JP2022151624A - foam paper laminate - Google Patents

Abstract

To provide a method for producing a foam paper laminate which is excellent in foam property and has good appearance even when a printing layer is formed by aqueous flexographic printing.SOLUTION: A method for producing a foam paper laminate has foam paper has a thermoplastic resin layer (A), a paper base material, and a foam thermoplastic resin layer (B) in this order, and a printing layer formed on the surface of the foam thermoplastic resin layer (B) of the foam paper by flexographic printing, wherein the printing layer has at least a white ink layer and the white ink layer is formed by at least two overprinting.SELECTED DRAWING: None

Description

Embodiments of the present invention relate to a method of making a foamed paper laminate having a printed layer.

Foam containers are widely used as containers for containing foods containing high-temperature or low-temperature liquids because of their excellent heat insulation properties, and are indispensable especially for cup noodles. Styrofoam containers and foamed paper containers are known as foamed containers for cup noodles. In recent years, foamed paper containers have attracted attention from the viewpoint of environmental load and safety.

A foamed paper container is manufactured using a foamed paper material having a paper substrate layer and a thermoplastic resin layer that foams when heated during container manufacturing to form a heat insulating layer. Generally, a printed layer on which a decorative pattern, a company name, a bar code, and the like are drawn is formed on the surface of a foamed paper container (foamed paper material). Therefore, when the thermoplastic resin layer of the foamed paper material is heated to foam and form a heat insulating layer, the printed layer preferably does not hinder foaming and has excellent foam followability. In addition, the surface (printed surface) of the printed layer in the foamed paper laminate after the thermoplastic resin layer has been foamed is smooth, free from cracks and blisters, and has an excellent appearance (hereinafter referred to as "foamed appearance"). It is desirable to have

Conventionally, oil-based inks such as oil-based gravure ink and oil-based flexographic ink have been used to form a printed layer on foamed paper containers. Oil-based ink usually contains an organic solvent such as toluene from the viewpoint of the solubility and drying properties of the binder resin. However, in recent years, from the viewpoint of environmental load and occupational safety and health, regulations on the use of organic solvents have become stricter.

For example, Patent Literature 1 discloses a water-based flexo ink containing a binder resin, a pigment, and water as an ink forming a printing layer of a foamed paper laminate, and wherein the binder resin contains a polyurethane resin. Further, Patent Literature 2 discloses that the foaming of the foam layer is suppressed to a predetermined range by a coated portion (printed layer) of water-based flexographic ink. The water-based flexographic ink disclosed in Patent Document 2 contains a coloring agent and a binder resin, and 30% by mass or more of a urethane-based resin having a glass transition point of −20° C. or higher and 30° C. or lower in the resin solid content of the binder resin. contains. Furthermore, Patent Document 3 discloses a water-based flexographic ink containing an acrylic resin, an acrylic-urethane copolymer resin, a styrene-maleic acid copolymer resin, or a polyurethane resin as a binder resin.

JP 2018-058955 A International Publication No. 2018/066031 JP 2011-068381 A

The heat insulating property of a foamed paper container mainly depends on the foaming property of the thermoplastic resin layer that constitutes the foam layer (heat insulating layer) in the foamed paper laminate. However, as the foamability of the thermoplastic resin layer during heat processing increases, appearance defects such as cracks and blisters tend to occur on the surface of the printed layer formed on the thermoplastic resin layer. Therefore, it is desirable that the printed layer can uniformly suppress foaming so as to prevent appearance defects such as cracks and blisters.

However, in the process of completing the present invention, it has become clear that when water-based flexographic ink is used, poor appearance tends to occur depending on the lot, position, and temperature of the thermoplastic resin layer (film). Since the poor appearance may not occur depending on the film lot, etc., it is presumed that this poor appearance is not due to the physical properties of the water-based flexographic ink.

Therefore, in view of the above situation, it is an object of the present invention to provide a method for producing a foamed paper laminate that is excellent in foamability and has a good appearance even when a printed layer is formed by water-based flexographic printing.

The inventors of the present invention have investigated the cause of poor appearance depending on film lots and the like when using water-based flexo ink, and have earnestly studied methods for solving the problem. As a result, it was found that the amount of water-based flexographic ink applied varies depending on the film lot, etc., causing poor appearance. came to.

That is, one embodiment of the present invention includes a foamed paper having a thermoplastic resin layer (A), a paper base layer, and a foamed thermoplastic resin layer (B) in this order, and the foamed thermoplastic resin layer of the foamed paper ( A method for producing a foamed paper laminate having a printed layer formed by flexographic printing on the surface of B), wherein the printed layer has at least a white ink layer, and the white ink layer is formed at least twice. The present invention relates to a method for manufacturing a foamed paper laminate, which is characterized by being formed by overprinting.

One embodiment relates to the method for producing the foamed paper laminate, wherein the number of foamed cells per unit area of the foamed thermoplastic resin layer (B) is 800/1 cm ² or more. .

One embodiment relates to the method for producing the foamed paper laminate, wherein the number of foamed cells per unit area of the foamed thermoplastic resin layer (B) is 1,000 cells/1 cm ² or more. is.

One embodiment relates to the method for producing the foamed paper laminate, wherein the print layer has a thickness of 1.0 to 2.5 μm.

