Title of Invention

HIGHLY CONDUCTING RESIN-COATED METAL SHEET

Abstract

ABSTRACT Disclosed is a resin-coated metal sheet formed by coating a surface of a metal sheet with a resin film, wherein a surface of the resin-coated metal sheet has a number of peaks per 2.54 cm (PPI) of 10 or above when a peak count level 2H is 2.54 µm, and a Kurtosis (Rku) of 5.0 or below in a surface roughness profile indicating a surface roughness of the resin-coated metal sheet. The resin-coated metal sheet can stably exhibit satisfactory conductivity under a light-contact condition that applies a light pressure in the range of 10 to 12 gf/mm2 to the resin-coated metal sheet.

Full Text

HIGHLY CONDUCTING RESIN-COATED METAL SHEET BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a highly conducting resin-coated metal sheet. Description of the Related Art. Galvanized steel sheets have been widely used in the fields of domestic electric utensils, automobiles and building materials. A resin film of about 1 um in thickness is formed on a zinc coating of the galvanized steel sheet for those uses to improve the corrosion resistance, fingerprint preventive property and coating film adhesion. When the galvanized steel sheet is intended for forming parts of domestic electric utensils, the surface of the galvanized steel sheet is required to have, in addition to the foregoing properties, a conductive property (grounding property) to prevent unstable operations of electronic units and to shield the domestic electric utensils from noise. Generally, a resin film has an insulating property and hence the resin film reduces the conductivity of the surface of the resin-coated steel sheet. If the resin film is formed in a very small thickness or if the resin film is omitted to ensure conductivity for the galvanized steel sheet, the corrosion resistance and fingerprint preventive property of the galvanized steel sheet are unsatisfactory. Parts having complicated shapes have increased with the recent progressive miniaturization and increase in functions of electronic parts, and spatial restrictions in various devices have been imparted on parts. Consequently, surf aces of parts are required to have conductivity capable of ensuring satisfactory grounding even if the surfaces are brought simply into contact with each other or into light, elastic contact with each other. Research and development activities have been made to provide a steel sheet satisfactory in the foregoing properties. For example, a precoated steel sheet excellent in back conductivity disclosed in JP-A H7-265791 has a back surface having a specific arithmetical average roughness Ra and a specific PPI and coated with a film containing a Ni filler. A metal material satisfactory in conductivity disclosed in JP-A 2D05-139551 has a surface having sharp projections having height in the range of 0.5 to 30 um distributed in a pre¬determined density. The technique disclosed in JP-A H7-265791 ensures conductivity principally by the Ni filler contained in the film coating of the steel sheet. It is possible that the Ni filler deteriorates the corrosion resistance of the steel sheet, and the Ni filler increases the cost of the steel sheet. The conductivity of the metal material disclosed in JP-A 2005-139551 is satisfactory. However, the conductivity of the metal material is evaluated on the basis of surface insulation resistance specified in JIS C2550, and electric resistance weldability. Surface insulation resistance is measured by pressing a terminal against a specimen by a pressure of 2 MPa±5% approximately equal to 204 gf/mm2. The conductivity measured under suc'h a contact condition does not correspond to conductivity under a light-contact condition of a pressure between 10 and 12 gf/mm2. Techniques for ensuring conductivity under a light-contact condition are disclosed in JP-A 2005-238535 and JP-A 2004-277876. The technique mentioned in JP-A 2005-238535 proposes processing a plated sheet by temper rolling using rollers having a predetermined surface roughness (Ra) and a predetermined PPI on the basis of a knowledge that the texture of the surface of the base sheet affects the surface con¬ductivity. The technique mentioned in JP-A 2004-277876 properly controls the surface condition represented by arithmetical average roughness Ra and filtered centerline waviness (Wca) of a surface-treated galvanized steel sheet to ensure grounding property. It is mentioned in JP-A 2005-238535 that conductivity can be improved by using a plated base sheet having a high PPI. However, actually specified in JP-A 2005-238535 is the surface roughness (Ra and Wca) of the rollers for rolling. The surface roughness of the rollers is transferred to the surface of a steel sheet rolled by using those rollers. However, the surface roughness of the rollers cannot be exactly transferred to the surface of a steel sheet. Whereas the PPI of the surface of the rollers is counted at (Count level) ±0.638 urn, the thickness of an organic film formed on a surface of the steel sheet is in the range of 0.1 to 5 um. Therefore, there will be a case where any projections capable of exhibiting electrical conductivity are not found under a light-contact condition. Although JP-A 2004-277876 specifies Ra and Wca, in some cases, it will be difficult to know whether or not a surface has a surface property capable of conducting electricity under a light-contact condition from only those parameters. Moreover, it is hardly considered that, con¬ductivity evaluating methods employed in JP-A 2005-238535 and JP-A 2004-277876 are intended to evaluate conductivity under a light-contact condition. Although studies of resin-coated metal sheets have been made to ensure conductivity in addition to properties in¬cluding corrosion resistance, there has not been provided a resin-coated metal sheet meeting light-contact conductivity required by progressively miniaturized electronic parts having multiple functions. SUMMARY OF THE INVENTION The present invention has been made in view of those problems and it is therefore an object of the present invention to provide a resin-coated metal sheet capable of stably exhibiting satisfactory conductivity under a light-contact condition that applies a light pressure in the range of 10 to 12 gf/mm2 to the resin-coated metal sheet. A resin-coated metal sheet according to the present invention formed by coating a surface of a metal sheet with a resin film, wherein a surface of the resin-coated metal sheet has a number of peaks per 2.54 cm (PPI) of 10 or above when a peak count level 2H is 2.54 urn, and a Kurtosis (Rku) of 5.0 or below in a surface roughness profile Z (x) indicating a surface roughness of the resin-coated metal sheet. The inventors of the present invention made studies to improve the conductivity of a resin-coated metal sheet coated with a resin film of a thickness on the order of 1 urn under a light-contact condition and found that differences each between adjacent ones of ridges and valleys, and the shapes of ridges and valleys, as well as the number of irregularities in the surface of the resin-coated metal sheet, have a significant influence on stable conductivity under a light-contact condition. The inventors of the present in¬vention made further studies on the basis of the knowledge and found that a resin-coated metal sheet having a PPI of 10 or above and a Rku of 5.0 or below exhibits stable conductivity under a light-contact condition. The present invention has been made on the basis of those findings. The value of PPI is determined by a measuring method specified in SAE J911 JUN8 6 (Automotive Engineering Standards, USA) . PPI is an index of the surface quality of a resin-coated metal sheet indicating the number of irregularities