JP4341471B2 - Resin-coated metal plate for containers - Google Patents

Description

  The present invention relates to a resin-coated metal plate for containers used in cans and lids for canned foods.

  Conventionally, metal plates such as tin-free steel (TFS) and aluminum, which are materials for metal cans used in food cans, have been painted. This painting technique not only has a complicated baking process, but also requires a lot of processing time, and has a problem of discharging a large amount of solvent. Many methods for laminating a resin film on a heated metal plate have been proposed.

Many of the above proposals relate to improvements in adhesion and formability between a film and a metal plate as a base material. For example, Patent Document 1 uses a film having a polar group (polyester resin or the like) as a film. However, in Patent Document 2, in order to ensure the post-processing adhesion of the polyethylene resin-coated metal plate, it is activated by performing a treatment such as corona discharge on the film surface, and the film surface free energy is increased. 38-54) × 10 ⁻³ N / m (38-54 dyn / cm).

  However, when the techniques described in Patent Document 1 and Patent Document 2 are used for food canning, when the contents are taken out from the container, the contents are firmly attached to the inner surface of the container, and the contents are difficult to take out. There is a problem. Since this problem is closely related to the consumer's willingness to purchase, it is a very important problem in securing the consumer's willingness to purchase. Nevertheless, in Patent Document 1 and Patent Document 2, no consideration is given to improving the ease of taking out the contents.

Therefore, as a result of intensive studies to ensure the contents take-out property, the present inventors added a specific wax (carnauba wax) into the polyester resin as shown in Patent Document 3, and applied to the resin surface. By making it exist, it was possible to secure good characteristics with respect to the content containing a large amount of fat (content with poor adhesion, such as a commercially available mixture of minced meat and eggs).
JP-A-63-236640, claims, etc. Japanese Patent Laid-Open No. 5-200961 and claims JP 2001-328204 A, Claims, etc.

  However, with the technique described in Patent Document 3, it was not possible to secure good content take-out properties for low fat and high protein content due to its strong adhesion. This is probably because the protein has many polar groups and thus easily forms hydrogen bonds with PET molecules. In the future, coupled with the growing health boom, demand for low-fat foods will increase further, and the importance of this technology will increase.

  The present invention has been made to solve the above problems, and has excellent moldability and adhesion required for container processing as well as ensuring excellent take-out properties even for low fat and high protein contents. It aims at providing the resin-coated metal plate for containers.

  As a result of intensive studies to solve the above-mentioned problems, the present inventors have found that a resin layer having a resin layer on the inner surface side of the container after the container molding of the resin-coated metal plate for a container is made of an olefinic wax in a resin mainly composed of polyester. It has been found that a resin-coated metal sheet for containers having moldability and adhesion required for container processing can be obtained by using a resin layer to which an appropriate amount has been added, while ensuring the contents take-out property.

  The present invention has been made based on the above findings, and the gist thereof is as follows.

(1) A resin-coated metal plate for a container having a metal plate and a polyester resin layer formed on at least an inner surface when the metal plate is molded into a container,
A) The polyester resin layer formed on the inner surface side has an upper polyester resin layer in contact with the internal space of the container and a lower polyester resin layer in contact with the metal plate,
B) The upper polyester resin layer contains 0.1 to 5% by mass of an olefin wax,
C) The upper layer polyester resin has a plane orientation coefficient of 0.06 to 0.15 or less, and the lower polyester resin layer has a plane orientation coefficient of 0.03 or less.
D) The upper polyester resin layer has a thickness of 0.5 to 10 μm, and the lower polyester resin layer has a thickness of 5 to 20 μm.

  (2) The resin-coated metal sheet for containers according to (1), wherein the olefin wax is polyethylene wax.

  (3) The resin-coated metal sheet for containers according to (1) or (2), wherein the lower polyester resin layer contains titanium dioxide particles.

  (4) The upper polyester resin layer is polyethylene terephthalate or a copolymerized polyethylene terephthalate copolymerized with isophthalic acid as an acid component at a ratio of 6 mol% or less, and the lower polyester resin has 10 parts of isophthalic acid as an acid component. The resin-coated metal sheet for containers according to any one of (1) to (3), which is a copolymerized polyethylene terephthalate copolymerized at a ratio of from mol% to 22 mol%.

