Resin-coated metal plate, metal container, and method for producing resin-coated metal plate

Abstract

Provided is a resin-coated metal plate having excellent slipperiness, moldability and ink adhesion of a resin coating layer. A resin-coated metal plate according to the present invention comprises, on at least one surface of a metal plate, a resin coating layer including a polyester resin and satisfying a predetermined requirement, wherein: the outermost surface layer of the resin coating layer contains a polyolefin satisfying a predetermined requirement; and on the surface of the outermost layer of the resin coating layer, the average area m of N Voronoi polygons obtained by performing Voronoi partitioning using the center of gravity of each polyolefin particle as a seed point is 200 μm2 to 10,000 μm2, and σ2 defined by formula (1) is at most 1.0. Here, N is the total number m of Voronoi polygons, m is the average area (μm2)of N Voronoi polygons, Si is the area (μm2) of the i-th Voronoi polygon among the N Voronoi polygons, and i is an integer in the range of 1 to N.

Description

Resin-coated metal sheet, metal container, and method for manufacturing resin-coated metal sheet

[0001] The present invention relates to a resin-coated metal sheet, a metal container, and a method for producing a resin-coated metal sheet.

[0002] Laminated metal sheets have been developed, in which the surface of a metal sheet, such as tin-free steel (TFS) or aluminum, used as a material for metal containers, is coated with a thermoplastic resin film. Laminated metal sheets are widely used in the fields of beverage cans and food cans, which require strict forming processes.

[0003] In recent years, from the viewpoint of resource conservation and material cost reduction, the materials used for metal containers, particularly the metal plates and resin coating layers, have been made thinner by tightening processing conditions. As a result, the degree of processing during can body production increases, and there is a possibility that the resin coating layer located on the outer surface of the metal container may break or be chipped, particularly after forming. In light of this, there is a need for a material design that suppresses breakage or chipping of the resin coating layer during can body production. As a technology for suppressing breakage or chipping of the resin coating layer, Patent Document 1 proposes a method of adding a lubricating component to the resin coating layer to improve the slip properties and formability of the resin coating layer.

[0004] Furthermore, the resin coating layer located on the outer surface of the container after molding is subjected to a printing process to improve design. If the affinity between the printing ink applied by the printing process and the resin coating layer is low, sufficient ink adhesion cannot be ensured, and the printing ink may peel off during can processing, potentially damaging the design and aesthetic appeal of the can's appearance.

[0005] International Publication No. 2019 / 116706

[0006] Although the lubricating component added to the resin coating layer prevents the resin coating layer from breaking or being scraped, it may inhibit the affinity between the printing ink applied in the printing process and the surface of the resin coating layer, which may cause the ink to peel off during processing. Therefore, there is a demand for a resin-coated metal sheet that has excellent ink adhesion in addition to the lubricity and formability of the resin coating layer.

[0007] SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide a resin-coated metal sheet having a resin coating layer excellent in slipperiness, formability, and ink adhesion, and a method for producing the same.

[0008] The present inventors conducted extensive research to solve the above-mentioned problems and discovered the following. Conventionally, even if only the amount and particle size of the lubricating component contained in the resin coating layer were specified, the uneven distribution of the lubricating component would prevent excellent properties from being achieved. Therefore, by controlling the dispersion state of the lubricating component present on the outermost surface of the resin coating layer, a resin coating layer with excellent slip properties, formability, and ink adhesion can be obtained. Furthermore, the dispersion state of the lubricating component can be controlled by setting the stretching conditions of the sheet-shaped molded body when producing a laminate film to be thermocompressed to a metal plate to predetermined conditions and by setting the temperatures of the metal plate and laminating rolls within appropriate ranges when thermocompressing the laminate film to the metal plate.

[0009] That is, the gist and configuration of the present invention are as follows.

[0010] [1] A resin-coated metal sheet having a resin coating layer containing a polyester resin on at least one surface of a metal sheet, wherein the resin coating layer has at least a three-layer structure including a surface layer, an intermediate layer, and a bottom layer, the melting point of the resin coating layer is 230°C or higher and 260°C or lower, the surface layer contains 1.0 mass% or higher and 5.0 mass% or lower of a polyolefin, the polyolefin being at least one of an acid-modified polyolefin and an oxidized polyolefin, the melting point of the polyolefin being 70°C or higher and 145°C or lower, the polyolefin being dispersed in a particulate form in the surface of the surface layer, and the dispersion state of the polyolefin particles is such that, on the surface of the surface layer, N Voronoi polygons obtained by Voronoi tessellation using the center of gravity of each polyolefin particle as a kernel point have an average area m of 200 μm ２ 10000 μm or more ２ is less than or equal to the value of σ defined by the following equation (1): ２ A resin-coated metal sheet that satisfies the requirement that the coefficient of friction is 1.0 or less. where N: total number of Voronoi polygons, m: average area (μm ２ ) S ｉ: the area (μm) of the i-th Voronoi polygon among the N Voronoi polygons ２ ) where i is an integer ranging from 1 to N.

[0011] [2] The resin-coated metal sheet according to [1] above, wherein the weight average molecular weight of the polyolefin is 2,000 or more and 50,000 or less.

[0012] [3] The resin-coated metal sheet according to [1] or [2] above, wherein the acid value of the polyolefin is 40 mg KOH / g or more and 80 mg KOH / g or less.

[0013] [4] The resin-coated metal sheet according to any one of [1] to [3] above, wherein the outermost layer and the lowermost layer each contain 0.010 mass% to 1.0 mass% of lubricating inorganic particles.

[0014] [5] The resin-coated metal sheet according to any one of [1] to [4] above, wherein the intermediate layer contains 10% by mass or more and 35% by mass or less of inorganic particles.

[0015] [6] The outermost layer and the lowermost layer each have a thickness of 1.0 μm or more and 5.0 μm or less, and the intermediate layer has a thickness of 6.0 μm or more and 30 μm or less. [7] The resin-coated metal sheet according to any one of [1] to [5] above.

[0016] [7] A metal container made using the resin-coated metal sheet according to any one of [1] to [6] above, wherein the resin coating layer is located on the outside of the metal container.

