Polyester resin coated seamless can and its manufacturing method

Abstract

To provide a polyester resin-coated seamless can capable of suppressing occurrence of whitening such as white spots and white streaks even when the entire can is exposed to high temperature conditions during retort sterilization or a heating step in a can-making process, and having also excellent adhesion of a resin coating.SOLUTION: A seamless can made by forming a polyester resin coating on at least an inner surface of a metal plate has a reduced diameter portion formed by neck processing within a distance of 0 to 15% from the top of the can body with respect to an entire height of the can from the top 6 of the can body to the can bottom 7. A melting point of the polyester resin is 250°C or higher. Before undergoing a sterilization treatment to prevent deterioration of the contents, an orientation index MOu of the inner polyester resin coating surface at the most reduced diameter portion 8 is 3.60 or less as measured by micro-Raman spectroscopy, and a ratio of the orientation index MOm, measured by micro-Raman spectroscopy on the surface of the polyester resin coating at a position within a distance of 45 to 60% from the top of the can body, to the orientation index MOu, MOm / MOu, is 1.80 or more.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a polyester resin-coated seamless can and a manufacturing method thereof, and more particularly to a polyester resin-coated seamless can having excellent adhesion of a polyester resin coating and excellent corrosion resistance and retort resistance, and a manufacturing method thereof. [Background technology]

[0002] Seamless cans are widely used, which are made by using polyester resin-coated metal sheets, which are made by covering a metal sheet with polyester resin, and subjecting them to forming processes such as drawing, drawing and re-drawing, drawing and ironing, and thin-walled drawing and ironing. Seamless cans used for beverages and food products are subjected to retort sterilization after being filled with the contents. Because this retort sterilization is carried out under high temperature and high humidity conditions, a problem called retort blushing (white spots) occurs in which water droplets on the bottom of the can or other parts of the can crystallize and whiten. In order to solve the problem of retort brushing, it has been proposed to use a copolymer polyester containing polybutylene terephthalate as the polyester resin film.

[0003] For example, Patent Document 1 listed below describes a stretched polyester film for metal lamination processing that has excellent retort resistance and is made of 99 to 60% by weight of polyester having a melting point of 210 to 245°C and a glass transition temperature of 60°C or higher, and 1 to 40% by weight of polyester mainly composed of polybutylene terephthalate having a melting point of 180 to 223°C, and has a free monomer content of 300 ppm or less. Furthermore, the following Patent Document 2 describes a metal laminate sheet for metal can bodies and can lid materials that does not whiten even when the film is crystallized, and that contains 10 to 70% by weight of polyethylene terephthalate resin and 90 to 30% by weight of polybutylene terephthalate resin and has two or more melting point peaks. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 7-330924 [Patent Document 2] JP 2008-143184 A Summary of the Invention [Problem to be solved by the invention]

[0005] However, in the polyester film containing a polyethylene terephthalate resin (hereinafter sometimes referred to as "PET") and a polybutylene terephthalate resin (hereinafter sometimes referred to as "PBT") as described in the above-mentioned prior art, when the content of PBT is low, whitening such as retort blushing that occurs on the bottom of the can with the outer resin coating cannot be sufficiently suppressed when the can is exposed to high temperature and high humidity conditions after filling the contents, such as sterilization such as retort sterilization or steaming. In addition, when the entire can is exposed to high temperature conditions in a heating process for removing lubricant and mitigating molding distortion during the can-making process, the occurrence of appearance defects called white streaks, in which the bottom of the can is whitened in a linear manner, cannot be sufficiently suppressed. On the other hand, when the content of PBT is high, the resin coating may be scraped off during drawing and ironing, causing metal exposure and deteriorating the appearance characteristics after printing. In addition, a new problem occurs in which the adhesion to the metal plate is reduced and the resin coating peels off (delamination) from the neck during retort sterilization, and the inclusion of PBT alone is not yet sufficient.

[0006] Accordingly, an object of the present invention is to provide a polyester resin-coated seamless can in which the occurrence of white spots is suppressed even when the can is exposed to high-temperature and high-humidity conditions such as those of a retort sterilization process, the occurrence of whitening such as white streaks is suppressed even when the entire can is exposed to high-temperature conditions in a heating process, and the can has excellent adhesion of the resin coating, and a method for producing the same. [Means for solving the problem]

[0007] According to the present invention, a seamless can having a polyester resin coating formed on at least the inner surface of a metal plate has a reduced diameter portion due to necking within a distance of 0 to 15% from the top of the can body with respect to the entire height of the seamless can from the top of the can body to the can bottom, the melting point of the polyester resin is 250°C or higher, the orientation index MOu of the inner surface polyester resin coating surface at the maximum reduced diameter portion measured by micro-Raman spectroscopy is 3.60 or less before the seamless can is subjected to retort sterilization treatment, and the ratio of the orientation index MOm of the polyester resin coating surface at a position within a distance of 45 to 60% from the top of the can body to the orientation index MOu, that is, MOm / MOu, is 1.80 or more. Before retort sterilization A seamless can is provided.

[0008] According to the present invention, there is also provided a seamless can having a polyester resin coating formed on at least the inner surface of a metal plate, the seamless can having a reduced diameter portion formed by neck processing within a distance of 0 to 15% from the top of the can body relative to the total height of the can from the top of the can body to the bottom, the polyester resin has a melting point of 250° C. or higher, and the seamless can is retort the orientation index MOu of the inner surface polyester resin coating surface at the maximum diameter reduction portion, as determined by micro-Raman spectroscopy, is 3.60 or less before the seamless can is subjected to a sterilization treatment; retort The seamless can is characterized in that, after being subjected to a sterilization treatment, the orientation index MOu is 4.10 or less, and the ratio of the orientation index MOm, determined by micro-Raman spectroscopy on the surface of the polyester resin coating at a position within a distance of 45 to 60% from the top of the can body, to the orientation index MOu, MOm / MOu, is 1.37 or more.

[0009] In the seamless can of the present invention, it is preferable that the maximum reduced diameter portion is reduced in diameter by a reduction rate of 15% or more relative to the diameter of the can body at a position 15 to 60% from the top of the can body.

