KR101803828B1 - Sheet for shielding electromagnetic wave with flexibility and preparation methods thereof - Google Patents

Abstract

Disclosed is a sheet for shielding an electromagnetic wave to shield electromagnetic waves that may cause harmful effects on a human body or malfunction of other electronic equipment and a manufacturing method thereof. To this end, provided is a sheet for shielding an electromagnetic wave which includes metal sheet including a sendust (Fe-Si-Al alloy) flake, and aluminum sheet provided on the lower surface of the metal sheet. According to the present invention, it is possible to provide an average shielding rate of 70 to 80 dB or more in the frequency band of 300 MHz to 1.5 GHz. It is possible to effectively shield electromagnetic waves even at a thin thickness of 100 m or less to lower the SAR of the human body. The sheet for shielding an electromagnetic wave according to the present invention is applicable to a plane or a curved surface requiring electromagnetic wave shielding.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electromagnetic wave shielding sheet having a bending property,

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electromagnetic wave shielding sheet for shielding electromagnetic waves generated in an electronic device and a method of manufacturing the same, and more particularly to an electromagnetic wave shielding sheet capable of shielding electromagnetic waves that may cause harmful effects on a human body or malfunction of other electronic equipment And a method for producing the same.

BACKGROUND ART Electromagnetic wave damping materials having an electromagnetic interference (EMI) shielding function are required due to miniaturization, high integration, data capacity, and high speed, which are led by mobile devices such as mobile phones, notebooks and iPads. Particularly, the entry of advanced foreign competitors into the domestic market is accelerating. Domestic production of materials for securing domestic market and technological competitiveness, development of alternative products, and response to harmonized environment are urgently required.

Here, the electromagnetic wave damping material is generally defined as a functional material that changes incident electromagnetic wave energy into heat energy. Further, the mechanism of the electromagnetic wave-damping material is fundamentally caused by the loss characteristics of the material, and classified largely into a conductive loss material, a dielectric loss material, a magnetic loss material and a composite material containing two or more of them depending on the material used. In addition, the performance of the electromagnetic wave attenuating material is evaluated by the matching frequency, the matching thickness, the reflection loss, and the bandwidth.

As the electromagnetic wave attenuating material, a magnetic metal powder or a non-magnetic powder is generally used. Representative magnetic metal powders include ferrite, permalloy, or sendust. Examples of the non-magnetic powder include metal powders such as copper (Cu), aluminum (Al), molybdenum (Mo) or tungsten (W), and ceramics such as AiN, BN, Si3N4, SiC, Al2O3 or BeO. In addition, the electromagnetic wave-damping material is often used as a flake-shaped flake rather than a sphere in order to realize a high permeability.

The magnetic metal powder will be described in more detail as follows.

The ferrite is a solid solution in which alloying elements or impurities are dissolved in iron of the body-centered cubic crystal stable at 900 DEG C or less. The ferrite mainly uses magnetic loss due to resonance phenomenon appearing in a frequency band of several MHz or more. And is most widely used in industry.

The permalloy is a trade name of an alloy having a high magnetic permeability and refers to a binary alloy of 78.5% nickel and 21.5% iron. Permyloy is a superior magnetic material that exhibits high magnetic permeability when subjected to proper heat treatment.

The sandust is a high permeability alloy having a composition of 5% aluminum, 10% silicon, and 85% iron. From the ternary composition of Al-Si-Fe, the magnetic anisotropy constant and the self- .

However, when electromagnetic waves radiated from various electronic devices are to be reduced by using the above-described materials, a reduction effect of a certain degree is exhibited in a high frequency band of 2 GHz or more. However, since the reflection attenuation of the materials is low, And the thickness is increased, so that there is a disadvantage that practical limitations are large.

For example, Japanese Patent Application Laid-Open No. 2000-114440 discloses a heat-dissipating sheet having a heat dissipation property and an electromagnetic wave attenuation property, wherein a soft magnetic substance such as rubber or resin is mixed with a spherical magnetic powder and a flat magnetic powder, Is added. However, the sheet exhibits some effect in the high frequency band of 2 GHz or more, but does not show the effect in the low frequency band of 1 GHz or less.

