Methods for manufacturing polyvinyl alcohol-based resins, dispersants for suspension polymerization, ethylene-based polymer particles, and methods for manufacturing polyvinyl alcohol-based resins.

Abstract

A polyvinyl alcohol-based resin, which substantially contains only polyvinyl alcohol in terms of polymer composition, and whose absorbance Abs 320 at an optical path distance of 10 mm and a wavelength of 320 nm when prepared as a 0.1% by mass aqueous solution is 0.09 or more, the ratio of the absorbance Abs 320 to the absorbance Abs 370 at an optical path distance of 10 mm and a wavelength of 370 nm when prepared as a 0.1% by mass aqueous solution (Abs 320 / Abs 370) is 4.9 or less, and the total content of divalent and trivalent metal elements is less than 30 μmol / g.

Description

Technical Field

[0001] This invention relates to a polyvinyl alcohol-based resin, a dispersant for suspension polymerization, a method for manufacturing ethylene-based polymer particles, and a method for manufacturing a polyvinyl alcohol-based resin. Prior Technology

[0002] In the industrial manufacturing of vinylidene polymers such as polyvinyl chloride (PVC), suspension polymerization is widely used, in which vinylidene compounds such as vinyl chloride are dispersed in an aqueous medium in the presence of a dispersant (sometimes also called a dispersion stabilizer) and polymerized using an oil-soluble catalyst. Generally speaking, the main factors affecting the quality of vinylidene polymers obtained by suspension polymerization of vinylidene compounds include polymerization rate, water-to-monomer ratio, polymerization temperature, type and amount of oil-soluble catalyst, type of polymerization tank, stirring speed of the contents in the polymerization tank, and type of dispersant. Among these, the type of dispersant has the greatest impact.

[0003] Regarding the required properties of dispersants for suspension polymerization of ethylene compounds, examples include the ability to stably obtain ethylene polymer particles with small amounts of additives. To date, dispersants for suspension polymerization of ethylene compounds, in addition to cellulose derivatives such as methylcellulose and carboxymethylcellulose, can be used alone or in suitable combinations with partially saponified polyvinyl alcohol.

[0004] Polyvinyl alcohol (hereinafter also referred to as "PVA"), which can be used as a dispersant for suspension polymerization, is described in Patent Document 1 as follows: "A polyvinyl alcohol resin, when prepared as a 0.1% by weight aqueous solution, has an absorbance (X) of 320 nm or more in an ultraviolet absorption spectrum, and the ratio (Y / X) of absorbance (Y) at 380 nm to absorbance (X) at 320 nm is 0.09 or more." [Previous Technical Documents] [Patent Literature]

[0005] [Patent Document 1] International Publication No. 2020 / 184397 Summary of the Invention

[0006] [The problem the invention aims to solve]

[0007] Patent Document 1 describes that by heat treatment, a PVA-based resin with a specified absorbance can be obtained as a result of fully forming the [-CO-(CH=CH) 4-] structure (paragraph

[0011] ); and, since the PVA-based resin contains at least one of a divalent or trivalent metal salt and a hydroxide, heat treatment can be effectively performed (paragraph

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

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

[0060] ).

[0008] However, when PVA-based resins with high levels of divalent and trivalent metal elements are used as dispersants in suspension polymerization, the insulation properties of the resulting ethylene-based polymers decrease, sometimes affecting their performance as wire sheathing materials. Furthermore, high levels of divalent and trivalent metal elements can sometimes cause discoloration of the PVA-based resin itself and the ethylene-based polymers obtained when used as dispersants in suspension polymerization. Moreover, high levels of divalent and trivalent metal elements, acting as catalysts, may cause deterioration in both the PVA-based resin itself and the ethylene-based polymers obtained when used as dispersants in suspension polymerization. Therefore, it is desirable for PVA-based resins used as dispersants in suspension polymerization to have low levels of these metal elements.

[0009] Furthermore, dispersants used in suspension polymerization to date, especially when added in small amounts, are difficult to stably carry out suspension polymerization, resulting in an undesirable situation where the particle size of the obtained ethylene-based polymer particles increases. In the aforementioned Patent Document 1, there is no evaluation of the specific performance of the actually obtained PVA-based resin as a dispersant for suspension polymerization.

[0010] Based on the above-mentioned circumstances, the present invention aims to provide: a polyvinyl alcohol resin having low content of divalent and trivalent metal elements, which, when used as a dispersant for suspension polymerization, can produce ethylene polymer particles with small particle size even with a small amount of addition; a dispersant for suspension polymerization using this polyvinyl alcohol resin; a method for manufacturing ethylene polymer particles; and a method for manufacturing this polyvinyl alcohol resin. Simple Explanation of the Diagram

[0011] none. Implementation

[0012] [The form in which the invention is carried out]

[0013] The above objective is achieved by providing any of the following: [1] A polyvinyl alcohol-based resin, which substantially contains only polyvinyl alcohol in terms of polymer composition, and whose absorbance Abs 320 at an optical path length of 10 mm and a wavelength of 320 nm when prepared as a 0.1% by mass aqueous solution is 0.09 or more, and the ratio of the absorbance Abs 320 to the absorbance Abs 370 at an optical path length of 10 mm and a wavelength of 370 nm when prepared as a 0.1% by mass aqueous solution (Abs 320 / Abs 370) is 4.9 or less, and the total content of divalent and trivalent metal elements is less than 30 μmol / g; [2] Polyvinyl alcohol resins such as [1], wherein the above ratio (Abs 320 / Abs 370) is 2.0 or higher; [3] Polyvinyl alcohol resins such as [1] or [2], wherein the absorbance Abs 320 is less than 0.30; [4] The polyvinyl alcohol resin of any one of [1] to [3] has a viscosity of 4.5 mPa·s or more and 8.0 mPa·s or less when it is made into a 4% by mass aqueous solution at 20°C; [5] A dispersant for suspension polymerization, comprising a polyvinyl alcohol resin as described in any one of [1] to [4]; [6] A method for manufacturing ethylene-based polymer particles, comprising the step of suspending polymerizing an ethylene-based compound using a suspension polymerization dispersant as described in [5]; [7] A method for manufacturing a polyvinyl alcohol-based resin includes a step of stirring and heating a saponified polyvinyl ester in a container, wherein gas is circulated in the container while the stirring and heating is performed at a temperature of 90°C to 180°C for 350 minutes to 1,300 minutes or more, wherein the gas flow rate is 2 parts by mass to 11 parts by mass per hour relative to 100 parts by mass of the saponified polyvinyl ester. [Effects of the Invention]

