Patent Description:
Packaging materials for the packaging of foods, pharmaceutical products, or similar articles are expected to prevent the spoiling of the contents, oxidization caused by oxygen in particular. To cope with this demand, the industry has used a barrier film made of resin, which is acknowledged to have relatively high oxygen barrier properties, or a laminate made using such a barrier film as a film substrate (multilayer film).

The oxygen barrier resin has been one that contains a hydrogen-bonding group, which is highly hydrophilic, in its molecule, typified by polyacrylic acid or polyvinyl alcohol. Packaging materials made of such resins deliver excellent oxygen barrier properties under dry conditions. In conditions of high humidity, however, they have the disadvantage that the hydrophilicity of the resins causes a great decrease in their oxygen barrier properties.

To overcome such disadvantages, a known method for preparing a gas barrier packaging material includes stacking a layer of a polycarboxylic acid-based polymer and a layer containing a polyvalent metal compound on top of each other on a substrate and allowing the two layers to react to form a polyvalent metal salt of polycarboxylic acid. The production of such a gas barrier packaging material, however, is cumbersome; it requires multiple coating solutions and multiple rounds of coating.

PTL <NUM> discloses a gas barrier layer formed from a gas barrier layer-forming coating material containing a polyalcohol-based polymer (A) and a polycarboxylic acid-based polymer (B); and a resin layer formed from a resin coating material containing either a monovalent metal compound, or a monovalent metal compound and a bivalent or higher metal compound.

PTL <NUM> and PTL <NUM> disclose gas barrier laminates wherein the oxygen barrier layer is formed from a composition containing a carboxyl group-containing polymer, a polyvalent metal compound particle, a surfactant, and an organic solvent, and the water content is <NUM>,<NUM> ppm or less.

The present invention addresses the problem of providing a composition that delivers high barrier properties in fewer coating steps.

After extensive research, the inventors found that a particular composition for gas barrier purposes that contains a carboxyl-containing resin, a divalent metal compound, and an alcohol can solve this problem.

That is, the present invention is one that provides a composition for gas barrier purposes consisting of a carboxyl-containing resin (A), a divalent metal compound (B), and an alcohol (C), wherein alcohol (C) content of the composition is between <NUM> and <NUM> wt%, and water content of the composition is <NUM> wt% or less the carboxyl-containing resin (A) is a homopolymer of a monomer selected from acrylic acid, methacrylic acid, maleic acid, and itaconic acid; and the divalent metal compound (B) is selected from zinc oxide, magnesium oxide and calcium oxide.

The present invention, furthermore, is one that provides a gas barrier coating agent containing this composition for gas barrier purposes and a laminate having a substrate and a coat layer obtained by applying this coating agent.

In addition, the present invention is one that provides a packaging material having this laminate and the use of this packaging material in heat sterilization.

The composition according to the present invention is superior in storage stability because divalent metal compound(s) is kept stable therein. The composition, furthermore, is suitable for use as a coating agent for gas barrier purposes. The applied coating film exhibits high barrier properties in a one-component system, which means the composition delivers high barrier properties in fewer coating steps.

The laminate obtained by applying this composition to a substrate is suitable for use as a packaging material by virtue of its superior gas barrier properties. In particular, the laminate is suitable for use as a packaging material for which barrier properties are essential, such as one for foods, daily necessities, or electronic materials or for medical purposes.

The laminate, furthermore, is highly resistant to heat and wet heat, making it also suitable for use as a packaging material in heat sterilization, such as boiling or retorting.

The present invention provides a composition for gas barrier purposes consisting of a carboxyl-containing resin (A), a divalent metal compound (B), and an alcohol (C). The alcohol (C) content of the composition is between <NUM> and <NUM> wt%, and the water content of the composition is wt% or less.

According to the present invention, the carboxyl-containing resin (A) is a homopolymer of a monomer selected from acrylic acid, methacrylic acid, maleic acid, and itaconic acid.

Preferably, the carboxyl-containing resin (A) is one whose acid value is between <NUM> to <NUM> KOH/g because this leads to improved barrier performance. It is particularly preferred that the acid value be between <NUM> and <NUM> KOH/g. When the acid value of the resin (A) is <NUM> KOH/g or more, ionic bonding will proceed to a sufficient extent that the composition will achieve high barrier performance.

Acid value is the amount of potassium hydroxide in mg required to neutralize acid present in <NUM> of the sample. Specifically, the acid value can be measured by the method of dissolving a weighed sample in any solvent in which the sample dissolves, e.g., the solvent of toluene/methanol = <NUM>/<NUM> by volume, adding some drops of a <NUM>% alcoholic solution of phenolphthalein, and then adding a <NUM> mol/L alcoholic solution of potassium hydroxide dropwise and observing the point where the color changes. The acid value can be determined by the following equation for calculation.

