Method for producing microcapsule

Abstract

To provide a method for producing a microcapsule having an outer shell containing a melamine resin which can form a microcapsule under a polymerization condition in a neutral region at a low temperature.SOLUTION: A method for producing a microcapsule including an outer shell containing a melamine resin, and an inside which is located in the outer shell and stores an organic solvent includes the steps of: mixing a first mixture containing a maleic anhydride copolymer and water with the organic solvent to obtain a first emulsion in which the first mixture is a continuous phase and the organic solvent is a dispersion phase; mixing the first emulsion with an addition phase containing a melamine aldehyde prepolymer and water to obtain a second emulsion; and polymerizing the second emulsion to obtain the microcapsule.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a method for producing microcapsules. [Background technology]

[0002] Microcapsules have been used in many fields, such as agricultural chemicals, medicines, fragrances, liquid crystals, adhesives, electronic material components, and building materials, as microcapsules encapsulating skin care ingredients, fragrance ingredients, dye ingredients, analgesic ingredients, deodorizing ingredients, antioxidant ingredients, bactericidal ingredients, heat storage ingredients, and the like, or as hollow microcapsules having a hollow interior.

[0003] In particular, in recent years, the use of hollow microcapsules has been investigated for the purpose of providing pores in chemical mechanical polishing (CMP) polishing pads made of polyurethane (urea) used for wafer polishing.

[0004] For example, Patent Document 1 discloses an invention relating to microhollow particles that are formed with a resin film made of a melamine-based resin, the resin film being composed of a plurality of small pieces and bonding portions that bond them together. It also describes that such microhollow particles have good compatibility with polyurethane resins, and therefore when used in CMP polishing pads, the microhollow particles are less likely to fall off and have excellent scratch resistance. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] International Publication No. 2022 / 163781 Summary of the Invention [Problem to be solved by the invention]

[0006] The above-mentioned Patent Document 1 describes microcapsules suitable for use in CMP polishing pads. In an example, the microcapsules described in Patent Document 1 are produced by adding an alkaline aqueous solution of methylol melamine, which is obtained by an addition reaction of melamine with formaldehyde, to an oil-in-water emulsion formed from an aqueous phase containing an ethylene-maleic anhydride copolymer as a surfactant and an oil phase consisting of toluene, and then reacting the mixture at 80°C under acidic conditions of pH 4 or less. As described above, the method for producing microcapsules disclosed in Patent Document 1 involves complicated production steps, and there is a need for improvement in terms of production costs as well.

[0007] Therefore, an object of the present invention is to provide an efficient method for producing microcapsules. [Means for solving the problem]

[0008] The present inventors have conducted extensive research to achieve the above object, and as a result have found that the above object can be achieved by a method for producing microcapsules, the method comprising: mixing a first mixture containing a maleic anhydride copolymer and water with an organic solvent to obtain a first emulsion in which the first mixture is a continuous phase and the organic solvent is a dispersed phase; mixing the first emulsion with an additive phase containing a melamine aldehyde prepolymer and water to obtain a second emulsion; and polymerizing the second emulsion to obtain the microcapsules, thereby completing the present invention.

