Polyvinyl alcohol resin, dispersant for suspension polymerization, method for producing vinyl polymer particles, and method for producing polyvinyl alcohol resin

Abstract

Provided is a polyvinyl alcohol resin in which, as a polymer component, substantially only a polyvinyl alcohol is contained, wherein the absorbance Abs320 of a 0.1 mass% aqueous solution of the polyvinyl alcohol resin when optical path length is 10 mm and wavelength is 320 nm is not less than 0.09, the ratio (Abs320 / Abs370) of the absorbance Abs320 to the absorbance Abs370 of a 0.1 mass% aqueous solution of the polyvinyl alcohol resin when optical path length is 10 mm and wavelength is 370 nm is not greater than 4.9, and the combined content of bivalent metallic elements and trivalent metallic elements is less than 30 μmol / g.

Description

Polyvinyl alcohol resin, dispersant for suspension polymerization, method for producing vinyl polymer particles, and method for producing polyvinyl alcohol resin

[0001] The present invention relates to a polyvinyl alcohol resin, a dispersant for suspension polymerization, a method for producing vinyl polymer particles, and a method for producing a polyvinyl alcohol resin.

[0002] In the industrial production of vinyl polymers such as polyvinyl chloride, suspension polymerization is widely practiced, in which a vinyl compound such as vinyl chloride is dispersed in an aqueous medium in the presence of a dispersant (sometimes called a dispersion stabilizer) and polymerized using an oil-soluble catalyst. Factors that generally govern the quality of a vinyl polymer obtained by suspension polymerization of a vinyl compound include the conversion rate, water-monomer ratio, polymerization temperature, type and amount of oil-soluble catalyst, model of polymerization vessel, stirring speed of the contents in the polymerization vessel, and type of dispersant, among which the type of dispersant has a significant effect.

[0003] The performance required of a dispersant for suspension polymerization of vinyl compounds includes the ability to stably obtain vinyl polymer particles with a small amount added, etc. Conventionally, as dispersants for suspension polymerization of vinyl compounds, cellulose derivatives such as methyl cellulose and carboxymethyl cellulose, as well as partially saponified polyvinyl alcohol, etc. have been used alone or in appropriate combination.

[0004] As polyvinyl alcohol (hereinafter also referred to as "PVA") used as a dispersant for suspension polymerization, Patent Document 1 describes "a polyvinyl alcohol-based resin in which, when made into a 0.1 wt % aqueous solution, the absorbance (X) at a wavelength of 320 nm in an ultraviolet absorption spectrum is 0.3 or more, and the ratio (Y / X) of the absorbance (Y) at a wavelength of 380 nm to the absorbance (X) at a wavelength of 320 nm is 0.09 or more."

[0005] International Publication No. 2020 / 184397

[0006] Patent Document 1 describes a method for converting a compound of [—CO—(CH═CH) ４It is also described that the PVA-based resin contains at least one of a salt and a hydroxide of a divalent or trivalent metal, and thus the PVA-based resin can be effectively heat-treated (paragraph

[0042] , etc.). For this reason, Patent Document 1 states that the content of the salt and / or hydroxide of a divalent or trivalent metal in the PVA-based resin is preferably 30 μmol / g or more (paragraph

[0044] ), and specifically, a PVA-based resin containing 141 μmol / g of magnesium acetate is produced (paragraph

[0060] ).

[0007] However, when a PVA-based resin having a high content of divalent metal elements and trivalent metal elements is used as a dispersant for suspension polymerization, the insulating properties of the resulting vinyl-based polymer may be reduced, which may affect the performance of the vinyl-based polymer as a wire coating material, etc. In addition, when the content of divalent metal elements and trivalent metal elements is high, discoloration may occur in the PVA-based resin itself and in the vinyl-based polymer obtained when used as a dispersant for suspension polymerization. Furthermore, when the content of divalent metal elements and trivalent metal elements is high, these elements may act as a catalyst and cause undesirable effects such as deterioration in the PVA-based resin itself and in the vinyl-based polymer obtained when used as a dispersant for suspension polymerization. For this reason, it is desirable for PVA-based resins used as dispersants for suspension polymerization, etc. to have a low content of these metal elements.

[0008] Furthermore, when conventional dispersants for suspension polymerization are used, particularly when the amount added is small, it is difficult to carry out stable suspension polymerization, and there is a disadvantage that the particle size of the obtained vinyl polymer particles becomes large. In the above-mentioned Patent Document 1, there is no evaluation at all of the specific performance of the actually obtained PVA-based resin as a dispersant for suspension polymerization.

[0009] The present invention has been made in light of the above circumstances, and an object of the present invention is to provide a polyvinyl alcohol-based resin which has a low content of divalent metal elements and trivalent metal elements and which, when used as a dispersant for suspension polymerization, enables the production of vinyl polymer particles having a small particle size even with a small amount added; a dispersant for suspension polymerization using such a polyvinyl alcohol-based resin; a method for producing vinyl polymer particles; and a method for producing such a polyvinyl alcohol-based resin.

[0010] The above object is to provide a method for producing a 0.1% by mass aqueous solution of a polyvinyl alcohol-based polymer containing substantially only polyvinyl alcohol as a polymer component, with an absorbance Abs of 10 mm at a wavelength of 320 nm. ３２０ The absorbance Abs of a 0.1% by mass aqueous solution at a wavelength of 370 nm and an optical path length of 10 mm is 0.09 or more. ３７０ The absorbance Abs ３２０ The ratio (Abs ３２０ / Abs ３７０ [2] a polyvinyl alcohol-based resin having a ratio (Abs) of 4.9 or less and a total content of divalent metal elements and trivalent metal elements of less than 30 μmol / g; ３２０ / Abs ３７０ [3] the polyvinyl alcohol-based resin according to [1], wherein the absorbance Abs ３２０ [4] the polyvinyl alcohol-based resin of any of [1] to [3], having a viscosity of 4.5 mPa·s or more and 8.0 mPa·s or less at 20°C when made into a 4% by mass aqueous solution; [5] a dispersant for suspension polymerization containing the polyvinyl alcohol-based resin of any of [1] to [4]; [6] a method for producing vinyl polymer particles, comprising a step of suspension polymerizing a vinyl compound using the dispersant for suspension polymerization of [5]; [7] a method for producing a polyvinyl alcohol-based resin, comprising a step of stirring and heating a saponified polyvinyl ester in a vessel, the stirring and heating being carried out at 90°C or more and 180°C or less for 350 minutes or more and 1,300 minutes or less while circulating a gas through the vessel, and the amount of the gas circulating is 2 parts by mass or more and 11 parts by mass or less per 100 parts by mass of the saponified polyvinyl ester.

