METHOD FOR MANUFACTURING ELECTRICAL STEEL SHEET PRODUCT

Abstract

A method for manufacturing an electrical steel sheet product according to an exemplary embodiment of the present invention includes: preparing an adhesive coating composition; forming an adhesive coating layer by applying and then curing the adhesive coating composition onto a surface of an electrical steel sheet; forming a heat-fused layer by laminating and heat-fusing a plurality of electrical steel sheets on which the adhesive coating layers are formed; and cooling the heat-fused electrical steel sheets at a cooling rate of 0.05 to 20° C./min.

Claims

1. A method for manufacturing an electrical steel sheet product, the method comprising: preparing an adhesive coating composition; forming an adhesive coating layer by applying and then curing the adhesive coating composition onto a surface of an electrical steel sheet; forming a heat-fused layer by laminating and heat-fusing a plurality of electrical steel sheets on which the adhesive coating layers are formed; and cooling the heat-fused electrical steel sheets at a cooling rate of 0.05 to 20° C./min.

2. The method of claim 1, wherein: in the cooling of the heat-fused electrical steel sheets at the cooling rate of 0.05 to 20° C./min, a holding pressing force is 1,000 N/mm.sup.2 or less.

3. The method of claim 2, wherein: the holding pressing force is 500 N/mm.sup.2 or less and the cooling rate is 0.05 to 1° C./min.

4. The method of claim 1, wherein: in the cooling of the heat-fused electrical steel sheets at the cooling rate of 0.05 to 20° C./min, a cooling finish temperature is 10 to 100° C.

5. The method of claim 1, wherein: the adhesive coating composition contains an organic/inorganic complex containing an organic resin and an inorganic nanoparticle substituted for the organic resin; and an inorganic substance.

6. The method of claim 5, wherein: the organic resin includes one or more selected from an epoxy-based resin, an ester-based resin, an acrylic resin, a styrene-based resin, a urethane-based resin, and an ethylene-based resin.

7. The method of claim 5, wherein: the organic resin is a water-soluble epoxy resin, and the water-soluble epoxy resin is a multifunctional epoxy resin having three or more epoxy groups, and the water-soluble epoxy resin has a weight average molecular weight of 1,000 to 50,000, a softening temperature (Tg) of 70 to 120° C., and a solid fraction of 10 to 50 wt %.

8. The method of claim 5, wherein: the inorganic nanoparticle includes one or more of SiO.sub.2, TiO.sub.2, ZnO, Al.sub.2O.sub.3, MgO, CaO, and ZrO.sub.2.

9. The method of claim 5, wherein: the inorganic nanoparticle is substituted in the organic/inorganic complex in an amount of 1 to 60 parts by weight with respect to 100 parts by weight of the organic resin.

10. The method of claim 5, wherein: the inorganic substance is phosphoric acid (H.sub.3PO.sub.4) or sodium hydroxide (NaOH).

11. The method of claim 5, wherein: the inorganic substance is contained in an amount of 1 to 70 parts by weight with respect to 100 parts by weight of a solid content of the organic/inorganic complex.

12. The method of claim 1, wherein: the forming of the adhesive coating layer is performed in a temperature range of 200 to 600° C.

13. The method of claim 5, wherein: in the forming of the adhesive coating layer, a ratio of the inorganic substance to an organic substance in the coating layer is 0.05 to 0.6.

14. The method of claim 1, wherein: in the forming of the heat-fused layer, a pressing force is 1 to 2,000 N/mm.sup.2, a pressing time is 1 to 180 minutes, and a pressing temperature is 100 to 300° C.

Description

DESCRIPTION OF THE DRAWINGS

[0032] FIG. 1 is a flow chart of a method for manufacturing an electrical steel sheet product according to an exemplary embodiment of the present invention.

[0033] FIG. 2 is a photograph captured with a transmission electron microscope (TEM) after a cross section of a coating layer of an electrical steel sheet is processed with focus ion beam (FIB), the coating layer being formed of a composition in which sodium hydroxide (NaOH) as an inorganic substance is contained in an epoxy (molecular weight≈30,000)-SiO.sub.2 (25 nm, 20 parts by weight) complex, and is a photograph showing a distribution of nanoparticles (one of SiO.sub.2, TiO.sub.2, and ZnO) in the adhesive coating layer.

