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

Technical Field

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

Background Art

[0002] In the industrial production of vinyl polymers such as polyvinyl chloride, suspension polymerization is widely carried out in which vinyl compounds such as vinyl chloride are dispersed in an aqueous medium in the presence of a dispersant (sometimes called a dispersion stabilizer), and polymerization is carried out using an oil-soluble catalyst. Generally, 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 influence of the type of dispersant is great.

[0003] Performance required for a dispersant for suspension polymerization of vinyl compounds includes that vinyl polymer particles can be stably obtained with a small addition amount. Conventionally, as a dispersant for suspension polymerization of vinyl compounds, in addition to cellulose derivatives such as methyl cellulose and carboxymethyl cellulose, partially saponified polyvinyl alcohol, etc. are 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 the absorbance (X) at a wavelength of 320 nm in the ultraviolet absorption spectrum when it is a 0.1% by weight aqueous solution 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".

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0006] In Patent Document 1, as a result of sufficiently forming the [-CO-(CH=CH)4-] structure by heat treatment, a PVA-based resin having a predetermined absorbance can be obtained (paragraph

[0011] ), and at least one of a salt and a hydroxide of a divalent to trivalent metal is contained in the PVA-based resin, so that heat treatment can be effectively performed (paragraph

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

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

[0060] ).

[0007] However, when a PVA-based resin having a high content of divalent and trivalent metal elements is used as a dispersant for suspension polymerization, the insulation of the obtained vinyl-based polymer may be reduced, which may affect the performance of the vinyl-based polymer as an electric wire coating material or the like. In addition, when the content of divalent and trivalent metal elements is high, it may cause coloring of the PVA-based resin itself and the vinyl-based polymer obtained when used as a dispersant for suspension polymerization. Further, when the content of divalent and trivalent metal elements is high, these may act as a catalyst and may have an unfavorable influence such as deterioration in the PVA-based resin itself and the vinyl-based polymer obtained when used as a dispersant for suspension polymerization. Therefore, it is desired that the content of these metal elements in the PVA-based resin used as a dispersant for suspension polymerization or the like is low.

[0008] In addition, conventional dispersants for suspension polymerization are particularly difficult to perform suspension polymerization stably when the addition amount is small, and there is a disadvantage that the particle diameter of the obtained vinyl-based polymer particles becomes large. In the above Patent Document 1, no specific performance of the actually obtained PVA-based resin as a dispersant for suspension polymerization has been evaluated at all.

[0009] The present invention has been made based on the circumstances described above, and aims to provide a polyvinyl alcohol-based resin that has a low content of divalent and trivalent metal elements and, when used as a dispersant for suspension polymerization, can produce vinyl polymer particles with small particle sizes even with a small amount of additive; 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. [Means for solving the problem]

[0010] The above purpose is, [1] The polymer component consists substantially only of polyvinyl alcohol, and the absorbance Abs of a 0.1% by mass aqueous solution at a path length of 10 mm and wavelength of 320 nm is measured. 320 The value is 0.09 or higher, and the absorbance Abbs is measured at a path length of 10 mm and wavelength of 370 nm when the solution is a 0.1 mass% aqueous solution. 370 Absorbance Abs for the above 320 The ratio (Abs 320 / Abs 370 A polyvinyl alcohol-based resin having a concentration of 4.9 or less and a total content of divalent and trivalent metal elements of less than 30 μmol / g; [2] The above ratio (Abs 320 / Abs 370 A polyvinyl alcohol-based resin of [1] having a coefficient of 2.0 or higher; [3] Absorbance Abs 320 Polyvinyl alcohol resins of [1] or [2] having a ratio of less than 0.30; [4] A polyvinyl alcohol-based resin of any of the following types [1] to [3], having a viscosity of 4.5 mPa·s or more and 8.0 mPa·s or less at 20°C when prepared as a 4% by mass aqueous solution; A suspension polymerization dispersant containing any of the polyvinyl alcohol-based resins listed in [5][1] to [4]; A method for producing vinyl polymer particles, comprising the step of suspension polymerization of a vinyl compound using the suspension polymerization dispersant [6][5]; [7] A step of stirring and heating the saponified polyvinyl ester in a container, wherein the stirring and heating 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 allowing a gas to flow through the container, and the flow rate of the gas is 2 parts by mass or more and 11 parts by mass or less per hour with respect to 100 parts by mass of the saponified polyvinyl ester, a method for producing a polyvinyl alcohol-based resin; It is achieved by providing any of them.

Effects of the Invention

[0011] According to the present invention, there are provided a polyvinyl alcohol-based resin having a low content of divalent metal elements and trivalent metal elements and capable of producing vinyl polymer particles having a small particle diameter even with a small addition amount when used as a dispersant for suspension polymerization, a dispersant for suspension polymerization using such a polyvinyl alcohol-based resin, and a method for producing such a polyvinyl alcohol-based resin.

