-based ceramic containing a compound selected from titanium oxide, chromium oxide, silicon oxide, manganese oxide, nickel oxide, and iron oxide, or an aluminum-based ceramic containing 5 to 17 weight percent of titanium oxide.

[0038] (7) The ceramic material comprises at least one selected from aluminum nitride-based, silicon carbide-based, and silicon nitride-based materials, and an organic or inorganic binder, and

[0039] (8) The thickness of the outermost layer is 50 to 500 μm, and

[0040] (9) The centerline average surface roughness Ra of the outermost layer is 0.01 to 5 μm, and

[0041] (10) The internal electrode layer is made of a conductive material including tungsten or molybdenum, and

[0042] (11) The thickness of the internal electrode layer is 5 to 50 μm, and

[0043] (12) The insulating layer comprises at least one high insulating material selected from aluminum oxide, aluminum oxide-based, magnesium oxide-based, beryllium oxide-based, aluminum nitride-based, or silicon nitride-based ceramic materials containing 2 to 4 weight percent of titanium oxide, porcelain, and enamel, and

[0044] (13) The volume resistivity of the insulating layer is 10 13 It is greater than Ωcm, and

[0045] (14) The thickness of the insulating layer is 50 to 500 μm,

[0046] It is characterized by the fact that. Effects of the invention

[0047] The present invention relates to an electrostatically assisted coating method using a highly stable backing roll in a process of applying a liquid composition (particularly a water-soluble composition) to a web that is supported by a roll and runs continuously, wherein spark discharge to surrounding equipment does not occur and short-circuit current is not generated in the web even when a DC high voltage of up to 6 kV is applied, and thus electric shock does not occur even if a human body comes into contact with the roll surface. This backing roll uses a high-insulating ceramic in the bottom layer, a conductive metal in the inner electrode layer, and a high-resistance ceramic-based material layer in the outermost layer to form a gapless three-layer structure consisting of an outermost layer, an inner electrode layer, and an insulating layer on the roll surface. The inner electrode layer is electrically completely shielded from the core of the backing roll and surrounding equipment by the outermost layer and the bottom layer. Since the dielectric breakdown voltage of the outermost layer and the bottom layer is 6 kV or higher, spark discharge to surrounding equipment is completely prevented even when a DC high voltage of up to 6 kV is applied to the inner electrode layer.

[0048] The volume resistivity of the outermost high-resistance semiconductor ceramic is 10 7 ~10 13Since it is 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 becomes 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 rear side is charged with a polarity opposite to the potential of the internal electrode, and the front side is charged with the same polarity. Then, due to the electrostatic charge with the same polarity as the internal electrode arranged on the web surface side, the coating liquid is attracted to the web surface and adheres to the web, eliminating the air layer associated with the front side of the web and promoting the wetting of the web by the coating liquid. At the same time, the charge with the opposite polarity to the internal electrode arranged on the rear side of the web is attracted to the free charge with the same polarity as the internal electrode charged on the surface of the high-resistance semiconductor ceramic, which is the outermost layer of the backing roll, so the web adheres to the surface of the backing roll. In this way, in the present invention, since the influence of the accompanying air layer on both the front and back sides of the web is eliminated, it becomes possible to significantly increase the coating speed on the web.

[0049] The following effects can be obtained by the present invention.

[0050] 1. By applying a DC high voltage of up to 6 kV to an internal electrode installed on the outer surface of a backing roll, it is possible to coat a liquid composition (especially a water-soluble composition) with electrostatic assistance without causing spark discharge to the surrounding substrate or damage to the web by short-circuit current.

[0051] 2. While a high DC voltage is applied to the internal electrode of the backing roll, an external auxiliary electrode is installed in the upper space of the front side of the web at the same time, and a DC voltage of 1.5 to 6 kV with a polarity opposite to that of the internal electrode is applied, thereby enabling electrostatic auxiliary coating that imparts a charge with a polarity opposite to that of the internal electrode to the front side of the web.

[0052] 3. Even if the diameter of the backing roll is 100 mm or more, it is possible to completely adhere the web to the backing roll using electrostatic force generated by electrostatic charge on the surface of the high-resistance semiconductor ceramic on the outermost surface, and effectively perform electrostatic assist coating on the web surface, and also prevent shaking or slip caused by the buoyancy of the web, thereby making it possible to increase the web conveying speed to 400 m / min or more.