[0010] The seamless can of a first embodiment and the seamless can of a third embodiment described below are reference examples, and the invention according to the present application is the seamless can of the second embodiment described below. is preferred. Effect of the Invention

[0011] In the resin-coated seamless can of the first embodiment of the present invention, the ratio Im / Iu in the aforementioned outer polyester resin coating (hereinafter sometimes referred to as "outer resin coating") is 1.0 or more. This prevents whitening such as white streaks from occurring even when the can is exposed to high temperature and high humidity conditions after filling with contents, such as sterilization treatment such as retort sterilization or steaming, or when the entire can is exposed to high temperature conditions in a heating process for removing lubricant and reducing molding distortion during the can-making process. Peeling of the outer resin coating at the neck portion is also effectively prevented, and the can has excellent hot water-resistant adhesion. The above-mentioned effects can also be judged by the condition of the inner surface polyester resin coating (hereinafter sometimes referred to as the "inner surface resin coating"), and the condition of the outer surface resin coating can be judged to be good when the ratio Wm / Wu in the inner surface polyester resin coating is 0.85 or less, the orientation index MOu of the polyester resin coating surface at the maximum diameter reduction portion is 2.40 or less, and the ratio of orientation indices MOm / MOu measured by micro-Raman spectroscopy is 2.10 or more.

[0012] That is, as mentioned above, in order to prevent whitening of the outer resin coating such as retort blushing and white streaks, it is preferable to use a polyester resin with a high PBT content, but since a resin coating with a high PBT content has high crystallinity, oriented crystals are generated in the can axis direction, which is the rolling direction of the resin-coated metal sheet in the can-making process, and adhesion to the metal sheet tends to decrease. In addition, when the PBT content increases, the PET resin content decreases relatively, and especially when the content of PET resin with a high isophthalic acid content decreases, adhesion to the metal sheet tends to decrease. On the other hand, wrinkle impressions (defects caused during seaming) are likely to occur in the neck part where seaming is performed, and it is thought that when exposed to high temperature and high humidity conditions such as retort sterilization, water vapor penetrates through the wrinkle impressions, causing peeling of the resin coating with poor adhesion (sometimes called "neck delamination"). Furthermore, in the neck portion of a seamless can, the compressive strain of the resin film in the circumferential direction of the can increases due to the drawing and ironing processes during the can-making process, and it is believed that the adhesion between the resin film and the metal sheet tends to be weak in this region. In contrast, in the seamless can of the first embodiment of the present invention, the oriented crystals of the outer resin coating in the neck portion are broken down so that Im / Iu falls within the above range, which is believed to improve the adhesion of the outer resin coating to the metal sheet and effectively prevent the intrusion of water vapor even if wrinkle impressions are formed. On the other hand, the oriented crystallization of the outer resin coating is maintained in areas other than the neck portion of the seamless can, ensuring the barrier properties of the outer resin coating, providing excellent corrosion resistance, and effectively preventing the occurrence of whitening such as retort blushing and white streaks.

[0013] On the other hand, in seamless cans with an inner resin coating, defects may occur in the neck portion as in the outer resin coating due to the seaming process performed on the top of the can body, handling, or foreign matter being caught in the can. If the inner resin coating is subjected to a sterilization process such as retort sterilization or a steaming process in a state where such defects have been formed, peeling of the inner resin coating (sometimes called "delamination of defects") may occur in the inner resin coating, which is affected by the internal pressure and contents, as in the outer resin coating. The adhesion between the resin coating and the metal plate tends to be weaker in the neck portion than in other regions of the entire can, and this is also true for the inner resin film. In particular, when the maximum diameter reduction portion with a diameter reduction rate of 15% or more with respect to the can body is present within a distance of 0 to 15% from the top of the can body with respect to the entire can height from the top of the can body to the bottom of the can, delamination of defects is likely to occur in the inner polyester resin coating. That is, even in the seamless can in which a polyester resin coating is formed on the inner surface of the above-mentioned outer resin-coated seamless can, the oriented crystals are broken down so that the ratio Wm / Wu in the inner resin coating is 0.85 or less. This improves the adhesion of the inner resin coating to the metal sheet, as in the case of the outer resin coating. Even when the inner resin coating is subjected to a sterilization treatment or steaming treatment after filling with contents, peeling of the inner resin coating at the neck portion is effectively prevented, and the inner resin coating has excellent hot water resistance and corrosion resistance.

[0014] In the second embodiment of the seamless can with an inner resin coating, the melting point of the polyester resin constituting the inner resin coating is 250° C. or higher. Before the seamless can is subjected to a sterilization treatment for preventing deterioration of the contents, the orientation index MOu is 3.60 or less, and the ratio of the orientation index MOm, MOm / MOu, is 1.80 or more. After the seamless can is subjected to a sterilization treatment for preventing deterioration of the contents, the orientation index MOu is 4.10 or less, and the ratio MOm / MOu is 1.37 or more. In the seamless can with an inner surface resin coating according to the third embodiment of the present invention, which has a melting point of 220°C or less, the MOm / MOu ratio after sterilization is 1.37 or more, the metal sheet is an aluminum sheet that has not been subjected to a surface treatment, and the orientation index MOu is 3.10 or less. This improves the adhesion of the inner surface resin coating to the metal sheet, and even when the diameter reduction rate of the maximum diameter reduction part is as large as 15% or more or when the can is subjected to a sterilization treatment or steam boiling treatment after filling with contents, peeling of the inner surface resin coating at the neck part is effectively prevented, and the inner surface resin coating has excellent hot water resistance and corrosion resistance.

[0015] According to the method for producing seamless cans of the present invention, the portion that will become the neck of the seamless can is heated at the specific timing as described above, thereby making it possible to control the oriented crystals in the portion that will become the neck of the seamless can as described above. If the neck is heated at a high temperature, a defective appearance having a plurality of small irregularities occurs only in the overlapping portion of the finishing varnish of the seamless can, but if such partial heating is performed at a temperature in the range of 185 to 230°C, a defective appearance does not occur in the overlapping portion of the finishing varnish of the seamless can, and a seamless can that is excellent in hot water resistant adhesion and appearance properties can be formed. [Brief description of the drawings]

[0016] [Figure 1] FIG. 1 is a side view showing an example of a seamless can of the present invention, with the right half being a side cross-sectional view. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0017] (Seamless can) FIG. 1 shows an example of a seamless can according to the present invention. The seamless can generally designated by 1 has a body 2 having a straight outer surface and a bottom 3 closing the lower part of the body 2. The upper part of the body 2 is connected to a neck 4 (sometimes called a "reduced diameter part") that has been reduced in diameter by necking (diameter reduction processing), and a flange 5 is formed at the upper end of the neck 4 via a maximum reduced diameter part 8, which is the part of the neck where the diameter reduction rate is the greatest. In this seamless can, as is clear from the enlarged cross-sectional view showing part X of the seamless can body, an outer resin coating 11 and an inner resin coating 12 are formed on the inner and outer surfaces of a metal plate 10. A printed layer 13 and a finishing varnish layer 14 are formed on the outer resin coating 11.

