KR20230135211A - Surface Treatment INCLUDING PERFLUOROPOLYETHER-BASED SILATRANE COMPOUND and METHOD FOR FORMING A COATING LAYER - Google Patents

Abstract

In a surface treating agent including a perfluoropolyether-based modified silatrane compound which can be applied to vacuum deposition by electron beam heating and a method of forming a coating layer using the same, the surface treating agent comprises a surface treating compound containing perfluoropolyether-based modified silatrane of chemical formula 1 and a solvent, and is applied to form a coating layer by a vacuum deposition method.

Description

Surface treatment agent containing a perfluoropolyether-based modified silatrane compound and method of forming a coating layer using the same {Surface Treatment INCLUDING PERFLUOROPOLYETHER-BASED SILATRANE COMPOUND and METHOD FOR FORMING A COATING LAYER }

It relates to a surface treatment agent containing a modified silatrane compound and a method of forming a coating layer using the same. Specifically, a surface treatment agent containing a perfluoropolyether-based modified silatrane compound applied to form a coating layer of a fluorine-based compound on the surface of a target material. and a method of forming a coating layer using the same.

In general, perfluoropolyether-containing compounds have physical properties such as water and oil repellency, chemical resistance, lubricity, release property, and stain resistance due to their very low surface energy. In particular, perfluoropolyether-containing compounds modified with trimethoxysilane have excellent adhesion to glass, metal, etc., and are widely used as mold release agents and to form protective coating films for optical materials. In general, coupling agents such as glycidoxypropyltrialkoxysilane are widely known as a means of establishing a tight bond between the surface of a material such as glass or metal and an organic compound. The silane coupling agent has radicals and moisture-reactive radicals that act with organic reactors in its own molecules, and some moisture-reactive radicals that are not combined with the functional groups of the substrate undergo a self-condensation reaction to form siloxane using moisture in the air as a catalyst. It is converted to form a hard coating layer.

Recently, perfluoropolyether-based modified trimethoxysilane compounds that utilize the low surface energy of perfluoropolyether and the adhesion ability of the silane coupling agent to various substrates have been used as a surface treatment agent with anti-fouling function. It is becoming.

In Republic of Korea Patent Registration No. 10-0959850, in manufacturing a perfluoropolyether-modified alkoxysilane compound capable of wet coating and vacuum deposition, a spacer such as polyalkyl ether is used between the perfluoropolyether functional group and the alkoxysilane functional group. A method of increasing durability against the external environment is proposed by imparting functional groups.

US Patent No. U5945555A introduces that mixing a small amount of a silatrane compound containing a vinyl or allyl functional group with a silicone resin capable of hydrosilylation and then heat-curing it at 120°C for 30 minutes improves adhesion to various materials. However, there are no records showing that silatrane compounds have excellent adhesion to substrates when used in a vacuum deposition cutting process. However, the compounds disclosed in the above-described technology have a limitation in that they do not sufficiently lower the coefficient of friction, which is a basic property required for anti-pollution coatings for display windows.

Patent Application No. 2021-0125972, the applicant's technology, discloses a surface treatment agent using perfluoropolyether-based modified silane as a surface treatment compound for preventing contamination. In the case of the patent, by coating the object to be treated with a surface treatment agent containing perfluoropolyether-based modified silane, which is the surface treatment compound described above, a coating layer with a low coefficient of friction and excellent physical properties such as wear resistance and chemical resistance can be formed. You can. The surface treatment process can be done by spraying with a solvent and then heat curing at 130°C or higher for more than 10 minutes, or by indirect heating using a heat resistor in a vacuum evaporator to evaporate and settle.

However, the above surface treatment compounds require process changes when applied to the vacuum deposition process. It cannot be applied to the method used in the existing vacuum deposition method of impregnating a ceramic carrier and then directly heating it with an electron beam. After impregnating a carrier with high thermal conductivity such as steel wool, it is placed on a heating boat, which is an electric resistance, and indirectly heated. It must be evaporated by heating. When directly irradiated with an electron beam, the thermal decomposition of the alkoxysilane functional group portion of the perfluoropolyether-based modified alkoxysilane compound is so severe that there is a problem in that the coating surface is severely contaminated with decomposition products.

