Coating agent for food packaging sheets and food packaging sheets

Abstract

To provide a food packaging sheet which has good stability of a coating agent, is excellent in blocking properties and heat sealability, and is also excellent in food product resistance such as seasoning resistance such as catsup and sauce, and microwave oven resistance.SOLUTION: A coating agent for a food packaging sheet contains resin fine particles (A) that are a polymer of an ethylenically unsaturated monomer (a), in which the resin fine particles (A) have glass transition temperatures in each range of -40 to 30°C and 40 to 140°C, and an acid value of 20-40 mgKOH / g, and contain 85-97 mass% of an ethylenically unsaturated monomer (a-1) having a straight-chain alkyl group having 1 to 4 carbon atoms based on the total mass of the ethylenically unsaturated monomer (a).SELECTED DRAWING: Figure 1

Description

[Technical field]

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

[0002] Conventionally, food packaging sheets have used laminated sheets in which a resin film such as polyethylene is laminated on a base sheet such as thin paper, cardboard, or nonwoven fabric. The laminated resin film has functions such as preventing oil stains on the base sheet caused by oil from food and preventing a decrease in the strength of the base sheet. On the other hand, in recent years, the recycling of paper has accelerated due to the global trend of reducing environmental load. However, the above-mentioned laminated sheets are difficult to recycle, and expensive capital investment is required in the process of removing the laminated resin film layer, which has been an obstacle to promoting recycling. Against this background, coating-type food packaging sheets that are easier to recycle are being actively studied. Even for these resin coating agents, materials with less environmental load are required, and for example, styrene-free materials that do not contain styrene are required. However, when the resin particles contained in the coating agent for food packaging are made styrene-free, the dispersion stability of the particles deteriorates, and the stability of the coating agent also deteriorates accordingly. The precipitates and aggregates generated by the deterioration of the stability of the coating agent adversely affect the coatability and film-forming property of the coating agent.

[0003] As an example of such a styrene-free type food packaging sheet, Patent Document 1 discloses a coating agent using acrylic resin fine particles that contain a large amount of components with a high acid value and low molecular weight and that is mainly composed of 2-ethylhexyl acrylate, with the aim of imparting oil resistance and water resistance to a paper base material, and a food packaging sheet formed by applying the coating agent.

[0004] However, such coating agents show a certain level of oil resistance against fats and oils contained in foods at room temperature, such as snack foods, and have a certain level of water resistance when used alone, but when packaging foods that contain multiple ingredients such as oil, salt, acid, and amino acids and have a high water content, such as seasonings such as ketchup and sauce, the coating agents are not stable and the resin composition affects the seasoning ingredients, causing them to permeate the base sheet and seep out to the outside, resulting in poor appearance (hereinafter referred to as seasoning resistance). Furthermore, when it is assumed that the packaged food will be heated in a microwave oven, oil resistance and water resistance at high temperatures for a certain period of time or more are required, but conventional food packaging sheets are not resistant to steam generated from the food when heated in a microwave oven, which causes moisture and oil to easily permeate the base sheet and seep out to the outside, resulting in a problem of blocking the food and the sheet (hereinafter referred to as microwave resistance).

[0005] In addition, poor coating properties and poor film-forming properties due to poor stability of the coating agent can lead to reduced heat sealing properties, and the resin coating layer can cause blocking when the food packaging sheet is rewound into a roll for storage. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] JP 2001-303475 A Summary of the Invention [Problem to be solved by the invention]

[0007] The present invention aims to provide a food packaging sheet having a coating agent with good stability, excellent blocking properties and heat sealability, and further having excellent food resistance, such as resistance to seasonings such as ketchup and sauce, and resistance to microwave ovens. [Means for solving the problem]

[0008] The present inventors have conducted extensive research to solve the above problems and have arrived at the present invention. That is, the present invention relates to a coating agent for food packaging sheets, which comprises resin fine particles (A) which are polymers of ethylenically unsaturated monomers (a), the resin fine particles (A) having glass transition temperatures in the ranges of -40 to 30°C and 40 to 140°C and an acid value of 20 to 40 mgKOH / g, and which contains 85 to 97 mass% of an ethylenically unsaturated monomer (a-1) having a linear alkyl group having 1 to 4 carbon atoms, based on the total mass of the ethylenically unsaturated monomers (a).

[0009] The present invention relates to the coating agent for food packaging sheets, wherein the polymer of the ethylenically unsaturated monomer (a) contains a polymer having a structure represented by general formula (1) at one terminal. [ka] R is a linear or branched alkyl group having 8 to 16 carbon atoms.

[0010] The present invention relates to the coating agent for food packaging sheets, wherein the ethylenically unsaturated monomer (a) contains methacrylic acid.

[0011] The present invention relates to the coating agent for food packaging sheets, wherein the resin fine particles (A) have an average particle size of 30 to 300 nm.

[0012] The present invention relates to the coating agent for food packaging sheets, which contains at least one of lauryl sulfate and dioctyl sulfosuccinate in a total amount of 0.1 to 2 parts by mass based on 100 parts by mass of the ethylenically unsaturated monomer (a).

[0013] The present invention relates to the coating agent for food packaging sheets, wherein the resin fine particles (A) have a mass average molecular weight of 100,000 or more.

[0014] 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 coating agent for food packaging sheets.

[0015] The present invention relates to the above food packaging sheet, wherein the base sheet is paper. Effect of the Invention

[0016] The coating agent for food packaging sheets of the present invention makes it possible to provide a food packaging sheet that has good stability and is resistant not only to seasonings such as ketchup and sauce, and to food when heated in a microwave oven, but also to blocking resistance and heat sealing properties. [Brief description of the drawings]

[0017] [Figure 1] FIG. 2 is a schematic diagram for explaining the glass transition temperature of resin fine particles (A) by DSC. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0018] The present invention will be described in detail below. Needless to say, other embodiments are also included in the scope of the present invention as long as they are consistent with the gist of the present invention. In addition, in this specification, a numerical range specified using "~" includes the numerical values ​​written before and after "~" as the range of the lower and upper limits. In addition, in this specification, "film" and "sheet" are not distinguished by thickness. In other words, in this specification, "sheet" includes thin film-like objects, and "film" includes thick sheet-like objects. Unless otherwise noted, the various components appearing in this specification may be used independently as a single type or as a combination of two or more types.

[0019] In this specification, the terms "(meth)acrylic acid" and "(meth)acrylate" refer to "acrylic acid or methacrylic acid", respectively, unless otherwise specified. Furthermore, the term "(meth)acrylic acid ester monomer" refers collectively to "acrylic acid ester monomer" and "methacrylic acid ester monomer". The monomer refers to an ethylenically unsaturated double bond-containing monomer.

[0020] In this specification, the ethylenically unsaturated monomer (a-1) having a linear alkyl group having 1 to 4 carbon atoms, the ethylenically unsaturated monomer (a-2) having a carboxy group, and the other ethylenically unsaturated monomer (a-3) copolymerizable with the ethylenically unsaturated monomers (a-1) and (a-2) may be abbreviated as the ethylenically unsaturated monomer (a-1), the ethylenically unsaturated monomer (a-2), and the ethylenically unsaturated monomer (a-3), respectively.

[0021] In this specification, "Mw" is the weight average molecular weight in terms of polystyrene measured by gel permeation chromatography (GPC). "Mn" is the number average molecular weight in terms of polystyrene measured by GPC. These can be measured by the method described in the [Examples] section.