[0034] FIG. 3 is a photograph captured with a transmission electron microscope (TEM) after a cross section of a coating layer of an electrical steel sheet is processed with focus ion beam (FIB), the coating layer being formed of a composition in which sodium hydroxide (NaOH) as an inorganic substance is contained in an epoxy (molecular weight≈30,000)-SiO.sub.2 (25 nm, 20 parts by weight) complex, and is a photograph showing a distribution of an inorganic substance (one of NaOH and H.sub.3PO.sub.4) dissolved in the adhesive coating layer.

MODE FOR INVENTION

[0035] In the present specification, the terms “first”, “second”, “third”, and the like are used to describe various parts, components, regions, layers, and/or sections, but are not limited thereto. These terms are only used to differentiate a specific part, component, region, layer, or section from another part, component, region, layer, or section. Accordingly, a first part, component, region, layer, or section which will be described hereinafter may be referred to as a second part, component, region, layer, or section without departing from the scope of the present invention.

[0036] In the present specification, unless explicitly described to the contrary, “comprising” any components will be understood to imply the inclusion of other components rather than the exclusion of any other components.

[0037] In the present specification, technical terms used are to mention only a specific exemplary embodiment, and are not intended to limit the present invention. Singular forms used herein include plural forms as long as phrases do not clearly indicate an opposite meaning. The term “comprising” used in the present specification concretely indicates specific properties, regions, integers, steps, operations, elements, and/or components, and is not to exclude the presence or addition of other specific properties, regions, integers, steps, operations, elements, and/or components.

[0038] In the present specification, the term “combination thereof” included in Markush type expression means a mixture or combination of one or more selected from the group consisting of components described in Markush type expression, and means comprising one or more selected from the group consisting of the above-described components.

[0039] In the present specification, when any part is positioned “on” or “above” another part, it means that the part may be directly on or above the other part or another part may be interposed therebetween. In contrast, when any part is positioned “directly on” another part, it means that there is no part interposed therebetween.

[0040] Unless defined otherwise, all terms including technical terms and scientific terms used herein have the same meanings as understood by those skilled in the art to which the present invention pertains. Terms defined in a generally used dictionary are additionally interpreted as having the meaning matched to the related technical document and the currently disclosed contents and are not interpreted as ideal or very formal meanings unless otherwise defined.

[0041] In addition, unless otherwise stated, % means wt %, and 1 ppm is 0.0001 wt %.

[0042] In an exemplary embodiment of the present invention, a method for manufacturing an electrical steel sheet product is provided.

[0043] The method for manufacturing an electrical steel sheet product according to an exemplary embodiment of the present invention includes preparing an adhesive coating composition; forming an adhesive coating layer by applying and then curing the adhesive coating composition onto a surface of an electrical steel sheet; forming a heat-fused layer by laminating and heat-fusing a plurality of electrical steel sheets on which the adhesive coating layers are formed; and cooling the heat-fused electrical steel sheets at a cooling rate of 0.05 to 20° C./min. In an exemplary embodiment of the present invention, the electrical steel sheet is a non-oriented or oriented electrical steel sheet, and more specifically, may be a non-oriented electrical steel sheet.

[0044] The adhesive coating composition may contain an organic/inorganic complex containing an organic resin and an inorganic nanoparticle substituted for the organic resin; and an inorganic substance.

[0045] First, the organic resin will be described.

[0046] Specifically, the organic resin may be a thermoplastic adhesive resin. More specifically, the organic resin may include one or more selected from an epoxy-based resin, an ester-based resin, an acrylic resin, a styrene-based resin, a urethane-based resin, and an ethylene-based resin. Still more specifically, the organic resin may be a water-soluble epoxy resin. A molecular weight of the water-soluble epoxy resin may be adjusted while changing a ratio of biphenyl A (BPA) and epichlorohydrin (ECH). More specifically, the water-soluble epoxy resin may be a multifunctional epoxy resin having three or more epoxy groups. This is to improve heat resistant adhesiveness. The water-soluble resin may have a molecular weight of 1,000 to 50,000 g/mol, a softening temperature (Tg) of 70 to 120° C., and a solid fraction of 10 to 50 wt %. When a molecular weight of epoxy is too small, curability may deteriorate and physical properties of a coating film, such as strength, may deteriorate. On the contrary, when the molecular weight of epoxy is too large, phase separation may occur in an aqueous dispersion resin and compatibility with the inorganic nanoparticles may deteriorate. Specifically, the molecular weight of the epoxy resin may be 5,000 to 40,000 g/mol, and more specifically, the molecular weight of the epoxy resin may be 5,000 to 30,000 g/mol. In addition, the epoxy resin consists of a combination of bisphenol and epoxide; however, a part of a structural formula may be substituted with a polar group in order to be present in an aqueous dispersion state, and the epoxy resin may have a stable state without the phase separation such as deposition or precipitation in the aqueous dispersion state.

