Resin powder

Abstract

Provided is a resin powder having excellent reactivity and excellent properties of passing through a pipe or a silo. This resin powder contains a vinyl alcohol-based polymer, wherein the average particle diameter is 100-2,000 μm, the average value PA of the roundnesses P, which are represented by expression (1), of 50 particles arbitrarily extracted from particles having a particle diameter of 100-1,000 μm is 0.1-0.8, and the total of the methanol content and the methyl acetate content is at most 0.5 mass%. In expression (1), ri is the radius of curvature for each angle of a particle. R is the radius of the maximum inscribed circle of a particle. N is the number of angles of a particle. Here, when the number of angles of a particle is at least 9, the radii of curvature of eight angles in an order from the smallest radius of curvature are employed, and N is 8.

Description

[Technical Field]

[0001] The present disclosure relates to a resin powder. [Background technology]

[0002] Polyvinyl alcohol (PVA) is a synthetic resin obtained by saponifying polyvinyl ester. PVA is water-soluble and is used as a synthetic fiber material, a film material, an emulsifying dispersant, an adhesive, and more.

[0003] PVA-containing resin powders are usually obtained by pulverizing and drying a solid PVA obtained through saponification. For example, in Patent Document 1, PVA powder with a controlled particle size is obtained by changing the size of the sieve at the outlet of a pulverizer. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] International Publication No. 2015 / 186745 Summary of the Invention [Problem to be solved by the invention]

[0005] Resin powders are transported through pipes and packed into bags in factories, etc. However, during these processes, resin powders containing PVA can clog pipes, silos, etc. Resin powders containing PVA are prone to clogging, particularly in humid environments, possibly due to the water solubility and hygroscopicity of PVA.

[0006] Furthermore, the resin powder contains some residual organic solvent used in the manufacturing process, and the residual solvent may become a problem depending on the intended use.

[0007] The present invention has been made to solve the above problems, and an object of the present invention is to provide a resin powder that has excellent passability through pipes and silos and excellent reactivity. [Means for solving the problem]

[0008] The above purpose is [1] A resin powder containing a vinyl alcohol polymer, having an average particle size of 100 to 2,000 μm, in which 50 particles randomly selected from particles having a particle size of 100 to 1,000 μm have an average value PA of 0.1 to 0.8 for the roundness P represented by the following formula (1):

number

number

[0009] The resin powder of the present disclosure has excellent passability through pipes and silos and excellent reactivity. DETAILED DESCRIPTION OF THE INVENTION

[0010] <Resin powder> The resin powder of the present disclosure contains a vinyl alcohol polymer (PVA), has an average particle size of 100 to 2,000 μm, has an average value PA (hereinafter also referred to as "average roundness") of the roundness P expressed by the above formula (1) of 50 particles randomly selected from particles having a particle size of 100 to 1,000 μm, and is 0.1 to 0.8, and has a total methanol content and methyl acetate content of 0.5 mass% or less.

[0011] In this specification, a numerical range indicated using "to" means that the numerical values ​​before and after "to" are included as the lower and upper limits. In addition, in this specification, "ppm" indicates a content by mass.

[0012] The total content of methanol and methyl acetate in the resin powder of the present disclosure is 0.5% by mass or less, preferably 0.4% by mass or less, more preferably 0.3% by mass or less, even more preferably 0.2% by mass or less, and even more preferably 0.1% by mass or less. When the total content of methanol and methyl acetate is within the above range, for example, when the resin powder is reacted with a modified species to produce a modified PVA, methanol and methyl acetate are less likely to inhibit the reaction, resulting in good reactivity of the resin powder. The total content of methanol and methyl acetate in the resin powder of the present disclosure may be 0% by mass, or, from the viewpoint of production efficiency, may be 0.001% by mass or more, or 0.01% by mass or more.

[0013] Furthermore, the organic volatile content of the resin powder of the present disclosure is preferably 0.5% by mass or less, more preferably 0.4% by mass or less, even more preferably 0.3% by mass or less, even more preferably 0.2% by mass or less, and particularly preferably 0.1% by mass or less. The organic volatile content of the resin powder of the present disclosure varies depending on the production conditions of the resin powder, but may be organic substances used in the resin powder production process or by-products generated during the production process, such as methanol, methyl acetate, acetaldehyde, crotonaldehyde, acetic acid, etc. The organic volatile content may also be organic substances with a boiling point of 260°C or less. The organic volatile content of the resin powder of the present disclosure may be 0% by mass, or, from the viewpoint of production efficiency, may be 0.001% by mass or more, or 0.01% by mass or more.

