Coating agent for food packaging sheets and food packaging sheets

Abstract

To provide a coating agent for food packaging sheets that can form a coating layer with excellent low-temperature heat sealability, blocking resistance, and oil resistance, and a food packaging sheet using the same. [Solution] A coating agent for food packaging sheets comprising an acrylic polymer (A) having a glass transition temperature of -50 to 10°C and lacking acid groups, an acrylic polymer (B) having a glass transition temperature of 40 to 150°C, and a plant-derived wax, wherein the coating agent contains 1 to 100 parts by mass of the plant-derived wax per 100 parts by mass of the total of the acrylic polymer (A) and the acrylic polymer (B), and neither the acrylic polymer (A) nor the acrylic polymer (B) has structural units derived from olefin monomers.

Description

Technical Field

[0001] The present invention relates to a coating agent for food packaging sheets and food packaging sheets.

Background Art

[0002] In food packaging sheets, conventionally, laminate sheets in which a resin film is laminated on a paper base material have been widely used for the purpose of preventing oil stains caused by oil components derived from foods. Further, a method of processing into a form that is easy to package food by heat-sealing these openings (hereinafter, also referred to as "heat seal") is frequently used.

[0003] On the other hand, in recent years, efforts to reduce environmental impact have been demanded globally, and efforts such as reducing the use of plastics and designing recyclable products have been carried out. Along with this, in food packaging sheets, instead of conventional laminate sheets, the development of paper coat agent type food packaging sheets in which a coating agent is applied on a paper base material, which is easy to recycle and can also reduce the amount of plastic used, has been actively carried out.

[0004] As such a packaging sheet, for example, in Patent Document 1, a water-resistant paper in which a water-resistant layer containing an ethylene-methacrylic acid copolymer and organic particles having a melting point or softening point of 100°C or higher at 40% by mass or less is provided on a paper base material is disclosed.

[0005] In a sheet having such a coat layer, it has excellent heat sealability and does not cause sticking between sheets (hereinafter, also referred to as "blocking") when the sheets are stacked and stored, but there is a problem that oil resistance cannot be exhibited and oil stains from food occur. Also, from the viewpoints of reducing environmental impact and safety, a lower heating temperature during heat sealing is preferable.

[0006] When packaging sheets are manufactured by coating them with polymers that form a flexible coating film to achieve heat sealability and oil resistance at low temperatures of 80-120°C, blocking resistance deteriorates. When sheets are peeled apart after being stored in stacks, defects occur in the coating layer and substrate at the blocking points, and oil stains easily form in these defective areas, making them unsuitable for use as food packaging sheets. On the other hand, using polymers that form a robust coating film improves blocking resistance, but reduces heat sealability at low temperatures, weakening the seal strength at the opening. This can lead to problems such as the opening of the sealed packaging material opening during transportation or handling of packaged food. In addition, there is the issue of reduced oil resistance and increased susceptibility to oil stains. It is generally recognized among those skilled in the art that blocking resistance is a property that conflicts with low-temperature heat sealability and oil resistance.

[0007] Furthermore, while olefin-based materials derived from petroleum have traditionally been used as the primary raw material for coatings used in food packaging sheets, research is underway to manufacture these coatings using plant-derived materials, similar to paper substrates, from an environmental perspective. [Prior art documents] [Patent Documents]

[0008] [Patent Document 1] Japanese Patent Publication No. 2022-186782 [Overview of the Initiative] [Problems that the invention aims to solve]

[0009] The problem that this invention aims to solve is to provide a coating agent for food packaging sheets that can form a coating layer with excellent low-temperature heat sealability, blocking resistance, and oil resistance, and a food packaging sheet using the same. [Means for solving the problem]

[0010] The inventors of this invention have diligently conducted research to solve the above problems, and as a result, have arrived at the present invention. The present invention relates to a coating agent for food packaging sheets comprising an acrylic polymer (A) having a glass transition temperature of -50 to 10°C and lacking acid groups, an acrylic polymer (B) having a glass transition temperature of 40 to 150°C, and a plant-derived wax, wherein the coating agent contains 1 to 100 parts by mass of the plant-derived wax per 100 parts by mass of the total of the acrylic polymer (A) and the acrylic polymer (B), and neither the acrylic polymer (A) nor the acrylic polymer (B) has structural units derived from olefin monomers.

[0011] Preferably, the plant-derived wax is carnauba wax.

[0012] Preferably, the acrylic polymer (B) has an acidic group.

