JP5978945B2 - Polylactic acid resin-coated metal plate for containers - Google Patents

Description

  The present invention relates to a polylactic acid-based resin-coated metal plate for containers used for, for example, can bodies 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 coated for the purpose of improving corrosion resistance, durability and weather resistance. However, this coating process has a problem that not only the baking process is complicated, but also a long processing time is required and a large amount of solvent is discharged.

  Therefore, in order to solve these problems, a film laminated metal plate in which a thermoplastic resin film is laminated on a heated metal plate has been developed instead of a painted metal plate, and is currently used industrially as a food canning material. ing.

In addition to basic properties such as processability and adhesion, food canning materials have a variety of functions such as deep drawability, adhesion after processing and retorting, corrosion resistance, and impact resistance for 2-piece can applications. Desired. Therefore, as the thermoplastic resin film, a polyester film excellent in mechanical properties and chemical stability is widely used, and among them, a biaxially stretched polyester film made of polyethylene terephthalate or polyethylene naphthalate is most frequently used.
However, in recent years, with the depletion of petroleum resources, disposal of plastic waste, global warming, etc., measures to escape from the extreme dependence on oil in energy and resources have begun to be sought. In particular, a shift from finite fossil resources to renewable resources such as biomass is underway. If the plant uses biomass resources created by assimilating carbon dioxide in the atmosphere by photosynthesis, it will not give a load of carbon dioxide in the atmosphere even if it is disposed of. This property of not increasing carbon dioxide in the environment is called carbon neutral.

  Under such circumstances, polylactic acid is a plant-derived thermoplastic resin such as corn, sugar, and radish, and generally has good mechanical properties, transparency, and processability. It is expected to be used in various fields such as equipment.

  However, polylactic acid has high drawbacks such as high hygroscopicity and low hydrolysis resistance. The hydrolysis chain reaction of polylactic acid is initiated from the carboxyl group at the end of the molecular chain and proceeds significantly under high temperature and high humidity. As a result, the mechanical properties such as molecular weight, strength, and elastic modulus decrease. End up. Therefore, it cannot endure the retort sterilization process which is one of the general manufacturing processes of food canned food. This is because, in a retort environment exposed to high temperature (120 ° C. or higher) water vapor, the hydrolysis reaction of the ester bond part advances rapidly and the molecular chain is broken.

  In order to solve this problem, a technique for blocking the carboxyl group at the end of the molecular chain, which is the starting point of the hydrolysis chain reaction, has been studied. For example, Patent Documents 1 and 2 propose a method of blocking a terminal carboxyl group of polylactic acid by a condensation reaction of an aliphatic alcohol and a carboxyl group. However, in the dehydration condensation reaction of alcohol and carboxyl group, water is generated as a by-product simultaneously with the end-capping. Therefore, although the terminal carboxyl group is blocked, moisture remains in the resin, resulting in deterioration in appearance due to partial decomposition of the ester bond and reduction in mechanical properties.

  Patent Documents 3 and 4 disclose the use of an addition reaction type carboxyl group end-capping agent such as epoxy and isocyanate. These compounds have the merit that no by-product is produced when an addition reaction with a carboxyl group occurs, but reacts not only with a carboxyl group but also with a hydroxyl group. Therefore, a part is taken off at the hydroxyl terminal end of polylactic acid, and the efficiency of blocking the carboxyl group end is deteriorated. For this reason, the amount added is increased more than necessary, not only reducing the economic effect of the technology, but also easily inducing gelation, which may impair the production stability.

  On the other hand, in Patent Document 5, the upper layer of the resin layer having a two-layer structure contains an isocyanate compound mainly composed of polyester, and the lower layer is composed mainly of polylactic acid having a residual orientation degree of 5% to 30%. The polylactic acid contains a polycarbodiimide compound in an amount of 0.01 to 10 mass%, and a polylactic acid resin-coated metal plate for a container having excellent hydrolysis resistance is disclosed. According to this, the hydrolysis resistance of the polylactic acid resin-coated metal plate is certainly improved.

JP 7-316273 A Japanese Patent Laid-Open No. 9-21017 JP 2001-323056 A JP 2002-30208 A JP 2010-158821 A

  However, in Patent Document 5, an isocyanate compound is used, and a highly toxic unreacted monomer may remain, which is not necessarily a safe substance.

  The present invention has been made in view of such circumstances, and is excellent in hydrolysis resistance in a retort processing environment, and has a polylactic acid resin-coated metal for containers that satisfies many characteristics required for food canned materials. The purpose is to provide a board.

As a result of intensive studies by the present inventors for solving the problems, the following knowledge was obtained.
A polycarbodiimide compound is added to the resin layer containing polylactic acid as a main component at a specific ratio, and the residual orientation degree of the resin layer is controlled. And the polyester resin layer is formed in the upper layer. Furthermore, the polyester resin layer as the upper layer is added with one or more selected from the group consisting of polyamine resin, polyamidoamine resin, and polyamide resin at a specific ratio. By using a resin layer with such a two-layer structure, in addition to basic properties such as excellent deep-drawing formability and post-processing adhesion, hydrolysis resistance that can suppress performance degradation under retort processing environments A polylactic acid-based resin-coated metal plate for a container excellent in the above can be obtained.