[0009] The gist of the present invention is the following [1] to [8]. [1] A method for producing a microcapsule comprising an outer shell containing a melamine resin and an interior portion located within the outer shell and containing an organic solvent, comprising: mixing a first mixture containing a maleic anhydride copolymer and water with the organic solvent to obtain a first emulsion in which the first mixture is a continuous phase and the organic solvent is a dispersed phase; mixing the first emulsion with an additive phase comprising a melamine aldehyde prepolymer and water to obtain a second emulsion; and polymerizing the second emulsion to obtain the microcapsules. [2] The method for producing the melamine aldehyde prepolymer according to the above [1], wherein the melamine aldehyde prepolymer contains a compound represented by the following formula (I): [ka] In the formula (I), R 1 is a hydrogen atom or a methylol group, n is a number between 2 and 10. [3] The method according to [1] or [2] above, wherein the pH of the addition phase is 5 or more and 14 or less. [4] The method according to any one of the above [1] to [3], wherein the pH of the first emulsion when the additive phase is mixed is 3 or more and 14 or less. [5] The method according to any one of the above [1] to [4], wherein the pH of the second emulsion during the polymerization is 6.5 or more and 7.5 or less. [6] The method according to any one of the above [1] to [5], wherein the temperature of the second emulsion during the polymerization is 10°C or higher and 60°C or lower. [7] The method according to any one of [1] to [6] above, wherein the maleic anhydride copolymer comprises at least one selected from the group consisting of styrene-maleic anhydride copolymer, ethylene-maleic anhydride copolymer, and isobutylene-maleic anhydride copolymer. [8] A method for producing microballoons, comprising producing microcapsules by any of the methods [1] to [7] above, and then removing the organic solvent contained in the microcapsules to obtain microballoons. [Effects of the Invention]

[0010] The present invention can provide an efficient method for producing microcapsules. [Brief explanation of the drawings]

[0011] [Figure 1]1 shows images of the microcapsules obtained in Examples 1 to 3 observed with a scanning electron microscope. [Figure 2] 1 shows images of the microcapsules obtained in Examples 4 and 5 observed with a scanning electron microscope. [Figure 3] 1 shows images of the microcapsules obtained in Examples 6 and 7 observed with a scanning electron microscope. DETAILED DESCRIPTION OF THE INVENTION

[0012] [Method of manufacturing microcapsules] The method for producing a microcapsule of the present invention is a method for producing a microcapsule including an outer shell containing a melamine resin and an interior portion located within the outer shell and containing an organic solvent, mixing a first mixture containing a maleic anhydride copolymer and water with the organic solvent to obtain a first emulsion in which the first mixture is a continuous phase and the organic solvent is a dispersed phase; mixing the first emulsion with an additive phase comprising a melamine aldehyde prepolymer and water to obtain a second emulsion; and polymerizing the second emulsion to obtain the microcapsules.

[0013] <First emulsion> In the production method of the present invention, a first mixture containing a maleic anhydride copolymer and water is mixed with an organic solvent to obtain a first emulsion in which the first mixture is a continuous phase and the organic solvent is a dispersed phase.

[0014] (1st mixture) The first mixture is a solution containing the maleic anhydride copolymer and water. The maleic anhydride copolymer functions as a surfactant and contributes to the formation of stable emulsions and microcapsules. The maleic anhydride copolymer is a copolymer of maleic anhydride and a monomer copolymerizable with maleic anhydride. Among them, the maleic anhydride copolymer preferably contains at least one selected from the group consisting of a styrene-maleic anhydride copolymer, an ethylene-maleic anhydride copolymer, and an isobutylene-maleic anhydride copolymer, and more preferably contains a styrene-maleic anhydride copolymer.

[0015] The average molecular weight of the maleic anhydride copolymer is not particularly limited, but from the viewpoint of forming more stable emulsions and microcapsules, it is preferably from 30,000 to 1,000,000, more preferably from 100,000 to 600,000. The average molecular weight is a weight average molecular weight measured by gel permeation chromatography (GPC) and is expressed as a standard polystyrene equivalent value.

[0016] The content of the maleic anhydride copolymer in the first mixture is not particularly limited, but is, for example, 0.1 to 15 parts by mass, and preferably 1 to 10 parts by mass, relative to 100 parts by mass of water. The pH of the first mixture is not particularly limited, but is, for example, from 3 to 14, preferably from 6 to 11, and more preferably from 8 to 11. The pH may be adjusted by adding an alkaline aqueous solution such as an aqueous sodium hydroxide solution as appropriate. Furthermore, by adding an alkaline aqueous solution to the first mixture, which is an aqueous solution of the maleic anhydride copolymer, at least a portion of the maleic anhydride copolymer is hydrolyzed to generate a carboxylic acid, which is thought to facilitate the condensation reaction at the droplet interface of the melamine aldehyde prepolymer, which will be described later.