One embodiment relates to the method for producing the foamed paper laminate, wherein the thickness of the foamed thermoplastic resin layer (B) is 500 to 950 μm.

INDUSTRIAL APPLICABILITY According to the present invention, it is possible to manufacture a foamed paper container having a good foamed appearance regardless of the film lot or the like.

The present invention will be specifically described below. However, the present invention is not limited to the embodiments described below.

<1> Foamed paper laminate The foamed paper laminate includes a foamed paper having a thermoplastic resin layer (A), a paper base layer, and a foamed thermoplastic resin layer (B) in order, and the foamed thermoplastic resin of the foamed paper. and a printed layer formed on the surface of the resin layer (B), wherein the number of foamed cells per unit surface area of the foamed thermoplastic resin layer (B) is 1,000/1 cm ² or more. and

<Expanded thermoplastic resin layer (B)>
(Number of foamed cells per unit surface area of foamed thermoplastic resin layer (B))
Regarding the number of foamed cells per unit surface area of the foamed thermoplastic resin layer (B) (hereinafter referred to as foamed layer (B)), the larger the number, the smaller the cells present in the foamed layer (B), and the smaller the number, the smaller the foamed cells. It means that the bubbles present in layer (B) are large. If the bubbles existing in the foam layer (B) become large, appearance defects such as cracks and blisters on the printed surface are likely to occur.

The number of foam cells per unit surface area of the foam layer (B) must be at least 800/1 cm ² or more, preferably 1,000/1 cm ² or more, and 1,250/1 cm 2 or more. It is more preferably ² or more. When the number of foamed cells is within the above range, the printed layer formed on the foamed layer (B) can easily obtain an excellent foam appearance without cracks and blisters. On the other hand, the upper limit of the number of foamed cells is not particularly limited. In one embodiment, the number of foamed cells may be 1,600/1 cm ² or less from the viewpoint of manufacturing conditions.

Here, the "number of foamed cells per unit area" refers to closed cells (bubbles) existing within a range partitioned with a certain length in the vertical and horizontal (XY) directions on the surface of the foam layer (B). means the value calculated as the number of independent cells per 1 cm ² . The number of closed cells is determined by removing the printed layer of the foamed paper laminate with a solvent to expose the surface of the foamed layer (B), and then observing the surface of the foamed layer (B) using an optical microscope. be.

(Thickness of foamed thermoplastic resin layer (B))
The thickness of the foam layer (B) is preferably 500 µm or more, more preferably 630 µm or more, from the viewpoint of heat insulation. If the thickness of the foam layer (B) is 500 μm or more, even if the foamed paper laminate is molded into a cup-shaped container and hot water of about 100° C. is poured into the container, the container can be continued with bare hands. easier to hold. On the other hand, from the viewpoint of resource saving, it is preferable that the amount of resin used is as small as possible. Moreover, from the viewpoint of heat insulation, the heat insulating layer does not need to be so thick as to be of excessive quality. Therefore, the thickness of the foam layer (B) is preferably 950 μm or less, more preferably 900 μm or less, and extremely preferably 800 μm.

From the above viewpoint, in one embodiment, the foam layer (B) preferably has a thickness of 500 to 950 μm, more preferably 500 to 900 μm, even more preferably 500 to 800 μm. The thickness of the foam layer (B) is determined by observing the cross section of the foamed paper laminate with an optical microscope photograph and measuring the height from the upper surface of the paper substrate to the lower surface of the printed layer.

(Total thickness of foamed paper laminate)
In the above embodiment, the total thickness of the foamed paper laminate is preferably 1,000 to 1,450 μm, more preferably 1,000 to 1,300 μm. Here, the total thickness refers to the height from the upper surface of the printed layer constituting the foamed paper laminate to the lower surface of the thermoplastic resin layer (A). Determined by measuring the height.

In the foamed paper laminate of the above embodiment, the foamed layer (B) of the foamed paper means a state after the thermoplastic resin layer is foamed by heating. That is, the foam layer (B) is formed by heating and foaming an unfoamed thermoplastic resin layer (foamed thermoplastic resin layer-forming layer (B ₀ )) as a precursor. In one embodiment, a thermoplastic resin layer (A), a paper substrate, and a thermoplastic resin layer (A) having a lower melting point than the thermoplastic resin layer (A) and foamed by heat treatment are used to form the foamed paper laminate. A foamed paper material (preheated foamed paper) having a foamed thermoplastic resin layer forming layer (B ₀ ) in order can be used. The foamed paper material can be constructed from materials known in the art. Constituent materials of the foamed paper laminate will be specifically described below.

<Paper base layer>
(Basis Weight and Moisture Content of Paper Base Material)
The paper substrate layer constituting the foamed paper laminate is composed of a paper substrate, and the type thereof is not particularly limited. For example, kraft paper or fine paper can be used. From the viewpoint of achieving sufficient toughness when used as a container, the basis weight of the paper substrate is preferably 150-450 g/m ² , more preferably 250-400 g/m ² . The thickness of the paper base layer is preferably 110-860 μm, more preferably 140-640 μm. From the viewpoint of obtaining suitable foaming properties of thermoplastic resins such as polyethylene, the water content in the paper substrate layer is preferably 4 to 10% by mass, more preferably 5 to 8% by mass.