The present invention is based on the above findings, and the features thereof are as follows.
[1] A polylactic acid-based resin-coated metal plate for containers having at least a two-layered resin layer on one side or both sides, wherein the two-layered resin layer satisfies the following (A) and (B) A polylactic acid-based resin-coated metal sheet for containers.
(A) The upper resin layer (a) is a resin layer mainly composed of a polyester having an adhesion amount of 0.1 g / m ² or more and 5.0 g / m ² or less, and (a) a polyamine resin or a polyamidoamine. 1.0-50 mass% of any one or more chosen from the group which consists of resin and a polyamide resin is contained.
(B) The lower resin layer (b) is a resin layer mainly composed of polylactic acid having a residual orientation degree of 5% or more and 30% or less, and the polylactic acid contains 0.01 to 10% by mass of a polycarbodiimide compound. contains.
[2] The polylactic acid resin-coated metal sheet for containers according to [1], wherein the resin layer (a) further contains (b) an epoxy resin.
[3] The polylactic acid resin for containers according to [1] or [2], wherein the resin layer (a) further contains (c) a metal alkoxide compound and / or a metal chelate compound. Coated metal plate.

  ADVANTAGE OF THE INVENTION According to this invention, the polylactic acid-type resin coating metal plate for containers excellent in hydrolysis resistance which can respond to many characteristics requested | required of a food canned raw material is obtained. And in accordance with the policy to get out of the extreme dependence on petroleum in energy and resources, it becomes an industrially useful invention as a new polyester resin-coated metal plate for containers that can easily add many functions required for food canning. .

It is a figure which shows the principal part of the laminating apparatus of a metal plate. (Example) It is a figure which shows the cross-section of the polylactic acid-type resin coating metal plate for containers. (Example)

  Hereinafter, the polylactic acid resin-coated metal plate for containers of the present invention will be described in detail.

First, the metal plate used by this invention is demonstrated.
As the metal plate of the present invention, an aluminum plate or a mild steel plate widely used as a can material can be used. In particular, a surface-treated steel sheet (hereinafter referred to as TFS) on which a two-layer film is formed, in which the lower layer is made of metallic chromium and the upper layer is made of chromium hydroxide, 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, 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.}

Next, the resin layer that covers the metal plate will be described. The resin layer of the present invention is a resin layer having at least a two-layer structure. The lower resin layer is a resin layer mainly composed of polylactic acid having a residual orientation degree of 5% or more and 30% or less, and the polylactic acid contains 0.01 to 10% by mass of a polycarbodiimide compound. The upper resin layer is a resin layer mainly composed of a polyester having an adhesion amount of 0.1 g / m ² or more and 5.0 g / m ² or less, and is selected from the group consisting of a polyamine resin, a polyamidoamine resin, and a polyamide resin. Any one or more of the above are contained in an amount of 1.0 to 50 mass%.

First, the lower layer will be described.
In the present invention, a resin layer (b) containing polylactic acid as a main component (hereinafter sometimes simply referred to as a polylactic acid-based resin layer (b)) is a lower layer of a two-layer structure, and an adhesion layer that is in close contact with a metal plate And The polylactic acid as a main component is a polylactic acid-based resin containing 50% by mass to 100% by mass of polylactic acid. As a resin other than polylactic acid, for example, a polyolefin can be secured within the range in which the function defined in the present invention can be secured. If it is within, it may be added.

  The composition of the polylactic acid resin layer (b) is polylactic acid mainly composed of L-lactic acid and / or D-lactic acid, but copolymerized with other monomer components having ester-forming ability. It may be a copolymerized polylactic acid. For example, a copolymer of L-lactic acid and another hydroxycarboxylic acid, or a blend thereof, or a polyester component obtained by ester reaction of dicarboxylic acid and diol is copolymerized with a lactic acid component. Among them, a polylactic acid resin containing L-lactic acid as a main constituent is preferable and has high production stability.

  The molecular weight of the polylactic acid-based resin is preferably 50,000 to 500,000 in terms of weight average molecular weight because the balance between mechanical properties and moldability is good.

  The melting point of the polylactic acid resin is preferably 165 ° C. to 175 ° C. from the viewpoint of retort resistance.

  In the present invention, a polycarbodiimide compound is added to a polylactic acid resin. As the polycarbodiimide compound, a polymer obtained by polymerizing a diisocyanate compound can be suitably used. Among these, a polymer of zinc cyclohexyl methane carbodiimide, a polymer of tetramethylxylylene carbodiimide, and a polymer whose end is blocked with ethylene glycol or the like are desirable.

  The addition amount of the polycarbodiimide compound is 0.01 to 10 mass%, preferably 0.05 to 5 mass%, based on the total amount of the polylactic acid resin. If the addition amount is less than 0.01 mass%, a sufficient hydrolysis inhibiting effect cannot be obtained, and molecular chain breakage under a retort environment cannot be inhibited. On the other hand, if it exceeds 10 mass%, the decrease in melt viscosity becomes remarkable, resulting in deterioration of moldability.

