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

Abstract

Provided is a resin-coated metal plate with a resin coating layer having excellent slipperiness, abrasion resistance and ink adhesion. A resin-coated metal plate according to the present invention comprises a resin coating layer on at least one surface of the metal plate, the resin coating layer containing a polyester resin and satisfying a prescribed requirement. The outermost surface layer of the resin coating layer contains a polyolefin satisfying a prescribed requirement, satisfying the condition that, on the surface of the outermost surface layer of the resin coating layer, the number density of polyolefin particles is 50 particles / cm2 or more, and m defined by formula (1) is 0.20 or more. Here, when the total number of polyolefin particles on the surface of the outermost surface layer is defined as N, Ai1 is the number of polyolefin particles with the center of gravity positioned within 50 μm of the i-th particle, and Ai2 is the number of polyolefin particles with the center of gravity positioned within greater than 50 μm and up to 100 μm of the i-th particle, wherein 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, which is 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 scraped, particularly after forming processing. In light of this, there is a need for material design that suppresses breakage or scraping of the resin coating layer during can body production. As a technology for suppressing breakage or scraping 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 scraping resistance 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 slipperiness and scrape resistance 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 that is excellent in slipperiness, abrasion resistance, 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, if the distribution position of the lubricating component were biased, excellent properties could not be ensured. Therefore, by controlling the number density and dispersion state of the lubricating component present on the outermost surface of the resin coating layer, a resin coating layer with excellent slip properties, abrasion resistance, and ink adhesion can be obtained. Furthermore, by setting the stretching conditions of the sheet-shaped molded body when producing a laminate film to be thermocompressed to a metal plate to specified 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, the dispersion state of the lubricating component can be controlled.

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

[0010] [1] A resin-coated metal sheet comprising a resin coating layer containing a polyester resin on at least one surface of a metal sheet, wherein the resin coating layer has a three-layer structure including an outermost layer, an intermediate layer, and a bottom layer, the resin coating layer has a melting point of 230°C or higher and 260°C or lower, the outermost layer contains a polyolefin which is at least one of an acid-modified polyolefin and an oxidized polyolefin, the melting point of the polyolefin is 70°C or higher and 145°C or lower, the polyolefin is dispersed in a particulate form in the outermost layer, and the dispersion state of the polyolefin particles is such that the number density of the polyolefin particles on the surface of the outermost layer is 50 particles / cm. ２ The resin-coated metal sheet satisfies the above, and m defined by the following formula (1) is 0.20 or more. Here, the total number of polyolefin particles on the surface of the outermost layer is N, and A ｉ１ A: the number of polyolefin particles whose center of gravity is located within 50 μm around the i-th particle among the N polyolefin particles ｉ２: the number of polyolefin particles whose center of gravity is located within a range of more than 50 μm and 100 μm from the circumference of the i-th particle among the N polyolefin particles, 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 1.0 mg KOH / g or more and 90 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 1.0 mass% to 10.0 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 of co-extruding a first composition containing a polyester resin and a polyolefin which is at least one of an acid-modified polyolefin and an oxidized polyolefin and has a melting point of 70°C or more and 145°C or less, 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 the first composition, the second composition, and the third composition as a first layer, a second layer, and a third layer, respectively; a process of cooling and solidifying the molded body to obtain a laminated film; and a process of stretching the laminated film at least once under conditions of a stretching temperature of 85°C or more and 100°C or less and a stretching ratio of 3.5 times or more and 5.0 times or less to obtain a resin film including the first layer, the second layer, and the third layer. Next, using a laminating roll controlled to a temperature of 110°C or higher and 120°C or lower, the resin film is thermocompression-bonded to at least one surface of a metal plate controlled to a temperature of 260°C or higher and 290°C or lower, to obtain a resin-coated metal plate having 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, on at least one surface of the metal plate, and comprising 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 1.0 mg KOH / g or more and 90 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 1.0 mass% or more and 10.0 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 resistance, abrasion resistance, 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 ２ It 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 ２It 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 can be prevented from softening during molding, preventing breakage or scraping. Therefore, the melting point of the resin coating layer is 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, preventing breakage or scraping of the resin coating layer during molding. Therefore, the melting point of the resin coating layer is 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 resin coating layer is peeled off by dissolving the metal sheet. The resin coating layer is also peeled off in the same manner in the following measurement methods. Heat flow measurements are performed on the obtained resin coating layer using a differential scanning calorimeter DSCQ100 manufactured by TA Instruments under the following conditions: N2 atmospheric gas, flow rate 50 ml / min, temperature range from room temperature to 290°C, and heating rate 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 slip properties and abrasion resistance can be ensured, and breakage or abrasion 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] When the polyolefin content of the outermost layer is 0.010% by mass or more, the slipperiness and abrasion resistance can be suitably ensured during molding, and the resin coating layer can be suitably prevented from breaking or being abraded. Therefore, the polyolefin content of the outermost 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, when the polyolefin content of the outermost layer is 1.0% by mass or less, the resin coating layer can be suitably prevented from being abraded due to the dispersed particles of coarse polyolefin during molding. Therefore, the polyolefin content of the outermost layer is preferably 1.0% by mass or less, more preferably 0.80% by mass or less, and even more preferably 0.60% 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 less 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 the 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 abrasion resistance 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 dispersed in the form of particles in the outermost layer. Dispersion in the form of particles is a prerequisite for specifying the polyolefin particles in defining the dispersion state described below.