Embodiments for Carrying Out the Invention

[0012] <Polyvinyl alcohol-based resin> The polyvinyl alcohol-based resin (PVA-based resin) of the present invention substantially contains only PVA as a polymer component, and the absorbance Abs at an optical path length of 10 mm and a wavelength of 320 nm when it is a 0.1 mass% aqueous solution 320 is 0.09 or more, and the absorbance Abs at an optical path length of 10 mm and a wavelength of 370 nm when it is a 0.1 mass% aqueous solution 370 with respect to 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-based resin has a low content of divalent and trivalent metal elements, and when used as a dispersant for suspension polymerization, it is possible to produce vinyl polymer particles with small particle sizes even with a small amount of additive. The reason why this PVA-based resin can produce vinyl polymer particles with small particle sizes even with a small amount of additive is not entirely clear, but the following is speculated: PVA, which is generally used as a dispersant for suspension polymerization, is 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 above terminal structure is formed by a dehydration reaction caused by heat treatment of PVA. On the other hand, regarding the absorption spectrum of PVA, the absorption at a wavelength of 320 nm is attributed to the -CO-(CH=CH)3- structure, and the absorption at a wavelength of 370 nm is attributed to the -CO-(CH=CH)4- structure. That is, in this PVA-based resin, the absorbance Abs 320 The ratio of absorbance (Abs) is 0.09 or higher. 320 / Abs 370 A value of 4.9 or less means that the resin has a sufficient amount of -CO-(CH=CH)3- and also a sufficient amount of -CO-(CH=CH)4-. Thus, the PVA-based resin has a large amount of relatively long polyene structures (n=3, 4), and as a result, sufficient surface activity is generated. Therefore, when used as a dispersant for suspension polymerization, polymerization stability is increased, and it is thought that vinyl polymer particles with small particle sizes can be produced even with a small amount of additive. 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 stirring and heating under predetermined conditions while circulating a gas, as will be described in detail later. This is presumed to be because stirring and heating while circulating a gas facilitates the dehydration reaction described above, resulting in a longer polyene structure. Therefore, by using this method, a good end structure can be introduced by heat treatment without using divalent or trivalent metal salts and hydroxides. For these reasons, even though the total content of divalent and trivalent metal elements in this PVA-based resin is less than 30 μmol / g, when used as a dispersant for suspension polymerization, it is possible to produce vinyl polymer particles with small particle size and good plasticizer absorption capacity even with a small amount of additive.

[0015] Furthermore, "polyvinyl alcohol (PVA)" refers to a polymer having vinyl alcohol units (-CH2CHOH-) as structural units. PVA usually further contains vinyl ester units and may also contain other structural units. Also, "PVA-based resin" refers to a resin that contains PVA as a polymer component and may also contain other optional components.

[0016] "Containing substantially only PVA as the polymer component" means that other polymer components other than PVA may be included to the extent that they do not affect the effects of the present invention. A polymer component refers to a compound formed by the polymerization of monomers. A polymer component may be an organic substance with a molecular weight of 500 or more. The PVA content in the polymer component may be 95% by mass or more, or 99% by mass or more.

[0017] A "divalent metal element" refers to a metal element that can form a divalent cation. A divalent metal element may also be a metal element that can form ions of other valencies, as long as it can form a divalent cation. Similarly, a "trivalent metal element" refers to a metal element that can form a trivalent cation. A trivalent metal element may also be a metal element that can form ions of other valencies, as long as it can form a trivalent cation. Divalent and trivalent metal elements include those that exist in any form, such as compounds, elements, or ions. Divalent and trivalent metal elements may form salts with the polar groups of 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 vinyl alcohol units as structural units. PVA is usually obtained by saponifying polyvinyl esters.

[0020] The lower limit of the degree of saponification 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 degree of saponification may be 100 mol% or 99 mol%, but preferably 98 mol%, more preferably 95 mol%, even more preferably 90 mol%, even more preferably 85 mol%, and particularly preferably 80 mol%. By having the degree of saponification of PVA 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 capacity can be produced. The degree of saponification 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. Monomers that give the above other structural units include α-olefins such as ethylene, propylene, 1-butene, isobutene, and 1-hexene; acrylic acid and 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 hydroxyl groups such as ethylene glycol vinyl ether, 1,3-propanediol vinyl ether, and 1,4-butanediol vinyl ether. Examples include vinyl ethers containing vinyl ethers; allyl acetates; 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 above-mentioned other structural units in the total structural units of PVA is preferably 20 mol% or less, and more preferably 10 mol% or less, 5 mol% or less, or 1 mol% or less. On the other hand, the proportion of the above-mentioned 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 for PVA is preferably 300, more preferably 500, and even more preferably 700. A viscosity-average degree of polymerization above this lower limit enhances protective colloidal properties, allowing for the production of smaller vinyl polymer particles when the PVA-based resin is used as a dispersant for suspension polymerization. On the other hand, the upper limit of this viscosity-average degree of polymerization is preferably 3,000, more preferably 2,000, and even more preferably 1,500. A viscosity-average degree of polymerization below this upper limit enhances surface activity, increasing the plasticizer absorption capacity of the resulting vinyl polymer particles when the PVA-based resin is used as a dispersant for suspension polymerization, and allowing for the production of smaller vinyl polymer particles. The viscosity-average degree of polymerization is measured according to JIS K6726:1994. In other words, the intrinsic viscosity [η] (unit: liters / g) of PVA, measured in water at 30°C after resaponification to a degree of 99.5 mol% or higher and purification, can be determined by the following formula. Viscosity average degree of polymerization=([η]×10 4 (8.29) (1 / 0.62)

