Aqueous coating agent

Abstract

Disclosed is an aqueous coating agent including: (A) a heat-expandable microcapsule; and (B) a thermoplastic resin, wherein the aqueous coating agent contains component (A) in an amount of less than 15 parts by mass based on a total of 100 parts by mass of components (A) and (B).

Description

Technical Field

[0001] This invention relates to: an aqueous coating agent for coating paper substrates; and a paper substrate coated with said aqueous coating agent. The invention also relates to packaging materials (e.g., bags, containers, boxes, etc.) obtained by processing said paper substrates. Background Technology

[0002] Given the environmental concerns, a global reduction in plastic products is recommended. These plastics cannot decompose naturally and are difficult to dispose of. Furthermore, burning plastics can produce dioxins and potentially cause air pollution.

[0003] In addition, in recent years, the issue of plastic waste being dumped in the ocean and breaking down into micron-sized fragments, the ingestion of these fragments by marine fish, and the potential for humans to consume these fish have been recognized as problems.

[0004] Given this background, the use of paper-based materials to replace plastic-based materials has been studied in various fields, particularly in the food industry, where paper-based materials have been processed to replace plastic-based materials in the production of packaging (e.g., food bags, containers such as paper cups, boxes, etc.). In the construction industry, paper-based materials have been processed to produce packaging (e.g., powder packaging bags for cement, etc.).

[0005] Laminated paper is known to be used as a form of packaging. Typically, laminated paper is obtained by laminating a polyethylene film or similar material onto a paper substrate. In recent years, due to increased environmental awareness, there has been a demand for the recycling of laminated paper. However, since the recycling of laminated paper requires specialized equipment, water-based resin coating technology has been used.

[0006] Patent document 1 teaches that an aqueous vinyl resin dispersion comprising an ethylene-acrylic acid copolymer neutralized with ammonia or amine and another olefin thermoplastic resin can be used as a heat sealant for paper or aluminum foil substrates (see [claims],

[0025] ,

[0027] ,

[0028] ,

[0029] ,

[0030] ,

[0035] ,

[0044] , etc.).

[0007] Patent document 1 discloses that by applying the above-mentioned aqueous vinyl resin dispersion to a paper substrate or aluminum foil substrate, drying the material, and then heat-sealing the applied portion at 80 or 120°C, excellent heat-sealing properties are provided (see

[0027] ,

[0030] ,

[0035] ,

[0044] , etc.).

[0008] Patent document 2 discloses the preparation of an acrylic pressure-sensitive adhesive comprising thermally expandable microspheres and a copolymer of butyl acrylate and acrylic acid (see

[0039] to

[0040] ), using the pressure-sensitive adhesive to glue a paper lid attached to a tray made of polymer containing a small amount of water (at room temperature) to produce a food package, and then placing the food package in a microwave oven and heating it at 120°C for 2 minutes, thereby causing the adhesive strength of the adhesive layer to be lost and the lid to be peeled off (see [claims],

[0003] ,

[0004] ,

[0040] to

[0041] ).

[0009] However, when using aqueous dispersions or pressure-sensitive adhesives according to Patent Documents 1 to 2 as coating agents, it is difficult to heat-seal the packaging where the contents adhere to the sealing portion. In particular, when the contents of the packaging are easily dispersed items such as powders, such items may float and adhere to the sealing portion of the packaging, potentially deteriorating the heat-sealing performance. Therefore, there is a need for aqueous coating agents capable of heat-sealing the packaging even where the contents adhere to the sealing portion. Furthermore, Patent Document 2 discloses a pressure-sensitive adhesive, which cannot be used as a heat-sealing agent.

[0010] Reference List Patent documents PTL 1: JP 2000-7860 A PTL 2: JP 2000-302178 A Summary of the Invention

[0011] Technical issues This invention was made to solve the aforementioned problems. The object of this invention is to provide an aqueous coating agent that exhibits excellent heat-sealing properties on paper substrates, particularly on packages comprising a sealed portion of the paper substrate (where the contents (more specifically, powder) adhere to the sealed portion). Furthermore, the object of this invention is to provide a paper substrate coated with said coating agent.

[0012] Solution to the problem As a result of repeated and in-depth research, the inventors discovered that by including a thermoplastic resin and specific microcapsules in an aqueous coating agent and limiting the amount of the microcapsules to a specific range, an aqueous coating agent with excellent heat-sealing properties, especially for packaging bodies including paper-based sealed portions (where the contents (more specifically, powder) adhere to the sealed portion), was obtained, thus completing the present invention.

[0013] In other words, this specification includes the following implementation schemes.

[0014] 1. An aqueous coating agent comprising: (A) heat-expandable microcapsules; and (B) a thermoplastic resin, The total amount of components (A) and (B) is 100 parts by mass, with component (A) being less than 15 parts by mass.

[0015] 2. The water-based coating agent according to 1, wherein the glass transition temperature of the thermoplastic resin in (B) is 40°C or lower.

[0016] 3. The aqueous coating agent according to 1 or 2, wherein the heat-expandable microcapsules (A) and the thermoplastic resin (B) are dispersed in an aqueous medium.

[0017] 4. The aqueous coating agent according to any one of 1 to 3, wherein the heat-expandable microcapsule (A) comprises a shell and a foaming agent contained in the shell and vaporized by heating.

