Chalkboard

Abstract

To enable high erasability of board writing while maintaining high visibility of board writing with chalk.SOLUTION: Provided is a chalk board having at least one layer of enamel on a metal substrate. The outermost surface enamel layer among the enamel layers satisfies at least one of the following conditions 1 and 2 and a condition 3. Condition 1: a developed area ratio (Sdr) of an interface of the surface enamel layer is 30% or less. Condition 2: an average principal curvature (Spc) of top points of the surface enamel layer is 4,000 μm-1 or less. Condition 3: a 75-degree specular gloss (Gs (75°)) of the surface enamel layer is 17% or more.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a chalkboard. [Background technology]

[0002] Blackboards (hereinafter also referred to as "chalkboards") having an enamel layer formed on their surfaces are known. Chalkboards have a surface on which chalk writing and characters written with chalk can be erased, and are also used as screens for projecting prepared materials using various projectors such as slides and overhead projectors. Enameled chalkboards that combine excellent blackboard and screen functions are widely available (see, for example, Patent Document 1).

[0003] Such chalkboards are widely used as display media in classes at educational facilities such as schools, and in assemblies and conferences at public facilities. For this reason, as specified in JIS S 6007, chalkboards have an appropriately rough surface to make it easy to write or draw on the board with chalk (hereinafter simply referred to as "chalkboard writing") and to leave clear marks on the lines, letters, figures, etc. written on the board with chalk (hereinafter simply referred to as "chalkboard writing"). [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Patent No. 4771880 Summary of the Invention [Problem to be solved by the invention]

[0005] However, when the outermost surface of a chalkboard has a surface roughness within a predetermined range, chalk powder accumulates in the recesses of the chalkboard, which reduces the ability of an eraser to erase writing on the board, leading to significant contamination of the chalkboard.

[0006] Therefore, the present invention has been made in consideration of the above-mentioned problems, and aims to provide a technology that enables high erasability of chalk-based writing on a board while maintaining high visibility of the writing on the board. [Means for solving the problem]

[0007] In order to solve the above problems, the chalkboard of the present invention is a chalkboard having at least one enamel layer on a metal substrate, and is characterized in that the outermost surface enamel layer of the enamel layers satisfies at least one of the following conditions 1 and 2, and condition 3: Condition 1: The developed area ratio (Sdr) of the interface of the surface enamel layer is 30% or less. Condition 2: The average principal curvature (Spc) of the peaks of the surface enamel layer is 4000 μm -1 The following is the result. Condition 3: The 75° specular gloss (Gs(75°)) of the surface enamel layer is 17% or more.

[0008] In the chalkboard according to one aspect of the present invention, it is preferable that the developed surface area ratio (Sdr) of the interface of the surface enamel layer is 20% or less.

[0009] In one embodiment of the chalkboard of the present invention, the average principal curvature (Spc) of the peaks of the surface enamel layer is 3000 μm. -1 It is preferable that:

[0010] In the chalkboard according to one aspect of the present invention, the 75° specular gloss (Gs(75°)) of the surface enamel layer is preferably 20% or more.

[0011] In the chalkboard according to one aspect of the present invention, it is preferable that the conditions 1, 2, and 3 are satisfied. [Effects of the Invention]

[0012] The present invention maintains high visibility of chalk writing on the board and also enables high erasability of the writing on the board. DETAILED DESCRIPTION OF THE INVENTION

[0013] The chalkboard according to the present invention is a dual-purpose blackboard that combines the functions of writing on a board and projecting an image from a projector. The chalkboard has at least one enamel layer on a metal substrate. The chalkboard is characterized in that the outermost surface enamel layer of the enamel layers satisfies at least one of the following conditions 1 to 3: Condition 1: The developed surface area ratio (Sdr) of the interface of the surface enamel layer is 30% or less. Condition 2: The average principal curvature (Spc) of the peaks of the surface enamel layer is 4000 μm -1 The following is the result. Condition 3: The 75-degree specular gloss (Gs(75°)) of the surface enamel layer is 17% or more. The chalkboard according to the present invention will be specifically described below.