Japanese Patent Application Laid-Open No. 2002-299112 discloses a method of producing a sheet by dispersing a soft magnetic metal powder on a flake in a matrix material. However, since the above-mentioned method uses press molding, particularly when a hot press is used, cross-linking hardening proceeds and the orientation of the powder is hindered, so that the effect of improving the permeability can not be expected. In addition, the sheet does not exhibit the attenuation effect of electromagnetic waves in a low frequency band of 1 GHz or less.

As described above, conventionally used electromagnetic wave damping materials do not exhibit sufficient electromagnetic wave damping effect, or exhibit a certain degree of damping effect in the high frequency band, but have a problem of almost no damping effect in the low frequency band. Further, the electromagnetic wave-damping materials conventionally used are mainly made of a sheet, and then a separate pressure-sensitive adhesive layer is formed on the sheet to be attached to various electronic apparatuses. In this case, however, a step of applying the pressure- The manufacturing process becomes complicated, and the manufacturing cost increases.

Korean Patent No. 10-0863579 (published on October 15, 2008) Korean Patent Publication No. 10-2011-0095448 (published on August 25, 2011) Korean Patent No. 10-1311341 (Announcement of Sep. 25, 2013)

Accordingly, it is a first object of the present invention to provide an electromagnetic wave shielding sheet having a high permeability real number in an initial frequency band and a high electromagnetic wave shielding ability even in a thin thickness having a bending property in a band below 1.5 GHz frequency.

A second object of the present invention is to provide a method of manufacturing an electromagnetic wave shielding sheet in which a sandstroke flake contained in a sheet is arranged flat in the horizontal direction, and generation of bubbles and wrinkles is suppressed to obtain a homogeneous sheet have.

In order to accomplish the first object of the present invention, in one embodiment of the present invention, a metal sheet including a sandstock (Fe-Si-Al alloy) flake and an aluminum sheet provided on a lower surface of the metal sheet And an electromagnetic shielding sheet.

In order to achieve the second object of the present invention, there is provided a method for producing a slurry, comprising: a slurry producing step of mixing a sandstock flake with an acrylic resin and a solvent to produce a slurry; A metal sheet production step of producing a sheet, a cutting step of cutting the metal sheet to a specified standard, and an aluminum sheet on the lower surface of the cut metal sheet, and a lamination process of attaching the metal sheet and the aluminum sheet to each other through a lamination process And a method of manufacturing an electromagnetic wave shielding sheet comprising the steps of:

The electromagnetic wave shielding sheet according to the present invention can provide an average shielding ratio of 70 to 80 dB or more in the frequency band of 300 MHz to 1.5 GHz.

In addition, the electromagnetic wave shielding sheet according to the present invention effectively shields electromagnetic waves even at a thin thickness of 100 탆 or less, thereby lowering the SAR of the human body.

In addition, since the electromagnetic shielding sheet according to the present invention has a thin thickness to provide a bending property, it can be used in various products such as an appearance of an electric control box with many electromagnetic waves, a cellular phone, and a body attaching accessory.

1 is a cross-sectional view illustrating an electromagnetic wave shielding sheet according to an embodiment of the present invention.
2 is a cross-sectional view showing a configuration of an electromagnetic wave shielding sheet according to the present invention.
3 is a flow chart for explaining a method of manufacturing an electromagnetic wave shielding sheet according to the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, an electromagnetic wave shielding sheet (hereinafter abbreviated as "electromagnetic wave shielding sheet") having a bending property according to preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings.

1 is a cross-sectional view illustrating an electromagnetic wave shielding sheet according to an embodiment of the present invention.

1, an electromagnetic wave shielding sheet according to the present invention includes a metal sheet 10 including a sandstroke flake 12, and an aluminum sheet 20 attached to a lower surface of the metal sheet 10.

Hereinafter, each component will be described in more detail with reference to the drawings.

Referring to FIG. 1, an electromagnetic wave shielding sheet according to the present invention includes a metal sheet 10.

The metal sheet 10 is a sheet including a sandstop flake therein, and comprises a sandstock flake and an acrylic resin.

Specifically, the metal sheet 10 may be composed of 86.2 to 91.4 wt% sandstorm flakes and 8.6 to 13.8 wt% of acrylic resin.

At this time, the sandstroke flakes are included in the metal sheet so as to be arranged flat in the horizontal direction as shown in FIG. In addition, the sandstroke flakes 12 are formed to have a flake shape having a particle diameter of 10 to 30 mu m and a thickness of 1 to 3 mu m.