  According to the present invention, among the polyester resin layers formed on at least the inner surface side when the metal plate is formed into a container, the upper resin layer in contact with the internal space of the container (that is, in contact with the contents) The polyester resin layer containing a wax and the surface orientation coefficient of the upper polyester resin layer is more than 0.06 and not more than 0.15, so that the contents can be taken out and the moldability is good. By limiting the thickness of the polyester resin layer and the plane orientation coefficient in the thickness direction, the adhesion with the metal plate is good, and further, by these limitations, the impact resistance of the entire polyester resin layer formed on the inner surface side is also improved. A good resin-coated metal sheet for containers can be obtained.

  The resin-coated metal plate according to the present invention is suitable as a container material for drawing processing, particularly as a food can container material.

  Hereinafter, the present invention will be described in detail.

  The resin-coated metal plate for containers according to the present invention has a metal plate and a polyester resin layer formed on the surface of the metal plate, and this polyester resin layer becomes at least the inner surface when molded into a container. Formed on the side. As this polyester resin layer, a resin mainly composed of polyester is used. Polyester-based resin is a polymer composed of dicarbononic acid and glycol component. As dicarboxylic acid component, terephthalic acid, isophthalic acid, naphthalenic acid dicarboxylic acid, diphenyldicarboxylic acid, etc. can be used. However, terephthalic acid and isophthalic acid can be preferably used. Examples of the glycol component include ethylene glycol, propanediol, butanediol, etc. Among them, ethylene glycol is preferable. These dicarboxylic acid components and glycol components may be used in combination of two or more. Moreover, antioxidants, heat stabilizers, ultraviolet absorbers, plasticizers, pigments, antistatic agents, crystal nucleating agents, and the like can be blended as necessary.

  Polyester composed of the above has excellent mechanical properties such as tensile strength, elastic modulus, impact strength, and polarity, so by using this as a main component, the resin layer can have adhesiveness and moldability that can withstand container processing. It is possible to improve the impact resistance after processing the container.

  Further, the polyester resin layer has two layers of an upper resin layer in contact with the internal space of the container (that is, in contact with the container contents) and a lower resin layer in contact with the metal plate. The upper resin layer in contact with the internal space of the container is a polyester resin layer containing an olefin wax. This is the most important requirement in the present invention. Olefin-based waxes are generally inactive and poor in reactivity, and therefore have a feature that they are difficult to adhere to contents such as food. By adding an appropriate amount of olefinic wax to the polyester resin, it becomes possible for the olefinic wax to be present on the surface of the resin, which inactivates the surface of the resin layer with the olefinic wax and causes the contents to adhere to each other. It becomes difficult, and it becomes possible to dramatically improve the content take-out property.

  Examples of the olefin wax to be added include homopolymers and copolymers of olefins, other olefins and other copolymerizable monomers such as copolymers of vinyl monomers, and modified polymers thereof. Etc. can be illustrated. Specifically, polyethylene (high density, low density, high molecular weight, low molecular weight, etc.), linear low density polyethylene, linear ultra low density polyethylene, polypropylene, ethylene / propylene copolymer, poly 4-methylene pentene. -1, Ionomer resin, ethylene / vinyl acetate copolymer, ethylene / acrylic acid copolymer, ethylene / methyl methacrylate copolymer, modified polyolefin (olefin homopolymer or copolymer, maleic acid or fumar An unsaturated carboxylic acid such as an acid, a reaction product with an acid anhydride, an ester, or a metal salt). These polyolefins can be used alone or in combination of two or more.

  Among these, in the present invention, it is preferable to use polyethylene wax as the olefin wax from the viewpoint of compatibility with the polyester resin and the ability to effectively inactivate the resin layer surface. Furthermore, the use of a low molecular weight polyethylene wax having a number average molecular weight (Mn) of 1,000 to 10,000 is more effective and preferable.

