JP4725026B2 - Laminated metal plate for cans - Google Patents

Description

  The present invention relates to a laminated metal plate for cans used as a can or lid material for beverage cans and food cans.

  In recent years, the resin coating of can materials has been laminated, and for example, for beverage cans, the bottom and can body of a two-piece can and the can body of a three-piece can have been laminated. Reasons for the lamination of can materials in this way include rationalization of manufacturing processes by omitting painting and baking, lower environmental impact by omitting solvent drying processes (baking processes), and inclusion in paints And avoidance of elution of environmental hormones such as BPA. In particular, regarding environmental hormones, there is a report that a very small amount of BPA affects the human body. In general, food cans have more BPA elution than beverage cans, and it is expected that laminating will continue in the field of food cans.

As a resin film for laminating, a polyester film, in particular, a polyethylene terephthalate film has attracted attention as a film having balanced characteristics, and several proposals based on this have been made.
For example, a technology in which a biaxially oriented polyethylene terephthalate film is laminated to a metal plate via a low melting point polyester adhesive layer and used as a material for cans (Patent Documents 1 and 2), amorphous or extremely low crystalline aromatic A technique in which a polyester film is laminated on a metal plate and used as a can material (Patent Documents 3 and 4). There is a technology (Patent Document 5) that uses a biaxially oriented polyethylene terephthalate film that is heat-fixed in a low orientation and is laminated on a metal plate as a material for cans. Also, as a film that compensates for the shortcomings of polyethylene terephthalate, Resin films made of a blend of polybutylene terephthalate have also been proposed (Patent Documents 6 to 8).

JP-A-56-10451 JP-A-1-192546 JP-A-1-192545 JP-A-2-57339 Japanese Unexamined Patent Publication No. 64-22530 JP-A-6-234188 JP 7-314625 A Japanese Patent Laid-Open No. 10-100350

  A major characteristic of a polyethylene terephthalate-based laminate film is that the amount of oriented crystals greatly affects the characteristics, and other factors have a small influence. By making use of this feature and controlling the amount of oriented crystals to an appropriate amount according to the required performance, it is possible to make a laminated steel sheet having a desired basic performance. As a specific method, a biaxially oriented crystal film is used, the lamination conditions in the thermocompression bonding method are precisely controlled, and the residual amount of oriented crystals is controlled. This method is very convenient industrially, and it is possible to make various varieties that meet the required performance using the same raw materials. It can be said that laminated steel sheets in the beverage can field have been developed mainly based on this technology. However, this technique is not easy to achieve when it is desired to satisfy both of the characteristics contradicting the amount of oriented crystals.

For example, the processability and impact resistance of cans are characteristics that conflict with the amount of oriented crystals, but the biaxially oriented polyethylene terephthalate laminate cans currently available on the market are optimal oriented crystals that satisfy both characteristics. The amount is adjusted. However, when stricter can-making workability and impact resistance are required in the future, there is no suitable amount of oriented crystals that satisfies both characteristics. In addition, as long as the biaxial orientation is used, it is a great restriction that biaxial stretching is essential in film production. In the manufacturing method such as direct lamination, since the oriented crystal does not exist originally, this technique cannot be used in the first place.
Under such circumstances, many proposals have been made regarding resin films based on polyester and blended with polyolefin (for example, Patent Documents 9 to 16).

JP-A-7-195617 JP-A-10-315389 JP 2001-341257 A JP 2001-341258 A JP 2001-353814 A JP 2002-347176 A JP 2001-253032 A JP 2002-114897 A

These proposals can be positioned as new technologies that do not depend on biaxially oriented crystals, although the composition and contents of the resin film differ depending on the purpose and target. The polyester base resin used in these proposals uses a polyethylene terephthalate-polyethylene isophthalate copolymer as in the biaxially oriented film. Some of these resins are broadly defined simply as a polyester resin, but preferred resins include polyethylene terephthalate-polyethylene isophthalate copolymers. As far as the examples are concerned, polyethylene terephthalate-polyethylene isophthalate copolymer Coalescence is the base. For can materials that may have a long life cycle, it is natural to design them with a proven resin system, and in that sense, the above technology can be said to be excellent. Actually, products having these coating compositions are gradually spreading in the market.
However, the reason why these blend-based laminated steel sheets do not spread rapidly is considered to be due to some technical problems. The largest of these is the problem of resistance to retort whitening.