[0014] In addition, another embodiment of the resin powder of the present disclosure contains a vinyl alcohol-based polymer, has an average particle size of 100 to 2,000 μm, has an average value PA of the roundness P represented by the above formula (1) of 50 particles randomly selected from particles having a particle size of 100 to 1,000 μm, and has a methanol content of 1,000 ppm or less.

[0015] In the resin powder of the above embodiment, the methanol content is preferably 500 ppm or less, more preferably 400 ppm or less, even more preferably 300 ppm or less, and even more preferably 200 ppm or less. When the methanol content is within the above range, the reactivity of the resin powder becomes better. The methanol content in the resin powder may be 0 ppm, or may be 1 ppm or more or 10 ppm or more from the viewpoint of production efficiency, etc.

[0016] In addition, in the resin powder of the above embodiment, the crotonaldehyde content is preferably 10 ppm or less, more preferably 5 ppm or less, even more preferably 4 ppm or less, even more preferably 3 ppm or less, particularly preferably 2 ppm or less, and even more particularly preferably 1 ppm or less. By having the crotonaldehyde content within the above range, the safety of the resin powder can be improved. The crotonaldehyde content in the resin powder may be 0 ppm, or may be 0.1 ppm or more from the viewpoint of production efficiency, etc.

[0017] Furthermore, from the viewpoint of improving the safety and reactivity of the resin powder, it is preferable that the total of the methanol content and the methyl acetate content in the resin powder of the present disclosure is 0.5 mass% or less, and the methanol content is 1,000 ppm or less, and in this case, it may be preferable that the total of the methanol content and the methyl acetate content, and the methanol content are each within the above-mentioned ranges.

[0018] PVA is typically the main component of the resin powder of the present disclosure. 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 of the present disclosure is preferably 50% by mass, more preferably 70% by mass, even more preferably 90% by mass, and even more preferably 99% by mass. The upper limit of the PVA content in the nonvolatile content of the resin powder of the present disclosure may be 100% by mass. Nonvolatile components other than PVA that may be contained in the resin powder of the present disclosure include resins other than PVA, additives such as surfactants and plasticizers, various compounds used during production, and by-products generated during production.

[0019] PVA is a polymer having vinyl alcohol units as monomer units. PVA is usually obtained by saponifying polyvinyl ester. The lower limit of the proportion of vinyl alcohol units in all monomer units in PVA is preferably 35 mol%, more preferably 50 mol%, even more preferably 70 mol%, and in some cases even more preferably 80 mol% or 90 mol%. By setting the proportion of vinyl alcohol units at or above the above lower limit, passability is improved, particularly in high humidity environments, and the resin powder of the present disclosure can be easily produced efficiently by a production method that involves crushing and surface treatment. On the other hand, the upper limit of the proportion of vinyl alcohol units may be 100 mol%, but in some cases it is preferably 99.99 mol%, and more preferably 99.00 mol%.

[0020] The lower limit of the saponification degree of PVA is preferably 35 mol%, more preferably 50 mol%, even more preferably 70 mol%, and in some cases, even more preferably 80 mol% or 90 mol%. By setting the saponification degree at or above the lower limit, passability is improved, particularly in high humidity environments, and the resin powder of the present disclosure can be more efficiently produced by a production method that involves crushing and surface treatment. On the other hand, the upper limit of the saponification degree may be 100 mol%, but 99.99 mol% is preferred, and 99.00 mol% is more preferred. The saponification degree is a value measured by the method described in JIS K6726:1994.

[0021] PVA may contain other monomer units in addition to vinyl alcohol units and vinyl ester units. Examples of monomers 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; 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; and hydroxy groups such as ethylene glycol vinyl ether, 1,3-propanediol vinyl ether, and 1,4-butanediol vinyl ether. Examples of the hydroxyl group-containing vinyl ether include vinyl ethers, 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, and methacrylic acid esters are preferred.

[0022] The proportion of the other monomer units in the total monomer units in the PVA may preferably be 20 mol% or less, more preferably 10 mol% or less. On the other hand, the proportion of the other monomer units may be, for example, 0.1 mol% or more, or even 1 mol% or more. Furthermore, the proportion of ethylene units in the total monomer units in the PVA may preferably be less than 10 mol%, more preferably 1 mol% or less, and even more preferably 0 mol%.

