Method for producing resin powder

Abstract

The present invention provides a method for producing a resin powder containing a vinyl alcohol polymer, said method making it possible to obtain a resin powder which has an improved drying efficiency and good handleability. Provided is a method for producing a resin powder which contains a vinyl alcohol polymer and which has an average particle size of not less than 100 μm, said method comprising a step for obtaining a solid which contains the vinyl alcohol polymer and in which bubbles are formed, wherein the bubbles are formed by introducing a gas into the system.

Description

Resin powder manufacturing method

[0001] The present invention relates to a method for producing a resin powder.

[0002] Vinyl alcohol polymers (hereinafter also referred to as "PVA") are synthetic resins generally obtained by saponifying polyvinyl esters (hereinafter also referred to as "PVAc"). PVA is water-soluble and is used in applications such as synthetic fiber raw materials, film raw materials, emulsifying dispersants, and adhesives.

[0003] PVA is usually obtained by radically polymerizing vinyl acetate in a methanol solvent to obtain a methanol solution of vinyl acetate polymer, followed by saponification with the addition of an alkali catalyst. The PVA composition obtained through saponification, which contains the solvent, is then subjected to a process such as drying to obtain a PVA resin powder (see Patent Document 1, etc.).

[0004] JP 2013-28712 A

[0005] Conventional resin powders containing PVA generally have the disadvantage of poor drying efficiency. In order to shorten the drying process, save energy, and reduce production costs in the production of resin powders, it is desirable to improve the drying efficiency of resin powders. Furthermore, PVA resin powders obtained through drying may be re-dried after being subjected to a liquid treatment (e.g., synthesis of modified PVA, known as post-modification), or moisture-absorbed PVA resin powders may be re-dried as needed. Even in such re-drying cases, it is desirable for the PVA resin powder to have high drying efficiency. To obtain resin powders with high drying efficiency, it is possible to consider micronizing the resin powder. However, micronized resin powders have another disadvantage, namely, poor handling.

[0006] The present invention aims to provide a method for producing a resin powder containing a vinyl alcohol-based polymer, which method has improved drying efficiency and can produce a resin powder that is easy to handle.

[0007] The object is to provide the following: [1] a method for producing a resin powder containing a vinyl alcohol polymer and having an average particle size of 100 μm or more, the method comprising a step of obtaining a solid material containing the vinyl alcohol polymer and having gas bubbles formed therein, the gas bubbles being formed by introducing a gas into a system; [2] a method for producing a resin powder according to [1], wherein the gas is introduced when obtaining the vinyl alcohol polymer by saponifying a vinyl ester polymer; [3] a method for producing a resin powder according to [1] or [2], wherein the gas is nitrogen; [4] a method for producing a resin powder according to any one of [1] to [3], wherein the resin powder has an average particle size of 100 to 2,000 μm, and the number of particles having gas bubbles of 1 μm or more among 100 particles randomly extracted from particles having a particle size of 106 to 1,000 μm in the resin powder is 10 or more; [5] a method for producing a resin powder according to any one of [1] to [4], further comprising a step of pulverizing the solid material into a resin powder having an average particle size of 100 μm or more; This can be solved by:

[0008] According to the present invention, a method for producing a resin powder containing a vinyl alcohol-based polymer can be provided, which method can produce a resin powder that has improved drying efficiency and good handleability.

[0009] In this specification, a range of values ​​stated using "to" means that the range of values ​​stated before and after "to" includes the lower and upper limits.

[0010] <Method for producing resin powder> A method for producing a resin powder according to one embodiment of the present invention is a method for producing a resin powder containing a vinyl alcohol-based polymer (PVA: polyvinyl alcohol) and having an average particle size of 100 μm or more, the method comprising the step of obtaining a solid material containing the vinyl alcohol-based polymer and having bubbles formed therein, and the bubbles are formed by introducing a gas into a system.

[0011] According to a method for producing a resin powder according to one embodiment of the present invention, it is possible to obtain a resin powder with improved drying efficiency compared to conventional resin powders. While the reason for this is not entirely clear, it is speculated that the resin powder obtained by this production method contains particles containing bubbles derived from the introduced gas, resulting in an increased surface area. Furthermore, in the case of particles without bubbles, the center of the particle is unlikely to be sufficiently heated during drying by heating, making it difficult for the volatile components at the center of the particle to volatilize. In contrast, in the case of particles containing bubbles, the presence of the bubbles suppresses this phenomenon, which is speculated to be another reason for the improved drying efficiency.

[0012] The ease of drying of resin powders containing PVA is greatly influenced by the physical properties of the PVA itself (such as the degree of saponification, degree of polymerization, the presence or absence of modified species and the amount of modification) and the particle size of the resin powder. Therefore, in this specification, "improved drying efficiency" means that the drying time is sufficiently shortened compared to a resin powder of substantially the same particle size using the same type of PVA. "Same type of PVA" refers to PVA with the same degree of saponification, degree of polymerization, presence or absence of modified species, and amount of modification. Furthermore, "modified species" refers to monomers other than vinyl esters. "Amount of modification" refers to the content of monomer units derived from the other monomers relative to the total monomer units in the PVA.

[0013] Furthermore, the resin powder obtained by the method for producing a resin powder according to one embodiment of the present invention has an average particle size of 100 μm or more, and therefore is easy to handle. However, in the case of a resin powder having an average particle size of less than 100 μm, the handling is poor because the resin powder has low wettability, making it difficult to prepare a solution such as an aqueous solution, is prone to scattering and adhesion to walls, and there is a high risk of dust explosion.

