JP2005089789A - Electrode in liquid, and its production method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a compact electrode capable of performing treatment to a high-volume liquid at a high speed, and to provide a liquid treatment device and a liquid treatment method using the electrode. <P>SOLUTION: The electrode comprises: (1) an electrically conductive base material; (2) a coating layer coating the electrically conductive base material; and (3) electrically conductive diamond particles fixed to the coating layer, and in which a part of each electrically conductive diamond particle comes into contact with the electrically conductive material, and the other one part is exposed to the surface of the coating layer. By the method, the compact electrode capable of treating a high-volume liquid at a high speed can be obtained. Further, by using the liquid treatment device provided with the electrode, the compact treatment device of high performance can be provided. Further, by using the liquid treatment method in the invention, a high-volume liquid can be treated at a high speed. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to an electrode characterized in that conductive diamond particles are fixed in contact with a conductive substrate, a method for manufacturing the electrode, a liquid processing apparatus using the electrode, and a liquid processing method.

  Conductive diamond is deposited in the form of a film on the surface of a base material to form an electrode, and when this is energized, electrolysis occurs in the liquid and the liquid is altered. This is because when conductive diamond is used as an electrode, there is a property (wide potential window) that the lower limit voltage until electrolysis of water can be significantly higher than other electrodes.

  The characteristics of causing an electrochemical reaction in a liquid using conductive diamond as an electrode are the following five points: (1) a wide potential window, (2) a small background current, (3) physical, chemical (4) low electron mobility with respect to the redox system, and (5) selectivity of electrode reaction (see Non-Patent Documents 1 to 3).

Among these, (1) is due to the fact that this diamond is formed of so-called sp ³ carbon, and there are very few adsorption sites of surface chemical species. For this reason, the overvoltage generated when used as an electrode for water electrolysis is as high as 1.0 V for hydrogen and 1.2 V for oxygen, and the entire potential window is as high as 3.5 V. For example, Non-Patent Document 2 shows a wide potential window of a diamond electrode when compared with other electrode materials. That is, for example, the potential window is 1.6 to 2.2 V when a platinum electrode is used, and about 2.8 V when a glassy carbon electrode is used, whereas it is 3.2 when a diamond electrode is used. ~ 3.5V.

As for (2), since this diamond has characteristics that are much closer to semiconductors than ordinary conductive materials and has a structure with few surface functional groups, the electric double layer capacity on the surface is several μF / This is due to cm ^2, which is two orders of magnitude less than glassy carbon. This is because the background current density is influenced by the current density required to move carriers for forming the electric double layer to the electrode surface. As a result, since a high signal current / background current ratio can be obtained, an electrode using conductive diamond, for example, a highly sensitive sensor for redox substances, metals contained in aqueous solutions, and trace amounts of ecosystem substances. Can be a sensor.

  As for (5), when a diamond electrode is used, the oxidation and reduction of water is suppressed, but the solute oxidation-reduction reaction occurs very easily. Useful for quality. Patent Document 1 describes a waste water-soluble treatment method in which an anode containing conductive diamond is used to electrolyze a solution to oxidize a solute.

Furthermore, the specific resistance can be freely changed by changing the doping amount of boron (B) which is a P-type impurity contained in the diamond coating. By doping boron to the maximum allowable concentration of CVD diamond coating of about 10 ⁴ ppm, the specific resistance can be reduced to about 10 ⁴ μΩcm (see Non-Patent Documents 3 and 4). Non-Patent Document 2 discloses, as an example of the change in potential window when the boron doping amount is changed, in a 0.1 mol / L Na ₂ SO ₄ solution when the boron introduction amount is 10 ² ppm. It is shown that a particularly wide potential window of 5.0 V or more can be obtained.

  However, in order to perform electrolysis of large volumes of liquid and oxidation-reduction treatment and decomposition of substances in the liquid with high efficiency using conductive diamond as an electrode, a diamond surface with a large area and no defects is secured. There is a need. Usually, when producing a diamond-coated electrode, as shown in FIG. 1, a conductive substrate (metal or an insulator doped with impurities) is used as a base, and conductive diamond is deposited on at least a part of the surface of the substrate. Yes. A technique for applying a base material with diamond by, for example, hot filament CVD, microwave CVD, or the like has been established. Then, in order to impart conductivity to diamond, which is originally an insulator, a doping gas containing boron or the like is introduced into the CVD film formation atmosphere.