If the particle size of the sander -stay flakes 12 is less than 30 mu m, the meaning of the flake shape is lost, and a suitable frequency band that can be absorbed becomes higher than 3 GHz and absorption of the low frequency band becomes difficult, Loses.

Since the acrylic resin serves as a binder, the acrylic resin melts the resin in the heat applied during the lamination operation and is useful for sheet composite, and the aluminum sheet 20 is bonded to the metal sheet 10 It becomes easy to attach to one side surface. As such an acrylic resin, it is preferable to use a copolymer of a (meth) acrylic acid ester monomer having an alkyl group having 1 to 12 carbon atoms and a polar monomer copolymerizable with the monomer.

Specific examples of the (meth) acrylate monomer include butyl (meth) acrylate, hexyl (meth) acrylate, n-octyl (meth) acrylate, isooctyl (meth) acrylate, ) Acrylate, and isononyl (meth) acrylate may be used, but the present invention is not limited thereto.

The polar monomer acts to improve adhesion by imparting a high cohesive force. The polar monomer is not particularly limited as long as it can exhibit such a function. Examples thereof include a carboxyl group-containing monomer such as (meth) acrylic acid, maleic acid or fumaric acid, Nitrogen-containing monomers such as pyrrolidone or N-vinylcaprolactam may be used alone or in admixture of two or more.

The ratio of the (meth) acrylic acid ester monomer to the polar monomer can be appropriately selected within a range that can achieve the desired effect and is not particularly limited. However, when the polar monomer is 1 To 20 parts by weight.

Further, the metal sheet 10 may further comprise a dispersing agent. Such a dispersant is added so that the sandstock flakes 12 and the acrylic resin can be uniformly dispersed. Any dispersant may be used as long as it is used to constitute the slurry. In particular, a non-polar dispersant is preferably used. Specifically, for example, as the dispersing agent, an alkyl ether, a sorbitan ester, an alkyl polyether amine, a polymer, or the like may be used alone or two or more of them may be used in combination. At this time, the injecting agent may include 5 to 10 parts by weight based on 100 parts by weight of the acrylic resin.

The metal sheet 10 is so formed that a plurality of sandwich flakes 12 are horizontally arranged and a metal sheet 10 is formed under an applied pressure of 140 to 160 MPa until a density of 2.6 to 2.8 kg / Is preferably used. At this time, if the applied pressure is less than 140 MPa, bubbles or wrinkles may be generated in the metal sheet 10, and the sandstroke flakes 12 may not be arranged horizontally. In addition, if the applied pressure exceeds 160 MPa, there may occur a problem that the sandstroke flakes 12 in the metal sheet 10 are broken.

Further, the metal sheet 10 is formed to have a thickness of 15 to 25 mu m. If necessary, the electromagnetic shielding sheet may be formed by laminating a plurality of metal sheets 10. In other words, the metal sheet 10 is basically formed to have a thickness of 15 to 25 mu m, and may be formed to have a thickness of 30 to 90 mu m by stacking various thicknesses according to the purpose of application to the user. At this time, if the total thickness of the metal sheet 10 exceeds 90 占 퐉, the bending property of the electromagnetic wave shielding sheet may be lowered, which may cause partial breakage or the like.

Referring to FIG. 1, the electromagnetic shielding sheet according to the present invention includes an aluminum sheet 20.

The aluminum sheet 20 is attached to the lower surface of the metal sheet 10 in a composite manner in order to utilize the properties of the dielectric material in the concept of magnetization, and is preferably formed to a thickness of 15 to 25 탆, but is not limited thereto . At this time, the aluminum sheet, which is a paramagnetic material, is not magnetized, and the electric wave constituting the electromagnetic wave can be efficiently attenuated, so that the electromagnetic wave attenuating effect is improved to a high frequency.

Referring to FIG. 1, the electromagnetic shielding sheet according to the present invention may further include a protective tape 30 and a double-sided tape 40.

The protection tape 30 is provided on the upper surface of the metal sheet 10 to maintain the commercial property of the electromagnetic shielding sheet and protects the metal sheet 10 from damage by external force . As the protective tape 30, a PET film or the like is used, and the electromagnetic wave shielding sheet is removed after attaching the electromagnetic wave shielding sheet to a product requiring shielding of electromagnetic waves.