having predetermined values of height and depth in a unit length in a surface roughness profile Z(x) indicating the surface quality of a surface of the resin-coated metal sheet. Ac¬cording to the present invention, the peak count level 2H for calculating PPI is 2.54 cm. Kurtosis (Rku) is a value measured by a measuring method specified in JIS B0601, (ISO 4284, 1997) and is an index of the sharpness of ridges or valleys in a unit length in a surface roughness profile indicating the surface quality of a re¬sin-coated metal sheet. Preferably, the mean thickness Y of the resin film of the resin-coated metal sheet is 1.2 urn or below, and the mean-thickness Y and the value of PPI meet a condition expressed by Expression (1). Y Preferably, the resin film contains an organic resin and inorganic particles. The resin-coated metal sheet of the present invention ensures high conductivity when the resin-coated metal sheet is pressed against another metal sheet by a low contact pressure. The resin-coated metal sheet of the present invention is useful for forming cases of electronic devices and such. BRIEF DESCRIPTION OF THE DRAWINGS The above and other objects features and advantages of ■ the present invention will become more apparent from the following description taken in connection with the accom¬panying drawings, in which: Fig. 1 is a diagram of assistance in explaining the concept of PPI; Fig. 2 is a diagram of assistance in explaining the concept of Rku; Fig. 3 is a graph showing experimental data showing the dependence of the conductivity of a resin-coated metal sheet on film thickness and PPI; and Fig. 4 is a view of assistance in explaining a con¬ductivity test method. DESCRIPTION OF THE PREFERRED EMBODIMENTS A resin-coated metal sheet in a preferred embodiment according to the present invention is formed by coating a surface of a metal sheet with a resin film. In a surface roughness profile Z(x) indicating the surface quality of the resin-coated metal sheet, PPI, namely, the number of ridges ' per 2.54 cm (1 in.) at a peak count level 2H of 2.54 urn is 10 or above, and Kurtosis (Rku) in the surface roughness profile Z (x) is 5.0 or below. The surface roughness profile Z (x) indicates measured values measured by a measuring method specified in JIS B0601, where cutoff value is 0.8 mm. As mentioned above, PPI is an index of the surface quality of a resin-coated metal sheet. PPI is measured by a measuring method specified in SAE Standards J911-1986. As shown in Fig. 1, a positive reference line b is at a distance +H from a mean line, a negative reference line a is at a distance -H from the mean line, and the distance between the positive reference line b and the negative reference line a is 2H. PPI is a ridge-valley count, namely, the sum of the number of ridges exceeding the positive reference line a at +H from the mean line, and the number of valleys exceeding the negative reference line a at -H from the mean line in 2.54 cm. The distance 2H between the positive reference line b and the negative reference line a is called a peak count level. According to SAE Standards, 2H = 50 uin. (1.27 una) . In this specification, 2H = 2.54 um (100 uin.). The arithmetical average roughness Ra of a surface of a metal sheet has been used as an index of the surface roughness of a resin-coated metal sheet formed by coating the surface of the metal sheet with a resin film. It was found through studies that in a resin-coated metal sheet having a resin film of a thickness on the order of 1 um intended to improve conductivity under a light-contact condition, the ridge-valley count PPI has a higher correlation with con¬ductivity than the arithmetical average roughness Ra. It is considered that the PPI has high correlation with conductivity for the following reasons. Usually, ridges and valleys originating from manu¬facturing conditions are formed in a surface of a metal sheet. Generally known resins are insulating materials. Parts of a resin film corresponding to ridges in a surface of a metal sheet are thinner than those of the resin film corresponding to valleys in the surface of the metal sheet. It is conjectured that a terminal, such as a grounding terminal, comes into contact with the ridges and is electrically connected to the metal sheet through the ridges to ensure conductivity. As shown by Expression (2), the arithmetical average roughness Ra is an index indicating the mean of absolute values represented by a roughness profile, and does not directly reflect the respective numbers of ridges and valleys in a surface of a metal sheet. Suppose, for example, that a roughness profile is a triangular wave. Then, the arith¬metical average roughness Ra of a roughness profile having one wave in a unit length and that of a roughness profile having 100 waves in a unit length are the same, provided that those roughness profiles have the same amplitude. It will be readily understood that the surface of a metal sheet having roughness represented by the former roughness profile has one contact point in a unit length and hence such a surface has difficulty in ensuring electrical continuity as compared with the surface of a metal sheet having roughness represented by the latter roughness profile having 100 contact points in a unit length. Thus, in some cases, there has been a tendency for surfaces having the same arithmetical average roughness Ra to have different conductivities. As mentioned above, PPI is the number of ridges having heights exceeding a predetermined height and valleys having depths exceeding a predetermined depth. A measured PPI can be an indirect index of contact points. Thus it is conjectured that PPI is highly correlated with conductivity. As mentioned above, the peak count level 2H is 2.54 um (100 uin.) in this specification. In most cases, the peak count level 2H = 1.27 um. specified in SAE Standards is employed, and there has not been any idea of employing a peak count level other than the peak count level specified in SAE Standard for improving conductivity. The inventors of the present in¬vention have found during the process of studies made by the inventors that the tendency of conductivity can be stably gripped by using the peak count level 2H = 2 . 54 urn when a coated metal sheet is coated with a coating film having a thickness on the order of 1 urn. Although reasons are not explicitly known, the inventors of the present invention infer that the use of the peak count level 2H = 2.54 um is proper for the following reasons. Generally, a resin film is formed on a surface of a metal sheet by a coating method including a coating process for coating a surface of the metal sheet with a film of a resin composition solution, and a drying process for evaporating moisture and solvent contained in the film of the resin composition solution. Although a resin film formed on the metal sheet is leveled to some extent during the drying process, the resin film is not perfectly leveled in a flat shape, and the irregularities in the surface of the metal sheet are reflected on the surface of the resin film. It was confirmed through experiments that the arithmetical average roughness Ra of the surface of the resin film is smaller than that of the surface of the metal sheet by 10% to 20%. Therefore, it is inferred that the surface of the metal sheet coated with the resin film having irregularities outside apeak count level 2H of 1.27 um have irregularities outside a peak count level 2H between 1.4 and 1.6 um. Suppose that irregularities in the surface of the metal sheet are represented by a triangular wave, and the coating film is perfectly leveled by the drying process . Then, theoretically, irregularities within a peak count level 2H between 1.4 and 1.6 um in the