Dispersant for suspension polymerization, method for producing dispersant for suspension polymerization, and method for producing vinyl polymer particles

Abstract

Provided are a dispersant for suspension polymerization that can produce vinyl polymer particles with a small particle size with a small addition amount, a method for producing such a dispersant for suspension polymerization, and a method for producing vinyl polymer particles using such a dispersant for suspension polymerization. The dispersant for suspension polymerization is a powder comprising a vinyl alcohol polymer having a structure represented by the following formula 1, and an average particle size of 100 μm or more and 2,000 μm or less, wherein the powder contains particles A having a particle size of 500 μm or more and particles B having a particle size of 220 μm or less, and the ratio (A / B) of the absorbance A of a 0.1% by mass aqueous solution of the particles A at an optical path length of 10 mm and a wavelength of 320 nm to the absorbance B of a 0.1% by mass aqueous solution of the particles B at an optical path length of 10 mm and a wavelength of 320 nm is 0.7 or more and 1.2 or less. In formula 1, R 1 is an alkyl group having 1 to 4 carbon atoms. n is an integer of 1 to 10. * represents a site bonded to a portion other than the structure represented by the above formula 1 in the above vinyl alcohol polymer.

Description

Technical Field

[0001] The present invention relates to a dispersant for suspension polymerization, a method for producing the dispersant for suspension polymerization, and a method for producing vinyl polymer particles. Background Art

[0002] In the industrial production of vinyl polymers such as vinyl chloride resins, suspension polymerization is widely practiced, in which vinyl compounds such as vinyl chloride are dispersed in an aqueous medium in the presence of a dispersant (sometimes also referred to as a dispersion stabilizer), and then polymerized using an oil-soluble catalyst. In general, factors that control the quality of vinyl polymers obtained by suspension polymerization of vinyl compounds include polymerization rate, water-monomer ratio, polymerization temperature, type and amount of oil-soluble catalyst, type of polymerization tank, stirring speed of the contents in the polymerization tank, type of dispersant, etc. Among them, the type of dispersant has a great influence.

[0003] The properties required of a dispersant for suspension polymerization of a vinyl compound include: [1] obtaining vinyl polymer particles stably with a small amount of addition; [2] obtaining vinyl polymer particles with high uniformity and porosity. Vinyl polymer particles with high uniformity and porosity have high plasticizer absorption and excellent processability. In the past, as a dispersant for suspension polymerization of a vinyl compound, in addition to cellulose derivatives such as methyl cellulose and carboxymethyl cellulose, partially saponified polyvinyl alcohol and the like have been used alone or in appropriate combination.

[0004] As a dispersant for suspension polymerization containing polyvinyl alcohol, Patent Document 1 describes "a dispersion stabilizer for suspension polymerization of a vinyl compound containing polyvinyl alcohol, wherein the ratio of the weight average degree of polymerization to the number average degree of polymerization is 3.0 or less, the polyvinyl alcohol has a carbonyl group in the molecule, and has a vinylene group adjacent thereto, the absorbance coefficients of a 0.1% aqueous solution of the polyvinyl alcohol at 280 nm and 320 nm based on an ultraviolet absorption spectrum are 0.30 or more and 0.15 or more, respectively, and the ratio of the absorbance coefficient at 320 nm to 280 nm is 0.30 or more". Patent Document 2 describes "a dispersion stabilizer for suspension polymerization of vinyl chloride containing polyvinyl alcohol, wherein the degree of saponification of the polyvinyl alcohol is 75 to 85 mol%, the absorbance of a 0.1 wt % aqueous solution at a wavelength of 280 nm is 0.1 or more, the content of carboxyl groups is 0.01 to 0.15 mol%, and the cloud point of a 0.1 wt % aqueous solution is 50° C. or more".

[0005] Prior art literature

[0006] Patent Literature

[0007] Patent Document 1: Japanese Patent Application Laid-Open No. 61-108602

[0008] Patent document 2: Japanese Patent Application Laid-Open No. 5-105702. Summary of the invention

[0009] Problems to be solved by the invention

[0010] Conventional dispersants for suspension polymerization (dispersion stabilizers for suspension polymerization) may not always provide satisfactory performance in accordance with the requirements [1] and [2] above. In particular, when the amount of dispersant added is small, it is difficult to stably carry out suspension polymerization, and there is a disadvantage that the particle size of the obtained vinyl polymer particles becomes larger.

[0011] The present invention is made to solve the above-mentioned problems, and its purpose is to provide a dispersant for suspension polymerization that can produce vinyl polymer particles with a small particle size with a small addition amount, a method for producing such a dispersant for suspension polymerization, and a method for producing vinyl polymer particles using such a dispersant for suspension polymerization.

[0012] Solutions for solving problems

[0013] The above object is achieved by providing any one of the following solutions.

[0014] [1] A dispersant for suspension polymerization, comprising a vinyl alcohol polymer having a structure represented by the following formula 1, and having an average particle size of 100 μm or more and 2,000 μm or less, the powder comprising particles A having a particle size of 500 μm or more and particles B having a particle size of 220 μm or less, wherein the ratio (A / B) of the absorbance A of a 0.1% by mass aqueous solution of the particles A at an optical path length of 10 mm and a wavelength of 320 nm to the absorbance B of a 0.1% by mass aqueous solution of the particles B at an optical path length of 10 mm and a wavelength of 320 nm is 0.7 or more and 1.2 or less;

[0015] [Chemistry 1]

[0016]

[0017] In the above formula 1, R 1 is an alkyl group having 1 to 4 carbon atoms. n is an integer of 1 to 10. * represents a site bonded to a portion other than the structure represented by the above formula 1 in the above vinyl alcohol polymer.

[0018] [2] The dispersant for suspension polymerization according to [1], wherein an average value PA of the roundness P represented by the following formula 2 of 50 particles randomly selected from particles having a particle size of 100 μm or more and 1,000 μm or less is 0.1 or more and 0.8 or less;

[0019] [Mathematical formula 1]

[0020]

[0021] In the above formula 2, r i is the radius of curvature of each corner of the particle. R is the radius of the largest inscribed circle of the particle. N is the number of corners of the particle. When the number of corners of the particle is 9 or more, the radius of curvature of the 8th corner in the order of the radius of curvature from small to large is used, and N is set to 8.