[0014] According to the present invention, the following can be provided: a polyvinyl alcohol resin having low content of divalent and trivalent metal elements, which, when used as a dispersant for suspension polymerization, can produce ethylene polymer particles with small particle size even with a small amount of addition; a dispersant for suspension polymerization using such a polyvinyl alcohol resin; and a method for manufacturing such a polyvinyl alcohol resin. [The form in which the invention is implemented]

[0015] Polyvinyl alcohol resins The polyvinyl alcohol-based resin (PVA-based resin) of the present invention substantially contains only PVA in terms of polymer composition. When prepared as a 0.1% by mass aqueous solution, the absorbance Abs 320 at an optical path distance of 10 mm and a wavelength of 320 nm is 0.09 or higher. The ratio of the absorbance Abs 320 to the absorbance Abs 370 at an optical path distance of 10 mm and a wavelength of 370 nm when prepared as a 0.1% by mass aqueous solution (Abs 320 / Abs 370) is 4.9 or lower. The total content of divalent and trivalent metal elements is less than 30 μmol / g.

[0016] This PVA-based resin, with low levels of divalent and trivalent metal elements, is used as a dispersant in suspension polymerization. Even with a small addition, it can produce ethylene-based polymer particles with small particle sizes. The reason why this PVA-based resin can produce ethylene-based polymer particles with small additions in suspension polymerization is not definitively established, but it is speculated to be as follows: Generally, PVA used as a dispersant in suspension polymerization has a terminal structure represented by R-CO-(CH=CH)n- (R is an alkyl group, n is a natural number). The polyene structure in this terminal structure is formed through a dehydration reaction that occurs through heat treatment of PVA. Furthermore, regarding the absorption spectrum of PVA, the absorption at 320 nm is considered to be attributed to the -CO-(CH=CH)3- structure, and the absorption at 370 nm is attributed to the -CO-(CH=CH)4- structure. In other words, in this PVA-based resin, the absorbance Abs 320 being 0.09 or higher and the absorbance ratio (Abs 320 / Abs 370) being 4.9 or lower means that it sufficiently possesses the -CO-(CH=CH) 3- structure and, more importantly, the -CO-(CH=CH) 4- structure. This PVA-based resin contains a large amount of relatively long polyene structures (n=3, 4), resulting in sufficient interfacial activity. Therefore, when used as a dispersant for suspension polymerization, the polymerization stability is increased, and even with a small addition, small-sized ethylene polymer particles can be produced. Furthermore, the ethylene polymer particles obtained by using this PVA-based resin as a dispersant for suspension polymerization also exhibit good plasticizer absorption capacity.

[0017] Furthermore, the PVA-based resin exhibiting the aforementioned absorption spectrum, as detailed later, can be obtained by stirring and heating while allowing gas to flow under specified conditions. It is presumed that this is because stirring and heating while allowing gas flow facilitates the aforementioned dehydration reaction, resulting in a longer polyene structure. Therefore, using this method, even without using divalent or trivalent metal salts and hydroxides, a good terminal structure can be introduced through heat treatment. Thus, this PVA-based resin, even with a combined content of divalent and trivalent metal elements below 30 μmol / g, can produce ethylene-based polymer particles with small particle sizes and good plasticizer absorption capacity even with a small amount added when used as a dispersant for suspension polymerization.

[0018] Furthermore, "polyvinyl alcohol (PVA)" refers to a polymer that has vinyl alcohol units (-CH 2CHOH-) as structural units. PVA usually further contains vinyl ester units, and may also contain other structural units. In addition, "PVA-based resin" refers to a resin that contains PVA as a polymer component, and may also contain any other components.

[0019] The phrase "contains essentially only PVA in terms of polymer composition" means that other polymer components besides PVA may be included, without affecting the effectiveness of the present invention. Polymer components refer to compounds formed by the polymerization of monomers. Polymer components can be organic compounds with a molecular weight of 500 or higher. The content of PVA in the polymer component can be 95% by mass or higher, or 99% by mass or higher.

[0020] The term "divalent metal element" refers to a metal element that can become a divalent cation. If a divalent metal element can become a divalent cation, it can also become an ion with other valences. Similarly, the term "trivalent metal element" refers to a metal element that can become a trivalent cation. If a trivalent metal element can become a trivalent cation, it can also become an ion with other valences. Divalent and trivalent metal elements are included in any form, including compounds, elements, and ions. Divalent and trivalent metal elements can also form salts with the polar groups present in PVA.

[0021] The PVA-based resin of the present invention will be described in detail below.

[0022] The PVA of this invention is the PVA contained in the resin, which is a polymer having vinyl alcohol units as structural units. PVA is usually obtained by saponifying polyethylene ester.

[0023] Regarding the lower limit of the degree of saponification of PVA, it is preferably 50 mol%, more preferably 55 mol%, further preferably 60 mol%, even more preferably 65 mol%, and especially preferably 68 mol%. On the other hand, the upper limit of the above-mentioned degree of saponification can be 100 mol% or 99 mol%, but is preferably 98 mol%, more preferably 95 mol%, further preferably 90 mol%, even more preferably 85 mol%, and especially preferably 80 mol%. Since the degree of saponification of PVA is within the above range, the interfacial activity is optimized, thereby enabling the production of vinyl polymer particles with smaller particle size and better plasticizer absorption capacity when using PVA-based resins as dispersants for suspension polymerization. The degree of saponification is measured according to the method described in JIS K6726:1994.

[0024] PVA may also have structural units other than vinyl alcohol units and vinyl ester units. Examples of monomers supplying these other structural units include α-olefins such as ethylene, propylene, 1-butene, isobutene, and 1-hexene; acrylic acid and methacrylic acid; acrylates such as methyl acrylate and ethyl acrylate; methacrylates such as methyl methacrylate and ethyl methacrylate; acrylamide derivatives such as N-methacrylamide 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-containing vinyl ethers such as ethylene glycol vinyl ether, 1,3-propanediol vinyl ether, and 1,4-butanediol vinyl ether. Allyl ethers; allyl acetate; propyl allyl ether, butyl allyl ether, hexyl allyl ether, and other allyl ethers; monomers having an oxoalkyl group; isopropyl acetate; hydroxyl-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; vinyltrimethoxysilane, vinylmethyldimethoxysilane, vinyldimethylmethoxysilane, vinyltriethoxysilane, vinylmethyldiethoxysilane, vinyldimethylethoxysilane, 3-(meth)acrylamidepropyltrimethoxysilane, and 3-(meth)acrylamidepropyltriethoxysilane, etc. Among these, α-olefins, acrylic acid, methacrylic acid, acrylates, and methacrylates are preferred.