surface of the metal sheet are buried in the coating film if the coating film has a thickness between 0.7 and 0.8 um, and any irregularities are not found at all in the surface of the coating film. As mentioned above, the surface quality of the coating film reflects the roughness of a surface underlying the coating film. Therefore, since there are irregularities outside the peak count level 2H of 1.27 urn in the surface of the coating film, it is inferred that there are not any ridges capable of serving as contact point in most cases even though parts of the coating film corresponding to ridges are thinner than the mean film thickness. It is inferred on the basis of the foregoing knowledge that, theoretically, the surface of a metal sheet is required to have irregularities outside a peak count level 2H of 2 um at a minimum, and the surface of a coating film formed on the surface of the metal sheet is required to have irregularities outside a peak count level 2H between 1.6 and 1.8 um to form ridges capable of forming contact points on the surface of a resin-coated metal sheet. In view of such facts and thickness irregularities in a film formed by a production process, the present invention employs a peak count level 2H of 2.54 um (100 pin.). The PPI is 10 or above, preferably, 30 or above because contact points for ensuring conductivity under a light-contact condition is likely to be deficient if PPI is excessively small. There is not any particular upper limit to PPI. Generally, the surface quality of a metal sheet is principally dependent on the surface roughness of rolling rollers used for man¬ufacturing the metal sheet. Therefore, it is preferable that the value of PPI is 250 or below from the view point of preventing the reduction of surface roughness and extending the life of the rollers. More preferably, the value of PPI is 200 or below. The resin-coated metal sheet of the present invention-has the PPI mentioned above and a Kurtosis Rku of 5.0 or below. It was found during the process of studies for completing the present invention that conductivity is unstable in some cases even if the PPI meets the foregoing condition. It was found through further studies that the shape of ridges in the surface of a resin-coated metal sheet affects conductivity. Kurtosis Rku is a value measured by a method specified in JIS B0601 (ISO 4287, 1997) . Kurtosis Rku is calculated by dividing the mean biquadrate of heights of ridges in a reference length lr in a roughness profile Z(x) expressed by Expression (3) by the biquadrate of the root mean square roughness Rq of the roughness profile calculated by using Expression (4). The value of Kurtosis Rku indicates the sharpness of the distribution of values expressed by a probability density function over the roughness profile. The Kurtosis Rku of a normal distribution is 3. As shown in Fig. 2, a larger Rku indicates a sharper peak of a distribution of heights of ridges and valleys forming a roughness profile, and a smaller Rku indicates a gentler peak of a distribution of ridges and valleys of nearly equal heights. The value of Kurtosis Rku reflects sharp ridges or sharp valleys in the surface of a resin-coated metal sheet. A Kurtosis Rku of 5.0 or below indicates that ridges and valleys in the surface of a resin-coated metal sheet are not extremely sharp. Although the reason for Kurtosis Rku being an effective index of conductivity is not clearly known, it is inferred as follows. A conventional conductivity measuring method presses a grounding terminal against a resin-coated metal sheet by a comparatively high contact pressure and hence ridges projecting from the surface of the resin-coated metal sheet come first into contact with the grounding terminal. The pressure applied to the grounding terminal is concentrated on the ridges and the ridges collapse under a high pressure. Consequently, the grounding terminal can come into contact also with other ridges lower than those projecting from the surface of the resin-coated metal sheet, increasing con- ductivity. Thus it is considered that the influence of-the shape of the peaks in a roughness profile on conductivity is not significant because the conventional conductivity measuring method presses the grounding terminal against the resin-coated metal sheet by a high pressure. It is inferred that ridges greatly projecting from the surface of the resin-coated metal sheet impede contact of the grounding terminal with ridges lower than those projecting from the surface of the resin-coated metal sheet, and hence contact points sufficient for satisfactory conductivity cannot be formed. Preferably, the value of Rku is 3.5 or below. Although -there is no particular lower limit to Rku, it is desirable that the value of Rku is 2.0 or above. When the value of Rku is above 5.0, the surface has sharp ridges or extremely high ridges (extremely deep valleys) and ridges effective in forming contact points decrease. Consequently, conductivity is low. When the value of Rku is excessively small, the surface has gently curved ridges, and hence parts of the resin film corresponding to the ridges are thick. Therefore it is possible that those gently curved ridges cannot form contact points under a light-contact condition. A resin film for a resin-coated metal sheet of the present invention may be any one of resin films for conventional resin-coated metal sheets. More concretely, possible ma¬terials as a base resin, namely, a principal component of a resin film according to the present invention, include acrylic resins, melamine resins, phenolic resins, epoxy resins, urethane resins, polyester resins, polyamide resins, alkyd resins, polyolefin resins, silicone resins, fluorocarbon resins, aminoplast resins, vinyl chloride resins, poly¬carbonate resins. Those resins may be used individually or some of those resins may be used in combination. Preferably, the resin film of the present invention contains, as a base resin, one of the polyester resins or a composite emulsion containing a polyolefin resin. Polyester resins of plentiful types of the VYLON® series (TOYOBOCO., LTD.) are suitable polyester resins . A polyester resin crosslinked by a melamine resin or the line may be used. Commercially available melamine resins are those of Sumimal® series (SUMITOMO CHEMICAL CO., LTD.), and Cymel® series (MITSUI CYTEC LTD.). A preferable composite emulsion of a resin contains an ethylene-unsaturated carboxylic acid copolymer (including a neutral state) as a principal component, 0.2 to 0.8 mol (20 ■ to 80% by mol) of amide having a boiling pint of 100°c or below for 1 mol of carboxyl groups of the ethylene-unsaturated carboxylic acid copolymer, 0.02 to 0.4 mol (2 to 40% by mol of a compound of a monovalent metal for 1 mol of carboxyl groups of the ethylene-unsaturated carboxylic acid copolymer, 0.5 to 20% by mass of a crosslinking agent having at least two functional groups reactive with carboxyl groups in one molecule for 100% by mass of the nonvolatile matter of the composite emulsion, and does not contain an amine having a boiling point above 100°C and ammonia. • A resin film formed by using the composite emulsion has excellent properties including corrosion resistance, coating property, lubricity, workability and grounding property. Those excellent properties are mentioned in JP-A 2005-264312 . The ethylene-unsaturated carboxylic acid copolymer is a copolymer of ethylene, and an unsaturated carboxylic acid, such as methacrylic acid. The ethylene-unsaturated car¬boxylic acid copolymer can be obtained by a known, high-temperature, high-pressure polymerization process. Although a random copolymer is the most desirable copolymer, the ethylene-unsaturated carboxylic acid copolymer may be a block