[0047] Next, the inorganic nanoparticle will be described. The inorganic nanoparticle may include one or more of SiO.sub.2, TiO.sub.2, ZnO, Al.sub.2O.sub.3, MgO, CaO, and ZrO.sub.2. Specifically, the inorganic particle may be SiO.sub.2, TiO.sub.2, or ZnO. In order to secure a high-temperature adhesive strength and high-temperature oil resistance, a nanoparticle in a colloidal state may be substituted for the above-described thermoplastic resin to modify the thermoplastic resin into an organic/inorganic complex form. In a case where the inorganic particle is SiO.sub.2, an average particle size of the inorganic particles may be 3 to 50 nm. In addition, the amount of SiO.sub.2 may be 1 to 40 parts by weight, and specifically, 3 to 40 parts by weight, with respect to 100 parts by weight of the organic resin. In a case where the inorganic particle is TiO.sub.2, an average particle size of the inorganic particles may be 20 to 100 nm. In addition, the amount of TiO.sub.2 may be 5 to 30 parts by weight with respect to 100 parts by weight of the organic resin. In a case where the inorganic nanoparticle is ZnO, an average particle size of the inorganic nanoparticles may be 3 to 100 nm, and specifically, 10 to 60 nm. In addition, the amount of ZnO may be 3 to 60 parts by weight with respect to 100 parts by weight of the organic resin. When an average particle size of inorganic nanoparticles in a colloidal state is too small, not only does it take a long time for the thermoplastic resin to be modified into a complex but it is not cost-effective because a substituted complex is expensive. On the contrary, when the average particle size of the inorganic nanoparticles is too large, compatibility with an adhesive resin deteriorates, and a weak boundary layer (WBL) is also generated due to a large particle size at an interface of an applied sample, resulting in deterioration of the adhesive strength due to introduction of oil or moisture through the weak boundary layer. In addition, when the amount of inorganic nanoparticles is too small, a ratio of the nanoparticles to a ratio of the adhesive resin in the coating layer is too low, and thus, heat resistance deteriorates. As a result, the high-temperature adhesive strength and oil resistance deteriorate. On the contrary, when the amount of inorganic nanoparticles is too large, heat resistance of the complex is improved, but a ratio of the organic resin in the coating layer is low, and thus, the high-temperature adhesive strength and oil resistance tend to deteriorate.

[0048] Next, the inorganic substance will be described. The inorganic substance may be phosphoric acid (H.sub.3PO.sub.4) or sodium hydroxide (NaOH). In order to maximize the high-temperature adhesive strength and high-temperature oil resistance of the above-described organic/inorganic complex, an inorganic substance having excellent solubility may be dissolved in the organic/inorganic complex. The amount of inorganic substance dissolved may be 1 to 70 parts by weight with respect to 100 parts by weight of a solid content of the organic/inorganic complex. In particular, in a case where the inorganic substance is phosphoric acid, the amount of inorganic substance dissolved may be 1 to 70 parts by weight, and specifically, 1 to 50 parts by weight. In addition, in a case where the inorganic substance is sodium hydroxide, the amount of inorganic substance dissolved may be 1 to 15 parts by weight, and specifically, 1 to 10 parts by weight. When the amount of inorganic substance dissolved is too small, a ratio of the inorganic substance in the complex is too low, and thus, the high-temperature adhesive strength and high-temperature oil resistance cannot be improved. On the contrary, when the amount of inorganic substance dissolved is too large, a deposition phenomenon occurs according to excessive dissolution of the inorganic substance in the complex, which improves heat resistance, but the high-temperature adhesive strength and oil resistance tend to deteriorate due to a relatively low ratio of the adhesive resin.