  As a method for adding the polycarbodiimide compound, it is desirable to add the polycarbodiimide compound directly during the melt-kneading of the polylactic acid resin. For example, it is a method of adding polycarbodiimide at the melting part of the polylactic acid resin, or kneading the separately melted polycarbodiimide compound and the polylactic acid resin with a static kneader.

In the present invention, in addition to adding the polycarbodiimide compound to the polylactic acid-based resin, the residual orientation degree of the resin layer (b) containing polylactic acid as a main component can be controlled in the range of 5% to 30%. is important. Here, the residual orientation degree is a value obtained by an X-ray diffraction method and is defined as follows.
(1) For the oriented polylactic acid resin (or oriented polylactic acid resin film) before lamination and the oriented polylactic acid resin (or film) after lamination, the X-ray diffraction intensity is in the range of 2θ = 5 to 30 °. taking measurement.
(2) The X-ray diffraction intensities at 2θ = 5 ° and 2θ = 30 ° are connected by a straight line to obtain a baseline.
(3) The height of the highest peak appearing around 2θ = 17 ° is measured from the baseline.
(4) When the height of the highest peak of the film before lamination is P1, and the highest peak of the film after lamination is P2, P2 / P1 × 100 is the residual orientation degree (%).

  As a result of intensive studies by the present inventors, it has been found that the amount of water vapor entering the inside of the polylactic acid resin decreases as the residual orientation degree increases. By making the residual orientation degree 5% or more, the amount of water vapor reaching the inside of the polylactic acid-based resin is reduced, and in addition to the effect of adding the polycarbodiimide compound, the hydrolysis chain reaction in the retort sterilization treatment environment Can be suppressed. As the degree of residual orientation increases, the amount of water vapor that penetrates into the resin further decreases and the hydrolysis resistance becomes better, while the flexibility and elongation characteristics of the resin decrease. When the residual orientation degree exceeds 30%, follow-up to the can manufacturing process becomes insufficient, and peeling of the resin or film or tearing of the material occurs. From the above, by controlling the residual orientation degree of the resin layer (b) containing polylactic acid as a main component in the range of 5% to 30%, both hydrolysis resistance and resin moldability can be achieved.

  Next, the upper layer will be described.

  In this invention, the resin layer (a) which has polyester as a main component is formed in the upper layer of the resin layer (b) which has polylactic acid as a main component. Further, any one selected from the group consisting of (a) a polyamine resin, a polyamidoamine resin, and a polyamide resin in the resin layer (a) containing the polyester as a main component (hereinafter also referred to simply as a polyester resin layer (a)). By containing one or more kinds, hydrolysis resistance can be further improved. Details will be described below. The polyester as a main component means a polyester resin containing 50 mass% or more of a polyester resin in the resin component, and examples of the resin other than the polyester include a polyolefin resin.

  The composition of the resin layer (a) mainly composed of polyester as the upper layer is represented by terephthalic acid as a carboxylic acid component and polyethylene terephthalate made of ethylene glycol as a glycol component, but as other carboxylic acid component, isophthalic acid, Also included are phthalic acid, naphthalenedicarboxylic acid, adipic acid and the like, and copolymer resins in which the components are replaced with diethylene glycol, propylene glycol, butanediol, neopentyl glycol and the like as other glycol components. As an acid component, terephthalic acid is essential because of mechanical strength, heat resistance, chemical resistance, and the like, but further, by copolymerizing with isophthalic acid, flexibility, tear strength, and the like are improved. By copolymerizing the isophthalic acid component with the terephthalic acid component in the range of 10.0 mol% or more and 60.0 mol% or less, it becomes easy to ensure the thermophysical properties specified in the present invention, and deep drawability, after processing This is preferable because it functions to improve adhesion. As the glycol component, a low Tg (Tg = glass transition temperature) component having excellent flexibility such as ethylene glycol and propanediol and a rigid high Tg component having a ring structure such as neopentyl glycol and cyclohexanedimethanol are copolymerized. It is desirable to make it. While ensuring the thermophysical property prescribed | regulated by this invention, intensity | strength and a softness | flexibility can be controlled. Preferable examples include polyester resins in which the acid component is composed of 10 to 30 mol% isophthalic acid and 70 to 90 mol% terephthalic acid, and the glycol component is composed of 30 to 50 mol% ethylene glycol and 50 to 70 mol% propanediol. be able to.

The adhesion amount of the upper polyester resin layer (a) is specified in the range of 0.1 g / m ^{2 to} 5.0 g / m ² . If it is less than 0.1 g / m ² , the lower resin layer surface cannot be uniformly coated, and the effect prescribed in the present invention cannot be expected. On the other hand, if it exceeds 5.0 g / m ² , the residual stress inside the resin becomes excessive, and the strength of the resin layer may decrease. As a result, at the time of can manufacturing, the resin layer is easily broken, and the can body portion is torn starting from the resin layer. Therefore, the adhesion amount is in the range of 0.1 g / m ^{2 to} 5.0 g / m ² , and preferably in the range of 1.0 g / m ^{2 to} 2.0 g / m ² .