  The retort treatment is a process that is mainly performed for the purpose of sterilization and is generally performed in the process of canning. However, a can body (including a lid) using a laminated steel plate has a phenomenon that the film is whitened by this treatment. This retort whitening not only impairs the design, but also has the adverse effect of making it difficult to see the print of the production number and the like, which is very undesirable. Also in the beverage can field, whitening due to retort processing is a problem, and for this problem, a method of concealing using a white film has been selected and commercialized. Certainly, the white film is useful because it has the effect of clearly showing the printed layer applied on the upper surface thereof. In addition, concealment by printing is also effective, and there is a case where there is no particular problem in a can body portion that requires design. When looking at the market, the two-piece can for beverages and the three-piece can body using white film are mainly being laminated. It is not unrelated to the above technical problems that lamination is not progressing in the two-piece can other than the white film, the top lid, the bottom lid, and the food can in the bottom lid and beverage can of the three-piece can.

Originally, the method of concealing the discoloration of the film with the pigment has a problem that the concealing power is low depending on the type of the pigment, and it is not possible to meet the needs of diversified colors, and the cost of the pigment is high. Nothing is the most desired thing.
There are also disclosures such as Patent Document 17 and Patent Document 18 as films for laminated steel sheets that suppress retort whitening. However, such a prior art film does not provide a sufficient effect under any conditions, and it is not possible to retort whitening for the first time by selecting appropriate lamination conditions while considering other necessary characteristics. It is known that it can be suppressed. And even if it laminates appropriately, when strict processing such as an easy open end lid or DRD processing is performed, it is impossible to achieve both can-making processability and retort whitening resistance.

Japanese Patent Laid-Open No. 5-331302 JP, 7-145252, A In view of the above-mentioned problem of the prior art, the present invention is excellent in retort whitening resistance and has excellent film properties such as can-making processability and impact resistance. The purpose is to provide.

The present inventors investigated the mechanism of retort whitening, and based on the results, found a film structure capable of suppressing retort whitening in a simple and rational manner compared to the prior art.
First, as a result of investigating the phenomenon in which whitening occurs during retort processing, the present inventors have found the following facts. That is, (A) retort whitening is a phenomenon in which water bubbles (or bubbles) generated in the film diffusely reflect light and the film looks white, and (B) water bubbles (or bubbles) are extremely retorted. The water vapor that entered the film in the initial stage was cooled by the contents and condensed (or then vaporized), so it was likely to occur near the metal plate surface. (C) Water bubbles (or bubbles) Revealed the fact that it grows by expanding (deforming) the surrounding resin. Therefore, suppression of retort whitening is to prevent the generation of water bubbles (or bubbles) in the film, and an effective method for suppressing this is (1) preventing water vapor from permeating into the film during retort. It was found that (2) water bubbles (or bubbles) were not grown.

As is well known in the prior art, polyester-based resins are very well-balanced materials for designing coatings that satisfy various required characteristics of can materials. However, it is difficult for amorphous polyester to effectively suppress water vapor permeation during retort, and it is difficult to make improvements to suppress retort whitening.
On the other hand, when examining a method for preventing the growth of water bubbles (or bubbles), it is considered that water bubbles grow due to the softness of the resin derived from the amorphous structure, and therefore, a metal that easily generates bubbles. If the resin layer in the vicinity of the plate can be hardened (not easily deformed), the growth of bubbles may be suppressed. The problem of retort whitening cannot be ignored even in a biaxially stretched film, but water bubbles (or bubbles) grow in the molten layer. Water bubbles (or bubbles) grow in the molten layer because the molten layer, which is an amorphous layer, is soft, and conversely, if the molten layer is as hard as the oriented crystal layer, water bubbles (or bubbles) do not grow. It is done. It is considered that the oriented crystal layer is hard (not easily deformed) because the oriented crystal existing in the amorphous functions to suppress the movement of the amorphous molecules. Then, the inventor came up with the idea that a soft amorphous layer (melted layer) immediately after lamination could be hardened if crystallized thereafter. Crystalline polyesters such as polyethylene terephthalate tend to form spherulites at temperatures above the glass transition point. Therefore, it is predicted that if any heat treatment is performed after lamination, crystallization proceeds and the amorphous layer becomes hard.