[0017] [8] A process for preparing a masterbatch by kneading a polyester resin containing 4.0 mol % to 10.0 mol % of isophthalic acid as an acid component with 2.0 mass % to 10.0 mass % of a polyolefin which is at least one of an acid-modified polyolefin and an oxidized polyolefin and has a melting point of 70°C to 145°C; a process for mixing the masterbatch with an additional polyester resin to prepare a first composition having a polyolefin content of 1.0 mass % to 5.0 mass %; a process for co-extruding the first composition, a second composition containing a polyester resin, and a third composition containing a polyester resin using a kneading extruder and discharging the resultant mixture through a T-die to obtain a sheet-like molded body having a first layer, a second layer, and a third layer composed of the first composition, the second composition, and the third composition, respectively; and a process for cooling and solidifying the molded body to obtain a laminated film. a step of stretching the laminated film at least once under conditions of a stretching temperature of 80°C or higher and 95°C or lower and a stretching ratio of 4.0 times or higher and 6.5 times or lower, thereby obtaining a resin film including the first layer, the second layer, and the third layer; and a step of thermocompression-bonding the resin film to at least one surface of a metal plate controlled to 250°C or higher and 280°C or lower, using a laminating roll controlled to 100°C or higher and 110°C or lower, thereby obtaining a resin-coated metal plate provided on at least one surface of the metal plate with at least a three-layer structure in which the first layer, the second layer, and the third layer are the outermost layer, the middle layer, and the bottom layer, respectively, and with a resin coating layer having a melting point of 230°C or higher and 260°C or lower.

[0018] [9] The method for producing a resin-coated metal sheet according to the above [8], wherein the weight average molecular weight of the polyolefin is 2,000 or more and 50,000 or less.

[0019]

[10] The method for producing a resin-coated metal sheet according to the above [8] or [9], wherein the acid value of the polyolefin is 40 mg KOH / g or more and 80 mg KOH / g or less.

[0020]

[11] The method for producing a resin-coated metal sheet according to any one of [8] to

[10] above, wherein the first composition and the third composition each contain 0.010 mass% or more and 1.0 mass% or less of lubricating inorganic particles.

[0021]

[12] The method for producing a resin-coated metal sheet according to any one of [8] to

[11] above, wherein the second composition contains 10% by mass or more and 35% by mass or less of inorganic particles.

[0022]

[13] The outermost layer and the lowermost layer each have a thickness of 1.0 μm or more and 5.0 μm or less, and the intermediate layer has a thickness of 6.0 μm or more and 30 μm or less.

[14] The method for producing a resin-coated metal sheet according to any one of [8] to

[12] above.

[0023] According to the present invention, it is possible to provide a resin-coated metal sheet having a resin coating layer excellent in slip property, formability, and ink adhesion, a method for producing the same, and a metal container made using the resin-coated metal sheet.

[0024] FIG. 2 is a diagram showing an outline of a cross section of a resin-coated metal plate.

[0025] Hereinafter, embodiments of the resin-coated metal sheet, the metal container, and the method for manufacturing the resin-coated metal sheet according to the present invention will be described. Note that the embodiment described below is an example of the present invention, and the configuration of the present invention is not limited to this specific example.

[0026] (Resin-coated metal sheet) Fig. 1 is a cross-sectional view showing the configuration of a resin-coated metal sheet according to one embodiment of the present invention. As shown in Fig. 1, the resin-coated metal sheet 1 includes a metal sheet 2, a resin coating layer 3 formed on the front surface side of the metal sheet 2, and a resin coating layer 4 formed on the back surface side of the metal sheet 2. The resin coating layer 3 has at least a three-layer structure, and in Fig. 1, it has a three-layer structure consisting of an outermost layer 3a, an intermediate layer 3b, and a bottom layer 3c. The resin coating layer 4 may also have a similar three-layer structure. The resin coating layer 4 is optional, and only the resin coating layer 3 may be provided on one side of the metal sheet 2.

[0027] [Metal Sheet] The metal sheet is preferably a steel sheet, and is preferably tinplate or tin-free steel (TFS). For tinplate, the plating amount per side is 0.5 g / m ２ 15g / m or more ２It is preferable to use the following tinplate: In the case of TFS, the surface of the TFS should have a thickness of 50 mg / m² per side. ２ More than 200g / m ２ The following metal chromium layer and 3 mg / m2 per side in terms of metal chromium thereon ２ 30g / m or more ２ It is preferable that the metal sheet has the following chromium oxide layer. The type of metal sheet is not particularly limited as long as it can be formed into the desired shape, but metal sheets with the following components and manufacturing methods are preferred. (1) A metal sheet obtained by recrystallization annealing low-carbon steel having a C (carbon) content of 0.010% by mass or more and 0.10% by mass or less through continuous annealing. (2) A metal sheet obtained by recrystallization annealing and overaging treatment low-carbon steel having a C content of 0.010% by mass or more and 0.10% by mass or less through continuous annealing. (3) A metal sheet obtained by recrystallization annealing low-carbon steel having a C content of 0.010% by mass or more and 0.10% by mass or less through box annealing. (4) A metal sheet obtained by recrystallization annealing low-carbon steel having a C content of 0.010% by mass or more and 0.10% by mass or less through continuous annealing or box annealing, followed by secondary cold rolling (double reduced rolling). (5) A metal plate obtained by recrystallization annealing an IF (Interstitial Free) steel, which is an ultra-low carbon steel having a C content of 0.003 mass% or less and to which an element that fixes the dissolved C, such as Nb or Ti, is added, through continuous annealing.

[0028] The mechanical properties of the metal sheet are not particularly limited as long as they can be formed into the desired shape. In order to obtain favorable workability and maintain favorable can body strength, the yield point (YP) of the metal sheet is preferably 220 MPa or more and 580 MPa or less. Furthermore, the Lankford value (r value), which is an index of plastic anisotropy, is preferably 0.8 or more. Furthermore, the absolute value of the in-plane anisotropy Δr of the r value is preferably 0.7 or less.

[0029] The components of the metal plate that satisfy the above mechanical properties are not particularly limited, but may contain, for example, components such as Si, Mn, P, S, Al, and N. The Si content is preferably 0.001 mass% or more and preferably 0.1 mass% or less. The Mn content is preferably 0.01 mass% or more and preferably 0.6 mass% or less. The P content is preferably 0.002 mass% or more and preferably 0.05 mass% or less. The S content is preferably 0.002 mass% or more and preferably 0.05 mass% or less. The Al content is preferably 0.005 mass% or more and preferably 0.100 mass% or less. The N content is preferably 0.0005 mass% or more and preferably 0.020 mass% or less. In addition, the metal plate may contain other components such as Ti, Nb, B, Cu, Ni, Cr, Mo, and V. However, from the viewpoint of ensuring corrosion resistance, etc., it is preferable that the total content of these component elements be 0.02 mass% or less.