[0049] Next, the forming of the adhesive coating layer by applying and then curing the adhesive coating composition onto the surface of the electrical steel sheet will be described. The organic/inorganic complex adhesive solution in which the inorganic nanoparticles are substituted and the inorganic substance is dissolved as described above may be subjected to heat treatment in a temperature range of 200 to 600° C. for 5 to 40 seconds to be applied at a thickness of 0.5 to 10.0 μm per one surface. In such a case, the electrical steel sheet may have excellent surface properties (for example, an insulating property, corrosion resistance, and adhesion). In this case, the inorganic nanoparticles and the inorganic substance may be uniformly distributed.

[0050] Next, the forming of the heat-fused layer by laminating and heat-fusing the plurality of electrical steel sheets on which the adhesive coating layers are formed will be described. The coated sample may be heat-fused at a pressing force of 1 to 1,000 N/mm.sup.2 and a temperature of 120 to 300° C. for 5 to 180 minutes.

[0051] Next, the cooling of the heat-fused electrical steel sheets at the cooling rate of 0.05 to 20° C./min will be described. In this case, a holding pressing force may be 1,000 N/mm.sup.2 or less, and specifically, 500 N/mm.sup.2 or less. In addition, the cooling rate may be 0.05 to 1° C./min. Meanwhile, the cooling finish temperature may be 10 to 1,000° C., and specifically, room temperature. Such a bonding core cooling method is performed to further improve a fastening force of the adhesive composition. As a result of evaluating an adhesive strength (tensile strength, peeling-off) at room temperature, an adhesive strength at a high temperature (for example, 150° C.), and oil resistance at a high temperature (for example, 170° C.) of the sample manufactured by cooling the electrical steel sheet after heat fusion to room temperature, an electrical steel sheet product of which adhesive strength is significantly excellent and in which automatic transmission fluid (ATF) oil does not permeate into interfaces between single cores or the adhesive coating layer is not melted by oil is obtained.

[0052] Hereinafter, examples of the present invention will be described in detail so that those skilled in the art to which the present invention pertains may easily practice the present invention. However, the present invention may be implemented in various different forms and is not limited to examples described herein.

EXAMPLES

[0053] In order to maximize a high-temperature adhesive strength and high-temperature oil resistance of each of three types of organic/inorganic complexes (epoxy-SiO.sub.2-based, epoxy-TiO.sub.2-based, and epoxy-ZnO-based organic/inorganic complexes) shown in Table 1, a predetermined amount of sodium hydroxide (NaOH) or phosphoric acid (H.sub.3PO.sub.4) was dissolved in each of the organic/inorganic complexes. The amounts of sodium hydroxide (NaOH) or phosphoric acid (H.sub.3PO.sub.4) dissolved were 1 to 10 parts by weight and 1 to 70 parts by weight, respectively, with respect to 100 parts by weight of a solid content of the organic/inorganic complex. A molecular weight of the used epoxy adhesive resin was about 30,000, average particle sizes of SiO.sub.2, TiO.sub.2, and ZnO substituted for the epoxy adhesive resin were 25 nm, 20 nm, and 10 nm, respectively, and amounts of particles were 20 parts by weight, 15 parts by weight, and 30 parts by weight, respectively, with respect to 100 parts by weight of the epoxy resin. Basic surface properties (an insulating property, corrosion resistance, adhesion, and the like) of a non-oriented electrical steel sheet after coating were significantly excellent, and processability (slitting property and punchability) was also excellent.