  However, it is very difficult to form a high soundness diamond coating without defects on a large-area substrate, and particularly when an electrode is used in a highly erodible liquid, the coating life is significantly reduced due to the presence of defects. In order to cope with this inconvenience, the electrode is formed by laminating a large number of segments with a small coating area, and the coating thickness is increased more than necessary, thereby suppressing damage to the coating caused by defects. Although it is considered to make the service life as long as possible, it is not an essential measure.

  Furthermore, as the biggest problem of electrochemical processing using a conductive diamond electrode, the electrode size is inevitably large, and there are many cases where it is confronted with difficulty in device design and manufacturing cost. This is because the reaction takes place in a region limited to the surface of the electrode, unlike the case where the reaction takes place in the whole area of the fluid as in a normal chemical plant. In other words, this is because it can be said that the fate of the electrochemical reaction.

The above problems will be described with specific examples. For example, consider a case where pulp waste liquid is decolorized by electrolysis with a diamond electrode. According to our laboratory experiment, as an extreme case, the amount of electricity required to completely decolorize 1 L of pulp waste liquid (black liquor) by diamond electrode treatment is 400 Ah (interelectrode voltage 7. 5V) is required. Therefore, considering the case of a plant that performs decoloring operation at a low processing speed of 1 L / min, and calculating the required power,
1 L / min × 400 Ah / L × 7.5 V = 3 kWh / min = 3 × 3,600 kWs / 60 s = 180 kW
It becomes a huge thing.
Along with the consumption of large electric power as described above, there is an adverse effect that the electrode area is apt to increase in size.

Now, as a necessary value of the electrode area, even if the current density on the electrode surface is set to the upper limit value = 40 mA / cm ² in soda industry,
180 × 10 ³ W / 7.5V ÷ (40.0 × 10 ⁻³ A / cm ² ) = 60 m ²
And requires a remarkably huge area. Although it is an extreme case of completely decolorizing the stock solution of black liquor, it can be said that it is very inconvenient and disadvantageous to require such a large area to process only 1 L of waste liquid per minute.
Japanese Patent Laid-Open No. 7-299467 Kensuke Honda, Kazo Yagi, Akira Fujishima, Catalysts, 41, 4 (1999) P.C. H.264 Kensuke Honda, Junya Shimizu, Meiden Times, 271, 2000, No. 2 (2000) P.I. (29) Notsu et al., Electrochemistry, 67, 4 (1999) Yagi et al., Surface Technology, 50, 6 (1999)

  An object of the present invention is to provide a small electrode capable of processing a large volume of liquid at high speed, a liquid processing apparatus using the electrode, and a liquid processing method.

  The present inventors can solve the above-mentioned problem by contacting and fixing conductive diamond particles to a conductive substrate, unlike the conventional method of depositing film-like diamond on a flat substrate. The present invention was completed.

That is, the present invention is an electrode comprising (1) a conductive substrate, (2) a coating layer covering the conductive substrate, and (3) conductive diamond particles fixed to the coating layer, The electrode is characterized in that a part of the conductive diamond particles is in contact with the conductive base material and the other part is exposed on the surface of the coating layer.
Moreover, this invention is a liquid processing apparatus provided with the said electrode.
Furthermore, the present invention is a liquid processing method characterized by using the above electrode.
The present invention also includes (1) a step of forming a coating layer on a conductive material, (2) a step of placing conductive diamond particles on the coating layer, and (3) contacting the conductive diamond particles with a conductive substrate. And (4) curing the coating layer and fixing the conductive diamond particles to the coating layer.
Furthermore, the present invention includes (1) a step of bringing conductive diamond particles into contact with a conductive substrate,
(2) A method for producing an electrode, comprising: a step of forming a coating layer on the surface of a conductive material; and (3) a step of curing the coating layer and fixing conductive diamond particles to the coating layer.

  According to the present invention, a small electrode capable of processing a large volume of liquid at high speed can be obtained. Furthermore, by using a liquid processing apparatus provided with the electrode of the present invention, a small and high-performance processing apparatus can be provided. Further, by using the liquid processing method of the present invention, a large volume of liquid can be processed at high speed.

