Description

COATING METHOD OF METAL SHEET USING MIXTURE OF

INK AND URUSHI LACQUER

Technical Field

[1] The present invention relates to a method of coating a metal sheet using a mixture of ink and urushi lacquer, and more particularly, to a method of coating a metal sheet with a mixture of ink and urushi lacquer, comprising pretreating the surface of the metal sheet, printing the metal sheet through silk screen printing using the mixture of ink and urushi lacquer, drying the printed metal sheet, coating the metal sheet using a filtered urushi lacquer mixture, and drying the coated metal sheet. Background Art

[2] In general, raw urushi lacquer, or urushi lacquer collected from the lacquer tree, is known to be composed of 55-66% urushiol, 25-30% water, 5-10% monosaccharides and polysaccharides, 3-5% of nitrogen-containing material, and small amounts of enzymes, such as laccase, stellacyanin, and peroxidase. Urushi lacquer, which is a kind of natural resin, has been used in large amounts, mainly by being applied on wood to produce articles for living, such as wardrobes, vessels, spoons, chopsticks, and tables, in Korea, China and Japan for thousands of years. Further, it is also known that urushi lacquer has advantages such as high flame resistance, heat resistance, rot resistance, insect resistance, and electrical insulation properties, and furthermore, that the preservative quality thereof is good.

[3] Recently, various attempts to apply urushi lacquer to a synthetic polymer (Korean

Patent Laid-open Publication No. 2000-0024493) and to a ceramic and metal container (Korean Patent Laid-open Publication No. 2002-0072903), in addition to the wood products, have been made.

[4] However, application of the urushi lacquer while keeping variously printed patterns clear has not yet been disclosed. Moreover, since the uncured urushi lacquer coating causes an allergic reaction in the human body, general users are reluctant to employ such lacquer. For these reasons, urushi lacquer has numerous difficulties in use. In addition, the urushi lacquer coating begins to cure from the time that its surface comes into contact with moisture and oxygen, and has very low oxygen permeability and moisture permeability. Thus, in order to cure the inner portion of the coating, there is the need for a UV drying process or a process of repeatedly conducting application and curing of urushi lacquer through a manual process, which is complicated. Thereby, the production cost increases, and economic benefits are negated, and consequently, it is difficult to realize industrial production of urushi lacquer products.

Disclosure of Invention

Technical Problem

[5] Accordingly, the present invention has been made keeping in mind the above problems occurring in the related art, and an object of the present invention is to provide a method of coating a metal sheet with a mixture of ink and urushi lacquer, suitable for use in the production of the metal sheet, having excellent far infrared emission and electromagnetic wave -blocking effects, and superior antibacterial activity, on an industrial scale, by subjecting a metal sheet to silk screen printing using a mixture of ink and urushi lacquer to realize various colors and then to silk screen printing using a mixture of urushi lacquer and varnish.

[6] Another object of the present invention is to provide a method of coating a metal sheet with a mixture of ink and urushi lacquer, suitable for use in the production of the metal sheet, having various glosses and colors and high hardness, on an industrial scale, by subjecting a metal sheet to silk screen printing using a mixture of ink and urushi lacquer to realize various colors and then to silk screen printing using a mixture of urushi lacquer and lacquer. Technical Solution

[7] In order to achieve the above objects, the present invention provides a method of coating a metal sheet using a mixture of ink and urushi lacquer, comprising pretreating the surface of the metal sheet, printing the metal sheet through silk screen printing using the mixture of ink and urushi lacquer, drying the printed metal sheet, coating the metal sheet with a mixture of filtered urushi lacquer and varnish, and drying the coated metal sheet. As such, the method may further comprise coating the coated and dried metal sheet with lacquer containing PMMA (polymethyl methacrylate) and MMA (methyl methacrylate) and then drying the metal sheet coated with the lacquer.

[8] Hereinafter, a detailed description will be given of the present invention.

[9] The metal sheet used in the present invention is formed of a metal, such as titanium steel, stainless steel, aluminum, or an aluminum alloy. In particular, aluminum is preferable. The metal sheet, which is surface polished and water washed, has a thickness of 0.3-2 mm. The surface of the metal sheet is pretreated using a physical process, a chemical process, or a primer process. Examples of the physical process include sand blasting, shot blasting, dry-ice blasting, hairline, and vibration, and examples of the chemical process include aluminum deposition, anodizing, elec- trocoloring, and alkali degreasing for final washing in all tests.

[10] The primer treatment is used in the case where it is difficult to adhere inorganic material, such as plastic, to the metal sheet, and is also used for silk screen printing for a heater or various decorations and marks. Such a pretreatment process functions to

increase adhesion upon printing, and to enhance corrosion resistance. In the present invention, anodizing treatment, which aims to prevent the oxidation of material via chromate treatment, is particularly preferred.

