Uniaxially oriented polyester film for metal plate coating, resin-coated metal plate, and method for manufacturing the uniaxially oriented polyester film for metal plate coating

Abstract

The present invention provides a uniaxially oriented polyester film for metal sheet coating that combines lamination properties, can-making processability, and feathering resistance, and a resin-coated metal sheet using the said film. The polyester film contains 70% by mass or more of polyester resin and an inorganic lubricant, has a crystallinity of 20-50%, a movable amorphous ratio of 1.0-50%, and the higher of the Young's modulus in the longitudinal direction and the Young's modulus in the width direction is E H , the lower one is E L The ratio of Young's modulus (E) in this case H / E L A uniaxially oriented polyester film for coating metal plates, wherein the ratio is 1.1 to 10.

Description

[Technical Field]

[0001] The present invention relates to a uniaxially oriented polyester film for coating metal plates, a resin-coated metal plate, and a method for manufacturing a uniaxially oriented polyester film for coating metal plates. [Background technology]

[0002] Traditionally, the inner and outer surfaces of metal containers made of tin-free steel (hereinafter referred to as TFS) or aluminum have been widely coated with paint for corrosion protection. Various thermosetting resins, such as epoxy resins and phenolic resins, have been used for such coatings. However, coating methods using thermosetting resins have problems: the long drying time required for the paint reduces productivity, and it consumes a great deal of energy and discharges large amounts of solvent.

[0003] Therefore, in order to solve these problems, a method has been proposed in which a thermoplastic resin is laminated onto a metal plate. One method for laminating a thermoplastic resin onto a metal plate is to heat a metal plate that has undergone various surface treatments such as plating, and then heat-press a thermoplastic resin film onto the metal plate to laminate it.

[0004] However, conventional thermoplastic resin films have problems such as thermal shrinkage when exposed to high-temperature atmospheres during heat-pressing, and the formation of air bubbles between the film and the metal plate, resulting in insufficient lamination properties. Furthermore, applying high tension to the film to suppress thermal shrinkage and air bubble formation also causes wrinkles in the film, which is another problem. When thermoplastic resin films shrink due to heat, the width of the product decreases, resulting in reduced productivity. In addition, the inclusion of defects such as air bubbles and wrinkles degrades the quality of the product.

[0005] In order to prevent the thermal shrinkage of the thermoplastic film, it is conceivable to lower the temperature of the metal plate during thermocompression bonding. However, when the temperature of the metal plate during thermocompression bonding is lowered, there is a problem that the adhesion between the metal plate and the thermoplastic film is poor. In addition, particularly when attempting to form a can that requires high processing, there is also a problem that the can manufacturing processability is poor. When the can manufacturing processability is poor, in addition to the film being shaved, broken, peeled off, etc., in some cases, the entire metal plate may break.

[0006] Therefore, various techniques have been proposed for the purpose of solving the problems caused by the low laminating property of the thermoplastic resin film and improving the adhesion between the metal plate and the thermoplastic resin film.

[0007] For example, in Patent Document 1, a polyester composite film having a structure in which three layers of an A layer, a B layer, and a C layer are laminated is proposed. The A layer is a layer using a resin having a high crystallization temperature responsible for heat resistance, the B layer is a layer using a high melting point resin responsible for impact resistance and deformation resistance, and the C layer is a layer using a resin having a low glass transition temperature responsible for adhesion to the metal plate.

[0008] In addition, in Patent Documents 2 to 4, laminating methods for suppressing the entrainment of bubbles are proposed. Specifically, in Patent Document 2, the diameter of the laminating roll and the rubber lining thickness are controlled. In Patent Document 3, a gas is sprayed near the laminating roll onto the traveling metal strip. In Patent Document 4, when continuously laminating, the space upstream of the pressure bonding roll is depressurized.

[0009] Furthermore, in Patent Document 5, a method of optimizing the Young's modulus and the thermal shrinkage rate by controlling the film manufacturing conditions to suppress bubbles and wrinkles is proposed.

[0010] Generally, metal containers are roughly classified into two-piece cans composed of two parts, a can body integrated with the can bottom and a lid, and three-piece cans composed of three parts, a can body, an upper lid, and a bottom lid. Two-piece cans have a beautiful appearance because they do not have a welded part on the can body, but generally require high processing performance.

[0011] On the other hand, in recent years, from the perspective of reducing material costs and conserving resources, the thickness of resin-coated metal sheets used in metal containers has been decreasing. When manufacturing cans of the same shape using these thinned resin-coated metal sheets, the degree of processing increases, which can lead to breakage or abrasion, particularly in the resin coating layer located on the outer surface of the metal container.

[0012] Furthermore, various types of printing are applied to the resin coating layer on the outer surface of the metal container to enhance its aesthetic appeal. However, cracks may occur in the resin coating layer when the can is heat-treated during the printing process. It is believed that these cracks occur because the crystals in the resin coating layer become oriented due to the processing performed during can manufacturing, reducing its adhesion to the metal plate, and the resulting shrinkage of the crystal-oriented resin coating layer due to heat treatment.

[0013] Therefore, in order to manufacture highly processed two-piece cans, material design is required to prevent rupture, abrasion of the resin coating layer, and crack formation during heat treatment of the can body.

[0014] Patent documents 6 to 8 propose techniques such as deep drawing and ironing for manufacturing two-piece can bodies using resin-coated metal sheets as the material. Furthermore, patent document 9 proposes a technique of adding organic lubricating components to the resin coating layer as a way to suppress rupture or abrasion of the resin coating layer when manufacturing highly processed two-piece can bodies.

[0015] Furthermore, a phenomenon called feathering can occur in which the coating resin remains on the surface during processes such as shearing resin-coated metal sheets, blank punching during the manufacturing of two-piece cans, and opening cans with EOE (Easy Open End) lids. When feathering occurs, the remaining coating resin can fall off and become mixed into the can product or the mold used for can manufacturing. Therefore, as a means to improve feathering resistance, Patent Document 10 proposes a method for controlling the thickness, elongation, degree of crystallinity, and heat of fusion of the film covering the metal sheet. [Prior art documents] [Patent Documents]

[0016] [Patent Document 1] Japanese Patent Application Publication No. 07-290666 [Patent Document 2] Japanese Patent Application Publication No. 06-008335 [Patent Document 3] Japanese Patent Application Publication No. 07-214724 [Patent Document 4] Japanese Patent Application Publication No. 07-186353 [Patent Document 5] International Publication No. 2020 / 090552 [Patent Document 6] Japanese Patent Application Publication No. 02-303634 [Patent Document 7] Japanese Patent Application Publication No. 04-091825 [Patent Document 8] Japanese Patent Publication No. 2004-148324 [Patent Document 9] Japanese Patent Publication No. 2017-30210 [Patent Document 10] Japanese Patent Application Publication No. 06-190965 [Overview of the Initiative] [Problems that the invention aims to solve]

[0017] However, the reality was that a resin-coated metal sheet possessing lamination properties, processability, and feathering resistance had yet to be realized.

[0018] Lamination For example, in order to suppress the inclusion of defects such as air bubbles and wrinkles when heat-press laminating a thermoplastic resin film onto a metal plate, it is necessary to suppress the thermal shrinkage of the film or to improve the stiffness of the film itself, as mentioned above.

[0019] Although Patent Document 1 improves heat resistance by using a three-layer polyester composite film, the heat resistance assumed in Patent Document 1 refers to heat resistance during can manufacturing and heat treatment after can manufacturing. Therefore, it is not possible to sufficiently suppress thermal shrinkage at high temperatures during lamination.

[0020] Similarly, the methods described in Patent Documents 2-4 cannot essentially suppress the thermal shrinkage of the film and are ineffective under high lamination temperature conditions.

[0021] Furthermore, while the method described in Patent Document 5 is effective in suppressing thermal shrinkage of the film and improving its rigidity, it cannot suppress cracks in the resin coating layer caused by the heat treatment of the can body during the printing process when manufacturing a two-piece can. This is because, in the aforementioned method, the crystallization of the film progresses too much, and a completely random molecular chain structure (movable amorphous), which is important for adhesion to the metal plate and processability during can manufacturing, does not exist. In addition, if one attempts to laminate at an even higher temperature to melt the crystals in the film that have crystallized too much, the film will ultimately shrink due to heat and defects will be introduced. Moreover, the film may over-melt and fuse to the laminating roll, significantly increasing the frequency of laminating roll replacement. Furthermore, a lot of energy is required to heat the metal plate to an even higher temperature.

[0022] • Can manufacturing processability Furthermore, in order to suppress rupture and abrasion of the resin coating layer when manufacturing highly processed two-piece can bodies, it is necessary to improve the processability of the can by adding a lubricant to the resin coating layer to ensure surface sliding properties or by reducing the crystallinity of the resin coating layer to ensure elongation.

[0023] However, the addition of lubricants can hinder the adhesion between the resin coating layer and the printing ink, potentially compromising the aesthetic appearance. Therefore, conventional technology has not been able to achieve both the required processability and ink adhesion, making it impossible to obtain a resin-coated metal sheet that combines processability for can manufacturing with an aesthetically pleasing appearance after printing.

[0024] For example, the method described in Patent Document 9 attempts to achieve both processability and ink adhesion by adding a lubricant to the resin coating layer and controlling the water contact angle of the resin coating layer. However, on hydrophobic surfaces with a water contact angle of 80° or more, sufficient affinity between the resin coating layer and the printing ink may not be obtained, and it is thought that the printing ink may peel off during various processes such as filling the contents after printing, sealing the lid, and transportation. Furthermore, methods that reduce the crystallinity of the resin coating layer are difficult to achieve in conjunction with the feathering resistance described later.

[0025] • Feathering resistance To improve feathering resistance, there is a method of controlling the crystallinity of the film to make it easier to tear, as described in Patent Document 10. However, in the case of resin-coated metal sheets that require a high degree of processing, such as two-piece cans, the design is such that the crystallinity of the coating resin is reduced, as mentioned above. As a result, when the resin-coated metal sheet is sheared or punched out, the film stretches and remains. Therefore, it can be said that there is a trade-off between can-making processability and feathering resistance.