[0022] <Coating agent for food packaging sheets> The coating agent for food packaging sheets of the present invention is applied to a base sheet such as paper or nonwoven fabric and dried to form a coating layer, and can be suitably used to form a sheet for packaging food (hereinafter, abbreviated as food packaging sheet). In addition, the food packaging sheet of the present invention is not only excellent in oil resistance and water resistance for snack foods, but also excellent in seasoning resistance and microwave resistance, so it can be suitably used in applications where the coating layer comes into direct contact with food.

[0023] The coating agent for food packaging sheets of the present invention is characterized in that it contains resin fine particles (A) which are polymers of ethylenically unsaturated monomers (a), the resin fine particles (A) having glass transition temperatures Tg in the ranges of -40 to 30°C and 40 to 140°C, an acid value of 20 to 40 mgKOH / g, and containing 85 to 97 mass% of ethylenically unsaturated monomers (a-1) having linear alkyl groups with 1 to 4 carbon atoms based on the total mass of the ethylenically unsaturated monomers (a). The coating agent for food packaging sheets containing such resin fine particles (A) is excellent in stability, and food packaging sheets prepared using this coating agent exhibit excellent performance in food resistance (seasoning resistance, microwave resistance), blocking resistance, and heat sealability, which were difficult to achieve with conventional resin water-based coating agents.

[0024] <Resin fine particles (A)> First, the resin fine particles (A) used in the present invention will be described. The resin fine particles (A) are a polymer of an ethylenically unsaturated monomer (a), and the ethylenically unsaturated monomer (a) is an ethylenically unsaturated monomer having a linear alkyl group having 1 to 4 carbon atoms. (a-1) an ethylenically unsaturated monomer having a carboxy group (a-2); and (a-3) another ethylenically unsaturated monomer copolymerizable with the ethylenically unsaturated monomers (a-1) and (a-2). The resin fine particles (A) can contain a polymer having a structure represented by general formula (1) at one end by polymerizing an ethylenically unsaturated monomer (a) in the presence of a chain transfer agent (X) described later. Such resin fine particles (A) have excellent dispersion stability and significantly improve the wettability and coatability of the coating agent. Therefore, they are preferable for further improving the food resistance (seasoning resistance, microwave resistance) and heat sealability of the food packaging sheet.

[0025] [ka] R is a linear or branched alkyl group having 8 to 16 carbon atoms.

[0026] The resin fine particles (A) contain 85 to 97 mass % of the ethylenically unsaturated monomer (a-1) having a linear alkyl group having 1 to 4 carbon atoms, based on 100 mass % of the total mass of the ethylenically unsaturated monomer (a), and more preferably in the range of 90 to 96 mass %. By including the above range, the dispersion stability of the resin particles during synthesis is greatly improved, and a coating agent with excellent coatability and film-forming properties can be obtained. The coating layer formed by this coating agent is strong, and is not easily damaged even when exposed to food ingredients containing multiple components such as moisture, oil, salt, and acid. In addition, even under microwave heating conditions, the coating layer suppresses the penetration of food ingredients, and there is no risk of blocking between the sheet and food. Therefore, the food packaging sheet exhibits excellent food resistance (seasoning resistance, microwave resistance), blocking resistance, and heat sealability.

[0027] [Ethylenically unsaturated monomer (a-1)] The ethylenically unsaturated monomer (a-1) is an ethylenically unsaturated monomer having a linear alkyl group having 1 to 4 carbon atoms, and examples of the ethylenically unsaturated monomer include methyl (meth)acrylate, ethyl (meth)acrylate, n-propyl (meth)acrylate, and n-butyl (meth)acrylate.

[0028] [Ethylenically unsaturated monomer (a-2)] The ethylenically unsaturated monomer (a-2) is an ethylenically unsaturated monomer having a carboxy group, and the carboxy group is introduced onto the surface of the resin fine particles for the purpose of improving the dispersion stability of the resin fine particles (A), controlling the rheology during application of the coating agent, and promoting film formation. The carboxy group can be introduced by copolymerizing the ethylenically unsaturated monomer (a-2) having a carboxy group with the ethylenically unsaturated monomer (a). The acid value of the resin fine particles (A) depends on the amount of the ethylenically unsaturated monomer (a-2) having a carboxy group introduced.

[0029] The acid value of the resin fine particles (A) used in the present invention is in the range of 20 to 40 mgKOH / g, more preferably in the range of 25 to 40 mgKOH / g. When the acid value is 20 mgKOH / g or more, the dispersion stability of the resin fine particles during synthesis is improved. The coating agent containing these resin fine particles has excellent stability, coatability and film-forming properties, and therefore forms a coating layer that is strong and has excellent resistance. Therefore, the food packaging sheet exhibits excellent food resistance (resistance to seasonings, resistance to microwave ovens). In addition, the interaction between the carboxy groups of the resin is also strong, so that the heat sealability is also improved. On the other hand, when the acid value is 40 mgKOH / g or less, the coating resistance against food ingredients including moisture, oil, salt, acid, etc. is improved, and the penetration of steam emitted from the food into the coating layer during microwave heating can be suppressed, and blocking between the sheet and the food does not occur. Therefore, the food packaging sheet exhibits excellent food resistance (resistance to seasonings, resistance to microwave ovens). Furthermore, the interaction between the carboxy groups in the coating layers is also reduced, improving the blocking resistance of the food packaging sheet.

[0030] Examples of the ethylenically saturated monomer (a-2) having a carboxyl group include acrylic acid, methacrylic acid, crotonic acid, itaconic acid, maleic acid, fumaric acid, citraconic acid, and cinnamic acid, but the ethylenically saturated monomer (a-2) having a carboxyl group preferably contains methacrylic acid, because it can obtain a resin microparticle (A) having better stability, and the coating property of the coating agent and the coating resistance of the coating layer against food-derived components are good. By containing methacrylic acid, the dispersion stability of the resin microparticles is further improved, and the stability of the coating agent is further improved. In addition, the food resistance (seasoning resistance, microwave resistance) of the food packaging sheet is further improved.

[0031] The ethylenically saturated monomer (a-2) having a carboxyl group is preferably contained in an amount of 2 to 5 mass %, more preferably 4 to 4.9 mass %, based on the total mass of the ethylenically unsaturated monomer (a). By containing the ethylenically saturated monomer (a-2) in the above range, the dispersion stability of the resin fine particles (A) is improved, and the coatability and film-forming property of the coating agent are improved. Therefore, a food packaging sheet having excellent food resistance (resistance to seasonings, resistance to microwave ovens) and heat sealability can be obtained.

[0032] [Ethylenically unsaturated monomer (a-3)] The ethylenically unsaturated monomer (a-3) is another ethylenically unsaturated monomer copolymerizable with the ethylenically unsaturated monomer (a-1) having a linear alkyl group having 1 to 4 carbon atoms and the ethylenically unsaturated monomer (a-2) having a carboxy group. In order to control the mass average molecular weight, average particle size, etc. of the resin fine particles (A), the ethylenically unsaturated monomer (a-1) and the ethylenically unsaturated monomer (a-2) can be further copolymerized with an ethylenically unsaturated monomer (a-3) as necessary.

[0033] Based on the total mass of the ethylenically unsaturated monomer (a), the ethylenically unsaturated monomer (a-3) is preferably contained in an amount of 0 to 13 mass%, more preferably in the range of 0.5 to 10 mass%. By containing the ethylenically unsaturated monomer (a-3) in the above range, the dispersion stability of the resin fine particles (A) is improved, and the coatability and film-forming property of the coating agent are improved. Therefore, a food packaging sheet having excellent food resistance (resistance to seasonings, resistance to microwave ovens) and heat sealability can be obtained.