[0014] Furthermore, the resin powder obtained by the resin powder manufacturing method according to one embodiment of the present invention has a low content of organic volatile matter and a high dissolution rate in water. This is presumably due to the fact that the resin powder contains particles containing bubbles, resulting in a sufficiently increased surface area. Furthermore, since the surface area of ​​the resin powder is sufficiently increased, it is thought that the reactivity of the resin powder when subjected to post-modification, etc. is also good.

[0015] A method for producing a resin powder according to one embodiment of the present invention may typically include a step of polymerizing a vinyl ester monomer (polymerization step), a step of saponifying the resulting vinyl ester polymer to obtain a solid material containing PVA (saponification step), a step of washing the resulting solid material (washing step), a step of drying the washed solid material (drying step), and a step of pulverizing the dried solid material (pulverization step). In this production method, a gas is introduced into the system during the saponification step or the like to obtain a solid material containing PVA and having gas bubbles formed therein. Note that "in system" may refer to a liquid, gel, or solid mixture containing PVA. "In system" may refer to a liquid, gel, or solid mixture containing at least one of the reactants and the product during the production of PVA. For example, the "in system" in the saponification step (saponification reaction) may refer to the reaction liquid during the saponification reaction. A portion of the reaction liquid in the saponification reaction liquid may be in a gel or solid state.

[0016] The introduction of gas into the system is preferably carried out during the saponification step. That is, it is preferable to introduce gas into the reaction solution when saponifying a vinyl ester polymer to obtain PVA. In other words, the step of obtaining a solid material containing PVA and having gas bubbles may be a step of saponifying a vinyl ester polymer to obtain a solid material containing PVA, in which a gas is introduced into the reaction solution of the saponification reaction. Alternatively, the saponification step carried out while introducing a gas into the system may be a step of obtaining a solid material containing PVA and having gas bubbles. By saponifying a vinyl ester polymer while introducing a gas into the reaction solution during the saponification reaction in this way, a solid material containing PVA and having gas bubbles can be efficiently obtained.

[0017] The introduction of a gas into the system may be carried out in a step other than the saponification step. For example, a PVA solution obtained through saponification or the like may be prepared, and a gas may be introduced into the PVA solution, followed by drying in a state in which air bubbles remain.

[0018] Hereinafter, each step in the method for producing a resin powder according to one embodiment of the present invention will be described in detail.

[0019] In the polymerization step, a vinyl ester monomer is polymerized to obtain a vinyl ester polymer. Methods for polymerizing a vinyl ester monomer include known methods such as bulk polymerization, solution polymerization, suspension polymerization, and emulsion polymerization. Among these methods, bulk polymerization carried out without a solvent and solution polymerization carried out using a solvent such as alcohol are preferred, and solution polymerization carried out in the presence of a lower alcohol is more preferred. The lower alcohol is preferably an alcohol having 3 or fewer carbon atoms, more preferably methanol, ethanol, n-propanol, and isopropanol, and even more preferably methanol. When carrying out the polymerization reaction by bulk polymerization or solution polymerization, either a batch or continuous reaction system can be used.

[0020] Examples of vinyl ester monomers include vinyl formate, vinyl acetate, vinyl propionate, vinyl valerate, vinyl caprate, vinyl laurate, vinyl stearate, vinyl benzoate, vinyl pivalate, vinyl versatate, etc. Of these, vinyl acetate is preferred.

[0021] Examples of initiators used in the polymerization reaction include known initiators such as azo initiators such as 2,2'-azobisisobutyronitrile, 2,2'-azobis(2,4-dimethylvaleronitrile), and 2,2'-azobis(4-methoxy-2,4-dimethylvaleronitrile); and organic peroxide initiators such as benzoyl peroxide and n-propyl peroxycarbonate. There are no particular restrictions on the polymerization temperature when the polymerization reaction is carried out, but a range of 5°C to 200°C is appropriate.

[0022] When polymerizing a vinyl ester monomer, other copolymerizable monomers (modified species) can be further copolymerized within the scope of the present invention. Examples of such other monomers include α-olefins such as ethylene, propylene, 1-butene, isobutene, and 1-hexene; acrylic acid, methacrylic acid; acrylic acid esters such as methyl acrylate and ethyl acrylate; methacrylic acid esters such as methyl methacrylate and ethyl methacrylate; maleic acid and derivatives thereof such as maleic acid, monomethyl maleate, and dimethyl maleate; acrylamide derivatives such as N-methylacrylamide and N-ethylacrylamide; methacrylamide derivatives such as N-methylmethacrylamide and N-ethylmethacrylamide; vinyl ethers such as methyl vinyl ether, ethyl vinyl ether, n-propyl vinyl ether, isopropyl vinyl ether, and n-butyl vinyl ether; ethylene glycol vinyl ether, 1,3-propanediol vinyl ether, and 1,4-butanediol vinyl ether. Examples of suitable monomers include hydroxyl group-containing vinyl ethers such as allyl vinyl ether; allyl acetate; allyl ethers such as propyl allyl ether, butyl allyl ether, and hexyl allyl ether; monomers having an oxyalkylene group; isopropenyl acetate; hydroxyl group-containing α-olefins such as 3-buten-1-ol, 4-penten-1-ol, 5-hexen-1-ol, 7-octen-1-ol, 9-decen-1-ol, and 3-methyl-3-buten-1-ol; and monomers having a silyl group such as vinyltrimethoxysilane, vinylmethyldimethoxysilane, vinyldimethylmethoxysilane, vinyltriethoxysilane, vinylmethyldiethoxysilane, vinyldimethylethoxysilane, 3-(meth)acrylamidopropyltrimethoxysilane, and 3-(meth)acrylamidopropyltriethoxysilane. Among these, α-olefins, acrylic acid, methacrylic acid, acrylic acid esters, methacrylic acid esters, maleic acid, and derivatives of maleic acid are preferred. The upper limit of the amount of these other monomers used varies depending on the purpose and application of the use, etc., but is preferably 20 mol %, more preferably 10 mol %, based on the total amount of monomers, and may be 5 mol %, 3 mol %, 1 mol %, or 0.1 mol %.On the other hand, the lower limit of the amount of the other monomer used may be, for example, 0.1 mol % or 1 mol %.