[0026] This invention has been made in view of the above circumstances. Its purpose is to provide a uniaxially oriented polyester film for metal sheet coating that combines lamination properties, can-making processability, and feathering resistance, and a resin-coated metal sheet using the said film. [Means for solving the problem]

[0027] As a result of their diligent research to solve the above problems, the authors obtained the following findings.

[0028] (1) When a thermoplastic resin film is applied to a metal plate using a thermocompression lamination method at high temperatures, it is important to include an inorganic lubricant in the polyester resin that makes up the film in order to suppress the inclusion of air bubbles and wrinkles due to the thermal shrinkage of the film. In addition, it is important to optimize the film manufacturing conditions and control the balance between the degree of crystallinity of the film, the movable amorphous ratio, and the Young's modulus in the longitudinal and width directions.

[0029] (2) In order to suppress the crystal orientation of the film during severe processing such as that of a two-piece can, that is, in order to suppress the occurrence of cracks in the resin coating layer during heat treatment of the can body, it is important to have a certain amount of movable amorphous material in the film before lamination.

[0030] In general film manufacturing, crystallization can be promoted by biaxial stretching or further promoted by heat fixation. Films manufactured in this way have strong molecular chain entanglement and do not contain movable amorphous material, so even if the crystals melt during lamination, the molecular chain entanglement may not be completely undone. Therefore, crystal orientation progresses significantly with harsh can manufacturing processes. On the other hand, when a film manufactured with controlled crystallinity so that a certain amount of movable amorphous material is present is coated onto a metal plate using a thermocompression lamination method, the degree of crystal orientation progresses only slightly even with harsh can manufacturing processes. As a result, crack formation in the resin coating layer during can heat treatment can be suppressed.

[0031] (3) When performing highly processed can molding, it is necessary to reduce the crystallinity of the resin coating and impart ductility in order to suppress rupture and abrasion of the resin coating layer. However, while resin coatings with low crystallinity have excellent can-making processability, their ductility causes feathering during shearing and punching processes.

[0032] Therefore, by adjusting the Young's modulus in the longitudinal and width directions of the film before heat-pressure lamination and controlling the balance of anisotropy, excellent feathering resistance can be achieved in the resin-coated metal plate after heat-pressure lamination. The mechanism by which excellent feathering resistance is achieved is not clear, but it is thought to be as follows: Even if the crystals melt and become amorphous-rich during heat-pressure lamination, there are amorphous regions with orientation (rigid amorphous regions). In such a state, the film is prone to tearing in the direction of strong rigid amorphous orientation, and has low elongation in the direction perpendicular to that orientation, thus suppressing feathering.

[0033] The present invention has been completed based on the above findings, and the gist thereof is as follows.

[0034] (1) A polyester film containing 70% by mass or more of a polyester resin and an inorganic lubricant, having a crystallinity of 20 to 50%, a mobile amorphous ratio of 1.0 to 50%, wherein, among the Young's modulus in the longitudinal direction and the Young's modulus in the width direction, the higher one is E L , H , the lower one is E L , and the ratio of the Young's modulus (E H / E L ) is 1.1 to 10, a uniaxially stretched polyester film for metal plate coating.

[0035] (2) The uniaxially stretched polyester film for metal plate coating according to (1) above, wherein both E H and E L are 1000 to 19000 MPa.

[0036] <00​​​​​​​​​​​​​​​​​​​​​​​

[0040] (7) The uniaxially oriented polyester film for metal plate coating according to (6) above, wherein the content of the inorganic pigment contained in the A layer is 7.0 to 28% by mass of the total components of the uniaxially oriented polyester film for metal plate coating.

[0041] (8) The uniaxially oriented polyester film for coating metal plates according to any one of (5) to (7) above, wherein the B layer contains wax.

[0042] (9) The uniaxially oriented polyester film for coating metal plates according to (8), wherein the wax contained in the B layer is a polyolefin wax.

[0043] (10) The uniaxially oriented polyester film for coating metal plates according to (9) above, wherein the polyolefin wax is any of polyethylene wax, polypropylene wax, acid-modified polyethylene wax, acid-modified polypropylene wax, oxidized polyethylene wax, and oxidized polypropylene wax, or a mixture thereof.

[0044] (11) The uniaxially oriented polyester film for coating metal plates according to (9) or (10) above, wherein the acid value of the polyolefin wax is 1.0 to 120 mg KOH / g.

[0045] (12) The uniaxially oriented polyester film for coating metal plates according to any one of (8) to (11) above, wherein the content of the wax contained in the B layer is 0.050 to 1.2% by mass of the total components of the B layer.

[0046] (13) A uniaxially oriented polyester film for coating metal plates according to any one of (1) to (12) above, wherein the film thickness is 10 to 50 μm.

[0047] (14) Total thickness t of the B layer B The total thickness t of layer A relative to the above A The ratio (t A / tB A uniaxially oriented polyester film for coating metal plates according to any one of (5) to (12) above, wherein the ratio is 2.5 to 7.5.

[0048] (15) A uniaxially oriented polyester film for coating metal plates according to any one of (1) to (14) above, wherein the stretching direction is transverse.

[0049] (16) A uniaxially oriented polyester film for coating metal plates, as described in any one of (1) to (15) above, for use as a surface coating for metal plates for containers.

[0050] (17) A resin-coated metal sheet having at least one side of a uniaxially oriented polyester film for metal sheet coating as described in any one of (1) to (16) above.

[0051] (18) A process of melting and kneading a mixture containing polyester resin and an inorganic lubricant in an extruder to obtain a molten resin, The process involves extruding the molten resin in a sheet-like form from a T-die and cooling and solidifying it on a casting drum to form an unstretched film. A step of stretching the unstretched film in the longitudinal or transverse direction under the conditions that the stretching temperature is above the glass transition temperature of the polyester resin and below the glass transition temperature + 50°C, and the stretching ratio is 4.5 to 8.0 times. A step of heat-fixing the stretched film while relaxing it in the stretching direction, and A method for producing a uniaxially oriented polyester film for coating a metal plate according to any one of (1) to (16) above, comprising the step of cooling the heat-fixed film to room temperature. [Effects of the Invention]

[0052] The uniaxially oriented polyester film for metal plate coating according to the present invention provides a resin-coated metal plate that combines lamination properties, can-making processability, and feathering resistance. Specifically, according to the present invention, wrinkles and thermal shrinkage of the film can be suppressed, especially at high temperatures, in the thermocompression lamination method. Furthermore, according to the present invention, in the manufacture of two-piece cans requiring a high degree of processability, rupture and abrasion of the resin coating layer during can-making, as well as crack formation in the resin coating layer during the can body heat treatment process, can be suppressed. Moreover, according to the present invention, in addition to these properties, excellent feathering resistance can also be achieved. [Modes for carrying out the invention]

[0053] The present invention will be described in detail below.

[0054] [Uniaxially oriented polyester film for metal plate coating] The uniaxially oriented polyester film for metal plate coating in one embodiment of the present invention is a polyester film containing 70% by mass or more of polyester resin and an inorganic lubricant. The content of polyester resin in the total components of the polyester film is preferably 80% by mass or more, and more preferably 85% by mass or more. On the other hand, there is no particular upper limit to the content of polyester resin, but it is preferably 99% by mass or less, more preferably 95% by mass or less, and even more preferably 90% by mass or less.

[0055] (Polyester resin) The polyester resin is preferably a resin obtained by polymerization of monomers mainly composed of aromatic dicarboxylic acids or aliphatic dicarboxylic acids and diols, or a mixture thereof.

[0056] Examples of the aforementioned aromatic dicarboxylic acids include terephthalic acid, isophthalic acid, phthalic acid, naphthalenedicarboxylic acid, diphenyldicarboxylic acid, diphenyletherdicarboxylic acid, diphenylsulfondicarboxylic acid, diphenoxyethanedicarboxylic acid, and 5-sodium sulfisophthalic acid.

[0057] Examples of the aliphatic dicarboxylic acids include oxalic acid, succinic acid, adipic acid, suberic acid, sebacic acid, dimer acid, maleic acid, fumaric acid, dodecanedionic acid, cyclohexanedicarboxylic acid and their ester derivatives.

[0058] These dicarboxylic acids may be used individually, or two or more may be used in combination. Furthermore, oxycarboxylic acids such as p-oxybenzoic acid may be copolymerized.

[0059] Examples of the aforementioned diol components include ethylene glycol, propanediol, butanediol, pentanediol, hexanediol, neopentyl glycol, cyclohexanedimethanol, diethylene glycol, triethylene glycol, polyalkylene glycol, 2,2-bis(4-hydroxyethoxyphenyl)propane, isosorbide (1,4:3,6-dianhydroglucitol, 1,4:3,6-dianhydro-D-sorbitol), spiroglycol, bisphenol A, and bisphenol S. Among these, it is preferable to use at least one of ethylene glycol and butanediol. Only one of these diol components may be used, or two or more may be used in combination.

[0060] As the polyester resin constituting the uniaxially oriented polyester film for metal plate coating of the present invention, it is preferable to use at least one selected from the group consisting of polyethylene terephthalate and its copolymers, and polybutylene terephthalate and its copolymers, from the viewpoint of cost, ease of molding as a film, and adhesion to the metal plate. Mixtures thereof can also be preferably used.