[0025] The proportion of the aforementioned other structural units in all structural units of the PVA is sometimes preferably 20 mol% or less, and sometimes more preferably 10 mol% or less, 5 mol% or less, or 1 mol% or less. On the other hand, the proportion of the aforementioned other structural units may, for example, be 0.1 mol% or more, or 1 mol% or more.

[0026] Regarding the lower limit of the viscosity-average degree of polymerization of PVA, it is preferably 300, more preferably 500, and even more preferably 700. Since the viscosity-average degree of polymerization is above this lower limit, the protective colloidal property is increased, and when this PVA-based resin is used as a dispersant for suspension polymerization, it is possible to produce vinyl polymer particles with smaller particle sizes. On the other hand, regarding the upper limit of this viscosity-average degree of polymerization, it is preferably 3,000, more preferably 2,000, and even more preferably 1,500. Since the viscosity-average degree of polymerization is below this upper limit, the interfacial activity is increased, and when this PVA-based resin is used as a dispersant for suspension polymerization, the plasticizer absorption capacity of the obtained vinyl polymer particles is increased. Furthermore, it is possible to produce vinyl polymer particles with smaller particle sizes. The viscosity-average degree of polymerization is the value measured according to JIS K6726:1994. That is, PVA can be resaponified to a degree of saponification of 99.5 moles or more and refined, and the limiting viscosity [η] (unit: liters / g) measured in water at 30°C can be obtained by the following formula. Viscosity-average degree of polymerization = ([η] × 10⁴ / 8.29) (1 / 0.62)

[0027] PVA typically has a terminal structure represented by R-CO-(CH=CH)n- (R is an alkyl group, n is a natural number). R is preferably an alkyl group with 1 to 4 carbon atoms, more preferably methyl or ethyl. n can be, for example, an integer from 1 to 4. The PVA-based resin system 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-. The terminal structure represented by R-CO-(CH=CH)n- can be effectively introduced into the polymer by using aldehydes, ketones, etc., as chain transfer agents (modifiers) during the polymerization of polyethylene terephthalate, which is a precursor, and by subjecting the polymer to heat treatment.

[0028] In PVA, the lower limit of the block character of the residual vinyl ester units is 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 can be, for example, 1.

[0029] The aforementioned block characterization refers to the numerical values ​​representing the distribution of residual esters (usually alkoxycarbonyl groups) and hydroxyl groups generated through ester saponification, ranging from 0 to 2. 0 indicates a completely blocky distribution of residual esters or hydroxyl groups, with alternation increasing with the value; 1 indicates a completely random presence of residual esters and hydroxyl groups; and 2 indicates a completely alternating presence of residual esters and hydroxyl groups. The aforementioned residual ester refers to the ester (-OC(=O)-Q (Q represents the hydrocarbon group other than the CH 2=CH-OC(=O) portion in the ethylene ester monomer) contained in the ethylene ester unit of PVA obtained through saponification). In other words, the block characterization represents the numerical values ​​representing the distribution of residual ethylene ester units and ethylene alcohol units. Furthermore, the block characterization can be determined by 13C-NMR measurement. In cases where PVA contains repeating units other than ethylene ester units and / or ethylene alcohol units, the block characterization is calculated based on all continuous sites of ethylene ester units and / or ethylene alcohol units in the PVA.

[0030] The aforementioned block characteristics can be adjusted by factors such as the type of ethylene ester monomer, saponification conditions including catalyst and solvent, and post-saponification heat treatment. Furthermore, when heat treatment is performed after saponification, the block characteristics tend to be 0.40 or higher.

[0031] Regarding the lower limit of the PVA content in the polymer composition of the PVA-based resin, it is preferably 99% by mass, more preferably 99.9% by mass. The upper limit of the PVA content in the polymer composition of the PVA-based resin can be 100% by mass or 99.99% by mass. Regarding polymer components other than PVA that may be contained in the PVA-based resin, examples include polyethylene ester.

[0032] Regarding the lower limit of PVA content in the non-volatile components of the PVA-based resin, it 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 PVA content in the non-volatile components of the PVA-based resin may be 100% by mass or may also be 99.99% by mass. Regarding non-volatile components other than PVA that may be contained in the PVA-based resin, examples include polymers other than PVA, various compounds used in the manufacturing process, and other impurities. Compounds typically containing divalent and trivalent metal elements are also considered non-volatile components other than PVA. Furthermore, regarding the lower limit of PVA content in the PVA-based resin, it 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 PVA content in the PVA-based resin may be 100% by mass or may also be 99.99% by mass.

[0033] This PVA-based resin may also contain volatile components. Because PVA is hygroscopic, residual solvents may remain if not dried sufficiently, while PVA-based resins can contain volatile components in any amount. Examples of volatile components include water and alcohol. Volatile components can be, for example, components with a standard boiling point below 250°C. The content of volatile components in this PVA-based resin is typically 10% by mass or less, preferably 1% by mass or less.

[0034] When the PVA-based resin is prepared as a 0.1% by mass aqueous solution, the lower limit of the absorbance Abs 320 at a wavelength of 320 nm with an optical path distance of 10 mm is 0.09, more preferably 0.15, more preferably 0.20, and even more preferably 0.25. The absorption at 320 nm is attributed to the -CO-(CH=CH) 3- structure in PVA. Because the absorbance Abs 320 is above the above-mentioned lower limit, sufficient interfacial activity is generated, and when using the PVA-based resin as a dispersant for suspension polymerization, ethylene-based polymer particles with small particle size and good plasticizer absorption capacity can be produced. The absorbance Abs 320 is preferably 0.5 or less, more preferably 0.4 or less, even more preferably less than 0.40, even more preferably less than 0.35, and even more preferably less than 0.30 or less. When the absorbance Abs 320 is above the lower limit and below the upper limit, or even lower than the upper limit, the interfacial activity can be further optimized, and when this PVA-based resin is used as a dispersant for suspension polymerization, ethylene-based polymer particles with smaller particle size and better plasticizer absorption capacity can be produced. The absorbance of a 0.1% by mass aqueous solution of the PVA-based resin at a path length of 10 mm can be measured by known methods, such as those described in the examples.