[0013] Furthermore, the present invention relates to a food packaging sheet having a base sheet and a coating layer formed on the base sheet from the above-mentioned food packaging sheet coating agent. [Effects of the Invention]

[0014] The present invention makes it possible to provide a coating agent for food packaging sheets that can form a coating layer with excellent low-temperature heat sealability, blocking resistance, and oil resistance, as well as a food packaging sheet using the same. [Modes for carrying out the invention]

[0015] The present invention will now be described in detail. In this specification, numerical ranges specified using "~" include the values ​​before and after "~" as the lower and upper limits. Unless otherwise noted, the various components described in this specification may be used independently or in combination of two or more. Furthermore, an acrylic polymer (A) with a glass transition temperature of -50 to 10°C and without acid groups may be abbreviated as "polymer (A)", and an acrylic polymer (B) with a glass transition temperature of 40 to 150°C may be abbreviated as "polymer (B)".

[0016] Furthermore, in this specification, when "(meth)acrylic acid" and "(meth)acrylate" are used, unless otherwise specified, they refer to "acrylic acid or methacrylic acid" and "acrylate or methacrylate," respectively. Also, "(meth)acrylic acid ester monomer" refers to the collective term for "acrylic acid ester monomer" and "methacrylic acid ester monomer." A monomer means an ethylenically unsaturated double-bond-containing monomer.

[0017] <Coating agent for food packaging sheets> The coating agent for food packaging sheets of the present invention (hereinafter sometimes abbreviated as "coating agent") is suitable for forming a coating layer (hereinafter sometimes referred to as "coating layer") on a substrate such as paper or nonwoven fabric to produce a coated sheet, and is particularly suitable for producing coated sheets for packaging food. Furthermore, the coating agent of the present invention not only has excellent heat sealability and blocking resistance at low temperatures, but can also form a coating layer that is oil-resistant, making it suitable for use in applications where the coating layer and food come into direct contact. In addition, "painting," "coating," and "application" shall be treated as synonymous.

[0018] The coating agent for a food packaging sheet of the present invention is a coating agent having a glass transition temperature of -50 to 10°C, comprising an acrylic polymer (A) having no acid group, an acrylic polymer (B) having a glass transition temperature of 40 to 150°C, and a plant-derived wax. With respect to a total of 100 parts by mass of the acrylic polymer (A) and the acrylic polymer (B), the plant-derived wax is contained in an amount of 1 to 100 parts by mass. The acrylic polymer (A) and the acrylic polymer (B) are each characterized by having no structural unit derived from an olefin monomer. The coated sheet using this coating agent can exhibit excellent low-temperature heat sealability, blocking resistance, and oil resistance, which were difficult to achieve with conventional aqueous coating agents.

[0019] <Acrylic polymer> The coating agent for a food packaging sheet of the present invention has a glass transition temperature of -50 to 10°C and comprises an acrylic polymer (A) having no acid group and an acrylic polymer (B) having a glass transition temperature of 40 to 150°C. Neither the acrylic polymer (A) nor the acrylic polymer (B) has a structural unit derived from an olefin monomer.

[0020] By using an acrylic polymer having no structural unit derived from an olefin monomer, a flexible coating film can be formed and coating film defects can be reduced. Also, it is presumed that the oil resistance of the coating layer is improved due to the poor compatibility between the ester groups or carboxy groups in the acrylic polymer and the oil components.

[0021] [Acrylic polymer (A)] The acrylic polymer (A) is a polymer of monomers containing a (meth)acrylate monomer having no acid group and having a glass transition temperature of -50 to 10°C. Further, it preferably has a glass transition temperature of -30 to 10°C. If the glass transition temperature is -50°C or higher, the blocking resistance of the formed coating layer is excellent. If the glass transition temperature is 10°C or lower, low-temperature heat sealability is exhibited and the film-forming property is excellent, so the oil resistance is also good. Also, by having no acid group, the intermolecular interaction is reduced, and the blocking resistance of the obtained food packaging sheet is improved.

[0022] Examples of the acid group-free monomers that can be used in the production of the acrylic polymer (A) include, but are not particularly limited to, the monomers shown below.