[0030] The thickness of the metal plate is not particularly limited, but is preferably 0.20 mm or more and 0.25 mm or less.

[0031] [Resin Coating Layer] The resin coating layer contains a polyester resin, which is a polymer composed of dicarboxylic acid units and glycol units.

[0032] As the dicarboxylic acid unit, units derived from aromatic dicarboxylic acids such as terephthalic acid, isophthalic acid, naphthalenedicarboxylic acid, diphenyldicarboxylic acid, diphenylsulfonedicarboxylic acid, diphenoxyethanedicarboxylic acid, 5-sodiumsulfoisophthalic acid, and phthalic acid; aliphatic dicarboxylic acids such as oxalic acid, succinic acid, adipic acid, sebacic acid, dimer acid, maleic acid, and fumaric acid; alicyclic dicarboxylic acids such as cyclohexanedicarboxylic acid; and oxycarboxylic acids such as p-oxybenzoic acid can be used.

[0033] As the glycol unit, units derived from aliphatic glycols such as ethylene glycol, propanediol, butanediol, pentanediol, hexanediol, and neopentyl glycol; alicyclic glycols such as cyclohexanedimethanol; aromatic glycols such as bisphenol A and bisphenol S; and diethylene glycol can be used.

[0034] The above-mentioned dicarboxylic acids and glycols may be used in combination in such a manner that the heat resistance and processability are not impaired.

[0035] If the melting point of the resin coating layer is 230°C or higher, the resin is prevented from softening during molding, which prevents breakage or abrasion, and excellent ink adhesion is obtained. Therefore, the melting point of the resin coating layer is set to 230°C or higher, preferably 235°C or higher, and more preferably 240°C or higher. On the other hand, if the melting point of the resin coating layer is 260°C or lower, the crystallinity of the polyester resin contained therein is within a suitable range, which prevents breakage or abrasion of the resin coating layer during molding, and excellent ink adhesion is obtained. Therefore, the melting point of the resin coating layer is set to 260°C or lower, preferably 258°C or lower, and more preferably 255°C or lower.

[0036] The melting point of the resin coating layer can be measured by the following method. First, the resin-coated metal sheet is immersed in a mixed solution of concentrated hydrochloric acid (30 wt%) and distilled water = 1:1 at room temperature, and the metal sheet is dissolved to peel off the resin coating layer. Note that the resin coating layer is peeled off in the same manner in the following measurement methods. The melting point of the obtained resin coating layer is measured using a differential scanning calorimeter DSCQ100 manufactured by TA Instruments in an atmosphere gas of N ２ The heat flow is measured under the following conditions: a flow rate of 50 ml / min, a temperature range of room temperature to 290°C, and a heating rate of 10°C / min. In the obtained heat flow, the peak top temperature of the endothermic peak in the range of 200°C to 280°C is taken as the melting point of the resin coating layer.

[0037] The resin coating layer has a three-layer structure consisting of a top layer, an intermediate layer, and a bottom layer. Each layer will be described below.

[0038] [Outermost layer of resin coating layer] The outermost layer of the resin coating layer contains a polyolefin as a lubricating component. By containing a polyolefin in the outermost layer, excellent lubricity and formability can be ensured, and breakage or scraping of the resin coating layer can be suppressed even during can body forming under severe processing conditions.

[0039] The polyolefin is at least one of an acid-modified polyolefin and an oxidized polyolefin. The acid-modified polyolefin or the oxidized polyolefin has a polar group and a high acid value, which improves the ink adhesion of the resin coating layer. The polyolefin may be an acid-modified polyolefin such as an acid-modified polyethylene or an ethylene-maleic anhydride copolymer, an oxidized polyolefin such as an oxidized polyethylene, or a mixture of an acid-modified polyolefin and an oxidized polyolefin.

[0040] If the polyolefin content of the outermost layer is less than 1.0% by mass, excellent moldability cannot be ensured. Therefore, the polyolefin content of the outermost layer is set to 1.0% by mass or more, preferably 1.5% by mass or more, and more preferably 2.0% by mass or more, calculated as solid content. On the other hand, if the polyolefin content of the outermost layer exceeds 5.0% by mass, the polyolefin will concentrate on the surface of the outermost layer, making it impossible to ensure excellent ink adhesion. Therefore, the polyolefin content of the outermost layer is set to 5.0% by mass or less, preferably 4.5% by mass or less, and more preferably 4.0% by mass or less, calculated as solid content. The polyolefin content of the outermost layer can be adjusted by the amount added to the resin before extrusion during production. The bottom layer may also contain polyolefin.

[0041] If the melting point of the polyolefin in the outermost layer is lower than 70°C, the polyolefin tends to thicken on the surface of the resin coating layer due to the heat treatment performed during the process of coating the metal plate with the resin coating layer and during the molding of the resin-coated metal plate. The polyolefin thickened on the surface may inhibit the adhesion of printing ink, which may cause the printing ink to peel off during molding. Therefore, the melting point of the polyolefin is set to 70°C or higher, preferably 85°C or higher, and more preferably 95°C or higher. On the other hand, if the melting point of the polyolefin exceeds 145°C, sufficient slip properties and moldability cannot be ensured, and the resin coating layer may break or chip during molding. Therefore, the melting point of the polyolefin is set to 145°C or lower, preferably 135°C or lower, and more preferably 125°C or lower.

[0042] The melting point of polyolefin can be measured by the following method. First, the resin coating layer is peeled off by the method described above. The peeled resin coating layer is dissolved in hexafluoro-2-propanol (HFIP) as a solvent. The dissolved resin coating layer is centrifuged and then pressure-filtered through filters with pore sizes of 1 μm and 0.1 μm, successively, to extract the polyolefin contained in the outermost and lowermost layers of the resin coating layer. Furthermore, Soxhlet extraction is performed on the filter using xylene as a solvent, and after concentration, reprecipitation, and centrifugation, additional polyolefin is extracted. Note that in the various measurements described below, the extraction of polyolefin refers to the procedure up to the above. The extracted polyolefin is measured using a differential scanning calorimeter DSCQ100 manufactured by TA Instruments in an atmosphere gas of N ２ The heat flow is measured under the conditions of a flow rate of 50 ml / min, a temperature range of room temperature to 290°C, and a heating rate of 10°C / min. The peak top temperature of the endothermic peak in the obtained heat flow, which is in the range of 60°C to 150°C, is taken as the melting point of the polyolefin.