[0035] The lower limit of absorbance Abs 370 at a wavelength of 370 nm with an optical path distance of 10 mm when the PVA-based resin is prepared as a 0.1% by mass aqueous solution is preferably 0.01, more preferably 0.03, further preferably 0.05, and particularly preferably 0.06. The upper limit of absorbance Abs 370 is preferably 0.3, more preferably 0.2, and further preferably 0.1. The absorption at a wavelength of 370 nm is attributed to the -CO-(CH=CH) 4- structure in PVA. With absorbance Abs 370 within the above range, the interfacial activity can be further optimized, and when the PVA-based resin is used as a dispersant for suspension polymerization, ethylene polymer particles with smaller particle size and better plasticizer absorption capacity can be produced.

[0036] The upper limit of the absorbance ratio (Abs 320 / Abs 370) is 4.9, preferably 4.7, more preferably 4.5, further preferably 4.3, even more preferably 4.1, and there are also cases where it is even more preferably 3.8 or 3.5. Because the absorbance ratio (Abs 320 / Abs 370) is below the above-mentioned upper limit, when the PVA-based resin is used as a dispersant for suspension polymerization, it is possible to produce vinyl polymer particles with small particle size and good plasticizer absorption capacity. In particular, when the absorbance ratio (Abs 280) is high (e.g., 0.30 or higher), the absorbance ratio (Abs 320 / Abs 370) is 3.8 or less, and further 3.5 or less, when the PVA-based resin is used as a dispersant for suspension polymerization, there is a tendency to produce vinyl polymer particles with even smaller particle size. Regarding the lower limit of the absorbance ratio (Abs 320 / Abs 370) mentioned above, it is preferably 2.0, more preferably 2.4, and even more preferably 2.8. When the absorbance ratio (Abs 320 / Abs 370) is above the aforementioned lower limit, when the PVA-based resin is used as a dispersant for suspension polymerization, it is possible to produce ethylene-based polymer particles with smaller particle size and better plasticizer absorption capacity.

[0037] Regarding the lower limit of absorbance Abs 280 at an optical path distance of 10 mm and a wavelength of 280 nm when the PVA-based resin is prepared as a 0.1% by mass aqueous solution, it is preferably 0.1, more preferably 0.2, further preferably 0.26, and particularly preferably 0.28, 0.30, or 0.32. The upper limit of absorbance Abs 280 is preferably 0.5, more preferably 0.4. The absorption at a wavelength of 280 nm is attributed to the -CO-(CH=CH) 2- structure in PVA. With absorbance Abs 280 within the above range, by further optimizing the interfacial activity, it is possible to produce ethylene-based polymer particles with smaller particle sizes when using the PVA-based resin as a dispersant for suspension polymerization.

[0038] The total content of divalent and trivalent metal elements in this PVA-based resin is less than 30 μmol / g. This PVA-based resin may also be free of divalent and trivalent metal elements. When the PVA-based resin contains divalent and trivalent metal elements, the total content of these elements is less than 30 μmol / g. By ensuring that the total content of divalent and trivalent metal elements is less than 30 μmol / g, when using this PVA-based resin as a dispersant for suspension polymerization, a vinyl polymer with excellent insulation properties and suppressed coloration can be obtained. Furthermore, by ensuring that the total content of divalent and trivalent metal elements is less than 30 μmol / g, the coloration of the PVA-based resin itself is also suppressed. Moreover, by ensuring that the total content of divalent and trivalent metal elements is less than 30 μmol / g, the degradation of the PVA-based resin caused by these metal elements acting as catalysts is also suppressed. Furthermore, even when monovalent metal elements such as sodium are present at a level of approximately 30 μmol / g, they are less prone to discoloration and degradation compared to the presence of divalent and trivalent metal elements. The upper limit of the total content of the aforementioned divalent and trivalent metal elements is preferably 10 μmol / g, more preferably 1 μmol / g, further preferably 0.1 μmol / g, and even more preferably 0.05 μmol / g. The lower limit of this total content of divalent and trivalent metal elements can be 0 μmol / g, 0.0001 μmol / g, or 0.001 μmol / g. This PVA-based resin can unavoidably contain divalent and trivalent metal elements. Alternatively, this PVA-based resin may not contain divalent and trivalent metal elements. The total content of divalent and trivalent metal elements in PVA-based resins can be determined by known methods, such as ICP luminescence analysis described in the examples. Furthermore, the total content of divalent and trivalent metal elements is based on the content of the entire PVA-based resin, which also includes, and may contain, volatile components.

[0039] Divalent and trivalent metal elements can be any metal element other than alkali metal elements. Divalent and trivalent metal elements can be Al, Ba, Ca, Cd, Co, Cr, Cu, Fe, Mg, Mn, Mo, Ni, Pb, Sb, Ti, V, and Zn.

[0040] Regarding the lower limit of the viscosity at 20 °C when the PVA-based resin is made into a 4% by mass aqueous solution, it is preferably 4.5 mPa·s, more preferably 5.0 mPa·s, further preferably 5.5 mPa·s, and even further preferably 6.0 mPa·s. On the other hand, regarding the upper limit of this viscosity, it is preferably 8.0 mPa·s, more preferably 7.5 mPa·s, and in some cases, it is further preferably 7.0 mPa·s. When the above viscosity is within the above range, when the PVA-based resin is used as a dispersant for suspension polymerization, vinyl polymer particles with a smaller particle size can be produced. The above viscosity is a value measured by the method described in JIS K6726:1994.

[0041] Regarding the lower limit of the yellowness index (YI) when the PVA-based resin is made into a 1% by mass aqueous solution, it is preferably 10, more preferably 20, and further preferably 30. On the other hand, regarding the upper limit of this yellowness index (YI), it is preferably 70, more preferably 60, and further preferably 50. When the yellowness index (YI) is within the above range, sufficient interfacial activity ability can be exhibited while suppressing coloring, and when the PVA-based resin is used as a dispersant for suspension polymerization, vinyl polymer particles with 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.

[0042] The shape of the PVA-based resin is not particularly limited, but it is usually preferably in the form of particles (powder form). Regarding the average particle diameter of the PVA-based resin in the case of being in the form of particles, it is preferably, for example, 100 μm or more and 1,000 μm or less. The average particle diameter of the PVA-based resin is a value measured by the method described in JIS K7369:2009.

[0043] The PVA-based resin can be used in various applications similar to those of the hitherto 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, adhesives for inorganic substances, etc. Among these, as will be described in detail later, it can be particularly suitably used as a dispersant for suspension polymerization of vinyl compounds and the like.