1. Republic of Korea Patent No. 10-0959850 2. US registered patent US 5945555A

The present invention is intended to solve the above problems, and provides a surface treatment agent containing a perfluoropolyether-based silatrane compound applicable to electron beam direct heating vacuum deposition suitable for forming an anti-contamination coating film for display windows, etc. there is.

In addition, the present invention provides a method of forming a coating layer using a surface treatment agent containing a perfluoropolyether-based silatrane compound capable of vacuum deposition using the electron beam heating method.

The surface treatment agent according to an embodiment of the present invention includes a surface treatment compound of the following formula (1).

--[Formula 1]

(Formula 1 is a perfluoropolyether-modified silatrane (2,8,9-trioxaa-5-aza-1-silabicyclo[3,3,3]undecane) compound, and perfluoropolyether is an important functional group. The portion has a number average molecular weight of 500 to 4,000, Rf is a monovalent or divalent perfluoroalkyl group having 1 to 4 carbon atoms, the p/q ratio is in the range of 0.8 to 1.2, and m is 0 or 1 or 2, and n is 3 or 5.)

In one embodiment, the surface treatment agent includes the surface treatment compound of Formula 1 and a solvent soluble therein.

A method of forming a coating layer according to an embodiment of the present invention includes preparing a tablet for forming an anti-contamination coating layer by impregnating a porous tablet made of ceramic with a surface treatment agent containing a surface treatment compound of the following formula (1); and forming a coating layer by directly heating the tablet with an electron beam and depositing the vaporized surface treatment compound on the surface of the substrate.

--[Formula 1]

(Formula 1 is a compound with a perfluoropolyether-modified silatrane (2,8,9-trioxaa-5-aza-1-silabicyclo[3,3,3]undecane) structure, and contains perfluoropoly, which is an important functional group. The ether portion has a number average molecular weight of 500 to 4,000, Rf is a monovalent or divalent perfluoroalkyl group having 1 to 4 carbon atoms, the p/q ratio is in the range of 0.8 to 1.2, and m is 0, 1. or 2, and n is 3 or 5.)

As an example, the coating layer forming method is preferably performed immediately in the same vacuum evaporator after forming a ceramic multi-coating layer on a substrate by directly heating it with an electron beam.

As an example, the substrate may include a substrate formed with a ceramic multi-coating layer for non-reflection, a substrate formed with a multi-coating layer for expressing a specific color, and a substrate formed with a half-mirror coating layer for reflecting light. As an illustrative example, the substrate is preferably a display window or a touch screen panel.

According to the present invention, it is possible to manufacture a surface treatment agent for deposition to form an anti-contamination coating layer with a low coefficient of friction for the coating layer, excellent wear resistance, and excellent chemical resistance by vaporizing and depositing it by direct heating with an electron beam. Using the surface treatment agent of the present invention has the advantage of being able to immediately apply an anti-pollution coating in a batch process in the same vacuum evaporator after applying a ceramic multi-layer coating with anti-reflection properties to the object (substrate) to be treated.

Hereinafter, the surface treatment agent of the present invention and the method of forming a coating layer using the same will be described in more detail. Since the present invention can make various changes and take various forms, specific embodiments will be described in detail in the text. However, this is not intended to limit the present invention to a specific disclosed form, and should be understood to include all changes, equivalents, and substitutes included in the spirit and technical scope of the present invention.

In the present application, terms such as “comprise” or “have” are intended to designate the presence of features, components, steps, processes, compositions, or combinations thereof described in the specification, and are intended to indicate the presence of one or more other features or components, It should be understood that this does not exclude in advance the possibility of the presence or addition of steps, processes, compositions or combinations thereof.

Additionally, unless otherwise defined, all terms used herein, including technical or scientific terms, have the same meaning as generally understood by a person of ordinary skill in the technical field to which the present invention pertains. Terms defined in commonly used dictionaries should be interpreted as having a meaning consistent with the meaning in the context of the related technology, and unless explicitly defined in the present application, should not be interpreted in an ideal or excessively formal sense. No.

Surface treatment agent for forming a coating layer

The surface treatment agent according to an embodiment of the present invention includes a surface treatment compound of Formula 1 and a solvent.

