Polyester film for metal plate laminate

Abstract

To provide a polyester film for metal plate laminate that maintains excellent surface smoothness even after laminating the polyester film to the metal plate, exhibits superior conveyability of the metal plate in a high speed transportation process, and possesses good corrosion resistance.SOLUTION: There is provided a polyester film for a metal plate laminate comprising: a layer A containing a polyester having an alkylene terephthalate unit as the main repeating unit; and a layer B containing a copolymerized polyester having an alkylene terephthalate unit and an alkylene isophthalate unit as the main repeating unit. The arithmetic mean height Sa of the surface on the A-layer side is 35 nm or more and less than 100 nm, the surface developed interfacial area ratio Sdr is 0.5 to 2.5%, and the plane orientation coefficient of the B layer is 0.140 or more and 0.155 or less.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a polyester film for laminating metal sheets, which is suitable for resin coating of metal sheets, and to a resin-coated metal sheet (sometimes referred to as a "laminate sheet") and a container using the same. [Background technology]

[0002] Conventionally, paints have been applied to prevent corrosion on the inside and outside of metal cans, and thermosetting resins have been used as the paint. The method of coating the surface of a metal can with thermosetting resin generally requires applying a paint made by dissolving the thermosetting resin in a solvent to the surface of the metal can, followed by heating at high temperatures of 190°C or higher for several minutes for a long period of time. Furthermore, since a large amount of organic solvent is released during the baking process, there is a demand for improvements that simplify the process and prevent pollution.

[0003] For this reason, a method has been widely adopted in which a metal plate is coated with a thermoplastic resin film and then processed to produce containers such as metal cans. As an example of such a polyester film for laminating with a metal plate, as disclosed in Patent Document 1, a multilayer polyester film with two or more layers is known, in which one surface (A) is composed of a polyester layer having a melting point of 180 to 220°C and the other surface (B) is composed of a polyester layer having a melting point of 240°C or higher and whose main repeating unit is ethylene terephthalate, and the centerline average surface roughness of surface (B) is 0.10 to 0.40 μm.

[0004] However, in the invention of Patent Document 1, although a lubricant is added to the layer on the surface (B) side, which will become the surface side after lamination, to adjust the surface roughness, the film surface design does not take into account the slipperiness after lamination to a metal plate.

[0005] Patent Document 2 proposes a film designed to take into account the sliding properties during can-making. The film is for use in drawing and ironing cans. The thermoplastic polyester A layer, which forms the surface, contains 0.50 to 1.50 weight % of inert particles with a particle size of 3 to 5 μm, and the thermoplastic polyester B layer, which is the thermally bonded side, contains 0.4 weight % or less of inert particles with a particle size of 3 to 5 μm. The film is laminated to a metal substrate, remelted above its melting point, and quenched to form a can. The film surface exhibits a dynamic friction coefficient of 0.20 or less when a steel ball with a load of 2 kg is applied as a sliding element in an environment of 150°C.

[0006] In this way, the invention of Patent Document 2 effectively improves the slipperiness of the film during can-making by keeping the dynamic friction coefficient of the film surface below a certain level when a load is applied in a 150°C environment. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Application Laid-Open No. 2000-071406 [Patent Document 2] Patent No. 5458618 Summary of the Invention [Problem to be solved by the invention]

[0008] However, in recent years, there has been a tendency to transport laminated sheets at high speeds, with an emphasis on productivity of products using laminated sheets. The inventors have found through their research that polyester films for metal sheet lamination having surface characteristics such as those described in Patent Document 2 are too slippery when transported at high speeds after lamination, making them prone to problems such as poor stacking and rubbing.

[0009] Furthermore, due to the recent diversification of can contents, the level of corrosion resistance required has increased, and for polyester films for metal plate lamination, which have a moderate level of lubricity achieved by adding a large amount of particles, achieving both corrosion resistance and transportability has become an important issue.

[0010] In view of the above circumstances, the present invention aims to provide a polyester film for laminating metal sheets, which has good surface smoothness even after lamination on a metal sheet, is excellent in transportability in a high-speed transport process of the metal sheet, and has good corrosion resistance. Another object of the present invention is to provide a resin-coated metal sheet and a container using such a polyester film for laminating metal sheets. [Means for solving the problem]

[0011] The inventors discovered that the above problem can be solved by not only reducing the arithmetic mean height as a surface characteristic of a polyester film for metal plate lamination, but also adjusting the developed interface area ratio, which is an indicator of the rate of increase in the area of ​​fine surfaces per projected area, to a predetermined range, and thus completed the present invention.

[0012] That is, the present invention includes the following:

[0013] [1] A polyester film for metal sheet lamination, comprising: an A layer containing a polyester having an alkylene terephthalate unit as a main repeating unit; and a B layer containing a copolymerized polyester having an alkylene terephthalate unit and an alkylene isophthalate unit as a main repeating unit, the arithmetic mean height Sa of the surface on the Layer A side is 35 nm or more and less than 100 nm, and the developed interface area ratio Sdr of the surface is 0.5 to 2.5%, A polyester film for laminating with a metal plate, wherein the plane orientation coefficient of the layer B is 0.140 or more and 0.155 or less.

[0014] [2] The polyester film for laminating with a metal plate according to [1], wherein the layer A contains 0.1% by weight or more and less than 0.5% by weight of inert particles having a particle size of 1.0 μm or more and less than 4.0 μm.

[0015] [3] The polyester film for laminating to a metal plate according to [1] or [2], wherein the thickness ratio of the layer A is 10% or more and 40% or less relative to the total thickness of the layer A and the layer B (100%).

[0016] [4] The polyester film for metal plate lamination according to any one of [1] to [3], wherein the repeating units of the polyester contained in the layer A contain 95 mol % or more of ethylene terephthalate units.