[0018] 5. The aqueous coating agent according to any one of 1 to 4, wherein the thermally expandable microcapsule (A) has an expansion initiation temperature lower than the maximum expansion temperature, the expansion initiation temperature being 65 to 200°C, and the maximum expansion temperature being 100 to 350°C.

[0019] 6. The aqueous coating agent according to any one of 1 to 5, wherein the thermoplastic resin (B) comprises at least one structural unit selected from structural units derived from (meth)acrylate polymers and structural units derived from styrene polymers.

[0020] 7. The aqueous coating agent according to any one of 1 to 6, wherein the thermoplastic resin (B) comprises both a copolymer of methyl methacrylate and 2-ethylhexyl acrylate and a styrene-butadiene copolymer.

[0021] 8. A paper substrate coated with an aqueous coating agent according to claims 1 to 7.

[0022] 9. A packaging body comprising a paper substrate according to claim 8.

[0023] 10. The packaging body according to claim 9, wherein the packaging body comprises powder.

[0024] Beneficial effects of the invention The aqueous coating agent of this invention comprises (A) heat-expandable microcapsules and (B) a thermoplastic resin, wherein, based on a total of 100 parts by weight of components (A) and (B), the amount of component (A) is less than 15 parts by weight. Therefore, the aqueous coating agent significantly improves the heat-sealing properties of paper substrates (particularly for packaging comprising a sealed portion of the paper substrate to which the contents adhere).

[0025] The low content of component (A) enables the aqueous coating agent of the present invention to be uniformly coated on the sealing portion of the paper substrate of the packaging bag, resulting in improved adhesion and peel strength, and also allows the swelling percentage to be maintained at a certain level.

[0026] The aqueous coating agent of this invention expands upon heating. Therefore, even if the contents (powder, more specifically, flour, cement powder, etc.) adhere to the sealing surface, the aqueous coating agent encapsulates the contents and improves adhesion to the sealing portion.

[0027] Even if contents such as powder adhere to the sealed portion of the packaging, the packaging can preferably be heat-sealed by coating the sealed portion with an aqueous coating agent according to an embodiment of the present invention.

[0028] As described above, the aqueous coating agent of the present invention can be used in various packaging fields (e.g., food, construction, etc.) without limiting the contents of the packaging to liquids and powders, because the aqueous coating agent can preferably be used to heat seal the sealed portion to which the contents of the packaging are adhered.

[0029] The paper substrate of the present invention is coated with the aqueous coating agent of the above-described embodiments to form a packaging body that has excellent heat-sealing properties even if the contents of food packaging bags, packaging bags containing powder such as cement, etc., may adhere to the sealing parts.

[0030] Description of the implementation plan The aqueous coating agent (also referred to as "coating agent") of the present invention comprises a heat-expandable microcapsule (A) (also referred to as "component (A)" or "microcapsule (A)") and a thermoplastic resin (B) (also referred to as "component (B)").

[0031] In the aqueous coating agent of the present invention, based on a total of 100 parts by mass of components (A) and (B), the amount of component (A) is less than 15 parts by mass. Because the content of component (A) is less than 15 parts by mass, the aqueous coating agent of the present invention exhibits excellent heat-sealing properties on paper substrates, and can heat-seal the sealing portion even if powder or the like adheres to it.

[0032] In this invention, based on a total of 100 parts by mass of components (A) and (B), the content of component (A) is more preferably 1 to 13 parts by mass, still more preferably 1 to 10 parts by mass, and most preferably 2 to 9 parts by mass.

[0033] In the embodiments of the present invention, the content of component (A) in the aqueous coating agent is 1 to 13 parts by mass. Therefore, the aqueous coating agent can maintain the heat sealing performance at a higher level and can more preferably heat seal the sealing part with powder or liquid substances adhering to it.

[0034] The aqueous coating agent of the present invention refers to a coating agent in which the polymer (e.g., component (B) etc.) can be dispersed and / or dissolved in an aqueous medium, and preferably is an aqueous emulsion (aqueous dispersion) obtained by dispersing the polymer in an aqueous medium. When the aqueous coating agent of the present invention is an aqueous emulsion, the aqueous coating agent can even be used in fields with strict hygiene requirements, such as the food industry.

[0035] In this document, "aqueous medium" refers to ordinary water, such as tap water, distilled water, or ion-exchanged water, and may contain organic solvents that are soluble or dispersible in water and are related to the resins of this invention, such as acetone, ethyl acetate, etc., which have poor monomer reactivity. It may also contain monomers, oligomers, prepolymers, resins, and / or similar substances that are soluble or dispersible in water, and may contain emulsifiers, polymerizable emulsifiers, polymerization initiators, chain extenders, various additives, and / or similar substances that are commonly used in the production of aqueous or water-soluble resins as described below.

[0036] The compositional characteristics of the aqueous coating agent according to embodiments of the present invention are described below.

[0037] <(A) Thermoexpandable microcapsules> In this paper, the thermoplastic microcapsule (A) comprises a microcapsule structure including an outer shell (shell) and a foaming agent contained within the outer shell. By heating the foaming agent contained within the outer shell (shell), the foaming agent vaporizes, the thermoplastic shell expands, and the thermoplastic microcapsule (A) can expand.

