CHEMICALLY DURABLE, LOW-E COATING COMPATIBLE BLACK ENAMEL COMPOSITIONS

Abstract

A black enamel composition includes a glass frit, a black pigment and an organic vehicle, wherein the glass frit includes 50 to 70 wt % of Bi.sub.2O.sub.3, 7.0 to 10.0 wt % of SiO.sub.2, 6.0 to 8.0 wt % of B.sub.2O.sub.3, 10.0 to 15.0 wt % of ZnO, 1.0 to 2.0 wt % of Al.sub.2O.sub.3, 3.2 to 10.9 wt % of the total of Co.sub.3O.sub.4, NiO.sub.2 and Fe.sub.2O.sub.3, based on the total weight of the glass frit, wherein the black pigment is 3 to 10 wt % relative to the total weight of the glass frit.

Claims

1. A black enamel composition comprising a glass frit, a black pigment and an organic vehicle, wherein the glass frit comprises 50 to 70 wt % of Bi.sub.2O.sub.3, 7.0 to 10.0 wt % of SiO.sub.2, 6.0 to 8.0 wt % of B.sub.2O.sub.3, 10.0 to 15.0 wt % of ZnO, 1.0 to 2.0 wt % of Al.sub.2O.sub.3, 3.2 to 10.9 wt % of the total of Co.sub.3O.sub.4, NiO.sub.2 and Fe.sub.2O.sub.3, based on the total weight of the glass frit, wherein the black pigment is 3 to 10 wt % relative to the total weight of the glass frit.

2. The black enamel composition according to claim 1, wherein the Co.sub.3O.sub.4 content is 3.0 to 6.0 wt %, the NiO.sub.2 content is 0.1 to 3.0 wt % and the Fe.sub.2O.sub.3 content is 0.1 to 5.0 wt % relative to the total weight of the glass frit.

3. The black enamel composition according to claim 1, wherein the glass frit further comprises at least one selected from TiO.sub.2 and Na.sub.2O in an amount of 0.1 to 3.0 wt % relative to the total weight of the glass frit.

4. The black enamel composition according to claim 1, wherein the black pigment comprises Cr and one or more compounds are selected from compounds comprising at least one of Zn, Fe, and Cu.

5. The black enamel composition according to claim 3, wherein the glass frit comprises 50 to 70 wt % of Bi.sub.2O.sub.3, 7.0 to 10.0 wt % of SiO.sub.2, 6.0 to 8.0 wt % of B.sub.2O.sub.3, 10.0 to 15.0 wt % of ZnO, 1.0 to 2.0 wt % of Al.sub.2O.sub.3, 3.0-6.0 wt % of Co.sub.3O.sub.4, 0.1-3.0 wt % of NiO.sub.2, 0.1-5.0 wt % of Fe.sub.2O.sub.3, 0.1-3.0 wt % of TiO.sub.2 and 0.1-3.0 wt % of Na.sub.2O relative to the total weight of the glass frit, wherein the black pigment is 3 to 10 wt % relative to the total weight of the glass frit.

6. A coated article comprising a substrate, a Low-E coating formed on the substrate, and a pattern portion having a black enamel coating formed in a predetermined pattern on at least a portion of the Low-E coated substrate, wherein the black enamel coating is formed of the black enamel composition of claim 1, and the Low-E coating of the portion where the enamel coating is formed can be at least partially removed by chemical reaction with the enamel coating.

7. The coated article according to claim 6, wherein a surface roughness (Ra) of the black enamel coating is less than 1 μm.

8. The coated article according to claim 6, wherein a glass side reflection color of the black enamel coating has CIELAB color coordinates a* and b* from −1.0 to 1.0 respectively.