[0044] The present inventors investigated the influence of the distribution of polyolefin in the outermost layer on the slip properties, abrasion resistance, and ink adhesion of the resin coating layer.

[0045] The present inventors conducted an analysis focusing on the number density and distribution state of the polyolefin particles observed on the surface of the outermost layer. As a result, it was found that the number density of the polyolefin particles was 50 particles / cm ２ It has been found that the properties of the resin coating layer are improved when m, as defined by the following formula (1), is 0.20 or greater. When the number density and m are within the above ranges, the polyolefin concentration range in the outermost layer is suitable, and the polyolefin is distributed without agglomeration. As a result, the resin coating layer can achieve a higher level of both slip resistance, abrasion resistance, and ink adhesion. Here, the total number of polyolefin particles on the surface of the outermost layer is N, and A ｉ１ A: the number of polyolefin particles whose center of gravity is located within 50 μm around the i-th particle among N polyolefin particles ｉ２ i: the number of polyolefin particles whose center of gravity is located within 50 μm to 100 μm around the i-th particle among N polyolefin particles, where i is an integer ranging from 1 to N.

[0046] The number density of polyolefin particles on the surface of the outermost layer is 50 particles / cm ２ If the number density of the polyolefin particles is less than 50 particles / cm, the concentration of the added polyolefin is low or the added polyolefin is in a state of agglomeration. In either state, excellent properties cannot be ensured. Therefore, the number density of the polyolefin particles should be 50 particles / cm. ２ or more, 100 pieces / cm ２ More than 300 particles / cm is preferable. ２ On the other hand, from the viewpoint of ink adhesion, the number density of polyolefin particles is preferably 1500 particles / cm. ２ The following is preferred:

[0047] When m defined in the above formula (1) is less than 0.20, it means that the number of polyolefin particles in the vicinity of a certain polyolefin particle in the outermost layer is greater than the number of polyolefin particles present at a position at a certain distance or more. In other words, it indicates that there are localized high-density regions where the added polyolefin is excessively dense, and in this case, excellent ink adhesion cannot be obtained. Therefore, m is set to 0.20 or more, preferably 0.30 or more, and more preferably 0.50 or more. On the other hand, from the viewpoint of abrasion resistance, m is set to 0.95 particles / cm ２ The following is preferred:

[0048] The number density of polyolefin particles and m defined in the above formula (1) can be determined by the following method. Raman spectrum measurement is performed on the outermost surface of a resin-coated metal sheet 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 from the obtained mapping. Measurements are performed on three randomly selected visual fields for each sample, and all dispersed polyolefin particles within the measurement visual field are targeted. The center of gravity of the polyolefin can be identified by the naked eye or using image processing software. The number density is determined by dividing the total number of polyolefin particles in each visual field by the visual field area, and m is then calculated from the above formula (1). The average values ​​of number density and m are calculated from the results of the three visual fields, and these are used as the number density and m of the polyolefin particles on the surface of the outermost layer of the resin-coated metal sheet.