[0017] (organic solvent) The organic solvent is not particularly limited, but examples thereof include aliphatic hydrocarbons having 7 to 11 carbon atoms, alicyclic hydrocarbons such as cycloheptane and cyclooctane, butyl acetate, dibutyl ether, 1,2-dichloroethane, toluene, xylene, benzaldehyde, chlorobenzene, and dichlorobenzene. Among these, the organic solvent is preferably an aliphatic hydrocarbon having 8 to 11 carbon atoms, cycloheptane, cyclooctane, toluene, xylene, or chlorobenzene, more preferably at least one selected from the group consisting of toluene, xylene, and chlorobenzene, and even more preferably chlorobenzene. These organic solvents may be used alone or in combination of two or more.

[0018] (Emulsion formation) The first mixture is mixed with an organic solvent to form a first emulsion. The first emulsion is an O / W emulsion in which the first mixture is the continuous phase and the organic solvent is the dispersed phase. The mixing ratio of the first mixture to the organic solvent is not particularly limited, but when the organic solvent is taken as 100 parts by mass, the first mixture is preferably 100 to 500 parts by mass, and more preferably 150 to 300 parts by mass.

[0019] A suitable method is to mix the first mixture with an organic solvent and then disperse the mixture using a known disperser such as a high-speed shear type, friction type, high-pressure jet type, or ultrasonic type to form an O / W emulsion. Of these, a high-speed shear type is preferred. When a high-speed shear type disperser is used, the rotation speed is preferably 500 to 20,000 rpm, more preferably 1,000 to 10,000 rpm. The dispersion time is preferably 0.1 to 30 minutes, and more preferably 1 to 10 minutes. The dispersion temperature is preferably 20 to 80°C.

[0020] <Second emulsion> In the production method of the present invention, after the first emulsion is obtained as described above, the first emulsion is mixed with an additive phase containing a melamine aldehyde prepolymer and water to obtain a second emulsion.

[0021] (Additional phase) The additive phase comprises a melamine aldehyde prepolymer and water. The melamine aldehyde prepolymer is an initial polymer of a compound obtained by reacting melamine with an aldehyde, and examples thereof include an initial polymer of monomethylolated melamine, an initial polymer of dimethylolated melamine, etc. The initial polymer refers to a polymer of at least a dimer and at most a decamer. In the production method of the present invention, it is important to use a melamine aldehyde prepolymer. By using a melamine aldehyde prepolymer, the production conditions for microcapsules can be easily controlled. For example, microcapsules can be formed at low temperatures (e.g., 10°C to 60°C) in a neutral range (e.g., pH 6.5 to 7.5), simplifying the production process and reducing production costs. The reason for this is not clear, but it is thought that by using a melamine aldehyde prepolymer in which polymerization has progressed to a certain extent, the polycondensation reaction during microcapsule formation can proceed more easily under mild conditions.

[0022] The melamine aldehyde prepolymer used in the production method of the present invention preferably contains a compound represented by the following formula (I). [ka] In the formula (I), R 1 is a hydrogen atom or a methylol group (-CH2OH), and n is a number between 2 and 10. In formula (I), R 1 is preferably a hydrogen atom. 1When a melamine aldehyde prepolymer containing a compound in which is a hydrogen atom is used, the polymerization reaction during microcapsule formation (polymerization reaction of the second emulsion) tends to proceed more gently, making it easier to obtain microcapsules with fewer aggregates and excellent surface smoothness.

[0023] In the additive phase, the content of the melamine aldehyde prepolymer is preferably 10 parts by mass or more and 300 parts by mass or less, more preferably 20 parts by mass or more and 200 parts by mass or less, and even more preferably 30 parts by mass or more and 150 parts by mass or less, per 100 parts by mass of water.