9. The coated article according to claim 6, wherein the enamel coated article is prepared by tempering at 700° C. for 200˜600 seconds; and the chemical durability of the enamel coated article is less than or equal to Grade 3 when the coated article is exposed to 0.1N HCl at 25° C. for 3 minutes and washed with deionized water and then the acid resistance of the coated article is evaluated with reference to Standard Test Method for Acid Resistance of Ceramic Decorations on Architectural Type Glass (ASTM C724-91).

Description

BRIEF DESCRIPTION OF DRAWINGS

[0031] FIG. 1 shows a color change of an enamel according to tempering time, while a glass pane printed with an enamel composition including a glass frit having the composition of Table 1 was tempered at 700° C.

[0032] FIG. 2 shows a comparison of a yellow shift effect of an enamel coating on single Low-E glass and double Low-E glass.

[0033] FIG. 3 shows a comparison of color and surface state of enamel coated on Low-E glass according to the content of black pigment.

[0034] FIG. 4 is a schematic representation showing a phenomenon in which a transition metal oxide becomes brittle due to unstable grain boundaries generated between networks of glass frit.

[0035] FIG. 5 is a graph showing the values of Rg a* or Rg b* (Y-axis) according to tempering time (X-axis) of each coated enamel of the compositions shown in Table 3.

MODE FOR THE INVENTION

[0036] Hereinafter, the present invention will be described in detail. However, the scope of the present invention is not limited thereto.

[0037] In the present description, “including” or “comprising” means that other components may be further included unless otherwise specified.

[0038] Black enamel is often applied to a substrate or article with a silver-based Low-E coating. As shown in Example 1, while an enamel is formed on the Low-E coating and performed tempering heat treatment, as a heat treatment time increases, the color of the enamel changes to yellowish-brown, making it impossible to produce a required black enamel in some cases. This is due to a yellow shift phenomenon in which silver in the Low-E coating dissolves toward the enamel during tempering and becomes yellowish-brown due to redox reaction. The yellow shift effect may appear more severely in enamel on multi-Low-E coating in which a plurality of silver layers are present, which was confirmed in Example 2 of the present description.

[0039] Accordingly, the present invention provides enamel compositions capable of having a black color and opacity within a required range even after tempering when forming an enamel on a Low-E coated substrate or article.

[0040] A simple way to make a black enamel would be mixing a glass frit and a black pigment with an organic vehicle to make a composition for enamel formation. As shown in Example 3, when the enamel was formed on the Low-E coated glass, if a black pigment was added to an enamel-forming composition, the yellow shift phenomenon could be suppressed. However, the quality of the surface of the formed enamel was deteriorated and the chemical durability was weakened in case that the content of the black pigment in the enamel composition increased.

[0041] The inventors of the present invention solved this problem by introducing transition metal oxides, which make enamel color, into a frit network of other metal oxides for forming the glass frit. As shown in Example 4, the enamel coatings with the compositions in which the metal oxides Co.sub.3O.sub.4, NiO.sub.2, and Fe.sub.2O.sub.3 were introduced into the glass frit suppressed the yellow shift, so that the CIELAB color coordinate values were within the acceptable range, and the surface roughness(Ra) as well as the chemical durability were excellent.

[0042] However, when a black pigment was not included in the compositions, the transmittance was higher than the allowable range and the opacity was beyond the allowable range, and as shown in Example 5, the process margin was very small in terms of color change as the tempering time increased, which results in limitations.

[0043] Therefore, based upon the above idea and in consideration of the color, opacity, surface quality, chemical durability, and process margin of the enamel, the inventors of the present invention found enamel compositions containing the metal oxides Co.sub.3O.sub.4, NiO.sub.2, and Fe.sub.2O.sub.3 that were introduced into a glass frit while using the minimum amount of black pigment as possible.

[0044] The enamel compositions according to the present invention are characterized in that, to express a black color, they include transition metal oxides added as a member of a network of glass frit and a black pigment that is physically mixed with the glass frit.

