JPS6157873B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to coating compositions, and more particularly to dispersion coatings for metal containers. Food and beverage cans have traditionally been made of iron, tin,
Containers made of materials such as aluminum have been used. In order to protect the contents packed in these containers from contamination by the container, i.e. metal, the interior of the container is coated with an organic paint that is essentially inert to the contents and provides an effective barrier between the container and the contents. It needs to be covered with. Currently, phenol-epoxy resins, epoxy-epoxy resins dissolved in organic solvents, and
Organic paints such as urea resins are used as protective coatings. These resins exhibit excellent adhesion to metal materials, act as an effective barrier between the container and the contents, and have high barrier properties against water and other contents. In this sense, it is suitable as an inner surface coating for metal containers (hereinafter referred to as can inner surface coating). However, in reality, various external forces are applied during the can manufacturing process, so the resin that serves as the base of the paint must be highly flexible in addition to the above-mentioned properties. When a thermosetting resin such as a phenol-epoxy resin or an epoxy-urea resin is heated on a metal plate, its properties such as barrier properties, adhesion to contents, and flexibility of the resin change greatly depending on the heating conditions. It is difficult to maintain all of these properties at appropriate levels simply by adjusting the heat treatment conditions. In fact, during the can manufacturing process, when forming processes such as press molding of can lids, flanging of can bodies, and seaming are performed, damage to the paint film and partial peeling are observed. In this case, the workability of the coating film is not sufficient, and even the slightest damage causes areas with poor adhesion, and corrosion of the exposed metal progresses. As a result, the flavor and freshness of the contents are impaired, and the original function of the food packaging material, which is to protect the food over a long period of time, is not fully fulfilled. Therefore, in the actual can manufacturing process, the above-mentioned phenol-epoxy resin or epoxy-urea resin is applied to the metal as an undercoat paint, and then another paint is coated over it to avoid the drawbacks of these thermosetting resin single coatings. The current situation is that they are making up for it. In this case, the resin used for surface coating is vinyl chloride-vinyl acetate copolymer,
Examples include vinyl chloride-vinyl acetate-maleic anhydride copolymer. Furthermore, although a single coating film of these PVC resins has excellent barrier properties and processability, it has the disadvantage of poor adhesion to metal materials. From the above results, regardless of whether thermosetting resin or thermoplastic resin is used, it has a wide range of properties required for can interior coatings, including barrier properties against contents, adhesion, processability, hygiene, flavor, and boiling resistance. It can be said that there are almost no resins that can maintain properties at a high level. In this sense, the types of resins that can be used as paints for the inside of cans are extremely limited, but there is currently a strong desire for the emergence of resins with the above-mentioned suitability. Recently, it has been proposed to use a thermoplastic polyester resin consisting of a combination of an aromatic dibasic acid or an aliphatic dibasic acid and a diol component as a paint for the inside of a can, which improves adhesion to metal materials and corrosion of metal materials. It has a wide range of suitability for combinations of properties such as metal elution suppression effect, workability of coated metal materials, hygiene properties such as extraction resistance of coating films, workability of coating, and wide applicability to various contents. Are known. However, in general, when considering the properties of thermoplastic polyester resins from the viewpoint of composition, among aromatic dibasic acids, for example, as the amount of terephthalic acid increases, properties such as boiling resistance improve, but conversely, the solubility or dispersion in solvents improves. Properties such as hardness, film-forming properties, adhesion to metals, and processability tend to decrease. On the other hand, resins with a large amount of amorphous components such as isophthalic acid and orthophthalic acid have low solubility or dispersibility in solvents.
Properties such as film-forming properties and adhesion to metals improve, but on the other hand, properties of the coating film such as boiling resistance deteriorate.