[0018] [First aspect] In the seamless can of the first embodiment of the present invention, as shown in FIG. 1 , at least a part of a neck portion 4 formed by necking is located within a distance of 0 to 15% of the total height of the can from the can body top 6 (0%) to the can bottom 7 (100%). It is important that the ratio Im / Iu of the maximum value of the peak intensity between 2Θ=15 to 19° as measured by X-ray diffraction method of the outer resin coating at the maximum diameter reduction portion 8 of the neck portion is divided by the thickness of the outer resin coating at the measurement site, Iu (cps / μm), to the maximum value of the peak intensity between 2Θ=15 to 19° as measured by X-ray diffraction method of the outer resin coating at a position within a distance of 45 to 60% from the can body top 6 (hereinafter, a position at least a part of the position within this distance may be referred to as the "can body center"), is 1.0 or more, and it is particularly preferable that the ratio is in the range of 1.0 to 4.5. Within 2Θ = 15 to 19° by X-ray diffraction of the polyester resin coating, there are peaks related to the crystallinity of PET and PBT, such as diffraction peaks with plane index (0-11) and plane index (010). In the seamless can with an outer resin coating of the present invention, the maximum peak intensity Iu of the outer resin coating near the neck is smaller than the maximum peak intensity Im near the center of the body, indicating that the oriented crystals of the outer resin coating in the neck are disrupted.

[0019] As described above, the oriented crystal state of the outer resin coating can also be judged from the state of the inner resin coating of a seamless can. -1 Nearby peak half-width (FWHM) Wu (cm -1 ) and the 1730 cm -1 Nearby peak half-width (FWHM) Wm (cm -1 ) is preferably 0.85 or less, and more preferably in the range of 0.59 to 0.70. In the analysis of polyester resin-coated PET by micro-Raman spectroscopy, -1 A Raman band attributable to C=O stretching vibration appears near the neck, and since PET takes on a resonance-stable planar structure when crystallized, the width of the measured C=O band narrows as crystallization progresses, and the half-width decreases as the oriented crystals progress. In the seamless can of the first embodiment of the present invention, the half-width Wu of the inner resin coating near the neck is larger than the half-width Wm near the center of the body, indicating that the oriented crystals of the inner resin coating at the neck are broken down.

[0020] Furthermore, in the seamless can of the first embodiment of the present invention, it is preferable that the orientation index MOu of the inner surface polyester resin coating surface at the maximum diameter reduction part 8, as determined by micro-Raman spectroscopy, is 2.40 or less, particularly in the range of 1.00 to 2.00, and that the ratio of the orientation index MOm of the polyester resin coating surface at a position within a distance of 45 to 60% from the top of the can body, as determined by micro-Raman spectroscopy, to the orientation index MOu, MOm / MOu, is 2.10 or more, particularly in the range of 2.50 to 5.00. In the above-mentioned PET analysis by micro-Raman spectroscopy, -1 The band at 632 cm is a peak due to the C=C stretching mode and is most highly correlated with the orientation, while the band at 632 cm -1 The band at 1615 cm can be used as an internal standard band because it is not affected by orientation. -1 Maximum peak intensity at I 1615 , 632cm -1 Peak top intensity at I 632 Relative intensity I by dividing 1615 / I 632 The ratio Ix / Iy of the relative intensities measured in the can height direction (Y) and the direction perpendicular to the can height direction (X) is the orientation index MO, which indicates the orientation of the PET including the amorphous portion. The orientation index MOu of the inner resin coating in the neck portion is 2.40 or less, the inner resin coating in the neck portion has a greater orientation in the can height direction (Y) than in the can vertical direction (X), and the ratio of the orientation index MOm at the center of the can body to the orientation index MOu, MOm / MOu, is 2.10 or more. This indicates that the inner resin coating in the neck portion is less oriented than the inner resin coating in the center of the can body, and the orientation of the inner resin coating in the neck portion is disrupted. Therefore, the inner surface resin coating of the seamless can of the first embodiment having such an oriented crystal state is effectively prevented from peeling (delamination) of the inner surface resin coating at the neck portion even when it is subjected to retort sterilization or the like, like the seamless cans of the second and third embodiments described later, and the inner surface resin coating has excellent hot water resistance and corrosion resistance.

[0021] In the seamless can according to the first aspect of the present invention, it is preferable that the diameter of the can body is reduced by a diameter reduction rate of 6% or less at the maximum diameter reduction portion relative to the diameter at a position 15 to 60% from the top of the can body. By reducing the diameter by the above value or less, it is possible to favorably maintain the oriented crystal state of the outer resin coating in the neck portion.

[0022] [Second and third aspects] An important feature of the seamless can according to the second embodiment of the present invention is that the melting point of the polyester resin constituting the inner surface resin coating is 250° C. or higher, and the orientation index MOu of the surface of the inner surface polyester resin coating at the maximum diameter reduction part, as determined by micro-Raman spectroscopy, is 3.60 or less before the seamless can is subjected to a sterilization treatment for preventing deterioration of the contents. In the second embodiment, it is preferable that the ratio of the orientation index MOm of the surface of the polyester resin coating measured by micro-Raman spectroscopy at a position within a distance of 45 to 60% from the top of the can body to the orientation index MOu, MOm / MOu, is 1.80 or more, particularly in the range of 1.90 to 4.50, and that after the seamless can has been subjected to a sterilization treatment to prevent deterioration of the contents (hereinafter sometimes referred to as "after sterilization treatment"), the orientation index MOu is 4.10 or less, particularly in the range of 1.00 to 4.00, and that the MOm / MOu is 1.37 or more, particularly in the range of 1.40 to 5.00. The seamless can according to the third aspect of the present invention is characterized in that the melting point of the polyester resin constituting the inner surface resin coating is 220° C. or lower, and the MOm / MOu after sterilization is 1.37 or higher, particularly in the range of 1.40 to 2.00. In the third aspect, when a multi-layer structure is adopted for the inner surface polyester resin coating, it is sufficient that the melting point of the resin constituting the lower layer on the metal sheet side is 220° C. or lower. In this embodiment, it is preferable that the orientation index MOu of the inner surface resin coating is 3.10 or less, and particularly preferably in the range of 1.40 to 2.00. In this specification, "the state of a seamless can before it has been subjected to a sterilization treatment to prevent deterioration of the contents" refers to the state of an empty can after it has been produced by the production method of the present invention described above and before it is filled with the contents, and "the state of a seamless can after it has been subjected to a sterilization treatment to prevent deterioration of the contents" refers to the state of a can filled with the contents that has been subjected to a conventionally known sterilization treatment such as retort sterilization or pasteurization which is carried out after the contents have been filled.