[0054] As can be seen from Table 1, the solution stability tends to deteriorate as the amount of sodium hydroxide (NaOH) or phosphoric acid (H.sub.3PO.sub.4) dissolved increases. The room temperature adhesive strength is generally excellent regardless of the type and size of the substituted inorganic particles, but tends to deteriorate as the amount of sodium hydroxide (NaOH) or phosphoric acid (H.sub.3PO.sub.4) dissolved increases. This is because the amount of adhesive resin in the coating layer is relatively smaller than those of the nanoparticles and the inorganic substance. The high-temperature adhesive strength and the high-temperature oil resistance were similar in all the adhesive solutions, and in the case of the epoxy-SiO.sub.2-based complex adhesive solution, when the amount of sodium hydroxide (NaOH) or phosphoric acid (H.sub.3PO.sub.4) dissolved was 1 to 15 wt %, all the two properties were excellent. In addition, in the cases of the epoxy-TiO.sub.2-based complex adhesive solution and the epoxy-ZnO-based complex adhesive solution, when the amount of sodium hydroxide (NaOH) or phosphoric acid (H.sub.3PO.sub.4) dissolved was at an appropriate level, all the two properties were also excellent. It can be appreciated that this is related to not only dissolved sodium hydroxide (NaOH) or phosphoric acid (H.sub.3PO.sub.4) but also the size and amount of the substituted nanoparticles. In general, when the amount of the entire substituted and dissolved inorganic substances (the nanoparticles and the inorganic substance) is too small, a ratio of the adhesive resin contained in the adhesive solution is relatively low, resulting in deterioration of the high-temperature adhesive strength and the high-temperature oil resistance. On the contrary, when the amount of the inorganic substances (the nanoparticles and the inorganic substance) is too large, the heat resistance is improved by the inorganic substance, but the ratio of the adhesive resin contained in the adhesive solution is relatively low, and thus, the high-temperature adhesive strength and the high-temperature oil resistance deteriorate. Therefore, in the present invention, when the ratio of the inorganic substance to the organic substance in the organic/inorganic complex adhesive solution was 0.05 to 0.6, adhesive properties at a high temperature were excellent.

[0055] Table 1 shows the solution and adhesive properties depending on the amount of sodium hydroxide (NaOH) or phosphoric acid (H.sub.3PO.sub.4) dissolved.

TABLE-US-00001 TABLE 1 Solution and adhesive properties Room High- Dissolved temperature temperature High- Organic/inorganic Type of amount adhesive adhesive temperature complex inorganic (parts by Solution strength strength oil adhesive solution substance weight) stability (25° C.) (150° C.) resistance Inventive Epoxy (molecular NaOH 1 ⊚ ⊚ ◯ ◯ Example weight ≈ 30,000)- 5 ⊚ ⊚ ◯ ◯ SiO.sub.2 (25 nm, 20 10 ◯ ◯ ⊚ ⊚ parts by weight) 15 Δ Δ Δ Δ complex H.sub.3PO.sub.4 1 ⊚ ⊚ ◯ ◯ 30 ⊚ ◯ ◯ ◯ 50 ◯ ◯ ◯ ◯ 70 Δ Δ Δ Δ Epoxy (molecular NaOH 1 ⊚ ⊚ ◯ ◯ weight ≈ 30,000)- 5 ⊚ ⊚ ◯ ◯ TiO.sub.2 (20 nm, 15 10 ◯ ⊚ ⊚ ⊚ parts by weight) 15 Δ Δ Δ Δ complex H.sub.3PO.sub.4 1 ⊚ ⊚ ◯ ◯ 30 ⊚ ◯ ◯ ◯ 50 ◯ ◯ ◯ ◯ 70 Δ Δ Δ Δ Epoxy (molecular NaOH 1 ◯ ⊚ Δ Δ weight ≈ 50,000)- 5 ◯ ⊚ ⊚ ⊚ ZnO (10 nm, 30 10 ◯ ◯ ◯ ◯ parts by weight) 15 Δ Δ Δ Δ complex H.sub.3PO.sub.4 1 ◯ ⊚ Δ Δ 30 ◯ ◯ ⊚ ⊚ 50 ◯ ◯ ◯ ◯ 70 Δ Δ Δ Δ

[0056] Physical property determination is expressed by the symbols shown in Table 1 as follows: significantly excellent: ⊚, excellent: ◯, normal: Δ, and deteriorative: X.

[0057] In addition to the experiment shown in Table 1, cooling with a specific cooling condition after heat fusion was performed to further improve the fastening force of the adhesive composition.

[0058] FIG. 1 illustrates a flow chart of a method for manufacturing an electrical steel sheet product according to an exemplary embodiment of the present invention. FIG. 1 illustrates a flow chart of the heat fusion and cooling conditions (the holding pressing force and the cooling rate) for improving the adhesive strength of the electrical steel sheet, in particular, a peeling adhesive strength (T-Peel off) at room temperature.