Moreover, the same thing is recognized about the diol component. For example, when a crystalline component such as ethylene glycol or 1,4-butanediol is used as a diol component, the resulting coating film improves properties such as boiling resistance, but conversely, it improves solubility or dispersibility in solvents, Properties such as film properties and processability deteriorate. Even when other diols are used, they often exhibit contradictory properties, such as boiling resistance and solubility or dispersibility in solvents. This problem is solved by using a certain amount of aliphatic dibasic acid or polyether type diol in combination with the acid. However, when combining these dibasic acid and diol components to obtain a resin that maintains the desired properties in a well-balanced manner, the monomers that will become the thermoplastic polyester resin component must be selected very strictly. Not only is this inconvenient, but the use of aliphatic dibasic acids, polyether diols, etc. can improve the boiling resistance, film forming properties, solubility or dispersibility in solvents, processability, etc. of the coating film. Trying to maintain a well-balanced property is actually quite difficult. The present inventors have noted that thermoplastic polyester resin has extremely good properties such as hygiene, adhesion to metal containers, and processability. We have carefully considered how to maintain a good balance between contradictory properties such as dispersibility and boiling resistance. As a result, as a method to take advantage of the advantageous properties that arise because thermoplastic polyester resin is linear and has a high molecular weight, it is necessary to react the terminal hydroxyl group of thermoplastic polyester resin with a polymerizable unsaturated carboxylic acid or an acid anhydride thereof. A stable organic solvent-dispersed polyester copolymer with improved dispersibility in organic solvents was obtained by carrying out a radical polymerization reaction using polymerizable unsaturated bonds.
Confirm that the obtained copolymer is free of extractables and has good hygiene properties, and also shows good results in terms of boiling resistance, adhesion to metal containers, processability, chemical resistance, etc. The present invention was achieved by confirming the following. In the present invention, after carrying out an esterification reaction () of a thermoplastic polyester resin having a terminal hydroxyl group and a polymerizable unsaturated carboxylic acid or its acid anhydride,
A polyester copolymer obtained by radical polymerization of a mixture of a (meth)acrylic acid derivative, (meth)acrylic acid and/or styrene, or a mixture of this and liquid polybutadiene is dispersed in an organic solvent. The present invention relates to a paint for metal cans characterized by the following. The thermoplastic polyester resin according to the present invention has good properties as a resin film itself such as boiling resistance, but it is a resin with poor properties such as solubility or dispersibility in solvents and stability as a paint, that is, a resin with a low molecular weight. Thermoplastic polyester resins that are large, highly crystalline, and have few functional groups are preferred. on the other hand,
Some thermoplastic polyester resins are known to dissolve or disperse in various solvents at room temperature, but these types of resins often have poor properties as coatings, such as boiling resistance. One of the reasons for this is the low crystallinity of this resin. One way to improve this drawback is to introduce functional groups into the resin so that it is possible to take measures to improve the physical properties. According to the present invention, even if the thermoplastic polyester resin itself is not resistant, by appropriately selecting a monomer or liquid polymer having a polymerizable unsaturated bond to be reacted, carboxyl groups and hydroxyl groups can be added to the resin. A coating film with excellent physical properties can be obtained by self-crosslinking or reaction with various resins such as melamine resin, epoxy resin, phenol resin, and urea resin. Therefore, in the present invention, the thermoplastic polyester resin is not limited to only high molecular weight and highly crystalline resins. Examples of aromatic dibasic acids that can be used in the synthesis of the thermoplastic polyester resin used in the present invention include dibasic acids such as terephthalic acid, isophthalic acid, and orthophthalic acid. In particular, as the content of terephthalic acid increases, the boiling resistance of the coating film becomes better, but the adhesion to metal becomes poorer. As the content of isophthalic acid and orthophthalic acid increases, the adhesion to metal,
