Foamed paper laminate

Abstract

The present invention aims to provide a method for manufacturing a foamed paper laminate, which is excellent in foaming property even when a printing layer is formed by using water-based flexographic printing, and which is also good in appearance. The present inventors have found that the above problem can be solved by a method for manufacturing a foamed paper laminate, which is characterized by being a method for manufacturing a foamed paper laminate having a foamed paper having a thermoplastic resin layer (A), a paper base material, and a foamed thermoplastic resin layer (B) in this order, and a printing layer formed on the surface of the foamed thermoplastic resin layer (B) of the foamed 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 2 times of overprinting.

Description

Technical Field

[0001] Embodiments of the present invention relate to a method for manufacturing a foamed paper laminate having a printed layer. Background Art

[0002] Foamed containers are widely used as containers for accommodating foods containing liquids at high or low temperatures due to their excellent heat insulation properties, and are particularly indispensable for use in cup noodles. As foamed containers for cup noodles, foamed styrene containers and foamed paper containers are known. However, in recent years, foamed paper containers have attracted attention from the viewpoints of environmental burden and safety.

[0003] Foamed paper containers are manufactured using a foamed paper material having a paper base material layer and a thermoplastic resin layer that foams due to heating during container manufacturing or the like to form a heat insulation layer. Usually, a printed layer on which decorative patterns, company names, barcodes, etc. are depicted is formed on the surface of the foamed paper container (foamed paper material). Therefore, for the printed layer, it is desired that when the thermoplastic resin layer of the foamed paper material foams due to heating to form a heat insulation layer, foaming is not hindered and the foaming followability is excellent. In addition, it is desired that the surface (printing surface) of the printed layer of the foamed paper laminate after the thermoplastic resin layer foams is smooth, without cracks and blisters (Japanese: 火脤れ), etc., and has an excellent appearance (hereinafter referred to as "foamed appearance"). <U+

[0004] Conventionally, when forming the printed layer of a foamed paper container, an oil-based ink such as an oil-based gravure ink and an oil-based flexographic ink has been used. From the viewpoints of the solubility and drying properties of the binder resin, oil-based inks generally contain organic solvents such as toluene. However, in recent years, restrictions on the use of organic solvents have gradually become stricter from the viewpoints of environmental burden and labor safety and hygiene, etc., and therefore, a conversion to a water-based flexographic ink is being studied.

[0005] For example, in Patent Document 1, a water-based flexographic ink containing a binder resin, a pigment, and water, and the binder resin containing a polyurethane resin has been disclosed as an ink for forming the printed layer of a foamed paper laminate. In addition, in Patent Document 2, it has been disclosed that foaming of the foamed layer is suppressed within a given range by a coating portion (printed layer) using a water-based flexographic ink. The water-based flexographic ink disclosed in Patent Document 2 contains a colorant and a binder resin, and in the resin solid content of the binder resin, a urethane resin having a glass transition temperature of -20°C or higher and 30°C or lower is contained at 30% by mass or more. Further, in Patent Document 3, a water-based flexographic ink containing an acrylic resin, an acrylic-urethane copolymer resin, a styrene-maleic copolymer resin, or a polyurethane resin as a binder resin has been disclosed.

[0006] Prior Art Documents<00000U+ <000004U+

[0007] Patent Documents

[0008] Patent Document 1: Japanese Patent Application Publication No. 2018-058955

[0009] Patent Document 2: International Publication No. 2018 / 066031

[0010] Patent Document 3: Japanese Patent Application Publication No. 2011-068381 Summary of the Invention

[0011] The problem that the invention aims to solve

[0012] The thermal insulation of foamed paper containers primarily depends on the foaming properties of the thermoplastic resin layer that constitutes the foamed layer (insulation layer) in the foamed paper laminate. However, as the foaming properties of the thermoplastic resin layer increase during heat processing, appearance defects such as cracks and blistering are prone to occur on the surface of the printed layer formed on the thermoplastic resin layer. Therefore, for the printed layer, it is desirable to uniformly suppress foaming in order to prevent appearance defects such as cracks and blistering.

[0013] However, during the development of this invention, it was discovered that when using water-based flexographic inks, appearance defects are prone to occur under certain batch, location, and temperature conditions of a particular thermoplastic resin layer (film). It should be noted that there are also cases where appearance defects do not occur under certain batch and temperature conditions of a particular film; therefore, it is presumed that these appearance defects are not caused by the coating properties of the water-based flexographic ink.

[0014] Therefore, in view of the above situation, the objective is to provide a method for manufacturing a foamed paper laminate that exhibits excellent foaming properties and good appearance even when the printed layer is formed using water-based flexographic printing.

[0015] Methods for solving problems

[0016] The inventors have investigated the causes of appearance defects occurring under certain batch conditions of a specific film when using water-based flexographic ink, and have conducted in-depth research on solutions to this problem. The results showed that differences in the coating amount of the water-based flexographic ink under certain batch conditions of a specific film cause appearance defects. Eliminating the uneven ink coating amount eliminates the appearance defects, thus completing this invention.

[0017] That is, one embodiment of the present invention relates to a method for manufacturing a foamed paper laminate, characterized in that it is a method for manufacturing a foamed paper laminate having a foamed paper having a thermoplastic resin layer (A), a paper substrate layer and a foamed thermoplastic resin layer (B) in sequence, and a printing layer formed on the surface of the foamed thermoplastic resin layer (B) of the foamed 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 repeated printings.

[0018] One embodiment relates to a method for manufacturing the above-described foamed paper laminate, characterized in that the number of foaming units per unit area of ​​the foamed thermoplastic resin layer (B) is 800 units / cm². 2 above.