[0023] Chain alkyl (meth)acrylates such as methyl (meth)acrylate, ethyl (meth)acrylate, n-propyl (meth)acrylate, n-butyl (meth)acrylate, pentyl (meth)acrylate, iso -butyl (meth)acrylate, tert-butyl (meth)acrylate, pentyl (meth)acrylate, isoamyl (meth)acrylate, hexyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, octyl (meth)acrylate, nonyl (meth)acrylate, decyl (meth)acrylate, undecyl (meth)acrylate, lauryl (meth)acrylate, tridecyl (meth)acrylate, tetradecyl (meth)acrylate, pentadecyl (meth)acrylate, hexadecyl (meth)acrylate, heptadecyl (meth)acrylate, stearyl (meth)acrylate, isostearyl (meth)acrylate, behenyl (meth)acrylate, etc.;

[0024] Cyclic alkyl (meth)acrylates such as cyclohexyl (meth)acrylate, isobornyl (meth)acrylate, dicyclopentanyl (meth)acrylate, etc.;

[0025] Amide group-containing monomers such as (meth)acrylamide, N-methoxymethyl-(meth)acrylamide, N-butoxymethyl-(meth)acrylamide, N-pentoxymethyl-(meth)acrylamide, N,N-dimethylaminopropyl (meth)acrylamide, N,N-dimethyl (meth)acrylamide, N,N-diethyl (meth)acrylamide, etc.; Ketone group-containing monomers such as diacetone (meth)acrylamide, acetoacetoxy (meth)acrylate, etc.;

[0026] Epoxy group-containing monomers such as glycidyl (meth)acrylate, 3,4-epoxycyclohexylmethyl (meth)acrylate, and allyl glycidyl ether; amino group-containing (meth)acrylates such as aminomethyl (meth)acrylate, dimethylaminomethyl (meth)acrylate, dimethylaminoethyl (meth)acrylate, and dimethylaminopropyl (meth)acrylate; Nitrile group-containing monomers such as (meth)acrylonitrile;

[0027] Monomers containing isocyanate groups, such as (meth)acryloyloxyethyl isocyanate; Heterocyclic monomers other than epoxy groups, such as tetrahydrofurfuryl (meth)acrylate and (meth)acryloylmorpholine; Alkoxyalkyl (meth)acrylates such as methoxyethyl (meth)acrylate and their alkylene oxide adducts; Aryloxyalkyl (meth)acrylates such as phenoxyethyl (meth)acrylate and their alkylene oxide adducts;

[0028] Methylol group-containing monomers such as N-methylol(meth)acrylamide, N,N-dimethylol(meth)acrylamide, and alkyl etherified N-methylol(meth)acrylamide; Alkoxysilyl group-containing monomers such as γ-methacryloxypropyltrimethoxysilane, γ-methacryloxypropyltriethoxysilane, γ-methacryloxypropyltributoxysilane, γ-methacryloxypropylmethyldimethoxysilane, γ-methacryloxypropylmethyldiethoxysilane, γ-acryloxypropyltrimethoxysilane, γ-acryloxypropyltriethoxysilane, γ-acryloxypropylmethyldimethoxysilane, γ-methacryloxymethyltrimethoxysilane, γ-acryloxymethyltrimethoxysilane, vinyltrimethoxysilane, vinyltriethoxysilane, vinyltributoxysilane, vinylmethyldimethoxysilane; Fluorine atom-containing monomers such as trifluoroethyl (meth)acrylate and heptadecafluorodecyl (meth)acrylate;

[0029] Allyl (meth)acrylate, 2-butenyl (meth)acrylate, 3-butenyl (meth)acrylate, 1-methyl-3-butenyl (meth)acrylate, o-allylphenyl (meth)acrylate, 2-(allyloxy)ethyl (meth)acrylate, citronellyl (meth)acrylate, cinnamyl (meth)acrylate, diallyl maleate, diaryl lutaconic acid, vinyl (meth)acrylate, vinyl crotonate, vinyl oleate, 2-(2'-vinyl Monomers having two or more ethylenically unsaturated groups, such as xyethoxyethyl (meth)acrylate, ethylene glycol di(meth)acrylate, triethylene glycol di(meth)acrylate, tetraethylene glycol di(meth)acrylate, trimethylolpropane tri(meth)acrylate, pentaerythritol tri(meth)acrylate, divinylbenzene, divinyl adipate, diallyl isophthalate, diallyl phthalate, diallyl maleate, etc.

[0030] Furthermore, styrenes such as styrene, α-methylstyrene, o-methylstyrene, p-methylstyrene, and m-methylstyrene may also be used, to the extent that they do not interfere with the effect.

[0031] [Acrylic polymer (B)] Acrylic polymer (B) has a glass transition temperature of 40 to 150°C. Acrylic polymer (B) is preferably a polymer of an acid-containing monomer, and more preferably a polymer of a (meth)acrylate monomer or a monomer containing (meth)acrylic acid. It is also preferable that it has a glass transition temperature of 40 to 125°C. If the glass transition temperature is 40°C or higher, the formed coating layer exhibits excellent blocking resistance. If the glass transition temperature is 150°C or lower, it exhibits excellent heat sealability.

[0032] Monomers that can be used to produce acrylic polymer (B) include, in addition to the monomers that can be used to produce acrylic polymer (A), monomers having acidic groups as shown below, but are not limited to these.