  As described above, the electrode of the present invention is an electrode including (1) a conductive substrate, (2) a coating layer covering the conductive substrate, and (3) conductive diamond particles fixed to the coating layer. A part of each conductive diamond particle is in contact with the conductive base material, and the other part is exposed on the surface of the coating layer. FIG. 2 shows an example of the electrode of the present invention as a cross-sectional view.

  Here, the conductive substrate used in the present invention is not particularly limited as long as it has conductivity and can contact and fix the conductive diamond particles. However, a metal such as gold or titanium; Examples thereof include conductive organic polymer materials such as plastic and rubber; conductive inorganic materials such as conductive ceramics, glass, and carbon. Among these, metals are preferable from the viewpoint of assembly work and the like.

  As the conductive diamond particles used in the present invention, any conductive diamond can be used without particular limitation.

  The size of the conductive diamond particles used in the present invention is not particularly limited as long as it can be brought into contact with and fixed to the conductive base material, and examples thereof include those having an average particle diameter of about 1 to 900 μm. it can.

  Here, the general production method of diamond particles can be broadly divided into two methods: high-pressure synthesis method and low-pressure synthesis method (vapor phase growth method). Impurity element doping is performed to provide conductivity. A low-pressure synthesis method is preferred. That is, as an advantage of the low-pressure synthesis method, an impurity element can be easily doped in the produced diamond by blending a gas containing a desired impurity in the gas raw material. This makes it possible to impart conductivity to diamond, which is originally an insulator. Here, examples of the impurity element to be doped include boron, phosphorus, arsenic, antimony, and bismuth. For example, boron is particularly preferable when a P-type conductor is manufactured. The doping ratio is not particularly limited as long as it can impart conductivity to diamond. For example, in the case of boron, it is preferable to dope several thousand to 10,000 ppm with respect to diamond. The conductivity of diamond imparted with conductivity is appropriately determined according to the characteristics of the electrode used, and examples thereof include 7,000 to 20,000 S / m. In addition, there is also known an example in which conductive diamond particles are produced by an ultra-high pressure method, thereby producing an electrode for industrial electrolysis (for example, http://www.jst.go.jp/giten/announce/ (See result / res-11 / res_11 — 139.pdf).

  In the low-pressure synthesis method, a diamond film can be formed on a substrate using, for example, a mixed gas of methane and hydrogen as a raw material at a pressure of 20 to 50 Torr and a substrate temperature of 900 ° C. (for example, Naoto Otake, Yoshikawa (See Masanori, “Gas Phase Synthetic Diamond”, 1995 5.6, Ohmsha, p. 31). In the low-pressure synthesis method, it is generally considered that a synthesis of a diamond film and a reaction in which once generated diamond is etched by the action of hydrogen radicals to return to the original hydrocarbon state occur simultaneously. In the low-pressure synthesis method, the behavior of hydrogen plasma is the key point, and in order to effectively link this to the production of diamond, hot filament CVD in which a high-temperature filament of 2,000 ° C or higher is provided in the gas phase, and microwaves are applied. Two methods of microwave plasma CVD are industrially successful.

  And according to the low-pressure synthesis method, not only can a continuous film be formed on a substrate, but also particulate conductive diamond used in the present invention can be produced.

For example, it is possible to produce diamond particles (twin crystals) having a particle diameter of ˜100 μm by direct reaction by DC plasma CVD method (Michio Otsuka, Atsuhito Sawabe, “Diamond Thin Film” (1987.9, Industrial Books) P131, 132). By adding a doping gas such as diborane (B ₂ H ₆ ) to the source gas to be supplied, boron as an impurity element can be introduced into the generated diamond particles. And since the diamond particle can be grown greatly by lengthening the reaction time, the particle size of the diamond can be changed by adjusting the reaction time. Thus, conductive diamond particles having a desired particle diameter can be manufactured.