[0043] The polyolefin is assumed to be dispersed in the form of particles in the outermost layer. Dispersion in the form of particles is a prerequisite for identifying the center of gravity of the polyolefin when performing Voronoi tessellation, which will be described below.

[0044] The inventors applied Voronoi tessellation to investigate the influence of the distribution of polyolefin in the outermost layer on the slipperiness, moldability, and ink adhesion of the resin coating layer. Voronoi tessellation is a technique for dividing a plane into regions based on the proximity of multiple points (kernel points) at any position on the plane to other points on the same plane. In a two-dimensional plane, the perpendicular bisector between adjacent kernel points is defined as the Voronoi boundary between those kernel points, and each divided region surrounded by the Voronoi boundary is defined as a Voronoi polygon.

[0045] The inventors performed Voronoi tessellation on the surface of the outermost layer by drawing a perpendicular bisector between the centers of gravity of adjacent polyolefin particles, using the center of gravity of each polyolefin particle as a kernel point. As a result, the average area m of the obtained N Voronoi polygons was 200 μm ２ 10000 μm or more ２ is less than or equal to the value of σ defined by the following equation (1): ２ It has been found that when m and σ are 1.0 or less, the properties of the resin coating layer are improved. ２ When the polyolefin concentration is within the above range, the polyolefin concentration in the outermost layer is suitable and the polyolefin is uniformly present, thereby enabling the resin coating layer to achieve a higher level of smoothness, moldability, and ink adhesion. where N: total number of Voronoi polygons, m: average area (μm ２ ) S ｉ : the area (μm) of the i-th Voronoi polygon among the N Voronoi polygons ２ ) where i is an integer ranging from 1 to N.

[0046] The average area m of N Voronoi polygons is 200 μm ２ If the average area m is less than 200 μm, i.e., if the area of ​​the Voronoi polygon obtained using the center of gravity of each polyolefin particle as the kernel point is small, the amount of polyolefin added is excessive. In such a dispersed state, the polyolefin inhibits the affinity between the printing ink and the surface of the resin coating layer, and excellent ink adhesion cannot be obtained. Therefore, the average area m is 200 μm ２ or more, 250 μm ２More than 300 μm is preferable. ２ More preferably, m is 10,000 μm or more. ２ If the average area m is more than 10,000 μm, the amount of polyolefin added is insufficient and excellent slip properties and moldability cannot be obtained. ２ Less than 8000 μm ２ Preferably, 6000 μm or less ２ The following is more preferred:

[0047] σ defined by the above formula (1) ２ When σ exceeds 1.0, that is, when the variance of the Voronoi polygon area is large, the distribution of polyolefin is non-uniform. In this case, polyolefin agglomerates and deficiencies are formed on the surface of the resin coating layer, and excellent moldability and ink adhesion cannot be obtained. Therefore, σ ２ is set to 1.0 or less, preferably 0.8 or less, and more preferably 0.6 or less. ２ The lower limit of is not particularly limited, and σ ２ may be 0.0.

[0048] The above m and σ ２ The Raman spectrum of the outermost surface of the resin-coated metal sheet is measured by Raman spectroscopy using a microscopic laser Raman spectrometer LabRAM HR VIS-NIR manufactured by Horiba, Ltd. The measurement conditions are a confocal laser, laser power 50%, aperture 25 μm, exposure time 0.05 sec, exposure count 1, grating 300 lines / mm, objective lens 100x, and wavenumber range 310 to 3400 cm －１ The measurement range is 200 μm (lamination direction of the resin-coated metal plate) × 200 μm (direction perpendicular to the lamination direction within the plate surface of the resin-coated metal plate). The measurement pitch is 0.5 μm in both directions. Based on the obtained Raman spectroscopy data, a CH stretching vibration peak (2850 cm) derived from polyethylene was detected. －１ ) and a CH stretching vibration peak (2960 cm ) derived from polyester, which is the main component of the resin coating layer. －１) is calculated, and mapping is performed. The center of gravity of the polyolefin is identified for the obtained mapping, and Voronoi division is performed using this position as the generating point, and Voronoi polygons are drawn. Measurements are performed on three randomly selected fields for each sample, and Voronoi division is performed on all dispersed polyolefin particles within the measurement field. The identification of the center of gravity of the polyolefin and the drawing of the Voronoi polygons can be performed with the naked eye or using image processing software. From the results of the drawing, the total number N of Voronoi polygons, the area of ​​each Voronoi polygon, and the average area m of the N Voronoi polygons are each calculated, and σ is calculated from the above formula (1). ２ Calculate m and σ from the results of the three fields of view. ２ The average values ​​of m and σ on the surface of the outermost layer of the resin-coated metal sheet were calculated. ２ Let's say.

[0049] If the weight-average molecular weight of the polyolefin is 2000 or more, the polyolefin can be suitably prevented from thickening on the surface of the resin coating layer, and ink adhesion can be suitably improved. Therefore, the weight-average molecular weight of the polyolefin is preferably 2000 or more, more preferably 3000 or more, and even more preferably 3500 or more. On the other hand, if the weight-average molecular weight of the polyolefin is 50,000 or less, suitable moldability can be ensured during molding processing. Therefore, the weight-average molecular weight of the polyolefin is preferably 50,000 or less, more preferably 45,000 or less, and even more preferably 40,000 or less.

[0050] The weight-average molecular weight of a polyolefin can be measured by the following method. First, the polyolefin in the resin coating layer is extracted by the method described above. The extracted polyolefin is measured using an Agilent gel permeation chromatograph analyzer PL-GPC220 under the conditions of two Agilent PLgel Olexis columns + Guard and an o-dichlorobenzene eluent. A calibration curve is also created from the molecular weight and elution time of the standard polystyrene. Using the created calibration curve, the weight-average molecular weight of the polyolefin is calculated based on the elution time of the polyolefin.