  The weight average molecular weight of the polyester resin layer (a) as the upper layer is preferably 10,000 or more and 40,000 or less, and more preferably 15,000 to 20,000. A polyester resin having a weight average molecular weight in such a range has an excellent balance between processability and strength, and has good deep drawability and adhesion after molding. By setting the molecular weight to 10,000 or more, the strength of the resin increases, and the resin follows the deformation without tearing during the deep drawing. Also in the subsequent retorting process, when a film is further formed on the upper layer, delamination from the trim end or the like can be suppressed against the thermal shrinkage of the film. Also, with respect to impact resistance after canning, the occurrence of defects can be suppressed and good performance can be obtained. On the other hand, if the molecular weight exceeds 40,000, the strength of the resin becomes excessive, and conversely, flexibility may be impaired. By setting it to 40000 or less, the balance between strength and flexibility can be maintained.

  Furthermore, the polyester resin layer (a) as an upper layer has a glass transition point of 30 ° C. or higher, a softening point when the resin layer is made of an amorphous resin, and a melting point of 130 ° C. when the resin layer is made of a crystalline resin. The above is preferable. When the resin-coated metal plate is stored and transported, it may be kept at a temperature of about 40 ° C. for a long time, so that the glass transition point needs to be 30 ° C. or higher. In addition, the retort sterilization treatment for food cans may take 1 hour or more at a high temperature of 120 ° C. or higher and is required to have sufficient heat resistance. Therefore, when the resin layer is an amorphous resin, JIS The softening point specified in K2425 needs to be 130 ° C. or higher. When the resin layer is a crystalline resin, the melting point specified in JIS K7121 needs to be 130 ° C. or higher, and more preferably 150 ° C. or higher. .

The polyester resin layer (a) contains the following component (a) in addition to the main component polyester resin.
(I) Any one or more selected from the group consisting of polyamine resin, polyamidoamine resin, and polyamide resin: 1.0 to 50 mass%
Preferably,
(B) Epoxy resin: 0.5 to 30 mass%
(C) Metal alkoxide compound and / or metal phyrate compound: 0.01 to 10 mass%
(A) One or more selected from the group consisting of polyamine resins, polyamide amine resins, and polyamide resins Polyamine resins, polyamide amine resins, and polyamide resins have a faster curing speed and form a tough film compared to melamine resins, etc. It is excellent in that it can be done. Because it has excellent curing characteristics compared to resin compositions made of polyester / melamine, epoxy / melamine, etc., cross-linking reaction between resins even in heat treatment of extremely short time (less than 1 second) such as heat fusion laminating. It becomes possible to polymerize.

  By forming a three-dimensional network by a crosslinking reaction, gas permeability and water vapor permeability can be significantly reduced. That is, (a) a polyester resin layer (a) containing at least one selected from the group consisting of a polyamine resin, a polyamidoamine resin, and a polyamide resin is formed on the resin layer (b) containing polylactic acid as a main component. By forming, permeation | transmission of the water vapor | steam in a retort sterilization process can be interrupted | blocked inside a polyester resin layer (a). Therefore, a drastic improvement in hydrolysis resistance can be expected by significantly reducing the amount of water vapor reaching the polylactic acid resin layer (b). In addition, since the cross-linked structure also improves the mechanical properties of the resin, a significant improvement in moldability can be expected.

  Specific examples of the polyamine resin include diethylenetriamine, triethylenetriamine, and triethylenepentamine as the aliphatic amine, and isophoronediamine as the alicyclic polyamine. In addition, an epoxy resin or acrylonitrile may be added to an aliphatic polyamine or modified by reacting formaldehyde and phenol to improve workability, reduce irritation, or improve mechanical properties. Examples of the aromatic polyamine include metaphenylenediamine, diaminodiphenylsulfonic acid, and diaminodiphenylmethane. Commercially available products include EPICRON EXB-353 manufactured by DIC Corporation, Ancamine 2596 manufactured by Air Products Japan Co., Ltd., Ancamine 2605, and the like.

  The polyamide amine resin and the polyamide resin are compounds synthesized by, for example, a dehydration condensation reaction between fat and fatty acid and polyamine. Commercially available products include Sanyo Kasei Polyamide L-15-3, Polyamide L-45-3, Air Products Japan Co., Ltd. Ancamide 2137, Sunmide 330, Sunmide X-2000, and the like.

  The addition amount of any one or more selected from the group consisting of a polyamine resin, a polyamidoamine resin, and a polyamide resin is in the range of 1.0 to 50 mass%. If the addition amount is less than 1 mass%, the curability is insufficient and the retort resistance is inferior, and if it exceeds 50 mass%, the workability may be deteriorated. More preferably, it is the range of 3.0-30 mass%.