<Thermoplastic resin layer (A), foam layer-forming layer (B ₀ )>
In one embodiment, the thermoplastic resin layer (A) and the foam layer-forming layer (B ₀ ) may each be a film made of a resin material conventionally known as a container material. For example, a film (thermoplastic resin film) made of at least one thermoplastic resin selected from the group consisting of stretched and unstretched polyolefins such as polyethylene and polypropylene, polyester, nylon, cellophane, and vinylon can be used. can. In one embodiment, a polyethylene film can be preferably used because of its excellent lamination suitability and foamability.

The foamed paper material can be constructed by laminating thermoplastic resin films having different melting points to a paper substrate. Here, the thermoplastic resin film provided on the other side of the paper base layer as the foam layer forming layer (B ₀ ) rather than the thermoplastic resin film provided on one side of the paper base layer as the thermoplastic resin layer (A). The material is selected so that the melting point (Mp) of is low. In the foamed paper material, water in the paper base layer evaporates during heat treatment, and the evaporated water is pushed out to the softened foamed layer forming layer (B ₀ ) (low Mp resin film) side. As a result of such extrusion, the low-Mp resin film swells (foams) outward, forming a foamed layer (B). The foam layer (B) thus formed functions as a heat insulating layer in the container. On the other hand, for the thermoplastic resin layer (A) (high Mp resin film), a material is selected that does not melt or soften when the low Mp resin film is foamed by heat treatment.

From the viewpoint of manufacturing a foam paper container using a foam paper laminate, the foam paper material is, for example, a high Mp polyethylene having a melting point of about 125° C. to 140° C. on one side of the paper base layer (inside the container). A film (thermoplastic resin layer (A)) and a low Mp polyethylene film (foaming layer forming layer (B ₀ )) having a melting point of about 105° C. to 120° C. on the other surface (outside of the container) of the paper base layer may have a laminated structure. The low Mp polyethylene film foams to form a foam layer when heated, such as during the manufacture of a foamed paper container. On the other hand, the high Mp polyethylene film preferably functions as a covering layer. That is, the coating layer can suppress the evaporation of water in the paper base layer to the outside during foaming of the low-Mp polyethylene film, and allow the water in the paper base layer to efficiently contribute to foaming. be.

(Density of foam forming layer (B ₀ ))
In one embodiment, the material of the foamed layer forming layer (B ₀ ) preferably contains low-density polyethylene resin (density 910-925 kg/m ³ , melting point 105-120° C.) among polyethylene resins. The density of the low-density polyethylene resin is more preferably 910-922 kg/m ³ , still more preferably 910-918 kg/m ³ . On the other hand, as materials for the foam layer forming layer (B ₀ ), medium density polyethylene resin (density 925 to 940 kg/m ³ , melting point 115 to 130° C.) and high density polyethylene resin (density 940 to 970 kg/m ³ , melting point 125 ~140°C), the melting point tends to be high, making it difficult to obtain sufficient foamability. The density of polyethylene resin is a value measured according to JIS K 6922-1 (1997).

(Melt flow rate of foam layer forming layer (B ₀ ))
Further, from the viewpoint of obtaining a uniformly foamed layer, the melt flow rate (hereinafter referred to as "MFR") of the polyethylene resin is preferably 8 to 28 g/10 minutes, more preferably 10 to 20 g/10 minutes. more preferred. The MFR of polyethylene resin is a value measured according to JIS K 6922-1 (1997).

(Film thickness of foam layer forming layer (B ₀ ))
Although not particularly limited, in one embodiment, the thickness of the foamed layer-forming layer (B ₀ ) is preferably 40 μm or more, more preferably 60 μm or more. By adjusting the film thickness to 40 μm or more, sufficient heat insulating properties can be obtained after the heat foaming treatment.

On the other hand, from the viewpoint of resource saving, it is preferable that the amount of resin used is as small as possible. Also, from the viewpoint of heat insulation, it is not necessary to have a thickness that would result in excessive quality. Therefore, the thickness of the foamed layer-forming layer (B ₀ ) is preferably 150 μm or less, more preferably 100 μm or less, and extremely preferably 80 μm or less.

<Print layer>
In the foamed paper laminate of the above embodiment, the print layer is a coating film obtained by applying an aqueous flexographic ink composition to the surface of the foamed layer forming layer (B ₀ ) of the foamed paper material. The main component of the coating film forming the print layer is the binder resin in the water-based flexographic ink composition, and the binder resin preferably contains a polyurethane resin.

(Thickness of printed layer)
The film thickness of the printed layer (coating film after drying) is preferably 1.0 to 2.5 μm from the viewpoint of suppressing foaming by the printed layer. The thickness of the printed layer may be more preferably 1.2 to 2.5 μm, more preferably 1.5 to 2.5 μm.

<2> Aqueous Flexographic Ink The aqueous flexographic ink of the present invention contains a polyurethane resin (X), water, an alcoholic solvent, a coloring agent and other materials.

By using the flexible polyurethane resin (X) as the main component, overfoaming of the foamed layer forming layer (B ₀ ) can be suppressed, and cracking of the foamed printed layer can be suppressed.

One embodiment of the polyurethane resin (X) includes a polyurethane resin (X1) obtained by reacting at least a polyol and a polyisocyanate. Another embodiment includes a polyurethane urea resin (X2) obtained by chain-extending a prepolymer of a polyurethane resin.