[0023] The viscosity-average degree of polymerization of the PVA is not particularly limited, but is preferably 200 or more, more preferably 250 or more, even more preferably 400 or more, and particularly preferably 600 or more. The viscosity-average degree of polymerization is preferably 5,000 or less, more preferably 4,500 or less, and even more preferably 3,500 or less. By setting the viscosity-average degree of polymerization within the above range, industrial production of resin particles having the average roundness described below becomes easier. The viscosity-average degree of polymerization is a value measured in accordance with JIS K6726:1994. That is, the viscosity-average degree of polymerization can be calculated from the intrinsic viscosity [η] (unit: liter / g) measured in water at 30°C after resaponifying PVA to a saponification degree of 99.5 mol% or more and purifying it, using the following formula: Viscosity average degree of polymerization=([η]×10 4 / 8.29) (1 / 0.62)

[0024] The PVA is preferably not graft-copolymerized. Furthermore, the resin powder of the present disclosure preferably does not contain graft-copolymerized PVA. However, the PVA may be modified with one or more graft-copolymerizable monomers, as long as the effects of the resin powder of the present disclosure are not significantly impaired. Graft copolymerization can be carried out on at least one of polyvinyl ester and PVA obtained by saponifying the polyvinyl ester. The proportion of monomer units derived from graft-copolymerizable monomers in the polyvinyl ester or PVA is preferably 5 mol % or less relative to the total monomer units constituting the polyvinyl ester or PVA.

[0025] The lower limit of the average particle diameter of the resin powder of the present disclosure is 100 μm, preferably 150 μm, and more preferably 300 μm. By setting the average particle diameter at or above the lower limit, dust explosions are less likely to occur, thereby improving safety. 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. By setting the average particle diameter at or below the upper limit, the fluidity of the particles can be improved, thereby improving passability.

[0026] The average particle size of the resin powder is a value measured in accordance with the method described in JIS K7369:2009.

[0027] The average roundness (PA) of 50 particles randomly selected from particles having a particle diameter of 100 to 1,000 μm is 0.1 or more, preferably 0.15 or more, more preferably 0.2 or more, even more preferably more than 0.2, particularly preferably 0.22 or more, and sometimes extremely preferably 0.25 or more. By setting the average roundness at or above the lower limit, the fluidity of the particles can be improved, and passability can be enhanced. On the other hand, the average roundness is 0.8 or less, preferably 0.7 or less. By setting the average roundness at or below the upper limit, the productivity of the resin powder of the present disclosure can be increased. Furthermore, resin powders having an average roundness at or below the upper limit can be effectively produced by a production method that involves crushing and surface treatment.

[0028] The average roundness of a resin powder can be determined by the following method. 50 particles are randomly extracted from particles having a particle diameter of 100 to 1,000 μm (particle diameter 106 to 1,000 μm) in the resin powder. Particles having a particle diameter of 100 to 1,000 μm can be selected as particles that pass through a sieve with a nominal opening of 1,000 μm (16 mesh) but do not pass through a sieve with a nominal opening of 106 μm (150 mesh) in sieving. The mechanical sieving can be performed, for example, by the method described in JIS K7369:2009. For one extracted particle, the radius of curvature r iThe eight smallest corners (if there are fewer than eight corners, all of them) are extracted, and the radius of curvature r i Also, based on the projection drawing that maximizes the apparent area, measure the radius R of the largest inscribed circle of the particle. Let N be the number of corners the particle has (if the number of corners the particle has is 9 or more, N is 8). i Based on and R, the roundness P of a particle can be calculated using the following formula (1): A low roundness P indicates that the particle has many sharp corners, while a high roundness P indicates that the particle is rounded.

[0029]

number

[0030] The roundness P is measured for the 50 extracted particles, and the average value PA of the roundness P of these 50 particles is calculated. This average value PA is the average roundness.

[0031] The resin powder of the present disclosure, in particular, has an average roundness within the above-mentioned specific range, and therefore, compared to conventional resin powders, is less likely to clog pipes or silos when transported through pipes or packed into bags through a silo, etc. The reason for this is not entirely clear, but it is presumed that the rounded corners of each particle reduce friction when particles come into contact with each other or with a wall surface, etc., resulting in improved particle flowability.