[0038] The housing comprises a thermoplastic resin obtained by polymerizing polymerizable component (a), and the thermoplastic resin may be the same as or different from the thermoplastic resin (B) described later.

[0039] The polymerizable component (a) includes a polymerizable monomer (a1) as an essential component, and may include a crosslinking agent (a2).

[0040] The polymerizable monomer (a1) is a monomer having a free radical polymerizable carbon-carbon double bond. This monomer is an addition polymerizable monomer. Furthermore, the crosslinking agent refers to a monomer having at least two free radical polymerizable carbon-carbon double bonds and is a component capable of introducing a crosslinked structure into the thermoplastic resin.

[0041] Examples of polymerizable monomers (a1) include: Alkoxy polyoxyalkylene mono(meth)acrylates, such as methoxy polyethylene glycol mono(meth)acrylate, methoxy polypropylene glycol mono(meth)acrylate, methoxy polybutylene glycol mono(meth)acrylate, methoxy polyethylene glycol-polypropylene glycol mono(meth)acrylate, methoxy polyethylene glycol-polybutylene glycol mono(meth)acrylate, methoxy polypropylene glycol-polybutylene glycol mono(meth)acrylate, octyl polyethylene glycol mono(meth)acrylate, lauroxy polyethylene glycol mono(meth)acrylate, and... Stearoxylated polyethylene glycol mono(meth)acrylate; phenoxylated polyoxyalkylene mono(meth)acrylate, such as phenoxylated polyethylene glycol mono(meth)acrylate; polyalkylene glycol mono(meth)acrylate, such as polyethylene glycol mono(meth)acrylate, polypropylene glycol mono(meth)acrylate, polybutylene glycol mono(meth)acrylate, polyethylene glycol-polypropylene glycol mono(meth)acrylate, polyethylene glycol-polybutylene glycol mono(meth)acrylate and polypropylene glycol-polybutylene glycol mono(meth)acrylate; Mono(2-acryloyloxyethyl) succinate; and Polylactone mono(meth)acrylates, such as ω-carboxylated polycaprolactone mono(meth)acrylate.

[0042] In this document, the term "(meth)acrylate" refers to acrylate or methacrylate. Such polymerizable monomers can be used alone or in combination of two or more.

[0043] In addition to the monomer components described above as examples, the polymerizable monomer (a1) may further include nitrile monomers. Including nitrile monomers improves the solvent resistance of the thermally expandable microcapsules. When the polymerizable component includes nitrile monomers, the weight ratio of the nitrile monomers to the polymerizable component (A) is preferably 5 to 95% by mass, more preferably 10 to 90% by mass, still more preferably 15 to 85% by mass, particularly preferably 20 to 80% by mass, and most preferably 25 to 75% by mass.

[0044] The polymerizable component (A) contains a polymerizable monomer (a1) of 5 to 100% by mass, more preferably 10 to 90% by mass, particularly preferably 15 to 80% by mass, and most preferably 20 to 75% by mass.

[0045] When the content of polymerizable monomer (a1) is 5 to 100% by mass, the shell of thermoplastic microcapsules (A) is more likely to contain a foaming agent.

[0046] Examples of crosslinking agents (a2) include: Aromatic divinyl compounds, such as divinylbenzene; and Polyfunctional (meth)acrylate compounds, such as allyl methacrylate, triacryloylformaldehyde, triaryl isocyanate, ethylene glycol di(meth)acrylate, diethylene glycol di(meth)acrylate, triethylene glycol di(meth)acrylate, neopentyl glycol di(meth)acrylate, polytetramethylene glycol diacrylate, 1,4-butanediol di(meth)acrylate, 1,6-hexanediol di(meth)acrylate, 1,9-nonanediol di(meth)acrylate, PEG #200 di(meth)acrylate, PEG #400 di(meth)acrylate, PEG #600 di(meth)acrylate, trimethylolpropane tri(meth)acrylate, pentaerythritol tri(meth)acrylate, pentaerythritol tetraacrylate, dipentaerythritol hexaacrylate, 2-butyl-2-ethyl-1,3-propanediol diacrylate, and tricyclodecanediethanol di(meth)acrylate. These crosslinking agents can be used alone, or in combination of two or more.

[0047] The content of crosslinking agent (a2) contained in polymerizable component (a) is preferably 0 to 5.0 parts by weight, more preferably 0.01 to 3.0 parts by weight, still more preferably 0.02 to 2.0 parts by weight, and particularly preferably 0.05 to 1.5 parts by weight, based on 100 parts by weight of polymerizable component (a).

[0048] Since the content of crosslinking agent (a2) is within the above range, the thermally expandable microcapsules (A) have excellent expansion properties.

[0049] In this document, the foaming agent is a component that vaporizes upon heating and is contained within the shell of the thermally expandable microcapsule (A). The thermally expandable microcapsule (A) contains the foaming agent, thereby causing the shell (lid) to swell upon heating, and the thermally expandable microcapsule possesses thermal expansion properties.