-[Formula 1]

The surface treatment compound of Formula 1 is a perfluoropolyether-based modified silatrane (2,8,9-trioxaa-5-aza-1-silabicyclo[3,3,3]undecane) structure compound, which is an important functional group. The perfluoropolyether portion has a number average molecular weight of 500 to 4,000, Rf is a monovalent or divalent perfluoroalkyl group having 1 to 4 carbon atoms, the p/q ratio is in the range from 0.8 to 1.2, and m is 0, 1, or 2, and n is 3 or 5.

In the polymer constituting the fluoropolyether functional group of Formula 1 of the present invention, all hydrogen atoms of polyalkylene oxide are replaced with fluorine atoms. If the hydrogen atom of polyalkylene oxide is replaced with a fluorine atom, it can exhibit the function of preventing contamination, so there are no restrictions on the specific structure, such as the structure and composition ratio of monomers such as methylene oxide, ethylene oxide, and propylene oxide.

As an example, the polymer may have a silatran functional group at one end or a silatran functional group at both ends. However, when considering the friction factor of the coated surface, it is better for the composition ratio of ethylene oxide and methylene oxide to be similar.

As an example, the polymer preferably has a number average molecular weight of 500 g/mol to 4,000 g/mol. If the molecular weight is too less than 500 g/mol, there is a problem that the compound is discharged outside the vacuum evaporator rather than being deposited on the object to be treated during the vacuum deposition process. If the molecular weight is greater than 4,000 g/mol, the evaporation rate of the compound is slow and the compound is exposed to the electron beam. There is a problem that the rate of decomposition increases.

An appropriate spacer functional group of ethylene oxide or alkylene exists between the polymer and silatrane. This is necessary to connect the silatrane functional group to the perfluoropolyether polymer, but spacer groups of various structures are possible to improve the adhesion and tactile feel of the final surface treatment agent. An addition reaction of ethylene oxide is possible, so the addition of 1 or 2 moles provides improved tactile feel, or a structure of propylene or pentylene is possible. Propylene has an easy manufacturing process, and pentylene improves adhesion.

A feature of the present invention is that the p/q ratio is in the range between 0.8 and 1.2. If the p/q ratio is less than 0.8, excessive manufacturing costs occur due to instability during the production of perfluoropolyether, and if the ratio is greater than 1.2, there is a problem in that the wear resistance of the vacuum-deposited coating layer is reduced.

The surface treatment agent may be prepared by mixing a surface treatment compound and a solvent, or by mixing the surface treatment compound, a perfluoropolyether-based modified trialkoxysilane, and a solvent. The solvent includes at least one of the following solvents. namely, fluorine-modified aliphatic hydrocarbon solvents (perfluoroheptane, perfluorooctane and perfluorodecane), fluorine-modified aromatic hydrocarbon solvents (m-xylene hexafluoride and benzotrifluoride), and fluorine-modified ether solvents ( Methylperfluorobutylether and perfluoro(2-butyltetrahydrofuran)), fluorine-modified alkylamine solvents (perfluorotributylamine and perfluorotripentylamine), hydrocarbon solvents (petroleum benzine, mineral spirits, toluene) and xylene), and ketone solvents (acetone, methyl ethyl ketone, and methyl isobutyl ketone).

The surface treatment agent containing the above-described perfluoropolyether-based modified silatrane compound can form a coating layer with excellent anti-fouling properties and excellent wear resistance even on plastic-based substrates. In particular, when using the surface treatment agent of the present invention, a ceramic multi-layer coating (silica or alumina deposition) with anti-reflection properties is applied to the object to be treated (substrate), and then an anti-contamination coating layer using the surface treatment agent is immediately applied in the same vacuum evaporator. Since it can be carried out as a batch process, it has the advantage of simplifying the deposition and coating process and managing the process efficiently.

Method of forming a coating layer using a surface treatment agent for forming a coating layer

In order to form an anti-fouling coating layer according to the present invention, first, prepare a surface treatment agent containing a surface treatment compound of the following formula (1) and then impregnate it into a porous tablet made of ceramic to prepare a tablet for forming an anti-pollution coating layer.

The surface treatment agent can be prepared by mixing a perfluoropolyether-based modified silatrane compound represented by the following formula (1) and a solvent dissolving it.