[0023] During polymerization of vinyl ester monomers, a chain transfer agent may be present to adjust the degree of polymerization of the resulting PVA. Examples of chain transfer agents include aldehydes such as acetaldehyde, propionaldehyde, butylaldehyde, and benzaldehyde; ketones such as acetone, methyl ethyl ketone, hexanone, and cyclohexanone; mercaptans such as 2-hydroxyethanethiol; thiocarboxylic acids such as thioacetic acid; and halogenated hydrocarbons such as trichloroethylene and perchloroethylene. Among these, aldehydes and ketones are preferred. The amount of chain transfer agent added is determined depending on the chain transfer constant of the chain transfer agent and the desired degree of polymerization of the PVA, but is generally 0.1 to 10% by mass based on the vinyl ester used.

[0024] In the saponification step, for example, a vinyl ester polymer is saponified in a solution (generally an alcohol solution) using an alkaline catalyst to obtain PVA. The saponification reaction of a vinyl ester polymer can be carried out by alcoholysis or hydrolysis using a conventionally known basic catalyst such as sodium hydroxide, potassium hydroxide, or sodium methoxide. Examples of solvents used in the saponification reaction include alcohols such as methanol and ethanol; esters such as methyl acetate and ethyl acetate; ketones such as acetone and methyl ethyl ketone; and aromatic hydrocarbons such as benzene and toluene. These solvents can be used alone or in combination of two or more. Among these, it is preferable to use methanol or a mixed solution of methanol and methyl acetate as the solvent and carry out the saponification reaction in the presence of sodium hydroxide, as this is simple and convenient.

[0025] As described above, it is preferable to introduce a gas into the system (reaction liquid) in the saponification step to obtain a solid product containing PVA and having gas bubbles formed. Examples of methods for introducing a gas into the system in the saponification step include a method in which the vinyl ester polymer and the alkali catalyst are mixed while mixing them, and a method in which gas bubbles are directly introduced into the mixture of the vinyl ester polymer and the alkali catalyst just before the formation of a PVA gel. It is not necessary to continuously introduce a gas into the system from the beginning to the end of the saponification step; it is sufficient to introduce a gas into the system in at least a part of the saponification step.

[0026] The gas introduced into the system, i.e., the gas contained in the formed bubbles, is not particularly limited, but from the viewpoint of safety, etc., it is preferable that it contains an inert gas. Furthermore, from the viewpoint of handleability, etc., it is preferable that the gas introduced into the system is an inorganic substance. From these viewpoints and from the viewpoint of production cost, the gas introduced into the system is preferably air, carbon dioxide, or nitrogen, and more preferably nitrogen.

[0027] The number of particles containing bubbles in the resulting resin powder can be adjusted, for example, by the amount of gas introduced into the system. The gas can be introduced into the system through a foaming filter having a predetermined pore size. This facilitates adjustment of the size of the bubbles formed. The pore size of the filter is preferably, for example, 1 to 300 μm, and more preferably 5 to 200 μm.

[0028] The PVA-containing solid obtained through the saponification step is typically washed, dried, and pulverized as described above to obtain a resin powder. The washing can be performed using an alcohol such as methanol. The drying conditions are not particularly limited, but the drying temperature can be, for example, 50°C to 120°C, or 60°C to 100°C. The drying time can be, for example, 1 hour to 24 hours, or 2 hours to 20 hours, or 16 hours or less, 12 hours or less, or 6 hours or less.

[0029] The dried solid can be pulverized by a known method. In this pulverization, it is preferable to pulverize so that the average particle size of the resulting resin powder is 100 μm or more. The pulverization can be performed using a known pulverizer. As the pulverizer, a device capable of controlling the degree of pulverization, such as the pulverization intensity, is preferred in order to adjust the average particle size of the resulting resin powder. The preferred average particle size of the resulting resin powder will be described in detail below. After pulverization, drying or the like may be performed again.

[0030] The production method may further include a step of removing fine powder from the resin powder obtained by pulverization. The removal of fine powder can be performed by a known method using, for example, an air classifier, a sieve, etc. The fine powder may be, for example, particles having a particle diameter of 200 μm or less, particles having a particle diameter of 100 μm or less, or particles having a particle diameter of 50 μm or less.

[0031] <Resin Powder> A preferred embodiment of the resin powder obtained by the resin powder manufacturing method according to one embodiment of the present invention will be described below. The resin powder contains PVA and is composed of particles having an average particle size of 100 μm or more. Preferably, the resin powder has an average particle size of 100 to 2,000 μm, and the number of particles having bubbles of 1 μm or more among 100 particles randomly selected from particles having a particle size of 106 to 1,000 μm is 10 or more. Hereinafter, "particles having bubbles of 1 μm or more" will also be referred to as "bubble-containing particles." "Bubbles of 1 μm or more" refers to bubbles having a bubble size of 1 μm or more, as described below. The bubble size may also be referred to as the diameter of the bubbles. Furthermore, the "number of particles having bubbles of 1 μm or more (bubble-containing particles) among 100 particles randomly selected from particles having a particle size of 106 to 1,000 μm" will also be referred to as the "bubble-containing particle number." The resin powder is an aggregate of a plurality of particles (PVA particles), and these plurality of particles (PVA particles) include a plurality of bubble-containing particles. The resin powder may be composed only of these plurality of particles (PVA particles).