[0023] In the seamless cans according to the second and third aspects of the present invention, the inner resin coating has the above-mentioned characteristics, and therefore the adhesion to the metal plate is significantly improved. Therefore, even if the seamless can has a maximum diameter reduction portion with a large diameter reduction rate of 15% or more, particularly 20% or more, or if a metal plate without surface treatment is used, when the seamless can is subjected to a sterilization treatment or steaming treatment, etc., performed to prevent deterioration of the contents after filling, the inner resin coating at the neck portion is effectively prevented from peeling off (delamination), and the inner resin coating has excellent hot water resistance and corrosion resistance.

[0024] [Metal plate] In the present invention, various surface-treated steel sheets and light metal sheets such as aluminum sheets, which have been conventionally used for forming seamless cans, can be used as the metal sheets. The surface-treated steel sheet may be a cold-rolled steel sheet which has been annealed, temper-rolled or secondary cold-rolled, and then subjected to one or more surface treatments such as zinc plating, tin plating, nickel plating, electrolytic chromate treatment, chromate treatment, and zirconium compound treatment. As the light metal plate, in addition to so-called aluminum plate, aluminum alloy plate is used. Specifically, aluminum alloy plates of the 3000, 5000, and 6000 series in "JIS H 4000" can be preferably used. These light metal plates are preferably subjected to an inorganic surface treatment such as chromate phosphate treatment, zirconium phosphate treatment, and zirconium treatment, but since the polyester resin coating in the present invention has excellent adhesion to the metal plate, untreated aluminum plates that have not been subjected to the above-mentioned surface treatment can also be preferably used. The thickness of the metal sheet varies depending on the type of metal, the purpose or size of the container, but is generally preferably 0.10 to 0.50 mm. In the case of surface-treated steel sheets, the thickness is preferably 0.10 to 0.30 mm from the viewpoint of the strength and formability of the resulting seamless can, and in the case of light metal sheets, the thickness is preferably 0.15 to 0.40 mm.

[0025] [Polyester outer surface resin coated] In the seamless can of the first embodiment of the present invention, the polyester resin constituting the outer resin coating preferably contains 20 to 60 mass%, particularly 45 to 55 mass%, of polybutylene terephthalate homopolymer or copolymer resin mainly composed of butylene terephthalate units (hereinafter, these may be collectively referred to as "PBT"), with the remaining components being polyethylene terephthalate homopolymer (PET) or copolymer resin mainly composed of ethylene terephthalate units (for example, PET containing 5 mol% isophthalic acid may be expressed as "PETIA5"). If the PBT content is less than the above range, whitening such as retort blushing or white streaks may occur at the bottom of the can, and the appearance characteristics may be deteriorated. On the other hand, if the PBT content is more than the above range, the adhesion of the outer resin coating may be deteriorated, as described above, and the outer resin coating of the neck may peel off due to retort sterilization treatment or the like. Furthermore, there is a risk that the resin coating will be scraped off during the drawing and ironing process, resulting in exposure of the metal, and there is also a risk that the appearance properties after printing will be deteriorated. The PBT copolymer resin or PET copolymer resin preferably contains butylene terephthalate units or ethylene terephthalate units in an amount of 50 mol % or more, particularly preferably 80 mol % or more.

[0026] Furthermore, examples of the carboxylic acid component other than the terephthalic acid component as the copolymerization component include, but are not limited to, isophthalic acid, naphthalenedicarboxylic acid, p-β-oxyethoxybenzoic acid, biphenyl-4,4'-dicarboxylic acid, diphenoxyethane-4,4'-dicarboxylic acid, 5-sodium sulfoisophthalic acid, hexahydroterephthalic acid, adipic acid, sebacic acid, trimellitic acid, pyromellitic acid, hemimellitic acid, 1,1,2,2-ethanetetracarboxylic acid, 1,1,2-ethanetricarboxylic acid, 1,3,5-pentanetricarboxylic acid, 1,2,3,4-cyclopentanetetracarboxylic acid, biphenyl-3,4,3',4'-tetracarboxylic acid, and dimer acid. On the other hand, examples of alcohol components other than 1,4-butanediol or ethylene glycol include, but are not limited to, propylene glycol, neopentyl glycol, 1,6-hexylene glycol, diethylene glycol, triethylene glycol, cyclohexanedimethanol, bisphenol A ethylene oxide adduct, glycerol, trimethylolpropane, pentaerythritol, dipentaerythritol, sorbitan, and the like. Although not limited thereto, the content of isophthalic acid (IA) in the polyethylene terephthalate resin is preferably 2 to 15 mol%, particularly preferably 5 to 13 mol%. Also, it is preferable that the polyester resin coating is made of a blend of polyethylene terephthalate (PET) and polybutylene terephthalate (PBT) in a mass ratio of PET:PBT=80:20 to 40:60.

[0027] In the second and third seamless cans of the present invention, the above-mentioned polyester resin can be used as the outer resin coating, but when the diameter reduction rate of the maximum diameter reduction portion is set to 15% or more, it is preferable to use a polyester resin containing ethylene terephthalate units in an amount of more than 80 mol %. The above-mentioned copolymerization components can be used as the copolymerization components other than ethylene glycol and terephthalic acid. In the seamless cans of the second and third embodiments, it is preferable that the outer surface is resin-coated in the same manner as the inner surface, as in the examples described later.