[0014] The chalkboard according to the present invention is a blackboard having at least one enamel layer on a metal substrate. In the present invention, the type of metal substrate used is not particularly limited, but a cold-rolled steel sheet is preferred. The cold-rolled steel sheet used as the metal substrate is preferably Ni-plated to improve adhesion between the metal substrate and the glaze. Examples of cold-rolled steel sheets include low-carbon steel sheets and stainless steel sheets. The C content of the low-carbon steel sheet is preferably 0.0200% or less, more preferably 0.0100% or less, and even more preferably 0.0050% or less, by mass.

[0015] The metal substrate may be an aluminum-plated steel sheet or a Zn-Al alloy-plated steel sheet. The Zn-Al alloy-plated steel sheet may be, for example, a plated steel sheet having a plating layer containing 4 to 70 mass% Al, the remainder being Zn and unavoidable impurities. The plating layer may contain additives to improve the properties of the plating film.

[0016] Of the enamel layers formed on the metal substrate, the upper limit of the developed interface area ratio (Sdr) of the outermost surface enamel layer as defined in ISO 25178 is, for example, 30%, preferably 25%, more preferably 20%, and even more preferably 15%. Furthermore, the lower limit of the developed interface area ratio (Sdr) is, for example, 2%, preferably 5%, and even more preferably 10%. The developed interface area ratio (Sdr) is an index that indicates how much the actual interface area is larger than the area of ​​the metal substrate when viewed in plan.

[0017] The developed surface area ratio (Sdr) tends to be large when there are many small irregularities or when the slope of the irregularities is steep. By keeping the developed surface area ratio (Sdr) within the above upper and lower limits, it is possible to reduce the resistance of the chalk getting caught on the peaks of the chalkboard when writing, suppressing writing defects and improving the visibility of the writing, as well as suppressing the accumulation of chalk powder in the recesses when writing, thereby improving the erasability of the writing.

[0018] Of the enamel layers formed on the metal substrate, the upper limit of the average principal curvature of the peaks (Spc) defined in ISO 25178 in the outermost surface enamel layer is, for example, 4000 μm. -1 and preferably 3500 μm -1 and more preferably 3000 μm -1 Furthermore, the lower limit of the average principal curvature (Spc) of the peaks of the surface enamel layer is, for example, 500 μm -1 and preferably 1000 μm -1 and more preferably 1500 μm -1 The mean principal curvature (Spc) is an index that represents the curvature of the apex of a protrusion. The larger the mean principal curvature (Spc) value, the sharper the apex, and the smaller the value, the smoother the apex.

[0019] If the average principal curvature (Spc) of the peaks of the surface enamel layer is within the above upper and lower limits, the resistance of the chalk to the peaks of the chalkboard when writing on the board can be reduced, which reduces writing defects and improves the visibility of the writing on the board, as well as preventing excessive adhesion of chalk powder to the peaks when writing on the board, improving the erasability of the writing on the board.

[0020] The developed surface area ratio (Sdr) of the interface and the average principal curvature of the peaks (Spc) can be adjusted to an appropriate range by setting the weight of the dried residue (obtained by passing 100cc of the finishing enamel glaze described below through a 325-mesh sieve and drying the residue) to 0.5g to 2.5g, preferably 0.8g to 1.7g, and more preferably 1.0g to 1.5g, as a guide for particle size distribution. Furthermore, when calculating the average principal curvature (Spc), negative values ​​may be obtained depending on the software used, but the above ranges can be applied as absolute values.

[0021] Of the enamel layers formed on the metal substrate, the upper limit of the arithmetic mean height (Sa), which represents the average of the absolute values ​​of the differences in height at each point from the average plane of the surface, for the surface texture defined in ISO 25178 in the outermost surface enamel layer, is, for example, 3.000 μm, preferably 2.500 μm, and more preferably 2.350 μm. Furthermore, the lower limit of the arithmetic mean height (Sa) is, for example, 1.000 μm, preferably 1.250 μm, and more preferably 1.500 μm.

[0022] When the arithmetic mean height (Sa) of the surface enamel layer is within the range between the upper and lower limits, the dust-proof property is excellent.

[0023] Of the enamel layers formed on the metal substrate, the upper limit of the maximum height (Sz) of the outermost surface enamel layer as defined in ISO 25178 is, for example, 20,000, preferably 19,700, and more preferably 19,500. Furthermore, the lower limit of the maximum height (Sz) is, for example, 13,000, preferably 13,250, and more preferably 13,500.