copolymer or a copolymer obtained by grafting un¬saturated carboxylic acid on ethylene. An olefin monomer obtained by modifying ethylene, such as propylene or 1-butene, may be used. A material obtained by copolymerizing a part on the order of 10% by mass or.below of a known vinyl monomer may be used without adversely affecting the object of the present invention. Preferably, the monomer contains 10 to 40% by mass unsaturated carboxylic acid. Since the ethylene-unsaturated carboxylic acid co¬polymer has carboxyl groups, the ethylene-unsaturated carboxylic acid copolymer can be emulsified in a water dispersion by neutralization using an organic base or metal ions . The organic base is amine having a boiling point of 100°C or below. Amines having a boiling point above 100°C are liable to remain on a steel sheet after a resin film formed on the steel sheet has been dried, and increases the water absorption property of a top coat. It is possible that increase in water absorption property cause the deterioration of the corrosion resistance. The foregoing boiling points are those at the atmospheric pressure. Amines having a boiling point not higher than 100°C are tertiary amines such as triethylamine, N,N-dimethylbutylamine, N,N-dimethylarylamine, N-methylpyrrolidine, tetrame-thyldiaminomethane, and trimethylamine; secondary amines including N-methylethyulamine, diisopropylamine, and di¬ethylamide; and primary amines including propylamine, t-butyl amine, sec-butyl amine, isobutyl amine, 1,2-dibutylpropyl amine, and 3-pentyl amine. Those amines may be individually used or some of those amines are used in combination. Tertiary amines are preferable. Triethylamine is the most desirable one. A preferable amount of the amine for 1 mol of carboxyl groups of the ethylene-unsaturated carboxylic acid copolymer is 0.2 to 0.8 mol (20 to 80% by mol). Corrosion resistance is satisfactory when the amount of the amine is in the range mentioned above. If the amount of the amine is below 0.2 mol, large resin particles are contained in the emulsion and the resin film cannot exercise the foregoing effects. If the amount of the amine is above 0.8 mol, the composite emulsion has a high viscosity and tends to adversely gelate. Preferably, the upper limit of the amount of the amine is 0 . 6 mol, desirably, 0.5 mol,. more desirably, 0.3 mol. Monovalent metal ions are used for preparing the composite emulsion. The monovalent metal ions are effective in improving solvent resistance and the hardness of a film. Preferably, the composite emulsion contains one or some of monovalent metals, such as sodium, potassium, and lithium. Hydroxides, carbonates or oxides of those metals are pre¬ferable. Above all those substances, NaOH, KOH, and LiOH are preferable. NaOH is the most efficient, desirable material. The use of compounds of bivalent metals and metals having valences greater than two are bit effective and hence compounds containing those metals are not used. The amount of a monovalent metal compound for 1 mol of carboxyl groups of the ethylene-unsaturated carboxylic acid copolymer is 0.02 to 0.4 mol (2 to 40% by mol). The metal compound emulsion is unstable when the amount of the metal compound is below 0.02 mol. The moisture absorption property (particularly, the property of absorbing alkali solution) increases, and corrosion resistance after a degreasing process deteriorates when the amount of the metal compound is above 0 . 4 mol. Preferably, the lower limit of the amount of the metal compound is 0.03 mol, desirably, 0.1 mol. Preferably, the upper limit of amount of the meal compound is 0.5 mol, desirably, 0.2 mol. The respective preferable amounts of the amine and the metal compound are in the foregoing ranges. The amine and the metal compound neutralize the carboxyl groups of the eth¬ylene-unsaturated carboxylic acid copolymer for emulsifi- cation. The viscosity of the composite emulsion rises sharply, it is possible that the composite emulsion solidified, and an excessive alkali deteriorates corrosion resistance if the sum of those amounts (neutralization amounts) is excessively large. Thus the excessively large sum of those amounts requires a large amount of volatilization energy and is undesirable. Sat¬isfactory emulsification cannot be achieved if the neu¬tralization amounts are excessively small. Thus it is preferable that the sum of the respective amounts of the amine and the monovalent metal compound for 1 mol of the carboxyl groups of the ethylene-unsaturated carboxylic acid copolymer is in the range of 0.3 to 1.0 mol. In a neutralization process (emulsification process) for the neutralization of the ethylene-unsaturated carboxylic acid copolymer by the amine and the monovalent metal ions, it is desirable to add the amine having a boiling point not higher than 100°C and the monovalent metal compound substantially simultaneously to the ethylene-unsaturated carboxylic acid copolymer or the amine having a boiling point not higher than 100°C is added first to the ethylene-unsaturated carboxylic acid copolymer. When the amine is added to the ethyl¬ene-unsaturated carboxylic acid copolymer after the mono¬valent metal compound, in some cases, corrosion resistance improving effect is unsatisfactory. The composite emulsion contains a crosslinking agent having at least two functional groups capable of reacting with carboxyl groups in one molecule for the chemical crosslinking of the ethylene-unsaturated carboxylic acid copolymer to form a strong resin film. Preferably the crosslinking agent concentration of the composite emulsion is in the range of 1 to 20% by mass, desirably, 5 to 10% by mass. Crosslinking effect of chemical bonding is insufficient and corrosion resistance improving effect is unsatisfactory when the crosslinking agent concentration is below 1% by mass. When the crosslinking agent concentration is above 20% by mass, the crosslink density of the resin film is excessively high, the hardness of the resin film is excessively high, the resin film cannot deform according to the deformation of the metal sheet and the resin film cracks when the resin-coated metal sheet is subjected to press work, and, consequently, the corrosion resistance and covering property of the resin film deteriorate. Desirably, the ratio of the amount of the crosslinking agent to that of the ethylene-unsaturated carboxylic acid copolymer is changed properly according to the amount of the carboxyl groups of the ethylene-unsaturated carboxylic acid copolymer by changing the amount of the crosslinking agent. Preferably, the composite emulsion contains 100 parts by mass of the ethylene-unsaturated carboxylic acid copolymer and 0.5 to 50 parts by mass, desirably, 5 to 20 parts by mass of the crosslinking agent. There is no particular restriction on the crosslinking agent having at least two functional groups capable of reacting with carboxyl groups in one molecule. Possible crosslinking agents are glycidyl group-containing linking agents including polyglycidylethers, such as sorbitol polyglycidylether, (poly)glycerol polyglycidylether, pentaerythritol poly¬glycidylether, trimethylolpropane polyglycidylether, neo-pentylglycol diglycidylether, and (poly)ethyleneglycol diglycidylether, and polyglycidyl amines; bifunctional aziridine compounds including 4,4'-bis(ethyleneimine car-bonylamino) diphenylmethane, N,N'-hexamethylene-1,6-bis(1-aziridine carboxyamide), N,N'-diphenylmethane-4,4'-bis(1-aziridine carboxyamide), toluene bisaziridine carboxyamide; and azirinyl group-containing crosslinking agents such as trifunctional or more aziridine compounds or derivatives of those compounds -including tri-1-aziridinylphosphineoxide, tris[1-(2-methyl) aziridnyl] phosphineoxide, trimethylolpropane