If the sample is a solution of the resin, the acid value of the resin (mg KOH/g) can be determined by the following equation for calculation. <MAT> NV: Nonvolatile content (%).

If the sample does not dissolve well in an organic solvent but separates to make the measurement difficult, the acid value can be measured by the following method instead.

Acid value (mg KOH/g-resin) is a value calculated by the following equation using an FT-IR (JASCO, FT-IR <NUM>) and a factor (f) obtained from a calibration curve constructed with a solution of maleic anhydride in chloroform and the absorbance (I) of the peak for the expansion of the anhydrous ring of maleic anhydride (<NUM>-<NUM>) and that (II) of the peak for the expansion of the carbonyl groups of maleic acid (<NUM>-<NUM>) in a solution of a maleic anhydride-modified polyolefin.

Molecular weight of maleic anhydride, <NUM>; molecular weight of potassium hydroxide, <NUM>.

The molecular weight of a carboxyl-containing resin (A) according to the present invention is not critical. Preferably, the number-average molecular weight is between <NUM> and <NUM>,<NUM>,<NUM>; this ensures the composition will form a coating well. It is particularly preferred that the number-average molecular weight be between <NUM> and <NUM>,<NUM>,<NUM>.

The weight-average molecular weight of a carboxyl-containing resin (A) according to the present invention can be calculated by measuring it by the method of gel permeation chromatograph (GPC).

Examples of carboxyl-containing vinyl resins include polymers of polymerizable unsaturated monomers having carboxyl group(s). Examples of polymerizable unsaturated monomers having carboxyl group(s) include unsaturated carboxylic acids, such as (meth)acrylic acid, <NUM>-carboxyethyl (meth)acrylate, crotonic acid, itaconic acid, maleic acid, and fumaric acid;.

The carboxyl-containing resin (A) can be obtained by just polymerizing the monomer using a process that is known and commonly used. The resin (A) can be produced by addition polymerization in the presence of a catalyst (polymerization initiator) and known polymerization techniques can be used, such as bulk polymerization, solution polymerization, suspension polymerization, and emulsion polymerization.

A metal compound (B) according to the present invention is characterized in that it is a divalent metal compound.

A divalent metal compound (B) is a compound of a divalent metal.

According to the present invention, the divalent metal compound (B) is selected from zinc oxide, magnesium oxide, and calcium oxide. Zinc oxide and magnesium oxide are particularly preferred.

Preferably, the divalent metal compound (B) is in particulate form. More preferably, the divalent metal compound (B) is fine particles having an average diameter of <NUM> or less and <NUM> or more. It is particularly preferred that the divalent metal compound (B) be fine particles having an average diameter between <NUM> and <NUM>.

The average diameter of particles in this context is measured using a dynamic-light-scattering particle size distribution analyzer, such as LB-<NUM> (HORIBA).

An alcohol (C) according to the present invention can be an alcohol that is known and commonly used. Specific examples include methanol, ethanol, propanol, butanol, hexanol, and pentanol. Methanol, ethanol, propanol, butanol are preferred, and propanol is particularly preferred.

A composition according to the present invention for gas barrier purposes is characterized in that it consists of a carboxyl-containing resin (A), a divalent metal compound (B), and an alcohol (C), all as described above. Of these, the percentage of the alcohol (C) is between <NUM> and <NUM> wt%, and the water content of the composition is <NUM> wt% or less. When the alcohol (C) and water are in these ranges, the divalent metal compound (B) is stable in the composition. The divalent metal compound (B), therefore, forms ionic bonds with the carboxyl-containing resin (A) and produces gas barrier properties only after the composition is applied and dried. Stable when stored in its normal state, the composition is eminently suitable for use as a coating agent for gas barrier purposes. The coating agent forms a gas barrier coat layer in a one-component system.

For the gas barrier composition according to the present invention, the nonvolatile content is between <NUM> wt% and <NUM> wt% of the composition. The percentage of the carboxyl-containing resin (A) and divalent metal compound (B) combined to the total nonvolatile content is <NUM> wt%. When this percentage is in this range, the composition produces sufficient gas barrier properties.

As for the proportion of the carboxyl-containing resin (A) to the divalent metal compound (B), it is preferred that the divalent metal compound (B) constitute <NUM> to <NUM> wt% of the carboxyl-containing resin (A) and divalent metal compound (B) combined. When this percentage is in this range, the composition combines good gas barrier properties with spreadability. It is particularly preferred that this percentage be between <NUM> and <NUM> wt%.

Applying a coating agent that contains a composition according to the present invention for gas barrier purposes to a substrate gives a laminate having gas barrier properties. Once the coating agent is applied to a substrate, its volatile components are eliminated, causing the carboxyl-containing resin (A) and the divalent metal compound (B) to form ionic bonds. The resulting crosslinked structure gives the coating barrier properties.