[0011] According to the present invention, it is possible to provide a polyvinyl alcohol-based resin which has a low content of divalent metal elements and trivalent metal elements and which, when used as a dispersant for suspension polymerization, can produce vinyl polymer particles having a small particle size even when added in a small amount; a dispersant for suspension polymerization using such a polyvinyl alcohol-based resin; and a method for producing such a polyvinyl alcohol-based resin.

[0012] <Polyvinyl alcohol-based resin> The polyvinyl alcohol-based resin (PVA-based resin) of the present invention contains substantially only PVA as a polymer component, and when prepared as a 0.1% by mass aqueous solution, has an absorbance Abs of 10 mm at a wavelength of 320 nm. ３２０ The absorbance Abs of a 0.1% by mass aqueous solution at a wavelength of 370 nm and an optical path length of 10 mm is 0.09 or more. ３７０ The absorbance Abs ３２０ The ratio (Abs ３２０ / Abs ３７０ ) is 4.9 or less, and the total content of divalent metal elements and trivalent metal elements is less than 30 μmol / g.

[0013] The PVA resin has a low content of divalent metal elements and trivalent metal elements, and when used as a dispersant for suspension polymerization, vinyl polymer particles having a small particle size can be produced even with a small amount added. The reason why vinyl polymer particles having a small particle size can be produced even with a small amount added when the PVA resin is used as a dispersant for suspension polymerization is not clear, but the following is presumed. PVA, which is generally used as a dispersant for suspension polymerization, has a structure of R—CO—(CH═CH) ｎ It has a terminal structure represented by - (R is an alkyl group, and n is a natural number). The polyene structure in the terminal structure is formed by a dehydration reaction that occurs when PVA is heat-treated. On the other hand, in the absorption spectrum of PVA, the absorption at a wavelength of 320 nm is represented by -CO-(CH=CH) ３ The absorption at a wavelength of 370 nm is attributed to the structure of -CO-(CH=CH) ４ In other words, in the PVA-based resin, the absorbance Abs ３２０is 0.09 or more, and the absorbance ratio (Abs ３２０ / Abs ３７０ ) is 4.9 or less, it means that -CO-(CH=CH) ３ - structure sufficiently, and further -CO-(CH=CH) ４ This means that the PVA-based resin also contains a sufficient amount of the - structure. This PVA-based resin contains a large amount of relatively long polyene structures (n = 3, 4), which results in sufficient surfactant activity. Therefore, when used as a dispersant for suspension polymerization, it is believed that polymerization stability is enhanced and that even a small amount of the resin can produce vinyl polymer particles with small particle sizes. Furthermore, vinyl polymer particles obtained by using the PVA-based resin as a dispersant for suspension polymerization also have good plasticizer absorption capacity.

[0014] Furthermore, the PVA-based resin having the above absorption spectrum can be obtained by performing a stirring and heating treatment under predetermined conditions while passing a gas through the resin, as described in detail below. This is presumably because the stirring and heating treatment under gas passage facilitates the above-mentioned dehydration reaction, thereby lengthening the polyene structure formed. Therefore, by using this method, a favorable terminal structure can be introduced by heat treatment without using salts and hydroxides of divalent or trivalent metals. For these reasons, even when the PVA-based resin has a total content of divalent and trivalent metal elements of less than 30 μmol / g, even a small amount of the resin can be used as a dispersant for suspension polymerization to produce vinyl polymer particles with small particle size and good plasticizer absorption capacity.

[0015] "Polyvinyl alcohol (PVA)" refers to a polymer having a vinyl alcohol unit (-CH) as a structural unit. ２ PVA generally further contains vinyl ester units and may further contain other structural units. The term "PVA-based resin" refers to a resin that contains PVA as a polymer component and may also contain other optional components.

[0016] The phrase "containing substantially only PVA as the polymer component" means that other polymer components besides PVA may be contained as long as the effects of the present invention are not affected. The polymer component refers to a compound formed by polymerization of monomers. The polymer component may be an organic substance having a molecular weight of 500 or more. The content of PVA in the polymer component may be 95% by mass or more, or may be 99% by mass or more.

[0017] The term "divalent metal element" refers to a metal element that can become a divalent cation. The divalent metal element may be a metal element that can become an ion of another valence, provided that it can become a divalent cation. Similarly, the term "trivalent metal element" refers to a metal element that can become a trivalent cation. The trivalent metal element may be a metal element that can become an ion of another valence, provided that it can become a trivalent cation. The divalent metal element and the trivalent metal element include those that exist in any form, such as a compound, a simple substance, or an ion. The divalent metal element and the trivalent metal element may form a salt with a polar group of the PVA.

[0018] The PVA-based resin of the present invention will be described in detail below.

[0019] The PVA contained in the PVA-based resin of the present invention is a polymer having a vinyl alcohol unit as a structural unit, and is usually obtained by saponifying a polyvinyl ester.

[0020] The lower limit of the saponification degree of PVA is preferably 50 mol%, more preferably 55 mol%, even more preferably 60 mol%, even more preferably 65 mol%, and particularly preferably 68 mol%. On the other hand, the upper limit of the saponification degree may be 100 mol% or 99 mol%, but is preferably 98 mol%, more preferably 95 mol%, even more preferably 90 mol%, even more preferably 85 mol%, and particularly preferably 80 mol%. When the saponification degree of PVA is within the above range, the surfactant properties are optimized, and when the PVA-based resin is used as a dispersant for suspension polymerization, vinyl polymer particles with smaller particle size and better plasticizer absorption ability can be produced. The saponification degree is a value measured by the method described in JIS K6726:1994.

[0021] PVA may have structural units other than vinyl alcohol units and vinyl ester units. Examples of monomers that provide the other structural 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 α-olefins 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 structural units in the total structural units of the PVA may be preferably 20 mol % or less, more preferably 10 mol % or less, 5 mol % or less, or 1 mol % or less, while the proportion of the other structural units may be, for example, 0.1 mol % or more, or 1 mol % or more.