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

[0025] In PVA, the lower limit of the block character of residual vinyl ester units may be preferably 0.40, and more preferably 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 mentioned above is a numerical value representing the distribution of residual ester (usually an alkoxycarbonyl group) and hydroxyl groups produced by saponification of the ester, and takes a value between 0 and 2. A value of 0 indicates that the residual ester or hydroxyl groups are distributed in a completely block-like manner, and as the value increases, the alternation increases, with 1 indicating that residual ester and hydroxyl groups are present in a completely random manner, and 2 indicating that residual ester and hydroxyl groups are present in a completely alternating manner. The residual ester mentioned above refers to the ester (-OC(=O)-Q (where Q represents a hydrocarbon group other than the CH2=CH-OC(=O) portion contained in the vinyl ester monomer)) contained in the vinyl ester unit in the PVA obtained after saponification. In other words, the block character is a numerical value representing the distribution of residual vinyl ester units and vinyl alcohol units. 13 This can be determined by 13C-NMR measurement. If PVA contains repeating units other than vinyl ester units and / or vinyl alcohol units, the block character is calculated based on all consecutive sites of vinyl ester units and / or vinyl alcohol units in the PVA.

[0027] The block character described above can be adjusted by the type of vinyl ester monomer, saponification conditions such as catalysts and solvents, and post-saponification heat treatment. Note that when heat treatment is performed after saponification, the block character tends to be 0.40 or higher.

[0028] The lower limit of the PVA content in the polymer component of the PVA-based resin is preferably 99% by mass, and 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 nonvolatile components of the PVA-based resin is preferably 95% by mass, more preferably 98% by mass, and may also be 99% by mass or 99.9% by mass. The upper limit of the PVA content in the nonvolatile components of the PVA-based resin may be 100% by mass or 99.99% by mass. Nonvolatile 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. Compounds containing divalent and trivalent metal elements are also typically nonvolatile components other than PVA. Furthermore, the lower limit of the PVA content in the PVA-based resin is preferably 90% by mass, more preferably 95% by mass, and may also 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 volatile components. Because PVA is hygroscopic and solvents may remain if drying is insufficient, the PVA-based resin may contain volatile components in any amount. Examples of volatile components include water and alcohol. The volatile components may be, for example, components with a standard boiling point of 250°C or lower. The content of volatile components in the PVA-based resin is usually 10% by mass or less, and preferably 1% by mass or less.

[0031] The absorbance (Abs) of the PVA-based resin when it is an aqueous solution of 0.1% by mass, with a path length of 10 mm and a wavelength of 320 nm. 320The lower limit is 0.09, preferably 0.15, more preferably 0.20, and even more preferably 0.25. The absorption at a wavelength of 320 nm is attributed to the -CO-(CH=CH)3- structure in PVA. Absorbance Abs 320 When the value is above the lower limit mentioned above, sufficient surface activity is generated, and when the PVA-based resin is used as a dispersant for suspension polymerization, vinyl polymer particles with small particle size and good plasticizer absorption capacity can be produced. Absorbance Abs 320 Absorbance Abs 320 When the value is above the lower limit and below or less than the upper limit, the surfactant properties are further optimized, and when the PVA resin is used as a dispersant for suspension polymerization, vinyl polymer particles with smaller particle size and better plasticizer absorption capacity can be produced. The absorbance at a path length of 10 mm when the PVA resin is an aqueous solution of 0.1% by mass can be measured by a known method, for example, the method described in the examples.

[0032] The absorbance (Abs) of the PVA-based resin when it is an aqueous solution of 0.1% by mass, with a path length of 10 mm and a wavelength of 370 nm. 370 The lower limit is preferably 0.01, more preferably 0.03, even more preferably 0.05, and particularly preferably 0.06. Absorbance Abs 370 The upper limit is preferably 0.3, more preferably 0.2, and even more preferably 0.1. The absorption at a wavelength of 370 nm is attributed to the -CO-(CH=CH)4- structure in PVA. Absorbance Abs 370 When the above range is achieved, the surfactant properties are further optimized, and when the PVA resin is used as a dispersant for suspension polymerization, vinyl polymer particles with smaller particle size and better plasticizer absorption capacity can be produced.