[0053] 4. Since the diameter of the backing roll can be increased according to the length of the roll, it is possible to increase the rigidity of the roll and make it easier to make the web wider. Brief explanation of the drawing

[0054] FIG. 1 is a cross-sectional view showing an example of a method of the present invention for performing electrostatically assisted coating using only a backing roll to which a high voltage is applied to an internal electrode. Figure 2 is a schematic diagram showing the charge distribution state in part A of Figure 1. FIG. 3 is an external view of the method of the present invention seen from an inclined direction. FIG. 4 is a cross-sectional view of a method of a modified example of the present invention that performs electrostatic assist coating using a backing roll and a web in combination with a method of applying a high voltage to an internal electrode and imparting an electrostatic charge to the web. Figure 5 is a schematic diagram showing the charge distribution state in part B of Figure 4. FIG. 6 is an external view of a method of a modified example of the present invention seen from an inclined direction. Figure 7 is a graph showing the change in web buoyancy with respect to tension and web speed in a backing roll with a diameter of φ200 mm. Figure 8 is a graph showing the change in web buoyancy with respect to backing roll diameter and web speed at a web tension of 150 N / m. Specific details for implementing the invention

[0055] FIGS. 1 to 3 illustrate the method of the present invention. FIG. 2 schematically illustrates the state of charge distribution in part A shown in FIG. 1.

[0056] (1) In order to increase coating capacity, in addition to increasing the web return speed, it is important to make the web wider by lengthening the backing roll. However, when the web tension is constant, lengthening the backing roll causes the amount of roll deflection to increase rapidly. Therefore, to prevent the backing roll from deflecting, it is necessary to increase the backing roll diameter at the same time to increase the rigidity of the roll (the amount of roll deflection is proportional to the cube of the roll length and inversely proportional to the cube of the second moment of area (approximately proportional to the roll diameter)). However, as shown in the above equation (1), if the roll diameter is increased, the accompanying air layer on the back side of the web also increases proportionally. The increase in the accompanying air layer makes it impossible to increase the backing roll diameter to lower the upper limit of the web return speed, and thus the roll cannot be lengthened. On the other hand, since the present invention can eliminate the influence of the accompanying air layer on the back side of the web, it becomes possible to widen the web by increasing the backing roll diameter and roll length as needed. Judging from FIG. 8, since the influence of the accompanying air layer becomes significant when the diameter of the web conveying roll is 100 mm or more, the present invention is more effective when applied to a backing roll with a diameter of 100 mm or more.

[0057] The inner electrode layer (3) of the backing roll (1) of the present invention is a conductive layer that is in contact with the outermost layer (4) and is adjacent to the outermost layer (4) without a gap on the inside. The inner electrode layer (3) is a single-pole type and is connected to a DC high-voltage power source (5) so that a DC voltage can be applied, but the inner electrode layer (3) is completely insulated from the earth and the core (16) of the backing roll (1) by an insulating layer (2) adjacent to the inside.

[0058] Generally, as for the internal electrode layer (3), there are two types, unipolar and bipolar, depending on the method of applying voltage. A unipolar type is a method in which only a single internal electrode, either positive (+) or negative (-), is used, and voltage is applied only between this single electrode with respect to ground. A bipolar type is a method in which two or more internal electrodes with different polarities are used, but when the web to be adsorbed is connected to ground, charge (current) leaks toward the ground side, causing the electrostatic adsorption force to become unstable. In the present invention, a unipolar type is used for the internal electrode layer (3). The internal electrode layer (3) is installed continuously and uniformly over a range wider than the width of the web (7) in the axial direction of the backing roll (1), and furthermore, over the entire circumference of the backing roll (1) in the rotational direction of the backing roll (1). If the internal electrode (3) is in a comb-like, strip-like, or spiral shape, shape non-uniformity occurs in the coating layer. Also, at both ends of the cylindrical surface of the backing roll (1) that are outside the support range of the web (7), the internal electrode layer (3) does not need to be formed.

[0059] The outermost layer (4) is a high-resistance ceramic material, and the volume resistivity is 10 at 25 to 100°C. 7 ~10 13 It is Ωcm, and more preferably 10 8 ~10 12 It is Ωcm. In "fields dealing with static electricity," the volume resistivity is 10 13 Materials with a resistivity of Ωcm or higher are classified as insulators because static electricity has difficulty moving through them. A volume resistivity of 10 6 Materials with a resistivity lower than Ωcm are called electrostatic conductors; static electricity moves easily, and they become equipotential when voltage is applied. A volume resistivity of 10 7 ~10 13These materials are classified as an intermediate region between conductors and insulators and are categorized as "electrostatic semiconductors." It is known that when voltage is applied, a minute current flows, causing free charges to move and become charged on the surface, just like in conductors. Furthermore, the amount of these free charges depends on the applied voltage. Unlike conductive metal surfaces, on the surface of these high-resistance semiconductor ceramic materials, even when a high voltage of 3.5 to 6 kV is applied, only a minute current of about 0.001 to 10 μA / cm² flows locally. For this reason, the discharge caused by free charges on the surface becomes a minute corona and does not develop into an arc discharge, nor does a spark discharge occur toward surrounding equipment.