[0024] The pH of the addition phase is not particularly limited, but is preferably 5 or more and 14 or less, more preferably 7 or more and 14 or less, even more preferably 8 or more and 13 or less, and even more preferably 12 or less. The additive phase may further contain an acidic compound. The additive phase is preferably prepared by preparing an aqueous solution of a melamine aldehyde prepolymer and then adding an acidic aqueous solution to the aqueous solution. The additive phase may be prepared by adding the melamine aldehyde prepolymer to the acidic aqueous solution. By maintaining the pH of the additive phase below 13, it becomes easier to adjust the pH to a neutral range when polymerizing the second emulsion. Examples of the acidic compound include citric acid, fumaric acid, maleic acid, adipic acid, tartaric acid, and hydrochloric acid.

[0025] The second emulsion can be prepared by adding an additive phase containing a melamine aldehyde prepolymer and water to the first emulsion described above. The pH of the first emulsion when the additive phase is added is preferably from 3 to 14, more preferably from 6 to 11, and even more preferably from 8 to 11. By adjusting the pH of the first emulsion in this manner, it becomes easier to adjust the pH to a neutral range when the second emulsion is obtained and polymerized.

[0026] In order to facilitate the formation of microcapsules, the mixing ratio of the first emulsion to the additive phase is preferably such that the amount of melamine aldehyde prepolymer in the additive phase is 15 to 100 parts by mass, more preferably 20 to 70 parts by mass, per 100 parts by mass of organic solvent in the first emulsion.

[0027] <Polymerization of the second emulsion> In the production method of the present invention, the second emulsion is obtained as described above, and the second emulsion is polymerized to obtain microcapsules. Specifically, in the polymerization of the second emulsion, the condensation reaction of the melamine aldehyde prepolymer described above proceeds, and microcapsules are formed in which the condensate serves as the outer shell. The mechanism of microcapsule formation is presumed to be as follows: droplets of an oil phase consisting of an organic solvent are dispersed in an aqueous phase, and the melamine aldehyde prepolymer forms an acid amide bond with the hydrophilic group (carboxyl group if hydrolyzed) of the maleic anhydride copolymer at the droplet interface, resulting in the droplets being surrounded by the melamine aldehyde prepolymer. In this surrounding state, adjacent melamine aldehyde prepolymers undergo a dehydration condensation reaction between the methylol groups, forming the outer shell of the microcapsule.

[0028] The pH of the second emulsion during polymerization is preferably 6.5 or more and 7.5 or less. When the pH of the second emulsion during polymerization is 6.5 or higher, the polymerization reaction can be prevented from proceeding too quickly, and the formation of microcapsule aggregates can be suppressed. When the pH of the second emulsion during polymerization is 7.5 or lower, the polymerization reaction can proceed easily, making it easier to obtain microcapsules. The pH of the second emulsion during polymerization is more preferably 6.7 or higher and 7.3 or lower. The pH may be adjusted by adding an acidic aqueous solution such as an aqueous citric acid solution, an aqueous fumaric acid solution, an aqueous maleic acid solution, an aqueous adipic acid solution, an aqueous tartaric acid solution, or hydrochloric acid.

[0029] The temperature of the second emulsion during polymerization is preferably 10° C. or higher and 60° C. or lower. In the present invention, as described above, microcapsules can be obtained even if the temperature during polymerization is low. The temperature of the second emulsion during polymerization is more preferably 20°C or higher and 40°C or lower, more preferably 25°C or higher and 35°C or lower, from the viewpoint of obtaining microcapsules with little breakage or aggregation. The polymerization time is not particularly limited, but may be, for example, in the range of 1 to 48 hours, and preferably 1 to 10 hours. As described above, by polymerizing the melamine aldehyde prepolymer, it is possible to obtain microcapsules each having an outer shell containing a melamine resin and an interior portion located within the outer shell and containing an organic solvent. As described above, according to the production method of the present invention, suitable microcapsules can be obtained by polymerization at a neutral pH and at a low temperature, the production process can be simplified, and costs can be reduced.