[0051] If the acid value of the polyolefin is 40 mgKOH / g or more, the affinity between the resin coating layer and the printing ink can be suitably ensured, and ink adhesion can be suitably improved. Therefore, the acid value of the polyolefin is preferably 40 mgKOH / g or more, more preferably 45 mgKOH / g or more, and even more preferably 50 mgKOH / g or more. On the other hand, if the acid value of the polyolefin is 80 mgKOH / g or less, the polyolefin is not compatible with the resin coating layer, and therefore suitable moldability can be ensured during molding processing. Therefore, the acid value of the polyolefin is preferably 80 mgKOH / g or less, more preferably 75 mgKOH / g or less, and even more preferably 70 mgKOH / g or less.

[0052] The acid value of polyolefin can be measured by the following method. First, the polyolefin in the resin coating layer is extracted using the method described above. According to JIS K5902, a predetermined amount of polyolefin corresponding to the estimated acid value is weighed into a flask and dissolved in 100 ml of a neutral solvent. Next, using phenolphthalein as an indicator, the neutralization endpoint is determined when the indicator changes color for 30 seconds. The acid value is calculated from the titration results using the following formula (2): (Acid value) = 5.611 × A × F / B (2) where A: Amount (ml) of 0.1 mol / L potassium hydroxide standard solution used; B: Amount (g) of sample collected; and F: Factor of 0.1 mol / L potassium hydroxide standard solution.

[0053] The outermost layer and the bottom layer preferably contain lubricating inorganic particles. If the amount of lubricating inorganic particles contained in the outermost layer and the bottom layer is 0.010% by mass or more, respectively, the roll transportability and winding property during film formation of the resin coating layer are improved, and blocking between the resin coating layers is suppressed, and roll payout property is also improved. Therefore, the amount of lubricating inorganic particles contained in the outermost layer and the bottom layer is preferably 0.010% by mass or more, more preferably 0.050% by mass or more, and even more preferably 0.100% by mass or more, calculated as solid content. On the other hand, if the amount of lubricating inorganic particles contained in the outermost layer and the bottom layer is 1.0% by mass or less, respectively, the number of dispersed lubricating inorganic particles present on the surface of the outermost layer and the bottom layer falls within a suitable range, the adhesion between the resin coating layer and the metal plate is improved, and suitable formability is obtained. Therefore, the amount of lubricating inorganic particles contained in the outermost layer and the bottom layer is preferably 1.0 mass % or less, more preferably 0.9 mass % or less, and even more preferably 0.8 mass % or less, calculated as solid content. The amount of lubricating inorganic particles contained in the outermost layer and the bottom layer may be the same or different. The amount of lubricating inorganic particles contained in the outermost layer and the bottom layer can be adjusted by the amount added to the resin before extrusion during production.

[0054] The lubricating inorganic particles contained in the outermost and lowermost layers are not particularly limited, but silica, lithium fluoride, kaolin, clay, calcium carbonate, aluminum oxide, calcium phosphate, etc. can be used.

[0055] [Intermediate layer of resin coating layer] The resin coating layer may be required to be white in order to improve the design and beauty of the appearance of the can body after printing. In this case, it is preferable that the intermediate layer of the resin coating layer contains inorganic particles.

[0056] The inorganic particles contained in the intermediate layer are not particularly limited, but the inorganic particles are preferably titanium oxide. By containing titanium oxide in the intermediate layer, the resin coating layer can be made white. The titanium oxide used as the inorganic particles is more preferably rutile titanium oxide having a purity of 90% or more. When the intermediate layer contains rutile titanium oxide having a purity of 90% or more, the titanium oxide exhibits good dispersibility when mixed with the polyester resin, resulting in a uniform whiteness and improving the design and beauty of the appearance.

[0057] If the inorganic particles contained in the intermediate layer are 10% by mass or more, sufficient whiteness can be ensured. Therefore, the inorganic particles contained in the intermediate layer are preferably 10% by mass or more, more preferably 12% by mass or more, and even more preferably 15% by mass or more, calculated as solid content. On the other hand, if the inorganic particles contained in the intermediate layer are 35% by mass or less, fracture or scraping of the resin coating layer can be suitably prevented even under more severe processing conditions. Therefore, the inorganic particles contained in the intermediate layer are preferably 35% by mass or less, more preferably 30% by mass or less, and even more preferably 25% by mass or less, calculated as solid content. The content of inorganic particles in the intermediate layer can be adjusted by the amount added to the resin before extrusion during production.

[0058] When the thickness of the outermost layer and the bottom layer is 1.0 μm or more and 5.0 μm or less, favorable moldability can be ensured during molding, and breakage or scraping of the resin coating layer during molding can be favorably prevented. Therefore, the thickness of the outermost layer and the bottom layer is preferably 1.0 μm or more, more preferably 1.2 μm or more, and even more preferably 1.5 μm or more. Similarly, the thickness of the outermost layer and the bottom layer is preferably 5.0 μm or less, more preferably 4.0 μm or less, and even more preferably 3.0 μm or less. The thicknesses of the outermost layer and the bottom layer may be equal to or different from each other.

[0059] When the thickness of the intermediate layer is 6.0 μm or more and 30 μm or less, there is no need to vary the thickness of the outermost layer and the bottom layer, and the absolute amount of polyolefin is suppressed from increasing or decreasing. This keeps the dispersion state within an appropriate range, and can suitably prevent the resin coating layer from breaking or being scraped during molding. In addition, the ink adhesion of the resin coating layer can be suitably improved. Therefore, the thickness of the intermediate layer is preferably 6.0 μm or more, more preferably 8.0 μm or more, and even more preferably 10 μm or more. Similarly, the thickness of the intermediate layer is preferably 30 μm or less, more preferably 25 μm or less, and even more preferably 20 μm or less.

[0060] The thicknesses of the outermost layer, intermediate layer, and bottom layer can be measured by the following method. First, the resin coating layer is peeled off using the method described above. A Pt coating is applied to the outermost surface of the obtained resin coating layer, and the resin is embedded in resin. A cross section is then prepared using an ion milling device EM TIC 3X manufactured by Leica Microsystems. Backscattered electron images of the prepared cross section are observed using a scanning electron microscope (SEM) Regulus 8220 manufactured by Hitachi High-Technologies Corporation at two magnifications: 3500x and 8000x. From the observation results, the thicknesses of the outermost layer, intermediate layer, and bottom layer of the resin coating layer can be determined.