[0061] The polyester resin constituting the uniaxially oriented polyester film for metal plate coating of the present invention may be copolymerized with polyfunctional compounds such as trimellitic acid, trimesic acid, and trimethylolpropane, as long as it does not hinder the effects of the present invention. Furthermore, resin components other than polyester may be added for the purpose of imparting functionality. Examples of resin components other than polyester include chain-like polyolefins such as polyethylene, polypropylene, poly(4-methylpentene-1), and polyacetal; alicyclic polyolefins which are ring-opening metathesis polymerization, addition polymerization, or addition copolymers with other olefins of norbornene; biodegradable polymers such as polylactic acid and polybutyl succinate; polyamides such as nylon 6, nylon 11, nylon 12, and nylon 66; aramids; polymethyl methacrylate; polyvinyl chloride; polyvinylidene chloride; polyvinyl alcohol; and poly Vinyl butyral, ethylene vinyl acetate copolymer, polyacetal, polyglycolic acid, polystyrene, styrene copolymer polymethyl methacrylate, polycarbonate, polyethersulfone, polyetheretherketone, modified polyphenylene ether, polyphenylene sulfide, polyetherimide, polyimide, polyarylate, tetrafluoroethylene resin, trifluoroethylene resin, trifluoroethylene chloride resin, tetrafluoroethylene-hexafluoropropylene copolymer, polyvinylidene fluoride, etc. can be used. These may be copolymers or mixtures.

[0062] (Inorganic lubricant) The uniaxially oriented polyester film for metal plate coating of the present invention contains an inorganic lubricant. By including an inorganic lubricant, the film can be given slipperiness, and wrinkle formation during heat-seal lamination can be suppressed.

[0063] Any inorganic lubricant that can impart slipperiness when added to a polyester film can be used. Examples of inorganic lubricants include oxide ceramics such as silicon dioxide, aluminum oxide, titanium dioxide, zirconium oxide, magnesium oxide, zinc oxide, barium titanate, and lead zirconate titanate, as well as talc, mica, calcium carbonate, and barium sulfate, and mixtures thereof may also be used.

[0064] The content of the inorganic lubricant is not particularly limited, but from the viewpoint of further suppressing wrinkle formation during heat-seal lamination, it is preferable that it be 50 ppm or more relative to the total components of the film. More preferably 80 ppm or more, and even more preferably 100 ppm or more. On the other hand, from the viewpoint of suppressing the rise in resin pressure due to filter clogging in the melt extrusion process of film manufacturing and reducing manufacturing costs, it is preferable that the content of the inorganic lubricant be 3000 ppm or less relative to the total components of the film. More preferably 2000 ppm or less, and even more preferably 1000 ppm or less.

[0065] The method for adding the inorganic lubricant to the uniaxially oriented polyester film for metal plate coating according to the present invention is not particularly limited and can be added by any method. For example, the inorganic lubricant may be added together with the polyester resin to the extruder used for melt-extruding the raw materials during film production, and then mixed and dispersed in the film-making extruder. Alternatively, the polyester resin and inorganic lubricant may be mixed in advance using a separate extruder to prepare a masterbatch, and then the masterbatch of the inorganic lubricant may be added together with the polyester resin to the extruder used for melt-extruding the raw materials during film production. It should be noted that the method of adding the inorganic lubricant as a masterbatch is preferred because it allows for the uniform and well-dispersible addition of a small amount of inorganic lubricant to the polyester resin.

[0066] • Crystallinity: 20-50% The uniaxially oriented polyester film for metal plate coating of the present invention has a crystallinity of 20 to 50%. By setting the crystallinity to 20% or more, it is possible to suppress the incorporation of air bubbles when coating the metal plate with the film using the thermocompression lamination method. The crystallinity is preferably 25% or more, and more preferably 30% or more. On the other hand, by setting the crystallinity to 50% or less, it is possible to suppress the rupture and abrasion of the resin coating layer during the can manufacturing process of the resin-coated metal plate, as well as the occurrence of cracks in the resin coating layer during the can body heat treatment process. The crystallinity is preferably 45% or less, and more preferably 40% or less.

[0067] The degree of crystallinity can be measured using a differential scanning calorimeter. More specifically, it can be measured by the method described in the examples.

[0068] To achieve a degree of crystallinity within the above range, it is crucial to optimize the type of polyester resin used in the film and the film manufacturing conditions. This can be achieved, in particular, by stretching the film in one direction to induce oriented crystallization, or by thermal crystallization through heat fixation.

[0069] ·Movable amorphous ratio: 1.0~50% The uniaxially oriented polyester film for metal plate coating of the present invention has a movable amorphous ratio of 1.0 to 50%. By setting the movable amorphous ratio to 1.0% or more, it is possible to suppress the rupture and abrasion of the resin coating layer during the can manufacturing process of resin-coated metal plates, as well as the occurrence of cracks in the resin coating layer during the can body heat treatment process. The movable amorphous ratio is preferably 5.0% or more, and more preferably 10% or more. On the other hand, by setting the movable amorphous ratio to 50% or less, it is possible to suppress welding to the laminating roll when coating the metal plate with the film using the thermocompression lamination method. The movable amorphous ratio is preferably 40% or less, and more preferably 30% or less.

[0070] The aforementioned movable amorphous ratio can be measured by a differential scanning calorimeter. More specifically, it can be measured by the method described in the examples.

[0071] To keep the movable amorphous ratio within the above range, it is important to optimize the type of polyester resin constituting the film and the film manufacturing conditions, and in particular, it is necessary to avoid excessive film stretching and heat setting. For example, the above movable amorphous ratio can be achieved by stretching the film in only one direction and adjusting the heat setting temperature and time so as not to crystallize the movable amorphous material.

[0072] When stretching a film in only one direction, stretching in the longitudinal direction (the direction of the machine line) is preferable from the viewpoint of production speed and equipment size. On the other hand, if a wide product width is required, stretching in the width direction (a direction perpendicular to the direction of the machine line) is preferable.

[0073] ·E H / E L :1.1~10 The uniaxially oriented polyester film for metal plate coating of the present invention has a Young's modulus in the longitudinal direction and a Young's modulus in the width direction, with the higher of these being E H , the lower one is E L The ratio of Young's modulus (E) in this case H / E L ) is 1.1 to 10. E H / E L By setting this to 1.1 or higher, feathering resistance can be improved. H / E L It is preferably 2.0 or higher, more preferably 3.6 or higher, and even more preferably 5.1 or higher. On the other hand, E H / E L By setting this value to 10 or less, it is possible to suppress the unidirectional shrinkage of the film at high temperatures during heat sealing. As a result, it is possible to suppress a reduction in product width, wrinkle formation and film breakage caused by film shrinkage, and the formation of air bubbles between the film and the metal plate. It is also possible to suppress the occurrence of wrinkles caused by the film becoming too weak in one direction. H / E L It is preferably 9.0 or less, more preferably 8.0 or less, and even more preferably 7.0 or less.

[0074] Here, "longitudinal direction" is defined as the direction in which the film travels during its formation. For example, in the case of a film manufactured by winding it into a roll, the winding direction of the roll corresponds to the longitudinal direction. Furthermore, here, "width direction" is defined as the direction perpendicular to the longitudinal direction.

[0075] The above Young's modulus can be measured by a tensile test. The tensile test should be performed under the conditions of a temperature of 23°C, a raw length of 50 mm, and a tensile speed of 300 mm / min. More specifically, it can be measured by the method described in the examples.

[0076] E H / E L To keep the above range within limits, the polyester film can be uniaxially stretched in the longitudinal or widthwise direction under the conditions described later, and then heat-fixed.

[0077] As described above, in the present invention, E L E for H The desired effect is achieved by controlling the ratio of E H and E L The value itself is not limited. However, from the perspective of improving the stiffness of the film and making it easier to keep the film smooth during transport and heat sealing, E H and E L It is preferable that all of these be 1000 MPa or higher. More preferably 1400 MPa or higher, even more preferably 1800 MPa or higher, and most preferably 2000 MPa or higher. On the other hand, from the viewpoint of further improving the processability of sheet metal fabrication, E H and E L It is preferable that all of these values ​​be 19,000 MPa or less. More preferably, they are 18,000 MPa or less, even more preferably 17,000 MPa or less, and most preferably 15,000 MPa or less.

[0078] E H and E LTo bring both of these within the above range, the polyester film can be uniaxially stretched in the longitudinal or widthwise direction under the conditions described later, and then heat-fixed.

[0079] The uniaxially oriented polyester film for metal plate coating of the present invention is E H and E L The sum of (E H +E L ) is preferably 3000 to 20000 MPa. H +E L If the pressure is 3000 MPa or higher, the stiffness of the film will be further improved, making it easier to keep the film smooth during transport and heat sealing. H +E L It is more preferably 8000 MPa or more, even more preferably 10000 MPa or more, and most preferably 12000 MPa or more. On the other hand, E H +E L If the pressure is 20,000 MPa or less, the processability of the resin-coated metal sheet for metal fabrication will be further improved. H +E L It is more preferably 19,000 MPa or less, even more preferably 18,000 MPa or less, and most preferably 17,000 MPa or less.

[0080] E H and E L The sum of (E H +E L To keep all of the above values ​​within the specified range, the polyester film can be uniaxially stretched in the longitudinal or widthwise direction under the conditions described later, and then heat-fixed.

[0081] The uniaxially oriented polyester film for metal plate coating of the present invention preferably has a heat shrinkage rate of 0 to 20% in both the longitudinal and width directions at 160°C. If the heat shrinkage rate at 160°C is 0% or more in both the longitudinal and width directions, stretching of the film during heat-pressing with the metal plate can be suppressed, further suppressing wrinkles and film breakage due to film stretching. On the other hand, if the heat shrinkage rate at 160°C is 20% or less in both the longitudinal and width directions, thermal shrinkage during heat-pressing with the metal plate can be suppressed, further suppressing wrinkles and film breakage due to film shrinkage, and the inclusion of air bubbles between the film and the metal plate. It is more preferable that the heat shrinkage rate at 160°C be 18% or less in both the longitudinal and width directions, even more preferable that it be 16% or less, and most preferable that it be 15% or less.

[0082] The thermal shrinkage rate at 160°C is defined as the displacement at 160°C when the temperature is increased from 25°C at a rate of 5°C / min, using a thermomechanical analyzer. More specifically, it can be measured by the method described in the examples.