The double-sided tape 40 is provided on the lower surface of the aluminum sheet 20 and provides an adhesive force so that the electromagnetic shielding sheet can be fixed smoothly to a product to be attached.

3 is a flow chart for explaining a method of manufacturing an electromagnetic wave shielding sheet according to the present invention.

Referring to FIG. 3, the method for producing an electromagnetic wave shielding sheet according to the present invention includes a slurry producing step (SlOO) for producing a slurry including a sandstock flake 12, a metal producing step A step S300 of cutting the metal sheet 10 to a specified size and a lamination step S400 of attaching the aluminum sheet 20 to the lower surface of the cut metal sheet 10 .

First, a method of manufacturing an electromagnetic wave shielding sheet according to the present invention includes a slurry generating step (S100).

The slurry producing step S100 is a step of producing a slurry by mixing the sandstock flakes 12 with an acrylic resin and a solvent and adjusting the content of the acrylic resin and the solvent mixed in the mixture to a viscosity of 5,000 to 10,000 cP. < / RTI > At this time, if the viscosity is less than 5,000 cP, the sheet is too thin to form the sheet. If the viscosity exceeds 10,000 cP, the sheet may crack on the surface during the drying process.

Since the acrylic resin serves as a binder, the acrylic resin melts the resin in the heat applied during the lamination operation, so that it is useful for compounding the sheet, and the aluminum sheet 20 is easily attached to one side of the metal sheet 10 .

The solvent is added to the mixture to control the viscosity of the slurry together with the acrylic resin, and any of water, alcohols, acetone, ethylacetate, n-methylpyrrolidone and butyl cellosolve may be used.

Such a slurry is produced by mixing 75 to 85 wt% of a sandstorm flake, 8 to 12 wt% of an acrylic resin, and 7 to 14 wt% of a solvent.

If the solvent is used in an amount less than 7% by weight, smooth dispersion and synthesis of the acrylic resin and the sand-struck flakes are not achieved, and the flatness of the surface is reduced during the production of the metal sheet 10, There arises a problem in structure formation. If the amount of the solvent is more than 14% by weight, the drying of the metal sheet 10 becomes difficult and the drying time is increased.

If necessary, the method for producing an electromagnetic wave shielding sheet according to the present invention may further comprise a sandstroke flaking step (S50) before the slurry producing step (S100).

The sandstroke flake forming step S50 is a step of fabricating the sander -stay flakes 12 using a sandstead (Fe-Si-Al alloy) powder, and includes a bead forming process and a flake producing process.

The bead forming process is a process of melting a sandersk powder and dispersing it to produce spherical sandstead beads.

In the flake generation process, an external force is applied to the sandstead beads produced through the bead production process to produce sandstiff flakes having a particle size of 10 to 30 탆 and a flake shape having a thickness of 1 to 3 탆.

The sandstrue is generated by vortexing due to the skin depth of the electromagnetic wave and the permeability is lowered. Therefore, the sandstrue is formed into a flaky shape.

Next, a method of manufacturing an electromagnetic wave shielding sheet according to the present invention includes a metal sheet production step (S200).

The metal sheet production step (S200) is a step of producing a metal sheet (10) having a thickness of 15 to 25 mu m with the slurry produced through the slurry production step (S100), and includes a pressing process and a drying process.

The sandstroke flakes 12 included in this slurry are irregularly arranged in the slurry state as shown in Fig. 2, but are regularly arranged inside the metal sheet 10 through a special manufacturing process. For example, the sandstroke flakes 12 may be arranged vertically within the metal sheet 10 through a special manufacturing process. However, this vertical arrangement of the sander -stay flakes 12 causes a problem that electromagnetic waves can not pass through between the sandstroke flakes 12 and the sander -stay flakes 12 to shield the electromagnetic waves.

Thus, in this step S200, a plurality of sandstroke flakes 12 are horizontally arranged inside the metal sheet 10. [ Here, the horizontal arrangement means a horizontal arrangement with reference to the upper and lower surfaces of the metal sheet 10. In other words, it is preferable that the plurality of sandstroke flakes 12 are formed to be vertically arranged with respect to the thickness direction of the metal sheet 10.