[0044] <Method for Producing PVA-Based Resin> The manufacturing method of the PVA-based resin of the present invention is not particularly limited, but the following method is preferred. That is, the manufacturing method of the PVA-based resin of the present invention includes a step (step C) of stirring and heating saponified polyethylene ester (untreated PVA) in a container. In step C, the stirring and heating is performed at 90°C to 180°C for 350 to 1300 minutes while allowing gas to circulate within the container. Furthermore, the gas flow rate in step C is 2 to 11 parts by mass per hour relative to 100 parts by mass of the saponified polyethylene ester. By this manufacturing method, a PVA-based resin can be obtained, which has low content of divalent and trivalent metal elements, and when used as a dispersant for suspension polymerization, small-sized ethylene polymer particles can be produced even with a small amount of addition.

[0045] The manufacturing method may also further include the following steps before step C: The step of obtaining polyethylene ester by polymerizing ethylene ester monomer (step A); and The above-mentioned step of saponifying polyethylene ester (step B).

[0046] The manufacturing method of this PVA-based resin is described in detail below in step-by-step order.

[0047] (Step A) In step A, polyethylene ester is obtained by polymerizing ethylene ester monomers. Polyethylene ester can be a homopolymer of ethylene ester, or a copolymer of several ethylene esters or ethylene esters with other monomers. Regarding the method of polymerizing ethylene ester monomers, known methods such as bulk polymerization, solution polymerization, suspension polymerization, and emulsion polymerization can be cited. Among these methods, solvent-free bulk polymerization and solution polymerization using solvents such as alcohols are preferred, and solution polymerization in the presence of lower alcohols is even more preferred. Lower alcohols are preferably alcohols with 3 or fewer carbon atoms, more preferably methanol, ethanol, n-propanol, and isopropanol, and even more preferably methanol. When carrying out the polymerization reaction by bulk polymerization or solution polymerization, either batch or continuous reaction methods can be used.

[0048] Examples of vinyl ester monomers include vinyl formate, vinyl acetate, vinyl propionate, vinyl valerate, vinyl decanoate, vinyl laurate, vinyl stearate, vinyl benzoate, vinyl pivalate, and vinyl versatate. Among these, vinyl acetate is preferred.

[0049] Regarding polymerization initiators used in polymerization reactions, examples include azo-based initiators such as 2,2'-azobisisobutyronitrile, 2,2'-azobis(2,4-dimethylpentanonitrile), and 2,2'-azobis(4-methoxy-2,4-dimethylpentanonitrile); and known polymerization initiators such as benzoyl peroxide, n-propyl peroxycarbonate, and diisopropyl peroxydicarbonate. There are no particular restrictions on the polymerization temperature at the time of the polymerization reaction, but a range of 5°C to 200°C is generally considered appropriate.

[0050] During the polymerization of ethylene ester monomers, copolymerizable monomers may be further copolymerized without compromising the spirit of the present invention. Examples of copolymerizable monomers include vinyl alcohol units that may be contained in PVA and monomers that supply other structural units besides ethylene ester units, as described above.

[0051] From the viewpoint of efficiently obtaining PVA with the aforementioned terminal structure, it is preferable to coexist a specified chain transfer agent (modifier) ​​during the polymerization of ethylene ester monomers. Examples of such chain transfer agents include alkyl aldehydes such as acetaldehyde, propionaldehyde, and butyraldehyde, and alkyl ketones such as acetone and methyl ethyl ketone. One of these chain transfer agents may be used alone, or two or more may be used in combination. Furthermore, during the polymerization of ethylene ester monomers, chain transfer agents other than the aforementioned alkyl aldehydes or alkyl ketones can be coexisted to adjust the degree of polymerization of the obtained PVA. Examples of such chain transfer agents include aldehydes other than alkyl aldehydes; ketones other than alkyl ketones; thiols such as 2-hydroxyethanethiol; thiocarboxylic acids such as thioacetic acid; and halogenated hydrocarbons such as trichloroethylene and perchloroethylene. The amount of chain transfer agent added depends on the chain transfer constant of the chain transfer agent to be added and the degree of polymerization of the target PVA, but generally it is preferred to be 0.1 to 10 by mass relative to the ethylene ester used.

[0052] (Step B) In step B, polyethylene ester is saponified, for example, using an alkaline or acidic catalyst in an alcohol solution. The saponification reaction of polyethylene ester can be carried out using alcoholysis or hydrolysis reactions, employing alkaline catalysts such as sodium hydroxide, potassium hydroxide, and sodium methoxylate, or acidic catalysts such as p-toluenesulfonic acid. Examples of solvents that can be 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 alone or in combination of two or more. Among these, using methanol or a mixture of methanol and methyl acetate as the solvent, and carrying out the saponification reaction in the presence of sodium hydroxide as an alkaline catalyst, is simpler and preferred.

[0053] Saponification can be carried out using a belt reactor, a kneading reactor, a tower reactor, etc. After step B, a solid product of saponified polyethylene ester is obtained. This solid product can also be pulverized before being supplied to step C. That is, the manufacturing method can also include a step between step B and step C to pulverize the solid product of saponified polyethylene ester. Furthermore, the solid product before or after pulverization can be washed or dried.

[0054] (Step C) In step C, the saponified polyethylene ester (untreated PVA) is stirred and heated in a container. This container can also be a heat treatment furnace. This container is an open container with a structure that allows gas to flow.

[0055] The shape of the saponified polyethylene ester supplied to step C is not particularly limited, but it is preferably in particulate form, and more preferably in particulate form with an average particle size of 100 μm to 1,000 μm. Since the average particle size of the saponified polyethylene ester particles is 100 μm or more, dispersion as dust due to the flowing gas is suppressed. On the other hand, since the average particle size of the saponified polyethylene ester particles is 1,000 μm or less, the water and organic solvent content in the particles tends to be low, thus suppressing particle agglomeration. The average particle size of the saponified polyethylene ester particles is determined according to the method described in JIS K7369:2009.

[0056] Regarding the apparatus used for heating and stirring in step C, examples include a rotary kiln with a rotating container, a planetary mixer with a screw blade that revolves within the container, and a mixer with a rotating screw and baffles within the container. In a rotary kiln, a rotating screw and baffles can also be installed within the container. In all these apparatuses, gas is configured to circulate within the container. The rotational speed of the container in step C, or the screw installed within the container, can be, for example, set to between 1 rpm and 20 rpm.