[0019] One embodiment relates to a method for manufacturing the above-described foamed paper laminate, characterized in that the number of foaming units per unit area of ​​the foamed thermoplastic resin layer (B) is 1000 units / cm². 2 above.

[0020] One embodiment relates to a method for manufacturing the above-described foamed paper laminate, characterized in that the film thickness of the printed layer is 1.0 to 2.5 μm.

[0021] One embodiment relates to a method for manufacturing the above-described foamed paper laminate, characterized in that the thickness of the foamed thermoplastic resin layer (B) is 500 to 950 μm.

[0022] Invention Effects

[0023] According to the present invention, foamed paper containers with good foaming appearance can be manufactured regardless of the batch of the membrane, etc. Detailed Implementation

[0024] The present invention will now be described in detail. However, the present invention is not limited to the embodiments described below.

[0025] <1> Foamed paper laminate

[0026] The foamed paper laminate is characterized by having foamed paper having sequentially formed a thermoplastic resin layer (A), a paper substrate layer, and a foamed thermoplastic resin layer (B), and a printing layer formed on the surface of the foamed thermoplastic resin layer (B) of the foamed paper, wherein the number of foaming units per unit surface area of ​​the foamed thermoplastic resin layer (B) is 1000 units / cm. 2 above.

[0027] <Foamed Thermoplastic Resin Layer (B)>

[0028] (Number of foaming units per unit surface area of ​​the foamed thermoplastic resin layer (B))

[0029] For the number of foaming units per unit surface area of ​​the aforementioned foamed thermoplastic resin layer (B) (hereinafter referred to as foamed layer (B)), a larger number means that the bubbles in the foamed layer (B) are smaller, and a smaller number means that the bubbles in the foamed layer (B) are larger. If the bubbles in the foamed layer (B) become larger, it is easy for appearance defects such as cracks and bubbling to occur on the printed surface.

[0030] The number of foaming units per unit surface area of ​​the foam layer (B) needs to be at least 800 per 1cm. 2 The above is preferably 1000 pieces / 1cm. 2 The above is preferred, with 1250 pieces / cm being even better. 2 The above describes how, when the number of foaming units is within the aforementioned range, an excellent foamed appearance free of cracks and bubbles can be easily obtained in the printed layer formed on the foamed layer (B). On the other hand, there is no particular upper limit to the number of foaming units. In one embodiment, from the viewpoint of manufacturing conditions, the aforementioned number of foaming units can be 1600 units / cm². 2 the following.

[0031] Here, "number of foaming units per unit area" refers to the value obtained as follows: on the surface of the foam layer (B), the number of independent units (bubbles) existing within a range divided by a certain length along the longitudinal and transverse (XY) directions is counted, and the number is expressed as per 1 cm. 2 The number of independent units is calculated in the form of the number of independent units. After removing the printed layer of the foamed paper laminate with a solvent to expose the surface of the foamed layer (B), the surface of the foamed layer (B) is observed using an optical microscope to determine the number of independent units.

[0032] (Thickness of the foamed thermoplastic resin layer (B))

[0033] From the viewpoint of thermal insulation, the thickness of the foam layer (B) is preferably 500 μm or more, and more preferably 630 μm or more. If the thickness of the foam layer (B) is 500 μm or more, even when the foamed paper laminate is formed into a cup-shaped container and hot water at around 100°C is poured into it, the container can be easily and continuously held in place by bare hands. On the other hand, from the viewpoint of resource conservation, the amount of resin used is preferably as small as possible. Furthermore, from the viewpoint of thermal insulation, it is unnecessary for the insulation layer to reach an excessively high quality. Therefore, the thickness of the foam layer (B) is preferably 950 μm or less, more preferably 900 μm or less, and most preferably 800 μm or less.

[0034] From the above viewpoints, in one embodiment, the thickness of the foamed layer (B) is preferably 500 to 950 μm, more preferably 500 to 900 μm, and still more preferably 500 to 800 μm. The thickness of the foamed layer (B) is determined by observing a cross-section of the foamed paper laminate using an optical microscope photograph and measuring the height from the upper surface of the paper substrate to the lower surface of the printing layer.

[0035] (Total thickness of the foamed paper laminate)

[0036] In the above embodiment, the total thickness of the foamed paper laminate is preferably 1000 to 1450 μm, more preferably 1000 to 1300 μm. Here, the total thickness refers to the height from the upper surface of the printing layer constituting the foamed paper laminate to the lower surface of the thermoplastic resin layer (A), and is determined by observing a cross-section of the foamed paper laminate using an optical microscope photograph and measuring the above height.

[0037] In the foamed paper laminate of the above embodiment, the foamed layer (B) of the foamed paper refers to the state in which the thermoplastic resin layer is foamed by heating. That is, the unfoamed thermoplastic resin layer (foamed thermoplastic resin layer forming layer (B0)) that is the precursor is heated to be foamed, thereby forming the foamed layer (B). In one embodiment, in order to constitute the above foamed paper laminate, a foamed paper material (pre-heated foamed paper) having a thermoplastic resin layer (A), a paper substrate, and a foamed thermoplastic resin layer forming layer (B0) having a melting point lower than that of the thermoplastic resin layer (A) and being foamed by heat treatment can be used in this order. The foamed paper material can be composed of materials well-known in the art. Hereinafter, the constituent materials of the foamed paper laminate will be specifically described.