Specifically, in the pressing process, a vertical pressure is applied to the prepared slurry by a pressing device such as a press to produce a sheet having a thickness of 15 to 25 탆. At this time, the pressurizing device pressurizes the slurry at a pressure of 140 to 160 MPa.

Particularly, when the thickness of the metal sheet 10 is as large as 50 to 60 탆 or more, the drying speed of the metal sheet 10 is different from that of the surface of the metal sheet 10, thereby causing cracking of the metal sheet 10. In addition, when the metal sheet 10 is formed to a thickness of 26 to 50 탆, it becomes difficult to produce electromagnetic shielding sheets of various thicknesses when a plurality of metal sheets 10 are laminated. Thus, the metal sheet 10 is produced with a thickness of 15 to 25 탆 so as to prevent cracking of the metal sheet 10 and to be applied to various thicknesses. For example, when two sheets of metal sheets 10 having a thickness of 20 占 퐉 are stacked and then passed through a lamination step, the total thickness of the metal sheet 10 is 30 占 퐉.

In the drying process, the metal sheet 10 having passed the pressing process is dried at a temperature of 130 to 160 ° C. for 1 to 30 minutes to volatilize the solvent present in the metal sheet 10.

The pressing process and the drying process may be repeatedly performed several times, preferably one to five times.

Next, a manufacturing method of an electromagnetic wave shielding sheet according to the present invention includes a cutting step (S300).

The cutting step S300 is a step of cutting the metal sheet 10 produced through the metal sheet producing step S200 through a tape casting method to produce a metal sheet 10 having a specified standard. Here, the tape casting method is a method of forming a sheet on a moving blade or moving carrying film with a predetermined thickness according to a desired bar.

Finally, the manufacturing method of the electromagnetic wave shielding sheet according to the present invention includes a lamination step (S400).

 The lamination step S400 includes an aluminum sheet 20 on the lower surface of the metal sheet 10 manufactured through the metal sheet production step S300 and the metal sheet 10 and the aluminum sheet 20 The acrylic resin contained in the metal sheet 10 is dissolved in the heat applied during the lamination operation to serve as an adhesive for adhering the aluminum sheet to the metal sheet 10.

Such a lamination process can be conducted at a temperature of 150 to 170 DEG C and a pressure of 140 to 160 MPa.

When a general adhesive is used for the metal sheet 10 to attach the aluminum sheet 20, the adhesive is not uniformly applied to the metal sheet 10, which causes non-uniformity of the thickness of the electromagnetic shielding sheet. Furthermore, since the fixation for applying the adhesive is added, the manufacturing cost of the electromagnetic wave shielding sheet also increases.

A step of attaching a protective tape 30 to the upper surface of the metal sheet 10 and attaching the double-sided tape 40 to the lower surface of the aluminum sheet 20 after the lamination step S400 S500) may be further included.

In order to maintain the marketability of the electromagnetic shielding sheet, the protective tape 30 is attached to the upper surface of the metal sheet 10 so that the electromagnetic shielding sheet can be fixed to the product to be attached smoothly The double-sided tape 40 is attached to the lower surface of the sheet.

Hereinafter, specific examples and experimental examples of the present invention will be described in more detail. It should be understood, however, that the embodiments and examples are for the purpose of promoting understanding of the specific examples of the invention described above, and the scope of rights and the like should not be construed thereby.

Example

[Example 1]

1. 75 to 85% by weight of a sandstreaflex [FM-76H, Sanyo, Japan], 8 to 12% by weight of an acrylic resin [SEN3100, TTT Chemical Korea, Korea] and 7 to 14% by weight of toluene as a solvent were mixed, For 20 minutes to prepare a slurry.

2. The prepared slurry was passed through a press machine (Hot Press, Dongwon Hydraulics, Korea) applying a pressure of 160 MPa and then dried at a temperature of 150 DEG C for 10 minutes to produce a metal sheet 10 having a thickness of 15 mu m.

3. The metal sheet was cut through a tape caster.

4. Two cut metal sheets of 15 탆 in thickness were laminated, a 20 탆 thick aluminum sheet was brought into contact with the bottom of the metal sheet, and a metal sheet and an aluminum sheet were laminated through a lamination process to form a 50 탆 thick electromagnetic wave A shielding sheet was prepared.

[Example 2]

Instead of laminating two metal sheets having a thickness of 15 占 퐉, two metal sheets having a thickness of 25 占 퐉 were laminated to prepare an electromagnetic wave shielding sheet having a thickness of 70 占 퐉.

