Resin powder

Abstract

The present invention provides a resin powder which contains a vinyl alcohol-based polymer and has improved drying efficiency. The present invention specifically provides a resin powder which contains a vinyl alcohol-based polymer and has an average particle diameter of 100 to 2,000 μm, wherein the number of particles having air bubbles of 1 μm or more among 100 particles that are arbitrarily selected from the particles that have a particle diameter of 106 to 1,000 μm is 10 or more.

Description

Resin powder

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

[0002] Vinyl alcohol polymer (hereinafter also referred to as "PVA") is a synthetic resin obtained by saponifying polyvinyl ester (hereinafter also referred to as "PVAc"). PVA is water-soluble and is used for 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 and save energy during the production of resin powders, thereby reducing production costs, it is desirable to improve the drying efficiency of the resin powder. 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 cases of re-drying, it is desirable for the PVA resin powder to have high drying efficiency.

[0006] An object of the present invention is to provide a resin powder containing a vinyl alcohol polymer, which has improved drying efficiency.

[0007] The object is to provide: [1] a resin powder containing a vinyl alcohol polymer, having an average particle size of 100 to 2,000 μm, wherein 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; [2] the resin powder of [1], wherein the viscosity-average degree of polymerization of the vinyl alcohol polymer is 200 to 5,000 and the degree of saponification is 35 to 99.99 mol%; [3] the resin powder of [1] or [2], wherein the content of the particles having a particle size of 106 to 1,000 μm is 50 mass% or more; [4] the resin powder of any of [1] to [3], wherein the bulk density is 0.4 to 0.7 g / mL; [5] the resin powder of any of [1] to [4], wherein the product of the average particle size (μm) and the bulk density (g / mL) is 700 μm g / mL or less; [6] The resin powder according to any one of [1] to [5], which has an organic volatile content of 3.0 mass % or less.

[0008] According to the present invention, it is possible to provide a resin powder containing a vinyl alcohol polymer, which has improved drying efficiency.

[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] <Resin Powder> A resin powder according to one embodiment of the present invention contains a vinyl alcohol polymer (PVA: polyvinyl alcohol), has an average particle size of 100 to 2,000 μm, and has 10 or more particles having bubbles of 1 μm or larger among 100 particles randomly selected from particles having a particle size of 10 to 1,000 μm. Hereinafter, "particles having bubbles of 1 μm or larger" will also be referred to as "bubble-containing particles." "Bubbles of 1 μm or larger" refers to bubbles having a bubble size of 1 μm or larger, 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 larger (bubble-containing particles) among 100 particles randomly selected from particles having a particle size of 10 to 1,000 μm" will also be referred to as the "number of bubble-containing particles." 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 solely of a plurality of these particles (PVA particles).

[0011] The resin powder according to one embodiment of the present invention has improved drying efficiency compared to conventional resin powders. While the reason for this is not entirely clear, it is speculated that the resin powder contains a sufficient amount of bubble-containing particles, thereby increasing the 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 in the center of the particle to volatilize. In contrast, in the case of bubble-containing particles, the presence of 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 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 a sufficient amount of bubble-containing particles, resulting in a sufficiently increased surface area. Furthermore, the resin powder is thought to have good reactivity when undergoing post-modification, etc., because the surface area is sufficiently increased.

[0014] PVA is usually the main component of the resin powder according to one embodiment of the present invention. 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.

[0015] PVA is a polymer having vinyl alcohol units as monomer units. PVA is typically obtained by saponifying polyvinyl ester. 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 resin powder containing PVA with a high content of vinyl alcohol units, the advantage of improving drying efficiency is particularly pronounced. 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.

[0016] 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 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.

[0017] PVA may have other monomer units in addition to vinyl alcohol units and vinyl ester units. Examples of monomers (modified species) that provide the other monomer units 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, 1,4 hydroxyl group-containing vinyl ethers such as 3-butanediol 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.