[0060] In the present invention, as shown in FIG. 2, when a DC voltage is applied to the inner electrode layer (3), free charges (17) of the same polarity as the inner electrode (3) move to the surface of the outermost layer (4) and become charged. Then, when the second side (13) (rear side) of the web (7) comes into contact with the surface of the outermost layer (4), the dipole (18) inside the web (7) is directed toward the surface of the outermost layer (4) with a polarity opposite to that of the free charges (17) due to dielectric polarization caused by the electric field of the inner electrode. Then, due to the electrostatic force between the charges (17) on the surface of the outermost layer (4) and the reverse charges of the dipoles directed toward the second side (13) of the web (7), the web (7) adheres to the surface of the outermost layer (4). At this time, the first surface (12) (front side) of the web, which is the coating surface, is polarized and charged with the same polarity as the internal electrode by the electrostatic field of the internal electrode (3). By this "polar charge," the coating liquid is attracted toward the first surface (12) of the web (7) that is being rotated and transported, so that a uniform coating layer (11) can be formed on the first surface (12) of the web (7) without intruding an air layer between the coating liquid (9) and the first surface (12) of the web (7). The higher the applied voltage to the internal electrode (3), the greater the amount of polar charge on the first surface (12) that is accumulated by dielectric polarization of the web (7), and since the electric field becomes stronger, it becomes possible to greatly increase the coating speed.

[0061] (2) FIGS. 4 and 5 are side views showing an example of a method of a modified example of the present invention. FIG. 5 schematically shows the state of charge distribution in part B of FIG. 4.

[0062] A method of a variation of the present invention is a method in which, in a device configuration similar to the method of the present invention, a space 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 (3) is applied to generate a corona discharge, thereby charging a first surface (12) of the web (7) with a free charge (19) of opposite polarity to the internal electrode layer (3).

[0063] The withstand voltage of the outermost layer (4) of the backing roll (1) is a maximum of 6 kV, and the volume resistivity is 10 7 ~10 13 Because it is Ωcm, even if there are pinholes or places with weak insulation resistance in the web to which the charge (19) is applied, it prevents the occurrence of short-circuit current from the coating liquid at those defect points toward the backing roll (1). Therefore, it is possible to raise the corona discharge potential of the external auxiliary electrode (14) installed in the space upstream of the coating point as needed, thereby increasing the amount of free charge (19) charged to the first surface (12) of the web (7), and it is possible to further increase the coating speed by electrostatic assistance. The applied voltage to the external auxiliary electrode is sufficiently effective at DC(-) 3.5~6kV, and if it is installed while maintaining an appropriate distance from peripheral equipment, spark discharge does not occur.

[0064] In addition, due to the electrostatic force between the charge (17) on the surface of the outermost layer (4) and the charge (19) charged on the first surface of the web, the web (7) adheres more strongly to the surface of the outermost layer (4), and the intrusion of accompanying airflow to the rear side of the web (7) can be completely prevented. Therefore, in accordance with the increase in coating speed, the conveying speed of the web can also be significantly increased as needed.

[0065] (3) Since the web (7) of the present invention, which is the adsorbent, is flexible, a uniform and strong adhesion force is required on all contact surfaces between the web (7) and the backing roll (1) without any gaps in adhesion. To obtain the electrostatic force required for electrostatic adhesion of the web (7), the voltage applied to the inner electrode layer (3) is usually sufficient at DC(+) 0.3 to 3.5 kV. The higher the applied voltage, the stronger the electrostatic assisting effect, and the electrostatic force is insufficient when the applied voltage is 0.3 kV or less. Since the withstand voltage of the outermost layer (4) is at least 6 kV, there is no spark discharge even when DC+3.5 kV is applied.

[0066] (4) At least one of the outermost layer (4), inner electrode layer (3), and insulating layer (2) of the backing roll (1) is a material formed by a thermal spraying method, or a material using either an inorganic or organic binder, and at least one is treated with a hole sealing process.

[0067] The term "spraying" as used in the present invention refers to a coating technology in which a coating material is melted or softened by heating to form fine particles, accelerated to collide with the surface of an object to be coated, and then solidified and deposited to form flattened particles. Although there are various methods of spraying and they are classified according to the materials used or the type of heat source, the atmospheric plasma spraying method is particularly suitable for the present invention.