[0024] Of the enamel layers formed on the metal substrate, the upper limit of the aspect ratio (Str) of the outermost surface enamel layer, which is defined in ISO 25178 as being related to the isotropy and anisotropy of the surface texture, is, for example, 0.700, preferably 0.650. Furthermore, the lower limit of the aspect ratio (Str) is, for example, 0.605, preferably 0.610.

[0025] Of the enamel layers formed on the metal substrate, the outermost surface enamel layer has a brightness V, as defined in JIS Z 8721-1993, of, for example, 3.0 to 7.0. The surface enamel layer is preferably an enamel layer of a dark color tone, such as black, gray, green, brown, or navy blue, and more preferably black, green, or brown. If the brightness V is below the above range, the image becomes too dark, reducing image recognition when projected. On the other hand, if the brightness V is above the above range, the image becomes too bright, reducing chalk character recognition.

[0026] The color tone and brightness of the surface enamel layer can be adjusted by the type and amount of pigment added. To adjust the brightness V to 3.0 to 7.0, it is preferable to add Co, Cr, Ti metals or their oxides alone or in combination as pigments. Furthermore, it is more preferable that the brightness V is 4.0 to 6.0.

[0027] In addition, on the chalkboard L * a * b * When displayed using a color system, brightness L * It is preferable that the brightness is, for example, 40 to 60. L * is 40 or more but less than 50.

[0028] Furthermore, in the chalkboard of the present invention, in order to further ensure the ease of writing on the board with chalk and the ease of erasing writing on the board, the surface characteristics of the surface enamel layer are adjusted so that the surface roughness Rz specified in JIS B 0601-2001 is, for example, 5 to 25 μm, preferably 10 to 22 μm, and more preferably 14 to 20 μm.

[0029] If the surface roughness Rz of the surface enamel layer falls outside the above range and approaches flatness, the chalk will slip, reducing the ease of writing on the board with chalk and the visibility of the writing on the board. On the other hand, if the surface roughness falls outside the above range and the unevenness becomes too large, the ease of erasing writing on the board with chalk and the visibility of the writing on the board will decrease. The above surface characteristics of the surface enamel layer can be achieved by adjusting the composition of the glaze applied to the metal substrate and the firing conditions after glaze application to an appropriate range, as described below.

[0030] In the chalkboard of the present invention, the surface enamel layer has the above-mentioned surface characteristics and color tone, maintains excellent erasability and visibility of writing on the board, and has reflective characteristics with a peak gain of, for example, 0.28 or more, and preferably a half gain of 13 or more.

[0031] If the peak gain is less than 0.28, the projected image will be dark and the visibility of the image will be poor, making it impossible to satisfy the requirements for the projection function of the chalkboard. Therefore, in the chalkboard of the present invention, the lower limit of the peak gain is set to, for example, 0.28. The peak gain is, for example, preferably 0.35 to 1.0, more preferably 0.35 to 0.60. In addition, to obtain a wide viewing angle, it is preferable that the half gain is, for example, 13 or more. Therefore, in the present invention, the surface enamel layer has reflective properties with a peak gain of, for example, 0.28 or more and a half gain of preferably 13 or more.

[0032] The term "peak gain" refers to the ratio of the reflective luminance of a blackboard measured under the same conditions to the reflective luminance of a standard whiteboard (perfectly diffusing board: a pure white board coated with barium sulfate) when light is shone on it. In this case, the reflective luminance at a viewing angle of 5° is usually used. In addition, "half gain" in this invention refers to the viewing angle width (degrees) when the reflective luminance in the reflection characteristic curve is half of its peak.

[0033] Of the enamel layers formed on the metal substrate, the lower limit of the 75° surface gloss (Gs(75°)) of the outermost surface enamel layer as defined in JIS Z 8471-1997 is, for example, 17%, preferably 17.5%, and more preferably 18%. Furthermore, the upper limit of the 75° surface gloss (Gs(75°)) is, for example, 25%, preferably 24%, and more preferably 23.5%.

[0034] If the 75-degree specular gloss (Gs(75°)) is less than 17%, the ability to erase chalk writing on the board will decrease, the reflective brightness will decrease, and the writing on the board and projected images will become unclear.On the other hand, if the 75-degree specular gloss (Gs(75°)) is more than 25%, the ability to write on the board will decrease, halation will occur more easily, and the viewing angle will become narrower.