[0033] (Meth)acrylic acid, maleic acid, fumaric acid, itaconic acid, citraconic acid, cinnamic acid, β-carboxyethyl (meth)acrylate and other carboxyl group-containing monomers and their anhydrides;

[0034] Sulfo group-containing monomers such as sodium 2-acrylamido-2-methylpropanesulfonate, methallyl sulfonic acid, sodium methallyl sulfonate, allyl sulfonic acid, sodium allyl sulfonate, ammonium allyl sulfonate, vinyl sulfonic acid, and their salts; 2-(meth)acryloyloxyethyl acid phosphate and other phosphate-containing monomers.

[0035] Furthermore, with respect to the acrylic polymer (B), styrenes such as styrene, α-methylstyrene, o-methylstyrene, p-methylstyrene, and m-methylstyrene may also be used, to the extent that they do not interfere with the effect.

[0036] Acrylic polymers can be obtained by appropriately selecting monomers such that acrylic polymer (A) has a glass transition temperature of -50 to 10°C and acrylic polymer (B) has a glass transition temperature of 40 to 150°C, and polymerizing them individually or copolymerizing two or more types in the presence of a radical polymerization initiator. Polymerization can be carried out by conventionally known methods such as bulk polymerization, solution polymerization, emulsion polymerization, and suspension polymerization. Furthermore, a composite resin may be formed from acrylic polymer (A) and acrylic polymer (B).

[0037] As radical polymerization initiators, known oil-soluble polymerization initiators and water-soluble polymerization initiators can be used, and these may be used individually or in combination of two or more. Examples of oil-soluble polymerization initiators include organic peroxides such as benzoyl peroxide, tert-butyl peroxybenzoate, tert-butyl hydroperoxide, tert-butyl peroxy(2-ethylhexanoate), tert-butyl peroxy-3,5,5-trimethylhexanoate, and di-tert-butyl peroxide; Azobis compounds such as 2,2'-azobisisobutyronitrile, 2,2'-azobis-2,4-dimethylvaleronitrile, 2,2'-azobis(4-methoxy-2,4-dimethylvaleronitrile), and 1,1'-azobis-cyclohexane-1-carbonitride; Examples include: Examples of water-soluble polymerization initiators include ammonium persulfate (abbreviated as APS), potassium persulfate (abbreviated as KPS), hydrogen peroxide, and 2,2'-azobis(2-methylpropionamidine) dihydrochloride. It is preferable to use 0.1 to 4 parts by mass of the radical polymerization initiator per 100 parts by mass of the monomer.

[0038] When carrying out emulsion polymerization, a water-soluble polymerization initiator is preferably used, but a reducing agent may also be used in combination with it. The use of reducing agents offers advantages such as accelerating the emulsion polymerization rate and facilitating emulsion polymerization at low temperatures. Examples of reducing agents include reducing organic compounds such as ascorbic acid, erythorbic acid, tartaric acid, citric acid, glucose, formaldehyde sulfoxylate, rongalit, and thiourea dioxide. Reducing inorganic compounds such as sodium thiosulfate, sodium sulfite, sodium bisulfite, sodium metabisulfite, and ferrous chloride; These are some examples.

[0039] The coating agent of the present invention only needs to contain acrylic polymer (A) and acrylic polymer (B) as acrylic polymers, and the method for obtaining them is not particularly limited. For example, acrylic polymer (A) and acrylic polymer (B) can be compounded by polymerizing a monomer that forms acrylic polymer (A) in the presence of acrylic polymer (B). This method is advantageous in terms of manufacturing cost and efficiency and is therefore preferably used.

[0040] <Plant-derived wax> Examples of plant-derived waxes include carnauba wax, candelilla wax, rice wax, wood wax, and soy wax, with carnauba wax being particularly preferred.

[0041] The amount of plant-derived wax added is 1 part by mass or more and 100 parts by mass or less, preferably 1 part by mass or more and 70 parts by mass or less, per 100 parts by mass of the total of acrylic polymer (A) and acrylic polymer (B). If the amount is 1 part by mass or more, blocking resistance can be achieved when sheets coated with the coating agent are stacked and stored. On the other hand, if the amount is 100 parts by mass or less, good low-temperature heat sealability is achieved without hindering the low-temperature heat sealability of acrylic polymer (A).

[0042] In addition to the plant-derived wax mentioned above, it is preferable to use a crude oil-derived wax in combination with the coating agent of the present invention. By using a crude oil-derived wax in combination, it fuses with the acrylic polymer during heat sealing at low temperatures, further improving the low-temperature heat sealability.