[0038] <Paper substrate layer>

[0039] (Unit area weight (Japanese: tsubo-ryo) and moisture content of the paper substrate)

[0040] The paper substrate layer constituting the foamed paper laminate is formed of a paper substrate, and its type is not particularly limited. For example, kraft paper or high-quality paper can be used. From the viewpoint of achieving sufficient toughness when used as a container, the unit area weight of the paper substrate is preferably 150 to 450 g / m 2 , more preferably 250 to 400 g / m 2 . The thickness of the paper substrate layer is preferably 110 to 860 μm, more preferably 140 to 640 μm. In addition, from the viewpoint of obtaining suitable foaming properties of thermoplastic resins such as polyethylene, the moisture content contained in the paper substrate layer is preferably 4 to 10% by mass, more preferably 5 to 8% by mass.

[0041] <Thermoplastic resin layer (A), foamed layer forming layer (B0)>

[0042] In one embodiment, the thermoplastic resin layer (A) and the foaming layer forming layer (B0) may each be a film comprising a resin material conventionally known as a container material. For example, a film comprising at least one thermoplastic resin selected from stretched and unstretched polyolefins, polyester, nylon, cellophane, and vinylon (thermoplastic resin film) may be used. In one embodiment, a polyethylene film may be suitably used from the perspective of excellent lamination adaptability and foaming properties.

[0043] Foamed paper material can be formed by laminating thermoplastic resin films with different melting points onto a paper substrate. Here, the material is selected such that the melting point (Mp) of the thermoplastic resin film, which forms the foaming layer (B0), is lower than that of the thermoplastic resin film on one side of the paper substrate layer (A). In the foamed paper material, during heat treatment, moisture in the paper substrate layer evaporates, and this evaporated moisture is extruded towards the softened foaming layer forming layer (B0) (low Mp resin film). Furthermore, along with this extrusion, the low Mp resin film expands (foams) outward, forming the foaming layer (B). The foaming layer (B) thus formed functions as an insulation layer in the container. On the other hand, for the thermoplastic resin layer (A) (high Mp resin film), a material that does not melt or soften when the low Mp resin film foams due to heat treatment is selected.

[0044] From the viewpoint of manufacturing foamed paper containers using foamed paper laminates, the foamed paper material can, for example, have the following structure: a high-Mp polyethylene film (thermoplastic resin layer (A)) with a melting point of approximately 125°C to 140°C is laminated on one side of the paper substrate layer (the inner side of the container), and a low-Mp polyethylene film (foaming layer forming layer (B0)) with a melting point of approximately 105°C to 120°C is laminated on the other side of the paper substrate layer (the outer side of the container). The low-Mp polyethylene film foams to form the foamed layer through heating during the manufacturing of the foamed paper container. On the other hand, the high-Mp polyethylene film preferably functions as a coating layer. That is, the coating layer can suppress the evaporation (evaporation) of moisture from the paper substrate layer to the outside during the foaming of the low-Mp polyethylene film, allowing the moisture in the paper substrate layer to participate efficiently in the foaming process.

[0045] (Density of the foamed cambium (B0))

[0046] In one embodiment, regarding the material of the foamed layer forming layer (BO), in polyethylene resin, it preferably includes low-density polyethylene resin (density 910-925 kg / m³). 3 (Melting point 105-120℃). The density of low-density polyethylene resin is more preferably 910-922 kg / m³. 3A further preferred value is 910–918 kg / m³. 3 On the other hand, in the use of medium-density polyethylene resin (density 925~940kg / m³) 3 Melting point 115~130℃) and high-density polyethylene resin (density 940~970kg / m³) 3 When polyethylene resin (with a melting point of 125–140°C) is used as the foaming layer forming layer (BO), it tends to have a high melting point and is difficult to achieve sufficient foaming properties. It should be noted that the density of the polyethylene resin is a value determined using JIS K 6922-1 (1997).

[0047] (Mel flow rate of the foamed layer forming layer (B0))

[0048] Furthermore, 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 min, more preferably 10 to 20 g / 10 min. It should be noted that the MFR of the polyethylene resin is a value measured using JIS K 6922-1 (1997).

[0049] (Film thickness of the foamed layer (B0))

[0050] While not particularly limited, in one embodiment, the film thickness of the foamed layer forming layer (BO) is preferably 40 μm or more, more preferably 60 μm or more. By adjusting the film thickness to 40 μm or more, sufficient thermal insulation can be obtained after the heat foaming treatment.

[0051] On the other hand, from the viewpoint of resource conservation, the amount of resin used is preferably as small as possible. Furthermore, from the viewpoint of thermal insulation, a thickness that reaches an excessive level of quality is not necessary. Therefore, the film thickness of the foamed layer forming layer (BO) is preferably 150 μm or less, more preferably 100 μm or less, and extremely preferably 80 μm or less.

[0052] <Printed Layer>

[0053] In the foamed paper laminate of the above embodiments, the printing layer is a coating film obtained by coating the surface of the foamed layer forming layer (BO) of the foamed paper material with an aqueous flexographic ink composition. The main component of the coating film constituting the printing layer is the binder resin in the aqueous flexographic ink composition, and the binder resin preferably contains polyurethane resin.

[0054] (film thickness of the printed layer)

[0055] From the viewpoint of suppressing foaming caused by the printed layer, the thickness of the printed layer (the dried coating) is preferably 1.0 to 2.5 μm. More preferably, the thickness of the printed layer is 1.2 to 2.5 μm, and even more preferably, 1.5 to 2.5 μm.

[0056] <2> Water-based flexographic ink

[0057] The aqueous flexographic ink of the present invention comprises polyurethane resin (X), water, alcohol solvent, colorant and other materials.