(B) Epoxy resin (preferred conditions)
The epoxy resin mainly improves the adhesion of the polyester resin layer (a). The type of the epoxy resin is not particularly limited. However, in recent years, bisphenol A type epoxy resins are concerned about endocrine disrupting action. Therefore, the resin is preferably free from such concerns and does not contain bisphenol A. It is preferable to use an epoxy resin. Examples of the epoxy resin that does not contain bisphenol A include novolak type epoxy resins and biphenyl type epoxy resins, and a novolak type epoxy resin is particularly preferable. Examples of the novolak type epoxy resin include a cresol novolak type and a phenol novolak type. As a commercial product of a novolak type epoxy resin, Epicron N-665, 670, 673, 680, 690, 695, 730, 740, 770, 865, 870 manufactured by DIC Corporation, manufactured by Dow Chemical Co., Ltd. XD-7855, ECN-1273, 1299 manufactured by Asahi Kasei Epoxy Co., Ltd. and the like. Examples of the biphenyl type epoxy resin include YL6121H and YX7399 manufactured by Mitsubishi Chemical Corporation.

  The addition amount of the epoxy resin is preferably in the range of 0.5 to 30 mass%. When the ratio of the epoxy resin is lower than 0.5 mass%, the effect of improving the adhesion is poor, and conversely, when it exceeds 30 mass%, the workability may be deteriorated. More preferably, it is the range of 5-25 mass%.

(C) Metal alkoxide compounds and / or metal chelate compounds (preferred conditions)
The metal alkoxide compound and / or the metal chelate compound is at least one selected from the group consisting of a polyester resin as a main component of the polyester resin layer (a), (a) a polyamine resin, a polyamidoamine resin, and a polyamide resin, (B) Reacts with epoxy resin. A crosslinking reaction proceeds between the functional group of each resin and the metal alkoxide compound and / or metal chelate compound. This cross-linking reaction has a significantly faster curing rate compared to the case where there is no metal alkoxide compound and / or metal chelate compound, and as a result, excellent adhesion, workability, Retort resistance and corrosion resistance can be expressed. For example, existing laminated cans are baked at 180 ° C. or higher for several seconds to several minutes after laminating the film, and then the post-heating is used to cure the resin film to ensure the above various required performances. . However, in the present invention, the polyester resin layer (a) containing a metal alkoxide compound and / or a metal chelate compound can be formed by only heating for a short time of about 1 second when performing heat fusion lamination. Performance sufficiently equal to or higher than that obtained by sufficiently curing and post-heating can be obtained. Therefore, the post-heating step in the manufacturing process is not necessary, and the manufacturing efficiency is greatly improved. In addition, carbon dioxide emissions can be reduced, which can be a very useful technique in practice. Furthermore, by incorporating a metal into the film, the strength of the film is improved, and as a result, impact resistance and corrosion resistance are greatly improved. For the above reasons, the polyester resin layer (a) preferably further contains a metal alkoxide compound and / or a metal chelate compound.

  Examples of the metal alkoxide compound and / or metal chelate compound include alkoxide metal compounds such as aluminum, titanium, tin, and zirconium, and metal chelate compounds in which acetoacetic acid is coordinated to the metal. Among these, it is preferable to use a titanium alkoxide compound and / or a titanium chelate compound.

  The addition amount of the metal alkoxide compound and / or the metal chelate compound is preferably in the range of 0.01 to 10 mass%. If it is less than 0.01 mass%, the expected effect such as fast curability cannot be obtained. Conversely, if it exceeds 10 mass%, the resin film becomes hard and the workability becomes inferior. Sometimes it causes gelation. More preferably, it is the range of 0.1-7 mass%.

  Furthermore, by adding colorants such as dyes and pigments, the underlying metal plate can be concealed and various colors unique to the resin can be imparted. For example, by adding carbon black as a black pigment, it is possible to conceal the metal color of the base and to impart a high-class feeling of black to food cans. In addition, in this application, when adding a coloring agent in the resin layer (b1), the total of components, such as outer split (polyester resin, (i)-(e) related to resin performance, is 100 mass parts. In parts by weight of colorant (PHR)). That is, the amount of carbon black added is desirably 5 PHR or more and 40 PHR or less from the viewpoint of achieving both design properties and other required characteristics. If it is less than 5 PHR, the blackness is insufficient, the color tone of the base metal cannot be concealed, and a high-quality design may not be imparted. On the other hand, even if it exceeds 40 PHR, the blackness does not change, so the effect of improving the design is not obtained, and the structure of the polyester resin becomes brittle, so that the resin layer may be easily broken during can manufacturing. is there. The layer to which the colorant is added is preferably a polylactic acid resin layer (b) that is an adhesion layer. This is because the polyester resin layer (a) that forms the upper layer protects the deterioration of the design property due to the processing flaw that occurs during can making processing or the like.

  The particle size of carbon black is preferably in the range of 5 to 50 nm, but in view of dispersibility and color developability in the polylactic acid resin or polyester resin, the range of 5 to 30 nm is preferable.