[0033] Absorbance Abs 370 Absorbance Abs 320 The ratio (Abs 320 / Abs 370The upper limit of the absorbance ratio (Abs) is 4.9, preferably 4.7, more preferably 4.5, still preferably 4.3, still preferably 4.1, and may also be 3.8 or 3.5. 320 / Abs 370 By keeping the above upper limit below, when the PVA-based resin is used as a dispersant for suspension polymerization, vinyl polymer particles with small particle size and good plasticizer absorption capacity can be produced. In particular, the absorbance Abs, which will be described later, can be obtained. 280 When the ratio of absorbance (Abs) is relatively high (for example, 0.30 or higher), 320 / Abs 370 When the ratio of absorbance (Abs) is 3.8 or less, or even 3.5 or less, it tends to be possible to produce vinyl polymer particles with smaller particle sizes 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 If the value is above the lower limit mentioned above, when the PVA resin is used as a dispersant for suspension polymerization, vinyl polymer particles with smaller particle size and better plasticizer absorption capacity can be produced.

[0034] The absorbance (Abs) of the PVA-based resin when it is an aqueous solution of 0.1% by mass is measured at a path length of 10 mm and a wavelength of 280 nm. 280 The lower limit is preferably 0.1, more preferably 0.2, even more preferably 0.26, and particularly preferably 0.28, 0.30, or 0.32. Absorbance Abs 280 The upper limit is preferably 0.5, and more preferably 0.4. The absorption at a wavelength of 280 nm is attributed to the -CO-(CH=CH)2- structure in PVA. Absorbance Abs 280 When the above range is met, the surfactant properties are further optimized, and when the PVA-based resin is used as a dispersant for suspension polymerization, vinyl polymer particles with smaller particle sizes can be produced.

[0035] The total content of divalent and trivalent metal elements in the PVA resin is less than 30 μmol / g. The PVA resin does not need to contain divalent and trivalent metal elements. If the PVA resin contains divalent and trivalent metal elements, the total content of divalent and trivalent metal elements in the PVA resin is less than 30 μmol / g. By having a total content of divalent and trivalent metal elements of less than 30 μmol / g, when the PVA resin is used as a dispersant for suspension polymerization, a vinyl polymer with excellent insulating properties and suppressed discoloration can be obtained. Furthermore, by having a total content of divalent and trivalent metal elements of less than 30 μmol / g, discoloration of the PVA resin itself is suppressed. In addition, by having a total content of divalent and trivalent metal elements of less than 30 μmol / g, degradation of the PVA resin due to these metal elements acting as catalysts is also suppressed. In the case of monovalent metal elements such as sodium, even if they are included in an amount of, for example, about 30 μmol / g, discoloration and deterioration are less likely to occur compared to cases where divalent and trivalent metal elements are included. The upper limit of the total content of the above divalent 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 these divalent and trivalent metal elements may be 0 μmol / g, 0.0001 μmol / g, or 0.001 μmol / g. In the PVA resin, divalent and trivalent metal elements may inevitably be included. The PVA resin may not contain divalent and trivalent metal elements. The total content of divalent and trivalent metal elements in the PVA resin can be measured by known methods, for example, by ICP emission spectrometry as described in the examples. The total content of divalent and trivalent metal elements is based on the total content of the PVA resin as a whole, including any volatile components that may be present.

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

[0037] When the PVA resin is prepared as a 4% by mass aqueous solution, the lower limit of viscosity 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 viscosity is 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 with smaller particle sizes can be produced when the PVA resin is used as a dispersant for suspension polymerization. The above viscosity values ​​are measured by the method described in JIS K6726:1994.

[0038] When the PVA-based resin is used as a 1% by mass aqueous solution, the lower limit of the yellowness index (YI) is preferably 10, more preferably 20, and still more preferably 30. On the other hand, the upper limit of the yellowness index (YI) is preferably 70, more preferably 60, and still more preferably 50. When the yellowness index (YI) is within the above range, sufficient surfactant activity can be exhibited while suppressing discoloration, 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 capacity 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 resin is not particularly limited, but it is generally preferable to be in particulate (powder) form. When the PVA resin is in particulate form, the average particle size is preferably, for example, 100 μm to 1,000 μm. The average particle size of the PVA resin is measured according to the method described in JIS K7369:2009.

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

[0041] <Method for Producing PVA-Based Resin> The method for producing the PVA-based resin of the present invention is not particularly limited, but the following method is preferred. That is, the method for producing the PVA-based resin of the present invention includes a step (step C) of stirring and heating the saponified polyvinyl ester (unheat-treated PVA) in a container. The stirring and heating in step C is performed 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 in the container. Further, the flow rate of the gas in step C is 2 parts by mass or more and 11 parts by mass or less per 100 parts by mass of the saponified polyvinyl ester. According to this production method, a PVA-based resin having a low content of divalent metal elements and trivalent metal elements and capable of producing vinyl-based polymer particles with a small particle diameter even with a small addition amount when used as a dispersant for suspension polymerization can be obtained.