[0058] By using a flexible polyurethane resin (X) as the main component, it is possible to suppress over-foaming of the foaming layer forming layer (B0) while also suppressing cracks in the foamed printed layer.

[0059] As one embodiment of polyurethane resin (X), a polyurethane resin (X1) obtained by reacting a polyol with a polyisocyanate can be cited. Alternatively, as another embodiment, a polyurethane urea resin (X2) obtained by chain extension of a prepolymer of a polyurethane resin can be cited.

[0060] First, a polyol (y1) with acidic groups and a polymeric polyol (y2) (or other polyols (y3) as needed) are reacted with a polyisocyanate (z1) to obtain a polyurethane resin (X1). Then, a polyol (y1) with acidic groups and a polymeric polyol (y2) (or other polyols (y3) as needed) are reacted with the polyisocyanate (z1), and a chain extender (z2) is reacted with the resulting isocyanate-terminated prepolymer to obtain a polyurethane urea resin (X2).

[0061] Examples of polyols (y1) that have an acid group include polyols with a carboxyl group and polyols with a sulfonic acid group.

[0062] Examples of polyols containing carboxyl groups include 2,2-dimethylolpropionic acid, 2,2-dimethylolbutyric acid, and 2,2-dimethylolvalerate. Among these, 2,2-dimethylolpropionic acid and 2,2-dimethylolbutyric acid, which have good dispersion stability, are preferred.

[0063] Examples of polyols containing sulfonic acid groups include polyester polyols obtained by reacting dicarboxylic acids such as 5-sulfoisophthalic acid, sulfoterephthalic acid, 4-sulfophthalic acid, and 5-(4-sulfophenoxy)isophthalic acid, or their salts, with low-molecular-weight polyols such as ethylene glycol, propylene glycol, 1,4-butanediol, 1,6-hexanediol, diethylene glycol, and neopentyl glycol. These polyols containing acid groups can be used alone or in combination of two or more.

[0064] The polyol (y1) having an acid group is preferably used in the range of 10 to 50, more preferably in the range of 10 to 35, where the acid value of the urethane resin (X) is within this range. Within this range, the urethane resin (X) can be dissolved in water or self-emulsified. Furthermore, by dissolving the urethane resin in water or enabling self-emulsification, the pigment dispersibility is also improved.

[0065] Examples of polymer polyols (y2) include polyether polyols, polyester polyols, and polycarbonate diols, with polyether polyols and / or polyester polyols being more preferred.

[0066] Examples of polyether polyols include polyethylene glycol, polypropylene glycol, and poly1,4-butanediol. Examples of polyester polyols include polyester polyol (PMPA) obtained from adipic acid and 3-methyl-1,5-pentanediol, and polyester diol (PPA) obtained from adipic acid and 1,2-propanediol. These polymer polyols can be used alone or in combination of two or more.

[0067] The number-average molecular weight of the polymeric polyol (y2) is not particularly limited, as it needs to be appropriately determined considering the characteristics of the polyurethane resin obtained as a reaction product, such as ethanol resistance, elongation, and stress. Generally speaking, the number-average molecular weight of the polymeric polyol (y2) is preferably in the range of 500 to 10,000, and more preferably in the range of 500 to 6,000.

[0068] In addition, other polyols (y3) besides the polyols mentioned above can be used in this invention.

[0069] Other polyols (y3) include, for example, cyclobutanediol, cyclopentanediol, 1,4-cyclohexanediol, cycloheptanediol, cyclooctanediol, cyclohexanediol, hydroxypropylcyclohexanol, dicyclohexanediol, butylcyclohexanediol, 1,1'-dicyclohexenediol, cyclohexanetriol, hydrogenated bisphenol A, and 1,3-adamantanediol, which are low molecular weight polyols containing alicyclic structures, ranging from approximately 100 to 500. These other polyols can be used alone or in combination of two or more.

[0070] Furthermore, in order to suppress the adhesion of printed materials, it is preferable to use the aforementioned polyol (y3) in the range of 1 to 20% by weight relative to the total amount of polyols. It should be noted that the total amount of polyols refers to the amount obtained by adding together the polyol with acid groups (y1), the polymer polyol (y2), and other polyols (y3).

[0071] As polyisocyanates (z), examples include various well-known aromatic diisocyanates, aliphatic diisocyanates, and alicyclic diisocyanates commonly used in the manufacture of polyurethane resins. Examples include 1,5-naphthalene diisocyanate, 4,4′-diphenylmethane diisocyanate (MDI), 4,4′-diphenyldimethylmethane diisocyanate, 4,4′-dibenzyl isocyanate, dialkyldiphenylmethane diisocyanate, tetraalkyldiphenylmethane diisocyanate, m-phenylene diisocyanate, terephthalene diisocyanate, toluene diisocyanate, butane-1,4-diisocyanate, hexamethylene diisocyanate, isopropylidene diisocyanate, methylene diisocyanate, 2,2,4-trimethylhexamethylene diisocyanate, lysine diisocyanate, cyclohexane-1,4-diisocyanate, phenylenediamine diisocyanate, isophorone diisocyanate, and dimer acid diisocyanate (Dimeryl Diisocyanates include dicyclohexylmethane-4,4′-diisocyanate, 1,3-bis(isocyanate methyl)cyclohexane, methylcyclohexane diisocyanate, norbornane diisocyanate, tetramethyl isophthalimide diisocyanate, 4,4-diphenylmethane diisocyanate, toluene diisocyanate, dichloromethyl-diphenylmethane-diisocyanate, 2,6-diisocyanate-benzyl chloride, and dimer diisocyanates obtained by converting the carboxyl group of a dimer acid to an isocyanate group. These diisocyanate compounds can be used alone or in combination of two or more.