[0059] The organic/inorganic complex (epoxy-SiO.sub.2-based+H.sub.3PO.sub.4) was prepared, and then, the organic/inorganic complex was coated and heat-fused onto a non-oriented electrical steel sheet strip. The results of measuring the adhesive strength depending on the cooling conditions (the holding pressing force and the cooling rate) are shown in Table 2. A molecular weight of the used epoxy adhesive resin was about 30,000, an average particle size of SiO.sub.2 substituted for the epoxy adhesive resin was 15 nm, and the amount of particles was 20 parts by weight with respect to 100 parts by weight of the epoxy resin. In order to improve the high-temperature adhesive strength, a bonding solution was prepared by dissolving 5 parts by weight of phosphoric acid (H.sub.3PO.sub.4) with respect to 100 parts by weight of a solid content of the organic/inorganic complex. The organic/inorganic complex adhesive solution in which the nanoparticles and the inorganic substance were substituted and dissolved as described above was applied at a thickness of 5.0 μm per one surface while being cured at 400° C. for 20 seconds, the coated samples were cut into a predetermined size and laminated, the laminated samples were heat-fused at a pressing force of 500 N/mm.sup.2 and a pressing temperature of 200° C. for a pressing time of 60 minutes, a holding pressing force was held at each of 0, 500 N/mm.sup.2, 1,000 N/mm.sup.2, and 1,500 N/mm.sup.2, cooling was performed to room temperature at a cooling rate of each of 0.5° C./min to 10° C./min and 30° C./min, and then, an adhesive strength was measured. It could be appreciated from Table 2 that there was a large difference in adhesive strength (a shearing method and a peeling-off method) depending on the cooling conditions (the holding pressing force and the cooling rate), and the lower the holding pressing force and the slower the cooling rate, the better the adhesive strength measured by each of the shearing method and the peeling-off method. In particular, the adhesive strength measured by each of the shearing method and the peeling-off method at a holding pressing force of 500 N/mm.sup.2 or less and a cooling rate of 1° C./min or less was excellent.

[0060] Table 2 shows the adhesive properties for each cooling condition.

TABLE-US-00002 TABLE 2 Adhesive properties Cooling condition Room temperature Bonding solution component Holding Cooling adhesive strength and initial heat-fusing pressing force rate Shearing Peeling-off condition (N/mm.sup.2) (° C./min) method method Inventive Epoxy-SiO.sub.2 Pressure 500 0 0.5 ⊚ ⊚ Example complex N/mm.sup.2, time 1 ⊚ ◯ (molecular 60 minutes, 5 ◯ Δ weight: about temperature 10 X X 30,000, SiO.sub.2 200° C. 500 0.5 ⊚ ◯ (15 nm 20 parts 1 ◯ ◯ by weight)) + 5 ◯ Δ H.sub.3PO.sub.4 (5 parts 10 Δ X by weight) 30 X X 1000 0.5 ◯ ◯ 1 ◯ Δ 5 Δ Δ 10 Δ X 1500 5 X X

[0061] Physical property determination is expressed by the symbols shown in Table 2 as follows: significantly excellent: ⊚, excellent: ◯, normal: Δ, and deteriorative: X.

[0062] FIG. 2 is a photograph captured with a transmission electron microscope (TEM) after a cross section of a coating layer of an electrical steel sheet is processed with focus ion beam (FIB), the coating layer being formed of a composition in which sodium hydroxide (NaOH) as an inorganic substance is contained in an epoxy (molecular weight 30,000)-SiO.sub.2 (25 nm, 20 parts by weight) complex, and is a photograph showing a distribution of nanoparticles (one of SiO.sub.2, TiO.sub.2, and ZnO) in the adhesive coating layer. In this case, white spots indicate the nanoparticles. It could be appreciated from FIG. 2 that the nanoparticles (one of SiO.sub.2, TiO.sub.2, and ZnO) were uniformly distributed in the coating layer, and the nanoparticles were uniformly distributed throughout the coating layer without a cohesion or aggregation phenomenon in the coating layer.