  Therefore, the present inventors performed heat treatment on the laminated steel sheet at 200 ° C. for 10 minutes, and examined whether crystallization by this heat treatment is effective in suppressing retort whitening. As a result, it was found that the laminated steel sheet subjected to the heat treatment can completely suppress the retort whitening of the film. In other words, in order to suppress retort whitening, it is sufficient if there is a crystal that restrains the amorphous layer, and not only oriented crystals but also spherulites that are considered to be generated by heat treatment are sufficiently effective. Was confirmed.

  However, it is not impossible to heat-treat after lamination, but it is not a reasonable method because it requires an extra step from an industrial viewpoint. Therefore, the present inventors have come up with the idea of using heat during retorting as a method for crystallizing an amorphous layer without adding an extra step. However, since retort whitening itself is a phenomenon that occurs very early during retort, in order to adopt the above method, it is necessary for the amorphous layer to crystallize faster than the retort whitening rate. A resin with a high crystallization rate must be placed in the amorphous layer.

  In the prior art, as the laminate resin layer, (a) ethylene terephthalate-ethylene isophthalate copolymer, (b) polyethylene terephthalate, (c) polyethylene terephthalate-polybutylene terephthalate copolymer (or polyethylene terephthalate and polybutylene terephthalate). Etc.), and as a result of investigation by the present inventors, the magnitude of the crystallization rate was (c)> (b)> (a). Further, when the crystallization rate of polybutylene terephthalate was measured, it was found that the crystallization rate was considerably higher than those of the resins (a) to (c).

  Based on the above results, the method of suppressing whitening of retort by crystallizing the metal plate vicinity layer (amorphous layer) in a short time using the heat at the time of retorting as described above is that polybutylene terephthalate is made of metal. It has been found that this can be realized by arranging it in the layer near the plate. Here, among the above-mentioned conventional materials (a) to (c), the one having the highest crystallization rate is a polyethylene terephthalate-polybutylene terephthalate copolymer (or a blend of polyethylene terephthalate and polybutylene terephthalate). Even when such a resin is used, it has been found that it is difficult to crystallize the metal plate vicinity layer in a desired short time, and it is necessary to use polybutylene terephthalate capable of obtaining a higher crystallization speed. That is, it has been found that the ratio of polybutylene terephthalate needs to be significantly higher than that of the conventional material.

  In general, polybutylene terephthalate is more expensive than polyethylene terephthalate, and it is desirable to suppress the amount of use as much as possible from the viewpoint of production cost. Therefore, a preferred film configuration using polybutylene terephthalate was examined. As a result, it is more reasonable to place the polybutylene terephthalate thinly only in the vicinity of the metal plate where high crystallization speed is required, and to design the coating on the upper layer according to other required characteristics. I got a conclusion. In addition, as a result of the investigation by the present inventors, it was confirmed that the properties other than the retort whitening resistance of polybutylene terephthalate are equal to or higher than those of the conventional materials as described above.

The present invention has been made on the basis of the above findings, and the features thereof are as follows.
[1] A resin blend in which a lower resin layer made of polybutylene terephthalate is formed on at least one surface of a metal plate so as to be in contact with the metal plate surface, and a polyester as a main component is blended thereon with a polyolefin. And a film structure in which an upper resin layer made of a resin blend having a polyolefin ratio of 2 to 30 mass% is formed, the lower resin layer having a thickness of 3 to 10 μm, and the upper resin layer having a thickness of 8 μm. A laminated metal plate for cans characterized by the above.

 [2] In the laminated metal sheet for cans according to [1] above, the polyester in the resin blend of the upper resin layer contains polyethylene terephthalate, ethylene terephthalate-ethylene isophthalate copolymer, polybutylene terephthalate, ethylene terephthalate-butylene terephthalate. The polymer is one or more selected from a blend of polyethylene terephthalate and polybutylene terephthalate, and the polyolefin in the resin blend of the upper resin layer is one or more selected from polyethylene, polypropylene and ionomer. A laminated metal plate for cans characterized by the above.

[3] A can body obtained by can-making the laminated metal plate for cans according to [1] or [2] .
[4] A can lid obtained by lid-processing the laminated metal plate for cans according to [1] or [2] .