[0083] To ensure that the thermal shrinkage rate at 160°C is within the above range in both the longitudinal and width directions, the polyester film should be stretched under the conditions described later and then heat-set.

[0084] In one embodiment of the present invention, the uniaxially oriented polyester film for coating a metal plate preferably has a laminated structure comprising a layer A containing 70% by mass or more of polyester resin, and a layer B laminated on at least one surface of the layer A, which contains 70% by mass or more of polyester resin and an inorganic lubricant. By adopting this laminated structure, the amount of inorganic lubricant can be efficiently reduced.

[0085] The B layer may be laminated on at least one surface of the A layer. Therefore, the polyester film may have a two-layer structure of A / B or a three-layer structure of B / A / B. Furthermore, it may have layers other than the A and B layers, as long as they do not hinder the effects of the present invention. The A layer may also contain an inorganic lubricant. In particular, when the polyester film has a two-layer structure of A / B, it is preferable that the A layer also contains an inorganic lubricant. However, from the viewpoint of reducing the inorganic lubricant content, it is preferable that the inorganic lubricant content in the A layer be less than the inorganic lubricant content in the B layer, more preferably 80% or less, and even more preferably 50% or less. On the other hand, when the polyester film has a three-layer structure of B / A / B, it is preferable that the A layer does not contain an inorganic lubricant.

[0086] Furthermore, if layer B is laminated on only one surface of layer A, for example, in a two-layer structure of A / B, layer A is the surface that is bonded to the metal plate, and layer B is the surface of the resin-coated metal plate.

[0087] There is no particular upper limit to the polyester resin content in the aforementioned A layer, but it is preferably 99% by mass or less, more preferably 95% by mass or less, and even more preferably 90% by mass or less.

[0088] Similarly, there is no particular upper limit to the polyester resin content in the B layer, but it is preferably 99.9% by mass or less, more preferably 99.8% by mass or less, and even more preferably 99.7% by mass or less.

[0089] The method for producing the polyester film having the above-described laminated structure is not particularly limited and can be produced by any method. For example, co-extrusion using a feed block system or a multi-manifold system, a method of bonding with other films, or a lamination method in which molten resin is directly laminated onto the film can be used. From the viewpoint of lamination accuracy and productivity, co-extrusion using a feed block system or a multi-manifold system is preferred.

[0090] (Inorganic pigments) From the viewpoint of providing design appeal through printing, it is preferable that the A layer contains an inorganic pigment.

[0091] The inorganic pigment is not particularly limited and any inorganic pigment can be used. Preferably, the inorganic pigment is one that turns white when added to a polyester film. For example, at least one selected from the group consisting of silicon dioxide, aluminum oxide, titanium dioxide, zirconium oxide, magnesium oxide, zinc oxide, barium titanate, lead zirconate titanate, and other oxide ceramics, talc, mica, calcium carbonate, and barium sulfate can be used. Among these, titanium dioxide is preferred in terms of dispersibility in polyester resin and whiteness, and rutile-type titanium dioxide is more preferred.

[0092] The amount of the inorganic pigment contained in layer A in the total components of the uniaxially oriented polyester film for coating the metal plate (hereinafter referred to as "inorganic pigment content in layer A") is not particularly limited. However, from the viewpoint of ensuring a whiteness suitable for a two-piece can for printing, it is preferable that the inorganic pigment content in layer A be 7.0% by mass or more, more preferably 10% by mass or more, and even more preferably 12% by mass or more. On the other hand, from the viewpoint of further suppressing breakage due to stretching during film manufacturing and breakage and abrasion of the resin coating layer during can manufacturing of the resin-coated metal plate, it is preferable that the inorganic pigment content in layer A be 28% by mass or less. It is more preferable that it be 22% by mass or less, and even more preferably 18% by mass or less.

[0093] The method for adding the inorganic pigment to layer A is not particularly limited. For example, the inorganic pigment may be added together with the polyester resin to the extruder for the raw material of layer A during film production, and then mixed and dispersed in the extruder for film formation. Alternatively, a masterbatch may be prepared in advance by mixing the polyester resin and the inorganic pigment using a separate extruder, and then the masterbatch of inorganic pigment may be added together with the polyester resin to the extruder for the raw material of layer A during film production. It should be noted that the method of adding the inorganic pigment as a masterbatch is preferred because it allows for uniform and well-dispersible addition of the inorganic pigment to the polyester resin.

[0094] (wax) From the viewpoint of further improving the processability of resin-coated metal sheets for can manufacturing, it is preferable that the B layer contains wax.

[0095] The wax contained in layer B is preferably a polyolefin wax, from the viewpoint of the processability of the resin-coated metal sheet for can manufacturing and its dispersibility in the polyester resin.

[0096] Furthermore, from the viewpoint of improving the processability of the resin-coated metal sheet for can manufacturing and its dispersibility in the polyester resin, the polyolefin wax is preferably one of polyethylene wax, polypropylene wax, acid-modified polyethylene wax, acid-modified polypropylene wax, oxidized polyethylene wax, or oxidized polypropylene wax, or a mixture thereof.

[0097] Acid-modified polyethylene wax and acid-modified polypropylene wax can be exemplified by acid-modified polyolefin wax obtained by random copolymerization, block copolymerization, or graft copolymerization of functional group-containing monomers such as acrylic acid, methacrylic acid, vinyl acetate, vinyl propionate, maleic acid, maleic anhydride, itaconic acid, monomethyl maleic acid, and unsaturated carboxylic acids having 3 to 8 carbon atoms, and metal salts obtained by neutralizing all or part of these acids with 1- to 2 valent metal cations such as sodium, potassium, lithium, zinc, magnesium, and calcium, methyl acrylate, methyl methacrylate, ethyl acrylate, ethyl methacrylate, n-propyl acrylate, n-propyl methacrylate, isobutyl acrylate, isobutyl methacrylate, n-butyl acrylate, n-butyl methacrylate, 2-hydroxyethyl acrylate, 2-hydroxyethyl methacrylate, monomethyl maleic acid, glycidyl acrylate, glycidyl methacrylate, vinyl acetate, acrylamine, acrylamide, etc., or mixtures thereof.

[0098] Oxidized polyethylene wax and oxidized polypropylene wax can be produced by introducing functional groups through oxidation reactions by introducing air into a polymer or its thermal decomposition product using ethylene or propylene as an olefin monomer while the polymer is in a molten state at 140°C to 180°C. The acid modification method is preferably maleic acid modification and / or maleic anhydride modification, and the functional groups introduced by oxidation are preferably carboxyl groups, ketone groups, and / or hydroxyl groups.

[0099] The acid value of the polyolefin wax contained in layer B is not particularly limited. However, from the viewpoint of improving the dispersibility of the polyolefin wax in the polyester resin and further improving the processability of the resin-coated metal sheet, it is preferable that the acid value of the polyolefin wax is 1.0 mg KOH / g or higher. It is more preferable that it is 30 mg KOH / g or higher, and even more preferable that it is 50 mg KOH / g or higher. On the other hand, from the viewpoint of suppressing excessive reaction between the polyolefin wax and the polyester resin, maintaining the dispersed state, and further improving the processability of the resin-coated metal sheet, it is preferable that the acid value of the polyolefin wax is 120 mg KOH / g or lower. It is more preferable that it is 110 mg KOH / g or lower, and even more preferable that it is 100 mg KOH / g or lower. The acid value of the polyolefin wax is measured in accordance with JIS K5902. More specifically, it can be measured by the method described in the examples.

[0100] The amount of the wax contained in the B layer in relation to the total components of the B layer (hereinafter referred to as "wax content in the B layer") is not particularly limited. However, from the viewpoint of well dispersing the wax within the polyester resin of the B layer and further improving the processability of can manufacturing, it is preferable that the wax content in the B layer be 0.050% by mass or more. More preferably 0.10% by mass or more, and even more preferably 0.20% by mass or more. On the other hand, from a similar viewpoint, it is preferable that the wax content in the B layer be 1.2% by mass or less, more preferably 1.0% by mass or less, and even more preferably 0.80% by mass or less.

[0101] The number-average molecular weight of the polyolefin wax contained in the B layer is not particularly limited, but is preferably between 500 and 20,000. If the number-average molecular weight of the polyolefin wax is between 500 and 20,000, the polyolefin wax disperses well within the polyester resin of the B layer, further improving the processability of the resin-coated metal sheet for can manufacturing. The number-average molecular weight is more preferably 1,000 or more, and even more preferably 1,500 or more. Furthermore, the number-average molecular weight is more preferably 10,000 or less, and even more preferably 5,000 or less. The number-average molecular weight is more preferably 1,000 or more and even more preferably 1,500 or more and even more preferably 5,000 or less. The number-average molecular weight of the polyolefin wax is measured by gel permeation chromatography (GPC). More specifically, it may be measured by the method described in the examples.

[0102] The method for adding wax to the uniaxially oriented polyester film for metal plate coating according to the present invention is not particularly limited. For example, the wax may be added together with the polyester resin in the extruder for the B-layer raw material during film production, and then kneaded and dispersed in the film-forming extruder. Alternatively, the polyester resin and wax may be kneaded separately in an extruder to prepare a masterbatch, and then the wax masterbatch may be added together with the polyester resin in the extruder for the B-layer raw material during film production. The method of adding the wax as a masterbatch is preferred because it allows for the uniform and well-dispersible addition of a small amount of wax to the polyester resin.

[0103] The uniaxially oriented polyester film for metal plate coating of the present invention is not particularly limited and may have any thickness (hereinafter referred to as "film thickness"). However, if the film thickness is 10 μm or more, the film has stiffness, exhibiting better film transportability, ensuring reliable coating during heat-press bonding with the metal plate, and providing even better corrosion resistance when used as a container. Therefore, the film thickness is preferably 10 μm or more, more preferably 12 μm or more, and even more preferably 15 μm or more. On the other hand, if the film thickness is 50 μm or less, cost increases can be suppressed without impairing the corrosion resistance when the resin-coated metal plate is used as a container. Therefore, the film thickness is preferably 50 μm or less, more preferably 30 μm or less, and even more preferably 25 μm or less. The film thickness can be measured using a dial gauge. More specifically, it can be measured by the method described in the examples.