First, the polyurethane resin (X1) can be obtained by reacting a polyol (y1) having an acid group and a polymer polyol (y2) (other polyol (y3) as necessary) with a polyisocyanate (z1). can. The polyurethane urea resin (X2) was obtained by reacting a polyol (y1) having an acid group and a polymer polyol (y2) (other polyol (y3) as necessary) with a polyisocyanate (z1). It can be obtained by reacting an isocyanate-terminated prepolymer with a chain extender (z2).

Examples of the acid group-containing polyol (y1) include a carboxyl group-containing polyol and a sulfonic acid group-containing polyol.

Examples of polyols having carboxyl groups include 2,2-dimethylolpropionic acid, 2,2-dimethylolbutanoic acid, 2,2-dimethylolvaleric acid and the like. Among them, 2,2-dimethylolpropionic acid and 2,2-dimethylolbutanoic acid are preferred because of their good dispersion stability.

Examples of the polyol having a sulfonic acid group include dicarboxylic acids such as 5-sulfoisophthalic acid, sulfoterephthalic acid, 4-sulfophthalic acid, 5-(4-sulfophenoxy)isophthalic acid, salts thereof, ethylene glycol and Examples include polyester polyols obtained by reacting with low-molecular-weight polyols such as propylene glycol, 1,4-butanediol, 1,6-hexanediol, diethylene glycol, and neopentyl glycol. These acid group-containing polyols may be used alone or in combination of two or more.

The acid group-containing polyol (y1) is preferably used within a range in which the urethane resin (X) has an acid value of 10 to 50, and more preferably within a range of 10 to 35. Within this range, the urethane resin (X) can be dissolved or self-emulsified in water. Further, dispersibility of the pigment is improved by dissolving or self-emulsifying the urethane resin in water.

Examples of the polymer polyol (y2) include polyether polyols, polyester polyols, polycarbonate diols, etc. Among them, it is more preferable to use polyether polyols and/or polyester polyols.

Polyether polyols include polyethylene glycol, polypropylene glycol, polytetramethylene glycol and the like. Polyester polyols include polyester polyols (PMPA) obtained from adipic acid and 3-methyl-1,5-pentadiol, and polyester diols (PPA) obtained from adipic acid and 1,2-propanediol. These polymer polyols can be used alone or in combination of two or more.

The number average molecular weight of the polymer polyol (y2) is not particularly limited because it is appropriately determined in consideration of the properties such as ethanol resistance, elongation and stress of the polyurethane resin obtained as a reaction product, but is usually , The number average molecular weight of the polymer polyol (y2) is preferably in the range of 500 to 10,000, more preferably in the range of 500 to 6,000.

Moreover, in the present invention, other polyols (y3) can be used in addition to the above polyols.

Other polyols (y3) include, for example, cyclobutanediol, cyclopentanediol, 1,4-cyclohexanediol, cycloheptanediol, cyclooctanediol, cyclohexanedimethanol, hydroxypropylcyclohexanol, dicyclohexanediol, butylcyclohexanediol, 1 ,1'-bicyclohexylidenediol, cyclohexanetriol, hydrogenated bisphenol A, 1,3-adamantanediol, and other low molecular weight polyols having an alicyclic structure of about 100 to 500. These other polyols can be used alone or in combination of two or more.

Further, in order to suppress blocking of printed matter, it is preferable to use the polyol (y3) in an amount of 1 to 20% by weight with respect to the total amount of polyols. In addition, the polyol total amount refers to the total amount of the polyol (y1) having an acid group, the polymer polyol (y2), and the other polyol (y3).

Examples of polyisocyanate (z) include various known aromatic diisocyanates, aliphatic diisocyanates, alicyclic diisocyanates, and the like, which are generally used in the production of polyurethane resins. For example, 1,5-naphthylene diisocyanate, 4,4′-diphenylmethane diisocyanate (MDI), 4,4′-diphenyldimethylmethane diisocyanate, 4,4′-dibenzyl isocyanate, dialkyldiphenylmethane diisocyanate, tetraalkyldiphenylmethane diisocyanate, 1 ,3-phenylene diisocyanate, 1,4-phenylene diisocyanate, tolylene diisocyanate, butane-1,4-diisocyanate, hexamethylene diisocyanate, isopropylene diisocyanate, methylene diisocyanate, 2,2,4-trimethylhexamethylene diisocyanate, lysine diisocyanate, Cyclohexane-1,4-diisocyanate, xylylene diisocyanate, isophorone diisocyanate, dimeryl diisocyanate, dicyclohexylmethane-4,4'-diisocyanate, 1,3-bis(isocyanatomethyl)cyclohexane, methylcyclohexane diisocyanate, norbornane diisocyanate, m-tetra Examples include methylxylylene diisocyanate, 4,4-diphenylmethane diisocyanate, tolylene diisocyanate, bis-chloromethyl-diphenylmethane-diisocyanate, 2,6-diisocyanate-benzyl chloride, and dimer diisocyanate obtained by converting the carboxyl group of dimer acid to an isocyanate group. be done. These diisocyanate compounds can be used alone or in combination of two or more.

Among the polyisocyanates described above, at least one selected from the group consisting of tolylene diisocyanate, isophorone diisocyanate, xylylene diisocyanate, bis(isocyanatomethyl)cyclohexane, hexamethylene diisocyanate, and a trimer of hexamethylene diisocyanate is preferred.