[0034] Examples of the other ethylenically unsaturated monomer (a-3) include aromatic ethylenically unsaturated monomers such as vinylnaphthalene, benzyl (meth)acrylate, phenoxyethyl (meth)acrylate, phenoxydiethylene glycol (meth)acrylate, phenoxytetraethylene glycol (meth)acrylate, phenoxyhexaethylene glycol (meth)acrylate, phenoxyhexaethylene glycol (meth)acrylate, and phenyl (meth)acrylate; pentyl (meth)acrylate, heptyl (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, phenyl (meth)acrylate, and the like. ethylenically unsaturated monomers containing a straight or branched alkyl group having 5 or more carbon atoms, such as dipentadecyl (meth)acrylate, hexadecyl (meth)acrylate, heptadecyl (meth)acrylate, stearyl (meth)acrylate, isostearyl (meth)acrylate, and behenyl (meth)acrylate; ethylenically unsaturated monomers containing an alicyclic alkyl group, such as cyclohexyl (meth)acrylate and isobornyl (meth)acrylate; ethylenically unsaturated monomers containing a fluorinated alkyl group, such as trifluoroethyl (meth)acrylate and heptadecafluorodecyl (meth)acrylate; ethylenically unsaturated monomers containing a sulfo group, such as sodium 2-acrylamido 2-methylpropanesulfonate, methallylsulfonic acid, methallylsulfonic acid, sodium methallylsulfonate, allylsulfonic acid, sodium allylsulfonate, ammonium allylsulfonate, and vinylsulfonic acid;(Meth)acrylamide, N-methoxymethyl-(meth)acrylamide, N-ethoxymethyl-(meth)acrylamide, N-propoxymethyl-(meth)acrylamide, N-butoxymethyl-(meth)acrylamide, N-pentoxymethyl-(meth)acrylamide, N,N-di(methoxymethyl)acrylamide, N-ethoxymethyl-N-methoxymethylmethacrylamide, N,N-di(ethoxymethyl)acrylamide, N-ethoxymethyl-N-propoxymethylmethacrylamide, N,N-di(propoxymethyl)acrylamide N-Butoxymethyl-N-(propoxymethyl)methacrylamide, N,N-Di(butoxymethyl)acrylamide, N-Butoxymethyl-N-(methoxymethyl)methacrylamide, N,N-Di(pentoxymethyl)acrylamide, N-Methoxymethyl-N-(pentoxymethyl)methacrylamide, N,N-Dimethylaminopropylacrylamide, N,N-Diethylaminopropylacrylamide, N,N-Dimethylacrylamide, N,N-Diethylacrylamide, Diacetone acrylamide ethylenically unsaturated monomers containing an amide group, such as 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, 4-hydroxybutyl (meth)acrylate, glycerol mono(meth)acrylate, 4-hydroxyvinylbenzene, 1-ethynyl-1-cyclohexanol, allyl alcohol, and other hydroxyl group-containing ethylenically unsaturated monomers; ethylenically unsaturated monomers containing a polyoxyethylene group, such as methoxypolyethylene glycol (meth)acrylate, polyethylene glycol (meth)acrylate, and other dioxyethylene group-containing ethylenically unsaturated monomers; Examples of the ethylenically unsaturated monomers include methylaminoethyl (meth)acrylate, diethylaminoethyl (meth)acrylate, methylethylaminoethyl (meth)acrylate, dimethylaminostyrene, and diethylaminostyrene. Examples of the ethylenically unsaturated monomers include amino group-containing ethylenically unsaturated monomers such as dimethylaminoethyl (meth)acrylate, diethylaminoethyl (meth)acrylate, and methylethylaminoethyl (meth)acrylate; epoxy group-containing ethylenically unsaturated monomers such as glycidyl (meth)acrylate and 3,4-epoxycyclohexyl (meth)acrylate;Ketone group-containing ethylenically unsaturated monomers such as diacetone (meth)acrylamide and acetoacetoxy (meth)acrylate; allyl (meth)acrylate, 1-methylallyl (meth)acrylate, 2-methylallyl (meth)acrylate, 1-butenyl (meth)acrylate, 2-butenyl (meth)acrylate, 3-butenyl (meth)acrylate, 1,3-methyl-3-butenyl (meth)acrylate, 2-chloroallyl (meth)acrylate, 3-chloroallyl (meth)acrylate, o-allylphenyl (meth)acrylate, 2-(allyloxy)ethyl (meth)acrylate, allyl lactyl (meth)acrylate, citronellyl (meth)acrylate, geranyl (meth)acrylate, rosinyl (meth)acrylate, cinnamyl (meth)acrylate, diallyl maleate, Ethylenically unsaturated monomers having two or more ethylenically unsaturated groups, such as lysyl itaconic acid, vinyl (meth)acrylate, vinyl crotonate, vinyl oleate, vinyl linoleate, 2-(2'-vinyloxyethoxy)ethyl (meth)acrylate, ethylene glycol di(meth)acrylate, triethylene glycol (meth)acrylate, tetraethylene glycol (meth)acrylate, trimethylolpropane tri(meth)acrylate, pentaerythritol tri(meth)acrylate, 1,1,1-trishydroxymethylethane diacrylate, 1,1,1-trishydroxymethylethane triacrylate, 1,1,1-trishydroxymethylpropane triacrylate, divinylbenzene, divinyl adipate, diallyl isophthalate, diallyl phthalate, and diallyl maleate;Alkoxysilyl group-containing ethylenically unsaturated 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, and vinylmethyldimethoxysilane; methylol group-containing ethylenically unsaturated monomers such as N-methylol(meth)acrylamide, N,N-dimethylol(meth)acrylamide, and alkyl etherified N-methylol(meth)acrylamide; but are not limited thereto.

[0035] In addition, styrenes such as styrene, α-methylstyrene, o-methylstyrene, p-methylstyrene, and m-methylstyrene may be used to the extent that the effect is not impaired, but it is preferable not to include them from the viewpoint of environmental friendliness.

[0036] Among the ethylenically unsaturated monomers (a-3) exemplified above, from the viewpoint of improving the stability of the resin microparticles (A) and improving the physical properties of the coating agent by increasing the molecular weight, it is more preferable that the ethylenically unsaturated monomer (a-3) is acrylamide, glycidyl methacrylate, ethylene glycol dimethacrylate, diethylene glycol dimethacrylate, or N-methylol acrylamide.

[0037] [Method for producing resin fine particles (A)] The polymerization method of resin microparticles (A) is not particularly limited, but it is preferable to use emulsion polymerization because it can easily obtain a resin microparticle dispersion with high molecular weight, low viscosity, and high solid content in an aqueous medium.As a method for preparing resin microparticles with different Tg, during emulsion polymerization, a dropping tank is divided into multiple stages, and the composition of ethylenically unsaturated monomers is changed and dropped so that the Tg of the polymer produced is different in each stage, or a method in which an acrylic resin is synthesized in advance by bulk polymerization or solution polymerization, dissolved or dispersed in an aqueous phase, and then an ethylenic monomer is dropped to polymerize so that a polymer with a Tg different from that of the previous acrylic resin is produced, etc. are listed, but considering that the process is not complicated and the dispersion stability of the resin microparticles produced is good, it is more preferable to synthesize resin microparticles (A) by multistage polymerization.

[0038] In the resin fine particles (A), the mass ratio of the total amount of polymers having a Tg in the range of -40 to 30°C to the total amount of polymers having a Tg in the range of 40 to 140°C is preferably in the range of 70 / 30 to 50 / 50, more preferably in the range of 60 / 40 to 55 / 45. By having the mass ratio of the polymers in the above range, the coating agent has excellent film-forming properties, while excessive fusion and thermal movement of the formed coating layer are suppressed, improving various coating film resistances. Therefore, the food packaging sheet exhibits better food resistance (seasoning resistance, microwave resistance), blocking resistance, and heat sealability. The total amount of the polymers refers to the total amount of ethylenically unsaturated monomers constituting the polymer.