  In addition to the black pigment, by adding a white pigment, the metallic luster of the base can be concealed, the printed surface can be sharpened, and a good appearance can be obtained. As a pigment to be added, it is necessary that an excellent design property can be exhibited after container molding. From such a viewpoint, inorganic pigments such as titanium dioxide can be used. Since the coloring power is strong and rich in spreadability, it is preferable because a good design property can be secured even after container molding. The amount of titanium dioxide added is desirably 5 to 40 PHR with respect to the target resin layer. If it is 5 PHR or more, sufficient whiteness can be obtained and good design properties can be secured. On the other hand, even if it is added in excess of 40 PHR, the whiteness is saturated, so it is desirable to make it 40 PHR or less for economic reasons. More preferably, it is the range of 10-30 PHR. In addition, the addition amount of a coloring agent is a ratio with respect to the resin layer which added the coloring agent.

When a bright color is desired on the container surface, use of an azo pigment is also suitable. Although it is excellent in transparency, it has a strong coloring power and a high spreadability, so that a bright appearance can be obtained even after the container is molded. As an azo pigment that can be used in the present invention, the color index (abbreviation: CI) is at least one of Pigment Yellow 12, 13, 14, 16, 17, 55, 81, 83, 139, 180, and 181. One type can be mentioned. In particular, from the viewpoints of vividness of color tone (bright color) and bleeding resistance in retort sterilization environment (inhibition ability against the phenomenon that pigments are deposited on the film surface), it has a large molecular weight and is soluble in PET resin. A poor pigment is desirable. For example, C.I. having a benzimidazolone structure having a molecular weight of 700 or more. I. Pigment Yellow 180 is more preferably used.
The addition amount of the azo pigment is desirably 10 to 40 PHR with respect to the target resin layer. If the addition amount is 10 PHR or more, it is preferable because coloring is excellent. When the color is 40 PHR or less, the transparency becomes high and the color tone is rich.

Next, a method for producing an example of a polylactic acid-based resin-coated metal plate for containers having the resin layer having the above-described at least two-layer structure on one side or both sides will be described.
In the polylactic acid resin-coated metal plate for containers of the present invention, first, the polyester resin layer (a) as the upper layer is formed on the surface of the polylactic acid resin layer (b) as the lower layer. Next, this multilayer resin layer (film) is laminated on the surface of the metal plate.

(Method for producing polylactic acid resin layer (b))
The polylactic acid-based resin layer (b) is preferably a film or sheet obtained by extrusion molding using a polylactic acid-based composition containing poly-L-lactic acid and poly-D-lactic acid, preferably twice or more in one direction. More preferably, by stretching 2 to 12 times, a polylactic acid-based stretched film having excellent heat resistance and transparency can be obtained. The upper limit of the stretching ratio is not particularly limited. However, if it exceeds 12 times, there is a risk that the film may be broken and cannot be stably stretched. The film or sheet obtained by extrusion molding is preferably at least twice in the vertical direction, at least twice in the horizontal direction, more preferably 2 to 7 times in the vertical direction and 2 to 7 times in the horizontal direction, and still more preferably. Is stretched 2.5 to 5 times in the longitudinal direction and 2.5 to 5 times in the transverse direction, whereby a polylactic acid-based stretched film (polylactic acid-based biaxially stretched film) having excellent heat resistance and transparency is obtained. The upper limit of the draw ratio is not particularly limited. However, if it exceeds 7 times, the film may be broken and may not be stably stretched. When the polylactic acid-based stretched film according to the present invention is subjected to heat treatment at 140 to 220 ° C., more preferably 150 to 200 ° C. for 1 second or more, more preferably 3 to 60 seconds after stretching, the heat resistance is further improved. .

(Method for forming polyester resin layer (a))
A method for forming the upper polyester resin layer (a) on the surface of the polylactic acid resin layer (b) will be described. A polyester resin having a resin composition defined in the present invention is dissolved in an organic solvent to form a coating solution. Next, the prepared coating solution is applied to the surface of the polylactic acid resin layer (b) (polylactic acid stretched film) and dried. The formation method is not particularly limited, but the above-described method is suitable because it is suitable for the purpose and application of the present invention.

  Examples of the organic solvent for dissolving the polyester resin defined in the present invention include aromatic hydrocarbon solvents such as toluene and xylene, ketone solvents such as methyl ethyl ketone and cyclohexanone, and ester solvents such as ethyl acetate and ethylene glycol monoethyl ether acetate. One or more of these can be appropriately selected and used. In addition, additives such as polyamine resin, polyamide amine resin, polyamide resin, epoxy resin, metal alkoxide compound, metal chelate compound, colorant, carbon black, and azo pigment as defined in the present invention are dissolved in an organic solvent. Alternatively, it is desirable to use in a dispersed manner. In this case, it is preferable to use a dispersant in combination because the uniformity of the additive can be imparted.

As a method of applying the coating liquid to the polylactic acid-based resin layer (b) (polylactic acid-based stretched film), known coating means such as a roll coater method, a die coater method, a gravure method, a gravure offset method, and a spray coating method can be used. Although applicable, the gravure roll coating method is most preferred. As drying conditions after application of the coating liquid, 80 to 170 ° C. for 20 to 180 seconds, particularly 80 to 120 ° C. for 60 to 120 seconds are preferable. The adhesion amount of the polyester resin layer (a) after drying is preferably in the range of 0.1 g / m ² or more and 5.0 g / m ² or less as defined in the present invention.
(Lamination method to metal plate and adjustment method of residual orientation)
In the present invention, for example, a metal plate is heated to a certain temperature or higher with a heating device, and a polylactic acid resin film is brought into contact with the metal plate using a pressure-bonding roll (hereinafter referred to as a laminate roll) on the surface thereof for heat fusion. A method of wearing can be used. In addition, the residual orientation degree of the polylactic acid-type resin layer (b) prescribed | regulated by this invention can be achieved by adjusting lamination conditions.