[0018] 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%.

[0019] 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 resin powder containing a PVA with a high viscosity-average degree of polymerization, the advantage of improving drying efficiency is particularly easily achieved. The upper limit of the viscosity-average degree of polymerization is preferably 5,000, more preferably 4,500, and 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）

[0020] The resin powder according to one embodiment of the present invention 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.

[0021] The upper limit of the organic volatile content in the resin powder according to one embodiment of the present invention 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 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.

[0022] The upper limit of the volatile content in the resin powder according to one embodiment of the present invention 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.

[0023] The organic volatile content and the volatile content in the resin powder according to one embodiment of the present invention can be measured in accordance with JIS K6726: 1994. For example, when the volatile content in 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.

[0024] The lower limit of the average particle diameter of the resin powder according to one embodiment of the present invention 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 this 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 2,000 μm, preferably 1,500 μm, more preferably 1,000 μm, and even more preferably 850 μm. Setting the average particle diameter at or below this 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.

[0025] A resin powder according to one embodiment of the present invention contains particles having a particle diameter of 106 to 1,000 μm. The lower limit of the number of bubble-containing particles (bubble particle count) among 100 particles randomly selected from the particles having a particle diameter of 106 to 1,000 μm contained in the resin powder is 10, preferably 15, more preferably 20, even more preferably 30, and even more preferably 40, 45, or 50. Having the number of bubble-containing particles equal to or greater than the lower limit can improve drying efficiency, etc. The upper limit of the number of bubble-containing particles may be 100, 90, 80, 70, or 60. Note that bubble-containing particles can be obtained by introducing a gas into a slurry or solid containing PVA, for example, in the saponification process or a subsequent process, as described in detail below. The number of bubble-containing particles can be adjusted by, for example, the amount of gas introduced.

[0026] The lower limit of the average bubble size in the bubble-containing particles of the resin powder according to one embodiment of the present invention is 1 μm, preferably 10 μm, more preferably 30 μm, even more preferably 40 μm, and even more preferably 50 μm. By setting the average bubble size at or above this 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 this upper limit, it is possible to increase the productivity of the resin powder. The average bubble size is the average value of the size (bubble size) of bubbles of 1 μm or more contained in the bubble-containing particles. 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.

[0027] 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 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.

[0028] From the viewpoint of safety, etc., the gas contained in the bubbles of the bubble-containing particles preferably contains an inert gas. Furthermore, from the viewpoint of handleability, etc., the gas contained in the bubbles of the bubble-containing particles is preferably an inorganic substance. From these viewpoints and from the viewpoint of production costs, the gas contained in the bubbles of the bubble-containing particles is preferably air or nitrogen, and more preferably nitrogen. The nitrogen content in the gas contained in the bubbles of the bubble-containing particles is preferably 78 vol% or more, more preferably 80 vol% or more, and even more preferably 90 vol% or more.

[0029] In a resin powder according to one embodiment of the present invention, 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 with a nominal mesh size of 1,000 μm (16 mesh) and a sieve with a nominal mesh size of 106 μm (150 mesh).

[0030] In the resin powder according to one embodiment of the present invention, the bulk density 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 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.

[0031] In a resin powder according to one embodiment of the present invention, 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 equal to or less than the upper limit, the bulk density is low relative to the particle size. Therefore, the smaller the value of the product, the larger the surface area, which tends to improve drying efficiency. The lower limit of the product may be, for example, 100 μm g / mL, 150 μm g / mL, or 200 μm g / mL.