[0104] When the uniaxially oriented polyester film for covering the metal plate includes layers A and B, the thicknesses of layers A and B are not particularly limited. However, the total thickness t of layer B is not limited. B The total thickness t of layer A relative to the above A The ratio (t A / t B If the ratio is between 2.5 and 7.5, the effect of the inorganic lubricant can be made even more efficient. In addition, the amount of inorganic lubricant can be reduced without causing wrinkles during heat-press bonding with the metal plate, thereby suppressing cost increases. Therefore, the total thickness t of the B layer B The total thickness t of layer A relative to the above A The ratio (t A / t B The total thickness t is preferably 2.5 to 7.5, more preferably 3.0 to 6.0, and even more preferably 4.0 to 5.0. A and total thickness t B These can be measured by observing the cross-section of the film with a scanning electron microscope. More specifically, they can be measured using the method described in the examples.

[0105] The uniaxially oriented polyester film for metal plate coating may further optionally contain an antioxidant. Adding an antioxidant can improve heat resistance. The content of the antioxidant is not particularly limited, but from the viewpoint of improving heat resistance, it is preferably 0.0001 to 1.0% by mass, and more preferably 0.001 to 1.0% by mass. Any antioxidant can be used without particular limitations. For example, at least one selected from the group consisting of hindered phenols, hydrazines, phosphites, etc., can be used as the antioxidant.

[0106] The uniaxially oriented polyester film for metal coating may contain various additives in addition to the antioxidant, as long as they do not impair the effects of the present invention. For example, at least one selected from the group consisting of nucleating agents, heat stabilizers, antistatic agents, antiblocking agents, fillers, viscosity modifiers, and coloring pigments can be used.

[0107] [Manufacturing method] Next, a preferred method for manufacturing a uniaxially oriented polyester film for metal plate coating according to one embodiment of the present invention will be described. However, the present invention is not limited to the following disclosure.

[0108] First, a mixture containing dried polyester resin and an inorganic lubricant is melt-kneaded in an extruder to form a molten resin. The form of the polyester resin is not particularly limited, but may be in the form of pellets, for example. The polyester resin is optionally dried and then supplied to the extruder together with additives such as an inorganic lubricant. In other words, it is preferable to dry the polyester resin to be used prior to the melt-kneading. The drying can be carried out by any method. For example, hot air drying or vacuum drying may be performed.

[0109] Inside the extruder, the polyester resin is heated above its melting point and kneaded to become a molten resin. When extruding the molten resin, it is preferable to equalize the extrusion amount using a gear pump or the like. It is also preferable to remove foreign matter and modified resin by extruding through a filter or the like.

[0110] When manufacturing a film with a laminated structure, the resin for layer A and the resin for layer B can be molten in separate extruders, and each resin can be supplied to the lamination device through different flow paths. For example, a feed block or a multi-manifold die can be used as the lamination device.

[0111] Next, the molten resin is extruded from the T-die in a sheet form and cooled and solidified on a casting drum to form an unstretched film. At this time, in order to improve the adhesion between the cooling body such as the casting drum and the molten sheet, it is preferable to use electrodes such as wire, tape, needle, or knife shapes to adhere them by electrostatic force and rapidly cool and solidify them. Methods of blowing air from a slit-shaped, spot-shaped, or surface-shaped device to adhere and rapidly cool and solidify, methods of adhering and rapidly cooling and solidifying using a nip roll, and methods combining these are also preferable.

[0112] Subsequently, the unstretched film is stretched. The stretching direction may be either longitudinal or transverse, but stretching in the transverse direction is preferable because it facilitates the manufacture of wide film products.

[0113] Here, longitudinal stretching refers to stretching the film to impart a longitudinal molecular orientation. Longitudinal stretching is usually performed using a pair of rolls with different peripheral speeds, and the stretching ratio is controlled by the difference in peripheral speeds. Longitudinal stretching may be performed in a single stage, or it may be performed in multiple stages using multiple pairs of rolls.

[0114] Furthermore, lateral stretching refers to stretching that imparts a molecular orientation in the width direction to the film. Lateral stretching is usually performed using a tenter stretcher. In a tenter stretcher, the film is introduced with its ends held by clips, and as the clips expand in the width direction, the film is stretched laterally.

[0115] ·Stretching ratio: 4.5~8.0 times If the stretching ratio in the stretching process is less than 4.5 times, stretching unevenness may occur, resulting in large thickness variations in the stretching direction and poor productivity. Therefore, the stretching ratio in the stretching process should be 4.5 times or more, preferably 4.8 times or more, and more preferably 5.0 times or more. On the other hand, if the stretching ratio exceeds 8.0 times, the orientation in the stretching direction may become too pronounced, making it prone to thermal shrinkage, or the stiffness in the direction perpendicular to the stretching direction may weaken, making it prone to wrinkles. Therefore, the stretching ratio in the stretching process should be 8.0 times or less, preferably 7.0 times or less, and more preferably 6.5 times or less.

[0116] ·Stretching temperature: Tg or higher, (Tg+50℃) or lower If the stretching temperature is too low, the film may break during stretching, or it may become too oriented in the stretching direction, making it prone to thermal shrinkage. Therefore, the stretching temperature should be equal to or above the glass transition temperature (Tg) of the polyester resin constituting the film, preferably (Tg + 10°C) or higher, and more preferably (Tg + 15°C) or higher. On the other hand, if the stretching temperature is too high, the unstretched film may undergo thermal crystallization, causing it to break during stretching, or conversely, to become too soft and stick to rolls or clips. Therefore, the stretching temperature should be (Tg + 50°C) or lower, preferably (Tg + 40°C) or lower, and more preferably (Tg + 30°C) or lower. When using multiple polyester resins with different glass transition temperatures, a single glass transition temperature can be observed due to the mixing of the multiple polyester resins. Therefore, the stretching temperature can be determined based on the glass transition temperature.

[0117] The glass transition temperature of polyester resin can be measured using a differential scanning calorimeter. More specifically, it can be measured by the method described in the examples.

[0118] When stretching in the longitudinal direction, if the temperature immediately after stretching is excessively high, the reduction in film width due to longitudinal stretching (neck-in) may be significant, or wrinkles may form on the film on the roll immediately after stretching. Therefore, it is preferable to cool the film to below the glass transition temperature of the polyester resin that makes up the film immediately after stretching. When stretching in the transverse direction, since the film ends are held by clips, no wrinkles form on the film during stretching or transport, so it is not necessary to cool the film from immediately after stretching until the heat setting described later.

[0119] Next, the stretched film is heat-fixed while being relaxed in the stretching direction. In the case of longitudinal stretching, the heat-fixing is preferably performed while the film is running on a roll heated to a high temperature inside a longitudinal stretching machine, and in the case of transverse stretching, it is preferably performed while the film is running in a heated oven inside a tenter-type stretching machine. The heat-fixing temperature is preferably above the stretching temperature and below the melting point of -50°C. More preferably, it is within the range of above the stretching temperature, below the stretching temperature of +60°C, and below the melting point of -50°C, and even more preferably, within the range of above the stretching temperature of +10°C, below the stretching temperature of +50°C, and below the melting point of -50°C. Particularly preferably, it is within the range of above the stretching temperature of +20°C and below the stretching temperature of +40°C, and below the melting point of -50°C.

[0120] Here, "melting point" refers to the melting point of the polyester resin used. The melting point of the polyester resin can be measured using a differential scanning calorimeter. More specifically, it can be measured using the method described in the examples.

[0121] When using multiple polyester resins with different glass transition temperatures, a single glass transition temperature may be observed due to the mixing of the multiple polyester resins. In that case, the stretching temperature can be determined based on the glass transition temperature. On the other hand, if the glass transition temperatures of individual polyester resins are observed, the stretching temperature is determined based on the lowest of those glass transition temperatures.

[0122] This heat setting process is important for reducing residual stress in uniaxially oriented films, but if the heat setting temperature is too low, the residual stress reduction effect may not be achieved. On the other hand, if the heat setting temperature is too high, the film shrinks excessively, which can cause wrinkles to form in the film during transport, or in some cases, even cause it to break.

[0123] Furthermore, the heat fixing may be performed while relaxing the film in the stretching direction. By relaxing the film simultaneously with the heat fixing, the residual stress of the uniaxially oriented film can be further reduced. When heat fixing is performed while relaxing the film in the stretching direction, the relaxation rate is preferably 0.5% to 5%, more preferably 0.8% to 4%, and even more preferably 1% to 3%. If the relaxation rate is too low, the residual stress reduction effect may not be obtained. On the other hand, if the relaxation rate is too high, the film may not shrink completely, and the film may sag during transport. The heat-fixed film is then cooled to obtain a uniaxially oriented film.

[0124] The uniaxially oriented polyester film for metal plate coating of the present invention can suppress wrinkles and thermal shrinkage of the film, especially at high temperatures, during the thermocompression lamination method. Furthermore, by controlling the degree of crystallinity and Young's modulus, it can also suppress the incorporation of air bubbles and wrinkles. In addition, it has excellent feathering resistance, and when used as a can body for two-piece cans where a high degree of processing is required, it can suppress rupture or abrasion of the resin coating layer during can manufacturing and crack formation in the resin coating layer during the can body heat treatment process. For this reason, the uniaxially oriented polyester film for metal plate coating of the present invention can be suitably used as a surface coating for metal plates for containers. Moreover, the uniaxially oriented polyester film for metal plate coating of the present invention can be suitably used as a resin-coated metal plate by coating at least one side of the metal plate. [Examples]

[0125] The present invention will be further described below based on the following examples. However, the present invention is not limited to the following examples.