[0035] As will be described later, the 75-degree specular gloss (Gs(75°)) can be achieved by adding an appropriate amount of titanium oxide-coated particles having an appropriate particle size distribution, preferably titanium oxide-coated mica particles, to the glaze to be applied to the metal substrate, and adjusting the firing temperature within an appropriate range.

[0036] The surface enamel layer is produced by blending a transparent matte glaze, which is a typical chalkboard glaze, with at least an appropriate amount of titanium oxide-coated particles, or with a titanium oxide glaze, and adjusting the firing conditions to an appropriate range. The surface enamel layer contains titanium oxide-coated particles, which improves the clarity of the chalkboard, and also forms unevenness on the surface, improving diffuse reflectivity, suppressing halation, and improving reflection characteristics.

[0037] In the present invention, the surface enamel layer preferably contains 2 to 40% by mass, more preferably 3 to 20% by mass, of titanium oxide-coated particles relative to the total amount of the surface enamel layer. If the titanium oxide-coated particles are contained in an amount less than 2% by mass, the effect of improving the reflectivity is small. If the titanium oxide-coated particles are contained in an amount greater than 40% by mass, the surface roughness Rz becomes excessively large, which deteriorates the erasability of chalk writing on the board, and the brightness becomes too high, reducing the visibility of chalk writing on the board.

[0038] Furthermore, the titanium oxide-coated particles preferably have an average particle size of, for example, 100 μm or less. More preferably, the titanium oxide-coated particles have a size of 5 to 80 μm. If the average particle size of the titanium oxide-coated particles is less than 5 μm, the diffuse reflectivity decreases. On the other hand, if the average particle size of the titanium oxide-coated particles exceeds 100 μm, the surface roughness of the surface enamel layer increases excessively, reducing the ability to erase chalk on the board. The average particle size of the particles referred to here is the 50% cumulative particle size measured using a laser diffraction particle size distribution analyzer. Furthermore, the titanium oxide-coated particles are preferably mica particles whose surfaces are coated with titanium oxide, and the mica particles are more preferably thin-plate mica particles.

[0039] The surface enamel layer having the above-mentioned properties is preferably formed using an enamel overglaze in which at least titanium oxide-coated particles, or a titanium oxide glaze, or a transparent gloss glaze are blended with a transparent matte glaze. From the viewpoint of peelability, the thickness of the surface enamel layer is preferably, for example, 200 μm or less, more preferably 150 μm or less.

[0040] The surface enamel layer formed using the above-mentioned enamel overglaze preferably contains, in mass %, 0.5 to 15% TiO2, more preferably 0.5 to less than 10%, and even more preferably 2 to 5% TiO2, where the total amount of titanium oxide, including titanium oxide from titanium oxide-coated particles, is TiO2, and further preferably contains 15 to 45% SiO2, 5 to 30% Al2O3, 2 to 20% of one or more selected from Na2O, K2O, and Li2O, and 2 to 20% B2O3. In the present invention, the surface enamel layer of the blackboard may contain up to 40% by mass of a pigment selected from black, gray, green, brown, yellow, and navy blue pigments to achieve the desired color.

[0041] Next, a preferred method for producing a chalkboard according to the present invention will be described. The chalkboard of the present invention is produced by applying an enamel underglaze to the surface of a metal substrate and firing it to form an enamel underlayer. After forming the enamel underlayer, or without forming the enamel underlayer, an enamel overglaze, which is a transparent matte glaze blended with at least titanium oxide-coated particles, or a titanium oxide glaze, or a transparent gloss glaze, is applied to the surface of the metal substrate, and fired at, for example, 400 to 850°C to form a surface enamel layer. The metal substrate is preferably, for example, a cold-rolled steel sheet, an aluminum-plated steel sheet, or a Zn-Al alloy-plated steel sheet.

[0042] For example, when a cold-rolled steel sheet is used as the metal substrate, the cold-rolled steel sheet is preferably Ni-plated, and then an enamel underglaze is applied to the surface of the cold-rolled steel sheet (metal substrate) and fired to form an enamel underlayer. The enamel underglaze is not particularly limited, and any of the enamel underglazes used in conventional enamel blackboards is suitable. The enamel underglaze can be applied by any known application method, such as spraying or roll coating.