[0030] <Separation of microcapsules> As described above, after the second emulsion is polymerized to obtain microcapsules, the microcapsules may be separated from the solution containing the microcapsules. The method for separating the microcapsules is not particularly limited and may be selected from common separation methods, specifically, methods such as filtration and centrifugation are used.

[0031] <Removal of organic solvents> The organic solvent contained inside the microcapsules obtained as described above can be removed to obtain microballoons, which are hollow microcapsules. The method for removing the organic solvent is not particularly limited and may be selected from common methods, and specifically, a circulating air dryer, a spray dryer, a fluidized bed dryer, a vacuum dryer, etc. The temperature condition for drying is preferably 40 to 250°C, more preferably 50 to 200°C.

[0032] [Application] The microcapsules obtained by the above-described manufacturing method can be used in a variety of applications, including in many fields such as agricultural chemicals, medicines, cosmetic materials, liquid crystals, adhesives, electronic materials and components, and building materials. In particular, they can be suitably used for shoe soles and insoles, heat insulation materials, soundproofing materials, and CMP polishing pads, and are particularly suitable for use in CMP polishing pads.

[0033] As a method for using such a CMP polishing pad, any known method can be adopted without limitation. For example, a resin containing the above-mentioned microcapsules (preferably hollow microcapsules) as a foaming agent can be cut and the surface polished to produce a CMP polishing pad having pores on the polishing surface of the resin.

[0034] The resin is not particularly limited, but polyurethane resin is preferred. That is, it is preferred to prepare a cured product in which the microcapsules described above are dispersed in polyurethane resin, and then use the cured product to prepare a CMP polishing pad. In particular, the microcapsules described above have good compatibility with polyurethane resin, so when used in a CMP polishing pad, they are less likely to fall off, and scratch resistance can be improved. In this specification, polyurethane resin is a general term for polyurethane resin, polyurea resin, polyurethane urea resin, and polythiourethane resin.

[0035] The density of the CMP polishing pad is 0.40 to 1.10 g / cm 3 It is preferable that the density is 0.50 to 1.05 g / cm 3It is more preferable that the microcapsules obtained by the above-mentioned method are combined with a known foaming method to form a foamed cured product, which can also be used as a CMP polishing pad. A known foaming method is, for example, a foaming agent foaming method in which water is added. In the foaming agent foaming method, if the resin used is a polyurethane resin, carbon dioxide and an amino group are generated after the reaction between water and an iso(thio)cyanate group, and the carbon dioxide becomes a foaming gas. The amino group further reacts with an iso(thio)cyanate group to form a urea bond and / or a thiourea bond.

[0036] The CMP polishing pad can have any suitable hardness. The hardness can be measured according to the Shore method, for example, according to JIS standard (hardness test) K6253. The Shore hardness of the CMP polishing pad is preferably 30A to 80D, and more preferably 40A to 70D (where "A" indicates hardness on the Shore "A" scale, and "D" indicates hardness on the Shore "D" scale). That is, for example, 30A to 80D means that the Shore A hardness is 30 or more and the Shore D hardness is 70 or less. The hardness can be adjusted to any desired value by changing the blending composition and blending amount as required.

[0037] The CMP polishing pad preferably has a compression ratio within the following range in order to achieve flatness of the object to be polished. The compression ratio can be measured by a method conforming to JIS L 1096. The compression ratio is preferably 0.5% to 50%. By keeping the compression ratio within the above range, it is possible to achieve excellent flatness of the object to be polished.

[0038] The abrasion resistance of the CMP polishing pad is preferably 60 mg or less, more preferably 50 mg or less, in the Taber abrasion test. A reduced Taber abrasion amount allows the pad to exhibit excellent abrasion resistance when used as a CMP polishing pad. The Taber abrasion amount can be measured by conducting a Taber abrasion test using a Taber 5130 device under the conditions of a load of 1 kg, a rotation speed of 60 rpm, a rotation number of 1000 rpm, and an abrasion wheel H-18.