[0126] In this example, the properties of the uniaxially oriented polyester film for metal plate coating were measured and evaluated using the following method.

[0127] (1) Crystallinity, mobile amorphous ratio Crystallinity and the ratio of movable amorphous material were measured using a differential scanning calorimeter (DSCQ100) manufactured by TA Instruments Inc. Temperature-modulated differential scanning calorimetry was performed in a nitrogen atmosphere at a temperature range of 0°C to 290°C, a heating rate of 2°C / min, an amplitude of ±0.5°C, and a period of 40 seconds. The ratio of movable amorphous material was calculated using formula (1) and the degree of crystallinity using formula (2).

[0128] Mobile amorphous ratio (%)={ΔCp / ΔCp(a)}×100…(1) ΔCp: Difference in specific heat before and after the glass transition temperature obtained by this measurement. ΔCp(a): Difference in specific heat before and after the glass transition temperature of a perfectly amorphous material In this invention, for products containing 89 mol% or more polyethylene terephthalate units, the theoretical specific heat difference of 0.4052 J / (g·℃) for perfectly amorphous polyethylene terephthalate was used as a reference.

[0129] Crystallinity (%)=(ΔHc / ΔHm)×100…(2) ΔHc: Cold crystallization heat (exothermic peak heat) obtained by this measurement. ΔHm: Heat of fusion (endothermic peak heat) obtained by this measurement.

[0130] (2) Young's modulus The Young's modulus was measured using a Shimadzu Corporation universal material testing machine (AG-X-5) at a temperature of 23°C. The sample used for measurement was cut from the film so that its length in the measurement direction (longitudinal or widthwise) was 130 mm and its length in the direction perpendicular to the measurement direction was 10 mm. The sample was stretched at a tensile speed of 300 mm / min with a length of 50 mm and the Young's modulus was measured. The above measurement was repeated five times in both the longitudinal and widthwise directions. The average of the five measurements in the longitudinal direction was taken as the longitudinal Young's modulus, and the average of the five measurements in the widthwise direction was taken as the widthwise Young's modulus. The higher of the two Young's moduli was taken as the E (Equation). H , the lower one is H L That's what I decided.

[0131] (3) Thermal shrinkage The thermal shrinkage rate in the longitudinal and widthwise directions was measured using a thermomechanical analyzer (TMA7100C) manufactured by Hitachi High-Tech Science Co., Ltd. The sample was cut so that the distance between the chucks in the measurement direction (longitudinal or widthwise) was 10 mm, and the distance in the direction perpendicular to the measurement direction was 4 mm, and then fixed between the chucks. The measurement mode was tensile mode (measurement load 29.4 mN), and the temperature was increased from 25°C to 200°C at a rate of 5°C / min under a nitrogen atmosphere with a flow rate of 100 ml / min. The TMA value (displacement) at 160°C was determined from the chart obtained from the measurement. Note that the displacement was defined as positive for shrinkage and negative for expansion or elongation. The thermal shrinkage rate after heat treatment at 160°C was calculated using the following formula, based on the distance between the chucks before measurement (L0) and the displacement at 160°C (L1). Thermal shrinkage rate (%) = (L1 / L0) × 100

[0132] (4) Inorganic pigment content In accordance with JIS K7250, a film with an initial mass W0 was placed in a platinum crucible and first thoroughly burned with a gas burner. Then, it was treated in an electric furnace at 750-800°C for 1 hour to completely ashify, and the mass W1 of the resulting ash was measured. Using the initial mass W0 and the ash mass W1, the inorganic pigment content was calculated using the following formula. The same measurement was performed three times for each film, and the average value was taken as the inorganic pigment content (i.e., the amount of inorganic pigment in the total components of the film). Although the film developed in this study contains an inorganic lubricant, the amount of inorganic lubricant is extremely small compared to the inorganic pigment; therefore, the effect of the inorganic lubricant is ignored in the measurement of the inorganic pigment content. Inorganic pigment content (%) = (W1 / W0) × 100

[0133] (5) Film thickness The film thickness was measured using a Mitutoyo Corporation dial gauge 2110S-10 (with a carbide ball measuring tip) mounted on a Mitutoyo Corporation dial gauge stand 7001-10. The measurement was performed 10 times at different locations, and the average value was taken as the film thickness.

[0134] (6) Total thickness The total thickness of layers A and B was measured by observing the film cross-section with a scanning electron microscope. Specifically, the film was embedded in epoxy resin, and the film cross-section was cut with a microtome. Then, the cross-section of the obtained sample was observed with a Hitachi High-Technologies Corporation S-4800 scanning electron microscope to determine the thickness of each layer. Ten measurements were taken at different locations, and the average value was taken as the total thickness of each layer.

[0135] (7) Thermal properties of the polyester resin constituting the film (melting point, glass transition temperature) The melting point and glass transition temperature of the polyester resin were measured using a differential scanning calorimeter (DSCQ100) manufactured by TA Instruments. First, the polyester resin alone was cooled to -50°C under a nitrogen atmosphere, and then heated to 290°C at a rate of 20°C / min (1st Run). After heating to 290°C in the 1st Run, it was held for 5 minutes and then rapidly cooled with liquid nitrogen. Subsequently, it was heated again from -50°C to 290°C at a rate of 20°C / min (2nd Run). The melting point and glass transition temperature were determined from the chart obtained from the 2nd Run measurement. The same measurement was performed three times for each, and the average values ​​were taken as the melting point and glass transition temperature of the polyester resin constituting the film, respectively.

[0136] (8) Acid value of polyolefin wax The acid value of polyolefin wax was measured in accordance with JIS K5902. A predetermined amount of polyolefin wax corresponding to the estimated acid value was measured into a flask in accordance with JIS K5902, dissolved in 100 ml of neutral solvent, and then titrated with 0.1 mol / L potassium hydroxide standard solution using phenolphthalein as an indicator. The neutralization endpoint was defined as the point at which the indicator color change continued for 30 seconds, and the acid value was calculated using the following formula. Acid value=5.611×A×F / B A: Amount of 0.1 mol / L potassium hydroxide standard solution to use (ml) B: Sample collection amount (g) F: Factor of 0.1 mol / L potassium hydroxide standard solution

[0137] (9) Number average molecular weight of polyolefin waxes The number-average molecular weight of polyolefin waxes was measured using a Waters GPC2000 gel permeation chromatograph under the following conditions. A calibration curve prepared using commercially available monodisperse standard polystyrene was used to calculate the number-average molecular weight. (Measurement conditions) Solvent: o-dichlorobenzene Columns: TSKgel GMH6-HT x 2, TSKgel GMH6-HTL column x 2 (manufactured by Tosoh Corporation) Flow rate: 1.0ml / min Sample: 0.15 mg / ml o-dichlorobenzene solution Temperature: 140℃

[0138] (10) Fabrication of resin-coated metal plates by pressure lamination A TFS with a thickness of 0.22 mm was used as the metal plate. The amount of metallic chromium layer deposited on the TFS was 120 mg / m² per side. 2 The amount of chromium oxide layer deposited is 10 mg / m² per side, in terms of metallic chromium. 2 The tempering degree of the original TFS plate was set to T3CA.

[0139] A uniaxially oriented polyester film for metal plate coating, manufactured in the examples and comparative examples described later, was coated onto at least one side of the metal plate by a thermocompression lamination method. The specific lamination conditions were that the metal plate temperature immediately before lamination was set to the same temperature as the melting point of the film, and the lamination roll temperature was set to 60°C. After that, by water cooling 1 second after thermocompression, a resin-coated metal plate was obtained in which a resin coating layer was applied to both sides of the metal plate.

[0140] (11) Laminating properties To evaluate the lamination properties, the percentage of air bubbles and the number of wrinkles in the resin-coated metal sheets obtained in item 10 above were checked. Based on these results, the lamination properties were evaluated according to the following criteria. If the evaluation is B or higher, there are no practical problems. A (Excellent): Air bubble content is less than 1%, and there are no wrinkles. B (Good): Air bubbles are present in an area of ​​1% or more but less than 5%, or there is one wrinkle. C (Unacceptable): Air bubbles or more are present in the product, or two or more wrinkles are present.

[0141] (12) Fabricability To evaluate the processability of the can manufacturing process, a can was manufactured using the resin-coated metal sheet obtained in item 8 above, following the procedure below. First, paraffin wax was applied to the surface of the resin-coated metal sheet, and then a blank with a diameter of 123 mm was punched out. Next, the blank was drawn into a cup with an inner diameter of 71 mm and a height of 36 mm using a cupping press, with the film facing outwards. The resulting cup was loaded into a DI (Drawing and Ironing) molding machine and subjected to redrawing and three-stage ironing to produce a can with an inner diameter of 52 mm and a can height of 90 mm. The redrawing was performed under conditions of a punch speed of 200 mm / second and a stroke of 560 mm. The total reduction rate in the ironing process was set to 51%, and the reduction rates for each step were set to 23%, 25%, and 25%, respectively.

[0142] The molded cans were placed in a hot air drying oven and heat-treated for 1 minute until the can body temperature reached the melting point of the resin coating - 5°. After that, the cans were removed from the hot air drying oven and cooled to room temperature.

[0143] For each can obtained using the above procedure, the resin coating layer on the outer surface was visually inspected, and its processability was evaluated according to the following criteria. If the evaluation is B or higher, there are no practical problems. A (Excellent): No abrasion or cracks observed. B (Good): Slight chipping or cracking occurs within 2mm of the can flange. C (Not acceptable): Abrasion or cracking occurs exceeding 2 mm from the can flange, or film breakage occurs.

[0144] (13) Feathering resistance To evaluate feathering resistance, an EOE lid was fabricated using the resin-coated metal plate obtained in item 8 above, following the procedure below. First, the resin-coated metal plate was punched out to a diameter of 68 mm from the opposite side of the film-coated surface to create a blank. At this time, if there was any remaining film, its length was measured. Next, the blank was press-molded using an EOE lid mold to form an EOE lid. For the score area, a V-shaped score mold was used, and the thickness of the metal plate in the score area was adjusted to 70 μm. The score area was formed on the surface opposite to the film-coated surface. Finally, a tab was attached to the EOE lid.