A coating agent according to the present invention is stable when stored, free of changes such as gelation. By virtue of the presence of alcohol(s) (C), ionic bonding between carboxyl-containing resin(s) (A) and divalent metal compound(s) (B) is blocked until the agent is applied.

In a two-layer gas barrier laminate like those that have existed, furthermore, the ionic bonding between acid groups and metal compound(s) is limited to the interface. Crosslinks, therefore, extend only in two dimensions, and the inventors presume this causes the coating to have only low gas barrier properties as a result. The coating agent according to the present invention, the inventors presume, delivers high barrier properties because it is of one-component type and, therefore, forms a coat layer inside which crosslinks extend three-dimensionally.

The material for the substrate is not critical; the manufacturer can choose a suitable material according to the purpose of use. Examples include wood, metal, metal oxides, plastic, paper, silicone, and modified silicone, and a substrate obtained by joining different materials together may also be used. The shape of the substrate is not critical; the substrate can be in any shape selected according to the purpose, such as flat-plate, sheet-shaped, or a three-dimensional shape having curvature throughout or in part of it. The hardness, thickness, etc., of the substrate are not critical either.

If the laminate is used as a packaging material, the substrate is, for example, a piece of paper, plastic, metal, or metal oxide.

It is not critical how to apply the coating agent; known and commonly used coating techniques can be used. Examples include spraying, spin coating, dipping, roll coating, blade coating, doctor roll coating, doctor blading, curtain coating, slit coating, screen printing, inkjet coating, and dispensing.

The coat layer obtained by applying the coating agent will have denser ionic bonds therein when the applied coating agent is dried. It is therefore preferred that the application be followed by a drying step. The drying step may be drying at room temperature or may be forced drying, such as heating, vacuum drying, or blow drying.

The laminate may be a multilayer one having a top layer on its substrate and coat layer. The top layer may be placed before the coating agent is dried or may be placed after the coating agent is dried. The top layer can be of any kind and can be, for example, a layer of wood, metal, metal oxide, plastic, paper, silicone, or modified silicone. Alternatively, an uncured resin solution may be applied over the coat layer and cured or dried into a top layer.

Examples of gases a resin composition according to the present invention or a laminate including this resin composition can intercept include inert gases, such as carbon dioxide, nitrogen, and argon, alcoholic substances, such as methanol, ethanol, and propanol, and phenols, such as phenol and cresol, as well as oxygen. Fragrance substances that are low-molecular-weight compounds, such as soy sauce, Worcestershire sauce, miso, limonene, menthol, methyl salicylate, coffee, cocoa shampoo, and conditioner.

Superior in gas barrier properties, the laminate according to the present invention is suitable for use as a packaging material for which gas barrier properties are a demand. In particular, foods, daily necessities, electronic materials, contents for medical purposes, etc., are suitable applications of the packaging material according to the present invention because in such applications have high barrier properties are required.

The following describes the present invention by examples. The present invention, however, is not limited to these examples. The units are by weight unless stated otherwise.

Definition: The molecular weight of the repeating unit of polyacrylic acid (hereinafter: sometimes abbreviated to PAA) is <NUM>. Usually, one molecule of zinc oxide (molecular weight, <NUM>) (hereinafter: sometimes abbreviated to ZnO) contributes to reaction with two molecules of the repeating unit of PAA (molecular weight, <NUM> × <NUM>) to form a salt. The formula in which PAA and ZnO are mixed in the proportions of PAA weight:ZnO weight = <NUM>/<NUM> = <NUM>/<NUM> is referred to as adding one equivalent of ZnO.

A PAA solution with a solids concentration: <NUM>% was obtained by dissolving, in a flask, a PAA powder having a number-average molecular weight of <NUM>,<NUM> (AC-10LHPK, Toagosei): <NUM> by stirring it in boiling isopropyl alcohol (hereinafter sometimes abbreviated to IPA), Kanto Chemical: <NUM>.

A PAA solution with a solids concentration: <NUM>% was obtained by dissolving, in a flask, a PAA powder having a number-average molecular weight of <NUM> (AC-10P, Toagosei): <NUM> by stirring it in boiling IPA: <NUM>. This solution was diluted with IPA to give <NUM>-Da PAA solutions with solids concentrations of <NUM>%, <NUM>%, <NUM>%, and <NUM>%.

For a liquid dispersion of ZnO, a ZnO solution with a solids concentration: <NUM>% was obtained by mixing ZnO having a diameter of primary particles of <NUM> (Sakai Chemical Industry Co. , FINEX-<NUM>): <NUM> and IPA: <NUM> together, dispersing the mixture in a bead mill (Kotobuki Co. : Ultra Aspec Mill UAM-<NUM>) using <NUM>-mm zirconia beads for <NUM> hour, and then isolating the beads by sieving. This solution was diluted with IPA to give liquid dispersions of ZnO in IPA with solids concentrations of <NUM>%, <NUM>%, <NUM>%, and <NUM>%. The diameter of particles of ZnO in these liquid dispersions was <NUM>.