[0039] The form of the CMP polishing pad is not particularly limited, and for example, a groove structure may be formed on the surface thereof. The groove structure of the CMP polishing pad is preferably a shape that retains and renews the slurry, and specific examples thereof include X (stripe) grooves, XY lattice grooves, concentric grooves, through holes, blind holes, polygonal columns, cylinders, spiral grooves, eccentric circular grooves, radial grooves, and combinations of these grooves.

[0040] The method for producing the groove structure of the CMP polishing pad is not particularly limited. For example, a method of producing the groove structure by pouring a composition for producing the CMP polishing pad into a mold having a predetermined groove structure and curing it, or a method of producing the groove structure using the obtained resin, such as a mechanical cutting method using a tool such as a cutting tool of a predetermined size, a method of pressing a resin with a press plate having a predetermined surface shape, a method using photolithography, a method using a printing method, or a method using laser light such as a carbon dioxide laser, can be mentioned.

[0041] The CMP polishing pad may also be composed of multiple layers. In this case, at least one of the layers may be made of a cured material containing microcapsules obtained by the manufacturing method of the present invention. For example, if the CMP polishing pad is composed of two layers, it will have a polishing layer (also referred to as the first layer) having a polishing surface that contacts the workpiece during polishing, and a base layer (also referred to as the second layer) that contacts the first layer on the surface opposite the polishing surface of the first layer. In this case, the characteristics of the CMP polishing pad can be adjusted by making the second layer and the first layer different in hardness and elastic modulus. In this case, it is preferable that the base layer has a lower hardness than the polishing layer. It is preferable to use a cured material containing microcapsules obtained by the manufacturing method of the present invention as the polishing layer, and further, the base layer may also be made of the cured material containing microcapsules.

[0042] The polyurethane resin used in the CMP polishing pad may be prepared by any known method without particular limitation, for example, by uniformly mixing and dispersing (B) a polyfunctional isocyanate compound (hereinafter also referred to as component (B)), (C) a compound having an isocyanate group and two or more curable active hydrogen groups (hereinafter also referred to as component (C)), and, if necessary, other compounding components, followed by curing. The polyfunctional isocyanate compound (B) is a compound having at least two isocyanate groups. The isocyanate group refers to an isocyanate group (NCO group) or an isothiocyanate group (NCS group). Of course, a compound having both an isocyanate group and an isothiocyanate group can also be selected as component (B). Examples of the active hydrogen group of the component (C) include groups selected from the group consisting of a hydroxyl group, a thiol group, and an amino group. In addition, (B) the polyfunctional isocyanate compound and (C) the compound having two or more active hydrogen groups curable with an isocyanate group, those disclosed in WO 2022 / 163781 can be used without particular limitation.

[0043] A cured product in which microcapsules are dispersed in a polyurethane resin can be produced by curing a curable composition containing components (B), (C), and microcapsules. Other components may be added to the curable composition as needed. The cured product thus produced can be processed into a desired shape and used as a CMP polishing pad.

[0044] The curing method is not particularly limited, and any known method may be used. For example, the conditions described in International Publication Nos. 2015 / 068798, 2016 / 143910, and 2018 / 092826 can be used. Specifically, dry methods such as the one-pot method and the prepolymer method, and wet methods using a solvent can be used. Among these, the dry method is preferably used.

[0045] The amount of microcapsules to be blended into the polyurethane resin is preferably 0.1 to 20 parts by mass, more preferably 0.2 to 10 parts by mass, and even more preferably 0.5 to 8 parts by mass, per 100 parts by mass of the total of components (B) and (C). By adjusting the amount within this range, it is possible to achieve excellent polishing properties.

[0046] Other ingredients that may be added to the curable composition as needed include, for example, a curing catalyst, abrasive grains, a surfactant, a flame retardant, a plasticizer, a filler, an antistatic agent, a foam stabilizer, a solvent, a leveling agent, and other additives. These other ingredients may be used alone or in combination of two or more.