  The laminated metal sheet for cans of the present invention is excellent in retort whitening resistance, excellent in film properties such as can processability and impact resistance, and imparts various film performances without depending on oriented crystals. be able to.

The laminated metal plate for cans of the present invention is a laminated metal plate having a resin layer laminated on at least one surface of the metal plate, and is in contact with the metal plate surface and contains polybutylene terephthalate or polybutylene terephthalate in a high blend. Forming a resin layer consisting of, that is, using polybutylene terephthalate having a high crystallization speed as a layer near the metal plate on which an amorphous layer is formed. By disposing it in contact with the surface, the basic performance is obtained that the amorphous layer crystallizes in a short time at the very initial stage of retort, and as a result, retort whitening is effectively suppressed. Additionally, cans laminated metal sheet of the present invention more blending an appropriate amount of polyolefin in the polyester layer disposed on the upper layer of polybutylene terephthalate layer, as laminated steel sheet for a can which does not depend on the orientation crystals are obtained It is a thing.

In the laminated metal plate for cans of the present invention, at least one surface of the metal plate is formed with a lower resin layer made of a polyester resin in which the ratio of polybutylene terephthalate or polybutylene terephthalate is 90 mass% or more in contact with the metal plate surface, The upper layer has a film structure in which an upper resin layer composed of a resin blend having a polyester as a main component and blended with a polyolefin and having a polyolefin ratio of 2 to 30 mass% is formed.

The lower resin layer (hereinafter simply referred to as “lower layer”) may be composed only of polybutylene terephthalate, but depending on the film design of the laminated metal plate, it may be desirable to set the lower melting point higher or Since it may be better to mix other resin components for the purpose, other polyester resin components may be included as long as the effects of the present invention are not impaired. However, in order not to impair the effects of the present invention, the proportion of polybutylene terephthalate in the lower layer resin needs to be 90 wt% or more. Therefore, as the lower layer resin, in addition to polybutylene terephthalate, the proportion of polybutylene terephthalate is 90 wt% or more, ethylene terephthalate-butylene terephthalate copolymer, polyethylene terephthalate-polybutylene terephthalate copolymer, polyethylene terephthalate and polybutylene terephthalate. 1 type or more, such as a blended product, can be used.
The thickness of the lower layer is 3 to 10 μm. The reason why the thickness of the lower layer is defined as 3 μm or more is that the region of the lower layer of 3 μm or less most affects retort whitening, that is, a region where water bubbles (or bubbles) are easily generated. From such a viewpoint, the more desirable thickness of the lower layer is 5 μm or more. On the other hand, if the thickness exceeds 10 μm, the effect of suppressing retort whitening is saturated, so the economy is impaired.

The upper resin layer (hereinafter simply referred to as “upper layer”) is composed of a resin blend obtained by blending polyester and polyolefin as main components.
There is no particular restriction on the type of polyester that is the main component of the upper resin layer. It is preferable to use at least one selected from a blend of polybutylene terephthalate and polybutylene terephthalate. This is because these resins have a track record of being used in conventional laminated metal plates due to their excellent film performance. That is, the laminated steel sheet for cans of the present invention using such a main component resin as an upper layer is provided with a retort whitening function without impairing the characteristics of the conventional can compound material.

The reason why the polyolefin is blended with the main component polyester of the upper layer is mainly to improve impact resistance. That is, the main component polyester phase is a phase mainly responsible for processability, strength, heat resistance, adhesion and the like, and the polyolefin phase blended therein is a phase responsible for impact resistance and the like.
As polyolefin to blend, 1 or more types chosen from polyethylene, a polypropylene, and an ionomer can be used, for example.

The ratio of polyolefin in the resin blend is 2 to 30 mass%. When the proportion of polyolefin is less than 2 mass%, the effect of blending polyolefin is not sufficiently exhibited, and particularly impact resistance is inferior. On the other hand, when the ratio of polyolefin exceeds 30 mass%, it is difficult to uniformly disperse the polyolefin particles in the polyester, which is not preferable.
The thickness of the upper layer is 8 μm or more. The thickness of the upper layer may be appropriately selected according to the required characteristics, but is set to 8 μm or more in consideration of workability and impact resistance when the thickness of the lower layer is close to the lower limit (3 μm).