[0032] PVA is usually the main component of the resin powder. The term "main component" refers to the component with the highest content by mass. The lower limit of the PVA content in the nonvolatile content of the resin powder is preferably 50% by mass, more preferably 70% by mass, even more preferably 90% by mass, and even more preferably 99% by mass in some cases. The upper limit of the PVA content in the nonvolatile content of the resin powder may be 100% by mass. The lower limit of the PVA content in the resin powder is preferably 60% by mass, more preferably 70% by mass, even more preferably 90% by mass, and even more preferably 99% by mass in some cases. The upper limit of the PVA content in the resin powder may be 100% by mass or 99.9% by mass.

[0033] PVA is a polymer having vinyl alcohol units as monomer units. PVA is typically obtained by saponifying a vinyl ester polymer. The lower limit of the content of vinyl alcohol units relative to the total monomer units in PVA is preferably 35 mol%, more preferably 50 mol%, even more preferably 70 mol%, and may be 80 mol%, 85 mol%, 90 mol%, or 95 mol%. As the content of vinyl alcohol units in PVA increases, drying generally tends to take longer. Therefore, when the present invention is applied to a method for producing a resin powder containing PVA with a high content of vinyl alcohol units, the advantage of improved drying efficiency is particularly easily achieved. On the other hand, the upper limit of the content of the vinyl alcohol units may be 100 mol%, but is preferably 99.99 mol%, more preferably 99 mol%, and may be 98 mol%, 95 mol%, or 90 mol%. When the content of vinyl alcohol units in PVA is equal to or less than the upper limit, drying time can be shortened.

[0034] The lower limit of the saponification degree of the PVA is preferably 35 mol%, more preferably 50 mol%, even more preferably 70 mol%, and may be 80 mol%, 85 mol%, 90 mol%, or 95 mol%. As the saponification degree of the PVA increases, drying generally tends to take longer. Therefore, when the present invention is applied to a method for producing a resin powder containing a PVA with a high saponification degree, the advantage of improving drying efficiency is particularly easily achieved. On the other hand, the upper limit of the saponification degree may be 100 mol%, but is preferably 99.99 mol%, more preferably 99 mol%, or may be 98 mol%, 95 mol%, or 90 mol%. When the saponification degree of the PVA is equal to or less than the upper limit, the drying time can be shortened. The saponification degree is a value measured by the method described in JIS K6726:1994.

[0035] The PVA may have other monomer units other than vinyl alcohol units and vinyl ester units. Examples of the monomers (modified species) that provide the other monomer units are as described above. The upper limit of the content (modification amount) of the other monomer units relative to the total monomer units in the PVA may be preferably 20 mol%, more preferably 10 mol%. The upper limit of the content of the other monomer units may be 5 mol%, 3 mol%, 1 mol%, or 0.1 mol%. On the other hand, the lower limit of the content of the other monomer units may be, for example, 0.1 mol% or 1 mol%.

[0036] The viscosity-average degree of polymerization of the PVA is not particularly limited, but its lower limit is preferably 200, more preferably 250, even more preferably 400, and particularly preferably 600, and may be 1,000, 1,500, or 2,000. As the viscosity-average degree of polymerization of the PVA increases, drying generally tends to take longer. Therefore, when the present invention is applied to a method for producing a resin powder containing a PVA with a high viscosity-average degree of polymerization, the advantage of improved drying efficiency is particularly easily achieved. The upper limit of the viscosity-average degree of polymerization is preferably 5,000, more preferably 4,500, even more preferably 3,500, and may be 3,000, 2,500, or 2,000. When the viscosity-average degree of polymerization of the PVA is equal to or less than the upper limit, drying time can be shortened. The viscosity-average degree of polymerization is a value measured in accordance with JIS K6726:1994. That is, PVA is resaponified to a degree of saponification of 99.5 mol% or more, purified, and then the intrinsic viscosity [η] (unit: liter / g) measured in water at 30°C can be calculated using the following formula: Viscosity average degree of polymerization = ([η] x 10,000 / 8.29) （１／０．６２）

[0037] The resin powder may contain components other than PVA. Examples of non-volatile components other than PVA that may be contained in the resin powder include resins other than PVA, additives such as surfactants and plasticizers, and various compounds used during production.

[0038] The upper limit of the organic volatile content in the resin powder may be, for example, 5.0% by mass or 4.0% by mass, but is preferably 3.0% by mass. By having the organic volatile content be equal to or less than the upper limit, handleability and the like can be improved. The lower limit of the organic volatile content may be, for example, 0.1% by mass, 0.5% by mass, 1.0% by mass, or 1.5% by mass.

[0039] The upper limit of the volatile content in the resin powder may be, for example, 5.0% by mass or 4.0% by mass, but is preferably 3.0% by mass. By having the volatile content be equal to or less than the upper limit, handleability can be improved. The lower limit of the volatile content may be, for example, 0.1% by mass, 0.5% by mass, 1.0% by mass, or 1.5% by mass.