[0063] In addition, FIG. 3 is a photograph captured with a transmission electron microscope (TEM) after a cross section of a coating layer of an electrical steel sheet is processed with focus ion beam (FIB), the coating layer being formed of a composition in which sodium hydroxide (NaOH) as an inorganic substance is contained in an epoxy (molecular weight 30,000)-SiO.sub.2 (25 nm, 20 parts by weight) complex, and is a photograph showing a distribution of an inorganic substance (one of NaOH and H.sub.3PO.sub.4) dissolved in the adhesive coating layer. In this case, white spots indicate the inorganic substance. It could be appreciated that the components (Na and P) contained in the inorganic substance were also uniformly distributed in the adhesive coating layer. The heat resistance of the adhesive resin was significantly improved by the uniform distribution of the nanoparticles and the inorganic substance described above, and thus, the high-temperature adhesive strength of the adhesive solution was improved to a level higher than that required in a drive motor for an environmentally friendly vehicle (hybrid electric vehicle (HEV) or electric vehicle (EV)).

[0064] In the present invention, as for the solution stability, an organic/inorganic complex in which nanoparticles were substituted or an organic/inorganic complex adhesive solution in which an inorganic substance was dissolved was strongly agitated with an agitator for 30 minutes, and the mixed solution was maintained for 30 minutes. Thereafter, the solution stability was determined by the presence or absence of precipitation or gelation in the coating composition.

[0065] As for the room temperature and high-temperature adhesive strength, samples to which the adhesive solution was applied at a predetermined thickness per one surface were laminated, the laminated samples were heat-fused, and then, a tensile adhesive strength at each of room temperature and a high temperature (150° C.) was measured. The adhesive strength measured at room temperature was expressed as “significantly excellent” when it was 6.0 MPa or more, was expressed as “excellent” when it was 3.0 MPa or more, was expressed as “normal” when it was 1.0 MPa or more, and was expressed as “deteriorative” when it was 0.5 MPa or less. On the other hand, the adhesive strength measured at a high temperature was expressed as “significantly excellent” when it was 3.0 MPa or more, was expressed as “excellent” when it was 1.0 MPa or more, was expressed as “normal” when it was 0.5 MPa or more, and was expressed as “deteriorative” when it was 0.5 MPa or less.

[0066] As for the high-temperature oil resistance, a heat-fused sample was maintained in automatic transmission fluid (ATF) oil at a high temperature (170° C.) for 3 hours or longer, the sample was slowly cooled, and a surface state and a tensile adhesive strength thereof at room temperature were measured. When observing the surface state, oil should not permeate into the interfaces of the single cores or the adhesive coating layer should not be melted by the ATF oil. As for determination criteria of the oil resistance in the present invention, the adhesive strength of the sample subjected to the ATF test at a high temperature was expressed as “significantly excellent” when it was 3.0 MPa or more, was expressed as “excellent” when it was 1.0 MPa or more, was expressed as “normal” when it was 0.5 MPa or more, and was expressed as “deteriorative” when it was 0.5 MPa or less.

[0067] As for the peeling adhesive strength, each of two coated samples was cut into a predetermined size (200 mm×30 mm), the prepared two samples were adhered to each other by 150 mm in a length direction, a non-adhesive portion corresponding to 50 mm was spread by 180° from both sides to form a T shape, the samples were fixed to upper/lower jigs with a predetermined force, and then, a tensile adhesive strength of the laminated sample was measured while pulling the sample at a predetermined speed. In this case, an average of values measured at points except for initial points corresponding to 10% and final points corresponding to 10% of total points with a predetermined force during peeling-off was measured as the tensile adhesive strength. The peeling-off adhesive strength measured at room temperature was expressed as “significantly excellent” when it was 50 N or more, was expressed as “excellent” when it was 30 N or more, was expressed as “normal” when it was 10 N or more, and was expressed as “deteriorative” when it was 10 N or less.

[0068] The present invention is not limited to the exemplary embodiments, but may be manufactured in various different forms, and it will be apparent to those skilled in the art to which the present invention pertains that various modifications and alterations may be made without departing from the spirit or essential feature of the present invention. Therefore, it is to be understood that the exemplary embodiments described hereinabove are illustrative rather than being restrictive in all aspects.