[11] The printing of the pretreated metal sheet is carried out through silk screen printing using the mixture of ink and urushi lacquer. Preferably, the metal sheet is subjected to silk screen printing using a mixture comprising 5~95 wt% of ink capable of representing various colors and designs and 95-5 wt% of filtered urushi lacquer. As such, however, an offset printing process is unsuitable because the viscosity increases due to the mixing of ink. In addition, the type of ink for silk screen printing is not particularly limited, and preferably, pigment powder for use as raw material for ink is used.

[12] Shortly after the printing process, a drying process is performed at 150~200°C for

30-90 min.

[13] The metal sheet thus printed and dried is coated with urushi lacquer in a manner such that a mixture of urushi lacquer and varnish is prepared and then applied on the metal sheet.

[14] In the present invention, urushi lacquer, native to Korea, China, Japan or Southeast

Asia, may be used without limitation. Further, although the composition of such lacquer may vary with the production region, it may be independent thereof. In addition, in the present invention, the urushi lacquer is used after a filtration process using filter paper to remove impurities therefrom. Thus, 95-98 wt% of the urushi lacquer thus filtered is mixed with 2-5 wt% of varnish, thus preparing the mixture. The mixture of urushi lacquer and varnish thus prepared is preferably applied though silk screen printing, but the present invention is not limited to such a printing process. Shortly after the application of the mixture of urushi lacquer and varnish, a drying process is performed at 130~180°C for 30-90 min.

[15] In addition, after the above drying process, it is preferred that coating the urushi lacquered metal sheet with lacquer containing PMMA (polymethyl methacrylate) and MMA (methyl methacrylate) and then drying the metal sheet coated with the lacquer be further performed so as to enhance gloss and weather resistance.

[16] In such a case, the PMMA is acryl resin including MMA as a main monomer, and the admixture containing PMMA and MMA is used in an amount of 95-99 wt% based on the total weight of the lacquer, in which the PMMA and MMA are preferably admixed at a ratio of 5-8:5-2. In particular, a ratio of PMMA and MMA of 6:4 is most preferable, in the interest of high transparency and desirable properties.

[17] The lacquer admixture is preferably applied through silk screen printing, but the present invention is not limited to such a printing process. Shortly after the application of the lacquer admixture, a drying process is conducted at 80~120°C for 30-90 min. In the present invention, such a drying process is performed at a temperature that is

optimal to prevent cracking or peeling of the urushi lacquer coating upon curing and also to increase adhesion thereof.

[18] As shown in FIG. 2, the metal sheet produced by the above method comprises a metal sheet 10, a primer applied layer 20, a design printed layer 30 of the mixture of ink and urushi lacquer, a urushi lacquered layer 40, and a lacquered layer 50, which are sequentially formed upwards.

[19] In addition, the present invention provides a method of coating a metal sheet with a mixture of ink and urushi lacquer, comprising pretreating the surface of the metal sheet, printing the metal sheet through silk screen printing using the mixture of ink and urushi lacquer, drying the printed metal sheet, coating the metal sheet with a mixture comprising filtered urushi lacquer and lacquer, and drying the coated metal sheet.

[20] In this case, pretreating the surface of the metal sheet, printing the metal sheet through silk screen printing using the mixture of ink and urushi lacquer, and drying the printed metal sheet are performed in the same manner as the processes mentioned above.

[21] The printed and dried metal sheet is coated with a mixture of urushi lacquer and a lacquer admixture. As such, the urushi lacquer, which is filtered using filter paper, is used in an amount of 5-90 wt%, and the lacquer admixture is used in an amount of 95-10 wt%, to thus prepare the mixture. When the amount of urushi lacquer is increased and the amount of lacquer is decreased, the surface gloss is decreased and the color is darkened. On the other hand, when the amount of urushi lacquer is decreased and the amount of lacquer is increased, the surface gloss is increased and the color becomes brightened. In this way, it is possible to perform the coating treatment in order to realize various glosses and colors according to the preferences of the consumer.

[22] Further, it is preferred that the urushi lacquer be mixed with 5 wt% of varnish. The mixture of the urushi lacquer and the lacquer admixture is preferably applied through silk screen printing, but the present invention is not limited to such a printing process.

[23] The lacquer admixture containing the PMMA and the MMA is used in an amount of 20-80 wt% based on the total weight of the mixture to be applied, in which the PMMA and MMA are preferably admixed at a ratio of 5-8:5-2. In particular, a ratio of PMMA to MMA of 6:4 is most preferable in the interest of high transparency and realizing desirable properties.

[24] Shortly after the application of the mixture of the urushi lacquer and the lacquer admixture, a drying process is conducted at 150~180°C for 30-90 min. In the present invention, the drying process is performed at a temperature that is optimal to prevent cracking or peeling of the urushi lacquer coating upon curing, and also to increase the adhesion thereof.