Examples of the chain extender (z2) include diamine compounds such as ethylenediamine, propylenediamine, hexamethylenediamine, diethylenetriamine, triethylenetetramine, isophoronediamine, dicyclohexylmethane-4,4′-diamine, 2-hydroxyethylethylenediamine, 2-hydroxy Ethylpropyldiamine, 2-Hydroxyethylpropylenediamine, Di-2-Hydroxyethylethylenediamine, Di-2-Hydroxyethylenediamine, Di-2-Hydroxyethylpropylenediamine, 2-Hydroxypyropyrethylenediamine, Di-2-Hydroxypyropyrethylenediamine , Di-2-hydroxypropylethylenediamine, etc., amine compounds having a hydroxyl group in the molecule, diethylenetriamine, iminobispropylamine: (IBPA, 3,3'-diaminodipropylamine), N-(3-aminopropyl)butane-1 ,4-diamine: (spermidine), polyfunctional amine compounds such as 6,6-iminodihexylamine, hydrazine, N,N'-dimethylhydrazine, 1,6-hexamethylenebishydrazine, succinic acid dihydrazide, adipic acid dihydrazide, glutaric acid dihydrazide, sebacic acid dihydrazide, isophthalic acid dihydrazide, β-semicarbazidepropionic acid hydrazide, 3-semicarbazide-propyl-carbazide, semicarbazide-3-semicarbazidemethyl-3,5,5-trimethylcyclohexane, etc. Hydrazine compounds are mentioned.

Also, a polymerization terminator (z3) can be used to terminate the excessive reaction. Specific examples include compounds having primary and secondary amino groups, dialkylamines such as di-n-butylamine, and aminoalcohols.

Furthermore, it is preferable to use a neutralizing agent (z4) in order to render the polyurethane resin having an acid value water-based by neutralization. Specific examples include sodium hydroxide, potassium hydroxide, ammonia, methylamine, ethylamine, propylamine, ethanolamine, propanolamine, diethanolamine, dimethylamine, diethylamine, and triethylamine. If the neutralizing agent remains in the ink film after printing, the water resistance tends to deteriorate and residual odor problems arise. Therefore, it is preferable to use ammonia, which quickly evaporates during printing, as the neutralizing agent.

Methods for synthesizing the polyurethane resin (X) include, for example, a method of polymerizing using a hydrophilic organic solvent that does not have reactivity with isocyanate (solvent method), a method of polymerizing without using a solvent ( non-solvent method).

Examples of hydrophilic organic solvents that do not have reactivity with isocyanate include ester solvents such as ethyl acetate, ketone solvents such as acetone, methyl ethyl ketone and cyclohexanone, and ether solvents such as diethyl ether and tetrahydrofuran. In the solvent method, it is necessary to add water and a neutralizing agent after the polymerization reaction and to remove the organic solvent. Therefore, it is preferable to use a solvent having a boiling point lower than that of water as the organic solvent.

(Content of polyurethane resin (X))
The solid content of the water-based flexographic ink preferably contains 20% by weight or more and 70% by weight or less of the polyurethane resin (X), and more preferably 40 to 60% by weight. Sufficient adhesiveness can be achieved by containing 20% by weight or more of the polyurethane resin (X). Moreover, by making the polyurethane resin (X) 70% by weight or less, it is easy to adjust the viscosity to be suitable for printing.

In the present invention, an alcohol-based solvent can be used in combination with water as a solvent for the water-based flexo ink. Drying property and wettability to the printing base material can be improved by using the alcohol-based solvent together. Specifically, methyl alcohol, ethyl alcohol, isopropyl alcohol, n-propyl alcohol and the like can be used.

(white pigment)
The water-based flexographic ink used in this embodiment contains various colorants. Colorants (white pigments) for white ink include titanium oxide, zinc oxide, zinc sulfide, barium sulfate, calcium carbonate, and aluminum hydroxide. The content of the white pigment should be 20% by weight or more and 50% by weight or less based on the total weight of the white ink, and a desired ink density can be obtained by setting it within this range. Titanium dioxide is preferable as the white pigment from the viewpoint of hiding power and light resistance.

(Colored pigment)
Colorants (colored pigments) for colored inks include inorganic pigments such as carbon black, aluminum, red iron oxide (iron oxide), azo pigments, phthalocyanine pigments, anthraquinone pigments, perylene pigments, perinone pigments, and quinacridone pigments. , thioindigo-based pigments, dioxazine-based pigments, isoindolinone-based pigments, quinophthalone-based pigments, and ditopyrrolopyrrole-based pigments. The content of the colored pigment should be 5% by weight or more and 30% by weight or less based on the total weight of the colored ink.

(constitution content)
The water-based flexographic ink used in this embodiment may further contain an extender. As described above, the water-based flexographic ink used in this embodiment contains a large amount of polyurethane resin. Therefore, the printing layer tends to cause blocking. However, in the present embodiment, by containing the extender, it is possible to suppress blocking of the printed layer while maintaining the flexibility of the polyurethane resin.

Examples of the extender pigment include silica (silicon dioxide). The particle size of silica is preferably 2 μm or more and 20 μm or less, more preferably 3 μm or more and 10 μm or less. By using silica having a size of 2 μm or more and 20 μm or less, blocking can be suppressed without causing poor printing.