[0045] In an embodiment according to the present invention, an enamel composition includes a glass frit, a black pigment, and a vehicle, wherein the glass frit contains 50 to 70 wt % of Bi.sub.2O.sub.3, 7.0 to 10.0 wt % of SiO.sub.2, 6.0 to 8.0 wt % of B.sub.2O.sub.3, 10.0 to 15.0 wt % of ZnO, 1.0 to 2.0 wt % of Al.sub.2O.sub.3 and 3.2 to 10.9 wt % of the total of Co.sub.3O.sub.4, NiO.sub.2 and Fe.sub.2O.sub.3 relative to the total weight of the glass frit, and the black pigment may be 3 to 10% by weight relative to the total weight of the glass frit.

[0046] More preferably, an enamel composition according to the present invention includes a glass frit, a black pigment, and a vehicle, wherein the glass frit contains 50 to 60 wt % of Bi.sub.2O.sub.3, 8.0 to 9.0 wt % of SiO.sub.2, 6.5 to 8.0 wt % of B.sub.2O.sub.3, 12.0 to 15.0 wt % of ZnO, 1.0 to 2.0 wt % of Al.sub.2O.sub.3 and 3.2 to 10.9 wt % of the total of Co.sub.3O.sub.4, NiO.sub.2 and Fe.sub.2O.sub.3 relative to the total weight of the glass frit, and the black pigment may be 3 to 10% by weight relative to the total weight of the glass frit.

[0047] In an embodiment of an enamel composition according to the present invention, Co.sub.3O.sub.4, NiO.sub.2 and Fe.sub.2O.sub.3 are contained in the glass frit, and the content of Co.sub.3O.sub.4 may be 3.0-6.0 wt %, the NiO.sub.2 content may be 0.1-3.0 wt %, and the Fe.sub.2O.sub.3 content may be 0.1 to 5.0 wt % with respect to the total weight of the glass frit.

[0048] In addition, in an embodiment of an enamel composition according to the present invention, at least one selected from TiO.sub.2 and Na.sub.2O may be further included in the glass frit in an amount of 0.1 to 3.0 wt % respectively with regard to the total weight of the glass frit.

[0049] Na.sub.2O plays a role in lowering the melting point of glass and increasing fluidity, but it is known that chemical durability decreases if an excessive amount is employed. Compositions of the present invention contain Na.sub.2O in an amount that does not lower chemical durability while increasing the fluidity reduced by transition metal oxides contained in glass frits.

[0050] Furthermore, in an embodiment of an enamel composition according to the present invention, the a black pigment in the enamel composition contains Cr, and it may include one or more compounds selected from compounds containing at least one of Zn, Fe, and Cu. Black pigments that can be used in enamel compositions are well known in the art and are commercially available. Examples are CuCr.sub.2O.sub.4, (Co, Fe) (Fe, Cr).sub.2O.sub.4, and the like. For example, there are *2991 pigment (copper chromite black pigment), *2980 pigment (cobalt chromium iron black pigment), *2987 pigment (nickel manganese iron chromium black pigment) available from Cerdec Corporation.

[0051] In an embodiment of a glass frit included in an enamel composition, the glass frit can be produced by melting components of the glass frit included in the enamel composition at a high temperature (about 900° C. to 1600° C.), and then rapidly cooling the molten glass by water or by pouring the molten glass between two cooling metal rolls rotating in opposite directions. Melting is generally carried out, for example, in ceramic or platinum crucibles or in suitably lined furnaces. The resulting frit fragments, chips, or flakes can be manufactured into fine grain sizes using a ball mill or the like.

[0052] An enamel composition may comprise a glass frit, a pigment and a vehicle. In one embodiment, the enamel composition may further include additives such as a dispersant, a leveling agent, an anti-bubble agent, or an anti-settling agent.

[0053] A glass frit can be mixed with a vehicle to make a printable enamel paste. The vehicle can be appropriately selected according to applications. In one embodiment, the vehicle properly suspends the particles and completely burns away when firing the paste on a substrate. The vehicle is usually an organic medium, for example, mineral oil, pine oil, vegetable oil, low-molecular petroleum fraction, and the like can be used.

