JP7734061B2 - Chalkboard - Google Patents

Description

The present invention relates to a chalkboard.

Blackboards (hereinafter also referred to as "chalkboards") with an enamel layer formed on their surface are known. Chalkboards have a surface on which chalk writing and chalk-written letters can be erased, and they can also be 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).

Such chalkboards are widely used as display media in classes at schools and other educational facilities, as well as in assemblies and meetings 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 ensure that lines, letters, figures, etc. written on the board with chalk (hereinafter simply referred to as "chalkboard writing") remain clearly visible.

Patent No. 4771880

However, because the outermost surface of a chalkboard has a surface roughness within a certain range, chalk powder accumulates in the recesses of the chalkboard. When chalk powder accumulates in the recesses, the ability of an eraser to erase writing on the board decreases, leading to significant contamination of the chalkboard.

The present invention was conceived in consideration of the above-mentioned problems, and aims to provide technology that enables chalk writing on a blackboard to be easily erased while maintaining high visibility.

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 or less.
Condition 3: The 75-degree specular gloss (Gs(75°)) of the surface enamel layer is 17% or more.

In a 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.

In the chalkboard according to one aspect of the present invention, it is preferable that the average principal curvature (Spc) of the peaks of the surface enamel layer is 3000 μm ⁻¹ or less.

In one aspect of the chalkboard of the present invention, it is preferable that the 75-degree specular gloss (Gs(75°)) of the surface enamel layer be 20% or more.

It is preferable that the chalkboard according to one aspect of the present invention satisfies conditions 1, 2, and 3.

This invention maintains high visibility of chalk writing on the board while also enabling high erasability of the writing.

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 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 or less.
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.

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, by mass, more preferably 0.0100% or less, and even more preferably 0.0050% or less.

The metal substrate may be an aluminum-plated steel sheet or a Zn-Al alloy-plated steel sheet. An example of a Zn-Al alloy-plated steel sheet is a plated steel sheet having a plating layer with an Al content of 4 to 70 mass%, the remainder being Zn and unavoidable impurities. The plating layer may also contain additives to improve the properties of the plating film.

Of the enamel layers formed on the metal substrate, the upper limit of the developed interface area ratio (Sdr) specified in ISO 25178 for the outermost surface enamel layer 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.

The developed surface area ratio (Sdr) tends to be larger 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, the resistance of the chalk to getting caught on the peaks of the chalkboard when writing can be reduced, preventing poor writing and improving the visibility of the writing, while also preventing chalk powder from accumulating in the recesses when writing, improving the erasability of the writing.

Of the enamel layers formed on the metal substrate, the upper limit of the mean principal curvature of the peaks (Spc) specified in ISO 25178 for the outermost surface enamel layer is, for example, 4000 μm ⁻¹ , preferably 3500 μm ⁻¹ , and more preferably 3000 μm ⁻¹ . Furthermore, the lower limit of the mean principal curvature of the peaks (Spc) of the surface enamel layer is, for example, 500 μm ⁻¹ , preferably 1000 μm ⁻¹ , and more preferably 1500 μm ⁻¹ . The mean principal curvature (Spc) is an index representing the curvature of the peaks of the protrusions. The larger the value of the mean principal curvature (Spc), the sharper the peaks, and the smaller the value, the smoother the peaks.

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 can be reduced. This reduces writing defects and improves the visibility of writing, while also preventing excessive chalk powder from adhering to the peaks when writing, improving the erasability of writing.

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 (100 cc of the finishing enamel overglaze described below, passed through a 325 MESH sieve and dried) to 0.5 g to 2.5 g, preferably 0.8 g to 1.7 g, and more preferably 1.0 g to 1.5 g, as a guideline 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.

Of the enamel layers formed on the above-mentioned 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 relative to the average plane of the surface, for the outermost surface enamel layer, regarding the surface texture as defined in ISO 25178, 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.

If the arithmetic mean height (Sa) of the surface enamel layer is within the above upper and lower limits, the dust-proofing properties will be excellent.

Of the enamel layers formed on the metal substrate, the upper limit of the maximum height (Sz) of the outermost surface enamel layer as specified 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.

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 specified in ISO 25178 as relating 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.

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 falls below the above range, the image will be too dark, reducing the recognizability of the projected image. On the other hand, if the brightness V falls above the above range, the image will be too bright, reducing the recognizability of the chalk characters.

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, or Ti metals or their oxides, either alone or in combination, as pigments. A brightness V of 4.0 to 6.0 is even more preferable.

When displayed on a chalkboard in accordance with the L ^* a ^* b ^* color system, the lightness L ^* is preferably, for example, 40 to 60. More preferably, the lightness L ^* is 40 or more and less than 50.

In addition, in the chalkboard of the present invention, to further ensure the ease of writing on the board with chalk and erasing writing, 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.

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 with chalk and the visibility of the writing on the board. On the other hand, if the surface falls outside the above range and becomes too uneven, the ease of erasing writing with chalk and the visibility of the writing on the board will decrease. The above-mentioned 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 the appropriate range, as described below.