When silica is contained in the present embodiment, silica is preferably 0.2% by mass or more and 5% by mass or less, based on the total weight of the aqueous flexo ink, and 0.5% by mass or more and 3.5% by mass. % or less is more preferable. By setting the silica content within the above range, blocking can be suppressed and the viscosity can be adjusted to be easy to handle as a water-based flexographic ink.

<3> Method for manufacturing foamed paper laminate One embodiment relates to a method for manufacturing a foamed laminate. That is, one embodiment includes a foamed paper having a thermoplastic resin layer (A), a paper base layer, and a foamed layer (B) in this order, and a printed layer formed on the surface of the foamed layer (B) of the foamed paper. and the number of foam cells per unit surface area of the foam layer (B) is 1,000/1 cm ² or more. This manufacturing method includes the following steps (i) to (iii): (i) lamination step (ii) flexographic printing step (iii) foaming step.

In the above manufacturing method, the (i) laminating step, (ii) flexographic printing step, and (iii) foaming step can be carried out according to methods well known in the art. Each step will be described below.

(Step (i): Lamination step)
The preparation of the foamed paper material can be carried out, for example, according to the extrusion lamination method. The constituent materials of the paper substrate, the thermoplastic resin layer (A), and the foam layer forming layer (B ₀ ), which constitute the foamed paper material, are as described above. As the extrusion lamination method, well-known methods such as a single lamination method, a tandem lamination method, a sandwich lamination method, and a co-extrusion lamination method can be appropriately selected. In one embodiment, polyethylene resins having different melting points can be suitably used as constituent materials of the thermoplastic resin layer (A) and the foamed layer-forming layer (B ₀ ).

The foam layer-forming layer (B ₀ ) is composed of a polyethylene resin (low Mp polyethylene resin) having an Mp lower than the melting point (Mp) of the polyethylene resin constituting the thermoplastic resin layer (A). The foamed paper material is obtained by extruding a low Mp polyethylene resin into a film on one side of the paper base layer through a T-die extruder, and extruding a high Mp polyethylene resin into a film on the other side of the paper base layer. It can be manufactured by extracting

The temperature of the polyethylene resin during lamination (the temperature immediately below the T-die) is preferably 300 to 350°C, more preferably 320 to 340°C. Within this temperature range, sufficient lamination strength can be achieved between each polyethylene resin layer (A, B ₀ ) and the paper base layer. It is preferable to control the surface temperature of the cooling roll passed after lamination within the range of 10 to 50°C.

In one embodiment, the lamination speed is preferably 50-130 m/min, more preferably 60-110 m/min. If the lamination speed is too slow, the productivity tends to be low. Neck-in is a phenomenon in which the width of the extruded polyethylene resin film becomes smaller than the effective width of the T-die when the polyethylene resin is extruded into a film by a T-die extruder. At this time, both ends of the film are thicker than the central portion. If the thickness of both ends is out of specification, it is common to cut and remove both ends.

In one embodiment, the air gap is preferably 300 mm or less, more preferably 200 mm or less. If the air gap is too wide, the polyethylene resin tends to neck in, resulting in a decrease in yield. The air gap refers to the distance from the extrusion port of the T-die to the nip rolls. It is preferable to surface-treat the polyethylene resin using ozone gas and/or oxygen gas while the polyethylene resin is passing through the air gap. By performing surface treatment using ozone gas and/or oxygen gas, the formation of an oxide film can be promoted and the adhesive force to the substrate layer can be improved. The amount of ozone gas and/or oxygen gas to be treated is not particularly limited, but is preferably 0.5 mg/m ² or more from the viewpoint of promoting oxidation of the polyethylene resin.

(Step (ii): flexographic printing step)
The printed layer is formed by flexographic printing. Flexographic printing is a printing method with little misregistration during printing and good production efficiency, but there is a limit to the amount of ink that can be applied because letterpress is used. For this reason, flexographic printing generally uses ink with a higher pigment concentration than other printing methods (for example, gravure printing). However, in the present invention, the printed layer (particularly the white ink layer) is required to have a function of suppressing excessive foaming of the foamed layer forming layer (B ₀ ), so an ink with a high pigment concentration and a low resin ratio is used. Can not do it.

Therefore, in the present invention, it is necessary to form the white ink layer by printing at least twice. By printing the white ink twice, it is possible to achieve both high designability (concealability) and suppression of overfoaming.

(Step (iii): foaming step)
Appropriate heating temperature and heating time for forming the foam layer (B) vary depending on the properties of the paper base layer and thermoplastic resin film used. A person skilled in the art can determine the optimal combination of heating temperature and heating time according to the material such as the thermoplastic resin film used. Although not particularly limited, the heat treatment is generally carried out in the process of forming the container. If the heating temperature during the heat treatment is too low, sufficient foamability cannot be obtained, and if the heating temperature is too high, the foamed cells are bound to easily cause blisters.