[0061] (Metal Container) By using the resin-coated metal plate described above, a metal container having a resin coating layer with excellent slip properties, formability, and ink adhesion can be manufactured. The metal container can be manufactured by a conventional method. In the metal container, it is preferable that the resin coating layer is located on the outer surface side of the metal container.

[0062] (Method for manufacturing resin-coated metal sheet) Next, a method for manufacturing a resin-coated metal sheet according to one embodiment of the present invention will be described. In the following embodiment, a manufacturing method in which the resin coating layer has a three-layer structure will be described, but the resin coating layer may have a four-layer or more structure.

[0063] In the production of the resin-coated metal sheet of the present invention, a resin film that will become the resin coating layer is first produced. First, a first composition, a second composition, and a third composition containing predetermined resins are co-extruded using a kneading extruder and discharged through a T-die to obtain a sheet-like molded body in which the first composition, the second composition, and the third composition form the first layer, the second layer, and the third layer, respectively. The molded body is preferably extruded onto a cooling body such as a casting drum.

[0064] In preparing the first composition, a masterbatch is first prepared by adding a polyolefin or the like to a polyester resin. The masterbatch is prepared by kneading 2.0% by mass or more and 10.0% by mass or less of a polyolefin with a polyester resin containing 4.0 mol% to 10.0 mol% of isophthalic acid as an acid component. The polyolefin is at least one of an acid-modified polyolefin and an oxidized polyolefin, and has a melting point of 70°C to 145°C. Then, during film formation, the masterbatch is mixed with additional polyester resin to prepare a first composition having a polyolefin content of 1.0% by mass to 5.0% by mass.

[0065] In the polyester resin of the masterbatch, if the isophthalic acid contained as an acid component is 4.0 mol% or more, the crystallinity of the polyester resin is favorable. As a result, when the masterbatch is used to form the outermost layer, the polyolefin is dispersed in a favorable state, ensuring smoothness and moldability even during severe molding, and preventing breakage or abrasion of the resin coating layer. Therefore, the isophthalic acid contained as an acid component in the masterbatch is 4.0 mol% or more, preferably 4.5 mol% or more, and more preferably 5.0 mol% or more. On the other hand, if the isophthalic acid contained as an acid component in the polyester resin of the masterbatch is 10.0 mol% or less, the crystallinity of the polyester resin is favorable and softening is suppressed. As a result, the polyolefin is dispersed in a favorable state when the outermost layer is formed using the masterbatch. In addition, since the resin coating layer does not soften, smoothness and moldability can be ensured even during severe molding, preventing breakage or abrasion of the resin coating layer. Therefore, the amount of isophthalic acid contained as an acid component in the masterbatch is 10.0 mol % or less, preferably 9.0 mol % or less, and more preferably 8.0 mol % or less.

[0066] The masterbatch is at least one of an acid-modified polyolefin and an oxidized polyolefin, and contains a polyolefin having a melting point of 70°C or higher and 145°C or lower. If the polyolefin content is 2.0% by mass or higher, the polyolefin will be well dispersed when the masterbatch is used to form the outermost layer, ensuring smoothness and moldability even during severe molding processes, and preventing breakage or abrasion of the resin coating layer. Therefore, the polyolefin content in the masterbatch is 2.0% by mass or higher, preferably 2.5% by mass or higher, and more preferably 3.0% by mass or higher. On the other hand, if the polyolefin content is 10.0% by mass or lower, the polyolefin will not coarsen when the masterbatch is used to form the outermost layer, ensuring smoothness and moldability even during severe molding processes, and preventing breakage or abrasion of the resin coating layer. Furthermore, the affinity between the resin coating layer and printing ink can be sufficiently ensured, improving ink adhesion. Therefore, the polyolefin content in the masterbatch is set to 10.0% by mass or less, preferably 8.0% by mass or less, and more preferably 6.0% by mass or less.

[0067] The second and third compositions also form the middle and bottom layers of the resin coating layer, respectively, and are therefore added so that each layer contains the above-mentioned additives. The method of addition is not particularly limited, but a preferred method involves preparing a masterbatch in which various additives are dispersed at high concentrations in a resin, and then mixing the masterbatch with additive-free resin pellets in a predetermined ratio during film formation and introducing the mixture into a kneading extruder. The additive content of each layer is as described above in the description of the resin coating layer. Specifically, the second composition contains a polyester resin, and the third composition contains a polyester resin.

[0068] The extrusion temperature is preferably 250°C or higher, which is about 20°C higher than the melting point of the polyester resin, while the extrusion temperature is preferably 290°C or lower.

[0069] The formed body is then cooled and solidified on a cooling body such as a casting drum to obtain an unstretched laminate film. Conventional cooling conditions can be used.

[0070] After preparing an unstretched laminated film, the laminated film is stretched one or more times to form a resin film. When the stretching temperature is 80°C or higher and 95°C or lower, welding or breakage of the resin film during film formation can be suppressed. Furthermore, coarse aggregation and a decrease in the number of polyolefin particles can be prevented, and the dispersion state can be kept within an appropriate range, so that m and σ ２ Therefore, the stretching temperature is set to 80° C. or higher, preferably 82° C. or higher, and more preferably 85° C. or higher. Similarly, the stretching temperature is set to 95° C. or lower, preferably 92° C. or lower, and more preferably 90° C. or lower.

[0071] Furthermore, when the stretching ratio during stretching is 4.0 times or more and 6.5 times or less, breakage during film formation can be suppressed, and the thickness of the resin film can be kept within an appropriate range while suppressing the occurrence of thickness unevenness. In addition, the aggregation and coarsening of polyolefin and the decrease in the number of particles can be prevented, and the dispersion state can be kept within an appropriate range, so that m and σ ２ Therefore, the stretching ratio is set to 4.0 times or more, preferably 4.5 times or more, and more preferably 5.0 times or more. Similarly, the stretching ratio is set to 6.5 times or less, preferably 6.0 times or less, and more preferably 5.8 times or less.