Although the solubility or dispersibility in solvents is good, the processability such as bending is not necessarily good. Aliphatic dibasic acids can also be used to balance various properties. Examples include dibasic acids such as adipic acid, sebacic acid, malonic acid, and succinic acid, but in general, the use of aliphatic dibasic acids is effective in increasing the solubility or dispersibility of the resin, but it also has the effect of increasing the solubility or dispersibility of the resin. The properties of the membrane deteriorate. Therefore, the types and amounts of aromatic dibasic acids and aliphatic dibasic acids must be appropriately combined. On the other hand, examples of diols include ethylene glycol, propylene glycol, 1,3-butanediol, 1,4-butanediol, neopentyl glycol, diethylene glycol, dipropylene glycol, 1,6
- Obtained from one type or a combination of two or more types of hexanediol and the like. It is also possible to use polyether glycols, but their type and amount are determined by the combination with other diols. Polymerizable unsaturated carboxylic acids or their acid anhydrides used in the reaction () of the present invention include monobasic acids such as acrylic acid and methacrylic acid, itaconic acid,
Examples include dibasic acids such as maleic acid, succinic acid, and fumaric acid, and acid anhydrides such as itaconic anhydride and maleic anhydride. The reaction () is carried out by heating and dissolving a thermoplastic polyester resin having a terminal hydroxyl group in a single or mixed solvent such as toluene, xylene, chloroform, cellosolve acetate, methyl ethyl ketone, dioxane, tetrahydrofuran, or cyclohexanone, and reacting with the hydroxyl group of the thermoplastic polyester resin. 0.5 to 2.5 equivalents of polymerizable unsaturated carboxylic acids or their acid anhydrides under a nitrogen stream
React for 1-5 hours at a temperature of 140-160°C. The reaction is allowed to proceed by distilling off water in a conventional manner, but at this time it is possible to use a small amount of an ordinary esterification catalyst and polymerization inhibitor in order to advance the esterification reaction. The end point of esterification is confirmed by measuring the hydroxyl value of the resin. The reaction () of the present invention can be carried out in the same manner as ordinary radical polymerization of the product obtained from the reaction () and a polymerizable unsaturated compound. Examples of polymerizable unsaturated compounds include acrylic acid derivatives such as methyl acrylate, ethyl acrylate, butyl acrylate, 2-ethylhexyl acrylate, and 2-hydroxyethyl acrylate, methyl methacrylate,
Ethyl methacrylate, isobutyl methacrylate,
2-ethylhexyl methacrylate, 2-hydroxyethyl methacrylate, lauryl methacrylate,
Methacrylic acid derivatives such as glycidyl methacrylate, styrene, vinyl acetate, acrylonitrile,
Examples include liquid 1,2-polybutadiene and liquid 1,4-polybutadiene. In the reaction (), a radical polymerization catalyst such as azobisisobutyronitrile, benzoyl peroxide, dicumyl peroxide, etc. is usually used. The amount of the polymerizable unsaturated compound to be used is 1 to 30% by weight based on the total amount including the thermoplastic polyester resin, the polymerizable unsaturated carboxylic acid or their acid anhydride, and the polymerizable unsaturated compound. It is desirable to use it in a range of 5 to 20% by weight. If it is less than 1% by weight, it is difficult to obtain the desired dispersion type paint, whereas if it is more than 30% by weight, although it is possible to obtain a dispersed type paint, the processability of the paint film may be reduced or the paint film may become opaque. Shortcomings arise. By the reactions () and () according to the present invention, a polyester copolymer, that is, a resin in which a polymer capable of improving dispersibility in organic solvents is bonded to the end of a thermoplastic polyester is obtained.