[0042] Before step C, the production method may further include a step (step A) of obtaining a polyvinyl ester by polymerizing a vinyl ester monomer, and a step (step B) of saponifying the polyvinyl ester. The production method of the PVA-based resin will be described in detail in the following order of steps.

[0043] (Step A)

[0044] (Step A) In step A, polyvinyl esters are obtained by polymerizing vinyl ester monomers. The polyvinyl ester may be a homopolymer of vinyl esters, or a copolymer of multiple vinyl esters or vinyl esters with other monomers. Known methods for polymerizing vinyl ester monomers include, for example, bulk polymerization, solution polymerization, suspension polymerization, and emulsion polymerization. Of these methods, bulk polymerization carried out without a solvent and solution polymerization carried out using a solvent such as an alcohol are preferred, and solution polymerization carried out in the presence of a lower alcohol is more preferred. As the lower alcohol, alcohols with 3 or fewer carbon atoms are preferred, methanol, ethanol, n-propanol, and isopropanol are more preferred, and methanol is even more preferred. When carrying out the polymerization reaction in bulk polymerization or solution polymerization, either batch or continuous reaction methods can be adopted.

[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, and vinyl versatate. Among these, vinyl acetate is preferred.

[0046] Examples of polymerization initiators used in polymerization reactions include azo-based 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-based initiators such as benzoyl peroxide, n-propyl peroxycarbonate, and diisopropyl peroxydicarbonate. There are no particular restrictions on the polymerization temperature when carrying out the polymerization reaction, but a range of 5°C to 200°C is appropriate.

[0047] When polymerizing vinyl ester monomers, copolymerizable monomers can be further copolymerized without impairing the spirit of the present invention. Examples of such copolymerizable monomers include those described above, which provide structural units other than vinyl alcohol units and vinyl ester units that PVA may contain.

[0048] From the viewpoint of efficiently obtaining PVA having the above-described terminal structure, it is preferable to include a predetermined chain transfer agent (modifier) ​​during the polymerization of vinyl ester monomers. Examples of such chain transfer agents include aldehydes having alkyl groups such as acetaldehyde, propionaldehyde, and butyraldehyde, and ketones having alkyl groups such as acetone and methyl ethyl ketone. These chain transfer agents may be used individually or in combination of two or more. Furthermore, during the polymerization of vinyl ester monomers, chain transfer agents other than the alkyl group aldehydes or alkyl group ketones mentioned above may be included for purposes such as adjusting the degree of polymerization of the resulting PVA. Examples of such chain transfer agents include aldehydes other than alkyl group aldehydes; ketones other than alkyl group ketones; mercaptans such as 2-hydroxyethanethiol; thiocarboxylic acids such as thioacetic acid; and halogenated hydrocarbons such as trichloroethylene and perchloroethylene. The amount of chain transfer agent to be added is determined according to the chain transfer constant of the chain transfer agent to be added and the degree of polymerization of the target PVA, but generally, 0.1 to 10% by mass relative to the vinyl ester used is preferred.

[0049] (Process B) In step B, for example, polyvinyl ester is saponified in an alcohol solution using an alkaline or acidic catalyst. For the saponification reaction of polyvinyl ester, conventionally known basic catalysts such as sodium hydroxide, potassium hydroxide, or sodium methoxide, or acidic catalysts such as p-toluenesulfonic acid, can be used for alcohol decomposition or hydrolysis reactions. 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 can be used individually or in combination of two or more. Among these, it is convenient and 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, which is a basic catalyst.

[0050] Saponification can be carried out using a belt reactor, a kneader reactor, a tower reactor, or the like. By going through step B, a solid of saponified polyvinyl ester is obtained. This solid may be crushed or otherwise subjected to pulverization before being used in step C. That is, the manufacturing method may include a step of crushing the solid of saponified polyvinyl ester between steps B and C. Furthermore, washing or drying treatments may be performed on the solid before or after crushing.

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

[0052] The shape of the saponified polyvinyl ester subjected to step C is not particularly limited, but particulate form is preferred, and particulate form with an average particle size of 100 μm or more and 1,000 μm or less is more preferred. An average particle size of 100 μm or more for the saponified polyvinyl ester particles suppresses scattering as dust by the flowing gas. On the other hand, an average particle size of 1,000 μm or less for the saponified polyvinyl ester particles tends to have a low water and organic solvent content in the particles, suppressing particle fusion. 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 equipment used for heating and stirring in process C include a rotary kiln with a rotating container, a planetary mixer equipped with a screw blade that rotates or orbits within the container, and a mixer with a rotating screw, baffles, etc., installed inside the container. In rotary kilns and the like, a rotating screw, baffles, etc., may be installed inside the container. All of these devices are configured to allow gas to flow through the container. The rotation speed of the container or the screw etc. installed inside the container in process C can be, for example, 1 rpm to 20 rpm.