[0043] The enamel underglaze is preferably an SiO2-based glaze containing, in mass % relative to the total solid content, 20 to 80% SiO2, with the remainder being 0 to 15% TiO2, 0 to 20% ZrO2, 0 to 25% B2O3, 0 to 25% Al2O3, 0 to 25% Na2O, 0 to 20% Li2O, 0 to 20% K2O, 0 to 40% PbO, 0 to 50% ZnO, 0 to 15% BaO, 0 to 15% CaO, 0 to 15% CaF2, 0 to 10% CoO, 0 to 20% NiO, 0 to 20% MnO, or the like.

[0044] An enamel overglaze is applied to the surface of the obtained base enamel layer, and then the surface enamel layer is formed by firing at 600 to 850° C. The enamel overglaze can be applied by any known application method such as spraying or roll coating.

[0045] The enamel glaze is preferably a glaze prepared by adding an appropriate amount of titanium oxide-coated particles to a transparent matte glaze (frit), or to a glaze prepared by adding 100 to 20 parts by weight of a titanium oxide glaze (frit) or 20 parts by weight or less of a transparent gloss glaze (frit) to 100 parts by weight of the transparent matte glaze (frit). The amount of titanium oxide-coated particles added is preferably 1 to 25 parts by weight per 100 parts by weight of the transparent matte glaze or the mixed glaze (slip) (calculated as the weight after firing).

[0046] The enamel overglaze is obtained as a viscous liquid by adding an appropriate amount of titanium oxide-coated particle powder to a slip obtained by grinding a mixture of a transparent matte glaze frit, or a transparent matte glaze, a titanium oxide glaze, or a transparent gloss glaze frit, as needed, and water, and then mixing the mixture while rotating and grinding.When 100 cc of the above enamel overglaze used for the surface enamel layer is passed through a 325 mesh sieve and dried, a dried residue weighing 0.5 to 1.5 g is obtained.

[0047] If the dried residue obtained from the enamel overglaze falls outside the above weight range, the materials used to prepare the enamel overglaze will not dissolve sufficiently, and surface properties such as the developed surface area ratio (Sdr) of the interface, the average principal curvature of the peaks (Spc), and the 75-degree specular gloss (Gs(75°)) will fall outside the ranges of the present invention. The weight range of the dried residue in the enamel overglaze is adjusted by the process time of rotary grinding.

[0048] The pigment must be black, gray, green, brown, yellow, or navy blue, either singly or in combination, depending on the desired color tone of the chalkboard. The amount of pigment added is preferably 20 parts by weight or less in total per 100 parts by weight of slip (weight after firing). If the amount added exceeds 20 parts by weight, the fluidity of the slip will decrease.

[0049] The transparent matte glaze (frit) preferably contains, in mass % relative to the total solid content, 30-45% SiO2, 20-35% Al2O3, 5-15% B2O2, 5-15% K2O, 10-25% Na2O, and the remainders include 0-15% BaO, 0-15% CaO, 0-10% F2, 0-10% TiO2, etc.

[0050] Titanium oxide glaze (frit) preferably contains, in mass % relative to the total solid content, 10-30% TiO2, with the remainder containing 0-80% SiO2, 0-20% Al2O3, 0-25% B2O3, 0-20% Na2O, 0-20% K2O, 0-20% Li2O, 0-10% P2O5, 0-20% ZrO2, 0-15% BaO, 0-15% CaO, 0-15% MgO, 0-5% PbO, 0-40% ZnO, 0-50% CaF2, etc. Note that instead of titanium oxide, which is the pigment in titanium oxide glaze, one or more pigments selected from zirconium oxide, antimony oxide, and zinc oxide may be used.

[0051] A preferred transparent gloss glaze (frit) contains, in mass % relative to the total solid content, 40-65% SiO2, with the remainder being 5-20% B2O3, 5-40% total of one or more of Na2O, K2O, and Li2O, 0-10% F2, 0-10% Al2O3, 0-10% CaO, 0-10% TiO2, 0-10% ZnO, and 0-5% MgO. Note that instead of titanium oxide, which is the pigment in titanium oxide glaze, one or more pigments selected from zirconium oxide, antimony oxide, and zinc oxide may be used.