[0050] Examples of foaming agents include: Straight-chain hydrocarbons, such as propane, butane, pentane, hexane, heptane, octane, nonane, decane, dodecane, tridecane, tetradecane, pentadecane, hexadecane, heptane, octadecane, and nonadecane; Branched hydrocarbons, such as isobutane, isopentane, isohexane, isoheptane, isooctane, isononane, isodecanane, isododecane, 3-methylundecane, isotriadecanane, 4-methyldodecane, isotetradecane, isopentadecanane, isohexadecanane, 2,2,4,4,6,8,8-heptamethylnonane, isohexadecanane, isooctadecane, isononadecanane, and 2,6,10,14-tetramethylpentadecanane; Hydrocarbons with cyclic structures, such as cyclododecane, cyclotridecane, hexylcyclohexane, heptylcyclohexane, n-octylcyclohexane, cyclopentadecane, nonylcyclohexane, decylcyclohexane, pentadecylcyclohexane, hexadecylcyclohexane, heptadecanylcyclohexane, and octadecylcyclohexane; Petroleum ether; Its halides; Fluorinated compounds, such as hydrofluoroethers; Tetraalkylsilane; and Compounds that decompose into gases through heating.

[0051] The blowing agent can be any of linear, branched, and alicyclic blowing agents, and is more preferably alicyclic blowing agents. Such blowing agents can be used alone or in combination of two or more of them.

[0052] The average particle size of the thermally expandable microcapsules (A) of the present invention is preferably 0.5 to 200 µm, particularly preferably 1 to 150 µm, still more preferably 2 to 75 µm, and most preferably 5 to 50 µm. When the average particle size of the thermally expandable microcapsules (A) is 0.5 to 200 µm, the balance between the thermal expansion and expansion stability of component (A) is better.

[0053] In this document, the average particle size of the thermally expandable microcapsules (A) can be measured according to the method described in

[0077] of the embodiment in JP 2023-35309 A.

[0054] The addition ratio of the foaming agent is defined as the percentage of the weight of the foaming agent contained in component (A) relative to 100 parts by mass of component (A). The addition ratio of the foaming agent is preferably 1 to 50% by mass, particularly preferably 2 to 45% by mass, still more preferably 5 to 40% by mass, and most preferably 10 to 35% by mass. The addition ratio of the foaming agent is calculated using the method described in

[0079] to

[0080] of the embodiments in JP 2023-35309 A.

[0055] The expansion initiation temperature (Ts) of the heat-expandable microcapsule (A) is preferably 65 to 200°C, more preferably 65 to 150°C, particularly preferably 70 to 150°C, and most preferably 70 to 100°C. When the expansion initiation temperature (Ts) is 65 to 200°C, the heat-expandable microcapsule (A) has more adequate heat resistance.

[0056] The maximum expansion temperature (Tmax) of the heat-expandable microcapsule (A) is not particularly limited, but is preferably 100 to 350°C, more preferably 100 to 190°C, particularly preferably 110 to 185°C, and most preferably 110 to 135°C. When the maximum expansion temperature (Tmax) is 100 to 350°C, the heat-expandable microcapsule (A) exhibits an excellent balance between heat resistance and expansion percentage.

[0057] Furthermore, the expansion initiation temperature (Ts) and maximum expansion temperature (Tmax) of the thermally expandable microcapsules are calculated using the method described in

[0078] of the embodiment in JP 2023-35309 A.

[0058] In this invention, the maximum volume expansion ratio of the thermally expandable microcapsule (A) is preferably 3 to 50 times, more preferably 5 to 20 times, particularly preferably 5 to 15 times, and most preferably 5 to 10 times. When the maximum volume expansion ratio of component (A) is 3 to 50 times, the aqueous coating agent of the embodiment of the present invention allows a larger amount of powder adhering to the sealing portion to enter the interior of the coating agent, and also maintains greater adhesion to the sealing portion.

[0059] In this invention, the thermally expandable microcapsule (A) differs from so-called hollow particles. Such hollow particles do not contain a foaming agent, or the foaming agent has been vaporized from such hollow particles. Therefore, such hollow particles do not thermally expand.

[0060] Commercially available products can be used as heat-expandable microcapsules (A). Examples of commercially available products include Microsphere F36D, F35D, F48D, F65D, FN100S, F100M, FN100M, FN180, F190D, F230D, F260D, F2800D, F2830D, and F2860D manufactured by Matsumoto Yushi-Seiyaku Co., Ltd.

[0061] Examples of commercially available hollow particle products include Microsphere F30E, F50E, F65E, F65DE, and F80DE manufactured by Matsumoto Yushi-Seiyaku Co., Ltd.

[0062] <(B) Thermoplastic Resins> In this article, thermoplastic resin (B) refers to a resin that has the property of softening and being moldable by heating and curing by cooling, and is obtained by polymerizing polymerizable unsaturated monomers (b).

[0063] In this embodiment, a “polymerizable unsaturated monomer” refers to a radical polymerizable monomer having an olefinic double bond. An “olefinic double bond” is a double bond between carbon atoms that can undergo polymerization (radical polymerization). Examples of functional groups having such olefinic double bonds may include vinyl (CH2=CH-), (meth)allyl (CH2=CH-CH2- and CH2=C(CH3)-CH2-), (meth)acryloyloxy (CH2=CH-COO- and CH2=C(CH3)-COO-), (meth)acryloyloxyalkyl (CH2=CH-COO-R- and CH2=C(CH3)-COO-R-), and -COO-CH=CH-COO-. The polymerizable unsaturated monomer (b) may be a single monomer or a combination of two or more monomers.