[0022] [3] The dispersant for suspension polymerization according to [1] or [2], wherein the vinyl alcohol polymer has a viscosity average degree of polymerization of 500 to 1,500, and a saponification degree of 60 mol % to 90 mol %;

[0023] [4] The dispersant for suspension polymerization according to any one of [1] to [3], wherein the absorbance of a 0.1 mass % aqueous solution of the vinyl alcohol polymer at an optical path length of 10 mm and a wavelength of 280 nm is 0.05 or more, and the absorbance at an optical path length of 10 mm and a wavelength of 320 nm is 0.05 or more;

[0024] [5] The dispersant for suspension polymerization according to any one of [1] to [4], wherein the content of particles having a particle size of 100 μm or more and 1,000 μm or less is 50% by mass or more;

[0025] [6] The method for producing a dispersant for suspension polymerization according to any one of [1] to [5], comprising:

[0026] a step of obtaining a coarse powder by pulverizing a resin solid containing a vinyl alcohol polymer; and

[0027] A step of processing the surface of the coarse powder while heating it;

[0028] [7] A method for producing vinyl polymer particles, comprising the step of subjecting a vinyl compound to suspension polymerization using the dispersant for suspension polymerization described in any one of [1] to [5].

[0029] Effects of the Invention

[0030] According to the present invention, there are provided a dispersant for suspension polymerization capable of producing vinyl polymer particles having a small particle size with a small addition amount, a method for producing such a dispersant for suspension polymerization, and a method for producing vinyl polymer particles using such a dispersant for suspension polymerization. DETAILED DESCRIPTION

[0031] <Dispersant for suspension polymerization>

[0032] The dispersant for suspension polymerization of the present invention (hereinafter also referred to as "dispersant") is a powder containing a vinyl alcohol polymer (hereinafter also referred to as "PVA (polyvinyl alcohol)") having a structure represented by the following formula 1 and having an average particle size of 100 μm or more and 2,000 μm or less. The dispersant contains particles A having a particle size of 500 μm or more and particles B having a particle size of 220 μm or less (212 μm or less). The ratio (A / B) of the absorbance A of a 0.1% by mass aqueous solution of the particles A at an optical path length of 10 mm and a wavelength of 320 nm to the absorbance B of a 0.1% by mass aqueous solution of the particles B at an optical path length of 10 mm and a wavelength of 320 nm is 0.7 or more and 1.2 or less.

[0033] [Chemistry 2]

[0034]

[0035] In the above formula 1, R 1 is an alkyl group having 1 to 4 carbon atoms. n is an integer of 1 to 10. * represents a site bonded to a portion other than the structure represented by the above formula 1 in the above vinyl alcohol polymer.

[0036] It should be noted that in the present specification, a numerical range described using “~” means that the numerical values ​​described before and after “~” are included as the lower limit and the upper limit.

[0037] According to the dispersant of the present invention, when performing suspension polymerization of vinyl compounds, vinyl polymer particles with small particle size can be produced with a small amount of addition. The reason for this effect has not yet been determined, but it can be inferred as follows. Generally speaking, the conventional dispersant containing PVA is produced by heat-treating the particles of PVA after producing particles (or powder) of PVA having a carbonyl group. -CO-(CH=CH) is formed in PVA by the dehydration reaction during the above heat treatment. n - structure. Therefore, in the dispersant obtained by the above-mentioned heat treatment, absorption at a wavelength of 320nm attributed to the -CO-(CH=CH)3- structure in PVA appears. Here, when using the conventional heat treatment method, it may be because the uniformity of the heat treatment is low, resulting in the -CO-(CH=CH)3- formed in the large-size PVA particles and the small-size PVA particles. n - The difference in the amount of structure is large. Specifically, PVA particles with small particle size are easily heat-treated to form -CO-(CH=CH) n- structure. When such a conventional dispersant is used, the dispersion state, reactivity, etc. of the vinyl compound are not uniform due to the difference in physical properties between the PVA contained in the particles with a large particle size and the PVA contained in the particles with a small particle size. It can be considered that as a result, the particle size (average particle size) of the obtained vinyl polymer particles becomes larger due to the formation of coarse polymer particles. In contrast, in the dispersant of the present invention, the absorbance A of the particles A with a larger particle size at a wavelength of 320nm is the same as the absorbance B of the particles B with a smaller particle size at a wavelength of 320nm, and the ratio of these two absorbances (A / B) is approximately 1. This means that the PVA contained in the particles A with a larger particle size and the PVA contained in the particles B with a smaller particle size have the same amount of -CO-(CH=CH) n - structure, physical properties, etc. are small. Therefore, when the dispersant of the present invention is used, a reaction with high uniformity will occur, so stable suspension polymerization can be carried out. It can be considered that as a result, the particle size (average particle size) of the obtained vinyl polymer particles is reduced due to the difficulty in producing coarse polymer particles, etc. It should be noted that the dispersant of the present invention can be effectively manufactured by the manufacturing method described below. Below, the dispersant of the present invention is described in detail.

[0038] (PVA having the structure shown in Formula 1)

[0039] The PVA having the structure shown in Formula 1 is an essential component of the dispersant of the present invention. 1 is an alkyl group having 1 to 4 carbon atoms, preferably an alkyl group having 1 to 3 carbon atoms, and more preferably an alkyl group having 1 to 2 carbon atoms. 1 The alkyl group having 1 to 4 carbon atoms shown in the formula may include methyl, ethyl, propyl, etc., preferably methyl and ethyl. n in formula 1 may be, for example, an integer of 1 to 6, or an integer of 1 to 4. As described later, the structure shown in formula 1 may be effectively introduced into the polymer by heating the vinyl ester polymer as a precursor by using an aldehyde or ketone as a chain transfer agent (modifier) ​​when polymerizing the polymer.

[0040] Above-mentioned PVA is a polymer having vinyl alcohol unit as a monomer unit. Above-mentioned PVA is usually obtained by saponifying a vinyl ester polymer. As the lower limit of the saponification degree of above-mentioned PVA, for example, it can be 40 mol % or 50 mol %, preferably 50 mol %, more preferably 60 mol %, and further preferably 68 mol %. On the other hand, the upper limit of above-mentioned saponification degree can be 100 mol % or 99 mol %, preferably 98 mol %, more preferably 95 mol %, further preferably 90 mol %, further preferably 85 mol %, and particularly preferably 80 mol %. By making the saponification degree of above-mentioned PVA be the above-mentioned range, so that surface activity performance is optimized, etc., it is possible to manufacture the vinyl polymer particles with smaller particle diameter. In addition, by making the saponification degree be below the above-mentioned upper limit, so that surface activity performance improves, the plasticizer absorbability of the obtained vinyl polymer particles improves. Saponification degree is the value measured according to the method recorded in JIS K6726:1994.