[0072] Among the above-mentioned polyisocyanates, preferably at least one is selected from toluene diisocyanate, isophorone diisocyanate, phenyl diisocyanate, bis(isocyanomethyl)cyclohexane, hexamethylene diisocyanate and trimers of hexamethylene diisocyanate.

[0073] Examples of chain extenders (z2) include diamine compounds such as ethylenediamine, propylenediamine, hexamethylenediamine, diethylenetriamine, triethylenetetramine, isophoronediamine, and dicyclohexylmethane-4,4′-diamine; amine compounds containing intramolecular hydroxyl groups such as 2-hydroxyethylethylenediamine, 2-hydroxyethylpropyldiamine, 2-hydroxyethylpropyldiamine, di(2-hydroxyethyl)ethylenediamine, di(2-hydroxy)ethylenediamine, di(2-hydroxyethyl)propylenediamine, 2-hydroxypropylethylenediamine, di(2-hydroxypropyl)ethylenediamine, and di(2-hydroxypropyl)ethylenediamine; and diethylenediamine. Polyfunctional amine compounds such as methyltriamine, iminodipropylamine (IBPA, 3,3′-diaminodipropylamine), N-(3-aminopropyl)-1,4-butanediamine (spermine), 6,6-iminodihexylamine, hydrazine, N,N'-dimethylhydrazine, 1,6-hexamethylenedihydrazine, succinic dihydrazine, adipic dihydrazine, glutaric dihydrazine, sebacate dihydrazine, isophthalic dihydrazine, β-aminourea propionic hydrazine, 3-aminourea-propyl-hydrazyl carbamate, and aminourea-3-aminourea-methyl-3,5,5-trimethylcyclohexane.

[0074] In addition, polymerization terminators (z3) can be used to stop excessive reactions. Specifically, examples include compounds with primary or secondary amino groups, dialkylamines such as di-n-butylamine, and amino alcohols.

[0075] Furthermore, to neutralize the acid-valued polyurethane resin into a water-based solution, a neutralizing agent (z4) is preferably used. Specifically, examples include sodium hydroxide, potassium hydroxide, ammonia, methylamine, ethylamine, propylamine, ethanolamine, propanolamine, diethanolamine, dimethylamine, diethylamine, and triethylamine. It should be noted that if a neutralizing agent remains in the ink coating after printing, water resistance can easily decrease, and residual odor may occur. Therefore, ammonia, which volatilizes rapidly during printing, is preferred as a neutralizing agent.

[0076] Examples of methods for synthesizing polyurethane resin (X) include polymerization using an organic solvent that is non-reactive to isocyanates and is hydrophilic (solvent method) and polymerization without using a solvent (solvent-free method).

[0077] Examples of hydrophilic organic solvents that are non-reactive to isocyanates 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. It should be noted that in solvent-based methods, water and a neutralizing agent need to be added after the polymerization reaction to remove the organic solvent. Therefore, solvents with boiling points lower than water are preferred as organic solvents.

[0078] (Content of polyurethane resin (X))

[0079] It should be noted that the solid components of the water-based flexographic ink preferably contain 20% to 70% by weight of polyurethane resin (X), more preferably 40% to 60% by weight. By containing 20% ​​or more of polyurethane resin (X), sufficient adhesion can be achieved. Furthermore, by setting the polyurethane resin (X) to 70% or less, the viscosity can be easily adjusted to a level suitable for printing.

[0080] In this invention, alcohol-based solvents can be used in addition to water as solvents for the water-based flexographic ink. By using alcohol-based solvents, drying properties and wettability onto the printing substrate can be improved. Specifically, methanol, ethanol, isopropanol, n-propanol, etc., can be used.

[0081] (White pigment)

[0082] The aqueous flexographic ink used in this embodiment contains various colorants. Examples of colorants (white pigments) for the white ink include titanium dioxide, zinc oxide, zinc sulfide, barium sulfate, calcium carbonate, and aluminum hydroxide. Regarding the content of the white pigment, based on the total weight of the white ink, it is acceptable to have a content of 20% by weight or more and 50% by weight or less; by setting it within this range, the desired ink concentration can be obtained. Furthermore, from the viewpoint of opacity and lightfastness, titanium dioxide is preferred as the white pigment.

[0083] (Colored pigments)

[0084] Examples of colorants (colored pigments) used in colored inks include inorganic pigments such as carbon black, aluminum, and red iron powder (iron oxide), as well as organic pigments such as azo pigments, phthalocyanine pigments, anthraquinone pigments, perylene pigments, violet ketone pigments, quinacridone pigments, indigo sulfide pigments, dioxazine pigments, isoindolinone pigments, quinophthalone pigments, and pyrrolopyrrole dione pigments. Regarding the content of colored pigments, based on the total weight of the colored ink, it is acceptable as long as it is between 5% and 30% by weight. By setting it within this range, the desired ink concentration can be obtained.

[0085] (Body pigment)

[0086] The aqueous flexographic ink used in this embodiment may further contain extender pigments. As mentioned earlier, the aqueous flexographic ink used in this embodiment contains a large amount of polyurethane resin. Therefore, the printed layer is prone to sticking. However, by including extender pigments in this embodiment, the sticking of the printed layer can be suppressed while maintaining the flexibility of the polyurethane resin.

[0087] Silica is an example of the aforementioned extender pigment. 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 with a particle size of 2 μm or more and 20 μm or less, printing defects are prevented, and adhesion can be suppressed.