-[Formula 1]

The formula 1 is a perfluoropolyether-modified silatrane (2,8,9-trioxaa-5-aza-1-silabicyclo[3,3,3]undecane) compound, and perfluoropolyether is an important functional group. The portion has a number average molecular weight of 500 to 4,000, Rf is a monovalent or divalent perfluoroalkyl group having 1 to 4 carbon atoms, the p/q ratio is in the range of 0.8 to 1.2, and m is 0 or 1 or 2, and n is 3 or 5. A detailed description of the surface treatment agent is omitted to avoid duplication since it has been described in detail above.

Next, the tablet is directly heated on the substrate with an electron beam to deposit the vaporized surface treatment compound on the surface of the substrate to form a coating layer.

The substrate may be a substrate on which a ceramic multi-coating layer is formed for anti-reflection, a multi-coating layer for expressing a specific color may be formed, or a substrate on which a half-mirror coating layer that reflects light may be formed. The silica or alumina deposition layer may be formed. It may be a formed substrate. As an example, the multi-coating layer may be a ceramic-based multi-coating layer formed by stacking SiO2 deposition, ZrO2 deposition, and SiO2 deposition in the order.

As an illustrative example, forming the coating layer includes directly heating the tablet impregnated with the surface treatment agent using an electron beam in a vacuum evaporator to evaporate the surface treatment compound contained in the surface treatment agent, and treating the evaporated surface. It includes depositing the compound on the surface of the object to be treated.

As an illustrative example, the substrate (object to be treated) is preferably a display window or a touch screen panel.

The above-described method of forming a coating layer simplifies the deposition coating process by applying a ceramic multi-layer coating with anti-reflection properties on the substrate and then immediately performing the anti-contamination coating layer using the surface treatment agent as a batch process in the same vacuum evaporator. It has the advantage of being able to manage efficiently.

In addition, the anti-pollution coating layer formed according to the present invention has excellent anti-pollution properties and is also excellent in durability.

Manufacturing Example 1

A compound having a perfluoropolyether-based modified trimethoxysilane structure of the following formula (2) is prepared. At this time, the important functional group in the compound of Formula 2 is the perfluoropolyether moiety, Rf is a monovalent perfluoroalkyl group having 1 to 4 carbon atoms, the molecular weight is about 1,000 g/mol, and the p/q ratio is 0.8. It ranges from 1.2 to 1.2, m is 0, and n is 5.

Rf-(OCF2CF2)p-(OCF2)q-CF2CH2O-(CH2CH2O)m-(CH2)n-Si(OCH3)3--[Formula 2]

Next, 50.00 g of the compound represented by Formula 2, 50.00 g of HFE-7200, a fluorine-based solvent, 7,50 g of Triethanolamine, and 50.00 g of methanol were added to a 250 ml three-necked flask equipped with a cooling pipe and a stirrer, and then stirred at room temperature for 30 minutes. Stir for a minute. Slowly increase the temperature and stir for 17 hours at a gentle reflux temperature of methanol, then add 50.00 g of FC-3283, a fluorine-based solvent, stir, and take the solution. Afterwards, the organic portion was extracted and removed with THF solvent, concentrated in vacuum, and perfluoropolyether-based modified silatrane (2,8,9-trioxaa-5-aza-1-silabicyclo[3) represented by the following formula (1): A compound with the structure ,3,3〕undecane) was obtained. In Formula 1, Rf is a monovalent perfluoroalkyl group, the molecular weight is about 1,000 g/mol, the p/q ratio is in the range of 0.8 to 1.2, m is 0, and n is 5.

-[Formula 1]

Production example 2

A compound having a perfluoropolyether-based modified trimethoxysilane structure of the following formula (2) is prepared. At this time, the important functional group in the compound of Formula 2 is the perfluoropolyether moiety, Rf is a monovalent perfluoroalkyl group having 1 to 4 carbon atoms, the molecular weight is about 1,000 g/mol, and the p/q ratio is 0.8. It ranges from to 1.2, m is 1, and n is 3.