[0032] The resin powder according to one embodiment of the present invention 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, rewettable 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-resistant agents, smoothness improvers, surface gloss improvers, sealing agents, barrier agents, light resistance agents, water resistance agents, dye and 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

[0033] <Method for Producing Resin Powder> The method for producing the resin powder of the present invention is not particularly limited, but it is preferable to incorporate gas bubbles into particles during any of the general PVA production processes, such as polymerization, saponification, washing, drying, and pulverization. In this production method, it is preferable to introduce a gas into the system to obtain a solid product containing PVA and having gas bubbles formed therein. Note that "in the system" may refer to a liquid, gel, or solid mixture containing PVA. "In the system" may refer to a liquid, gel, or solid mixture containing at least one of the reactants and products used in producing PVA. For example, the 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. It is preferable to introduce a gas into the system during the saponification step. That is, it is preferable to introduce a gas into the reaction liquid when saponifying a vinyl ester polymer to obtain PVA.

[0034] 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 system or a continuous system can be used.

[0035] 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.

[0036] 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.

[0037] When polymerizing a vinyl ester monomer, other copolymerizable monomers can be copolymerized within the scope of the present invention. Examples of such other monomers include the monomers (modified species) that provide the other monomer units described above. The upper limit of the amount of these other monomers used varies depending on the purpose and application of the monomers, but is preferably 20 mol % and more preferably 10 mol % relative to the total amount of monomers. The upper limit of the amount of the other monomer units used may be 5 mol %, 3 mol %, 1 mol %, or 0.1 mol %. Meanwhile, the lower limit of the amount of the other monomer used may be, for example, 0.1 mol % or 1 mol %.

[0038] 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.

[0039] In the saponification step, a vinyl ester polymer is saponified in 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.

[0040] 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.

[0041] The gas introduced into the system, that is, the gas contained in the formed bubbles, is not particularly limited, and examples thereof include air, nitrogen, carbon dioxide, etc., with nitrogen being preferred.

[0042] The number of bubble-containing particles 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.

[0043] The PVA-containing solid obtained through the saponification step is usually washed, dried, and pulverized 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.

[0044] 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.

[0045] [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）

[0046] [Degree of Saponification of PVA] The degree of saponification of PVA (including modified PVA) was determined by the method described in JIS K6726:1994.

[0047] [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.

[0048] [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.

[0049] [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.

[0050] [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.

[0051] 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.

[0052] [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

[0053] [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

[0054] [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.

[0055] 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.

[0056] 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.

[0057] Comparative Example 1 A resin powder of Comparative Example 1 (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 Comparative Example 1 (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. The resin powder of PVA1', the same as in Example 1, was used as the reference resin powder in the evaluation of drying efficiency.

[0058] Comparative Example 2 A resin powder of Comparative Example 2 (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 Comparative Example 2 (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. The resin powder of PVA2', the same as in Example 2, was used as the reference resin powder in the evaluation of drying efficiency.

[0059]

[0060]

[0061]

[0062] As shown in Tables 2 and 3, the drying efficiency of each of the resin powders in Examples 1 to 7 was evaluated as A to C, which means that the drying efficiency was sufficiently improved.

[0063] The resin powder of the present invention has excellent productivity due to improved drying efficiency, and can be used in a variety of applications such as synthetic fiber raw materials, film raw materials, emulsifying dispersants, adhesives, etc.

Claims

1. A resin powder containing a vinyl alcohol polymer, having an average particle size of 100 to 2,000 μm, and having 10 or more particles with bubbles of 1 μm or larger out of 100 particles randomly selected from particles having a particle size of 106 to 1,000 μm.

2. The resin powder according to claim 1, wherein the vinyl alcohol polymer has a viscosity-average degree of polymerization of 200 to 5,000 and a degree of saponification of 35 to 99.99 mol %.

3. The resin powder according to claim 1 or 2, wherein the content of particles having a particle diameter of 106 to 1,000 μm is 50 mass % or more.

4. The resin powder according to claim 1 or 2, having a bulk density of 0.4 to 0.7 g / mL.

5. A resin powder according to claim 1 or 2, wherein the product of the average particle size (μm) and the bulk density (g / mL) is 700 μm·g / mL or less.

6. The resin powder according to claim 1 or 2, having an organic volatile content of 3.0 mass% or less.