  When the olefin wax is contained, the content is 0.1 to 5% by mass with respect to the polyester resin layer to which the olefin wax is added. If it is less than 0.1%, the amount of olefinic wax produced on the resin surface is small, and the contents can be taken out poorly. If it exceeds 5%, the contents take-out property is almost saturated and a special effect cannot be obtained. In addition, the resin is technically difficult to manufacture, resulting in poor productivity and high cost. For the above reasons, in order to sufficiently cover the resin surface with the olefin wax and to ensure the productivity, the addition amount of the olefin wax is in the range of 0.1 to 5%, preferably 1.0 to 3%. And

  Further, the upper polyester resin layer has a plane orientation coefficient of more than 0.06 and 0.15 or less. If it is 0.06 or less, the content takeout property is insufficient. On the other hand, there is a possibility that a part of the resin layer exceeding 0.15 may break and cause a crack. The reason for this can be roughly estimated as follows.

  In general, a polyester film formed by a biaxial stretching method has oriented crystals oriented in the direction of the film plane, and the amount of the crystal can be quantified using the plane orientation coefficient of the film as an index. The olefinic wax in the polyester resin layer exists only in the amorphous region of the polyester resin layer and cannot exist in the region where the crystal structure is formed. Therefore, as the amount of crystal in the polyester resin layer increases, the amount of olefinic wax that can be present in the polyester resin decreases, and it is deposited on the surface of the polyester resin in a form that is pushed out of the system, leading to inactivation of the surface. become. Therefore, when the plane orientation coefficient, which is an index of the amount of oriented crystals, is 0.06 or less, the amount of olefinic wax remaining in the polyester resin layer increases, surface inactivation becomes insufficient, and the contents can be taken out easily. It seems to be lacking. Also, the impact resistance after processing the container becomes better as the amount of oriented crystals in the polyester resin layer increases, so it is desirable that the impact resistance exceeds 0.06. On the other hand, when the surface orientation coefficient of the upper polyester resin layer exceeds 0.15, the amorphous region rich in flexibility decreases, and the workability is insufficient. The resin layer cannot withstand the processing during the molding of the container, and a part of the resin layer is broken and cracks are generated. Therefore, the plane orientation coefficient of the upper polyester resin layer must be in the range of more than 0.06 and 0.15 or less.

  The plane orientation coefficient of the lower polyester resin layer is 0.03 or less. The reason for this is as follows.

  In the production of a resin-coated metal plate, the resin is usually brought into contact with a heated metal plate and subjected to pressure bonding to melt the resin at the interface of the metal plate and wet the metal plate to adhere to the resin. . Therefore, in order to ensure the adhesion between the resin and the metal plate, the resin needs to be melted, and the surface orientation coefficient of the resin in contact with the metal plate after the fusion is because the oriented crystals melt. Will be reduced. As specified in the present invention, if the surface orientation coefficient of the resin in this part is 0.03 or less, this indicates that the film melt wettability at the time of fusion was sufficient, that is, excellent adhesion was ensured. It becomes possible to do. When it exceeds 0.03, the adhesiveness is lowered, and there is a possibility that the resin layer may be peeled off at a portion tightened with the can lid after the high temperature and long time retort sterilization treatment applied to the food can.

  The resin layer of the resin-coated metal sheet for a container of the present invention is composed of an upper layer and a lower layer polyester resin layer, the thickness of the upper resin layer in contact with the contents is 0.5 μm to 10 μm, and the thickness of the lower resin layer Needs to be 5 μm to 20 μm.

  If the thickness of the upper layer is less than 0.5 μm, the high orientation region is insufficient and the impact resistance is deteriorated. If it exceeds 10 μm, the highly oriented region becomes excessive and the moldability is insufficient. The resin layer cannot withstand the processing during the molding of the container, and a part of the resin layer is broken and cracks are generated.

  On the other hand, if the thickness of the lower layer is less than 5 μm, the adhesiveness becomes poor, which is not suitable. On the other hand, if it exceeds 20 μm, the adhesiveness is saturated and further improvement in characteristics cannot be expected, resulting in an increase in cost, which is not suitable.

  The polyester resin layer has a composition in which the upper layer is polyethylene terephthalate or copolymer polyethylene terephthalate obtained by copolymerizing isophthalic acid as an acid component at a ratio of 6 mol% or less, and the lower layer is made of isophthalic acid as an acid component. It is desirable that the copolymerized polyethylene terephthalate is 10 mol% or more and 22 mol% or less.