In addition, in the reactions of () and (), for example, the monomer or polymer having a polymerizable unsaturated bond does not bond with the esterified product of the thermoplastic polyester resin and the polymerizable unsaturated carboxylic acid, and the monomer or polymer does not bond with the esterified product of the thermoplastic polyester resin and the polymerizable unsaturated carboxylic acid. Although it is possible that some of the polymer may be formed depending on the reaction conditions, there is no problem as long as it does not adversely affect the coating film obtained by coating. Commonly used additives can also be used in combination with the dispersion type paint of the present invention. The dispersion type paint according to the present invention has a milky white appearance, has a nonvolatile content of 15 to 40% by weight, and is an extremely stable paint that does not cause any separation or sedimentation of the resin even when stored at 40°C. be. The polymerizable unsaturated compound used in the reaction of () copolymerizes with the thermoplastic polyester resin end double bond introduced by the polymerizable unsaturated carboxylic acid or these acid anhydrides, thereby forming the polyester resin end end. It is thought that this part increases the affinity with the organic solvent and is stably dispersed in the organic solvent. Even if the thermoplastic polyester resin itself is not dissolved or dispersed in an organic solvent at room temperature, a stable dispersion type coating material can be obtained according to the present invention. Therefore, the present invention can be particularly effectively applied when the thermoplastic polyester resin itself has good properties as a coating film, such as boiling resistance, but lacks solubility or dispersibility. The dispersion-type paint according to the present invention has an extremely excellent coating film transparency, unlike a blend of a simple thermoplastic polyester resin and a polymer of the polymerizable unsaturated compound used in the reaction (2). When the dispersion type paint of the present invention is used as a can inner surface paint, it can be applied directly to the surface of the metal container material, but conventional epoxy-phenolic resin,
It can also be used as a top coating agent on epoxy-urea resins, melamine-epoxy resins, etc. In any of these cases, properties such as film-forming properties, transparency, adhesion, boiling resistance, consumption of potassium permanganate, and processability are good. Any method such as spray coating, roll coating, knife coating, or air knife coating may be used to apply the dispersion type coating of the present invention onto metal materials or various types of plastics. The coating method must enable the protective coating to function as an effective barrier between the container and its contents. In this sense, the resin coating thickness also plays an important role. Considering unavoidable pinholes, it is practically desirable to use the thermosetting resin in combination with the thermosetting resin. If the organic solvent-dispersed paint of the present invention is used as a paint for the inside of a can, not only will the characteristics of the thermoplastic polyester resin due to its high molecular weight be taken advantage of, but it will not only dissolve but also be dispersed in the organic solvent at room temperature. Even thermoplastic polyester resins that cannot be dispersed stably in solvents. Moreover, since it is a dispersed paint, it is also possible to synthesize a paint with a high resin concentration and low viscosity. Generally speaking, a certain coating thickness is required to impart barrier properties and contents protection functions to the coating for the inside of a can, but if a coating with a low resin concentration is used for this purpose, the coating may sag or the coating may sag. Problems arise, such as thick coating on the bottom. At the same time, this may cause brittleness to occur at the bottom of the can. The dispersion-type paint of the present invention not only allows for the preparation of a paint with a high resin concentration and low viscosity, but also makes it easier to obtain the desired coated product.
It is very advantageous in terms of operational safety, air pollution, and economy. Next, the present invention will be explained by examples. Example 1 A thermoplastic polyester resin was synthesized in advance as follows. For dimethyl terephthalate (0.75 mol), dimethyl isophthalate (0.25 mol), ethylene glycol (3 mol) and total monomers
Mix 0.3 wt% titanium tetrabutoxide,
The reaction is carried out at 180° C. for 1 to 2 hours under a nitrogen stream, and the reaction is allowed to proceed while measuring the amount of methanol removed.
After the demethanol reaction is completed, the reaction system is reduced in pressure (1 mmHg) and the temperature is raised to 220°C in about 30 minutes. The reaction is carried out at 220°C for about 3 hours to accelerate the deglycol reaction, increasing the viscosity of the resin and producing a thermoplastic polyester resin. The molecular weight and softening point of the obtained thermoplastic polyester resin were 23,000 and 73°C, respectively. The resulting thermoplastic polyester resin was dissolved in cyclohexanone at 120° C. and applied to a tin plate using a 6 mil applicator while hot in a 5 μm coating. A transparent coating film was obtained, and no whitening or other abnormalities were observed even when subjected to a boiling test at 100°C for 30 minutes. Table 1 shows the results of various tests shown in Table 1 regarding the obtained coating film. However, the polyester resin was not dissolved or dispersed at all in single or mixed solvents such as cyclohexanone, cellosolve acetate, xylene, and methyl ethyl ketone at room temperature. Next, 50 grams of the thermoplastic polyester resin was mixed with xylene/cellosolve acetate (weight ratio 1/
Mix with the mixed solvent of 1), then mix 3 grams of methacrylic acid and 0.05 grams of P-toluenesulfonic acid (catalyst), raise the temperature to 150 ° C., react for 2 hours, and distill off water. The esterification reaction proceeded. It can be confirmed from the measured value of the amount of dehydration that the esterification reaction proceeds almost quantitatively. After confirming that the esterification reaction has been completed, a monomer consisting of styrene/methyl methacrylate/ethyl acrylate/methacrylic acid (weight ratio 20/20/40/20) is added to an esterified product of thermoplastic polyester resin and methacrylic acid. It was added dropwise at a rate of 10% by weight.