[0041] The above-mentioned PVA may have other monomer units in addition to the vinyl alcohol unit and the vinyl ester unit. Examples of monomers providing the above-mentioned other monomer units include α-olefins such as ethylene, propylene, 1-butene, isobutylene, 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 vinyl groups containing hydroxyl groups such as ethylene glycol vinyl ether, 1,3-propylene glycol vinyl ether, and 1,4-butanediol vinyl ether. Ether; allyl acetate; allyl ethers such as propyl allyl ether, butyl allyl ether, and hexyl allyl ether; monomers having an oxyalkylene group; isopropenyl acetate; α-olefins containing a hydroxyl group such as 3-butene-1-ol, 4-pentene-1-ol, 5-hexene-1-ol, 7-octen-1-ol, 9-decen-1-ol, and 3-methyl-3-butene-1-ol; monomers having a silyl group such as vinyltrimethoxysilane, vinylmethyldimethoxysilane, vinyldimethylmethoxysilane, vinyltriethoxysilane, vinylmethyldiethoxysilane, vinyldimethylethoxysilane, 3-(meth)acrylamidepropyltrimethoxysilane, and 3-(meth)acrylamidepropyltriethoxysilane. Among these, α-olefins, acrylic acid, methacrylic acid, acrylic acid esters, and methacrylic acid esters are preferred.

[0042] The ratio of the other monomer units in the total monomer units of the PVA is preferably 20 mol% or less, and more preferably 10 mol% or less. On the other hand, the ratio of the other monomer units may be, for example, 0.1 mol% or more, or 1 mol% or more.

[0043] The lower limit of the viscosity average degree of polymerization of the PVA may be, for example, 200, 300 or 400, preferably 500, more preferably 600, and further preferably 700. By making the viscosity average degree of polymerization above the lower limit, the protective colloid property is improved, and vinyl polymer particles with a smaller particle size can be produced. On the other hand, the upper limit of the viscosity average degree of polymerization may be, for example, 3,000 or 2,000, preferably 1,800, more preferably 1,700, further preferably 1,500, further preferably 1,300, and particularly preferably 1,100. By making the viscosity average degree of polymerization below the upper limit, the surface active performance is improved, the plasticizer absorption of the obtained vinyl polymer particles is improved, and vinyl polymer particles with a smaller particle size can be produced. The viscosity average degree of polymerization is a value measured in accordance with JIS K6726:1994. That is, it can be determined by the following formula from the intrinsic viscosity [η] (unit: liter / g) measured in water at 30° C. after re-saponifying PVA to a saponification degree of 99.5 mol % or more and purifying it.

[0044] Viscosity average degree of polymerization = ([η] × 10 4 / 8.29) (1 / 0.62)

[0045] The lower limit of the absorbance (UV280nm) of the 0.1% by mass aqueous solution of the above-mentioned PVA at an optical path length of 10mm and a wavelength of 280nm is preferably 0.05, more preferably 0.1, and further preferably 0.2. The absorption at a wavelength of 280nm is attributed to the structure of -CO-(CH=CH)2- in the above-mentioned PVA. In addition, the lower limit of the absorbance (UV320nm) of the 0.1% by mass aqueous solution of the above-mentioned PVA at an optical path length of 10mm and a wavelength of 320nm is preferably 0.05, more preferably 0.1, and further preferably 0.2. As described above, the absorption at a wavelength of 320nm is attributed to the structure of -CO-(CH=CH)3- in the above-mentioned PVA. When the absorbance (UV280nm) and the absorbance (UV320nm) are above the above-mentioned lower limits, it can be said that a sufficient amount of the structure shown in the above-mentioned formula 1 is formed, and vinyl polymer particles with a smaller particle size can be manufactured. It should be noted that the upper limit of the absorbance (UV280nm) is not particularly limited, and is usually 10, and is sometimes preferably 8, 5, 3, 1 or 0.5. In addition, the upper limit of the absorbance (UV320nm) is not particularly limited, and is usually 10, and is sometimes preferably 8, 5, 3, 1 or 0.5.

[0046] As the lower limit of the content of the above-mentioned PVA in the non-volatile component of the dispersant of the present invention, it is sometimes preferably 30% by mass, more preferably 50% by mass, and further preferably 70% by mass, 90% by mass or 99% by mass. The upper limit of the content of the above-mentioned PVA in the non-volatile component of the dispersant of the present invention can be 100% by mass. The non-volatile components other than the above-mentioned PVA that can be contained in the dispersant of the present invention include PVA other than the PVA having the structure shown in Formula 1, resins other than PVA, additives such as surfactants and plasticizers, and various compounds used in the manufacture. As the lower limit of the content of all PVA (the above-mentioned PVA and other PVAs having the structure shown in Formula 1) in the non-volatile component of the dispersant of the present invention, it is sometimes preferably 50% by mass, more preferably 70% by mass, and further preferably 80% by mass, 90% by mass or 99% by mass. The upper limit of the content of all PVA in the non-volatile component of the dispersant of the present invention can be 100% by mass. In addition, the content of the volatile component in the dispersant of the present invention is usually 20% by mass or less, preferably 15% by mass or less, and more preferably 10% by mass or less. Examples of the volatile component that may be contained in the dispersant of the present invention include alcohol and water.

[0047] (shape, absorbance ratio, etc.)

[0048] The dispersant of the present invention is a powder having an average particle size of 100 μm or more and 2,000 μm or less. The lower limit of the above average particle size is preferably 150 μm, more preferably 200 μm. On the other hand, the upper limit of the above average particle size is preferably 1,500 μm, more preferably 1,000 μm, and further preferably 800 μm. When the average particle size is in the above range, it is possible to produce vinyl polymer particles having a smaller particle size due to the formation of a heat-treated powder with higher uniformity and good solubility. In addition, by making the average particle size above the above lower limit, dust explosion is less likely to occur, and safety is improved. The average particle size of the dispersant is a value measured according to the method described in JIS K7369:2009.