When the isophthalic acid copolymerization ratio of the upper layer is more than 6 mol%, the melting point of the resin layer is lowered, so that it is easily melted by heat. Therefore, when the resin layer is formed on the metal plate by heat fusion, it is specified by the present invention. It becomes difficult to realize the alignment state. On the other hand, when the isophthalic acid copolymerization ratio of the lower layer is less than 10 mol%, the melting point of the resin is high, so that it is difficult to dissolve with heat. When the resin layer is formed on the metal plate, if the orientation state of the upper layer is controlled within the specified range of the present invention, there is a concern that the melt wetting on the metal plate is insufficient and the adhesion is deteriorated. Further, as the isophthalic acid copolymerization ratio increases, the resin cost also increases. Therefore, it is desirable to suppress the lower layer isophthalic acid copolymerization ratio to 22 mol% or less.
Further, by adding a color pigment to the polyester resin layer, the underlying metal plate can be concealed and various colors unique to the resin can be imparted. Further, it is possible to give a bright color using the metallic luster of the base without making the concealment perfect, and an excellent design can be obtained. Furthermore, unlike printing on the surface of the resin layer, since the pigment is added directly to the resin and colored, there is no problem that the color tone is lost even in the container molding process, and a good appearance can be maintained.

  As the pigment to be added, it is necessary to be able to exhibit excellent design properties after molding the container. From such a viewpoint, inorganic pigments such as titanium dioxide and organic pigments can be used. Since these have strong coloring power and abundant malleability, they are suitable because good design properties can be secured even after container molding.

  In particular, in the case of a resin layer that becomes the inner surface of the container after the container is molded as defined in the present invention, it is desirable to use titanium dioxide. This is because after the container is opened, the color of the contents shines and a clean feeling can be imparted. In addition, since these pigments are rich in coloring power and spreadability and are safety and health substances approved by the FDA, they are also preferable from this point.

  In the polyester resin layer defined in the present invention, the pigment may be added to at least one of the upper layer and the lower layer, and is preferably added only to the lower layer. By including titanium dioxide, the resin layer becomes slightly brittle, and therefore, when applied to the upper layer, the resin may be scraped when it is rubbed against the mold during container molding.

  The amount of pigment added is not particularly specified, but generally, when the content of the resin layer is 30% or more by mass, the concealability is saturated and economically. Since it is disadvantageous, it is desirable to make the range less than 30%. In addition, the addition amount of the said pigment is a ratio with respect to the resin layer (lower layer when added to the lower layer) which added the pigment.

  Next, a manufacturing method will be described.

  First, the manufacturing method of the resin layer (film) containing a multilayer is demonstrated. Although not particularly limited, for example, after drying each polyester resin as necessary, each and / or each is supplied to a known melt lamination extruder, extruded from a slit-shaped die into a sheet, In close contact with the casting drum by the method, it is cooled and solidified to obtain an unstretched sheet.

  A biaxially stretched film is obtained by stretching this unstretched sheet in the longitudinal direction and the width direction of the film. The draw ratio can be arbitrarily set according to the degree of orientation, strength, elastic modulus, etc. of the target film, but is preferably a tenter method in terms of film quality, and after stretching in the longitudinal direction, A sequential biaxial stretching method of stretching in the width direction and a simultaneous biaxial stretching method of stretching the longitudinal direction and the width direction substantially the same are desirable.

  Next, a method for producing a resin-coated metal plate by laminating the film on a metal plate will be described. In the present invention, for example, a method can be used in which a metal plate is heated at a temperature exceeding the melting point of the film, and a resin film is brought into contact with both surfaces using a pressure-bonding roll (hereinafter referred to as a laminate roll) and heat-sealed.

The laminating conditions are appropriately set so that the resin layer defined in the present invention can be obtained. For example, it is preferable that the temperature at the start of laminating is 220 ° C. or higher, and the temperature history received by the film during laminating is the time of contact at a temperature equal to or higher than the melting point of the film in the range of 1 to 20 msec. In order to achieve such lamination conditions, it is necessary to cool during bonding in addition to lamination at high speed. The pressure applied at the time of lamination is not particularly specified, but the surface pressure is preferably 9.8 to 294N (1 to 30 kgf / cm ² ). If this value is too low, even if the temperature reached by the resin interface is equal to or higher than the melting point, the time is short, so that the melting is insufficient and it is difficult to obtain sufficient adhesion. In addition, if the pressure is large, there is no problem in the performance of the laminated metal plate, but the force applied to the laminate roll is large and equipment strength is required, resulting in an increase in the size of the apparatus, which is uneconomical.