[0068] In addition, when a thermal spray ceramic material is used for the outermost layer (4) and the insulating layer (2), it is preferable that at least one layer among the insulating layer (2), the inner electrode layer (3), and the outermost layer (4) undergoes a hole sealing treatment. A thermal spray ceramic film is a collection of flat thermal spray particles, but there are fine gaps between these thermal spray particles, and interconnected pores corresponding to 3 to 10 Vol% of the total volume of the thermal spray film are formed. In a normal atmospheric environment, air or liquid containing humidity penetrates into these pores, and moisture is adsorbed onto the inner walls of the pores, so the thermal spray film has significantly low insulation resistance and dielectric strength. By infusing the thermal spray ceramic with an insulating hole sealant, the interconnected pores disappear, the volume resistivity stabilizes, and the material becomes a ceramic material with high dielectric strength and excellent corrosion resistance. The hole sealing treatment after coating the ceramic material can be performed even with a large backing roll of 200 mm or more in diameter, which is used in actual equipment, and is a practical method. As materials used for the hole sealing treatment, liquid hole sealing agents such as low-viscosity silicon oligomers (e.g., viscosity 8-40 mPs, 25°C), low-viscosity epoxy resins (e.g., viscosity 80-400 mPs, 25°C), polyester resins, aqueous lithium silicate solutions forming inorganic films, and metal alkoxides forming inorganic sols can be used after being diluted with a solvent to achieve low viscosity. In the case of the present invention, low-viscosity silicon oligomers and low-viscosity epoxy resins are preferred because they exhibit excellent impregnation characteristics, insulation performance, and dielectric strength characteristics. Furthermore, regarding the thickness range of the hole sealing treatment layer, it is preferable that the entire thermal spray layer is hole-sealed. It is important to stabilize the volume resistivity and dielectric strength characteristics of the hole sealing agent by penetrating the hole sealing agent into the pores of the thermal spray film to seal the pores and then heat-curing the sealant. If the pores inside the thermal spray film are not properly sealed, both the insulating layer (2) and the outermost layer (4) cannot be adjusted to their original volume resistivity and withstand voltage.

[0069] The volume resistivity of the outermost layer (4) after hole sealing treatment at 25 to 100°C is 10 7 ~10 13 It is Ωcm, preferably 10 8 ~10 12 It is Ωcm, and more preferably 10 9 ~10 12 It is Ωcm. If the volume resistivity is less than 107Ωcm, it becomes impossible to prevent short-circuit current from the coating liquid connected to earth through the web (7) in contact with the surface of the backing roll (1) or spark discharge to a peripheral device. That is, the volume resistivity of the outermost layer (4) is 10 7 Below, it is undesirable because it becomes impossible to perform electrostatically assisted coating by applying a high voltage of 1.5 kV or more to the surface of the back roll and web.

[0070] Meanwhile, the volume resistivity of the outermost layer (4) is 10 13If it exceeds Ωcm, the outermost layer (4) becomes an insulating dielectric, and no substantial free charge (17) flows from the inner electrode layer (3). As a result, the surface of the outermost layer (4) cannot be charged with free charge (17) of the same polarity as the inner electrode, which is undesirable. When the electrical characteristics of the outermost layer (4) are an insulating dielectric, the dipole (18) is oriented such that the surface direction of the outermost layer (4) becomes a charge of the same polarity as the inner electrode (3) due to dielectric polarization caused by the voltage of the inner electrode (3). Here, the charging of the surface of the outermost layer (4) is due to the charge fixed to the dipole and cannot be neutralized and extinguished by free charge. When the web (7) moves away from the backing roll, reverse charge remains on the surface of the outermost layer (4) due to peeling charge. In this reversing state, when voltage is applied to the backing roll, the fixed charge at the dipole end of the outermost layer (4) is trapped, and although the apparent charge potential decreases, it does not disappear and remains as is. When the surface of the outermost layer (4) becomes like this, the web (7) bounces off the surface of the outermost layer (4). That is, the outermost layer (4) has a volume resistivity of 10 13 If it is higher than Ωcm, it becomes an insulating dielectric, and the peeling charge of the web remains on its surface without being neutralized, so the web cannot adhere to the surface of the outermost layer (4), and a problem occurs in the web's movement.

[0071] (5) The outermost layer (4) is made of a ceramic material, and the ceramic material is an alumina-based or zirconium-based or magnesium-based ceramic containing a compound selected from titanium oxide, chromium oxide, silicon oxide, manganese oxide, nickel oxide and iron oxide, or an aluminum-based ceramic containing 5 to 17 weight percent of titanium oxide.

[0072] As a material for the outermost layer (4) of the present invention, it is preferable to have a volume resistivity of 10 7 ~10 13It is a material that is adjustable in Ωcm and has excellent handling properties. In terms of adhesion to the internal electrode layer (3), density, stability of volume resistivity, and handling properties, an aluminum oxide-based ceramic material layer containing 5 to 17 weight%, preferably 7 to 15 weight%, titanium oxide produced by a thermal spraying method is most preferred.