The resin layer laminated on the metal plate may be composed only of the resin layer described above, but other resin layers (for example, the uppermost resin layer, metal plate, etc.) as long as the effects of the present invention are not impaired. And an adhesive layer may be provided. For example, by forming a thin polyethylene terephthalate layer as the uppermost layer, the flavor and the like can be improved.
In addition, the resin layer of the laminated metal plate of the present invention may contain additives such as an antioxidant, a heat stabilizer, an ultraviolet absorber, a plasticizer, a pigment, an antistatic agent, a lubricant, and a crystal nucleating agent as necessary. In addition, a wax component or the like can be added to the surface layer of the upper layer.

The resin layer can be formed by a method such as a co-extrusion method, a direct lamination method, or a thermocompression bonding method.
There is no special restriction | limiting in the base metal plate of the laminated metal plate of this invention. Tin-free steel is preferable in terms of being inexpensive and excellent in adhesion, but other surface-treated steel plates such as tinplate, aluminum plates, and the like may be used.
The laminated metal plate for cans according to the present invention is different from can containers such as food cans and beverage cans, apart from members such as can body materials and lid materials (top lid, bottom lid), cans such as 2-piece cans and 3-piece cans. Regardless of the form, it can be used as a can material for all uses.

As the base metal plate, a tin-free steel original plate having a thickness of 0.20 mm and a hardness of T4 was used. A resin layer was laminated on the base metal plate using a direct laminating method to produce laminated steel plates for cans of the present invention and comparative examples. However, in Comparative Examples 9 and 10, the resin layer was laminated using a film laminating method by thermocompression bonding to produce a laminated steel sheet for cans.
The measurement method of the plane orientation coefficient of the obtained laminated steel sheet and the evaluation methods of retort whitening resistance, can manufacturing processability and impact resistance are shown below.
(1) Measurement of plane orientation coefficient About the laminated steel sheets for cans of Comparative Examples 9 and 10, using an Abbe refractometer, the film surface under the conditions of light source: sodium / D line, intermediate solution: methylene iodide, temperature: 25 ° C The refractive index Nx in the longitudinal direction, the refractive index Ny in the lateral direction of the film surface, and the refractive index Nz in the thickness direction of the film were measured, and the plane orientation coefficient Ns was calculated by the following equation.
Planar orientation coefficient (Ns) = (Nx + Ny) / 2−Nz

(2) Retort whitening resistance (test)
A laminated steel plate was formed into a bottom lid shape and used for the test. First, the test piece was wound around a can body for a capacity of 350 cc, filled with 350 cc of 90 ° C. hot water, the upper lid was wrapped, and then a retort treatment was performed under conditions of 130 ° C. × 30 minutes. After this retort treatment, the resin layer that was not whitened was “◯”, and the one that was whitened was “x”. Furthermore, only for the laminated steel sheet in which no whitening was observed, a test can was newly produced and subjected to another test. In this test, a test piece formed of a laminated steel plate in the shape of a bottom lid is wrapped around a can body for 350 cc capacity, filled with 150 g of ice and 200 cc of water, the top lid is wrapped up, and then mounted on a retort tester. And retort treatment was performed under conditions of 130 ° C. × 30 minutes. After the retort treatment, the resin layer that was not whitened was designated as “◎”.

(3) Can manufacturing process (test)
The laminated steel sheet was sequentially subjected to first stage drawing and redrawing under the following conditions to obtain a thinned deep drawn can.
・ First stage drawing condition Blank diameter: 150-160mm
Aperture ratio with 1-stage aperture: 1.65
-Re-drawing condition First-drawing drawing ratio: 1.25
Aperture ratio of the second re-drawing: 1.25
Curvature radius of die corner in redrawing process: 0.4mm
Wrinkle holding weight during redrawing: 39227N (4000kg)
-Average thinning ratio of the can body: 40 to 55% of the thickness of the laminated steel sheet before forming
In the second redrawing process in the redrawing process, the case where film damage was detected was indicated as “x”, and the case where no film damage was detected was indicated as “◯”.