[0028] The polyester resin preferably has an intrinsic viscosity (IV) measured using a phenol / tetrachloroethane mixed solvent as a solvent in the range of 0.5 to 1.4 dL / g, particularly preferably in the range of 0.65 to 1.4 dL / g. If the intrinsic viscosity is greater than the above range, the melt viscosity when the resin is heated and melted becomes extremely high, which is undesirable since it becomes difficult to coat the metal plate with the resin. If the intrinsic viscosity is smaller than the above range, the resin cannot withstand severe processing such as drawing and ironing, and is also inferior in flavor and corrosion resistance, which is undesirable. The glass transition temperature (Tg) of the polyester resin is preferably in the range of 30 to 80° C., and more preferably in the range of 50 to 65° C. In particular, if the Tg is higher than the above range, the processability may be reduced. On the other hand, if the Tg is lower than the above range, the retort whitening resistance may be poor. The melting point (Tm) of the polyester resin is preferably in the range of 200 to 260° C., and particularly preferably in the range of 215 to 235° C. If the melting point is lower than the above range, there is a risk of the retort whitening resistance decreasing. Furthermore, known resin additives, for example, antiblocking agents such as amorphous silica, pigments such as titanium dioxide, antistatic agents, antioxidants, lubricants, etc., may be blended into the polyester resin in a known manner. The thickness of the outer resin coating is not limited to this, but is preferably in the range of 5 to 20 μm. The outer resin coating may be a single layer as shown in FIG. 1, but may be a multi-layer of two or more layers. In the case of a multi-layer, it is important that all layers except for at least the bottom layer are outer resin coatings containing the above-mentioned PBT. In the case of a multi-layer, it is preferable that the total thickness is in the above range.

[0029] [Polyester inner resin coating] In the seamless can of the present invention, the inner surface resin coating may be made of the same polyester resin as that used for the outer surface resin coating. From the viewpoints of the adhesion during drawing and ironing, corrosion resistance, etc., of the resin coating, it is preferable to use a copolymer resin mainly composed of ethylene terephthalate units. In particular, a polyethylene terephthalate homoresin (homo-PET) or a copolymer resin mainly composed of ethylene terephthalate units (co-PET) is preferred, and a copolymer PET containing 1 to 15 mol % of isophthalic acid can be suitably used. In the inner resin coating, as in the outer resin coating, the melting point (Tm) of the polyester resin is preferably in the range of 200 to 260° C., and more preferably in the range of 215 to 235° C. If the melting point is lower than the above range, there is a risk of reduced retort whitening resistance. The seamless can of the second embodiment is characterized in that a polyester resin having a melting point (Tm) of 250° C. or more is used as the inner resin coating, which makes it possible to realize a container having excellent container performance at low cost and excellent flavor characteristics.

[0030] The inner resin coating also has an intrinsic viscosity and a glass transition temperature within the same range as the outer resin coating, and can use conventionally known resin additives. The thickness of the inner resin coating is preferably in the range of 10 to 30 μm, although it is not limited thereto. The inner resin coating may be either single-layer or multi-layer, but it is particularly preferable to use PETIA with an isophthalic acid content of 9 to 15 mol% for the lower layer and PETIA or homo-PET with an isophthalic acid content of 9 mol% or less that of the lower layer for the surface layer. The thickness ratio of the lower layer to the surface layer is preferably in the range of 8:2 to 4:6, although it is not limited thereto.

[0031] (Polyester resin coated metal sheet) The polyester resin-coated metal sheet used in the production of the resin-coated seamless can of the present invention can be produced by coating a metal sheet with the polyester resin using the above-mentioned polyester resin by a conventionally known method such as extrusion lamination, heat fusion, dry lamination, etc. From the viewpoint of processability, etc., lamination by the extrusion lamination method is preferable, but a biaxially oriented polyester film having excellent corrosion resistance can also be laminated by the heat fusion method or dry lamination. It is preferable that the resin coating made of polyester resin is formed on both the inner and outer surfaces, but it is also possible to form a polyester resin coating on only one of the surfaces and to form a coating film made of a conventional paint composition on the other surface.

[0032] To further improve adhesion of the polyester resin coating to the metal sheet, an adhesion primer may be used. As a primer paint having excellent adhesion and corrosion resistance, conventionally known primers such as epoxy phenol-based primer paint and polyester phenol-based primer paint can be used. From the viewpoint of hygiene, however, it is preferable to use a polyester phenol-based primer paint consisting of polyester resin and a resol-type phenolic resin derived from m-cresol as a hardener.

[0033] (Seamless can manufacturing method) The seamless can of the present invention is produced by forming a shallow drawn can by a conventional forming method, such as a drawing-forming process in which a metal plate having the above-mentioned polyester resin coated on the inside and / or outside is used to form a shallow drawn can by drawing, and a re-drawing and ironing process in which the shallow drawn can is re-drawn and ironed to form a drawn and ironed can, and then carrying out a heating process in which the entire drawn and ironed can obtained is heated, a printing process in which printing is carried out on the outer surface of the body of the drawn and ironed can that has been subjected to the heating process, a drying and baking process in which the entire printed can is heated, and a printing process in which a printing agent is applied to the drawn and ironed can that has been subjected to the drying and baking process. The can is formed through a neck-and-flange processing process in which necking and flange processing are performed on the drawn and ironed can. As described above, an important feature of the present invention is that it has a partial heating process in which the area that will become the neck of a seamless can is heated to a temperature in the range of 185 to 230°C, particularly 190 to 210°C, at any of the following times: (a) between the drawing process and the re-drawing and ironing process, (b) between the heating process and the printing process, or (c) after the neck-and-flange processing process. This makes it possible to adjust Im / Iu, Wm / Wu, MOm / MOu, etc. within the above-mentioned ranges by reducing the oriented crystals of the polyester resin coating in the area that will become the neck portion without reducing the oriented crystals of the polyester resin coating in areas other than the neck portion.This prevents whitening such as retort brushing from occurring when the can is exposed to high temperature and high humidity conditions such as retort sterilization, and also prevents whitening such as white streaks from occurring when the entire can is exposed to high temperature conditions in a heating process for the purpose of removing lubricant and alleviating molding distortion during the can making process, and also effectively prevents peeling of the resin coating in the neck portion.

[0034] [Drawing and ironing process] The drawing and redrawing / ironing steps can be carried out by a conventionally known method. In the drawing process, a shallow drawn can is drawn using a drawing punch and a die so that the drawing ratio (=blank diameter / punch diameter) is 1.4 to 1.8. The shallow drawn can is then subjected to the re-drawing and ironing process. As described above, in the present invention, before the re-drawing and ironing process, a portion of the shallow drawn can that will become the neck portion of a seamless can, which is 0 to 25% of the can height from the top of the can body, may be partially heated (the timing (a) mentioned above: the heating method will be described later). The subsequent redrawing and ironing process can be omitted depending on the drawing ratio of the shallow drawn can, and ironing can be performed directly. Also, bending and stretching (stretching) can be performed by redrawing. The side wall is preferably thinned by redrawing and ironing. The thickness is preferably reduced to 30 to 65%, particularly 34 to 42%, of the thickness of the resin-coated metal sheet.