The reaction was carried out at 100° C. for 2 hours in the presence of 3% by weight of benzoyl peroxide based on the monomer. As the reaction progresses, the state changes from a transparent state to a suspended state. After the reaction was completed, the nonvolatile content was further adjusted to 35% by weight with a mixed solvent of xylene/cellosolve acetate (weight ratio 1/1) to obtain a dispersed paint. The obtained dispersed paint was stored at 40°C for 14 days, but no abnormalities such as resin separation or sedimentation were observed. Further, the dispersion type paint was applied to a tinplate plate with a film thickness of 5 μm using a 6 mil applicator, heat treated at 170° C. for 10 minutes, and the properties of the film were examined. The results are shown in Table 1.

[Table] The polyester copolymer according to the present invention is not only stable as a dispersion type paint, but also has good adhesion properties, which is inferior to that of the thermoplastic polyester resin itself. Examples 2 to 9 In the same manner as in Example 1, dispersion type paints were prepared with the compositions shown in Table 2, and the results of testing for each test method are shown in Table 3.

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[Table] Comparative Example 1 A resin was separately synthesized using the composition of the polymerizable unsaturated compound used in the second stage reaction shown in Example 1 (number average molecular weight 26,000). This resin was mixed at a ratio of 20% by weight based on the esterified thermoplastic polyester resin. When heated at 100° C. for 2 hours using xylene/cellosolve acetate (1/1) as a solvent, it appeared slightly cloudy in appearance. When it was further cooled to room temperature, it turned white. Similarly, when a coating was applied, the coating became completely cloudy and high transparency could not be obtained. Comparative Example 2 A separate resin was synthesized using the same types and amounts of polymerizable unsaturated compounds used in the first and second stage reactions shown in Example 2 (number average molecular weight
29000). When this resin was mixed in an amount of 10% by weight based on the thermoplastic polyester resin and heated to 100°C using cyclohexanone as a solvent, it gelated in about 40 minutes. A stable dispersion could not be obtained by simply mixing high molecular weight resins. Comparative Example 3 An attempt was made to dissolve or disperse a thermoplastic polyester resin obtained from the composition shown in Example 2 in cyclohexanone at room temperature. Of course, it almost dissolves,
It did not disperse and a stable dispersion could not be obtained. Next, the resin liquid was heated to 110°C, and the resin melted when heated. Apply the resin solution to a tin plate while hot.
It was coated with a film thickness of μ and heat treated at 170°C for 10 minutes. The coated board after heat treatment was subjected to a boiling test at 100°C for 30 minutes. No whitening of the paint film or other abnormalities were observed. Separately, the coated plate was examined for film forming properties, clear properties, adhesion, potassium permanganate consumption, processability, etc., and showed good properties except for adhesion. However, the thermoplastic polyester resin liquid obtained from Comparative Example 3 has a major drawback in that it cannot be applied directly to a substrate at room temperature.

Claims (1)

[Claims] 1 A mixture of a (meth)acrylic acid derivative, (meth)acrylic acid and/or styrene after an esterification reaction () of a thermoplastic polyester resin having a terminal hydroxyl group and a polymerizable unsaturated carboxylic acid or its acid anhydride, Or a metal can paint characterized by dispersing in an organic solvent a polyester copolymer obtained by radical polymerization () of a mixture of the same and liquid polybutadiene.