  The coating layer used in the present invention is not particularly limited as long as it can coat a conductive base material and fix conductive diamond particles. For example, the coating layer is composed of an organic polymer material and / or an inorganic material. Things can be mentioned. Examples of the organic polymer material include plastic and rubber. Plastics include thermoplastic resins such as polyethylene, polypropylene, polystyrene, vinyl chloride, vinylidene chloride, fluororesin, acrylic resin, polyvinyl acetate resin, polyamide resin, acetal resin, polycarbonate, polyphenylene oxide, polyester, polysulfone, and polyimide; phenol Thermosetting resins such as resins, urea resins, melamine resins, alkyd resins, unsaturated polyester resins, epoxy resins, silicon resins, polyurethane resins, etc. Among them, polyvinyl acetate resins used as adhesives And thermoplastic resins such as polyimide resins are preferred. As rubber, natural rubber, polyisoprene rubber, butadiene rubber, styrene-butadiene rubber, butyl rubber, ethylene-propylene rubber and other general-purpose rubbers; chloroprene rubber, acrylonitrile-butadiene rubber, hydrogenated nitrile rubber, chlorosulfonated polyethylene, Special rubbers such as epichlorohydrin rubber, chlorinated polyethylene, acrylic rubber, silicone rubber, fluororubber, polyether special rubber; styrene, olefin, vinyl chloride, urethane, ester, polyamide thermoplastic elastomers Further, a photocurable resin and an electron beam curable resin; and the like. In addition, examples of the inorganic material include ceramics, cement, and glass. These organic polymer materials and inorganic materials may be used alone or in combination of two or more. Further, it may be used as a composite material of an inorganic material and an organic polymer material. Moreover, as a coating layer, what coated metal, such as nickel, by plating may be used. The coating layer used in the present invention is preferably a material having a specific resistance larger than that of an insulating material or conductive diamond. When a metal is plated, the surface has a specific resistance higher than that of the insulating material or conductive diamond. It is preferable to coat with a large material.

  The thickness of the coating layer used in the present invention is appropriately determined according to the particle diameter of the conductive diamond particles to be fixed, as long as the conductive diamond particles can be fixed and exposed to the surface of the coating layer. Good.

  Next, the manufacturing method of the electrode of this invention is demonstrated. When the coating layer is a thermoplastic resin, a thermoplastic adhesive, a thermoplastic elastomer, or the like, the electrode of the present invention can be manufactured, for example, by a process as shown in FIG. That is, for example, a thermoplastic adhesive such as polyvinyl acetate is formed on a conductive substrate to a certain thickness by coating, lining, dipping, dripping, spin coating, spraying, doctor blade, baking, etc. From there, conductive diamond particles are placed in a desired interval, density and pattern. By raising the temperature of the coating layer to a predetermined temperature, the viscosity of the adhesive decreases, so that the diamond particles settle in the adhesive and come into contact with the surface of the conductive substrate. Thereafter, when the temperature of the coating layer is lowered, the adhesive is solidified to fix the conductive diamond particles to the coating layer, and to form a coating layer firmly bonded to the surface of the conductive substrate.

  Here, the adhesive directly adheres to the conductive diamond particles, and the coating layer surrounds and embeds the uneven particles (see FIG. 4). And as a result of the unevenness | corrugation of the surface of electroconductive diamond particle becoming a state which meshes with a coating layer (state similar to an anchor effect), electroconductive diamond particle is fixed in the state which contacted the electroconductive base material surface. Here, by utilizing the fact that the thickness of the coating layer after solidification can be adjusted by the application amount of the adhesive and the degree of temperature rise, as shown in FIG. Can be exposed to the outside. As a matter of course, the adhesive is not necessarily limited to the polyvinyl acetate type, and may be one that is liquid at room temperature, such as casting polyimide.

  When the coating layer is a thermosetting resin, for example, a thermosetting resin that is liquid at room temperature is formed on the conductive substrate in a certain thickness, and then conductive diamond particles are placed thereon. At this time, since the resin is in an uncured state, the conductive diamond particles come into contact with the conductive substrate as they are. Then, the conductive diamond particles can be fixed to the coating layer by raising the temperature of the coating layer and curing the resin.

  Furthermore, when the coating layer is a photo-curing resin or an electron beam-curing resin, the conductive diamond particles are placed after these resins are formed on the conductive base material to a certain thickness. At this time, since the resin is not cured, the conductive diamond particles come into contact with the conductive base material as it is. Then, the conductive diamond particles can be fixed to the coating layer by irradiating the coating layer with light or an electron beam to cure the resin.

  Moreover, as a manufacturing method of an electrode, after placing conductive diamond particles directly on a conductive substrate and bringing them into contact with each other, a coating layer is formed on the surface of the conductive substrate, and then the coating layer is cured. Also good.