[0035] [Heating process] The seamless can obtained by drawing and redrawing and ironing is subjected to a heat treatment after trimming the top of the can body to remove the lubricant used during forming and to relieve forming distortion of the polyester resin coating. This heat treatment is generally performed at a temperature of Tm-5°C or higher, particularly in the range of Tm-5 to Tm+20°C, based on the melting point (Tm) of the resin coating. The heating time differs depending on the heating method and cannot be generally specified, but is generally preferably in the range of 30 to 60 seconds. As a heating method, for example, a hot air circulating furnace, an infrared heating furnace, a high-frequency induction heating device, a dielectric heating device, or other conventionally known heating means can be used. The seamless can that has been cooled through the heating process is then subjected to a printing process. As described above, in the present invention, the portion of the heat-treated seamless can that will become the neck portion (a position that is 0 to 15% from the top of the can body relative to the height of the heated seamless can) may be partially heated before the printing process (the timing (b) described above: the heating method will be described later).

[0036] [Printing process and neck / flange processing] The seamless can that has been subjected to the heat treatment is then printed on the can body by a conventionally known printing method such as gravure printing, offset printing, flexographic printing, or inkjet printing, and then baked and dried to form a printed layer. A finishing varnish layer is formed on the printed layer to prevent scratches on the printed image. From the viewpoint of design, it is preferable that the printed layer is formed not only in the center of the can body but also in the part where the neck is to be processed. The seamless can on which the print layer and the finishing varnish layer are formed is then subjected to necking (diameter-reducing processing) to reduce the diameter of the neck portion, and flange processing to form a flange portion, completing the can as a seamless can. The diameter reduction rate due to necking is generally in the range of 5 to 25%, and in the seamless can of the first embodiment having the outer resin coating described above, the diameter reduction rate is preferably 6% or less, and in the seamless can of the second and third embodiment having the inner resin coating described above, the diameter reduction rate can be 15% or more. The diameter reduction rate in this specification is calculated by the following formula. Diameter reduction rate (%)=(D0-D1) / D0×100 In the formula, D0 is the inside diameter of the can body within a position that is 15 to 60% from the top of the can body, and D1 is the inside diameter of the can body at the maximum reduced diameter part.

[0037] In the present invention, as described above, the seamless can is finished by heating a portion located 0 to 15% from the top of the can body relative to its height (timing (c) above: the heating method will be described later). At least a portion of the region where necking is performed includes a printed layer and a finishing varnish layer, and if a defect occurs immediately below the seam on the outer surface of the can where the pigment is printed, peeling is likely to occur from the defect during retort sterilization.

[0038] [Partial heating process] As described above, in the present invention, the portion which will become the neck portion when the seamless can is manufactured is partially heated at any of the timings (a) to (c) described above to a temperature of 185 to 230°C, particularly 190 to 210°C, so that the Im / Iu value of the outer resin coating and the Wm / Wu, MOu and MOm / MOu values ​​of the inner resin coating can be adjusted to the above-mentioned values. The partial heating step may be performed at any of the timings (a) to (c) described above. However, by directly heating the neck portion, which is the portion where the adhesion with the metal sheet should be improved, in the state of the finished product, the oriented crystals can be controlled efficiently. Therefore, it is particularly preferable to perform the partial heating step at the timing (c) described above, which is performed on seamless cans after the neck / flange processing step.

[0039] The heating time cannot be generally determined depending on the heating temperature and heating method, but is preferably in the range of 0.05 to 40 seconds. In particular, when high-frequency induction heating is used, it is preferable to heat for less than 2.0 seconds, and more preferably in the range of 0.1 to 0.6 seconds. The heating method is not limited as long as the above temperature conditions are satisfied, but high-frequency induction heating is particularly preferable. The advantages of high-frequency induction heating over other heating methods such as ovens include the following: since high-frequency induction heating heats the metal plate, it heats the interface between the metal plate and the resin coating, so that the adhesive strength between the resin coating and the metal plate can be efficiently improved; the space required for installation of the equipment is small; it is possible to reach a high temperature in a short time; and it is possible to heat only a specific portion. In the present invention, when partial heating is performed by high-frequency induction heating, only the portion that will become the neck of the seamless can is selectively heated, and the above temperature is the temperature reached at the interface between the metal sheet and the film, and the heating time is the time it takes to reach this temperature. In the case of oven heating, the above temperature is the maximum temperature reached in the oven, and the heating time is the time the maximum temperature is maintained.

[0040] Although not limited thereto, a high frequency wave having a frequency of 10 to 200 KHz is used for the high frequency induction heating. For the high frequency induction heating, a heating device having a high frequency induction heating coil known per se can be used. This heating device generally comprises a high frequency induction heating coil, electrodes for connecting the coil to a power source, a magnetic member for strengthening the electromagnetic coupling between the coil and the seamless can and for regulating the heated portion of the seamless can, and a cooling mechanism for cooling the coil.

[0041] As described above, the seamless can of the present invention completed in this manner is inhibited from causing whitening such as white streaks even when exposed to high temperature and high humidity conditions such as those encountered in retort sterilization, and even when the entire can is exposed to high temperature conditions in a heating process for removing lubricant and reducing forming distortion during the can manufacturing process, and peeling of the resin coating at the neck portion is effectively prevented. EXAMPLES

[0042] The present invention will be described more specifically with reference to the following examples, but the present invention is not limited to the following examples. In the table, "after canning" means "before retort sterilization treatment." .