  The temperature of the metal plate at the start of heat fusion is set to a range of + 5 ° C. to + 20 ° C. based on the melting point of the polylactic acid resin of the polylactic acid resin layer (b). In order to ensure the interlaminar adhesion of the metal plate-polylactic acid resin layer (b) by the thermal fusion method, it is necessary to heat the resin at the adhesion interface. By setting the temperature of the metal plate to a temperature range of + 5 ° C. or higher based on the melting point of the polylactic acid-based resin layer (b), the polylactic acid-based resin is heat-fluidized and the wetting at the interface becomes mutually good. Excellent adhesion can be obtained. Therefore, the lower limit of the temperature at the start of heat fusion is set to the melting point of the polylactic acid resin + 5 ° C.

  On the other hand, the temperature at the start of heat-sealing was set to + 20 ° C. or less because even if it exceeds + 20 ° C., further improvement of adhesion cannot be expected, and the polylactic acid resin film is excessively melted, resulting in lamilol This is because there is a concern that problems such as surface roughness due to the embossing of the surface may occur. Therefore, the upper limit of the temperature at the start of heat fusion is set to the melting point + 20 ° C.

  At the time of heat-sealing, the polylactic acid resin layer (b) is heated at a temperature exceeding the melting point, so it melts not a little, the crystal structure is broken, and the degree of orientation is lowered. Therefore, the degree of residual orientation can be controlled by adjusting the amount of heat given from the metal plate to the polylactic acid resin layer (b) during heat fusion. If the amount of heat applied is large, the polylactic acid resin layer (b) is melted, so that the degree of residual orientation is lowered. If the amount of heat applied is small, melting of the polylactic acid resin layer (b) is suppressed, and the residual orientation The degree will increase.

  In order to set the residual orientation degree of the polylactic acid-based resin layer (b) to 30% or less, when the temperature at the start of thermal fusion is set to the melting point of the polylactic acid-based resin + 5 ° C., the fusion time is 10 msec or more. Necessary.

  On the other hand, in order to set the residual orientation degree to 5% or more, when the temperature at the start of thermal fusion is the melting point of the polylactic acid resin + 20 ° C., the fusion time needs to be 30 msec or less.

  In order to achieve such lamination conditions, in addition to high-speed operation of 150 mpm or more, cooling during heat sealing is also necessary. For example, when the laminate roll is an internal water-cooling type, it is possible to prevent each resin layer from being heated excessively by passing the cooling water. Furthermore, it is preferable because the heat history of the polylactic acid resin layer (b) and the upper polyester resin layer (a) can be controlled by changing the temperature of the cooling water.

The pressurization of the laminate roll is preferably 9.8 to 294 N / cm ² (1 to 30 kgf / cm ² ) as the surface pressure. When the temperature is less than 9.8 N / cm ² , even if the temperature at the start of thermal fusion is not less than the melting point of the polylactic acid resin + 5 ° C. and sufficient fluidity is ensured, the resin is not pushed onto the metal surface. Since the spreading force is weak, sufficient coverage cannot be obtained, and as a result, performance such as adhesion and corrosion resistance may be affected. On the other hand, if it exceeds 294 N / cm ^2, there is no inconvenience in the performance of the laminated metal plate, but the force applied to the laminating roll is large and equipment strength is required, resulting in an increase in size of the apparatus, which is uneconomical.

Examples of the present invention will be described below.
Preliminarily dried polylactic acid resin (Teramac (registered trademark)) and polycarbodiimide compound are premixed at the ratios shown in Tables 1 and 2 and then supplied to a twin-screw kneading extruder and melted at 250 ° C. After kneading, it was extruded and cooled and solidified on a cooling drum to obtain an unstretched film. Next, biaxial stretching and heat setting were performed to obtain a polylactic acid-based stretched film. The draw ratio was 4.5 times in the longitudinal direction and 4.5 times in the transverse direction.
Next, masses shown in Tables 1 and 2 are various additives such as a resin mainly composed of a polyester resin, a polyamine resin, a polyamidoamine resin, a polyamide resin, an epoxy resin, a metal alkoxide compound, a metal chelate compound, and a colorant. The coating solution was prepared by dissolving in a mixed solvent of toluene and methyl ethyl ketone at a ratio. This coating solution is applied to the upper layer of the polylactic acid film obtained above using a gravure roll coater and dried, the amount of the resin layer after drying is adjusted, and a film comprising a resin layer having a two-layer structure 4a and 4b were prepared. From Table 1, a film 4a that becomes the inner surface of the container after forming the container was prepared, and from Table 2, a film 4b that became the outer surface of the container after forming the container was prepared. The drying temperature was in the range of 80 to 120 ° C.