[0057] The lower limit of the processing temperature in step C is 90°C, preferably 100°C, more preferably 110°C, further preferably 120°C, and even more preferably 130°C. By setting the processing temperature above the aforementioned lower limit, sufficient heat treatment is performed, and the absorbance (Abs 370, etc.) becomes sufficiently high, resulting in a PVA-based resin. When used as a dispersant for suspension polymerization, even a small amount of this resin can produce vinyl polymer particles with small particle size and good plasticizer absorption capacity. On the other hand, the upper limit of this processing temperature is 180°C, preferably 170°C, more preferably 160°C, and even more preferably 150°C. By setting the processing temperature below the aforementioned upper limit, cross-linking in PVA can be suppressed. Furthermore, when using a cross-linked PVA-based resin as a dispersant for suspension polymerization, the amount of insoluble matter that causes "fish eye" defects in the obtained vinyl polymer increases.

[0058] The minimum 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 above the aforementioned minimum, sufficient heat treatment is performed, resulting in sufficiently high absorbance (Abs 370, etc.). When used as a dispersant for suspension polymerization, even a small amount can produce vinyl polymer particles with small particle size and good plasticizer absorption capacity. On the other hand, the maximum processing time is 1,300 minutes, preferably 1,200 minutes, more preferably 1,100 minutes, and even more preferably 1,000 minutes. By setting the processing time below the aforementioned maximum, there is a tendency to suppress cross-linking of PVA without dissolution and excessive coloring.

[0059] The lower limit for the gas flow rate in the container during step C is 2 parts by mass per hour, more preferably 2.5 parts by mass, and even more preferably 3 parts by mass, relative to 100 parts by mass of saponified polyethylene ester. By setting the gas flow rate above this lower limit, a sufficient polyolefination reaction can be carried out, resulting in a PVA-based resin with sufficiently high absorbance (Abs 370). Consequently, when using the obtained PVA-based resin as a dispersant for suspension polymerization, even a small amount can produce vinyl polymer particles with small particle size and good plasticizer absorption capacity. On the other hand, the upper limit for this gas flow rate is 11 parts by mass, more preferably 10 parts by mass, and even more preferably 9 parts by mass. By setting the gas flow rate below this lower limit, the coloring of the obtained PVA-based resin is suppressed, resulting in a good color of the vinyl polymer obtained when using the PVA-based resin as a dispersant for suspension polymerization.

[0060] Regarding the gas flowing in step C, examples include nitrogen, oxygen, and mixtures of such gases (such as air), and air can be used appropriately.

[0061] <Dispersants for suspension polymerization> The suspension polymerization dispersant of the present invention comprises the PVA-based resin of the present invention as 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 obtained 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 can be 100% by mass. The suspension polymerization dispersant of the present invention may also be composed solely of the PVA-based resin of the present invention. Other components besides the PVA-based resin that may be included in the suspension polymerization dispersant of the present invention include other resins, surfactants, plasticizers, and various compounds used in the manufacturing process. The shape of the suspension polymerization dispersant of the present invention is not particularly limited, but it is generally a powder.

[0062] The dispersant for suspension polymerization of the present invention is suitable as a dispersant for suspension polymerization of ethylene-based compounds. Using the dispersant for suspension polymerization of the present invention increases polymerization stability and allows for the efficient acquisition of polymer particles with small particle sizes. Furthermore, the polymer particles obtained by suspension polymerization using the dispersant for suspension polymerization of the present invention also exhibit good plasticizer absorption capacity.

[0063] <Methods for Manufacturing Vinyl Polymer Particles> The method for manufacturing ethylene-based polymer particles includes the step of suspending and polymerizing an ethylene-based compound using the suspension polymerization dispersant of the present invention. This manufacturing method is identical to known methods for manufacturing ethylene-based polymer particles, except that it uses the suspension polymerization dispersant of the present invention as the dispersant.

[0064] The method for manufacturing ethylene-based polymer particles uses the dispersant for suspension polymerization of the present invention, typically involving the suspension polymerization of ethylene-based compounds in an aqueous medium. Regarding the aqueous medium, in addition to pure water, aqueous solutions containing various additives or aqueous media containing other organic solvents can be used.

[0065] When performing suspension polymerization of ethylene-based compounds, the amount of dispersant used in suspension polymerization according to the present invention is not particularly limited, but in terms of the lower limit, relative to the ethylene-based compound, it is preferably 100 ppm, more preferably 300 ppm, and even more preferably 500 ppm by mass. On the other hand, in terms of the upper limit, it is preferably 50,000 ppm, more preferably 10,000 ppm, and 5,000 ppm, 2,000 ppm or 1,000 ppm are also more preferred. By using the dispersant used in suspension polymerization according to the present invention, even with such a small amount, ethylene-based polymer particles with small particle size can be obtained.

[0066] The dispersant for suspension polymerization of the present invention can be used alone or in combination with other dispersants. Other dispersants include water-soluble polymers such as methylcellulose, hydroxyethylcellulose, hydroxypropylcellulose, and hydroxypropylmethylcellulose, polyvinyl alcohol, and gelatin, commonly used in the suspension polymerization of ethylene compounds in an aqueous medium; oil-soluble emulsifiers such as sorbitan monolaurate, sorbitan trioleate, glycerol tristearate, and ethylene oxide-propylene oxide block copolymers; and water-soluble emulsifiers such as polyoxyethylene sorbitan monolaurate, polyoxyethylene glycerol oleate, and sodium laurate.

[0067] Regarding the polymerization initiator used in the method for manufacturing ethylene-based polymer particles, the same polymer initiator used in the polymerization of ethylene-based compounds to date can be used; specifically, the same polymer initiator exemplified in the polymerization of the aforementioned ethylene ester monomers can be used.

[0068] In the manufacturing process of vinyl polymer particles, various other additives can be added to the polymerization system as needed. Examples of additives include polymerization regulators such as aldehydes, halogenated hydrocarbons, and thiols, and polymerization inhibitors such as phenolic compounds, sulfur compounds, and N-oxide compounds. Furthermore, pH adjusters, scale inhibitors, and crosslinking agents can also be added. Multiple additives can also be used in combination.