[0040] The organic volatile content and the volatile content of the resin powder can be measured in accordance with JIS K6726: 1994. For example, if the volatile content of the resin powder measured in accordance with the above standard is 3.0% by mass, it is clear that the organic volatile content is 3.0% by mass or less.

[0041] The lower limit of the average particle diameter of the resin powder is 100 μm, preferably 150 μm, more preferably 300 μm, and even more preferably 400 μm. By setting the average particle diameter at or above the lower limit, the wettability of the resin powder is increased, making it easier to prepare solutions such as aqueous solutions, scattering and adhesion to wall surfaces are suppressed, and the risk of dust explosions is reduced, thereby improving handleability. Furthermore, as the average particle diameter of a resin powder increases, it generally tends to take longer to dry. Therefore, when the present invention is applied to a resin powder with a relatively large average particle diameter, the advantage of improving drying efficiency is particularly pronounced. On the other hand, the upper limit of the average particle diameter is preferably 2,000 μm, more preferably 1,500 μm, even more preferably 1,000 μm, and even more preferably 850 μm. Setting the average particle diameter at or below the upper limit can shorten the drying time, for example. The average particle diameter of the resin powder is a value measured in accordance with the method described in JIS K7369:2009.

[0042] The resin powder preferably contains particles having a particle diameter of 106 to 1,000 μm. The lower limit of the number of bubble-containing particles (bubble particle number) among 100 particles randomly selected from the particles having a particle diameter of 106 to 1,000 μm contained in the resin powder is preferably 10, more preferably 15, even more preferably 20, even more preferably 30, and even more preferably 40, 45, or 50. When the number of bubble-containing particles is equal to or greater than the lower limit, drying efficiency can be improved, etc. The upper limit of the number of bubble-containing particles may be 100, or may be 90, 80, 70, or 60. The number of bubble-containing particles can be adjusted by, for example, the amount of gas introduced.

[0043] The lower limit of the average bubble size in the bubble-containing particles of the resin powder is preferably 1 μm, more preferably 10 μm, even more preferably 30 μm, even more preferably 40 μm, and particularly preferably 50 μm. By setting the average bubble size at or above the lower limit, it is possible to further improve drying efficiency. On the other hand, the upper limit of the average bubble size is preferably 200 μm, more preferably 100 μm. By setting the average bubble size at or below the upper limit, it is possible to increase the productivity of the resin powder. The average bubble size is the average value of the size of bubbles of 1 μm or more contained in the bubble-containing particles (bubble size). The average bubble size can be adjusted, for example, by the pore size of a filter used when introducing gas into a slurry or solid containing PVA.

[0044] The number of bubble-containing particles and average bubble size of resin powder are determined by the following method. Randomly extract 100 particles from particles with a particle diameter of 106 to 1,000 μm in the resin powder. Particles with a particle diameter of 106 to 1,000 μm can be selected by sieving as particles that pass through a sieve with a nominal mesh size of 1,000 μm (16 mesh) but do not pass through a sieve with a nominal mesh size of 106 μm (150 mesh). The mechanical sieving can be performed, for example, by the method described in JIS K7369:2009. Each extracted particle is observed for the presence or absence of bubbles and the bubble size (bubble diameter) is measured using a microscope. Observation and measurement of each particle using a microscope are performed from directly above each particle placed on a horizontal plane. If the bubbles are circular, the bubble size is the diameter of the circle; if the bubbles are elliptical, the bubble size is the major axis. The average bubble size is the average value of the bubble sizes of bubbles with a size of 1 μm or more contained in the observed bubble-containing particles. Bubbles include not only completely closed bubbles (those present inside the particles) but also bubbles that are not completely closed (those with a circular or elliptical recessed surface shape that is thought to be caused by bubbles). Even for bubbles that are not completely closed (those with a circular or elliptical recessed surface shape that is thought to be caused by bubbles), the diameter or major axis is taken as the bubble size.

[0045] The gas contained in the bubbles of the bubble-containing particles is preferably air or nitrogen, more preferably nitrogen. The nitrogen content in the gas contained in the bubbles of the bubble-containing particles is preferably 78% by volume or more, more preferably 80% by volume or more, and even more preferably 90% by volume or more.

[0046] In the resin powder, the content of particles having a particle diameter of 106 to 1,000 μm is not particularly limited, but the lower limit is preferably 50% by mass, more preferably 60% by mass, and even more preferably 70% by mass. Meanwhile, the upper limit of the content of particles having a particle diameter of 106 to 1,000 μm may be 100% by mass, 99% by mass, or 95% by mass. By having the content of particles having a particle diameter of 106 to 1,000 μm within the above range, drying efficiency is further improved. The content of particles having a particle diameter of 106 to 1,000 μm in the resin powder can be determined according to the method described in JIS K7369:2009 using a sieve mesh with a nominal mesh size of 1,000 μm (16 mesh) and a sieve mesh with a nominal mesh size of 106 μm (150 mesh).

[0047] The bulk density of the resin powder is not particularly limited, but the upper limit is preferably 0.7 g / mL, more preferably 0.6 g / mL, and even more preferably 0.55 g / mL. Having a bulk density equal to or less than the upper limit further improves drying efficiency. On the other hand, the lower limit of the bulk density is preferably 0.4 g / mL. The bulk density of the resin powder is measured by filling a 100 mL measuring cylinder with the resin powder up to 100 mL under conditions of 25°C and 50% RH, and then calculating the volume (100 mL) and the mass (unit: g) of the resin powder placed in the measuring cylinder.