  In addition, as another method for manufacturing the electrode of the present invention, for example, there is a method using a technique used for fixing abrasive grains on a dresser surface of a CMP (Chemical Mechanical Polishing) apparatus (Japanese Patent Laid-Open No. 2001-150328, JP, 2000-127046 A). That is, after pre-plating directly on the surface of the conductive substrate or with Ni or the like, the conductive diamond particles are temporarily placed, and then Ni plating is performed until the total thickness is 30 to 80% of the particle diameter of the diamond particles. The conductive diamond particles are fixed by forming a film by the above method. Then, the surface is coated with an insulating material, and finally the surface is lightly polished, whereby a part of the conductive diamond particles is exposed to obtain the electrode of the present invention. In addition, as a means for forming a coating layer for fixing conductive diamond particles, various industrial surface coating methods such as coating with an inorganic substance such as ceramic, cement, glass, etc. can be used as effective means. .

  In the electrode of the present invention, even if the individual conductive diamond particles are arranged without being in contact with each other, they are in contact with the conductive base material, so that they are electrically connected to the conductive base material. As a matter of course, even if the conductive diamond particles are in contact with each other, the electrical action does not change at all. For this reason, it becomes possible to operate as an in-liquid electrode as shown in FIG.

  The liquid treatment apparatus of the present invention can be obtained by replacing an electrode of a normal liquid treatment apparatus such as a wastewater treatment apparatus with the electrode of the present invention. And the usage method of the liquid processing apparatus of this invention can also be used by a normal method.

  In addition, the liquid treatment method of the present invention will be specifically described in Examples, but can be performed by passing a current through the electrode of the present invention immersed in the liquid to be treated.

  With the electrode of the present invention described above, it is possible to mitigate the adverse effect of enlarging the electrode area in various electrochemical treatment processes using conductive diamond as an electrode. This will be outlined using the following example.

As an example, consider fixing or coating conductive diamond on the surface of a rectangular electrode substrate. FIG. 5A shows an electrode of the present invention, in which spherical conductive diamond particles having a diameter d are arranged on a conductive substrate of a rectangular plate so as to contact each other. Further, the particles are fixed so that the portion buried in the coating layer of the adhesive and the exposed portion have the same volume, and the exposed portion has a hemispherical shape. The substantial liquid contact surface area per apparent unit surface area of the substrate is expressed by the following equation.

  On the other hand, FIG.5 (b) shows what coated the electroconductive base material of the same area as (a) with the electroconductive diamond film. Therefore, when the substantial liquid contact areas in FIGS. 5A and 5B are compared, the area in (a) has an area of π / 2≈1.6 times. That is, when comparing electrodes having the same current density, the electrode of the present invention can reduce the electrode size (area) to about 1 / 1.6 compared with the case of using a conventional flat film electrode. It will be.

  In addition, assuming the same current density with the same electrode size, the electrode of the present invention can input 1.6 times the electric power compared to the conventional method, so that the efficiency and speed of the target electrochemical treatment can be reduced. It can be improved by minutes.

  Here, instead of the hemispherical shape shown in FIG. 5, the shape of the particles is made to be a rough rugged surface and the exposed portion is relatively increased, so that the substantial liquid contact surface area can be further increased. .

  As described above, in general, liquid processing and reforming based on electrochemical reactions are limited to the vicinity of the electrodes in the liquid processing / modification process, so it is unavoidable to increase the size of the electrodes when performing large capacity and high speed processing. It is easy to invite.

  Therefore, the advantage of suppressing and mitigating the increase in size of the electrode by utilizing the method of the present invention is very large in practical use.

<Manufacture of electrodes>
Conductive diamond particles having an average particle diameter of 0.2 mm were obtained by the low pressure synthesis method under the following conditions.
That is, a DC plasma CVD method was used for reaction for 11 hours at a substrate temperature of 900 ° C. and a pressure of 195 Torr with a reaction gas containing 5% CH ₄ and 0.3% B ₂ H ₆ in hydrogen.

A titanium plate having a size of 100 mm × 300 mm was used as the conductive base material, and a modified epoxy resin was used as a coating layer on the titanium plate, which was applied at a thickness of 30 μm. The conductive diamond particles were placed on the conductive base material at a density of 25 particles / mm ² and heated at 80 ° C. for 1 hour to contact and fix the conductive diamond particles to the conductive base material. Thereafter, the temperature was lowered to room temperature over 2 hours to obtain an electrode.