[0023] The lower limit of the viscosity-average degree of polymerization of the PVA is preferably 300, more preferably 500, and even more preferably 700. When the viscosity-average degree of polymerization is equal to or greater than the lower limit, protective colloid properties are enhanced, and when the PVA-based resin is used as a dispersant for suspension polymerization, vinyl polymer particles with smaller particle sizes can be produced. On the other hand, the upper limit of the viscosity-average degree of polymerization is preferably 3,000, more preferably 2,000, and even more preferably 1,500. When the viscosity-average degree of polymerization is equal to or less than the upper limit, surface activity is enhanced, and when the PVA-based resin is used as a dispersant for suspension polymerization, the plasticizer absorption capacity of the resulting vinyl polymer particles is enhanced, and vinyl polymer particles with smaller particle sizes can be produced. The viscosity-average degree of polymerization is a value measured in accordance with JIS K6726:1994. That is, after PVA is resaponified to a degree of saponification of 99.5 mol% or more and purified, the intrinsic viscosity [η] (unit: liter / g) measured in water at 30°C can be calculated by the following formula: Viscosity average polymerization degree = ([η] x 10 ４ / 8.29) （１／０．６２）

[0024] PVA is usually R—CO—(CH═CH) ｎ The PVA-based resin has a terminal structure represented by the formula: - (R is an alkyl group, and n is a natural number). R is preferably an alkyl group having 1 to 4 carbon atoms, more preferably a methyl group or an ethyl group. n may be, for example, an integer of 1 to 4. The PVA-based resin has a terminal structure represented by the formula: R-CO-(CH=CH) ３ PVA having a terminal structure represented by - and R-CO-(CH=CH) ４ and a PVA having a terminal structure represented by R—CO—(CH═CH) ｎ As will be described later, the terminal structure represented by - can be effectively introduced into a polymer by using an aldehyde, a ketone, or the like as a chain transfer agent (modifier) ​​when polymerizing a precursor polyvinyl ester and heat-treating the polymer.

[0025] In the PVA, the lower limit of the block character of the residual vinyl ester units may be preferably 0.40, and may more preferably be 0.42, 0.45, 0.50, or 0.54. On the other hand, the upper limit of this block character may be, for example, 1.

[0026] The block character is a numerical value that represents the distribution of residual esters (usually alkoxycarbonyl groups) and hydroxyl groups generated by saponification of the esters, and takes a value between 0 and 2. 0 indicates that the residual esters or hydroxyl groups are distributed completely in blocks, and as the value increases, the alternation increases, with 1 indicating that the residual esters and hydroxyl groups are completely randomly present, and 2 indicating that the residual esters and hydroxyl groups are completely alternately present. The residual esters are the esters (—O—C(═O)-Q (Q is the CH group contained in the vinyl ester monomer) contained in the vinyl ester unit in the PVA obtained through saponification treatment. ２ = represents a hydrocarbon group other than the CH-O-C(=O) moiety. In other words, the block character is a value that represents the distribution of the remaining vinyl ester units and vinyl alcohol units. １３ When the PVA contains repeating units other than vinyl ester units and / or vinyl alcohol units, the block character is calculated for all sites in the PVA where vinyl ester units and / or vinyl alcohol units are continuous.

[0027] The block character can be adjusted by the type of vinyl ester monomer, saponification conditions such as catalyst and solvent, heat treatment after saponification, etc. When heat treatment is performed after saponification, the block character tends to be 0.40 or more.

[0028] The lower limit of the PVA content in the polymer component of the PVA-based resin is preferably 99% by mass, more preferably 99.9% by mass. The upper limit of the PVA content in the polymer component of the PVA-based resin may be 100% by mass or 99.99% by mass. Examples of polymer components other than PVA that may be contained in the PVA-based resin include polyvinyl esters.

[0029] The lower limit of the PVA content in the non-volatile components of the PVA-based resin is preferably 95% by mass, more preferably 98% by mass, and may be 99% by mass or 99.9% by mass. The upper limit of the PVA content in the non-volatile components of the PVA-based resin may be 100% by mass or 99.99% by mass. Non-volatile components other than PVA that may be contained in the PVA-based resin include polymers other than PVA, compounds used during production, and other impurities. Typically, compounds containing divalent metal elements and trivalent metal elements are also non-volatile components other than PVA. The lower limit of the PVA content in the PVA-based resin is preferably 90% by mass, more preferably 95% by mass, and may be 98% by mass or 99% by mass. The upper limit of the PVA content in the PVA-based resin may be 100% by mass or 99.99% by mass.

[0030] The PVA-based resin may contain a volatile component. Because PVA is hygroscopic and solvents may remain if the PVA-based resin is not dried sufficiently, the PVA-based resin may contain a volatile component in any amount. Examples of the volatile component include water and alcohol. The volatile component may be, for example, a component having a standard boiling point of 250°C or less. The content of the volatile component in the PVA-based resin is usually 10% by mass or less, and preferably 1% by mass or less.

[0031] When the PVA-based resin was dissolved in a 0.1% by mass aqueous solution, the absorbance Abs was measured at a wavelength of 320 nm and an optical path length of 10 mm. ３２０ The lower limit of the absorption at a wavelength of 320 nm is -CO-(CH=CH) in PVA. ３ - structure. Absorbance Abs ３２０When the absorbance Abs is equal to or greater than the lower limit, sufficient surface activity is achieved, and when the PVA resin is used as a dispersant for suspension polymerization, vinyl polymer particles having a small particle size and good plasticizer absorption capacity can be produced. ３２０ is preferably 0.5 or less, more preferably 0.4 or less, even more preferably less than 0.40, even more preferably 0.35 or less, and in some cases even more preferably 0.30 or less or less than 0.30. ３２０ When the PVA-based resin is at least the lower limit and at most the upper limit or less, the surfactant activity is further optimized, and when the PVA-based resin is used as a dispersant for suspension polymerization, vinyl polymer particles having a smaller particle size and better plasticizer absorption ability can be produced. The absorbance of a 0.1% by mass aqueous solution of the PVA-based resin at an optical path length of 10 mm can be measured by a known method, for example, the method described in the Examples.