[0039] In the resin fine particles (A), the polymer having a Tg in the range of -40 to 30°C and the polymer having a Tg in the range of 40 to 140°C are not compatible with each other, and the particles can have a core-shell structure, a sea-island structure, a raspberry-shaped structure, or other irregular particle structure. From the viewpoint of better coating film properties, it is preferable to have a core-shell structure, and more preferably, the core part is a core-shell structure composed of a polymer having a Tg in the range of -40 to 30°C and the shell part is a polymer having a Tg in the range of 40 to 140°C. The core-shell structure significantly improves the film-forming properties of the resin fine particles, and the coating agent forms a dense and tough coating film even under low-temperature drying conditions. Therefore, the food packaging sheet made of the coating agent exhibits better food resistance (seasoning resistance, microwave resistance).

[0040] (Radical polymerization initiator) As the radical polymerization initiator used in the polymerization reaction of the ethylenically unsaturated monomer (a), a known oil-soluble polymerization initiator or water-soluble polymerization initiator can be used. These may be used alone or in combination of two or more kinds. The radical polymerization initiator is preferably used in an amount of 0.1 to 4 parts by mass, and more preferably 0.2 to 2 parts by mass, based on 100 parts by mass of the total amount of the ethylenically unsaturated monomer (a).

[0041] The oil-soluble polymerization initiator is not particularly limited, and examples thereof include organic peroxides such as benzoyl peroxide, tert-butyl peroxybenzoate, tert-butyl hydroperoxide, tert-butyl peroxy(2-ethylhexanoate), tert-butylperoxy-3,5,5-trimethylhexanoate, and di-tert-butyl peroxide; and 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-carbonitrile.

[0042] In the emulsion polymerization, it is preferable to use a water-soluble polymerization initiator. As the water-soluble polymerization initiator, for example, conventionally known ones such as ammonium persulfate (APS), potassium persulfate (KPS), hydrogen peroxide, and 2,2'-azobis(2-methylpropionamidine) dihydrochloride can be suitably used.

[0043] (reducing agent) In emulsion polymerization, a reducing agent may be used in combination with the polymerization initiator. The use of a reducing agent in combination accelerates the emulsion polymerization rate and facilitates emulsion polymerization at low temperatures. Examples of reducing agents include reducing organic compounds such as ascorbic acid, erythorbic acid, tartaric acid, citric acid, glucose, and metal salts of formaldehyde sulfoxylate; reducing inorganic compounds such as sodium thiosulfate, sodium sulfite, sodium bisulfite, and sodium metabisulfite; ferrous chloride, Rongalite, and thiourea dioxide. The reducing agent is preferably used in an amount of 0.05 to 5 parts by mass based on 100 parts by mass of the total amount of the ethylenically unsaturated monomer (a).

[0044] The polymerization temperature may be equal to or higher than the polymerization initiation temperature of the polymerization initiator, and for example, in the case of a peroxide-based polymerization initiator, the polymerization temperature is usually about 80° C. The polymerization time is not particularly limited, but is usually 2 to 24 hours. The ethylenically unsaturated monomer (a) may be polymerized by photochemical reaction or radiation exposure without using the above-mentioned polymerization initiator.

[0045] (Buffer or chain transfer agent) In the polymerization of the ethylenically unsaturated monomer (a), a buffer or a chain transfer agent may be further used as necessary. Examples of the buffer include sodium acetate, sodium citrate, and sodium bicarbonate. Examples of the chain transfer agent include n-hexyl mercaptan, n-heptyl mercaptan, t-hexyl mercaptan, t-heptyl mercaptan, n-octyl mercaptan, t-octyl mercaptan, n-nonyl mercaptan, t-nonyl mercaptan, n-decyl mercaptan, t-decyl mercaptan, n-undecyl mercaptan, t-undecyl mercaptan, n-dodecyl mercaptan, t-dodecyl mercaptan, n-tridecyl mercaptan, t-tridecyl mercaptan, n-tetradecyl mercaptan, and t-tetradecyl mercaptan. mercaptan, n-heptadecyl mercaptan, t-heptadecyl mercaptan, t-hexadecyl mercaptan, n-hexadecyl mercaptan, n-heptadecyl mercaptan, n-octadecyl mercaptan, t-heptadecyl mercaptan, t-octadecyl mercaptan, mercaptan 2-ethylhexyl mercaptoacetate, octyl mercaptoacetate, methoxybutyl mercaptopropionate, 2-ethylhexyl mercaptopropionate, octyl mercaptopropionate, and stearyl mercaptopropionate. The buffer is preferably used in an amount of 0 to 1 part by mass, more preferably 0.05 to 0.5 part by mass, based on 100 parts by mass of the total amount of the ethylenically unsaturated monomer (a). The chain transfer agent is preferably used in an amount of 0.4 to 3 parts by mass, more preferably 0.6 to 2 parts by mass, based on 100 parts by mass of the total amount of the ethylenically unsaturated monomer (a).

[0046] Among the above chain transfer agents, it is preferable to use a chain transfer agent (X) represented by the following general formula (2).

[0047] [ka] R is a linear or branched alkyl group having 8 to 16 carbon atoms.

[0048] By using a chain transfer agent (X) during polymerization, a structure represented by general formula (1) can be introduced to one end of a polymer of an ethylenically unsaturated monomer (a). A polymer having a structure represented by general formula (1) at one end exhibits surface activity due to an appropriate polarity contrast between the terminal alkyl group R, which is a low polarity site, and the highly polar acrylic resin bonded thereto, and further improves the dispersion stability of the resin fine particles (A). Therefore, the stability of the coating agent is improved, and the generation of sediments and aggregates that lead to poor coating and poor film formation is greatly reduced. It also has the effect of improving the wettability of the resin to the substrate. As a result, the coating agent components penetrate and bond firmly to the substrate, making the bond at the interface between the substrate and the resin stronger. Therefore, even if the coating layer comes into contact with food ingredients, there is less risk of the food ingredients penetrating the substrate side, and the food resistance (seasoning resistance, microwave resistance) and heat sealability of the food packaging sheet are further improved. The polymer having the structure represented by the above general formula (1) is more preferably included in the polymers having a Tg in the range of 40 to 140° C., from the viewpoint of more effectively improving the film-forming property and wettability of the resin.

[0049] Examples of the chain transfer agent (X) include n-octyl mercaptan, t-octyl mercaptan, n-nonyl mercaptan, t-nonyl mercaptan, n-decyl mercaptan, t-decyl mercaptan, n-undecyl mercaptan, t-undecyl mercaptan, n-dodecyl mercaptan, t-dodecyl mercaptan, n-tridecyl mercaptan, t-tridecyl mercaptan, n-tetradecyl mercaptan, t-tetradecyl mercaptan, n-heptadecyl mercaptan, t-heptadecyl mercaptan, t-hexadecyl mercaptan, and n-hexadecyl mercaptan.

[0050] The chain transfer agent (X) is preferably used in the range of 0.4 to 3 parts by mass, more preferably 0.6 to 2 parts by mass, based on 100 parts by mass of the total amount of the ethylenically unsaturated monomer (a). By using it in the above range, the effect of introducing an alkyl group to one end of the polymer described above can be sufficiently obtained, and at the same time, there is no risk of adversely affecting other physical properties. Therefore, a food packaging sheet that exhibits better food resistance (resistance to seasonings, resistance to microwave ovens) and heat sealability can be obtained. The introduction rate of the chain transfer agent of general formula (2) into the polymer is 100%.