[0052] If the firing temperature is less than 600°C, the surface becomes too rough and the reflectivity deteriorates, whereas if the firing temperature exceeds 850°C, the surface becomes too smooth and the visibility of chalk writing on the board, the ease of writing on the board with chalk, and the ease of erasing the chalk writing on the board deteriorate.

[0053] Furthermore, when an aluminum-plated steel sheet or a Zn-Al alloy-plated steel sheet is used as the metal substrate, it is preferable to apply an enamel overglaze to the surface of the metal substrate and then bake it at, for example, 400 to 600°C to form a surface enamel layer.

[0054] In this case, it is necessary to use an enamel overglaze with a low melting point to prevent the plating layer from melting. Therefore, instead of the transparent matte glaze described above, it is preferable to use an enamel overglaze that is made by blending titanium oxide-coated particles with a glaze containing phosphoric acid, or by further blending a titanium oxide glaze with the glaze. The phosphoric acid content in the glaze containing phosphoric acid is, for example, 40% by mass or more, preferably 80% by mass or less, and more preferably 70% by mass or less.

[0055] The glaze (frit) containing phosphoric acid preferably contains, in mass % relative to the total solid content, 40-80% P2O5, with the remainder being 0-40% SiO2, 0-50% Al2O3, 0-25% B2O3, 0-20% Na2O, 0-20% K2O, 0-20% Li2O, 0-30% TiO2, 0-25% Sb2O3, 0-20% ZnO, 0-15% BaO, 0-15% CaO, 0-5% MgO, 0-10% PbO, 0-20% SrO, etc.

[0056] When an aluminum-plated steel sheet or a Zn-Al alloy-plated steel sheet is used as the metal substrate, a low-melting-point enamel overglaze is used to prevent the plating layer from melting, improving adhesion and making it unnecessary to apply an underglaze. Furthermore, when an aluminum-plated steel sheet or a Zn-Al alloy-plated steel sheet is used as the metal substrate, if the firing temperature is less than 400°C, the adhesion of the enamel layer decreases. On the other hand, if the firing temperature exceeds 600°C, the plating layer will melt, which is a problem. [Example]

[0057] Samples (Examples 1 and 2) in which the developed surface area ratio (Sdr), average principal curvature (Spc), and 75-degree specular gloss (Gs(75°)) of the surface enamel layer were within the above upper and lower limits, respectively, and samples (Comparative Examples 1 to 3) in which the developed surface area ratio (Sdr), average principal curvature (Spc), and 75-degree specular gloss (Gs(75°)) of the surface enamel layer of each sample were prepared, and the relationship between the developed surface area ratio (Sdr), average principal curvature (Spc), and 75-degree specular gloss (Gs(75°)) of each sample and erasability was investigated. Examples 1 and 2 were produced using an enamel overglaze prepared so as to obtain a dried residue within the weight range described in the above embodiment.

[0058] (1) Developed area ratio (Sdr) The developed area ratio (Sdr) of each sample was determined in accordance with ISO 25178 using a laser microscope (VK-X300) equipped with a white light interferometer manufactured by Keyence Corporation.

[0059] (2) Mean principal curvature (Spc) The average principal curvature of the peaks (Spc) of each sample was determined in accordance with ISO 25178 using a laser microscope (VK-X300) equipped with a white light interferometer manufactured by Keyence Corporation.

[0060] (3) 75° surface gloss (Gs(75°)) The 75° surface gloss (Gs(75°)) of each sample was determined in accordance with JIS Z 8471-1997 using a portable gloss meter GMX203 manufactured by Murakami Color Research Laboratory.

[0061] The developed area ratio (Sdr), average principal curvature (Spc) and 75-degree specular gloss (Gs(75°)) of the surface enamel layer of each sample, as well as an evaluation of the erasability of each sample, are shown in Table 1 below.

[0062] In accordance with the JIS B 6007 standard, three rows of lines were drawn on each sample by lightly rubbing a piece of white, yellow, or red chalk across the surface of each sample. A 700g load was then applied to each chalkboard using a designated machine equipped with a felt eraser, and the chalk lines were rubbed crosswise three times to assess their erasability. After each stroke, the chalkboard was viewed from a distance of 1m from each chalkboard, and the erasability was quantified as follows: if the chalk lines on each sample were completely erased, a score of 3 was given; if the edge of the line was visible, a score of 2 was given; if the entire line was faintly visible, a score of 1 was given; and if the line was still visible, a score of 0 was given.