[0017] [5] The polyester film for metal plate lamination according to any one of [1] to [4], wherein ΔSdr calculated by the following formula (1) is 0.2 or less, where Sdr1 is the developed interface area ratio Sdr of the surface of the A layer, and Sdr2 is the developed interface area ratio Sdr of the surface of the A layer after the B layer side is laminated onto a metal substrate heated to 250°C, which is equal to or higher than the melting point of the B layer, using a rubber roll: ΔSdr=Sdr1-Sdr2 (1)

[0018] [6] A resin-coated metal plate obtained by laminating the polyester film for laminating metal plates according to any one of [1] to [5] on the metal plate by thermal bonding the layer B side of the polyester film.

[0019] [7] A container comprising a member obtained by laminating the polyester film for metal plate lamination according to any one of [1] to [6] on a metal plate by thermal bonding the layer B side of the polyester film. [Effects of the Invention]

[0020] According to the present invention, it is possible to provide a polyester film for laminating metal sheets, which has good surface smoothness even after lamination on a metal sheet, is excellent in transportability in a high-speed transport process of the metal sheet, and has good corrosion resistance.Furthermore, it is possible to provide a resin-coated metal sheet and a container using such a polyester film for laminating metal sheets.

[0021] The reason why not only reducing the arithmetic mean height but also adjusting the developed interface area ratio to fall within a predetermined range results in excellent transportability in the high-speed transport process is not clear, but is presumed to be as follows.

[0022] In conventional polyester films for metal sheet lamination, the arithmetic mean height becomes too large due to the inclusion of particles, which tends to deteriorate the transportability in the high-speed transport process of the metal sheet. Therefore, it is desirable to reduce the arithmetic mean height, but simply reducing the arithmetic mean height does not have much effect on improving the transportability in the high-speed transport process. In contrast, in the polyester film for metal sheet lamination of the present invention, not only is the arithmetic mean height reduced, but the developed interface area ratio, which is an index of the increase rate of the area of ​​the fine surface per projected area, is adjusted to a predetermined range, so that an appropriate air layer is formed between the film surface and the contact surface, which improves the pile-up state during high-speed transport and is thought to have a significant effect on improving the transportability.

[0023] Furthermore, although the details of why adjusting the plane orientation coefficient of layer B within a specified range improves corrosion resistance are not clear, it is speculated that this is because the molecular orientation of the copolymer polyester that makes up layer B is in a state that allows it to exhibit favorable thermal properties, thereby improving thermal adhesion to metal plates, and orienting the polymer chains shrinks the spacing between polymers, leading to a decrease in the ion diffusion rate and therefore improving corrosion resistance. DETAILED DESCRIPTION OF THE INVENTION

[0024] Hereinafter, embodiments of the present invention will be described in detail, but the present invention is not limited to these embodiments. In this specification, a numerical range expressed using "to" means a range that includes the numerical values ​​written before and after "to" as the lower and upper limits.

[0025] [Polyester film for laminating metal sheets] The polyester film for metal sheet lamination of the present invention is a polyester film used for laminating with a metal sheet, and has an A layer that will be the surface side after lamination and a B layer that will be thermally bonded to the metal sheet. An intermediate layer or the like may be provided between the A layer and the B layer to improve adhesion between them, but a polyester film for metal sheet lamination consisting of the A layer and the B layer is preferred. Each component will be described in detail below.

[0026] [A layer] Layer A contains a polyester (hereinafter referred to as "resin A") whose main repeating unit is an alkylene terephthalate unit, and contains resin A as a resin component having a higher melting point or softening point than the resin component of layer B. In this specification, the "main repeating unit" refers to 80 mol % or more of all repeating units, preferably 90 mol % or more, more preferably 95 mol % or more, and even more preferably 98 mol % or more.

[0027] Layer A can contain polyester other than resin A or a resin component other than polyester, but it is preferable that the resin component consists of polyester only, and it is particularly preferable that the resin component consists of resin A only.

[0028] Layer A may contain components other than the resin component, and it is particularly preferable to use particles to adjust the arithmetic mean height Sa of the surface on the Layer A side to 35 nm or more and less than 100 nm, and the developed interface area ratio Sdr of the surface to 0.5 to 2.5%. Note that the surface properties of the Layer A side may also be similarly adjusted by copolymerizing Resin A, adding a polyester component other than Resin A, or adding a resin component other than polyester.

[0029] (Resin A) Resin A is a polyester containing alkylene terephthalate units as the main repeating units. Examples of alkylene terephthalate units include repeating units obtained from terephthalic acid and alkylene glycol. Examples of alkylene glycols include those having 1 to 12 carbon atoms, and alkylene glycols having 1 to 4 carbon atoms are preferred from the viewpoints of corrosion resistance, surface properties, heat resistance, etc.

[0030] In particular, as the alkylene terephthalate unit, an ethylene terephthalate unit, a trimethylene terephthalate unit, or a polybutylene terephthalate unit is preferred, with an ethylene terephthalate unit being most preferred.

[0031] In the present invention, from the viewpoint of minimizing the change in the developed interface area ratio Sdr before and after lamination with a metal plate, the repeating units of the polyester contained in Layer A preferably contain ethylene terephthalate units at 95 mol% or more, more preferably 98 mol% or more, and most preferably 100 mol%.

[0032] Resin A may contain other repeating units other than alkylene terephthalate units, and examples of other units include repeating units containing a polycarboxylic acid component other than terephthalic acid and / or a polyhydric alcohol component other than alkylene glycol.

[0033] Examples of polycarboxylic acids other than terephthalic acid include aromatic polycarboxylic acids such as isophthalic acid, phthalic acid, naphthalenedicarboxylic acid, and biphenyldicarboxylic acid; aliphatic dicarboxylic acids such as adipic acid, azelaic acid, sebacic acid, decanedicarboxylic acid, dodecanedicarboxylic acid, and dimer acid; and alicyclic polycarboxylic acids such as cyclohexanedicarboxylic acid.