[0051] (Basic compounds) In order to enhance the dispersion stability of the resin fine particles (A) during polymerization of the ethylenically unsaturated monomer (a), a basic compound may be used as a neutralizing agent. Examples of the basic compound include various organic amines such as ammonia water, dimethylaminoethanol, diethanolamine, and triethanolamine; and inorganic alkali agents such as hydroxides of alkali metals such as sodium hydroxide, lithium hydroxide, and potassium hydroxide. The basic compound may be added after the completion of the first stage of dropwise addition, or after the completion of the reaction. The basic compound is preferably used in an amount of from 0.75 to 1.2 mol per 1 mol of carboxyl groups in the resin fine particles (A).

[0052] (Surfactant) In addition, when obtaining the resin fine particles (A), a surfactant can be used for the purpose of improving the dispersion stability of the resin fine particles, so long as it does not adversely affect the various physical properties of the coating agent or food packaging sheet. Examples of the surfactant include anionic surfactants, cationic surfactants, nonionic surfactants, and amphoteric surfactants, and from the viewpoint of safety and good coating film physical properties, anionic surfactants or nonionic surfactants are preferable. In terms of being less likely to adversely affect food resistance (seasoning resistance, microwave resistance), the surfactant is preferably contained in a range of 0.1 to 2.0 parts by mass, more preferably 0.4 to 1.0 parts by mass, based on 100 parts by mass of the total amount of the ethylenically unsaturated monomer (a). The surfactant can be used alone or in combination of two or more kinds.

[0053] Examples of anionic surfactants that can be used include higher fatty acid salts such as sodium oleate, alkylarylsulfonates such as dodecylbenzenesulfonic acid, alkyl sulfates such as sodium lauryl sulfate, polyoxyethylene alkyl ether sulfates such as sodium polyoxyethylene lauryl ether sulfate, alkyl sulfosuccinates such as sodium monooctyl sulfosuccinate, sodium dioctyl sulfosuccinate, and sodium polyoxyethylene lauryl sulfosuccinate, and derivatives thereof, and polyoxyethylene distyrenated phenyl ether sulfate. However, from the viewpoint of being less likely to adversely affect the food resistance (seasoning resistance, microwave resistance) of the food packaging sheet, among the above, it is preferable that the surfactant is an alkyl sulfate or an alkyl sulfosuccinate, and more preferably that the alkyl sulfate is lauryl sulfate and the alkyl sulfosuccinate is dioctyl sulfosuccinate.

[0054] Examples of nonionic surfactants include polyoxyethylene alkyl ethers such as polyoxyethylene lauryl ether and polyoxyethylene stearyl ether, polyoxyethylene alkyl phenyl ethers such as polyoxyethylene octyl phenyl ether and polyoxyethylene nonyl phenyl ether, sorbitan higher fatty acid esters such as sorbitan monolaurate, sorbitan monostearate, and sorbitan trioleate, polyoxyethylene sorbitan higher fatty acid esters such as polyoxyethylene sorbitan monolaurate and polyoxyethylene sorbitan monostearate, polyoxyethylene higher fatty acid esters such as polyoxyethylene monolaurate and polyoxyethylene monostearate, glycerin higher fatty acid esters such as oleic acid monoglyceride and stearic acid monoglyceride, polyoxyethylene-polyoxypropylene block copolymers, polyvinyl alcohol, polyvinylpyrrolidone, and polyoxyethylene distyrenated phenyl ether.

[0055] As the surfactant, a polymerizable surfactant having one or more radically polymerizable ethylenically unsaturated double bonds in the molecule can be used.

[0056] The polymerizable surfactant is preferably a polymerizable anionic surfactant or a polymerizable nonionic surfactant.

[0057] Examples of the polymerizable anionic surfactant include those having a main skeleton such as a sulfosuccinate ester, an alkyl ether, an alkyl phenyl ether, an alkyl phenyl ester, a (meth)acrylate sulfate ester, and a phosphate ester. Examples of the polymerizable nonionic surfactant include those having a main skeleton such as an alkyl ether, an alkyl phenyl ether, and an alkyl phenyl ester.

[0058] Among the above-mentioned surfactants, the surfactant contains at least one of lauryl sulfate and dioctyl sulfosuccinate, and the surfactant preferably contains 0.1 to 2.0 parts by mass of both in total, more preferably 0.4 to 1.0 parts by mass, based on 100 parts by mass of the total amount of the ethylenically unsaturated monomer (a). By containing lauryl sulfate or dioctyl sulfosuccinate in the above range, various physical properties of the food packaging sheet are not adversely affected, and the dispersion stability of the resin fine particles (A) is improved, and the film-forming property of the coating agent is also improved. In addition, the wetting of the coating agent to the substrate is improved, so that coating unevenness is reduced and the coating agent firmly penetrates and bonds to the substrate. Therefore, it is preferable because the food resistance (seasoning resistance, microwave resistance) and heat sealability of the food packaging sheet are further improved.

[0059] [Glass transition temperature (Tg)] The resin fine particles (A) have a glass transition temperature Tg in the low temperature range of -40 to 30° C. and a high temperature range of 40 to 140° C. Thus, the resin fine particles (A) of the present invention are characterized in that they have at least two glass transition temperatures Tg obtained by DSC within a specific temperature range, and may have an additional glass transition temperature Tg outside these temperature ranges. The lower temperature side Tg is more preferably in the range of -20 to 10°C, and the higher temperature side Tg is more preferably in the range of 70 to 140°C. When the Tg is in the range of -40 to 30°C, the film-forming properties of the coating agent are improved, forming a dense coating without gaps or cracks, eliminating the risk of food ingredients soaking into the coating layer. In addition, when heated in a microwave oven, the resin components of the heated coating layer are not likely to swell or dissolve due to steam or oil from the food. In addition, during heat sealing, the coating layers are firmly fused together, forming a strong intertwining between the resins. Therefore, the food packaging sheet exhibits excellent food resistance (seasoning resistance, microwave resistance) and heat sealing properties. If the Tg is in the range of 40 to 140°C, there is no risk of excessive fusion of the coating layers, resulting in blocking between the coating layers. In addition, the mobility of the low-temperature polymer (Tg of -40 to 30°C) coexisting in the coating layer is also suppressed, so that when heated in a microwave oven, the resin components of the heated coating layer are prevented from swelling due to steam and oil from the food. At the same time, the film-forming properties of the coating agent are not deteriorated when applied. Therefore, the food packaging sheet exhibits excellent food resistance (resistance to seasonings, resistance to microwave ovens) and blocking resistance. The resin fine particles (A) have a Tg in the low temperature range of -40 to 30°C and a high temperature range of 40 to 140°C, and this synergistic effect allows a food packaging sheet to be obtained that has excellent food resistance (resistance to seasonings and microwave ovens), blocking resistance, and heat sealability. The glass transition temperature of the resin fine particles (A) can be appropriately adjusted by the combination of the ethylenically unsaturated monomers (a) that constitute the resin fine particles (A).

[0060] The glass transition temperature of the resin fine particles (A) can be determined by performing DSC measurement on the solid obtained by drying the dispersion of the resin fine particles (A). When DSC is measured under temperature increasing conditions, the glass transition of the resin is observed as an endothermic phenomenon. The temperature at the intersection of the tangent line of the baseline on the low temperature side and the inflection point is taken as the glass transition temperature of the resin. For the resin fine particles (A), for example, as shown in Figure 1, Tg can be confirmed on the low temperature side of -40 to 30°C and on the high temperature side of 40 to 140°C.