tris(p-aziridinylpropionate), tris-2,4,6-(1-aziridinyl)-1, 3, 5-triazine, and tetrame-thylpropane tetraaziridinylpropionate. Those crosslinking agents may be individually used or some of those crosslinking agents may be used in combination. Azirid-inyl-group-containing crosslinking agents are particularly preferable . A multifunctional aziridine and a monofunctional aziridine, such as ethylene-imine, may be used in combination. The composite emulsion may contain a wax. When the composite emulsion contains, in solids content, 0.5 to 20% by mass wax, desirably, 0.5 to 10% by mass wax, more desirably, 0.5 to 5% by mass wax, a resin film formed from the composite emulsion is satisfactory in lubricity, scratch resistance, deep-drawability required by press working, blanking quality, die abrasion resistance, and resistance against blackening that may occur when the surface is rubbed during working. When the wax content is excessively high, the wax softens, liquefies or blooms, and concentrates in the interface between the resin film and a top coat or between the resin film and a surface modification layer, and corrosion resistance deteriorates after degreasing. The wax may be any suitable one. Possible waxes are natural waxes including microcrystalline waxes, and paraffin wax; synthetic waxes including polyethylene wax; and mixtures of some of those waxes. A wax having a softening point in the range of 80 to 140°C is preferable. A spherical polyethylene wax having a mean particle size in the range of 0.1 to 3 jam, preferably, in the range of 0.3 to 1.0 um is the most desirable wax. The wax remarkably improves lubricity, blanking quality, die abrasion resistance and deep-drawability. Commercially available spherical polyethylene waxes are, for example, DYEDIT E-17(G00 CHEMICAL CO., LTD.), KUE-1, KUE-5, and KUE-8 (SANYOU CHEMICAL INDUSTRIES, LTD.), W-100, W-200, W-300, W-400, W-500, W-640, and W-700 of the CHEM-IPEARL® series (MITSUI CHEMICALS, INC. ) , and Elepon E-20 (NICCA CHEMICAL CO. , LTD. ) . Preferably, the composite emulsion employed in the present invention contains the ethylene-unsaturated car-boxylic acid copolymer, the amine mentioned above, the monovalent metal compound and the crosslinking agent, such as an aziridine compound, as essential components, and a wax as the need arises. It is desired to determine the respective amounts of the-aziridine compound, the wax, and other additives so that the ethylene-unsaturated carboxylic acid copolymer content, in solids content, of the composite emulsion is 50% by mass or above. The composite emulsion is prepared by the following method. The ethylene-unsaturated carboxylic acid copolymer, namely, an essential component, and an aqueous medium are put in a homogenizer, the mixture of the ethylene-unsaturated carboxylic acid copolymer and the aqueous medium is heated at a temperature between 70 and 250°C when necessary. An aqueous solution of the amine and the monovalent metal compound is added to the mixture. The amine is added first to the mixture and then the monovalent metal compound is added to the mixture or the amine and the monovalent metal compound are added substantially simultaneously to the mixture. The mixture-is stirred by high-shear stirring. The wax and the crosslinking agent may be added to the mixture at any stage of a composite emulsion preparation process. It is desirable that the mixture is not heated after the crosslinking agent has been added to the mixture to prevent gelation due to the progress of a crosslinking reaction. Preferably, the mean thickness of the resin film of the present invention is 1.2 urn or below, desirably, between 0.1 and 1.0 um, more desirably, between 0.2 and 0.8 urn. Making sure of conductivity under a light-contact condition tends to be difficult when the resin film is excessively thick. In some cases, it is difficult for the resin film to exhibit desired effects, such as a corrosion-resistant effect and a decorative effect, when the resin film is excessively thin. The thickness of the resin film can be measured by the following method as well as by a measuring method which will be described later in connection with the description of the preferred embodiments. An area of 20 mm x 20 mm of the surface of a resin-coated metal sheet is coated with an Au film by Au evaporation. The resin-coated metal sheet is embedded in a resin, and the resin-coated metal sheet embedded in the resin is cut to exposure a section of the resin-coated metal sheet. Then, the section of the resin-coated metal sheet is polished to obtain a specimen. A surface layer of the section of the specimen is photographed by a SEM (scanning 'electron mi¬croscope) at an acceleration voltage of 20 kV and a x5000 magnification to obtain a microphotograph of the surface layer. The thickness of the resin film is determined on the basis of the thickness of the resin film in the microphotograph. Three microphotographs of three optional parts of the specimen are taken. The respective thicknesses of three parts of the resin film in each microphotograph are measured. The mean thickness of the resin film is calculated by using nine measurements. Preferably, the mean thickness Y and PPI of the resin film of the resin-coated metal sheet of the present invention meet a condition expressed by Expression (1). Y A generally known resin film has an insulating property and hence the thickness of a resin film has a significant influence on the■conductivity of a resin-coated metal sheet coated with the resin film. Similarly, PPI affects the conductivity. The correlation between the thickness and PPI of the resin film was studied and it was found through experiments that the resin-coated metal sheet has excellent, stable conductivity when the resin film meets the condition expressed by Expression (1). Preferably, the resin film contains inorganic particles . Inorganic particles contained in the resin film tend to harden the resin film. It is considered that contact points can be readily formed because micro-cracks develop around the inorganic particles when a grounding terminal is brought into contact with the resin film. Possible organic particles are those of silica (silicon dioxide) , Ca-ion exchange silica, Oxides and hydroxides of Al, Ti, Ce, Sb, Zr, Fe, Sn, Mg, Ca, and Zn, metallic salts, such as phosphates, sulfates, nitrates, and carbonates of Al, Mn, Mg, Ca, and Ni, molybdates, tungstates, banadates, and phosphomolybdates. Preferably, those inorganic particles have a 50%-volume mean particle size as measured by a laser diffraction method (dynamic light scattering) in the range of 1 to 100 nm, desirably, in the range of 2 to 20 run. Preferably, the inorganic particle content of the resin film is in the range of 5 to 80% by mass. It is difficult for inorganic particles to exhibit their effect when the inorganic particle content is low. When the inorganic particle content is excessively high, the resin content of the resin film is excessively low, and the resin film tends to crack. Desirably, the inorganic particle content of the resin film is in the range of 10 to 75% by mass, more desirably, in the range of 20 to 70% by mass. Possible metal sheets for the resin-coated metal sheet of the present invention are aluminum sheets, copper sheets, cold-rolled steel sheets, hot-dip galvanized steel sheets, and-electrogalvanized steel sheets. Galvanized steel sheets are particularly preferable. Electrogalvanized steel sheets are particularly preferable when the resin-coated metal sheet is intended for uses placing importance on corrosion resistance, aesthetic appearance and dimensional accuracy as well as conductivity, such as uses for domestic electric utensils. A galvanized steel sheet in which Zn and elements of the