[0034] Examples of polyhydric alcohols other than ethylene glycol include aliphatic polyhydric alcohols such as diethylene glycol, triethylene glycol, propanediol, butanediol, pentanediol, hexanediol, dodecamethylene glycol, and neopentyl glycol; alicyclic diols such as cyclohexanedimethanol and cyclohexanediethanol; aliphatic polyhydric alcohols such as trimethylolpropane and pentaerythritol; and aromatic polyhydric alcohols such as ethylene oxide adducts of bisphenol derivatives.

[0035] Preferred repeating units other than the alkylene terephthalate unit include an alkylene isophthalate unit and an alkylene naphthalate unit. Examples of the alkylene isophthalate unit include an ethylene isophthalate unit, a trimethylene isophthalate unit, and a polybutylene isophthalate unit. Examples of the alkylene naphthalate unit include an ethylene naphthalate unit, a trimethylene naphthalate unit, and a polybutylene naphthalate unit.

[0036] In addition, in Resin A, the amount of units derived from diethylene glycol generated as a by-product during polymerization is preferably 5 mol % or less, and more preferably 3 mol % or less, of all repeating units of the polyester.

[0037] Resin A may be a single polyester or a mixture of multiple polyesters.

[0038] Resin A is preferably a crystalline polyester from the viewpoint of minimizing the change in the developed interface area ratio Sdr before and after lamination with the metal plate. Crystalline polyester refers to a polyester that, in a differential scanning calorimetry (DSC) curve, exhibits an endothermic peak of 0.05 J / g or more associated with crystalline melting at a temperature higher than the temperature at which a baseline shift corresponding to the glass transition point occurs. Differential scanning calorimetry to obtain the DSC curve involves using a DSC-60 differential scanning calorimeter to scrape a sample from Layer A, heating 10 mg of the sample to 290°C at 20°C / min, holding the sample isothermally for 3 minutes, quenching at 200°C / min, and then heating to 290°C at 10°C / min.

[0039] The intrinsic viscosity of Resin A or the resin component constituting Layer A is preferably 0.55 to 0.80 dL / g, more preferably 0.57 to 0.75 dL / g, even more preferably 0.60 to 0.72 dL / g, and particularly preferably 0.62 to 0.70 dL / g. An intrinsic viscosity of 0.55 dL / g or higher improves film production operability and, in applications involving contact with food or beverages, improves aroma retention. Furthermore, an intrinsic viscosity of 0.80 dL / g or lower reduces the risk of thermal decomposition due to excessive heat or pressure when the resin is melted and extruded in an extruder during the film production process, thereby reducing the generation of thermal degradation products derived from low-molecular-weight components.

[0040] The content of Resin A is preferably 80% by mass or more, more preferably 90% by mass or more, and even more preferably 95% by mass or more, when Layer A is taken as 100% by mass. The content of Resin A is preferably 99.9% by mass or less, preferably 99.8% by mass or less, and more preferably 99.7% by mass or less, when Layer A is taken as 100% by mass.

[0041] (Polyester other than Resin A) The polyester other than Resin A that may be contained in Layer A does not contain an alkylene terephthalate unit in the repeating unit or contains less than 80 mol% of an alkylene terephthalate unit. Examples of such polyesters include polyesters having repeating units obtained by appropriately combining the polycarboxylic acid component and polyhydric alcohol component exemplified for Resin A.

[0042] In Layer A, the content of polyester other than Resin A is preferably 20% by mass or less, more preferably 10% by mass or less, and most preferably 0% by mass, of the total resin components, from the viewpoint of suitably adjusting the surface characteristics and corrosion resistance of Layer A.

[0043] (Other resin components) Resin components other than polyester that may be contained in layer A include compatible resins, finely dispersible resins, or resin particles described below that do not affect the surface properties of layer A. Examples of such resin components include polycarbonate, which is a resin that can be polymer-alloyed with aromatic polyester.

[0044] In Layer A, the content of resin components other than polyester is preferably 10% by mass or less, more preferably 5% by mass or less, and most preferably 0% by mass, of the total resin components, from the viewpoint of suitably adjusting the surface characteristics and corrosion resistance of Layer A.

[0045] (particle) Layer A preferably contains particles for adjusting surface properties, and the particles are preferably inactive particles such as inorganic fine particles, crosslinked polymer particles, etc. The inactive particles may be used alone or in combination of two or more kinds.

[0046] Examples of inorganic fine particles include metal oxides such as silica, alumina, zirconia, and titanium oxide; composite oxides such as kaolin, zeolite, sericite, and sepiolite; sulfates such as calcium sulfate and barium sulfate; phosphates such as calcium phosphate and zirconium phosphate; and carbonates such as calcium carbonate, among which metal oxides are preferred. These fine particles may be natural or synthetic.

[0047] The composition may also contain organic fine particles such as crosslinked polymer particles. Examples of crosslinked polymer particles include copolymers of acrylic monomers such as acrylic acid, methacrylic acid, acrylic acid esters, and methacrylic acid esters, styrene monomers such as styrene and alkyl-substituted styrenes, and crosslinkable monomers such as divinylbenzene, divinyl sulfone, ethylene glycol dimethacrylate, trimethylolpropane trimethyl acrylate, and pentaerythritol tetramethyl acrylate, melamine resins, benzoguanamine resins, phenolic resins, and silicone resins. Among these, (co)polymers of acrylic monomers are preferred.

[0048] In order to adjust the particle size and particle size distribution of the particles, pulverization, classification, etc. are possible. In the present invention, it is preferable to use inorganic fine particles having an irregular shape (a shape other than spherical or nearly spherical).