[Example 3]

Instead of laminating two metal sheets having a thickness of 15 탆, two metal sheets having a thickness of 25 탆 and one metal sheet having a thickness of 20 탆 were laminated to prepare an electromagnetic wave shielding sheet having a thickness of 90 탆 Respectively.

[Example 4]

Instead of stacking two metal sheets having a thickness of 15 탆, three metal sheets having a thickness of 25 탆 and one metal sheet having a thickness of 15 탆 were laminated to prepare an electromagnetic wave shielding sheet having a thickness of 110 탆 Respectively.

[Comparative Example]

An electromagnetic wave shielding sheet was produced in the same manner as in Example 1 except that an aluminum sheet was not attached.

Experimental Example

The shielding properties of the electromagnetic wave shielding sheets manufactured according to Examples 1 to 4 and Comparative Examples were evaluated. At this time, the equipment was measured using a network analyzer [E5071C, Agilent Technologies, Korea] and a test jig [EM-2107A, Electro-metrics, USA], and the results are summarized in Table 1 below.

According to the experimental results, it was confirmed that the shielding rate was 50 to 110 탆 for the sheet thickness (including 20 탆 of the aluminum sheet), and the shielding ratio was 70 to 80 dB or more. Especially, it was found that the performance is good from 300 MHz to 1.5 GHz.

It will be apparent to those skilled in the art that various modifications and variations can be made in the present invention without departing from the spirit and scope of the invention as defined in the appended claims. It can be understood that it is possible.

10: metal sheet 12: sander stripe
20: aluminum sheet 30: protective tape
40: Double-sided tape

Claims (10)

A metal sheet comprising 86.2 to 91.4% by weight of Sandhurst (Fe-Si-Al alloy) flakes and 8.6 to 13.8% by weight of acrylic resin and having a density of 2.6 to 2.8 kg / m3; And
And an aluminum sheet provided on a lower surface of the metal sheet,
Wherein the sandstroke flakes are contained in the metal sheet so as to be arranged flat in the horizontal direction and are formed to have a flake shape having a particle diameter of 10 to 30 mu m and a thickness of 1 to 3 mu m,
Wherein the acrylic resin is a copolymer of 100 parts by weight of an acrylate ester monomer having an alkyl group of 1 to 12 carbon atoms and 100 parts by weight of an acrylic ester monomer, Caprolactam, or a mixture thereof, and 1 to 20 parts by weight of a polar monomer composed of a mixture thereof. delete delete The metal sheet according to claim 1, wherein the metal sheet
Wherein the electromagnetic shielding sheet is formed by laminating a plurality of metal sheets having a thickness of 15 to 25 占 퐉. delete Mixing a sandstock flake formed to have a flake shape having a particle size of 10 to 30 mu m and a thickness of 1 to 3 mu m, an acrylic resin and a solvent to produce a slurry having a viscosity of 5,000 to 10,000 cP;
The sand-struck flakes are arranged flat in the horizontal direction and molded at a temperature of 130 to 160 DEG C and an application pressure of 140 to 160 MPa until the density becomes 2.6 to 2.8 kg / m < 3 > so as to obtain a homogeneous coating, A metal sheet production step of producing a metal sheet having a thickness of 15 to 25 탆;
A cutting step of cutting the metal sheet through a tape casting method; And
And a lamination step of laminating the metal sheet and the aluminum sheet to each other by melting an acrylic resin contained in the metal sheet at a temperature of 150 to 170 DEG C through a lamination process,
Wherein the acrylic resin is a copolymer of 100 parts by weight of an acrylate ester monomer having an alkyl group of 1 to 12 carbon atoms and 100 parts by weight of an acrylic ester monomer, Caprolactam, or a mixture thereof, and 1 to 20 parts by weight of a polar monomer composed of a mixture thereof. delete 7. The method according to claim 6,
Wherein the slurry is produced by mixing 75 to 85% by weight of sandstone spray flakes, 8 to 12% by weight of an acrylic resin, and 7 to 14% by weight of a solvent. delete 7. The method of claim 6, wherein after the lamination step
Further comprising a step of attaching a protective tape to the upper surface of the metal sheet and attaching a double-sided tape to the lower surface of the aluminum sheet.

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