iron family, such as Fe, Co and Ni, are alloyed is an example of the galvanized steel sheet. From the viewpoint of for-mability, it is preferable that each of iron-family element contents is in the range of about 5 to about 2 0% by mass. Apref erable mass of deposit per unit area is, for example, 50 g/m2 or below, desirably, 40 g/m2 or below, more desirably, 35 g/m2 or below. Generally, in electrogalvanized steel sheets, the mass of deposit per unit area is 20 g/m2. Although a lower limit mass of deposit per unit area is not specified, a preferable lower limit mass of deposit per unit area is 5 g/m2 in view of corrosion resistance, desirably, 10 g/mz. A method of manufacturing the resin-coated metal sheet of the present invention will be described. Preferably, the resin-coated metal sheet of the present invention is formed by preparing a composite material for forming the resin film, coating a surface of a metal sheet with a film of the composite material, and drying the film of the composite material. The composite material is prepared by dispersing a matrix resin, and a crosslinking agent, which is used as the occasion demands, in water or diluting the composite material with an organic solvent. The concentration or viscosity of the composite material is adjusted properly such that the composite material can be easily applied to the metal sheet. Although any suitable organic solvent may be used, possible organic solvents are aromatic hydrocarbons including toluene, and xylene; aliphatic esters including ethyl acetate, and butyl acetate; alicyclic hydrocarbons including cyclohexane; aliphatic hydrocarbons including hexane, and pentane; and ketones including methylethyl ketone, and cyclohexanone. Preferably, the composite material has a solids content in the range of about 5 to about 35% by mass. The composite material may contain, in contents that will not spoil the object of the present invention, known additives ' generally used in the resin-coated metal sheet field, such as a flatting agent, a loading pigment, a rust preventive agent, an antisettling agent, a wax, and/or such. The composite material may be applied to the metal sheet by any suitable coating method. Possible coating methods are a roller coating method, a roller coating method, a spray method, and a curtain coating method. The metal sheet coated with a film of the composite material is subj ected to a drying process . It is preferable, when the composite material contains a crosslinking agent, that the drying process heats the workpiece at a temperature at which the crosslinking agent can react with the matrix resin. Desirably, the drying process heats the metal sheet coated with the film of the composite material at a temperature in the range of 40 to 250°C for a time in the range of 1 to 60 s . The metal sheet may be pretreated by a known surface treatment (base preparation), such as a chromate treatment or a phosphate treatment to improve corrosion resistance and to improve adhesion to a resin film. In view of preventing environmental pollution, it is pre¬ferable to use a metal sheet processed by a non-chromate treatment. To obtain a resin-coated metal sheet having a surface quality represented by the values of PPI and Rku (sometimes referred to as "surface quality" hereafter) meeting the foregoing condition, it is recommended to apply a surface roughness adjusting method to the manufacture of a metal sheet, such as a steel sheet or a galvanized steel sheet. When the metal sheet is an electrogalvanized steel sheet, it is preferable to adjust the surface quality of a steel sheet to be coated with a zinc coating beforehand because the surface quality of the zinc coating reflects the surface quality of the steel sheet. More concretely, a surface quality adjusting method for adjusting the surface quality represented by PPI and Rku processes a metal sheet by a tandem rolling process, a reverse rolling process or a temper rolling process by using rolling rollers having surfaces dull-finished by a shot blasting process, an electrical discharge texturing process or an etching process or processes the metal sheet by a shot blasting process or an etching process. The most desirable surface quality adjusting method processes a cold-rolled metal sheet by an annealing process, and then, rolls the metal sheet by a temper rolling process using rolling rollers having specified PPI under rolling conditions for rolling the metal sheet in a desired thickness. The Rku of the surface of a metal sheet needs to be controlled by controlling the Rku of the surfaces of the rolling rollers to transfer irregularities in the surface of the rolling rollers to the surface of the metal sheet. The Rku of the surfaces of the rolling rollers can be adjusted to a desired Rku by increasing the PPI of the rolling rollers to distribute irregularities uniformly over the surfaces of the rolling rollers. A preferable Rku of the surfaces of the rolling rollers is on the order of 3. Preferably, the surfaces of the rolling rollers have a PPI (peak count level 2H = 2.54 um) in the range of 100 to 300. Desirably, the surfaces of the rolling rollers have a PPI of 190 or above. It is preferable to finish the surface of the rolling rollers by a discharge dulling process to control the surface quality of the rolling rollers. The electrical discharge texturing process immerses a roller having a polished surface in an oil, disposes an electrode at a predetermined distance from the surface of the roller, and causes electric discharge between the roller and the electrode to form irregularities in the surface of the roller. The electrical discharge texturing process can easily form ridges of uniform height and valleys of uniform depth by properly adjusting the distance between the surface of the roller and the electrode, current, and voltage, and can form a surface having a large PPI. Other possible processes for processing rolling rollers are a shot blasting process, a laser beam texturing process, and an electron beam texturing process . ' Generally used processes are an electrical discharge texturing process, and a shot blasting process. Recommended temper rolling conditions for forming, for example, a metal sheet (steel sheet) having a thickness between 0.4 and 2.0 mm are a rolling reduction in the range of 0.5 to 3% (preferably, in the range of 0.8 to 2.5%), a unit tension in the range of 1 to 15 kgf/mm2 (preferably, in the range of 3 to 13 kgf/mm2, and a line load in the range of 100 to 650 kgf/mm (preferably, in the range of 150 to 600 kgf/mm). To permit the surface quality of the rolling roller to be satisfactorily transferred to the surface of the metal sheet, it is recommended to have a condition where (unit tension)/ (line load) is less than 0.030. Unit tension is tensile force acting on a metal sheet in the moving direction of the metal sheet. Line load is a force applied by a roller to a metal sheet. The condition where (unit tension)/ (line load) A steel sheet processed by temper rolling may be directly subjected to a resin film forming process. However, it is preferable to form a coating layer by plating on a surface of the steel sheet to improve the corrosion resistance of the steel sheet. To electroplate a steel sheet processed by temper rolling, the steel sheet is processed in a horizontal position by a horizontal electroplating system including a cur¬rent-carrying unit provided with a conductor roller of a metal and backup roller of rubber disposed in a vertical arrangement so as to hold the steel sheet horizontally. The horizontal electroplating system processes the metal sheet by an alkali scrubber degreasing process, an electrolytic degreasing process, a water washing process, and a sulfuric acid pickling