Rf-(OCF2CF2)p-(OCF2)q-CF2CH2O-(CH2CH2O)m-(CH2)n-Si(OCH3)3--[Formula 2]

Add 50.00 g of the compound represented by Chemical Formula 2, 50.00 g of HFE-7200, a fluorine-based solvent, 7.50 g of Triethanolamine, and 50.00 g of methanol into a 250 ml three-necked flask equipped with a cooling pipe and a stirrer, and stir at room temperature for 30 minutes. . Slowly increase the temperature and stir for 17 hours at a gentle reflux temperature of methanol, then add 50.00 g of FC-3283, a fluorine-based solvent, stir, and take the solution. Afterwards, the organic portion was extracted and removed with THF solvent, concentrated in vacuum, and perfluoropolyether-based modified silatrane (2,8,9-trioxaa-5-aza-1-silabicyclo[3) represented by the following formula (1): A compound with the structure ,3,3〕undecane) was obtained. In Formula 1, Rf is a monovalent perfluoroalkyl group, the molecular weight is about 1,000 g/mol, the /q ratio is in the range from 0.8 to 1.2, m is 1, and n is 3.

-[Formula 1]

Example 1

Perfluoropolyether-modified silatrane prepared in Preparation Example 2 (represented by Chemical Formula 1, where Rf is a monovalent perfluoroalkyl group, molecular weight is about 1,000 g/mol, m is 1, and n is 3) Compound) was dissolved in a fluorine solvent (FC-3283) to make 20 parts by weight to prepare a surface treatment coating agent. A tablet for vacuum deposition was prepared by adding 0.4 g of the surface treatment coating agent to a tablet made of alumina and then volatilizing the solvent. After plasma treatment on an acrylic hard coating-coated acrylic sheet in a vacuum evaporator, ceramic-based multi-coating is performed in the order of SiO2 deposition, ZrO2 deposition, and SiO2 deposition, and then the vacuum deposition tablet is heated with an electron beam, An anti-pollution coating layer was formed to a thickness of about 0.2 nanometers.

Example 2

Perfluoropolyether-modified silatrane prepared in Preparation Example 2 (represented by Chemical Formula 1, where Rf is a monovalent perfluoroalkyl group, molecular weight is about 1,000 g/mol, m is 1, and n is 3) Compound) was dissolved in a fluorine solvent (FC-3283) to make 20 parts by weight to prepare a surface treatment coating agent. A tablet for vacuum deposition was prepared by adding 0.4 g of the surface treatment coating agent to a tablet made of alumina and then volatilizing the solvent. After plasma treatment on an acrylic hard coating-coated acrylic sheet in a vacuum evaporator, ceramic-based multi-coating is performed in the order of SiO2 deposition, ZrO2 deposition, and SiO2 deposition, and then the vacuum deposition tablet is heated with an electron beam, An anti-pollution coating layer was formed to a thickness of about 0.2 nanometers.

Comparative Example 1

20 weight of perfluoropolyether-modified trimethoxysilane (a compound represented by Chemical Formula 2 where Rf is a monovalent perfluoroalkyl group, molecular weight is about 1,000 g/mol, m is 0, and n is 5) It was dissolved in a fluorine solvent (FC-3283) to prepare a surface treatment coating agent. A tablet for vacuum deposition was prepared by adding 0.4 g of the surface treatment coating agent to a tablet made of alumina and then volatilizing the solvent. After plasma treatment on an acrylic hard coating-coated acrylic sheet in a vacuum evaporator, ceramic-based multi-coating is performed in the order of SiO2 deposition, ZrO2 deposition, and SiO2 deposition, and then the vacuum deposition tablet is heated with an electron beam, An anti-pollution coating layer was formed to a thickness of about 0.2 nanometers.

Comparative Example 2

20 weight of perfluoropolyether-modified trimethoxysilane (compound represented by Chemical Formula 2, where Rf is a monovalent perfluoroalkyl group, molecular weight is about 1,000 g/mol, m is 1, and n is 3) It was dissolved in a fluorine solvent (FC-3283) to prepare a surface treatment coating agent. A tablet for vacuum deposition was prepared by adding 0.4 g of the surface treatment coating agent to a tablet made of alumina and then volatilizing the solvent. After plasma treatment on an acrylic hard coating-coated acrylic sheet in a vacuum evaporator, ceramic multi-coating is performed in the order of SiO2 deposition, ZrO2 deposition, and SiO2 deposition, and then the vacuum deposition tablet is heated with an electron beam, An anti-pollution coating layer was formed to a thickness of about 0.2 nanometers.

The physical properties of the coating layer obtained in Examples 1 and 2 and Comparative Examples 1 and 2 described above were evaluated in the following manner.