[0061] [Mass average molecular weight] The mass average molecular weight of the resin fine particles (A) is preferably 100,000 or more. More preferably, it is 200,000 or more. Since the mass average molecular weight is 100,000 or more, the coating layer forms a tough coating film, so that even when exposed to food ingredients containing a mixture of moisture, oil, salt, acid, etc., swelling and elution of the resin are suppressed, and the coating layer is less likely to be damaged. In addition, even under microwave heating conditions, the coating layer is less likely to be damaged by steam and oil from the food. Therefore, the food resistance (seasoning resistance, microwave resistance) of the food packaging sheet is further improved. In addition, excessive fusion between the coating layers is also suppressed, so the blocking resistance of the food packaging sheet is also improved. Since a tough coating layer is formed with a high molecular weight resin, the resins are firmly fused and entangled during heat sealing, and the heat sealability is also greatly improved. The mass average molecular weight of the resin fine particles (A) can be appropriately adjusted by the amount of initiator and the amount of chain transfer agent used in polymerization. Preferably, it is 3 million or less. The mass average molecular weight refers to a value calculated in terms of polystyrene by GPC (gel permeation chromatography).

[0062] [Average particle size] The average particle size of the resin fine particles (A) is preferably in the range of 30 to 300 nm. More preferably, it is 35 to 200 nm. By having a particle size in the above range, problems such as the particle size being too small and the coating agent penetrating too much into the substrate, and not fully exhibiting heat sealability, or the particle size being too large and not penetrating enough into the substrate, and not being able to bond firmly between the substrate and the coating layer, and the coating agent causing film formation defects, etc., can be prevented, and the like, in which heat sealability and food resistance are not fully exhibited. Therefore, a coating agent with excellent coatability and film formation properties can be obtained, and a food packaging sheet with excellent food resistance (seasoning resistance, microwave resistance) and heat sealability can be obtained.

[0063] <Optional ingredients> The coating agent for food packaging sheets of the present invention may contain additives such as extender pigments, antifoaming agents, leveling agents, preservatives, solvents, waxes, etc. It is preferable that the optional components do not have adverse effects on health if they remain on the food packaging sheet after coating.

[0064] The body pigment is preferably one approved as a food or food additive, such as talc, silica, calcium carbonate, barium sulfate, titanium oxide, diatomaceous earth (white carbon), cellulose powder, etc.

[0065] Examples of the solvent include ethanol, isopropyl alcohol, propylene glycol, and glycerin from the viewpoint of oral safety.

[0066] Examples of the wax include carnauba wax, beeswax, paraffin wax, modified paraffin wax, polyethylene wax, oxidized polyethylene wax, modified polyethylene wax, polypropylene wax, oxidized polypropylene wax, modified polypropylene wax, ethylene-vinyl acetate copolymer wax, modified ethylene-vinyl acetate copolymer wax, fatty acid amide, microcrystalline wax, Fischer-Tropsch wax, and polytetrafluoroethylene.

[0067] <Food packaging sheet> The food packaging sheet of the present invention has a coating layer formed from the coating agent for food packaging sheets on a base sheet.

[0068] The substrate sheet is a sheet having a porous surface condition for paper, nonwoven fabric, and woven fabric, but is preferably paper. By using a paper substrate as the substrate, the coating agent is sufficiently wetted with the substrate and penetrates into the substrate, so that the substrate and the coating layer are more firmly bonded. Therefore, the food packaging sheet can exhibit better food resistance (seasoning resistance, microwave resistance) and heat sealability. After forming the coating layer, the food packaging sheet coating agent can be applied several times to improve food resistance (resistance to seasonings, resistance to microwave ovens) and heat sealability. The coating layer may be impregnated into the base sheet, or a non-impregnated embodiment may be used.

[0069] The coating can be performed by a known coating method, but flexographic and gravure methods are preferred. The flexographic method is a method in which the coating agent is transferred from an intaglio plate, called an anilox roll, to a resin plate or rubber plate, and then the coating agent on the resin plate or rubber plate is applied to the original roll. It is also possible to pattern the resin plate or rubber plate. The gravure method includes a method in which the coating agent is transferred directly from the intaglio plate to the original roll, and a so-called gravure offset method in which the coating agent is transferred from the intaglio plate to a flat plate and then transferred to the original roll. It is also possible to pattern the intaglio plate. In the case of the gravure method, it is preferable to perform a press treatment with a smoothing roll after coating.

[0070] The food packaging sheet of the present invention can be used for packaging not only confectionery such as popcorn, chocolate, caramel, and fruits, but also burgers such as hamburger steak, hot dog, hamburger, rice burger, fried foods such as fried chicken, french fries, fried chicken, tempura, fried bread, grilled chicken, yakitori, grilled meat and seafood such as saury, meat buns, red bean buns, dumplings, shumai, spring rolls, and can be used for foods that contain multiple ingredients such as oil, salt, acid, amino acid, and have a high water content, such as ketchup, sauce, and sauce. It can also be used as a sheet for packaging various foods such as frozen foods and paper trays for lunch boxes, and can be used for applications such as heating in a microwave oven from a packaged state. The food packaging sheet of the present invention is suitable for use in applications in which the coating layer directly contacts the various foods described above. EXAMPLES

[0071] The present invention will be described in more detail below with reference to examples, but the following examples are not intended to limit the scope of the invention. In the examples, "parts" means "parts by mass," "%" means "% by mass," the values ​​in the tables are solid content masses, and blanks indicate that no space is used. The glass transition temperature, acid value, average particle size, and mass average molecular weight of the resin fine particles are measured as follows.

[0072] <Glass transition temperature> The glass transition temperature of the resin microparticles was measured using a DSC (differential scanning calorimeter manufactured by TA Instruments). Specifically, an aluminum pan containing about 3 mg of dried resin precisely weighed out and an empty aluminum pan as a reference were set in a DSC measurement holder, and the temperature at the intersection of the baseline on the low-temperature side of the endothermic phenomenon and the tangent at the inflection point in the DSC curve obtained by measurement under a temperature rise condition of 10°C / min was taken as the glass transition temperature (Tg).

[0073] <Acid value> The acid value of a resin is the number of milligrams of potassium hydroxide required to neutralize the acidic components contained in 1 g of dried resin. It was calculated by potentiometric titration of the dried water-soluble resin with a potassium hydroxide-ethanol solution according to the method described in JIS K2501.

[0074] <Average particle size> The average particle size was measured by diluting the resin fine particle dispersion 500 times with water and measuring about 5 ml of the diluted solution using a dynamic light scattering measurement method (measuring device manufactured by Nanotrack UPA Co., Ltd., Microtrack Bell Co., Ltd.). The peak of the volume particle size distribution data (histogram) obtained at this time was taken as the average particle size.