[0054] To make an enamel composition, glass frit and other solid materials are mixed, liquid components are added thereto, and then they are thoroughly mixed or kneaded to form a paste. This paste can be further dispersed using a typical apparatus such as a disperser or a roll mill. The enamel composition can be applied to a substrate by screen printing, spraying, brushing, roller coating, sputtering coating, pyrolytic coating, and the like. After applying the enamel paste to the substrate in a desired pattern, it is fired so that the enamel adheres to the substrate. The firing temperature is generally determined according to the frit ripening temperature, and in one embodiment, it may be in the range of 600 to 760° C.

[0055] In one embodiment of an enamel-coated article according to the present invention, it includes a substrate, a Low-E coating formed on the substrate, and a pattern portion in which a black enamel coating is formed in a predetermined pattern on at least a part of the Low-E coated substrate, where the black enamel coating is formed of the enamel composition according to the present invention, and the Low-E coating of the portion on which the enamel coating is formed may be removed by a chemical reaction with the enamel coating.

[0056] In addition, in one embodiment, the thickness of a black enamel coating of an enamel-coated article according to the present invention may be 5 μm to 15 μm, and the surface roughness(Ra) of the enamel coating may be less than 1 μm. The surface roughness of the enamel coating can be measured using, for example, a stylus type surface roughness meter or a non-contact surface roughness meter.

[0057] In addition, in one embodiment, the glass side reflection color of the enamel coating of an enamel-coated article according to the present invention is in the range of −1.0 to 1.0 respectively of the CIELAB color coordinates a* and b*.

[0058] In the context of the present invention, the term “chemical durability” refers to the ability to resist degradation upon exposure to specified chemical conditions. Specifically, the chemical durability of an enamel-coated article described in this description was evaluated by an acid resistance test. After the specimen was immersed in 0.1N HCl at 25° C. for 3 minutes, and washed with deionized water, the grade was evaluated according to the test method as defined in the Standard Test Method for Acid Resistance of Ceramic Decorations on Architectural Type Glass (ASTM C724-91) as follows:

[0059] Grade 1:No damage

[0060] Grade 2:Loss of glossing

[0061] Grade 3:Obvious matting, color changed but not severe

[0062] Grade 4:Severe color change, chocking available, scratch not resistant or wiped off when washing

[0063] Grade 5:Full dissolution/delamination of surface

[0064] In one embodiment of an enamel-coated article according to the present invention, the enamel-coated article prepared by tempering at 700° C. for 200-600 seconds was immersed in 0.1 N HCl at 25° C. for 3 minutes, and then washed with deionized water. The chemical durability of the coated article after washed was evaluated as grade 3 or less according to the above-described grades.

[0065] In one embodiment, the present invention relates to a method of manufacturing an enamel-coated article comprising the step of printing a black enamel composition according to the present invention to have a predetermined pattern on at least a portion of the Low-E coated substrate; and the step of forming a pattern portion including a black enamel coating by heat treatment of the substrate on which the enamel composition is printed. The heat treatment may be performed for 150 seconds to 600 seconds at a temperature of 500° C. to 760° C. The heat treatment may be a tempering process of the substrate.

[0066] Hereinafter, the present invention will be described in more detail based on examples. However, the present invention is not limited by the following examples. Embodiments of the present invention may be modified in various forms unless the gist of the invention is changed.

Example 1 Changes in Color of an Enamel on Low-E Coating According to Tempering Time

[0067] Oxides with the weight ratio shown in Table 1 below are melted at a high temperature (1000° C. or higher) to produce a glass frit, and then the frit is milled using a ball mill to produce 8-12 micrometers-sized particles.