[0032] When the PVA-based resin was made into a 0.1% by mass aqueous solution, the absorbance Abs at a wavelength of 370 nm and an optical path length of 10 mm was ３７０ The lower limit of the absorbance Abs is preferably 0.01, more preferably 0.03, even more preferably 0.05, and particularly preferably 0.06. ３７０ The upper limit of the absorption at a wavelength of 370 nm is -CO-(CH=CH) in PVA. ４ - structure. Absorbance Abs ３７０ Within the above range, the surfactant activity is further optimized, and therefore, when the PVA resin is used as a dispersant for suspension polymerization, vinyl polymer particles having a smaller particle size and better plasticizer absorption capacity can be produced.

[0033] Absorbance Abs ３７０ Absorbance Abs ３２０ The ratio (Abs ３２０ / Abs ３７０ The upper limit of the absorbance ratio (Abs) is 4.9, preferably 4.7, more preferably 4.5, even more preferably 4.3, even more preferably 4.1, and in some cases even more preferably 3.8 or 3.5. ３２０ / Abs ３７０) is equal to or less than the upper limit, vinyl polymer particles having a small particle size and a good plasticizer absorption capacity can be produced when the PVA resin is used as a dispersant for suspension polymerization. ２８０ When the absorbance ratio (Abs ３２０ / Abs ３７０ When the absorbance ratio (Abs ) is 3.8 or less, or even 3.5 or less, vinyl polymer particles having a smaller particle size tend to be produced when the PVA resin is used as a dispersant for suspension polymerization. ３２０ / Abs ３７０ The lower limit of the absorbance ratio (Abs) is preferably 2.0, more preferably 2.4, and even more preferably 2.8. ３２０ / Abs ３７０ ) is equal to or greater than the lower limit, vinyl polymer particles having a smaller particle size and better plasticizer absorption capacity can be produced when the PVA resin is used as a dispersant for suspension polymerization.

[0034] When the PVA-based resin was dissolved in a 0.1% by mass aqueous solution, the absorbance Abs was measured at a wavelength of 280 nm and an optical path length of 10 mm. ２８０ The lower limit of the absorbance Abs is preferably 0.1, more preferably 0.2, even more preferably 0.26, and particularly preferably 0.28, 0.30, or 0.32. ２８０ The upper limit of the absorption at a wavelength of 280 nm is -CO-(CH=CH) in PVA. ２ - structure. Absorbance Abs ２８０ When the PVA resin is in the above range, the surfactant ability is further optimized, and when the PVA resin is used as a dispersant for suspension polymerization, vinyl polymer particles having a smaller particle size can be produced.

[0035] The total content of divalent metal elements and trivalent metal elements in the PVA-based resin is less than 30 μmol / g. The PVA-based resin does not necessarily contain divalent metal elements and trivalent metal elements. When the PVA-based resin contains divalent metal elements and trivalent metal elements, the total content of divalent metal elements and trivalent metal elements in the PVA-based resin is less than 30 μmol / g. When the PVA-based resin is used as a dispersant for suspension polymerization, a vinyl-based polymer having excellent insulating properties and reduced coloration can be obtained. Furthermore, when the total content of divalent metal elements and trivalent metal elements is less than 30 μmol / g, coloration of the PVA-based resin itself is also suppressed. Furthermore, by having the total content of divalent metal elements and trivalent metal elements be less than 30 μmol / g, deterioration of the PVA-based resin due to the catalytic action of these metal elements is suppressed. In the case of monovalent metal elements such as sodium, even when the content is, for example, about 30 μmol / g, discoloration and deterioration are less likely to occur compared to when divalent metal elements and trivalent metal elements are contained. The upper limit of the total content of the divalent metal elements and trivalent metal elements is preferably 10 μmol / g, more preferably 1 μmol / g, even more preferably 0.1 μmol / g, and even more preferably 0.05 μmol / g. The lower limit of the total content of the divalent metal elements and trivalent metal elements may be 0 μmol / g, 0.0001 μmol / g, or even 0.001 μmol / g. The PVA-based resin may inevitably contain divalent metal elements and trivalent metal elements. The PVA-based resin may not contain any divalent metal element or trivalent metal element. The total content of the divalent metal element and the trivalent metal element in the PVA-based resin can be measured by a known method, for example, by ICP emission spectrometry as described in the Examples. The total content of the divalent metal element and the trivalent metal element is based on the entire PVA-based resin, including any volatile components that may be contained.

[0036] The divalent metal element and the trivalent metal element may be any metal element other than alkali metal elements, and may be Al, Ba, Ca, Cd, Co, Cr, Cu, Fe, Mg, Mn, Mo, Ni, Pb, Sb, Ti, V, and Zn.

[0037] The lower limit of the viscosity of a 4% by mass aqueous solution of the PVA-based resin at 20°C is preferably 4.5 mPa·s, more preferably 5.0 mPa·s, even more preferably 5.5 mPa·s, and even more preferably 6.0 mPa·s. On the other hand, the upper limit of this viscosity may be preferably 8.0 mPa·s, more preferably 7.5 mPa·s, and even more preferably 7.0 mPa·s. When the viscosity is within the above range, vinyl polymer particles having a smaller particle size can be produced when the PVA-based resin is used as a dispersant for suspension polymerization. The viscosity is a value measured by the method described in JIS K6726:1994.

[0038] The lower limit of the yellowness index (YI) when the PVA-based resin is prepared into a 1% by mass aqueous solution is preferably 10, more preferably 20, and even more preferably 30. On the other hand, the upper limit of the yellowness index (YI) is preferably 70, more preferably 60, and even more preferably 50. When the yellowness index (YI) is within the above range, sufficient surfactant activity can be exhibited while suppressing coloration, and when the PVA-based resin is used as a dispersant for suspension polymerization, vinyl polymer particles having a smaller particle size and better plasticizer absorption ability can be produced. The yellowness index (YI) is a value measured by the method described in ASTM E313-05.

[0039] The shape of the PVA-based resin is not particularly limited, but is usually preferably particulate (powdered). When particulate, the PVA-based resin preferably has an average particle size of, for example, 100 μm or more and 1,000 μm or less. The average particle size of the PVA-based resin is a value measured in accordance with the method described in JIS K7369:2009.