[0075] <Mass average molecular weight> The mass average molecular weight of the resin was measured by dissolving the dried resin in tetrahydrofuran to prepare a 0.2% solution, filtering the solution with a membrane filter (13HP045AN, pore size 0.45 μm, manufactured by ADVANTEC), and measuring the molecular weight using the following equipment and measurement conditions. Note that resins that are not completely soluble in THF, or that dissolve but do not pass through the filter, are considered to have a sufficiently high molecular weight, and their molecular weight is set to 2 million or more. Equipment: HLC-8320-GPC system (Tosoh Corporation) Column: TSKgel-Super Multipore HZ-M0021488 4.6 mm I.D. x 15 cm x 3 (molecular weight measurement range: 2,000 to approximately 2 million) Elution solvent: tetrahydrofuran Standard material: Polystyrene (Tosoh Corporation) Flow rate: 0.6mL / min Amount of sample solution used: 10μL Column temperature: 40℃

[0076] <Preparation of coating agent for food packaging sheet> [Example 1] A reaction vessel (reaction tank) equipped with a stirrer, a thermometer, a dropping funnel, and a reflux condenser was charged with 83.6 parts of water and 0.35 parts of EMALE 0 (100% active ingredient, sodium lauryl sulfate, manufactured by Kao Corporation). Separately, 30.0 parts of ethyl acrylate, 53.0 parts of methyl methacrylate, 7.0 parts of n-butyl acrylate, 10.0 parts of methacrylic acid, 3.9 parts of t-dodecyl mercaptan, 0.80 parts of EMALE 0, and 52.9 parts of water were mixed and stirred in advance to prepare an emulsion of ethylenically unsaturated monomers (first-stage dropping tank) to be dropped into the first stage. The internal temperature of the reaction vessel was raised to 80°C and nitrogen substitution was sufficiently performed, and then 5.5 parts of a 5% aqueous solution of potassium persulfate was added as an initiator to start emulsion polymerization. While maintaining the internal temperature at 80°C, the emulsion of the ethylenically unsaturated monomer to be dropped in the first stage and 5.5 parts of a 5% aqueous solution of potassium persulfate were added dropwise over 2 hours. After the first stage ethylenically unsaturated monomer emulsion was dropped, 30 minutes were left, and the emulsion of the second stage ethylenically unsaturated monomer (second stage dropping tank) and 11.1 parts of a 10% aqueous solution of ammonium persulfate were dropped over 2 hours. After the drop was completed, the mixture was reacted at 80°C for another 3 hours. After the reaction was completed, 9.8 parts of 25% aqueous ammonia was added with stirring to neutralize the mixture, thereby obtaining an aqueous dispersion of resin fine particles. The Tg of the obtained resin microparticles was 8.4°C and 56.4°C, the acid value was 33.8mgKOH / g, and the average particle size was 200nm. Since the resin was insoluble in THF, the mass average molecular weight was set to 2 million or more. Water was further added to the resin microparticle water dispersion to adjust the non-volatile content to 40%, and the intended coating agent for food packaging sheets was obtained.

[0077] [Examples 2 to 12, Comparative Examples 1 to 6] A water dispersion of resin particles was prepared in the same manner as in Example 1 with the blending composition shown in Tables 1 and 2. For Example 6 and Comparative Example 3, the amount of water charged in the second stage dropping tank was changed from 160.8 parts to 250.8 parts, and for Examples 7, 8, 11, 12 and Comparative Examples 4 to 6, it was changed to 130.8 parts. After the reaction was completed, 25% ammonia water was added to the resin so as to be equimolar to the carboxyl group in the resin to neutralize it. For the obtained resin particles, Tg, acid value, average particle size, and mass average molecular weight were measured in the same manner as in Example 1. Water was further added to the resin particle water dispersion to adjust the non-volatile content to 40%, and the intended coating agent for food packaging sheets was obtained.

[0078] [Example 13] In a reaction vessel (reaction tank) equipped with a stirrer, a thermometer, a dropping funnel, and a reflux condenser, 237.8 parts of water and 0.86 parts of PELLEX OT-P (Kao Corporation dioctyl sulfosuccinate sodium active ingredient 70%) were charged. Separately, 69.5 parts of methyl methacrylate, 18.0 parts of n-butyl acrylate, 12.5 parts of methacrylic acid, 3.0 parts of t-dodecyl mercaptan, 2.01 parts of PELLEX OT-P, and 65.8 parts of water were mixed and stirred in advance to prepare an emulsion of ethylenically unsaturated monomers (first-stage dropping tank) to be dropped into the first stage. The internal temperature of the reaction vessel was raised to 80°C and nitrogen substitution was sufficiently performed, and then 12.1 parts of a 5% aqueous solution of potassium persulfate was added as an initiator to start emulsion polymerization. While maintaining the internal temperature at 80°C, the emulsion of the ethylenically unsaturated monomer to be dropped in the first stage and 5.5 parts of a 5% aqueous solution of potassium persulfate were added dropwise over 2 hours. After the dropping was completed, the mixture was reacted for 2 hours, and 9.9 parts of 25% aqueous ammonia was added to neutralize the mixture while maintaining the temperature at 80°C. Next, a second-stage ethylenically unsaturated monomer solution (second-stage dropping tank) prepared by mixing 50.0 parts of methyl acrylate and 102.0 parts of n-butyl acrylate in advance and 24.2 parts of a 10% aqueous solution of ammonium persulfate were dropped over 2 hours. After the dropping was completed, the mixture was reacted for another 3 hours while maintaining the temperature at 80°C, and an aqueous dispersion of resin fine particles was obtained. The Tg of the obtained resin microparticles was -18.8℃ and 70.6℃, the acid value was 31.2mgKOH / g, and the average particle size was 35nm. Since the resin was insoluble in THF, the mass average molecular weight was set to 2 million or more. Water was further added to the resin microparticle water dispersion to adjust the non-volatile content to 40%, and the intended coating agent for food packaging sheets was obtained.

[0079] [Examples 13 to 20, Comparative Examples 7 and 8] An aqueous dispersion of resin particles was prepared in the same manner as in Example 13, with the composition shown in Table 3. 25% aqueous ammonia was added to neutralize the resin so that the molar ratio was equimolar to the carboxyl groups in the resin. The Tg, acid value, average particle size, and mass average molecular weight of the resulting resin particles were measured in the same manner as in Example 13. Water was further added to the aqueous dispersion of resin particles to adjust the non-volatile content to 40%, and the intended coating agent for food packaging sheets was obtained.

[0080] [Table 1]

[0081] [Table 2]

[0082] [Table 3]

[0083] The abbreviations in the table are as follows. <Surfactant> Emal 0: Sodium lauryl sulfate (Kao Corporation, active ingredient 100% by weight) Pelex O-TP: Sodium dioctyl sulfosuccinate (Kao Corporation, active ingredient 70% by weight) Noigen TDS-120: Polyoxyethylene tridodecyl ether (manufactured by Daiichi Kogyo Seiyaku Co., Ltd., active ingredient 100% by mass)

[0084] [Example 21] A reaction vessel (reaction tank) equipped with a stirrer, a thermometer, a dropping funnel, and a reflux condenser was charged with 143.0 parts of n-propanol. Separately, 71.0 parts of methyl methacrylate, 15.0 parts of n-butyl acrylate, and 14.0 parts of methacrylic acid were mixed in advance to prepare a mixed solution of ethylenically unsaturated monomers to be dropped. The internal temperature of the reaction vessel was raised to 90°C and nitrogen substitution was sufficiently performed, and then 6.0 parts of azobisisobutyronitrile (AIBN) was added as an initiator to start radical polymerization. While maintaining the internal temperature at 90°C, the mixed solution of ethylenically unsaturated monomers was dropped over 2 hours. Then, while maintaining the temperature at 90°C, the reaction was continued for another 4 hours. After the reaction was completed, 11.5 parts of 25% ammonia water and 660 parts of water were added and stirred. From there, the solvent was removed under reduced pressure conditions to obtain an aqueous solution of acrylic resin with a non-volatile content adjusted to 22.0%. Next, the internal temperature of the reaction vessel was raised again to 80°C and nitrogen substitution was thoroughly performed, after which 70.0 parts of methyl acrylate, 82.0 parts of n-butyl acrylate, and 15.0 parts of a 10% aqueous solution of ammonium persulfate as an initiator were dropped over 2 hours. After completion of the dropwise addition, the reaction was continued for another 3 hours while maintaining 80°C, to obtain an aqueous dispersion of resin fine particles. The content of the ethylenically unsaturated monomer (a-1) in the obtained resin fine particles was 94.4% by mass. The content of the surfactant per 100 parts by mass of the ethylenically unsaturated monomer (a) was 0 parts, and the content of the chain transfer agent per 100 parts by mass of the ethylenically unsaturated monomer (a) was 0 parts. In addition, the Tg, acid value, average particle size, and mass average molecular weight were measured in the same manner as in Example 13, and the Tg of the resin microparticles was -10.4°C and 78.8°C, the acid value was 35.4 mgKOH / g, the average particle size was 102 nm, and the mass average molecular weight was 900,000. Water was further added to the resin microparticle aqueous dispersion to adjust the non-volatile content to 40%, and the intended coating agent for food packaging sheets was obtained.