[0072] Next, the formed resin film is heated to a temperature equal to or higher than the melting initiation temperature and thermocompression bonded to a metal plate using a laminating roll (thermocompression film lamination method). The thermocompression film lamination method is preferable in that it reduces production costs and enables energy-saving production.

[0073] When the temperature of the metal plate during thermocompression bonding is 250°C or higher and 280°C or lower, i.e., when the temperature of the metal plate is about 20 to 50°C higher than the melting point of the resin coating layer, the adhesion between the laminated film and the metal plate is improved and welding of the film to the laminating roll can be suppressed. In addition, the aggregation and coarsening of the polyolefin in the resin coating layer and the decrease in the number of particles are prevented, improving the dispersion state, and therefore m and σ ２Therefore, the temperature of the metal plate is set to 250°C or higher, preferably 255°C or higher, and more preferably 260°C or higher. Similarly, the temperature of the metal plate is set to 280°C or lower, preferably 278°C or lower, and more preferably 275°C or lower.

[0074] When the temperature of the laminating roll during thermocompression bonding is 100°C or higher and 110°C or lower, that is, 20 to 30°C higher than the glass transition temperature of the laminated film (about 80°C), the film is prevented from welding to the laminating roll, and further, the aggregation and coarsening of the polyolefin in the resin coating layer and the decrease in the number of particles are prevented, improving the dispersion state, and therefore m and σ ２ Therefore, the laminating roll temperature is set to 100° C. or higher, preferably 101° C. or higher, and more preferably 103° C. or higher. Similarly, the laminating roll temperature is set to 110° C. or lower, preferably 108° C. or lower, and more preferably 106° C. or lower.

[0075] For steps and conditions not described in the present invention, conventional methods can be used.

[0076] Plate thickness: 0.22 mm, metal chromium layer: 120 mg / m per side ２ , the chromium oxide layer is 10 mg / m per side in terms of metallic chromium ２ The metal plate used was tin-free steel (TFS) with a temper of T3CA. The mechanical properties of this metal plate were a YP of 400 MPa, an r-value of 1.0, and a Δr of 0.5.

[0077] In each example, masterbatches were prepared for the first, second, and third compositions, each comprising a polyester resin with various additives dispersed at high concentrations. In this example, the first and third compositions were the same masterbatches. In each example, the polyester resins in each composition had the same resin composition, as shown in Tables 1 and 2. Furthermore, lubricating components and lubricating inorganic particles were added to the masterbatches of the first and third compositions, while inorganic particles were added to the masterbatches of the second composition. The type, weight average molecular weight, acid value, acid component ratio of the polyester resin in the masterbatches of the first and third compositions, and the amount of lubricating component added are shown in Tables 1 and 2. Silica was used as the lubricating inorganic particles, and rutile-type titanium oxide was used as the inorganic particles.

[0078] In each example, the prepared master batch and resin pellets made of polyester resin having the aforementioned resin composition were mixed so that the content in each layer after molding would be the values ​​shown in Tables 1 and 2, and then introduced into a kneading extruder to produce a laminated film. The extrusion temperature was 278°C. Furthermore, the laminated film was stretched by uniaxial stretching to form a resin film. Thereafter, the metal plate was heated, and the resin film was thermocompression-bonded to both the front and back surfaces of the metal plate by a thermocompression film lamination method. The stretching conditions (stretching temperature and stretch ratio) and thermocompression-bonding conditions (metal plate temperature and laminating roll temperature) for each example are shown in Tables 1 and 2. The pressure of the laminating roll during thermocompression bonding was 400 kgf, and the film was water-cooled in a water-cooled tank at 50°C 0.7 seconds after thermocompression bonding.

[0079] The resin-coated metal sheet thus obtained was subjected to the above-described methods to determine the melting point of the resin coating layer, the thickness of each layer, the melting point of the polyolefin, the weight-average molecular weight of the polyolefin, the acid value of the polyolefin, and m and σ on the surface of the resin coating layer. ２ The measurement results are shown in Tables 1 and 2.

[0080]

[0081]

[0082] In each example, the resin coating layer was evaluated for slipperiness, formability, and ink adhesion by the following methods. The evaluation results are shown in Table 3.

[0083] [Evaluation of Slipperiness] The resin-coated metal plate of each example was punched into a circular shape with a diameter of 68 mm, and a sliding test was performed using a rotary abrasion tester manufactured by Takachiho Seiki Co., Ltd. The test conditions were a load of 44 N, a sample temperature of 145±3°C, a rotation speed of 370 rpm, a rotation radius of 20 mm, and a carbide ball indenter (Φ10 mm). The test surface was the resin coating layer located on the outer surface of the container after molding, and four samples were prepared in each example and tested. From the obtained results, the coefficient of friction at the time of the maximum static friction force was taken as the static friction coefficient, and the slipperiness was evaluated according to the following criteria.

[0084] Evaluation criteria for slipperiness Evaluation "◎": Static friction coefficient is 0.080 or less. Evaluation "〇": Static friction coefficient is more than 0.080 and less than 0.110. Evaluation "△": Static friction coefficient is more than 0.110 and less than 0.130. Evaluation "×": Static friction coefficient is more than 0.130.

[0085] [Evaluation of formability] After applying wax to each resin-coated metal sheet, the sheet was punched into a disk having a diameter of 123 mm and then drawn into a cup shape at a drawing ratio of 1.7 using a cupping press. The resulting cup was inserted into a DI forming device and subjected to redrawing and DI processing at a drawing ratio of 1.3 to form a can having an inner diameter of 52 mm and a can height of 90 mm. The surface of the resin coating layer of each formed can was visually observed, and formability was evaluated according to the following criteria.

[0086] Evaluation criteria for formability Evaluation "◎": No chipping is observed visually. Evaluation "◯": Minor chipping is observed visually within a range of 5 mm in the height direction from the flange of the can. Evaluation "△": Chips are observed visually within a range of more than 5 mm and up to 15 mm in the height direction from the flange of the can. Evaluation "×": Chips or broken bodies are observed visually within a range of more than 15 mm in the height direction from the flange of the can.