[0049] In the dispersant of the present invention, the content of particles with a particle size of 100 to 1,000 μm (specifically, a particle size of 106 to 1,000 μm) is not particularly limited, and is preferably 50% by mass or more, more preferably 55% by mass or more, further preferably 60% by mass or more, and further preferably 65% ​​by mass or more. On the other hand, the upper limit of the content of particles with a particle size of 100 to 1,000 μm can be 100% by mass or 95% by mass. When the content of particles with a particle size of 100 to 1,000 μm is within the above range, it is possible to produce vinyl polymer particles with a smaller particle size by forming a heat-treated powder with higher uniformity and good solubility. The content of particles having a particle size of 100 to 1,000 μm in the dispersant can be determined according to the method described in JIS K7369:2009 using a sieve with a nominal opening of 1,000 μm (16 mesh) and a sieve with a nominal opening of 106 μm (150 mesh).

[0050] The dispersant of the present invention contains particles A having a particle size of 500 μm or more and particles B having a particle size of 220 μm or less (specifically, 212 μm or less). Particles B may be particles having a particle size of 212 μm or less. The lower limit of the ratio (A / B) of the absorbance A of a 0.1% by mass aqueous solution of the particles A at an optical path length of 10 mm and a wavelength of 320 nm to the absorbance B of a 0.1% by mass aqueous solution of the particles B at an optical path length of 10 mm and a wavelength of 320 nm is 0.7, preferably 0.75, more preferably 0.8, further preferably 0.82, further preferably 0.85, particularly preferably 0.88, and most preferably 0.90. In addition, the upper limit of the above ratio (A / B) is 1.2, preferably 1.1, more preferably 1.05, further preferably 1.03, and particularly preferably 1.0. When the absorbance ratio (A / B) is within the above range, it can be said that the heat treatment is carried out with high uniformity, and the deviation in the amount of the structure represented by the above formula 1 formed in the PVA of each dispersant particle is small. Therefore, by making the absorbance ratio (A / B) within the above range, it is possible to produce vinyl polymer particles with a small particle size.

[0051] The lower limit of the absorbance A of a 0.1 mass % aqueous solution of the particles A at an optical path length of 10 mm and a wavelength of 320 nm is preferably 0.05, more preferably 0.1, further preferably 0.15, and particularly preferably 0.2. In addition, the upper limit of the absorbance A is not particularly limited, but is usually 10, and is sometimes preferably 8, 5, 3, 1 or 0.5.

[0052] The lower limit of the absorbance B of a 0.1 mass % aqueous solution of the particles B at an optical path length of 10 mm and a wavelength of 320 nm is preferably 0.05, more preferably 0.1, further preferably 0.15, and particularly preferably 0.2. In addition, the upper limit of the absorbance B is not particularly limited, but is usually 10, and is sometimes preferably 8, 5, 3, 1 or 0.5.

[0053] It should be noted that the particles A having a particle size of 500 μm or more can be selected in the form of particles that do not pass through a sieve with a nominal mesh size of 500 μm during screening. The particles B having a particle size of 220 μm or less (specifically, 212 μm or less) can be selected in the form of particles that pass through a sieve with a nominal mesh size of 212 μm during screening. The above-mentioned mechanical screening can be performed by the method described in, for example, JIS K7369:2009.

[0054] In the dispersant of the present invention, the average PA of the circularity P represented by the following formula 2 of 50 particles randomly selected from particles having a particle size of 100 μm or more and 1,000 μm or less (hereinafter also referred to as “average circularity”) is preferably 0.1 or more and 0.8 or less.

[0055] [Mathematical formula 2]

[0056]

[0057] In the above formula 2, r i is the radius of curvature of each corner of the particle. R is the radius of the largest inscribed circle of the particle. N is the number of corners of the particle. When the number of corners of the particle is 9 or more, the radius of curvature of the 8th corner in the order of the radius of curvature from small to large is used, and N is set to 8.

[0058] The present inventors have found that by subjecting a coarse powder containing PVA described later to a heat treatment (surface processing while heating) in which the average circularity increases, a dispersant for suspension polymerization having the absorbance ratio (A / B) close to 1 can be obtained regardless of the particle size. Therefore, when the average circularity of the dispersant of the present invention is preferably 0.1 or more, more preferably 0.15 or more, further preferably 0.20 or more, further preferably 0.22 or more, further preferably 0.25 or more, further preferably 0.28 or more, and particularly preferably 0.30 or more, vinyl polymer particles with a smaller particle size can be produced. On the other hand, from the viewpoint of productivity, the average circularity is preferably 0.8 or less, and more preferably 0.7 or less.

[0059] In addition, when the average roundness of the dispersant of the present invention is greater than the above lower limit, the dispersant is less likely to be clogged in the pipes or silos when transported through pipes or bagged through silos. The reason for this is not necessarily clear, but it is presumed that this is because the corners of each particle are curved, so the friction when the particles contact each other and the friction when the particles contact the wall surface are small, resulting in improved fluidity of the particles.

[0060] The average circularity of the dispersant can be determined by the following method. Randomly extract 50 particles from particles with a particle size of 100 to 1,000 μm (specifically, a particle size of 106 to 1,000 μm) in the dispersant. Particles with a particle size of 100 to 1,000 μm can be selected in the form of particles that pass through a sieve with a nominal mesh size of 1,000 μm but do not pass through a sieve with a nominal mesh size of 106 μm during screening. The above screening can be performed by the method described in JIS K7369:2009, for example. For one particle extracted, with respect to the projection diagram that reaches the maximum apparent area, the curvature radius r is calculated. i Select 8 angles from small to large (if the number of angles is less than 8, select all the angles) and measure the radius of curvature r of each angle. i In addition, the radius R of the maximum inscribed circle of the particle is measured based on the projection diagram where the apparent area is the largest. The number of corners of the particle is set to N (when the number of corners of the particle is 9 or more, N is set to 8), and the measured r i The roundness P of a particle is calculated by using the above formula 2, using the values ​​of R and R. When the roundness P is low, it means that there are many protruding corners in the particle, and when the roundness is high, it means that the particle has a curvature. The roundness P is measured for 50 particles extracted, and the average value PA of the roundness P of these 50 particles is calculated. The average value PA is the average roundness.

[0061] <Method for producing a dispersant for suspension polymerization>

[0062] The method for producing the dispersant of the present invention is not particularly limited, but is preferably the following method. That is, the method for producing the dispersant of the present invention comprises:

[0063] a step of obtaining a coarse powder by pulverizing a resin solid containing PVA (step B); and

[0064] A step of processing the surface of the coarse powder while heating the coarse powder (step C).