[0032] In the resin powder of the present disclosure, the content of particles having a particle diameter of 100 to 1,000 μm (particle diameter 106 to 1,000 μm) is not particularly limited, but is preferably 50% by mass or more, more preferably 55% by mass or more, and even more preferably 60% by mass or more. Meanwhile, the upper limit of the content of particles having a particle diameter of 100 to 1,000 μm may be 100% by mass. By keeping the content of particles having a particle diameter of 100 to 1,000 μm within the above range, the passability is further improved. The content of particles having a particle diameter of 100 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).

[0033] The resin powder of the present disclosure preferably satisfies the following formula (2), and more preferably satisfies the following formula (2) and has an average particle size of 100 to 1,000 μm. In such a case, permeability can be further improved, particularly under high humidity conditions. According to the inventor's studies, the higher the average roundness, the higher the permeability tends to be. On the other hand, particularly under high humidity conditions, resin powders containing PVA with a low degree of saponification tend to have a lower permeability, possibly due to the influence of hygroscopicity, etc. Therefore, by setting the product (PA×S) of the average value PA (average roundness) of the roundness P and the degree of saponification S of the PVA to a predetermined value or more, better permeability can be exhibited even under high humidity conditions. PA×S≧18 (2) In formula (2), PA is the average value of the roundness P. S is the degree of saponification (mol %) of PVA.

[0034] The product (PA×S) of the average value PA (average roundness) of the roundness P and the degree of saponification S of PVA is more preferably 19 or more, and even more preferably 20 or more. On the other hand, the upper limit of this product (PA×S) may be, for example, 80 or 60.

[0035] The angle of repose of the resin powder of the present disclosure, measured after conditioning it for one week in an atmosphere at 20°C and 30% RH, is preferably less than 38°, more preferably less than 35°. Furthermore, the angle of repose of the resin powder of the present disclosure, measured after conditioning it for one week in an atmosphere at 20°C and 65% RH, is preferably less than 40°, more preferably less than 38°. When the angle of repose of the resin powder of the present disclosure is this low, it can exhibit excellent passability even in a high-humidity environment. The lower limit of these angles of repose may be, for example, 25° or 30°. The angle of repose of the resin powder is a value measured according to the method described in JIS R 9301-2-2:1999.

[0036] The average area circularity of the resin powder of the present disclosure may be less than 0.80 or less than 0.76. Typically, when a resin powder containing PVA is produced through a pulverization process, the area circularity is often less than 0.80, and such conventional resin powders tend to have poor passability through pipes and silos. Therefore, when the technology of the present disclosure is applied to a resin powder with an average area circularity of less than 0.80, the effect of improving passability through pipes and silos is particularly significant. The lower limit of the average area circularity of the resin powder of the present disclosure may be, for example, 0.1 or 0.2. The area circularity is a numerical value expressed by the following formula (3), and its average value can be the average value of 50 arbitrarily selected particles. The 50 particles may be selected from particles with a particle diameter of 10 to 1,000 μm, or may be particles selected when calculating the roundness P. The area circularity can be calculated by image analysis using, for example, a particle image analyzer "Morphologi G3S" manufactured by Malvern. Area circularity = 4 x π x area / (perimeter) 2 ···(3)

[0037] The resin powder of the present disclosure can be used in a variety of applications, including, but not limited to, the following examples. (1) Vinyl chloride dispersant Use: Dispersion stabilizer and dispersing aid for suspension polymerization of vinyl chloride and vinylidene chloride (2) Coating agent applications: sizing agents, textile processing agents, leather finishing agents, paints, anti-fogging agents, metal corrosion inhibitors, zinc plating brighteners, anti-static agents (3) Adhesive and binder applications: adhesives, pressure sensitive adhesives, re-moistening adhesives, various binders, additives for cement and mortar (4) Dispersion stabilizer Applications: Dispersion stabilizer for organic and inorganic pigments in paints and adhesives, dispersion stabilizer for emulsion polymerization of various vinyl compounds, post-emulsifier for bitumen, etc. (5) Paper processing applications: paper strength enhancers, oil and solvent resistance agents, smoothness improvers, surface gloss improvers, sealing agents, barrier agents, light resistance agents, water resistance agents, dye and developer dispersants, adhesive strength improvers, binders (6) Agricultural uses: pesticide binders, pesticide spreaders, agricultural coating agents, soil conditioners, erosion inhibitors, pesticide dispersants (7) Medical and cosmetic applications: granulation binders, coating agents, emulsifiers, patches, binders, film preparation base materials, film-forming agents (8) Viscosity modifier Uses: Thickener, rheology modifier (9) Flocculant Use: Flocculant for suspended and dissolved matter in water, metal flocculant (10) Film applications: water-soluble film, polarizing film, barrier film, textile packaging film, seed protection sheet, vegetation sheet, seed tape, moisture-absorbing film (11) Molded product applications: fibers, pipes, tubes, leak-proof membranes, water-soluble fibers for chemical lace, sponges (12) Resin raw material applications: raw material for polyvinyl butyral, raw material for photosensitive resin, raw material for graft polymer, raw material for various gels (13) Post-reaction applications: Post-reaction applications with low molecular weight organic compounds, high molecular weight organic compounds, and inorganic compounds