[0048] In the resin powder, the upper limit of the product of its average particle size (μm) and its bulk density (g / mL) is preferably 700 μm·g / mL, more preferably 500 μm·g / mL, even more preferably 400 μm·g / mL, and even more preferably 350 μm·g / mL, 320 μm·g / mL, or 300 μm·g / mL. When the product of the average particle size (μm) and bulk density (g / mL) of the resin powder is below the upper limit, it means that the bulk density is low relative to the particle size. Therefore, the smaller the value of the product, the larger the surface area, and the more the drying efficiency tends to be improved. The lower limit of the product may be, for example, 100 μm·g / mL, 150 μm·g / mL, or 200 μm·g / mL.

[0049] The resin powder can be used in a variety of applications, including, but not limited to, the following: (1) Vinyl chloride dispersant applications: dispersion stabilizers and dispersion aids for suspension polymerization of vinyl chloride and vinylidene chloride; (2) Coating agent applications: sizing agents, fiber processing agents, leather finishing agents, paints, anti-fogging agents, metal corrosion inhibitors, zinc plating gloss agents, antistatic agents; (3) Adhesive and binder applications: adhesives, pressure-sensitive adhesives, re-wet adhesives, various binders, additives for cement and mortar; (4) Dispersion stabilizer applications: dispersion stabilizers for organic and inorganic pigments in paints and adhesives, dispersion stabilizers for emulsion polymerization of various vinyl compounds, post-emulsifiers for bitumen, etc.; (5) Paper processing applications: paper strength agents, oil and solvent resistance agents, smoothness improvers, surface gloss improvers, fillers, barrier agents, light resistance agents, water resistance agents, dye and color developer dispersants, adhesion improvers, binders. (6) Agricultural uses: pesticide binders, pesticide spreaders, agricultural covering agents, soil conditioners, erosion inhibitors, pesticide dispersants (7) Medical and cosmetic uses: granulation binders, coating agents, emulsifiers, patches, binders, film formulation base materials, film formers (8) Viscosity adjuster uses: thickeners, rheology adjusters (9) Flocculant uses: flocculants for suspended and dissolved substances in water, metal flocculants (10) Film uses: water-soluble films, polarizing films, barrier films, textile packaging films, seed protection sheets, vegetation sheets, seed tapes, moisture-absorbing films (11) Molded products uses: fibers, pipes, tubes, leak-proof membranes, water-soluble fibers for chemical lace, sponges (12) Resin raw material uses: raw materials for polyvinyl butyral, raw materials for photosensitive resins, raw materials for graft polymers, raw materials for various gels (13) Post-reaction uses: post-reaction uses with low-molecular-weight organic compounds, high-molecular-weight organic compounds, and inorganic compounds

[0050] The present invention will be described in more detail with reference to the following examples, but the present invention is not limited to these examples. The measurement methods used in the following examples and comparative examples are shown below.

[0051] [Viscosity-average degree of polymerization of PVA] The viscosity-average degree of polymerization of PVA (including modified PVA) was measured in accordance with JIS K6726:1994. Specifically, when the saponification degree of PVA was less than 99.5 mol%, it was saponified until the saponification degree reached 99.5 mol% or more, and the viscosity-average degree of polymerization of the resulting PVA was calculated using the intrinsic viscosity [η] (liters / g) measured in water at 30°C according to the following formula: Viscosity-average degree of polymerization = ([η] × 10,000 / 8.29) （１／０．６２）

[0052] [Degree of Saponification of PVA] The degree of saponification of PVA (including modified PVA) was determined by the method described in JIS K6726:1994.

[0053] [Content of Organic Volatile Matter] The content of organic volatile matter in the resin powder was determined by the method described in JIS K6726:1994.

[0054] [Average particle size of resin powder and content of particles with a particle size of 106 to 1,000 μm] The average particle size of the resin powder and the content of particles with a particle size of 106 to 1,000 μm were determined using a JIS standard sieve according to the method described in JIS K7369:2009.

[0055] [Number of bubble-containing particles and average bubble size] Particles having a particle diameter of 106 to 1,000 μm were selected by the sieving described above, and 100 particles were randomly selected from these particles. The number of bubble-containing particles and the average bubble size were measured for these particles based on images taken at a magnification of 100 times using a digital microscope VHX-900 manufactured by Keyence Corporation.

[0056] [Bulk density] Resin powder was poured into a 100 mL measuring cylinder up to 100 mL under conditions of 25°C and 50% RH, and bulk density was calculated from the volume (100 mL) and the mass (unit: g) of the resin powder put into the measuring cylinder.