[0043] Examples of crude oil-derived waxes include paraffin wax, microcrystalline wax, and slacks wax, with paraffin wax being particularly preferred.

[0044] The amount of crude oil-derived wax blended is preferably 10 parts by mass or more and 100 parts by mass or less, and more preferably 20 parts by mass or more and 80 parts by mass or less, per 100 parts by mass of the total of acrylic polymer (A) and acrylic polymer (B). If the amount is 10 parts by mass or more, it will fuse with the acrylic polymer even when heat-sealed under low temperature conditions, resulting in excellent heat-sealability. On the other hand, if the amount is 100 parts by mass or less, good blocking resistance will be achieved when sheets coated with the coating agent are stacked and stored.

[0045] The melting points of plant-derived waxes and crude oil-derived waxes are preferably in the range of 50°C to 100°C, and more preferably in the range of 55°C to 90°C. If the melting point is 50°C or higher, excellent blocking resistance at high temperatures is achieved. On the other hand, if the melting point is 100°C or lower, excellent heat sealing properties can be achieved even when heat sealing is performed under low-temperature conditions.

[0046] From the viewpoint of the long-term stability of the coating agent, plant-derived waxes and crude oil-derived waxes are preferably used in the form of aqueous dispersions. In aqueous dispersions of plant-derived waxes and crude oil-derived waxes, the average particle size of the wax particles is preferably 1 μm or less. If it is 1 μm or less, the coating agent will have excellent film-forming properties and the oil resistance of the coating layer will be easily exhibited. In this specification, the average particle size refers to the median diameter obtained by dynamic light scattering measurement.

[0047] The coating agent of the present invention may optionally contain additives such as liquid media, preservatives, leveling agents, defoaming agents, fillers, colorants, and crosslinking agents.

[0048] <Liquid media> The coating agent of the present invention preferably contains a liquid medium, and an aqueous medium is preferred as the liquid medium. Here, an aqueous medium means water or an organic solvent that is miscible with water. Examples of aqueous media include water, alcohols, etc. Among these, water is preferred as the liquid medium.

[0049] Examples of organic solvents miscible with water include monohydric alcohols such as ethanol, 1-propanol, 2-propanol, 1-butanol, 2-methyl-1-propanol, 2-butanol, and 2-methyl-2-propanol; glycols such as ethylene glycol, 1,3-propanediol, propylene glycol, 1,2-butanediol, 1,4-butanediol, pentylene glycol, 1,2-hexanediol, 1,6-hexanediol, diethylene glycol, triethylene glycol, and tetraethylene glycol; ethylene glycol monomethyl ether, diethylene glycol monomethyl ether, triethylene glycol monoethyl ether, ethylene glycol monoethyl ether, diethylene glycol monoethyl ether, triethylene glycol monoethyl ether, ethylene glycol monoisopropyl ether, and diethylene Examples of glycol ethers include glycol monoisopropyl ether, triethylene glycol monoisopropyl ether, ethylene glycol monobutyl ether, diethylene glycol monobutyl ether, triethylene glycol monobutyl ether, ethylene glycol monoisobutyl ether, diethylene glycol monoisobutyl ether, triethylene glycol monoisobutyl ether, ethylene glycol monohexyl ether, diethylene glycol monohexyl ether, ethylene glycol dimethyl ether, diethylene glycol dimethyl ether, triethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, propylene glycol monomethyl ether, dipropylene glycol monomethyl ether, dipropylene glycol dimethyl ether, and tripropylene glycol monomethyl ether.

[0050] <Preservatives> To prevent spoilage during storage of the coating agent, it is preferable to add a preservative at a concentration of approximately 0.001 to 5% by mass of the total non-volatile content of the coating agent (100% by mass).

[0051] The preservatives are not particularly limited and include, for example, sodium dehydroacetate, dichlorophene, sorbic acid, sodium benzoate, p-hydroxybenzoic acid esters, and isothiazolinone compounds.

[0052] <Leveling agent> To improve coating suitability, it is preferable to add a leveling agent in an amount of approximately 0.01 to 10% by mass of the total non-volatile content of the coating agent, as needed.

[0053] Examples of leveling agents include acrylic, vinyl, silicone, fluorine, and acetylene glycol-based agents, but acetylene glycol-based agents are preferred from the viewpoint of oral safety.

[0054] <Antifoaming agent> Defects in the coating film may occur due to bubbles generated during the coating process. Therefore, it is preferable to add an antifoaming agent in an amount of approximately 0.001 to 10% by mass of the total non-volatile content of the coating agent, as needed.