[0145] The EOE lid was opened by raising the tab and separating the panel section from the outer ring section. At this time, if there was any remaining film, its length was measured. The longer of the remaining film length at the time of blank punching and the remaining film length at the time of EOE opening was used to evaluate the feathering resistance according to the following criteria. If the evaluation was B or higher, there would be no practical problems. A (Excellent): Film remaining length is less than 0.1 mm. B (Good): Film remaining length is 0.1mm or more but less than 0.2mm. C (Not acceptable): Film remaining length is 0.2mm or more.

[0146] (Example 1) Resin composition A, to be used as a raw material resin for the A layer of the film, was prepared by the following procedure. First, pellets a and b were prepared as shown below. Pellet a: Pellet of polyethylene terephthalate with a melting point of 250°C Pellet b: A masterbatch pellet containing 60% by mass of rutile-type titanium dioxide as an inorganic pigment in polyethylene terephthalate with a melting point of 255°C.

[0147] Each of the pellets a and b was thoroughly dried under vacuum and high temperature to remove any moisture. Then, pellets a and b were blended to form resin composition A. The amount of pellet b in resin composition A was 25% by mass.

[0148] Resin composition B, used as the raw material resin for layer B, was prepared by the following procedure. First, pellets c, d, and e were prepared as shown below. Pellet c: Pellet of polyethylene terephthalate with a melting point of 250°C Pellet d: A masterbatch pellet containing 8000 ppm of inorganic lubricant in polyethylene terephthalate with a melting point of 255°C. Pellet e: A masterbatch pellet containing 3.0% by mass of acid-modified polyethylene wax in polyethylene terephthalate with a melting point of 250°C.

[0149] Silicon dioxide was used as the inorganic lubricant. Furthermore, an acid-modified polyethylene wax with an acid value of 60 mgKOH / g was used.

[0150] Each of the pellets c, d, and e was thoroughly dried under vacuum and high temperature to remove any moisture. Then, the pellets c, d, and e were blended to form resin composition B. The amount of pellet d in resin composition B was 6.3% by mass, and the amount of pellet e was 6.7% by mass. The wax content in layer B was 0.20% by mass.

[0151] The resin compositions A and B were each fed into two different single-screw extruders and melt-kneaded at 270°C. Next, after removing foreign matter through a 25 μm cut sintered filter, the layers were merged in a feed block designed with a layering ratio of 1 (layer B):8 (layer A):1 (layer B) to form three layers in the thickness direction. The laminated molten resin was extruded from a T-die and cooled and solidified on a casting drum with a surface temperature controlled to 30°C to obtain an unstretched film.

[0152] Next, the film was preheated to 95°C using a heated ceramic roll, stretched 5.0 times in the longitudinal direction, and immediately cooled to 70°C. Then, it was heat-set at 120°C using a heated mirror-finish HCr plated roll. During this process, a 1% relaxation in the longitudinal direction was applied by utilizing the speed difference between two consecutive mirror-finish HCr plated rolls. Finally, the film was slowly cooled to room temperature, the edges were removed, and the film was wound on a winding machine to obtain a uniaxially oriented polyester film for metal plate coating with a thickness of 15 μm, a melting point of 250°C, and a glass transition temperature of 80°C. The physical properties of the obtained film are shown in Table 1.

[0153] The content of inorganic lubricant and inorganic pigment in the total components can be calculated from the blending ratio of each masterbatch to resin composition A and resin composition B, and the thickness ratio of layer A to layer B. In Example 1, the inorganic lubricant content is 100 ppm, and the inorganic pigment content is 12% by mass. In addition, the wax content in the total components is 0.04% by mass (calculated from the content in layer B, which is 0.2% by mass). Therefore, the polyester resin content in the total components is the value obtained by subtracting the inorganic lubricant content of 100 ppm, the inorganic pigment content of 12% by mass, and the wax content of 0.04% by mass from 100% by mass, which in Example 1 is 87.95% by mass.

[0154] (Example 2) A uniaxially oriented polyester film for metal plate coating was prepared under the same conditions as in Example 1, except for the points described below. Acid-modified polypropylene wax with an acid value of 50 mgKOH / g was used as the wax. The unoriented film was introduced into a tenter stretcher with its ends held by clips, and stretched 6.5 times in the width direction at 110°C. Then, while heat-fixing at 120°C in the same tenter stretcher, it was relaxed by 1% in the width direction. The uniaxially oriented polyester film for metal plate coating had a thickness of 15 μm, a melting point of 250°C, and a glass transition temperature of 80°C. The physical properties of the obtained film are shown in Table 1.

[0155] (Example 3) A uniaxially oriented polyester film for metal plate coating was prepared under the same conditions as in Example 2, except that polyethylene wax was used as the wax and the heat-fixing temperature was set to 200°C. The uniaxially oriented polyester film for metal plate coating had a thickness of 15 μm, a melting point of 250°C, and a glass transition temperature of 80°C. The physical properties of the obtained film are shown in Table 1.

[0156] (Example 4) A uniaxially oriented polyester film for metal plate coating was prepared under the same conditions as in Example 1, except that polypropylene wax was used as the wax, the stretching temperature was 110°C, and the stretching ratio was 4.5 times. The uniaxially oriented polyester film for metal plate coating had a thickness of 15 μm, a melting point of 250°C, and a glass transition temperature of 80°C. The physical properties of the obtained film are shown in Table 1.

[0157] (Example 5) A uniaxially oriented polyester film for metal plate coating was prepared under the same conditions as in Example 1, except that polyethylene oxide wax with an acid value of 1.0 mg KOH / g was used as the wax, the stretching temperature was 105°C, and the stretching ratio was 6.0 times. The uniaxially oriented polyester film for metal plate coating had a thickness of 15 μm, a melting point of 250°C, and a glass transition temperature of 80°C. The physical properties of the obtained film are shown in Table 1.

[0158] (Example 6) A uniaxially oriented polyester film for metal plate coating was prepared under the same conditions as in Example 1, except that the amount of pellet d in resin composition B was 3.1% by weight, and an oxidized polypropylene wax with an acid value of 12 mg KOH / g was used as the wax. The uniaxially oriented polyester film for metal plate coating had a thickness of 15 μm, a melting point of 250°C, and a glass transition temperature of 80°C. The physical properties of the obtained film are shown in Table 1.

[0159] (Example 7) Resin composition A, to be used as a raw material for the film, was prepared using the following procedure. First, pellets a and b were prepared as shown below. Pellet a: Pellet of polyethylene terephthalate with a melting point of 250°C Pellet b: A masterbatch pellet containing 8000 ppm silicon dioxide as an inorganic lubricant in polyethylene terephthalate with a melting point of 255°C.

[0160] The pellets a and b were thoroughly dried under vacuum and high temperature to remove all moisture. Then, the pellets a and b were blended to obtain a resin composition in which the inorganic lubricant content in the total components was 4000 ppm.

[0161] The resin composition was fed into a uniscrew extruder and melted and kneaded at 270°C. Next, after removing foreign matter through a 25 μm cut sintering filter, the molten resin was extruded from a T-die and cooled and solidified on a casting drum with a surface temperature controlled to 30°C to obtain an unstretched film. From the unstretched core film, a uniaxially oriented polyester film for metal plate coating was prepared using the same procedure as in Example 1. The uniaxially oriented polyester film for metal plate coating had a thickness of 15 μm, a melting point of 250°C, and a glass transition temperature of 80°C. The physical properties of the obtained film are shown in Table 1.

[0162] (Comparative Example 1) A biaxially oriented polyester film for metal plate coating was prepared under the same conditions as in Example 1, except that the stretching was performed under the following conditions. Specifically, the unstretched film obtained under the same conditions as in Example 1 was preheated to a film temperature of 95°C using a heated ceramic roll, stretched 5.0 times in the longitudinal direction of the film, and immediately cooled to 70°C. Then, the film was introduced into a tenter stretcher with its ends held by clips and stretched 5.0 times in the width direction at 110°C. While heat-fixed at 160°C, it was relaxed by 2% in the width direction. Finally, it was slowly cooled to room temperature, the ends were removed, and the film was wound on a winding machine to obtain a biaxially oriented polyester film for metal plate coating. The biaxially oriented polyester film for metal plate coating had a thickness of 15 μm, a melting point of 250°C, and a glass transition temperature of 80°C. The physical properties of the obtained film are shown in Table 1.

[0163] (Comparative Example 2) A uniaxially oriented polyester film for metal plate coating was prepared under the same conditions as in Example 4, except that the stretching ratio was 4.0 times. The uniaxially oriented polyester film for metal plate coating had a thickness of 15 μm, a melting point of 250°C, and a glass transition temperature of 80°C. The physical properties of the obtained film are shown in Table 1.

[0164] (Comparative Example 3) A uniaxially oriented polyester film for metal plate coating was prepared under the same conditions as in Example 5, except that the stretching temperature was 100°C and the stretching ratio was 2.5 times. The uniaxially oriented polyester film for metal plate coating had a thickness of 15 μm, a melting point of 250°C, and a glass transition temperature of 80°C. The physical properties of the obtained film are shown in Table 1.

[0165] (Comparative Example 4) A uniaxially oriented polyester film for metal plate coating was prepared under the same conditions as in Example 6, except that an inorganic lubricant was not added. Specifically, when preparing resin composition B, pellet d containing an inorganic lubricant was not added, and only pellets c and e were blended. The uniaxially oriented polyester film for metal plate coating had a thickness of 15 μm, a melting point of 250°C, and a glass transition temperature of 80°C. The physical properties of the obtained film are shown in Table 1.