[0069] Examples of vinyl compounds that can be suspension polymerized in methods for manufacturing vinyl polymer particles include vinyl halides such as vinyl chloride; vinyl esters such as vinyl acetate and vinyl propionate; acrylic acid, methacrylic acid, and their esters and salts; maleic acid, fumaric acid, and their esters and anhydrides; styrene; acrylonitrile; vinylidene chloride; and vinyl ethers. Among these vinyl compounds, vinyl chloride is preferred. Methods for manufacturing vinyl polymer particles are particularly suitable for suspension polymerizing vinyl chloride alone or together with monomers capable of copolymerizing with vinyl chloride. Examples of monomers capable of copolymerizing with vinyl chloride include vinyl esters such as vinyl acetate and vinyl propionate; (meth)acrylates such as methyl (meth)acrylate and ethyl (meth)acrylate; α-olefins such as ethylene and propylene; unsaturated dicarboxylic acids such as maleic anhydride and itconic acid; acrylonitrile; styrene; vinylidene chloride; and vinyl ethers.

[0070] In the method for manufacturing ethylene-based polymer particles, when ethylene-based compounds are subjected to suspension polymerization, the feed ratios of each component, polymerization temperature, polymerization time, etc., can be set to the same conditions as those used to date in the suspension polymerization of ethylene-based compounds such as vinyl chloride. Furthermore, there are no restrictions on the feeding sequence and ratio of ethylene-based compounds, polymerization initiators, dispersants, aqueous media, and other additives. [Example]

[0071] The present invention is specifically illustrated by the following examples, but the invention is not limited by these examples. Furthermore, the various measurement and evaluation methods used in the following examples and comparative examples are presented below.

[0072] [Analysis of PVA-based resins] The following analysis was performed using powder samples reduced according to the sample collection method specified in JIS K6726:1994.

[0073] (1) Degree of saponification The determination was performed according to JIS K6726:1994.

[0074] (2) Viscosity of 4% aqueous solution The determination was performed according to JIS K6726:1994.

[0075] (3) Total content of divalent and trivalent metal elements The total content of divalent and trivalent metal elements was determined using an ICP-luminescence analyzer (iCAP-6500Duo) manufactured by Thermo Fisher Scientific. Specifically, for Al, Ba, Ca, Cd, Co, Cr, Cu, Fe, Mg, Mn, Mo, Ni, Pb, Sb, Ti, V, and Zn, the mass of the detected elements was converted to atomic weight and the total value was defined as the total content of divalent and trivalent metal elements. Furthermore, in all the examples and comparative examples, no divalent or trivalent metal elements other than Al, Ca, Cr, Cu, Fe, Mg, Mn, and Zn were detected.

[0076] (4) Yellowness of the aqueous solution 1 g of PVA resin was precisely weighed and dissolved in distilled water to prepare a 1.0% (w / w) aqueous solution. The yI of the aqueous solution was determined using a Color meter ZE6000 (manufactured by Nippon Denshoku Kogyo) according to ASTM E313-05.

[0077] (5) Absorbance of aqueous solution 0.1 g of PVA resin was precisely weighed and dissolved in distilled water to prepare a 0.1% (w / w) aqueous solution. The UV-Vis absorption spectrum of this aqueous solution was measured using a Shimadzu UV-1800 UV-Vis spectrophotometer with a quartz cell (10 mm optical path distance).

[0078] [Evaluation of vinyl chloride polymer particles] (1) Average particle size The particle size distribution was determined by dry sieving analysis using a metal mesh based on a Tyler mesh, and the average particle size was calculated.

[0079] (2) Plasticizer Absorption Capacity (CPA) The absorbance of dioctyl phthalate at 23°C was determined by the method described in ASTM-D3367-75.

[0080] [Example 1] (Manufacturing PVA-based resins) 10 kg of saponified polyethylene ester (resin) in particulate form with a saponification degree of 72.4 mol%, a 4% aqueous solution viscosity of 6.2 mPa·s, a total content of divalent and trivalent metal elements of 0.009 μmol / g, and an average particle size of 570 μm was prepared as raw material. Furthermore, this resin was obtained by partially saponifying polyethylene ester obtained using acetaldehyde as a chain transfer agent with sodium hydroxide as a catalyst. The above raw material was added to a heat treatment furnace (container), and air was circulated through 100 parts by weight of resin (raw material) at a rate of 3.0~9.0 parts by weight / hr·100 parts by weight while heat treatment was performed at a furnace speed of 5 rpm, a treatment temperature of 135°C, and a treatment time of 1200 minutes to obtain the PVA-based resin of Example 1.

[0081] The heat treatment furnace is a rotary kiln type device with an internal volume of 50L, equipped with a jacket and baffles for internal resin stirring. The heat treatment furnace is configured with its rotation axis horizontal. The device has gas inlets at both ends of the rotation axis, and air flows in a predetermined amount along the rotation axis inside the heat treatment furnace (container). Furthermore, steam is used as the heat source.

[0082] The saponification degree, viscosity of a 4% aqueous solution, total content of divalent and trivalent metal elements, yellowness of the aqueous solution, and absorbance (Abs 280, Abs 320, and Abs 370) of the obtained PVA-based resin were determined. These results and absorbance ratios (Abs 320 / Abs 370) are presented in Table 1.

[0083] (Manufacturing vinyl polymer particles) 1,390 g of an aqueous solution containing 0.94 g (1,000 ppm relative to the vinyl chloride monopolymer) of PVA-based resin was fed into a 5 L autoclave. Then, 0.6 g of cumyl peroxyneodecanate (PERCUMYL ND-R, Nippon Oil Manufacturing Co., Ltd.) and 0.9 g of tributyl peroxyneodecanate (PERBUTYL ND-R, Nippon Oil Manufacturing Co., Ltd.) were fed into the autoclave. Nitrogen gas was introduced until the pressure inside the autoclave reached 0.3 MPa for degassing, and this process was repeated six times to remove oxygen. Subsequently, 940 g of vinyl chloride was fed, and the contents of the autoclave were heated to 57°C, and suspension polymerization was initiated with stirring. The initial pressure inside the autoclave was 0.83 MPa. Polymerization was stopped 4 hours after the start of suspension polymerization, when the pressure inside the autoclave reached 0.70 MPa, and unreacted vinyl chloride was removed. Subsequently, the polymer slurry was removed and dried at 65°C overnight to obtain vinyl chloride polymer particles.

[0084] The average particle size and plasticizer absorption capacity of the obtained vinyl chloride polymer (PVC) particles were determined. The results of these determinations are presented in Table 1.