<Production of liquid processing apparatus>
A liquid processing apparatus having the process flow shown in FIG. 6 was produced. Five electrodes manufactured in Example 1 were used.

<Waste water treatment>
Using the liquid treatment apparatus prepared in Example 2, the sample solution collected from the leach wastewater at the waste landfill was oxidized to confirm the organic matter decomposition / decolorization effect. The treatment was carried out continuously for 20 hours with the current density on the electrode surface being nominally constant at 50 mA / cm ² . An example of the results is shown in Table 1. For comparison, results of activated sludge treatment and coagulation sedimentation treatment, which are other conventional treatment methods, are also shown.
From Table 1, in the treatment with the electrode of the present invention, when viewed in terms of the removal rate with respect to the leachate wastewater (raw water), the chromaticity, COD, and BOD were 92, 74, and 88%, respectively, which satisfied the water quality target of the discharged water there were.

Further, as a comparative example, a treatment was performed using an electrode in which a flat film-like conductive diamond (deposited to a thickness of 20 μm) was formed on a substrate of the same size operated at the same current density and time. Table 2 shows a comparison under the same conditions as in Table 1.
As is clear from Table 2, the water removal rate for raw material according to Comparative Example 3 is 74, 51, and 57% for chromaticity, COD, and BOD, respectively, and 92, 74, and 88% for the present invention (in the same order). It is quite low, achieving only 80, 69 and 65% respectively. As described above, this is considered to be caused by the substantial difference between the wetted areas of the two electrodes, and it can be seen that the electrode of the present invention is clearly effective in practice.

FIG. 1 is a conceptual diagram showing conductive diamond coating on a substrate. FIG. 2 is a diagram showing an electrode of the present invention. FIG. 3 is a diagram showing an example of the manufacturing flow of the electrode of the present invention. FIG. 4 is a diagram showing an electrode of the present invention. FIG. 5 is a diagram showing a comparison between the electrode of the present invention and a diamond film electrode. FIG. 6 is a diagram showing a process flow of the liquid processing method of the present invention.

Claims (15)

(1) conductive substrate,
(2) a coating layer for coating the conductive substrate;
(3) conductive diamond particles fixed to the coating layer;
An electrode comprising: a portion of each conductive diamond particle in contact with the conductive substrate, and the other portion exposed to the surface of the coating layer. The electrode according to claim 1, wherein the covering layer is made of an insulating material. The electrode according to claim 1, wherein the coating layer is composed of an organic polymer material and / or an inorganic material. The electrode according to claim 3, wherein the organic polymer material is plastic and / or rubber. The electrode according to claim 3 or 4, wherein the inorganic material is at least one selected from the group consisting of ceramics, cement, and glass. The electrode according to claim 1, wherein the conductive diamond particles are produced by a low-pressure synthesis method. The electrode according to claim 1, wherein the electrode is a submerged electrode used in a liquid. A liquid processing apparatus comprising the electrode according to claim 1. A liquid processing method using the electrode according to claim 1. (1) forming a coating layer on the surface of the conductive material;
(2) a step of placing conductive diamond particles on the coating layer;
(3) a step of bringing the conductive diamond particles into contact with the conductive substrate;
(4) curing the coating layer and fixing the conductive diamond particles to the coating layer;
A method for producing an electrode, comprising: The covering layer is a thermoplastic resin and / or a thermoplastic elastomer, and step (3) is performed by increasing the temperature, and step (4) is performed by decreasing the temperature. Method. The method according to claim 10, wherein the coating layer is a thermosetting resin and step (4) is performed by increasing the temperature. (1) a step of bringing conductive diamond particles into contact with a conductive substrate;
(2) forming a coating layer on the surface of the conductive material;
(3) curing the coating layer and fixing the conductive diamond particles to the coating layer;
A method for producing an electrode, comprising: The method according to claim 13, wherein the coating layer is a thermoplastic resin and / or a thermoplastic elastomer, and step (3) is carried out by lowering the temperature. The method according to claim 13, wherein the coating layer is a thermosetting resin and step (3) is performed by increasing the temperature.

Images