[0065] The method for producing a dispersant for suspension polymerization of the present invention may further include a step of synthesizing PVA to obtain a resin solid containing PVA (step A).

[0066] (Process A)

[0067] Step A may include, for example, a polymerization step, a saponification step, and the like.

[0068] In the polymerization step, vinyl ester monomers are polymerized to obtain vinyl ester polymers. As methods for polymerizing vinyl ester monomers, known methods such as bulk polymerization, solution polymerization, suspension polymerization, and emulsion polymerization can be cited. Among these methods, bulk polymerization under solvent-free conditions and solution polymerization using solvents such as alcohol are preferred, and solution polymerization in the presence of lower alcohols is more preferred. As the above-mentioned lower alcohol, alcohols with a carbon number of 3 or less are preferred, and methanol, ethanol, n-propanol and isopropanol are more preferred, and methanol is further preferred. When the polymerization reaction is carried out by bulk polymerization or solution polymerization, the reaction method can also be any of intermittent and continuous methods.

[0069] 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, and vinyl versatate. Among them, vinyl acetate is preferred.

[0070] Examples of the polymerization initiator used in the polymerization reaction include azo initiators such as 2,2'-azobisisobutyronitrile, 2,2'-azobis(2,4-dimethylvaleronitrile), and 2,2'-azobis(4-methoxy-2,4-dimethylvaleronitrile); and known polymerization initiators such as organic peroxide initiators such as benzoyl peroxide, n-propyl peroxycarbonate, and diisopropyl peroxydicarbonate. The polymerization temperature during the polymerization reaction is not particularly limited, but is preferably within a range of 5°C to 200°C.

[0071] When the vinyl ester monomer is polymerized, a copolymerizable monomer may be further copolymerized within the scope that does not impair the gist of the present invention. As the copolymerizable monomer, the monomers described above as monomers that provide other monomer units optionally included in the PVA represented by Formula 1 except the vinyl alcohol unit and the vinyl ester unit can be cited.

[0072] The PVA synthesized in step A preferably contains 1 -CO-(R 1 In order to efficiently obtain the PVA, it is preferred that R 2 -CHO(R 2 is an alkyl group having 1 to 4 carbon atoms) or an aldehyde represented by R 3 -CO-R 4 (R 3 and R 4Each independently represents an alkyl group having 1 to 4 carbon atoms). As the chain transfer agent, aldehydes such as acetaldehyde, propionaldehyde, butyraldehyde, etc.; ketones such as acetone and methyl ethyl ketone. These chain transfer agents can be used alone or in combination of two or more. In addition, during the polymerization of the vinyl ester monomer, a chain transfer agent other than the above-mentioned aldehyde or ketone may be further coexisted for the purpose of adjusting the degree of polymerization of the obtained PVA. As the chain transfer agent, aldehydes other than the above-mentioned aldehydes; ketones other than the above-mentioned ketones; thiols such as 2-hydroxyethanethiol; thiocarboxylic acids such as thioacetic acid; halogenated hydrocarbons such as trichloroethylene and perchloroethylene, etc. The amount of the chain transfer agent added is determined according to the chain transfer constant of the added chain transfer agent and the degree of polymerization of the target PVA, and is generally preferably 0.1 to 10% by mass relative to the vinyl ester used.

[0073] In the saponification process, a vinyl ester polymer is saponified in an alcohol solution using an alkali catalyst or an acid catalyst to obtain PVA. In the saponification reaction of the vinyl ester polymer, the alcoholysis or hydrolysis reaction using an alkaline catalyst such as sodium hydroxide, potassium hydroxide, sodium methoxide or an acidic catalyst such as p-toluenesulfonic acid, which is known in the art, can be applied. As the solvent used in the saponification reaction, alcohols such as methanol and ethanol; esters such as methyl acetate and ethyl acetate; ketones such as acetone and methyl ethyl ketone; aromatic hydrocarbons such as benzene and toluene, etc. can be listed. They can be used alone or in combination of two or more. Among these, it is convenient to use methanol or a mixed solution of methanol and methyl acetate as a solvent, and to carry out the saponification reaction in the presence of sodium hydroxide as an alkaline catalyst, so it is preferred.

[0074] The saponification process can be performed using a belt reactor, a kneader reactor, a tower reactor, etc. A resin solid containing PVA is obtained by undergoing the saponification process. The content ratio of PVA in the resin solid in the non-volatile component is, for example, 50% by mass or more, sometimes preferably 70% by mass or more, more preferably 90% by mass or more, and further preferably 99% by mass or more. The non-volatile component in the resin solid can be substantially PVA as the main component, and can also contain impurities such as sodium acetate, by-products, etc.

[0075] (Process B)

[0076] In step B, the resin solid material containing PVA is pulverized. The PVA in the resin solid material used in step B is preferably a PVA containing a carbonyl group in the terminal structure. Specifically, it is preferably a PVA having R 1 -CO-(R 1 By using PVA having a carbonyl group in the terminal structure, the PVA having the structure represented by Formula 1 can be effectively obtained by heat treatment in step C.1 -CO-(R 1 The PVA having a structure represented by (i) an alkyl group having 1 to 4 carbon atoms) can be obtained by saponifying a vinyl ester polymer obtained by using an aldehyde or a ketone as a chain transfer agent (modifier).

[0077] By crushing the resin solids containing PVA, a coarse powder containing PVA is obtained. The above-mentioned crushing can be carried out using a known crusher. As a crusher, in order to adjust the average particle size of the obtained coarse powder and the dispersant finally obtained, it is preferably a device that can control the degree of crushing such as crushing strength. In addition to adjusting the crushing strength, the average particle size of the obtained coarse powder can also be controlled by processing time. It should be noted that the obtained coarse powder can also be saponified again. In addition, the obtained coarse powder can also be subjected to a cleaning process for reducing impurities such as sodium acetate, by-products, etc. and a drying process for reducing volatile components. The resin solids before crushing can also be cleaned and dried.

[0078] (Process C)

[0079] In step C, the surface of the coarse powder is processed while being heated. When the resin solid containing PVA is crushed, the resulting coarse powder has a shape with very sharp corners. When such a coarse powder is simply heat-treated, the difference in the degree of heat treatment between each particle and within the particle is large, and it is difficult to obtain a dispersant for suspension polymerization of the present invention in which the ratio of absorbance A to absorbance B is close to 1. Therefore, by utilizing step C, the coarse powder is heat-treated while being processed (grinding, etc.) to round the corners, thereby performing a heat treatment with high uniformity. It can also be said that step C is a step of heating the coarse powder while being processed such as grinding to round the corners. In addition, a dispersant with a high average roundness is obtained by undergoing such step C.