[0064] In one embodiment of the invention, the polymerizable unsaturated monomer (b) contained in the thermoplastic resin (B) preferably has a chemical structure derived from a carboxylic acid ester polymer.

[0065] "Chemical structure derived from carboxylic acid ester polymer" means the chemical structure of a polymer (whether homopolymer or copolymer) containing a carboxylic acid ester having an olefinic double bond, and any modified product of that polymer. A "carboxylic acid ester polymer" is obtained by polymerizing a polymerizable unsaturated monomer (b) containing a carboxylic acid ester (b1) having an olefinic double bond.

[0066] In this article, examples of "carboxylic acid esters (b1) having olefinic double bonds" (also simply "carboxylic acid esters (b1)") may include: (Meth)acrylates, such as methyl (meth)acrylate, ethyl (meth)acrylate, butyl (meth)acrylate and 2-ethylhexyl (meth)acrylate; Vinyl carboxylate, such as vinyl acetate; and Allyl carboxylate, such as allyl acetate.

[0067] In this article, (meth)acrylate refers to both acrylate and methacrylate.

[0068] In this invention, the "carboxylic acid ester (b1) having an olefinic double bond" is preferably methyl methacrylate, butyl acrylate or vinyl acetate, and is particularly preferably vinyl acetate.

[0069] In this embodiment, the carboxylic acid ester polymer can be a copolymer of a carboxylic acid ester (b1) having an olefinic double bond and a polymerizable unsaturated monomer (b2) other than a carboxylic acid ester having an olefinic double bond (also referred to as "other monomers (b2)").

[0070] Other monomers (b2) are not particularly limited, and examples include: olefins such as ethylene and propylene; dienes such as butadiene and isoprene; vinyl aromatic compounds such as styrene and α-methylstyrene; vinyl alcohol; and acrylonitrile.

[0071] In this invention, (b1) preferably includes (meth)acrylate, and particularly preferably includes methyl methacrylate, methyl acrylate or 2-ethylhexyl acrylate, and (b2) preferably includes styrene.

[0072] In this invention, component (B) preferably includes at least one structural unit selected from structural units derived from (meth)acrylate polymers and structural units derived from styrene polymers, particularly preferably including both structural units derived from (meth)acrylate polymers and structural units derived from styrene polymers, and more preferably including both copolymers of methyl methacrylate and 2-ethylhexyl acrylate, and styrene-butadiene copolymers.

[0073] When component (B) has the above composition, the excellent peel strength of the aqueous coating agent of the present invention is maintained, and the aqueous coating agent is also excellent in terms of adhesion.

[0074] In this document, "structural units derived from (meth)acrylate polymers" have chemical structures comprising homopolymers or copolymers of (meth)acrylates. "Structural units derived from styrene polymers" have chemical structures derived from homopolymers or copolymers of styrene.

[0075] In this invention, the glass transition temperature of the thermoplastic resin (B) is preferably 40°C or lower, particularly preferably -50°C to 35°C, still more preferably -40°C to 35°C, and most preferably -0°C to 35°C. When the glass transition temperature of component (B) is within the above range, the aqueous coating agent of this invention has excellent heat-sealing properties.

[0076] In this paper, the glass transition temperature of thermoplastic resin (B) is calculated based on the glass transition temperature (hereinafter also referred to as "homogene Tg") of the homopolymer obtained by homopolymerization of polymerizable unsaturated monomer (b) as raw material of thermoplastic resin (B).

[0077] The glass transition temperature of the thermoplastic resin (B) is determined by considering the homopolymer Tg of each polymerizable unsaturated monomer (b) (monomer) and the mixing ratio (parts by mass). Specifically, the Tg of the copolymer can be determined by calculation using the following equation.

[0078] 1 / Tg = C1 / Tg1 + C2 / Tg2 + ... + Cn / Tgn [In the calculation equation, Tg represents the theoretical Tg of the copolymer, Cn represents the mass fraction of the nth monomer n contained in the monomer mixture, Tgn represents the homopolymer Tg of the nth monomer n, and n is the number of monomers contained in the copolymer, and is a positive integer.] The values ​​described in the literature can be used as the Tg of the homopolymer. Such literature is, for example, "POLYMER HANDBOOK" (4th edition, published by John Wiley & Sons, Inc.). As an example, the Tg values ​​of the monomers for homopolymers described in "POLYMER HANDBOOK" are as follows.

[0079] Methyl methacrylate (“MMA”, Tg = 105℃) n-Butyl acrylate (“n-BA”, Tg = -54℃) 2-Ethylhexyl acrylate (“2EHA”, Tg = -70℃) Styrene (“St”, Tg = 100℃) Acrylic acid (“AA”, Tg = 106℃) Methacrylic acid (“MAA”, Tg = 130℃) n-Butyl methacrylate (“BMA”, Tg = 20℃) In addition to components (A) and (B), the aqueous coating agent of this invention may also contain viscosity modifiers, plasticizers, defoamers, preservatives, colorants, etc., as additives. These additives can be blended after the thermoplastic resin (B) is synthesized, can be blended into the monomers that are raw materials for the thermoplastic resin (B), or can be added to the aqueous coating agent after mixing components (A) and (B).