Coating agent <NUM> was obtained by mixing <NUM> of the <NUM>% PAA solution obtained in Preparation Example <NUM> and <NUM> of the <NUM>% ZnO solution obtained in Preparation Example <NUM> together. Coating agent <NUM> was subjected to an appearance test of the coating agent.

In addition to this, laminate <NUM> was prepared by applying the coating agent to a substrate. The resulting laminate was subjected to an appearance test of the coat layer and a gas barrier test of the laminate.

The appearance test of the coating agent was performed visually. The grades were as follows.

The appearance test of the coat layer was performed visually. The substrate was a piece of polyethylene terephthalate film (TOYOBO ESTER Film's E5100; thickness, <NUM>), and laminate <NUM> was prepared by applying coating agent <NUM> thereto by the application method described below. For the resulting coat layer, its appearance test was conducted visually. The grades were as follows.

A barrier coat film was obtained by preparing Matsuo Sangyo Co. : K303 bar No. <NUM>, yellow/<NUM>, K303 bar No. <NUM>, red/<NUM>, K303 bar No. <NUM>, green/<NUM>, and K303 bar No. <NUM>, black/ <NUM>, applying the mixed solution to a piece of PET film (TOYOBO ESTER Film's: E5100: thickness: <NUM>), and drying the coating at <NUM>: <NUM> minute. The bar was selected so that the weight of the barrier coat material on the dried sample would be about <NUM>/m2. The weight of applied coating was calculated by making ten coating samples under each set of conditions, weighing a <NUM> × <NUM> cutout of each sample and averaging the measured weights, and subtracting the average weight of ten substrate sheets of the same area from the determined average. Based on the results, the right coating bar was chosen.

Oxygen permeability was tested using the barrier coat film on PET (thickness: <NUM>) obtained in the previous section, including the substrate. The measurement of oxygen permeability was carried out in accordance with JIS-K7126 (equal-pressure method) using MOCON's OX-TRAN <NUM>/<NUM> oxygen transmission rate analyzer in <NUM> temperature and <NUM>%RH humidity and <NUM> temperature and <NUM>%RH humidity atmospheres. RH stands for relative humidity. The unit of oxygen permeability is cc/day·atm·m2.

For Examples <NUM> to <NUM> and Comparative Examples <NUM> to <NUM>, a coating agent and a laminate were tested as in Example <NUM> except that the formula was changed to that in Table <NUM>. The results are presented in Tables <NUM> to <NUM>.

In Examples <NUM> to <NUM>, the <NUM>-kDa PAA and ZnO solutions were mixed to make the ZnO equivalence factor vary from <NUM> to <NUM>. The mixed solution and the coating were in good appearance, and the oxygen permeability was also good. In Examples <NUM> and <NUM>, water was added to a percentage of <NUM>% or <NUM>% of the mixed solution as a whole. The appearance of the mixed solution and the coating, although somewhat worse, was practically acceptable, and the oxygen permeability was also good. In Examples <NUM> to <NUM>, solids content levels of <NUM>% to <NUM>% were studied using <NUM>-Da PAA and a constant ZnO equivalence factor of <NUM>. The mixed solution and the coating were in good appearance, and the oxygen permeability was also good.

In Comparative Examples <NUM> and <NUM>, water was added to a percentage of <NUM>% of the entire system with each of the <NUM>-kDa and <NUM>-Da PAA solutions and the ZnO solution. The appearance of the mixed solution and the coating and the oxygen permeability were degraded significantly. In Comparative Example <NUM>, the <NUM>-Da PAA and ZnO solutions were studied at a solids concentration of <NUM>%. The appearance of the mixed solution of the coating and the oxygen permeability were poor.

Overall, the results indicated that in the context of a composition for gas barrier purposes containing a carboxyl-containing resin, a divalent metal compound, and an alcohol, good appearance of the mixed solution and the coating is combined with good oxygen permeability when the alcohol content of the composition is between <NUM> and <NUM> wt% and when the water content of the composition is <NUM> wt% or less.

Claim 1:
A composition for gas barrier purposes consisting of a carboxyl-containing resin (A), a divalent metal compound (B), and an alcohol (C), wherein:
the alcohol (C) content of the composition is between <NUM> and <NUM> wt%;
the water content of the composition is <NUM> wt% or less;
the carboxyl-containing resin (A) is a homopolymer of a monomer selected from acrylic acid, methacrylic acid, maleic acid, and itaconic acid; and
the divalent metal compound (B) is selected from zinc oxide, magnesium oxide and calcium oxide.