[0080] As the device used in step C, there can be listed a device that can grind the surface of the coarse powder while heating the coarse powder, for example, a converter that grinds the surface by rotating the powder filling container and contacting the powders with each other; a planetary motion mixer that can apply three-dimensional motion to the contents by using the screw blades that rotate in the container; a mixer that rotates the blades and screws in the container and grinds the internal powder by the rotation, etc. As a mixer, a loedige mixer, a PAM APEX mixer, a ribbon mixer, etc. can be listed. Among these, from the viewpoint of processing efficiency, a mixer is preferred, and a loedige mixer is more preferred.

[0081] The treatment time and treatment temperature in step C are appropriately adjusted according to the device used, etc. The treatment time can be, for example, 10 minutes or more and 20 hours or more, or 30 minutes or more and 12 hours or less. The treatment temperature can be, for example, 60°C or more and 160°C or less, or 80°C or more and 140°C or less. It should be noted that if the treatment time is long and the treatment temperature is high, there is a tendency that the heat treatment is aggravated, the absorbance at a wavelength of 280nm, and the absorbance at a wavelength of 320nm become higher.

[0082] The method for producing a dispersant for suspension polymerization of the present invention may include a sieving step for adjusting the average particle size, etc. Furthermore, after step C, a washing treatment or a drying treatment may be performed.

[0083] <Method for producing vinyl polymer particles>

[0084] The method for producing vinyl polymer particles of the present invention comprises the step of performing suspension polymerization on a vinyl compound using the dispersant of the present invention. The method is the same as a known method for producing vinyl polymer particles except that the dispersant of the present invention is used as the dispersant.

[0085] In the method for producing vinyl polymer particles of the present invention, the vinyl compound is usually subjected to suspension polymerization in an aqueous medium using the dispersant of the present invention. As the aqueous medium, in addition to pure water, an aqueous solution containing various additives or an aqueous medium containing other organic solvents can be used.

[0086] When performing suspension polymerization of a vinyl compound, the amount of the dispersant of the present invention added is not particularly limited, but is sometimes preferably 100 ppm or more and 50,000 ppm or less, more preferably 200 ppm or more and 20,000 ppm or less, and further preferably 10,000 ppm or less, 5,000 ppm or less, or 2,000 ppm or less, based on mass, relative to the vinyl compound. By using the dispersant of the present invention, vinyl polymer particles with a small particle size can be obtained even with such a small amount of the dispersant added.

[0087] The dispersant of the present invention can be used alone or in combination with other dispersants. Other dispersants include water-soluble cellulose ethers such as methylcellulose, hydroxyethylcellulose, hydroxypropylcellulose, and hydroxypropylmethylcellulose, which are commonly used when suspension polymerization of vinyl compounds in aqueous media; water-soluble polymers such as polyvinyl alcohol and gelatin; oil-soluble emulsifiers such as sorbitan monolaurate, sorbitan trioleate, glyceryl tristearate, and ethylene oxide-propylene oxide block copolymers; water-soluble emulsifiers such as polyoxyethylene sorbitan monolaurate, polyoxyethylene glyceryl oleate, and sodium laurate.

[0088] As the polymerization initiator used in the method for producing the vinyl polymer particles of the present invention, polymerization initiators conventionally used for polymerization of vinyl compounds can be used. Specifically, polymerization initiators similar to those exemplified in the polymerization of the above-mentioned vinyl ester monomers can be used.

[0089] In the method for producing vinyl polymer particles of the present invention, various other additives may be added to the polymerization system as needed. Examples of additives include polymerization regulators such as aldehydes, halogenated hydrocarbons, and mercaptans; polymerization inhibitors such as phenol compounds, sulfur compounds, and N-oxide compounds. In addition, pH regulators, antioxidants, crosslinking agents, and the like may also be added. The above-mentioned additives may be used in combination of multiple types.

[0090] As the vinyl compounds that can be subjected to suspension polymerization in the method for producing vinyl polymer particles of the present invention, there can be listed halogenated vinyls such as vinyl chloride; vinyl esters such as vinyl acetate and vinyl propionate; acrylic acid, methacrylic acid, their esters and salts; maleic acid, fumaric acid, their esters and anhydrides; styrene; acrylonitrile; vinylidene chloride; vinyl ether, etc. Among these vinyl compounds, vinyl chloride is preferred. The method for producing vinyl polymer particles of the present invention can be particularly suitably used when vinyl chloride is subjected to suspension polymerization alone or when vinyl chloride is subjected to suspension polymerization together with a monomer copolymerizable with vinyl chloride. As monomers copolymerizable with vinyl chloride, there can be listed vinyl esters such as vinyl acetate and vinyl propionate; (meth)acrylic acid esters such as methyl (meth)acrylate and ethyl (meth)acrylate; α-olefins such as ethylene and propylene; unsaturated dicarboxylic acids such as maleic anhydride and itaconic acid; acrylonitrile; styrene; vinylidene chloride; vinyl ether, etc.

[0091] In the method for producing vinyl polymer particles of the present invention, when the vinyl compound is subjected to suspension polymerization, the charging ratio of each component, the polymerization temperature, the polymerization time, etc. can be the same as the conditions used in the suspension polymerization of vinyl compounds such as vinyl chloride. In addition, there is no limitation on the charging order and ratio of the vinyl compound, the polymerization initiator, the dispersant, the aqueous medium and other additives. Example

[0092] 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. It should be noted that the measurement methods and evaluation methods used in the following examples and comparative examples are shown below.

[0093] [Viscosity average degree of polymerization of PVA]

[0094] The viscosity average polymerization degree of PVA was measured in accordance with JIS K6726: 1994. Specifically, when the saponification degree of PVA is less than 99.5 mol %, the PVA is saponified until the saponification degree reaches 99.5 mol % or more, and the obtained PVA is measured in water at 30° C. Using the intrinsic viscosity [η] (liter / g) thus obtained, the viscosity average polymerization degree is calculated using the following formula.

[0095] Viscosity average degree of polymerization = ([η] × 10 4 / 8.29) (1 / 0.62)

[0096] [Saponification degree of PVA]

[0097] The saponification degree of PVA was determined according to the method described in JIS K6726:1994.