  As the metal plate, an aluminum plate or a mild steel plate that is widely used as a material for cans can be used, and in particular, a surface-treated steel plate on which a two-layer film is formed in which the lower layer is made of metal chromium and the upper layer is made of chromium hydroxide. (So-called TFS) is optimal.

The amount of adhesion of the metal chromium layer and chromium hydroxide layer of TFS is not particularly limited, but from the viewpoint of adhesion after processing and corrosion resistance, both are in terms of Cr, and the metal chromium layer is 70 to 200 mg / m ² , chromium. hydroxide layer is preferably in the range of 10 to 30 mg / ^{m 2.}

  In the present invention, the resin layer is formed into a film and coated on a metal plate in principle. However, if the resin layer is within the scope of the present invention, the resin layer is not formed into a film. It is also possible to apply melt extrusion lamination that melts the resin layer and coats the surface of the metal plate.

Examples of the present invention will be described below.
A steel sheet that had been cold-rolled, annealed, and temper-rolled with a thickness of 0.18 mm and a width of 977 mm was degreased and pickled, and then chrome-plated to produce a chromium-plated steel sheet (TFS). Chromium plating was performed by chromium plating in a chromium plating bath containing CrO ₃ , F ⁻ , SO ₄ ²⁻ , intermediate rinsing, and then electrolyzed with a chemical conversion treatment solution containing CrO ₃ and F ⁻ . At this time, electrolysis conditions adjusted to metallic chromium adhering amount and chromium hydroxide deposition amount (current density, the quantity of electricity, etc.), respectively Cr terms was adjusted to ^{120mg / m 2, 15mg / m} 2.

  Next, using the metal band laminating apparatus shown in FIG. 1, the chrome-plated steel sheet 1 obtained above is heated by the metal band heating apparatus 2, and the container is formed on one surface of the chrome-plated steel band 1 by the laminating roll 3. Various resin films 4a shown in Table 1 as the resin film to be the container inner surface later, and various films 4b as the resin film to be the container outer surface side after container molding are laminated (heat-sealed) on the other surface to form a resin-coated metal plate. Manufactured.

  The laminating roll 3 was an internal water-cooling type, and cooling water was forcibly circulated during the laminating to perform cooling during film adhesion. When laminating the resin film on the metal plate, the time during which the film temperature at the interface in contact with the metal plate was equal to or higher than the melting point of the film was set in the range of 1 to 20 msec.

  The characteristics of the resin-coated metal plate obtained by the above method and the film (resin layer) on the metal plate were measured and evaluated by the following methods (1) to (5), respectively. The obtained results are shown in Table 2.

(1) Plane orientation coefficient of polyester resin Using sodium D line (wavelength 589 nm) as a light source and using an Abbe refractometer, the refractive index in the longitudinal direction, the width direction, and the thickness direction (Nx, Ny, Nz, respectively) is measured. The plane orientation coefficient Ns was calculated by Ns = (Nx + Ny) / 2−Nz. In addition, the measurement was performed about the arbitrary positions: 10 places about the film after coating, and the average value was used.

(2) Content take-out property Using a drawing machine, a laminated metal plate was cup-formed in a drawing process with a blank diameter of 100 mm and a drawing ratio (diameter before molding / diameter after molding): 1.88. Subsequently, the cup is filled with high protein / low fat content (highly adherent with 100% reddish ground beef and mixed with oatmeal), and the lid is wrapped and retort processed ( 130 ° C. × 90 minutes). Then, remove the lid, observe the extent of the contents remaining inside the cup when the cup is turned upside down, and shake the hand a few times to remove the contents. The degree of ease of taking out the contents was evaluated by observing the degree of the contents remaining in the container.

(About the score)
A: The contents can be taken out by simply turning the cup upside down (without shaking by hand), and there is no deposit on the inner surface of the cup after removal.

○: The contents remain on the inside of the cup just by turning the cup upside down, but if the hand is shaken a few times, there will be no deposit on the inner surface of the cup.

X: It is difficult to take out the contents even if shaken by hand a few times.