[0055] Examples of defoaming agents include polysiloxane-based defoaming agents, mineral oil-based defoaming agents, and nonionic surfactants. Among these, polysiloxane-based defoaming agents are preferred due to their strong defoaming power. Polysiloxane-based defoaming agents with a polydimethylsiloxane structure can be used. Polysiloxane-based defoaming agents mixed with hydrophobic silica or mineral oil may be used as needed.

[0056] Other additives include fillers, colorants, and crosslinking agents. Fillers are not particularly limited as long as they are known, and examples include talc, silica, calcium carbonate, barium sulfate, titanium dioxide, diatomaceous earth (white carbon), and cellulose powder. As colorants, known organic pigments, inorganic pigments, dyes, etc., can be used. As crosslinking agents, polyfunctional compounds or metal oxides that are reactive to carboxyl groups are preferred, and examples include polyfunctional carbodiimide, polyfunctional isocyanate, polyfunctional epoxy, polyfunctional oxazoline, magnesium oxide, and zinc oxide.

[0057] <Base material> There are no restrictions on the substrate to which the coating agent of the present invention is applied, but paper is suitable. If the coating is applied to only one side of the substrate, the other side may be coated with another coating agent.

[0058] <Coating Method> Coating equipment includes knife coaters, comma coaters, roll coaters, bar coaters, gravure coaters, and flexo coaters.

[0059] Furthermore, the coating agent of the present invention may be applied in multiple layers as needed to obtain the desired low-temperature heat sealability and oil resistance.

[0060] As described above, after applying the coating agent to the substrate, the food packaging sheet of the present invention is obtained by removing volatile components by means such as heating as necessary to form a coating layer. [Examples]

[0061] The present invention will be specifically described below with reference to examples. In the examples, unless otherwise specified, "parts" means "parts by mass," and "%" means "percent mass."

[0062] <Glass transition temperature> The glass transition temperature (hereinafter also referred to as "Tg") was measured using a DSC (Differential Scanning Calorimeter, manufactured by TA Instruments). Specifically, an aluminum pan containing 3 mg of dried polymer, accurately weighed, and an empty aluminum pan serving as a reference were placed in a DSC measurement holder. The temperature at the intersection of the low-temperature baseline of the endothermic phenomenon and the tangent line at the inflection point in the resulting DSC curve, obtained under a heating condition of 10°C / min, was defined as the glass transition temperature (Tg). In the case of a two-step polymerization process, a sample was taken after the completion of the first polymerization reaction and DSC measurement was performed, and the resulting Tg was designated as Tg1. Then, after the completion of the second polymerization reaction, a sample was taken again and DSC measurement was performed, and the newly appearing Tg, which was different from Tg1, was designated as Tg2.

[0063] <Heat seal strength> Two test pieces were prepared by cutting the obtained food packaging sheet to a width of 15 mm. The coated surface of one test piece was placed on top of the coated surface of the other test piece, and the temperature was set to three levels of 80°C, 100°C, and 120°C using a HEATSHEEL & IMPULSE TESTER (manufactured by Nichiri Kagaku Kogyo Co., Ltd., with upper and lower plate heating), and the temperature was set to 2 kgf / cm². 2 Samples crimped under the -1 second condition were used for evaluation. For this evaluation sample, the peel strength was measured under the following conditions, and the heat seal strength (heat sealability) was evaluated. Testing equipment: Tensile testing machine (manufactured by Tester Industries Co., Ltd.) Peeling conditions: 180° peeling, 300 mm / min [Evaluation Criteria] S: 3N or higher (extremely good) A: 2N or more, less than 3N (good) B: 1N or more, less than 2N (usable) C: Less than 1N (Not usable)

[0064] <Blocking resistance> Two food packaging sheets were prepared. The coated surface of one sheet was placed on top of the coated surface of the other sheet, and the evaluation was performed using the following test equipment and conditions. Test equipment: CO-201 permanent strain tester (manufactured by Tester Industries Co., Ltd., upper and lower plate heating) Pressure: 4 kg / cm² 2 [Evaluation Criteria] S: After compression for 24 hours in a 40℃-80%RH environment, there was no resistance during delamination, and the laminate was intact (extremely good). A: After compression for 24 hours in a 40℃-40%RH environment, there was no resistance during delamination, and the laminate was intact (good). B: After compression for 24 hours in a 40℃-40%RH environment, there is some resistance during delamination, but the laminate is not damaged (usable). C: After compression for 24 hours in a 40℃-40%RH environment, there was resistance during delamination and the laminate was damaged (unusable).