[0080] Examples of viscosity modifiers may include nitrogen-containing substances such as urea, urea compounds and dicyandiamide, calcium hydroxide, calcium oxide, sodium carbonate, trisodium phosphate, diammonium hydrogen phosphate, borax, sodium fluoride, water glass and ammonia.

[0081] Examples of plasticizers may include: glycerin; polyols such as ethylene glycol and propylene glycol; sugars such as sucrose and sorbitol; and organic solvents such as cellosolves.

[0082] Examples of defoamers may include: Silicone-based defoamers, such as dimethyl polysiloxane, polyoxyalkylene modified silicone, organic modified polysiloxane, and fluorosilicone; Oil and fat-based defoamers, such as castor oil, sesame oil, flaxseed oil, and animal and vegetable oils; Fatty acid-based defoamers, such as stearic acid, oleic acid and palmitic acid; Defoamers based on fatty acid esters, such as isopentyl stearic acid, diethylene glycol lauric acid, distearate succinic acid, distearate, sorbitol monolauric acid, glycerol fatty acid esters, polyoxyethylene sorbitol, butyl monolaurate stearate, sucrose fatty acid esters, ethyl acetate alkyl ester of sulfonated castor oil, and natural waxes. Alcohol-based defoamers, such as polyoxyalkylene glycols and their derivatives, polyoxyalkylene glycol hydrates, dipentylphenoxyethanol, 3-heptanol and 2-ethylhexanol; Ether-based defoamers, such as 3-heptyl cellosolve and nonyl cellosolve-3-heptylcarbitol; Defoamers, such as phosphate esters, including tributyl phosphate, sodium octyl phosphate and tri(butoxyethyl) phosphate; Amine-based defoamers, such as dipentylamine; Amide-based defoamers, such as polyalkylene amides, acylated polyamines, and bis(octadecyl)piperidine; Metal soap-based defoamers, such as aluminum stearate, calcium stearate, potassium oleate, and calcium lanolinate; and Sulfonate-based defoamers, such as sodium lauryl sulfonate and sodium dodecyl sulfonate.

[0083] The surface of a paper substrate can be coated with the water-based coating agent of the present invention. As a method for coating a paper substrate with the water-based coating agent of the present invention, conventional coating methods can be used. For example, the paper substrate can be coated with the coating agent of the present invention using known coating machines such as benchtop coating machines, rod coating machines, two-roll sizing press coating machines, gate roller coating machines, blade metaling coaters, rod metaling coaters, doctor blade coating machines, air knife coating machines, roller coating machines, brush coating machines, kiss-type coating machines, extrusion coating machines, curtain coating machines, die coating machines, gravure coating machines, or dip coating machines.

[0084] There is no particular limitation on the amount of aqueous coating agent used for coating paper substrates; for example, as a solid content (dry weight), it is preferably 5 to 100 g / m³. 2 More preferably 5 to 50 g / m 2 And particularly preferred is 10 to 20 g / m 2 Here, the solids content of the coating agent refers to the solids content obtained after drying the coating agent at 105°C for 3 hours.

[0085] In another embodiment, the present invention relates to a paper substrate whose surface is coated with the aforementioned aqueous coating agent. The paper substrate of the present invention has significantly superior heat-sealing properties and can be used for packaging powders including food (retort pouches), building materials, etc. (e.g., food powders such as tea powder, flour or rice powder, building material powders such as cement, etc.).

[0086] In this document, "powder" is considered to be an aggregate of solid particles (tiny solids). Powders containing solid particles with a particle size of 5 mm or smaller are preferred.

[0087] When the powder contains solid particles with a particle size of 5 mm or smaller, the aqueous coating agent of the present invention can more preferably heat seal the sealing portion to which the powder adheres.

[0088] Because the water-based coating agent of the present invention exhibits significantly superior heat-sealing properties, the sealed portion of the packaging is less susceptible to adhesion of the packaging contents. In particular, the sealed portion of packaging containing easily dispersed items such as powder may become contaminated by these items during heat sealing, potentially leading to a deterioration of the seal. However, by coating the sealed portion with the water-based coating agent of the present invention, the heat-sealing properties of the packaging bag can be maintained.

[0089] There are no particular restrictions on the paper substrate; known paper or synthetic paper made from chemical pulps such as hardwood or softwood kraft pulp, mechanical pulps such as GP (groundwood pulp), RGP (refined groundwood pulp), or TMP (thermomechanical pulp) can be used as the paper substrate. In addition, high-quality paper, medium-quality paper, alkaline paper, glassine paper, semi-glassine paper, or paperboard or white paperboard, white-lined core paperboard, core paperboard, etc., used for corrugated fiberboard, building materials, etc., can be used as the aforementioned paper substrate. The paper substrate may contain organic or inorganic pigments, or papermaking additives such as paper strengthening agents, sizing agents, or yield improvers.

[0090] In yet another embodiment, the present invention relates to a paper product having a paper substrate with its surface coated with the aforementioned aqueous coating agent. The paper product of this invention can also be used for paper straws, toilet paper, paper cups, food packaging bags, cement bags, etc.

[0091] [Example] The present invention will now be described in detail with reference to embodiments and comparative examples. Each of these embodiments is merely one implementation of the invention, and the invention is not limited to these embodiments in any way. Unless otherwise stated, portions not taking solvents into account are based on parts by mass or percentages by mass in the example descriptions.