(3) Formability After applying wax to the laminated metal plate, a 179 mm diameter disc was punched out to obtain a shallow drawn can with a drawing ratio of 1.80. Next, the drawn cup was redrawn with a drawing ratio of 2.20 and 2.90. Then, after performing doming shaping | molding in accordance with the conventional method, it trimmed and then gave the neck-in-flange process and shape | molded the deep drawing can. Focusing on the neck-in portion of the deep-drawn can thus obtained, the degree of film damage was visually observed.

(About the score)
A: No damage to the film after molding, and no film whitening is observed.

○: Molding is possible, but film whitening is observed.

X: The can is broken and cannot be molded.

(4) Adhesion A peel test sample (width 15 mm × length 120 mm) was cut out from the can body portion of the can that could be molded in the above (3). Part of the film is peeled off from the long side end of the cut sample, and the part of the film peeled off by the tensile tester is opened in the opposite direction (angle: 180 °) from the chrome-plated steel sheet from which the film has been peeled. A peel test was performed at a speed of 30 mm / min to evaluate the adhesion per 15 mm width. In addition, the contact | adhesion power measurement object surface was made into the can inner surface side.

(About the score)
A: 1.47 N / 15 mm or more (0.15 kgf / 15 mm or more).
○: 0.98N / 15mm or more, less than 1.47N / 15mm (0.10kgf / 15mm or more, less than 0.15kgf / 15mm).
X: Less than 0.98N / 15mm (less than 0.10kgf / 15mm).
(5) Impact resistance The can that was moldable in (3) above was filled with normal-temperature tap water in the can, and then the lid was wrapped and sealed. For each test, 10 cans were dropped from a height of 1.25 m onto the PVC tile floor, and then the lid and the tap water in the can were removed, and the film on the top of the can was scraped at one place to expose the steel plate surface. Then, 5% saline solution was filled in the can, and a platinum electrode was immersed in this (the immersed position is the central portion of the can) as a cathode, and the upper end portion (exposed portion of the steel plate) of the can was used as an anode. Subsequently, a voltage of 6 V was applied to the platinum electrode and the can, the current value after 3 seconds was read, and the average value after 10 cans was measured.

(About the score)
A: Less than 0.01 mA.

A: 0.01 mA or more and less than 0.1 mA.

X: 0.1 mA or more.

  From Table 2, the examples within the scope of the present invention have good contents takeout properties and moldability, and also good adhesion and impact resistance. On the other hand, the comparative example outside the scope of the present invention is inferior in any of the characteristics.

Other than containers for which moldability, adhesion and impact resistance are required, for example, it is also suitable for household appliances.

The figure which shows the outline | summary of the manufacturing apparatus of the resin-coated metal plate for containers which concerns on Example 1. FIG.

Explanation of symbols

1 Metal plate (chrome plated steel plate)
2 Metal belt heating device 3 Laminate roll 4a, 4b Film

Claims (4)

A resin-coated metal plate for a container having a metal plate and a polyester resin layer formed on at least the inner surface when the metal plate is molded into a container,
A) The polyester resin layer formed on the inner surface side has an upper polyester resin layer in contact with the internal space of the container and a lower polyester resin layer in contact with the metal plate,
B) The upper polyester resin layer contains 0.1 to 5% by mass of an olefin wax,
C) The upper layer polyester resin has a plane orientation coefficient of 0.06 to 0.15 or less, and the lower polyester resin layer has a plane orientation coefficient of 0.03 or less.
D) A resin-coated metal sheet for containers, wherein the upper polyester resin layer has a thickness of 0.5 to 10 μm, and the lower polyester resin layer has a thickness of 5 to 20 μm. 2. The resin-coated metal sheet for containers according to claim 1, wherein the olefin wax is polyethylene wax. The resin-coated metal sheet for containers according to claim 1 or 2, wherein the lower polyester resin layer contains titanium dioxide particles. The upper polyester resin layer is polyethylene terephthalate or copolymer polyethylene terephthalate copolymerized with isophthalic acid as an acid component at a ratio of 6 mol% or less, and the lower polyester resin is 10 mol% or more of isophthalic acid as an acid component. The resin-coated metal sheet for containers according to any one of claims 1 to 3, which is a copolymerized polyethylene terephthalate copolymerized at a ratio of 22 mol% or less.

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