[25] As shown in FIG. 5, the metal sheet produced by the above method comprises a metal sheet 10, a primer- applied layer 20, a design-printed layer 30 of the mixture of ink and urushi lacquer, and a layer 40 of the mixture of urushi lacquer and lacquer, which are sequentially formed upwards. Advantageous Effects

[26] In the method of coating the metal sheet according to the present invention, since the mixture of ink and urushi lacquer is applied through silk screen printing to represent various colors and patterns, a urushi lacquered metal sheet having various patterns can be easily produced on an industrial scale. Further, the urushi lacquer component is superior with respect to the absorption of harmful electromagnetic waves, far infrared emissivity, antibacterial activity, and durability, and therefore such a metal sheet can be variously applied to mobile phones, notebook computers, automobile interior materials, construction materials, etc.

Brief Description of the Drawings [27] FIG. 1 is a flowchart showing the process of coating a metal sheet through silk screen printing using a mixture of ink and urushi lacquer, according to a first embodiment of the present invention; [28] FIG. 2 is a sectional view of the metal sheet, produced through silk screen printing using the mixture of ink and urushi lacquer according to the first embodiment of the present invention; [29] FIG. 3 is a photograph of an aluminum sheet, which is subjected to silk screen printing using a mixture of 50 wt% of ink and 50 wt% of urushi lacquer and then to coating using a mixture of urushi lacquer and varnish; [30] FIG. 4 is a flowchart showing the process of coating a metal sheet through silk screen printing using a mixture of ink and urushi lacquer, according to a second embodiment of the present invention; [31] FIG. 5 is a sectional view of the metal sheet, produced through silk screen printing using the mixture of ink and urushi lacquer according to the second embodiment of the present invention; and [32] FIG. 6 is a photograph of an aluminum sheet, which is subjected to silk screen printing using a mixture of 50 wt% of ink and 50 wt% of urushi lacquer and then to coating using a mixture of urushi lacquer and lacquer.

Mode for the Invention [33] A better understanding of the present invention may be obtained through the following examples and test examples, which are set forth to illustrate, but are not to be construed as the limit of the present invention. [34] Example 1. Coating of Metal sheet using Urushi Lacquer

[35] FIG. 1 illustrates the process of coating a metal sheet through silk screen printing using a mixture of ink and urushi lacquer, according to a first embodiment of the present invention. Specifically, an aluminum sheet (H-26, available from Alcoa) weighing 0.25-0.30 kg and having a thickness of 0.5 mm was surface polished and wa ter washed. Thereafter, the aluminum sheet was surface cleaned with a degreasing agent and was then pretreated with 2 kg of an acidic solution in which chromate was diluted with water, to thus form an oxide film 1.5-2.5 D thick.

[36] Subsequently, the aluminum sheet was subjected to silk screen printing using each of a first mixture of 50 wt% of black ink for metal (available from Cromtryck, Sweden) and 50 wt% of urushi lacquer and a second mixture of 50 wt% of blue ink for metal (available from Cromtryck, Sweden) and 50 wt% of urushi lacquer, and was then dried in a box oven (available from Taeyang Electronic Company, Korea) at 16O ⁰ C for 1 hour.

[37] Raw urushi lacquer native to China was filtered with 300 mesh filter paper and then mixed with 5 wt% of varnish based on the total weight of the filtered urushi lacquer to thus prepare a urushi lacquer mixture, which was then applied to a thickness of 1.5-2.5 D on the above aluminum sheet through silk screen printing. Thereafter, the sheet was dried in a box oven at 16O ⁰ C for 1 hour.

[38]

[39] Example 2

[40] Coating of Urushi Lacquered Metal Sheet using Lacquer

[41] On the urushi lacquered aluminum sheet of Example 1, lacquer (plex6788, available from ROHM), containing PMMA and MMA admixed at 6:4, was applied through silk screen printing. Thereafter, the sheet was dried in a box oven at 100 ⁰ C for 1 hour, therefore producing urushi lacquered aluminum sheets (A and B of FIG. 3).

[42]

[43] Example 3

[44] Coating of Metal Sheet using Mixture of Urushi Lacquer and Lacquer

[45] FIG. 4 illustrates the process of coating a metal sheet through silk screen printing using a mixture of ink and urushi lacquer, according to a second embodiment of the present invention. Specifically, an aluminum sheet (H-26, available from Alcoa) weighing 0.25-0.30 kg and having a thickness of 0.5 mm was surface polished and water washed. Thereafter, the aluminum sheet was surface cleaned with a degreasing agent, and was then pretreated with 2 kg of an acidic solution in which chromate was diluted with water, to thus form an oxide film 1.5-2.5 D thick.