[0043] (Examples 1 to 12, Comparative Examples 1 to 5) An aluminum plate (A3104 material) with a thickness of 0.26 mm was used, and a 10 μm thick film made of polyester resin (glass transition temperature 56°C, melting point I: 218°C, melting point II: 232°C) composed of a blend of polybutylene terephthalate resin (PBT) and polyethylene terephthalate resin copolymerized with 11 moles of isophthalic acid (PETIA11) in a mass ratio of 60:40 was used as the outer resin coating, and a 12 μm thick two-layer film made of polyethylene terephthalate resin containing isophthalic acid (IA) (surface layer: glass transition temperature 79°C, melting point 247°C, lower layer: glass transition temperature 75°C, melting point 215°C) was used as the inner resin coating.The aluminum plate was heated to 250°C and heat-pressed with a laminating roll to obtain an aluminum plate with inner and outer resin coatings. The obtained resin-coated aluminum sheet was punched out into a circular blank with a diameter of 126.5 mm by a drawing machine to form a shallow-drawn cup with an average body wall height of 38 mm (drawing process). The shallow-drawn cup was then ironed (redrawing and ironing process), and the obtained drawn and ironed can was heated in an oven at a set temperature of 190 to 210°C for 30 to 60 seconds (heating process). A printed layer and a finishing varnish layer were then formed on the can body (printing process). After that, the can body diameter was reduced from 54.0 mm to 50.8 mm (reduced diameter amount: 5.9%) to form the neck, and then a flange was formed (neck and flange processing process), to produce a seamless can with a can height of 133.215 mm.

[0044] The partial heating step was performed between the drawing step and the redrawing and ironing step (indicated as "a" in Table 1), between the heating step and the printing step (indicated as "b" in Table 1), or after the neck and flange processing step (indicated as "c" in Table 1). Heating was performed for 0.30 seconds using a high-frequency induction heating device under the temperature conditions shown in Table 1.

[0045] (Examples 13 to 15, Comparative Examples 6 to 7) An aluminum plate (A3104 material) with a thickness of 0.27 mm was used, and a 12 μm thick film made of polyethylene terephthalate resin (glass transition temperature 80°C, melting point 254°C) was thermocompressed by a laminating roll to the inner and outer sides of the aluminum plate heated to 290°C to obtain an aluminum plate with inner and outer resin coatings. The obtained resin-coated aluminum sheet was punched out into a circular blank with a diameter of 142.0 mm by a drawing machine to form a shallow-drawn cup with an average body wall height of 33.5 mm (drawing process). The shallow-drawn cup was then ironed (redrawing and ironing process), and the obtained drawn and ironed can was heated in an oven at a set temperature of 190 to 210°C for 30 to 60 seconds (heating process). A printed layer and a finishing varnish layer were then formed on the can body (printing process). After that, the can body diameter of 68.3 mm was reduced to 54.0 mm (reduction rate 20.9%) to form the neck, and then a flange was formed (neck and flange processing process), to produce a seamless can with a can height of 122.2 mm. The partial heating step was carried out after the neck and flange processing (denoted by "c" in Table 4), and high-frequency induction heating was carried out in the same manner as in Example 1 under the temperature conditions shown in Table 4.

[0046] (Examples 16 to 19, Comparative Examples 8 to 9) An aluminum plate (A3104 material) with a thickness of 0.27 mm was used, and a two-layer film (surface layer: glass transition temperature 79°C, melting point 247°C, lower layer: glass transition temperature 75°C, melting point 215°C) with a thickness of 12 μm and made of polyethylene terephthalate resin containing isophthalic acid (IA) was attached to the inner and outer sides of the aluminum plate heated to 250°C using a laminating roll, thereby obtaining an aluminum plate with inner and outer resin coatings. The obtained inner resin-coated aluminum plate was punched out into a circular blank with a diameter of 142.0 mm by a drawing machine to form a shallow drawn cup with an average body wall height of 33.5 mm (drawing process). The drawn cup was then ironed (redrawing and ironing process), and the obtained drawn and ironed can was heated in an oven at a set temperature of 190 to 210°C for 30 to 60 seconds (heating process). A printed layer and a finishing varnish layer were then formed on the can body (printing process). After that, the can body diameter of 68.3 mm was reduced to 54.0 mm (reduction rate 20.9%) to form the neck, and then a flange was formed (neck and flange processing process), to produce a seamless can with a can height of 122.2 mm. The partial heating step was carried out after the neck and flange processing (denoted by "c" in Table 5), and high-frequency induction heating was carried out in the same manner as in Example 1 under the temperature conditions shown in Table 5.

[0047] (Creating a sample) Samples of approximately 4 mm x 20 mm were cut out from the seamless cans obtained in Examples 1 to 19 and Comparative Examples 1 to 9, with the short side in the height direction of the can and the center at the maximum reduced diameter part and a position approximately 53% from the top of the can body (center of the can body). The samples were cut out so that the rolling 0° direction of the plate was the center of the long side of the sample.

[0048] (Maximum peak intensity between 2Θ=15~19° by X-ray diffraction method) Using a fully automatic multipurpose horizontal X-ray diffraction apparatus, the samples of Examples 1 to 12 and Comparative Examples 1 to 5 cut out by the above-mentioned method were set horizontally on the stage, and measurements were performed with the short side facing the X-ray source and detector, and the long side facing the top of the can toward the inside of the apparatus with respect to the can height direction. Note that, with the outer resin coating on the top, measurements were performed near the center of the sample with the aluminum plate remaining on the sample. The measurement conditions are as follows. Equipment: Rigaku Corporation's fully automated multipurpose horizontal X-ray diffraction equipment SmartLab Tube: Target: Cu, Output: 9kW Tube voltage: 40kV Tube current: 20mA Scan axis: 2Θ / Θ Scanning range: 15~30° Step: 0.02deg

[0049] For the outer resin coating, the X-ray diffraction angle 2Θ of the polyester resin was measured from 15 to 19°, with the X-ray incidence angle and reflection angle Θ being the diffraction angle 2Θ, and the diffraction angle 2Θ was scanned from 15 to 30° while keeping the incidence angle and reflection angle always equal, and the X-ray diffraction spectrum was measured by the reflection method. Note that when measuring, the sample was fixed on the stage using tweezers, etc., because there is a risk that the analysis will not be performed correctly if there is oil on the sample. The peak intensity at 2Θ=15° and the peak intensity at 2Θ=30° were connected with a straight line to form the background, and the maximum peak intensity between 2Θ=15-19° was obtained by the X-ray diffraction method of the outer resin coating at the neck after subtracting the background, and the value Iu (cps / μm) was calculated by dividing the maximum peak intensity between 2Θ=15-19° by the thickness of the outer resin coating at the measurement site. Similarly, the maximum peak intensity between 2Θ=15-19° was obtained by the X-ray diffraction method of the outer resin coating at the center of the can body after subtracting the background, and the value Im (cps / μm) was calculated by dividing the maximum peak intensity by the thickness of the outer resin coating at the measurement site. From these results, the ratio Im / Iu was calculated. The measurements were performed before and after retort sterilization at 125°C for 30 minutes. The results are shown in Tables 1 and 2.