[0073] (6) Also in the same sense, in the present invention, the outermost layer (4) and the insulating layer (2) may be a non-oxide ceramic material layer using an organic or inorganic binder by a coating method. The outermost layer (4) and the insulating layer (2) may be a coating layer formed by applying a coating material comprising an organic binder such as polyimide, or an inorganic binder such as aluminum phosphate, water glass, or silicon, and at least one selected from aluminum nitride, silicon carbide, and silicon nitride that is blended to have a volume resistivity corresponding to the present invention, and an organic or inorganic binder.

[0074] (7) The thickness of the outermost layer (4) is preferably 50 μm or more, and furthermore, 80 to 500 μm. The lower limit of this thickness is the lower film thickness required to ensure that there are no pinholes penetrating the inner electrode layer (3) in the outermost layer (4), and it is a film thickness that can secure withstand voltage for the minimum applied voltage required for stable adhesion of the web (7). The upper limit of this thickness is set considering the maximum applied voltage. To obtain a stronger electrostatic assist effect, a higher applied voltage is required, and it is necessary to increase the thickness of the outermost layer (4) to increase the withstand voltage. However, since a sufficient effect is obtained with an applied voltage of 3.5 kV to the inner electrode layer, a maximum withstand voltage of 6 kV is sufficient considering the safety factor. It is not desirable for the thickness of the outermost layer (4) to increase beyond 500 μm, which is the thickness at which the withstand voltage is obtained, as this only increases the spraying time.

[0075] (8) The surface roughness of the outermost layer (4) of the backing roll (1) of the present invention is such that the centerline average surface roughness Ra is 0.01 to 5 μm. A smooth surface of less than 0.01 μm is difficult to obtain with the polishing technology of the ceramic layer in reality. The larger the surface roughness of the outermost layer (4), the more stable the web travel at high speed becomes, but if it exceeds 5 μm, it is undesirable because the surface roughness of the outermost layer (4) is transferred to the back side of the web.

[0076] (9) The material of the inner electrode layer (3) can be a sintered conductive paste such as tungsten, molybdenum, high-performance activated carbon, copper, silver, etc., but a tungsten or molybdenum thermal spray coating applied by plasma spraying is preferred because it has high thermal conductivity and is also easy to handle.

[0077] (10) The volume resistivity of the outermost layer (4) is 10 7 ~10 13 Since the voltage is high in Ωcm, even when a high voltage of 6kV is applied to the internal electrode, the total current flowing from the internal electrode to the web surface is very small, less than a few mA, so the thickness of the internal electrode layer (3) is sufficient to be 5 to 50 μm, and it is desirable to make the boundary at both ends of the roll of the internal electrode (3) as thin as possible.

[0078] (11) As a high-insulating material for the insulating layer (2), a ceramic spray film made of high-purity alumina of 99.6% or higher 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 preferred. In addition, a polymer resin selected from polyimide, polyphenylene oxide, polytetrafluoroethylene (Teflon (registered trademark)), polytrifluorochloroethylene, polyethylene, polypropylene, polystyrene, etc., or SiO2-based glass film such as porcelain or enamel may be used, but in the case of the present invention, a ceramic spray film made of high-purity alumina of 99.6% or higher or alumina containing 2-4 wt% titanium oxide is most preferred due to insulation performance, thermal conductivity, handling, and cost.

[0079] (12) The volume resistivity of the insulating layer (2), which is the innermost layer in contact with the core (16) of the backing roll (1) of the present invention, is 10 13 It is preferable that it be greater than Ωcm. This is to reduce the leakage current from the inner electrode layer (3) through the insulating layer (2) to the core (16) to a level that has no effect.

[0080] (13) The thickness of the insulating layer (2) is preferably 50 to 500 μm, and if it is less than 50 μm, there is insufficient withstand voltage to apply the minimum voltage of 0.3 kV to the inner electrode layer (3) to obtain the required adhesion force. In order to enable the application voltage of up to 6 kV to the inner electrode layer, it is desired that the withstand voltage of the insulating layer (2) be 6 kV or higher, but the thickness required for this withstand voltage is 500 μm.

[0081] (14) Webs used in the present invention include paper, plastic film, resin-coated paper, synthetic paper, etc. For example, the material of the plastic film is polyolefin such as polyethylene and polypropylene, vinyl polymer such as polyvinyl acetate, polyvinyl chloride, and polystyrene, polyamide such as 6,6-nylon and 6-nylon, polyester such as polyethylene terephthalate and polyethylene-2,6-naphthalate, polycarbonate, cellulose acetate such as cellulose austoracetate and cellulose diacetate, etc. Also, polyolefins including polyethylene are typical of the resins used in resin-coated paper, but are not necessarily limited to these. The web may have one or several pre-coated layers.

[0082] (15) In addition, the term “coating solution” includes various liquid compositions depending on the application, and can be used to form an ink absorption layer, for example, a photosensitive emulsion layer, undercoat layer, protective layer, backing layer, antistatic layer, or anti-halation layer of a photosensitive material, or an ink absorption layer in the case of an inkjet receiving medium. Magnetic layer, undercoat layer, lubricating layer, protective layer, backing layer, etc., adhesive layer, coloring layer, anti-corrosion layer, etc. of a magnetic recording medium. The coating solutions may include a water-soluble binder or an organic binder.