[0040] The PVA-based resin can be used in various applications similar to those of conventionally known PVA-based resins, such as raw materials for films and fibers, additives for paper processing and fiber processing, adhesives, dispersants for emulsion polymerization and suspension polymerization, binders for inorganic materials, etc. Among these, the PVA-based resin can be particularly suitably used as a dispersant for suspension polymerization of vinyl compounds, etc., as will be described in detail later.

[0041] <Method for Producing PVA-Based Resin> The method for producing a PVA-based resin of the present invention is not particularly limited, but the following method is preferred. That is, the method for producing a PVA-based resin of the present invention includes a step (step C) of stirring and heating a saponified polyvinyl ester (unheat-treated PVA) in a container. The stirring and heating in step C is carried out at 90°C or higher and 180°C or lower for 350 minutes or longer and 1,300 minutes or shorter while circulating a gas through the container. The flow rate of the gas in step C is 2 parts by mass or higher and 11 parts by mass or lower per hour relative to 100 parts by mass of the saponified polyvinyl ester. This production method can produce a PVA-based resin that has low contents of divalent metal elements and trivalent metal elements and, when used as a dispersant for suspension polymerization, can produce vinyl polymer particles with small particle sizes even when added in a small amount.

[0042] The production method may further include, before step C, a step (step A) of polymerizing a vinyl ester monomer to obtain a polyvinyl ester, and a step (step B) of saponifying the polyvinyl ester.

[0043] The method for producing the PVA-based resin will be described in detail below in the order of steps.

[0044] (Step A) In step A, a polyvinyl ester is obtained by polymerizing a vinyl ester monomer. The polyvinyl ester may be a homopolymer of a vinyl ester, or a copolymer of multiple types of vinyl esters or a vinyl ester with another monomer. Methods for polymerizing a vinyl ester monomer include known methods such as bulk polymerization, solution polymerization, suspension polymerization, and emulsion polymerization. Among these methods, bulk polymerization carried out without a solvent and solution polymerization carried out using a solvent such as alcohol are preferred, and solution polymerization carried out in the presence of a lower alcohol is more preferred. As the lower alcohol, an alcohol having 3 or less carbon atoms is preferred, with methanol, ethanol, n-propanol, and isopropanol being more preferred, and methanol being even more preferred. When carrying out the polymerization reaction by bulk polymerization or solution polymerization, either a batch system or a continuous system can be used.

[0045] Examples of vinyl ester monomers include vinyl formate, vinyl acetate, vinyl propionate, vinyl valerate, vinyl caprate, vinyl laurate, vinyl stearate, vinyl benzoate, vinyl pivalate, vinyl versatate, etc. Of these, vinyl acetate is preferred.

[0046] Examples of the polymerization initiator used in the polymerization reaction include known polymerization initiators such as azo initiators such as 2,2'-azobisisobutyronitrile, 2,2'-azobis(2,4-dimethylvaleronitrile), 2,2'-azobis(4-methoxy-2,4-dimethylvaleronitrile), etc.; and organic peroxide initiators such as benzoyl peroxide, n-propyl peroxycarbonate, diisopropyl peroxydicarbonate, etc. The polymerization temperature when carrying out the polymerization reaction is not particularly limited, but is suitably in the range of 5°C to 200°C.

[0047] When polymerizing the vinyl ester monomer, a copolymerizable monomer can be further copolymerized within the scope of the present invention. Examples of such copolymerizable monomers include those mentioned above as monomers that provide structural units other than vinyl alcohol units and vinyl ester units that may be contained in PVA.

[0048] From the viewpoint of efficiently obtaining a PVA having the above-described terminal structure, it is preferable to allow a predetermined chain transfer agent (modifier) ​​to coexist during polymerization of the vinyl ester monomer. Examples of such chain transfer agents include aldehydes having an alkyl group, such as acetaldehyde, propionaldehyde, and butylaldehyde, and ketones having an alkyl group, such as acetone and methyl ethyl ketone. These chain transfer agents may be used alone or in combination. Furthermore, during polymerization of the vinyl ester monomer, a chain transfer agent other than the above-described aldehydes having an alkyl group or ketones having an alkyl group may be further allowed to coexist, for the purpose of adjusting the degree of polymerization of the resulting PVA. Examples of such chain transfer agents include aldehydes other than aldehydes having an alkyl group; ketones other than ketones having an alkyl group; mercaptans such as 2-hydroxyethanethiol; thiocarboxylic acids such as thioacetic acid; and halogenated hydrocarbons such as trichloroethylene and perchloroethylene. The amount of the chain transfer agent to be added is determined depending on the chain transfer constant of the chain transfer agent to be added and the desired degree of polymerization of the PVA, but is generally preferably 0.1 to 10% by mass based on the vinyl ester used.

[0049] (Step B) In step B, for example, polyvinyl ester is saponified in an alcohol solution using an alkali catalyst or an acid catalyst. The saponification reaction of polyvinyl ester 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 of two or more. Among these, it is preferable to use methanol or a mixed solution of methanol and methyl acetate as the solvent and carry out the saponification reaction in the presence of sodium hydroxide as a basic catalyst, as this is simple and convenient.

[0050] The saponification can be carried out using a belt reactor, a kneader reactor, a column reactor, or the like. A saponified polyvinyl ester solid is obtained through step B. This solid may be subjected to pulverization or the like before being subjected to step C. That is, the production method may include a step of pulverizing the saponified polyvinyl ester solid between steps B and C. Furthermore, the solid may be subjected to a washing treatment or a drying treatment before or after pulverization.

[0051] (Step C) In step C, the saponified polyvinyl ester (unheat-treated PVA) is stirred and heated in a container. This container may be a heat treatment furnace. This container is an open container that has a structure that allows gas to pass through.

[0052] The shape of the saponified polyvinyl ester subjected to step C is not particularly limited, but is preferably particulate, and more preferably particulate with an average particle size of 100 μm or more and 1,000 μm or less. When the average particle size of the saponified polyvinyl ester particles is 100 μm or more, scattering as dust by the flowing gas is suppressed. On the other hand, when the average particle size of the saponified polyvinyl ester particles is 1,000 μm or less, the water and organic solvent contents in the particles tend to be low, and fusion of the particles is suppressed. The average particle size of the saponified polyvinyl ester particles is a value measured in accordance with the method described in JIS K7369:2009.