[0063] Samples with a total erasability score of over 20 were rated as "○", samples with a total score of 15 to 19 were rated as "△", and samples with a total score of 14 or less were rated as "×".

[0064] [Table 1]

[0065] As can be seen from Table 1, for Examples 1 and 2 of the present invention, the total erasability score was "24 points," exceeding "20 points," and the evaluation was "Good," proving that they are excellent in erasing chalk lines written on the board. In particular, for Examples 1 and 2, after one stroke, they each received a rating of "2 points" for all white, yellow, and red chalk, and after two strokes and beyond, they received a rating of "3 points" for all lines. In other words, for Examples 1 and 2, the chalk lines had completely disappeared by the second stroke.

[0066] In contrast, in Comparative Example 1, the evaluation after one stroke was "1 point" for all white, yellow, and red chalk, the evaluation after two strokes was "2 points" for all lines, and the evaluation after three strokes was "3 points" for all lines, for a total score of "18" and a rating of "△". The erasability up to two strokes was inferior to Examples 1 and 2, and it took three strokes for the board lines of each chalk to completely disappear.

[0067] In Comparative Example 2, the evaluation after one stroke was "0 points" for all white, yellow, and red chalk, after two strokes was "2 points" for all lines, and after three strokes was "2 points" for the white line and "3 points" for both the yellow and red lines, for a total score of "14 points" and an evaluation of "X." The erasability up to two strokes was inferior to Examples 1 and 2, and by the third stroke the yellow and red lines had disappeared, but a faint white line remained.

[0068] In Comparative Example 3, the evaluation after one stroke was "0 points" for all white, yellow, and red chalk, the evaluation after two strokes was "1 point" for all lines, and the evaluation after three strokes was "2 points" for all lines, for a total score of "9 points" and an evaluation of "X." The erasability up to two strokes was inferior to Examples 1 and 2, and even after the third stroke, all lines remained faintly, showing a significant deterioration in erasability compared to Examples 1 and 2.

[0069] As described above, Examples 1 and 2, whose developed area ratio (Sdr), average principal curvature (Spc) and gloss (Gs(75°)) were within the range of the present invention, were superior in terms of erasability of blackboard writing compared to Comparative Examples 1 to 3, which were outside the range of the present invention.

[0070] <Other> Although the preferred embodiments of the present invention have been described above, the present invention is not limited to the above embodiments and includes all aspects encompassed by the concept of the present invention and the scope of the claims. Furthermore, each configuration may be appropriately and selectively combined to achieve at least some of the above-described problems and effects. Furthermore, for example, the shape, material, arrangement, size, etc. of each component in the above embodiments may be appropriately modified depending on the specific use of the present invention.

[0071] For example, the chalkboard in the above embodiment is a chalkboard that can be used for both writing and projection, but it may also be a chalkboard that is used exclusively for writing, as long as the developed area ratio (Sdr), average principal curvature (Spc), and gloss (Gs (75°)) of the surface enamel layer are within the scope of the present invention.

[0072] In addition, the chalkboard may be an embodiment in which at least one of the developed area ratio (Sdr), average principal curvature (Spc), and gloss (Gs(75°)) of the surface enamel layer is within the scope of the present invention.

Claims

1. A chalkboard having at least one enamel layer on a metal substrate, A chalkboard characterized in that the outermost surface enamel layer of the enamel layers satisfies the following conditions 1, 2, and 3. Condition 1: The developed area ratio (Sdr) of the upper surface of the surface enamel layer is 2% or more and 30% or less. Condition 2: The average principal curvature (Spc) of the peaks of the surface enamel layer is 500 μm −1 or more and 4000 μm or less －１ The following is the result. Condition 3: The 75-degree specular gloss (Gs(75°)) of the surface enamel layer is 17% or more.

2. 2. The chalkboard according to claim 1, wherein the developed area ratio (Sdr) of the upper surface of the surface enamel layer is 20% or less.

3. The average principal curvature (Spc) of the peaks of the surface enamel layer is 3000 μm －１ 3. The chalkboard according to claim 1, wherein:

4. 4. The chalkboard according to claim 1, wherein the 75-degree specular gloss (Gs(75°)) of the surface enamel layer is 20% or more.

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