process, and then coats the surface of the metal sheet with a coating layer in a plating bath by cathodic electrolysis. The plated surface of the metal sheet is cleaned by water washing, then, the composite material for forming a resin film is applied to the surface of the metal sheet by a roller coater or the like to form a film of the composite material, and then a solvent contained in the film of the composite material is evaporated and the film is dried by a dryer to complete a resin-coated metal sheet having a resin film coating the plated surface of the metal sheet. The resin-coated metal- sheet of the present invention has the foregoing surface quality. When the resin-coated metal sheet is used for forming boxes of electronic devices, the resin-coated metal sheet is processed such that the surface having the specific surface quality faces inside the boxes. Embodiments Resin-coated metal sheets in preferred embodiments of the present invention will be described. The following embodiments are only examples, are intended to illustrate the invention, and are not to be construed to limit the scope of the invention. Examples 1 to 30 A slab of an Al-killed low-carbon steel was cold-rolled to obtain steel sheets respectively having thicknesses of 0.5 mm, 0 . 8 mm and 1. 0 mm. The steel sheets were processed by temper rolling under conditions shown in Table 1 after being degreased, cleaned and annealed. PPIs of rolling rollers used for temper rolling are shown in Table 1. Surfaces of the rolling rollers respectively having PPIs of 190 and 220 were finished by an electrical discharge texturing process, and the surfaces of the rolling rollers having a PPI of 165 were finished by a shot blasting process. The steel sheets processed by temper rolling were passed through a horizontal electroplating system to process the steel sheets by an electrogalvanization process under conditions shown below. Masses of deposit per unit area are shown in Table 2. (i) Alkali scrubber degreasing process A surface of the steel sheet was decreased by using a 3% orthosodium silicate solution heated at 60°C. (ii) Electrolytic degreasing and water washing processes • The surface of the steel sheet was degreased by electrolytic degreasing by using a 3% orthosodium silicate solution heated at 60°C, and supplying current in 20 A/dm2, and then the surface was cleaned by water washing. (iii) Pickling and water washing processes The steel sheet was pickled by using a 5% sulfuric acid solution of an ordinary temperature, and then the steel sheet was cleaned by water washing. (iv) Electrogalvanizing and water washing processes The Electrogalvanizing process was carried out by using a plating bath having the following composition under the following conditions. Plating bath ZnSo4-7H20: 300 to 400 g/1 Na2S04: 50 to 100 g/l H2S04: 25 to 35 g/1 Current density: 50 to 200 A/dm2 Temperature of the plating bath: 60°C Flowing speed of the plating bath: 0.8 to 2.4 m/s The thus galvanized steel sheet was coated with resin films A and B of different composition, respectively. Resin film A (Organic film) The electrogalvanized steel sheets were processed by the water washing process. Then, a processing solution A was applied to the surfaces of the steel sheets by a roller coating method. Films of the processing solution A were baked to form films A having mean thicknesses shown Table 2 . The composition of the processing solution A and baking conditions are as follows (Resin A). Preparation of the processing solution A A polyester resin, VYLON®245 (TOYOBOCO., LTD.) was used. The processing solution was prepared by mixing 100% by mass in solid of the polyester resin, 20% by mass melamine crosslinking agent (Sumimal® M-40ST, SUMITOMO CHEMICAL CO., LTD. ) to obtain a composite mixture, and diluting the composite mixture with a mixed solvent containing 1 part by mass xylene and 1 part by mass cyclohexanone. The processing solution had a solid content of 10% by mass. Baking conditions The processing solution A was applied to the surfaces of the steel sheets processed by the electrogalvanizing process, the water washing process and the drying process by a roller coating method to form a film of the processing solution A on the steel sheets. Then, the steel sheets were held in a hot-air drying furnace for 50 s to heat the steel sheets up to 230°C to dry the films of the processing solution A. Thus the surfaces of the steel sheets were coated with films A, respectively. (Examples 1 and 2). Film B (Organic-inorganic film) The electrogalvanized steel sheets were processed by the water washing process. Then, a processing solution B (composite emulsion) was applied to the surfaces of the steel sheets by a roller coating method. Water was evaporated from films of the processing solution B and the films of the processing solution B were dried to form films B having mean thicknesses shown Table 2. The processing solution B was prepared by the following method, and the films of the processing solution B was dried under the following conditions. Preparation of the processing solution B An ethylene-acrylic acid copolymer emulsion was prepared by mixing 626 parts by mass of water (hereinafter, referred to simply as "parts"), 160 parts of an ethyl¬ene-acrylic acid copolymer containing 20% by mass acrylic acid and having a melt index Ml of 300, 40% by mol of triethylamine for 1 mol of carboxyl groups of the ethylene-acrylic acid copolymer, and 15% by mol of NaOH for 1 mol of carboxyl groups of the ethylene-acrylic acid copolymer in an autoclave by high-speed stirring in an atmosphere of 150°C and 0.5 MPa. Then, 5% by mol in solid content of 4, 4'-bis (ethyle- neiminocarbonylamino)diphenyl methane, namely, a crosslinking agent, (CHEMITITE® DZ-22E, NIPPON SHOKUBAI CO., LTD.) (hereinafter, the solid content of .the composite emulsion is 100% by mass), 5% by mass in solid content of a glycidyl-group-containing compound (EPICLON® CR5L (abbre¬viated to "CR5L") , DAINIPPON INK AND CHEMICALS INCORPORATED) , 30% by mass in solid content of silica particles having particle sizes in the range of 10 to 20 nm (SNOWTEX 40, NISSAN CHEMICAL INDUSTRIES, LTD.) , and 5% by mass in solid content of spherical polyethylene wax (CHEMIPEARL® W-700, MITSUI CHEMICALS, INC.) having a softening point of 120°C and a mean particle size of 1 urn were added to the emulsion and a mixture of those materials were stirred to obtain a composite emulsion, namely, the processing solution B. The composite emulsion had a solid content of 15% by mass. Drying conditions The composite emulsion was applied to the surfaces of the steel sheets processed by the electrogalvanizing process, and the water washing process by a roller coating method to form films of the composite emulsion on the surfaces, re¬spectively. Then, the films were dried by heating with hot air of 200°C flowing at 53 m/s for 1 to 2 s. Thus or¬ganic-inorganic films were formed on the steel sheets, respectively (Examples 3 to 30). Specimens of the resin-coated steel sheets in examples were evaluated by the following evaluation methods. Results of evaluation are shown in Table 2. Fig. 3 shows the relation among film thickness, PPI and conductivity. In Fig. 3, blank triangles, solid circles, solid triangles and crosses represent Examples 1 and 2, Examples 3 to 18, Examples 19 to 25 and Examples 26 to 30, respectively. Evaluation Methods (1) PPI PPI was measured by a method specified in SAE (Society of Automotive Engineers) J911-1986. Apeak count level 2H of 2.54 urn was used. A surface roughness tester (SURFCOM 1400A-3DF, TOKYO SEIMITSU CO., LTD.) was used for measurement. A small surface roughness tester (Surftest, SJ-301, MITUTOYO CORPORATION) was used for measuring the PPI of the surfaces