Test method 1

Abrasion resistance test:

Measure the water contact angle before the abrasion test using a contact angle meter (CEO, Phoenix 150). Using an abrasion tester (Daesung Precision Co., Ltd.), a load of 500g is applied to a rubber eraser (diameter 6mm, hardness 81A), and the rubber eraser (diameter 6mm, hardness 81A) is subjected to 3,000 round trips at a speed of 40 times per minute with a round trip distance of 20mm. Afterwards, the water contact angle after the abrasion resistance test is measured. This can be said to be an abrasion test using a rubber eraser.

Test method 2

Chemical resistance test:

Measure the water contact angle before the chemical resistance test using a contact angle meter (CEO, Phoenix 150). Using an abrasion tester (Daesung Precision Co., Ltd.), a load of 500g was applied to a rubber eraser (diameter 6MM, hardness 81A), and ethanol of 99.5% purity was instilled at a rate of one drop per 20 seconds, with a reciprocating distance of 20mm and 40 times per minute. It makes 1,000 round trips at speed. After drying at room temperature for more than 4 hours, the water contact angle after the chemical resistance test was measured. This can be said to be a chemical resistance test using ethanol and a rubber eraser.

Referring to Table 1, looking at the amount of change in contact angle after the abrasion test, it can be seen that Examples 1 and 2 showed the same or relatively small amount of change in contact angle compared to Comparative Example 1 and Comparative Example 2. You can.

Looking at the amount of change in contact angle after the chemical resistance test, it can be seen that Examples 1 and 2 show a significantly smaller amount of change in contact angle compared to Comparative Example 1 and Comparative Example 2. Looking at the above results, the coated surface deposited using the perfluoropolyether-based modified silatrane surface treatment compound prepared in Preparation Example 1 and Preparation Example 2 was coated using a perfluoropolyether-based modified trimethoxysilane surface treatment agent. It can be confirmed that it exhibits a level of durability that is superior to that of the deposited coated surface.

In particular, it can be seen that chemical resistance is relatively excellent. Therefore, it can be seen that durability is improved when a trialkoxysilane compound is transformed into a silatrane compound.

The surface treatment agent according to the present invention can be applied to form the final protective coating of optical products such as display windows or ceramic-based anti-reflective coatings, and can be used as an anti-pollution coating method with excellent workability.

Claims (5)

A surface treatment agent comprising a perfluoropolyether-modified silatrane compound having a perfluoropolyether-modified silatrane structure of the following formula (1), and applicable to vacuum deposition using electron beam direct heating.
-[Formula 1]
(Formula 1 is a compound with a perfluoropolyether-modified silatrane (2,8,9-trioxaa-5-aza-1-silabicyclo[3,3,3]undecane) structure, and contains perfluoropoly, which is an important functional group. The ether portion has a number average molecular weight of 500 to 4,000, Rf is a monovalent or divalent perfluoroalkyl group having 1 to 4 carbon atoms, the p/q ratio is in the range of 0.8 to 1.2, and m is 0, 1. or 2, and n is 3 or 5.) The surface treatment agent according to claim 1, wherein the surface treatment agent contains the surface treatment compound of Formula 1 and a solvent soluble therein. Preparing a tablet for forming a coating layer by impregnating a porous tablet made of ceramic with a surface treatment agent containing a surface treatment compound of the following formula (1); and
A coating layer forming method comprising forming a coating layer by directly heating the tablet with an electron beam and depositing the vaporized surface treatment compound directly on the surface of the substrate.
--[Formula 1]
(Formula 1 is a compound with a perfluoropolyether-modified silatrane (2,8,9-trioxaa-5-aza-1-silabicyclo[3,3,3]undecane) structure, and contains perfluoropoly, which is an important functional group. The ether portion has a number average molecular weight of 500 to 4,000, Rf is a monovalent or divalent perfluoroalkyl group having 1 to 4 carbon atoms, the p/q ratio is in the range of 0.8 to 1.2, and m is 0, 1. or 2, and n is 3 or 5.) The method of claim 3, wherein the coating layer forming method is
A coating layer forming method characterized in that a ceramic multi-coating layer is formed on a substrate by direct heating with an electron beam and then immediately performed in the same vacuum evaporator. 4. The method of claim 3, wherein the substrate includes a substrate formed with a ceramic multi-coating layer for non-reflection, a substrate formed with a multi-coating layer for expressing a specific color, and a substrate formed with a half-mirror coating layer for reflecting light. method.