In addition, the chrome plating steel plate was used as a metal plate. A steel sheet having a thickness of 0.18 mm and a width of 977 mm that had been subjected to cold rolling, annealing, and temper rolling was manufactured by degreasing and pickling, followed by chrome plating. 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 with the metal band heating apparatus 2, and the container roll is formed on one surface of the chrome-plated steel sheet 1 with the laminating roll 3. The film 4a prepared from Table 1 is laminated (heat-sealed) as a film that later becomes the inner surface of the container, and the film 4b prepared from Table 2 is laminated as the film that becomes the outer surface of the container after forming the container on the other surface. (Heat fusion).
The laminating start temperature was in the range of the melting point of the film + 5 ° C. to 20 ° C., and the fusing time was in the range of 10 msec to 30 msec. The degree of remaining orientation of the polylactic acid-based stretched film was adjusted by changing the temperature and time and increasing or decreasing the amount of heat applied to the film.
Thereafter, the metal band cooling device 5 was used for water cooling to produce a polylactic acid resin-coated metal plate. FIG. 2 shows a cross-sectional structure of a polylactic acid resin-coated metal plate.
The laminating roll 3 was an internal water-cooling type, and cooling water was forcibly circulated during laminating to perform cooling during film adhesion.

Next, the properties of the polylactic acid resin-coated metal plate obtained above and the resin layer on the metal plate were measured and evaluated by the following methods (1) to (3), respectively.
(1) Formability After applying wax to a polylactic acid resin-coated metal plate, a disc having a diameter of 200 mm was punched out to obtain a shallow drawn can with a drawing ratio of 2.00. Next, the drawn can was processed at a drawing ratio of 2.20, and further drawn again so as to obtain a drawing ratio of 2.50. Then, after performing doming forming according to a conventional method, trimming was performed, and then neck-in-flange processing was performed to form a deep drawn can. Focusing on the neck-in portion of the deep-drawn can obtained in this way, the degree of damage was visually observed on the inner and outer surfaces of the can.
(About the score)
◎: No damage to the film after molding for any of the inner and outer film ○: Can be molded to any of the inner and outer film, but partially damaged film ×: Can broken Canned and not moldable (2) Hydrolysis resistance Cans that were moldable (greater than or equal to) in the moldability evaluation of (1) above were targeted. The retort sterilization treatment was performed at 130 ° C. for 90 minutes, and then the damage state of the can outer film was visually observed.
(About score)
◎: No damage to the film ○: No damage is observed, but partial discoloration (whitening) of the film is observed ×: The film is hydrolyzed and a broken portion is observed (3) Adhesiveness after processing Cans that were moldable (◯ or higher) in the moldability evaluation of (1) above were targeted. After filling the inside of the can with tap water, the lid was wrapped and sealed. Subsequently, retort sterilization was performed at 130 ° C. for 90 minutes, and a peel test sample (width 15 mm, length 120 mm) was cut out from the can body. Part of the film is peeled off from the long side end of the cut sample. The peeled film is opened in the direction opposite to the peeled direction (angle: 180 °), and using a tensile tester, a tensile speed of 30 mm / min. A peel test was conducted to evaluate the adhesion per 15 mm width. The film to be measured is a can inner surface film.
(Score)
A: 10.0 (N) / 15 (mm) or more B: 5.0 (N) / 15 (mm) or more, less than 10.0 (N) / 15 (mm) x: 5.0 (N) / Table 3 shows the results obtained as described above.

  From Table 3, it can be seen that the examples of the present invention have good performance with respect to moldability, hydrolysis resistance, and adhesion after molding required for the container material. On the other hand, the comparative example outside the scope of the present invention is inferior in any of the characteristics.

  The polylactic acid-based resin-coated metal plate for containers of the present invention is excellent in hydrolysis resistance in a retort processing environment and satisfies many characteristics required for food canned materials. It is a material suitable for container use.

1 Metal plate (chrome plated steel plate)
2 Metal belt heating device 3 Laminate roll 4a Film 4b Film 5 Metal belt cooling device

Claims (3)

A polylactic acid-based resin-coated metal plate for containers having at least a two-layered resin layer on one or both surfaces, wherein the two-layered resin layer satisfies the following (A) and (B): A polylactic acid resin-coated metal plate for containers.
(A) The upper resin layer (a) is a resin layer mainly composed of a polyester having an adhesion amount of 0.1 g / m ² or more and 5.0 g / m ² or less, and (a) a polyamine resin or a polyamidoamine. 1.0-50 mass% of any one or more chosen from the group which consists of resin and a polyamide resin is contained.
(B) The lower resin layer (b) is a resin layer mainly composed of polylactic acid having a residual orientation degree of 5% or more and 30% or less, and the polylactic acid contains 0.01 to 10% by mass of a polycarbodiimide compound. contains.   The polylactic acid resin-coated metal sheet for containers according to claim 1, wherein the resin layer (a) further contains (b) an epoxy resin.   The polylactic acid resin-coated metal plate for containers according to claim 1 or 2, wherein the resin layer (a) further contains (c) a metal alkoxide compound and / or a metal chelate compound.