[0049] In the present invention, from the viewpoint of adjusting the arithmetic mean height Sa of the surface of Layer A and the developed interface area ratio Sdr within a predetermined range, it is preferable that the layer contains 0.1 wt % or more and less than 0.5 wt % of inert particles having a particle size of 1.0 μm or more and less than 4.0 μm, and it is preferable that the layer contains 0.2 to 0.4 wt % of inert particles having a particle size of 1.2 μm or more and 3.0 μm or less.

[0050] When the particles are blended into the resin component, the particles may be added during the polyester production process, or the particles may be added after the polyester is produced and melt-kneaded. Alternatively, a polyester containing a high concentration of particles may be produced, and this may be used as a masterbatch to be melt-kneaded with a polyester-based resin that does not contain or contains a small amount of the above-mentioned components.

[0051] (Other components other than particles) Layer A may contain various additives as needed, such as antioxidants, heat stabilizers, UV absorbers, plasticizers, pigments, antistatic agents, lubricants, crystal nucleating agents, etc. The content of components other than particles in Layer A is preferably 5% by mass or less, more preferably 3% by mass or less, and even more preferably 1% by mass or less, based on 100% by mass of the composition constituting Layer A.

[0052] (Characteristics of Layer A) In the polyester film for metal plate lamination of the present invention, the arithmetic mean height Sa of the surface on the A layer side is 35 nm or more, and is preferably 35 nm or more, more preferably 40 nm or more, from the viewpoint of providing appropriate slipperiness during stacking of metal plates in a high-speed conveying process and suppressing variation in the stacked state.

[0053] In addition, the arithmetic mean height Sa of the surface on the A layer side is less than 100 nm, and from the viewpoint of preventing misalignment due to excessive slippage, particularly when stacking metal plates in a high-speed transport process, it is preferably 80 nm or less, and more preferably 65 nm or less.

[0054] The arithmetic mean height Sa of the surface on the Layer A side can be adjusted by the particle size and amount of added particles, particle shape, thickness of Layer A, resin composition, stretching conditions, etc. For example, increasing the particle size or amount of added particles can increase the arithmetic mean height Sa.

[0055] In the present invention, by setting not only the arithmetic mean height Sa of the surface on the A layer side but also the developed interface area ratio Sdr, which is an index of the increase rate of the area of ​​the fine surface per projected area, within a predetermined range, a polyester film for laminating to metal plates is obtained that has good surface smoothness even after lamination to a metal plate and is also excellent in transportability in a high-speed transport process of the metal plate.

[0056] The arithmetic mean height Sa represents the average of the absolute values ​​of Z(x, y) (height difference from the average plane) in the measurement area, while the developed interface area ratio Sdr represents the rate of increase in surface area, and is calculated using the following formula from the surface area A1 of the measurement area and the area A0 when that surface is projected onto the XY plane. Sdr = ({A1 / A0)-1} x 100(%)

[0057] In this way, by keeping the developed interface area ratio Sdr, which is an indicator of the rate of increase in the area of ​​the fine surface per projected area, within a specified range, an appropriate air layer is formed between the surface of layer A and the contact surface (metal plate), which improves the stacking condition during high-speed conveyance and is thought to have a significant effect on improving conveyability.

[0058] In the polyester film for laminating with a metal plate of the present invention, the developed interface area ratio Sdr of the surface on the A-layer side is 0.5% or more, and is preferably 0.7% or more, and more preferably 0.9% or more, from the viewpoint of imparting appropriate slipperiness particularly when stacking metal plates in a high-speed conveying process and suppressing variations in the stacked state.

[0059] In addition, the developed interface area ratio Sdr of the surface on the A layer side is 2.5% or less, and from the viewpoint of preventing displacement due to excessive slippage, particularly when stacking metal plates in a high-speed transport process, it is preferably 2.0% or less, and more preferably 1.8% or less.

[0060] The developed interface area ratio Sdr can be adjusted by the particle size and amount of added particles, particle shape, thickness of layer A, resin composition, stretching conditions, heating method, etc., but an effective method for adjusting the developed interface area ratio Sdr without significantly changing the arithmetic mean height Sa is, for example, to employ a radiant heating method as the heating method during stretching, and in particular to heat and stretch using an infrared (IR) heater or the like. This is a preferred method for adjusting the developed interface area ratio Sdr because it is easy to obtain high temperatures and to easily apply localized heating.

[0061] When the developed interface area ratio Sdr of the surface of Layer A is Sdr1 and the developed interface area ratio Sdr of the surface of Layer A after the B layer is bonded with a rubber roll to a metal substrate heated to 250°C, which is above the melting point of Layer B, is Sdr2, ΔSdr calculated by the following formula (1) is preferably 0.3 or less, and more preferably 0.2 or less. ΔSdr=Sdr1-Sdr2 (1)

[0062] In this way, by reducing the change ΔSdr in the developed interface area ratio Sdr before and after lamination, the developed interface area ratio Sdr after lamination can be suitably controlled, thereby improving the transportability in the high-speed transport process of the metal plate.

[0063] Effective methods for reducing the change ΔSdr in the developed interface area ratio Sdr include using a resin A with a large content of ethylene terephthalate units and adjusting the thickness ratio of layer A within a predetermined range, or heat treating the film at a high temperature after stretching.

[0064] The coefficient of static friction between the surfaces of the A layers at 23°C is preferably 0.23 to 0.35, more preferably 0.25 to 0.33, from the viewpoint of film handling properties such as winding properties of the polyester film. The coefficient of dynamic friction between the surfaces of the A layers at 23°C is also important in view of the processability of the metal sheets after lamination and the like. From the viewpoint of film winding properties, the thickness is preferably 0.20 to 0.33, and more preferably 0.25 to 0.31.

[0065] The plane orientation coefficient Ns of the A layer is preferably 0.148 to 0.160, and more preferably 0.153 to 0.158, from the viewpoint of increasing the crystallinity of the A layer and reducing the change ΔSdr in the developed interface area ratio Sdr.