[0083] (16) The surface tension and coating suitability of the coating solution can be changed by using a surfactant. The surfactant may include nonionic surfactants such as polyalkylene oxide and water-soluble adducts of glycidol and alkylphenol, anionic surfactants such as alkylaryl polyether sulfate and sulfonate, amphoteric surfactants such as arylalkyl taurine, N-alkyl and N-acyl β-aminopropionate, saponin, alkylammonium sulfonic acid betaine, etc.

[0084] (17) Thickeners can be used to adjust the viscosity of the coating solution.

[0085] (18) Coating applicators to which the present invention is applicable are bead coating applicators, curtain coating applicators, extrusion coating applicators, and slide extrusion coating applicators.

[0086] [measurement method]

[0087] The method for measuring physical property values ​​used in the present invention is as follows.

[0088] 1. Centerline average roughness Ra

[0089] For measuring the surface roughness of the backing roll, if measurement is possible while the roll is in a rolled state, the average surface roughness Ra was measured using a portable surface roughness meter. In accordance with JIS B0651, a conical tip with an angle of 60 degrees and a radius of curvature of 2 μm at the tip was used as the measuring instrument, and in accordance with JIS B0601-2013, the measurement was performed under the condition of a cutoff value of 0.8 mm to obtain the centerline average roughness Ra. As the measuring instrument, the measurement was performed using the Kosaka Genkyusho Surfcoder SE1700α.

[0090] 2. Volume resistivity

[0091] The volume resistivity of the outermost layer (4) on the surface of the backing roll (1) is measured in accordance with the test method for electrical insulating ceramic materials JISC 2141-1992. From a conductive adhesive sheet, 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, and the guard electrode and the main electrode are attached to the curved surface of the outermost layer (4) so ​​that they form concentric circles in accordance with the three-terminal method of JISC 2141. A wire with a terminal attached with conductive adhesive tape is used at the contact point on the surface of the main electrode and the guard electrode. The inner electrode layer (3) of the backing roll is used as the counter electrode. The value taken 1 minute after the start of the measurement is adopted. The super-insulating R-503 manufactured by Kawaguchi Denki Seisakusho was used as the measuring instrument. A DC voltage of 500V is applied between the main electrode, the inner electrode layer (3), and the guard electrode, and at that time, from the current value I (A) flowing through the main electrode and the counter electrode (inner electrode layer (3)), the volume resistance R V Calculate (=V / I), and the volume resistivity ρ using the following formula. V Calculate.

[0092] ρ V (Ωcm)=R V ×A / d

[0093] A=π×D 2 / 4

[0094] Here,

[0095] V: Applied voltage (V)

[0096] R V : Volume resistance (Ω)

[0097] ρ V : Volume resistivity (Ωcm)

[0098] A: Area of ​​the main electrode (㎠)

[0099] D: Main electrode outer diameter (cm)

[0100] d: Outermost layer thickness (cm)

[0101] In addition, the thickness (d) is measured using a magnetic or eddy current thickness gauge. As a measuring instrument, the FMP20 magnetic and eddy current film thickness gauge manufactured by Fisher Instruments Co., Ltd. was used.

[0102] Examples

[0103] (Example 1)

[0104] In the method of Fig. 1, the web (7) was continuously conveyed to the backing roll (1), and a DC voltage was applied only to the internal electrode (3) of the backing roll to perform electrostatic assist curtain coating according to the method of the present invention.

[0105] The backing roll (1) used here was manufactured as follows.

[0106] On the entire surface of the middle section of the body of a cylindrical core (16) made of steel with a diameter of 200 mm, after sandblasting, an 80 wt% Ni / 20 wt% Cr alloy with a thickness of 50 μm was deposited by plasma spraying as a bonding layer to improve the adhesion of the ceramic layer. Subsequently, on the surface of the bonding layer, aluminum oxide (alumina) (99.6 wt% Al2O3) was deposited by plasma spraying to a thickness of 250 μm as a high-insulating material, and then a hole sealing treatment was performed on this sprayed alumina layer using a low-viscosity epoxy resin, so that the volume resistivity was 10 14An insulating layer (2) with a resistance of Ωcm or higher was formed. Tungsten (W) with a thickness of 30 μm was deposited on the insulating layer (2) by plasma spraying and formed as an inner electrode layer (3). At this time, a masking was performed at a width of 20 mm from both ends in the width direction of the backing roll (1), so that the inner electrode layer (3) was not formed within this range. After removing the masks at both ends, an alumina-based ceramic material containing 10 wt% titanium oxide (TiO2) and the remaining 90 wt% alumina (Al2O3) was deposited as the outermost layer (4) with a thickness of 400 μm by plasma spraying on these ends and additionally on the upper surface of the inner electrode layer (3), that is, on the entire cylindrical surface of the backing roll (1). Likewise, the alumina-based ceramic material layer, together with the tungsten layer of the inner electrode layer (3), was treated with a hole sealing process using a low-viscosity epoxy resin, so that the volume resistivity was 5.6 × 10⁻⁶. 10 The outermost layer (4) of Ωcm was obtained.