[0088] In this embodiment, when silica is present, the silica content is preferably set to 0.2% by mass or more and 5% by mass or less, more preferably 0.5% by mass or more and 3.5% by mass or less, based on the total weight of the water-based flexographic ink. By setting the silica content within the above range, adhesion can be suppressed, and the viscosity can be adjusted to be easily handled as a water-based flexographic ink.

[0089] <3> Method for manufacturing foamed paper laminate

[0090] One embodiment relates to a method for manufacturing a foamed laminate. Specifically, one embodiment relates to a method for manufacturing a foamed paper laminate having foamed paper having sequentially formed a thermoplastic resin layer (A), a paper substrate layer, and a foamed layer (B), and a printed layer formed on the surface of the foamed layer (B) of the foamed paper, wherein the number of foaming units per unit surface area of ​​the foamed layer (B) is 1000 units / cm². 2 The above. This manufacturing method includes the following steps (i) to (iii), namely...

[0091] (i) Lamination process,

[0092] (ii) Flexographic printing process,

[0093] (iii) Foaming process.

[0094] In the above manufacturing method, (i) the lamination process, (ii) the flexographic printing process, and (iii) the foaming process can each be implemented according to methods well known in the art. The following describes each process.

[0095] (Process (i): Lamination process)

[0096] The preparation of the foamed paper material can be carried out, for example, by extrusion lamination. The constituent materials of the paper substrate, thermoplastic resin layer (A), and foamed layer forming layer (B0) constituting the foamed paper material are as described above. As the extrusion lamination method, well-known methods such as single lamination, tandem lamination, sandwich lamination, and co-extrusion lamination can be appropriately selected. In one embodiment, polyethylene resins with different melting points can be appropriately used as the constituent materials of the thermoplastic resin layer (A) and the foamed layer forming layer (B0).

[0097] The foamed layer forming layer (B0) is composed of a polyethylene resin (low-Mp polyethylene resin) having a melting point (Mp) lower than that of the polyethylene resin constituting the thermoplastic resin layer (A). The foamed paper material can be manufactured by extruding low-Mp polyethylene resin in a film form onto one side of the paper substrate layer through a T-die extruder, and simultaneously extruding high-Mp polyethylene resin in a film form onto the other side of the paper substrate layer.

[0098] The temperature of the polyethylene resin during lamination (the temperature immediately below the T-die) is preferably 300–350°C, more preferably 320–340°C. Within this temperature range, sufficient lamination strength can be achieved between each polyethylene resin layer (A, B0) and the paper substrate layer. The surface temperature of the cooling roller after lamination is preferably controlled within the range of 10–50°C.

[0099] 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, productivity is low; on the other hand, if the lamination speed is too fast, the yield tends to decrease due to necking. Necking refers to the phenomenon where, when polyethylene resin is extruded and film-formed using a T-die extruder, the width of the extruded polyethylene resin film becomes smaller than the effective width of the T-die. In this case, the ends of the film become thicker than the central portion. When the thickness at both ends deviates from the standard, the ends are usually cut off; however, in cases of severe necking, the yield decreases because the area deviating from the standard increases.

[0100] In one embodiment, the air gap (Japanese: エアギヤップ) is preferably 300 mm or less, more preferably 200 mm or less. If the air gap is too large, the polyethylene resin will neck, tending to reduce the yield. The air gap refers to the distance from the extrusion port of the T-die to the clamping roller. During the passage of the polyethylene resin through the air gap, it is preferable to perform surface treatment on the polyethylene resin using ozone gas and / or oxygen. By using ozone gas and / or oxygen for surface treatment, the formation of an oxide film can be promoted, and the adhesion to the substrate layer can be improved. There is no particular limitation on the amount of ozone gas and / or oxygen used, however, from the viewpoint of promoting the oxidation of polyethylene resin, 0.5 mg / m³ is preferred. 2 above.

[0101] (Process (ii): Flexographic printing process)

[0102] The printing layer is formed using flexographic printing. Flexographic printing is a printing method with minimal scale deviation and high production efficiency. However, due to the use of a letterpress, the amount of ink applied is limited. Therefore, compared to other printing methods (such as gravure printing), flexographic printing typically uses inks with high pigment concentration. However, in this invention, because the printing layer (especially the white ink layer) needs to have the function of suppressing over-foaming of the foaming layer (BO), inks with high pigment concentration and low resin ratio cannot be used.

[0103] Therefore, in this invention, at least two printing passes are required to form the white ink layer. By performing two printing passes on the white ink, both high design flexibility (coverage) and suppression of over-foaming can be achieved.

[0104] (Process (iii): Foaming process)

[0105] In the formation of the foamed layer (B), the appropriate heating temperature and heating time vary depending on the characteristics of the paper substrate layer and the thermoplastic resin film used. Those skilled in the art can determine the optimal combination of heating temperature and heating time based on the materials used, such as the thermoplastic resin film. Although not particularly limited, heat treatment is generally performed during the container forming process. If the heating temperature is too low, sufficient foaming cannot be obtained; if the heating temperature is too high, the foaming units may bond together, easily leading to foaming.

[0106] While not particularly limited, when the foamed layer is formed from a low-density polyethylene film, the heating temperature is preferably 100–125°C, more preferably 110–120°C. The heating time can be appropriately adjusted according to the heating temperature, but is preferably 3–10 minutes, more preferably 5–7 minutes. In one embodiment, when the foamed layer is formed from a low-density polyethylene film and a printing layer is formed thereon using the previously described aqueous flexographic ink composition, the heating temperature is preferably adjusted to 110–123°C and the heating time to 5–7 minutes. More preferably, the heating temperature is adjusted to 115–121°C and the heating time to 5–7 minutes. Under the above conditions, it is easy to appropriately control the foaming of the foamed layer during the heating process using the printing layer.