[0065] <Oil resistance> The oil resistance of the coated surface of the obtained food packaging sheets was evaluated using the TAPPIT 559cm-02 method. Higher values ​​indicate better oil resistance. [Evaluation Criteria] S: Oil resistance of 5 or higher (extremely good) A: Oil resistance of 3 or higher, less than 5 (good) B: Oil resistance of 1 or higher, less than 3 (usable) C: Oil resistance 0 (unusable)

[0066] [Example 1] In a reaction vessel (reaction tank) equipped with a stirrer, thermometer, dropping funnel, and reflux apparatus, 72.0 parts of water and 0.4 parts of Perex OT-P (sodium dioctyl sulfosuccinate, 70% active ingredient, manufactured by Kao Corporation) as a surfactant were charged. Separately, 25.3 parts of methyl methacrylate, 10.7 parts of n-butyl acrylate, 4.0 parts of methacrylic acid as monomers having acid groups, 0.8 parts of t-dodecyl mercaptan as a chain transfer agent, 1.0 part of Perex OT-P, and 23.0 parts of water were pre-mixed and stirred to prepare an emulsion of monomers to be dropped into the first stage (first stage dropping solution), and this was charged into the dropping funnel. After raising the internal temperature of the reaction vessel to 85°C and thoroughly purging it with nitrogen, 3.5 parts of a 10% aqueous solution of ammonium persulfate was added to the reaction vessel as a polymerization initiator. While maintaining the internal temperature of the reaction vessel at 85°C, the emulsion of the monomers to be added in the first stage was added dropwise to the reaction vessel over 2 hours to carry out emulsion polymerization. After the addition of the monomers in the first stage was completed, the temperature was maintained at 85°C for 1 hour, and then 3.2 parts of 25% aqueous ammonia was added while stirring to neutralize the mixture and obtain a resin solution of acrylic polymer (B). The Tg of the obtained acrylic polymer (B) was 50°C. Next, 24.9 parts of methyl methacrylate, 35.1 parts of n-butyl acrylate, 0.4 parts of Perex OT-P, and 35.0 parts of water were pre-mixed and stirred to prepare the emulsion of the second monomer (second dropping solution), which was then placed in a dropping funnel. 5.5 parts of a 10% aqueous solution of ammonium persulfate was added to the reaction vessel, and the emulsion of the second monomer was added dropwise to the reaction vessel over 2 hours while maintaining the internal temperature of the reaction vessel at 80°C to carry out emulsion polymerization. After the addition was complete, the reaction was continued for another 2 hours at 80°C to obtain a dispersion of composite resin particles of acrylic polymer (A) and acrylic polymer (B). The Tg of the acrylic polymer (A) portion of the obtained composite resin was 0°C. To a dispersion of composite resin particles of the obtained acrylic polymer (A) and acrylic polymer (B), 66.7 parts of AQUACER2650 (modified carnauba wax manufactured by BYK, non-volatile content concentration: 30%, melting point: 85°C) and a predetermined amount of water were added to adjust the non-volatile content concentration to 30% to obtain the desired coating agent. The resulting coating agent is applied to commercially available paper (basis weight 100g / m²) as a base material. 2 One side of the material was coated using a bar coater #18. After coating, a food packaging sheet was obtained by drying in a hot air oven at 100°C for 60 seconds.

[0067] [Examples 2, 3, 5-7, 10-12, Comparative Examples 1-6] A coating agent was prepared using the same method as in Example 1, with the formulation shown in Table 1. The amount of water added was adjusted so that the non-volatile content of the coating agent was 30%. The Tg of the resulting composite resin of acrylic polymer (A) and acrylic polymer (B) was measured in the same manner as in Example 1. Furthermore, in Table 1, blank spaces indicate that an ingredient is not included or is not present. Note that in Table 1, the information regarding water and organic solvents used in the formulation has been omitted. In Examples 2, 4-6, 8, 11, and 12, in addition to plant-derived waxes, AQUACER497 (modified paraffin wax manufactured by BYK, non-volatile content concentration: 60%), a crude oil-derived wax, was used in combination. In Example 11, EXP SW166HS-B (soy wax manufactured by Moriroku Chemicals, non-volatile content concentration: 40%, melting point: 68°C) was used instead of AQUACER2650 as the plant-derived wax.