[0053] Examples of the apparatus used for heating and stirring in step C include a rotary kiln in which the container rotates, a planetary motion mixer equipped with screw blades that rotate and revolve within the container, and a mixer in which a rotating screw, baffle, or the like is provided within the container. In a rotary kiln in which the container rotates, a rotating screw, baffle, or the like may be provided within the container. All of these apparatuses are configured to allow gas to circulate within the container. The rotation speed of the container or the screw, or the like, provided within the container in step C can be, for example, 1 rpm or more and 20 rpm or less.

[0054] The lower limit of the treatment temperature in step C is 90° C., preferably 100° C., more preferably 110° C., even more preferably 120° C., and even more preferably 130° C. By setting the treatment temperature to the above lower limit or higher, sufficient heat treatment can be performed, and the absorbance Abs ３７０ As a result, when used as a dispersant for suspension polymerization, even a small amount of the PVA-based resin can be added to produce vinyl polymer particles having a small particle size and good plasticizer absorption capacity. The upper limit of the treatment temperature is 180°C, preferably 170°C, more preferably 160°C, and even more preferably 150°C. By setting the treatment temperature at or below the upper limit, crosslinking of the PVA can be suppressed. When a crosslinked PVA-based resin is used as a dispersant for suspension polymerization, the resulting vinyl polymer will contain a large amount of insoluble matter, which can cause fisheyes.

[0055] The lower limit of the treatment time in step C is 350 minutes, preferably 400 minutes, more preferably 500 minutes, and even more preferably 600 minutes. By setting the treatment time to the above lower limit or more, sufficient heat treatment is performed, and the absorbance Abs ３７０ As a result, when used as a dispersant for suspension polymerization, vinyl polymer particles having a small particle size and good plasticizer absorption capacity can be produced even with a small amount added. On the other hand, the upper limit of this treatment time is 1,300 minutes, preferably 1,200 minutes, more preferably 1,100 minutes, and even more preferably 1,000 minutes. By setting the treatment time to the above upper limit or less, crosslinking of the PVA and insolubilization, excessive coloration, etc. tend to be suppressed.

[0056] The lower limit of the amount of gas flowing into the vessel in step C is 2 parts by mass, preferably 2.5 parts by mass, and more preferably 3 parts by mass per hour relative to 100 parts by mass of the saponified polyvinyl ester. By setting the amount of gas flowing to the lower limit or more, the polyenation reaction proceeds sufficiently, and the absorbance Abs ３７０A PVA-based resin having sufficiently high properties such as high viscosity, high viscosity, and low viscosities can be obtained. As a result, when the obtained PVA-based resin is used as a dispersant for suspension polymerization, vinyl polymer particles having a small particle size and good plasticizer absorption capacity can be produced even with a small amount added. On the other hand, the upper limit of the amount of gas flow is 11 parts by mass, preferably 10 parts by mass, and more preferably 9 parts by mass. By setting the amount of gas flow to the above lower limit or less, coloration of the obtained PVA-based resin can be suppressed, and as a result, when the PVA-based resin is used as a dispersant for suspension polymerization, the color of the vinyl polymer obtained can be made good.

[0057] Examples of the gas to be circulated in step C include nitrogen, oxygen, and a mixture of these gases (air, etc.), and air is preferably used.

[0058] <Suspension Polymerization Dispersant> The suspension polymerization dispersant of the present invention contains the PVA-based resin of the present invention described above. The suspension polymerization dispersant is an additive used to improve the dispersibility and polymerization stability of monomers during suspension polymerization and to control the particle size of the resulting polymer particles. The lower limit of the content of the PVA-based resin of the present invention in the suspension polymerization dispersant of the present invention is preferably 50% by mass, more preferably 70% by mass, and even more preferably 90% by mass or 99% by mass. The upper limit of the content of the PVA-based resin of the present invention in the suspension polymerization dispersant of the present invention may be 100% by mass. The suspension polymerization dispersant of the present invention may consist solely of the PVA-based resin of the present invention. Components other than the PVA-based resin that may be contained in the suspension polymerization dispersant of the present invention include other resins, additives such as surfactants and plasticizers, and various compounds used during production. The shape of the suspension polymerization dispersant of the present invention is not particularly limited, but is usually a powder.

[0059] The dispersant for suspension polymerization of the present invention is suitable as a dispersant for suspension polymerization of vinyl compounds. By using the dispersant for suspension polymerization of the present invention, polymerization stability is improved and polymer particles with small particle sizes can be efficiently obtained. Furthermore, the polymer particles obtained by suspension polymerization using the dispersant for suspension polymerization of the present invention also have good plasticizer absorption capacity.

[0060] <Method for producing vinyl polymer particles> A method for producing vinyl polymer particles includes a step of suspension polymerizing a vinyl compound using the dispersant for suspension polymerization of the present invention. The production method is the same as known methods for producing vinyl polymer particles, except that the dispersant for suspension polymerization of the present invention is used as the dispersant.

[0061] The method for producing vinyl polymer particles typically involves suspension polymerization of a vinyl compound in an aqueous medium using the dispersant for suspension polymerization of the present invention. The aqueous medium may be pure water, an aqueous solution containing various additives, or an aqueous medium containing other organic solvents.

[0062] When carrying out suspension polymerization of a vinyl compound, the amount of the dispersant for suspension polymerization of the present invention to be added is not particularly limited, but the lower limit is preferably 100 ppm, more preferably 300 ppm, and even more preferably 500 ppm, based on the mass of the vinyl compound. On the other hand, the upper limit is preferably 50,000 ppm, more preferably 10,000 ppm, and even more preferably 5,000 ppm, 2,000 ppm, or 1,000 ppm. By using the dispersant for suspension polymerization of the present invention, vinyl polymer particles having a small particle size can be obtained even with such a small amount added.

[0063] The dispersant for suspension polymerization of the present invention may be used alone or in combination with other dispersants. Examples of other dispersants include water-soluble cellulose ethers such as methyl cellulose, hydroxyethyl cellulose, hydroxypropyl cellulose, and hydroxypropylmethyl cellulose, which are commonly used when a vinyl compound is suspended in an aqueous medium, water-soluble polymers such as polyvinyl alcohol and gelatin, oil-soluble emulsifiers such as sorbitan monolaurate, sorbitan trioleate, glycerin tristearate, and ethylene oxide-propylene oxide block copolymers, and water-soluble emulsifiers such as polyoxyethylene sorbitan monolaurate, polyoxyethylene glycerin oleate, and sodium laurate.