[0087] [Evaluation of Ink Adhesion] Each resin-coated metal plate was placed in a hot air drying oven and heat-treated to 240°C for 2 minutes, then cooled to room temperature. For each cooled sample, a melamine-based printing ink was printed on the resin coating layer located on the outer surface of the container after molding using a universal printing tester manufactured by Kumagai Riki Kogyo Co., Ltd. After printing, the sample was left to stand for 1 minute, then placed in a hot air drying oven and heat-treated to 230°C for 3 minutes, then cooled to room temperature. A scratch test was performed on the ink-printed surface of each sample along the length of the sample using a load-varying friction and wear tester (HHS2000) manufactured by Shinto Scientific Co., Ltd. The test conditions were as follows: a sapphire indenter (Φ0.6 mm) was used, a continuous load of 10 to 1000 gf from the edge of the print, a movement speed of 1.0 mm / sec, and a movement distance of 30 mm. Three samples were prepared for each sample, and the test was performed five times on each sample, each at different locations. The ink peeling load was calculated from the ink peeling length in each test, and the average value of the peeling loads for 15 tests was calculated to evaluate the ink adhesion according to the following criteria.

[0088] Evaluation criteria for ink adhesion Evaluation "◎": Average peel load is 400 g or more. Evaluation "◯": Average peel load is 300 g or more and less than 400 g. Evaluation "△": Average peel load is 200 g or more and less than 300 g. Evaluation "×": Average peel load is less than 200 g.

[0089]

[0090] According to the present invention, it is possible to provide a resin-coated metal sheet having a resin coating layer excellent in slip property, formability, and ink adhesion, a method for producing the same, and a metal container made using the resin-coated metal sheet.

[0091] 1 Resin coated metal plate 2 Metal plate 3 Resin coating layer 3a Outermost layer 3b Intermediate layer 3c Bottom layer 4 Resin coating layer

Claims

1. A resin-coated metal plate having a resin coating layer containing a polyester resin on at least one side of the metal plate, wherein the resin coating layer has a three-layer structure including a top layer, an intermediate layer, and a bottom layer, the melting point of the resin coating layer is 230°C or higher and 260°C or lower, the top layer contains 1.0% by mass or higher and 5.0% by mass or lower of a polyolefin, the polyolefin being at least one of an acid-modified polyolefin and an oxidized polyolefin, the melting point of the polyolefin being 70°C or higher and 145°C or lower, the polyolefin being dispersed in a particulate form in the top layer, and the dispersion state of the polyolefin particles is such that, on the surface of the top layer, Voronoi tessellation is performed using the center of gravity of each polyolefin particle as a kernel point, and the average area m of N Voronoi polygons obtained is 200 μm ２ 10000 μm or more ２ is less than or equal to the value of σ defined by the following equation (1): ２ A resin-coated metal sheet that satisfies the requirement that the resistance is 1.0 or less. where N: total number of Voronoi polygons, m: average area (μm ２ ) S ｉ : the area (μm) of the i-th Voronoi polygon among the N Voronoi polygons ２ ) where i is an integer ranging from 1 to N.

2. The resin-coated metal sheet according to claim 1, wherein the weight-average molecular weight of the polyolefin is 2,000 or more and 50,000 or less.

3. A resin-coated metal sheet according to claim 1 or 2, wherein the acid value of the polyolefin is 40 mg KOH / g or more and 80 mg KOH / g or less.

4. A resin-coated metal sheet according to any one of claims 1 to 3, wherein the outermost layer and the lowermost layer each contain 0.010% by mass or more and 1.0% by mass or less of lubricating inorganic particles.

5. A resin-coated metal sheet according to any one of claims 1 to 4, wherein the intermediate layer contains 10% by mass or more and 35% by mass or less of inorganic particles.

6. A resin-coated metal sheet according to any one of claims 1 to 5, wherein the thickness of the outermost layer and the bottom layer is 1.0 μm or more and 5.0 μm or less, respectively, and the thickness of the intermediate layer is 6.0 μm or more and 30 μm or less.

7. A metal container made using the resin-coated metal sheet according to any one of claims 1 to 6, wherein the resin coating layer is located on the outside of the metal container.

8. A process for preparing a masterbatch by kneading a polyester resin containing 4.0 mol % to 10.0 mol % of isophthalic acid as an acid component with 2.0 mass % to 10.0 mass % of a polyolefin which is at least one of an acid-modified polyolefin and an oxidized polyolefin and has a melting point of 70°C to 145°C; a process for mixing the masterbatch with an additional polyester resin to prepare a first composition having a polyolefin content of 1.0 mass % to 5.0 mass %; a process for co-extruding the first composition, a second composition containing a polyester resin, and a third composition containing a polyester resin using a kneading extruder and discharging the resultant through a T-die to obtain a sheet-like molded body having a first layer, a second layer, and a third layer made of the first composition, the second composition, and the third composition, respectively; and a process for cooling and solidifying the molded body to obtain a laminated film. a step of stretching the laminated film at least once under conditions of a stretching temperature of 80°C or higher and 95°C or lower and a stretching ratio of 4.0 times or higher and 6.5 times or lower, thereby obtaining a resin film including the first layer, the second layer, and the third layer; and a step of thermocompression-bonding the resin film to at least one surface of a metal plate controlled to 250°C or higher and 280°C or lower, using a laminating roll controlled to 100°C or higher and 110°C or lower, thereby obtaining a resin-coated metal plate provided on at least one surface of the metal plate with at least a three-layer structure in which the first layer, the second layer, and the third layer are the outermost layer, the middle layer, and the bottom layer, respectively, and with a resin coating layer having a melting point of 230°C or higher and 260°C or lower.

9. The method for producing a resin-coated metal sheet according to claim 8, wherein the weight average molecular weight of the polyolefin is 2,000 or more and 50,000 or less.

10. The method for producing a resin-coated metal sheet according to claim 8 or 9, wherein the acid value of the polyolefin is 40 mg KOH / g or more and 80 mg KOH / g or less.

11. A method for producing a resin-coated metal sheet according to any one of claims 8 to 10, wherein the first composition and the third composition each contain 0.010 mass % or more and 1.0 mass % or less of lubricating inorganic particles.

12. A method for producing a resin-coated metal sheet according to any one of claims 8 to 11, wherein the second composition contains 10% by mass or more and 35% by mass or less of inorganic particles.

13. A method for producing a resin-coated metal sheet according to any one of claims 8 to 12, wherein the thickness of the outermost layer and the bottom layer is 1.0 μm or more and 5.0 μm or less, respectively, and the thickness of the intermediate layer is 6.0 μm or more and 30 μm or less.

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