TABLE-US-00001 TABLE 1 SiO.sub.2 B.sub.2O.sub.3 Bi.sub.2O.sub.3 ZnO Al.sub.2O.sub.3 9.0 wt % 7.0 wt % 68.4 wt % 13.6 wt % 2.0 wt %

[0068] The milled glass frit was mixed with alcohol and ethyl cellulose to form a paste, and then applied to a single Low-E glass by screen printing. The glass plate on which an enamel composition was applied was tempered at 700° C., and the color of the glass on which the enamel was applied was observed according to tempering time (seconds) (FIG. 1).

[0069] Before tempering (0 s in FIG. 1), the coated enamel was transparent and was the color of the glass itself, but it can be seen that the color changes as the tempering time is increased, and the color is clearly a yellowish brown color in 400 s.

[0070] The lower right blue panel of FIG. 1 shows the color change with each concentration when the silver colloid concentration is increased toward the right test tube, from which it can be seen that the silver nanoparticles have a yellowish brown color.

[0071] When Bi-based enamel paste is printed on Low-E glass and fired, silver in the silver layer of the Low-E coating is dissolved out and a redox reaction occurs as the tempering time increases. During tempering, silver ions dissolve into a soft enamel phase.

[0072] Ag->Ag+(Dissolved, Fast, Unstable)

[0073] Since Ag has a high reduction potential, silver ions are relatively unstable even in the enamel phase. Since the solubility and dissolution capacity of silver in the enamel should be very low, a slow and gradient redox reaction occurs as the total heating time increases, and the silver becomes yellowish brown, and other silver will dissolve at the same time.

[0074] Ag+->Ag (redox, slow, yellow shift) Therefore, the yellow shift effect may appear more severely in the enamel on a multi Low-E coating in which a plurality of silver layers exist, which was confirmed in Example 2 below.

Example 2 Comparison of the Yellow Shift Effect of an Enamel Coating on Single Low-E Glass and Double Low-E Glass

[0075] The enamel composition prepared in Example 1 was applied to single Low-E glass and double Low-E glass, then tempered at 700° C. for 210 seconds, 260 seconds, and 310 seconds, and CIELAB color coordinates a* and b* were measured with Minolta CM600 and shown as a graph (FIG. 2).

[0076] Within the range of the experimental conditions, the CIELAB color coordinate values of the enamel coated on the single Low-E glass were between −1.0 to 1.0, respectively which are within the allowable ranges of Rg a* and Rg b*, but the color coordinate values of the enamel coated on the double Ro-E glass were outside the allowable ranges of Rg a* and Rg b*, at a tempering time of 260 seconds and 310 seconds.

Example 3 Effect of Adding Black Pigment to an Enamel Coated on Low-E Glass

[0077] In order to suppress the yellow shift phenomenon due to tempering of the enamel coated on the Low-E glass, the glass frit having the composition of Table 1 and 6 wt %, 10 wt %, or 20 wt % of black pigment (mainly composed of CuCr.sub.2O.sub.4, spinel structure) relative to the total weight of the glass frit, was applied to a single Low-E glass according to the method of Example 1 and tempered at 700° C. for 230 seconds. FIG. 3 is a comparison after tempering the enamel coating surfaces comprising each amount of black pigment as above.

[0078] As shown in FIG. 3, when a black pigment was added, the color of the enamel after tempering appeared vivid black. However, the coating surface formed of the enamel composition containing 6% by weight of the black pigment based on the total weight of the glass frit was smooth ((1) in FIG. 3), and in the condition when containing 10% by weight of the black pigment the surface roughness (Ra) was maintained within the permissible range ((2) in FIG. 3), and bubbles were severely formed on the surface in the condition when containing 20% by weight of the black pigment ((3) in FIG. 3). Most of the pigments are made of transition metal oxides, which increase the viscosity of the glass frit when the temperature increases, so bubbles are generated on the surface and the surface is not slippery.