Although not particularly limited, when the foam layer-forming layer is composed of a low-density polyethylene film, the heating temperature may preferably be 100 to 125°C, more preferably 110 to 120°C. The heating time can be appropriately adjusted according to the heating temperature, preferably 3 to 10 minutes, more preferably 5 to 7 minutes. In one embodiment, when the foamed layer-forming layer is composed of a low-density polyethylene film and the printed layer is formed thereon using the water-based flexographic ink composition described above, the heating temperature is 110 to 123°C. It is preferable to adjust the time to 5-7 minutes. It is more preferable to adjust the heating temperature to 115 to 121° C. and the heating time to 5 to 7 minutes. When the heating is performed under the above conditions, it becomes easy to appropriately control the foaming of the foamed layer forming layer during heat processing by the printed layer.

Any means such as hot air, electric heat, and electron beams can be used as the heating means. A large amount of heat treatment can be performed at low cost by heating with hot air or electric heat in a tunnel equipped with conveying means using a conveyor.

<4> Foamed Paper Container One embodiment relates to a foamed paper container provided with the foamed paper laminate of the above embodiment. The foamed paper container comprises a container body member and a bottom plate member, and is characterized in that the container body member is formed from the foamed paper laminate of the above embodiment.

The foamed paper container can be molded by applying a well-known technique. For example, the foamed paper laminate (the foamed paper laminate before heating) on which the printed layer is first formed is punched into a predetermined shape along a mold to obtain a container body member. Similarly, the bottom plate material is punched into a predetermined shape to obtain a bottom plate member. Next, using a conventional container manufacturing apparatus, the container body member and the bottom plate member are assembled and formed into the shape of the container. The container is assembled and molded by the container manufacturing apparatus so that the high-Mp resin film of the container body member forms the inner wall surface, the low-Mp resin film forms the outer wall surface, and the laminated surface of the bottom plate member faces the inside. to implement. After the container is assembled and molded by the container manufacturing apparatus in this way, the low-Mp resin film is foamed by performing heat treatment to form a foam layer (heat insulation layer), thereby obtaining a foamed paper container having heat insulation properties. can.

In one embodiment, when the inner wall surface of the body portion and the outer wall surface of the body portion of the foamed paper container are each made of a polyethylene film, one surface of the paper substrate (the inner wall surface of the container) is made of a medium-density or high-density polyethylene film. It is preferable to laminate and laminate the other surface (outer wall surface of the container) with a low-density polyethylene film. The thickness of each film laminated on the paper substrate is not particularly limited. However, the thickness of the low-Mp resin film constituting the outer wall surface of the container body is appropriately set so that when the film is foamed, the film after foaming has a sufficient thickness to function as a heat insulating layer. preferably.

For example, when the outer wall surface of the container body is composed of a low-density polyethylene film, the thickness of the film to be laminated on the paper substrate may be 40 to 150 μm. On the other hand, when the inner wall surface of the container body is composed of a medium-density or high-density polyethylene film, the thickness of the film to be laminated on the paper substrate is not particularly limited. However, it is preferable to appropriately set the thickness of the film so as to ensure the penetration resistance of the contents when used as a heat-insulating foamed paper container. Since the thickness of the film to be laminated on the paper substrate varies depending on the resin material of the film used, it is desirable for those skilled in the art to appropriately set the thickness in consideration of the properties of the resin material.

EXAMPLES The present invention will be described in more detail below with reference to Examples, but the present invention is not limited to these Examples. "Parts" and "%" described below represent "parts by weight" and "% by weight" unless otherwise specified.

(Prototype example 1)
A medium-density polyethylene resin was extrusion-laminated on one side of the paper base layer, and a low-density polyethylene resin was extrusion-laminated on the opposite side of the paper base layer (lamination step). Next, water-based flexographic ink was printed on the surface of the low-density polyethylene side using a CI-type flexographic printing machine (printing process). After printing, it was heated in an oven at 121° C. for 120 seconds to produce a foamed paper laminate 1 (prototype example 1) (foaming step).

The details of the lamination process are as follows.

(Paper substrate)
Paper substrate: moisture content 23 g/m ² , basis weight 320 g/m ²

(Thermoplastic resin layer (A): medium density polyethylene resin layer)
Medium-density polyethylene resin (M): “Petrothene LW04-1” manufactured by Tosoh Corporation, MFR 4.3 g/10 minutes, density 940 kg/m ³
Extrusion temperature (T die outlet temperature): 320°C
Take-up speed (laminating speed): 50 m/min Air gap: 130 mm
Thickness: 40 µm (thickness at center of polyethylene resin layer)

(Foam layer-forming layer (B ₀ ): low-density polyethylene resin layer)
Low-density polyethylene resin (L): "Petrothene 07C03C" manufactured by Tosoh Corporation, MFR 15 g/10 minutes, density 918 kg/m ³
Extrusion temperature (T die outlet temperature): 310°C
Take-up speed (laminating speed): 60 m/min Air gap: 130 mm
Thickness: 70 μm (thickness at center of polyethylene resin layer)

The details of the printing process are as follows.
Printing machine: CI type flexo printing Water-based flexo ink: 20% by weight of white pigment, 30% by weight of urethane resin, 45% by weight of water, 5% by weight of ethanol
Anilox linearity (lpi): See Table 1 Anilox capacity (ml/m ² ): See Table 1

The layer structure of the foamed paper laminate of this trial example is expressed as follows.
Mark/LDPE 70/Paper/MDPE 40
"/" represents the boundary between layers. The layer on the left end is the layer forming the outer surface of the foamed paper laminate, and the layer on the right end is the layer forming the innermost surface of the foamed paper laminate. "Mark" means printed layer. "LDPE" means low density polyethylene resin layer. "Paper" means a paper base layer. "MDPE" means medium density polyethylene resin layer. The number means the layer thickness (unit: μm).