[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 abrasion resistance 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 1.0 mgKOH / g or more, the affinity between the resin coating layer and the printing ink can be suitably ensured, and the ink adhesion can be suitably improved. Therefore, the acid value of the polyolefin is preferably 1.0 mgKOH / g or more, more preferably 2.0 mgKOH / g or more, and even more preferably 3.0 mgKOH / g or more. On the other hand, if the acid value of the polyolefin is 90 mgKOH / g or less, the polyolefin is not compatible with the resin coating layer, and therefore suitable abrasion resistance can be ensured during molding. Therefore, the acid value of the polyolefin is preferably 90 mgKOH / g or less, more preferably 80 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 lubricating inorganic particles contained in the outermost layer and the bottom layer are each 1.0% by mass or more, 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 the roll payout property is also improved. Therefore, the lubricating inorganic particles contained in the outermost layer and the bottom layer are preferably 1.0% by mass or more, more preferably 2.5% by mass or more, and even more preferably 4.0% by mass or more, calculated as solid content. On the other hand, if the lubricating inorganic particles contained in the outermost layer and the bottom layer are each 10.0% by mass or less, the dispersed number of 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 abrasion resistance is obtained. Therefore, the lubricating inorganic particles contained in the outermost layer and the bottom layer are preferably 10.0% by mass or less, more preferably 9.0% by mass or less, and even more preferably 8.0% by mass or less, calculated as solid content. The content of the lubricating inorganic particles in the outermost layer and the innermost layer may be the same or different, and 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, it is possible to ensure suitable abrasion resistance during molding and to suitably prevent the resin coating layer from breaking or being abraded during molding. 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, abrasion resistance, 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] The first composition, second composition, and third composition form the outermost, middle, and bottom layers of the resin coating layer, respectively, and are added so that each layer contains the polyolefin, lubricating inorganic particles, and inorganic particles. The addition method is not particularly limited, but a preferred method involves preparing a masterbatch in which various additives are dispersed at a high concentration in a resin, mixing the masterbatch with additive-free resin pellets at a predetermined ratio, and introducing the mixture into a kneading extruder. The content of the additives in each layer is as described above in the description of the resin coating layer. Specifically, the first composition contains a polyester resin and at least one of an acid-modified polyolefin and an oxidized polyolefin, having a melting point of 70°C or higher and 145°C or lower. The second composition contains a polyester resin, and the third composition also contains a polyester resin.

[0065] 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.

[0066] 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.

[0067] After producing an unstretched laminated film, the laminated film is stretched one or more times to form a resin film. When the stretching temperature is 85°C or higher and 100°C or lower, welding or breakage of the resin film during film formation is suppressed, and further, aggregation and coarsening of the polyolefin and a decrease in the number of polyolefins can be prevented, so that the dispersion state, such as number density and m, can be kept within an appropriate range. Therefore, the stretching temperature is 85°C or higher, preferably 88°C or higher, and more preferably 92°C or higher. Similarly, the stretching temperature is 100°C or lower, preferably 98°C or lower, and more preferably 96°C or lower.

[0068] Furthermore, when the stretching ratio during stretching is 3.5 times or more and 5.0 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, since it is possible to prevent the polyolefin from agglomerating and becoming coarse and the number reduction, it is possible to keep the dispersion state, such as number density and m, within an appropriate range. Therefore, the stretching ratio is 3.5 times or more, preferably 3.8 times or more, and more preferably 4.0 times or more. Similarly, the stretching ratio is 5.0 times or less, preferably 4.8 times or less, and more preferably 4.6 times or less.

[0069] 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.

[0070] When the temperature of the metal plate during thermocompression bonding is 260°C or higher and 290°C or lower, i.e., when the temperature of the metal plate is about 30 to 60°C higher than the melting point of the resin coating layer, the adhesion between the laminate film and the metal plate is improved and welding of the film to the laminating roll can be suppressed. In addition, the polyolefin in the resin coating layer does not aggregate or coarsen, and the dispersion state is improved, so the number density and m can be kept within appropriate ranges. Therefore, the temperature of the metal plate is set to 260°C or higher, preferably 265°C or higher, and more preferably 270°C or higher. Similarly, the temperature of the metal plate is set to 290°C or lower, preferably 288°C or lower, and more preferably 285°C or lower.

[0071] When the laminating roll temperature during thermocompression bonding is 110°C or higher and 120°C or lower, i.e., 30 to 40°C higher than the glass transition temperature of the laminated film (approximately 80°C), welding of the film to the laminating roll is suppressed, and the polyolefin in the resin coating layer does not aggregate or coarsen, improving the dispersion state, thereby allowing the number density and m to be kept within appropriate ranges. Therefore, the laminating roll temperature is 110°C or higher, preferably 111°C or higher, and more preferably 113°C or higher. Similarly, the laminating roll temperature is 120°C or lower, preferably 119°C or lower, and more preferably 118°C or lower.

[0072] For steps and conditions not described in the present invention, conventional methods can be used.

[0073] 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.

[0074] In each example, masterbatches were prepared for the first composition, second composition, and third composition, in which various additives were dispersed at high concentrations in a polyester resin. In this example, the first composition and the third composition were the same masterbatch. In each example, the polyester resin in each composition had the same resin composition, and the resin compositions are shown in Tables 1 and 2. Furthermore, lubricating components and lubricating inorganic particles were added to the masterbatches of the first and third compositions, and inorganic particles were added to the masterbatch of the second composition. The type, weight average molecular weight, and acid value of the lubricating component added to the first and third compositions in each example are shown in Tables 1 and 2. Furthermore, silica was used as the lubricating inorganic particles, and rutile-type titanium oxide was used as the inorganic particles.