[0107] As a heating method, any method can be used, such as hot air, electric heating, or electron beam. If hot air or electric heating is used in a tunnel equipped with a conveyor belt-based transport vehicle, large-scale heating processes can be carried out inexpensively.

[0108] <4> Foamed paper containers

[0109] One embodiment relates to a foamed paper container having the foamed paper laminate of the above embodiment. The foamed paper container is characterized in that it is formed of a container body component and a base plate component, wherein the container body component is formed of the foamed paper laminate of the above embodiment.

[0110] The forming process of foamed paper containers can be carried out using well-known techniques. For example, a foamed paper laminate with an initial printed layer (the foamed paper laminate before heating) is punched into a given shape along a die to obtain a container body component. Similarly, a base plate material is punched into a given shape to obtain a base plate component. Then, using a common container manufacturing apparatus, the container body component and the base plate component are assembled into the shape of a container. The assembly and forming of the container using the container manufacturing apparatus is carried out as follows: the aforementioned high Mp resin film of the container body component forms the inner wall surface, the aforementioned low Mp resin film forms the outer wall surface, and the laminated surface of the base plate component becomes the inner side. After the container is assembled and formed using the container manufacturing apparatus in this way, a heat treatment is performed, thereby causing the low Mp resin film to foam, forming a foamed layer (insulating layer), and a foamed paper container with heat insulation properties can be obtained.

[0111] In one embodiment, when the inner wall surface and outer wall surface of the main body of the foamed paper container are formed of polyethylene film respectively, it is preferable that one side of the paper substrate (the inner wall surface of the container) is laminated with a medium-density or high-density polyethylene film, and the other side (the outer wall surface of the container) is laminated with a low-density polyethylene film. The thickness of each film laminated to the paper substrate is not particularly limited. However, the thickness of the low-MPa resin film constituting the outer wall surface of the container main body is preferably appropriately set such that, when the film is foamed, the foamed film reaches a thickness sufficient to function as an insulation layer.

[0112] For example, when the outer wall of the container body is formed with a low-density polyethylene film, the thickness of the film laminated to the paper substrate can be 40–150 μm. On the other hand, when the inner wall of the container body is formed with a medium-density or high-density polyethylene film, the thickness of the film laminated to the paper substrate is not particularly limited. However, it is preferable to appropriately set the film thickness to ensure resistance to permeation of contents when used as an insulated foamed paper container. Since the thickness of the film laminated to the paper substrate varies depending on the resin material of the film used, those skilled in the art will preferably consider the characteristics of the resin material to set it appropriately.

[0113] Example

[0114] The present invention will be further described in detail below with reference to embodiments; however, the present invention is not limited to these embodiments. It should be noted that, unless otherwise specified, "parts" and "%" in the following description refer to "parts by weight" and "% by weight".

[0115] (Trial Production Example 1)

[0116] Medium-density polyethylene resin is extruded and laminated onto one side of the paper substrate layer, and low-density polyethylene resin is extruded and laminated onto the opposite side of the paper substrate layer (lamination process). Then, water-based flexographic ink is printed on the low-density polyethylene side using a CI-type flexographic printing press (printing process). After printing, the paper is heated in an oven at 121°C for 120 seconds to produce foamed paper laminate 1 (prototype 1) (foaming process).

[0117] The details of the lamination process are shown below.

[0118] (Paper substrate)

[0119] Paper substrate: Moisture content 23g / m 2 320g / m² 2 .

[0120] (Thermoplastic resin layer (A): Medium-density polyethylene resin layer)

[0121] Medium density polyethylene resin (M): TOSOH "Petrocene LW04-1", MFR 4.3g / 10min, density 940kg / m³ 3 .

[0122] Extrusion temperature (T-die exit temperature): 320℃.

[0123] Traction speed (lamination speed): 50m / minute.

[0124] Air gap: 130mm.

[0125] Thickness: 40μm (thickness of the central part of the polyethylene resin layer).

[0126] (Foaming layer (B0): Low-density polyethylene resin layer)

[0127] Low-density polyethylene resin (L); TOSOH "Petrocene 07C03C", MFR 15g / 10min, density 918kg / m³ 3 .

[0128] Extrusion temperature (T-die exit temperature): 310℃.

[0129] Traction speed (lamination speed): 60m / minute.

[0130] Air gap: 130mm.

[0131] Thickness: 70μm (thickness of the central part of the polyethylene resin layer).

[0132] The details of the printing process are shown below.

[0133] Printing press: CI type flexographic printing.

[0134] Water-based flexographic ink: 20% by weight white pigment, 30% by weight urethane resin, 45% by weight water, and 5% by weight ethanol.

[0135] Line count (lpi): Refer to Table 1.

[0136] Cell volume (ml / m²) 2 (Refer to Table 1)

[0137] The layer structure of the foamed paper laminate in this prototype is shown below:

[0138] Printing / LDPE 70 / Paper / MDPE 40.

[0139] The slash " / " indicates the boundary between layers. The layer on the left forms the outer surface of the foamed paper laminate, and the layer on the right forms the innermost surface. "Print" indicates the printing layer. "LDPE" indicates the low-density polyethylene resin layer. "Paper" indicates the paper substrate layer. "MDPE" indicates the medium-density polyethylene resin layer. The numbers indicate the layer thickness (in μm).

