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Laminated metal sheet and process for producing the sameRelated Patent Categories: Stock Material Or Miscellaneous Articles, Composite (nonstructural Laminate), Of MetalLaminated metal sheet and process for producing the same description/claimsThe Patent Description & Claims data below is from USPTO Patent Application 20060210814, Laminated metal sheet and process for producing the same. Brief Patent Description - Full Patent Description - Patent Application Claims
TECHNICAL FIELD [0001] The present invention relates to a laminated metal sheet used as a material for food cans and other containers and comprised of a metal sheet laminated on the front and back with two types of resins with different melting points and a method of production of the same. BACKGROUND ART [0002] As materials for food cans, beverage cans, aerosol cans, and other containers, metal sheets covered on their surfaces with a polyester, polyolefin, or other thermoplastic resin are being made much use of. In this case, the metal sheet used is generally steel sheet or aluminum sheet. Among these, for example, polyester-based resins are generally superior in corrosion resistance, flaw resistance, and printability and can be used for both the inner surface and outer surface of cans in some cases, but when the content is alkaline, the resin is insufficient in durability. Further, when used for meat-based food cans, there is the problem that the meat releasability is poor. To solve this problem, two-sided laminated metal sheets covered with different types of resin at the can inside surface side and the can outer surface side are being used. [0003] In general, as the can outer surface side film, a relatively hard polyester-based resin film is preferably used, while as the can inside surface side film, a lower melting point, excellent meat releasability and alkali durability, relatively soft polyolefin-based resin film is preferably used. Metal sheet laminated with different films on its two surfaces is disclosed in Japanese Patent Publication (A) No. 63-231926, Japanese National Publication (A) No. 2-501644, and Japanese Patent Publication (A) No. 2002-120324. Note that the terms "high melting point film" and "low melting point film" used in the present description do not mean films with melting points of absolute values. A relatively high melting point side film is called a "high melting point film", while a relatively low melting point side film is called a "low melting point film". [0004] In general, a laminated metal sheet is produced by the method of superposing a heated metal sheet and resin films and using lamination rolls to apply pressure to bond them (heat lamination method). To make them bond, the temperature of a film surface contacting the metal sheet has to be at least its melting start point Tsm (normally a temperature about 0 to 30.degree. C. lower than the melting point), more preferably at least the melting point MP, but on the other hand if the temperature of a film surface contacting a lamination roll becomes the melting start point or more, the film will stick to the lamination roll making production impossible. [0005] For this reason, the temperature of the metal sheet at the lamination part has to be strictly controlled in relation with the film melting points, but when using a polyester-based resin film as the high melting point film and using a polyolefin-based resin film as the low melting point film, since the melting points MP of the two greatly differ (for example, polyethylene terephthalate (polyester-based) has a melting point of 265.degree. C., while polypropylene (polyolefin-based) has one of 168.degree. C.), there is the problem that if setting the temperature of the metal sheet to match with one of the films, the other film will not bond well. [0006] Therefore, as shown in Japanese Patent Publication (A) No. 63-231926, the method of first laminating the high melting point resin film, then laminating the low melting point resin film in a later step where the temperature of the metal sheet falls, that is, a two-step lamination method, and, as shown in Japan National Publication (A) No. 2-501644, the method of reheating after the lamination step by the press-bonding of the lamination rolls so as to melt-bond the resins have been proposed, but both of these have the problems of swelling capital costs. Further, Japanese Patent Publication (A) No. 2002-120324 proposes a simultaneous lamination method matching the temperature of the metal sheet with the low melting point resin film, but it is believed that a low melting point resin for bonding purposes is required at the metal sheet side of the high melting point resin film, so again the cost rises. [0007] Further, in the prior art disclosed in Japanese Patent Publication (A) No. 63-231926 and Japanese Patent Publication (A) No. 2002-120324, since the low melting point resin film is kept from melt-bonding with the lamination roll by setting the temperature of the metal sheet to match with the low melting point resin film, the crystallization degree will not be lowered to an extent enabling the laminated low melting point resin film to be sufficiently worked. For this reason, when bending a two-sided laminated metal sheet to work it to a food can etc., the phenomenon of the worked part of the low melting point film whitening appears. It looks like the can contains foreign matter. Therefore, users and can makers would be liable to raise complaints. DISCLOSURE OF THE INVENTION [0008] The present invention solves the above problems of the prior art and provides a laminated metal sheet enabling simultaneous lamination of two types of film with different melting points, without sticking on the lamination rolls, on the two surfaces of a metal sheet and a method of production of the same. Further, the present invention provides more inexpensively a two-sided laminated metal sheet free from the low melting point resin film whitening even when working the sheet to produce a food can etc. The present invention has as its gist the following. [0009] (1) A laminated metal sheet obtained by laminating two types of film including a high melting point film and low melting point film on the two sides of a metal sheet, said laminated metal sheet characterized by having a low melting point film thickness d2 defined by the following equation 1: d2(.mu.m).gtoreq.k(.DELTA.MP-.DELTA.T)/V [0010] where, .DELTA.MP=MP1-MP2 [0011] MP1: melting point of high melting point film (.degree. C.) [0012] MP2: melting point of low melting point film (.degree. C.) [0013] k: constant determined by low melting point film's heat conductivity, heat capacity, and temperature, k.gtoreq.2 [.mu.mm/(.degree. C.s)]0<.DELTA.T=MP1-.PHI.Ti.ltoreq.50 (.degree. C.) [0014] Ti: metal sheet temperature at lamination roll inlet side (.degree. C.) .PHI.: constant determined by heat removal conditions at time of lamination (0.75.ltoreq..PHI.<1) [0015] V: sheet running speed (m/s) [0016] (2) A laminated metal sheet as set forth in (1), characterized in that said high melting point film is comprised of a polyester-based resin and said low melting point film is comprised of a polyolefin-based resin. [0017] (3) A laminated metal sheet as set forth in (2), characterized in that said polyester-based resin is selected from polyethylene terephthalate, a polyethylene terephthalate/isophthalate copolymer, polyethylene naphthalate, a polyethylene terephthalate/naphthalate copolymer, a mixed resin of polybutylene terephthalate and polyethylene terephthalate, a mixed resin of polybutylene terephthalate and a polyethylene terephthalate/isophthalate copolymer, and further one of these resins including a pigment or dye. [0018] (4) A laminated metal sheet as set forth in (2), characterized in that said olefin-based resin is selected from a polypropylene, polyethylene, a polypropylene/polyethylene random copolymer, or these resins including a pigment or dye. [0019] (5) A laminated metal sheet as set forth in (1), characterized in that a difference .DELTA.Hz between a haze value Hz2 of a low melting point film after bending a sheet of a thickness of 1 mm by 180 degrees across said high melting point film side and a haze value Hz1 of the low melting point film before bending is made not more than 20%. [0020] (6) A laminated metal sheet as set forth in (5), characterized in that the haze value Hz1 of the low melting point film before bending is made not more than 60%. [0021] (7) A laminated metal sheet as set forth in (1) or (2), characterized by using for said high melting point film a biaxially stretched polyester-based resin film and leaving crystal orientation at that surface layer. 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