[0066] Here, the plane orientation coefficient Ns is a value calculated from the refractive index in each direction of each surface layer of the film according to JIS-K7105 and the following formula: Ns=(nMD+nTD) / 2-nZ

[0067] The plane orientation coefficient Ns can be adjusted by the type of resin used, the stretching conditions in each direction, etc. For example, the plane orientation coefficient Ns can be increased as the area stretching ratio, which is the product of the stretching ratios in the MD and TD directions, increases.

[0068] [B layer] Layer B contains a copolymerized polyester (hereinafter referred to as "resin B") whose main repeating units are alkylene terephthalate units and alkylene isophthalate units, and contains resin B as a resin component having a lower melting point or softening point than resin A that constitutes layer A.

[0069] The total amount of alkylene terephthalate units and alkylene isophthalate units in Resin B is 80 mol % or more, preferably 90 mol % or more, more preferably 95 mol % or more, and even more preferably 98 mol % or more of all repeating units.

[0070] Layer B may contain polyester other than resin B or a resin component other than polyester, but it is preferable that the resin component consists of polyester only, and it is particularly preferable that the resin component consists of resin B only.

[0071] Examples of polyesters other than resin B include resins A that do not fall under resin B, and other polyesters exemplified for layer A. Layer B can contain components other than the resin component.

[0072] (Resin B) Resin B is a copolymerized polyester having alkylene terephthalate units and alkylene isophthalate units as main repeating units. Examples of the alkylene terephthalate units of Resin B include the same as those of Resin A, such as ethylene terephthalate units, trimethylene terephthalate units, and polybutylene terephthalate units, with ethylene terephthalate units being preferred.

[0073] Examples of the alkylene isophthalate unit in Resin B include an ethylene isophthalate unit, a trimethylene isophthalate unit, and a polybutylene isophthalate unit, with the ethylene isophthalate unit being preferred.

[0074] Resin B may contain other units as repeating units. Examples of other units include those exemplified for Layer A, but alkylene naphthalate units such as ethylene naphthalate units, trimethylene naphthalate units, and polybutylene naphthalate units are preferred.

[0075] When the total repeating units of Resin B is taken as 100 mol%, the alkylene isophthalate units are preferably 4 mol% or more, more preferably 6 mol% or more, even more preferably 8 mol% or more, and still more preferably 10 mol% or more. When the total repeating units of Resin B is taken as 100 mol%, the alkylene isophthalate units are preferably 40 mol% or less, more preferably 35 mol% or less, even more preferably 30 mol% or less, and still more preferably 25 mol% or less.

[0076] Of the resin components or polyesters contained in Layer B, the content of Resin B is preferably 70% by mass or more, more preferably 80% by mass or more, and even more preferably 90% by mass or more, from the viewpoint of thermal adhesion to metal plates and corrosion resistance.

[0077] (Characteristics of Layer B) The plane orientation coefficient of layer B is 0.140 or more and 0.155 or less, and from the viewpoint of heat adhesion to metal plates and heat resistance in particular, it is preferably 0.142 or more and 0.152 or less, and more preferably 0.145 or more and 0.150 or less.

[0078] The melting point of the resin B or resin component constituting the layer B is preferably 220 to 235°C, more preferably 225 to 235°C, and even more preferably 225 to 233°C, from the viewpoint of thermal adhesion to the metal plate and heat resistance.

[0079] The method for laminating Layer A and Layer B is not particularly limited, but it is preferable to laminate them by co-extrusion when producing the film of the present invention.

[0080] The intrinsic viscosity of the resin constituting Layer B is preferably 0.55 to 0.75 dl / g, more preferably 0.60 to 0.70 dl / g, from the viewpoint of film-forming properties and aroma retention.

[0081] The temperature at which the layer B and the metal plate can be laminated is preferably 250° C. or lower, more preferably 240° C. or lower, and even more preferably 235° C. or lower. By thermally bonding the layer B and the metal plate at a low temperature, film-laminated metal plates and metal containers can be produced at low cost.

[0082] (Properties of polyester film) From the viewpoints of gas barrier properties, corrosion resistance, and production costs, the thickness of the polyester film for metal plate lamination of the present invention is preferably 5 μm or more and 30 μm or less, more preferably 8 μm or more and 25 μm or less, and even more preferably 12 μm or more and 20 μm or less.

[0083] Furthermore, from the viewpoint of making it easier to control the surface characteristics of Layer A within a predetermined range, the thickness ratio of Layer A is preferably 10% or more and 40% or less, more preferably 12% or more and 30% or less, and even more preferably 15% or more and 25% or less, relative to the total thickness of Layer A and Layer B (100%).

[0084] [Film manufacturing method] The polyester film for metal plate lamination of the present invention may be produced by laminating Layer A and Layer B by coextrusion and then biaxially stretching the laminate, or by producing Layer A and Layer B as separate films and then laminating them. Layer A and Layer B can be produced by similar melt extrusion and stretching methods, but it may also be produced by producing Layer A as a film and then laminating Layer B by extrusion lamination.

[0085] As a method for producing a film consisting of Layer A or Layer B, or a laminated film by co-extrusion, a melt extrusion method can be used in which raw material chips of each polyester used are dried using a dryer such as a hopper dryer or paddle dryer, or a vacuum dryer, preferably so that the residual moisture content is 150 ppm or less, and then extruded into a film using an extruder at a temperature of 260 to 300°C.

[0086] As a melt extrusion method, undried polyester raw material chips are extruded into a film at a temperature of 260 to 300°C in a vented extruder while removing moisture. Any known method such as a T-die method, an I-die method, or a tubular method may be used for extrusion. After extrusion, the extrusion is rapidly cooled to obtain an unstretched film.