[0064] As the polymerization initiator used in the method for producing vinyl polymer particles, those conventionally used for the polymerization of vinyl compounds can be used. Specifically, the same polymerization initiators as those exemplified for the polymerization of the vinyl ester monomers can be used.

[0065] In the method for producing vinyl polymer particles, 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, and polymerization inhibitors such as phenol compounds, sulfur compounds, and N-oxide compounds. pH adjusters, scale inhibitors, and crosslinking agents may also be added. Multiple additives described above may be used in combination.

[0066] Examples of vinyl compounds that can be suspension polymerized in the method for producing vinyl polymer particles include vinyl halides such as vinyl chloride; vinyl esters such as vinyl acetate and vinyl propionate; acrylic acid, methacrylic acid, esters and salts thereof; maleic acid, fumaric acid, esters and anhydrides thereof; styrene; acrylonitrile; vinylidene chloride; vinyl ethers, etc. Among these vinyl compounds, vinyl chloride is preferred. The method for producing vinyl polymer particles is particularly suitable for use in suspension polymerizing vinyl chloride alone or together with vinyl chloride and a monomer copolymerizable with vinyl chloride. Examples of monomers that can be copolymerized with vinyl chloride include 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 ethers, etc.

[0067] In the method for producing vinyl polymer particles, when a vinyl compound is suspension polymerized, the charging ratio of each component, polymerization temperature, polymerization time, etc. may be the same as those conventionally used in the suspension polymerization of vinyl compounds such as vinyl chloride. There are also no limitations on the charging order or ratio of the vinyl compound, polymerization initiator, dispersant, aqueous medium, and other additives.

[0068] The present invention will be described in more detail with reference to the following examples, but the present invention is not limited to these examples. The measurement and evaluation methods employed in the following examples and comparative examples are shown below.

[0069] [Analysis of PVA-Based Resins] The following analyses were carried out using powder samples divided according to the sampling method specified in JIS K6726:1994.

[0070] (1) Degree of Saponification: Measured in accordance with JIS K6726:1994.

[0071] (2) Viscosity of 4% aqueous solution: Measured in accordance with JIS K6726:1994.

[0072] (3) Total Content of Divalent Metal Elements and Trivalent Metal Elements The total content of divalent metal elements and trivalent metal elements was measured by ICP atomic emission spectroscopy using a Thermo Fisher Scientific ICP atomic emission spectrometer "iCAP-6500Duo." Specifically, the detected masses of Al, Ba, Ca, Cd, Co, Cr, Cu, Fe, Mg, Mn, Mo, Ni, Pb, Sb, Ti, V, and Zn were converted into the amount of substance in atomic weight, and the sum was taken as the total content of divalent metal elements and trivalent metal elements. Note that in all examples and comparative examples, no divalent metal elements or trivalent metal elements other than Al, Ca, Cr, Cu, Fe, Mg, Mn, and Zn were detected.

[0073] (4) Yellowness Index of Aqueous Solution 1 g of PVA-based resin was precisely weighed and dissolved in distilled water to prepare a 1.0 mass % aqueous solution. The YI of this aqueous solution was measured using a Color Meter ZE6000 color difference meter manufactured by Nippon Denshoku Kogyo Co., Ltd. in accordance with ASTM E313-05.

[0074] (5) Absorbance of Aqueous Solution 0.1 g of a PVA-based resin was precisely weighed and dissolved in distilled water to prepare a 0.1% by mass aqueous solution. The ultraviolet-visible absorption spectrum of this aqueous solution was measured using a Shimadzu UV-1800 ultraviolet-visible spectrophotometer in a quartz cell container with an optical path length of 10 mm.

[0075] [Evaluation of Vinyl Chloride Polymer Particles] (1) Average Particle Size The particle size distribution was measured by dry sieve analysis using a wire mesh based on a Tyler mesh standard, and the average particle size was determined.

[0076] (2) Plasticizer Absorption Capacity (CPA) The amount of dioctyl phthalate absorbed at 23° C. was measured by the method described in ASTM-D3367-75.

[0077] Example 1 (Production of PVA-Based Resin) 10 kg of particulate saponified polyvinyl ester (resin) was prepared as a raw material. The saponification degree was 72.4 mol%, the viscosity of a 4% aqueous solution was 6.2 mPa·s, the total content of divalent and trivalent metal elements was 0.009 μmol / g, and the average particle size was 570 μm. This resin was obtained by partially saponifying polyvinyl ester obtained using acetaldehyde as a chain transfer agent with sodium hydroxide as a catalyst. The raw material was placed in a heat treatment furnace (container), and heat treatment was carried out at a rotation speed of 5 rpm, a treatment temperature of 135°C, and a treatment time of 1,200 minutes while circulating air at a rate of 3.0 to 9.0 parts by mass / hr·100 parts by mass per 100 parts by mass of the resin (raw material). This produced the PVA-based resin of Example 1.

[0078] The heat treatment furnace used was a rotary kiln-type device with an internal volume of 50 L, equipped with a jacket and baffles for stirring the resin inside. The heat treatment furnace was positioned so that its rotation axis was horizontal. The device was configured so that gas flow ports were provided at both ends of the rotation axis, allowing a predetermined amount of air to flow inside the heat treatment furnace (container) in the direction of the rotation axis. Steam was used as the heat source.

[0079] The saponification degree, 4% aqueous solution viscosity, total content of divalent metal elements and trivalent metal elements, yellowness of the aqueous solution, and absorbance (Abs) of the aqueous solution of the obtained PVA-based resin were measured. ２８０ , Abs ３２０ and Abs ３７０ ) were measured. These measurement results and the absorbance ratio (Abs ３２０ / Abs ３７０ ) are shown in Table 1.