[0085] [Examples 2-5, Comparative Examples 1-2] Except for the raw materials and heat treatment conditions as described in Table 1, the procedures were performed in the same manner as in Example 1 to obtain the PVA-based resins of Examples 2-5 and Comparative Examples 1-2. The degree of saponification, viscosity of a 4% aqueous solution, total content of divalent and trivalent metal elements, yellowness of the aqueous solution, and absorbance (Abs 280, Abs 320, and Abs 370) of each obtained PVA-based resin were measured. These measurement results and absorbance ratios (Abs 320 / Abs 370) are presented in Table 1.

[0086] 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, only 1,200 ppm of the PVA-based resin of Comparative Example 2 was used relative to the vinyl chloride monomer. Otherwise, vinyl chloride polymer particles were obtained in the same manner as in Example 1. Moreover, when using the PVA-based resin of Comparative Example 2 and manufacturing vinyl chloride polymer particles under the same conditions as in the other examples, particle coarsening and scale formation occurred, which was significantly worse than 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 presented in Table 1.

[0087] [Table 1] Example 1 Example 2 Example 3 Example 4 Example 5 Comparative Example 1 Comparative Example 2 raw material degree of saponification mol% 72.4 71.9 72.0 71.8 72.1 71.8 72.4 4% aqueous solution viscosity mPa·s 6.2 6.6 6.6 6.6 6.6 6.6 5.5 Total content of divalent and trivalent metal elements µmol / g 0.009 0.010 0.010 0.010 0.010 0.010 0.010 Heat treatment conditions Processing temperature ℃ 135 137 139 137 140 136 124 Processing time min 1200 760 740 800 830 720 330 Gas (air) flow rate 100 parts per hr 3.0 - 9.0 3.0 - 9.0 3.0 - 9.0 3.0 - 9.0 3.0 - 9.0 <1.5 3.0 - 9.0 PVA-based resins degree of saponification mol% 72.4 71.9 72.0 71.8 72.1 72.1 72.4 4% aqueous solution viscosity mPa·s 7.1 6.3 6.3 6.3 6.8 6.3 5.5 Total content of divalent and trivalent metal elements µmol / g 0.009 0.010 0.010 0.010 0.010 0.010 0.010 Yellowness of the aqueous solution - 42.9 30.5 35.5 37.4 50.7 33.7 8.0 Abs 280 - 0.275 0.342 0.325 0.355 0.378 0.355 0.710 Abs 320 - 0.287 0.266 0.298 0.292 0.331 0.243 0.258 Abs 370 - 0.092 0.065 0.079 0.078 0.102 0.049 0.013 Abs 320 / Abs 370 - 3.1 4.1 4.1 3.7 3.2 5.0 19.8 Evaluation PVC average particle size µm 165 156 150 145 130 172 168 ※ PVC plasticizer absorption capacity % 27.9 28.6 29.0 28.7 27.1 27.7 32.6 ※ ※: Comparative Example 2 was evaluated at an addition amount of 1200 ppm due to its poor polymerization stability (other examples and comparative examples were evaluated at 1000 ppm).

[0088] As shown in Table 1, the PVA-based resins of Examples 1-5, with an absorbance Abs 320 of 0.09 or higher and an absorbance ratio (Abs 320 / Abs 370) of 4.9 or lower, can produce vinyl polymer particles with an average particle size of 165 μm or lower when used as dispersants for suspension polymerization, even at an addition amount of 1,000 ppm relative to vinyl chloride monomer. The vinyl polymer particles obtained by using the PVA-based resins of Examples 1-5 as dispersants for suspension polymerization also have a plasticizer absorption capacity of 25% or higher, which is a good value. Furthermore, a comparison between Examples 1-5 and Comparative Examples 1 and 2 shows that the effect of an absorbance ratio (Abs 320 / Abs 370) of 4.9 or lower is, among the various performance requirements for dispersants for suspension polymerization, the point at which the particle size of the obtained vinyl polymer particles can be reduced. Furthermore, the PVA-based resins in Examples 1-5 have a total content of divalent and trivalent metal elements of less than 30 μmol / g, which is a very low content.

[0089] Furthermore, as in Examples 1-5, it was confirmed that by circulating gas within the container at a specified flow rate while performing heat treatment (stirring and heating) at 90°C to 180°C for 350 to 1300 minutes, a PVA-based resin with an absorbance Abs 320 of 0.09 or higher and an absorbance ratio (Abs 320 / Abs 370) of 4.9 or lower can be obtained, even with a low total content of divalent and trivalent metal elements. Comparative Example 1 could not obtain a PVA-based resin with an absorbance Abs 320 of 0.09 or higher and an absorbance ratio (Abs 320 / Abs 370) of 4.9 or lower due to a low gas flow rate, and Comparative Example 2 could not obtain a PVA-based resin with an absorbance Abs 320 of 0.09 or higher and an absorbance ratio (Abs 320 / Abs 370) of 4.9 or lower due to a short processing time. [Potential for industrial application]

[0090] The PVA-based resin of this invention can be used as a dispersant during the suspension polymerization of ethylene-based compounds.

[0091] none

Claims

1. A polyvinyl alcohol-based resin, which substantially contains only polyvinyl alcohol in terms of polymer composition, and has an absorbance Abs320 of 10 mm and 320 nm when prepared as a 0.1% by mass aqueous solution, which is 0.09 or more, and the ratio of this absorbance Abs320 to the absorbance Abs370 of 10 mm and 370 nm when prepared as a 0.1% by mass aqueous solution (Abs320 / Abs370) is 4.9 or less, and the total content of divalent and trivalent metal elements is less than 30 μmol / g.

2. The polyvinyl alcohol resin of claim 1, wherein the ratio (Abs320 / Abs370) is 2.0 or higher.

3. The polyvinyl alcohol resin of claim 1 or 2, wherein the absorbance Abs320 is less than 0.

30.

4. The polyvinyl alcohol resin of claim 1 or 2 has a viscosity of 4.5 mPa·s or more and 8.0 mPa·s or less when it is made into a 4% by mass aqueous solution at 20°C.

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

6. A method for manufacturing ethylene-based polymer particles, comprising the step of suspending polymerizing an ethylene-based compound using a suspension polymerization dispersant as described in claim 5.

7. A method for manufacturing a polyvinyl alcohol-based resin, comprising the step of stirring and heating saponified polyvinyl ester in a container, wherein gas is circulated within the container while the stirring and heating is performed at a temperature of 90°C to 180°C for 350 minutes to 1,300 minutes, wherein the gas flow rate is 2 to 11 parts by mass per hour relative to 100 parts by mass of the saponified polyvinyl ester; wherein the gas is nitrogen, oxygen, or a mixture thereof.