[0038] <Method for producing resin powder> The method for producing the resin powder of the present disclosure is not particularly limited, but the following method is preferred. A step (step B) of obtaining a coarse powder by pulverizing a resin solid containing PVA; A step of processing the surface of the coarse powder (step C) Equipped with.

[0039] The method for producing a resin powder according to the present disclosure may further include a step (step A) of synthesizing PVA to obtain a resin solid containing PVA.

[0040] (Process A) Step A can include, for example, a polymerization step, a saponification step, and the like.

[0041] 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 performed without a solvent and solution polymerization performed using a solvent such as alcohol are preferred, with solution polymerization in the presence of a lower alcohol being 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 using bulk polymerization or solution polymerization, either a batch or continuous reaction system can be used.

[0042] Examples of the vinyl ester monomer 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.

[0043] 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 temperature in the range of 5°C to 200°C is appropriate.

[0044] When polymerizing a vinyl ester monomer, a copolymerizable monomer can be copolymerized within the scope of the present invention. During polymerization of the vinyl ester monomer, 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. Generally, a range of 0.1 to 10% by mass of the vinyl ester used is preferred.

[0045] In the saponification step, a vinyl ester polymer is saponified in an alcohol solution using an alkali or acid 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, or an acidic catalyst such as p-toluenesulfonic acid. 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. 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 a basic catalyst, as this is simple and convenient.

[0046] The saponification step can be carried out using a belt reactor, kneader reactor, tower reactor, or the like. A resin solid containing PVA is obtained through the saponification step. The PVA content of the nonvolatile content in the resin solid is, for example, 50% by mass or more, preferably 70% by mass or more, more preferably 90% by mass or more, and even more preferably 99% by mass or more. The nonvolatile content in this resin solid may essentially be composed mainly of PVA, but may also contain impurities such as sodium acetate, by-products, and the like.

[0047] (Process B) In step B, the resin solid containing PVA is pulverized. This results in a coarse powder containing PVA. The pulverization can be carried out using a known pulverizer. The pulverizer is preferably a device capable of controlling the degree of pulverization, such as the pulverization intensity, in order to adjust the average particle size of the resulting coarse powder and ultimately the final resin powder. In addition to adjusting the pulverization intensity, the average particle size of the resulting coarse powder can also be controlled by the processing time. The resulting coarse powder may be subjected to a second saponification treatment. The resulting coarse powder may also be subjected to a washing treatment to reduce impurities such as sodium acetate and by-products, and a drying treatment to reduce volatile content. The resin solid before pulverization may also be subjected to a washing treatment or a drying treatment.

[0048] (Process C) In step C, the surface of the coarse powder is processed (surface processing treatment). When a resin solid containing PVA is crushed, the resulting coarse powder has very sharp corners. Therefore, step C rounds the corners, allowing for efficient production of a resin powder with an average roundness within a specified range.

[0049] The equipment used in step C is not particularly limited as long as it can polish the surface of the coarse powder, but examples include a rotary kiln in which a container filled with powder rotates and the powder particles come into contact with each other, thereby polishing the surface; a planetary motion mixer that can impart three-dimensional motion to the contents using screw blades that rotate and revolve within the container; and a mixer in which paddles or screws rotate within the container, polishing the internal powder through this rotation. Examples of mixers include high-speed mixers, Henschel mixers, turbulizers, Loedige mixers, Pam Amex mixers, ribbon mixers, and Nauta mixers. Among these, mixers are preferred from the viewpoint of processing efficiency, and turbulizers and Loedige mixers are more preferred.