[0140] (evaluate)

[0141] The foamed paper laminate was evaluated for the film thickness of the printed layer, hiding power, number of units in the foam layer (B), total thickness of the foamed paper laminate, and appearance (bubbling and cracking). Details are as follows.

[0142] (film thickness of the printed layer)

[0143] The thickness of the printed layer was determined using scanning electron microscopy (SEM) images (5000x magnification) of the cross-section of the foamed paper laminate. It should be noted that measurements were taken at five randomly selected locations, and their average values ​​are recorded in Table 1.

[0144] (Coverage)

[0145] When the printed layer (amount of pigment) is thin, the covering power of the printed layer is insufficient, and the foaming unit may be visible through it. Therefore, the covering power of the printed layer is evaluated according to the following criteria.

[0146] ○: The foaming unit is not visible, and the coverage is good.

[0147] ×: The foam unit can be seen through it, indicating poor coverage.

[0148] (Total thickness of the foamed paper laminate)

[0149] The total thickness of the foamed paper laminate, as described above, is determined by observing the cross-section of the foamed paper laminate using 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).

[0150] (Number of foaming units in foam layer (B))

[0151] The printed layer of the foamed paper laminate was removed with methyl ethyl ketone (MEK), exposing the surface of the foamed layer (B). Then, the surface of the foamed layer (B) was observed using an optical microscope (Nikon AZ100M) (25x magnification) to determine the number of independent units existing within a range divided by a certain length along the longitudinal and transverse (XY) directions, and subsequently, the number of units per 1 cm was obtained. 2 The values ​​are calculated in the form of the number of independent units. It should be noted that observations were made on 5 randomly selected locations, and their average values ​​are recorded in Table 1.

[0152] (Thickness of foam layer (B))

[0153] The thickness of the foam layer (B) was determined by observing the cross-section of the foamed paper laminate using an optical microscope and measuring the height from the upper surface of the paper substrate to the lower surface of the printed layer. Furthermore, the film thickness before foaming corresponds to the film thickness of the foam layer forming layer. Therefore, a value obtained by measuring the film thickness of the low-density polyethylene resin formed as the foam layer forming layer was used.

[0154] (Appearance Evaluation)

[0155] <Bubble>

[0156] The printed surface of the foamed paper laminate was visually observed. The evaluation criteria are shown below. It should be noted that the results shown in Table 1 are the most frequent values ​​when 10 samples of the foamed paper laminate were randomly selected and evaluated. In cases where multiple most frequent values ​​exist, the value with the lower evaluation value is used.

[0157] (Evaluation Criteria)

[0158] ○: No bubbles at all (bubbling cannot be confirmed).

[0159] △: per 100cm 2 There are 1 to 2 bubbles with a major diameter of less than 5 mm.

[0160] ×: per 100cm 2 There are more than 3 bubbles with a major diameter less than 5 mm in the sample. Or per 100 cm 2 There is one bubble with a major diameter of 5mm or more.

[0161] It should be noted that when multiple bubbles with different lengths are mixed together, a lower evaluation score is used. Specifically, per 100cm...2 If there are two bubbles with a major diameter less than 5 mm and one bubble with a major diameter of 5 to 20 mm, the evaluation is "×".

[0162] <Crack>

[0163] For foamed paper laminates, the evaluation method is the same as for bubble formation, involving visual observation of the printed surface. The evaluation criteria are shown below. It should be noted that the results shown in Table 1 are the most frequent values ​​obtained from observing and evaluating 10 randomly prepared samples of foamed paper laminates. In cases where multiple most frequent values ​​exist, the lower-ranking value is used.

[0164] (Evaluation Criteria)

[0165] ○: No cracks at all (cracks cannot be identified).

[0166] △: per 100cm 2 There are 1 to 4 cracks less than 2 mm in length.

[0167] ×: per 100cm 2 There are more than 5 cracks less than 2mm in length within 100cm. 2 There is one or more cracks with a length of more than 2mm.

[0168] It should be noted that a lower evaluation is used when multiple cracks of varying lengths are mixed together. Specifically, for every 100cm... 2 If there is one crack with a length of 1 mm and one crack with a length of 4 mm, the evaluation is "×".

[0169] Table 1

[0170]

[0171] It should be noted that in the foamed paper laminates of Test Examples 1 to 7, the thickness of the foam layer (B) is 500 to 950 μm. In addition, the thickness of the foam layer (B) in the foamed paper laminates of Test Examples 3 to 7 is 500 to 800 μm.

Claims

1. A method for manufacturing a foamed paper laminate, characterized in that, The present invention relates to a method for manufacturing a foamed paper laminate comprising a foamed paper having a thermoplastic resin layer A, a paper substrate layer, and a foamed thermoplastic resin layer B in sequence, and a printed layer formed on the surface of the foamed thermoplastic resin layer B of the foamed paper by flexographic printing. The printed layer has at least one water-based white ink layer, and the white ink layer is formed by at least two repeated printing processes. The number of foaming units per unit area of ​​the foamed thermoplastic resin layer B is 800 per 1cm. 2 above, The thickness of the printed layer is 1.0 μm to 2.5 μm. The white ink layer contains polyurethane resin as its main component.

2. The method for manufacturing the foamed paper laminate according to claim 1, characterized in that, The number of foaming units per unit area of ​​the foamed thermoplastic resin layer B is 1000 per 1cm. 2 above.

3. The method for manufacturing a foamed paper laminate according to claim 1 or 2, characterized in that, The thickness of the printed layer is 1.2 μm to 2.5 μm.

4. The method for manufacturing a foamed paper laminate according to claim 1 or 2, characterized in that, The thickness of the foamed thermoplastic resin layer B is 500μm to 950μm.

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