[0107] After the hole sealing treatment, polishing was performed on the outermost layer (4) using a diamond grinding stone, so that the residual thickness was 300 μm and the surface roughness Ra after polishing was 0.05 μm. In addition, to enable electrical connection to the inner electrode layer (3), during the thermal spraying of the outermost layer (4), a rectangular masking of 20 mm × 30 mm was performed on one end of the roll, and after the outermost layer (4) was sprayed, the inner electrode layer (3) was exposed at the end of the roll with a width of 20 mm × 10 mm. A slip ring (6) was connected to the exposed inner 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 inner electrode (3). At this time, while the voltage was applied to the inner electrode, no current was felt by the human body even when touching the surface of the outermost layer (4) with a finger.

[0108] The web (7) is a polyethylene terephthalate film with a thickness of 100 μm and forms a gelatin underlayer with a thickness of 0.3 μm. The web (7) was previously heated in a furnace at a temperature of 90 to 100°C, and the charge was removed by alternately contacting the back side and the surface with a roll connected to earth until the surface potential was ±50V or less, and then cooled to 25°C.

[0109] The coating liquid (9) was applied with a curtain height of 10 cm, a coating angle of 30° forward from the apex of the roll, a web (7) tension of 150 N / m, and a web (7) conveying speed of 400 m / min, so that the thickness of the wet coating layer (11) was 60 μm. The coating liquid used was a 15% gelatin aqueous solution with 0.1% dodecyl benzene sodium sulfonate added, the low shear viscosity was adjusted to 100 mPa·s by a thickener, and the flow rate of the coating liquid was set to 4 cc / sec per 1 cm of coating width.

[0110] The back surface (12) of the web (7) was in close contact with the surface of the backing roll (1), and the movement of the web was stable. Coating could be continued without any coating failure caused by the air layer accompanying the surface of the web (7) being rolled under the coating liquid, and the uniformity of the thickness of the wet coating layer (11) was good, and the surface was smooth. No spark discharge occurred from the back roll to the surroundings, and no short-circuit current occurred penetrating the web (7) from the coating liquid (9) toward the roll surface.

[0111] (Comparative Example 1)

[0112] In the same configuration and conditions as in Example 1, when coating was performed with the voltage to the internal electrode (3) set to 0 (zero), the curtain of the coating liquid (9) alone could not exclude the accompanying air layer, and since the coating liquid did not wet the surface (12) of the web (7), uniform coating was not possible. In addition, because the web (7) was lifted from the backing roll, the traction force of the web (7) by the backing roll (1) decreased, and the tension and speed of the web (7) fluctuated, and the driving became unstable, so coating could not be continued.

[0113] (Example 2)

[0114] In a coating apparatus such as Example 1, a web (7) is conveyed to a backing roll (1) with a diameter of 200 mm, and the applied voltage to the internal electrode (3) is set to +3.5 kV. Additionally, as shown in FIG. 2, a tungsten wire electrode (14) with a diameter of φ0.5 mm is installed at a distance of 10 mm from the web, and a voltage of -3 kV is applied, and a negative charge (-) is attached to the first surface of the web (7) from the electrode (14), thereby performing electrostatic assist curtain coating according to the method of the modified example of the present invention.

[0115] The web (7) is a polyethylene coated paper with a thickness of 100 μm and forms a gelatin underlayer with a thickness of 0.6 μm. The web (7) was previously heated in a furnace at a temperature of 90 to 100°C, and the charge was removed by alternately contacting the back side and the surface with a roll connected to earth until the surface potential was ±50V or less, and then cooled to 25°C.

[0116] With a curtain height of 10 cm, an application angle of 30° forward from the apex of the roll, a web (7) tension of 150 N / m, and a web conveying speed of 400 m / min, the gelatin composition was applied to achieve a wet thickness of 35 μm. The coating liquid (9) used was a 12% gelatin aqueous solution with 0.1% dodecyl benzene sodium sulfonate added, the low shear viscosity was adjusted to 21 mPa·s by a thickener, and the flow rate of the coating liquid was set to 2.3 cc / sec per 1 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 run was stable. The coating could be sustained stably without causing coating failures due to the first surface (12) of the web (7) being rolled into the coating liquid of the air layer, and both the homogeneity of the thickness of the wet coating layer (11) and the smoothness of the surface were good. No spark discharge or glow discharge occurred from the back roll to the surroundings, and no short-circuit current occurred penetrating the web (7) from the coating liquid (9) toward the roll surface.