[0050] From the viewpoint of reducing the content of methanol and methyl acetate in the resin powder, the surface treatment in step C is preferably carried out at a processing temperature of higher than 30°C, more preferably 50°C or higher, even more preferably 80°C or higher, even more preferably 100°C or higher, and in some cases particularly preferably 110°C or higher. The processing temperature is preferably 200°C or lower, more preferably 150°C or lower. The processing time for the surface treatment is not particularly limited, but is preferably 1 hour or longer and 13 hours or shorter. The surface treatment is preferably carried out in a nitrogen atmosphere.

[0051] The method for producing a resin powder according to the present disclosure may further include a sieving step for adjusting the average particle size, etc. After step C, a washing treatment or a drying treatment may be performed. [Example]

[0052] 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 evaluation methods used in the following examples and comparative examples are shown below.

[0053] [Viscosity average degree of polymerization of PVA] The viscosity-average degree of polymerization of PVA was measured in accordance with JIS K6726: 1994. Specifically, when the degree of saponification of PVA was less than 99.5 mol%, it was saponified until the degree of saponification reached 99.5 mol% or more, and the viscosity-average degree of polymerization of the resulting PVA was calculated by the following formula using the intrinsic viscosity [η] (liter / g) measured in water at 30°C. Viscosity average degree of polymerization=([η]×10 4 / 8.29) (1 / 0.62)

[0054] [Saponification degree of PVA] The degree of saponification of PVA was determined by the method described in JIS K6726:1994.

[0055] [Average particle size of resin powder and content of particles with a particle size of 100 to 1,000 μm] Using a JIS standard sieve, the average particle size of the resin powder and the content of particles with a particle size of 100 to 1,000 μm (particle size 106 to 1,000 μm) were determined according to the method described in JIS K7369:2009.

[0056] [Average roundness of resin powder] Particles with a particle diameter of 100 to 1,000 μm (particle diameter 106 to 1,000 μm) were selected by the above-mentioned sieving, and 50 particles were randomly selected from these particles. The radius of curvature r was measured for these particles based on images taken at a magnification of 100 times using a digital microscope VHX-900 manufactured by Keyence Corporation. i The radius R of the largest inscribed circle was calculated, and the roundness P of each particle was determined. The average value PA of the roundness P of 50 particles was calculated as the average roundness. PA x S was calculated from the product of the average roundness (PA) and the degree of saponification (S).

[0057] [Repose angle of resin powder] The resin powder was conditioned for one week in an atmosphere of 20°C and 30% RH or 20°C and 65% RH. Thereafter, the angle of repose of the resin powder was measured using a multi-tester MT-1001 manufactured by Seishin Enterprise Co., Ltd. The angle of repose was measured according to the method described in JIS 9301-2-2:1999. The inventors have confirmed that the smaller the angle of repose of a resin powder, the better its passability through pipes and silos.

[0058] [Total of methanol content and methyl acetate content of resin powder] The total of the methanol content and the methyl acetate content of the resin powder is the sum of the methanol content and the methyl acetate content measured by the following method using headspace gas chromatography.

[0059] <Creating a calibration curve> Using isopropanol as an internal standard, three aqueous solutions with known methanol contents and three aqueous solutions with known methyl acetate contents were prepared. Measurements were performed using a gas chromatograph (GC-2014, Shimadzu Corporation) equipped with a headspace sampler (HS20, Shimadzu Corporation) to create a calibration curve. <Measurement of methanol and methyl acetate content in resin powder> Distilled water was taken to the markings of a 1,000 mL volumetric flask, and 0.1 mL of the internal standard solution, isopropanol, was added using a measuring pipette. The mixture was stirred thoroughly, and this liquid was designated the "solution." Next, 500 mg of resin powder was weighed into a vial for headspace gas chromatography measurement. After adding a stirrer, 10 mL of the solution was measured using a volumetric pipette and poured into the vial. The cap was attached to the vial and tightened until it locked. The vial was then placed on a hot stirrer, and the resin powder sample was heated and dissolved. After visually confirming that the resin powder was completely dissolved, headspace gas chromatography measurement was performed. The methanol and methyl acetate contents (mass%) in the resin powder were calculated from the calibration curves prepared as described above, and the values ​​were added together.