[0092] The values ​​for the amounts of each blending component (A) to (B) listed in Table 1 represent the number of parts of "solids content excluding solvent (materials excluding water)" in parts by mass. The total amount of components (A) to (B) is converted to 100 parts by mass, and the parts by mass of each component are listed in Table 1. Details of components (A) to (B) used in the examples and comparative examples are described below.

[0093] (A) Thermoplastic microcapsules (A1) Thermally expandable microcapsules (Matsumoto Microsphere F48D, manufactured by Matsumoto Yushi-Seiyaku Co., Ltd., expansion initiation temperature 90 to 100°C, maximum expansion temperature 125 to 135°C, average particle size 12 to 19 µm) (A2) Thermally expandable microcapsules (Matsumoto Microsphere F36D, manufactured by Matsumoto Yushi-Seiyaku Co., Ltd., with an initial thermal expansion temperature of 70 to 85°C, a maximum expansion temperature of 120 to 130°C, and an average particle size of 12 to 19 µm). (A'3) Hollow particles (Matsumoto Microsphere F50E, manufactured by Matsumoto Yushi-Seiyaku Co., Ltd., with an average particle size of 40 to 60 µm) (B) Thermoplastic resin (B1) Acrylic emulsion (AQUENCE EPIX BC9210, manufactured by Henkel Japan Ltd., glass transition temperature (Tg) 34°C) (B2) Styrene-acrylic emulsion (AQUENCE EPIX BC900F, manufactured by Henkel Japan Ltd., glass transition temperature (Tg) is -28℃) (B3) Styrene-butadiene emulsion (SB Latex A7301, manufactured by Asahi-Kasei Chemicals Corporation, glass transition temperature (Tg) of -8°C) (B4) Olefin dispersion (AQUECNE EPIX BC9220HS, manufactured by Henkel Japan Ltd., glass transition temperature (Tg) of -37°C) (B5) Ethylene-vinyl acetate emulsion (SUMIKAFLEX S400HQ, manufactured by Sumika Chemtex Company, Limited, glass transition temperature (Tg) of 0℃) <Preparation of water-based coating agents and production of paper substrates> Example 1 <Preparation of Waterborne Coating Agents> 1000 g of (B1) acrylic emulsion (trade name: AQUECNE EPIXBC9210, manufactured by Henkel Japan Ltd., solids content 50% by mass), which is the thermoplastic resin (B), was placed in a 2000 mL separable flask equipped with a stirring blade with a diameter of 150 mm. 50 g of (A1) heat-expandable microcapsules (trade name: Matsumoto Microsphere F48D, manufactured by Matsumoto Yushi-Seiyaku Co., Ltd.) were added with stirring. The contents of the flask were stirred for 30 minutes to obtain the aqueous coating agent of Example 1.

[0094] <Paper Substrate Production> The aqueous coating agent of Example 1 was applied using a rod coater to a basis weight of 104.7 g / m³. 2 On 126 µm thick, 94% opacity white premium grade paper (trade name: Premium Grade (manufactured by Chuetsu Pulp & Paper Co., Ltd.)), the coating weight of the water-based coating agent after drying is 20 g / m². 2 .

[0095] The paper substrate coated with the coating agent was placed in a dryer and dried at 75 to 90°C to obtain the paper substrate of Example 1.

[0096] Example 2 The aqueous coating agent and paper substrate of Example 2 were obtained under conditions similar to those of Example 1, except that (B2) (trade name: AQUECNE EPIX BC900F, manufactured by Henkel Japan Ltd., with a solid content of 46% by mass) was used as the thermoplastic resin (B).

[0097] Examples 3 to 10 and Comparative Examples 1 to 6 As components (A) and (B) used in Example 1, the components and their proportions listed in Tables 1 and 2 were added to separable flasks, and aqueous coating agents and paper substrates were obtained under conditions similar to those in Example 1.

[0098] For the papers coated with the water-based coatings of these embodiments and comparative examples, the heat-sealing properties (adhesion, peel strength, and swelling properties) of the water-based coatings were evaluated as shown in Tables 1 and 2.

[0099] The details of the heat-sealing performance evaluation test are as follows.

[0100] <Adhesion of the sealing parts with adhered powder> The paper substrates obtained in Examples 1 to 10 and Comparative Examples 1 to 6 were cut into 25 mm × 100 mm sizes to prepare test subjects A and B for adhesion testing. Cement powder was sprinkled onto the coating surface of test subject A. The amount of cement powder sprinkled on the coating surface of test subject A per square meter was 15 to 20 g.

[0101] The coated surfaces of test specimen A (with cement powder adhering to them) and the coated surfaces of test specimen B (without cement powder adhering to them) were stacked and placed in a press. The pressing conditions were: temperature 130℃, pressure 0.6 MPa, and time 0.6 seconds.

[0102] The pressed test specimens were cured at room temperature (23°C) for 2 hours, and then the coated surface was peeled off using a T-type peel tester (TENSILON) at a speed of 300 mm / min. The peeled parts were visually observed and evaluated. The evaluation criteria are as follows.

[0103] In this article, material failure refers to the failure of the paper substrate after peeling.