[0087] The stretching method for the unstretched film is not particularly limited, but is preferably a uniaxially stretched film or a biaxially stretched film. Stretching a polyester film can further improve the aroma retention and corrosion resistance of the polyester film, and biaxially stretched films are particularly superior in aroma retention and corrosion resistance. When biaxial stretching is performed, either sequential biaxial stretching or simultaneous biaxial stretching may be used, but sequential biaxial stretching is preferred because it broadens the range of thickness that can be produced. In this case, the stretching ratio in the longitudinal direction is preferably 2 to 5 times, more preferably 2.5 to 4 times, and the stretching temperature is preferably 80 to 120°C, more preferably 90 to 110°C. The stretching ratio in the transverse direction is preferably 2 to 5 times, more preferably 3 to 4.5 times, and the stretching temperature is preferably 80 to 140°C, more preferably 90 to 130°C.

[0088] When forming a laminate film, it is preferable that the residual shrinkage stress caused by biaxial stretching of Layer A is reduced or eliminated by a heat setting method or the like. This reduces dimensional changes in the laminate film due to thermal history during processes such as can making. Furthermore, when Layer A is heat set or the like to reduce or eliminate the residual shrinkage stress, Layer B is preferably made amorphous or non-oriented by the thermal history or the like. This allows sufficient lamination adhesion to be obtained when laminating the laminate film to a preheated metal plate without preheating the metal plate to the melting point of Layer B, thereby speeding up the lamination process.

[0089] To reduce or eliminate the residual shrinkage stress due to biaxial stretching of Layer A and to amorphize or de-orient Layer B, Layer A is preferably heat-set at a temperature that is at least 5°C lower than the melting point of the polyester that constitutes Layer B and not higher than 15°C lower than the melting point of the polyester that constitutes Layer A, more preferably at a temperature that is at least 2°C lower than the melting point of the polyester that constitutes Layer B and not higher than 20°C lower than the melting point of the polyester that constitutes Layer A. By reducing or eliminating the residual shrinkage stress of Layer A and amorphizing or de-orienting Layer B, it is possible to achieve impact strength and ensure so-called lamination operability or handleability.

[0090] In the case of a laminated film, the melting points of Layer A and Layer B are preferably temperatures at which the above-mentioned suitable heat setting temperature can be selected. When a plurality of polyester resins constitute the layer, the melting point of Layer A means the melting point with the largest crystalline melting peak area measured by DSC, and when a plurality of polyester resins constitute the layer, the melting point of Layer B means the melting point with the largest crystalline melting peak area measured by DSC.

[0091] [Resin-coated metal sheet] The resin-coated metal sheet of the present invention is prepared by thermally bonding the layer B side of the polyester film for laminating metal sheets to at least one side of a metal sheet. This results in a resin-coated metal sheet with good surface smoothness, excellent transportability in high-speed metal sheet transport processes, and good corrosion resistance. The resin-coated metal sheet is particularly suitable for use in the manufacture of metal cans.

[0092] The metal plate to be used is not particularly limited, but examples thereof include tinplate, tin-free steel, aluminum, etc. The thickness is also not particularly limited, but is preferably 100 to 500 μm, more preferably 150 to 400 μm, from the viewpoints of economic efficiency, represented by material cost and can-making processing speed, etc., and on the other hand, ensuring material strength.

[0093] Furthermore, the method for laminating the film on at least one side of the metal plate can be any known method, and is not particularly limited, but a thermal bonding method (thermal lamination method) is preferred, and a method in which the metal plate is heated by electrical current to achieve thermal bonding is particularly preferred. The film may be laminated on both sides of the metal plate. When the film is laminated on both sides of the metal plate, the films may be laminated simultaneously or sequentially.

[0094] The resin-coated metal sheet can be formed into various shapes as required and used for applications such as various containers, exterior components for various products, wall components, and housing components.

[0095] [container] The container of the present invention includes a member obtained by laminating the above-described polyester film for metal plate lamination to at least one side of a metal plate by thermal bonding of the B layer side of the film, thereby providing a container using a resin-coated metal plate that has good surface smoothness, excellent transportability in a high-speed metal plate transport process, and good corrosion resistance.

[0096] The container of the present invention can be obtained using a member formed by molding the resin-coated metal sheet, and can be a metal container made partially or entirely of metal. The shape of the metal container is not particularly limited, but can be, for example, can-shaped, bottle-shaped, barrel-shaped, etc. The method for forming the metal container is also not particularly limited, but known methods such as drawing, ironing, and drawing and ironing can be used. For example, a three-piece can can be obtained by laminating a polyester film for metal sheet lamination to the inner surface of the can for the purpose of preventing corrosion of metal containers for food and beverages such as soft drinks, coffee, and canned goods. The film can also be used for applications such as food cans and beverage cans and their lids for food use, as well as aerosol cans and aerosol can bottom lids for household goods. [Example]

[0097] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples. The physical properties and characteristics in the examples were measured or evaluated by the following methods. Furthermore, "parts" means "parts by mass."

[0098] (1) Film thickness Measurement was carried out in accordance with JIS K7310-1999 Method A using a dial gauge (manufactured by Mitutoyo).

[0099] (2) Measurement method for thickness of each layer of laminated film The film sample was cut into a length of 2 mm and a width of 20 mm, fixed in an embedding capsule, and then embedded in epoxy resin (Epomount, manufactured by Refine Tech Co., Ltd.). The embedded sample was cut perpendicular to the width direction using a microtome (ULTRACUT UCT, manufactured by LEICA) to create 5 mm-thick thin slices. Images were taken using a transmission electron microscope (Hitachi S-4300) at an accelerating voltage of 100 kV, and the thickness ratio of each layer relative to the combined thickness of layers A and B (100%) was measured from the photographs.