[0079] Also, transition metal oxides are easily soluble in an acidic environment because ionic bonds are formed inside of the transition metal oxides, thereby generating an unstable grain boundary between the transition metal oxides and a network of glass frit, thus becoming brittle (FIG. 4). Therefore, when the content of the black pigment is increased, the chemical durability of the enamel is weakened.

[0080] An enamel composition containing 15% by weight of a glass frit having the composition of Table 1 and a black pigment (mainly composed of CuCr.sub.2O.sub.4, spinel structure) based on the total weight of the glass frit was applied to a single Low-E glass according to the method of Example 1, then tempered at 700° C. for 230 seconds. Within the range of the experimental conditions, the CIELAB color coordinate values of the enamel coated on the Low-E glass were measured to be −0.5 and −0.2, respectively, within the allowable ranges of Rg a* and Rg b* (Minolta CM600), and the surface roughness(Ra) was good with 0.1 μm (measured with Sufcom JIS094 standard, 0.15 mm/s, 3.0 mm).

[0081] However, the chemical durability evaluated by the acid resistance was measured very low with Grade 5 (complete dissolution/peeling of the surface). To evaluate the chemical durability, the specimen was immersed in 0.1N HCl at 25° C. for 3 minutes, washed with deionized water, and a grade was evaluated with reference to the evaluation criteria as described in the Standard Test Method for Acid Resistance of Ceramic Decorations on Architectural Type Glass Test Method (ASTM C724-91).

[0082] Grade 1:No damage

[0083] Grade 2:Loss of glossing

[0084] Grade 3:Obvious matting, color changed but not severe

[0085] Grade 4:Severe color change, chocking available, scratch not resistant or wiped off when washing

[0086] Grade 5:Full dissolution/delamination of surface

Example 4 Enamel Compositions in which Transition Metal Oxides were Directly Inserted into Frit Network

[0087] In order for an enamel to exhibit a predetermined color, there is a method of physically mixing a pigment with a glass frit as in Example 3, and another method is to directly insert transition metal oxides into a frit network. When metal oxides are melted above the melting point with other compounds in a glass frit, it not only contributes to the color of the enamel, but also replaces the role of ZnO to make it chemically stronger.

[0088] By including metal oxides Co.sub.3O.sub.4, NiO.sub.2, and Fe.sub.2O.sub.3 into a glass frit, the glass frit was prepared according to the method of Example 1 with the compositions shown in Table 2, and enamel compositions were prepared with a vehicle, etc., and coated on a single Low-E glass, and tempered at 700° C. for 230 seconds.

TABLE-US-00002 TABLE 2 Sample 1 Sample 2 Sample 3 Glass frit SiO.sub.2 (wt % in the glass frit) 8.7 8.6 8.6 B.sub.2O.sub.3 (wt % in the glass frit) 6.8 7.7 7.7 Bi.sub.2O.sub.3 (wt % in the glass frit) 66 51.5 51.5 ZnO (wt % in the glass frit) 10.7 14.9 14.9 A1.sub.2O.sub.3 (wt % in the glass frit) 1.9 2.0 2.0 Co.sub.3O.sub.4 (wt % in the glass frit) 3.3 5.8 5.8 NiO.sub.2 (wt % in the glass frit) 1.3 1.2 1.2 Fe.sub.2O.sub.3 (wt % in the glass frit) 1.1 3.9 3.9 Na.sub.2O (wt % in the glass frit) 3.0 3.0 TiO.sub.2 (wt % in the glass frit) 1.6 1.6 Pigment (wt % of total glass frit weight) 0.0 0.0 6.3

[0089] The transmittance (Perkin-Elmer Lambda1050), CIELAB color coordinate values (Minolta CM600), and surface roughness(Ra) (Sufcom JIS-94 standard, 0.15 mm/s, 3.0 mm) of the enamel-coated Low-E glass were measured. In addition, chemical durability was evaluated according to the method described in Example 3.