[0054] The lower limit of the processing 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 processing temperature above the above lower limit, sufficient heat treatment is performed, and the absorbance Abs 370 As a result of sufficiently high levels of these properties, when used as a dispersant for suspension polymerization, a PVA-based resin can be obtained that produces vinyl polymer particles with small particle size and good plasticizer absorption capacity even with a small amount of additive. On the other hand, the upper limit of this treatment temperature is 180°C, preferably 170°C, more preferably 160°C, and even more preferably 150°C. By keeping the treatment temperature below the above upper limit, it is possible to suppress the formation of crosslinking in the PVA. Note that when a PVA-based resin with crosslinking has been formed is used as a dispersant for suspension polymerization, the resulting vinyl polymer will contain a large amount of insoluble matter that causes fish eyes.

[0055] The lower limit of the processing time in step C is 350 minutes, preferably 400 minutes, more preferably 500 minutes, and even more preferably 600 minutes. By setting the processing time to be above the lower limit, sufficient heat treatment is performed, and the absorbance Abs 370 As a result of obtaining materials with sufficiently high properties, when used as a dispersant for suspension polymerization, even a small amount of additive can be used to produce vinyl polymer particles with small particle size and good plasticizer absorption capacity. 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 keeping the treatment time below the above upper limit, crosslinking and insolubilization of PVA, as well as excessive discoloration, tend to be suppressed.

[0056] The lower limit of the gas flow rate into the container in step C is 2 parts by mass per hour per 100 parts by mass of saponified polyvinyl ester, preferably 2.5 parts by mass, and more preferably 3 parts by mass. By setting the gas flow rate above the above lower limit, a sufficient polyene reaction proceeds, and the absorbance Abs 370 A PVA-based resin with sufficiently high properties can be obtained. As a result, when the obtained PVA-based resin is used as a dispersant for suspension polymerization, vinyl polymer particles with small particle size and good plasticizer absorption capacity can be produced even with a small amount of additive. On the other hand, the upper limit of the gas flow rate is 11 parts by mass, preferably 10 parts by mass, and more preferably 9 parts by mass. By keeping the gas flow rate below the above lower limit, discoloration of the obtained PVA-based resin can be suppressed, and as a result, the hue of the vinyl polymer obtained when this PVA-based resin is used as a dispersant for suspension polymerization can be made good.

[0057] Examples of gases to be circulated in process C include nitrogen, oxygen, and mixtures thereof (such as air), with air being particularly suitable.

[0058] <Dispersant for suspension polymerization> The dispersant for suspension polymerization of the present invention contains the PVA-based resin of the present invention described above. The dispersant for suspension polymerization 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 dispersant for suspension polymerization of the present invention is preferably 50% by mass, more preferably 70% by mass, and in some cases 90% by mass or 99% by mass is even more preferred. The upper limit of the content of the PVA-based resin of the present invention in the dispersant for suspension polymerization of the present invention may be 100% by mass. The dispersant for suspension polymerization of the present invention may consist only of the PVA-based resin of the present invention. Other components that may be included in the dispersant for suspension polymerization of the present invention include other resins, surfactants, plasticizers and other additives, and various compounds used during production. The shape of the dispersant for suspension polymerization of the present invention is not particularly limited, but it is usually in powder form.

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

[0060] <Method for producing vinyl polymer particles> A method for producing vinyl polymer particles comprises a step of suspension polymerization of a vinyl compound using the suspension polymerization dispersant of the present invention. This method is the same as known methods for producing vinyl polymer particles, except that the suspension polymerization dispersant 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 suspension polymerization dispersant of the present invention. As the aqueous medium, in addition to pure water, an aqueous solution containing various additive components or an aqueous medium containing other organic solvents can be used.

[0062] When performing suspension polymerization of vinyl compounds, there are no particular restrictions on the amount of the suspension polymerization dispersant of the present invention added. However, as a lower limit, 100 ppm is preferred, 300 ppm is more preferred, and 500 ppm is even more preferred, based on mass relative to the vinyl compound. On the other hand, as an upper limit, 50,000 ppm is preferred, 10,000 ppm is more preferred, and 5,000 ppm, 2,000 ppm, or 1,000 ppm may also be even more preferred. By using the suspension polymerization dispersant of the present invention, vinyl polymer particles with small particle sizes can be obtained even with such small amounts of additive.