[0075] 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.

[0076] The resin-coated metal sheets thus obtained were analyzed for 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 the number density and m of the polyolefin particles present on the surface of the resin coating layer, using the methods described above. The measurement results are shown in Tables 1 and 2.

[0077]

[0078]

[0079] In each example, the slipperiness, abrasion resistance, and ink adhesion of the resin coating layer were evaluated by the following methods. The evaluation results are shown in Table 3.

[0080] [Evaluation of Sliding Properties and Abrasion Resistance] 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. 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 defined as the friction coefficient, and the sliding properties were evaluated according to the following criteria. In addition, the abrasion resistance was visually evaluated according to the following criteria based on the degree of abrasion of the resin coating layer after the test.

[0081] Evaluation criteria for slipperiness Evaluation "◎": Friction coefficient is 0.110 or less. Evaluation "〇": Friction coefficient is more than 0.110 and less than 0.135. Evaluation "△": Friction coefficient is more than 0.135 and less than 0.160. Evaluation "×": Friction coefficient is 0.160 or more.

[0082] Evaluation criteria for abrasion resistance Evaluation "◯": No abrasion was observed visually in all four tests. Evaluation "Δ": Abrasion was observed visually in one of the four tests. Evaluation "×": Abrasion was observed visually in two or more of the four tests.

[0083] [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. Immediately after printing, each sample was placed in a hot air drying oven and heat-treated to 230°C for 1 minute, 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 0.5 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.

[0084] 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.

[0085]

[0086] From Table 3, it is clear that in the inventive examples that satisfy the conditions specified in the present invention, a suitable number density and dispersion state of the lubricating component can be obtained, and a resin-coated metal sheet having excellent slip properties, abrasion resistance, and ink adhesion of the resin coating layer can be obtained. On the other hand, in the comparative examples that do not satisfy the conditions specified in the present invention, one or more of the slip properties, abrasion resistance, and ink adhesion of the resin coating layer were evaluated as "X". In particular, in comparative examples 7 and 8, depending on the combination of the stretching temperature, stretching ratio, metal sheet temperature, and laminating roll temperature relative to the amount of lubricating component added, the lubricating component aggregated and coarsened, and the number of lubricating components decreased, and a suitable dispersion state was not obtained.

[0087] According to the present invention, it is possible to provide a resin-coated metal sheet having a resin coating layer excellent in slip resistance, abrasion resistance, and ink adhesion, a method for producing the same, and a metal container made using the resin-coated metal sheet.

[0088] 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 sheet having a resin coating layer containing a polyester resin on at least one surface of the metal sheet, 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 a polyolefin which is at least one of an acid-modified polyolefin and an oxidized polyolefin, the melting point of the polyolefin is 70°C or higher and 145°C or lower, the polyolefin is dispersed in the top layer in a particulate form, and the dispersion state of the polyolefin particles is such that the number density of the polyolefin particles on the surface of the top layer is 50 particles / cm. ２ The resin-coated metal sheet satisfies the above, and m defined by the following formula (1) is 0.20 or more. Here, the total number of polyolefin particles on the surface of the outermost layer is N, and A ｉ１ A: the number of polyolefin particles whose center of gravity is located within 50 μm around the i-th particle among the N polyolefin particles ｉ２ : the number of polyolefin particles whose center of gravity is located within a range of more than 50 μm and 100 μm from the circumference of the i-th particle among the N polyolefin particles, 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 1.0 mgKOH / g or more and 90 mgKOH / 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 1.0 mass % to 10.0 mass % 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 of co-extruding a first composition containing a polyester resin and a polyolefin which is at least one of an acid-modified polyolefin and an oxidized polyolefin and has a melting point of 70°C or more and 145°C or less, 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 the first composition, the second composition, and the third composition as a first layer, a second layer, and a third layer, respectively; a process of cooling and solidifying the molded body to obtain a laminated film; and a process of subsequently stretching the laminated film at least once under conditions of a stretching temperature of 85°C or more and 100°C or less and a stretching ratio of 3.5 times or more and 5.0 times or less to obtain a resin film containing the first layer, the second layer, and the third layer. Next, using a laminating roll controlled to a temperature of 110°C or higher and 120°C or lower, the resin film is thermocompression-bonded to at least one surface of a metal plate controlled to a temperature of 260°C or higher and 290°C or lower, to obtain a resin-coated metal plate having 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, on at least one surface of the metal plate, and comprising 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 1.0 mgKOH / g or more and 90 mgKOH / 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 1.0 mass % or more and 10.0 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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