[0050] (Measurement of the inner resin coating by micro-Raman spectroscopy) The samples of Examples 4, 9, 12 to 19 and Comparative Examples 1, 4, 6 to 9 cut out by the above-mentioned method were set horizontally on the stage, and measurements were performed near the center of the sample with the inner resin coating facing up and aluminum remaining on the sample. A. Measurement of half-width The apparatus used and the measurement conditions are as follows. Equipment: Thermo Fisher Scientific DXR2xi Raman Imaging Microscope Laser, spectrometer: Wavelength 532nm, 5mW Non-polarized, full range grating Aperture: 25μm confocal pinhole Analyzer: None Objective lens: x100 Measurement position: Sample surface (measurement is performed within 2 μm in depth from the top surface focus position) 1730cm under the above conditions -1 The half-width W of the peak near the peak was obtained. In addition, when calculating the half-width (FWHM) and peak intensity, the peak intensity after subtracting the background was used. The measurements were performed before and after retort sterilization at 125°C for 30 minutes. The results are shown in Tables 3 to 5.

[0051] B. Measurement of the orientation index MO The laser polarization and analyzer were aligned along the height direction of the sample, and aligned perpendicular to the sample. -1 Peak top intensity around 1615 cm -1 Ratio of peak top intensities near I 1615 / I 632 Calculate the ratio of the height direction to the vertical direction (I 1615 / I 632 )yy / (I 1615 / I 632 The procedure was the same as in A, except that xx was the orientation index MO. The results are shown in Tables 3 to 5.

[0052] (Evaluation of delamination of the neck part of the outer resin coating) Each of the seamless cans obtained in Examples 1 to 12 and Comparative Examples 1 to 5 was filled with water, and then the lid was sealed and subjected to retort sterilization treatment under conditions of a sterilization temperature of 130° C. and a sterilization time of 30 minutes. For seamless cans after retort sterilization, if any film lift occurred directly below the seam, regardless of the magnitude of peeling, it was evaluated as having peeling, and the number of cans for which peeling occurred was counted out of 90 cans. The evaluation criteria were as follows. The results are shown in Table 1 (the numbers in parentheses in the table are the number of cans with peeling). ◎: 0 cans with peeling ○: Peeling occurred 1 to 3 cans ×: Peeling occurred for 4 or more cans

[0053] (Evaluation of neck flaws and delamination of inner resin coating) The seamless cans obtained in Examples 4, 9, and 12 to 19 and Comparative Examples 1, 4, and 6 to 9 were each cut in the inner resin coating of the neck in the circumferential direction with a cutter, and then subjected to retort sterilization treatment at a sterilization temperature of 125°C for a sterilization time of 30 minutes. The evaluation criteria were as follows. The results are shown in Tables 3 to 5. 〇: 0 out of 10 cans had film lifting ×: Film floating occurred in 1 or more cans out of 10 cans

[0054] (Appearance evaluation of finishing varnish overlap) The appearance of the overlapping portion of the finishing varnish of each of the seamless cans obtained in Examples 1 to 12 and Comparative Examples 1 to 5 was visually observed. The evaluation criteria were as follows. The results are shown in Table 1. 〇: No abnormalities ×: Poor appearance occurs

[0055] (comprehensive evaluation) The criteria for the overall evaluation in Table 1 are as follows: ○: Appearance evaluation of the overlapping part and delamination evaluation of the neck part of the resin coating after retort were both ○ ×: At least one of the appearance evaluation of the overlapping portion or the delamination evaluation of the neck portion of the resin coating after retort is ×

[0056] [Table 1]

[0057] [Table 2]

[0058] [Table 3]

[0059] [Table 4]

[0060] [Table 5] [Industrial Applicability]

[0061] The seamless can of the present invention is effectively inhibited from whitening such as white streaks even when exposed to high temperature and high humidity conditions such as retort sterilization, and even when the entire can is exposed to high temperature conditions in a heating process for removing lubricant and easing forming distortion during the can manufacturing process, and peeling of the resin coating at the neck portion is also effectively prevented. Therefore, the seamless can of the present invention can be suitably used as a container for filling contents that require retort sterilization, steaming, etc. [Explanation of symbols]

[0062] 1 seamless can, 2 body, 3 bottom, 4 neck, 5 flange, 6 top of can body, 7 bottom of can, 8 maximum diameter reduction section, 10 metal plate, 11 outer resin coating, 12 inner resin coating, 13 printing layer, 14 finishing varnish layer.

Claims

1. A seamless can having a polyester resin coating formed on at least the inner surface of a metal plate, the seamless can has a reduced diameter portion formed by neck processing within a distance of 0 to 15% from the top of the can body relative to the entire can height from the top of the can body to the bottom of the can, the melting point of the polyester resin is 250° C. or higher, and the orientation index MOu of the inner surface polyester resin coating surface at the maximum diameter reduction portion, as determined by micro-Raman spectroscopy, is 3.60 or less before the seamless can is subjected to a retort sterilization treatment; A seamless can before retort sterilization treatment, characterized in that the ratio of the orientation index MOm of the polyester resin coating surface at a position within a distance of 45 to 60% from the top of the can body, as determined by micro-Raman spectroscopy, to the orientation index MOu, i.e., MOm / MOu, is 1.80 or more.

2. A seamless can having a polyester resin coating formed on at least the inner surface of a metal plate, the seamless can has a reduced diameter portion formed by neck processing within a distance of 0 to 15% from the top of the can body relative to the entire can height from the top of the can body to the bottom of the can, the melting point of the polyester resin is 250° C. or higher, and the orientation index MOu of the inner surface polyester resin coating surface at the maximum diameter reduction portion, as determined by micro-Raman spectroscopy, is 3.60 or less before the seamless can is subjected to a retort sterilization treatment; The seamless can is characterized in that, in a state after the seamless can has been subjected to a retort sterilization treatment, the orientation index MOu is 4.10 or less, and the ratio of the orientation index MOm, determined by micro-Raman spectroscopy on the surface of the polyester resin coating at a position within a distance of 45 to 60% from the top of the can body, to the orientation index MOu, i.e., MOm / MOu, is 1.37 or more.

3. 3. The seamless can according to claim 1, wherein the maximum reduced diameter portion is reduced in diameter by a reduction rate of 15% or more with respect to the diameter of the can body at a position 15 to 60% from the top of the can body.

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