[0063] The suspension polymerization dispersant of the present invention may be used alone or in combination with other dispersants. Other dispersants include water-soluble polymers such as methylcellulose, hydroxyethylcellulose, hydroxypropylcellulose, hydroxypropylmethylcellulose, and other water-soluble cellulose ethers commonly used when suspend polymerization of vinyl compounds in an aqueous medium, as well as polyvinyl alcohol and gelatin; oil-soluble emulsifiers such as sorbitan monolaurate, sorbitan triolate, glycerin tristearate, and ethylene oxide-propylene oxide block copolymer; and water-soluble emulsifiers such as polyoxyethylene sorbitan monolaurate, polyoxyethylene glycerin oleate, and sodium laurate.

[0064] In the method for producing vinyl polymer particles, polymerization initiators conventionally used in the polymerization of vinyl compounds can be used. Specifically, polymerization initiators similar to those exemplified in the polymerization of vinyl ester monomers can be used.

[0065] In a 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 may be used in combination.

[0066] Examples of vinyl compounds that can be subjected to suspension polymerization in a 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, 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 is particularly suitable for suspension polymerization of vinyl chloride alone or together with monomers capable of copolymerizing vinyl chloride. Examples of monomers capable of copolymerizing 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 ether, etc.

[0067] In the method for producing vinyl polymer particles, when a vinyl compound is subjected to suspension polymerization, the charging ratio of each component, polymerization temperature, polymerization time, etc., can be the same as the conditions conventionally used for suspension polymerization of vinyl compounds such as vinyl chloride. Furthermore, there are no restrictions on the charging order or ratio of the vinyl compound, polymerization initiator, dispersant, aqueous medium, and other additives. [Examples]

[0068] The present invention will be specifically described by the following examples, but the present invention is not limited in any way by these examples. The measurement methods and evaluation methods used in the following examples and comparative examples are shown below.

[0069] [Analysis of PVA-based resins] The following analyses were performed using powder samples that were reduced in accordance with the sampling method specified in JIS K6726:1994.

[0070] (1) Degree of saponification Measurements were taken in accordance with JIS K6726:1994.

[0071] (2)4% aqueous solution viscosity Measurements were taken in accordance with JIS K6726:1994.

[0072] (3) Total content of divalent and trivalent metal elements The total content of divalent and trivalent metal elements was measured using ICP emission spectroscopy with a Thermo Fisher Scientific iCAP-6500Duo ICP emission spectrometer. Specifically, for Al, Ba, Ca, Cd, Co, Cr, Cu, Fe, Mg, Mn, Mo, Ni, Pb, Sb, Ti, V, and Zn, the detected mass was converted to a molar amount using atomic weights, and the sum was considered the total content of divalent and trivalent metal elements. In all examples and comparative examples, no divalent or trivalent metal elements other than Al, Ca, Cr, Cu, Fe, Mg, Mn, and Zn were detected.

[0073] (4) Yellowness of the aqueous solution One g of PVA resin was accurately weighed and dissolved in distilled water to prepare a 1.0% by mass aqueous solution. The YI of this aqueous solution was measured using a Color meter ZE6000 manufactured by Nippon Denshoku Industries Co., Ltd., in accordance with ASTM E313-05.

[0074] (5) Absorbance of aqueous solution 0.1 g of PVA resin was accurately 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 a path length of 10 mm.

[0075] [Evaluation of polyvinyl chloride polymer particles] (1) Average particle size The particle size distribution was measured by dry sieving analysis using a wire mesh based on the Tyler mesh standard, and the average particle size was determined.

[0076] (2) Plasticizer absorption capacity (CPA) The absorption of dioctyl phthalate at 23°C was measured using the method described in ASTM-D3367-75.

[0077] [Example 1] (Manufacturing of PVA-based resins) As raw materials, 10 kg of particulate saponified polyvinyl ester (resin) with a degree of saponification of 72.4 mol%, a viscosity of 6.2 mPa·s in a 4% aqueous solution, a total content of divalent and trivalent metal elements of 0.009 μmol / g, and an average particle size of 570 μm was prepared. This resin was obtained by partially saponifying polyvinyl ester obtained using acetaldehyde as a chain transfer agent, with sodium hydroxide as a catalyst. The above raw materials were added to a heat treatment furnace (container), and heat treatment was performed 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 resin (raw material), to obtain the PVA-based resin of Example 1.

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

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

[0080] (Manufacturing of vinyl polymer particles) 1,390 g of an aqueous solution containing 0.94 g of the obtained PVA resin (1,000 ppm relative to vinyl chloride monomer) was placed in a 5 L autoclave. Next, 0.6 g of cumyl peroxyneodecanoate (Permil ND-R, manufactured by NOF Corporation) and 0.9 g of t-butyl peroxyneodecanoate (Perbutyl ND-R, manufactured by NOF Corporation) were placed in the autoclave. The process of filling the autoclave with nitrogen gas and degassing was repeated six times until the pressure inside the autoclave reached 0.3 MPa to remove oxygen. After that, 940 g of vinyl chloride was placed in the autoclave, and the contents were heated to 57°C. Suspension polymerization was started under stirring. The pressure inside the autoclave at the start of polymerization was 0.83 MPa. After 4 hours from the start of suspension polymerization, when the pressure inside the autoclave reached 0.70 MPa, the polymerization was stopped and unreacted vinyl chloride was removed. The polymerization slurry was then removed 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 polymer (PVC) particles were measured. These measurement results are shown in Table 1.