[0060] [Measurement of crotonaldehyde content] The crotonaldehyde content (ppm) was measured using headspace gas chromatography in the same manner as above.

[0061] [Decrease in degree of saponification during re-acetylation reaction] The resin powder was subjected to a reacetylation reaction by adding 20 mol% of acetic anhydride and 400 mol% of pyridine relative to the vinyl alcohol units of the resin powder, and the degree of saponification of the powder obtained after the reaction was determined by the method described in JIS K6726:1994. The difference between the degrees of saponification before and after the reaction (degree of saponification before the reaction - degree of saponification after the reaction) was defined as the decrease in the degree of saponification. The greater the decrease in the degree of saponification, the more excellent the reactivity of the resin powder.

[0062] [Example 1] A 250 L reactor equipped with a stirrer, reflux condenser, nitrogen inlet, and initiator addition port was charged with 112.5 kg of vinyl acetate and 37.5 kg of methanol (75% vinyl acetate by mass: 25% methanol by mass). The system was then purged with nitrogen for 30 minutes while bubbling with nitrogen. The reactor was heated, and when the internal temperature reached 60°C, 35 g of 2,2'-azobisisobutyronitrile (AIBN) was added to initiate polymerization. The polymerization was terminated by cooling when the conversion reached 50%. The solids concentration at the time of termination was 37.0%. Subsequently, unreacted vinyl acetate monomer was removed at 30°C under reduced pressure with occasional addition of methanol, yielding a methanol solution of polyvinyl acetate (PVAc) (35% concentration). Furthermore, 54.05 kg of a methanol solution of PVAc (20 kg of PVAc in solution) was prepared by adding methanol to the PVAc solution. To this solution, 1.86 kg 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 PVAc was 0.02 mol%). Approximately 1 minute after the addition of the alkaline solution, a gel-like substance (resin solid) was formed, which was then crushed in a grinder. 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 neutralization was confirmed using a phenolphthalein indicator, the white solid was filtered off, and 200 kg of methanol was added and left to wash at room temperature for 3 hours. This washing procedure was repeated three times, and the white solid obtained by centrifugal dewatering was left in a dryer at 65 °C for 2 days to obtain a crude powder of PVA1. The degree of polymerization of PVA1 was 1,700, and the degree of saponification was 98.5 mol%. Next, the PVA1 coarse powder was loaded into a Lödige mixer "FKM130D" equipped with a Becker-type shovel manufactured by Chuo Kiko Co., Ltd., and subjected to a surface treatment treatment in a nitrogen atmosphere at 120°C and a rotation speed of 160 rpm for 2 hours. This yielded resin powder 1 with an average particle size of 630 μm, an average roundness of 0.27, and a total methanol content and methyl acetate content of 0.48 mass%.

[0063] [Examples 2 to 3, 6, and 7] Resin powders 2 to 3, 6 and 7 were obtained in the same manner as in Example 1 except that the conditions shown in Table 1 were used.

[0064] [Example 4] A 250 L reactor equipped with a stirrer, reflux condenser, nitrogen inlet, comonomer dropping port, and initiator addition port was charged with 120.0 kg of vinyl acetate and 30.0 kg of methanol (80% by mass of vinyl acetate: 20% 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, 2.5 kg of acetaldehyde and 35 g of 2,2'-azobisisobutyronitrile (AIBN) were added to initiate polymerization. The polymerization was terminated by cooling when the conversion reached 50%. The solids concentration at the time of termination of polymerization was 39.3%. The subsequent saponification step and subsequent steps were carried out under the conditions listed in Table 1. The same procedure as in Example 1 was followed, except that PVA4 resin powder 4 was obtained.

[0065] [Example 5] Resin powder 5 of PVA5 was obtained in the same manner as in Example 2, except that the pulverization using the pulverizer was carried out more coarsely than in Example 2.

[0066] [Examples 8 to 11] Resin powders 8 to 11 of PVA 8 to 11, etc. were obtained in the same manner as in Example 2, except that the surface treatment was performed under the surface treatment conditions (apparatus, treatment temperature and treatment time) shown in Table 1 instead of using a Loedige mixer.

[0067] [Comparative Example 1] Resin powder 1' of PVA1' was obtained in the same manner as in Example 2, except that the pulverization using the pulverizer was carried out more coarsely than in Example 5.