[0057] Example 1: Production of PVA1 A 3-L reactor equipped with a stirrer, reflux condenser, nitrogen inlet, and initiator addition port was charged with 1,125 g of vinyl acetate and 375 g of methanol (75% by mass of vinyl acetate: 25% by mass of methanol), and the system was purged with nitrogen for 30 minutes while bubbling with nitrogen. The reactor was heated, and when the internal temperature reached 60°C, 0.5 g of 2,2'-azobisisobutyronitrile (AIBN) was added to initiate polymerization. When the conversion reached 50%, the mixture was cooled and the polymerization was terminated. The solids concentration at the time of termination of polymerization was 37.0%. Subsequently, unreacted vinyl acetate monomer was removed at 30°C under reduced pressure while occasionally adding methanol, yielding a methanol solution of polyvinyl acetate (PVAc) (concentration 35%). Further, 1,429 g of a methanol solution of PVAc (500 g of PVAc in the solution) was prepared by adding methanol to the PVAc solution. To this solution, 46.5 g of an alkaline solution (10% sodium hydroxide in methanol) was added for saponification (the PVAc concentration in the saponification solution was 30%, and the molar ratio of sodium hydroxide to vinyl acetate units in the PVAc was 0.02 mol%). Approximately 3 minutes after the addition of the alkaline solution, nitrogen was introduced into the reaction solution at 0.3 L / min through a 40 μm pore filter during the saponification reaction. Approximately 1 minute later, a solid (gel-like substance) was formed, which was then crushed in a crusher. The crushed substance was left at 40°C for 1 hour to allow saponification to proceed, and then 50 kg of methyl acetate was added to neutralize the remaining alkali. After confirming completion of neutralization using a phenolphthalein indicator, the solution was filtered to obtain a white solid. 1,000 g of methanol was added to the white solid, which was then left to wash at room temperature for 3 hours. The above washing procedure was repeated three times, and the resulting white solid obtained by centrifugal deliquor was dried in a dryer at 70°C for 16 hours. The resulting coarse powder was loaded into a juicer mixer and pulverized at room temperature in a nitrogen atmosphere to obtain the resin powder of Example 1 (PVA1 resin powder). The viscosity-average degree of polymerization of PVA1 was 1,700 and the degree of saponification was 98.5 mol%. The organic volatile content of the PVA1 resin powder was 2.7% by mass, the average particle size was 550 μm, the content of particles (predetermined particles) with particle diameters of 106 to 1000 μm was 85% by mass, the number of bubble-containing particles was 55, the average bubble size was 50 μm, the bulk density was 0.49 g / mL, and the product of the average particle size and bulk density was 269.5 μm g / mL.Furthermore, for the purpose of evaluating the drying efficiency described below, the PVAc obtained by the above method was saponified without introducing nitrogen, and then the same treatment was carried out to obtain a resin powder of PVA1'. The viscosity-average degree of polymerization and degree of saponification of PVA1', as well as the average particle size and content of particles with particle sizes of 106 to 1000 μm of the resin powder of PVA1', were substantially equal to those of PVA1 and PVA1 powder. The organic volatile content of the resin powder of PVA1' was 6.1% by mass, and the number of bubble-containing particles was less than 5.

[0058] [Evaluation of Drying Efficiency] 100 g of each of the PVA1 resin powder and the PVA1' resin powder was immersed in 500 mL of methanol at room temperature overnight to swell the resin powder. Each swollen resin powder was dried in a dryer at 100°C, and the drying time was measured as the time until the methanol content of each resin powder became less than 3% by mass. The drying time (T) for the PVA1 resin powder was 2.5 hours, and the drying time (T') for the PVA1' resin powder was 6.0 hours. The ratio (T / T') of the drying time (T) for the PVA1 resin powder to the drying time (T') for the reference PVA1' resin powder was calculated as an index of improvement in drying efficiency. The value of the ratio (T / T') for the PVA1 resin powder was 0.42. The ratio (T / T') was evaluated according to the following criteria. The resin powder of PVA1 was evaluated as A, and the inclusion of air bubbles was recognized to have an excellent effect of improving drying efficiency. (Criteria) A: Less than 0.50 B: 0.50 or more and less than 0.65 C: 0.65 or more and less than 0.75 D: 0.75 or more and less than 1.00 E: 1.00 or more

[0059] [Evaluation of Wettability] A 500 mL beaker was filled with 300 mL of distilled water, and a 30 mm rotor was placed inside the beaker. The beaker was placed on a magnetic stirrer and stirred at 100 rpm. 10 g of PVA1 resin powder was added all at once to the beaker while stirring, and the particle dispersion state was visually observed. 95% or more of the particles were dispersed in water. The particle dispersion state was also evaluated according to the following criteria. The PVA1 resin powder was evaluated as A, meaning that the particles were easily dispersed in water, the aqueous solution was easy to prepare, and it was easy to handle. (Criteria) A: 90% or more of the particles were dispersed in water B: 50% or more but less than 90% of the particles were dispersed in water C: Less than 50% of the particles were dispersed in water

[0060] [Examples 2 to 5] Resin powders of Examples 2 to 5 (resin powders of PVA2 to 5) were obtained under the same conditions as in Example 1, except that the production conditions were changed to those shown in Table 1. Furthermore, saponification was performed under the same production conditions as in Example 1 for resin powders of PVA2 to 5, but without introducing nitrogen, to obtain resin powders of PVA2' to 5'. The number of bubble-containing particles in each of the resin powders of PVA2' to 5', which served as the standard for evaluating drying efficiency, was less than 5. Measurements and evaluations were performed on each of the resin powders of Examples 2 to 5 (resin powders of PVA2 to 5) in the same manner as in Example 1. The results are shown in Tables 2 and 3.

[0061] Example 6 A 5-L pressurized reactor equipped with a stirrer, a nitrogen inlet, an ethylene inlet, an initiator addition inlet, and a delay solution addition inlet was charged with 2,400 g of vinyl acetate and 600 g of methanol. The temperature was raised to 60°C, and the system was then purged with nitrogen for 30 minutes while bubbling with nitrogen. Ethylene was then introduced so that the pressure inside the reactor was 0.50 MPa. The internal temperature of the reactor was adjusted to 60°C, and 1.0 g of 2,2'-azobisisobutyronitrile (AIBN) was then injected to initiate polymerization. During polymerization, the polymerization temperature was maintained at 60°C while adjusting the ethylene pressure. After 3 hours, the polymerization was terminated by cooling when the conversion reached 30%. Subsequently, saponification was carried out using the same method as in Example 1 and the conditions listed in Table 1 to produce the resin powder of Example 6 (PVA6 resin powder). Furthermore, a PVA6' resin powder was obtained under the same production conditions as for the PVA6 resin powder, but without introducing nitrogen. In the resin powder of PVA6', which is used as a standard for evaluating drying efficiency, the number of bubble-containing particles was less than 5. The resin powder of Example 6 (resin powder of PVA6) was measured and evaluated in the same manner as in Example 1. The results are shown in Tables 2 and 3. PVA6 and PVA6' are ethylene-modified PVAs, and the modification amount was １ Determined by H-NMR.