[0082] [Examples 2-5, Comparative Examples 1-2] Examples 2-5 and Comparative Examples 1-2 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 degree of saponification, viscosity of the 4% aqueous solution, total content of divalent and trivalent metal elements, yellowness of the aqueous solution, and absorbance (Abs) of the aqueous solution of each obtained PVA resin were measured. 280 , Abs 320 and Abs 370 The absorbance ratio (Abs) was measured. These measurement results and the absorbance ratio (Abs) were measured. 320 / Abs 370 ) are shown in Table 1.

[0083] Using the PVA-based resins obtained in Examples 2-5 and Comparative Example 1, vinyl chloride polymer particles were obtained in the same manner as in Example 1. Furthermore, because 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 1,200 ppm of the PVA-based resin of Comparative Example 2 was used 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 the other examples, coarse particle formation and scale generation of the vinyl chloride polymer occurred, resulting in clearly undesirable results 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. These measurement results are shown in Table 1.

[0084] [Table 1]

[0085] As shown in Table 1, the PVA resins of Examples 1 to 5 have an absorbance of Abs 320 The ratio of absorbances (Abs) is 0.09 or higher, and the ratio of absorbances (Abs) is 0.09 or higher. 320 / Abs 370 The ratio of absorbance (Abs) was 4.9 or less, and when used as a dispersant for suspension polymerization, vinyl polymer particles with an average particle size of 165 μm or less could be produced even with an addition amount of 1,000 ppm relative to vinyl chloride monomer. The vinyl polymer particles obtained 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. Furthermore, the ratio of absorbance (Abs) was determined from a comparison between Examples 1 to 5 and Comparative Examples 1 and 2. 320 / Abs 370 The effect of having a ratio of 4.9 or less is that, among the various properties required for dispersants for suspension polymerization, it is possible to reduce the particle size of the resulting vinyl polymer particles. Furthermore, the PVA resins of Examples 1 to 5 had a total content of divalent and trivalent metal elements of less than 30 μmol / g, which is a very low content.

[0086] Furthermore, as in Examples 1 to 5, by performing heat treatment (stirring and heating) at a temperature of 90°C to 180°C for 350 minutes to 1,300 minutes while circulating gas in the container at a predetermined flow rate, even if the total content of divalent and trivalent metal elements is low, the absorbance Abs can be increased. 320 The ratio of absorbances (Abs) is 0.09 or higher, and the ratio of absorbances (Abs) is 0.09 or higher. 320 / Abs 370 It was confirmed that a PVA-based resin with an absorbance of 4.9 or less could be obtained. In Comparative Example 1, the amount of gas flow was small, and in Comparative Example 2, the processing time was short, so the absorbance Abs 320 The ratio of absorbances (Abs) is 0.09 or higher, and the ratio of absorbances (Abs) is 0.09 or higher. 320 / Abs 370 No PVA-based resin with a coefficient of 4.9 or less was obtained. [Industrial applicability]

[0087] The PVA-based resin of the present invention can be used as a dispersant during the suspension polymerization of vinyl compounds, etc.

Claims

1. The polymer component contains virtually only polyvinyl alcohol. Absorbance Abs when prepared as a 0.1 mass% aqueous solution with a path length of 10 mm and wavelength of 320 nm ３２０ The value is 0.09 or higher, Absorbance Abs when the solution is a 0.1% by mass aqueous solution with a path length of 10 mm and a wavelength of 370 nm ３７０ Absorbance Abs for the above ３２０ The ratio (Abs ３２０ / Abs ３７０ ) is 4.9 or less, A polyvinyl alcohol-based resin having a total content of divalent and trivalent metal elements of less than 30 μmol / g.

2. The above ratio (Abs ３２０ / Abs ３７０ The polyvinyl alcohol-based resin according to claim 1, wherein the ratio is 2.0 or higher.

3. Absorbance Abs ３２０ The polyvinyl alcohol-based resin according to claim 1, wherein the ratio is less than 0.

30.

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

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

6. A method for producing vinyl polymer particles, comprising the step of suspend polymerization of a vinyl compound using the suspension polymerization dispersant described in claim 5.

7. A process of stirring and heating saponified polyvinyl ester in a container. Equipped with, The above stirring and heating is performed at a temperature of 90°C to 180°C for 350 minutes to 1,300 minutes while circulating gas within the container. A method for producing a polyvinyl alcohol-based resin, wherein the flow rate of the above gas is 2 parts by mass or more and 11 parts by mass or less per hour per 100 parts by mass of the above saponified polyvinyl ester.

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