[0068] Comparative Example 2 Resin powder 2' of PVA2' was obtained in the same manner as in Example 2, except that no surface treatment was carried out.

[0069] Comparative Example 3 Resin powder 3' of PVA 3' was obtained in the same manner as in Example 2, except that the surface treatment was carried out at room temperature (30°C).

[0070] Table 2 shows the degree of polymerization, degree of saponification (S), total methanol content and methyl acetate content (total content of methanol + methyl acetate), methanol content and crotonaldehyde content of each PVA obtained in Examples 1 to 11 and Comparative Examples 1 to 3, as well as the average roundness (PA), average particle size, content of particles with a particle size of 100 to 1,000 μm, and the product PA×S of the average roundness (PA) and degree of saponification (S) of each resin powder.

[0071] [evaluation] For each of the resin powders obtained in Examples 1 to 11 and Comparative Examples 1 to 3, the angle of repose was measured using the method described above after humidity conditioning in an atmosphere of 20°C and 30% RH and 20°C and 65% RH. Furthermore, the resin powders were subjected to a reacetylation reaction, and the decrease in the degree of saponification of the resin powders was measured. The respective results are shown in Table 2.

[0072] [Table 1]

[0073] [Table 2]

[0074] As shown in Table 2, the resin powders of Examples 1 to 11 had angles of repose of less than 38° after humidity conditioning in an atmosphere with a humidity of 30% RH, and angles of repose of less than 40° in an environment with a humidity of 65% RH. Therefore, it was confirmed that the resin powders of Examples 1 to 11 had small angles of repose and excellent passability through pipes and silos. Furthermore, the decrease in the degree of saponification during the re-acetylation reaction in Examples 1 to 11 was 6 mol% or more, suggesting that acetic anhydride reacted efficiently and exhibited excellent reactivity. Furthermore, Examples 6, 9 to 11, which had a methanol content of 1,000 ppm or less, exhibited particularly good reactivity. On the other hand, the resin powder of Comparative Example 1, which had a large average particle size, had a large angle of repose, resulting in poor passability through pipes and silos. Furthermore, the resin powder of Comparative Example 2, which had a small average roundness, a high total content of methanol and methyl acetate, and a high methanol content, had a large angle of repose, resulting in poor passability through pipes and silos, a small decrease in the degree of saponification during the re-acetylation reaction, and poor reactivity. Furthermore, the total of the methanol content and the methyl acetate content was high, and Comparative Example 3, which had a high methanol content, had excellent processability but poor reactivity. [Industrial Applicability]

[0075] The resin powder of the present disclosure can be used for various 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, having an average value PA of 0.1 to 0.8 for the roundness P expressed by the following formula (1) of 50 particles randomly selected from particles having a particle size of 100 to 1,000 μm, and having a total methanol content and methyl acetate content of 0.5 mass% or less: [Equation 1] (In formula (1), r ｉ is the radius of curvature for each corner of the particle. R is the radius of the largest inscribed circle of the particle. N is the number of corners the particle has. However, if the number of corners of a particle is 9 or more, the radii of curvature of the 8 corners in order of smallest radius of curvature are used, and N is 8.)

2. 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. 3. The resin powder according to claim 1, which satisfies the following formula (2) and has an average particle size of 100 to 1,000 μm: PA × S≧18 (2) (In formula (2), S is the degree of saponification (mol %) of the vinyl alcohol polymer.)

4. 4. The resin powder according to claim 1, wherein the content of particles having a particle diameter of 100 to 1,000 μm is 50% by mass or more.

5. A resin powder containing a vinyl alcohol polymer, having an average particle size of 100 to 2,000 μm, having an average value PA of 0.1 to 0.8 of roundness P, expressed by the following formula (1), of 50 particles randomly selected from particles having a particle size of 100 to 1,000 μm, and having a methanol content of 1,000 ppm or less: [Equation 2] (In formula (1), r ｉ is the radius of curvature for each corner of the particle. R is the radius of the largest inscribed circle of the particle. N is the number of corners the particle has. However, if the number of corners of a particle is 9 or more, the radii of curvature of the 8 corners in order of smallest radius of curvature are used, and N is 8.)

6. 6. The resin powder according to claim 5, wherein the content of crotonaldehyde is 10 ppm or less.

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