[0062] Example 7 A 3 L reactor equipped with a stirrer, reflux condenser, nitrogen inlet, comonomer dropping port, and initiator addition port was charged with 1,200 g of vinyl acetate, 300 g of methanol, and 0.71 g of monomethyl maleate (MMM), and the system was purged with nitrogen for 30 minutes while bubbling with nitrogen. Furthermore, a comonomer solution with a concentration of 10% by mass was prepared by dissolving MMM in methanol as a delay solution, and nitrogen was purged by bubbling with nitrogen gas. The reactor temperature was then increased, and when the internal temperature reached 60°C, 1.0 g of 2,2'-azobisisobutyronitrile (AIBN) was added to initiate polymerization. The delay solution was added dropwise to maintain a constant monomer composition (ratio of vinyl acetate and MMM) in the polymerization solution, and the polymerization was continued for 3 hours at 60°C. After cooling, the polymerization was terminated. Subsequently, saponification was carried out using the same method as in Example 1 and the conditions listed in Table 1, to produce the resin powder of Example 7 (PVA7 resin powder). Furthermore, a resin powder of PVA7' was obtained under the same manufacturing conditions as the resin powder of PVA7, but without introducing nitrogen. In the resin powder of PVA7', which serves as a standard for evaluating drying efficiency, the number of bubble-containing particles was less than 5. The resin powder of Example 7 (resin powder of PVA7) was measured and evaluated in the same manner as in Example 1. The results are shown in Tables 2 and 3. Note that PVA7 and PVA7' are MMM-modified PVAs, and the amount of modification was １ Determined by H-NMR.

[0063] [Example 8] A resin powder of Example 8 (a resin powder of PVA8) was produced under the same conditions as in Example 1, except that the amount of nitrogen introduced was changed to 0.05 L / min. The resin powder of Example 8 (a resin powder of PVA8) was measured and evaluated in the same manner as in Example 1. The results are shown in Tables 2 and 3. Note that the resin powder of PVA1', the same as in Example 1, was used as the reference resin powder in the evaluation of drying efficiency.

[0064] [Example 9] A resin powder of Example 9 (a resin powder of PVA9) was produced under the same conditions as in Example 2, except that the amount of nitrogen introduced was changed to 0.05 L / min. The resin powder of Example 9 (a resin powder of PVA9) was measured and evaluated in the same manner as in Example 1. The results are shown in Tables 2 and 3. Note that the resin powder of PVA2', the same as in Example 2, was used as the reference resin powder in the evaluation of drying efficiency.

[0065] The respective production methods in which resin powder was obtained by saponification without introducing nitrogen (the production methods for resin powders PVA1' to PVA7') correspond to comparative examples of the present invention.

[0066] As another comparative example, the resin powder of PVA1 obtained in Example 1 was sieved through a sieve with 0.075 mm openings to collect particles that passed through a 200 mesh, yielding a resin powder with an average particle size of 50 μm. When the wettability of this resin powder was evaluated, it was rated C, indicating that the particles were difficult to disperse in water, making it difficult to prepare an aqueous solution, and making it difficult to handle.

[0067]

[0068]

[0069]

[0070] As shown in Tables 2 and 3, the drying efficiency of each of the resin powders of Examples 1 to 9 was evaluated as A to D, indicating that the drying efficiency was improved. Among them, the resin powders of Examples 1 to 7, in which the average particle size of the resin powder was 100 to 2,000 μm and the number of bubble-containing particles in the resin powder was 10 or more, were evaluated as A to C, indicating that the drying efficiency was sufficiently improved. Furthermore, each of the resin powders of Examples 1 to 9 had an average particle size of 100 μm or more, and was excellent in wettability, was resistant to scattering, and was easy to handle.

[0071] The resin powder obtained by the method for producing a resin powder of the present invention has improved drying efficiency and good handleability, and therefore has excellent productivity and can be used for various applications such as synthetic fiber raw materials, film raw materials, emulsifying dispersants, adhesives, etc.

Claims

1. A method for producing a resin powder containing a vinyl alcohol polymer and having an average particle size of 100 μm or more, comprising a step of obtaining a solid material containing the vinyl alcohol polymer and having bubbles formed therein, wherein the bubbles are formed by introducing a gas into the system.

2. The method for producing a resin powder according to claim 1, wherein the gas is introduced when the vinyl alcohol polymer is obtained by saponifying a vinyl ester polymer.

3. The method for producing resin powder according to claim 1 or 2, wherein the gas is nitrogen.

4. A method for producing a resin powder according to claim 1 or claim 2, wherein the resin powder has an average particle size of 100 to 2,000 μm, and the number of particles having bubbles of 1 μm or more out of 100 particles randomly selected from particles having a particle size of 106 to 1,000 μm in the resin powder is 10 or more.

5. The method for producing a resin powder according to claim 1 or 2, further comprising a step of pulverizing the solid material into a resin powder having an average particle size of 100 μm or more.