[0085] Evaluation items and evaluation methods The evaluation items and evaluation methods for the coating agent for food packaging sheets and the food packaging sheets are as follows. <Stability of coating agents for food packaging sheets> The coating agent for food packaging sheets obtained above was placed in a screw bottle and left to stand at 50°C for one month, and the presence or absence of sedimentation and the rate of change in average particle size over time were confirmed. The average particle size of the coating agent before and after aging was measured in the same manner as the average particle size of the resin fine particles described above. [Evaluation Criteria] S: No sedimentation occurs and the average particle size remains unchanged (Very good) A: No sedimentation has occurred, but the average particle size has changed slightly. (Change rate is less than ±20%) (Good) B: No sedimentation occurs, but the average particle size has changed. (Change rate is between ±20% and ±50%) (usable) C: Sedimentation has occurred or the average particle size has changed significantly (Change rate is ±50% or more) (Not available)

[0086] <Production of food packaging sheets> (Food packaging sheet 1) The obtained coating agent for food packaging sheets was applied to the glossy surface of a commercially available sanitary paper (weighing 21 g, gloss-treated on one side) using a bar coater #4. After coating, the paper was dried in a hot air oven at 80°C for 30 seconds to obtain food packaging sheet 1. (Food packaging sheet 2) The obtained coating agent for food packaging sheets was applied to one side of a commercially available thick paper (weighing 300 g) using a bar coater #8. After coating, the sheet was dried in a hot air oven at 100° C. for 60 seconds to obtain food packaging sheet 2 for contact with frozen foods.

[0087] <Blocking resistance test> The coated surface and the uncoated surface of the obtained food packaging sheet 1 were placed together, and the blocking resistance was measured using the following device and under the following measurement conditions. Equipment: CO-201 permanent deformation tester (manufactured by Tester Sangyo Co., Ltd., upper and lower plate heating) Condition: 2kg / cm 2 -40℃-24hr [Evaluation Criteria] S: No resistance when peeling off, no damage to the sanitary paper (very good) A: There is some resistance when peeling, but the sanitary paper is not damaged (good) B: There is some resistance when peeling off, but the sanitary paper is not damaged (usable) C: There is resistance when peeling off, and the sanitary paper is also damaged (unusable)

[0088] <Heat sealability> The obtained food packaging sheet 1 was cut into a width of 15 mm, and the heat seal strength was measured using the test piece as described below under the following measuring device and measuring conditions. (Heat seal) Equipment: HEATSHEEL&IMPULSE TESTER (manufactured by Nichiri Kagaku Kogyo Co., Ltd., upper and lower plate heating) Heat seal conditions: 130℃-2kgf / cm 2 -1 second (peel off) Equipment: Tensile testing machine (manufactured by Tester Sangyo Co., Ltd.) Peeling conditions: 180° peeling, 300mm / min [Evaluation Criteria] S: 2N or better (very good) A: 1.5N or more, less than 2N (good) B: 1N or more, less than 1.5N (usable) C: Less than 1N (unusable)

[0089] <Food resistance> (Seasoning resistance test) In an environment of 23°C, a drop of a mixture of ketchup / vinegar / salad oil (1 / 1 / 1) was placed on the coating agent-coated surface of the obtained food packaging sheet 1, and the time it took for the drop on the surface to pass through the sanitary paper and reach the back surface was measured. [Evaluation Criteria] S: 90 minutes or more (very good) A: 60 minutes or more, less than 90 minutes (good) B: 30 minutes or more, less than 60 minutes (usable) C: Less than 30 minutes (very poor)

[0090] (Microwave oven resistance test) A piece of "Tokukara Fried Young Chicken" manufactured by Nichirei Foods, Inc. was placed on the coating surface of the obtained food packaging sheet 2, heated in a microwave oven at 500W for 180 seconds, and then left for 20 minutes to return to room temperature.The fried chicken was then removed and the state of penetration of oil and moisture into the sheet surface and the presence or absence of the fried chicken sticking to the sheet (blocking between the food and the sheet) were checked. [Evaluation Criteria] S: Oil and water do not penetrate the sheet, and the fried chicken does not stick to the sheet. (Very good) A: There is some oil and water penetration, but it does not penetrate to the back side, and the fried chicken does not stick to the sheet. (Good) B: Oil and water have penetrated the sheet, but not to the other side, and the fried chicken does not stick to the sheet. (Can be used) C: Oil or moisture has penetrated to the back side, or the fried chicken is stuck to the sheet. (Not for use)

[0091] [Table 4]

[0092] [Table 5]

[0093] [Table 6]

[0094] As can be seen from Tables 4 to 6, the food packaging sheet coating agents obtained in Examples 1 to 21 had good stability, and the food packaging sheets produced using them had excellent food resistance (seasoning resistance, microwave resistance), blocking resistance, and heat sealability, and exhibited performance that fully met the practical level. On the other hand, the food packaging coating agents of Comparative Examples 1, 5, and 6 were significantly inferior in stability, and the physical properties of the food packaging sheets produced using them were also significantly inferior. In addition, the food packaging coating agents of Comparative Examples 2 to 4, 7, and 8 also had extremely poor physical properties of any of the food packaging sheets, and the results were difficult to say that they met the practical level.

Claims

1. A food packaging sheet having a base sheet and a coating layer formed on the base sheet from a coating agent for food packaging sheets, the coating agent for food packaging sheets containing resin fine particles (A) which are polymers of ethylenically unsaturated monomers (a), the resin fine particles (A) having glass transition temperatures in the ranges of -40 to 30°C and 40 to 140°C and an acid value of 20 to 40 mgKOH / g, and containing 85 to 97 mass% of an ethylenically unsaturated monomer (a-1) having a linear alkyl group having 1 to 4 carbon atoms based on the total mass of the ethylenically unsaturated monomer (a), and the polymer of the ethylenically unsaturated monomer (a) contains a polymer having a structure represented by general formula (1) at one end. 【Chemistry 1】 R is a straight or branched alkyl group having 8 to 16 carbon atoms.

2. The food packaging sheet according to claim 1 , wherein the ethylenically unsaturated monomer (a) contains methacrylic acid.

3. 3. The food packaging sheet according to claim 1, wherein the resin fine particles (A) have an average particle size of 30 to 300 nm.

4. The food packaging sheet according to any one of claims 1 to 3, comprising at least one of lauryl sulfate and dioctyl sulfosuccinate, the total amount of both of which is 0.1 to 2.0 parts by mass based on 100 parts by mass of the total amount of the ethylenically unsaturated monomer (a).

5. The food packaging sheet according to any one of claims 1 to 4, wherein the resin fine particles (A) have a mass average molecular weight of 100,000 or more.

6. The food packaging sheet according to any one of claims 1 to 5, wherein the base sheet is paper.

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