TABLE-US-00003 TABLE 3 Properties Allowable range Sample 1 Sample 2 Sample 3 Transmittance(T) T < 0.1% 3.5 0.09 a*Rg −1.0 < a*Rg < 1.0 0.38 0.6 0.1 b*Rg −1.0 < b*Rg < 1.0 0.03 −0.3 −0.3 Ra Ra < 0.5 μm 0.21 0.1 0.1 Chemical durability Grade 3 or less Grade 3 Grade 3 Grade 3

[0090] The enamel coating of the compositions of Table 2 in which metal oxides Co.sub.3O.sub.4, NiO.sub.2 and Fe.sub.2O.sub.3 were introduced into the glass frit was excellent in chemical durability as well as CIELAB color coordinate values and surface roughness(Ra). However, when the pigment was not included, the transmittance was higher than the allowable range.

Example 5 Formulations of Enamel Compositions and Process Margin

[0091] Enamel compositions were prepared according to the method of Example 4 with the compositions shown in Table 4, coated on a single Low-E glass, tempered at 700° C. for 230 seconds, and the properties of each enamel coating were measured.

TABLE-US-00004 TABLE 4 Sample 1 Sample 3 Sample 4 Glass frit SiO.sub.2 (wt % in the glass frit) 8.7 8.6 9.0 B.sub.2O.sub.3 (wt % in the glass frit) 6.8 7.7 7.0 Bi.sub.2O.sub.3 (wt % in the glass frit) 66 51.5 68.4 ZnO (wt % in the glass frit) 10.7 14.9 13.6 Al.sub.2O.sub.3 (wt % in the glass frit) 1.9 2.0 2.0 Co.sub.3O.sub.4 (wt % in the glass frit) 3.3 5.8 NiO.sub.2 (wt % in the glass frit) 1.3 1.2 Fe.sub.2O.sub.3 (wt % in the glass frit) 1.1 3.9 Na.sub.2O (wt % in the glass frit) 3.0 TiO.sub.2 (wt % in the glass frit) 1.6 Pigment (wt % of total glass frit 0.0 6.3 6.5 weight)

[0092] Further, the enamel coating prepared with the same compositions was tempered for 200 seconds, 230 seconds, 260 seconds, 300 seconds, 420 seconds, and 600 seconds at 700° C. in a Northglass furnace. The process margin was calculated in the range of the tempering time with an absolute value less than 1 of the CIELAB color coordinate values (Rg a* or Rg b*).

TABLE-US-00005 TABLE 5 Properties Allowable range Sample 1 Sample 3 Sample 4 Transmittance(T) T < 0.1% 0.09 0.09 a*Rg −1.0 < a*Rg < 1.0 0.38 0.1 −0.49 b*Rg −1.0 < b*Rg < 1.0 0.03 −0.3 −0.96 Ra Ra < 0.5 μm 0.21 0.1 0.11 Chemical durability Grade 3 or less Grade 3 Grade 3 Grade 4 Process margin(sec) 60 400 60

[0093] FIG. 5 is a graph showing CIELAB color coordinate values Rg a* or Rg b* (Y-axis) according to the tempering time (X-axis) of each coated enamel.

[0094] From the graph of FIG. 5, it can be seen that Samples 1 and 4 deviate from the allowable area in +(yellow direction) the Rg b* area as the tempering time increases, or the margin is significantly smaller with 60 seconds. On the other hand, Sample 3 maintained a stable color during test from 200 seconds to 600 seconds, regardless of the tempering time, and the process margin was significantly larger with 400 seconds than Samples 1 and 4.

[0095] The present invention has been described in detail above, and the scope of the present invention is not limited to the above-described embodiments. The basic concept of the present invention and the invention defined in the detailed description and claims, and modifications and improvements using the same are also included in the scope of the present invention.

[0096] The black enamel composition according to the present invention can be used in various articles that need to form a black enamel on a Low-E coated substrate based on silver, and thus can be used in various fields such as automobiles and construction.