of a rolling roller used for temper rolling. In the surface roughness test of the resin-coated steel sheets, cutoff value was 0.8 mm, the radius R of the spherical tip of the probe was 2 urn, and measuring length was 25.4 mm. Measuring length in the surface roughness test of the rolling roller was 4 mm and measured value was converted into a value for 25.4 mm. PPIs of two parts in a first direction, and two parts in a second direction perpendicular to the first direction on the re¬sin-coated steel sheet were measured. PPIs of parts of the roller along the length of the roller were measured. The respective mean PPIs of the surfaces of the resin-coated steel sheets, and the mean PPI of the surface of the roller were calculated using measured PPIs.. (2) Kurtosis Rku Kurtosis Rku was measured by a method specified in JIS B0601, (ISO 4287, 1997) . The same surface roughness tester as for PPIs (SURFCOM 1400A-3DF, TOKYO SEIMITSU CO., LTD.) was used for measurement. In the Kurtosis measurement of the resin-coated steel sheets, cutoff value was 0.8 mm, the radius R of the spherical tip of the probe was 2 jam, and measuring length was 25.4 mm. Kurtosis Rku of two parts in a first direction, and two parts in a second direction perpendicular to the first direction on the resin-coated steel sheet were measured. The respective means of Rku of the surfaces of the resin-coated steel sheets were calculated using measured values of Rku. The Kurtosis of only the resin-coated steel sheets coated with the organic resin film were measured. (3) Mass of Zn deposit per unit area An x-ray fluorescence analyzer (MXF-2100, SHIMADZU CORPORATION) was used for the measurement of the mass of Zn deposit per unit area on each of the steel sheets. A ca¬libration curve representing the relation between the amount of Zn and fluorescent x-ray intensity was determined be¬forehand, and the mass of Zn deposit per unit area was determined with reference to the calibration curve. (4) Mean thickness of the resin film (4-1) The filmAwas removed from the surface of the steel sheet after swelling the film A with N-methyl-2-pyrroidone, namely, a solvent. A mass of deposit per unit area was calculated by using the difference in mass between the steel sheet coated with the film A and the steel sheet not coated with the film A, and the area of the film A removed from the steel sheet. The calculated mass of deposit per unit area was divided by the specific gravity of the film A to determine the mean thickness t (urn) of the film A. (4-2) The amount of Si contained in the silica particles (silicon dioxide particles) contained in the film B was measured by x-ray fluorescence analysis using an x-ray fluorescence analyzer (MXF-2100, SHIMADZU CORPORATION). A calibration curve representing the relation between the amount of Si and fluorescent x-ray intensity was determined be¬forehand, and the Si content of the film B was determined with reference to the calibration curve. The mass of the film B was calculated on the basis of the measured Si content (intensity of fluorescent x-ray) by specific gravity con¬version, and the mean thickness t (urn) of the film B was determined. The following formula was used for the con¬version. t (urn) = (Si content (mg/m2) ) /{ (Si/Si02) * (Mass ratio of SiOa) x (Specific gravity of the resin film (kg/m3) ) where (Si/Si02) = 28/60, and (Mass ratio of Si02) = 0.3 (5) Conductivity Surface electrical resistances of the resin-coated metal sheets were measured by the following procedure using a tester (Multitester CX-250, CUSTOM CORPORATION). As shown in Fig. 4, two electrodes were held at 45° to the surface of the resin film. The two electrodes were pressed in light contact with the resin film and were moved along the surface of the resin film in the direction of the length of a specimen at a mean speed of 10 mm/s by a distance of 30 mm. The electrodes were kept in light contact with the resin film such that each electrode exerts a pressure of about 11 gf/mm2 equivalent to its weight of 7 g on the resin film. An indicated electrical resistance was read after the passage of 1 s or longer, in which the measurement was stabilized, after the start of measurement. Electrical resistances of five parts of the resin film were measured, and the mean electrical resistance of the five measured electrical resistance was calculated. It was decided that the mean electrical resistances not higher than 50 Q were excellent, the mean electrical resistances not higher than 100 Q were good, the mean electrical resistances not higher than 200 Q were fair, and the mean electrical resistances above 200 Q were bad. It is known from Table 2 that the resin-coated steel sheets in Examples 1 to 25 have PPIs not smaller than 10, values of Kurtosis (Rku) not greater than 5, which proves that the resin-coated steel sheets in Examples 1 to 25 are satisfactory in conductivity. It is known that the resin-coated steel sheet has particularly satisfactory conductivity when the mean thickness Y of the resin film and the PPI meet the condition expressed by Expression (1) and the resin film contains the organic resin and the inorganic particles. It is inferred that the resin-coated steel sheets in Examples 26 to 28 are unsatisfactory in conductivity because those resin-coated steel sheets have values of Rku above 5, and ridges in the surf acesthereof are not in a shape effective in contributing to conductivity. It is known from the values of Rku that ridges of a shape effective in contributing to conductivity are formed in the surfaces of the resin-coated steel sheets in Examples 29 and 30. However, it is inferred that the resin-coated steel sheets in Examples 29 and 30 are unsatisfactory in conductivity because the number of the ridges formed therein is small, i.e., the values of PPI are below 10, and hence the number of contact points are in¬sufficient . It is known from Table 1 that a resin-coated steel sheet having satisfactory values of PPI and Rku can be obtained when (Unit tension)/(line load) is below 0.030. The resin-coated metal sheet of the present invention exhibits excellent conductivity even in a light-contact condition where the contact pressure is in the range of 10 to 12 gf/mm2. Therefore, the resin-coated metal sheet of the present invention is suitable for forming boxes of electronic devices, electric devices and optical devices, and for forming domestic electric appliances. Although the invention has been described in its examples with a certain degree of particularity, obviously many changes and variations are possible therein. It is therefore to be understood that the present iriveiiLJ-on may ua pracucea otherwise than as specifically described herein without departing from the scope and spirit thereof. What Is Claimed Is: 1. A resin-coated metal sheet formed by coating a surface of a metal sheet with a resin film, wherein a surface of the resin-coated metal sheet has a number of peaks per 2.54 cm (PPI) of 10 or above when a peak count level 2H is 2.54 µm, and a Kurtosis (Rku) of 5.0 or below in a surface roughness profile indicating a surface roughness of the resin-coated metal sheet. 2. The resin-coated metal sheet according to claim 1, wherein a mean thickness Y of the resin film of the resin-coated metal sheet is 1.2 µm or below, and the mean thickness Y and the value of PPI meet a condition expressed by Expression (1). Y 3. The resin-coated metal sheet according to claim 1 or 2, wherein the resin film contains an organic resin and inorganic particles.

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2390-CHE-2008 AMENDED CLAIMS 03-12-2013.pdf

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2390-che-2008 abstract.pdf

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