(4) Impact resistance (test)
The can after the second redrawing produced in the evaluation test of can manufacturing processability was subjected to a heat treatment for strain removal for 2 minutes under a heat treatment temperature condition of [film melting point -15 ° C.] and then rapidly cooled with cold air. The can was filled with 350 cc of water and the lid was wrapped, and then placed in a refrigerator to stabilize the can body temperature at 4 ° C. Next, an impact was given by dropping a 0.5 kg iron ball from a height of 30 cm onto the bottom of the can. Next, the lid is opened, 1% saline solution is filled so that the impacted part is immersed inside the can, and after 5 minutes immersion, a 6 V load is applied to the platinum electrode and the can metal part immersed in the solution, and after 5 seconds. The current value of was measured. The case where the current value was 10 mA or more was “X”, and the case where the current value was less than 10 mA was “◯”. The can after the second re-drawing was tested after confirming that no film damage was observed at all levels at the bottom.

Tables 1 and 2 show the film configuration of the laminated steel sheet of each example, and Table 3 shows the results of performance evaluation of each example.
Inventive Examples 1 to 7 are obtained by variously changing the thickness of the lower layer, the main component resin type of the upper layer, the blended additive resin type and the blending ratio, etc. under the conditions of the present invention. It has been. In Invention Examples 6 and 7, since the thickness of the lower layer is not particularly preferably 5 μm or more, the evaluation of retort whitening resistance remains “◯”.
Comparative Examples 1 to 3 have a structure in which polybutylene terephthalate is not disposed in contact with the steel plate surface, and are conventional compound materials. Therefore, the can-making processability and impact resistance are good, but the retort whitening resistance is inferior.

Comparative Example 4 has a two-layer structure, but is an example in which polybutylene terephthalate is not used in the lower layer, and has the same results as Comparative Examples 1 to 3.
Comparative Example 5 is an example in which the thickness of the lower layer is lower than the conditions of the present invention, and the retort whitening resistance is poor.
Comparative Example 6 is an example in which the ratio of polybutylene terephthalate in the lower layer was lower than the conditions of the present invention, and the retort whitening resistance was poor.
Comparative Example 7 is an example in which the blending ratio of the polyolefin in the upper resin blend was lower than the conditions of the present invention, and the retort whitening resistance and can manufacturing processability were good, but the impact resistance was inferior. In addition, about what the compounding ratio of polyolefin in the upper layer resin blend exceeded the conditions of this invention, since it was not able to form a film, it was not able to use for a test.

Comparative Example 8 is an example in which the blend resin in the upper layer resin is not a polyolefin, and the lower layer satisfies the conditions of the present invention, so the retort whitening resistance is good, but the can-making processability and impact resistance are both inferior. Yes.
Comparative Examples 9 and 10 are obtained by laminating biaxially oriented films by a thermocompression bonding method. Among them, Comparative Example 9 has a high plane orientation coefficient, and Comparative Example 10 has a low plane orientation coefficient and has each set lamination conditions. It is. In both cases, the lower layer satisfies the conditions of the present invention, so the retort whitening resistance is good. However, since the upper layer does not contain polyolefin, comparative example 9 has can-making processability, and comparative example 10 has impact resistance. Each is inferior.
Comparative Example 11 is an example in which polypropylene is used for the upper layer, but since the lower layer satisfies the conditions of the present invention, the retort whitening resistance is good, but the can-making processability is inferior. Polypropylene is a resin having excellent extensibility, but it is considered that the molding was not continued because of its low strength.

Claims (4)

A lower resin layer made of polybutylene terephthalate is formed on at least one surface of the metal plate in contact with the metal plate surface, and the upper layer is a resin blend composed mainly of polyester and blended with polyolefin. It has a film structure in which an upper resin layer made of a resin blend having a ratio of 2 to 30 mass% is formed, the thickness of the lower resin layer is 3 to 10 μm, and the thickness of the upper resin layer is 8 μm or more. A laminated metal plate for cans characterized by the above.   The polyester in the resin blend of the upper resin layer is selected from polyethylene terephthalate, ethylene terephthalate-ethylene isophthalate copolymer, polybutylene terephthalate, ethylene terephthalate-butylene terephthalate copolymer, and a blend of polyethylene terephthalate and polybutylene terephthalate. 2. The laminated metal for cans according to claim 1, wherein the polyolefin in the resin blend of the upper resin layer is at least one selected from polyethylene, polypropylene, and ionomer. Board.   A can body obtained by can-making the laminated metal plate for cans according to claim 1 or 2.   A can lid obtained by lid-making the laminated metal plate for cans according to claim 1.