[0100] (3) Film surface roughness (arithmetic mean height Sa and surface interface area ratio Sdr) The obtained film was cut into an area of ​​100 mm in the longitudinal direction and 100 mm in the width direction, and a Zygo white laser interferometer (NEW VIEW8300) was used to measure the area by scanning with a 20x lens attached to the interferometer. Measurements were taken at 10 random points on the 100 mm x 100 mm sample, over a range of 0.82 μm in the MD direction and 0.82 μm in the width direction on one surface, and the arithmetic mean height Sa (nm) and surface interface area ratio Sdr (%) were measured.

[0101] (4) Film friction coefficient The resulting film was cut into a 400mm x 100mm sample. This was then aged for 12 hours at 23°C and 65% RH. The sample was divided into a 300mm x 100mm slide and a 100mm x 100mm slide. The slide was attached to the slide, and the load was attached to the surface of the 1.5kg metal load with tape, ensuring contact between the slide and load samples. The static and dynamic friction coefficients were measured using an AND Corporation Tensilon (RTG-1210) at a metal load tension speed of 200mm / min, 23°C, and 65% RH. The average of three measurements was calculated.

[0102] (5) Plane orientation coefficient The refractive index of each surface layer of the obtained film was measured using an Abbe refractometer (light source: sodium D line 589 nm, mounting solution: methylene iodide) in accordance with JIS-K7105. From the refractive index in each direction obtained, the plane orientation coefficient Ns was calculated according to the following formula. Ns=(nMD+nTD) / 2-nZ

[0103] (6) Preparation of film-laminated metal sheets The degreased tin-free steel sheet was 0.21 mm thick and 0.3 m wide (metallic chromium content: 120 mg / m 2 Chromium hydroxide content: 15 mg / m as chromium 2 The film (surface roughness: 0.3-0.5 μm) was preheated to 250°C. The metal plate and one side of the film were bonded together, and the bonded assembly was passed between two rubber rolls at a pressure of 500 N / cm and a speed of 9 m / min, and then rapidly cooled in water to obtain a film-laminated metal plate. During lamination, Layer B was positioned on the metal plate side.

[0104] The obtained film-laminated metal sheet was measured under the conditions described below. The arithmetic mean height Sa and surface interface area ratio Sdr were also measured under the same conditions as in (3) using the obtained film-laminated metal sheet, and the difference in Sdr before and after lamination was taken as ΔSdr and calculated using the following formula (1). ΔSdr=Sdr1-Sdr2 (1)

[0105] (7) Transportability The film-laminated metal plate was cut into 100 mm x 100 mm pieces to prepare 100 samples for transport. The samples were placed on a driving belt and transported sequentially at a transport speed of 100 m / min. The samples were then dropped into a storage box, and the 100 samples were stacked sequentially (metal plate on the bottom). The stacking conditions were observed and evaluated according to the following criteria. Judgment A: There was absolutely no misalignment of the metal plates when they were stacked. Judgment B: When stacking, misalignment of the metal plates occurred at a frequency of 1 to 5 sheets. Judgment C: Five or more metal plates were misaligned during stacking, or the stacking could not be completed.

[0106] (8) Corrosion resistance The film side of the obtained film-laminated metal sheet was brought into contact with a sufficient amount of an aqueous acetic acid solution of pH 3, and the film and metal sheet were evaluated according to the following criteria after 2 weeks at 80°C. Judgment A: No peeling or cracking of the film was observed, and no corrosion of the metal was observed. Rating B: Peeling of the film and cracking were observed, but no metal corrosion was observed. Rating C: Peeling and cracking of the film occurred, and metal corrosion was observed.

[0107] (9) Composition analysis of polyester resins Approximately 30 mg of sample was dissolved in a 10:1 (volume ratio) mixture of chloroform D (Eurisop) and trifluoroacetic acid D1 (Eurisop) to prepare a sample solution. Then, using a nuclear magnetic resonance (NMR) analyzer (GEMINI-200, Varian) at 23°C and 64 cycles, the proton NMR of the sample solution was measured. In the NMR measurement, the peak intensity of a specific proton was calculated, and the content (mol %) of terephthalic acid and isophthalic acid components in 100 mol % of the acid component was calculated.

[0108] (10) Intrinsic viscosity (IV) The raw material polyester was dissolved in a mixed solvent of phenol (60% by mass) and 1,1,2,2-tetrachloroethane (40% by mass) to a concentration of 0.4 g / dL, and the intrinsic viscosity was measured using an Ubbelohde viscometer at 30° C. The unit of intrinsic viscosity is dL / g. (11) Melting point Measurement was performed using a Shimadzu DSC-60 differential scanning calorimeter. The raw material polyester was heated and melted at 290°C for 3 minutes, and then quenched with liquid nitrogen. 10 mg of the quenched polyester was used as a sample, and the endothermic peak temperature (melting point) due to crystalline melting that appeared when the temperature was increased at a rate of 10°C / min was measured.

[0109] (12) Average particle size (diameter) of fine particles (12-1) Method for measuring the average particle size (diameter) of silica particles The average particle size of the inorganic fine particles was measured using a particle size distribution analyzer (Horiba, Ltd., SZ-100). (12-2) Measurement method for average particle size (diameter) of calcium carbonate particles The film was dissolved in hexafluoroisopropanol to separate the inorganic fine particles. Thirty of the resulting particles were observed under a scanning electron microscope (Hitachi S-3100). The diameter of the circle equivalent to the area of ​​each particle was measured as the particle diameter, and the average value was taken as the average particle diameter.