The chalkboard of the present invention has a surface enamel layer with the above-described surface characteristics and color tone, maintaining excellent erasability and visibility of writing on the board, while also exhibiting reflective characteristics with a peak gain of, for example, 0.28 or more, and preferably a half gain of 13 or more.

If the peak gain is less than 0.28, the projected image will be dark and the visibility of the image will be poor, failing to meet the requirements for the chalkboard's projection function. For this reason, 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 preferably, for example, 0.35 to 1.0, and more preferably 0.35 to 0.60. Furthermore, to obtain a wide viewing angle, it is preferable to set the half gain to, for example, 13 or more. For this reason, the present invention uses a surface enamel layer with reflective properties that has, for example, a peak gain of 0.28 or more and a half gain of preferably 13 or more.

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 (a perfect diffuser: a pure whiteboard 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 on the reflection characteristic curve is half of its peak.

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

If the 75-degree specular gloss (Gs(75°)) is less than 17%, the ability to erase chalk writing on the board will decrease, the reflected 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 with chalk will decrease, halation will occur more easily, and the viewing angle will become narrower.

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

The surface enamel layer is produced by blending a transparent matte glaze, a typical chalkboard glaze, with at least an appropriate amount of titanium oxide-coated particles, or possibly even 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 creates an uneven surface, improving diffuse reflectivity, suppressing halation, and improving reflective properties.

In the present invention, the surface enamel layer preferably contains 2 to 40% by mass, and 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 less than 2% by mass, the effect of improving reflectivity is minimal. If the titanium oxide-coated particles are contained in an amount exceeding 40% by mass, the surface roughness Rz becomes excessively large, which reduces the ability to erase chalk writing on the board, and the brightness becomes too high, reducing the visibility of chalk writing on the board.

Furthermore, it is preferable that the titanium oxide-coated particles 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. Note that the average particle size here refers to the 50% cumulative particle size measured using a laser diffraction particle size analyzer. Furthermore, it is preferable that the titanium oxide-coated particles are mica particles whose surfaces are coated with titanium oxide, and it is more preferable that the mica particles are thin-plate mica particles.

A surface enamel layer having the above-mentioned properties is preferably formed using an enamel top glaze that combines a transparent matte glaze with at least titanium oxide-coated particles, or with a titanium oxide glaze, or with a transparent gloss glaze. From the standpoint of peelability, the thickness of the surface enamel layer is preferably, for example, 200 μm or less, and more preferably 150 μm or less.

The surface enamel layer formed using the above-mentioned enamel overglaze preferably contains, by 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 of the titanium oxide-coated particles, is expressed as _TiO2 , and further preferably contains 15 to 45% _SiO2 , 5 to 30% _Al2O3 , 2 to 20% _total 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 in order to obtain a desired color.

Next, a preferred method for manufacturing the chalkboard of the present invention will be described. The chalkboard of the present invention is manufactured 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 composed of 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.

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. There are no particular restrictions on the enamel underglaze, and any of the enamel underglazes used in conventional enamel blackboards are suitable. The enamel underglaze can be applied using any known application method, such as spraying or roll coating.

As an enamel underglaze, it is particularly preferable to use a _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 20% PbO, 0 to 40%, ZnO, 0 to 50%, BaO, 0 to 15%, CaO, 0 to 15%, _CaF2 , 0 to 10%, CoO, 0 to ₂₀ %, NiO, 0 to 20%, MnO, etc.

An enamel overglaze is applied to the surface of the resulting base enamel layer, and then fired at 600-850°C to form a surface enamel layer. The enamel overglaze can be applied using any known application method, such as spraying or roll coating.

The enamel top glaze is preferably a glaze made by adding an appropriate amount of titanium oxide-coated particles to a transparent matte glaze (frit), or to a glaze made by adding 100 to 20 parts by weight of titanium oxide glaze (frit) or up to 20 parts by weight of transparent gloss glaze (frit) to 100 parts by weight of transparent matte glaze (frit). Furthermore, the amount of titanium oxide-coated particles added is preferably 1 to 25 parts by weight per 100 parts by weight (converted to weight after firing) of the transparent matte glaze or mixed glaze (slip).

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 transparent matte glaze frit, or transparent matte glaze, titanium oxide glaze, or transparent gloss glaze frit, as needed, and water, and then mixing while rotating and grinding. When 100 cc of the above-mentioned enamel overglaze used for the surface enamel layer is sieved through a 325 MESH sieve and dried, a dried residue weighing 0.5 to 1.5 g is obtained.

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 of the interface (Sdr), 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.

The pigment must be black, gray, green, brown, yellow, or navy blue, either alone or in combination, depending on the desired color tone of the chalkboard. The amount of pigment added should preferably be 20 parts by weight or less in total per 100 parts by weight of slip (weight after firing). Adding more than 20 parts by weight will reduce the fluidity of the slip.

It is preferable that the transparent matte glaze (frit) contains, in mass % relative to the total solid content, 30-45% SiO ₂ , 20-35% Al ₂ O ₃ , 5-15% B ₂ O ₂ , 5-15% K ₂ O, 10-25% Na ₂ O, with the remainder being 0-15% BaO, 0-15% CaO, 0-10% F ₂ , 0-10% TiO ₂ , etc.