[0098] [Average particle size of the dispersant for suspension polymerization and content of particles with a particle size of 100 to 1,000 μm]

[0099] The average particle size of the dispersant for suspension polymerization and the content of particles having a particle size of 100 to 1,000 μm (specifically, a particle size of 106 to 1,000 μm) were determined by the method described in JIS K7369:2009 using a JIS standard sieve.

[0100] [Average circularity of dispersant for suspension polymerization]

[0101] Particles with a particle size of 100 to 1,000 μm (specifically, a particle size of 106 to 1,000 μm) were selected by the above sieving, and 50 particles were randomly selected from these particles. The curvature radius r was calculated from an image of these particles with a magnification of 100 times obtained using a digital microscope VHX-900 manufactured by Keyence Corporation. i and the radius R of the largest inscribed circle, and the roundness P of each particle is calculated. The average PA of the roundness P of 50 particles is calculated as the average roundness.

[0102] [Absorbance]

[0103] A 0.1% by mass aqueous solution of PVA (dispersant for suspension polymerization) was prepared, and a UV spectrophotometer ("UV2100" manufactured by Shimadzu Corporation) was used to measure the absorbance (UV280nm) at a wavelength of 280nm and a wavelength of 320nm of the measurement sample with an optical path length of 10mm. The PVA that can confirm these absorptions is regarded as the substance having the structure shown in Formula 1.

[0104] Through the above sieving, particles A having a particle size of 500 μm or more and particles B having a particle size of 220 μm or less were selected from the dispersant for suspension polymerization. A 0.1 mass % aqueous solution of each of particles A and particles B was prepared, and the absorbance (absorbance A and absorbance B) of the measurement sample at an optical path length of 10 mm and a wavelength of 320 nm was measured using a UV spectrophotometer ("UV2100" manufactured by Shimadzu Corporation).

[0105] [Evaluation method]

[0106] (1) Average particle size (MGS)

[0107] The obtained vinyl polymer particles were measured for particle size distribution using a JIS standard sieve by the dry sieving method described in JIS Z 8815: 1994. The results were plotted in the Rosin-Rammler distribution to calculate the average particle size. The average particle size measured was evaluated according to the following criteria.

[0108] 5: Less than 150μm

[0109] 4: 150μm or more and less than 160μm

[0110] 3: 160μm or more and less than 170μm

[0111] 2: 170μm or more and less than 180μm

[0112] 1: 180μm or more.

[0113] (2) Plasticizer absorption (CPA)

[0114] The obtained vinyl polymer particles were measured for dioctyl phthalate absorption at 23° C. by the method described in ASTM-D3367-75. The measured plasticizer absorption was evaluated according to the following criteria.

[0115] 5: More than 28%

[0116] 4: 26% or more and less than 28%

[0117] 3: 24% or more and less than 26%

[0118] 2: 22% or more and less than 24%

[0119] 1: Less than 22%.

[0120] [Example 1] Preparation of Dispersant 1 for Suspension Polymerization Containing PVA1

[0121] 80 kg of vinyl acetate and 20 kg of methanol (80% by mass of vinyl acetate: 20% by mass of methanol) were added to a 250 L reactor equipped with a stirrer, a reflux condenser, a nitrogen inlet, a comonomer addition port, and an initiator addition port. The system was purged with nitrogen for 30 minutes while bubbling nitrogen. The temperature of the reactor was raised. When the internal temperature reached 60°C, 1.85 kg of acetaldehyde (2.3% by mass relative to vinyl acetate) and 43 g of 2,2'-azobisisobutyronitrile (AIBN) were added to start polymerization. When the polymerization rate reached 50%, the polymerization was stopped by cooling. The solid content concentration at the time of stopping the polymerization was 39.3% by mass. Next, unreacted vinyl acetate monomer was removed while adding methanol at any time under reduced pressure at 30°C to obtain a methanol solution of polyvinyl acetate (PVAc). Furthermore, 0.465 kg of an alkali solution (10% by mass methanol solution of sodium hydroxide) was added to 57.1 kg of a methanol solution of PVAc prepared by adding methanol (the concentration was 35% by mass, and the PVAc in the solution was 20.0 kg), and saponification was performed (the PVAc concentration of the saponified solution was 30% by mass, and the molar ratio of sodium hydroxide to the vinyl acetate unit in PVAc was 0.005). A gel-like substance (resin solid) was generated about 1 minute after the addition of the alkali solution, so it was crushed with a crusher. After the crushed substance was placed at 40°C for 1 hour for saponification, 50 kg of methyl acetate was added to neutralize the remaining alkali. After confirming that the neutralization was completed using a phenolphthalein indicator, a white solid was obtained by filtration, 200 kg of methanol was added thereto, and it was placed at room temperature for washing for 3 hours. After repeating the above washing operation 3 times, the white solid obtained by centrifugal dehydration was placed in a dryer at 65°C for 2 days to obtain a coarse powder of PVA1. The degree of polymerization of PVA1 was 800, and the degree of saponification was 72.0 mol %.

[0122] Next, the coarse powder of PVA1 was filled into a Lödige mixer "FKM130D" equipped with a Becker-type shovel manufactured by Chuo Machinery Co., Ltd., and heat treated at a rotation speed of 160 rpm, a treatment time of 3.5 hours, and a treatment temperature of 120° C. in a nitrogen atmosphere. Thus, a dispersant 1 for suspension polymerization having an average particle size of 650 μm and an average circularity of 0.35 was obtained.

[0123] [Examples 2 to 16, Comparative Examples 1 to 4]

[0124] Except having adopted the conditions described in Table 1, the same operation as in Example 1 was carried out to obtain dispersants 2 to 16 for suspension polymerization containing PVAs 2 to 16, respectively, and dispersants 1' to 4' for suspension polymerization containing PVAs 1' to 4', respectively.

[0125] The polymerization degree (viscosity average polymerization degree) and saponification degree of each PVA obtained in Examples 1 to 16 and Comparative Examples 1 to 4, the average particle size of each suspension polymerization dispersant (PVA), the content of particles with a particle size of 100 to 1,000 μm, and the average circularity were measured by the above method. In addition, for each suspension polymerization dispersant (PVA), the absorbance (UV280), absorbance (UV320), absorbance A, and absorbance B were measured by the above method. The measured values ​​and the ratio of absorbance A to absorbance B (A / B) are shown in Table 2.