[46] Subsequently, the aluminum sheet was subjected to silk screen printing using each of a first mixture of 50 wt% of red ink for metal (available from Cromtryck, Sweden) and 50 wt% of urushi lacquer, a second mixture of 50 wt% of silver ink for metal

(available from Cromtryck, Sweden) and 50 wt% of urushi lacquer, a third mixture of 50 wt% of white ink for metal (available from Cromtryck, Sweden) and 50 wt% of urushi lacquer, and a fourth mixture of 50 wt% of yellow ink for metal (available from Cromtryck, Sweden) and 50 wt% of urushi lacquer, and was then dried in a box oven (available from Taeyang Electronic Company, Korea) at 16O ⁰ C for 1 hour.

[47] Raw urushi lacquer was filtered with 300 mesh filter paper and then mixed with 5 wt% of varnish based on the total weight of the filtered urushi lacquer, to thus prepare a urushi lacquer mixture. Thereafter, 10 wt% of the urushi lacquer thus mixed was further mixed with 90 wt% of a lacquer admixture containing PMMA and MMA (plex6788, available from ROHM) admixed at 6:4, thus preparing a mixture of urushi lacquer and lacquer. Subsequently, the mixture thus obtained was applied to a thickness of 1.5—2.5 D on the above aluminum sheet through silk screen printing. Then, the sheet was dried in a box oven at 16O ⁰ C for 1 hour, thereby yielding aluminum sheets coated with the urushi lacquer mixture (A, B, C, and D of FIG. 6).

[48] [49] Test Example 1. Measurement of Hardness and Gloss [50] The thickness, hardness and gloss of the urushi lacquer coating of the aluminum sheet of each of Examples 1 to 3 were measured.

[51] The hardness was evaluated using a pencil according to JIS D 0202.8.10, and the gloss was evaluated at an incident angle of 60 using a glossmeter, available from BYK. The results are shown in Table 1 below.

[52] Table 1

[53] As is apparent from Table 1 , it was possible to mass produce the aluminum sheet having high hardness and gloss even with a thin urushi lacquer coating.

[54] [55] Test Example 2. Test of Specific Absorption Rate [56] Mobile phone cases were manufactured by the present inventors using the aluminum sheet of Example 2. Such mobile phone cases and commercially available mobile phone cases from manufacturers A and B were measured by Hyundai Calibration & Certification Technologies Co., Ltd., with respect to their specific absorption rate for evaluating the absorption of electromagnetic waves under conditions given in Table 2 below

[57] Table 2

[58] Table 3

[59] As is apparent from Table 3, the three mobile phone cases exhibited a specific absorption rate of 1.6 W/kg or less, satisfying the standard for exposure to electromagnetic waves. In particular, of these mobile phone cases, the mobile phone case manufactured using the aluminum sheet produced by the method of the present invention could be seen to have a superior specific absorption rate of 0.097.

[60] [61] Test Example 3. Test of Far Infrared Emission [62] The emission of far infrared rays from the aluminum sheet of Example 2 was measured by the Korea Institute of Far Infrared Applied Estimation at the present inventor's request.

[63] Specifically, the emissivity of the aluminum sheet was determined at 37 ⁰ C using an FT-IR spectrometer, and compared to that of a black body. The results are given in Table 4 below. As is apparent from Table 4, high far infrared emissivity was obtained.

[64] Table 4

[65] [66] Test Example 4. Test of Antibacterial Activity [67] The test for antibacterial activity of the aluminum sheet of Example 2 was conducted by Korea Testing and Research Institute for the Chemical Industry at the present inventor's request. In this case, the test strains were Escherichia CoIi (ATCC 8739) and Staphylococcus aureus (ATCC 6538).

[68] Table 5

[69] Note: A is the bacterial cell number (average) of the control sample shortly after inoculation, B is the bacterial cell number (average) of the control sample after 24 hours of incubation, and C is the bacterial cell number (average) of the polyethylene terephthalate resin of the present invention after 24 hours of incubation. In addition, an antibacterial value (log(B/C)) results from the subtraction of log(C/A) from log(B/A).

[70] As is apparent from Table 5, the urushi lacquered metal sheet of the present invention could be seen to exhibit excellent antibacterial activity. [71] Although the preferred embodiments of the present invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims. Industrial Applicability

[72] As described hereinbefore, in the method of coating the metal sheet according to the

present invention, since the mixture of ink and urushi lacquer is applied through silk screen printing to represent various colors and patterns, the urushi lacquered metal sheet having various patterns can be easily produced on an industrial scale. Further, the urushi lacquer component is superior with respect to the absorption of harmful electromagnetic waves, far infrared emissivity, antibacterial activity, and durability, and therefore such a metal sheet can be variously applied to mobile phones, notebook computers, automobile interior materials, construction materials, etc.