[0117] (Comparative Example 2)

[0118] In 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) could form a wet coating layer (11) that adhered to the entire surface of the first side (12) of the web. However, because an accompanying air layer intruded between the back side (13) of the web and the backing roll and the web (7) floated, the tension and speed of the web (7) became unstable and oscillated, and the movement became unstable, making it impossible to continuously form a wet coating layer (11) with a smooth surface. Explanation of the symbols

[0119] 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; Strip 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 side (surface) 13; Web 2nd page (back side) 14; Space auxiliary electrode(-) 15; DC high voltage power supply (for space auxiliary electrode) 16; Roll heartstrings 17; Free charge(+) 18; Dipole inside the web 19; Surface charge(-)

Claims

Claim 1 A method for applying a coating solution comprising a liquid composition by flowing it from a grounded applicator to the first surface of a continuously moving flexible plastic web having opposing first and second surfaces, comprising: a process of conveying the web along a path to a backing roll for coating, and passing the web through a coating point while supporting the second surface of the web by pressing it against a part of the surface of the rotating backing roll to which a DC voltage is applied using the electrostatic field of the backing roll; and a process of attracting the coating solution at the coating point and applying it to the first surface of the web by means of an electrostatic force generated by coordinating charges of the same polarity as the DC voltage applied to the backing roll to the first surface of the web, wherein the backing roll comprises an outermost layer to which the web is in contact, a conductive unipolar inner electrode layer adjacent to the inner side of the outermost layer, and an insulating layer adjacent to the inner side of the inner electrode layer, wherein the backing roll is configured to apply a predetermined voltage to the inner electrode layer, and wherein the outermost layer has a volume resistivity 10 at 25~100℃ 7 ~10 13 A coating method characterized by a ceramic material layer of Ωcm, wherein, when the surface of the backing roll is in a state where the predetermined voltage is applied to the inner electrode layer, the outermost layer is charged with a charge having the same sign as the voltage applied to the inner electrode layer, the second surface of the web in contact with the outermost layer is pressed against the outermost layer by the electrostatic force of the charge and rotated and transported, and the coating liquid is pulled and brought into contact with the first surface of the web being rotated and transported, and immersed in the first surface of the web. Claim 2 A coating method according to claim 1, wherein a charge having a polarity opposite to the DC voltage applied to the inner electrode layer is applied to a first surface opposite to the second surface of the web before the coating liquid comes into contact with the web being rotated and transported. Claim 3 A coating method according to claim 1, wherein the DC voltage applied to the internal electrode layer is 0.3 to 6.0 KV. Claim 4 A coating method according to claim 1, wherein the diameter of the backing roll is 100 mm or more. Claim 5 A coating method according to claim 1, wherein at least one of the insulating layer, the inner electrode layer, and the outermost layer is a material formed by a thermal spraying method or a material using a binder of either an inorganic or an organic type, and at least one of the insulating layer, the inner electrode layer, and the outermost layer of the backing roll is subjected to a hole sealing treatment. Claim 6 A 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-based, or magnesium-based ceramic containing a compound selected from titanium oxide, chromium oxide, silicon oxide, manganese oxide, nickel oxide, and iron oxide, or an aluminum-based ceramic containing 5 to 17 weight percent of titanium oxide. Claim 7 A coating method according to claim 1, wherein the outermost layer is made of a ceramic-based material, and the ceramic-based material comprises at least one selected from aluminum nitride-based, silicon carbide-based, and silicon nitride-based materials, and an organic or inorganic binder. Claim 8 A coating method according to claim 1, wherein the thickness of the outermost layer is 50 to 500 μm. Claim 9 A coating method according to claim 1, wherein the centerline average surface roughness Ra of the outermost layer of the backing roll is 0.01㎛ to 5㎛. Claim 10 A coating method according to claim 1, wherein the inner electrode layer is made of a conductive material comprising tungsten or molybdenum. Claim 11 A coating method according to claim 1, wherein the thickness of the internal electrode layer is 5 to 50 μm. Claim 12 A coating method according to claim 1, wherein the insulating layer comprises at least one high-insulating material selected from ceramic materials containing aluminum oxide, aluminum oxide-based, magnesium oxide-based, beryllium oxide-based, aluminum nitride-based, or silicon nitride containing 2 to 4 weight percent of titanium oxide, porcelain, and enamel. Claim 13 In claim 1, the volume resistivity of the insulating layer is 10 13 Application method with a thickness of Ωcm or more. Claim 14 A coating method according to claim 1, wherein the thickness of the insulating layer is 50 to 500 μm.

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