[0047] As the curing catalyst, a urethane or urea reaction catalyst can be used to rapidly accelerate the curing. Specific examples of the urethane or urea reaction catalyst that can be suitably used include those described in WO 2015 / 068798.

[0048] These urethane or urea reaction catalysts can be used either alone or in combination of two or more. The amount used may be a so-called catalytic amount, for example, in the range of 0.001 to 10 parts by mass, particularly 0.01 to 5 parts by mass, per 100 parts by mass of the total of components (B) and (C).

[0049] Examples of the abrasive grains include particles made of a material selected from cerium oxide, silicon oxide, alumina, silicon carbide, zirconia, iron oxide, manganese dioxide, titanium oxide, and diamond, or particles made of two or more of these materials. [Example]

[0050] EXAMPLES The present invention will be explained in more detail below with reference to examples, but the present invention is not limited to these examples.

[0051] [Evaluation method] The obtained microcapsules were observed under a field emission scanning electron microscope (JEOL Ltd., JSM-7800FPrime).

[0052] [Melamine aldehyde prepolymer] The melamine aldehyde prepolymers used in the examples are as follows: "Nikaresin S-260" manufactured by Nippon Carbide Industries Co., Ltd., in formula (I), R 1 is a methylol group. "Nikaresin S-176" manufactured by Nippon Carbide Industries Co., Ltd., in formula (I), R 1 A compound in which is a hydrogen atom.

[0053] Example 1 3.5 g of styrene-maleic anhydride copolymer (average molecular weight 350,000, "scripset520" manufactured by Solenis) as a surfactant was dissolved in 55 mL of 2.0 mass % aqueous sodium hydroxide solution while heating to prepare a first mixture at 80°C and pH 9.7. 34.9 g of chlorobenzene was added to the first mixture, and the mixture was stirred at 1500 rpm for 10 minutes using a homogenizer to prepare a first emulsion (O / W emulsion) at 25° C. and pH 9.6. Separately, 8.87 g of melamine aldehyde prepolymer (Nikaresin S-260 manufactured by Nippon Carbide Industries Co., Ltd.) and 2 g of 10% by mass citric acid aqueous solution were added to 18 g of distilled water and stirred to prepare an addition phase at a temperature of 80°C and a pH of 6.0. The additive phase was added to the first emulsion and mixed to obtain a second emulsion with a pH of 8.0. A 10% by weight aqueous citric acid solution was then added to the second emulsion to adjust the pH to 7.0. The mixture was then stirred and mixed at 150 rpm at a reaction temperature of 40°C for 5 hours to produce microcapsules with a melamine resin shell and an organic solvent housed within the shell. The solution containing the microcapsules was then centrifuged three times: at 8000 rpm for 10 minutes, 30 minutes, and 40 minutes. The aqueous phase was removed, and the mixture was then freeze-dried for 48 hours to remove the encapsulated organic solvent, yielding hollow microcapsules (microballoons). The microcapsules obtained were observed with a field-emission scanning electron microscope, and the results are shown in Figure 1. In each figure, the upper image is an image of multiple microcapsules observed over a wide field of view, and the lower image is an enlarged image of a single microcapsule.

[0054] <Examples 2 to 7> Microcapsules were obtained in the same manner as in Example 1, except that the composition of the additive phase, each pH, ​​reaction temperature, and reaction time were adjusted as shown in Table 1. The microcapsules obtained in Examples 2 to 7 were observed with a field emission scanning electron microscope, and the results are shown in Figures 1 to 3, respectively.