The titanium oxide glaze (frit) preferably contains, in mass % relative to the total solid content, 10 to 30% _TiO2 , with the remainder being _SiO2 : 0 to 80%, _{Al2O3 :} 0 to ₂₀ %, _B2O3 : 0 to 25%, _Na2O : 0 to 20%, _K2O : 0 to 20%, _Li2O : 0 to 20%, _P2O5 : 0 to 10%, _ZrO2 : 0 to 20%, _BaO : 0 to 15%, CaO: 0 to 15%, MgO: 0 to 5%, PbO: 0 to 40%, ZnO: 0 to 50%, _CaF2 : 0 to 10%, etc. Instead of titanium oxide, which is the pigment of the titanium oxide glaze, one or more pigments selected from the group consisting of zirconium oxide, antimony oxide, and zinc oxide may be used.

A preferred transparent gloss glaze (frit) is one that contains, in mass % relative to the total solid content, 40 to 65% SiO ₂ , with the remainder containing 5 to 20% B ₂ O ₃ , 5 to 40% in total of one or more of Na ₂ O, K _{2 O} , and Li ₂ O, 0 to 10% F 2 , 0 to 10% Al ₂ O ₃ , 0 to 10% CaO, 0 to 10% TiO ₂ , 0 to 10% ZnO, 0 to 10% MgO, 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.

If the firing temperature is below 600°C, the surface will become too rough and the reflectivity will decrease. On the other hand, if the firing temperature exceeds 850°C, the surface will become too smooth, reducing the visibility of chalk writing on the board, the ease of writing on the board with chalk, and the ease of erasing writing on the board.

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.

In this case, it is necessary to use an enamel overglaze with a low melting point to prevent the plating layer from melting. For this reason, 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 phosphoric acid-blended glaze is, for example, 40% by mass or more, preferably 80% by mass or less, and more preferably 70% by mass or less.

A 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, 0-15% MgO, 0-5% PbO, 0-10% SrO, etc.

When using an aluminum-plated steel sheet or a Zn-Al alloy-plated steel sheet 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 using an aluminum-plated steel sheet or a Zn-Al alloy-plated steel sheet as the metal substrate, if the firing temperature is below 400°C, the adhesion of the enamel layer will decrease. On the other hand, if the firing temperature exceeds 600°C, the plating layer will melt, which can be a problem.

Samples (Examples 1 and 2) were prepared 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, and samples (Comparative Examples 1 to 3) were prepared 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 outside these limits. The relationship between the erasability and the developed surface area ratio (Sdr), average principal curvature (Spc), and 75-degree specular gloss (Gs(75°)) of each chalkboard sample was investigated. Examples 1 and 2 were manufactured using an enamel overglaze prepared to obtain a dried residue within the weight range described in the above embodiment.

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

(2) Mean principal curvature (Spc)
The average principal curvature (Spc) of the peaks 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.

(3) 75° surface gloss (Gs(75°))
The 75-degree 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.

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.

In accordance with JIS B 6007, 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 with a horizontal stroke. A 700g load was then applied using a specified 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 position 1m away from each chalkboard, and erasability was quantified as follows: if the chalk line for each sample had completely disappeared, 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.

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

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 at erasing chalk lines written on the blackboard. 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.

In contrast, in Comparative Example 1, after one stroke, the evaluation was "1 point" for all white, yellow, and red chalk lines, after two strokes the evaluation was "2 points" for all lines, and after three strokes the evaluation 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 each chalk line to completely disappear.

In addition, in Comparative Example 2, after one stroke, the evaluation was "0 points" for all white, yellow, and red chalk, after two strokes the evaluation was "2 points" for all lines, and after three strokes the evaluation 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 a rating 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.

In addition, in Comparative Example 3, after one stroke, the evaluation was "0 points" for all white, yellow, and red chalk, after two strokes the evaluation was "1 point" for all lines, and after three strokes the evaluation was "2 points" for all lines, for a total score of "9 points" and a rating of "X." The erasability up to two strokes was inferior to Examples 1 and 2, and even after the third stroke all lines were faintly remaining, showing a significantly inferior erasability to Examples 1 and 2.

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

<Others>
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.

For example, while the chalkboard in the above embodiment is a chalkboard that can be used for both writing and projection, 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.

Furthermore, in the case of a chalkboard, at least one of the developed area ratio (Sdr), average principal curvature (Spc), and gloss (Gs (75°)) of the surface enamel layer may be within the scope of the present invention.

Claims (4)

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 ⁻¹ or less.
Condition 3: The 75-degree specular gloss (Gs(75°)) of the surface enamel layer is 17% or more. 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. The chalkboard according to claim 1 or 2, characterized in that the average principal curvature (Spc) of the peaks of the surface enamel layer is 3000 μm ⁻¹ or less. 4. The chalkboard according to claim 1, wherein the 75-degree specular gloss (Gs(75°)) of the surface enamel layer is 20% or more.