(evaluation)
The foamed paper laminate was evaluated for the film thickness of the printed layer, the concealability, the number of cells of the foamed layer (B), the total thickness of the foamed paper laminate, and the appearance (blisters and cracks). Details are as follows.

(Thickness of printed layer)
The film thickness of the printed layer was measured from a scanning electron microscope (SEM) photograph (magnification 5,000) of the cross section of the foamed paper laminate. In addition, measurements were performed at arbitrary five points, and the average values of these measurements are shown in Table 1.

(Concealability)
When the print layer (the amount of pigment) is thin, the foam cells may be seen through because the print layer has insufficient hiding power. Therefore, the hiding property of the printed layer was evaluated according to the following criteria.
◯: Foam cells are not visible, and the concealability is good.
x: The foam cells are seen through, and the concealability is poor.

(Total thickness of foamed paper laminate)
As described above, the total thickness of the foamed paper laminate is obtained by observing the cross section of the foamed paper laminate with an optical microscope photograph and measuring the height from the upper surface of the printed layer to the lower surface of the thermoplastic resin layer (A). determined by

(Number of foam cells in foam layer (B))
The printed layer of the foamed paper laminate was removed with methyl ethyl ketone (MEK) to expose the surface of the foamed layer (B). Next, the surface of the foam layer (B) was observed using an optical microscope (Nikon, AZ100M) (magnification: 25 times). The number of independent cells was determined, and a value calculated as the number of independent cells per 1 cm ² was obtained. Observations were made at arbitrary five points, and the average values of these observations are shown in Table 1.

(Foam layer (B) thickness)
The thickness of the foamed layer (B) was determined by observing the cross section of the foamed paper laminate with an optical micrograph and measuring the height from the upper surface of the paper substrate to the lower surface of the printed layer. Further, the film thickness before foaming corresponds to the film thickness of the foamed layer forming layer. Therefore, the value was obtained by measuring the film thickness of the low-density polyethylene resin formed as the foam layer-forming layer.

(Appearance evaluation)
<Blister>
The printed surface of the foamed paper laminate was visually observed. Evaluation criteria are as follows. The results shown in Table 1 are the most frequent values when 10 foamed paper laminates were casually sampled and evaluated. When there were multiple modes, the value with the lower evaluation was adopted.
(Evaluation criteria)
O: No blisters at all (no blisters can be confirmed).
Δ: 1 to 2 blisters having a major diameter of less than 5 mm exist per 100 cm ² .
x: There are 3 or more blisters with a major diameter of less than 5 mm per 100 cm ² . Alternatively, there is one blister with a major diameter of 5 mm or more per 100 cm ² .
If a plurality of blisters with different major diameters coexist, a lower evaluation is adopted. Specifically, when there are two blisters with a major diameter of less than 5 mm and one blister with a major diameter of 5 to 20 mm per 100 cm ² , the evaluation is "x".

<crack>
Regarding the foamed paper laminate, the printed surface of the foamed paper laminate was visually observed in the same manner as in the evaluation of the blisters. Evaluation criteria are as follows. The results shown in Table 1 are the mode values obtained by randomly preparing 10 foamed paper laminate samples and observing and evaluating each sample. When there were multiple modes, the value with the lower evaluation was adopted.
(Evaluation criteria)
O: No cracks at all (cracks cannot be confirmed).
Δ: 1 to 4 cracks having a length of less than 2 mm exist per 100 cm ² .
x: 5 or more cracks having a length of less than 2 mm exist per 100 cm ² . Alternatively, one or more cracks with a length of 2 mm or more exist per 100 cm ² .
If multiple cracks with different lengths coexist, a lower evaluation is adopted. Specifically, when one crack with a length of 1 mm and one crack with a length of 4 mm exist per 100 cm ² , the evaluation is "x".

In addition, in the foamed paper laminates of Test Examples 1 to 7, the thickness of the foamed layer (B) is 500 to 950 μm. The height was 500-800μ.

Claims (5)

Formed by flexographic printing on a foamed paper having a thermoplastic resin layer (A), a paper base layer, and a foamed thermoplastic resin layer (B) in order, and on the surface of the foamed thermoplastic resin layer (B) of the foamed paper A method for manufacturing a foamed paper laminate having a printed layer and a printed layer, comprising:
The printing layer has at least a white ink layer, and the white ink layer is formed by overprinting at least twice,
A method for producing a foamed paper laminate. The number of foamed cells per unit area of the foamed thermoplastic resin layer (B) is 800/1 cm ² or more,
A method for producing a foamed paper laminate according to claim 1. The number of foamed cells per unit area of the foamed thermoplastic resin layer (B) is 1,000/1 cm ² or more,
The method for manufacturing the foamed paper laminate according to claim 1 or 2. characterized in that the film thickness of the printed layer is 1.0 to 2.5 μm,
A method for producing a foamed paper laminate according to any one of claims 1 to 3. characterized in that the foamed thermoplastic resin layer (B) has a thickness of 500 to 950 μm,
A method for producing a foamed paper laminate according to any one of claims 1 to 4.