[0080] (Production of vinyl polymer particles) 1,390 g of an aqueous solution dissolving 0.94 g of the obtained PVA-based resin (1,000 ppm relative to the vinyl chloride monomer) was charged into a 5 L autoclave. Next, 0.6 g of cumyl peroxy neodecanoate (Percumyl ND-R, manufactured by NOF Corp.) and 0.9 g of t-butyl peroxy neodecanoate (Perbutyl ND-R, manufactured by NOF Corp.) were charged into the autoclave. The autoclave was filled with nitrogen gas and degassed six times until the pressure inside the autoclave reached 0.3 MPa, thereby removing oxygen. Thereafter, 940 g of vinyl chloride was charged, and the contents inside the autoclave were heated to 57°C, and suspension polymerization was initiated with stirring. The pressure inside the autoclave at the start of polymerization was 0.83 MPa. Four hours after the start of suspension polymerization, when the pressure inside the autoclave reached 0.70 MPa, the polymerization was terminated, and unreacted vinyl chloride was removed. Thereafter, the polymerized slurry was taken out and dried overnight at 65° C. to obtain vinyl chloride polymer particles.

[0081] The average particle size and plasticizer absorption capacity of the obtained polyvinyl chloride (PVC) particles were measured. The results are shown in Table 1.

[0082] [Examples 2 to 5, Comparative Examples 1 and 2] PVA-based resins of Examples 2 to 5 and Comparative Examples 1 and 2 were obtained in the same manner as in Example 1, except that the raw materials and heat treatment conditions were as shown in Table 1. The saponification degree, viscosity of 4% aqueous solution, total content of divalent metal elements and trivalent metal elements, yellowness of the aqueous solution, and absorbance (Abs) of the aqueous solution of each of the obtained PVA-based resins were measured. ２８０ , Abs ３２０ and Abs ３７０ ) were measured. These measurement results and the absorbance ratio (Abs ３２０ / Abs ３７０ ) are shown in Table 1.

[0083] Vinyl chloride polymer particles were obtained in the same manner as in Example 1 using the PVA-based resins obtained in Examples 2 to 5 and Comparative Example 1. Furthermore, since the PVA-based resin of Comparative Example 2 had low polymerization stability, vinyl chloride polymer particles were obtained in the same manner as in Example 1, except that the PVA-based resin of Comparative Example 2 was used in an amount of 1,200 ppm relative to the vinyl chloride monomer. Note that when vinyl chloride polymer particles were produced using the PVA-based resin of Comparative Example 2 under the same conditions as in the other Examples, the vinyl chloride polymer became coarse and scale formed, resulting in results that were clearly unfavorable compared to the results of Comparative Example 2 in Table 1. The average particle size and plasticizer absorption capacity of the obtained vinyl chloride polymer (PVC) particles were measured. The measurement results are shown in Table 1.

[0084]

[0085] As shown in Table 1, the PVA-based resins of Examples 1 to 5 had an absorbance Abs ３２０ is 0.09 or more, and the absorbance ratio (Abs ３２０ / Abs ３７０ ) was 4.9 or less, and when used as a dispersant for suspension polymerization, vinyl polymer particles having an average particle size of 165 μm or less could be produced even at an addition amount of 1,000 ppm relative to vinyl chloride monomer. The vinyl polymer particles obtained by using the PVA resins of Examples 1 to 5 as dispersants for suspension polymerization also had a plasticizer absorption capacity of 25% or more, which was a good value. In addition, by comparing Examples 1 to 5 with Comparative Examples 1 and 2, the absorbance ratio (Abs ３２０ / Abs ３７０ It can be seen that the effect of having a ρ of 4.9 or less is that the particle size of the obtained vinyl polymer particles can be reduced, among other properties required of a dispersant for suspension polymerization. Furthermore, the PVA-based resins of Examples 1 to 5 had a very low total content of divalent metal elements and trivalent metal elements of less than 30 μmol / g.

[0086] Furthermore, as in Examples 1 to 5, by carrying out the heat treatment (stirring and heating) at 90°C or higher and 180°C or lower for 350 minutes or longer and 1,300 minutes or shorter while circulating gas in the container at a predetermined flow rate, it is possible to obtain a high absorbance Abs３２０ is 0.09 or more, and the absorbance ratio (Abs ３２０ / Abs ３７０ It was confirmed that a PVA-based resin having an absorbance Abs of 4.9 or less was obtained. In Comparative Example 1, the amount of gas flow was small, and in Comparative Example 2, the treatment time was short. ３２０ is 0.09 or more, and the absorbance ratio (Abs ３２０ / Abs ３７０ ) of 4.9 or less could not be obtained.

[0087] The PVA resin of the present invention can be used as a dispersant in the suspension polymerization of vinyl compounds.

Claims

1. The polymer component contains substantially only polyvinyl alcohol, Absorbance Abs when made into a 0.1% by mass aqueous solution at an optical path length of 10 mm and a wavelength of 320 nm ３２０ is 0.09 or more, Absorbance Abs when made into a 0.1% by mass aqueous solution at an optical path length of 10 mm and a wavelength of 370 nm ３７０ The absorbance Abs ３２０ The ratio of (Abs ３２０ / Abs ３７０ ) is 4.9 or less, A polyvinyl alcohol-based resin having a total content of divalent metal elements and trivalent metal elements of less than 30 μmol / g.

2. The above ratio (Abs ３２０ / Abs ３７０ 2. The polyvinyl alcohol resin according to claim 1, wherein the molecular weight ratio (Mw) is 2.0 or more.

3. The absorbance Abs ３２０ The polyvinyl alcohol-based resin according to claim 1 , wherein the polyvinyl alcohol-based resin has a molecular weight of less than 0.

30.

4. The polyvinyl alcohol-based resin according to claim 1, wherein the polyvinyl alcohol-based resin has a viscosity of 4.5 mPa·s or more and 8.0 mPa·s or less at 20° C. when made into a 4% by mass aqueous solution.

5. A dispersant for suspension polymerization comprising the polyvinyl alcohol resin according to any one of claims 1 to 4.

6. A method for producing vinyl polymer particles, comprising a step of suspension polymerizing a vinyl compound using the dispersant for suspension polymerization according to claim 5.

7. A step of stirring and heating the saponified polyvinyl ester in a container Equipped with The stirring and heating is carried out at 90° C. or more and 180° C. or less for 350 minutes or more and 1,300 minutes or less while circulating a gas through the container; the amount of the gas flowing is 2 parts by mass or more and 11 parts by mass or less per hour relative to 100 parts by mass of the saponified polyvinyl ester.