[0166] (Example 8) A uniaxially oriented polyester film for metal plate coating was prepared under the same conditions as in Example 1, except that the stretching ratio was set to 5.5 times and the heat-fixing temperature was set to 180°C. The uniaxially oriented polyester film for metal plate coating had a thickness of 15 μm, a melting point of 250°C, and a glass transition temperature of 80°C. The physical properties of the obtained film are shown in Table 2.

[0167] (Example 9) A uniaxially oriented polyester film for metal plate coating was prepared under the same conditions as in Example 1, except for the following: The amount of pellet b containing an inorganic pigment in resin composition A was set to 36% by mass. The amount of pellet d containing an inorganic lubricant in resin composition B was set to 85% by mass. For the preparation of the unoriented film by extrusion, a feed block with a lamination ratio of 1 (layer B):2.5 (layer A):1 (layer B) was used. The uniaxially oriented polyester film for metal plate coating had a thickness of 15 μm, a melting point of 250°C, and a glass transition temperature of 80°C. The physical properties of the obtained film are shown in Table 2.

[0168] (Example 10) A uniaxially oriented polyester film for metal plate coating was prepared under the same conditions as in Example 1, except that the amount of pellet b containing an inorganic pigment in resin composition A was set to 14.5% by mass. The uniaxially oriented polyester film for metal plate coating had a thickness of 15 μm, a melting point of 250°C, and a glass transition temperature of 80°C. The physical properties of the obtained film are shown in Table 2.

[0169] (Example 11) A uniaxially oriented polyester film for metal plate coating was prepared under the same conditions as in Example 1, except that the amount of pellet b containing an inorganic pigment in resin composition A was set to 58% by mass. The uniaxially oriented polyester film for metal plate coating had a thickness of 15 μm, a melting point of 250°C, and a glass transition temperature of 80°C. The physical properties of the obtained film are shown in Table 2.

[0170] (Example 12) A uniaxially oriented polyester film for metal plate coating was prepared under the same conditions as in Example 1, except that the amount of wax-containing pellet e in resin composition B was set to 4.0% by mass. The uniaxially oriented polyester film for metal plate coating had a thickness of 15 μm, a melting point of 250°C, and a glass transition temperature of 80°C. The physical properties of the obtained film are shown in Table 2.

[0171] (Example 13) A uniaxially oriented polyester film for metal plate coating was prepared under the same conditions as in Example 1, except that an acid-modified polyethylene wax with an acid value of 120 mgKOH / g was used as the wax. The uniaxially oriented polyester film for metal plate coating had a thickness of 15 μm, a melting point of 250°C, and a glass transition temperature of 80°C. The physical properties of the obtained film are shown in Table 2.

[0172] (Example 14) A uniaxially oriented polyester film for metal plate coating was prepared under the same conditions as in Example 1, except that the melt extrusion amounts of resin compositions A and B were adjusted. The uniaxially oriented polyester film for metal plate coating had a thickness of 10 μm, a melting point of 250°C, and a glass transition temperature of 80°C. The physical properties of the obtained film are shown in Table 2.

[0173] (Example 15) A uniaxially oriented polyester film for metal plate coating was prepared under the same conditions as in Example 1, except that the melt extrusion amounts of resin compositions A and B were adjusted. The uniaxially oriented polyester film for metal plate coating had a thickness of 50 μm, a melting point of 250°C, and a glass transition temperature of 80°C. The physical properties of the obtained film are shown in Table 2.

[0174] (Example 16) A uniaxially oriented polyester film for metal plate coating was prepared under the same conditions as in Example 1, except for the following: The amount of pellet b containing an inorganic pigment in resin composition A was set to 28% by mass. The amount of pellet d containing an inorganic lubricant in resin composition B was set to 4.5% by mass. For the preparation of the unoriented film by extrusion, a feed block with a lamination ratio of 1 (layer B):5 (layer A):1 (layer B) was used. Furthermore, the melt extrusion amounts of resin compositions A and B were adjusted. The uniaxially oriented polyester film for metal plate coating had a thickness of 25 μm, a melting point of 250°C, and a glass transition temperature of 80°C. The physical properties of the obtained film are shown in Table 2.

[0175] (Example 17) A uniaxially oriented polyester film for metal plate coating was prepared under the same conditions as in Example 1, except for the following: The amount of pellet b containing an inorganic pigment in resin composition A was set to 22.5% by mass. The amount of pellet d containing an inorganic lubricant in resin composition B was set to 10.5% by mass. For the preparation of the unoriented film by extrusion, a feed block with a lamination ratio of 1 (layer B):15 (layer A):1 (layer B) was used. Furthermore, the melt extrusion amounts of resin compositions A and B were adjusted. The uniaxially oriented polyester film for metal plate coating had a thickness of 25 μm, a melting point of 250°C, and a glass transition temperature of 80°C. The physical properties of the obtained film are shown in Table 2.

[0176] (Example 18) A uniaxially oriented polyester film for metal plate coating was prepared under the same conditions as in Example 1, except that a feed block designed with a layering ratio of 4 (layer A):1 (layer B) was used. The uniaxially oriented polyester film for metal plate coating had a thickness of 15 μm, a melting point of 250°C, and a glass transition temperature of 80°C. The physical properties of the obtained film are shown in Table 2.

[0177] (Comparative Example 5) A uniaxially oriented polyester film for metal plate coating was prepared under the same conditions as in Example 3, except that the stretching ratio was increased to 8.5 times. The uniaxially oriented polyester film for metal plate coating had a thickness of 15 μm, a melting point of 250°C, and a glass transition temperature of 80°C. The physical properties of the obtained film are shown in Table 2.

[0178] [Table 1]

[0179] [Table 2]

[0180] As can be seen from the results shown in Tables 1 and 2, the film that satisfies the conditions of the present invention possesses lamination properties, can-making processability, and feathering resistance, and can be suitably used as a coating film for metal plates, especially metal plates for containers. Furthermore, a resin-coated metal plate comprising the uniaxially oriented polyester film for metal plate coating of the present invention can be suitably used as a material for manufacturing containers, especially two-piece cans that require a high degree of processability.

Claims

1. A polyester film containing 70% or more by mass of polyester resin and an inorganic lubricant, The degree of crystallinity is 20-50%. The mobile amorphous ratio is 1.0 to 50%. Of the Young's modulus in the longitudinal direction and the Young's modulus in the width direction, the higher one is E. Ｈ , the lower one is E Ｌ The ratio of Young's modulus (E) in this case Ｈ / E Ｌ A uniaxially oriented polyester film for coating metal plates, wherein the ratio is 1.1 to 10.

2. E Ｈ and E Ｌ The uniaxially oriented polyester film for coating metal plates according to claim 1, wherein all of these values ​​are 1,000 to 19,000 MPa.

3. E Ｈ and E Ｌ The sum of (E Ｈ + E Ｌ ) is 3000 to 20000 MPa. The uniaxially stretched polyester film for metal plate coating according to claim 1 or 2.

4. A uniaxially oriented polyester film for coating metal plates according to claim 1 or 2, wherein the thermal shrinkage rate at 160°C is 0 to 20% in both the longitudinal and width directions.

5. Layer A contains 70% or more by mass of polyester resin, The uniaxially oriented polyester film for metal plate coating according to claim 1 or 2, comprising a layer B containing 70% by mass or more of polyester resin and an inorganic lubricant, laminated on at least one surface of the A layer.

6. The uniaxially oriented polyester film for coating metal plates according to claim 5, wherein the A layer contains an inorganic pigment.

7. The uniaxially oriented polyester film for metal plate coating according to claim 6, wherein the inorganic pigment contained in the A layer is present in an amount of 7.0 to 28% by mass of the total components of the uniaxially oriented polyester film for metal plate coating.

8. The uniaxially oriented polyester film for coating metal plates according to claim 5, wherein the B layer contains wax.

9. The uniaxially oriented polyester film for coating metal plates according to claim 8, wherein the wax contained in the B layer is a polyolefin wax.

10. The uniaxially oriented polyester film for coating metal plates according to claim 9, wherein the polyolefin wax is any of polyethylene wax, polypropylene wax, acid-modified polyethylene wax, acid-modified polypropylene wax, oxidized polyethylene wax, and oxidized polypropylene wax, or a mixture thereof.

11. The uniaxially oriented polyester film for coating metal plates according to claim 9, wherein the acid value of the polyolefin wax is 1.0 to 120 mg KOH / g.

12. The uniaxially oriented polyester film for coating metal plates according to claim 8, wherein the content of the wax contained in the B layer is 0.050 to 1.2% by mass of the total components of the B layer.

13. A uniaxially oriented polyester film for coating a metal plate according to claim 1 or 2, wherein the film thickness is 10 to 50 μm.

14. The total thickness t of the aforementioned B layer Ｂ The total thickness t of the A layer relative to the above Ａ The ratio (t Ａ / t Ｂ The uniaxially oriented polyester film for metal plate coating according to claim 5, wherein the ratio is 2.5 to 7.

5.

15. A uniaxially oriented polyester film for coating a metal plate according to claim 1 or 2, wherein the stretching direction is transverse.

16. A uniaxially oriented polyester film for coating metal plates, according to claim 1 or 2, for surface coating of metal plates for containers.

17. A resin-coated metal plate having at least one side a uniaxially oriented polyester film for metal plate coating as described in claim 1 or 2.

18. A process of melting and kneading a mixture containing polyester resin and an inorganic lubricant in an extruder to form a molten resin. The process involves extruding the molten resin in a sheet-like form from a T-die and cooling and solidifying it on a casting drum to form an unstretched film. A step of stretching the unstretched film in the longitudinal or transverse direction under the conditions that the stretching temperature is above the glass transition temperature of the polyester resin and below the glass transition temperature + 50°C, and the stretching ratio is 4.5 to 8.0 times. A step of heat-fixing the stretched film while relaxing it in the stretching direction, and A method for producing a uniaxially oriented polyester film for coating a metal plate according to claim 1 or 2, comprising the step of cooling the heat-fixed film to room temperature.