[0068] [Example 4] 12.0 parts of butyl alcohol were charged into a reaction vessel (reaction tank) equipped with a stirrer, thermometer, dropping funnel, and reflux apparatus. Separately, 8.0 parts of styrene, 6.0 parts of α-methylstyrene, and 6.0 parts of acrylic acid were mixed as monomers containing (meth)acrylate monomer or (meth)acrylic acid. Furthermore, 1.2 parts of azoisobutyronitrile, a polymerization initiator, were charged and dissolved to prepare the monomer solution to be added dropwise in the first stage (first stage dropwise solution), and this was charged into the dropping funnel. The internal temperature of the reaction vessel was raised to 110°C while purging with nitrogen, and the first stage dropwise solution was added to the reaction vessel dropwise over 2 hours while maintaining the internal temperature at 110°C. After the dropwise addition was complete, the reaction was allowed to proceed at 110°C for 4 hours. After the reaction, the solution was cooled to 80°C, neutralized with 5.7 parts of 25% aqueous ammonia, diluted with 44.0 parts of water, and the butyl alcohol was removed by distillation to adjust the non-volatile content to 30% to obtain an aqueous solution of acrylic polymer (B). The Tg of the obtained acrylic polymer (B) was 120°C. Next, the internal temperature of the reaction vessel was adjusted to 80°C, and 0.5 parts of Perex OT-P were added to the reaction vessel. Then, 32.4 parts of methyl methacrylate, 47.6 parts of 2-ethylhexyl acrylate, 1.0 part of Perex OT-P, and 25.0 parts of water were pre-mixed and stirred to prepare the emulsion of the second monomer (second dropping solution), which was then placed in a dropping funnel. After maintaining the internal temperature of the reaction vessel at 80°C and ensuring sufficient nitrogen purging, 13.5 parts of a 10% aqueous solution of ammonium persulfate were added to the reaction vessel, and while maintaining the internal temperature of the reaction vessel at 80°C, the second dropping solution was added dropwise to the reaction vessel over 2 hours to carry out emulsion polymerization. After the addition was complete, the reaction was continued for another 3 hours at 80°C to obtain a dispersion of composite resin particles of acrylic polymer (A) and acrylic polymer (B). The Tg of the acrylic polymer (A) portion of the resulting composite resin was -23°C. To a dispersion of composite resin particles of the obtained acrylic polymer (A) and acrylic polymer (B), 66.7 parts of AQUACER2650 and 80.0 parts of AQUACER497 as plant-derived waxes, along with a predetermined amount of water, were added to adjust the non-volatile content concentration to 30% to obtain the desired coating agent. The resulting coating agent is applied to commercially available paper (basis weight 100g / m²) as a base material. 2 One side of the material was coated using a bar coater #18. After coating, a food packaging sheet was obtained by drying in a hot air oven at 100°C for 60 seconds.

[0069] [Examples 8 and 9] A coating agent was prepared using the same method as in Example 4, with the formulation shown in Table 1. The amount of water was adjusted so that the non-volatile content of the coating agent was 30%. The Tg of the obtained acrylic polymers (A) and (B) was measured in the same manner as in Example 1.

[0070] [Comparative Example 7] A predetermined amount of water was added to MFX-003 (an ethylene-based heat sealant manufactured by Maruyoshi Chemical Co., Ltd., with a non-volatile content concentration of 35%) to adjust the non-volatile content concentration to 30% and obtain the desired coating agent. The resulting coating agent is applied to commercially available paper (basis weight 100g / m²) as a base material. 2One side of the material was coated using a bar coater #18. After coating, a food packaging sheet was obtained by drying in a hot air oven at 100°C for 60 seconds.

[0071] [Table 1]

[0072] [Table 1]

[0073] As can be seen from Table 1, the food packaging sheets using the coatings obtained in Examples 1 to 12 exhibited excellent heat sealability, blocking resistance, and oil resistance at 80 to 120°C, demonstrating performance that fully met practical requirements. On the other hand, the food packaging sheets using the coatings obtained in Comparative Examples 1 to 7 showed extremely poor performance in one or more of the following properties at 80 to 120°C: heat sealability, blocking resistance, and oil resistance. The results were not satisfactory and could not be said to meet practical requirements.

Claims

1. A coating agent for food packaging sheets comprising an acrylic polymer (A) having a glass transition temperature of -50 to 10°C and lacking acid groups, an acrylic polymer (B) having a glass transition temperature of 40 to 150°C, and a plant-derived wax, wherein the coating agent contains 1 to 100 parts by mass of the plant-derived wax per 100 parts by mass of the total of the acrylic polymer (A) and the acrylic polymer (B), and neither the acrylic polymer (A) nor the acrylic polymer (B) has structural units derived from olefin monomers.

2. The coating agent for food packaging sheets according to claim 1, wherein the plant-derived wax is carnauba wax.

3. The coating agent for food packaging sheets according to claim 1, wherein the acrylic polymer (B) has an acid group.

4. A food packaging sheet having a base sheet and a coating layer formed on the base sheet from a food packaging sheet coating agent according to any one of claims 1 to 3.

5. The food packaging sheet according to claim 4, wherein the base sheet is paper.

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