[0055] <Comparative Example 1> 3.5 g of styrene-maleic anhydride copolymer (average molecular weight 350,000, "scripset520" manufactured by Solenis) as a surfactant was dissolved in 55 mL of 2.0 mass % aqueous sodium hydroxide solution while heating to prepare a first mixture at 80°C and pH 9.7. 34.9 g of chlorobenzene was added to the first mixture, and the mixture was stirred at 1500 rpm for 10 minutes using a homogenizer to prepare a first emulsion (O / W emulsion) at 80° C. and pH 9.6. Separately, 4.54 g of melamine, 11.69 g of a 37 mass % aqueous formaldehyde solution, and 5 mL of a 10 mass % aqueous sodium hydroxide solution were added to 7.12 g of distilled water, and the mixture was stirred at 80°C for 2 hours to obtain an added phase containing methylol melamine. The additive phase was added to the first emulsion and mixed to obtain a second emulsion with a pH of 11.0. A 10% by weight aqueous citric acid solution was added to the second emulsion to adjust the pH to 3.8. The mixture was then stirred and mixed at 150 rpm at a reaction temperature of 80°C for 3 hours to produce microcapsules with a melamine resin shell and an organic solvent housed within the shell. The solution containing the microcapsules was centrifuged three times: at 8000 rpm for 10 minutes, 30 minutes, and 40 minutes. The aqueous phase was removed, and the mixture was then freeze-dried for 48 hours to remove the encapsulated organic solvent, yielding hollow microcapsules (microballoons).

[0056] [Table 1]

[0057] The results of each example show that the manufacturing method of the present invention, which uses an additive phase containing melamine aldehyde prepolymer, makes it possible to form microcapsules at a low reaction temperature of approximately 20 to 60°C and at a neutral pH. In contrast, in the manufacturing method of Comparative Example 1, which used an additive phase that did not contain melamine aldehyde prepolymer, high temperature (80°C) and acidic conditions (pH 3.8) were required to form microcapsules. Note that, in the manufacturing method of Comparative Example 1, when the reaction temperature was lowered (20 to 60°C) or the pH was changed to the neutral range (6.5 to 7.5), microcapsules could not be formed.

[0058] Among the examples, it was found that the examples in which the reaction temperature was relatively low, at 20 to 40°C (Examples 1 to 3, Examples 6 and 7), resulted in less aggregation of microcapsules and made it easier to obtain good microcapsules than the example in which the reaction temperature was relatively high, at 60°C (Example 4). In addition, Example 3 differs from the other Examples in that the melamine formaldehyde prepolymer used in Example 3 is R 1 This is an example in which a compound in which is a hydrogen atom was used, and the aggregation of the microcapsules was extremely small, and the microcapsules obtained had excellent surface smoothness.

Claims

1. 1. A method for producing a microcapsule comprising an outer shell comprising a melamine resin and an interior portion located within the outer shell and containing an organic solvent, the method comprising: mixing a first mixture containing a maleic anhydride copolymer and water with the organic solvent to obtain a first emulsion in which the first mixture is a continuous phase and the organic solvent is a dispersed phase; mixing the first emulsion with an additive phase comprising a melamine aldehyde prepolymer and water to obtain a second emulsion; and polymerizing the second emulsion to obtain the microcapsules.

2. The method according to claim 1 , wherein the melamine aldehyde prepolymer comprises a compound represented by the following formula (I): 【Chemistry 1】 In the formula (I), R １ is a hydrogen atom or a methylol group, n is a number between 2 and 10.

3. The method according to claim 1 , wherein the pH of the addition phase is 5 or more and 14 or less.

4. The method according to claim 1 , wherein the pH of the first emulsion when the additive phase is mixed is 3 or more and 14 or less.

5. The method according to claim 1 , wherein the pH of the second emulsion during the polymerization is 6.5 or more and 7.5 or less.

6. The method according to claim 1 , wherein the temperature of the second emulsion during the polymerization is 10° C. or higher and 60° C. or lower.

7. 2. The method according to claim 1, wherein the maleic anhydride copolymer comprises at least one selected from the group consisting of a styrene-maleic anhydride copolymer, an ethylene-maleic anhydride copolymer, and an isobutylene-maleic anhydride copolymer.

8. A method for producing microballoons, comprising producing microcapsules by the method of any one of claims 1 to 7, and then removing the organic solvent contained in the microcapsules to obtain microballoons.

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