[0110] The compositions of the polyesters and particle-containing polyesters used in the examples and comparative examples are as follows. [P1] Polyethylene terephthalate (PET homopolymer, IV = 0.67, melting point = 254°C) [P2] Polyethylene terephthalate copolymerized with 10 mol% isophthalic acid (IV = 0.70, melting point = 228 ° C) [P3] Masterbatch (particle concentration 5% by mass) containing aggregated silica particles (Fuji Silysia Chemical, Sylysia 730, average particle size 4.0 μm) in P1 [P4] Masterbatch (particle concentration 5% by mass) containing agglomerated silica particles (Fuji Silysia Chemical, Sylysia SY23, average particle size 2.3 μm) in P1 [P5] Masterbatch (particle concentration 5% by mass) containing spherical silica particles (Nippon Shokubai Co., Ltd., Seahoster S150, average particle size 1.5 μm) in P1 [P6] A masterbatch containing 1% by mass of agglomerated silica particles (Fuji Silysia Chemical, Sylysia 310, average particle size 2.7 μm) and 5% by mass of amorphous calcium carbonate particles (Maruo Calcium, Caltex 5, average particle size 1.0 μm) in P1 (total particle concentration 6% by mass). [P7] Masterbatch (particle concentration 5% by mass) containing agglomerated silica particles (Fuji Silysia Chemical, Sylysia 27, average particle size 2.7 μm) in P1

[0111] [Example 1] The raw material for Layer A was a 91 / 5 (wt%) P1 / P5 polyester mixture dried at 160°C for 4 hours. The raw material for Layer B was a 100% (wt%) polyester P2 mixture dried at 160°C for 4 hours. After melting each mixture at 270°C in a twin-screw extruder, the resulting mixture was laminated in two layers through a T-die so that the thickness ratio of Layer A was 20%. The resulting unstretched sheet was stretched 3.5 times in the longitudinal direction at a preheating temperature of 80°C and a stretching temperature of 95°C, and then stretched 3.7 times in the transverse direction at a preheating temperature of 95°C and a stretching temperature of 120°C in a tenter. The sheet was then heat-treated at 180°C for 10 seconds and then relaxed 5% in the transverse direction at 170°C to obtain a 16 μm thick polyester film. The evaluation results of the resulting polyester film are shown in Table 1.

[0112] [Examples 2 to 5] Biaxially stretched polyester films were obtained by changing the type of polyester used in Layer A as shown in Table 1 and maintaining the subsequent film-forming conditions the same as in Example 1. Table 1 shows the evaluation results of the obtained polyester films.

[0113] [Comparative Examples 1 to 3 and 5] Biaxially stretched polyester films were obtained by changing the type of polyester used in Layer A as shown in Table 1, and maintaining the same film-forming conditions as in Example 1. In each Comparative Example, the particle content in Layer A was kept at the same level as in Example 1, and in Comparative Example 3, particles with a relatively large particle size were used. The evaluation results of the obtained polyester films are shown in Table 1.

[0114] Comparative Example 4 Biaxially stretched polyester films were obtained in the same manner except that the type of polyester used in Layer A was changed as shown in Table 1, and the longitudinal stretching temperature was 135° C. and the transverse stretching temperature was 145° C. The evaluation results of the obtained polyester films are shown in Table 1.

[0115] [Table 1]

[0116] As shown in Table 1, in Examples 1 to 5, polyester films for laminating to metal sheets having good transportability and corrosion resistance were obtained by setting the arithmetic mean height and developed interface area ratio of the surface of Layer A within predetermined ranges and the plane orientation coefficient of Layer B within predetermined ranges. In contrast, in Comparative Examples 1 to 3 and 5, in which the developed interface area ratio of the surface of Layer A was greater than the predetermined range, transportability was deteriorated. Furthermore, in Comparative Example 4, in which the developed interface area ratio of the surface of Layer A was smaller than the predetermined range and the plane orientation coefficient of Layer B was smaller than the predetermined range, both transportability and corrosion resistance were deteriorated. [Industrial Applicability]

[0117] According to the present invention, it is possible to provide a polyester film for laminating metal sheets, which has good surface smoothness even after lamination on a metal sheet, excellent transportability in a high-speed transport process of the metal sheet, and good corrosion resistance. Therefore, the polyester film for laminating metal sheets of the present invention is useful as a laminating film for producing resin-coated metal sheets and various containers.

Claims

1. A polyester film for metal sheet lamination, comprising: an A layer containing a polyester having an alkylene terephthalate unit as a main repeating unit; and a B layer containing a copolymerized polyester having an alkylene terephthalate unit and an alkylene isophthalate unit as a main repeating unit, the arithmetic mean height Sa of the surface on the Layer A side is 35 nm or more and less than 100 nm, and the developed interface area ratio Sdr of the surface is 0.5 to 2.5%, A polyester film for metal plate lamination, wherein the plane orientation coefficient of the layer B is 0.140 or more and 0.155 or less.

2. 2. The polyester film for metal plate lamination according to claim 1, wherein the layer A contains 0.1 wt % or more and less than 0.5 wt % of inert particles having a particle size of 1.0 μm or more and less than 4.0 μm.

3. 2. The polyester film for metal plate lamination according to claim 1, wherein the thickness ratio of the layer A is 10% or more and 40% or less with respect to 100% of the total thickness of the layers A and B.

4. 2. The polyester film for metal plate lamination according to claim 1, wherein the repeating units of the polyester contained in the layer A contain 95 mol % or more of ethylene terephthalate units.

5. 5. The polyester film for metal plate lamination according to claim 4, wherein ΔSdr calculated by the following formula (1) is 0.2 or less, where Sdr1 is the developed interface area ratio Sdr of the surface of the A layer, and Sdr2 is the developed interface area ratio Sdr of the surface of the A layer after the B layer side is bonded to a metal substrate heated to 250°C, which is equal to or higher than the melting point of the B layer, using a rubber roll: ΔSdr=Sdr1-Sdr2 (1)

6. A resin-coated metal plate obtained by laminating the polyester film for metal plate lamination according to any one of claims 1 to 5 to a metal plate by thermal bonding the layer B side of the polyester film to the metal plate.

7. A container comprising a member obtained by laminating the polyester film for metal plate lamination according to any one of claims 1 to 5 to a metal plate by thermal bonding the layer B side of the polyester film to a metal plate.

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