[0126] [evaluate]

[0127] Using each of the dispersants for suspension polymerization obtained in Examples 1 to 16 and Comparative Examples 1 to 4, particles of polyvinyl chloride as a vinyl polymer were obtained by the following method.

[0128] 1390 g of an aqueous solution in which 0.94 g of each dispersant for suspension polymerization shown in Table 2 (1000 ppm relative to vinyl chloride monomer) is dissolved is placed in a 5 L autoclave. Next, 1.5 g of a 70% by mass toluene solution of diisopropyl peroxydicarbonate is placed in the autoclave. Degas until the pressure in the autoclave reaches 0.0067 MPa to remove oxygen. Thereafter, 940 g of vinyl chloride is added, the temperature of the contents in the autoclave is raised to 57°C, and suspension polymerization is started under stirring. The pressure in the autoclave at the start of the polymerization is 0.83 MPa. After 4 hours from the start of the suspension polymerization, the polymerization is stopped when the pressure in the autoclave reaches 0.65 MPa, and the unreacted vinyl chloride is removed. Thereafter, the polymerization slurry is taken out and dried at 65°C overnight to obtain vinyl chloride polymer particles.

[0129] The average particle size (MGS) and plasticizer absorption (CPA) of the obtained vinyl chloride polymer particles (vinyl-based polymer particles) were evaluated by the above-mentioned methods.

[0130]

[0131]

[0132] As shown in Table 2, the absorbance ratio (A / B) of the dispersants for suspension polymerization of Examples 1 to 16 was 0.7 or more and 1.2 or less. When these dispersants for suspension polymerization were used, small-diameter vinyl polymer particles having an average particle size (MGS) evaluation of 2 or more were obtained.

[0133] On the other hand, when the dispersants for suspension polymerization of Comparative Examples 1 to 3 having an absorbance ratio (A / B) less than 0.7 and the dispersant for suspension polymerization of Comparative Example 4 not containing PVA having the structure shown in Formula 1 were used, the evaluation of the average particle size (MGS) became 1, and vinyl polymer particles with a small particle size could not be obtained. In addition, by heat-treating the PVA having a carbonyl group using a specific device, a dispersant for suspension polymerization having an absorbance ratio (A / B) of 0.7 or more and 1.2 or less can be obtained. The dispersant for suspension polymerization having an absorbance ratio (A / B) of 0.7 or more and 1.2 or less when heat-treated using a specific device has an average roundness of 0.1 or more.

[0134] As shown in Table 2, the particle size of the dispersant for suspension polymerization, the degree of polymerization and saponification degree of PVA also affect the performance of the dispersant for suspension polymerization. For example, in Examples 1 to 4, 8, 14, and 15, where the degree of polymerization of PVA is 700 to 1,500, the degree of saponification is 68 mol % to 85 mol %, and the average particle size is 200 μm to 1,000 μm, the average particle size (MGS) is evaluated as 4 or 5, and vinyl polymer particles with a smaller particle size can be obtained. Furthermore, in Examples 1 to 3 and 14 in which the degree of polymerization of PVA is 700 to 1,100, the degree of saponification is 68 mol % to 85 mol %, and the content of particles with a particle size of 100 μm to 1,000 μm is 65 mass % or more, or the average particle size is 200 μm to 800 μm, the average particle size (MGS) is evaluated as 5, and vinyl polymer particles with particularly small particle sizes can be obtained. In addition, it can also be said that in Examples 1 to 3 and 14 in which the average particle size (MGS) is evaluated as 5, the absorbance ratio (A / B) is 0.9 or more, the degree of polymerization of PVA is 700 to 1,100, the degree of saponification is 68 mol % to 85 mol %, and the content of particles with a particle size of 100 μm to 1,000 μm is 60 mass % or more.

[0135] Furthermore, it was found that the plasticizer absorption of the obtained vinyl polymer particles was also improved by adjusting the degree of polymerization, degree of saponification, etc. of PVA.

[0136] Industrial Applicability

[0137] The dispersant for suspension polymerization of the present invention can be used as a dispersant in suspension polymerization of a vinyl compound, etc.

Claims

1. A dispersant for suspension polymerization, comprising a powder containing a vinyl alcohol polymer having a structure represented by the following formula 1 and having an average particle size of 200 μm or more and 1,000 μm or less, It contains particles A with a particle size of 500 μm or more and particles B with a particle size of 220 μm or less. The ratio A / B of the absorbance A of a 0.1 mass % aqueous solution of the particles A at an optical path length of 10 mm and a wavelength of 320 nm to the absorbance B of a 0.1 mass % aqueous solution of the particles B at an optical path length of 10 mm and a wavelength of 320 nm is 0.7 or more and 1.2 or less, In Formula 1, R 1 is an alkyl group having 1 to 4 carbon atoms; n is an integer of 1 to 10; * represents a site bonded to a portion of the vinyl alcohol polymer other than the structure represented by Formula 1, The vinyl alcohol polymer has a viscosity average degree of polymerization of 700 to 1,500, a saponification degree of 68 mol % to 85 mol %, The average value PA of the roundness P represented by the following formula 2 of 50 particles randomly selected from particles having a particle size of 100 μm or more and 1,000 μm or less is 0.1 or more and 0.8 or less, The content of particles having a particle size of 100 μm or more and 1,000 μm or less is 50% by mass or more, [Mathematical formula 1] In the formula 2, r i is the radius of curvature of each corner of the particle; R is the radius of the largest inscribed circle of the particle; N is the number of corners of the particle, where When the number of corners of a particle is 9 or more, the curvature radius of the eighth corner in ascending order of curvature radius is used, and N is set to 8.

2. The dispersant for suspension polymerization according to claim 1, wherein The absorbance of a 0.1 mass % aqueous solution of the vinyl alcohol polymer at an optical path length of 10 mm and a wavelength of 280 nm is 0.05 or more, and the absorbance at an optical path length of 10 mm and a wavelength of 320 nm is 0.05 or more.

3. The method for producing a dispersant for suspension polymerization according to claim 1 or 2, comprising: A step of obtaining a coarse powder by pulverizing a resin solid containing a vinyl alcohol polymer; and A step of processing the surface of the coarse powder while heating it.

4. A method for producing vinyl polymer particles, comprising: A step of performing suspension polymerization on a vinyl compound using the dispersant for suspension polymerization according to claim 1 or 2.

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