[0104] Excellent: Material failure occurs in 90% or more of the heat-sealed portion. Good: Material failure occurs in 50% or more but less than 90% of the heat-sealed portion. Acceptable: Material failure occurs in 30% or more but less than 50% of the heat-sealed portion. Poor: Less than 30% of the heat-sealed portion experiences material failure or lacks adhesion. Peel strength of the sealed portion with adhering powder The paper substrates obtained in each example and comparative example were cut into 25 mm × 100 mm dimensions to prepare test specimens A and B for peel strength testing. Cement powder was sprinkled onto the coating surface of test specimen A. The amount of cement powder sprinkled on the coating surface of test specimen A per square meter was 15 to 20 g.

[0105] The coated surfaces of test specimen A (with cement powder adhering to them) and the coated surfaces of test specimen B (without cement powder adhering to them) were stacked and placed in a press. The pressing conditions were: temperature 130℃, pressure 0.6 MPa, and time 0.6 seconds.

[0106] The pressed test specimens were cured at room temperature (23°C) for 2 hours, and then the coated surface was peeled off using a T-type peel tester (TENSILON) at a speed of 300 mm / min for evaluation. The evaluation criteria are as follows.

[0107] Excellent: Peel strength of the heat-sealed portion is 2 N / 25 mm or higher. Good: The peel strength of the heat-sealed portion is 1 N / 25 mm or higher and less than 2 N / 25 mm. Acceptable: The peel strength of the heat-sealed portion is 0.5 N / 25 mm or higher but less than 1 N / 25 mm. Poor: The peel strength of the heat-sealed portion is less than 0.5 N / 25 mm, or there is no adhesion. <Adhesion of the sealed portion without powder adhering> The paper substrates obtained in Examples 1 to 10 and Comparative Examples 1 to 6 were cut into 25 mm × 100 mm sizes to prepare test specimens A and B for adhesion testing.

[0108] The coated surfaces of test specimen A and test specimen B were stacked and placed in a press. The pressing conditions were: temperature 130℃, pressure 0.6 MPa, and time 0.6 seconds.

[0109] The pressed test specimens were cured at room temperature (23°C) for 2 hours, and then the coated surface was peeled off using a T-type peel tester (TENSILON) at a speed of 300 mm / min. The peeled parts were visually observed and evaluated. The evaluation criteria are as follows.

[0110] Excellent: Material failure occurs in 90% or more of the heat-sealed portion. Good: Material failure occurs in 50% or more but less than 90% of the heat-sealed portion. Acceptable: Material failure occurs in 30% or more but less than 50% of the heat-sealed portion. Poor: Less than 30% of the heat-sealed sections experience material failure or are not sealed properly. <Percentage of expansion of the sealed portion without adhering powder> The heat-sealing paper obtained in each embodiment and comparative example was cut into 25 mm × 100 mm dimensions to prepare test specimens A and B for adhesion testing.

[0111] The coated surfaces of test specimen A and test specimen B were stacked and placed in a press. The pressing conditions were: temperature 130℃, pressure 0.6 MPa, and time 0.6 seconds.

[0112] The substrate thickness of test subjects A and B was measured before pressing, and the substrate thickness of test subjects A and B was measured after pressing. The percentage of expansion after heat sealing was calculated. The evaluation criteria are as follows.

[0113] Excellent: The expansion percentage of the heat-sealed portion is 200% or higher. Good: The percentage of expansion of the heat-sealed portion is 150% or higher but less than 200%. Acceptable: The expansion percentage of the heat-sealed portion is 110% or higher but less than 150%. Poor: The expansion percentage of the heat-sealed portion is less than 110%. [Table 1]

[0114] [Table 2]

[0115] As shown in Tables 1 and 2, the aqueous coatings of each embodiment exhibit significantly superior heat-sealing properties because, based on a total of 100 parts by weight of components (A) and (B), the content of component (A) in each embodiment is less than 15 parts by weight. This demonstrates that even when cement powder adheres to the sealed portion, the aqueous coatings of the embodiments maintain high levels of adhesion and peel strength.

[0116] In contrast to the aqueous coatings of the embodiments, the comparative aqueous coatings exhibited poor heat-sealing properties because none of the comparative aqueous coatings contained component (A), or the content of component (A) in each comparative aqueous coating was excessively high. Specifically, the overall peel strength of the sealed portions with adhered cement powder in Comparative Examples 1 to 6 was poor.

[0117] [Industrial Applicability] This invention provides an aqueous coating agent. A paper substrate is coated with the aqueous coating agent according to an embodiment of this invention to produce packaging. Examples of packaging include food packaging bags, paper cups, and powder packaging bags for cement, etc.

[0118] [Cross-references to related applications] This application claims priority under Article 4 of the Paris Convention based on Japanese Patent Application No. 2023-210165, filed in Japan on December 13, 2023. That priority patent application is incorporated herein by reference in its entirety.

Claims

1. An aqueous coating agent comprising: (A) Thermally expandable microcapsules; (A) and (B) thermoplastic resin, wherein, based on a total of 100 parts by weight of components (A) and (B), the aqueous coating agent contains less than 15 parts by weight of component (A).

2. The aqueous coating agent according to claim 1, wherein the heat-expandable microcapsule (A) comprises a shell and a foaming agent contained in the shell and vaporized by heating.

3. A paper substrate coated with an aqueous coating agent according to claim 1 or 2.

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